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A CDC25 homologue from rice functions as an arsenate reductase
New Phytologist
Abstract
Enzymatic reduction of arsenate to arsenite is the first step in arsenate metabolism in all organisms studied. The rice genome contains two ACR2-like genes, OsACR2.1 and OsACR2.2, which may be involved in regulating arsenic metabolism in rice. Here, we cloned both OsACR2 genes and expressed them in an Escherichia coli strain in which the arsC gene was deleted and in a yeast (Saccharomyces cerevisiae) strain with a disrupted ACR2 gene. OsACR2.1 complemented the arsenate hypersensitive phenotype of E. coli and yeast. OsACR2.2 showed much less ability to complement. The gene products were purified and demonstrated to reduce arsenate to arsenite in vitro, and both exhibited phosphatase activity. In agreement with the complementation results, OsACR2.1 exhibited higher reductase activity than OsACR2.2. Mutagenesis of cysteine residues in the putative active site HC(X)(5)R motif led to nearly complete loss of both phosphatase and arsenate reductase activities. In planta expression of OsACR2.1 increased dramatically after exposure to arsenate. OsACR2.2 was observed only in roots following arsenate exposure, and its expression was less than OsACR2.1.
Supplementary resources (2)
Nucleotide Sequence
December 2004
Nucleotide Sequence
December 2004
... Rice can also alleviate As toxicity by accumulating osmoregulatory compounds and their derivatives (such as organic acids, amino acids, and hormones) [110,112] ( Figure 2). The reported functional genes related to As detoxification in rice are presented in Table 2. OsHAC1;1 LOC_Os02g01220 Root, shoot Cytoplasm, nucleus Reduces As(V) to As(III) [113,114] OsHAC1;2 LOC_Os04g17660 Root Cytoplasm, nucleus Reduces As(V) to As(III) [113] OsHAC4 LOC_Os02g06290 Root Cytoplasm, nucleus Reduces As(V) to As(III) [115] OsACR2.1 LOC_Os10g39860 Root, shoot Cytoplasm, nucleus Reduces As(V) to As(III) [116] OsPCS1 LOC_Os05g34290 Root, shoot, leaf, grain Cytosol Catalyzes the formation of phytochelatins that form As(III)-phytochelatin conjugates [117,118] OsPCS2 LOC_Os06g01260 Root, shoot, leaf, grain Cytosol Catalyzes the formation of phytochelatins that form As(III)-phytochelatin conjugates [117][118][119] OsGrx_C2.1 LOC_Os02g40500 Root, shoot / Involved in redox regulation and protection under oxidative stress/alters the transcripts of AtNIPs in Arabidopsis ...
... In rice, As(V) reduction is mainly carried out by OsHAC1;1, OsHAC1;2, OsHAC4, OsACR2.1, OsGrx, etc. [113][114][115][116], among which a key role is played by the As(V) reductase OsHAC1;1, located in the epidermis, root hairs, and pericycle cells, and OsHAC1;2, located in the epidermis and endodermis/exodermis cells [113]. OsACR2.1 has a dual function as a phosphatase and arsenate reductase, and its expression level was significantly increased in rice roots and shoots treated with As(V) [116]. ...
... OsGrx, etc. [113][114][115][116], among which a key role is played by the As(V) reductase OsHAC1;1, located in the epidermis, root hairs, and pericycle cells, and OsHAC1;2, located in the epidermis and endodermis/exodermis cells [113]. OsACR2.1 has a dual function as a phosphatase and arsenate reductase, and its expression level was significantly increased in rice roots and shoots treated with As(V) [116]. After As(V) is reduced, As(III) can be released back to the external environment through the efflux system. ...
Arsenic (As) is a metalloid environmental pollutant ubiquitous in nature that causes chronic and irreversible poisoning to humans through its bioaccumulation in the trophic chain. Rice, the staple food crop for 350 million people worldwide, accumulates As more easily compared to other cereal crops due to its growth characteristics. Therefore, an in-depth understanding of the molecular regulatory mechanisms underlying As uptake, transport, and detoxification in rice is of great significance to solving the issue of As bioaccumulation in rice, improving its quality and safety and protecting human health. This review summarizes recent studies on the molecular mechanisms of As toxicity, uptake, transport, redistribution, regulation, and detoxification in rice. It aims to provide novel insights and approaches for preventing and controlling As bioaccumulation in rice plants, especially reducing As accumulation in rice grains.
... ACR2 genes from A. thaliana (AtACR2), rice (OsACR2) Holcus lanatus (HlACR2) and P. vittata (PvACR2) are also CDC25-like genes. AtACR2, OsACR2, and PvACR2 genes are able to restore resistance to arsenate (Dhankher et al., 2006, Ellis et al., 2006, Duan et al., 2007. Furthermore, ACR2 proteins purified from E. coli overexpressing OsACR2;1, OSACR2;2, PvACR2, or HlACR2 (also named HlAsr) could catalyze GSH/Grx-dependent arsenate reduction (Bleeker et al., 2006, Ellis et al., 2006, Duan et al., 2007. ...
... AtACR2, OsACR2, and PvACR2 genes are able to restore resistance to arsenate (Dhankher et al., 2006, Ellis et al., 2006, Duan et al., 2007. Furthermore, ACR2 proteins purified from E. coli overexpressing OsACR2;1, OSACR2;2, PvACR2, or HlACR2 (also named HlAsr) could catalyze GSH/Grx-dependent arsenate reduction (Bleeker et al., 2006, Ellis et al., 2006, Duan et al., 2007. Plants synthesize PCs utilizing both cysteine (Cys) and GSH (Hasan et al., 2017, Kumar & Trivedi, 2018 during the stress and with the help of GST (glutathione-S-transfarase), PCs make complexes with As (Ahmad et al., 2022;Mohd et al., 2017;Shukla et al., 2022Shukla et al., , 2023. ...
... Accumulated metals or metalloids such as As, which is transformed from arsenate to arsenite to reduce toxicity by arsenite reductase, is bound with a thiolic compound such as glutathion or phytochelatines and sequestrated in vacuoles [35,36]. In plants, it was observed that dual-specificity phosphatase (CDC25) may functionally mediate arsenate reductase activity involved in arsenate resistance [37][38][39][40][41][42]. ...
... Levels of 5-mC in roots were higher than in leaves, but the increase was more gradual and the increase in CDC25 expression was not affected; and with increasing As contamination, the level of CDC25 expression also increased, as did the (As III) ratio. Arsenate reduction in plants by CDC25 was described in [37,38]. The growth of the As (III) ratio was in leaves too, but the values of accumulated As were much lower than in the roots and the reduction of arsenate by CDC25 may have worked as the activity of the enzyme may have been sufficient despite the lower expression. ...
Experimental spinach plants grown in soil with (5, 10 and 20 ppm) arsenic (As) contamination were sampled in 21 days after As(V) contamination. Levels of As in spinach samples (from 0.31 ± 0.06 µg g−1 to 302.69 ± 11.83 µg g−1) were higher in roots and lower in leaves, which indicates a low ability of spinach to translocate As into leaves. Species of arsenic, As(III) and As(V), were represented in favor of the As (III) specie in contaminated variants, suggesting enzymatic arsenate reduction. In relation to predominant As accumulation in roots, changes in malondialdehyde levels were observed mainly in roots, where they decreased significantly with growing As contamination (from 11.97 ± 0.54 µg g−1 in control to 2.35 ± 0.43 µg g−1 in 20 ppm As). Higher values in roots than in leaves were observed in the case of 5-methylcytosine (5-mC). Despite that, a change in 5-mC by As contamination was further deepened in leaves (from 0.20 to 14.10%). In roots of spinach, expression of the CDC25 gene increased by the highest As contamination compared to the control. In the case of total phenolic content, total flavonoid content, total phenolic acids content and total antioxidant capacity were higher levels in leaves in all values, unlike the roots.
... OsACR2.1 and OsACR2.2 are identified in rice having similar functions of the yeast ACR2 genes. OsACR2.1 is much active than OsACR2.2 for arsenate reduction (Duan et al. 2007). The severity of As accumulation is higher in rice plants than other crops. ...
... 6. Mutation of OsACR2.1 and OsACR2.2 genes led to decreased arsenate reduction in root, decreased arsenite efflux, and increased As accumulation in root and grain (Duan et al. 2007). 7. Overexpression of OsPIP2;4, OsPIP2;6, and OsPIP2;7 had increased As 3+ tolerance in transgenic Arabidopsis during short-term exposure (Mosa et al. 2012). ...
Rapid industrialization and urbanization gradually shrink the cultivable land worldwide. To meet the growing demand for food, excessive application of fertilizer/pesticide and irrigation with sewage/industry effluent result in contamination of arable land with heavy metals. Rice, a major staple food used worldwide specially in the Asian countries, is also affected by heavy metal (HM) toxicity. These HMs affect plant growth and metabolism negatively and decrease crop quality and productivity. They are also able to enter the food chain and affect human health. Lead (Pb), zinc (Zn), copper (Cu), nickel (Ni), cadmium (Cd), chromium (Cr), mercury (Hg), arsenic (As), and antimony (Sb) are some of the metals and metalloids found in contaminated soil. Both the essential and nonessential elements are taken up by the plants through different metal transporters. As more HMs are being transported into the plants, deficiencies of essential elements and oxidative stress may occur. As a defense mechanism, plants employ different strategies to mitigate the oxidative stress. Genetically engineered plants are developed in such a way that they are equipped with the production of enhanced stress tolerance protein/metal sequestration mechanism/efficient metal efflux system/modified metal transporters, which can make them more adaptable in HM stress condition. Cultivation of tolerant/genetically engineered genotypes can help to eliminate metal toxicity and accumulation in grains. Rice grown in As/Cd-contaminated sites causes accumulation of these metals in grains, which is deleterious for health. Transgenic rice engineered with As(III)-S-adenosyl methyl transferase (arsM) decreased As accumulation in rice grains, and by gene manipulation, Cd entry in rice grain can also be blocked.
... The dominance of trivalent As in plant tissues is found when arsenic is supplied to the plants in the form of arsenate (Dhankher et al., 2002, Pickering et al., 2000, Xu et al., 2007, Zhao et al., 2009) indicates a higher capacity of arsenate reduction. Both roots and shoots of rice exhibit arsenate reduction activities (Duan et al., 2007), but roots may be quantitatively more important because arsenite is the main form found in the xylem sap of a number of plant species (Zhao et al., 2009). The plant homologues of the yeast arsenate reductase Acr2p have recently been isolated from A. thaliana (Dhankher et al., 2006), Holcus lanatus (Bleeker et al., 2006), rice (Duan et al., 2007), and P. Vittata (Ellis et al., 2006). ...
... Both roots and shoots of rice exhibit arsenate reduction activities (Duan et al., 2007), but roots may be quantitatively more important because arsenite is the main form found in the xylem sap of a number of plant species (Zhao et al., 2009). The plant homologues of the yeast arsenate reductase Acr2p have recently been isolated from A. thaliana (Dhankher et al., 2006), Holcus lanatus (Bleeker et al., 2006), rice (Duan et al., 2007), and P. Vittata (Ellis et al., 2006). The plant ACR2 proteins are CDC25-like (cell division cycle) tyrosine phosphatises that have both phosphatase and arsenate reductase activities; PvACR2 from Pteris vittata appears to be an exception with only the activity of an arsenate reductase (Ellis et al., 2006). ...
EFFECTS OF DIFFERENT DOSES OF ARSENIC LEVELS IN GROWTH AND YIELD OF RICE ABSTRACT
The pot experiment was conducted at the Sher-e-Bangla Agricultural University, Dhaka, Bangladesh during the period from December 2016 to March 2017 to study the effect of arsenic on growth and yield of rice. The experiment comprised of two factors. Factor A: Arsenic level (4 levels): A1: Control (No arsenic); A2: 1 ppm As pot-1, A3: 2 ppm As pot-1 and A4:4 ppm As pot-1. Factor B: Nitrogen doses: (2 Nos.); T1: recommended dose of nitrogen; T2: 50% more than recommended dose of N.The experiment was laid out in Randomized Complete Block Design (RCBD) with three replications. In case of arsenic, at harvest,the tallest rice plant (92.00 cm), highest effective tillers per plant (24.0), the largest panicle length (23.93 cm), the highest filled grain per plant (880.67) , the highest filled grain per panicle (43.66),the highest 1000-grain weight (28.26 g), the highest straw yield (28.10 g), the highest grain yield (28.00 g) were found in A1 treated pot & the highest un-filled grain per panicle (36.00),the highest un-filled grain per plant (298.33) were found in A4 treated pot.The shortest rice plant (61.90 cm) , lowest effective tillers per plant (12.0) , smallest panicle length (11.48 cm),and lowest filled grain per plant (99.67), and lowest straw yield (8.3 g), lowest grain yield (13.00 g) were found in A4 treated pot and lowest un-filled grain per plant (188.67),lowest un-filled grain per panicle (22.66) were found in A1 treated pot and lowest filled grain per panicle (31.00),lowest 1000-grain weight (27.30 g) were found in A3 treatment.In case of nitrogen, tallest rice plant(92.00), highest effective tillers(24.00), largest panicle length(23.93), , highest straw yield(28.10)were found in T1 treatment & highest filled grain per plant(880.67) ,highest un-filled grain per plant(298.33), highest filled grain per panicle(43.66), highest un-filled grain per panicle(36.00) ,highest 1000-grain weight(28.26), highest grain yield(28.00) were found in T2 treatment.On the other hand ,shortest rice plant (61.90 cm), smallest panicle length (11.48 cm),lowest 1000-grain weight (27.30 g), lowest straw yield (8.3 g), lowest grain yield (13.00 g) were found in T2 treatment & lowest effective tillers per plant (12.0), lowest filled grain per plant (99.67), lowest un-filled grain per plant (188.67), lowest filled grain per panicle (31.00),lowest un-filled grain per panicle (22.66) were found in T1 treatment.Interaction effect of arsenic & nitrogen doses,highest plant height(92.00 cm), highest effective tillers per plant (24.0), largest panicle length (23.93 cm), highest filled grain per plant (880.67), highest straw yield (28.10 g) found in A1T1. The highest filled grain per panicle (43.66),highest 1000-grain weight (28.26 g), highest grain yield (28.00 g) found in A1T2.The highest un-filled grain per panicle (36.00), highest un-filled grain per plant (298.33) were found in A4T2. On the other hand shortest rice plant (61.90 cm) , lowest effective tillers per plant (12.0) , smallest panicle length (11.48 cm), lowest straw yield (8.3 g), lowest grain yield (13.00 g) were found in A4T2 & lowest un-filled grain per plant (188.67), lowest un-filled grain per panicle (22.66) found in A1T1.
