Research published in Proceedings of the National Academy of Sciences recently has warned that the type of microbes in rice paddies can determine whether arsenic, a notorious carcinogen and plant toxin, builds up in rice grains and triggers drastic crop losses.
The study has identified an important balance between arsenic-methylating bacteria, which convert inorganic arsenic into the toxic organic forms, versus demethylating archaea, which can undo this process. Where the methylating bacteria dominate, rice plants absorb the compound dimethylarsinic acid (DMA) and its more toxic derivative, dimethylated monothioarsenate (DMMTA). These compounds pose health risks to humans as well as induce straighthead disease.
“Straighthead needs to be considered as a physiological disorder rather than a disease in the absence of any infectious agent,” rice pathologist Sridhar Ranganathan, who wasn’t involved in the study, said.
“The symptoms are erect panicles with unfilled grains, often remaining green. Due to empty grains affecting the weight of the panicles, the-ear bearing tillers don’t droop down and remain green and upright, as can be seen with unaffected healthy plants bearing normally filled matured grains in which the plants droop down showing symptoms of initiation of senescence of the leaves and the grains.”
While long dismissed as a local agronomic issue, straighthead disease is now recognised as a global threat. In parts of the US and China, farmers have reported significant outbreaks, often in newly established or rotated paddies. West Bengal in India and Bangladesh have also previously reported straighthead disease.
The condition can result in up to 70% yield losses in severely affected areas. It occurs even when the total arsenic level in the soil is relatively low because the real problem is arsenic speciation, i.e. the chemical form arsenic takes in the soil and plant. The new study has found that the microbial communities that dominate in the paddies determine this speciation.
The research team, led by Peng Wang at Nanjing Agricultural University in China, analysed rice paddies of different ages in China, unearthing a surprising pattern. Soils younger than 700 years were dominated by arsenic-methylating bacteria, so the rice grown there accumulated more DMA and DMMTA and the fields were more prone to straighthead disease outbreaks. Soils older than 700 years had more demethylating archaea, which broke down DMA and reduced the compounds’ buildup.
The researchers combined these field data with controlled soil incubation tests, genetic analyses, and a global survey of 801 paddy soil microbiomes. Eventually, they identified 11 methylating microbes and six demethylating archaea whose abundance could accurately predict arsenic risk.
In the paper, the team also reported that newly cultivated paddy regions like the US, southern Europe, and northeast China showed high ratios of methylating to demethylating microbes, rendering them particularly vulnerable to straighthead outbreaks. Ancient rice-growing regions in South and Southeast Asia had stronger demethylating communities instead. When the ratio of methylating to demethylating microbes exceeded 1.5, the risk of straighthead disease was found to rise sharply.
India is the world’s second-largest producer and consumer of rice. While much of the country’s rice farming occurs in old, legacy paddies with relatively balanced microbial communities, several States — particularly in East and South India — have had new or reclaimed paddy fields established in the last few decades. These fields may be at greater risk, per the new study.
Arsenic contamination in groundwater already poses a severe challenge in West Bengal, Bihar, and Assam, compounding the potential hazard.
Experts said the research also intersects with climate change. Higher temperatures and altered flooding regimes are expected to increase the soils’ arsenic content (whether from natural sources or anthropogenic), and could tip the microbial balance towards the more harmful varieties. For a country where rice contributes nearly 40% of the population’s caloric intake, the crop’s safety and productivity are crucial.
Dr. Ranganathan said that even if the crop can’t be saved in a single cropping season, agronomic interventions can mitigate risks. According to the research paper, draining the rice fields midseason can ‘suppress’ the methylating microbes by reintroducing oxygen into the soils. Silicon fertilisation has also been known to reduce rice plants’ arsenic uptake. He also said crop rotation strategies can be adjusted to avoid destabilising microbial communities.
At the policy level, the findings highlight the need to monitor arsenic speciation, and not just total arsenic levels, as part of food safety regulations. Current standards, including those of the UN Food and Agriculture Organisation’s ‘Codex Alimentarius’, focus on inorganic arsenic, leaving gaps around methylated species like DMMTA.
Ashmita Gupta is a science writer.
Published – October 18, 2025 09:06 pm IST
