AtTIP2;2 facilitates resistance to zinc toxicity via promoting zinc immobilization in the root and limiting root-to-shoot zinc translocation in Arabidopsis thaliana

Abstract

Zinc (Zn) is an essential micronutrient for plants. However, excess Zn is toxic to non-accumulating plants like Arabidopsis thaliana. To cope with Zn toxicity, non-accumulating plants need to keep excess Zn in the less sensitive root tissues and restrict its translocation to the vulnerable shoot tissues, a process referred to as Zn immobilization in the root. However, the mechanism underlying Zn immobilization is not fully understood. In Arabidopsis, sequestration of excess Zn to the vacuole of root cells is crucial for Zn immobilization, facilitated by distinct tonoplast-localized transporters. As some members of the aquaporin superfamily have been implicated in transporting metal ions besides polar but non-charged small molecules, we tested whether Arabidopsis thaliana tonoplast intrinsic proteins (AtTIPs) could be involved in Zn immobilization and resistance. We found that AtTIP2;2 is involved in retaining excess Zn in the root, limiting its translocation to the shoot, and facilitating its accumulation in the leaf trichome. Furthermore, when expressed in yeast, the tonoplast-localized AtTIP2;2 renders glutathione (GSH)-dependent Zn resistance to yeast cells, suggesting that AtTIP2;2 facilitates the across-tonoplast transport of GSH-Zn complexes. Our findings provide new insights into aquaporins' roles in heavy metal resistance and detoxification in plants.