Iron and zinc-mediated regulation of cadmium transport in the rice (Oryza sativa L.) stem-leaf-grain pathway

Iron (Fe) and zinc (Zn) effectively inhibit cadmium (Cd) translocation in rice, although the exact mechanisms underlying this are not fully understood. This study investigated the effects of Fe/Zn co-application on cadmium (Cd) accumulation and translocation in rice using pot and hydroponic experiments. Fe-Zn application at the tillering stage reduced grain Cd by 66 % (from 0.38 to 0.13 mg/kg, P < 0.01), surpassing the 17 % reduction achieved by seedling-stage pre-treatment alone. Fe-Zn co-treatment decreased labile Cd fractions in foliage, with H₂O-extractable Cd (hydro-soluble organic acid-bound) and NaCl-extractable Cd (pectinate/protein-bound) declining by 67 % and 50 %, respectively, compared to that in control. The presence of Fe and Zn enhanced Fe plaque formation and fibrous root development, expanding root cross-sectional area by 51 % (increased by 337 cm²), increasing fibrous root number by 27 %, and length by 3.9 folds. Metabolic analysis revealed that phytohormone pathways alterations, specifically a 63 % increase in allantoin and 2,3-dihydroxybenzoic acid (P < 0.05), coupled with 30 % and 46 % decreases in threonine and hydroxyphenylacetic acid (P < 0.05), which potentially mitigated Cd toxicity. The strategy of combining Fe-Zn pre-treatment and its application at the tillering stage offer dual advantages by effectively restricting Cd mobility through physical barriers (Fe plaques), chemical immobilization (labile Cd reduction), and metabolic regulation, while simultaneously improving micronutrient content. This approach provides a practical agronomic solution for minimising Cd-related food safety risks in contaminated paddy fields.