Stage-dependent zinc detoxification and allocation in rice revealed by Zn isotopes and transporter expression

Zinc (Zn) is essential for rice but becomes toxic under excess supply, yet the processes governing Zn uptake, long-distance transport, and organ allocation remain unclear. Here we combine Zn stable isotopes (δ⁶⁶Zn), transport efficiencies (φ), and expression of OsZIP4 and OsZIP1 (ZIP-family influx/efflux transporters), and OsHMA2 (a heavy metal ATPase mediating xylem loading) to quantify Zn homeostasis across a Zn gradient at tillering and maturity. Under sufficient Zn, whole-plant δ⁶⁶Zn was lighter than the nutrient, indicating net uptake favoring light isotopes. Under high Zn, whole-plant δ⁶⁶Zn shifted heavier, consistent with preferential exclusion/export of light Zn and supported by strong OsZIP1 induction. Shoots were consistently lighter than roots across treatments and stages, evidencing persistent kinetic fractionation during xylem loading. Development further modulated bulk flux independently of isotopic selectivity: at tillering, high Zn sharply reduced root-to-stem transport efficiency (φ₂/φ₁) while the Zn isotope fractionation factor between root and plant sap (ε₂) changed little, indicating restricted translocation and enhanced root detoxification; at maturity, φ₂/φ₁ remained high with pronounced OsHMA2 upregulation, sustaining long-distance Zn delivery. Meanwhile, Zn excess rerouted Zn away from grains and toward vegetative pools, coinciding with strong OsZIP4 suppression in leaves. These patterns reveal a stage-dependent switch from "restricted transport and root detoxification" at tillering to "maintained transport but altered allocation" at maturity under Zn excess.