Overexpression of SmZIP11 gene can enhance the absorption and transfer of heavy metals in poplar

Objective Woody plants are cost-effective and friendly sources for the remediation of heavy metal-contaminated soils. Investigating the function and the effect of gene SmZIP11, a member of zinc-regulated transporter and iron-regulated transporter-like protein family, on the uptake and translocation of heavy metals will provide valuable gene resources for plant remediation strategies.

Methods The 60-day-old rooted cuttings of willow (Salix matsudana var. matsudana f. umbraculifera Rehd.) were subjected to nutrient solutions containing 200 µmol/L ZnSO₄, 100 µmol/L CuSO₄, and 100 µmol/L CdSO₄, respectively. At the 0, 1, 4, 7, 14, and 21 days of stress, root, stem, and leaf samples were collected for the RNA extraction and determination of the expression levels of SmZIP11 by quantitative real-time PCR (qRT-PCR). Then, the SmZIP11 genes were transfected into Populus × canescens through the leaf disk method, and eight transgenic Populus × canescens plants (OEs) were constructed. Three OE plants with high SmZIP11 expression levels and the wild Populus × canescens plant (WT) were subjected to the heavy metal stress for 14 days, then the biomass and heavy metal contents were determined for the calculation of tolerance index and translocation factor.

Results The coding region of the SmZIP11 gene spans 1053 bp, encoding 351 amino acids with a molecular weight of 37.71 kDa. It harbors nine conserved transmembrane domains, and encoded protein was located in the cell membrane. Phylogenetic analysis revealed that SmZIP11 was closely related to Salicaceae, while showing more distant relationship to ZIP11 in Euphorbiaceae, Malvaceae, and Umbelliferaceae. SmZIP11 expression in leaves and stems were up-regulated under Zn, Cu, and Cd stress compared to the control. Meanwhile, the expression levels of SmZIP11 in roots, stems, and leaves showed a significant increase under Cd and Zn stress. In transgenic poplars, the SmZIP11 gene significantly enhanced the tolerance and transfer capabilities of Zn and Cu, facilitated Cd transfer from roots to stems and Zn to leaves, and accumulated Cu in roots. Compared to WT, OE7 plants exhibited higher tolerance to Zn, as well as Zn concentration and translocation factor in the aboveground parts.

Conclusion SmZIP11 enhanced tolerance to Cu and Zn, and improved the ability to transfer Cd and Zn from the root system to the aboveground parts. These findings will provide genetic resources and theoretical guidance for phytoremediation of soil heavy metal pollution.