Straighthead disease is a physiological disorder in rice with symptoms of sterile spikelets, distorted husks, and erect panicles. Methylated arsenic species have been implicated as the causal agent of the disease, but direct evidence is lacking. Here, we investigated whether dimethylarsinic acid (DMA) causes straighthead disease and its effect on the transcriptome of young panicles. DMA addition caused typical straighthead symptoms in hydroponic culture, which were alleviated by silicon addition. DMA addition to soil at the tillering to flowering stages induced straighthead disease. Transgenic rice expressing a bacterial arsenite methyltransferase gene gained the ability to methylate arsenic to mainly DMA, with the consequence of inducing straighthead disease. Field surveys showed that seed setting rate decreased with increasing DMA concentration in the husk, with an EC50 of 0.18 mg kg−1. Transcriptomic analysis showed that 364 and 856 genes were significantly up- and down-regulated, respectively, in the young panicles of DMA-treated plants compared with control, whereas Si addition markedly reduced the number of genes affected. Among the differentially expressed genes, genes related to cell wall modification and oxidative stress responses were the most prominent, suggesting that cell wall metabolism is a sensitive target of DMA toxicity and silicon protects against this toxicity.

Arsenic species transformation in paddy soils has important implications for arsenic accumulation in rice grains and its safety to the consumers. Methylated thioarsenates including highly toxic dimethylated monothioarsenate (DMMTA) have been detected in paddy soils, but their production and dynamics remain poorly understood. In the present study, we first optimized a HPLC-ICP-MS method to quantify methylated thioarsenate species. Using this method together with 10 mM diethylenetriamine pentaacetate (DTPA) to preserve As speciation, we investigated methylated thioarsenate species in porewaters of seven As-contaminated soils incubated under flooded conditions and of two paddy fields. DMMTA was the main methylated thioarsenate species in the porewaters in both incubated soils and paddy fields, with concentrations ranging from 0.2 to 36.2 μg/L and representing ca. 58% of its precursor dimethylarsenate (DMA). The temporal production and dynamics of DMMTA were linked with the DMA concentrations. When soils were drained, DMMTA was converted to DMA. In the two paddy fields, DMMTA concentrations in rice grains were 0.4–10.1 μg/kg. Addition of sulfur fertilizer and rice straw incorporation increased grain DMMTA by 9–28%. These results suggest that DMMTA is an important As species in paddy soils and can accumulate in rice grains, presenting a risk to food safety and human health.

Agronomic biofortification of rice with the triple foliar spray of Fe, Zn, and pesticide is a rapidly effective and cost-effectiveness pathway to alleviate Fe and Zn deficiency for rice-based dietary populations.

Arsenic (As) accumulation in rice is of global concern for human health and international trade. Rice is typically reported to contain inorganic As (iAs) and dimethylated arsenate (DMA), with current food guidelines limiting toxic iAs but not less-toxic DMA. Here, we show that the highly toxic dimethylated monothioarsenate (DMMTA) is also found in rice worldwide and has been unknowingly determined as less-toxic DMA by previous routine analytical methods. Using enzymatic extraction followed by high-performance liquid chromatography−inductively coupled plasma−mass spectrometry (HPLC-ICP-MS) analysis with a C18 column, DMMTA was detected in rice grains (n = 103) from a field survey from China and in polished rice grains (n = 140) from a global market-basket survey. Concentration ranged from <0.20 to 34.8 μg/kg (median 10.3 μg/kg), accounting for 0 to 21% of total As. A strong linear correlation was observed in all rice samples between DMA and DMMTA (being 30 ± 8% of DMA) concentrations. This robust relationship allows an estimation of DMMTA in rice grains from the DMA data reported in previous market-basket surveys, showing a general global geographical pattern with DMMTA concentration increasing from the equator toward high-latitude regions. Based on the global occurrence and potential high toxicity, DMMTA in rice should be considered in health risk assessments and for setting food regulations.

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