Molecular Characterization of the IRT Gene Family in Poplar (Populus tremula ‘Erecta')

Iron chlorosis, a common problem in plants grown in the alkaline/calcareous soils, causes yield loss, reduces fruit quality, and destroys aesthetic beauty of plants. Research has verified that iron chlorosis is not directly related to the supply of iron in the soil, but related to the plant’s ability to acquire iron from the soil and to efficiently transport iron in plants. It was demonstrated that iron transport is controlled by a series of iron-regulator transporter genes (IRT). The IRT genes encode iron transporter protein to transport Fe (II) from the root to the leaf where iron is used for synthesis of chlorophyll. In this study, 18 IRT sequences were selected from the genome sequence database of Populus trichocarpa based on the conserved ZIP family domain derived from Arabidopsis AtIRT1 gene. Using homology cloning strategy, candidate genes were amplified from two phenotypes of Populus tremula ‘Erecta’.  One phenotype (wild type) has the classic iron chlorosis symptom (bright yellow leaf with green veins) and the other shows tolerance to iron-deficiency (mutant) under the same soil condition. To date, one candidate gene (PtIRT1) was isolated from the genomic DNA of ‘Erecta’. This PtIRT1 gene is composed of two introns and three extrons. Putative protein structure analysis showed that the PtIRT1 gene contains the conserved ZIP family domain and a signal peptide, suggesting that the PtIRT1 gene has the same gene structure and functional domain as AtIRT1 and AtIRT2 genes. Compared with the IRT genes of other species, the PtIRT1 has 64% homology with the AtIRT1and 77% with the OsIRT1. Expression analysis showed that the PtIRT1 gene did not express in young leaves of ‘Erecta’. Putative protein sequence alignment of the PtIRT1 gene of the wild type to that of the mutant showed that these two genes share 98.05% identity. Hydrophobicity and hydrophilicity analyses indicated that six different regions corresponding to the different amino acid regions in the two PtIRT1 genes were observed. This research will help understand mechanisms of iron uptake and utilization and address iron chlorosis in plants, particularly in woody species in the alkaline/calcareous and waterlogged soils.