... The reduction of As(V) follows enzymatic and non-enzymatic pathways. The non-enzymatic reduction of As(V) require two glutathione molecules (Delnomdedieu et al., 1994) while, enzymatic pathway is triggered by As(V)-reductase which is much speedy processes (Duan et al., 2007;Bleeker et al., 2006). In addition to As reductase enzymes, the enzymes from other systems also have the ability to reduce As(V) i.e. glyceraldehyde-3-phosphate dehydrogenase, and F1Fo ATP-synthase in the mitochondria (N� emeti et al., 2010;Gregus and N� emeti, 2002). ...
... Highly toxic inorganic species are converted to less toxic organic As species within the plants shoots (Zhang et al., 2015). In addition to this, arbuscular mycorrhizae colonization increases the ratio of As(III)/As(V) (Zhang et al., 2015) as this form readily complexes with phytochelatins and compartmentalized to the vacuole (Duan et al., 2007). Under As contaminations, plant association with arbuscular mycorrhizae could improve phosphorus nutrition and growth resulting in the ''dilution effect'' on As in the plant tissues Chen et al., 2007;Xia et al., 2007). ...
... On upregulating their expression in transgenic rice plants, they lessened the As translocation from root to shoot and internode to shoot (Deng et al., 2018). Further studies conducted in tobacco, rice, and Pteris plants have demonstrated that As (V) was reduced to As (III) by the enzyme As reductase (ACR) in different plants Duan et al., 2007;Nahar et al., 2017). Transgenic tobacco plants overexpressing the Arabidopsis ACR2 gene exhibited increased tolerance to high As stress levels. ...
Arsenic (As) toxicity in crops is a major global concern, adversely affecting sustainable agricultural practices, and serving as a potential carcinogenic pollutant. As contamination in soil poses a significant threat to plant health and productivity, adversely impacting growth, photosynthesis, and the antioxidant system. To address this issue, plants endogenously regulate the levels of various phytohormones, and the exogenous application of phytohormones to mitigate As-induced stress has gained significant attention. Phytohormones act as secondary (2°) messengers, participating in diverse signaling cascades under As stress. As uptake in plants leads to the As-accumulation and generation of excessive reactive oxygen species (ROS) which can be alleviated by phytohormones. Numerous studies have highlighted the role of phytohormones, such as auxins, methyl jasmonates, salicylic acid, brassinosteroids, and Mel, in regulating pathways that enhance plant growth, bio-mass accumulation, ROS scavenging, antioxidative enzyme and photosynthesis under As stress. This review summarizes the detailed mechanism of As phytotoxicity, its detoxification mechanism, and the exogenous application of phytohormones to alleviate As stress. Additionally, we provide insights into recent findings on the possible roles of various genes, proteins, transgenic factors, and genome editing approaches in phytohor-mone-mediated As-stress tolerance.
... On upregulating their expression in transgenic rice plants, they lessened the As translocation from root to shoot and internode to shoot (Deng et al., 2018). Further studies conducted in tobacco, rice, and Pteris plants have demonstrated that As (V) was reduced to As (III) by the enzyme As reductase (ACR) in different plants Duan et al., 2007;Nahar et al., 2017). Transgenic tobacco plants overexpressing the Arabidopsis ACR2 gene exhibited increased tolerance to high As stress levels. ...
... The reduction is usually carried out by enzymes, e.g. arsenate reductase (ACR) [11,58,59]. The non-enzymatic reduction of As (V) to As (III) is mediated by two glutathione molecules that are oxidized to glutathione disulfide, which can be rapidly recycled to reduced GSH molecules by GSH reductase [16]. ...
A potential plant species suitable for As bioaccumulation and phytoremediation of water environments could be the macrophyte Berula erecta. The objective of the present study was to investigate the effect of arsenate (As(V), C2H6AsNaO2∙3 H2O), predominant in freshwater systems, on the growth, development and low molecular weight thiols of Berula erecta under controlled tissue culture conditions in vitro. Uptake of total arsenate increased with increasing arsenate treatments, at a higher percentage in the roots than in the aboveground parts of the plants. Lower concentrations of As(V) (0.1, 1, 10 mg L−1) had a positive effect on growth, dry weight, length of roots and shoots and number of buds. High concentrations of arsenate (50 and 100 mg As(V) L−1) significantly inhibited all growth parameters and decreased the photochemical efficiency of PSII. Evaluation of thiols revealed the critical As level (146 µg g−1 DW; 50 mg As(V) L−1 treatment) above which the As concentration can be toxic.
... The inoculation of B5.12 accelerated the oxidation of As(III) to As(V), resulting in the decrease of As(III) concentration in roots. As(V) in rice root might further reduced to As(III) by the activity of As(V) reducer Sh2.11 and then followed by its efflux into the medium by As(III) extrusion pump (Duan et al., 2007;Yang et al., 2015). The As (III) discharged out might again oxidized into As(V) by the As(III) oxidizing activity of B5.12 to generate a dynamic cycle of coupled oxidation-reduction (redox) process of As detoxification (Anand et al., , 2023Navarro et al., 2022;Yang et al., 2015). ...
The study aimed to explicate the role of microbial co-inoculants for the mitigation of arsenic (As) toxicity in rice. Arsenate (AsV) reducer yeast Debaryomyces hansenii NBRI-Sh2.11 (Sh2.11) with bacterial strains of different biotransformation potential was attempted to develop microbial co-inoculants. An experiment to test their efficacy (yeast and bacterial strains) on plant growth and As uptake was conducted under a stressed condition of 20 mg kg-1 of arsenite (AsIII). A combination of Sh2.11 with an As(III)-oxidizer, Citrobacter sp. NBRI-B5.12 (B5.12), resulted in ∼90% decrease in grain As content as compared to Sh2.11 alone (∼40%). Reduced As accumulation in rice roots under co-treated condition was validated with SEM-EDS analysis. Enhanced As expulsion in the selected combination under in vitro conditions was found to be correlated with higher As content in the soil during their interaction with plants. Selected co-inoculant mediated enhanced nutrient uptake in association with better production of indole acetic acid (IAA) and gibberellic acid (GA) in shoot, support microbial co-inoculant mediated better biomass under stressful condition. Boosted defense response in association with enhanced glutathione-S-transferase (GST) and glutathione reductase (GR), activities under in vitro and in vivo conditions were observed. These results indicated that the As(III) oxidizer-B5.12 accelerated the As detoxification property of the As(V) reducer-Sh2.11. Henceforth, the results confer that the coupled reduction-oxidation process of the co-inoculant reduces the accumulation of As in rice grain. These co-inoculants can be further developed for field trials to achieve higher biomass with alleviated As toxicity in rice.
... Although the uptake mechanism of As is well known in higher plants, but a specific As efflux system is yet to be discovered. After uptake, As(V) is reduced to As(III) by intracellular As(V) reductases (Duan et al., 2007;Shi et al., 2016). Reduction of As(V) in roots cells is important as the As (III) is a prominent As species found in the xylem sap. ...
... However, the enzymatic pathway is comparatively faster, wherein, As(V) to As(III) reduction by arsenate reductase have been reported in many species as a detoxification measure, like AtACR2 (Arabidopsis), OsACR2 (rice), HlAsr (Holcus lanatus) and PvACR2 (P. vittata) (Duan et al. 2007;Dhankher et al. 2006;Ellis et al. 2006). An arsenate reductase ATQ1 in Arabidopsis has been identified to catalyze As(V) to As(III) reduction in the outer cell layer of root which also helps in efflux of As(III) back into the soil (Sanchez-Bermejo et al. 2014;Chao et al. 2014). ...
Arsenic (As), a ubiquitous metalloid in the Earth’s crust, is one of the most toxic soil constituents affecting the plants. Accumulating from geogenic and anthropogenic sources, As piles up through the trophic levels of the food chain, leading to human health risk. As-uptake through the roots occurs through the apoplast, enters into the cell cytoplasm by phosphate transporters or aquaporin channels and can be transported to the above-ground tissues also through xylem. As-led toxicity is mediated by oxidative stress, severely impacting the staple food crops worldwide, including rice, wheat, pulses, etc. Decreased germination and vitality index, chlorophyll content, biomass content, etc. massively reduce yield percentage. In response to As-stress, plants have evolved several defense mechanisms, such as the production of antioxidant enzymes, like superoxide dismutase, catalase, glutathione, etc., and induction of phytochelatin synthesis. Phytochelatin-triggered As-detoxification is regulated by the transport of As-phytochelatin complex to the vacuole, leading to metal ion sequestration. Hence, to resolve the adverse effects of As on the whole ecosystem, genetic engineering can be used to develop As-tolerant plants. Overexpression of phytochelatin synthase genes, either alone or together with γ-ECS can render tolerance to As-stress. Thus, significant research has been carried out in recent years concerning As-transport, speciation and detoxification. However, the gaps to be filled include the mechanism of As-compartmentalization in the vacuoles, transport through the xylem, and accumulation in grains. Therefore, studies of cellular mechanism of As-detoxification may open up new prospects to develop tolerant crop cultivars for sustainable As-free agriculture practice. The present book chapter focuses on As-toxicity and detoxification with special emphasis on phytochelatin regulation and genetic engineering approaches to enhance stress tolerance.
... Obviously, these results were not due to a direct competition in uptake between Si and arsenate because they do not share the same transporters (Meharg and Macnair, 1992;Ma et al., 2008). Duan et al. (2007) cloned and expressed both ACR2-like genes from the rice genome, which can catalyze arsenate reduction in rice. Although rice plants were supplied with arsenate, both As(III) and As(V) were detected in the xylem sap (Fig. 5), showing that As(V) can be reduced to As(III) and then transported to the xylem within rice. ...
Possible mechanisms of the effects of silicon (Si) on arsenic (As) uptake were explored using a wild-type rice and its low-Si mutant (lsi1). Hydroponic experiments were carried out to investigate the effects of internal and external Si on the As accumulation and uptake by rice in excised roots (28 d–old seedlings) and xylem sap (61 d–old plants). The presence of Si significantly decreased the As concentrations in both shoots and roots of the wild type but not in the mutant with 13.3 μM–arsenite or 10/20 μM–arsenate treatments. The Si-defective mutant rice (lsi1) also showed a significant reduction in arsenite or arsenate uptake. Moreover, As concentrations in xylem sap of the wild type were reduced by 51% with 1 mM Si– and 15 μM–arsenate treatments, while Si had no effect on As concentrations in the xylem sap of the mutant. Arsenic-species analysis further indicated that the addition of 1 mM Si significantly decreased As(III) concentrations but had little effect on As(V) concentrations in the xylem sap of the wild type with 15 μM–arsenate treatments. These results indicated that external Si-mediated reduction in arsenite uptake by rice is due to the direct competition between Si and arsenite during uptake. This is because both share the same influx transporter Lsi1. In addition, internal Si-mediated reduction in arsenite uptake by rice is due to competition of the Si/arsenite efflux transporter Lsi2 during the As(III)-transportation process. Silicon also inhibited arsenate uptake by rice. It is proposed that this could actually be due not to the inhibition of arsenate uptake per se but rather the inhibition of arsenite transformed from arsenate, either in the external solution or in rice roots.
... Once inside the plant cells, As(V) interferes with oxidative phosphorylation and ATP production due to structural similarity with phosphate (Finnegan and Chen, 2012). However, due to the presence of As(III) efflux pump, intracellular As(V) is immediately detoxified to As (III) by arsenate reductase and effluxed out of the plant cell (Duan et al., 2007). On the other hand, As(III) exerts its toxic effects by interacting with the thiol groups in proteins, diminishing the enzyme activity and causing lipid peroxidation (Tripathi et al., 2007). ...
Arsenic is a toxic metalloid categorized under class 1 carcinogen and is detrimental to both plants and animals. Agricultural land in several countries is contaminated with arsenic, resulting in its accumulation in food grains. Increasing global food demand has made it essential to explore neglected lands like arsenic-contaminated lands for crop production. This has posed a severe threat to both food safety and security. Exploration of arsenic-resistant plant growth-promoting rhizobacteria (PGPR) is an environment-friendly approach that holds promise for both plant growth promotion and arsenic amelioration in food grains. However, their real-time performance is dependent upon several biotic and abiotic factors. Therefore, a detailed analysis of associated mechanisms and constraints becomes inevitable to explore the full potential of available arsenic-resistant PGPR germplasm. Authors in this review have highlighted the role and constraints of arsenic-resistant PGPR in reducing the arsenic toxicity in food crops, besides providing the details of arsenic transport in food grains.
https://authors.elsevier.com/a/1eCtu15DSlK5VW
... Over-expression of OsGSTL2 increase As chelation and tolerance (Kumar et al., 2013) Defective in anther dehiscence (DAD)OsDAD1;2, lipoxygenases (LOX)OsLOX2;1, allene oxide synthase (AOS) OsAOS2, and jasmonate Involved in Jasmonic acid biosynthesis Over-expression increase As stress tolerance (Huang et al., 2012) Reduces As (V) to As (III) (Duan et al., 2007) Os03g09970 ...
