Heavy metal uptake in rice is regulated by pH-dependent iron plaque formation and the expression of the metal transporter genes

The uptake of metal ions in soil depends primarily on the soil pH, but the effects of pH on individual heavy metals are not well understood. We found the effects of pH on rice uptake of heavy metals were quite different and the behavior of Zn and Cd were comparable, similar to Mn, but opposite of Fe. The concentration of Zn and Cd increased in plants when the pH was raised from 4.5 to 6.5, and then decreased at a pH of 7.5. Concentrations of Mn increased significantly when the pH was increased to 7.5. Iron plaque on root surfaces induced by high pH was likely related to the absorption and uptake of heavy metal elements, such as Cd and Zn, by the plant. Shoot Cd, Mn, and Zn concentrations were strongly positively correlated with DCB-extractable Cd (R-2 = 0.98), Mn (R-2 = 0.97), and Zn (R-2 = 0.76) on the root surface. By contrast, Fe deposition on the root surface inhibited Fe uptake. Additionally, the pH-dependent expression of OsZIPs and OsNRAMPs affected metal uptake under different pH conditions, including OsZIP1, OsIRT1 and OsNRAMP5. Using a genetic approach that used Osnramp 5 mutants, we revealed the regulation of OsNRAMP 5 expression is the primary factor affecting the concentration of Cd and Mn in rice under different pH. Collectively, our research indicates that the uptake of heavy metals was regulated by pH-depended iron plaque formation and expression of metal transporter genes in rice.