Monothioarsenate uptake and reduction in Arabidopsis thaliana

Monothioarsenate (MTA) is a newly discovered arsenic species, formed under sulfur-reducing conditions in paddy soil pore waters. It is structurally analogues of arsenate-As(V). Given the food safety risks associated with As accumulation in rice it is important to understand how thiolated As species are taken up and metabolized by plants. In this work, MTA reduction was evaluated using the model system Arabidopsis thaliana . The analysis was carried out in the accessions Col-0 (Wild type = WT) as well as in two hac1 null alleles Krefeld (Kr-0) and a AtHAC1 mutant. Total arsenic accumulation and speciation assays in shoots for those plants showed that hac1 null alleles plants accumulated more arsenic than wild type (Col-0) under As(V) conditions. HAC1 represents the known arsenate reductase. However, HAC1 mutant plants did not show this phenotype under MTA exposition. In spite of this, Shoot-As-speciation showed mainly As(III), followed by As(V) and few amounts of MTA, which could indicate that most of the MTA that is translocated to the shoots is first reduced to As(III), although a minor part remains as MTA. Arsenic speciation in roots showed that the dominant As species was arsenite for both plants in MTA/As(V) treatments. Nevertheless, a reduction in the percentage of the total accumulated arsenic present as arsenite was observed, hac1 null alleles has lower As(III) amount than Col-0, under As(V) conditions. Similarly, in plants exposed to MTA, the dominant As species in the roots was arsenite for all genotypes, followed by As(V) and a lower MTA proportion. The reduction of MTA to arsenite in roots gave evidence that MTA is actively taken up by plants and reduced inside the plant to As(III). A. thaliana exposed to MTA showed higher rates of As translocation from roots to shoots than plants under As(V) treatment. A better understanding of thioarsenate translocation will be especially important in order to prevent/reduce the T-As accumulation in edible plant parts.