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Photosystem II and photosynthetic oxidation of water: An overview
Philosophical Transactions B
Abstract
Conceptually, photosystem II, the oxygen-evolving enzyme, can be divided into two parts: the photochemical part and the catalytic part. The photochemical part contains the ultra-fast and ultra-efficient light-induced charge separation and stabilization steps that occur when light is absorbed by chlorophyll. The catalytic part, where water is oxidized, involves a cluster of Mn ions close to a redox-active tyrosine residue. Our current understanding of the catalytic mechanism is mainly based on spectroscopic studies. Here, we present an overview of the current state of knowledge of photosystem II, attempting to delineate the open questions and the directions of current research.
... This process relies on the same fundamentals of plant and algal photosynthesis, however leading to biohydrogen production instead of biomass accumulation (Das and Veziroǧlu 2001). As discussed in previous reviews (Ghirardi et al. 2014;Goussias et al. 2002), the biohydrogen production of green algae and cyanobacteria depends on the photosystem II PSII-dependent pathway (direct biophotolysis) or PSII-independent pathway (indirect biophotolysis). In these photosystems, the light absorbed by antenna pigments of photosynthetic microorganisms is transferred to pigment-protein complexes called reaction centers (Antal et al. 2012), where a photosynthetic electron transfer chain is generated (Goussias et al. 2002). ...
... As discussed in previous reviews (Ghirardi et al. 2014;Goussias et al. 2002), the biohydrogen production of green algae and cyanobacteria depends on the photosystem II PSII-dependent pathway (direct biophotolysis) or PSII-independent pathway (indirect biophotolysis). In these photosystems, the light absorbed by antenna pigments of photosynthetic microorganisms is transferred to pigment-protein complexes called reaction centers (Antal et al. 2012), where a photosynthetic electron transfer chain is generated (Goussias et al. 2002). The PSII photosystem is at the start of this chain and uses light energy to generate the strongly oxidizing cation radical P 680 , which catalyzes the water oxidation, thus providing the system with a source of electrons that is essentially unlimited (Goussias et al. 2002). ...
... In these photosystems, the light absorbed by antenna pigments of photosynthetic microorganisms is transferred to pigment-protein complexes called reaction centers (Antal et al. 2012), where a photosynthetic electron transfer chain is generated (Goussias et al. 2002). The PSII photosystem is at the start of this chain and uses light energy to generate the strongly oxidizing cation radical P 680 , which catalyzes the water oxidation, thus providing the system with a source of electrons that is essentially unlimited (Goussias et al. 2002). ...
Hydrogen is a promising green alternative to fossil fuel due to its low environmental impact and high energy density, which is nearly three times higher than current transportation fuels such as gasoline and diesel. Renewable means of hydrogen production can be achieved by several different microbial routes and a variety of phototrophic microorganisms are endowed with genes and proteins for biohydrogen production. In this context, research has been focused on improving photobioreactor performance for practical and commercial applications, specifically regarding the low yield associated with it compared to other biofuels. This chapter provides an overview of the different types of photosynthetic microorganisms involved in biohydrogen production, the substrates utilized, the various photobioreactor designs employed, and the role of genetic engineering in enhancing hydrogen production rates. Moreover, the hurdles and limitations encountered during hydrogen production are discussed, along with future perspectives for advancing biohydrogen production.
... In the electron transfer chain, it serves as a protein cofactor and acts as a catalyst. Mn is an important component of essential metalloenzymes and photosynthesis, where Mn complex mediates the dissociation of water molecules into four electrons, four hydrogen ions and one oxygen (Goussias et al. 2002;Allen et al. 2006;Nickelsen and Rengstl 2013). Mn is also a crucial part of various biological processes in all living organisms. ...
... It is well known that Mn plays an important role in photosystem as an imperative element of the Mn cluster structure of the photosystem II (PSII) oxygen-evolving complex (Goussias et al. 2002;Nickelsen and Rengstl 2013;Alejandro et al. 2020). However, excessive Mn can cause necrotic spots on plants and damage to leaves. ...
Melatonin (MT) can effectively improve the resistance of plants to various abiotic stresses. However, the effects of exogenous melatonin on the growth of tobacco plants under high manganese (Mn) stress condition are still largely unknown. In this study, growth inhibition, antioxidant activity, ion accumulation, as well as photoinhibition, in tobacco plants treated with high concentration of Mn and different concentrations of melatonin were investigated. Exogenous MT application significantly increased the resistance of tobacco plants to high Mn stress. Tobacco plants treated with various concentrations of MT showed less growth inhibition by high Mn stress, and accumulated less malondialdehyde (MDA) and hydrogen peroxide (H2O2), accompanied with a higher antioxidant enzyme activity and antioxidative gene expression. An altered ion accumulation, an increased metal related gene expression and a higher net photosynthetic rate (Pn) were also detected in tobacco plants treated with exogenous MT. Our observations indicate that exogenous MT application improved Mn tolerance in tobacco plants through the scavenging of reactive oxygen species (ROS) and the regulation of ion homeostasis.
... Manganese (Mn) is one of the essential trace elements for plants. Mn is an integral component of the Mn-cluster structure, directly involved in plant photosynthesis [1,2]. Mn also acts as the cofactors of multiple enzymes such as superoxide dismutase (SOD), catalase (CAT), phosphoenolpyruvate carboxylase (PEPC) and so on, which are involved in diverse metabolic pathways in plants [3,4]. ...
... Mn is an essential element for several metabolic pathways in plants [1][2][3][4]. On the other hand, plants easily suffer from the toxicity of excess Mn, especially growing on acidic soils [5][6][7][8][9]12]. ...
As an essential micronutrient, manganese (Mn) participates in diverse processes during plant growth and development. Excess accumulation of Mn in plants is toxic, and soybean (Glycine max) growth and production are severely limited by Mn toxicity. However, the molecular basis in adaptation to Mn toxicity for soybean remains largely unknown. In this study, RNA-seq analysis on soybean leaves was conducted, more than 44 million reads were generated, and a total of 38,022 expressed genes were identified. Compared to control, 2258 differentially expressed genes (DEGs), including 744 up-regulated and 1514 down-regulated ones, were obtained. Cellular process, cell part and binding function were the most enriched terms by GO analysis. Furthermore, 49 DEGs were identified in plant hormone signal transduction pathways by KEGG analysis. Among them, Mn toxicity up-regulated AHK, PIF, JAZ and TGA family DEGs might play important roles in the adaptation of soybean leaves to Mn toxicity.
Supplemental data for this article is available online at https://doi.org/10.1080/13102818.2021.1950566 .
... All the Mn application methods were effective in enhancing the grain yield of rice in both production systems ( Table 6). Application of Mn improves the number and rate of tiller emergence and seed set due to improved pollen germination and fertilization (Longnecker et al. 1990;Goussias et al. 2002) similar to this study. Moreover, Mn application enhances the photosynthetic rate and assimilates supply toward grains during grain development (Marschner 1995;Goussias et al. 2002). ...
... Application of Mn improves the number and rate of tiller emergence and seed set due to improved pollen germination and fertilization (Longnecker et al. 1990;Goussias et al. 2002) similar to this study. Moreover, Mn application enhances the photosynthetic rate and assimilates supply toward grains during grain development (Marschner 1995;Goussias et al. 2002). It is well documented that Mn deficiency causes poor another development, infertility of pollens and lower down the assimilate translocation and ultimately results in poor grain setting and decline yield (Longnecker et al. 1991). ...
Manganese (Mn) deficiency in human nutrition is widespread in the rice-wheat cropping system where cereal grains are the staple food. Agronomic biofortification is an innovative and pragmatic approach to tackle Mn malnutrition. A 2-year field study was conducted to investigate the comparative effect of different Mn application methods: (1) No Mn application , (2) Mn seed coating (2 g kg −1 seed), (3) hydro-priming, (4) Mn seed priming (0.1 M Mn solution), (5) soil application (0.5 kg ha −1), (6) foliar water spray, and (7) foliar Mn spray (0.02 M Mn solution) in improving the productivity, profitability, biofortification and Mn-use efficiencies of rice in puddled transplanted flooded rice (FR) and direct-seeded aerobic rice (AR) production systems. Under AR system, soil physical and biological properties, as well as nutrient dynamics were improved. Averaged across 2 years, improvement in total soil porosity (9%), soil organic carbon (12%), soil microbial biomass carbon (6%), soil microbial biomass nitrogen (3%), total nitrogen (13.6%), available phosphorus (10.98%), and exchangeable potassium (7.1%) were noted in AR system over FR system. Regardless of application methods, Mn nutrition substantially improved the yield and related traits and grain Mn concentration in both production systems. Averaged across 2 years, the increase in grain yield under different Mn application treatments was in the order: foliar application (22%) > seed priming (16%) > soil application (12%) > seed coating (12%). Grain Mn concentration was the highest with foliar application of Mn (27% over control) in both production systems. Moreover, the maximum net benefits and benefit-cost ratio were obtained through Mn-foliar application in both production systems. In conclusion, Mn application by either method improved the productivity, profitability, biofortification and Mn-use efficiencies under FR and AR systems; nevertheless, foliar Mn application performed better for all the studied traits.
... For example, Mn forms a four-atom cluster 48 that catalyzes water oxidation in the water-splitting apparatus of photosystem II (PSII) and as a 49 cofactor of the Mn superoxide dismutase (MnSOD), located mainly in mitochondria (Zouni et al. 50 2001; Goussias et al. 2002;Millaleo et al. 2010). Plants take up Mn 2+ from the rhizosphere through 51 root cells, releasing H + or low-molecular-weight organic acids to acidify the local environment and 52 reduce Mn-oxides into bioavailable Mn (Rengel and Marschner 2005). ...
Plant litter is a well-defined pool of organic matter in which the influence of Mn on decomposition (both decomposition rate, and the mix of compounds ultimately transferred to soil organic matter) has been clearly demonstrated in temperate forests. However, no similar study exists on grasslands, and the effect of foliar Mn versus soil-derived Mn on litter decomposition is poorly known. We used 5-month and 12-month field and 10-month laboratory experiments to evaluate organic-matter decomposition on the Kohala rainfall gradient in areas with different foliar and soil Mn abundances, and on which a single plant species dominates primary production and the litter pool. Chemical imaging analyses of decomposed litter revealed that Mn2+ oxidized to Mn3+ and Mn4+ on grass litter during decompositions—hallmarks of Mn-driven OM oxidation. However, these transformations and Mn abundance did not predict greater litter mass loss through decomposition. These observations demonstrate that the importance of Mn to an ecosystem’s C cycle does not rely solely on the metal’s abundance and availability.
... The tillers/plant is a factor that affects grain yield because it not only indicates how well a crop is established but also results in a greater number of grains, which raises the yield of wheat crops. Because of improved pollen germination and fertilization, Mn application increased the productive tillers and seed set (Goussias et al., 2002). Manal et al. (2010) reported an increased number of tillers through the Mn application. ...
Manganese (Mn) is an essential micronutrient in plants, and it is necessary for hydrolysis in photosystem II, chlorophyll biosynthesis, and also chloroplast breakdown. Limited Mn availability in light soil resulted in interveinal chlorosis, poor root development, and the development of fewer tillers, particularly staple cereals including wheat, while foliar Mn fertilizers were found efficient in improving crop yield as well as Mn use efficiency. In the above context, a study was conducted in consecutive two wheat growing seasons for screening of the most effective and economical Mn treatment for improving the yield and Mn uptake in wheat and to compare the relative effectiveness of MnCO3 against the recommended dose of MnSO4 for wheat. To fulfill the aims of the study, three manganese products, namely, 1) manganese carbonate MnCO3 (26% Mn w/w and 3.3% N w/w), 2) 0.5% MnSO4·H2O (30.5% Mn), and 3) Mn-EDTA solution (12% Mn), were used as experimental treatments. Treatments and their combinations were as follows: two levels of MnCO3 (26% Mn) @ 750 and 1,250 ml ha⁻¹ were applied at the two stages (i.e., 25–30 and 35–40 days after sowing) of wheat, and three sprays each of 0.5% MnSO4 (30.5% Mn) and Mn-EDTA (12% Mn) solution were applied in other plots. The 2-year study showed that Mn application significantly increased the plant height, productive tillers plant⁻¹, and 1,000 grain weight irrespective of fertilizer source. The results of MnSO4 for grain yield wheat as well as uptake of Mn were statistically at par with both levels (750 and 1,250 ml ha⁻¹) of MnCO3 with two sprays at two stages of wheat. However, the application of Mn in the form of 0.5% MnSO4·H2O (30.5% Mn) was found more economical than MnCO3, while the mobilization efficiency index (1.56) was found maximum when Mn was applied in MnCO3 with two sprays (750 and 1,250 ml ha⁻¹) in the two stages of wheat. Thus, the present study revealed that MnCO3 can be used as an alternative to MnSO4 to enhance the yield and Mn uptake of wheat.
... Arabidopsis NRAMP family members NRAMP3 and NRAMP4 have homology with yeast SMF1 and SMF2, and are involved in the Mn-containing oxygen-evolving complex (OEC). OEC catalyzes the water-splitting reaction that produces oxygen and provides electrons for the photosynthetic electron transport chain (Goussias, Boussac, & Rutherford, 2002;Nickelsen & Rengstl, 2013). Disruption of vacuole-localized NRAMP3 and NRAMP4 decreased PSII amount in chloroplasts, but did not affect MnSOD activity in mitochondria (Allen, Kropat, Tottey, Del Campo, & Merchant, 2007;Lanquar et al., 2010). ...
... Mn was the only element which concentration was systematically enhanced in leaves ( Figure 6B). This element is known to be involved in several processes related to photosynthesis (Goussias et al., 2002;Schmidt et al., 2016), ATP synthesis (Pfeffer et al., 1986;Ahanger et al., 2016), RuBP carboxylase reaction (Houtz et al., 1988;Bloom and Lancaster, 2018), and biosynthesis of fatty acid, acyl lipid, and proteins (Ness and Woolhouse, 1980;Millaleo et al., 2010;Tripathi et al., 2015). Moreover, Mn plays a role in antioxidant response under water stress via its requirement for superoxide dismutase (Sevilla et al., 1980;Bridges and Salin, 1981;Sandmann and Böger, 1983;Polle et al., 1992). ...
Introduction
Peas, as legume crops, could play a major role in the future of food security in the context of worldwide human nutrient deficiencies coupled with the growing need to reduce consumption of animal products. However, pea yields, in terms of quantity and quality (i.e. grain content), are both susceptible to climate change, and more specifically to water deficits, which nowadays occur more frequently during crop growth cycles and tend to last longer. The impact of soil water stress on plant development and plant growth is complex, as its impact varies depending on soil water availability (through the modulation of elements available in the soil), and by the plant’s ability to acclimate to continuous stress or to memorize previous stress events.
Method
To identify the strategies underlying these plant responses to water stress events, pea plants were grown in controlled conditions under optimal water treatment and different types of water stress; transient (during vegetative or reproductive periods), recurrent, and continuous (throughout the plant growth cycle). Traits related to water, carbon, and ionome uptake and uses were measured and allowed the identification typical plant strategies to cope with water stress.
