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.