Arsenic (As) is a potentially toxic metalloid classified as a group 1 carcinogen, released in the soil environment because of natural as well as different anthropogenic activities. The presence of excess As content in soil and irrigation water enhances the As accumulation in rice grains. Millions of people who consume these contaminated grains are exposed to severe health issues. Increased arsenic uptake causes oxidative stress in plants, which combats by inducing the expression of several genes and signaling the biosynthesis of various antioxidants and phytochelatins. As toxicity reduces crop productivity, so it's critical to improve plant growth in As-contaminated environments while minimizing metal translocation to grains. Arbuscular mycorrhiza fungi (AMF) is considered a sustainable way to tolerate As toxicity. Organic pollutants metabolism by AMF, degradation of these soil contaminants by AMF exudation enzymes, and elimination of the pollutants by plant uptake and accumulation are the principal mechanisms of AMF mediated bioremediation. However, plant responses are established to vary with the host plant and the species of AMF. In our review, we showed that understanding the community composition, diversity, and gene regulation of AMF in the rice ecosystem played a critical role in maximizing As uptake and their potential in sustainable rice and other crops production. It has been reviewed that AMF has the potential to survive in an extremely As toxic condition and it potentially aids to improve the tolerance level of host plants.
... AtSTR5 or AtCDC25) and OsACR2.1 or OsACR2.2 characterized as arsenate reductases in Arabidopsis thaliana (Dhankher et al., 2006) and Oryza sativa (Duan et al., 2007), respectively, which all by homology belong to the sulfurtransferase (STR)/rhodanase family (Selles et al., 2019). As documented in Fig. 3A, deduced PcACR2 contains the features described in other plant ACR2 proteins. ...
Pollution and poisoning with carcinogenic arsenic (As) is of major concern globally. Interestingly, there are ferns that can naturally tolerate remarkably high As concentrations in soils while hyperaccumulating this metalloid in their fronds. Besides Pteris vittata in which As-related traits and molecular determinants have been studied in detail, the As hyperaccumulation status has been attributed also to Pteris cretica. We thus inspected two P. cretica cultivars, Parkerii and Albo-lineata, for As hyperaccumulation traits. The cultivars were grown in soils supplemented with 20, 100, and 250 mg kg⁻¹ of inorganic arsenate (iAsV). Unlike Parkerii, Albo-lineata was confirmed to be As tolerant and hyperaccumulating, with up to 1.3 and 6.4 g As kg⁻¹ dry weight in roots and fronds, respectively, from soils amended with 250 mg iAsV kg⁻¹. As speciation analyses rejected that organoarsenical species and binding with phytochelatins and other proteinaceous ligands would play any significant role in the biology of As in either cultivar. While in Parkerii, the dominating As species, particularly in roots, occurred as iAsV, in Albo-lineata the majority of the root and frond As was apparently converted to iAsIII. Parkerii markedly accumulated iAsIII in its fronds when grown on As spiked soils. Considering the roles iAsV reductase ACR2 and iAsIII transporter ACR3 may have in the handling of iAs, we isolated Albo-lineata PcACR2 and PcACR3 genes closely related to P. vittata PvACR2 and PvACR3. The gene expression analysis in Albo-lineata fronds revealed that the transcription of PcACR2 and PcACR3 was clearly As responsive (up to 6.5- and 45-times increase in transcript levels compared to control soil conditions, respectively). The tolerance and uptake assays in yeasts showed that PcACRs can complement corresponding As-sensitive mutations, indicating that PcACR2 and PcACR3 encode functional proteins that can perform, respectively, iAsV reduction and membrane iAsIII transport tasks in As-hyperaccumulating Albo-lineata.
... Non-enzymatic antioxidant defense involve low molecular weight compounds that remove ROS by their oxidation, maintaining the cellular redox state. GSH is a tripeptide with a sulfhydryl group (−SH) in the cysteine residue which can directly interact with ROS, producing GSSG, or act as an enzymatic co-factor (Duan et al. 2007;Das and Roychoudhury 2014;Choudhury et al. 2017). Moreover, GSH is the precursor of phytochelatins (PCs) that are nonprotein thiols synthesized by phytochelatin synthase (PCS) highly induced by metal(loid)s (Hernández et al. 2015;Bianucci et al. 2017a). ...
The Leguminosae family constitutes the second most important family of crop plants worldwide. Nowadays, legumes provide one-third of the entire amount of protein for human consumption, animal food and edible and industrial oils. Plants are prone to suffer stress episodes such as salinity, drought or the presence of metal(loid)s. On initial exposure to an abiotic stress, plants show alterations in metabolism, ionic balance, osmolarity and membrane stability, among others. An oxidative burst with consequent biomolecules damage aggravates the stress condition. Along the evolution, plants acquired stress-specific cellular sensing mechanisms that help in signal transduction, yielding the activation of transcription factors and genes to counteract the deleterious effects triggered by the stressful condition. Among the contributors to help the plant in re-establishing cellular homeostasis are ion balancing, compatible solutes accumulation, antioxidant defense, hormonal regulation. However, depending on the severity of the stress, plants can retard or cease growth and finally die. Thus, this can conduct to yield loss having a huge impact in agroeconomy.Therefore, the present chapter focuses on the tolerance mechanisms to salinity, drought stress and metal(loid)s and summarizes the human efforts that upraised in an exhaustive tentative for improve stress tolerance in legume crops. This chapter intends to increase the understanding of the tolerance mechanisms evoked by legumes exposed to abiotic stresses, hence avoiding yield loss. The biochemical, molecular and physiological responses triggered by plants to cope with abiotic stresses are presented. Besides, we discuss the last advances in legume improvement through transgenic, breeding or agronomic approaches.
... After taking up by plants, AsV is rapidly reduced to AsIII by arsenate reductase in the roots (Chao et al., 2014;). Arsenate reductases, ACR2 and HAC, both mediated AsV reduction in plants (Ellis et al., 2006;Duan et al., 2007;Chao et al., 2014;Sanchez-Bermejo et al., 2014;Shi et al., 2016). ACR2 from P. vittata PvACR2 mediates AsV reduction in yeast, which may play a role in AsV reduction in P. vittata (Ellis et al., 2006). ...
Arsenite (AsIII) antiporter ACR3 is crucial for arsenic (As) translocation and sequestration in As-hyperaccumulator Pteris vittata, which has potential for phytoremediation of As-contaminated soils. In this study, two new ACR3 genes PvACR3;2 and PvACR3;3 were cloned from P. vittata and studied in yeast (Saccharomyces cerevisiae) and tobacco (Nicotiana tabacum). Both ACR3s mediated AsIII efflux in yeast, decreasing As accumulation and enhancing As tolerance. In addition, PvACR3;2 and PvACR3;3 were expressed in model plant tobacco. Localized on the plasma membrane, PvACR3;2 mediated both AsIII translocation to the shoots and AsIII efflux from the roots in tobacco, resulting in 203−258% increase in shoot As after exposing to 5 μM AsIII under hydroponics. In comparison, localized to the vacuolar membrane, PvACR3;3 sequestrated AsIII in tobacco root vacuoles, leading to 18−20% higher As in the roots and 15−36% lower As in the shoots. Further, based on qRT-PCR, both genes were mainly expressed in P. vittata fronds, indicating PvACR3;2 and PvACR3;3 may play roles in AsIII translocation and sequestration in the fronds. This study provides not only new insights into the functions of new ACR3 genes in P. vittata, but also important gene resources for manipulating As accumulation in plants for phytoremediation and food safety.
... During the long-term evolutionary process, plants have also established a suite of mechanisms to detoxify As(V). When As(V) enters the roots of non-hyperaccumulators, it is quickly converted to arsenite [As (III)], whose chemical properties are very different than those of phosphate, by arsenate reductases, such as ARSENATE REDUCTASE 2 (ACR2) and HIGH ARSENIC CONTENT 1 (HAC1), and then As(III) is sequestered in the root vacuoles or extruded back into the soil from roots, thereby preventing the root-to-shoot translocation of As via the phosphate transport system (Chao et al., 2014;Dhankher et al., 2006;Duan et al., 2007). The mutation or RNAi silencing of arsenate reductase genes will increase the accumulation of As in the shoots, thus, harming plants grown under As(V) conditions (Chao et al., 2014;Dhankher et al., 2006;Shi et al., 2016). ...
Arsenic (As) contamination is a worldwide problem and threatens human health. Here, we found that Rhizobium symbiosis can improve the tolerance to arsenate [As(V)], and a wild type R. meliloti Rm5038 symbiosis can significantly decrease the accumulation of As in Medicago truncatula shoots. The As content in plants could be decreased by nitrogen and the mutation of nitrate transporter NRT3.1. The expression of M. truncatula NRT3.1-like gene NRT3.1L1 could reverse the As(V)-tolerance phenotype of the Arabidopsis nrt3.1 mutant. Rm5038 symbiosis significantly increased the level of nitrogen in the shoot and reduced the expression of NRT3.1Ls in plants afflicted by As(V). The genetic analyses of aba2-1, pyr1/pyl1/2/4/5/8, and abi1-2/abi2-2/hab1-1/pp2ca-1 mutants revealed that abscisic acid (ABA) signaling regulates the tolerance of plants to As(V). ABA and linalool could promote the expression of NRT3.1Ls, however, their root biosynthesis was inhibited by ammonium, the first form of nitrogen fixed by Rhizobium symbiosis. Moreover, ABA and linalool may also control As and nitrate accumulation in Rhizobium symbionts via signaling pathways other than ammonia and NRT3.1Ls. Thus, Rhizobium symbiosis modulates the accumulation of As in plants via nitrogen and NRT3.1Ls regulated by ABA and linalool, which provides novel approaches to reduce As accumulation in legume crops.
... Plants with mutant ACR2 gene showed higher As accumulation in stems, leaves or fruits leads the more susceptibility toward As stress (Dhankher et al. 2006). Further, overexpression of OsACR2;1 and OsACR2;2 in the rice plants shows higher resistance for As (Duan et al. 2007). ...
This review explains the transport, mobility, resistance and detoxification of toxic metalloid arsenic (As) in plants. Arsenic is ubiquitously present in Earth’s crust; however, numerous human interventions such as rapid industrialization use of As-based pesticides, insecticides and discharge of industrial wastes in water bodies leads to cumulative increase in As in the environment and has become a global challenge. Arsenic exists in different organic and inorganic forms, but inorganic forms such as pentavalent arsenate (AsV) and trivalent arsenite (AsIII) are more toxic and actively taken up by plants. Its toxicity is marked by generation of reactive oxygen species (ROS) that are capable of degrading various biomolecules of the cellular systems. To keep the ROS under the limit, plants have an array of enzymatic antioxidants such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR) and glutathione-S-transferase (GST); and non-enzymatic antioxidant like ascorbate, proline, and cysteine. Contrary to this, As-hyper-accumulator plants survive under high concentration of As through the strenuous action of Asv reduction into AsIII followed by the vacuolar compartmentalization of complex or inorganic As. Hence, this review focuses on the potential sources of As in the environment, its speciation and toxicity, and tolerance strategies in plants.
... 39,46,47 In rice roots, AsV can be reduced to AsIII, making AsIII a main As form in plant tissues. 24,25,48 To further understand As metabolism in PvACR3;1 transgenic rice, rice seedlings were grown in a solution containing 20 μM AsV for 1 d. Similar to AsIII, transgenic rice under AsV exposure also showed higher As levels in the roots, with similar As levels in the shoots and lower As translocation to the shoots ( Figure 2ABC). ...
Rice (Oryza sativa) is a major food crop in the world, feeding half of the world's population. However, rice is efficient in taking up toxic metalloid arsenic (As), adversely impacting human health. Among different As species, inorganic As is more toxic than organic As. Thus, it is important to decrease inorganic As in rice to reduce human exposure from the food chain. The arsenite (AsIII) antiporter gene PvACR3;1 from As-hyperaccumulator Pteris vittata decreases shoot As accumulation when heterologously expressed in plants. In this study, three homozygous transgenic lines (L2, L4, and L7) of T3 generation were obtained after transforming PvACR3;1 into rice. At 5 μM of AsIII, PvACR3;1 transgenic rice accumulated 127%−205% higher As in the roots, with lower As translocation than wild type (WT) plants. In addition, at 20 μM of AsV, the transgenic rice showed similar results, indicating that expressing PvACR3;1 increased As retention in the roots from both AsIII and AsV. Furthermore, PvACR3;1 transgenic rice plants were grown in As-contaminated soils under flooded conditions. PvACR3;1 decreased As accumulations in transgenic rice shoots by 72%−83% without impacting nutrient minerals (Mn, Zn, and Cu). In addition, not only total As in unhusked rice grain of PvACR3;1 transgenic lines were decreased by 28%−39%, but also inorganic As was 26%−46% lower. Taken together, the results showed that expressing PvACR3;1 effectively decreased both total As and inorganic As in rice grain, which is of significance to breed low-As rice for food safety and human health.
... Other study has reported aquaporins can slowly transport the arsenite through diffusion (Ma et al. 2008). Scientists have been found that accumulated arsenate by heperaccumulators instantly and rapidly oxidise to arsenite through rice (Duan et al. 2007), glutathione (Delnomdedieu et al. 1994), or Arabidopsis having Homologues genes (Dhankher et al. 2006), Pteris (Ellis et al. 2006) and Holcus (Bleeker et al. 2006). ...
This study presents a review conducted to critically evaluate the microbial fabrication and its subsequent effect on arsenic contamination in soil. Taking into consideration the global situation, the sources of arsenic, the microbial arsenic pathways and the impact of arsenic contamination on soil have been defined. Different published studies highlighted the biochemical and physiological molecular aspects of arsenic and evaluated their impacts on soil; in the current paper, the sources and extent of arsenic contamination and the translocation and speciation from microbes to soil are illustrated. This study also discusses the forms of water-soluble arsenic and whether these are more toxic than insoluble forms, as previously reported. In addition , this paper briefly illustrates the sources and coverage of arsenic contamination and arsenic speciation in contaminated soil globally. Some vital research findings are also discussed which relate to the scientific exploration of microbial arsenic systems in contaminated soil.
