SaZIP4, an uptake transporter of Zn/Cd hyperaccumulator Sedum alfredii Hance

Sedum alfredii Hance is a zinc (Zn)/cadmium (Cd) hyperaccumulator plant. However, the molecular mechanism of the Zn/Cd uptake of roots and shoots has not been thoroughly elucidated to date. In this work, two isoforms of the putative Zn/Cd transporter of S. alfredii Hance, SaZIP4, were investigated. SaZIP4h and SaZIP4n were cloned from the hyperaccumulating and non-hyperaccumulating ecotypes of S. alfredii, respectively. Transcriptional analysis, subcellular localization analysis, yeast functional complementation analysis and transgenic plants were used to characterize SaZIP4. The transcription levels of SaZIP4h are significantly and constitutively higher than those of SaZIP4n, and the expression levels of SaZIP4h and SaZIP4n in the roots were highly induced by Zn deficiency. A subcellular localization analysis indicated that both SaZIP4h and SaZIP4n are localized to the plasma membrane. In addition, expressing SaZIP4h and SaZIP4n in the yeast mutant ZHY3 can reverse the Zn uptake deficiency. However, expressing SaZIP4h alone in the yeast mutant Δzrc1 increased sensitivity to Cd. Transgenic Arabidopsis thaliana mutant zip4-2 expressing SaZIP4h reversed the Zn/Cd uptake defect, and wild-type A. thaliana ectopically overexpressing SaZIP4h displayed increased Zn accumulation both in roots and shoots. Together, these results suggest that SaZIP4 is an important Zn uptake transporter that takes up Zn in the roots and shoots of S. alfredii. These findings help to elucidate the molecular mechanisms of heavy metal and micronutrient uptake and accumulation in hyperaccumulator plants.