A novel strategy for arsenic phytoremediation

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Project Summary: The main objective of this supplemental project is to characterize arsenate reductase, ACR2, as a multifunctional enzyme for its role in arsenic tolerance, translocation, and limiting accumulation in food crops. Under aerobic conditions, plants absorb arsenate (AsV) from soil through phosphate transporters, and then AsV in roots is electrochemically reduced to arsenite (AsIII) by the activity of endogenous arsenate reductase, ACR2, or its homologs HAC1. AsIII is either extruded out of the roots or strongly binds to glutathione (GSH) and phytochelatins (PCs), which causes trapping most As belowground in the roots. Previously, to enhance the translocation of As to shoot tissues for phytoremediation purpose, we knocked down the AtACR2 expression using RNA interference (RNAi) in Arabidopsis. RNAi lines translocated 10- to 16-fold more As in shoots and retained less As in roots compared to wild-type plants. Therefore, arsenate reductases play a critical role in the translocation and accumulation of As in plants. Contrary to the results of RNAi knockdown of AtACR2, the overexpression of AtACR2 in Arabidopsis provided strong tolerance to AsV and caused a 50-75% reduction of As accumulation in aboveground shoot tissues. However, this multifunctional AtACR2 gene is not fully characterized and the mode of action in modulating the As tolerance/sensitivity and accumulation is not well understood. Arabidopsis AtACR2 contains a canonical arsenate reductase domain “HCX5R” and a highly cysteine- rich C-terminal domain. The canonical “HXC5R” and C-terminal Cys-rich domains are missing in the recently characterized alternate arsenate reductase HAC1. We speculate that “HCX5R” domain reduces AsV to AsIII, which binds to the Cys-rich C-terminal domain and hence provides AsIII tolerance. n the proposed supplemental training project, the PhD student trainee will characterize this AtACR2 gene to understand its role and mechanism of As tolerance and accumulation using a mutational approach by replacing the conserved Cys-residues in the C- terminus. Rice (Oryza sativa) is well known to accumulate high levels of As in edible grains. The ultimate goal of this supplemental project is to translate the knowledge into rice via overexpressing AtACR2 constitutively for increase tolerance and limiting As accumulation in the grains. The resulting transgenic rice lines overexpressing AtACR2 will be grown in As contaminated soils and will be analyzed for As tolerance and accumulation under greenhouse conditions. Therefore, the proposed project will lead to developing strategies for limiting As in rice to improve human health and thus will have a significant societal impact. This project is directly related, but not overlapped, to our R01 (Project ID: 1R01E032686-01) project “A novel strategy for arsenic phytoremediation”. The objective of the R01 project is to develop a genetics-based phytoremediation strategy for arsenic uptake, translocation, detoxification, and hyperaccumulation into the fast-growing, high biomass, non-food crop Crambe abyssinica.