Identification of the SAUR Gene Family in Pinus massoniana and Analysis of Its Expression Patterns Under Drought Stress

P. massoniana is an important native economic and ecological tree species in southern China, where seasonal drought has emerged as a critical factor limiting its productivity. The SAUR gene family, recognized as core early auxin-responsive genes, plays a crucial role in balancing plant growth, development, and stress adaptation; however, research related to this family in conifers remains limited. Utilizing the chromosome-level genome of P. massoniana, this study identified 73 SAUR genes (PmSAUR1~73) through bioinformatics methods, systematically analyzing the physicochemical properties of the encoded proteins, chromosomal localization, phylogenetic relationships, gene structures, and cis-acting elements. Combined with transcriptome sequencing and molecular experiments, the drought stress response patterns of these genes were further elucidated. The results indicated that PmSAUR genes predominantly encode alkaline proteins, primarily localized in mitochondria and nuclei, with an uneven distribution across nine chromosomes, where tandem duplication serves as the primary mechanism driving family expansion. Phylogenetic analysis classified these genes into seven subfamilies, which include both conserved clades homologous to angiosperms and branches specific to P. massoniana. All members contain the Auxin_inducible conserved domain, with motif1 identified as the core essential motif. Promoter regions were enriched with MeJA (methyl jasmonate)-responsive (56%), ABA-responsive, and drought stress-related cis-elements. Under drought stress, 38 PmSAUR genes exhibited diverse temporal expression patterns. Four key genes (PmSAUR14, PmSAUR28, PmSAUR54, and PmSAUR73), which are localized in the nucleus and exhibit high expression specifically in male cones or roots, were identified. These genes exhibit an expression pattern consistent with an auxin-negative response (i.e., repressed by IAA and induced by drought) and display a distinctive response pattern characterized by drought-induced upregulation coupled with IAA-mediated downregulation. This mechanism may contribute to the drought adaptation strategies of P. massoniana, involving regulatory processes for aboveground reproduction and adaptation of the underground root system. This study represents the first effort to elucidate the evolutionary characteristics and drought response patterns of the SAUR gene family in P. massoniana, thereby addressing the existing research gap regarding the functions of SAUR genes in coniferous trees. Furthermore, it offers candidate gene resources and theoretical support for the molecular breeding of stress resistance in P. massoniana. In addition, two auxin-induced SAUR genes (PmSAUR22 and PmSAUR37) were identified as contrasting examples, but the main focus of this study is on the four auxin-repressed genes.