Soil redox status governs within-field spatial variation in microbial arsenic methylation and rice straighthead disease.

Microbial arsenic (As) methylation in paddy soil produces mainly dimethylarsenate (DMA), which can cause physiological straighthead disease in rice. The disease is often highly patchy in the field, but the reasons remain unknown. We investigated within-field spatial variations in straighthead disease severity, As species in rice husks and in soil porewater, microbial composition and abundance of arsM gene encoding arsenite S-adenosylmethionine methyltransferase in two paddy fields. The spatial pattern of disease severity matched those of soil redox potential, arsM gene abundance, porewater DMA concentration, and husk DMA concentration in both fields. Structural equation modelling identified soil redox potential as the key factor affecting arsM gene abundance, consequently impacting porewater DMA and husk DMA concentrations. Core amplicon variants that correlated positively with husk DMA concentration belonged mainly to the phyla of Chloroflexi, Bacillota, Acidobacteriota, Actinobacteriota, and Myxococcota. Meta-omics analyses of soil samples from the disease and non-disease patches identified 5129 arsM gene sequences, with 71% being transcribed. The arsM-carrying hosts were diverse and dominated by anaerobic bacteria. Between 96 and 115 arsM sequences were significantly more expressed in the soil samples from the disease than from the non-disease patch, which were distributed across 18 phyla, especially Acidobacteriota, Bacteroidota, Verrucomicrobiota, Chloroflexota, Pseudomonadota, and Actinomycetota. This study demonstrates that even a small variation in soil redox potential within the anoxic range can cause a large variation in the abundance of As-methylating microorganisms, thus resulting in within-field variation in rice straighthead disease. Raising soil redox potential could be an effective way to prevent straighthead disease.