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Arsenate

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Fangjie Zhao – 1st expert on this subject based on the ideXlab platform

  • oshac1 1 and oshac1 2 function as Arsenate reductases and regulate arsenic accumulation
    Plant Physiology, 2016
    Co-Authors: Tao Wang, Ziru Chen, Zhong Tang, Zhongchang Wu, David E Salt, Daiyin Chao, Fangjie Zhao

    Abstract:

    Rice is a major dietary source of the toxic metalloid arsenic (As). Reducing its accumulation in rice (Oryza sativa) grain is of critical importance to food safety. Rice roots take up Arsenate and arsenite depending on the prevailing soil conditions. The first step of Arsenate detoxification is its reduction to arsenite, but the enzyme(s) catalyzing this reaction in rice remains unknown. Here, we identify OsHAC1;1 and OsHAC1;2 as Arsenate reductases in rice. OsHAC1;1 and OsHAC1;2 are able to complement an Escherichia coli mutant lacking the endogenous Arsenate reductase and to reduce Arsenate to arsenite. OsHAC1:1 and OsHAC1;2 are predominantly expressed in roots, with OsHAC1;1 being abundant in the epidermis, root hairs, and pericycle cells while OsHAC1;2 is abundant in the epidermis, outer layers of cortex, and endodermis cells. Expression of the two genes was induced by Arsenate exposure. Knocking out OsHAC1;1 or OsHAC1;2 decreased the reduction of Arsenate to arsenite in roots, reducing arsenite efflux to the external medium. Loss of arsenite efflux was also associated with increased As accumulation in shoots. Greater effects were observed in a double mutant of the two genes. In contrast, overexpression of either OsHAC1;1 or OsHAC1;2 increased arsenite efflux, reduced As accumulation, and enhanced Arsenate tolerance. When grown under aerobic soil conditions, overexpression of either OsHAC1;1 or OsHAC1;2 also decreased As accumulation in rice grain, whereas grain As increased in the knockout mutants. We conclude that OsHAC1;1 and OsHAC1;2 are Arsenate reductases that play an important role in restricting As accumulation in rice shoots and grain.

  • A novel pathway of Arsenate detoxification.
    Molecular Microbiology, 2016
    Co-Authors: Fangjie Zhao

    Abstract:

    Microorganisms have evolved various mechanisms to detoxify arsenic, an ubiquitous environmental toxin. Known mechanisms include arsenite efflux, Arsenate reduction followed by arsenite efflux and arsenite methylation. In this issue, Chen et al. describe a novel mechanism for Arsenate detoxification via synergistic interaction of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and a major facilitator superfamily protein (ArsJ). They propose that GAPDH catalyzes the formation of 1-arseno-3-phosphoglycerate, which is then extruded out of the cell by ArsJ. The significance of this pathway and questions for further research are discussed.

  • genome wide association mapping identifies a new Arsenate reductase enzyme critical for limiting arsenic accumulation in plants
    PLOS Biology, 2014
    Co-Authors: Daiyin Chao, Fangjie Zhao, Ziru Chen, Yi Chen, Jiugeng Chen, Chengcheng Wang, John Danku, David E Salt

    Abstract:

    Inorganic arsenic is a carcinogen, and its ingestion through foods such as rice presents a significant risk to human health. Plants chemically reduce Arsenate to arsenite. Using genome-wide association (GWA) mapping of loci controlling natural variation in arsenic accumulation in Arabidopsis thaliana allowed us to identify the Arsenate reductase required for this reduction, which we named High Arsenic Content 1 (HAC1). Complementation verified the identity of HAC1, and expression in Escherichia coli lacking a functional Arsenate reductase confirmed the Arsenate reductase activity of HAC1. The HAC1 protein accumulates in the epidermis, the outer cell layer of the root, and also in the pericycle cells surrounding the central vascular tissue. Plants lacking HAC1 lose their ability to efflux arsenite from roots, leading to both increased transport of arsenic into the central vascular tissue and on into the shoot. HAC1 therefore functions to reduce Arsenate to arsenite in the outer cell layer of the root, facilitating efflux of arsenic as arsenite back into the soil to limit both its accumulation in the root and transport to the shoot. Arsenate reduction by HAC1 in the pericycle may play a role in limiting arsenic loading into the xylem. Loss of HAC1-encoded arsenic reduction leads to a significant increase in arsenic accumulation in shoots, causing an increased sensitivity to Arsenate toxicity. We also confirmed the previous observation that the ACR2 Arsenate reductase in A. thaliana plays no detectable role in arsenic metabolism. Furthermore, ACR2 does not interact epistatically with HAC1, since arsenic metabolism in the acr2 hac1 double mutant is disrupted in an identical manner to that described for the hac1 single mutant. Our identification of HAC1 and its associated natural variation provides an important new resource for the development of low arsenic-containing food such as rice.