Conclusion
Our results highlighted (i) the common responses to the three types of water stress in shoots, involving manganese (Mn) in particular, (ii) the potential implications of boron (B) for root architecture modification under continuous stress, and (iii) the establishment of an “ecophysiological imprint” in the root system via an increase in nodule numbers during the recovery period.
... Recent studies have emphasized the biologically important influence of manganese (Mn) in litter decomposition and terrestrial C cycling Trum et al., 2015;Kranabetter et al., 2021;Subedi et al., 2021). Manganese is an essential micronutrient for plants (Goussias et al., 2002;Hakala et al., 2006;Socha and Guerinot, 2014) that is made available in soil through the weathering of rocks and minerals, as well as through atmospheric deposition from industrial activities (Herndon et al., 2011). The amount of bioavailable Mn (Mn 2+ ) in soil is influenced by factors including soil pH, aeration, soil moisture, and soil organic carbon (SOC) content (Rengel, 2014). ...
... S 1 is the dominant state in the dark, whereas S 2 and S 3 are metastable states that decay back to S 1 in a few minutes at room temperature (Forbush et al. 1971;Styring and Rutherford 1988). S 4 is a transient state that spontaneously progresses to S 0 concomitant with the release of O 2 (Goussias et al. 2002;Haumann et al. 2005). ...
Photosystem II (PSII) has a number of hydrogen-bonding networks connecting the manganese cluster with the lumenal bulk solution. The structure of PSII from Thermosynechococcus vulcanus (T. vulcanus) showed that D1-R323, D1-N322, D1-D319 and D1-H304 are involved in one of these hydrogen-bonding networks located in the interfaces between the D1, CP43 and PsbV subunits. In order to investigate the functions of these residues in PSII, we generated seven site-directed mutants D1-R323A, D1-R323E, D1-N322R, D1-D319L, D1-D319R, D1-D319Y and D1-H304D of T. vulcanus and examined the effects of these mutations on the growth and functions of the oxygen-evolving complex. The photoautotrophic growth rates of these mutants were similar to that of the wild type, whereas the oxygen-evolving activities of the mutant cells were decreased differently to 63–91% of that of the wild type at pH 6.5. The mutant cells showed a higher relative activity at higher pH region than the wild type cells, suggesting that higher pH facilitated proton egress in the mutants. In addition, oxygen evolution of thylakoid membranes isolated from these mutants showed an apparent decrease compared to that of the cells. This is due to the loss of PsbU during purification of the thylakoid membranes. Moreover, PsbV was also lost in the PSII core complexes purified from the mutants. Taken together, D1-R323, D1-N322, D1-D319 and D1-H304 are vital for the optimal function of oxygen evolution and functional binding of extrinsic proteins to PSII core, and may be involved in the proton egress pathway mediated by YZ.
... Iron (Fe) and manganese (Mn) are essential metals for all living organisms [1]. In plants, both metals have important roles in fundamental processes such as respiration and photosynthesis, among others [2,3]. In addition, Fe is involved in structural processes, participates in Fe-sulfur clusters, and acts as a cofactor in heme and other Fe-binding sites of proteins [4,5]. ...
Iron (Fe) and manganese (Mn) are two essential elements for plants that compete for the same uptake transporters and show conflicting interactions at the regulatory level. In order to understand the differential response to both metal deficiencies in plants, two proteomic techniques (two-dimensional gel electrophoresis and label-free shotgun) were used to study the proteome profiles of roots from tomato plants grown under Fe or Mn deficiency. A total of 119 proteins changing in relative abundance were confidently quantified and identified, including 35 and 91 in the cases of Fe deficiency and Mn deficiency, respectively, with 7 of them changing in both deficiencies. The identified proteins were categorized according to function, and GO-enrichment analysis was performed. Data showed that both deficiencies provoked a common and intense cell wall remodelling. However, the response observed for Fe and Mn deficiencies differed greatly in relation to oxidative stress, coumarin production, protein, nitrogen, and energy metabolism.
... PSII catalyzes light-induced water oxidation in oxygenic photosynthesis to convert light into chemical energy [56,57]. However, PSII is the primary target of photoinhibition [58,59]. ...
Zoysia japonica is a warm-season turfgrass with a good tolerance and minimal maintenance requirements. However, its use in Northern China is limited due to massive chlorophyll loss in early fall, which is the main factor affecting its distribution and utilization. Although ethephon treatment at specific concentrations has reportedly improved stress tolerance and extended the green period in turfgrass, the potential mechanisms underlying this effect are not clear. In this study, we evaluated and analyzed chlorophyll changes in the physiology and transcriptome of Z. japonica plants in response to cold stress (4 °C) with and without ethephon pretreatment. Based on the transcriptome and chlorophyll content analysis, ethephon pretreatment increased the leaf chlorophyll content under cold stress by affecting two processes: the stimulation of chlorophyll synthesis by upregulating ZjMgCH2 and ZjMgCH3 expression; and the suppression of chlorophyll degradation by downregulating ZjPAO, ZjRCCR, and ZjSGR expression. Furthermore, ethephon pretreatment increased the ratio of chlorophyll a to chlorophyll b in the leaves under cold stress, most likely by suppressing the conversion of chlorophyll a to chlorophyll b due to decreased chlorophyll b synthesis via downregulation of ZjCAO. Additionally, the inhibition of chlorophyll b synthesis may result in energy redistribution between photosystem II and photosystem I.
... It plays a significant role to readily start and completes the process of germination, which can lead to uniform crop standing even under adverse conditions [79] and leads to improved seed setting and grain weight [33]. Manganese nutrition improves the number of tillers and seeds set due to better pollen germination and fertilization [80]. After osmopriming, foliar-applied Mn enhanced the wheat yield for both WTs, because it is an efficient method of application owing to various reasons, including lower application rates, uniform application and distribution, and rapid plant response [81]. ...
Manganese is an important essential micronutrient, and its deficiency causes latent health issues in humans. Agronomic biofortification can promisingly improve the plant nutrient concentration without changing the genetic makeup of plants. This study was designed to assess the best method of Mn application to enhance productivity and grain Mn contents under conventional till-age (CT) and no tillage (NT) systems. Manganese was delivered through seed coating (250-mg kg −1 seed), osmopriming (0.1-M Mn solution), soil application (1 kg ha −1), and foliar application (0.25-M Mn solution). A general control with no seed Mn application was included, whereas hydropriming and water spray were used as positive control treatments for Mn seed priming and Mn foliar spray, respectively. No tillage had a higher total soil porosity (9%), soil organic carbon (16%), soil microbial biomass carbon (4%), nitrogen (2%), and soil nutrients in the CT system. Manganese nutrition through various methods significantly enhanced the yield, grain biofortification, and net benefits for CT and NT systems. Averaged across two years, the maximum improvement in grain productivity was recorded with osmopriming (28%) followed by foliar application (26%). The highest grain Mn concentration (29% over no application) was recorded with Mn foliar applications under both tillage systems. Moreover, the highest economic returns and marginal net benefits were recorded with osmopriming. To improve the wheat production, profitability, and grain Mn concentration, Mn application through priming and foliar application may be opted.
... 7 limiting the photosynthetic rate and efficiency. Manganese is essential for chlorophyll synthesis and photosynthesis, 8 nitrogen and carbohydrate metabolism, oxidation, and reduction and activation of some enzymes. However, a high level of Mn seems to damage the photosynthetic apparatus. ...
Salinity is one of the most important environmental factors which have a significant effect on the growth and fertility. Manganese is an essential element that plays a key role as a nutrient in many plant metabolic processes. The present study was carried out to investigate the effects of manganese (1ppm) on Yaghoti and Ghara uzum - two cultivars of grapevine - which were planted and grown under salt stress (0, 50 and 100mmol L-1 NaCl) using greenhouse and hydroponics methods. Results on saline concentration showed led to a significant decrease in length, fresh and dry weight, photosynthesis pigments content, K+ and nitrate content, and K+/Na+ ratio of the plants. It also caused a significant increase in Na+ and Cl- ion concentration and soluble sugar content in shoots and roots of both cultivars. Ghara uzum was more sensitive than Yaghoti to salinity stress. Applying manganese to the medium, resulted in low level of salinity toxicity and sodium ion accumulation in the shoots for Ghara uzum cultivar. For Yaghoti, however, the application of manganese did not reduce the sodium ion accumulation in the shoots, but increased the salinity toxicity. Our findings suggest that Ghara uzum and Yaghoti cultivars had different responses to salinity, manganese and their interaction.
... 32 Mn 2+ is an essential element and micronutrient in 33 Mn 2+ also plays an important role in the photosystem(II) for water splitting and oxygen evolution, a fundamental part of plant survival. 34,35 The sharp and intense line at g = 2.0 may attribute to the aromatic organic radical that is related to the semiquinone radical. [36][37][38] Mn 2+ with 3d 5 ( 6 S 5/2 ) configuration exists in high spin state (S = 5/2). ...
... Manganese, an essential micronutrient, is a component of the oxygen evolving complex of Photosystem II (Nickelsen & Rengstl, 2013), is a constituent of superoxide dismutase (MnSOD), and is an activator of numerous enzymes involved in several metabolic processes (Marschner, 1995), especially in photosynthesis (Goussias, Boussac, & Rutherford, 2002;Hatch & Kagawa, 1974;Millaleo, Reyes-Díaz, Ivanov, Mora, & Alberdi, 2010). ...
Manganese is an essential nutrient and plays key roles in photosynthetic processes, including in NAD‐malic enzyme (NAD‐ME)‐type C4 plants as an activator of NAD‐ME. However, little is known about the Mn requirements of switchgrass (Panicum virgatum L.). To study Mn requirements for optimum growth and biomass production, a lowland (‘Alamo’) and an upland (‘Cave‐in‐Rock’) switchgrass ecotype were grown with either washed sand, vermiculite, or perlite and fertilized with nutrient solutions ranging in [Mn] from 0 to 200 μM. In the perlite experiment, pearl millet [Pennisetum glaucum (L.) R. Br. ‘KGraze’] was also grown. Shoot [Mn] was highly responsive to increasing Mn in the nutrient solution. When grown in washed sand and vermiculite, tissue [Mn] remained above those normally considered deficient, even in the 0 μM Mn treatment, and no Mn treatment effects on biomass production were found. In perlite, end‐of‐season shoot [Mn] were <5 mg kg⁻¹ in all entries when no Mn was supplied, and a decrease in biomass production compared with 10–25 μM Mn treatments was observed for Alamo and KGraze, but not for Cave‐in‐Rock. Relative chlorophyll contents of switchgrass were lower in the 0 μM Mn treatment than in other treatments late in the season, but in KGraze, they were low early in the season and increased throughout the season, resulting in less pronounced (but still significant) differences at late stages. Overall, results indicated that switchgrass and pearl millet respond differently to low Mn availability and that even low levels of shoot tissue [Mn] allow switchgrass to maintain biomass production.
... Like other micronutrients, roots absorb Mn and distribute it throughout the plant to sink organelles (plastids and mitochondria) and storage sites (vacuole). It is integral for most photosynthetic organisms as a component of the oxygen-evolving complex in photosystem II, serving as an enzyme cofactor in the water-splitting reaction for producing oxygen and providing electrons to the photosynthetic electron transport chain [15,16]. Mn can, in some cases, be replaced by other metals as a cofactor, typically magnesium (Mg) [17,18]. ...
Symbioses with soil microorganisms are central in shaping the diversity and productivity of land plants and provide protection against a diversity of stresses, including metal toxicity. Arbuscular mycorrhizal fungi (AMF) can form extensive extraradical mycelial networks (ERM), which are very efficient in colonizing a new host. We quantified the responses of transcriptomes of wheat and one AMF partner, Rhizoglomus irregulare, to soil disturbance (Undisturbed vs. Disturbed) and to two different preceding mycotrophic species (Ornithopus compressus and Lolium rigidum). Soil disturbance and preceding plant species engender different AMF communities in wheat roots, resulting in a differential tolerance to soil manganese (Mn) toxicity. Soil disturbance negatively impacted wheat growth under manganese toxicity, probably due to the disruption of the ERM, and activated a large number of stress and starvation-related genes. The O. compressus treatment, which induces a greater Mn protection in wheat than L. rigidum, activated processes related to cellular division and growth, and very few related to stress. The L. rigidum treatment mostly induced genes that were related to oxidative stress, disease protection, and metal ion binding. R. irregulare cell division and molecular exchange between nucleus and cytoplasm were increased by O. compressus. These findings are highly relevant for sustainable agricultural systems, when considering a fit-for-purpose symbiosis.
... Mn is involved in a variety of metabolic processes, including photosynthesis, respiration, fatty acid and protein synthesis, as well as enzyme activation. For example, Mn is an indispensable constitutive element in the Mn cluster structure of the oxygen-evolving complex in photosystem II (PSII) that participates in the water-splitting process, providing necessary electrons for photosynthesis [9,10]. Mn acts as an important cofactor of various enzymes, including superoxide dismutase (MnSOD), catalase (MnCAT), decarboxylases of the tricarboxylic acid (TCA) cycle, and RNA polymerases [8,11]. ...
Manganese (Mn) is an essential element for plant growth due to its participation in a series of physiological and metabolic processes. Mn is also considered a heavy metal that causes phytotoxicity when present in excess, disrupting photosynthesis and enzyme activity in plants. Thus, Mn toxicity is a major constraint limiting plant growth and production, especially in acid soils. To cope with Mn toxicity, plants have evolved a wide range of adaptive strategies to improve their growth under this stress. Mn tolerance mechanisms include activation of the antioxidant system, regulation of Mn uptake and homeostasis, and compartmentalization of Mn into subcellular compartments (e.g., vacuoles, endoplasmic reticulum, Golgi apparatus, and cell walls). In this regard, numerous genes are involved in specific pathways controlling Mn detoxification. Here, we summarize the recent advances in the mechanisms of Mn toxicity tolerance in plants and highlight the roles of genes responsible for Mn uptake, translocation, and distribution, contributing to Mn detoxification. We hope this review will provide a comprehensive understanding of the adaptive strategies of plants to Mn toxicity through gene regulation, which will aid in breeding crop varieties with Mn tolerance via genetic improvement approaches, enhancing the yield and quality of crops.
... Phosphorus (P) and manganese (Mn) are essential nutrients for plant growth and development (Millaleo et al., 2010), participating both in important metabolic processes. As an essential micronutrient, Mn takes place actively in photosynthesis by forming part of the structure of proteins in the oxygen-evolving complex, participating in the H 2 O photolysis, electron transport and also as an enzyme antioxidant-cofactor (Goussias et al., 2002;Millaleo et al., 2010). Deficiency of Mn in plants affects the water-splitting systems of photosystem II (PSII), whereas Mn excess can lead to damages of the photosynthetic machinery, specifically PSII subunits (Millaleo et al., 2010). ...
