Genome-wide identification of the HIPP gene family in Populus alba and functional validation of PalHIPP26L in resistance to Dothiorella gregaria

As an economically and ecologically important tree genus, poplars ( Populus spp. ) play a critical role in environmental protection. However, poplars are highly susceptible to a wide range of pathogens, among which the poplar canker fungal pathogen, Dothiorella gregaria Sacc. (D. gregaria) , is one of the most destructive. The molecular mechanisms underlying poplar resistance to D. gregaria remain poorly understood. In this study, we performed a genome-wide identification and systematic characterization of the heavy metal–associated isoprenylated plant protein (HIPP) gene family in Populus alba , identifying a total of 42 members. Comprehensive bioinformatic analyses were performed to elucidate their phylogenetic relationships, exon/intron structures, conserved protein domain architectures, and cis-regulatory elements in promoter regions. Expression profiling based on transcriptomic data, together with qRT-PCR validation, revealed distinct gene-expression patterns in response to stress, particularly to infection with D. gregaria and Botrytis cinerea . Functional analyses also revealed key effects of phytohormones, including salicylic acid (SA), jasmonic acid (JA), and zeatin-riboside (ZR). We found that the HIPP gene, PalHIPP26L , was significantly down-regulated following D. gregaria infection. Overexpression of PalHIPP26L significantly enhanced susceptibility to D. gregaria , and transcriptomic analysis further demonstrated that it suppresses defense-related gene expression. We also showed that PalMYB44 directly repressed PalHIPP26L expression, thereby enhancing poplar resistance to D. gregaria . Collectively, these findings advance our understanding of fungal disease resistance mechanisms in poplar, identify candidate genes with potential applications in breeding programs, and establish a useful foundation for the genetic improvement of woody plant species.