Genome-Wide Identification and Expression Analysis of Aquaporin Gene Family in 84K Poplar

Aquaporins (AQPs) mediate the transmembrane transport of water and diverse small molecules, playing essential roles in plant growth and development and stress responses. However, their genome-wide organization and evolution at the subgenome level in hybrid 84K poplar (Populus alba × Populus tremula var. glandulosa) remain incompletely characterized. Based on the telomere-to-telomere genome of 84K poplar, we identified 94 PagAQPs, with 46 and 48 members in the A and G subgenomes, respectively. Phylogenetic analysis classified them into five subfamilies unevenly distributed across 29 chromosomes, with PagPIP and PagTIP being the two largest subfamilies. Most PagAQPs were retained as corresponding A/G allelic pairs, whereas few exhibit subgenome-specific retention. Valid Ka/Ks ratios for 38 allelic pairs and 59 paralogous gene pairs were all below 1, indicating predominant purifying selection. Gene structure, conserved motifs, and substrate-recognition residues showed conservation within subfamilies and divergence among subfamilies. The promoters contained predicted cis-acting elements associated with light, phytohormone and abiotic stress responses. Expression profiling showed that some PagPIP and PagTIP members were highly expressed in vascular tissues and exhibited changes in transcript abundance under nitrogen and drought treatments. Compared with wild-type plants, the 35S:PagPIP2;6A overexpression lines were shorter before treatment and exhibited higher leaf relative water content and lower malondialdehyde content after water withholding. This study systematically characterized the subgenomic distribution, evolutionary features and expression patterns of the PagAQP family in 84K poplar and identified candidate genes for further functional investigation.