Phosphate Transporters Mediate the Uptake of Monothioarsenate

Arsenic (As) is one of the most problematic environmental toxins. Exposure to As, predominantly via drinking water and the intake of food, represents a major human health threat. Various species of As exist in the environment, among them organic and inorganic thioarsenates. Their ubiquitous presence in rice paddy soil pore water has recently been established. Thioarsenates are taken up by plants and show high mobility within plants. They are efficiently translocated from roots to shoots and can be loaded into grains. To date, however, no information is available on the transporter proteins enabling the necessary membrane passages. We tested the hypothesis that the major inorganic thioarsenate, monothioarsenate (MTA), is a substrate for phosphate transporters in experiments with yeast and plant model systems. Short-term uptake assays demonstrated MTA transport, albeit at much lower rates than apparent for arsenate. Plant mutants with defects in phosphate transporters or regulators controlling phosphate deficiency responses were more tolerant to MTA as indicated by growth phenotypes and pigment concentrations. High external phosphate supply suppressed the MTA effects. Also, the mutants accumulated less As in roots and shoots upon MTA exposure. Inside plants, MTA was efficiently converted into arsenite and activated the phytochelatin pathway. Nonetheless, in light of the much lower relative uptake rate for MTA, we hypothesize that this As species exerts specific toxicity effects.