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The present model for chlororespiration
Abstract
The present model of chlororespiration deals with the dark reduction and oxidation of plastoquinone. Both stages are reviewed here for algae and higher plants. Recent data confirm the presence of a plastoquinone:oxygen oxidoreductase with features different from those of the mitochondrial oxidases. The possible involvement of the chloroplast oxidase in the pathway of carotenoid biosynthesis is discussed in view of various experimental data and on energetics considerations.
... Десатуразы имеют сходные свойства. Они являются ФАД-связывающими белками, ассоциированными с поверхностью тилакоидной мембраны, обращенной в строму хлоропластов [Bennoun, 2002;Cunningham, Gantt, 1998;Sieiro et al., 2003]. Акцепторами электронов для PDS и ZDS служат окисленные пластохиноны, которые необходимы для протекания реакции in vivo [Bennoun, 2002]; для поддержания необходимого размера пула окисленных хинонов в тилакоидной мембране функционируют оксидазы пластидной дыхательной цепи, таким образом, интенсивность хлородыхания является важным фактором, влияющим на эффективность накопления каротиноидов [Bennoun, 2002]. ...
... Они являются ФАД-связывающими белками, ассоциированными с поверхностью тилакоидной мембраны, обращенной в строму хлоропластов [Bennoun, 2002;Cunningham, Gantt, 1998;Sieiro et al., 2003]. Акцепторами электронов для PDS и ZDS служат окисленные пластохиноны, которые необходимы для протекания реакции in vivo [Bennoun, 2002]; для поддержания необходимого размера пула окисленных хинонов в тилакоидной мембране функционируют оксидазы пластидной дыхательной цепи, таким образом, интенсивность хлородыхания является важным фактором, влияющим на эффективность накопления каротиноидов [Bennoun, 2002]. Их аминокислотная последовательность очень консервативна [Cunningham, Gantt, 1998;Sieiro et al., 2003]. ...
... Они являются ФАД-связывающими белками, ассоциированными с поверхностью тилакоидной мембраны, обращенной в строму хлоропластов [Bennoun, 2002;Cunningham, Gantt, 1998;Sieiro et al., 2003]. Акцепторами электронов для PDS и ZDS служат окисленные пластохиноны, которые необходимы для протекания реакции in vivo [Bennoun, 2002]; для поддержания необходимого размера пула окисленных хинонов в тилакоидной мембране функционируют оксидазы пластидной дыхательной цепи, таким образом, интенсивность хлородыхания является важным фактором, влияющим на эффективность накопления каротиноидов [Bennoun, 2002]. Их аминокислотная последовательность очень консервативна [Cunningham, Gantt, 1998;Sieiro et al., 2003]. ...
... Several reviews have been devoted to PTOX (16,38,91,110,116,130,146), but its role(s) and interplay with the photosynthetic function remain enigmatic. ...
... The increase in PSI antenna size agrees well with biochemical and structural data and so does the complementary decrease in that of PSII. The extent of ST in microalgae is therefore higher than in vascular plants, and contrary to those recent reports that suggested an LHCII aggregation-quenching process [14][15][16] (see supplementary information for further discussion), it consists mostly of photochemical quenching by PSI. The antenna redistribution model suits well with the ecological niche the algae inhabit, Chlamydomonas being often exposed to hypoxic or anoxic conditions, when growing on soil in dense populations 18 , conditions that are never met by land plants This habitat also results in low light exposure, as well as in the presence of reduced carbon sources from the microbial environment. ...
... Upon reduction of the PQ pool, phosphorylation of PSII core and antenna occurs, leading to electrostatic repulsion from the stacked regions of the membrane13,46and their migration to the unstacked regions10,20, loosening the overall stacking16. In state II, PSI-9 LHCa supercomplex further increases its size by about 40% (this article) by attaching two LHCII trimers and a monomeric antenna such as CP26/CP29 or 5 antenna subunits28,29,33,47 . ...
Chlororespiration was initially described in Chlamydomonas reinhardtii. This electron transfer pathway, found in all photosynthetic lineages, consists of the action of a NAD(P)H:plastoquinone oxidoreductase and a plastoquinol oxidase (PTOX). Hence, because it uses plastoquinones for electron transport, chlororespiration constitutes an electron pathway potentially antagonistic to the linear photosynthetic electron flow from H2O to CO2 However, the limited flow these enzymes can sustain suggests that their relative contribution, at least in the light and in steady-state conditions, is limited. I thus focused on the involvement of PTOX in Chlamydomonas during transitions from dark to light and vice versa. I found that, following a brief illumination, the redox relaxation of the chloroplast in the dark was much affected when PTOX2, the major plastoquinol oxidase in Chlamydomonas, is lacking. Importantly, I show that this has a significant physiological relevance as the growth of a PTOX2- lacking mutant is markedly slower in intermittent light, which can be rationalized in terms of a decreased flux sustained by photosystem II. I also investigated the influence of chlororespiration on cyclic electron flow using novel experimental techniques combined with theoretical modelling. Last, I explored, in collaboration with Stefano Santabarbara, the mechanism for redistribution of light excitation energy between the two photosystems, a process triggered by changes in the redox state of plastoquinone pool. I showed that, contrarily to what has been suggested recently, this regulation mechanism corresponds to an actual transfer of light harvesting antenna between the two photosystems.
... One strategy is to dissipate excess absorbed light energy as heat, possibly involving the high-light inducible proteins , Havaux et al. 2003. A second strategy involves transferring electrons from the PQ pool to molecular oxygen via the action of terminal oxidases (Bennoun 1982, 2002, Berry et al. 2002, Hart et al. 2005. When these strategies are not sufficient to prevent photoinhibition, D1 protein synthesis is upregulated to replace damaged D1 proteins and restore function to the PSII reaction center (Schaefer & Golden 1989a,b, Clarke et al. 1993, Long et al. 1994, Campbell et al. 1998a. ...
... In the light, PTOX may mediate the re-oxidation of the PQ pool reduced by PSII (Cournac et al. 2000). In the dark, PTOX may oxidize the PQ pool reduced internally via NADP(H) dehydrogenase (NDH), PQ oxidoreductase (Bennoun 1982, 2002, Rumeau et al. 2007), or phytoene desaturase (Carol & Kuntz 2001) (Fig. 1). Putative ptox genes encoded by both Synechococcus WH8102 and Prochlorococcus MED4 have been characterized in natural cyanobacterial assemblages in ocean gyres (McDonald & Vanlerberghe 2005), and recent investigations in Prochlorococcus sp. have demonstrated that the ptox gene is regulated by high-light exposure (Steglich et al. 2006). ...
... The interpretation of the role of PTOX is complicated by the fact that this oxidase is at the intersection of many redox pathways associated with the PQ pool. These pathways include oxidation of the PQ pool reduced by PSII (Shahbazi et al. 2007, by NDH or other reductases (Bennoun 1982, Peltier et al. 1987, Bennoun 2002, Rumeau et al. 2007, and by phytoene desaturase, which is associated with the desaturation reactions of carotenoid biosynthesis (Wetzel et al. 1994, Carol et al. 1999, Josse et al. 2000, Kuntz 2004). It appears that the importance of PTOX in these various pathways differs among photosynthetic organisms. ...
Expression of 3 gene families involved with photoacclimation-psbA (encoding the photosystem II reaction center protein D1), hli (encoding the high-light inducible proteins), and ptox (encoding the plastid terminal oxidase)-was compared in the marine cyanobacteria Synechococcus WH8102 and Prochlorococcus MED4 acclimated to either low or high light. These 2 strains, adapted for growth in oligotrophic marine environments, have distinct light-harvesting systems and respond differently to changes in irradiance. In response to growth at higher irradiance, Synechococcus WH8102 increased expression of the psbA multigene family (psbA1-4) 5-fold. Within this gene family, the expression of psbA2 increased 60-fold. Expression of 4 hli genes increased 2- to 5-fold, whereas expression of the ptox gene decreased 3-fold. In comparison, expression of the psbA gene increased 2-fold in Prochlorococcus MED4 cultures grown at higher irradiances. Expression of the Prochlorococcus MED4 hli6-9 and hli16-19 operons increased 11- to 14-fold, while ptox expression increased 3-fold. Using psbA induction as a standard for acclimation to changes in irradiance, we observed that the induction ratio of ptox:psbA1 and hli: psbA1 was 144 and 70 times greater, respectively, in Prochlorococcus MED4 compared with Synechococcus WH8102. These observations suggest that induction of ptox and hli may play a key role in the phototolerance of Prochlorococcus MED4. Conversely, the induction of psbA, and the synthesis of the PSII reaction center protein D1, may be critical for the acclimation of Synechococcus WH8102 to high irradiances.
... Plants recruit various non-water electron donors such as proline and glycine betaine to compensate for electrons generated through photolysis of water at the donor side of PSII under salinity stress . Plants enhance several enzymes such as chloroplast oxidases (involved in carotenoid biosynthesis) and NAD(P)H-plastoquinone oxidoreductases to maintain plastoquinine redox homeostasis, which enables stable electron transport rate across thylakoid membranes under extreme saline conditions (Bennoun, 2002). The root is the primary organ that senses salt stress and acts as a physical barrier to restrict Na + ion distribution across plants (Marriboina and Attipalli, 2020a). ...
... However, limited electron transport from PSII to Q A and constant maintenance of active PQ pool under high saline conditions are quite arguable. The involvement of chloroplast oxidases in higher plants and algae could maintain or recycle the PQ pool under stressful conditions (Bennoun, 2002). ...
Cultivation of potential biofuel tree species such as Pongamia pinnata would rehabilitate saline marginal lands toward economic gains. We carried out a physiological, biochemical, and proteomic analysis to identify key regulatory responses which are associated with salt tolerance mechanisms at the shoot and root levels. Pongamia seedlings were grown at 300 and 500 mM NaCl (∼3% NaCl; sea saline equivalent) concentrations for 15 and 30 days, gas exchange measurements including leaf net photosynthetic rate (Asat), stomatal conductance (gs), and transpiration rate (E), and varying chlorophyll a fluorescence kinetics were recorded. The whole root proteome was quantified using the free-labeled nanoLC-MS/MS technique to investigate crucial proteins involved in signaling pathways associated with salt tolerance. Pongamia showed no visible salt-induced morphological symptoms. However, Pongamia showed about 50% decline in gas exchange parameters including Asat, E, and gs 15 and 30 days after salt treatment (DAS). The maximum potential quantum efficiency of photosystem (PS) II (Fv/Fm) was maintained at approximately 0.8 in salt-treated plants. The thermal component of PSII (DIo) was increased by 1.6-fold in the salt-treated plants. A total of 1,062 protein species were identified with 130 commonly abundant protein species. Our results also elucidate high abundance of protein species related to flavonoid biosynthesis, seed storage protein species, and carbohydrate metabolism under salt stress. Overall, these analyses suggest that Pongamia exhibited sustained leaf morphology by lowering net photosynthetic rates and emitting most of its light energy as heat. Our root proteomic results indicated that these protein species were most likely recruited from secondary and anaerobic metabolism, which could provide defense for roots against Na⁺ toxicity under salt stress conditions.
