ABSTRACT
Rice is a major dietary source of inorganic arsenic for the population who consumes rice as the staple food. Paddy rice accumulates much more arsenic than other cereal crops because the anaerobic conditions in submerged paddy soil is conducive to the mobilization of arsenite, and arsenite hitchhikes on the silicon uptake pathway which is highly expressed in rice roots. Although arsenite is the predominant arsenic species in submerged paddy soil, arsenate can still account for a significant portion of the soluble arsenic species. A number of denitrifying bacteria from paddy soils are able to mediate anaerobic arsenite oxidation by coupling with denitrification. Additions of nitrate enhanced anaerobic arsenite oxidation and decreased arsenic mobility in paddy soil. Anaerobic conditions also enhance arsenic methylation, whereas methylated arsenic species can be demethylated by some microbial populations. The dynamics of arsenic methylation and demethylation control the accumulation of dimethylarsenate, which is highly phytotoxic. Different arsenic species are taken up by rice via different transporters. Recent studies have identified a new class of enzymes that can reduce arsenate to arsenite, allowing arsenite to be extruded from rice roots to the external medium for detoxification.
