Methylated Thioarsenates and Monothioarsenate Differ in Uptake, Transformation, and Contribution to Total Arsenic Translocation in Rice Plants

Abstract

Methylated and inorganic thioarsenates have recently been reported from paddy fields besides the better-known oxyarsenates. Methylated thioarsenates are highly toxic for humans, yet their uptake, transformation, and translocation in rice plants is unknown. Here, hydroponic experiments with 20 day old rice plants showed that monomethylmonothioarsenate (MMMTA), dimethylmonothioarsenate (DMMTA), and monothioarsenate (MTA) were taken up by rice roots and could be detected in the xylem. Total arsenic (As) translocation from roots to shoots was higher for plants exposed to DMMTA, MTA, and dimethylarsenate (DMAV) compared to MMMTA and monomethylarsenate (MMAV). All thioarsenates were partially transformed in the presence of rice roots, but processes and extents differed. MMMTA was subject to abiotic oxidation and largely dethiolated to MMAV already outside the plant, probably due to root oxygen loss. DMMTA and MTA were not oxidized abiotically. Crude protein extracts showed rapid enzymatic reduction for MTA but not for DMMTA. Our study implies that DMMTA has the highest potential to contribute to total As accumulation in grains either as DMAV or partially as DMMTA. DMMTA has once been detected in rice grains using enzymatic extraction. By routine acid extraction, DMMTA is determined as DMAV and thus escapes regulation despite its toxicity.

Detection of Thioarsenates in Rice Grains and Rice Products

“…Method C used the same enzymatic extraction as method B with the exception that the saline solution was replaced by a phosphate buffer solution (PBS, 2 mmol L –1 of NaH 2 PO 4 + 0.2 mmol L –1 of Na 2 -EDTA, pH 2) to reduce the salt load and because the stability of MTA and DMMTA in PBS had previously been confirmed. For chromatographic separation, a method that had previously been developed to separate inorganic and methylated thioarsenates in aqueous samples was applied, using an AG/AS16 IonPac column (Dionex; 2.5–100 mmol L –1 of NaOH + 2.4% methanol eluent, gradient elution, a flow rate of 1.2 mL·min –1 , and 50 μL injection volume).…”

“…8 Uptake of thioarsenates by rice plants has been shown before. 11,12 The presence of MTA in rice roots and shoots has been confirmed by direct analytical evidence, 11 and hydroponic experiments have demonstrated that MTA, 11 MMMTA, 12 and DMMTA 12 can reach the xylem of rice plants. In rice grains, As speciation is dominated by iAs and DMA.…”

“…In rice grains, As speciation is dominated by iAs and DMA . Even though the DMA contribution to the total As in pore water typically is low, DMA can contribute up to 90% to total As in grains because it is translocated with greater efficiency in comparison to iAs. − Hydroponic experiments have revealed that the root to shoot translocation factors of MTA and DMMTA are on the same order of magnitude as those of DMA, , suggesting that these species can potentially contribute to As accumulation in rice grains.…”

The roles of membrane transporters in arsenic uptake, translocation and detoxification in plants

“…Rice plants can take up monothioarsenate and transport it in the xylem (Kerl et al, 2018). Dimethylmonothioarsenate has also been detected in rice grains and can account for up to 20% of the total As in the enzymatic extracts of commercial rice samples (Ackerman et al, 2005;Kerl et al, 2019). A hydroponic study found that rice roots accumulated about 10 time more dimethylmonothioarsenate than DMAs when plants were supplied with the same concentration of the two As species, indicating a high uptake and/or sequestration of dimethylmonothioarsenate in rice roots (Kerl et al, 2019).…”

Microbial Arsenic Methylation in Soil and Uptake and Metabolism of Methylated Arsenic in Plants: A Review

“…Outside the plant, MMMTA could be dethiolated to MMAs(Ⅴ) due to abiotic oxidation, but DMMTA was not oxidized abiotically. DMMTA had the highest potential to promote total As accumulation either as DMAs(Ⅴ) or potentially as DMMTA [12]. The concentration of arsenic in the human body is determined mainly by the arsenic concentration in drinking water and the seafood consumption in the diet.…”