... Arsenate reductase is represented by ACR2, which shows homology with yeast ASCR2 (ScACR2) (Landrieu et al., 2004a,b). Similarly, in Pteris vittata transporter PvACR2 was reported, and from O. sativa transporters OsACR2.1 and OsACR2.3 were reported (Ellis et al., 2006;Duan et al., 2007). In A. thaliana, ACR2 (CDC25) plays a role in As metabolism. ...
Understanding the functions of the major cellular components, including biomolecules such as proteins, nucleic acids, and various metabolites involved in the biological processes in an organism is crucial. Regulation of the elements uptake and distribution from soils is also indispensable for plants. Ionome represents an important inorganic nutrient of an organism and is desirable in small quantities. Ionome assists in performing several functions in plants, including censoring plant genome functions. Various genetic mapping approaches have contributed significantly in the prosperity of ionomics. Moreover, it has been well established that ionomics also play a crucial role in tolerance toward different abiotic/biotic stresses in plants. This chapter therefore discusses the ionomics approaches of abiotic stress factors, which effects plant’s growth and performance, including heavy metals, osmotic and high salinity stress.
... Among the important phenolic antioxidants anthocyanins belong [80], with the ability, besides free radical scavenging, to bind heavy metals and detoxify them in vacuoles [81,82]. The antioxidative properties of anthocyanins arise from their high reactivity as hydrogen or electron donors, from the ability of the polyphenol-derived radicals to stabilize and delocalize the unpaired electron, and from their ability to chelate transition metals [83]. In our study, the anthocyanins content exhibited stable levels in the tolerant WIS ( Figure 5A) but dramatically decreased in the sensitive SYL at higher As. ...
Arsenic (As) contaminates the food chain and decreases agricultural production through impairing plants, particularly due to oxidative stress. To better understand the As tolerance mechanisms, two contrasting tobacco genotypes: As-sensitive Nicotiana sylvestris and As-tolerant N.tabacum, cv. ‘Wisconsin’ were analyzed. The most meaningful differences were found in the carbohydrate status, neglected so far in the As context. In the tolerant genotype, contrary to the sensitive one, net photosynthesis rates and saccharide levels were unaffected by As exposure. Importantly, the total antioxidant capacity was far stronger in the As-tolerant genotype, based on higher antioxidants levels (e.g., phenolics, ascorbate, glutathione) and activities and/or appropriate localizations of antioxidative enzymes, manifested as reverse root/shoot activities in the selected genotypes. Accordingly, malondialdehyde levels, a lipid peroxidation marker, increased only in sensitive tobacco, indicating efficient membrane protection in As-tolerant species. We bring new evidence of the orchestrated action of a broad spectrum of both antioxidant enzymes and molecules essential for As stress coping. For the first time, we propose robust carbohydrate metabolism based on undisturbed photosynthesis to be crucial not only for subsidizing C and energy for defense but also for participating in direct reactive oxygen species (ROS) quenching. The collected data and suggestions can serve as a basis for the selection of plant As phytoremediators or for targeted breeding of tolerant crops.
... 93,94 In addition, this protein could also have an arsenate reductase activity as it is being discovered in several protein phosphatases in all kingdoms. 95,96 For instance, in Bacillus sp. CDB3, a protein phosphatase belonging to an As resistance operon shows certain homology with arsenate reductases. ...
Transgenic tobacco hairy roots expressing the bacterial arsenite efflux pump Acr3 from Ensifer medicae were generated. The gene product was targeted, either to the plasma membrane (ACR3 lines) or to...
... Rice presents two genes encoding for As(V)-reductase, viz., OsACR2.1 and OsACR2.2, which are involved in the reduction of As(V) (Duan et al. 2007). The products of these genes were purified from E. coli and demonstrate the capacity to reduce As(V) to As(III) in vitro, being OsACR2.1 the one which presented higher reductase activity comparing to OsACR2.2; ...
Arsenic (As) in food is a threat for human health, and among all cereal rice is the most important source of the metalloid through diet. The dynamic of the element and the natural ability of rice to uptake, transport, and accumulate the metalloid at grains have motivated important research to be carried out in this regard. Thus, the aim of this chapter is to draw a synthesis on the main factors which ultimately determines As content in rice, from its uptake from the soil solution to its transport, metabolism, and final accumulation in grain. The element is of natural occurrence, varying in concentration, and its chemical species are modified due to a range of factors, such as the soil redox state. Among all As species, arsenite [As (III)] is the most common in anoxic conditions, such as the ones found in flooded paddy rice fields, which form is preferred for rice root uptake. Rice is naturally efficient in As(III) uptake, compared to most crops already studied, which is mainly due to its improved ability to uptake and transport silicon, especially with the aid of aquaporins, as silicic acid and arsenite are chemically analogous. Similarly, the second most important As form, arsenate [As(V)], is mainly uptaken and transported through the phosphate way. After being uptaken, As is transported either via xylem or phloem and is loaded to the grain. Within rice tissues, the element can be metabolized, being reduced, biomethylated, complexed to other elements, or even sequestered into vacuoles. The knowledge regarding the mechanisms of As uptake, transport, and metabolization in rice allows one to draw strategies in order to mitigate the content of this element in the grains, either via management practices or also via breeding and biotechnological approaches.
... Once As(V) is inside the cell, several modifications can disrupt normal cellular processes such as photosynthesis and mitochondrial respiration, by alteration of ATP synthesis and phosphate-mediated cellular metabolism regulation (Tripathi et al. 2007). However, under reducing condition, most As(V) could be rapidly reduced to As(III) by arsenate reductase enzymes, constituting the first step of As detoxification in cells (Dhankher et al. 2006;Duan et al. 2007;Sánchez-Bermejo et al. 2014). Besides the already known As transporters, it was reported that there are transporters which allow As(III) passage into the xylem (OsLsi2 and OsLsi1) Mitani et al. 2009). ...
Arsenic (As) is a harmful metalloid that can be naturally found in soils and water. Consumption of contaminated groundwater is the most common source of human poisoning; furthermore, the ingestion of contaminated food is currently getting much attention. The main problem arises when crops incorporate the metalloid by direct absorption of groundwater or artificial irrigation containing high levels of the toxic element, or through contaminated soils, being the first stage of As distribution in the trophic chain. This issue constitutes not only an agronomic problem, due to the effect in grain quality and yield, but also a serious risk for human health. In this chapter, we aim to summarize the effect of As on growth and metalloid accumulation and distribution on cereal and legume crops used for human consumption. The comprehension of the responses evoked by plants under As exposure could contribute to a better understanding of metalloid toxicity and the possible risks of grain contamination as well to as assess mitigation strategies to prevent human poisoning.
... 39,46,47 In rice roots, AsV can be reduced to AsIII, making AsIII a main As form in plant tissues. 24,25,48 To further understand As metabolism in PvACR3;1 transgenic rice, rice seedlings were grown in a solution containing 20 μM AsV for 1 d. Similar to AsIII, transgenic rice under AsV exposure also showed higher As levels in the roots, with similar As levels in the shoots and lower As translocation to the shoots ( Figure 2ABC). ...
Rice (Oryza sativa L.) is the staple food for more than 50% of the world’s population. Owing to its growth characteristics, rice has more than 10-fold the ability to enrich the carcinogen arsenic (As) than other crops, which seriously affects world food security. The consumption of rice is one of the primary ways for humans to intake As, and it endangers human health. Effective measures to control As pollution need to be studied and promoted. Currently, there have been many studies on reducing the accumulation of As in rice. They are generally divided into agronomic practices and biotechnological approaches, but simultaneously, the problem of using the same measures to obtain the opposite results may be due to the different species of As or soil environments. There is a lack of systematic discussion on measures to reduce As in rice based on its mechanism of action. Therefore, an in-depth understanding of the molecular mechanism of the accumulation of As in rice could result in accurate measures to reduce the content of As based on local conditions. Different species of As have different toxicity and metabolic pathways. This review comprehensively summarizes and reviews the molecular mechanisms of toxicity, absorption, transport and redistribution of different species of As in rice in recent years, and the agronomic measures to effectively reduce the accumulation of As in rice and the genetic resources that can be used to breed for rice that only accumulates low levels of As. The goal of this review is to provide theoretical support for the prevention and control of As pollution in rice, facilitate the creation of new types of germplasm aiming to develop without arsenic accumulation or within an acceptable limit to prevent the health consequences associated with heavy metal As as described here.
Arsenic is one of the most detrimental heavy metals on the planet due to its bio-accumulative and carcinogenic nature. In addition to humans and animals, it also has a hazardous foot print on agriculture. Arsenic in its inorganic form is readily bio-available from water sources and taken up by the crop plants through phosphate transporters due to its structural similarity with inorganic phosphate. Bio-accumulation of arsenic in crop plants can lead to an arsenic-contaminated food chain and amplify manifold the hazardous effects of arsenic pollution. Apart from phosphate transporters, different aquaporins like Nodulin26-like intrinsic proteins, plasma membrane intrinsic proteins, tonoplast intrinsic proteins, and vacuolar transporters are responsible for arsenic uptake and accumulation in plants. To withstand a higher concentration of arsenic, bacterial and fungal systems use ArsB and Acr3 efflux transporter proteins, respectively, to pump out the arsenic from the cell. In plants, Acr3 (Arsenic Compound Resistance 3) efflux protein has been reported only in Pteridophytes to efflux arsenic from cells. In this review, we have discussed different transporters involved with arsenic uptake, translocation, sequestration, and efflux with special emphasis on Acr3. We have also explored the transgenic approaches performed with ACR3 to reduce arsenic accumulation in plants and discussed the future prospects of reducing arsenic from the food chain by lowering arsenic accumulation altogether from crop plants.
Arsenic (As) pollution in wetlands, mainly as As(III) and As(V), has threatened wetland plant growth. It has been well documented that arbuscular mycorrhizal (AM) fungi can alleviate As stress in terrestrial plants. However, whether AM fungi can protect natural wetland plants from As stress remains largely unknown. Therefore, three hydroponic experiments were conducted in which Iris tectorum Maxim. (I. tectorum) plants were exposed to As(III) or As(V) stresses, to investigate the effects of mycorrhizal inoculation on As uptake, efflux, and accumulation. The results suggested that short-term kinetics of As influx in I. tectorum followed the Michaelis–Menten function. Mycorrhizal inoculation decreased the maximum uptake rate (Vmax) and Michaelis constant (Km) of plants for As(III) influx, while yielding no significant difference in As(V) influx. Generally, mycorrhizal plants released more As into environments after 72 h efflux, especially under As(V) exposure. Moreover, mycorrhizal plants exhibited potential higher As accumulation capacity, probably due to more active As reduction, which was one of the mechanisms through which AM fungi mitigate As phytotoxicity. Our study has revealed the role of aerobic microorganism AM fungi in regulating As translocation in wetland plants and supports the involvement of AM fungi in alleviating plant As stress in anaerobic wetlands.
Arsenic (As) is a toxic metalloid, elevated levels of which in soils are becoming a major global environmental issue that poses potential health risks to humans. Pteris vittata, the first known As hyperaccumulator, has been successfully used to remediate As-polluted soils. Understanding why and how P. vittata hyperaccumulates As is the core theoretical basis of As phytoremediation technology. In this review, we highlight the beneficial effects of As in P. vittata, including growth promotion, elemental defense, and other potential benefits. The stimulated growth of P. vittata induced by As can be defined as As hormesis, but differs from that in non-hyperaccumulators in some aspects. Furthermore, the As coping mechanisms of P. vittata, including As uptake, reduction, efflux, translocation, and sequestration/detoxification are discussed. We hypothesize that P. vittata has evolved strong As uptake and translocation capacities to obtain beneficial effects from As, which gradually leads to As accumulation. During this process, P. vittata has developed a strong As vacuolar sequestration ability to detoxify overloaded As, which enables it to accumulate extremely high As concentrations in its fronds. This review also provides insights into several important research gaps that need to be addressed to advance our understanding of As hyperaccumulation in P. vittata from the perspective of the benefits of As.
Arsenic is a ubiquitous element in the environment, a source of constant evolutionary pressure on organisms. The arsenic resistance machinery is thoroughly described for bacteria. Highly resistant lineages are also common in eukaryotes, but evolutionary knowledge is much more limited. While the origin of the resistance machinery in eukaryotes is loosely attributed to horizontal gene transfer (HGT) from bacteria, only a handful of eukaryotes were deeply studied. Here we investigate the origin and evolution of the core genes in arsenic resistance in eukaryotes using a broad phylogenetic framework. We hypothesize that, as arsenic pressure is constant throughout Earth’s history, resistance mechanisms are probably ancestral to eukaryotes. We identified homologs for each of the arsenic resistance genes in eukaryotes and traced their possible origin using phylogenetic reconstruction. We reveal that: i. an important component of the arsenic-resistant machinery originated before the last eukaryotic common ancestor; ii. later events of gene duplication and HGT generated new homologs that, in many cases, replaced ancestral ones. Even though HGT has an important contribution to the expansion of arsenic metabolism in eukaryotes, we propose the hypothesis of ancestral origin and differential retention of arsenic resistance mechanisms in the group. Key-words: Environmental adaptation; resistance to toxic metalloids; detoxification; comparative genomics; functional phylogenomics.
Arsenic is a metalloid toxic to plants. Arsenate [As(V)], the prevalent chemical form of arsenic, is an analog to phosphate (Pi) and is incorporated into plant cells via Pi transporters. Here, we found that the MYB40 transcription factor played important roles in the control of Arabidopsis As(V)-resistance. The expression of MYB40 was induced by As(V) stress. The MYB40-overexpressing lines showed obvious As(V)-resistant phenotype and reduced As(V)/Pi uptake rate, whereas the myb40 mutants were sensitive to As(V) stress. With exposure to As(V), the MYB40 directly repressed the expression of PHT1;1, which encodes a main Pi transporter. The As(V)-resistant phenotypes of MYB40-overexpressing lines were impaired by overexpression of PHT1;1, demonstrating an epistatic genetic regulation between MYB40 and PHT1;1. Moreover, overexpression of MYB40 enhanced, and disruption of MYB40 reduced, thiol-peptide contents. On exposure to As(V), the MYB40 positively regulated the expression of PCS1, which encodes a phytochelatin synthase, and ABCC1 and ABCC2, which encode the major vacuolar phytochelatin transporters. Together, our data demonstrate that AtMYB40 acts as a central regulator of As(V) responses, providing a genetic strategy to enhance plant As(V)-tolerance and reduce As(V) uptake for improving food safety.