We evaluated whether phosphorus (P) ameliorates manganese (Mn) excess harmful effects on photosynthetic performance, growth, oxidative stress, and antioxidants in ryegrass. Two perennial ryegrass genotypes, Banquet-II as Mn-resistant and One-50 as Mn-sensitive genotype, were growth under hydroponic conditions subjected to increased P (25, 50, 100, 200 and 400 μM), excess (750 μM) and sufficient Mn (2.4 μM) for 15 days. Growth rate, lipid peroxidation (LP), enzymatic and non-enzymatic antioxidants, photosynthetic parameters, and pigments were determined. Significant reduction of photosynthesis and growth in One-50 was observed under Mn-excess combined with low and adequate P, recovering under greater P-doses. The P concentration of both genotypes was enhanced towards increased P-supply, regardless of Mn treatments. Shoots Mn-concentration remained constant in both genotypes under Mn-excess, independently of P-levels; meanwhile, Banquet-II roots Mn-concentration increased 23% by P-supply. Furthermore, Banquet-II roots showed higher superoxide dismutase (SOD) activity than One-50, which increased towards the highest P dose under sufficient and excess of Mn. A high dose of phosphorus amendment alleviated Mn-toxicity in Mn-sensitive genotype (One-50). Besides, in the Mn-resistant genotype, enhanced plant performance is highlighted, explained by a high Mn-accumulation in roots and increased SOD activity, decreasing Mn translocation to shoots and therefore protecting the photosynthetic apparatus.
... For example, Mn participated in the watersplitting system of photosystem II (PSII) and provided electrons necessary for photosynthetic electron transport. In addition, a group of four Mn atoms (Mn cluster) was associated with the oxygen-evolving complex (OEC) bound to the reaction center protein (D1) of PSII in water photolysis ( Goussias et al. 2002). Mn also intervened in activating enzyme-catalyzed reactions, including phosphorylation, decarboxylation, reduction, and hydrolysis reaction. ...
Manganese (Mn) is an essential microelement in cottonseeds, which is usually determined by the techniques relied on hazardous reagents and complex pretreatment procedures. Therefore a rapid, low-cost, and reagent-free analytical way is demanded to substitute the traditional analytical method. The Mn content in cottonseed meal was investigated by near-infrared spectroscopy (NIRS) and chemometrics techniques. Standard normal variate (SNV) combined with first derivatives (FD) was the optimal spectra pre-treatment method. Monte Carlo uninformative variable elimination (MCUVE) and successive projections algorithm method (SPA) were employed to extract the informative variables from the full NIR spectra. The linear and nonlinear calibration models for cottonseed Mn content were developed. Finally, the optimal model for cottonseed Mn content was obtained by MCUVE-SPA-LSSVM, with root mean squares error of prediction (RMSEP) of 1.994 6, coefficient of determination (R2) of 0.949 3, and the residual predictive deviation (RPD) of 4.370 5, respectively. The MCUVE-SPA-LSSVM model is accuracy enough to measure the Mn content in cottonseed meal, which can be used as an alternative way to substitute for traditional analytical method.
... PS II is a multi-subunit protein found in the photosynthetic membranes of plants, algae, and cyanobacteria (Barber, 2002). It utilizes light-induced electron transfer and water-splitting reactions to produce protons, electrons, and molecular oxygen (Goussias et al., 2002). The domain contains 17 features, seven of which are binding sites and 11 are interfaces (Kawakami et al., 2009). ...
... Manganese (Mn) is an essential plant micro-nutrient participating in several metabolic pathways, including photosynthesis [1], and as a cofactor of several enzymes [2]. However, when accumulated in higher quantity, Mn can have phytotoxic effects [3]. ...
Manganese (Mn) is an essential micro-nutrient for plants, but flooded rice fields can accumulate high levels of Mn²⁺ leading to Mn toxicity. Here, we present a genome-wide association study (GWAS) to identify candidate loci conferring Mn toxicity tolerance in rice (Oryza sativa L.). A diversity panel of 288 genotypes was grown in hydroponic solutions in a greenhouse under optimal and toxic Mn concentrations. We applied a Mn toxicity treatment (5 ppm Mn²⁺, 3 weeks) at twelve days after transplanting. Mn toxicity caused moderate damage in rice in terms of biomass loss and symptom formation despite extremely high shoot Mn concentrations ranging from 2.4 to 17.4 mg g⁻¹. The tropical japonica subpopulation was more sensitive to Mn toxicity than other subpopulations. Leaf damage symptoms were significantly correlated with Mn uptake into shoots. Association mapping was conducted for seven traits using 416741 single nucleotide polymorphism (SNP) markers using a mixed linear model, and detected six significant associations for the traits shoot manganese concentration and relative shoot length. Candidate regions contained genes coding for a heavy metal transporter, peroxidase precursor and Mn²⁺ ion binding proteins. The significant marker SNP-2.22465867 caused an amino acid change in a gene (LOC_Os02g37170) with unknown function. This study demonstrated significant natural variation in rice for Mn toxicity tolerance and the possibility of using GWAS to unravel genetic factors responsible for such complex traits.
... During the second year of the study, maximum 123 water productivity was observed with Mn foliar application, which was followed by Mn seed priming in DSAR at the both experimental sites (Table 3). Deficiency of Mn reduces the number and rate of tiller emergence (Longnecker et al. 1990), seed setting due to decrease in pollen germination and fertilization (Sharma et al. 1991), reduction in rate of photosynthesis (Goussias et al. 2002) and decrease in assimilate supply during the grain development (Longnecker et al. 1990;Marschner 1995). In fact, there was Mn deficiency at both experimental sites, which suppressed the tillering, fertilization and grain development resulting in low paddy yield. ...
... Zn is an important component of many enzymes, and a structural stabilizer of proteins and plant membranes [12]. Mn is an active component of the water-splitting system of photosystem II, which provides the electrons necessary for photosynthesis [13]. In addition, Mn plays an important role in the biosynthesis of secondary metabolites such as flavonoids and lignin [14]. ...
The control of micronutrient application in cucumber cultivation has great importance as they participate in many functions of metabolism. In addition, micronutrient application efficiency is fundamental to avoid periods of overconsumption or deficits in the crop. To determine micronutrient accumulation using a dynamic model, two cycles of Vitaly and Luxell cucumber crops were grown. During the development of the crop, micronutrient content (Fe, B, Mn, Cu, and Zn) in the different organs of the cucumber plant was quantified. The model dynamically simulated the accumulation of biomass and micronutrients using climatic variables recorded inside the greenhouse as inputs. It was found that a decrease in photosynthetically active radiation and temperature significantly diminished the accumulation of biomass by the cucumber plants. On the other hand, the results demonstrated that the model efficiently simulated both the accumulation of biomass and micronutrients in a cucumber crop. The efficiency evaluation showed values higher than R 2 > 0.95. This dynamic model can be useful to define adequate strategies for the management of cucumber cultivation in greenhouses as well as the application of micronutrients.
... It is of particular importance for most photosynthetic organisms as an indispensable component in the oxygen-evolving complex of the PSII, where a cluster of Mn atoms acts as the catalytic center for light-induced water oxidation. In addition, Mn is required as a cofactor of many enzymes, such as the Mn-dependent superoxide dismutase (MnSOD), a principal antioxidant enzyme in mitochondria (Goussias et al. 2002;Morgan et al. 2008;Holley et al. 2011). Manganese deficiency occurs in plants grown on calcareous or alkaline soils that favor Mn oxidation and immobilization of Mn 2+ . ...
Plants require an adequate balance of mineral nutrients in each stage of development to achieve maximum yield. Deficiencies of mineral nutrients are common in crops worldwide. To solve this problem in modern agriculture, the fertilizer applications are necessary, but this practice may be associated with undesirable environmental impacts as well as the high cost of fertilizers. However, improving nutrient use efficiency (NUE) via genetic manipulation may result in increased plant capacity to capture and utilize nutrients. In this chapter, we presented the advances made through genetic engineering and molecular strategies in a range of plant species aimed at enhancing uptake, translocation, and remobilization of nutrients as a sustainable way to increase crop productivity and quality.
... The dichotomy we invoke is plausible because the physiological role of each nutrient, while somewhat versatile, shows signs of our metabolic categorisation. Manganese, for example, is a crucial anabolic catalyst because four of its ions form the centre of the oxygen-evolving complex (OEC), which is the site of water oxidation in Photosystem II (PSII), the first protein complex in photosynthesis [79,80,81]. The exact structure of the OEC remains uncertain, but there is little doubt that Mn ions are central to it [77,82,83]. ...
The causes of the worldwide problem of encroachment of woody plants into grassy vegetation are elusive. The effects of soil nutrients on competition between herbaceous and woody plants in various landscapes are particularly poorly understood. A long-term experiment of 60 plots in a South African savanna, comprising annual applications of ammonium sulphate (146–1166 kg ha⁻¹ yr⁻¹) and superphosphate (233–466 kg ha⁻¹ yr⁻¹) over three decades, and subsequent passive protection over another three decades, during which indigenous trees encroached on different plots to extremely variable degrees, provided an opportunity to investigate relationships between soil properties and woody encroachment. All topsoils were analysed for pH, acidity, EC, water-dispersible clay, Na, Mg, K, Ca, P, S, C, N, NH4, NO3, B, Mn, Cu and Zn. Applications of ammonium sulphate (AS), but not superphosphate (SP), greatly constrained tree abundance relative to control plots. Differences between control plots and plots that had received maximal AS application were particularly marked (16.3 ± 5.7 versus 1.2 ± 0.8 trees per plot). Soil properties most affected by AS applications included pH (H2O) (control to maximal AS application: 6.4 ± 0.1 to 5.1 ± 0.2), pH (KCl) (5.5 ± 0.2 to 4.0 ± 0.1), acidity (0.7 ± 0.1 to 2.6 ± 0.3 cmol kg⁻¹), acid saturation (8 ± 2 to 40 ± 5%), Mg (386 ± 25 to 143 ± 15 mg kg⁻¹), Ca (1022 ± 180 to 322 ± 14 mg kg⁻¹), Mn (314 ± 11 to 118 ± 9 mg kg⁻¹), Cu (3.6 ± 0.3 to 2.3 ± 0.2 mg kg⁻¹) and Zn (6.6 ± 0.4 to 3.7 ± 0.4 mg kg⁻¹). Magnesium, B, Mn and Cu were identified using principal component analysis, boundary line analysis and Kruskal-Wallis rank sum tests as the nutrients most likely to be affecting tree abundance. The ratio Mn/Cu was most related to tree abundance across the experiment, supporting the hypothesis that competition between herbaceous and woody plants depends on the availability of anabolic relative to catabolic nutrients. These findings, based on more than six decades of experimentation, may have global significance for the theoretical understanding of changes in vegetation structure and thus the practical control of invasive woody plants.
... Manganese plays a role in the charge accumulating process in the active site in the water-splitting system of photosystem II (PSII) (Goussias et al., 2002) and acts as a cofactor activating many different enzymes (Marschner, 1995). ...
Low plant-availability of iron (Fe), zinc (Zn) and manganese (Mn) leads to micronutrient deficiency, causing significant yield reductions of crops throughout the world, especially in calcareous soils. This study was performed in order to evaluate the efficiency of xylem sap analysis in the determination of Fe, Zn and Mn availability in plants (Cucumis sativus L.) affected by calcium carbonate (CaCO3) levels. A soil with six levels of CaCO3 (0−10% DW) was used. We performed a combination approach, including analysis of the soil mobility of micronutrients using different extractants (water, DTPA-TEA and ammonium acetate), as well as xylem and shoot elemental analysis. Generally, application of CaCO3 resulted in a pH increase of the bulk soil of 1.4−2.2 pH units; extractability of all micronutrients was significantly decreased 1.4−4.2 times, irrespective of the extracting solution. Xylem sap Fe, Zn and Mn concentrations were significantly correlated with the respective concentrations in the soil extracting solutions. By contrast, only shoot concentrations of Zn and Mn, but not of Fe, were linearly correlated with their extractable forms. With electrothermal atomic absorption spectrometry, changes in xylem sap concentrations of micronutrients were detected without preliminary mineralization of plant material, in contrast to shoot analysis. Our results demonstrate that xylem sap analysis offers the advantages of a simple characterization of multi-microelement availability in plants under CaCO3 stress.
... During the second year of the study, maximum water productivity was observed with Mn foliar application, which was followed by Mn seed priming in DSAR at the both experimental sites (Table 3). Deficiency of Mn reduces the number and rate of tiller emergence (Longnecker et al. 1990), seed setting due to decrease in pollen germination and fertilization (Sharma et al. 1991), reduction in rate of photosynthesis (Goussias et al. 2002) and decrease in assimilate supply during the grain development (Longnecker et al. 1990;Marschner 1995). In fact, there was Mn deficiency at both experimental sites, which suppressed the tillering, fertilization and grain development resulting in low paddy yield. ...
Manganese (Mn) deficiency is prevalent in rice-growing regions resulting in poor paddy yield and human health. In this study, role of Mn, applied through various methods, in improving the productivity and grain biofortification of fine grain aromatic rice was evaluated. Manganese was delivered as soil application (SA) (0.5 kg ha−1), foliar spray (FA) (0.02 M Mn), seed priming (SP) (0.1 M Mn) and seed coating (SC) (2 g Mn kg−1 seed) in conventional (puddled transplanted flooded rice) and conservation (direct seeded aerobic rice) production systems at two different sites (Faisalabad, Sheikhupura) in Punjab, Pakistan. Manganese application, through either method, improved the grain yield and grain Mn contents of fine grain aromatic rice grown in both production systems at both sites. However, Mn application as SC and FA was the most beneficial and cost effective in improving the productivity and grain biofortification in this regard. Overall, order of improvement in grain yield was SC (3.85 t ha−1) > FA (3.72 t ha−1) > SP (3.61 t ha−1) > SA (3.36 t ha−1). Maximum net benefits and benefit–cost ratio were obtained through Mn SC in flooded field at Faisalabad, which was followed by Mn SP in direct seeded aerobic rice at the same site. However, maximum marginal rate of return was noted with Mn SC in direct seeded aerobic rice at both sites. In crux, Mn nutrition improved the productivity and grain biofortification of fine grain aromatic rice grown in both conventional and conservation production systems. However, Mn application as seed treatment (SC or SP) was the most cost effective and economical.
... Mn presence is indispensable as a component of the oxygen-evolving complex in photosystem II (PSII). It catalyzes the watersplitting reaction which produces oxygen and provides electrons for the photosynthetic electron transport chain (Goussias et al. 2002;Nickelsen and Rengstl 2013). Moreover, Mn is required for carbohydrate and lipid biosynthesis in plants and acts as a direct cofactor of a variety of enzymes, among others in Mn superoxide dismutase (MnSOD), a principal antioxidant enzyme in cellular redox reactions (Marschner 2012). ...