... Chlororespiration is a term that was coined by Bennoun (1982) !30 years ago in analogy to the mitochondrial respiration in Chlamydomonas reinhardtii. Chlororespiration refers to a respiratory electron transport chain in the thylakoid membrane of chloroplasts, which interacts with the photosynthetic electron transport chain and involves both non-photochemical reduction and oxidation of the PQ pool with the corresponding consumption of molecular oxygen (Nixon 2000, Bennoun 2002. Protons are transferred from stromal reduction to the PQ pool through the NDH complex, while plastid terminal oxidases (PTOX), which act as a plastoquinol (PQH 2 ) oxygen oxidoreductases and remove electrons from PQH 2 , reduce the accumulation of reactive oxygen species (ROS) via a non-electrogenic process (Bennoun 2002, Aluru and Rodermel 2004, Kuntz 2004, Aluru et al. 2006, Nawrocki et al. 2015. ...
... Chlororespiration refers to a respiratory electron transport chain in the thylakoid membrane of chloroplasts, which interacts with the photosynthetic electron transport chain and involves both non-photochemical reduction and oxidation of the PQ pool with the corresponding consumption of molecular oxygen (Nixon 2000, Bennoun 2002. Protons are transferred from stromal reduction to the PQ pool through the NDH complex, while plastid terminal oxidases (PTOX), which act as a plastoquinol (PQH 2 ) oxygen oxidoreductases and remove electrons from PQH 2 , reduce the accumulation of reactive oxygen species (ROS) via a non-electrogenic process (Bennoun 2002, Aluru and Rodermel 2004, Kuntz 2004, Aluru et al. 2006, Nawrocki et al. 2015. As a chloroplasttargeted quinol oxidase, homologous to mitochondrial alternative oxidase, PTOX is sensitive to propyl gallate (PG) , Bennoun 2001). ...
As an alternative electron sink, chlororespiration, comprised of the NAD(P)H dehydrogenase complex and plastid terminal plastoquinone oxidase, may play a significant role for sustaining the redox equilibrium between stroma and thylakoid membrane. This study identified a distinct role of chlororespiration in the marine angiosperm Zostera marina, whose oxygen evolving complex (OEC) is prone to photo-inactivation as a result of its inherent susceptibility to excess irradiation. The strong connectivity between OEC peripheral proteins and key chlororespiratory enzymes, as demonstrated in the interaction network of differentially expressed genes, suggested that the recovery of photo-inactivated OEC was connected with chlororespiration. Chlorophyll fluorescence, transcriptome, and Western blot data verified a new physiological role of chlororespiration to function as photoprotection and generate proton gradient across the thylakoid membrane for the recovery of photo-inactivated OEC. Chlororespiration was only activated in darkness following excess irradiation exposure, which might be related to the electron deficiency in the electron transport chain because of the continuous impairment of OEC. The activation of chlororespiration in Z. marina was prone to proactivity, which was also supported by the further activation of the oxidative pentose-phosphate pathway synthesizing NADPH to meet the demand of chlororespiration during darkness. This phenomenon is distinct from the common assumption that chlororespiration is prone to consuming redundant reducing power during the short transition phase from light to dark.
... Alors que la chaîne de transport des électrons de la photosynthèse est clairement limitée aux chloroplastes, la chaîne de transport des électrons de la respiration cellulaire, initialement limitée à la mitochondrie, est également présente(au moins partiellement) dans les chloroplastes (Bennoun, 2002;. ...
... La chlororespiration est donc une voie alternative d'électrons comportant la réduction de plastoquinones (PQ) par l'action de la NDH (NADH réductase du chloroplaste), l'oxydation des plastoquinones réduites par l'oxydase terminale plastidial (PTOX) avec la réduction de l'oxygène en eau (Aluru & Rodermel, 2004;Bennoun, 2002;Nixon, 2000).Il est connu que, dans des conditions normales de saturation de la chaîne de transport des électrons (régime «Z»),les chloroplastes gèrent un transport d'électrons cyclique dans le but de contrôler la lumière à travers un mécanisme de quenching non-photochimique de la fluorescence de la chlorophylle (NPQ) (Johnson, 2005 (Rumeau, Peltier, & Cournac, 2007). ...
Les interactions entre chloroplastes et mitochondries restent encore mal connues, notamment en réponse à des conditions de stress. Dans ces conditions, il est suggéré que les voies de transfert d'électrons liées à des protéines découplantes AOX ou pUCP (mitochondriales) et PTOX (plastidiale) pourraient limiter la formation de ROS afin d'atténuer les dommages oxydatifs dans ces organites. Dans le cadre de notre travail, nous avons retenu deux cultivars de Vigna unguiculata, cv 1183 et cv EPACE. Les deux cultivars n'ont pas montré de réelles différences de sensibilité à la sécheresse. Par contre le cultivar EPACE s'est montré plus tolérant à l'O3 sur la base du développement des nécroses et plusieurs paramètres physiologiques (Fv/Fm, [phi]PSII) et biochimiques (glutathion). Pour les profils d'expression des gènes codant pour l'AOX, la pUCP et la PTOX deux réponses ont été clairement identifiées chez le cultivar EPACE. Sur un court terme l'expression de ces protéines est généralement stimulée. Sur un plus long terme (14 jours), la réponse diffère en fonction de la contrainte. Sous O3, la plus forte expression des protéines mitochondriales est maintenue alors que le gène codant pour la PTOX est sousrégulé. Sous sécheresse seule la protéine plastidiale (PTOX) reste sur-régulée. Dans des conditions de combinaison de contrainte, la sécheresse a peu d'effet sur l'influx d'O3 dans les feuilles, et les gènes VuPTOX et VuUCP1b sont sur-régulés après 3 et 7 jours de contrainte. Cette sur-régulation, déjà observée en réponse à la sécheresse seule, pourrait jouer un rôle déterminant pour prévenir la formation de ROS à la fois dans la mitochondrie et dans le chloroplaste
... Tóth et al. 2007b) and leaves that show a high rate of chlororespiration. Chlororespiration refers to the non-photochemical reduction of the plastoquinone pool by reducing equivalents derived from Fd red or NADPH in the stroma (Bennoun 2002). Feild et al. (1998) showed a high chlororespiratory activity in light acclimated sunflower leaves following a light-to-dark transition leading to considerably higher F O 0 values. ...
... As noted in Question 16, a process called chlororespiration has been identified in higher plants (Bennoun 1982(Bennoun , 2002Rumeau et al. 2007). Cyanobacteria, which are thought to be the ancestors of the chloroplast, lack mitochondria; instead they have a respiratory chain that shares the PQpool with the photosynthetic ETC (Vermaas 2001;Schmetterer and Pils 2004;Hart et al. 2005). ...
... All phytoplankton use their photosynthetic membranes for respiratory electron transport in the dark (i.e., chlororespiration), and this mechanism is directly manifested as a nocturnal decrease in PSII (Behrenfeld et al. 2006). In the dark, an NADP-dependent plastoquinone oxidoreductase complex is capable of reducing plastoquinone, which is subsequently re-oxidized by molecular oxygen, a respiratory reaction that serves to establish a charge potential across the thylakoid membrane and thus allows the generation of energy (ATP) to fuel a variety of biological processes in the dark (Bennoun 2002). The physiological chlororespiration pathway is proposed to be involved with carotenoid biosynthesis (Bennoun 2002), as both chlororespiration and carotenoid biosyntheses have been shown to share the plastoquinone pool as a common intermediate. ...
... In the dark, an NADP-dependent plastoquinone oxidoreductase complex is capable of reducing plastoquinone, which is subsequently re-oxidized by molecular oxygen, a respiratory reaction that serves to establish a charge potential across the thylakoid membrane and thus allows the generation of energy (ATP) to fuel a variety of biological processes in the dark (Bennoun 2002). The physiological chlororespiration pathway is proposed to be involved with carotenoid biosynthesis (Bennoun 2002), as both chlororespiration and carotenoid biosyntheses have been shown to share the plastoquinone pool as a common intermediate. Given the strong potential for ultraviolet radiation damage to phytoplankton in Lake Erie waters because of its relatively low ultraviolet radiation attenuation relative to PAR attenuation (Hiriart et al. 2002), carotenoid biosynthesis by phytoplankton in surface waters of Lake Erie during summer thermal stratification is likely to be constantly engaged. ...
Unattended sensor networks are a cost-effective strategy to enhance the resolution of environmental datasets and are required to understand how large aquatic ecosystems respond to complex stressors (e.g., climate change). We made unattended and continuous measurements of the quantum yield of photosystem II (ΦPSII) photochemistry in the surface mixed layer of Lake Erie during three lake-wide cruises to observe how phytoplankton physiology varied across nutrient and taxonomic gradients. Three prominent diel ΦPSII patterns were noted. The diel maximum consistently occurred at sunrise or sunset, nocturnal measurements were consistently lower than diel maxima, and daytime values were strongly diminished by nonphotochemical quenching. The diurnal pattern was modeled as a function of irradiance to a mean accuracy of 0.03 to 0.04. Contrary to previously published reports in Lake Erie, ΦPSII was largely insensitive to indices of nutrient deficiency through space and time. This finding was consistent with much recent literature about ΦPSII and suggests that Lake Erie phytoplankton, like many others, can tune their photosynthetic machinery to maintain relatively high efficiency of photochemistry in photosystem II even when deficient in phosphorus or nitrogen. © 2015, National Research Council of Canada. All Rights Reserved.
... Tóth et al. 2007b) and leaves that show a high rate of chlororespiration. Chlororespiration refers to the non-photochemical reduction of the plastoquinone pool by reducing equivalents derived from Fd red or NADPH in the stroma (Bennoun 2002). Feild et al. (1998) showed a high chlororespiratory activity in light acclimated sunflower leaves following a light-to-dark transition leading to considerably higher F O 0 values. ...
... As noted in Question 16, a process called chlororespiration has been identified in higher plants (Bennoun 1982(Bennoun , 2002Rumeau et al. 2007). Cyanobacteria, which are thought to be the ancestors of the chloroplast, lack mitochondria; instead they have a respiratory chain that shares the PQpool with the photosynthetic ETC (Vermaas 2001;Schmetterer and Pils 2004;Hart et al. 2005). ...
