Genome-Wide Analysis of the ZRT, IRT-Like Protein (ZIP) Family and Their Responses to Metal Stress in Populus trichocarpa

The ZRT, IRT-like protein (ZIP) family plays an important role in the transport of zinc (Zn) and iron (Fe) across the cell membrane in many different species. However, studies on ZIP family are mainly limited in herbaceous species; hence, we investigated functional divergence of ZIP family in Populus trichocarpa. We identified 21 ZIP genes in P. trichocarpa and classified them into four groups based on phylogenetic analysis. Structural analyses revealed that most of the PtrZIP transporters have eight transmembrane domains (TMDs). PtrZIP members were unequally positioned in 19 P. trichocarpa linkage groups (LGs), with six tandem duplications and four segmental duplications. The promoter regions of PtrZIP genes contain Zn, Fe, copper (Cu), and other metal stress-related cis-elements. Additionally, tissue-specific expression of PtrZIP genes showed that most of them had relatively high expression levels in the root. Quantitative real-time RT-PCR (qRT-PCR) analysis revealed that the expression of PtrZIP genes were induced not only under deficiency or excess condition of Zn, Fe, Cu and manganese (Mn) but also under excess condition of cadmium (Cd) and lead (Pb) stress. These findings indicated that PtrZIP genes may have played potential roles in metal transporters. Genome-wide analysis of PtrZIP genes in P. trichocarpa provided more comprehensive insights on the structure and function of this gene family.