Arsenic poisoning affects millions of people worldwide. Human arsenic intake from rice consumption can be substantial because rice is particularly efficient in assimilating arsenic from paddy soils, although the mechanism has not been elucidated. Here we report that two different types of transporters mediate transport of arsenite, the predominant form of arsenic in paddy soil, from the external medium to the xylem. Transporters belonging to the NIP subfamily of aquaporins in rice are permeable to arsenite but not to arsenate. Mutation in OsNIP2;1 (Lsi1, a silicon influx transporter) significantly decreases arsenite uptake. Furthermore, in the rice mutants defective in the silicon efflux transporter Lsi2, arsenite transport to the xylem and accumulation in shoots and grain decreased greatly. Mutation in Lsi2 had a much greater impact on arsenic accumulation in shoots and grain in field-grown rice than Lsi1. Arsenite transport in rice roots therefore shares the same highly efficient pathway as silicon, which explains why rice is efficient in arsenic accumulation. Our results provide insight into the uptake mechanism of arsenite in rice and strategies for reducing arsenic accumulation in grain for enhanced food safety.efflux ͉ influx ͉ arsenic contamination ͉ silicon ͉ aquaporin A rsenic (As) is a human carcinogen, and there may be no threshold below which it does not cause cancer (1). More than 40 million people worldwide are at risk from drinking As-contaminated groundwater (2), and chronic inorganic As poisoning has reached a massive scale in Bangladesh and West Bengal, India (3). In these countries, As-contaminated groundwater is also widely used for irrigating crops during dry season rice production, adding Ͼ1,000 metric tons of As to soil per year in Bangladesh alone and resulting in As accumulation in soils and elevated As uptake by crops (4-6). Elevated As accumulation in rice has the potential to become a new disaster for the population in Southeast Asia (7). As concentrations in rice grain are often high enough to cause concern even in uncontaminated soils containing background levels of As, because paddy rice appears to be particularly efficient in As assimilation compared with other cereal crops (8). Worldwide market surveys show that rice grain contains considerably higher levels of inorganic As than other foods (9, 10). Human intake of As from consumption of rice can be substantial, especially for people who consume a lot of rice (11). It is therefore crucial that the mechanism of As accumulation in rice is understood to counteract this widespread contamination of the food chain.Plants take up arsenate, the predominant form of As in aerobic soils, through phosphate transporters (12, 13). However, in paddy soils, which are flooded during much of the rice growing season, arsenite becomes the predominant chemical species of As (14). It has been shown that arsenite is taken up via aquaglyceroporins in microbes (13,(15)(16)(17). Evidence from physiological studies suggests that arsenite may also be transp...

Rice is efficient at arsenic (As) accumulation, thus posing a potential health risk to humans and animals. Arsenic bioavailability in submerged paddy soil is enhanced due to mobilisation of arsenite, but rice may also have an inherently greater ability to take up and translocate arsenite than other cereal crops. To test this hypothesis, rice, wheat and barley were exposed to 5 µM arsenate or arsenite for 24 h. Arsenic uptake and distribution, and As speciation in the xylem sap and nutrient solution were determined. Regardless of the As form supplied to plants, rice accumulated more As in the shoots than wheat or barley. Arsenite uptake by rice was double of that by wheat or barley, whereas arsenate uptake was similar between rice and wheat and approximately a third smaller in barley. The efficiency of As translocation from roots to shoots was greater when plants were supplied with arsenite than with arsenate, and in both treatments rice showed the highest translocation efficiency. Arsenite was the main species of As (86-97%) in the xylem sap from arsenite-treated plants of all three species. In the arsenate-treated plants, 84%, 45% and 63% of As in the xylem sap of rice, wheat and barley, respectively, was arsenite. Arsenite efflux to the external medium was also observed in all three plant species exposed to arsenate. The results show that rice is more efficient than wheat or barley in arsenite uptake and translocation, probably through the highly efficient pathway for silicon.

Summary• The hyperaccumulator Pteris vittata translocates arsenic (As) from roots to fronds efficiently, but the form of As translocated in xylem and the main location of arsenate reduction have not been resolved.• Here, P. vittata was exposed to 5 µM arsenate or arsenite for 1-24 h, with or without 100 µM phosphate. Arsenic speciation was determined in xylem sap, roots, fronds and nutrient solutions by high-performance liquid chromatography (HPLC) linked to inductively coupled plasma mass spectrometry (ICP-MS).• The xylem sap As concentration was 18-73 times that in the nutrient solution. In both arsenate-and arsenite-treated plants, arsenite was the predominant species in the xylem sap, accounting for 93-98% of the total As. A portion of arsenate taken up by roots (30-40% of root As) was reduced to arsenite rapidly. The majority (c. 80%) of As in fronds was arsenite. Phosphate inhibited arsenate uptake, but not As translocation. More As was translocated to fronds in the arsenite-treated than in the arsenate-treated plants. There was little arsenite efflux from roots to the external solution.• Roots are the main location of arsenate reduction in P. vittata. Arsenite is highly mobile in xylem transport, possibly because of efficient xylem loading, little complexation with thiols in roots, and little efflux to the external medium.