Andrew A Meharg – 2nd expert on this subject based on the ideXlab platform

  • Arsenite transport into paddy rice (Oryza sativa) roots
    New Phytologist, 2020
    Co-Authors: Andrew A Meharg, Louise Jardine

    Abstract:

    Summary
    • Here the mechanism of arsenite transport into paddy rice ( Oryza sativa ) roots, uptake of which is described by Michaelis–Menten kinetics, is reported. A recent study on yeast ( Saccharomyces cerevisiae ) showed that undissociated arsenite (its pK a is 9.2) was transported across the plasma membrane via a glycerol transporting channel. To investigate whether the same mechanism of transport was involved for rice, competitive studies with glycerol, which is transported into cells via aquaporins, were performed.
    • Glycerol competed with arsenite for transport in a dose-dependent manner, indicating that arsenite and glycerol uptake mechanisms were the same. Arsenate transport was unaffected by glycerol, confirming that Arsenate and arsenite are taken up into cells by different mechanisms.
    • Antimonite, an arsenite analogue that is transported into S. cerevisiae cells by aquaporins, also competed with arsenite transport in a dose-dependent manner, providing further evidence that arsenite is transported into rice roots via glycerol transporting channels. Mercury (Hg 2+ ) inhibited both arsenite and Arsenate uptake, suggesting that inhibition of influx was due to general cellular stress rather than the specific action of Hg 2+ on aquaporins.
    • Arsenite uptake by pea ( Pisum sativum ) and wheat ( Triticum aestivum ) was also described by Michaelis–Menten kinetics.

  • arsenic as a food chain contaminant mechanisms of plant uptake and metabolism and mitigation strategies
    Annual Review of Plant Biology, 2010
    Co-Authors: S P Mcgrath, Andrew A Meharg

    Abstract:

    Arsenic (As) is an environmental and food chain contaminant. Excessive accumulation of As, particularly inorganic arsenic (Asi), in rice (Oryza sativa) poses a potential health risk to populations with high rice consumption. Rice is efficient at As accumulation owing to flooded paddy cultivation that leads to arsenite mobilization, and the inadvertent yet efficient uptake of arsenite through the silicon transport pathway. Iron, phosphorus, sulfur, and silicon interact strongly with As during its route from soil to plants. Plants take up Arsenate through the phosphate transporters, and arsenite and undissociated methylated As species through the nodulin 26-like intrinsic (NIP) aquaporin channels. Arsenate is readily reduced to arsenite in planta, which is detoxified by complexation with thiol-rich peptides such as phytochelatins and/or vacuolar sequestration. A range of mitigation methods, from agronomic measures and plant breeding to genetic modification, may be employed to reduce As uptake by food crops.

  • direct evidence showing the effect of root surface iron plaque on arsenite and Arsenate uptake into rice oryza sativa roots
    New Phytologist, 2004
    Co-Authors: Zheng Chen, Andrew A Meharg

    Abstract:

    Summary
    • The present study aimed to investigate the effects of root surface iron plaque on the uptake kinetics of arsenite and Arsenate by excised roots of rice (Oryza sativa) seedlings.
    • The results demonstrated that the presence of iron plaque enhanced arsenite and decreased Arsenate uptake.
    • Arsenite and Arsenate uptake kinetics were adequately fitted by the Michaelis–Menten function in the absence of plaque, but produced poor fits to this function in the presence of plaque.
    • Phosphate in the uptake solution did not have a significant effect on arsenite uptake irrespective of the presence of iron plaque; however phosphate had a significant effect on Arsenate uptake. Without iron plaque, phosphate inhibited Arsenate uptake. The presence of iron plaque diminished the effect of phosphate on Arsenate uptake, possibly through a combined effect of Arsenate desorption from iron plaque.