... Aerobic microbial R is the principal sink for O 2 in most natural aquatic environments (Stumm and Morgan, 1996;Wetzel, 2001). Aquatic community R is defined as the weighted mean of all O 2 consuming pathways, which include: (1) microbial R via the cytochrome oxidase pathway in eukaryotes and the equivalent pathway in prokaryotes; (2) microbial R via the alternative oxidase pathway in eukaryotic autotrophs; (3) photorespiration in photoautotrophs (Osmond, 1981); (4) the Mehler-peroxidase reaction in photoautotrophs (Laws et al., 2000); (5) chlororespiration in eukaryotes (Bennoun, 2002); (6) photochemical consumption of O 2 (Andrews et al., 2000;Miles and Brezonik, 1981); and (7) geochemical oxidation of reduced species such as Fe 2+ , NH + 4 , and S. The relative importance of each pathway will likely be different for aquatic systems of differing community structure. The first two pathways are the most prevalent and occur both in light and dark, consuming organic carbon and O 2 while producing CO 2 . ...
... All of these pathways preferentially consume the light O 2 isotopologue (cf. Andrews et al., 2000;Bennoun, 2002;Chomicki and Schiff , 2008;Laws et al., 2000;Miles and Brezonik, 1981;. The magnitude of the dissolved O 2 community R isotopic fractionation factor (α R ) is a function of the type of aquatic respiring community and the factors that affect R rates. ...
... In prokaryotic cyanobacteria, PETC and RETC coexist in the thylakoid membrane and share some intermediate components (Myers, 1986;Cooley et al., 2000). As the evolutionary remnants of cyanobacteria, chloroplasts also contain an O 2 -dependent electron transfer pathway, referred to as chlororespiration, which involves the NADH dehydrogenase complex, the PQ pool, and the plastid terminal oxidase (PTOX) (Bennoun, 2002). As a distant homolog of mitochondrial AOX (Carol et al., 1999;Wu et al., 1999), PTOX transfers electrons from PQH 2 to oxygen at the thylakoid membrane, mimicking the function of AOX at the mitochondrial inner membrane (Nawrocki et al., 2015). ...
Alternative oxidase (AOX) and plastid terminal oxidase (PTOX) are terminal oxidases of electron transfer in mitochondria and chloroplasts, respectively. Here, taking advantage of the variegation phenotype of the Arabidopsis PTOX deficient mutant (im), we examined the functional relationship between PTOX and its five distantly related homologs (AOX1a, 1b, 1c, 1d, and AOX2). When engineered into chloroplasts, AOX1b, 1c, 1d, and AOX2 rescued the im defect, while AOX1a partially suppressed the mutant phenotype, indicating that AOXs could function as PQH2 oxidases. When the full length AOXs were overexpressed in im, only AOX1b and AOX2 rescued its variegation phenotype. In vivo fluorescence analysis of GFP-tagged AOXs and subcellular fractionation assays showed that AOX1b and AOX2 could partially enter chloroplasts while AOX1c and AOX1d were exclusively present in mitochondria. Surprisingly, the subcellular fractionation, but not the fluorescence analysis of GFP-tagged AOX1a, revealed that a small portion of AOX1a could sort into chloroplasts. We further fused and expressed the targeting peptides of AOXs with the mature form of PTOX in im individually; and found that targeting peptides of AOX1a, AOX1b, and AOX2, but not that of AOX1c or AOX1d, could direct PTOX into chloroplasts. It demonstrated that chloroplast-localized AOXs, but not mitochondria-localized AOXs, can functionally compensate for the PTOX deficiency in chloroplasts, providing a direct evidence for the functional relevance of AOX and PTOX, shedding light on the interaction between mitochondria and chloroplasts and the complex mechanisms of protein dual targeting in plant cells.
... This phosphorylation-mediated NPQ mechanism [37] is particularly active in C. reinhardtii during the first few minutes of light-to-dark and dark-to-light acclimation and is affected in npq4 [38]. High light-treated cells are in state 1, but when subjected to darkness they transit to state 2 due to chlororespiration-induced phosphorylation of specific components of the antenna, including LHCII and LHCSR3 [39,40]. During transition to state 2, the majority of LHCII migrates energy transfer to PSI [41], and LHCSR3 also migrates as part of the LHCSR3-LHCII antenna of PSI [40]. ...
Natural light intensities can rise several orders of magnitude over subsecond time spans, posing a major challenge for photosynthesis. Fluctuating light tolerance in the green alga Chlamydomonas reinhardtii requires alternative electron pathways, but the role of nonphotochemical quenching (NPQ) is not known. Here, fluctuating light (10 min actinic light followed by 10 min darkness) led to significant increase in NPQ/qE-related proteins, LHCSR1 and LHCSR3, relative to constant light of the same subsaturating or saturating intensity. Elevated levels of LHCSR1/3 increased the ability of cells to safely dissipate excess light energy to heat (i.e., qE-type NPQ) during dark to light transition, as measured with chlorophyll fluorescence. The low qE phenotype of the npq4 mutant, which is unable to produce LHCSR3, was abolished under fluctuating light, showing that LHCSR1 alone enables very high levels of qE. Photosystem (PS) levels were also affected by light treatments; constant light led to lower PsbA levels and F v /F m values, while fluctuating light led to lower PsaA and maximum P700 + levels, indicating that constant and fluctuating light induced PSII and PSI photoinhibition, respectively. Under fluctuating light, npq4 suffered more PSI photoinhibition and significantly slower growth rates than parental wild type, whereas npq1 and npq2 mutants affected in xanthophyll carotenoid compositions had identical growth under fluctuating and constant light. Overall, LHCSR3 rather than total qE capacity or zeaxanthin is shown to be important in C. reinhardtii in tolerating fluctuating light, potentially via preventing PSI photoinhibition.
... As such, the previous conclusion (52) that the PIFR represents a very slow rate of PQ pool reduction should be reassessed, as these kinetics probably reflect, not a slow NDH, but the shifting of equilibrium between stromal reductants and the PQ pool as ⌬p is dissipated in the dark. PTOX activity, acting as a slow drain of electrons from the PQ pool under aerobic conditions (49,50), may also be expected to modify the kinetics of the fluorescence transients observed in Fig. 3, although the effect of anaerobiosis on the PIFR transients was observed to be relatively minor in S. oleracea and A. hybridus (supplemental Fig. S7). ...
Cyclic electron flow around photosystem I (CEF) is critical for balancing the photosynthetic energy budget of the chloroplast, by generating ATP without net production of NADPH. We demonstrate that the chloroplast NADPH dehydrogenase complex (NDH), a homolog to respiratory Complex I, pumps approximately two protons from the chloroplast stroma to the lumen per electron transferred from ferredoxin to plastoquinone, effectively increasing the efficiency of ATP production via CEF by two-fold compared to CEF pathways involving non-proton-pumping plastoquinone reductases. Under certain physiological conditions, the coupling of proton and electron transfer reactions within NDH should enable a non-canonical mode of photosynthetic electron transfer, allowing electron transfer from plastoquinol to NADPH to be driven by the thylakoid proton motive force possibly helping to sense or remediate mismatches in the photosynthetic budget.
... Wang et al. [31] found in the transcriptome of H. pluvialis two forms of PTOX-ptox1 and ptox2 involved in the biosynthesis of astaxanthin. The importance of this enzyme for protection against oxidative stress has been shown [32][33][34]. The membrane-bound β-lycopene cyclase catalyzes the formation of dicyclic rings containing two β-ionone rings from a symmetrical linear lycopene molecule [24]. ...
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... In this route, plants are able to increase their metabolism to create alternative sinks for the excess of electrons. This metabolic dissipation could be achieved by the cyclic electron flow around PSI, the photorespiration pathway or the chlororespiration (Osmond and Grace 1995, Asada 1999, Bennoun 2002. Moreover, the VAZ (violaxanthin, antheraxanthin, zeaxanthin) xanthophyl cycle can also be used to dissipate the excess of energy produced by the lightharvesting complexes into a harmless longer wavelength with the conversion of violaxanthin (V) to zeaxanthin (Z) via antheraxanthin (A) (Takahashi and Badger 2011). ...
High-mountain-ecosystems in the Mediterranean-type climate are exceptional because of their outstanding biodiversity but also because of their characteristic drought stress in summer. Still, plant functioning in these habitats has been largely understudied. Here, morphological, photochemical, and biochemical traits were seasonally assessed in six shrubs characterized by contrasting morphological traits, in the Teide mountain in the Canary Islands. Two adjacent populations, the first located in an open site and the second in the understorey of Pinus canariensis treeline forest, were evaluated. We aimed at disentangling (1) the role of morphological and biochemical photoprotective strategies and of their seasonal plasticity to cope with changing environmental conditions at this semiarid ecosystem, (2) how the interspecific differences in biochemical photoprotection are related to leaf morphology and phenology and (3) how living in the understory of the treeline may affect those responses. Our results indicate that both morphological and biochemical traits (particularly leaf habit and morphology and carotenoids from the β-branch) play an intricate role in photoprotection, and that a high interspecific variability exists. According to the down-regulation of photochemical activity and the upregulation of photoprotective molecules, species could be grouped into three types: (1) those more responsive to summer stress (e.g. Descurainia bourgeauana); (2) those more responsive to winter stress (e.g. Pterocephalus lasiospermus, Scrophularia glabrata and Adenocarpus viscosus); and (3) those showing rather constant behaviour across seasons (e.g. Spartocytisus supranubius and Erysimum scoparium). In all the species, plants in the open site showed a marked seasonal physiological response in most of the studied parameters. Pinus canariensis canopy buffers environmental abiotic constrains. On a global change scenario, and provided further functional studies are needed, our results pinpoints heterogeneity in the sensitivity of these species against for instance late-frost or summer-heat/drought events, which could easily shift current species distribution in the coming years. This article is protected by copyright. All rights reserved.
... Further adjustment is obtained by transferring LHCII units in order to cope with sudden changes in the light spectrum 8 , forming PSI-LHCI-LHCII supercomplexes with an enhanced antenna size 9,10 . The plastoquinone redox state is further affected by the rate of electron transport from reduced substrates; that is, from mitochondria through to the chlororespiratory pathway 11 . In mixotrophic organisms such as Chlamydomonas reinhardtii, changes in the PSI antenna size can be controlled through the supply of organic respiratory substrates that reduce plastoquinone and activate the STT7 kinase. ...