In order to understand the mechanisms of arsenic (As) accumulation and detoxification in aquatic plants exposed to different As species, a hydroponic experiment was conducted and the three aquatic plants (Hydrilla verticillata, Pistia stratiotes and Eichhornia crassipes) were exposed to different concentrations of As(III), As(V) and dimethylarsinate (DMA) for 10 days. The biomass, the surface As adsorption and total As adsorption of three plants were determined. Furthermore, As speciation in the culture solution and plant body, as well as the arsenate reductase (AR) activities of roots and shoots, were also analyzed. The results showed that the surface As adsorption of plants was far less than total As absorption. Compared to As(V), the plants showed a lower DMA accumulation. P. stratiotes showed the highest accumulation of inorganic arsenic but E. crassipes showed the lowest at the same As treatment. E. crassipes showed a strong ability to accumulate DMA. Results from As speciation analysis in culture solution showed that As(III) was transformed to As(V) in all As(III) treatments, and the oxidation rates followed as the sequence of H. verticillata>P. stratiotes>E. crassipes>no plant. As(III) was the predominant species in both roots (39.4-88.3%) and shoots (39-86%) of As(III)-exposed plants. As(V) and As(III) were the predominant species in roots (37-94%) and shoots (31.1-85.6%) in As(V)-exposed plants, respectively. DMA was the predominant species in both roots (23.46-100%) and shoots (72.6-100%) in DMA-exposed plants. The As(III) contents and AR activities in the roots of P. stratiotes and in the shoots of H. verticillata were significantly increased when exposed to 1 mg·L-1 or 3 mg·L-1 As(V). Therefore, As accumulation mainly occurred via biological uptake rather than physicochemical adsorption, and AR played an important role in As detoxification in aquatic plants. In the case of As(V)-exposed plants, their As tolerance was attributed to the increase of AR activities.
Developmental and environmental signals converge on cell cycle machinery to achieve proper and flexible organogenesis under changing environments. Studies on the plant cell cycle began 30 years ago, and accumulated research has revealed many links between internal and external factors and the cell cycle. In this review, we focus on how phytohormones and environmental signals regulate the cell cycle to enable plants to cope with a fluctuating environment. After introducing key cell cycle regulators, we first discuss how phytohormones and their synergy are important for regulating cell cycle progression and how environmental factors positively and negatively affect cell division. We then focus on the well-studied example of stress-induced G2 arrest and view the current model from an evolutionary perspective. Finally, we discuss the mechanisms controlling the transition from the mitotic cycle to the endocycle, which greatly contributes to cell enlargement and resultant organ growth in plants.
Expected final online publication date for the Annual Review of Plant Biology, Volume 72 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
High Arsenic Concentration 1 (HAC1), an Arabidopsis thaliana arsenate reductase, plays a key role in arsenate [As(V)] tolerance. Through conversion of As(V) to arsenite [As(III)], HAC1 enables As(III) export from roots, and restricts translocation of As(V) to shoots. To probe the ability of different root tissues to detoxify As(III) produced by HAC1 we generated A. thaliana lines expressing HAC1 in different cell-types. We investigated the As(V) tolerance phenotypes: root growth, As(III) efflux, arsenic translocation, and arsenic chemical speciation. We showed that HAC1 can function in the outer tissues of the root (epidermis, cortex and endodermis) to confer As(V) tolerance, As(III) efflux and to limit arsenic accumulation in shoots. HAC1 is less effective in the stele at conferring As(V) tolerance phenotypes. The exception is HAC1 activity in the protoxylem, which we found to be sufficient to restrict As translocation, but not to confer As(V) tolerance. In conclusion, we describe cell-type specific functions of HAC1 that separate spatially the control of As(V) tolerance and As translocation. Further, we identify a key function of protoxylem cells in As(V) translocation, consistent with the model where endodermal passage cells, above protoxylem pericycle cells, form a 'funnel' loading nutrients and potentially toxic elements into the vasculature.
Mineral elements play an extremely important role in human health, and are worthy of study in rice grain. Wild rice is an important gene pool for rice improvement including grain yield, disease, and pest resistance as well as mineral elements. In this study, we identified 33 quantitative trait loci (QTL) for Fe, Zn, Se, Cd, Hg, and As contents in wild rice Oryza longistaminata. Of which, 29 QTLs were the first report, and 12 QTLs were overlapped to form five clusters as qSe1/qCd1 on chromosome 1, qCd4.2/qHg4 on chromosome 4, qFe5.2/qZn5.2 on chromosome 5, qFe9/qHg9.2/qAs9.2 on chromosome 9, and qCd10/qHg10 on chromosome 10. Importantly, qSe1/qCd1, can significantly improve the Se content while reduce the Cd content, and qFe5.2/qZn5.2 can significantly improve both the Fe and Zn contents, they were delimited to an interval about 53.8 Kb and 26.2 Kb, respectively. These QTLs detected from Oryza longistaminata not only establish the basis for subsequent gene cloning to decipher the genetic mechanism of mineral element accumulation, but also provide new genetic resource for rice quality improvement.
Arsenic (As) is a toxic metalloid and inorganic arsenic is carcinogenic to humans and animals. About 50 ppb (μg/L) has been the standard for arsenic in drinking water in the USA since 1942. In the 1960s, published data from Taiwan indicated that arsenic in drinking water could cause skin cancer. In 2001, USEPA reduced the limit from 50 to 10 μg/L. According to WHO, the newly recommended guideline value for drinking water is 10 μg/L (Australia 7 μg/L). Arsenic enters in the plant system via contaminated irrigation water or soil. The toxic effect of arsenic in plants causes many metabolic disorders and often leads to death of the plants. Plants are equipped with multilayer defense weapons to encounter arsenic toxicity inside the system. Arsenic is analogous to phosphate and competes with phosphate molecules at transporter site. Once it enters in the plant cell, phytochelatins are ready for sequestration and accumulation of arsenic in vacuoles. Plants have enzymatic and nonenzymatic defense systems for arsenic-mediated oxidative damage. The chapter presents the latest research and findings for interaction between plant cell and arsenic.
Arsenic is a toxic metalloid. Arsenic pollution in soils affects food safety and threatens human health. Pteris vittata L. has enormous application value in phytoremediation of arsenic-contaminated soil for its high arsenic hyperaccumulation ability. Understanding the arsenic hyperaccumulation molecular mechanism of P. vittata is the core theoretical basis of phytoremediation technology. This review introduces the omics study on arsenic hyperaccumulation mechanisms, as well as important molecular component that is involved in arsenic hyperaccumulation of P. vittata. Further research directions and trends are also discussed.
This study presents a review conducted to critically evaluate the microbial fabrication and its subsequent effect on arsenic contamination in soil. Taking into consideration the global situation, the sources of arsenic, the microbial arsenic pathways and the impact of arsenic contamination on soil have been defined. Different published studies highlighted the biochemical and physiological molecular aspects of arsenic and evaluated their impacts on soil; in the current paper, the sources and extent of arsenic contamination and the translocation and speciation from microbes to soil are illustrated. This study also discusses the forms of water-soluble arsenic and whether these are more toxic than insoluble forms, as previously reported. In addition, this paper briefly illustrates the sources and coverage of arsenic contamination and arsenic speciation in contaminated soil globally. Some vital research findings are also discussed which relate to the scientific exploration of microbial arsenic systems in contaminated soil.
Arsenic is a highly toxic element that is not essential for plant and are readily absorbed by plant roots and placed into the human food chain. Principal components analysis of the expression of genetic variation, it can be used in many applications, a variety of patterns, multi-dimensional showed. Main components, people in more than one dimension and this is shown to be more commentary on the relationship between the optimum. The experiment was conducted at Agricultural Biotechnology Research institute of Zabol University in the hydroponic environment in 2013. The experiments was arranged as to Completely randomized block design, with three replicates and two treatments normal and Arsenic stress. The studied traits physiologic traits including: Chlorophyll concentration, Proline, The amount of soluble sugars, Relative water content, Membrane stability index and Chlorophyll fluorescence. Principal components analysis using the measured physiological traits barley 72 DHL derived of the cross “Steptoe/Morex” population. In The PCA (principal components analysis) analysis, the first 3 principal components explained 57.2% of the total variability in normal condition and the first two principal components explained 62% of the total variability in stress condition.
Arsenic occupies the first position in the list of toxic substances elaborated by the ATSDR (Agency for Toxic Substances and Disease Registry) based on criteria such as frequency of occurrence at NPL (National Priority List) sites, toxicity, carcinogenicity, and potential threat to human health. Thus the accumulation of this metalloid in crops, particularly in rice, is of great concern and it is currently considered as a global problem affecting 150 million people. Furthermore, As reduces plant growth and crop productivity, decreases chlorophyll content and photosynthesis, and induces damage to DNA, oxidative stress and lipid peroxidation. In the case of legumes, nodulation is one of the processes more affected by this toxic chemical, so the selection of suitable and As‐tolerant symbiotic partners is of outstanding importance. In this chapter, the molecular mechanisms involved in the effect of As on the legume‐rhizobia symbiotic interaction – which are being elucidated – are revised. Besides, novel approaches involving plant genetic manipulation and rhizosphere engineering aimed to decrease As accumulation in grains are reviewed.
Arsenic toxicity emerged as one of the major challenges for the human being localized in a certain region of the globe, including the eastern part of India, Bangladesh, and China. Rice, a staple crop growing in an arsenic contaminated area, accumulates high arsenic in the grains. The consumption of such rice grains associated with several detrimental effects on kidney, liver, cancer, and cardio-vascular diseases in human. Being as food crop of half of the world population, many reports showed that eating of arsenic-containing rice grains is one the major routes of arsenic toxicity to human. Given that contribution of arsenic toxicity by using arsenic-containing rice grains, various studies have been conducted in past years to understand the molecular mechanism of arsenic metabolism in rice, and greatly increases our knowledge about key molecular players. In this chapter, the role of membrane-bound transporters, which mediate arsenic uptake from soil and cellular as well as long-distance transport has been reviewed in the light of recent reports.
Arsenic (As) is a toxic and ubiquitously present element. It is present in some places of the world in excessively high concentrations in water and soil that threaten public health. North America constitutes one of the hotspots of As contamination. In Canada and the USA, a number of reports show the contamination of As in a number of food items including rice, rice-based products, fruits, beverages, animal food items, etc. Further, As contamination of water sources is also known to occur in different states of both Canada and the USA. Thus, the problem of As contamination is widespread and needs attention. This chapter provides an overview of As contamination of water and food sources of North America.
Metal toxicity and deficiency are both common abiotic problems faced by plants. While metal contamination around the world is a critical issue, the bioavailability of some essential metals like zinc (Zn) and selenium (Se) can be seriously low in other locations. The list of metals spread in high concentrations in soil, water and air includes several toxic as well as essential elements, such as arsenic (As), cadmium (Cd), chromium (Cr), aluminum (Al), and selenium (Se). The problems for some metals are geographically confined, while for others, they are widespread. For instance, arsenic is an important toxic metalloid whose contamination in Southeast Asia and other parts of world is well documented. Its threats to human health via food consumption have generated immense interest in understanding plants’ responses to arsenic stress.
Metals constitute crucial components of key enzymes and proteins in plants. They are important for the proper growth and development of plants. In turn, plants serve as sources of essential elements for humans and animals. Studies of their physiological effects on plants metabolism have led to the identification of crucial genes and proteins controlling metal uptake and transport, as well as the sensing and signaling of metal stresses. Plant-Metal Interactions sheds light on the latest development and research in analytical biology with respect to plant physiology. More importantly, it showcases the positive and negative impacts of metals on crop plants growth and productivity.
In Saccharomyces cerevisiae,expression of the ACR2 and ACR3 genes confers arsenical resistance. Acr2p is the first identified eukaryotic arsenate reductase. It reduces arsenate to arsenite,
which is then extruded from cells by Acr3p. In this study, we demonstrate that ACR2complemented the arsenate-sensitive phenotype of an arsCdeletion in Escherichia coli. ACR2 was cloned into a bacterial expression vector and expressed in E. coli as a C-terminally histidine-tagged protein that was purified by sequential metal chelate affinity and gel filtration chromatography.
Acr2p purified as a homodimer of 34 kDa. The purified protein was shown to catalyze the reduction of arsenate to arsenite.
Enzymatic activity as a function of arsenate concentration exhibited an apparent positive cooperativity with an apparent Hill
coefficient of 2.7. Activity required GSH and glutaredoxin as the source of reducing equivalents. Thioredoxin was unable to
support arsenate reduction. However, glutaredoxins from both S. cerevisiae and E. coli were able to serve as reductants. Analysis ofgrx mutants lacking one or both cysteine residues in the Cys-Pro-Tyr-Cys active site demonstrated that only the N-terminal cysteine
residue is essential for arsenate reductase activity. This suggests that during the catalytic cycle, Acr2p forms a mixed disulfide
with GSH before being reduced by glutaredoxin to regenerate the active Acr2p reductase.