S P Mcgrath – 3rd expert on this subject based on the ideXlab platform

  • knocking out acr2 does not affect arsenic redox status in arabidopsis thaliana implications for as detoxification and accumulation in plants
    PLOS ONE, 2012
    Co-Authors: Henk Schat, S P Mcgrath, David E Salt, Mathijs Bliek, Yi Chen, Graham N George, Fangjie Zhao

    Abstract:

    Many plant species are able to reduce Arsenate to arsenite efficiently, which is an important step allowing detoxification of As through either efflux of arsenite or complexation with thiol compounds. It has been suggested that this reduction is catalyzed by ACR2, a plant homologue of the yeast Arsenate reductase ScACR2. Silencing of AtACR2 was reported to result in As hyperaccumulation in the shoots of Arabidopsis thaliana. However, no information of the in vivo As speciation has been reported. Here, we investigated the effect of AtACR2 knockout or overexpression on As speciation, arsenite efflux from roots and As accumulation in shoots. T-DNA insertion lines, overexpression lines and wild-type (WT) plants were exposed to different concentrations of Arsenate for different periods, and As speciation in plants and arsenite efflux were determined using HPLC-ICP-MS. There were no significant differences in As speciation between different lines, with arsenite accounting for >90% of the total extractable As in both roots and shoots. Arsenite efflux to the external medium represented on average 77% of the Arsenate taken up during 6 h exposure, but there were no significant differences between WT and mutants or overexpression lines. Accumulation of As in the shoots was also unaffected by AtACR2 knockout or overexpression. Additionally, after exposure to Arsenate, the yeast (Saccharomyces cerevisiae) strain with ScACR2 deleted showed similar As speciation as the WT with arsenite-thiol complexes being the predominant species. Our results suggest the existence of multiple pathways of Arsenate reduction in plants and yeast.

  • arsenic as a food chain contaminant mechanisms of plant uptake and metabolism and mitigation strategies
    Annual Review of Plant Biology, 2010
    Co-Authors: S P Mcgrath, Andrew A Meharg

    Abstract:

    Arsenic (As) is an environmental and food chain contaminant. Excessive accumulation of As, particularly inorganic arsenic (Asi), in rice (Oryza sativa) poses a potential health risk to populations with high rice consumption. Rice is efficient at As accumulation owing to flooded paddy cultivation that leads to arsenite mobilization, and the inadvertent yet efficient uptake of arsenite through the silicon transport pathway. Iron, phosphorus, sulfur, and silicon interact strongly with As during its route from soil to plants. Plants take up Arsenate through the phosphate transporters, and arsenite and undissociated methylated As species through the nodulin 26-like intrinsic (NIP) aquaporin channels. Arsenate is readily reduced to arsenite in planta, which is detoxified by complexation with thiol-rich peptides such as phytochelatins and/or vacuolar sequestration. A range of mitigation methods, from agronomic measures and plant breeding to genetic modification, may be employed to reduce As uptake by food crops.

  • highly efficient xylem transport of arsenite in the arsenic hyperaccumulator pteris vittata
    New Phytologist, 2008
    Co-Authors: S P Mcgrath, Y H Su, Fangjie Zhao

    Abstract:

    Summary
    • The hyperaccumulator Pteris vittata translocates arsenic (As) from roots to fronds efficiently, but the form of As translocated in xylem and the main location of Arsenate reduction have not been resolved.
    • Here, P. vittata was exposed to 5 µM Arsenate or arsenite for 1–24 h, with or without 100 µM phosphate. Arsenic speciation was determined in xylem sap, roots, fronds and nutrient solutions by high-performance liquid chromatography (HPLC) linked to inductively coupled plasma mass spectrometry (ICP-MS).
    • The xylem sap As concentration was 18–73 times that in the nutrient solution. In both Arsenate– and arsenite-treated plants, arsenite was the predominant species in the xylem sap, accounting for 93–98% of the total As. A portion of Arsenate taken up by roots (30–40% of root As) was reduced to arsenite rapidly. The majority (c. 80%) of As in fronds was arsenite. Phosphate inhibited Arsenate uptake, but not As translocation. More As was translocated to fronds in the arsenite-treated than in the Arsenate-treated plants. There was little arsenite efflux from roots to the external solution.
    • Roots are the main location of Arsenate reduction in P. vittata. Arsenite is highly mobile in xylem transport, possibly because of efficient xylem loading, little complexation with thiols in roots, and little efflux to the external medium.