Novel Insights into Hg⁰ Oxidation in Rice Leaf: Catalase Functions and Transcriptome Responses

Rice leaves can assimilate atmospheric mercury (Hg⁰), which is accumulated by grains and causes health risks to rice consumers. However, the molecular mechanisms underlying Hg⁰ assimilation in rice leaves remain poorly understood. Here, we investigated catalase's (CAT) function in Hg⁰ oxidation within rice leaves, as well as the Hg speciation and transcriptomic profiles of rice leaves exposed to Hg⁰. The inactivation of catalase reduced Hg⁰ oxidation by 91% in the leaf homogenate and the Hg⁰ oxidation rate increased along with CAT activity, showing the CAT's function in Hg⁰ oxidation. Hg⁰ was converted to Hg(cysteine)₂ complexes in the leaf. Transcriptomic results revealed that the expression levels of both OsCATA and OsCATB (catalase-encoding genes) increased with Hg concentration, suggesting the involvement of catalase-related molecular network in Hg⁰ oxidation. Upstream transcription factors, including NAC (NAM-no apical meristem, ATAF-Arabidopsis transcription activation factor, and CUC-cup-shaped cotyledon), and ethylene-responsive transcription factor, are likely involved in catalase expression. Genes related to cysteine metabolism and amino acid transport appeared to regulate Hg accumulation. Our findings demonstrate the important function of catalase in Hg⁰ oxidation within rice and are fundamental for developing genetically modified rice cultivars to minimize human Hg exposure health risks.