The sodium pump of human red blood cells mediates a Rb:Rb exchange that is dependent for maximal rates upon the simultaneous presence of intracellular ATP (or ADP) and phosphate. We have measured ouabain-sensitive 86Rb uptake into resealed ghosts of human red cells containing ADP and show that arsenate will substitute for phosphate in supporting the Rb:Rb exchange transport mode. The concentration dependence of arsenate-supported Rb:Rb exchange in ghosts containing 2 mM ADP shows both activating and inhibiting phases; the dependence upon phosphate shows similar characteristics. Elevation of the external [Rb] lowers the apparent affinity for arsenate since there is a shift to higher concentrations of arsenate in the activating and inhibiting phases of the arsenate concentration dependence curve. Similarly, elevation of [ADP] substantially reduces the inhibition of Rb:Rb exchange observed at higher [arsenate]. These effects are also observed in phosphate-supported Rb:Rb exchange. The phosphate requirement for Rb:Rb exchange involves phosphorylation of the sodium pump protein; the close agreement between the effects of arsenate and phosphate in supporting Rb:Rb exchange makes it likely that arsenylation of the sodium pump occurs during Rb:Rb exchange. Arsenate efflux from red blood cell ghosts into arsenate-free chloride medium is partially inhibited (77-80%) by DNDS (4,4'-dinitro-2,2'-stilbenedisulfonic acid), this compares with 82-87% inhibition by DNDS of phosphate efflux under the same conditions. It appears that Band III, the red cell anion transport system, accepts arsenate in a similar fashion to phosphate and that a fraction of the flux of both anions may occur through pathways other than Band III. Thus, in human red blood cells, both the sodium pump and the anion exchange transport system will accept arsenate as a phosphate congener and the protein-arsenate interactions are very similar to those with phosphate.
In Escherichia coli ArsC catalyzes the reduction of arsenate to arsenite using GSH with glutaredoxin as electron donors. E. coli has three glutaredoxins: 1, 2, and 3, each with a classical -Cys-Pro-Tyr-Cys- active site. Glutaredoxin 2 is the major glutathione disulfide oxidoreductase in E. coli, but its function remains unknown. In this report glutaredoxin 2 is shown to be the most effective hydrogen donor for the reduction of arsenate by ArsC. Analysis of single or double cysteine-to-serine substitutions in the active site of the three glutaredoxins indicated that only the N-terminal cysteine residue is essential for activity. This suggests that, during the catalytic cycle, ArsC forms a mixed disulfide with GSH before being reduced by glutaredoxin to regenerate the active ArsC reductase.
Acr2p detoxifies arsenate by reduction to arsenite in Saccharomyces cerevisiae. This reductase has been shown to require glutathione and glutaredoxin, suggesting that thiol chemistry might be involved in the reaction mechanism. Acr2p has a HC(X)(5)R motif, the signature sequence of the phosphate binding loop of the dual-specific and protein-tyrosine phosphatase family. In Acr2p these are residues His-75, Cys-76, and Arg-82, respectively. Acr2p has another sequence, (118)HCR, that is absent in phosphatases. Acr2p also has a third cysteine residue at position 106. Each of these cysteine residues was changed individually to serine residues, whereas the histidine and arginine residues were altered to alanines. Cells of Escherichia coli heterologously expressing the majority of the mutant ACR2 genes retained wild type resistance to arsenate, and the purified altered Acr2p proteins exhibited normal enzymatic properties. In contrast, cells expressing either the C76S or R82A mutations lost resistance to arsenate, and the purified proteins were inactive. These results suggest that Acr2p utilizes a phosphatase-like Cys(X)(5)Arg motif as the catalytic center to reduce arsenate to arsenite.
Arsenic (As) finds its way into soils used for rice (Oryza sativa) cultivation through polluted irrigation water, and through historic contamination with As-based pesticides. As is known to be present as a number of chemical species in such soils, so we wished to investigate how these species were accumulated by rice. As species found in soil solution from a greenhouse experiment where rice was irrigated with arsenate contaminated water were arsenite, arsenate, dimethylarsinic acid, and monomethylarsonic acid. The short-term uptake kinetics for these four As species were determined in 7-d-old excised rice roots. High-affinity uptake (0-0.0532 mM) for arsenite and arsenate with eight rice varieties, covering two growing seasons, rice var. Boro (dry season) and rice var. Aman (wet season), showed that uptake of both arsenite and arsenate by Boro varieties was less than that of Aman varieties. Arsenite uptake was active, and was taken up at approximately the same rate as arsenate. Greater uptake of arsenite, compared with arsenate, was found at higher substrate concentration (low-affinity uptake system). Competitive inhibition of uptake with phosphate showed that arsenite and arsenate were taken up by different uptake systems because arsenate uptake was strongly suppressed in the presence of phosphate, whereas arsenite transport was not affected by phosphate. At a slow rate, there was a hyperbolic uptake of monomethylarsonic acid, and limited uptake of dimethylarsinic acid.
The use of arsenic (As) contaminated groundwater for irrigation of crops has resulted in elevated concentrations of arsenic in agricultural soils in Bangladesh, West Bengal (India), and elsewhere. Paddy rice (Oryza sativa L.) is the main agricultural crop grown in the arsenic-affected areas of Bangladesh. There is, therefore, concern regarding accumulation of arsenic in rice grown those soils. A greenhouse study was conducted to examine the effects of arsenic-contaminated irrigation water on the growth of rice and uptake and speciation of arsenic. Treatments of the greenhouse experiment consisted of two phosphate doses and seven different arsenate concentrations ranging from 0 to 8 mg of As L(-1) applied regularly throughout the 170-day post-transplantation growing period until plants were ready for harvesting. Increasing the concentration of arsenate in irrigation water significantly decreased plant height, grain yield, the number of filled grains, grain weight, and root biomass, while the arsenic concentrations in root, straw, and rice husk increased significantly. Concentrations of arsenic in rice grain did not exceed the food hygiene concentration limit (1.0 mg of As kg(-1) dry weight). The concentrations of arsenic in rice straw (up to 91.8 mg kg(-1) for the highest As treatment) were of the same order of magnitude as root arsenic concentrations (up to 107.5 mg kg(-1)), suggesting that arsenic can be readily translocated to the shoot. While not covered by food hygiene regulations, rice straw is used as cattle feed in many countries including Bangladesh. The high arsenic concentrations may have the potential for adverse health effects on the cattle and an increase of arsenic exposure in humans via the plant-animal-human pathway. Arsenic concentrations in rice plant parts except husk were not affected by application of phosphate. As the concentration of arsenic in the rice grain was low, arsenic speciation was performed only on rice straw to predict the risk associated with feeding contaminated straw to the cattle. Speciation of arsenic in tissues (using HPLC-ICP-MS) revealed that the predominant species present in straw was arsenate followed by arsenite and dimethylarsinic acid (DMAA). As DMAA is only present at low concentrations, it is unlikely this will greatly alter the toxicity of arsenic present in rice.
The findings of the accompanying paper (Németi and Gregus, Toxicol: Sci. 70, 4-12) indicate that the arsenate (AsV) reductase activity of rat liver cytosol is due to an SH enzyme that uses phosphate (or its analogue, arsenate, AsV) and a purine nucleoside (guanosine or inosine) as substrates. Purine nucleoside phosphorylase (PNP) is such an enzyme. It catalyzes the phosphorolytic cleavage of 6-oxopurine nucleosides according to the following scheme: guanosine (or inosine) + phosphate <--> guanine (or hypoxanthine) + ribose-1-phosphate. Therefore, we have tested the hypothesis that PNP is responsible for the thiol- and purine nucleoside-dependent reduction of AsV to AsIII by rat liver cytosol. AsIII formed from AsV was quantified by HPLC-hydride generation-atomic fluorescence spectrometry analysis of the deproteinized incubates. The following findings support the conclusion that PNP reduces AsV to AsIII, using AsV instead of phosphate in the reaction above: (1) Specific PNP inhibitors (CI-1000, BCX-1777) at a concentration of 1 microM completely inhibited cytosolic AsV reductase activity. (2) During anion-exchange chromatography of cytosolic proteins, PNP activity perfectly coeluted with the AsV reductase activity, suggesting that both activities belong to the same protein. (3) PNP purified from calf spleen catalyzed reduction of AsV to AsIII in the presence of dithiothreitol (DTT) and a 6-oxopurine nucleoside (guanosine or inosine). (4) AsV reductase activity of purified PNP, like the cytosolic AsV reductase activity, was inhibited by phosphate (a substrate of PNP alternative to AsV), guanine and hypoxanthine (products of PNP favoring the reverse reaction), mercurial thiol reagents (nonspecific inhibitors of PNP), as well as CI-1000 and BCX-1777 (specific PNP inhibitors). Thus, PNP appears to be responsible for the AsV reductase activity of rat liver cytosol in the presence of DTT. Further research should clarify the mechanism and the in vivo significance of PNP-catalyzed reduction of AsV to AsIII.
Reduction of the pentavalent arsenate (AsV) to the thiol-reactive arsenite (AsIII) toxifies this environmentally prevalent form of arsenic, yet its biochemical mechanism in mammals is incompletely understood. Purine nucleoside phosphorylase (PNP) has been shown recently to function as an AsV reductase in vitro, provided its substrate (inosine or guanosine) and an appropriate dithiol (e.g., dithiothreitol, DTT) were present. It was of interest to know if this ubiquitous enzyme played a significant role in reduction of AsV to AsIII in vivo. Two approaches were used to test this. First, it was determined if compounds that influenced AsV reduction by purified PNP (i.e., nucleosides, thiols, and PNP inhibitors) would similarly affect reduction of AsV by human erythrocytes. Erythrocytes were incubated with AsV, and the formed AsIII was quantified by HPLC-hydride generation-atomic fluorescence spectrometry. The red blood cells reduced AsV at a considerable rate, which could be enhanced by inosine or inosine plus DTT. These stimulated AsIII formation rates were PNP-dependent, as PNP inhibitors strongly inhibited them. In contrast, PNP inhibitors had little if any inhibitory effect on AsIII formation in the absence of exogenous inosine, indicating that this basal rate of AsV reduction is PNP-independent. Second, the role of PNP in reduction of AsV in vivo was also assessed by investigating the effect of the PNP inhibitor BCX-1777 on the biotransformation of AsV in control and DTT-treated rats with cannulated bile duct and ligated renal pedicles. Although it abolished hepatic PNP activity, BCX-1777 influenced neither the biliary excretion of AsIII and monomethylarsonous acid, nor the tissue concentration of AsV and its metabolites in either group of AsV-injected rats. Thus, despite its in vitro activity, PNP does not appear to play a significant role in AsV reduction in human erythrocytes and in rats in vivo. Further research should clarify the in vivo relevant mechanisms of AsV reduction in mammals.
Quantitative study of protein-protein and protein-ligand interactions in solution requires accurate determination of protein concentration. Often, for proteins available only in "molecular biological" amounts, it is difficult or impossible to make an accurate experimental measurement of the molar extinction coefficient of the protein. Yet without a reliable value of this parameter, one cannot determine protein concentrations by the usual uv spectroscopic means. Fortunately, knowledge of amino acid residue sequence and promoter molecular weight (and thus also of amino acid composition) is generally available through the DNA sequence, which is usually accurately known for most such proteins. In this paper we present a method for calculating accurate (to +/- 5% in most cases) molar extinction coefficients for proteins at 280 nm, simply from knowledge of the amino acid composition. The method is calibrated against 18 "normal" globular proteins whose molar extinction coefficients are accurately known, and the assumptions underlying the method, as well as its limitations, are discussed.
The dual-specificity CDC25 phosphatases are critical positive regulators of cyclin-dependent kinases (CDKs). Even though an antagonistic Arabidopsis thaliana WEE1 kinase has been cloned and tyrosine phosphorylation of its CDKs has been demonstrated, no valid candidate for a CDC25 protein has been reported in higher plants. We identify a CDC25-related protein (Arath;CDC25) of A. thaliana, constituted by a sole catalytic domain. The protein has a tyrosine-phosphatase activity and stimulates the kinase activity of Arabidopsis CDKs. Its tertiary structure was obtained by NMR spectroscopy and confirms that Arath;CDC25 belongs structurally to the classical CDC25 superfamily with a central five-stranded beta-sheet surrounded by helices. A particular feature of the protein, however, is the presence of an additional zinc-binding loop in the C-terminal part. NMR mapping studies revealed the interaction with phosphorylated peptidic models derived from the conserved CDK loop containing the phosphothreonine-14 and phosphotyrosine-15. We conclude that despite sequence divergence, Arath;CDC25 is structurally and functionally an isoform of the CDC25 superfamily, which is conserved in yeast and in plants, including Arabidopsis and rice.
The mechanisms of arsenic (As) hyperaccumulation in Pteris vittata, the first identified As hyperaccumulator, are unknown. We investigated the interactions of arsenate and phosphate on the uptake and distribution of As and phosphorus (P), and As speciation in P. vittata. In an 18-d hydroponic experiment with varying concentrations of arsenate and phosphate, P. vittata accumulated As in the fronds up to 27,000 mg As kg(-1) dry weight, and the frond As to root As concentration ratio varied between 1.3 and 6.7. Increasing phosphate supply decreased As uptake markedly, with the effect being greater on root As concentration than on shoot As concentration. Increasing arsenate supply decreased the P concentration in the roots, but not in the fronds. Presence of phosphate in the uptake solution decreased arsenate influx markedly, whereas P starvation for 8 d increased the maximum net influx by 2.5-fold. The rate of arsenite uptake was 10% of that for arsenate in the absence of phosphate. Neither P starvation nor the presence of phosphate affected arsenite uptake. Within 8 h, 50% to 78% of the As taken up was distributed to the fronds, with a higher translocation efficiency for arsenite than for arsenate. In fronds, 49% to 94% of the As was extracted with a phosphate buffer (pH 5.6). Speciation analysis using high-performance liquid chromatography-inductively coupled plasma mass spectroscopy showed that >85% of the extracted As was in the form of arsenite, and the remaining mostly as arsenate. We conclude that arsenate is taken up by P. vittata via the phosphate transporters, reduced to arsenite, and sequestered in the fronds primarily as As(III).
