Route and regulation of zinc, cadmium, and iron transport in rice plants ( L.) during vegetative growth and grain filling; Metal transporters, metal speciation, grain Cd reduction and Zn and Fe biofortification

Yoneyama, Tadakatsu*; Ishikawa, Satoru*; Fujimaki, Shu

Zinc (Zn), iron (Fe) concentrations in rice grains harvested under different levels in soil are known to change only within a small range, while cadmium (Cd) concentrations show greater changes. To clarify the mechanisms underlying such difference, we synthesized information on the routes of metal transport and accumulation in rice plants. At grain-filling, Zn and Cd ascending in xylem are transferred to the phloem at stem nodes. Grain Fe is largely derived from the leaves by remobilization. Zn and Fe concentrations in phloem-sap and grains are regulated within a small range, while Cd concentrations vary depending on xylem supply. To increase concentrations of the metal chelators are effective in increasing grain Zn and Fe concentrations. The elimination of OsNRAMP5 Cd-uptake transporter and the enhancement of root-cell vacuolar Cd sequestration reduce uptake and root-to-shoot transport, respectively, resulting in a reduction of grain Cd accumulation.