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Inorganic As in soil is classified based on the redox status of the soil: arsenite [As(III)] and arsenate [As(V)] (Abedin et al., 2002). As(III) is the predominant form in anaerobic paddy soil and As(V) in soil under aerobic conditions. Bioavailability of arsenic to rice roots follows the order AsIII> MMA >AsV > DMA, but the rate of organic species uptake is much lower than that of inorganic arsenic (Abedin et al., 2002). The toxicity of arsenic species is in the following order AsIII > AsV > MMA > DMA (Baig et al., 2010). In Rhizosphere, release of oxygen by rice roots, formation of iron plaque, and microbial oxidation contribute in oxidation of AsIII (arsenite) to AsV (arsenate) in soils (Liu et al., 2006; Jia et al., 2014). AsV is taken up by roots phosphate transporters, including OsPht1;8 (OsPT8) and OsPht1;1(OsPT1) in rice (Wu et al., 2011; Wang et al., 2016). Following uptake, AsV can be rapidly reduced to arsenite (AsIII) in plant cells by HAC1 (High Arsenic Content 1) arsenate reductases (Shi et al., 2016). AsIII enters plants via aquaporin channels, mainly the nodulin 26-like intrinsic proteins (NIPs, a subfamily of the aquaporin family) (Ma et al., 2008; Kamiya et al., 2009; Mitani-Ueno et al., 2011; Xu et al., 2015). In rice, the silicon influx transporter Lsi1 (Low silicon rice 1; OsNIP2;1) is responsible for AsIII uptake while Si efflux transporter Lsi2 (Low silicon rice 2) mediates AsIII efflux (Ma et al., 2006, 2007, 2008). Both Lsi1 and Lsi2 localize at the plasma membrane of exodermal and endodermal cells of the roots, but with different polar localization (Chen et al., 2017). Some plasma membrane intrinsic proteins (PIPs, another subfamily of the aquaporin family), including OsPIP2;4, OsPIP2;6 and OsPIP2;7, and rice NRAMP (Natural Resistance-Associated Macrophage Protein) transporter, OsNRAMP1 are also involved in AsIII transport (Mosa et al., 2012; Tiwari et al., 2014; Chen et al., 2017). Lsi1 uptakes both inorganic As (III) and organic As species (MMA and DMA) from the rhizosphere in pH dependent manner (Li et al., 2009). Lsi2 mediates the efflux of As (III) to the xylem. The OsNRAMP1 may also facilitate the xylem loading of As(III) (Tiwari et al., 2014). DMA translocates from root to rice grain more efficiently (~10-fold greater) than inorganic As species (Carey et al., 2010). As a step of As detoxification, formation of AsIII-PC complexes and OsABCC1 mediated sequestration of AsIII-PC in vacuoles limits the translocation of AsIII from root to shoot thus reducing As accumulation in rice grains.
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