Effects related to the adaptation of wheat to Mn excess were determined by structural and biochemical characterization of chloroplasts obtained from three-leaf seedlings of Mn-treated wheat. Chloroplasts were isolated from two wheat genotypes: sensitive (Raweta) and tolerant (Parabola) cultivated in hydroponic conditions in Hoagland nutrient supplemented with 0 (control), 5, 10, and 20 mmol dm⁻³ MnSO4. Microscopic observations of the chloroplast structure revealed differences in the size and starch presence between both objects. Changes indicating the stresogenic influence of Mn on Raweta seedlings appeared already at the Mn dose of 10 mmol dm⁻³, whereas on Parabola, only at 20 mmol dm⁻³. Biochemical analysis indicated the differences in starch content. Results of measurements of lipid peroxidation, polarity, and electrokinetic potential of chloroplasts point to Mn-stimulated modifications of chloroplast membranes which occurred to be larger for Raweta. The activation of antioxidative enzymes (SOD and POX) shows that ROS are generated under Mn-excess conditions. The content of Mn and Cu, Fe, Mo, and Zn (microelements) as well as Ca, Mg, K, P, and S (macroelements) in chloroplasts was determined by mass (ICP MS) and plasma optic emission (ICP OES) spectrometry. Raweta accumulated greater amount of Mn in comparison with Parabola at all Mn doses in media. Increased concentration of Mn was accompanied with a decrease of uptake other investigated elements (except for K).
... Manganese is a key element in restoring the chlorophyll molecule structure (Sarwar et al., 2010). In addition, Mn plays key role in the activity of enzymes that mediate the catalysis of the water-splitting reaction to produce oxygen and to provide electrons for the photosynthetic electron transport chain (Goussias et al., 2002; Nickelsen and Rengstl, 2013), and its deficiency directly affects the photosynthetic rate. Therefore, it is likely that the reduction of Mn concentrations observed in this study may be an additional factor inducing chlorosis symptoms and reducing plant growth. ...
A hydroponic greenhouse study was carried out to evaluate the effects of increasing cadmium (Cd) concentration on plant growth, mineral nutrition and Cd distribution of H-250 sunflower genotype. Exposure to increasing Cd concentrations reduced plant biomass by 40, 34, 47 and 42% of the total, leaves, stem and roots dry weights as compared to the control. Regardless of the treatment most of Cd uptake by the genotype was allocated in the root, followed by leaf and stem. The higher bioconcentration factors values in both above ground and underground plant tissues and low transfer factor value indicated that this genotype may be an alternative for use in phytostabilization programs. The results also showed that increasing Cd concentration disrupted plant homeostasis as it increased the concentration of some nutrients and had adverse effect on others, impacting plant growth. In this context, the results suggest that the low magnesium, iron and manganese concentrations in the leaves were the main cause for plant biomass reduction and leaf chlorosis and necrosis, as each one of these elements plays a key role on the chlorophyll molecule and on photosynthesis process.
... Ca 2+ is an essential cofactor in PSII-driven oxygenic photosynthesis and mediates the initial assembly of the PSII/oxygen-evolving complex and the repair of PSII reaction centers that have been damaged by photoinhibition ( Boussac et al., 1989;Mattoo et al., 1989;Krieger and Weis, 1993). Manganese (Mn) is also essential for most photosynthetic organisms as a component of the oxygen-evolving complex in PSII, which catalyzes the water-splitting reaction to produce oxygen and provides electrons for the photosynthetic electron transport chain ( Goussias et al., 2002;Schansker et al., 2002;Nickelsen and Rengstl, 2013). The evidence provided in Figures 5-7 suggested CCHA1 may facilitate uptake of Ca 2+ and Mn 2+ by chloroplasts and regulate stromal Ca 2+ and Mn 2+ levels, thus affecting the assembly of the PSII/oxygen-evolving complex. ...
Calcium is important for chloroplasts, including photosynthetic and non-photosynthetic functions. Multiple Ca(2+)/H(+) transporters and channels have been described and studied in the plasma membrane and organelle membranes of plant cells; however, the molecular identity and physiological roles of chloroplast Ca(2+)/H(+) antiporters have remained unknown. Here we identified a potential Ca(2+)/H(+) antiporter CCHA1 in Arabidopsis thaliana. CCHA1 localizes to the chloroplast and the ccha1 mutants showed pale green leaves and severely stunted growth along with impaired Photosystem II (PSII) function. The levels of the PSII core subunits and the oxygen-evolving complex decreased in the ccha1 mutants, compared with wild type. In high Ca(2+) concentrations, Arabidopsis CCHA1 partially rescued the growth defect of the yeast gdt1Δ null mutant, which could be defective in a Ca(2+)/H(+) antiporter. The ccha1 mutant also showed significant sensitivity to high concentrations of CaCl2 and MnCl2, as well as variation in pH. Based on these results, we propose that CCHA1 could be a putative chloroplast-localized Ca(2+)/H(+) antiporter that have critical functions in the regulation of PSII and in chloroplast Ca(2+) and pH homeostasis in Arabidopsis.
... In photosynthesis, excitation energy from the sun is transferred from antenna pigments of light-harvesting complexes of photosystem II to photoactive chlorophyll molecule (P680) of the reaction center. As a result, strongly oxidizing cation radical Downloaded by [Anoop Singh] at 19:58 06 November 2014 P680 + is formed, which catalyzes the oxidation of water in the oxygen-evolving complex of PS II through a series of redox-active components including the tyrozine Z residue (YZ), and the Mn 4 O 4 Ca cluster located at the luminal side of the thylakoid membrane (Barber 2008;Goussias et al. 2002;Kern and Renger 2007). Electrons released from P680 + were accepted by a negatively charged radical of the Mg-free chlorophyll pigment pheophytin (Pheo). ...
... The pigment-protein complex of photosystem II (PS II) embedded into the thylakoid membranes of cyanobacteria and chloroplasts functions as a light-driven water-plastoquinone oxidoreductase (Goussias et al. 2002;Wydrzynski and Satoh 2005). From a functional point of view, the PS II can be described in terms of three domains: central photochemical, plastoquinone-reducing, and water-oxidizing. ...
In a direct experiment, the rate constants of photochemical k
p and non-photochemical k
p+ quenching of the chlorophyll fluorescence have been determined in spinach photosystem II (PS II) membrane fragments, oxygen-evolving PS II core, as well as manganese-depleted PS II particles using pulse fluorimetry. In the dark-adapted reaction center(s) (RC), the fluorescence decay kinetics of the antenna were measured at low-intensity picosecond pulsed excitation. To create a “closed” P680+Q
A− state, RCs were illuminated by high-intensity actinic flash 8 ns prior to the measuring flash. The obtained data were approximated by the sum of two decaying exponents. It was found that the antennae fluorescence quenching efficiency by the oxidized photoactive pigment of RC P680+ was about 1.5 times higher than that of the neutral P680 state. These results were confirmed by a single-photon counting technique, which allowed to resolve the additional slow component of the fluorescence decay. Slow component was assigned to the charge recombination of P680+Pheo− in PS II RC. Thus, for the first time, the ratio k
p+/k
p ≅ 1.5 was found directly. The mechanism of the higher efficiency of non-photochemical quenching comparing to photochemical quenching is discussed.
... Manganese (Mn) is an essential micronutrient for plant growth due to participation in many aspects of physiology and metabolism [1]. For example, Mn is the main component of the Mn cluster structure of photosystem II, in which water is oxidized to generate O 2 and protons for photosynthesis [2]. Manganese also acts as a cofactor of a variety of enzymes, including Mn-superoxide dismutase (Mn-SOD), Mn-catalase (Mn-CAT) and phosphoenolpyruvate carboxylase (PEPC) [3,4]. ...
Significance:
This study highlighted the effects of Mn toxicity on soybean root growth and its proteome profiles. Excess Mn treatments inhibited root growth. Comparative proteomic analysis was performed to analyze the changes in protein profiles of soybean roots in response to Mn toxicity. A total of 31 root proteins with differential abundances were identified and predominantly associated with signal transduction and cell wall metabolism. Among them, the abundances of the GTP-binding nuclear protein Ran-3 and Ran-binding protein 1 were significantly increased, suggesting that the proteins could be involved in the signaling network in soybean roots responsive to Mn toxicity. Interestingly, three 14-3-3 proteins were decreased by excess Mn at protein but not mRNA levels, suggesting that these proteins could be regulated at post-transcriptional modification under Mn excess conditions. Furthermore, changes in abundances of expansin-like B1-like protein, peroxidase 5-like protein, dirigent protein 2-like protein and dirigent protein strongly suggested that Mn toxicity could influence root cell wall modification, and thus inhibit root growth. This study provided significant insights into the potential molecular mechanisms underlying soybean root adaptation to Mn toxicity, which was mainly through alteration of root cell wall structure and lignification.
Background
Manganese (Mn) deficiency due to nutrient mining by high yielding cereal–cereal cropping patterns and forgetfulness of Mn fertilizer applications becomes potential challenge in crop production.
Aim
Nano‐enabled Mn fertilizers can be safer and more nutrient efficient than conventional Mn fertilizers (nutrient use efficiency ≈ 1%–3%). However, studies about nano‐Mn fertilizer synthesis and their behaviour in soil–plant system are rare.
Methods
In this study, two novel nano‐Mn fertilizers, that is nano‐MnO 2 (NMO) and manganese nanoclay polymer composites (Mn‐NCPC), were synthesized, characterized (dynamic light scattering, X‐ray diffraction, Scanning electron microscopic and energy‐dispersive X‐ray, Fourier transform infrared spectroscopy etc.) and investigated for their impact on growth, yield and nutrient acquisition by wheat crop ( Triticum aestivum L., variety HD‐2967) in a pot culture experiment. Treatment comprised 25%, 50% and 100% of recommended dose of Mn (RDMn) through NMO along with 100% RDMn through MnSO 4 ∙H 2 O (MS). Effect of exposure route was also investigated using foliar spray of NMO at tillering stage. Mn‐NCPC was found to be most efficient Mn fertilizer in terms of yield, Mn uptake and use efficiency by wheat crop.
Results
Nano‐sized formulations improved the solubility of Mn in soil due to its higher active surface area (NMO) and slow‐release behaviour (Mn‐NCPC); thus, minimal losses happened due to the fixing of Mn in oxide/hydroxide forms. Application of 25% RDMn through NMO fertilizers maintained equitant diethylenetriamine pentaacetate Mn content to 100% RDMn through MnSO 4 ∙H 2 O. Mn‐NCPC stimulated the soil enzymatic activities, namely dehydrogenase, acid–alkaline phosphatase activities. Mn‐NCPC and NMO at 100% RDMn recorded 3.51% and 5.20% improvement in grain yield, respectively, when compared to MnSO 4 ∙H 2 O 100%.
Conclusions
Mn fertilizer doses can be reduced up to 25% of RDMn when applied through NMO or Mn‐NCPC fertilizers. However, effects of Mn‐NCPC and NMO need to be critically evaluated in long‐term field experiments in various cropping systems especially under cereal–cereal sequences for economic profitability and wide‐scale farmer's adaptability.
Manganese (Mn) is an essential micronutrient for plant growth that plays vital roles in a set of physiological and biochemical processes. To meet normal growth requirements, plants must cope with changes in environmental Mn status from deficiency to excess through regulation of Mn uptake and homeostasis, which is required for the involvement of various Mn transporters. Notable advances have been made in elucidating the mechanisms of Mn uptake and homeostasis at both the cellular and subcellular levels, but the precise functions of most Mn transporters remain fragmentary due to their transport substrate activity and specificity. Thus the aim of this chapter is to summarize and integrate the molecular characteristics of transporters implicated in Mn uptake, translocation, distribution, and sequestration in response to variable environmental Mn, which might facilitate a better understanding of the mechanism regulating Mn homeostasis in the plant cell.
Nitric oxide (NO) is an important signaling molecule in plants, which regulates many processes in the cell. The various studies have been tried to understand the role of NO in plant cells especially in the chloroplast by exogenous application of gaseous molecule or donor of NO. Here, we review the important target sites of NO in plastids which are directly affected by the exogenous application of NO gas or NO donor, e.g., sodium nitroprusside (SNP). The studies show the positive effect of NO at low concentration (nanomoles) and inhibitory effect of NO at higher concentrations (micro- or millimole) on photosynthetic machinery in chloroplast. NO, thus by interacting with plastids proteins/enzymes, influencing photophosphorylation, electron transport activity, and oxidoreduction state of the Mn clusters of the oxygen-evolving complex, RuBisCo activity, stomatal conductance, and thereby modulating the overall photosynthesis of the plant cells. The detailed modulation of NO-induced changes in the photosynthetic apparatus on its functions and sensitivity are discussed here in this chapter.
World demand for energy is rapidly increasing and finding sufficient supplies of clean energy for the future is one of the major scientific challenges of today. This book presents the latest knowledge and chemical prospects in developing hydrogen as a solar fuel. Using oxygenic photosynthesis and hydrogenase enzymes for bio-inspiration, it explores strategies for developing photocatalysts to produce a molecular solar fuel. The book begins with perspective of solar energy utilization and the role that synthetic photocatalysts can play in producing solar fuels. It then summarizes current knowledge with respect to light capture, photochemical conversion, and energy storage in chemical bonds. Following chapters on the natural systems, the book then summarizes the latest developments in synthetic chemistry of photo- and reductive catalysts. Finally, important future research goals for the practical utilization of solar energy are discussed. The book is written by experts from various fields working on the biological and synthetic chemical side of molecular solar fuels to facilitate advancement in this area of research.
This chapter discusses the current knowledge concerning the mitochondrial metal homeostasis in plants. The most important metals present in plant mitochondria are iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), molybdenum (Mo) and cobalt (Co), which constitute the mitochondrial metallome. In mitochondria, Fe plays its biological function essentially as an enzyme co‐factor and it is mainly bound to ligands, such as Fe‐heme groups and Fe‐sulfur (Fe‐S) clusters, which are components of the mitochondrial respiratory complexes. The loss of Cu suggests damage to soluble mitochondrial cuproproteins and the membrane‐bound electron transport chain, whereas the loss of Fe and Mn suggests that H 2 O 2 treatment damaged the matrix metalloproteins such as Fe‐S‐containing aconitase and Mn‐containing superoxide dismutase (MnSOD). The decrease in the membrane‐bound Cu may be linked to the decrease of Complex IV (COX) complexes, which are the major Cu user in mitochondria.
The concurrent occurrence of more than one abiotic stress in plants is a regular consequence in natural field environment. The heavy metals and deficiency of water are two different abiotic stress factors that affect plant system by their single or simultaneous existence. In recent times, due to the industrial revolution, the contamination of agricultural soil by different heavy metals has increased drastically. For plants, some of the heavy metals are considered as essential nutrients in microquantities, which help in plant growth and development, whereas some are considered nonessential and show a visible toxic effect even at low concentration. Water deficit is a condition of inadequate supply of water in plants due to limited water availability that results in damaging effects in plants. Both of these stresses, that is, metal and water deficit, when combined to a certain level, result in some consequences in plants that might have responses similar to that of individual stress, or these responses might completely differ from that of single stress. Previous relevant literature suggested that plants have unique and sometimes complex responses against different abiotic stress combinations that cannot be directly compared with those of individual stress effects. At present, very few reports are available on mutual and interactive effects of metal and water-deficit stress in plants. Hence, it is essentially required to analyze numerous stress responses in different plants to understand the underlying mechanism of interaction and develop resistant varieties against various stresses but maintaining high yields. This chapter reviewed the articles related to different abiotic stresses’ interaction with plants, specifically heavy metal and deficiency of water; with an attempt to elucidate the morphological, physiological, and biochemical effects on the plants. We have discussed the combined metal and water-deficit stress to understand the responses of plants against these two stresses individually and in combination.
