Cadmium distribution in ratoon rice: OsNramp5 mutant reduces accumulation while physiological factors modulate node-specific partitioning.

Cadmium (Cd) accumulation in ratoon rice poses significant food-safety risks and complicates the sustainable expansion of ratooning systems. This study elucidates the physiological mechanisms governing Cd accumulation patterns in ratoon rice by comparing two genotypes contrasting in Cd accumulation under controlled Cd exposure. OsNramp5 loss-of-function reduced grain Cd by 39.5-51.2% across the main and ratoon seasons, accompanied by downregulation of OsNramp1 (root uptake) and OsHMA2 (xylem loading) and enhanced leaf Cd chelation. Across the ratoon crop, upper-node tillers accumulated 30.8-59.5% more grain Cd than lower-node tillers, attributable to 13.4-40.4% higher transpiration rates and 41.0-45.5% greater grain-filling rates. Lower-node tillers exhibited late-season Cd accumulation surges coinciding with OsLCT1/OsCCX2 upregulation and peak transpiration rates (7.17-8.38 mmol·m⁻²·s⁻¹). Overall, our findings support OsNramp5 loss-of-function as a practical genetic option to reduce grain Cd in ratoon rice system. Upper-node tillers acted as preferential Cd sinks due to early-season vigor, prolonged grain-filling, and elevated gas-exchange capacity, amplifying ratoon grain contamination risks.