Photosystem I of the moss Physcomitrella patens has special properties, including the capacity to undergo non-photochemical fluorescence quenching. We studied the organization of photosystem I under different light and carbon supply conditions in wild-type moss and in moss with the lhcb9 (light-harvesting complex) knockout genotype, which lacks an antenna protein endowed with red-shifted absorption forms. Wild-type moss, when grown on sugars and in low light, accumulated LHCB9 proteins and a large form of the photosystem I supercomplex, which, besides the canonical four LHCI subunits, included a LHCII trimer and four additional LHC monomers. The lhcb9 knockout produced an angiosperm-like photosystem I supercomplex with four LHCI subunits irrespective of the growth conditions. Growth in the presence of sublethal concentrations of electron transport inhibitors that caused oxidation or reduction of the plastoquinone pool prevented or promoted, respectively, the accumulation of LHCB9 and the formation of the photosystem I megacomplex. We suggest that LHCB9 is a key subunit regulating the antenna size of photosystem I and the ability to avoid the over-reduction of plastoquinone: this condition is potentially dangerous in the shaded and sunfleck-rich environment typical of mosses, whose plastoquinone pool is reduced by both photosystem II and the oxidation of sugar substrates. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
... The non-photochemical reduction of the PQ pool can be estimated by the measurement of chlorophyll induction kinetics. It is generally accepted that the chlorophyll (Chl)a fluorescence rise kinetics reflect the progressive reduction of the photosynthetic electron transport chain [72,73], i.e., the reduction of Q A to Q A À and the reduction of the PQ pool. In particular, the increase in the J-step level is taken as an indication of a reduced transfer of electrons further than Q A and V J parameter and, thus, of an accumulation of reduced Q A À [70] (Fig. 3). ...
... In these plants, non-photochemical quenching of energy excitation (NPQ) remained similar to the values observed in control plants during the entire experimental period (figure 4). Such behavior is consistent with the light protection activity of the nicotinamide adenine dinucleotide (phosphate)dehydrogenase (Ndh), a component of the respiratory electron transport chain within the chloroplast thylakoid membrane (chlororespiration) (Bennoun, 1982;2002). In the light, the Ndh complex is able to oxidize stromal reductant and thus act as an emergency electron sink for photosynthetic electron transport, in order to avoid the generation of reactive oxygen species in the stroma (Nixon, 2000). ...
... The non-photochemical reduction of the PQ pool can be estimated by the measurement of chlorophyll induction kinetics. It is generally accepted that the chlorophyll (Chl)a fluorescence rise kinetics reflect the progressive reduction of the photosynthetic electron transport chain [72,73], i.e., the reduction of Q A to Q A À and the reduction of the PQ pool. In particular, the increase in the J-step level is taken as an indication of a reduced transfer of electrons further than Q A and V J parameter and, thus, of an accumulation of reduced Q A À [70] (Fig. 3). ...
... The cytochrome (cyt) b 6 /f complex is located between PSII and PSI at a crossroad of different electron pathways (linear electron transport, Q cycle, chlororespiration, cyclic electron transport) (Sacksteder et al. 2000;Bennoun 2002;Mulkidjanian 2010;Johnson 2011;Shikanai 2014) and is an important site for the regulation of electron flow and the control of regulatory mechanisms like state transitions (see Question 8) and q E . The Q cycle and cyclic electron transport increase the ATP to NADPH ratio by diverting electrons away from NADP ? ...
... The cytochrome (cyt) b 6 /f complex is located between PSII and PSI at a crossroad of different electron pathways (linear electron transport, Q cycle, chlororespiration, cyclic electron transport) (Sacksteder et al. 2000;Bennoun 2002;Mulkidjanian 2010;Johnson 2011;Shikanai 2014) and is an important site for the regulation of electron flow and the control of regulatory mechanisms like state transitions (see Question 8) and q E . The Q cycle and cyclic electron transport increase the ATP to NADPH ratio by diverting electrons away from NADP ? ...
Using chlorophyll (Chl) a fluorescence many aspects of the photosynthetic apparatus can be studied, both in vitro and, noninvasively, in vivo. Complementary techniques can help to interpret changes in the Chl a fluorescence kinetics. Kalaji et al. (Photosynth Res 122:121-158, 2014a) addressed several questions about instruments, methods and applications based on Chl a fluorescence. Here, additional Chl a fluorescence-related topics are discussed again in a question and answer format. Examples are the effect of connectivity on photochemical quenching, the correction of F V /F M values for PSI fluorescence, the energy partitioning concept, the interpretation of the complementary area, probing the donor side of PSII, the assignment of bands of 77 K fluorescence emission spectra to fluorescence emitters, the relationship between prompt and delayed fluorescence, potential problems when sampling tree canopies, the use of fluorescence parameters in QTL studies, the use of Chl a fluorescence in biosensor applications and the application of neural network approaches for the analysis of fluorescence measurements. The answers draw on knowledge from different Chl a fluorescence analysis domains, yielding in several cases new insights.
... Due to the large defect on q E in pgr5, an observation not made in the NDH deficient mutants, the case has been made that the FQR is the main route of CEF (Munekage et al. 2002;Munekage et al. 2004). However, as discussed above, this mutant has a much larger defect on thylakoid proton conductivity, which could also explain the inability to maintain a ΔpH (Avenson et al. 2005a (Bendall and Manasse 1995;Bennoun 2002)] was identified chloroplast genome sequence data and shown to be enzymatically active Burrows et al. 1998). ...
... PTOX is localized in the non-appressed regions of the thylakoid membrane (Lennon et al., 2003) and is involved in carotenoid biosynthesis, plastid development, and chlororespiration. Reviews have focused on the role of PTOX in chlororespiration (Bennoun, 2002;Rumeau et al., 2007), in chloroplast biogenesis (Putarjunan et al., 2013) and in stress responses (McDonald et al., 2011;Sun and Wen, 2011). A recent review by Nawrocki et al. (2015) has addressed the role of PTOX in poising the chloroplast redox potential in darkness. ...
... The occurrence of a partially reduced PQ pool in the dark is attributed to the transfer of reducing equivalents from mitochondria to chloroplasts, but also the degradation of chloroplastic starch reserves (Bult e et al., 1990;Wieckowski and Wojtczak, 1997). The reduction state of the PQ pool also involves chlororespiration, a process that requires NAD(P)H dehydrogenase (NDA2 in Chlamydomonas), which reduces PQ, and plastid terminal oxidase 2, which regenerates oxidized PQ through reduction of O 2 ; this occurs in Chlamydomonas and other photosynthetic organisms (Bennoun, 2002;Peltier and Cournac, 2002;Bailey-Serres and Voesenek, 2008;Jans et al., 2008;Houille-Vernes et al., 2011). Although it was once thought that chlororespiration was involved in dark ATP production in chloroplasts, there is now evidence suggesting that chlororespiration may not be electrogenic (i.e. ...
Chlamydomonas reinhardtii is a unicellular, soil-dwelling (and aquatic) green alga that has significant metabolic flexibility for balancing redox equivalents and generating ATP when it experiences hypoxic/anoxic conditions. The diversity of pathways available to ferment sugars is often revealed in mutants in which the activities of specific branches of fermentative metabolism have been eliminated; compensatory pathways having little activity in parental strains under standard laboratory fermentative conditions are often activated. The ways in which these pathways are regulated and integrated are little explored. In this text, we primarily discuss the intricacies of dark anoxic metabolism in Chlamydomonas, but also reference aspects of dark oxic metabolism, the utilization of acetate, and the relatively uncharacterized but critical interactions that link chloroplast and mitochondrial metabolic networks. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
... Several reviews have been devoted to PTOX (16,37,90,109,115,129,146), but its role(s) and interplay with the photosynthetic function remain enigmatic. Here, we describe some structural features of the enzyme in light of the recent structural data obtained for its mitochondrial counterpart. ...
Plastids have retained from their cyanobacterial ancestor a fragment of the respiratory electron chain comprising an NADPH dehydrogenase and a diiron oxidase, which sustain the so-called chlororespiration pathway. Despite its very low turnover rates compared with photosynthetic electron flow, knocking out the plastid terminal oxidase (PTOX) in plants or microalgae leads to severe phenotypes that encompass developmental and growth defects together with increased photosensitivity. On the basis of a phylogenetic and structural analysis of the enzyme, we discuss its physiological contribution to chloroplast metabolism, with an emphasis on its critical function in setting the redox poise of the chloroplast stroma in darkness. The emerging picture of PTOX is that of an enzyme at the crossroads of a variety of metabolic processes, such as, among others, the regulation of cyclic electron transfer and carotenoid biosynthesis, which have in common their dependence on the redox state of the plastoquinone pool, set largely by the activity of PTOX in darkness. Expected final online publication date for the Annual Review of Plant Biology Volume 66 is April 29, 2015. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
... The addition of DCMU, an inhibitor of PS II, also suppressed the light-induced evolution of oxygen by WT cells (data not shown). In the presence of 20 mM KCN, which should completely inhibit cyanide-sensitive terminal oxidases, the rates of oxygen consumption by both WT and PS II À cells (Fig. 7B) were slowed down by a factor of 6. Remaining respiration could be assigned to KCNÀresistant oxidases and/or to so-called chlororespiration process [36]. ...
In this work, we investigated electron transport processes in the cyanobacterium Synechocystis sp. PCC 6803, with a special emphasis focused on oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains. Redox transients of the photosystem I primary donor P700 and oxygen exchange processes were measured by the EPR method under the same experimental conditions. To discriminate between the factors controlling electron flow through photosynthetic and respiratory electron transport chains, we compared the P700 redox transients and oxygen exchange processes in wild type cells and mutants with impaired photosystem II and terminal oxidases (CtaI, CydAB, CtaDEII). It was shown that the rates of electron flow through both photosynthetic and respiratory electron transport chains strongly depended on the transmembrane proton gradient and oxygen concentration in cell suspension. Electron transport through photosystem I was controlled by two main mechanisms: (i) oxygen-dependent acceleration of electron transfer from photosystem I to NADP + , and (ii) slowing down of electron flow between photosystem II and photosystem I governed by the intrathylakoid pH. Inhibitor analysis of P700 redox transients led us to the conclusion that electron fluxes from dehydrogenases and from cyclic electron transport pathway comprise 20–30% of the total electron flux from the intersystem electron transport chain to P700 + .
... 2 and 3) and often occur at a similar O 2 level in apples (Peppelenbos and Rabbinge, 1996). Past research suggests that an anaerobic environment, even in darkness, leads to an over-reduction of the chloroplast's PQ pool (Harris and Heber, 1993;Bennoun, 2002;Tóth et al., 2005;Wright et al., 2011). Wright et al. (2011 speculated that low O 2 leads to an overabundance of cytosolic reductant and a loss of homeostasis as a result of a marked decline in the mitochondria's cytochrome oxidase; excess reductant is then transported into the chloroplast where it is used to reduce the plastoquinone (PQ) pool. ...
A link between the minimum fluorescence (Fo) and a metabolic shift from predominantly aerobic to fermentative metabolism [i.e. the lower oxygen limit (LOL)] is the foundation of dynamic controlled atmosphere (DCA). Current DCA technology uses pulse frequency modulated (PFM) sensors and employs a range of light intensities and extrapolation to measure Fα, an approximation of Fo. Like fruit mass, colour, sugar or acid levels, the LOL is inherently variable, even between apples (Malus domestica) (for example) from a given cultivar and tree or between the sun-exposed and shaded regions of a single fruit. The physiological link between metabolism and fluorescence has not been extensively studied. However, recent work suggests the low-O2-induced rise in Fα results from a shut down of mitochondrial function and a buildup of reductant that leads to an over-reduction of the plastoquinone (PQ) pool and a decrease in photochemical quenching. Hypoxic conditions above the LOL can decrease Fα slightly in some species, possibly as a result of zeaxanthin formation and increased non-photochemical quenching. Low-intensity light differentially affects Fα depending on the O2 level: light increases Fα when O2 levels are above the LOL due to light-induced reduction of the oxidized PQ pool, but decreases the elevated Fα signal below the LOL as a result of a PSI-driven oxidation of the over-reduced PQ pool. Temperature has a negative, primarily non-physiological correlation with the Fα baseline which seems unrelated to the PQ pool redox state. Understanding how O2 and other factors affect Fα may improve the utility and commercial application of DCA.
