Candidate Gene Involved in Methylmercury Translocation from Roots to Shoots in Rice
Methylmercury (MeHg), a potent neurotoxin, efficiently bioaccumulates in rice grains, serving as a major dietary exposure pathway in mercury-contaminated regions worldwide. This study elucidates the molecular mechanisms of MeHg uptake and root-to-shoot translocation by comparing high-accumulator (H699) and low-accumulator (H777) rice cultivars under realistic soil MeHg stress. Root transcriptome analysis revealed divergent adaptive responses: H699 showed extensive reprogramming of cell wall biogenesis and remodeling, detoxification pathways, transmembrane transport systems, and core metabolic networks to enhance tolerance, whereas H777 mainly activated stimulus perception and defense signaling cascades. Several transporter genes, notably OsAAP3, OsSULTR1.2, and OsYSL2, emerged as strong candidates for mediating MeHg flux across cellular compartments. Functional validation with aap3 mutants demonstrated that OsAAP3, an amino acid permease, critically facilitates root-to-shoot MeHg translocation. Under environmentally relevant exposure, the mutant maintained similar root MeHg concentrations but exhibited markedly reduced shoot and grain accumulation, resulting in alleviated growth inhibition, increased biomass production, and improved root system architecture. These findings delineate transporter-driven MeHg partitioning in rice, providing a mechanistic basis for breeding low-accumulation varieties and developing targeted agronomic interventions to minimize MeHg risks in the global rice food chain within Hg-impacted agroecosystems.