Fig. 1. Schematic illustrations of Cd and As(III) uptake via metal transporters in mammalian cells (A) and rice root (B). (A): Among the ZIP family members of Zn transporters, ZIP8 and ZIP14 participate in the transports of Cd(II) and Mn(II) as well as that of Zn(II). The functionalities of ZIP8 and ZIP14 for the transports of Cd(II) and Mn(II) have been recognized in a variety of mammalian cells including hepatic (), renal (,), cardiac (), neuronal (), intestinal (), and uterine () cells. Cellular uptake of As(III) in mammalian cells is mediated by aquaglyceroporin, a water channel used also for the uptake of glycerol (–). (B): In the root of rice, the uptake of Cd(II) is mediated by the Mn(II) transporter, OsNramp5 (), and that of As(III) is mediated by the Si(IV) transporter OsLsi1 (), which is an ortholog of aquaporin in mammals. Since rice is a hyperaccumulator of Si (), higher amounts of As(III) is accumulated in rice than other plants. The microenvironment of soil and water surrounding the rice root affect the release of soluble Cd(II) and As(III) from the soil in opposite ways. Under aerobic conditions, the insoluble CdS in soil, which is stable under anaerobic conditions, is oxidized to CdSO4, leading to the release of soluble Cd(II). In contrast, under anaerobic conditions, the complex of As(V)-Fe(III), which is stable under aerobic conditions, is reduced to As(III) and Fe(II), leading to the release of soluble As(III). To achieve the mitigation of Cd and As contaminations in rice by the management of irrigation water, this trade-off problem should be solved (–).
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