Reduction of Dimethylarsenate to Highly Toxic Dimethylarsenite in Paddy Soil and Rice Plants

Dimethylarsenate [DMAs­(V)] is a common methylated As species in soils and plants and can cause the physiological disorder straighthead disease in rice. Because DMAs­(V) is relatively noncytotoxic, we hypothesize that phytotoxicity of DMAs­(V) may arise from trivalent dimethylarsenite [DMAs­(III)]. DMAs­(III) has been detected in human urine samples but not in environmental samples, likely due to its instability under oxic conditions. We first established methods for preservation and detections of DMAs­(III) in soil and plant samples. We showed that DMAs­(III) was a major As species in soil solution from an anoxic paddy soil. Enrichment cultures for fermentative, sulfate-reducing, and denitrifying bacteria from the paddy soil could reduce DMAs­(V) to DMAs­(III). Twenty-two strains of anaerobic bacteria isolated from the soil showed some ability to reduce DMAs­(V). Rice plants grown in hydroponic culture with DMAs­(V) also showed the ability to reduce DMAs­(V) to DMAs­(III). Rice plants and grains grown in a flooded paddy soil contained both DMAs­(V) and DMAs­(III); their concentrations were higher in the spikelets with straighthead disease than those without. DMAs­(III) was much more toxic to the protoplasts isolated from rice plants than DMAs­(V). Taken together, the ability to reduce DMAs­(V) to highly toxic DMAs­(III) is common to soil anaerobes and rice plants.