ZINC TRANSPORTER5 and ZINC TRANSPORTER9 Function Synergistically in Zinc/Cadmium Uptake

Abstract

The elements Zinc (Zn) and cadmium (Cd) have similar chemical and physical properties, but contrasting physiological effects in higher organisms. In plants, Zn/Cd transport is mediated by various transporter proteins belonging to different families. In this study, we functionally characterized two Zn transporter genes in rice (<i>Oryza sativa</i>), <i>ZINC TRANSPORTER5</i> (<i>OsZIP5</i>) and <i>ZINC TRANSPORTER9</i> (<i>OsZIP9</i>), which are tandem duplicates and act synergistically in Zn/Cd uptake. Both genes encode plasma membrane-localized proteins with influx transporter activity. The expression profiles of <i>OsZIP5</i> and <i>OsZIP9</i> overlap in the root epidermis and respond to the local Zn status in the root. However, <i>OsZIP9</i> is also regulated by systemic signals of Zn status from the shoot. OsZIP5 functions redundantly to OsZIP9, but has a relatively weaker effect. Plants with the knockout mutations <i>oszip5</i>, <i>oszip9</i>, or <i>oszip5oszip9</i> show impaired Zn/Cd uptake. The decreased Zn/Cd levels and growth retardation in the <i>oszip5</i> mutant are less severe than in the <i>oszip9</i> mutant. However, the double mutant <i>oszip5oszip9</i> showed an enhanced Zn deficiency phenotype compared with the single mutants, and few double-knockout plants were able to survive the entire growth cycle without excessive Zn supply. Transgenic plants overexpressing <i>OsZIP9</i> had markedly enhanced Zn/Cd levels in the aboveground tissues and brown rice. The results of our study fill a gap in current knowledge of Zn uptake and improve our understanding of Zn/Cd accumulation in rice.