The mammalian enzymes responsible for reduction of the environmentally prevalent arsenate (AsV) to the much more toxic arsenite (AsIII) are unknown. In the previous paper (Nemeti and Gregus, 2005), we proposed that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and/or phosphoglycerate kinase (PGK) may catalyze reduction of AsV in human red blood cells (RBC), hemolysate, or rat liver cytosol. In testing this hypothesis, we show here that, if supplied with glutathione (GSH), NAD, and glycolytic substrate, the mixture of purified GAPDH and PGK indeed catalyzes the reduction of AsV. Further analysis revealed that GAPDH is endowed with AsV reductase activity, whereas PGK serves as an auxiliary enzyme, when 3-phosphoglycerate is the glycolytic substrate. The GAPDH-catalyzed AsV reduction required GSH, NAD, and glyceraldehyde-3-phosphate. ADP and ATP moderately, whereas NADH strongly inhibited the AsV reductase activity of the enzyme even in the presence of NAD. Koningic acid (KA), a specific and irreversible inhibitor of GAPDH, inhibited both the classical enzymatic and the AsV-reducing activities of the enzyme in a concentration-dependent fashion. To assess the contribution of GAPDH to the reduction of AsV carried out by hemolysate, rat liver cytosol, or intact erythrocytes, we determined the concentration-dependent effect of KA on AsV reduction by these cells and extracts. Inactivation of GAPDH by KA abolished AsV reduction in intact RBC as well as in the hemolysate and the liver cytosol, when GAPDH in the latter extracts was abundantly supplied with exogenous NAD and glycolytic substrate. However, despite complete inactivation of GAPDH by KA, the hepatic cytosol exhibited significant residual AsV-reducing activity in the absence of exogenous NAD and glycolytic substrate, suggesting that besides GAPDH, other cytosolic enzyme(s) may contribute to AsV reduction in the liver. In conclusion, the key glycolytic enzyme GAPDH can fortuitously catalyze the reduction of AsV to AsIII, if GSH, NAD, and glycolytic substrate are available. AsV reduction may take place during, or as a consequence of, the arsenolytic cleavage of the thioester bond formed between the enzyme's Cys149 and the 3-phosphoglyceroyl moiety of the substrate. Although GAPDH is exclusively responsible for reduction of AsV in human erythrocytes, its role in AsV reduction in vivo remains to be determined.
Root extracts from the arsenic (As) hyperaccumulating Chinese brake fern (Pteris vittata) were shown to be able to reduce arsenate to arsenite. An arsenate reductase (AR) in the fern showed a reaction mechanism similar to the previously reported Acr2p, an AR from yeast (Saccharomyces cerevisiae), using glutathione as the electron donor. Substrate specificity as well as sensitivity toward inhibitors for the fern AR (phosphate as a competitive inhibitor, arsenite as a noncompetitive inhibitor) was also similar to Acr2p. Kinetic analysis showed that the fern AR had a Michaelis constant value of 2.33 mM for arsenate, 15-fold lower than the purified Acr2p. The AR-specific activity of the fern roots treated with 2 mM arsenate for 9 d was at least 7 times higher than those of roots and shoots of plant species that are known not to tolerate arsenate. A T-DNA knockout mutant of Arabidopsis (Arabidopsis thaliana) with disruption in the putative Acr2 gene had no AR activity. We could not detect AR activity in shoots of the fern. These results indicate that (1) arsenite, the previously reported main storage form of As in the fern fronds, may come mainly from the reduction of arsenate in roots; and (2) AR plays an important role in the detoxification of As in the As hyperaccumulating fern.
In order to assess soil As contamination and potential risk for human, soil, paddy rice, vegetable and human hair samples from the areas near the industrial districts in Chenzhou, southern China were sampled and analyzed. The results showed that the anthropogenic industrial activities have caused in local agricultural soils to be contaminated with As in a range of 11.0-1217 mg/kg. The GIS-based map shows that soil contamination with As occurred on a large scale, which probably accounted for up to 30% of the total area investigated. Soil As concentration abruptly decreased with an increase in the distance from the polluting source. High As concentrations were found in the rice grain that ranged from 0.5 to 7.5 mg/kg, most of which exceed the maximal permissible limit of 1.0 mg/kg dry matter. Arsenic accumulated in significantly different levels between leafy vegetables and non-leafy vegetables. Non-leafy vegetables should be recommended in As-contaminated soils, as their edible parts were found in relatively low As level. Arsenic concentrations in 95% of the total human hair samples in the contaminated districts were above the critical value, 1.0 mg/kg, set by the World Health Organization. Arsenic could be enriched in human hair to very high levels without being affected by As containing water. The results revealed that the soils and plants grown on them are major contributors to elevate hair As in the industrial population. Therefore, the potential impact on human health of ingestion/inhalation of soil As around the industrial districts seems to be rather serious. Hence proper treatments for As contaminated soils are urgently needed to reduce the contamination.
Arsenic, which is ubiquitous in the environment and comes from both geochemical and anthropogenic sources, has become a worldwide
public health problem. Every organism studied has intrinsic or acquired mechanisms for arsenic detoxification. In Saccharomyces cerevisiae arsenate is detoxified by Acr2p, an arsenate reductase. Acr2p is not a phosphatase but is a homologue of CDC25 phosphatases.
It has the HCX5R phosphatase motif but not the glycine-rich phosphate binding motif (GXGXXG) that is found in protein-tyrosine phosphatases. Here we show that creation of a phosphate binding motif through the introduction
of glycines at positions 79, 81, and 84 in Acr2p resulted in a gain of phosphotyrosine phosphatase activity and a loss of
arsenate reductase activity. Arsenate likely achieved geochemical abundance only after the atmosphere became oxidizing,
creating pressure for the evolution of an arsenate reductase from a protein-tyrosine phosphatase. The ease by which an arsenate
reductase can be converted into a protein-tyrosine phosphatase supports this hypothesis.
Decreased arsenate [As(V)] uptake is the major mechanism of naturally selected As(V) hypertolerance in plants. However, As(V)-hypertolerant ecotypes also show enhanced rates of phytochelatin (PC) accumulation, suggesting that improved sequestration might additionally contribute to the hypertolerance phenotype. Here, we show that enhanced PC-based sequestration in As(V)-hypertolerant Holcus lanatus is not due to an enhanced capacity for PC synthesis as such, but to increased As(V) reductase activity. Vacuolar transport of arsenite-thiol complexes was equal in both ecotypes. Based on homology with the yeast As(V) reductase, Acr2p, we identified a Cdc25-like plant candidate, HlAsr, and confirmed the As(V) reductase activity of both HlAsr and the homologous protein from Arabidopsis thaliana. The gene appeared to be As(V)-inducible and its expression was enhanced in the As(V)-hypertolerant H. lanatus ecotype, compared with the non-tolerant ecotype. Homologous ectopic overexpression of the AtASR cDNA in A. thaliana produced a dual phenotype. It improved tolerance to mildly toxic levels of As(V) exposure, but caused hypersensitivity to more toxic levels. Arabidopsis asr T-DNA mutants showed increased As(V) sensitivity at low exposure levels and enhanced arsenic retention in the root. It is argued that, next to decreased uptake, enhanced expression of HlASR might act as an additional determinant of As(V) hypertolerance and As transport in H. lanatus.
Concern has been raised by Bangladeshi and international scientists about elevated levels of arsenic in Bengali food, particularly in rice grain. This is the first inclusive food market-basket survey from Bangladesh, which addresses the speciation and concentration of arsenic in rice, vegetables, pulses, and spices. Three hundred thirty aman and boro rice, 94 vegetables, and 50 pulse and spice samples were analyzed for total arsenic, using inductivity coupled plasma mass spectrometry (ICP-MS). The districts with the highest mean arsenic rice grain levels were all from southwestern Bangladesh: Faridpur (boro) 0.51 > Satkhira (boro) 0.38 > Satkhira (aman) 0.36 > Chuadanga (boro) 0.32 > Meherpur (boro) 0.29 microg As g(-1). The vast majority of food ingested arsenic in Bangladesh diets was found to be inorganic; with the predominant species detected in Bangladesh rice being arsenite (AsIII) or arsenate (AsV) with dimethyl arsinic acid (DMAV) being a minor component. Vegetables, pulses, and spices are less important to total arsenic intake than water and rice. Predicted inorganic arsenic intake from rice is modeled with the equivalent intake from drinking water for a typical Bangladesh diet. Daily consumption of rice with a total arsenic level of 0.08 microg As g(-1) would be equivalent to a drinking water arsenic level of 10 microg L(-1).
Isolation and study of metal tolerant and hypersensitive strains of higher plant (and yeast) species has greatly increased our knowledge of the individual pathways that are involved in tolerance. Plants have both constitutive (present in most phenotypes) and adaptive (present only in tolerant phenotypes) mechanisms for coping with elevated metal concentrations. Where studies on the mechanisms of tolerance fall down is in their failure to integrate tolerance mechanisms within cell or whole-plant function by not relating adaptive mechanisms to constitutive mechanisms. This failure often distorts the relative importance of a proposed tolerance mechanism, and indeed has confused the search for adaptive mechanisms. The fundamental goal of both constitutive and adaptive mechanisms is to limit the perturbation of cell homeostasis after exposure to metals so that normal or near-normal physiological function may take place. Consideration of the response to metals at a cellular rather than a biochemical level will lead to a greater understanding of mechanisms to withstand elevated levels of metals in both contaminated and uncontaminated environments. Recent advances in the study of Al, As, Cd, and Cu tolerance and hypersensitivity are reported with respect to the cellular response to toxic metals. The role of genetics in unravelling tolerance mechanisms is also considered.
The ACR2 gene of Saccharomyces cerevisiae was disrupted by insertion of a HIS3 gene. Cells with the disruption were sensitive to arsenate. This phenotype could be complemented by ACR2 on a plasmid. The ACR2 gene was cloned and expressed in Escherichia coli as a malE gene fusion with a C-terminal histidine tag. The combination of chimeric MBP-Acr2-6H protein and yeast cytosol from an ACR2-disrupted strain exhibited arsenate reductase activity.
In Holcus lanatus L. phosphate and arsenate are taken up by the same transport system. Short-term uptake kinetics of the high affinity arsenate
transport system were determined in excised roots of arsenate-tolerant and non-tolerant genotypes. In tolerant plants the
Vmax of ion uptake in plants grown in phosphate-free media was decreased compared to non-tolerant plants, and the affinity of
the uptake system was lower than in the non-tolerant plants. Both the reduction in Vmax and the increase in Km led to reduced arsenate influx into tolerant roots. When the two genotypes were grown in nutrient solution containing high
levels of phosphate, there was little change in the uptake kinetics in tolerant plants. In non-tolerant plants, however, there
was a marked decrease in the Vmax to the level of the tolerant plants but with little change in the Km. This suggests that the low rate of arsenate uptake over a wide range of differing root phosphate status is due to loss of
induction of the synthesis of the arsenate (phosphate) carrier.
Fourteen cDNA clones whose corresponding mRNAs accumulate during the hypersensitive reaction (HR) of tobacco leaves infiltrated with an incompatible strain of the bacterial pathogen Pseudomonas solanacearum have been subdivided by sequence homologies into 6 families. Studies on the accumulation of the mRNAs encoded by these genes in compatible and incompatible plant-bacterial interactions have been carried out and indicate that the 6 cDNA clones can be subdivided into 2 groups. In one group corresponding to 3 cDNA clones, the maximal level of mRNA accumulation is similar in both types of interaction, whereas in the other group, maximal mRNA accumulation in leaves undergoing an HR is 3- to 7-fold higher than in leaves infiltrated with the compatible strain. Within each group, the timing and kinetics of accumulation of the corresponding mRNAs differ for each individual cDNA clone. Run-on experiments indicate that transcriptional activation of these genes plays a major role in the control of their expression. Genomic hybridizations have been performed and indicate that the mRNAs corresponding to the cDNA clones are encoded by multigene families (6 to 20 genes).
Studies in Taiwan and Argentina suggest that ingestion of inorganic arsenic from drinking water results in increased risks of internal cancers, particularly bladder and lung cancer. The authors investigated cancer mortality in a population of around 400,000 people in a region of Northern Chile (Region II) exposed to high arsenic levels in drinking water in past years. Arsenic concentrations from 1950 to the present were obtained. Population-weighted average arsenic levels reached 570 microg/liter between 1955 to 1969, and decreased to less than 100 microg/liter by 1980. Standardized mortality ratios (SMRs) were calculated for the years 1989 to 1993. Increased mortality was found for bladder, lung, kidney, and skin cancer. Bladder cancer mortality was markedly elevated (men, SMR = 6.0 (95% confidence interval (CI) 4.8-7.4); women, SMR = 8.2 (95% CI 6.3-10.5)) as was lung cancer mortality (men, SMR = 3.8 (95% CI 3.5-4.1); women, SMR = 3.1 (95% CI 2.7-3.7)). Smoking survey data and mortality rates from chronic obstructive pulmonary disease provided evidence that smoking did not contribute to the increased mortality from these cancers. The findings provide additional evidence that ingestion of inorganic arsenic in drinking water is indeed a cause of bladder and lung cancer. It was estimated that arsenic might account for 7% of all deaths among those aged 30 years and over. If so, the impact of arsenic on the population mortality in Region II of Chile is greater than that reported anywhere to date from environmental exposure to a carcinogen in a major population.
The ACR2 gene of Saccharomyces cerevisiae was disrupted by insertion of a HIS3 gene. Cells with the disruption were sensitive to arsenate. This phenotype could be complemented by ACR2 on a plasmid. The ACR2 gene was cloned and expressed in Escherichia coli as a malE gene fusion with a C-terminal histidine tag. The combination of chimeric MBP-Acr2-6H protein and yeast cytosol from an ACR2-disrupted strain exhibited arsenate reductase activity.
