Nanostructured hydroxyapatite-zinc complex as a novel foliar system to reduce cadmium accumulation in wheat grain: Performance and mechanism

Foliar zinc (Zn) application offers a promising strategy for cadmium (Cd) mitigation and Zn biofortification in wheat, while the poor leaf adhesion of conventional Zn formulations limits their effectiveness. This study utilized hydroxyapatite nanoparticles (nHAP) as a nanocarrier and prepared the nHAP-Zn complex through simple reactions. The complex exhibited superior leaf adhesion compared to ZnSO₄ solution. Foliar application of nHAP-Zn increased grain Zn concentration by 38.8 % in the low-Cd-accumulating wheat cultivar Ningmai-11 (NM11) and 31.6 % in the high-Cd-accumulating cultivar Zhengmai-10 (ZM10), surpassing the effects of ZnSO₄. Notably, nHAP-Zn decreased grain Cd concentration by 31.5 % in NM11 and 32.9 % in ZM10. Mechanistically, nHAP-Zn suppressed Cd uptake by downregulating the expression of TaNramp5, TaIRT1, and TaZIP5 genes in the roots, and inhibited Cd translocation from node I to the grain by reducing xylem-to-phloem Cd transfer-related gene expression (i.e., TaHMA2, TaZIP3, and TaZIP7 in NM11, and TaHMA2, TaLCT1, TaZIP5, and TaZIP7 in ZM10). Furthermore, nHAP-Zn application did not compromise plant growth characteristics such as plant biomass and height. These findings highlight nHAP-Zn as a highly efficient foliar fertilizer for enhancing Zn biofortification while mitigating Cd accumulation in wheat cultivated in Cd-contaminated regions. This study presents a novel approach to improving wheat safety and nutritional quality.