Context-Dependent Zinc Regulation of Cadmium Uptake, Transport, and Accumulation in Rice

Zinc (Zn) is an essential micronutrient for plants and is often considered antagonistic to cadmium (Cd) because the two metals have similar chemical properties and partially overlapping transport pathways. However, Zn does not uniformly suppress Cd accumulation in rice. Depending on soil conditions, Zn nutritional status, application strategy, and genotype, Zn can decrease, exert little effect on, or promote Cd transfer to aboveground tissues and grain. This review examines the mechanistic basis of these contrasting responses across the soil–root–shoot–grain continuum through four control steps: (i) soil Cd supply, (ii) root uptake and sequestration, (iii) root-to-shoot transport, and (iv) grain-directed allocation. Competitive sorption, redox-dependent mineral transformations, and sulfide formation and oxidation modify soil Cd availability. In plants, Zn can restrict Cd entry through partially shared uptake pathways, while Cd entry through comparatively Zn-insensitive pathways can persist and increased Zn supply can facilitate root-to-shoot transport. Root sequestration and vascular redistribution further regulate Cd delivery to grain. Uptake antagonism can therefore coexist with transport facilitation, and the final grain Cd response reflects the net balance among these control steps. Soil pH and redox history, relative Zn/Cd availability, Zn dose and nutritional status, application route and timing, and genotype influence which step dominates. Controlled mechanistic evidence and field responses are distinguished when assessing agronomic applicability. This synthesis provides a basis for evaluating Zn management while recognizing potential trade-offs among grain Zn enrichment, grain Cd reduction, and yield maintenance.