Polyamines (PAs) are recognized as plant growth regulators that are involved in the stress management in various crops. In the current study, mitigative roles of spermidine (Spd) and putrescine (Put) were assessed in manganese (Mn) stressed Brassica juncea plants. Spd or Put (1.0 mM) were applied to the foliage of Brassica juncea at 35 days after sowing (DAS) grown in the presence of Mn (30 or 150 mg kg-1 soil). The higher levels of Mn (150 mg kg-1) diminished photosynthetic attributes and growth, enhanced the production of reactive oxygen species (ROS) like hydrogen peroxide (H 2 O 2) and superoxide anion (O 2 •¯) content, affected stomatal movement and increased the Mn concentration in roots and shoots of the plant at 45 DAS, whereas it enhanced activities of various antioxidant enzymes and proline content in the foliage of Brassica juncea plants. On the other hand, treatment of PAs (Spd or Put) to Mn stressed as well as non-stressed plants resulted in a remarkable improvement in the stomatal behaviour, photosynthetic attributes, growth and biochemical traits, decreased the production of ROS (H 2 O 2 and O 2 •¯) and concentration of Mn in different parts of plant. It is concluded that out of the two polyamines
Brassinosteroids and polyamines are generally used to surpass different abiotic stresses like heavy metal toxicity in plants. The current study was conducted with an aim that 24-epibrassinolide (EBL) and/or spermidine (Spd) could modify root morphology, movement of stomata, cell viability, photosynthetic effectiveness, carbonic anhydrase and antioxidant enzyme activities in Brassica juncea under manganese (Mn) stress (30 or 150 mg kg−1 soil). EBL (10−8 M) and/or Spd, (1.0 mM) were applied to the foliage of B. juncea plants at 35 days after sowing (DAS), grown in the presence of Mn (30 or 150 mg kg−1 soil). High Mn concentration (150 mg kg−1 soil) altered root morphology, affected stomatal movement, reduced the viability of cells and photosynthetic effectiveness and increased the production of reactive oxygen species (O
·−2
and H2O2) in the leaves and antioxidant defense system of B. juncea at 45 DAS. Furthermore, exogenous treatment of EBL and Spd under stress and stress- free conditions improved the aforesaid traits while decreased the O
·−2
and H2O2 production. Therefore, EBL and Spd could be applied to the foliage of B. juncea plants for the better growth under metal stress.
This chapter discusses the current knowledge concerning the mitochondrial metal homeostasis in plants. The most important metals present in plant mitochondria are iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), molybdenum (Mo) and cobalt (Co), which constitute the mitochondrial metallome. In mitochondria, Fe plays its biological function essentially as an enzyme co-factor and it is mainly bound to ligands, such as Fe-heme groups and Fe-sulfur (Fe-S) clusters, which are components of the mitochondrial respiratory complexes. The loss of Cu suggests damage to soluble mitochondrial cuproproteins and the membrane-bound electron transport chain, whereas the loss of Fe and Mn suggests that H2O2 treatment damaged the matrix metalloproteins such as Fe-S-containing aconitase and Mn-containing superoxide dismutase (MnSOD). The decrease in the membrane-bound Cu may be linked to the decrease of Complex IV (COX) complexes, which are the major Cu user in mitochondria.
One of the limiting factors in the growth and development of plants is soil acidity due to high aluminium (Al) content. The aim of this research was to determine the micronutrient contents, photosynthetic pigments, gas exchange and morphological parameters, and to explain the possible tolerance mechanisms involved in two species of the genus Eucalyptus that were exposed to low and high aluminium concentrations. The experiment was conducted in a factorial completely randomised design, with two aluminium concentrations, viz., 0.08 (low) and 1.60 (high) mM Al, and two species, i.e., Eucalyptus platyphylla and Eucalyptus grandis. High Al concentration increased the Al contents in E. platyphylla and E. grandis by 104 and 29%, respectively. Significant reductions of Fe, Zn and Mn contents were detected only in E. platyphylla. Reductions on chlorophyll b and total chlorophyll were observed in both the species, which were more intense in the E. platyphylla. Net photosynthetic rate and water use efficiency increased under high Al concentration, whereas stomatal conductance and transpiration rate decreased in E. grandis. Growth characteristics decreased under high Al concentration in E. platyphylla and increased in E. grandis, while opposite response was observed in both species under low Al concentration. Our results described clearly that E. platyphylla is sensitive, while E. grandis is tolerant to Al. The tolerance mechanism of E. grandis can be explained by the maintenance of the iron, zinc and manganese supplies, combined with an increase in the chlorophyll a, net photosynthetic rate and water use efficiency, resulting in the mitigation of the Al effects on growth parameters.
The reaction of MnCl2·4H2O with salicylaldoxime (H2salox) and the sodium salt of 1,3-bis(carboxypropyl)tetramethyldisiloxane (H2L) in a 1:1:1 molar ratio led to the self-assembly of {[Mn6O2(salox)6(H2salox)(H2O)3(μ-L)]H2salox·1.2H2O}n, a 1D coordination polymer consisting of hexamanganese(III) salicylaldoximate cluster as secondary building unit (SBU) and tetramethyldisiloxane-based dicarboxylate linker, namely, 1,3-bis(carboxypropyl)tetramethyldisiloxane. The structure of the compound was established by single crystal X-ray diffraction. The Mn(III) clusters consist of two staggered μ3-oxo-bridged Mn3 triangles held together by the oxygen atoms of the oxime groups. Because of Jahn–Teller distortion, the Mn–O distances reach 2.5 Å for the oxygen atoms located above and below the triangles mean planes. The compound showed a glass transition peak at around 14 °C in the differential scanning calorimetry (DSC) curve. The magnetic susceptibility data were fitted with a set of three intracluster antiferromagnetic exchange interaction coupling constants: J1 = −0.65 cm–1, J2 = −1.5 cm–1, and J3 = −0.9 cm–1. The ac magnetic susceptibility measurements in the 2–5 K temperature range reveal a frequency-dependent behavior indicative of a slow relaxation of magnetization at low temperature. The coexistence of the lypophilic 1,3-bis(propyl)tetramethyldisiloxane moieties and hydrophilic polar SBUs confers to the structure an amphiphilic character. Dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and transmission (TEM) and scanning (SEM) electron microscopies demonstrate that in dimethylformamide (DMF) the coordination polymer organizes as micelles, whereas in chloroform it tends to form inverse micelles and vesicles.
Nature's subtle systems drive essential reactions responsible for sustenance of our existence "reactions of life" through sophisticated mechanisms of charge transfer, energy harvest and conversion. The interconnectedness between living nature and technologically relevant electrochemical reactions, for example, oxygen reduction and evolution catalyzed by cytochrome c oxidases and photosystem II respectively, and hydrogen oxidation and evolution catalyzed by hydrogenases, does not only intrigue but also inspires us. To what extent therefore can our present understanding of electrocatalysis guide us to decipher nature's sophistication, or rather, can bioinspired electrocatalysis succeed to replicate and supersede nature's perfection "the exemplar paragon"? Herein, we present a harmonized perspective of the principle factors which govern electrocatalysis and bioelectrocatalysis featuring examples of technologically important electrochemical reactions catalyzed by both enzymes and inorganic electrocatalysts. Sound knowledge of the inter-relationships linking electrocatalysis and bioelectrocatalysis is essential for enabling a deeper understanding of nature's bioelectrochemical reactions, and for insightful design of functional catalysts inspired by models from living nature.
This article is an up-to-date review of the literature available on the subject of liquid biofuels. In search of a suitable fuel alternative to fast depleting fossil fuel and oil reserves and in serious consideration of the environmental issues associated with the extensive use of fuels based on petrochemicals, research work is in progress worldwide. Researchers have been redirecting their interests in biomass based fuels, which currently seem to be the only logical alternative for sustainable development in the context of economical and environmental considerations. Renewable bioresources are available globally in the form of residual agricultural biomass and wastes, which can be transformed into liquid biofuels. However, the process of conversion, or chemical transformation, could be very expensive and not worthwhile to use for an economical large-scale commercial supply of biofuels. Hence, there is still need for much research to be done for an effective, economical and efficient conversion process. Therefore, this article is written as a broad overview of the subject, and includes information based on the research conducted globally by scientists according to their local socio-cultural and economic situations.
There has been much speculation concerning the mechanism for water oxidation by Photosystem 11. Based on recent work on the biophysics of Photosystem I1 and our own work on the reactivity of synthetic manganese complexes, we propose a chemically reasonable mechanistic model for the water oxidation function of this enzyme. An essential feature of the model is the nucleophilic attack by calcium-ligated hydroxide on an electrophilic 0x0 group ligated to high-valent manganese to achieve the critical 0-0 bond formation step. We also present a model for S-state advancement as a series of proton-coupled electron transfer steps, which has been proposed previously [Hoganson et. al.], but for which we have developed model systems that allow us to probe the thermodynamics in some detail. One of the great unsolved mysteries in bioinorganic chemistry is the mechanism of water oxidation by the oxygen evolving complex (OEC) of Photosystem I1 (PS 11). This reaction is responsible for nearly all of the dioxygen on our planet and conceptually is the reverse reaction of respiration where dioxygen is converted back to water. Plants use an expansive airay of photopigments in Photosystem 11, four manganese ions, calcium and chloride to carry out these reactions. While intensively studied for many years, only now is a picture emerging as to how this fascinating and essential chemistry may result. The scope of this article is far too limited to allow for a detailed summary of previous studies in the field: therefore, interested readers are directed to
Electron spin echo electron-nuclear double resonance (ESE-ENDOR) experiments performed on a broad radical electron paramagnetic resonance (EPR) signal observed in photosystem II particles depleted of Ca2+ indicate that this signal arises from the redox-active tyrosine YZ. The tyrosine EPR signal width is increased relative to that observed in a manganese-depleted preparation due to a magnetic interaction between the photosystem II manganese cluster and the tyrosine radical. The manganese cluster is located asymmetrically with respect to the symmetry-related tyrosines YZ and YD. The distance between the YZ tyrosine and the manganese cluster is estimated to be approximately 4.5 A. Due to this close proximity of the Mn cluster and the redox-active tyrosine YZ, we propose that this tyrosine abstracts protons from substrate water bound to the Mn cluster.
A single flash given at − 15°C to chloroplasts results in charge separation in Photosystem II to form a stable state which, upon warming, recombines giving rise to luminescence. This recombination occurs at 25°C in untreated chloroplasts but is shifted to 0°C in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea or weak concentrations of a reducing agent. The luminescence at 0°C is attributed to recombination of the S2Q−A state while that at 25°C is attributed to recombination of S2QAQ−B (and S3QAQ−B upon further flash illumination). The identification of the thermoluminescence at 25°C is based upon the following experimental evidence: (1) illumination of chloroplasts in the presence of methyl viologen with 710 nm light before and after flash illumination has no effect on the extent or temperature of the thermoluminescence. This is taken as evidence that the plastoquinone pool is not involved in the recombination reaction. (2) Calculations of the extent of thermoluminescence expected after a number of flashes, assuming that S2QAQ−B and S3QAQ−B are the thermoluminescent reactants, give a good fit to the experimental results. (3) The effect of continuous illumination at 77 K (i.e., donation from cytochrome b-559 to QA and thence to QB or Q−B) results in predictable changes in the extent of flash-induced thermoluminescence.
The inhibition of the oxygen-evolving enzyme of photosystem II, induced by NaCl washing, has been studied by monitoring the yield of the EPR multiline signal arising from the Sâ charge storage state. After continuous illumination at 200 K, the Sâ-multiline signal was present with almost the same amplitude in inhibited and Ca/sup 2 +/-reconstituted membranes. Flash illumination given at room temperature to Ca/sup 2 +/-reconstituted membranes resulted in the usual period four oscillation pattern in the amplitude of the Sâ-multiline signal. In the absence of Ca/sup 2 +/, although the multiline signal was formed on the first flash and decreased on the second flash, no further increase in the signal amplitude occurred on the fifth flash. In addition, dark adaptation for 15 s of inhibited membranes that had been given three flashes resulted in the formation of the Sâ-multiline signal due to deactivation of the Sâ state. These results indicate that in NaCl-washed material inhibition of S-state turnover occurs at the Sâ to Sâ transition. These results disagree with several earlier reports. Attempts have therefore been made to reconcile these reports with the present work both experimentally and be reevaluation of the earlier data. Other apparent inconsistencies in the literature are discussed in terms of differential depletion of Ca/sup 2 +/ from sites with different binding affinities. The authors speculate that these complex phenomena may be simply explained within a model in which a single Ca/sup 2 +/ specific binding site is associated with oxygen evolution and that the affinity of this site is modulated by the S states. It is also shown that reactivation by Sr/sup 2 +/ instead of Ca/sup 2 +/ induced a modified multiline signal similar to that observed in NHâ-treated photosystem II membranes.
A study of electron paramagnetic resonance (EPR) signals from components on the electron donor side of photosystem II has been performed. By measurement of EPR signal II/sub slow/ (D/sup +/) it is shown that, after three flashes, D/sup +/ decays slowly in the dark at room temperature in the fraction of the centers that was in the Sâ state (tââ of 20 min in thylakoid membranes and 50 min in photosystem II enriched membranes). This reaction is accompanied by a conversion of Sâ to Sâ. The concentration of Sâ was estimated from the amplitude of the Sâ-state multiline EPR signal that could be generated by illumination at 200 K. These observations indicate that D/sup +/ accepts an electron from Sâ in dark reaction in which D and Sâ are formed. In addition, the reactions by which D donates an electron to Sâ or Sâ have been directly measured by monitoring both signal II/sub slow/ and the multiline signal. The redox interactions between the D/D/sup +/ couple and the S states are explained in terms of a model in which D/D/sup +/ has a midpoint potential between those of Sâ/Sâ and Sâ/Sâ. In addition, this model provides explanations for a number of previously unrelated phenomena, and the proposal is put forward that the reaction between D/sup +/ and Mn/sup 2 +/ is involved in the so-called photoactivation process.
Ca2+ and Cl- are obligatory cofactors in photosystem II PS-II the oxygen-evolving enzyme of plants. The sites of inhibition in both Ca2+ and Cl--depleted PS-II were compared using EPR and flash absorption spectroscopies to follow the extent of the photooxidation of the redox-active tyrosine (TyrZ) and of the primary electron donor chlorophyll (P680) and their subsequent reduction in the dark. The inhibition occurred after formation of the S3 State in Ca2+-depleted PS-II. In Cl--depleted photosystem II, the inhibition occurred after formation of the S3 state in about half of the centers and probably after S2TyrZ+ formation in the remaining centers. After the S3 state was formed in Ca2+- and Cl--depleted photosystem II, electron transfer from TyrZ to P680 was inhibited. This inhibition is discussed in terms of electrostatic constraints resulting from S3 formation in the absence of Ca2+ and Cl-.