... In addition to photosynthetic electron transport in the chloroplast, a respiratory electron flow (from NAD(P)H to PQ and O 2 ), named chlororespiration and involving both a nonphotochemical reduction and re-oxidation of PQ pool, was indicated to exist in the chloroplast of higher plants and originate from a bacterial ancestor (Garab et al., 1989;Gruszecki et al., 1994;Scherer, 1990). This concept is supported by the recent identification and characterization of several components that could function in the chlororespiratory pathway (see reviews by Bennoun, 2002;Peltier and Cournac, 2002). A membrane associated plastid NAD(P)H dehydrogenase (NDH) complex was shown to mediate non-photochemical reduction of the PQ pool by stromal reductants, which can be monitored by a far-red light quenchable postillumination chlorophyll (Chl) fluorescence increase after turning off actinic light Shikanai et al., 1998). ...
ABSTRACT A transient rise in chlorophyll fluorescence after turning off actinic light reflects non-
... In these plants, non-photochemical quenching of energy excitation (NPQ) remained similar to the values observed in control plants during the entire experimental period (figure 4). Such behavior is consistent with the light protection activity of the nicotinamide adenine dinucleotide (phosphate)dehydrogenase (Ndh), a component of the respiratory electron transport chain within the chloroplast thylakoid membrane (chlororespiration) (Bennoun, 1982;2002). In the light, the Ndh complex is able to oxidize stromal reductant and thus act as an emergency electron sink for photosynthetic electron transport, in order to avoid the generation of reactive oxygen species in the stroma (Nixon, 2000). ...
The physiological performance of acariquara (Minquartia guianensis) seedlings submitted to water deficit and the recovery of physiological parameters during rehydration were investigated in a greenhouse experiment. The analyzed parameters were: leaf water potential, gas exchange and chlorophyll a fluorescence. After thirty-five days, non-irrigated plants exhibited a leaf water potential 70 % lower compared to control plants (irrigated daily) and the stomatal conductance reached values close to zero, inducing a severe decrease in gas exchange (photosynthesis and transpiration). Six days after the beginning of the rehydration of drought-stressed plants, the results demonstrated that water stress did not irreversibly affect gas exchange and quantum efficiency of photosystem II (PSII) in M. guianensis seedlings, since four to six days after rehydration the plants exhibited total recovery of the photosynthetic apparatus. We conclude that M. guianensis presented good tolerance to water stress and good capacity to recover the physiological performance related to leaf water status, photosynthesis and photochemical efficiency of PS II under hydric stress, suggesting substantial physiological plasticity during the juvenile phase for this tree species.
... All of these pathways preferentially consume the light O 2 isotopologue (cf. Miles and Brezonik 1981, Andrews et al. 2000, Laws et al. 2000, Bennoun 2002, Venkiteswaran et al. 2007, Chomicki and Schiff 2008. The magnitude of the dissolved O 2 community R isotopic fractionation factor (a R ) is a function of the type of aquatic respiring community and the factors that affect R rates. ...
The spatial footprint over which municipal wastewater effluents cause changes to aquatic community structure and metabolism is key information required for the management of discharges into rivers. Longitudinal studies were undertaken on the Bow and South Saskatchewan rivers, Canada, to assess a new isotopic and modelling approach that combined O-2 and delta O-18-O-2 diel (24 h) response curves to quantify changes in integrated community aquatic metabolism as a result of point-source wastewater inputs. Diel samplings were conducted over four seasons along 50 km transects at Calgary (Bow River) and Saskatoon (South Saskatchewan River). Diel O-2 and delta O-18-O-2 cycles grew in magnitude downstream of effluent inputs in all seasons compared with upstream control sites. delta O-18-O-2 depletions clearly revealed the stimulating effect of effluent on aquatic photosynthesis. Diel isotopic mass balance modelling showed community metabolic responses to effluent inputs were most pronounced in the spring and summer when photosynthesis and respiration rates were about two-to three-fold higher than at upstream control sites. Our findings revealed that sewage treatment plant nutrient additions resulted in an enhanced metabolic footprint that extended beyond 50 km downstream.
... In addition to the listing provided in this paper, readers are encouraged to consult papers in Govindjee and Gest (2002a), Govindjee et al. (2003a) and the papers in this issue. To give just a few examples, see Belyaeva (2003) for chlorophyll biosynthesis, Bennoun (2002) for chlororespiration, Borisov (2003) for discoveries in biophysics of photosynthesis, de Kouchkovsky (2002) for research at CNRS in Gifsur-Yvette, Delosme and Joliot (2002) for photoaccoustics, Grossman (2003) for complementary chromatic adaptation, Heber (2002) for Mehler reaction, Joliot and Joliot (2003) for excitation energy transfer among Photosystem II units, Klimov (2003) for the history of the discovery of pheophytin as electron acceptor of Photosystem II, Krasnovsky (2003) for discoveries in photochemistry in Russia, Kuang et al. (2003) for discoveries in China, Larkum (2003) for contributions of Lundegardh, Lewin (2002) for the discovery of Prochlorophyta, Papageorgiou (2003) for discoveries in Greece, Pearlstein (2002) for a 1960 theory on excitation energy transfer, Raghavendra et al. (2003) for discoveries in India, and Vernon (2003) for discoveries at the Kettering Research Laboratory. ...
We present historic discoveries and important observations, related to oxygenic photosynthesis, from 1727 to 2003. The decision
to include certain discoveries while omitting others has been difficult. We are aware that ours is an incomplete timeline.
In part, this is because the function of this list is to complement, not duplicate, the listing of discoveries in the other
papers in these history issues of Photosynthesis Research. In addition, no one can know everything that is in the extensive literature in the field. Furthermore, any judgement about
significance presupposes a point of view. This history begins with the observation of the English clergyman Stephen Hales
(1677–1761) that plants derive nourishment from the air; it includes the definitive experiments in the 1960–1965 period establishing
the two-photosystem and two-light reaction scheme of oxygenic photosynthesis; and includes the near-atomic resolution of the
structures of the reaction centers of these two Photosystems, I and II, obtained in 2001–2002 by a team in Berlin, Germany,
coordinated by Horst Witt and Wolfgang Saenger. Readers are directed to historical papers in Govindjee and Gest [(2002a) Photosynth
Res 73: 1–308], in Govindjee, J. Thomas Beatty and Howard Gest [(2003a) Photosynth Res 76: 1–462], and to other papers in
this issue for a more complete picture. Several photographs are provided here. Their selection is based partly on their availability
to the authors (see Figures 1–15). Readers may view other photographs in Part 1 (Volume 73, Photosynth Res, 2002), Part 2
(Volume 76, Photosynth Res, 2003) and Part 3 (Volume 80 Photosynth Res, 2004) of the history issues of Photosynthesis Research. Photographs of most of the Nobel-laureates are included in Govindjee, Thomas Beatty and John Allen, this issue. For a complementary
time line of anoxygenic photosynthesis, see H. Gest and R. Blankenship (this issue).
... No strong signature of desert versus aquatic original habitat emerged from this small sampling of five species. The nonzero NPQ (and qN) after overnight dark adaptation (Figs. 2, 3, 4, 5) observed in four species may have been supported by several potentially interacting mechanisms, including persistence of zeaxanthin in the dark, generation of charge across the thylakoid membranes by chlororespiration or chloroplast-mitochondrial redox interactions, and/or charge-induced state transitions (Niyogi 2000;Bennoun 2002). There is precedence in the embryophyte literature for long-term persistence of zeaxanthin under stressful conditions, for example in needleand broad-leaved evergreens in winter (Adams III and Demmig-Adams 2004). ...
It has long been suspected that photoprotective mechanisms in green algae are similar to those in seed plants. However, exceptions have recently surfaced among aquatic and marine green algae in several taxonomic classes. Green algae are highly diverse genetically, falling into 13 named classes, and they are diverse ecologically, with many lineages including members from freshwater, marine, and terrestrial habitats. Genetically similar species living in dramatically different environments are potentially a rich source of information about variations in photoprotective function. Using aquatic and desert-derived species from three classes of green algae, we examined the induction of photoprotection under high light, exploring the relationship between nonphotochemical quenching and the xanthophyll cycle. In liquid culture, behavior of aquatic Entransia
fimbriata (Klebsormidiophyceae) generally matched patterns observed in seed plants. Nonphotochemical quenching was lowest after overnight dark adaptation, increased with light intensity, and the extent of nonphotochemical quenching correlated with the extent of deepoxidation of xanthophyll cycle pigments. In contrast, overnight dark adaptation did not minimize nonphotochemical quenching in the other species studied: desert Klebsormidium sp. (Klebsormidiophyceae), desert and aquatic Cylindrocystis sp. (Zygnematophyceae), and desert Stichococcus sp. (Trebouxiophyceae). Instead, exposure to low light reduced nonphotochemical quenching below dark-adapted levels. De-epoxidation of xanthophyll cycle pigments paralleled light-induced changes in nonphotochemical quenching for species within Klebsormidiophyceae and Trebouxiophyceae, but not Zygnematophyceae. Inhibition of violaxanthin–zeaxanthin conversion by dithiothreitol reduced high-light-associated nonphotochemical quenching in all species (Zygnematophyceae the least), indicating that zeaxanthin can contribute to photoprotection as in seed plants but to different extents depending on taxon or lineage.
Like all organisms performing oxygenic photosynthesis, Chlamydomonas captures light energy in two photochemical steps to drive linear electron flow from water to NADPH and to produce ATP. However, this process alone is not sufficient to drive CO2 fixation in the Calvin–Benson cycle and to respond to environmental and metabolic constraints, for example, light availability or metabolic needs in term of ATP and NADPH. A complex network of alternative electron flows, comprises cyclic electron flow around photosystem I and various water-to-water cycles that utilize O2 as an alternative electron acceptor, provide an additional degree of freedom to face this challenge. The present chapter describes the various alternative routes of photosynthetic electron transfer in Chlamydomonas, describes how they are coordinated for optimization of photosynthesis and gives a retrospective on the early physiology work on those alternative electron flows among model photosynthetic organisms.