Saccharomyces cerevisiae has two independent transport systems for the removal of arsenite from the cytosol. Acr3p is a plasma membrane transporter that confers resistance to arsenite, presumably by arsenite extrusion from the cells. Ycf1p, a member of the ABC transporter superfamily, catalyzes the ATP-driven uptake of As(III) into the vacuole, also producing resistance to arsenite. Vacuolar accumulation requires a reductant such as glutathione, suggesting that the substrate is the glutathione conjugate, As(GS)3. Disruption of either the ACR3 or YCF1 gene results in sensitivity to arsenite and disruption of both genes produces additive hypersensitivity. Thus, Acr3p and Ycf1p represent separate pathways for the detoxification of arsenite in yeast.
An arsenate (As(V)) reductase has been partially purified from human liver. Its apparent molecular mass is approximately 72 kDa. The enzyme required a thiol and a heat stable cofactor for activity. The cofactor is less than 3 kDa in size. The thiol requirement can be satisfied by dithiothreitol (DTT). However, the extent of stimulation of reductase activity by glutathione, thioredoxin, or reduced lipoic acid was negligible compared to that of DTT. The heat stable cofactor does not appear to be Cu(2+), Mn(2+), Zn(2+), Mg(2+), or Ca(2+). The enzyme does not reduce monomethylarsonic acid (MMA(V)). The isolation and characterization of this enzyme demonstrates that in humans, the reduction of arsenate to arsenite is enzymatically catalyzed and is not solely the result of chemical reduction by glutathione as has been proposed in the past.
An arsenate reductase has been partially purified from human liver using ion exchange, molecular exclusion, hydroxyapatite chromatography, preparative isoelectric focusing, and electrophoresis. When SDS-beta-mercaptoethanol-PAGE was performed on the most purified fraction, two bands were obtained. One of these bands was a 34 kDa protein. Each band was excised from the gel and sequenced by LC-MS/MS, and sequest analyses were performed against the OWL database SWISS-PROT with PIR. Mass spectra analysis matched the 34 kDa protein of interest with human purine nucleoside phosphorylase (PNP). The peptide fragments equal to 40.1% of the total protein were 100% identical to the corresponding regions of the human purine nucleoside phosphorylase. Reduction of arsenate in the purine nucleoside arsenolysis reaction required both PNP and dihydrolipoic acid (DHLP). The PNP rate of reduction of arsenate with the reducing agents GSH or ascorbic acid was negligible compared to that with the naturally occurring dithiol DHLP and synthetic dithiols such as BAL (British anti-lewisite), DMPS (2,3-dimercapto-1-propanesulfonate), or DTT (alpha-dithiothreitol). The arsenite production reaction of thymidine phosphorylase had approximately 5% of such PNP activity. Phosphorylase b was inactive. Monomethylarsonate (MMAV) was not reduced by PNP. The experimental results indicate PNP is an important route for the reduction of arsenate to arsenite in mammalian systems.
Establishment of the maximum contaminant level that regulates the concentration of arsenic in public water supplies has been
an extraordinarily protracted process. The U.S. Public Health Service set an interim standard of 50 mg per liter in 1942.
It was another 60 years before the U.S. Environmental Protection Agency lowered the standard to 10 mg per liter, despite extensive
epidemiological evidence of significant cancer risks.
Smith
et al.
of this Policy Forum consider how the regulatory process might interpret and respond more effectively to results from epidemiological
studies.
All living organisms have systems for arsenic detoxification. The common themes are (a) uptake of As(V) in the form of arsenate by phosphate transporters, (b) uptake of As(III) in the form of arsenite by aquaglyceroporins, (c) reduction of As(V) to As(III) by arsenate reductases, and (d) extrusion or sequestration of As(III). While the overall schemes for arsenic resistance are similar in prokaryotes and eukaryotes, some of the specific proteins are the products of separate evolutionary pathways.
In attempting to eliminate disease caused by drinking polluted surface water, millions of shallow surface wells were drilled into the Ganges delta alluvium in Bangladesh. The latest statistics indicate that 80% of Bangladesh and an estimated 40 million people are at risk of arsenic poisoning-related diseases because the ground water in these wells is contaminated with arsenic. The clinical manifestations of arsenic poisoning are myriad, and the correct diagnosis depends largely on awareness of the problem. Patients with melanosis, leuco-melanosis, keratosis, hyperkeratosis, dorsum, non-petting edema, gangrene and skin cancer have been identified. The present article reviews the current arsenic contamination of ground water, hydrological systems, groundwater potential and utilization and environmental pollution in Bangladesh. This paper concludes by clarifying the main actions required to ensure the sustainable development of water resources in Bangladesh.
The ubiquity of arsenic in the environment has led to the evolution of enzymes for arsenic detoxification. An initial step in arsenic metabolism is the enzymatic reduction of arsenate [As(V)] to arsenite [As(III)]. At least three families of arsenate reductase enzymes have arisen, apparently by convergent evolution. The properties of two of these are described here. The first is the prokaryotic ArsC arsenate reductase of Escherichia coli. The second, Acr2p of Saccharomyces cerevisiae, is the only identified eukaryotic arsenate reductase. Although unrelated to each other, both enzymes receive their reducing equivalents from glutaredoxin and reduced glutathione. The structure of the bacterial ArsC has been solved at 1.65 A. As predicted from its biochemical properties, ArsC structures with covalent enzyme-arsenic intermediates that include either As(V) or As(III) were observed. The yeast Acr2p has an active site motif HC(X)(5)R that is conserved in protein phosphotyrosine phosphatases and rhodanases, suggesting that these three groups of enzymes may have evolved from an ancestral oxyanion-binding protein.
Arsenic contaminated groundwater is used extensively in Bangladesh to irrigate the staple food of the region, paddy rice (Oryza sativa L.). To determine if this irrigation has led to a buildup of arsenic levels in paddy fields, and the consequences for arsenic exposure through rice ingestion, a survey of arsenic levels in paddy soils and rice grain was undertaken. Survey of paddy soils throughout Bangladesh showed that arsenic levels were elevated in zones where arsenic in groundwater used for irrigation was high, and where these tube-wells have been in operation for the longest period of time. Regression of soil arsenic levels with tube-well age was significant. Arsenic levels reached 46 microg g(-1) dry weight in the most affected zone, compared to levels below l0 microg g(-1) in areas with low levels of arsenic in the groundwater. Arsenic levels in rice grain from an area of Bangladesh with low levels of arsenic in groundwaters and in paddy soils showed that levels were typical of other regions of the world. Modeling determined, even these typical grain arsenic levels contributed considerably to arsenic ingestion when drinking water contained the elevated quantity of 0.1 mg L(-1). Arsenic levels in rice can be further elevated in rice growing on arsenic contaminated soils, potentially greatly increasing arsenic exposure of the Bangladesh population. Rice grain grown in the regions where arsenic is building up in the soil had high arsenic concentrations, with three rice grain samples having levels above 1.7 microg g(-1).
Arsenic contaminating groundwater in Bangladesh is one of the largest environmental health hazards in the world. Because of the potential risk to human health through consumption of agricultural produce grown in fields irrigated with arsenic contaminated water, we have determined the level of contamination in 100 samples of crop, vegetables and fresh water fish collected from three different regions in Bangladesh. Arsenic concentrations were determined by hydride generation atomic absorption spectrophotometry. All 11 samples of water and 18 samples of soil exceeded the expected limits of arsenic. No samples of rice grain (Oryza sativa L.) had arsenic concentrations more than the recommended limit of 1.0 mg/kg. However, rice plants, especially the roots had a significantly higher concentration of arsenic (2.4 mg/kg) compared to stem (0.73 mg/kg) and rice grains (0.14 mg/kg). Arsenic contents of vegetables varied; those exceeding the food safety limits included Kachu sak (Colocasia antiquorum) (0.09-3.99 mg/kg, n=9), potatoes (Solanum tuberisum) (0.07-1.36 mg/kg, n=5), and Kalmi sak (Ipomoea reptoms) (0.1-1.53 mg/kg, n=6). Lata fish (Ophicephalus punctatus) did not contain unacceptable levels of arsenic. These results indicate that arsenic contaminates some food items in Bangladesh. Further studies with larger samples are needed to demonstrate the extent of arsenic contamination of food in Bangladesh.
A study was undertaken to develop a method for the chemical speciation of As in rice on the basis of current knowledge in this field for use in preparing a certified reference material (CRM). Samples of the Arborio rice variety were ground to a fine powder, which was extracted under sonication with a water-methanol mixture (1 + 1, v/v). The resulting solutions were injected into a high-performance liquid chromatograph combined on-line with a quadrupole inductively coupled plasma-mass spectrometer. This hyphenated system allowed for the quantification of As species in one analytical step. Four forms of As were detected: inorganic As (III), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), and inorganic As (V) at concentrations of 88.2 +/- 7.1, 50.8 +/- 5.0, 15.2 +/- 1.7, and 51.2 +/- 3.5 ng/g, respectively. The concentration of total As was 211 +/- 7 ng/g. The limits of detection (3sigma criterion) and the quantitation (10sigma criterion) were, respectively, as follows (in ng/g): As (III), 0.095 and 0.320; As (V), 0.082 and 0.273; MMA, 0.110 and 0.367; and DMA, 0.145 and 0.480. Ten hours were needed for the extraction procedure, 6 h for the evaporation, and 30 min for quantification of the analytes. This investigation was performed in the frame of a European Commission Project on the feasibility of CRMs for As and Se species.
Arsenicals and antimonials are first line drugs for the treatment of trypanosomal and leishmanial diseases. To create the
active form of the drug, Sb(V) must be reduced to Sb(III). Because arsenic and antimony are related metalloids, and arsenical
resistant Leishmania strains are frequently cross-resistant to antimonials, we considered the possibility that Sb(V) is reduced by a leishmanial
As(V) reductase. The sequence for the arsenate reductase of Saccharomyces cerevisiae, ScAcr2p, was used to clone the gene for a homologue, LmACR2, from Leishmania major. LmACR2 was able to complement the arsenate-sensitive phenotype of an arsC deletion strain of Escherichia coli or an ScACR2 deletion strain of Saccharomyces cerevisiae. Transfection of Leishmania infantum with LmACR2 augmented Pentostam sensitivity in intracellular amastigotes. LmACR2 was purified and shown to reduce both As(V) and Sb(V).
This is the first report of an enzyme that confers Pentostam sensitivity in intracellular amastigotes of Leishmania. We propose that LmACR2 is responsible for reduction of the pentavalent antimony in Pentostam to the active trivalent form
of the drug in Leishmania.
A study by Yongguan Zhu and co-workers has added greatly to our understanding of arsenic dynamics in the rhizosphere of paddy rice. Their finding that arsenic is sequestered in iron plaque on root surfaces in plants, regulated by phosphorus status, and that there is considerable varietal variation in arsenic sequestration and subsequently plant uptake, offers a hope for breeding rice for the new arsenic disaster in South-East Asia - the contamination of paddy soils with arsenic.
CDC25 enzymes are dual-specificity phosphatases involved in the regulation of the cell cycle. No CDC25 enzymes have been described in higher plant organisms. We report here the characterization of an Arabidopsis thaliana CDC25 enzyme, constituted by a sole catalytic domain and devoid of the N-terminal regulatory region found in the human CDC25. We describe the recombinant expression in Escherichia coli of the Arath;CDC25 and its purification for activity assay and structure determination by NMR. The recombinant enzyme has a tyrosine phosphatase activity towards an artificial substrate, a NMR characterization equally concludes to its correct folding. The secondary structure of the protein was predicted on the basis of the assigned chemical shift of (1)H, (15)N, and (13)C backbone atoms of the protein. The presence of a metal ion in the C-terminus of this new protein points to a zinc finger, and sequence homology indicates that this new structural element might be conserved in related plant homologs.
The putative mitotic inducer gene, Arath;CDC25 cloned in Arabidopsis thaliana, was screened for cell cycle function by overexpressing it in Schizosaccharomyces pombe (fission yeast). The expression pattern of Arath;CDC25 was also examined in different tissues of A. thaliana. Fission yeast was transformed with plasmids pREP1 and pREP81 with the Arath;CDC25 gene under the control of the thiamine-repressible nmt promoter. Using reverse transcription-polymerase chain reaction (RT-PCR), the expression of Arath;CDC25 was examined in seedlings, flower buds, mature leaves and stems of A. thaliana; actin (ACT2) was used as a control. In three independent transformants of fission yeast, cultured in the absence of thiamine (T), pREP1::Arath;CDC25 induced a highly significant reduction in mitotic cell length compared with wild type, pREP::Arath;CDC25 +T, and empty vector (pREP1 +/- T). The extent of cell shortening was greater using the stronger pREP1 compared with the weaker pREP81. However, Arath;CDC25 was expressed at low levels in all tissues examined. The data indicate that Arath;CDC25 can function as a mitotic accelerator in fission yeast. However, unlike other plant cell cycle genes, expression of Arath;CDC25 was not enhanced in rapidly dividing compared with non-proliferative Arabidopsis tissues.
Ingestion of drinking water is not the only elevated source of arsenic to the diet in the Bengal Delta. Even at background levels, the arsenic in rice contributes considerably to arsenic ingestion in subsistence rice diets. We set out to survey As speciation in different rice varieties from different parts of the globe to understand the contribution of rice to arsenic exposure. Pot experiments were utilized to ascertain whether growing rice on As contaminated soil affected speciation and whether genetic variation accounted for uptake and speciation. USA long grain rice had the highest mean arsenic level in the grain at 0.26 microg As g(-1) (n = 7), and the highest grain arsenic value of the survey at 0.40 microg As g(-1). The mean arsenic level of Bangladeshi rice was 0.13 microg As g(-1) (n = 15). The main As species detected in the rice extract were AsIII, DMAV, and AsV. In European, Bangladeshi, and Indian rice 64 +/- 1% (n = 7), 80 +/- 3% (n = 11), and 81 +/- 4% (n = 15), respectively, of the recovered arsenic was found to be inorganic. In contrast, DMAV was the predominant species in rice from the USA, with only 42 +/- 5% (n = 12) of the arsenic being inorganic. Pot experiments show that the proportions of DMAV in the grain are significantly dependent on rice cultivar (p = 0.026) and that plant nutrient status is effected by arsenic exposure.