γ-Ray irradiation at liquid nitrogen temperature of a dimethylformamide solution of the tetranuclear complex [MnIV4O6(bipy)6]4+ (bipy = 2,2′-bipyridine) allowed the generation of the first mixed-valence tetranuclear system containing MnIII and MnIV ions and exhibiting a S = ½ ground state. The X-band EPR spectrum of this tetranuclear system has been obtained. Simulations have been undertaken and the Mn hyperfine coupling tensors determined clearly show a MnIIIMnIV3 composition for the EPR active species. A general approach for the analysis of the isotropic components of the Mn hyperfine tensors is presented in detail. This allowed the determination of the spin projection value for each Mn site. A three J coupling scheme assuming that the linear topology of the starting compound remains is able to reproduce these spin projection values if and only if the MnIII ion is located at a terminal position in a N4O2 environment. The EPR signal of this [Mn4O6(bipy)6]3+ species is compared with the multiline signal observed in the S2 state of the photosynthetic Oxygen Evolving Complex.
A functional model for the tetranuclear manganese cluster in the photosystem II of higher plants and cyanobacteria is now provided by binuclear dicationic manganese complexes such as 1. In the anodic oxidation of aqueous acetonitrile containing these complexes water is oxidized to oxygen by a four-electron process. An example of Ar is 4-tBuC6H4; counterion ClO.
New evidence on the chloride requirement for photosynthetic O 2 evolution has indicated that Cl - facilitates oxidation of the manganese cluster by the photosystem II (PSII) Tyr-Z + radical. Illumination above 250K of spinach PSII centers which are inhibited in O 2 evolution bu either Cl - depletion of F - substitution produces a new EPR signal which has magnetic characteristics similar to one recently discovered in samples inhibited by depletion of Ca 2+ only.
Oxygen evolution by the mangano-enzyme of photosystem II is inhibited by Ca2+ depletion induced by NaCl washing and restored by Ca2+ addition. The effectiveness of NaCl treatment in inhibiting oxygen evolution in photosystem II was studied after a series of preilluminating flashes. The susceptibility of the enzyme to NaCl treatment varied with the number of preilluminating flashes and this variation showed an oscillation pattern with a period of four. This pattern is characteristic of cycling through the four long-lived intermediate states in the enzyme cycle (i.e. the states, S0, S1, S2, S3). The relative extent of inhibition corresponding to each of the S states was as follows: S3 > S0≈S2 > S1. From these results it is concluded that Ca2+ binding is dependent on the S states and that Ca2+ probably plays a fundamental role in the mechanism of water splitting. The results also help to explain the conflicting reports of the extent of inhibition induced by NaCl washing and the controversy over which electron transfer step is inhibited by Ca2+ depletion.
— Using isolated chloroplasts and techniques as described by Joliot and Joliot[6] we studied the evolution of O2 in weak light and light flashes to analyze the interactions between light induced O2 precursors and their decay in darkness. The following observations and conclusions are reported: 1. Light flashes always produce the same number of oxidizing equivalents either as precursor or as O2. 2. The number of unstable precursor equivalents present during steady state photosynthesis is ∼ 1.2 per photochemical trapping center. 3. The cooperation of the four photochemically formed oxidizing equivalents occurs essentially in the individual reaction centers and the final O2 evolution step is a one quantum process. 4. The data are compatible with a linear four step mechanism in which a trapping center, or an associated catalyst, (S) successively accumulates four + charges. The S4+ state produces O2 and returns to the ground state S0. 5. Besides S0 also the first oxidized state S+ is stable in the dark, the two higher states, S2+ and S3+ are not. 6. The relaxation times of some of the photooxidation steps were estimated. The fastest reaction, presumably S*1←S2, has a (first) half time ≤ 200 μsec. The S*2 state and probably also the S*0 state are processed somewhat more slowly (˜ 300–400 μsec).
An experimental and theoretical photon echo (PE) study of the primary charge separation process in the photosystem II reaction center (PS II RC) at low temperature (T = 1.33 ± 0.01 K) is reported. Experiments were carried out at low excitation intensities of 5 × 1012 photons/cm2 with time and spectral resolution of about 0.5 ps and 1 nm, respectively, using the two-pulse photon echo technique (2PE). The data were interpreted in the framework of the exciton model. For that purpose the theory of the PE formation and energy transfer in an excitonically coupled system, including explicitly the electron-bath interaction, is developed. By comparing the measured and the simulated PE kinetics, we draw the conclusion that the accessory chlorophyll in the active branch of the RC core is the primary electron donor. The charge separation occurs with an intrinsic time constant of ≈1.5 ps, in good agreement with previously published data (Wasielewski, M. R.; Johnson, D. G.; Seibert, M.; Govindjee Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 524; Jankowiak, R.; Tang, D.; Small, G. J.; Seibert, M. J. Phys. Chem. 1989, 93, 1649; Tang, D.; Jankowiak, R.; Seibert, M.; Small, G. J. Photosynth. Res. 1991, 27, 19). However, the dipole−dipole interaction between pigments leads to a very wide distribution of the effective charge separation kinetics ranging from 1.5 ps up to a few nanoseconds. Thus, the experimentally observable effective distributive charge separation rate differs strongly from the intrinsic one. In this work the effect of the charge separation process in an excitonically coupled system is described for the first time. Energy transfer rates, calculated on the basis of developed theory, show that the energy transfer occurs in the 100−200 fs time domain in agreement with our own experimental observations, and previously published data. This fast energy transfer contributes to the intense and narrow peak at early delay times in the 2PE kinetics. In contrast, the slow dephasing observed in the 2PE kinetics at time delay above ca. 1 ps reflects mainly the primary charge separation process.
In a mononuclear MnIV and a trinuclear MnII complex, the ligands of which contain electron-rich phenols (coordinated to the Mn('s)) and covalently attached ruthenium(II) 2,2‘-trisbipyridyl(=bpy)-type groups, intramolecular electron transfer (ET) from the phenolate ligand (in the mononuclear MnIV complex) or from a MnII (in the trinuclear MnII complex) to the photochemically (λexc= 455 nm) generated RuIII takes place with k ≥ 5 × 107 s-1, giving rise to the corresponding phenoxyl radical (complexed to MnIV) or to MnIII, respectively. Thus, in the trinuclear MnII complex, the source of the electron that reduces the photogenerated RuIII(bpy•-) moiety is a MnII, in contrast to the situation with the mononuclear MnIV complex, where the electron stems from a phenolate. The half-life of the coordinated phenoxyl-type Ru(bpy)/Mn complex (as produced in the presence of [CoIII(NH3)5Cl]2+) is of the order 0.5−1 ms. The Ru(bpy) compound containing three (phenolate-ligated) MnII atoms is the first example of a photochemically induced intramolecular ET from a multinuclear Mn cluster to an attached “sensitizer”, and the Ru complex containing one (phenolate-ligated) MnIV is the first case of an ET from a synthetic MnIV-coordinated phenolate to a photochemically produced oxidant (RuIII).
The two forms of the g ≈ 4.1 signal recently identified in photosystem II (Smith, P. J.; Pace, R. J. Biochim. Biophys. Acta 1996, 1275, 213) have been simulated at several frequencies as near-axial spin 3/2 centers. In both cases, an explicit spin coupling model is assumed, involving two magnetically isolated Mn pairs, one for each signal type. For that pair assumed to give rise to the spin 1/2 multiline signal as the ground state, the modeling of the first-excited-state 4.1 signal gives estimates of the fine structure parameters for the individual Mn centers and the exchange coupling constant for the pair. The fine structure terms suggest that one Mn ion is a conventional MnIII ion in a highly axially distorted environment. The other Mn center, which is formally spin 3/2, is unlikely to be a conventional MnIV ion, but rather a MnIII−radical ligand pair, strongly antiferromagnetically coupled to give a net spin 3/2 state. The coupling between this Mn−radical center and the other MnIII is weak (J = −2.3 cm-1) in the absence of alcohol in the buffer medium, as determined earlier (Smith and Pace). The model is shown to be quantitatively consistent with the behavior of other signals proposed to arise from this coupled dimer. Comparison of our own data with those of others (Haddy, A.; et al. Biochim. Biophys. Acta 1992, 1099, 25−34) on one-dimensionally ordered photosystem II samples shows a generally consistent orientation of the molecular axis system for the dimer in the membrane plane. The second 4.1 signal, which exhibits ground-state behavior, may be simulated at X- and Q-band frequencies as an isolated system with D = +1.1 cm-1 and E/D = 0.037. The spin center is suggested to arise from a radical-bridged Mn homodimer, and the modeling parameters have been interpreted within this framework. The resulting proposal, involving two isolated dimers for the Mn organization within the oxygen evolving center, is critically examined in the light of recent work from other groups.
We have performed continuous-wave electron paramagnetic resonance (CW-EPR) and electron spin echo electron nuclear double resonance (ESE-ENDOR) experiments on the multiline form of the S2-state of untreated, MeOH-treated, and ammonia-treated spinach photosystem II (PS II) centers. Through simultaneously constrained simulations of the CW-EPR and ESE-ENDOR data, we conclude that four effective 55Mn hyperfine tensors (AX, AY, AZ) are required to properly simulate the experimental data [untreated and MeOH-treated PS II centers (MHz): −232, −232, −270; 200, 200, 250; −311, −311, −270; 180, 180, 240; ammonia-treated PS II centers (MHz): 208, 208, 158; −150, −150, −112; 222, 222, 172; −295, −315, −390]. We further show that these effective hyperfine tensors are best supported by a trimer/monomer arrangement of three Mn(IV) ions and one Mn(III) ion. In this topology, MnA, MnB, and MnC form a strongly exchange coupled core (JAB and JBC < −100 cm-1) while MnD is weakly exchange coupled (JCD) to one end of the trinuclear core. For untreated and MeOH-treated PS II centers, the Mn(III) ion is either MnA or MnC, with a zero-field-splitting of D = −1.25 to −2.25 cm-1. For ammonia-treated PS II centers, the Mn(III) ion is MnD, with a zero-field-splitting of D = +0.75 to +1.75 cm-1. The binding of the ammonia ligand results in a shift of the Mn(III) ion from the trinuclear core to the monomer Mn ion. This structural model can also account for the higher spin of the g = 4.1 signal and the magnetic properties of the S0-state.
The microwave power for half-saturation (P1/2) for the radical in photosystem II giving rise to signal IIslow (SIIs) has been measured by EPR in samples illuminated by a series of flashes. The charge storage state of the oxygen-evolving complex (S0-S4) was monitored by measuring the multiline EPR signal arising from the S2 state. The following results were obtained: (1) SIIs becomes easier to saturate after tris(hydroxymethyl)aminomethane (Tris) washing, a treatment that partially removes the Mn cluster. (2) P1/2 for SIIs oscillates with the flash number. P1/2 is lower in S1 (in dark-adapted material and after four flashes) than in S2, S3, or S0. (3) P1/2(S2) = P1/2(S3). (4) At 8 K P1/2(S2) > P1/2(S2), but at 20 K P1/2(S0) < P1/2(S2). (5) P1/2 for SIIs increases with temperature (8-70 K) in the S1 state. SHs is more difficult to saturate in S2, S3, and S0 than in S1 over the investigated temperature range. In addition, the increase in P1/2 is complex around 20-30 K in S2, S3, and S0. (6) In S0, P1/2 for SIIs decreases with time (decay half-time 30-60 s) to a stable level significantly above the dark level. The data are explained in terms of cross relaxation between the radical giving rise to SIIs and an efficient relaxer, which is suggested to be the Mn cluster. This relaxes more slowly in S1 than in the other S states. Since it is known that a mixed-valence Mn cluster is present in S2, and because P1/2 of SIIs in S3 and S0 is comparable to that in S2, it is suggested that mixed-valence Mn clusters are present in the S3 and S0 states also. Different models with these features can be proposed, the simplest of which is the following: S0 [Mn(H)-Mn(III)], S1 [Mn(III)-Mn(III)], S2 [Mn(III)-Mn(IV)], and S3 [Mn(III)-Mn(IV)].
Photosynthetic organisms are able to oxidize organic or inorganic compounds upon the absorption of light, and they use the extracted electron for the fixation of carbon dioxide. The most important oxidation product is oxygen due to the splitting of water. In eukaryotes these processes occur in photosystem II of chloroplasts. Among prokaryotes photosynthetic oxygen evolution is restricted to cyanobacteria and prochloron-type organisms. How water is split in the oxygen-evolving complex of photosystem II belongs to the most important question to be answered. The primary charge separation occurs in the reaction center of photosystem II. This reaction center is a complex consisting of peripheral and integral membrane proteins, several chlorophyll A molecules, two pheophytin A molecules, two and three plastoquinone molecules, and one non-heme iron atom. The location of the photosystem II reaction center is still a matter of debate. Nakatani et al. (l984) concluded from fluorescence measurements that a protein of apparent molecular weight 47,000 (CP47) is the apoprotein of the photosystem II reaction center. A different view emerged from work with the photosynthetic reaction centers from the purple bacteria. The amino acid sequence of the M subunit of the reaction center from Phodopseudomonas (Rps.) sphaeroides has sequence homologies with the D1 protein from spinach. A substantial amount of structural information can be obtained with the reaction center from Rhodopseudomonas viridis, which can be crystallized. Here the authors discuss the structure of the photosynthetic reaction center from the purple bacterium Rps. viridis and describe the role of those amino acids that are conserved between the bacterial and photosystem II reaction center.
A study of electron paramagnetic resonance (EPR) signals from components on the electron donor side of photosystem II has been performed. By measurement of EPR signal IIslow (D+) it is shown that, after three flashes, D+ decays slowly in the dark at room temperature in the fraction of the centers that was in the S0 state (t1/2 of 20 min in thylakoid membranes and 50 min in photosystem II enriched membranes). This reaction is accompanied by a conversion of S0 to S1. The concentration of S1 was estimated from the amplitude of the S2-state multiline EPR signal that could be generated by illumination at 200 K. These observations indicate that D+ accepts an electron from S0 in a dark reaction in which D and S1 are formed. In addition, the reactions by which D donates an electron to S2 or S3 have been directly measured by monitoring both signal IIslow and the multiline signal. The redox interactions between the D/D+ couple and the S states are explained in terms of a model in which D/D+ has a midpoint potential between those of S0/S1, and S1/S2. In addition, this model provides explanations for a number of previously unrelated phenomena, and the proposal is put forward that the reaction between D+ and Mn2+ is involved in the so-called photoactivation process.
The re-reduction course of P-680+, the photooxidized PS II primary donor, was measured as a function of excitation number in Cl−-depleted PS II membranes. After the 1st and 2nd excitations the signal amplitude of P-680+ is small, indicating a submicrosecond reduction of P-680+ by Z, the secondary donor of PS II. After the 3rd excitation, however, a larger P-680+ signal with a 40–50 μs half-life is observed. The slow decay of this signal is attributed to a back-reaction with a reduced acceptor in the presence of the Z+S2 state on the donor side. The state Z+S2 has a lifetime longer than 300 ms and its formation was found to depend on the presence of the abnormal S2 state created by the 1st excitation. The P-680 data and thermoluminescence measurements show that the S-state advancement beyond S2 is blocked in the absence of Cl− and that the Cl−-free abnormal S2 state has a lifetime about 10-times longer than the normal S2 state.