The evolution of oxygenic photosynthesis enabled organisms to use sunlight as an energy source, allowing them to colonize new niches. At the same time, life (as we know it) places severe constraints on photosynthesis. For example, the initial reactions of photosynthesis involve highly energetic intermediates that, if not controlled, can generate highly toxic side products (especially reactive oxygen species, ROS), that can damage other essential components of the organisms it powers. Photosynthesis must therefore be tightly regulated to balance the need for efficient energy conversion with the necessity of avoiding photodamage (Gust D, Kramer D, Moore A, Moore T, Vermaas W, Mater Res Bull 33:383–389, 2008). A related constraint on photosynthesis is the need to precisely balance how much energy is stored in ATP and NADPH to precisely meet biochemical demands. If this balancing does not occur, the system will fail, leading to photodamage (Kramer DM, Evans JR, Plant Physiol 155:70–78, 2011). Consideration of these requirements is essential for efforts to improve the efficiency of photosynthesis by introducing CO2 concentrating mechanisms, altering metabolism or biosynthetic pathways to shunt energy to alternative products (Kramer DM, Evans JR, Plant Physiol 155:70–78, 2011). These balancing processes must be extremely robust to contend with the rapid and unpredictable fluctuations in environmental conditions and metabolic demands that occur in nature. A large body of work has come from model systems, especially terrestrial higher plants and the green alga Chlamydomonas reinhardtii, leading to a model for the regulation of light reactions that involves 1) sensing of the pH gradient component of the thylakoid proton motive force (pmf), and 2) the redox state of the plastoquinone- and stromal pools. Over the short term, these sensors trigger regulation of light capture by altering the activity of ATP synthase leading to adjustments in lumen pH, which fine tunes light capture through nonphotochemical quenching (NPQ) and control of electron flow by adjusting the rate of PQH2 oxidation at the b6f complex. Simultaneously, this system controls the balance of ATP/NADPH by adjusting electron flux to linear and cyclic electron flow pathways to balance ATP/NADPH. This integrated “pmf paradigm” model explains much of the existing data on plants and green algae, but may not extend to other diverse organisms. This review considers how advances in our understanding of photosynthesis over the past 7–8 years, particularly in the discoveries of diverse biochemical/biophysical mechanisms in aquatic photosynthetic species, affects the view of energy balance, including the shunting of electrons to O2 through the flavodiiron proteins (FLV), the plastid terminal oxidase, the dissipation of electric field by ion movements, and the activation of alternative electron sinks. We will introduce the basic model that has been developed for higher plant chloroplasts, then contrast these with selected aquatic systems, focusing on how the differences impact the needs to re-balance both energy input and its partitioning into energy currencies.
Background: as a plasto quinol oxidase involved in plastoquinol oxidation in higher plants and microalgae, the plastid terminal oxidase (PTOX) was first recognized in the tomato mutant GHOST (GH) and Arabidopsis mutant IMMUTANS (IM). Genome sequence analysis revealed that duplication of the PTOX gene occurs in certain eukaryotic microalgae, but not in cyanobacteria and most higher plants. PTOX may also be involved in carotenoid synthesis and play a critical protective role against stress, such as high light, heat shock and hyperosmosis. However, the connections of PTOX with astaxanthin and bio-hydrogen production and their functional relationship between two PTOX genes in the model green microalga Chlamydomonas is unknown.
Results: we successfully knocked down two ptoxs through RNAi in Chlamydomonas, respectively. We demonstrated that expression levels of both PTOXs were increased under stress conditions, and interestingly when one PTOX was silenced the other’s transcriptional level was significantly raised.
Conclusions: this shows a complementary relationship under high light condition. In addition, the astaxanthin accumulation level was up-regulated in silenced ptox2 strain, compared to the wide type strain. What’s more, significantly increased hydrogen production was observed in silenced ptox1 strain. In conclusion, PTOXs in Chlamydomonas are connected with not only astaxanthin accumulation but also hydrogen production, and their knock-down strains provide new insights in manipulating microalgae for high light stress tolerant strains, carotenoid production and even biofuels.
Cyclic electron flow around photosystem I (CEF) is critical for balancing the photosynthetic energy budget of the chloroplast, by generating ATP without net production of NADPH. We demonstrate that the chloroplast NADPH dehydrogenase complex (NDH), a homolog to respiratory Complex I, pumps approximately two protons from the chloroplast stroma to the lumen per electron transferred from ferredoxin to plastoquinone, effectively increasing the efficiency of ATP production via CEF by two-fold compared to CEF pathways involving non-proton-pumping plastoquinone reductases. By virtue of this proton-pumping stoichiometry, we hypothesise that NDH not only efficiently contributes to ATP production, but operates near thermodynamic reversibility, with potentially important consequences for remediating mismatches in the thylakoid energy budget.
Carotenoids are a diverse group of terpenoid pigments ubiquitous in and essential for functioning of phototrophs. Most of the researchers in the field are focused on the primary carotenoids serving light harvesting, photoprotection and supporting the structural integrity of the photosynthetic apparatus (PSA) within the thylakoid membranes. A distinct group of the pigments functionally and structurally uncoupled from the PSA and accumulating outside of the thylakoids is called secondary carotenoids. Induction of the biosynthesis and massive accumulation of the latter termed as secondary carotenogenesis and carotenogenic response (CR), respectively is a major though insufficiently studied stress response discovered in many phototrophic organisms ranging from single-celled algae to terrestrial higher plants. The CR protects cell by means of optical shielding of cell structures vulnerable photodamage, consumption of potentially harmful dioxygen, augmenting sink capacity of photoassimilates, and exerting an antioxidant effect. The secondary carotenoids exhibit a remarkable photostability in situ. Therefore, the CR-based photoprotective mechanism, unlike e.g. antioxidant enzyme-based protection in the chloroplast, does not require continuous investment of energy and metabolites making it highly suitable for long-term stress acclimation in phototrophs. Capability of the CR determines the strategy of acclimation of photosynthetic organisms to different stresses such as excessive irradiance, drought, extreme temperatures and salinities. Build-up of the CR might be accompanied by gradual disengagement of ‘classical’ active (energy-dependent) photoprotective mechanisms such as non-photochemical quenching, NPQ. In addition to that, the CR has great ecological significance. Illustrious examples of this are extremely stress-tolerant ‘snow’ algae and conifer species developing red coloration during winter. The CR has also considerable practical implications since the secondary carotenoids exert a plethora of beneficial effects on human and animal health. The carotenogenic microalgae are the richest biotechnological sources of natural value-added carotenoids such as astaxanthin and β-carotene. In the present review, we summarize current functional, mechanistic, and ecological insights into the CR in a broad range of organisms suggesting that it is obviously more widespread and important stress response than it is currently thought to be.
We present historic discoveries and important observations, related to oxygenic photosynthesis, from 1727 to 2003. The decision to include certain discoveries while omitting others has been difficult. We are aware that ours is an incomplete timeline. In part, this is because the function of this list is to complement, not duplicate, the listing of discoveries in the other papers in these history issues of Photosynthesis Research. In addition, no one can know everything that is in the extensive literature in the field. Furthermore, any judgement about significance presupposes a point of view. This history begins with the observation of the English clergyman Stephen Hales (1677–1761) that plants derive nourishment from the air; it includes the definitive experiments in the 1960–1965 period establishing the two-photosystem and two-light reaction scheme of oxygenic photosynthesis; and includes the near-atomic resolution of the structures of the reaction centers of these two Photosystems, I and II, obtained in 2001–2002 by a team in Berlin, Germany, coordinated by Horst Witt and Wolfgang Saenger. Readers are directed to historical papers in Govindjee and Gest [(2002a) Photosynth Res 73: 1–308], in Govindjee, J. Thomas Beatty and Howard Gest [(2003a) Photosynth Res 76: 1–462], and to other papers in this volume for a more complete picture. Several photographs are provided here. Their selection is based partly on their availability to the authors (see Figures 1-15). Readers may view other photographs in Part 1 (Volume 73, Photosynth Res, 2002), Part 2 (Volume 76, Photosynth Res, 2003) and Part 3 (Volume 80, Photosynth Res, 2004) of the history issues of Photosynthesis Research. Photographs of most of the Nobel-laureates are included in Govindjee, Thomas Beatty and John Allen, this volume. For a complementary time line of anoxygenic photosynthesis, see H. Gest and R. Blankenship (this volume).
Mitochondria possess oxygen-consuming respiratory electron transfer chains (RETCs), and the oxygen-evolving photosynthetic electron transfer chain (PETC) resides in chloroplasts. Evolutionarily mitochondria and chloroplasts are derived from ancient α-proteobacteria and cyanobacteria, respectively. However, cyanobacteria harbor both RETC and PETC on their thylakoid membranes. It is proposed that chloroplasts could possess a RETC on the thylakoid membrane, in addition to PETC. Identification of a plastid terminal oxidase (PTOX) in the chloroplast from the Arabidopsis variegation mutant immutans (im) demonstrated the presence of a RETC in chloroplasts, and the PTOX is the committed oxidase. PTOX is distantly related to the mitochondrial alternative oxidase (AOX), which is responsible for the CN-insensitive alternative RETC. Similar to AOX, an ubiquinol (UQH2) oxidase, PTOX is a plastoquinol (PQH2) oxidase on the chloroplast thylakoid membrane.
This paper summarizes aspects of the history of photosynthetic hydrogen research, from the pioneering discovery of Hans Gaffron over 60 years ago to the potential exploitation of green algae in commercial H2-production. The trail started as a mere scientific curiosity, but promises to be a most important discovery, one that leads photosynthesis research to important commercial applications. Progress achieved in the field of photosynthetic hydrogen production by green algae includes elucidation of the mechanism, the ability to modify photosynthesis by physiological means and to produce bulk amounts of H2 gas, and cloning of the [Fe]-hydrogenase genes in several green algal species.
Nitrogen is an essential factor for normal plant and algal development. As a component of nucleic acids, proteins, and Chl molecules, it has a crucial role in the organisation of a functioning photosynthetic apparatus. Our aim was to study the effects of nitrogen starvation in cultures of the unicellular green alga, Chlamydomonas reinhardtii, maintained on nitrogenfree, and then on nitrogen-containing medium. During the three-week-long degreening process, considerable changes were observed in the Chl content, the ratio of Chl-protein complexes and the photosynthetic activity of the cultures as well as in the ultrastructure of the single chloroplast. The regreening process was much quicker then the degradation, total greening of the cells occurred within four days. The rate of regeneration depended on the nitrogen content. At least 50% of the normal nitrogen content of TAP medium was required in the medium for the complete regreening of the cells and regeneration of chloroplasts.
