Natural variation in OsZIP10 promoter contributes to grain zinc accumulation in rice

Zinc (Zn) biofortification of rice grain is critical for addressing global Zn deficiency and improving nutritional security. However, the molecular mechanisms governing grain Zn content (GZC) remain poorly understood. Here, we demonstrated that the plasma membrane-localized OsZIP10 is a key contributor to GZC and Zn deficiency tolerance (ZDT). OsZIP10 exhibits high expression in the phloem of flag leaf sheath, xylem and vascular bundle sheath of node I, and developing grain. Knockout (KO) of OsZIP10 impaired Zn root-to-shoot translocation and distribution and increased sensitivity to Zn deficiency. Conversely, OsZIP10 overexpression (OE) enhanced Zn root-shoot translocation and distribution to grain, thereby elevating GZC, without compromising yield and altering concentrations of other essential metal elements. Replacing the commonly used maize UBI promoter by the endosperm-specific EnhGt13a promoter to drive OsZIP10 greatly increases GZC in the transgenic lines. The F-group bZIP transcription factors (F-bZIPs) OsbZIP48/49/50 bind to the OsZIP10 promoter to activate its expression and regulate GZC. We identified a novel binding site in the promoter of OsZIP10 for F-bZIPs, B1-like motif, which is widely present in many genes involved in Zn homeostasis in rice and other cereal crops. A natural A-to-G variation in the B1-like motif of the OsZIP10 promoter enhances its binding by OsbZIP48/49/50, thereby increasing OsZIP10 expression and GZC. Collectively, OsZIP10 is a pivotal transporter for Zn distribution to grain with good potential for Zn biofortification in rice. The binding of F-bZIPs to the B1-like and/or ZDRE (ZDRE-like) motifs is likely a conserve molecular mechanism for Zn homeostasis regulation in cereal crops.