The primary electron donor of photosystem II is a special form of chlorophyll a known as P680. Its detection and subsequent biophysical characterisation has relied heavily on the technique of flash photolysis of Norrish and Porter [Nature 164 (1949) 658] and on the physical principles which emerged from photochemical studies of isolated chlorophyll a using this technique. When oxidised the P680 radical has a midpoint redox potential estimated to be 1.17 V or more which is needed to drive the oxidising reactions of the water-splitting process. Such a high oxidising potential dictates special properties of P680 which are discussed in terms of robustness and structural organisation of photosystem II. Of particular importance has been the recent finding that P680 is not a 'special pair' of chlorophyll molecules as is the case for the primary electron donors of other types of photosynthetic reaction centres. Instead P680 is composed of a cluster of four weakly coupled monomeric chlorophylls which together with the local protein environment enables this primary donor to generate a redox potential capable of oxidising water. © 2001 Elsevier Science B.V. All rights reserved.
Laser-flash-induced absorption changes at 830 nm, fluorescence-induction curves and the average oxygen yield per flash have been measured in spinach Photosystem II membrane fragments as a function of trypsin treatment and its modification by CaCl2. The following was found. (i) The relative contribution of the nanosecond relaxation to the overall decay kinetics of 830 nm absorption changes reflecting the P-680+-reduction decreases as a function of incubation time with trypsin. Simultaneously, mild treatment at pH = 6.0 markedly increases the extent of 200 μs kinetics that highly revert back to nanosecond kinetics by CaCl2 addition. After harsher trypsin treatment (pH = 7.5) pH-dependent 2–20 μs kinetics appear that cannot be reverted to nanosecond kinetics by CaCl2. (ii) The CaCl2-induced restoration of nanosecond kinetics is mainly due to a Ca2+-induced effect rather than to a functional role of Cl−. Sr2+ can substantially substitute for Ca2+, whereas Mg2+, Mn2+ and monovalent ions are almost inefficient. (iii) A quantitative correlation between the extent of the nanosecond kinetics and the average oxygen yield per flash was not observed. (iv) If CaCl2 is present in the assay medium for trypsin treatment the samples are markedly protected to proteolytic degradation. This effect mainly refers to the reaction pattern of the acceptor side. Other bivalent cations can substitute Ca2+ for its protective function. (v) The CaCl2-induced protection to proteolytic attack is extremely sensitive to a very short trypsin pretreatment that does hardly affect the shape of the fluorescence induction curve. The results are discussed in relation to the functional and structural organization of Photosystem II.
Conditions were defined for obtaining photoligation of Mn2+ and photoactivation of O2 evolution in NH2OH- and Tris-extracted PS II membranes (TMF-2) completely devoid of the CF0/CF1 complex and containing only approx. 2–3 PS II e− acceptor equivalents per PS II reaction center. At optiomal pH (pH 6.5) only light and Mn2+ were essential for Mn ligation by the apo-S-state complex (approx. 4 Mn/reaction center); however, Ca2+ addition was required for maximal expression of water oxidation activity of the photoligated Mn. In the absence of added PS II e− acceptors, the quantum efficiency and yield of photoactivation was diminished and was insensitive to atrazine. PS II e− acceptors increased the quantum efficiency/yield by approx. 2-fold and conferred sensitivity to atrazine. Kinetic analyses of the photoactivation process gave evidence for a rate constraint () and an unstable intermediate (half-life of approx. 1.0 s). Cl− was not absolutely essential for photoactivation. A 7-fold increase of rate of O2 evolution was obtained without Cl− addition to Cl−-depleted NH2OH-TMF-2; Cl− addition during photoactivation gave only a 2-fold additional increase. This Cl− effect (Km = 3.8 mM) was different from the Cl effect (Km = 1.5 mM) on O2 evolution. Weak light ageing of NH2OH-TMF-2 in the absence (but not in the presence) of Mn2+ inhibited photoactivation and photoreduction of DCIP by Mn2+; however, DCIP photoreduction by DPC or TPB was not diminished. Such weak light ageing also increased the VLP-form of Cyt b-559, but there was no apparent correlation between the increase in O2 evolution and the conversion of the VLP- and LP-Cyt b-559 to the HP-Cyt b-559 during photoactivation. Photoactivation is suggested to be a two-quantum process in which two Mn2+ are sequentially bound, photooxidized and ligated by the apo-S-state complex. This process facilitates ligation of two additional Mn ions to form the tetra-Mn S-state water-oxidizing complex.
Using thoroughly dark-adapted thylakoids and an unmodulated Joliot-type oxygen electrode, the following results were obtained. (i) At high flash frequency (4 Hz), the oxygen yield at the fourth flash (Y4) is lower compared to Y3 than at lower flash frequency. At 4 Hz, the calculated S0 concentration after thorough dark adaptation is found to approach zero, whereas at 0.5 Hz the apparent ratio increases to about 0.2. This is explained by a relatively fast donation () of one electron by an electron donor to S2 and S3 in 15–25% of the Photosystem II reaction chains. The one-electron donor to S2 and S3 appears to be rereduced very slowly, and may be identical to the component that, after oxidation, gives rise to ESR signal IIs. (ii) The probability for the fast one-electron donation to S2 and S3 has nearly been the same in triazine-resistant and triazine-susceptible thylakoids. However, most of the slow phase of the S2 decay becomes 10-fold faster () in the triazine-resistant ones. In a small part of the Photosystem II reaction chains, the S2 decay was extremely slow. The S3 decay in the triazine-resistant thylakoids was not significantly different from that in triazine-susceptible thylakoids. This supports the hypothesis that S2 is reduced mainly by Q−A, whereas S3 is not. (iii) In the absence of CO2/HCO−A and in the presence of formate, the fast one-electron donation to S2 and S3 does not occur. Addition of HCO−3 restores the fast decay of part of S2 and S3 to almost the same extent as in control thylakoids. The slow phase of S2 and S3 decay is not influenced significantly by CO2/HCO−3. The chlorophyll a fluorescence decay kinetics in the presence of DCMU, however, monitoring the Q−A oxidation without interference of QB, were 2.3-fold slower in the absence of CO2/HCO−3 than in its presence. (iv) An almost 3-fold decrease in decay rate of S2 is observed upon lowering the pH from 7.6 to 6.0. The kinetics of chlorophyll a fluorescence decay in the presence of DCMU are slightly accelerated by a pH change from 7.6 to 6.0. This indicates that the equilibrium Q−A concentration after one flash is decreased (by about a factor of 4) upon changing the pH from 7.6 to 6.0. When direct or indirect protonation of Q−B is responsible for this shift of equilibrium Q−A concentration, these data would suggest that the pKa value for Q−B protonation is somewhat higher than 7.6, assuming that the protonated form of Q−B cannot reduce QA.
The role of chloride on the S-state transition in spinach Photosystem II (PS II) particles was investigated by EPR spectroscopy at low temperature and the following results were obtained. (1) After excitation by continuous light at 200 K, chloride-depleted particles did not show the EPR multiline signal associated with the S2 state, but only showed the broad signal at g = 4.1. The S2 multiline signal was completely restored upon chloride repletion. (2) In the absence of chloride the S2 multiline signal was not induced by a single flash excitation at 0°C. However, upon addition of chloride after the flash the signal was developed in darkness. (3) The amplitude of the multiline S2 signal thus developed upon chloride addition after flash illumination did not show oscillations dependent upon flash number. These results indicate that the O2-evolving complex in chloride-depleted PS II membranes is able to store at least one oxidizing equivalent, a modified S2 state, which does not give rise to the multiline signal. Addition of chloride converts this oxidizing equivalent to the normal S2 state which gives rise to the multiline signal. The modified S2 state is more stable than the normal S2 state, showing decay kinetics about 20-times slower than those of the normal S2 state, and the formation of higher S states is blocked.
In examining the energetics of the elementary act of reactions, involving a change in the number of particles, one has to consider not the standard free reaction energy, or the free energy at given reagent concentrations, but only the configurational component of the free energy. It is this quantity, independent of the reagents concentration, which affects directly the probability of the elementary act of the reaction. Using this approach it is shown that the oxygen-evolution reaction from water corresponds to the configurational redox potential about + 1.4 V, i.e., to a higher one than the redox potential of the primary acceptor of Photosystem II-oxidized P-680. The subdivision of the overall reaction into steps cannot eliminate this energy deficiency. It is shown that the potential in question can be considerably decreased if protons detached from water during the elementary act undergo immediate binding with sufficiently strong bases. An increase in the pH value of these bases upon reduction of manganese as well as the ionization of water molecules preceding their oxidation favours the reaction. The favourable energetics of the process in the form of a single four-electron or of two two-electron elementary acts are pointed out.
Photosystem II (PS II) evolves oxygen from two bound water molecules in a four-stepped reaction that is driven by four quanta of light, each oxidizing the chlorophyll moiety P680 to yield P+680. When starting from its dark equilibrium (mainly state S1), the catalytic center can be clocked through its redox states (S0…S4) by a series of short flashes of light. The center involves at least a Mn4-cluster and a special tyrosine residue, named YZ, as redox cofactors plus two essential ionic cofactors, Cl− and Ca2+. Centers which have lost Ca2+ do not evolve oxygen. We investigated the stepped progression in dark-adapted PS II core particles after the removal of Ca2+. YZ was oxidized from the first flash on. The difference spectrum of YZ→YoxZ differed from the one in competent centers, where it has been ascribed to a hydrogen-bonded tyrosinate. The rate of the electron transfer from YZ to P+680 was slowed down by three orders of magnitude and its kinetic isotope effect rose up from 1.1 to 2.5. Proton release into the bulk was now a prerequisite for the electron transfer from YZ to P+680. On the basis of these results and similar effects in Mn-(plus Ca2+-)depleted PS II (M. Haumann et al., Biochemistry, 38 (1999) 1258–1267) we conclude that the presence of Ca2+ in the catalytic center is required to tune the apparent pK of a base cluster, B, to which YZ is linked by hydrogen bonds. The deposition of a proton on B within close proximity of YZ (not its release into the bulk!) is a necessary condition for the reduction in nanoseconds of P+680 and for the functioning of water oxidation. The removal of Ca2+ rises the pK of B, thereby disturbing the hydrogen bonded structure of YZB.
In plants, solar energy is used to extract electrons from water, producing atmospheric oxygen. This is conducted by Photosystem II, where a redox "triad" consisting of chlorophyll, a tyrosine, and a manganese cluster, governs an essential part of the process. Photooxidation of the chlorophylls produces electron transfer from the tyrosine, which forms a radical. The radical and the manganese cluster together extract electrons from water, providing the biosphere with an unlimited electron source. As a partial model for this system we constructed a ruthenium(II) complex with a covalently attached tyrosine, where the photooxidized ruthenium was rereduced by the tyrosine. In this study we show that the tyrosyl radical, which gives a transient EPR signal under illumination, can oxidize a manganese complex. The dinuclear manganese complex, which initially is in the Mn(III)/(III) state, is oxidized by the photogenerated tyrosyl radical to the Mn(III)/(TV) state. The redox potentials in our system are comparable to those in Photosystem II. Thus, our synthetic redox "triad" mimics important elements in the electron donor "triad" in Photosystem II, significantly advancing the development of systems for artificial photosynthesis based on ruthenium-manganese complexes.
It was shown recently [Goussias, C., Ioannidis, N., and Petrouleas, V. (1997) Biochemistry 36, 9261-9266] that incubation of photosystem II preparations with NO at -30 degrees C in the dark results in the formation of a new intermediate of the water-oxidizing complex. This is characterized by an EPR signal centered at g = 2 with prominent manganese hyperfine structure. We have examined the detailed structure of the signal using difference EPR spectroscopy. This is facilitated by the observations that NO can be completely removed without decrease or modification of the signal, and illumination at 0 degree C eliminates the signal. The signal spans 1600 G and is characterized by sharp hyperfine structure. 14NO and 15NO cw EPR combined with pulsed ENDOR and ESEEM studies show no detectable contributions of the nitrogen nucleus to the spectrum. The spectrum bears similarities to the experimental spectrum of the Mn(II)-Mn(III) catalase [Zheng, M., Khangulov, S. V., Dismukes, G. C., and Barynin, V. V. (1994) Inorg. Chem. 33, 382-387]. Simulations allowing small variations in the catalase-tensor values result in an almost accurate reproduction of the NO-induced signal. This presents strong evidence for the assignment of the latter to a magnetically isolated Mn(II)-Mn(III) dimer. Since the starting oxidation states of Mn are higher than II, we deduce that NO acts effectively as a reductant, e.g., Mn(III)-Mn(III) + NO--> Mn(II)-Mn(III) + NO+. The temperature dependence of the nonsaturated EPR-signal intensity in the range 2-20 K indicates that the signal results from a ground state. The cw microwave power saturation data in the range 4-8 K can be interpreted assuming an Orbach relaxation mechanism with an excited state at delta = 42 K. Assuming antiferromagnetic coupling, -2JS1.S2, between the two manganese ions, J is estimated to be 10 cm-1. The finding that an EPR signal from the Mn cluster of PSII can be clearly assigned to a magnetically isolated Mn(II)-Mn(III) dimer bears important consequences in interpreting the structure of the Mn cluster. Although the signal is not currently assigned to a particular S state, it arises from a state lower than S1, possibly lower than S0, too.
Ca2+ and Cl- are obligatory cofactors in photosystem II (PS-II), the oxygen-evolving enzyme of plants. The sites of inhibition in both Ca(2+)- and Cl(-)-depleted PS-II were compared using EPR and flash absorption spectroscopies to follow the extent of the photooxidation of the redox-active tyrosine (TyrZ) and of the primary electron donor chlorophyll (P680) and their subsequent reduction in the dark. The inhibition occurred after formation of the S3 state in Ca(2+)-depleted PS-II. In Cl(-)-depleted photosystem II, the inhibition occurred after formation of the S3 state in about half of the centers and probably after S2TyrZ+ formation in the remaining centers. After the S3 state was formed in Ca(2+)- and Cl(-)-depleted photosystem II, electron transfer from TyrZ to P680 was inhibited. This inhibition is discussed in terms of electrostatic constraints resulting from S3 formation in the absence of Ca2+ and Cl-.
The temperature dependence of the rate constants of the univalent redox steps YzoxSi----YzSi + 1 (i = 0,1,2) and YzoxS3----(YzS4)----YzSo + O2 in the water oxidase was investigated by measuring time resolved absorption changes at 355 nm induced by a laser flash train in dark adapted PS II membrane fragments from spinach. Activation energies of 5.0, 12.0 and 36.0 kJ/mol were obtained for the reactions YzoxSi----YzSi + 1 with i = 0,1 and 2, respectively. The reaction YzoxS3----(YzS4)----YzS0 + O2 exhibits a temperature dependence with a characteristic break point at 279 K with activation energies of 20 kJ/mol (T greater than 279 K) and 46 kJ/mol (T less than 279 K). Evaluation of the data within the framework of the classical Marcus theory of nonadiabatic electron transfer [(1985) Biochim. Biophys. Acta 811, 265-322] leads to the conclusion that the S2 oxidation to S3 is coupled with significant structural changes. Furthermore, the water oxidase in S3 is inferred to attain two different conformational states with populations that markedly change at a characteristic transition temperature.
