Accumulation and transformation of inorganic and organic arsenic in rice and role of thiol-complexation to restrict their translocation to shoot

Environmental contamination of arsenic (As) and its accumulation in rice (Oryza sativa L.) is of serious human health concern. In planta speciation of As is an important tool to understand As metabolism in plants. In the present study, we investigated root to shoot As translocation and speciation in rice exposed to inorganic and methylated As. Arsenate (As(V)) and methylarsonate (MA(V)) were efficiently reduced to arsenite (As(III)) and MA(III), respectively in rice root and shoot but no trivalent form of dimethylarsinate (DMA(V)) was detected. Further, up to 48 and 83% of root As in As(V) and MA(V) exposed plants, respectively were complexed with various thiols showing up to 20 and 16 As species, respectively. Several mixed As- and MA-complexes with hydroxymethyl-phytochelatin, DesGly-phytochelatin, hydroxymethyl-GSH and cysteine were identified in rice. Despite high complexation in roots, more As was translocated to shoots in MA(V) exposed plants than As(V), with shoot/root As transfer factor being in order DMA(V) > MA(V) > As(V). Moreover, in shoots 78% MA(III) and 71% As(III) were present as weakly bound species which is alarming, as MA(III) has been found to be more cytotoxic than As(III) for human and it could also be an important factor inducing straighthead (spikelet sterility disorder) in rice.