Investigating the origin and transport of methylated arsenic species in plants

Inorganic arsenic is a toxic element known to cause various diseases and cancers in humans. Arsenic contamination is widespread worldwide, particularly in South-East Asia where arsenic-contaminated groundwater is used for drinking and rice cultivation. Unlike other cereals, paddy rice can efficiently accumulate arsenic in the grain. Rice is a staple food for around 50% of the world's population, so arsenic accumulation in rice is of great concern. Arsenite, As(III), is the predominant form of arsenic within plants, but rice grains often contain significant proportions of organic arsenic species. The most common of these are dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA). A series of axenic experiments demonstrated that plants are unable to methylate arsenic, and instead take these species up from soil where they are produced by micro-organisms. The uptake of undissociated MMA by rice roots is predominantly facilitated by OsNIP2;1 (OsLsi1), a member of the NIP-subfamily of aquaporins, which also accounts for 50% of root DMA uptake. Expression of OsNIP1;1 and OsNIP3;3 in Xenopus oocytes demonstrated that these NIP aquaporins are permeable to pentavalent MMA, as well as arsenite, silicon and water. However, uptake of DMA was not observed for oocytes expressing any NIP gene, including OsNIP2;1. MMA and DMA have a pKa1 of 4.19 and 6.14 respectively, and so increasing the pH of the medium increases the proportion of dissociated complexes. In hydroponic culture, rice plants over-expressing the high-affinity phosphate transporter OsPT8, took up significantly more MMA and DMA than wild-type. Additionally, the presence of phosphate in the medium significantly decreased the uptake of both MMA and DMA by OsPT8-overexpression and wild-type rice plants. Therefore we have discovered that methylated arsenic species are not formed within plants, and can be transported by two different classes of transporters depending on the pH of the medium.