Differential roles of manganese and zinc on cadmium accumulation in rice: Mechanisms involving transport, antioxidant responses, and gene regulation

Cadmium (Cd) accumulation in rice poses a serious threat to food safety. While soil amendments with manganese (Mn) and zinc (Zn) can reduce Cd accumulation in rice, their distinct modes of action remain unclear, hindering the optimization of mitigation strategies. This study aimed to clarify the differential physiological and molecular mechanisms by which Mn and Zn mitigate Cd toxicity in rice. In a pot experiment using two rice genotypes with differing Cd-accumulation capacities, plants were exposed to Cd stress alongside MnSO₄ (150 and 300 mg.kg⁻1) or ZnSO₄ (50 and 100 mg.kg⁻1) amendments. Results showed that although soil-available Cd content was unaffected, both treatments differentially reduced Cd concentrations in plant tissues. Mn and Zn application reduced grain Cd by 26.46–33.11% and 43.67–52.60%, respectively. Notably, Mn strongly inhibited the root-to-shoot transport of Cd, reducing the translocation factor by up to 56.46% and 47.30% in the two genotypes. Conversely, a low dose of Zn increased this transport by 1.54- to 1.76-fold. Correlation analyses indicated that Zn levels in soil and rice tissues were negatively associated with root Cd, whereas Mn levels were negatively correlated with shoot Cd. Both amendments alleviated root oxidative stress and enhanced antioxidant enzyme activity. Furthermore, they differentially regulated the expression of key genes involved in Cd transport. This study demonstrates that while both Mn and Zn mitigate Cd accumulation, their dominant mechanisms differ: Zn primarily reduces Cd entry and enhances its retention in roots, while Mn predominantly restricts its upward movement to shoots. These insights provide a scientific foundation for developing tailored micronutrient management strategies to improve rice safety in Cd-contaminated environments.