Mutation of OsNramp5 reduces the partitioning of soil-derived Cd to grains of lower-node tillers in ratoon rice.

Enhanced cadmium (Cd) uptake in lower-node tillers increases the contamination risk in ratoon rice. Using OsNramp5 mutants and seasonal Cd treatments, we quantified the contributions of stubble-redistributed Cd and newly soil-derived Cd to ratoon tillers at different nodal positions. OsNramp5 loss-of-function reduced grain Cd content by 55.4%-66.8% in the ratoon season by decreasing Cd uptake and translocation to grains. Compared with their wild-type, the mutants exhibited 16.3%-26.3% lower grain Cd concentrations at lower nodes than at upper nodes under exogenous Cd treatment. Upper-node tillers accounted for 84.0%-97.7% of the stubble-redistributed Cd, whereas soil-derived Cd preferentially accumulated in lower-node tillers, increasing their share of Cd accumulation by 12.4-26.0 percentage points relative to stubble-redistributed Cd. The expression levels of OsNramp1, OsHMA2, and OsHMA3 were also downregulated in mutant adventitious roots, suggesting a possible feedback response associated with reduced Cd transport. Overall, OsNramp5 loss-of-function reduces both Cd uptake and translocation, thereby providing a genetic strategy for mitigating Cd contamination in ratoon rice systems.