Rice Cultivation Reshapes Mercury Accumulation in Soil-Plant Systems

Predicting the bioaccumulation of potent neurotoxic methylmercury (MeHg) in rice is essential for assessing and mitigating human exposure risks, particularly for Asians and infants. However, significant uncertainties lie in understanding and predicting MeHg accumulation in rice, partially due to the insufficient understanding of how rice plants regulate Hg biogeochemistry in soil-rice systems. Here, we conducted a pot experiment involving 32 commonly cultivated rice varieties under the same soil and ambient conditions. By analyzing MeHg contents in rice-paddy systems and amino acid profiles in grains, we demonstrate that rice varieties impact soil MeHg production through secreting differential root exudates and regulate MeHg uptake by modulating plant biomass and grain amino acids. Importantly, we provide evidence that rice cultivars impact MeHg accumulation by regulating the amounts of specific amino acids and antioxidant pigments, which quench reactive oxygen species and subsequently inhibit in vivo MeHg demethylation, reshaping tissue MeHg concentrations and MeHg translocation. Meanwhile, in vivo demethylation may serve as a primary source of grain inorganic Hg (IHg), which explains the positive correlation between grain MeHg and IHg. Our study underscores the critical yet overlooked role of plants in Hg transformations, offering valuable insights for elucidating MeHg bioaccumulation.