Screening for low-methylmercury rice cultivars and transcriptomic insight into underlying accumulation mechanisms

Methylmercury (MeHg) contamination in rice plants (Oryza sativa L.) poses significant health risks to consumers worldwide. Growing low-MeHg rice cultivars is a practical and efficient strategy to mitigate MeHg risks from paddy fields. However, the mechanistic understanding on the varietal differences in MeHg accumulation remains limited. Here, we performed a field trial in an Hg-polluted paddy field to evaluate MeHg uptake capacity of sixty rice cultivars commonly cultivated in Southwest China. Further, we selected high- and low-MeHg rice cultivars for comparative transcriptomic analysis to reveal molecular mechanisms governing MeHg accumulation in rice. Results revealed distinct varietal accumulation patterns among the studied cultivars. Among them, Oryza sativa L. cv. Xiangliangyouguifuzhan (XGZ) exhibited the lowest MeHg concentration in polished rice (11.89 μg kg^-1), whereas Oryza sativa L. cv. Leyou 891 (L891) showed a significantly higher concentration (41.85 μg kg^-1). Comparative transcriptome analyses between L891 and XGZ revealed that genes related to cysteine and methionine metabolism, glutathione metabolism, and transporter activity might be involved in MeHg accumulation. Specifically, glutathione S-transferases (e.g., GSTU1) and transporter activity-related genes (e.g., OsABCG36) were overexpressed exclusively in L891 upon MeHg exposure, but not in XGZ, suggesting their potential roles in cultivar-specific MeHg accumulation. Our findings enhance the understanding of the molecular basis underlying varietal differences in MeHg accumulation and will be helpful for the management of MeHg risks in Hg-polluted paddy fields by planting low-MeHg rice cultivars.