NtZIP11, a new Zn transporter specifically upregulated in tobacco leaves by toxic Zn level

Understanding the molecular mechanisms governing the uptake and accumulation of Zn in the leaves of tobacco plants exposed to high Zn concentrations is important from the perspective of phytoremediation of metal contaminated soil. This study identifies a new candidate gene, NtZIP11, which may contribute to Zn accumulation in tobacco leaves. NtZIP11 encodes a protein of 346 amino acids, with characteristic conserved sequences of the ZIP family of proteins. Phylogenetic analysis shows that NtZIP11 forms a distinct clade with other ZIP11 proteins. Transient expression of GFP-tagged ZIP11 in the abaxial epidermis of tobacco leaves demonstrates localization at the plasma membrane. Yeast complementation tests and growth assays indicate that NtZIP11 is involved in Zn but not Fe, Mn and Cd uptake. NtZIP11 complements the zrt1zrt2 mutant deficient in Zn uptake, but not the fet3fet4 mutant deficient in Fe uptake. Nor does it modify the sensitivity of wild-type yeast, DY1457, to increasing concentrations of Fe, Mn and Cd. NtZIP11 is expressed in both roots and leaves, with transcript abundance increasing with age. Noteworthy, the expression level of NtZIP11 is not modulated by Zn-deficiency but is highly upregulated especially in the older leaves by high Zn concentrations (50 and 200 µM). Our data indicate that the primary role for NtZIP11 is in the uptake of Zn by the cells of tobacco leaves specifically when experiencing high Zn concentrations. It also likely contributes to maintaining a basal supply of Zn to cells at the level of the whole plant. Thus the present study contributes to broadening our understanding of Zn homeostasis mechanisms in tobacco, and clarifying the role of ZIP11 genes