The aim of this educational review is to provide practical information on the hardware, methodology, and the hands on application of chlorophyll (Chl) a fluorescence technology. We present the paper in a question and answer format like frequently asked questions. Although nearly all information on the application of Chl a fluorescence can be found in the literature, it is not always easily accessible. This paper is primarily aimed at scientists who have some experience with the application of Chl a fluorescence but are still in the process of discovering what it all means and how it can be used. Topics discussed are (among other things) the kind of information that can be obtained using different fluorescence techniques, the interpretation of Chl a fluorescence signals, specific applications of these techniques, and practical advice on different subjects, such as on the length of dark adaptation before measurement of the Chl a fluorescence transient. The paper also provides the physiological background for some of the applied procedures. It also serves as a source of reference for experienced scientists.
Green algae have been treated for a long time as “free living choroplasts” and therefore used as model organisms in photosynthesis research. However, recent progress has provided evidence that they have a paraphyletic origin, resulting in a wide array of different evolutionary lineages. This diversity opens the opportunity to utilise green algae not only for the production of bulk biomass, but also for the extraction of specific biotechnological compounds. This chapter gives an overview of the taxonomic, structural, biochemical, molecular and physiological features of those species which are the most widely used in algal biomass technologies. Based on this description, we suggest how green algal biodiversity and metabolic pathways can be exploited in the future for biological energy generation.
Monitoring transmission changes at 820 nm, a measure of the redox states of plastocyanin (PC) and P700, is a good complementary technique for chlorophyll (chl) a fluorescence induction measurements. A thorough characterization of the properties of the 820-nm transmission kinetics during the first second after a dark-to-light transition is provided here for pea (Pisum sativum L.) leaves. The data indicate that plastocyanin in a dark-adapted leaf is in the reduced state. Three photosystem I (PSI)-related components, PC, P700 and ferredoxin, can contribute to the 820-nm transmission signal. The contribution of ferredoxin, however, is only approximately 5%, thus, it can be neglected for further analysis. Here, we show that by monitoring the sequential oxidation of PC and P700 during a far-red pulse and analysing the re-reduction kinetics it is possible to assign the three re-reduction components to PC (τ = 7-14 s) and P700 (τ = 35-55 ms and 1.2-1.6 s). Our data indicate that the faster re-reduction phase (τ = 35-55 ms) may represent a recombination reaction between P700+ and the acceptor side of PSI. This information made it possible to show that the ratio between the potential contributions of PC:P700 is 50:50 in pea and Camellia leaves and 40:60 in sugar beet leaves.
Using chloroplasts from barley leaves we attempt to purify and partially characterize the NADH dehydrogenase complex. The enzymatic activity was assayed as NADH-ferricyanide and NADH-nitro blue tetrazolium oxidoreductase. Analyzed by SDS-polyacrylamide gel electrophoresis and subsequent enzymatic renaturation, the chloroplast soluble fraction contains principally a 66 kDa enzyme. The membranous fraction solubilized with deoxycholate and analyzed by native electrophoresis and NADH-nitro blue tetrazolium staining revealed three enzymes: one with similar electrophoretic mobility to that described for the soluble enzyme, another one which is a complex separated in 3% polyacrylamide gel and a third one, another complex separated in the top of the 5-22% linear polyacrylamide gel gradient. The complex polypeptidic patterns were similar but different to those found for any thylakoidal proteinic complex known. Nine major polypeptides were detected in the complex polypeptidic patterns, four of them constituents of the small size thylakoid enzyme. The molecular masses of six polypeptides agreed with those indicated as encoded by 6 chloroplast ndh genes. All the enzymes, including the 66 kDa soluble enzyme, contained a 53 kDa polypeptide, which is probably the NADH-binding complex subunit. Isoelectric focusing of the thylakoidal enzyme points to a basic isoelectric point. Ion-exchange or hydroxylapatite column chromatography followed by native electrophoresis of the active fractions only separated the small size enzyme, which showed complex inactivation.
Preparations enriched in Chlamydomonas reinhardtii thylakoids have proven useful in the study of photosynthesis. Many of their polypeptides however remain unidentified. We report here on three of those, h1 (34 kDa), h2 (11 kDa), and P3 (63 kDa). h1, h2, and P3 are present in all tested mutants of C. reinhardtii lacking either one or several of the photosynthetic chain complexes or depleted in thylakoid membranes. h2 is an ascorbate-reducible, soluble c550-type cytochrome encoded in the nucleus. It cross-reacts immunologically with mitochondrial cytochromes c from various sources and contains a hexapeptide encoded in C. reinhardtii cytochrome c cDNA. P3, a nuclear-encoded peripheral protein, cross-reacts with various ATP synthase beta subunits. Its N-terminal sequence is encoded in C. reinhardtii mitochondrial beta subunit cDNA. h1 behaves as an integral hemoprotein; it is absent in a mitochondrial mutant that carries a deletion in apocytochrome b gene. We conclude that C. reinhardtii mitochondrial membranes copurify with thylakoid membranes. h1 is part of the cytochrome bc1 complex, h2 is cytochrome c, and P3 is the beta subunit of mitochondrial ATP synthase.
The plastid genomes of several plants contain homologues, termed ndh genes, of genes encoding subunits of the NADH:ubiquinone oxidoreductase or complex I of mitochondria and eubacteria. The functional significance of the Ndh proteins in higher plants is uncertain. We show here that tobacco chloroplasts contain a protein complex of 550 kDa consisting of at least three of the ndh gene products: NdhI, NdhJ and NdhK. We have constructed mutant tobacco plants with disrupted ndhC, ndhK and ndhJ plastid genes, indicating that the Ndh complex is dispensible for plant growth under optimal growth conditions. Chlorophyll fluorescence analysis shows that in vivo the Ndh complex catalyses the post-illumination reduction of the plastoquinone pool and in the light optimizes the induction of photosynthesis under conditions of water stress. We conclude that the Ndh complex catalyses the reduction of the plastoquinone pool using stromal reductant and so acts as a respiratory complex. Overall, our data are compatible with the participation of the Ndh complex in cyclic electron flow around the photosystem I complex in the light and possibly in a chloroplast respiratory chain in the dark.
The immutans (im) mutant of Arabidopsis shows a variegated phenotype comprising albino and green somatic sectors. We have cloned the IM gene by transposon tagging and show that even stable null alleles give rise to a variegated phenotype. The gene product has amino acid similarity to the mitochondrial alternative oxidase. We show that the IM protein is synthesized as a precursor polypeptide that is imported into chloroplasts and inserted into the thylakoid membrane. The albino sectors of im plants contain reduced levels of carotenoids and increased levels of the carotenoid precursor phytoene. The data presented here are consistent with a role for the IM protein as a cofactor for carotenoid desaturation. The suggested terminal oxidase function of IM appears to be essential to prevent photooxidative damage during early steps of chloroplast formation. We propose a model in which IM function is linked to phytoene desaturation and, possibly, to the respiratory activity of the chloroplast.
In Chlamydomonas reinhardtii mutants deficient in photosystem I because of inactivation of the chloroplast genes psaA or psaB, oxygen evolution from photosystem II occurs at significant rates and is coupled to a stimulation of oxygen uptake. Both
activities can be simultaneously monitored by continuous mass spectrometry in the presence of18O2. The light-driven O2 exchange was shown to involve the plastoquinone pool as an electron carrier, but not cytochrome b
6
f. Photosystem II-dependent O2 production and O2 uptake were observed in isolated chloroplast fractions. Photosystem II-dependent oxygen exchange was insensitive to a variety
of inhibitors (azide, carbon monoxide, cyanide, antimycin A, and salicylhydroxamic acid) and radical scavengers. It was, however,
sensitive to propyl gallate. From inhibitors effects and electronic requirements of the O2 uptake process, we conclude that an oxidase catalyzing oxidation of plastoquinol and reduction of oxygen to water is present
in thylakoid membranes. From the sensitivity of flash-induced O2 exchange to propyl gallate, we conclude that this oxidase is involved in chlororespiration. Clues to the identity of the
protein implied in this process are given by pharmacological and immunological similarities with a protein (IMMUTANS) identified
in Arabidopsis chloroplasts.
The immutans (im) mutant of Arabidopsis shows a variegated phenotype comprising albino and green somatic sectors. We have cloned the IM gene by transposon tagging and show that even stable null alleles give rise to a variegated phenotype. The gene product has amino acid similarity to the mitochondrial alternative oxidase. We show that the IM protein is synthesized as a precursor polypeptide that is imported into chloroplasts and inserted into the thylakoid membrane. The albino sectors of im plants contain reduced levels of carotenoids and increased levels of the caro-tenoid precursor phytoene. The data presented here are consistent with a role for the IM protein as a cofactor for caro-tenoid desaturation. The suggested terminal oxidase function of IM appears to be essential to prevent photooxidative damage during early steps of chloroplast formation. We propose a model in which IM function is linked to phytoene de-saturation and, possibly, to the respiratory activity of the chloroplast.
The Arabidopsis IMMUTANS gene encodes a plastid homolog of the mitochondrial alternative oxidase, which is associated with phytoene desaturation. Upon expression in Escherichia coli, this protein confers a detectable cyanide-resistant electron transport to isolated membranes. In this assay this activity is sensitive to n-propyl-gallate, an inhibitor of the alternative oxidase. This protein appears to be a plastid terminal oxidase (PTOX) that is functionally equivalent to a quinol:oxygen oxidoreductase. This protein was immunodetected in achlorophyllous pepper (Capsicum annuum) chromoplast membranes, and a corresponding cDNA was cloned from pepper and tomato (Lycopersicum esculentum) fruits. Genomic analysis suggests the presence of a single gene in these organisms, the expression of which parallels phytoene desaturase and ζ-carotene desaturase gene expression during fruit ripening. Furthermore, thisPTOX gene is impaired in the tomato ghostmutant, which accumulates phytoene in leaves and fruits. These data show that PTOX also participates in carotenoid desaturation in chromoplasts in addition to its role during early chloroplast development.
The changes of the redox state of plastoquinone and of the rate of back-reaction between the primary photoproducts of PS II centres have been studied following addition of myxothiazol to whole cells of C. reinhardtii. Using appropriate mutant strains it is shown that these changes, namely a slow reduction of plastoquinone and a comparatively fast increase in the rate of back-reaction, are a consequence of the inhibition of mitorespiration at the level of cytochrome bc1 complex. These new data are discussed in relation with our previous model of chlororespiration (Bennoun, P. (1982) Proc. Natl. Acad. Sci. USA 79, 4352–4356). In contrast to these effects, a decrease in oxygen concentration affecting only slightly the rate of back-reaction induces a fast reduction of plastoquinone, suggesting that the chloroplast oxidase has a low affinity for oxygen as compared to mitochondrial oxidases. The large variations in the rate of back-reaction that we could observe raise many questions relevant to their origin and to their possible implications, that are discussed in detail. If, as seems likely, these variations result from changes in the electrochemical gradient built up across thylakoid membranes, then the existence of a new gradient generator should be postulated.