Flash-induced absorption changes of pH-indicating dyes were investigated in photosystem II enriched membrane fragments, in order to retrieve the individual contributions to proton release of the successive transitions of the Kok cycle. These stoichiometric coefficients were found to be, in general, noninteger and to vary as a function of pH. Proton release on the S0----S1 step decreases from 1.75 at pH 5.5 to 1 at pH 8, while, on S1----S2 the stoichiometry increases from 0 to 0.5 in the same pH range and remains close to 1 for S2----S3. These findings are analyzed in terms of pK shifts of neighboring amino acid residues caused by electrostatic interactions with the redox centers involved in the two first transitions. The electrochromic shift of a chlorophyll, associated with the S transitions, responding to local electrostatic effects was investigated under similar conditions. The pH dependence of this signal upon the successive transitions was found correlated with the titration of the proton release stoichiometries, expressing the electrostatic balance between the oxidation and deprotonation processes.
The effect of redox-active amines NH2R (R = OH or NH2) on the period-four oscillation pattern of oxygen evolution has been analyzed in isolated spinach thylakoids as a function of the redox state Si (i = 0, ..., 3) of the water oxidase. The following results were obtained: (a) In dark-adapted samples with a highly populated S1 state, NH2R leads via a dark reaction sequence to the formal redox state "S-1"; (b) the reaction mechanism is different between the NH2R species; NH2OH acts as a one-electron donor, whereas NH2NH2 mainly functions as a two-electron donor, regardless of the interacting redox state Si (i = 0, ..., 3). For NH2NH2, the modified oxygen oscillation patterns strictly depend upon the initial ratio [S0(0)]/[S1(0)] before the addition of the reductant; while due to kinetic reasons, for NH2OH this dependence largely disappears after a short transient period. (c) The existence of the recently postulated formal redox state "S-2" is confirmed not only in the presence of NH2NH2 [Renger, G., Messinger, J., & Hanssum, B. (1990) in Current Research in Photosynthesis (Baltscheffsky, M., Ed.) Vol. 1, pp 845-848, Kluwer, Dordrecht] but also in the presence of NH2OH. (d) Activation energies, EA, of 50 kJ/mol were determined for the NH2R-induced reduction processes that alter the oxygen oscillation pattern from dark-adapted thylakoids. (e) Although marked differences exist between NH2OH and NH2NH2 in terms of the reduction mechanism and efficiency (which is about 20-fold in favor of NH2OH), both NH2R species exhibit the same order of rate constants as a function of the redox state Si in the nonperturbed water oxidase: kNH2R(S0) greater than kNH2R(S1) much less than kNH2R(S2) much greater than kNH2R(S3) The large difference between S2 and S3 in their reactivity toward NH2R is interpreted to indicate that a significant change in the electronic configuration and nuclear geometry occurs during the S2----S3 transition that makes the S3 state much less susceptible to NH2R. The implications of these findings are discussed with special emphasis on the possibility of complexed peroxide formation in redox state S3 postulated previously on the basis of theoretical considerations [Renger, G. (1978) in Photosynthetic Water Oxidation (Metzner, H., Ed.) pp 229-248, Academic Press, London].
New evidence on the chloride requirement for photosynthetic O2 evolution has indicated that Cl- facilitates oxidation of the manganese cluster by the photosystem II (PSII) Tyr-Z+ radical. Illumination above 250 K of spinach PSII centers which are inhibited in O2 evolution by either Cl- depletion or F- substitution produces a new EPR signal which has magnetic characteristics similar to one recently discovered in samples inhibited by depletion of Ca2+ only [Boussac et al. (1989) Biochemistry 28, 8984; Sivaraja et al. (1989) Biochemistry 28, 9459]. The physiological roles of Cl- and Ca2+ in water oxidation are thus linked. The characteristics include a nearly isotropic g = 2.00 +/- 0.005, a symmetric line shape with line width = 16 +/- 2 mT, almost stoichiometric spin concentration relative to Try-D+ = 0.6 +/- 0.3 spin/PSII, very rapid spin relaxation at all temperatures measured down to 6 K, and an undetectable change in magnetic susceptibility upon formation (less than 1 mu B2). The signal appears to originate from a spin doublet (radical) in magnetic dipolar contact with a transition-metal ion, most probably a photooxidized protein residue within 10 A of the Mn cluster (Mn-proximal radical). It is distinct from the three other protein-bound radical-type electron donors found in the PSII reaction center: Tyr-D+, Tyr-Z+, and C+. This signal photoaccumulates to a stable level under continuous illumination at 270 K and decays only after illumination stops.(ABSTRACT TRUNCATED AT 250 WORDS)
Photosystem II enriched membranes were depleted of Ca2+ and the 17- and 23-kDa polypeptides by treatment with NaCl and EGTA. The 17- and 23-kDa polypeptides were then reconstituted. This preparation was incapable of O2 evolution until Ca2+ was added. An EPR study revealed the presence of two new EPR signals. One of these is a modified S2 multiline signal with an isotropic g value of 1.96 with at least 26 hyperfine peaks (average spacing 55 G) distributed over approximately 1600 G. The other is a near-Gaussian signal with an isotropic g value of 2.004, which is attributed to a formal S3 state. Experiments involving the interconversion of these signals and the effect of Ca2+ and Sr2+ rebinding provide evidence for these assignments. From these results the following conclusions are drawn: (1) These results are consistent with our earlier demonstration that charge accumulation is blocked after formation of S3 when Ca2+ is deficient. (2) Binding of the 17- and 23-kDa polypeptides to photosystem II in the absence of Ca2+ results in the perturbation of the Mn cluster. This is taken as a further indication that the Ca2+-binding site is close to or even an integral part of the Mn cluster. (3) The S3 signal may arise from an organic free radical interacting magnetically with the Mn cluster. However, other possible origins for this signal, including the Mn cluster itself, must also be considered.
The light-driven water-splitting/oxygen-evolving enzyme remains one of the great enigmas of plant biology. However, due to the recent expansion of research efforts on this enzyme, it is grudgingly giving up some of its secrets.
We present a mechanism for photosynthetic O2 evolution based on a structural conversion of a Mn4O6 "adamantane"-like complex to a Mn4O4 "cubane"-like complex. EPR spectral data obtained from the S2 state of the O2-evolving complex are characteristic of a Mn4O4 cubane-like structure. Based on this structure for the manganese complex in the S2 state as well as a consideration of the other evidence available on the natural system and the coordination chemistry of manganese, structures are proposed for the five intermediate oxidation states of the manganese complex. A molecular mechanism for the formation of an O--O bond and the displacement of O2 from the S4 state is easily accommodated by the proposed model. The model is discussed in terms of recent EPR, x-ray, and UV spectral data obtained from the manganese site in the photosynthetic O2-evolving complex.
The Ca(2+)-binding properties of photosystem II were investigated with radioactive 45Ca2+. PS II membranes, isolated from spinach grown on a medium containing 45Ca2+, contained 1.5 Ca2+ per PS II unit. Approximately half of the incorporated radioactivity was lost after incubation for 30 h in nonradioactive buffer. About 1 Ca2+/PS II bound slowly to Ca(2+)-depleted membranes in the presence of the extrinsic 16- and 23-kDa polypeptides in parallel with restoration of oxygen-evolving activity. The binding was heterogeneous with dissociation constants of 60 microM (0.7 Ca2+/PS II) and 1.7 mM (0.3 Ca2+/PS II), respectively, which could reflect different affinities of the dark-stable S-states for Ca2+. The reactivation of oxygen-evolving activity closely followed the binding of Ca2+, showing that a single exchangeable Ca2+ per PS II is sufficient for the water-splitting reaction to function. In PS II, depleted of the 16- and 23-kDa polypeptides, about 0.7 exchangeable Ca2+/PS II binds with a dissociation constant of 26 microM, while 0.3 Ca2+ binds with a much weaker affinity (Kd > 0.5 mM). The rate of binding of Ca2+ in the absence of the two extrinsic polypeptides was significantly higher than with the polypeptides bound. The rate of dissociation of bound Ca2+ in the dark, which had a half-time of about 80 h in intact PS II, increased in the absence of the 16- and 23-kDa polypeptides and showed a further increase after the additional removal of the 33-kDa protein and manganese. The rate of dissociation was also significantly faster in weak light than in the dark regardless of the presence or absence of the 16- and 23-kDa polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)
The Ca{sup 2+}-binding properties of photosystem II were investigated with radioactive {sup 45}Ca{sup 2+}. PS II membranes, isolated from spinach grown on a medium containing {sup 45}Ca{sup 2+}, contained 1.5 Ca{sup 2+} per PS II unit. Approximately half of the incorporated radioactivity was lost after incubation for 30 h in nonradioactive buffer. About 1 Ca{sup 2+}/PS II bound slowly to Ca{sup 2+}-depleted membranes in the presence of the extrinsic 16- and 23-kDa polypeptides in parallel with restoration of oxygen-evolving activity. The binding was heterogeneous with dissociation constants of 60 {mu}M (0.7 Ca{sup 2+}/PS II) and 1.7 mM (0.3 Ca{sup 2+}/ PS II), respectively, which could reflect different affinities of the dark-stable S-states for Ca{sup 2+}. The reactivation of oxygen-evolving activity closely followed the binding of Ca{sup 2+}, showing that a single exchangeable Ca{sup 2+} per PS II is sufficient for the water-splitting reaction to function. In PS II, depleted of the 16- and 23-kDa polypeptides, about 0.7 exchangeable Ca{sup 2+}/PS II binds with a dissociation constant of 26 {mu}M, while 0.3 Ca{sup 2+} binds with a much weaker affinity (K{sub d} > 0.5 mM). The rate of binding of Ca{sup 2+} in the absence of the two extrinsic polypeptides wasmore » significantly higher than with the polypeptides bound. The rate of dissociation of bound Ca{sup 2+} in the dark, which had a half-time of about 80 h in intact PS H, increased in the absence of the 16-and 23-kDa polypeptides and showed a further increase after the additional removal of the 33-kDa protein and manganese. The rate of dissociation was also significantly faster in weak light than in the dark. Removal of the 33-kDa donor-side polypeptide together with the two lighter ones led to a reduction in the amount of bound Ca{sup 2+}, while practically no Ca{sup 2+} bound after treatments to dissociate also the manganese of the water-oxidizing site. 34 refs., 9 figs., 2 tabs.« less
The effects of different Cl- depletion treatments in photosystem II (PS-II)-enriched membranes have been investigated by electron paramagnetic resonance (EPR) spectroscopy and by measurements of oxygen-evolving activity. The results indicated that the oxygen-evolving complex of PS-II exhibits two distinct Cl(-)-dependent properties. (1) After Cl(-)-free washes at pH 6.3, a reversibly altered distribution of structural states of PS-II was observed, manifested as the appearance of a g = 4 EPR signal from the S2 state in a significant fraction of centers (20-40%) at the expense of the S2 multiline signal. In addition, small but significant changes in the shape of the S2 multiline EPR signal were observed. Reconstitution of Cl- to Cl(-)-free washed PS-II rapidly reversed the observed effects of the Cl(-)-free washing. The anions, SO4(2-) and F-, which are often used during Cl- depletion treatments, had no effect on the S2 EPR properties of PS-II under these conditions in the absence or presence of Cl-. Flash experiments and measurements of oxygen evolution versus light intensity indicated that the two structural states observed after the removal of Cl- at pH 6.3 originated from oxygen-evolving centers exhibiting a lowered quantum yield of water oxidation. (2) Depletion of Cl- in PS-II by pH 10 treatment reversibly inhibited the oxygen-evolving activity to approximately 15%. The pH 10 treatment depleted the Cl- from a site which is considered to be equivalent to that studied in most earlier work on Cl(-)-depleted PS-II. The S2 state in pH 10/Cl(-)-depleted PS-II was reversibly modified to a state from which no S2 multiline EPR signal was generated and which exhibited an intense S2 g = 4 EPR signal corresponding to at least 40% of the centers but possibly to a much larger fraction of centers. The state responsible for the intense S2 g = 4 signal generated under these conditions is unlike that observed after removal of Cl- from PS-II at pH 6.3, in that this state was more stable in the dark, showing a half-decay time of approximately 1.5 h at 0 degrees C, and was unable to undergo further charge accumulation. Nevertheless, a fraction of centers, probably different from those exhibiting the S2 g = 4 signal, was able to advance to the formal S3 state, giving rise to a narrow EPR signal around g = 2. Addition of the anions SO4(2-) or F- to pH 10/Cl(-)-depleted PS-II affected the properties of PS-II, resulting in EPR properties of the S2 state similar to those reported earlier following Cl- depletion treatment of PS-II in the presence of these anions. Surprisingly, after addition of F-, the g = 4 EPR signal showed a damped flash-dependent oscillation. In addition, a narrow signal around g = 2, corresponding to the formal S3 state, also showed a damped flash-dependent oscillation pattern. The presence of oscillating EPR signals (albeit damped) in F(-)-treated pH 10/Cl(-)-depleted PS-II indicates functional enzyme turnover. This was confirmed by measurements of the oxygen-evolving activity versus light intensity which indicated that in approximately 45% of oxygen-evolving centers the enzyme turnover was slowed by a factor of 2. The distinct Cl- depletion effects in PS-II observed under the two different Cl- depletion treatments are considered to reflect the presence of two distinct Cl(-)-binding sites in PS-II.
Photosystem II membranes, dialyzed against a Cl(-)-free buffer to remove bound Cl-, lost about 65% of the control activity. A light-intensity study of the Cl(-)-free membranes showed that all PS II centers were able to evolve oxygen at about 35% of the control rate when measured in Cl(-)-free medium. The Cl(-)-depleted membranes were immediately (< 15 s) reactivated to 85-90% of the original activity by the addition of fairly high concentrations of Cl- (Kd = 0.5 mM), but both Cl- and the activity were promptly lost when the membranes immediately after reactivation were diluted in a Cl(-)-free medium. However, stabilization of Cl(-)-binding could be accomplished by prolonged incubation in the presence of Cl-. The transition to stable binding, followed using 36Cl-, occurred over several minutes. The stable binding was further characterized by a Kd of 20 microM and a t1/2 for dissociation of about 1h [Lindberg et al. (1993) Photosynth. Res. 38, 401-408]. The effects on S2 signals of removal of Cl- were studied using EPR. The depletion of Cl- was accompanied by a shift in intensity toward the g = 4.1 signal at the expense of the multiline signal. When Cl- or Br- but not F- was added to the depleted PS II membranes, the original distribution of the signals was immediately (< 30 s) restored. We propose that Cl(-)-binding responsible for high oxygen-evolution activity and normal EPR properties of the S2 state may occur either as high affinity (Kd = 20 microM) and slowly exchanging (t1/2 = 1 h), or as low affinity (Kd = 0.5 mM) and rapidly exchanging (t1/2 < 15 s). Our results suggest that Br- but not F- has a mode of binding similar to that of Cl-. The high-affinity state is the normal state of binding, but once Cl- has been removed, it will first rebind as low-affinity, rapidly exchanging followed by conversion into a high-affinity, slowly exchanging mode of binding.