OsAAP3, a candidate gene governing methylmercury upward translocation in rice

Methylmercury (MeHg), a potent neurotoxin, efficiently bioaccumulates in rice grains, constituting a major dietary exposure pathway in mercury contaminated regions worldwide. This study elucidates the molecular mechanisms governing MeHg uptake and root to shoot translocation by comparing high accumulator (H699) and low accumulator (H777) rice cultivars under realistic soil MeHg stress (0.9 ug g-1). Root transcriptome analysis revealed starkly divergent adaptive responses. H699 displayed extensive reprogramming of cell wall biogenesis and remodeling, detoxification pathways, transmembrane transport systems, and core metabolic networks to confer enhanced tolerance, whereas H777 predominantly 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 via aap3 mutants conclusively demonstrated that OsAAP3, an amino acid permease, is a critical facilitator of root to shoot MeHg translocation. Under environmentally relevant exposure (1 ng g-1), the mutant maintained comparable 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 blueprint for breeding low accumulation varieties and developing targeted agronomic interventions to minimize MeHg risks in the global rice food chain within Hg impacted agroecosystems.