Citrate-Glutamate-Ca2+ Pathway Promoted Cd Sequestration in Rice Root Cell Walls

Cadmium (Cd) contamination in rice threatens food safety. Roots are the main barrier to Cd entry, and this study explored how citrate (CA) inhibits Cd uptake and transport in rice roots. Field and hydroponic experiments showed that CA markedly reduced Cd uptake and translocation, decreasing Cd concentrations in seedlings and grains by 40.2-47.9% and 36.0-54.6%, respectively, reducing noncancerous risks to human health and the ecological risks associated with incorporating Cd-contaminated straw into the soil. Mechanistically, CA enhanced the synthesis of the signaling molecule glutamate (Glu), furthermore triggering the citrate-glutamate-Ca2+ signaling pathway. This pathway upregulates key metabolites and genes involved in the synthesis of cell wall polysaccharides such as cellulose, lignin, and pectin, while reducing pectin methylation, thereby enhancing the Cd barrier and binding capacity in roots. In conclusion, the CA-Glu-Ca2+ signal pathway enhanced Cd immobilization in rice root cell walls, which limited Cd absorption and translocation and ultimately improved the rice grain safety.