Functional characterization of IlHMA2, a P1B2-ATPase in Iris lactea response to Cd

Heavy metal P1B2-ATPases (HMAs) are involved in transport or detoxification of heavy metals to maintain metal homeostasis in plants. Here we report the function of an uncharacterized member of HMAs, IlHMA2, from the monocotyledon Iris lactea under Cd stress. To uncover the role of IlHMA2 in I. lactea response to Cd, subcellular localization of IlHMA2 was determined by confocal imaging and its expression levels were investigated under Cd or other metals conditions by qRT-PCR, and then plant growth, accumulation of Zn and Cd and genes expression level were measured by RNAi-mediated gene silencing. IlHMA2 was predominantly expressed in roots with either excess Cd or Zn deficiency, and the protein was localized to the plasma membrane. IlHMA2-silenced lines were hypersensitive to Cd with severe growth retardation. Further investigations showed that IlHMA2-silenced lines exhibited a dramatic reduction in Cd and Zn concentration in the xylem sap than wild type, which resulted in significantly decrease the root-to-shoot translocation of Cd and Zn. Meanwhile, the expression levels of genes encoding transporters related to Zn homeostasis, including IlZIP2 and IlMTP1, was significantly downregulated in different tissues of IlHMA2-silenced lines compared to wild type. These findings suggest that the plasma membrane transporter IlHMA2 plays a key role in root-to-shoot translocation of Zn and Cd and the maintenance of Zn homeostasis via modulating Zn transport systems, thereby improving its Cd tolerance.