Pentavalent methylated arsenic (As) species such as monomethylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)] are used as herbicides or pesticides, and can also be synthesized by soil microorganisms or algae through As methylation. The mechanism of MMA(V) and DMA(V) uptake remains unknown. Recent studies have shown that arsenite is taken up by rice (Oryza sativa) roots through two silicon transporters, Lsi1 (the aquaporin NIP2;1) and Lsi2 (an efflux carrier). Here we investigated whether these two transporters also mediate the uptake of MMA(V) and DMA(V). MMA(V) was partly reduced to trivalent MMA(III) in rice roots, but only MMA(V) was translocated to shoots. DMA(V) was stable in plants. The rice lsi1 mutant lost about 80% and 50% of the uptake capacity for MMA(V) and DMA(V), respectively, compared with the wild-type rice, whereas Lsi2 mutation had little effect. The short-term uptake kinetics of MMA(V) can be described by a MichaelisMenten plus linear model, with the wild type having 3.5-fold higher V max than the lsi1 mutant. The uptake kinetics of DMA(V) were linear with the slope being 2.8-fold higher in the wild type than the lsi1 mutant. Heterologous expression of Lsi1 in Xenopus laevis oocytes significantly increased the uptake of MMA(V) but not DMA(V), possibly because of a very limited uptake of the latter. Uptake of MMA(V) and DMA(V) by wild-type rice was increased as the pH of the medium decreased, consistent with an increasing proportion of the undissociated species. The results demonstrate that Lsi1 mediates the uptake of undissociated methylated As in rice roots.
Summary• When supplied with arsenate (As(V)), plant roots extrude a substantial amount of arsenite (As(III)) to the external medium through as yet unidentified pathways. The rice (Oryza sativa) silicon transporter Lsi1 (OsNIP2;1, an aquaporin channel) is the major entry route of arsenite into rice roots. Whether Lsi1 also mediates arsenite efflux was investigated.• Expression of Lsi1 in Xenopus laevis oocytes enhanced arsenite efflux, indicating that Lsi1 facilitates arsenite transport bidirectionally.• Arsenite was the predominant arsenic species in arsenate-exposed rice plants. During 24-h exposure to 5 lM arsenate, rice roots extruded arsenite to the external medium rapidly, accounting for 60-90% of the arsenate uptake. A rice mutant defective in Lsi1 (lsi1) extruded significantly less arsenite than the wild-type rice and, as a result, accumulated more arsenite in the roots. By contrast, Lsi2 mutation had little effect on arsenite efflux to the external medium.• We conclude that Lsi1 plays a role in arsenite efflux in rice roots exposed to arsenate. However, this pathway accounts for only 15-20% of the total efflux, suggesting the existence of other efflux transporters.
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