Regulation of Tissue-Specific Expression and Screening of Candidate Genes in the HSF Gene Family of Populus wilsonii

Abstract

Heat shock transcription factor (HSF) is a pivotal regulator of plant response to adversity stress; however, its functional diversity and evolutionary mechanism in woody plants require further analysis. In this study, the HSF gene family was systematically identified for the first time in the genome of Populus wilsonii. A total of 30 members (PwiHSF) were identified, which were similar in number compared with that of Populus trichocarpa (31) and significantly higher than that of Iljinskaea planisiliqua (23). These findings reveal the species-specific expansion of the HSF family in Populus. Phylogenetic analysis of the 30 PwiHSFs revealed their classification into three distinct subfamilies: A (12), B (2), and C (16). Subfamily C exhibited the highest prevalence, accounting for up to 53.3% of the total. Subsequent analysis of gene structure and functional domains revealed subfamily differentiation: Subfamily A contains three to four introns and complete functional domains (DBD, OD, CTAD). The acidic amino acid cluster of motif 7 in CTAD may enhance transcriptional activation by recruiting RNA polymerase II. Subfamily B contains one to two introns and lacks CTAD, but retains repressor motif 5. Subfamily C has no introns and retains only DBD, OD, and CTAD. Subfamily B contains one to two introns and lacks the CTAD element but retains the repressor motif 5. In contrast, subfamily C lacks introns and retains only the DBD and OD elements, suggesting that it is dependent on synergistic regulation by interactions. Transcriptome analysis revealed that members of subfamily A (e.g., PwiHSF7/12) exhibited high levels of expression in various tissues, while certain genes (e.g., PwiHSF3/5/6) demonstrated low expression levels in phloem and root tissues, indicative of functional tissue specificity. The protein interaction network further revealed PwiHSF12 as the core hub node and PwiHSF14/30 as the secondary hub, suggesting that it enhances the flexibility of the network through synergistic regulation. This study is pioneering in its analysis of the significance of the HSF family in the context of tissue development.