The plastid genomes of higher plants contain eleven reading frames (ndhA-K) that are homologous to genes encoding subunits of the mitochondrial NADH-ubiquinone-oxidoreductase (complex I). The carboxyterminal end of the NDH-H subunit from rice (Oryza sativa L.) was expressed as a fusion protein in Escherichia coli and antibodies against the fusion protein were generated in rabbits. The antibody was used to study the expression of NDH-H, and the following results were obtained: (i) NDH-H is expressed in mono- and dicotyledonous plants, (ii) NDH-H is localized on the stroma lamellae of the thylakoid membrane and (iii) NDH-H is expressed in etioplasts. Together with the finding that two other ndh genes (ndhI and ndhK) are expressed in plastids, these results point to the existence of an NAD(P)H-plastoquinone-oxidoreductase on the thylakoid membrane. The possible function of the enzyme in plastids is discussed and it is suggested that it works in balancing the ATP/ADP and the NADPH/NADP ratios during changing external (i.e. light) or internal (i.e. ATP and NADPH demands of biosynthetic pathways of the plastid) conditions.
The non-linear light-saturation curve for oxygen production in both Chlorella vulgaris and Phormidium luridium at low light intensities, under anaerobic conditions is shown to be caused by the reduction of a pool of electron carriers coupled to both an endogenous reducing agent R, and to oxygen. The light dependence of oxygen production in these algae was studied by a repetitive-flash method, which allows a direct analysis of the steady-state kinetics of pool reduction. We propose a kinetic model which quantitatively accounts for these kinetics and several transient phenomena. This model centers on a novel cross reaction at the pool of photo and dark electron input and output, allowing a delicate poising of oxygen production by the environment. This model shows a positive feedback of oxygen on oxygen production.
An antibody against the NDH-K subunit of the NAD(P)H-dehydrogenase from the cyanobacterium Synechocystis sp. PCC6803 was used to isolate a subcomplex of the enzyme from Triton X-100 solubilized total membranes by immunoaffinity chromatography. The isolated subcomplex consisted of seven major polypeptides with molecular masses of 43, 27, 24, 21, 18, 14 and 7 kDa. The amino-terminal amino acid sequences of the polypeptides were determined. By comparing the sequences with the amino acid sequences deduced from DNA, three proteins were identified as NDH-H (43 kDa), NDH-K (27 kDa) and NDH-I (24 kDa). A fourth subunit (NDH-J, 21 kDa) was identified by Western blot analysis with an NDH-J antibody.
Plastids contain a NAD(P)H-plastoquinone-oxidoreductase (NDH complex) which is homologous to the eubacterial and mitochondrial NADH-ubiquinone-oxidoreductase (complex I), but the metabolic function of the enzyme is unknown. The enzyme consists of at least eleven subunits (A-K), which are all encoded on the plastid chromosome. We have mutagenized ndhC and ndhJ by insertion, and ndhK and ndhA-I by deletion and insertion, of a cassette which carried a spectinomycin resistance gene as a marker. The transformation was carried out by the polyethylene glycol-mediated plastid transformation method. Southern analysis revealed that even after repeated regeneration cycles each of the four different types of transformants had retained 1-5% of wild-type gene copies. This suggests that complete deletion of ndh genes is not compatible with viability. The transformants displayed two characteristic phenotypes: (i) they lack the rapid rise in chlorophyll fluorescence in the dark after illumination with actinic light for 5 min; in the wild-type this dark-rise reflects a transient reduction of the plastoquinone pool by reduction equivalents generated in the stroma; and (ii) transformants with defects in the ndhC-K-J operon accumulate starch, indicating inefficient oxidation of glucose via glycolysis and the oxidative pentose phosphate pathway. Both observations support the theory of chlororespiration, which postulates that the NDH complex acts as a valve to remove excess reduction equivalents in the chloroplast.
Using a new method of delayed luminescence digital imaging, mutants of Chlorella sorokiniana lacking the chloroplast CF0CF1 ATP synthase were isolated for the first time. Biochemical characterization of these strains indicates a lack of detectable synthesis and accumulation of the ATP synthase subunits alpha-CF1 and beta-CF1. Functional characterization indicates the presence of a permanent electrochemical gradient (DeltaMu) across the thylakoid membrane in the dark-adapted state, which is not suppressed under anaerobic conditions. Contrary to what is observed in the presence of the CF0CF1 ATP synthase, this gradient is essentially due to an electric field component DeltaPsi with no detectable DeltapH component, under both aerobic and anaerobic conditions. Neither the CF0CF1 ATP synthase nor a respiratory process can thus be responsible for a permanent gradient detected under these conditions. The previous proposal of a new ATP-dependent electrogenic pump in thylakoid membranes is supported by these results that, in addition, indicate a specificity of this new pump for ions other than protons.
The Arabidopsis IMMUTANS gene encodes a plastid homolog of the mitochondrial alternative oxidase, which is associated with phytoene desaturation. Upon expression in Escherichia coli, this protein confers a detectable cyanide-resistant electron transport to isolated membranes. In this assay this activity is sensitive to n-propyl-gallate, an inhibitor of the alternative oxidase. This protein appears to be a plastid terminal oxidase (PTOX) that is functionally equivalent to a quinol:oxygen oxidoreductase. This protein was immunodetected in achlorophyllous pepper (Capsicum annuum) chromoplast membranes, and a corresponding cDNA was cloned from pepper and tomato (Lycopersicum esculentum) fruits. Genomic analysis suggests the presence of a single gene in these organisms, the expression of which parallels phytoene desaturase and zeta-carotene desaturase gene expression during fruit ripening. Furthermore, this PTOX gene is impaired in the tomato ghost mutant, which accumulates phytoene in leaves and fruits. These data show that PTOX also participates in carotenoid desaturation in chromoplasts in addition to its role during early chloroplast development.
Inactivation of a plastid located quinone-oxygen oxidoreductase gene in the immutans Arabidopsis mutant leads to a photobleached phenotype because of a lack of photoprotective carotenoids. Inactivation of the corresponding gene in the ghost tomato mutant leads to a similar phenotype in leaves and to carotenoid deficiency in petals and ripe fruits. This plastid terminal oxidase (the first to be cloned and biochemically characterized) resembles the mitochondrial cyanide-insensitive alternative oxidase. Here, we propose a model integrating this novel oxidase as a component of an electron transport chain associated to carotenoid desaturation, as well as to a respiratory activity within plastids.
The plastoquinone pool during dark adaptation is reduced by endogenous reductants and oxidized at the expense of molecular oxygen. We report here on the redox state of plastoquinone in darkness, using as an indicator the chlorophyll fluorescence kinetics of whole cells of a Chlamydomonas reinhardtii mutant strain lacking the cytochrome b(6)f complex. When algae were equilibrated with a mixture of air and argon at 1.45% air, plastoquinol oxidation was inhibited whereas mitochondrial respiration was not. Consequently, mitochondrial oxidases cannot be responsible for the oxygen consumption linked to plastoquinol oxidation. Plastoquinol oxidation in darkness turned out to be sensitive to n-propyl gallate (PG) and insensitive to salicylhydroxamic acid (SHAM), whereas mitochondrial respiration was sensitive to SHAM and PG. Thus, both PG treatment and partial anaerobiosis allow to draw a distinction between an inhibition of plastoquinol oxidation and an inhibition of mitochondrial respiration, indicating the presence of a plastoquinol:oxygen oxidoreductase. The possible identification of this oxidase with an oxidase involved in carotenoid biosynthesis is discussed in view of various experimental data.
The most widely accepted mechanism of electron and proton transfer within the cytochrome (Cyt) b/f complex derives from the Q-cycle hypothesis originally proposed for the mitochondrial Cyt b/c1 complex by Mitchell [Mitchell, P. (1975) FEBS Lett. 57, 135-137]. In chloroplasts, the Cyt b/f complex catalyzes the oxidation of a plastoquinol at a site, Qo (the plastoquinol binding site), close to the inner aqueous phase and the reduction of a quinone at a site, Qi (the plastoquinone binding site), close to the stromal side of the membrane. In an alternative model, the semiquinone cycle [Wikström, M. & Krab, K. (1986) J. Bioenerg. Biomembr. 18, 181-193], a charged semiquinone formed at site Qo is transferred to site Qi where it is reduced into quinol. Flash-induced kinetics of the redox changes of Cyt b and of the formation of a transmembrane potential have been measured in Chlorella sorokiniana cells incubated in reducing conditions that induce a full reduction of the plastoquinone pool. The experiments were performed in the presence of an uncoupler that collapses the permanent electrochemical proton gradient and thus accelerates the rate of the electrogenic processes. The results show that the electrogenic reaction driven by the Cyt b/f complex precedes the processes of reduction or oxidation of the b-hemes. This electrogenic process is probably due to a transmembrane movement of a charged semiquinone, in agreement with the semiquinone-cycle hypothesis. This mechanism may represent an adaptation to reducing conditions when no oxidized quinone is available at the Qi site.
Chlororespiration has been defined as a respiratory electron transport chain (ETC) in interaction with the photosynthetic ETC in thylakoid membranes of chloroplasts. The existence of chlororespiration has been disputed during the last decade, with the initial evidence mainly obtained with intact algal cells being possibly explained by redox interactions between chloroplasts and mitochondria. The discovery in higher-plant chloroplasts of a plastid-encoded NAD(P)H-dehydrogenase (Ndh) complex, homologous to the bacterial complex I, and of a nuclear-encoded plastid terminal oxidase (PTOX), homologous to the plant mitochondrial alternative oxidase, brought molecular support to the concept of chlororespiration. The functionality of these proteins in non-photochemical reduction and oxidation of plastoquinones (PQs), respectively, has recently been demonstrated. In thylakoids of mature chloroplasts, chlororespiration appears to be a relatively minor pathway compared to linear photosynthetic electron flow from H2O to NADP+. However, chlororespiration might play a role in the regulation of photosynthesis by modulating the activity of cyclic electron flow around photosystem I (PS I). In non-photosynthetic plastids, chlororespiratory electron carriers are more abundant and may play a significant bioenergetic role.
Evidence is given for the existence of an electron transport pathway to oxygen in the thylakoid membranes of chloroplasts (chlororespiration). Plastoquinone is shown to be a redox carrier common to both photosynthetic and chlororespiratory pathways. It is shown that, in dark-adapted chloroplasts, an electrochemical gradient is built up across the thylakoid membrane by transfer of electrons through the chlororespiratory chain as well as by reverse functioning of the chloroplast ATPases. It is proposed that these mechanisms ensure recycling of the ATP and NAD(P)H generated by the glycolytic pathway converting starch into triose phosphates. Chlororespiration is thus an O(2)-uptake process distinct from photorespiration and the Mehler reaction. The evolutionary significance of chlororespiration is discussed.
Presence of large protein complex containing the ndhK gene product and possessing NADH-specific dehydrogenase activity in thylakoid membranes of higher plant chloroplasts
- La Sazanov
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Dordrecht, The Netherlands
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