No abstract

Projections of global rice yields account for climate change. They do not, however, consider the coupled stresses of impending climate change and arsenic in paddy soils. Here, we show in a greenhouse study that future conditions cause a greater proportion of pore-water arsenite, the more toxic form of arsenic, in the rhizosphere of Californian Oryza sativa L. variety M206, grown on Californian paddy soil. As a result, grain yields decrease by 39% compared to yields at today’s arsenic soil concentrations. In addition, future climatic conditions cause a nearly twofold increase of grain inorganic arsenic concentrations. Our findings indicate that climate-induced changes in soil arsenic behaviour and plant response will lead to currently unforeseen losses in rice grain productivity and quality. Pursuing rice varieties and crop management practices that alleviate the coupled stresses of soil arsenic and change in climatic factors are needed to overcome the currently impending food crisis.

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.

Thioarsenates form under sulfur-reducing conditions in paddy soil pore waters. Sulfur fertilization, recently promoted for decreasing total arsenic (As) grain concentrations, could enhance their formation. Yet, to date, thioarsenate toxicity, uptake, transformation, and translocation in rice are unknown. Our growth inhibition experiments showed that the toxicity of monothioarsenate (MTA) was similar to that of arsenate but lower than that of arsenite. Higher toxicity of MTA with lower phosphate availability might imply uptake through phosphate transporters similar to arsenate. To demonstrate direct uptake of MTA by rice plants, a species-preserving extraction method for plant samples was developed. When plants were exposed to 10 μM MTA for 72 h, up to 19% and 4% of total As accumulated in roots and shoots, respectively, was MTA. Monothioarsenate was detected in xylem sap and root exudates, and its reduction to arsenite in rice roots and shoots was shown. Total As uptake was lower upon exposure to MTA compared to arsenate, but root to shoot translocation was higher, resulting in comparable As shoot concentrations. Thus, before promoting sulfur fertilization, uptake and detoxifying mechanisms of thioarsenates as well as potential contribution to grain As accumulation need to be better understood.

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