Genome-Wide Identification, Evolutionary Analysis, and Expression Profiling of the β-D-Xylosidase Gene Family in Cotton (Gossypium hirsutum) Under PEG-Simulated Osmotic and Salt Stress

β-D-Xylosidases (BXLs) are members of glycoside hydrolase family 3. They play essential roles in cell wall remodeling, plant development, and abiotic stress responses. BXL gene families have been systematically characterized in several crop species. However, a comprehensive analysis of BXL genes in cotton (Gossypium hirsutum), a globally important fiber and oilseed crop, is still lacking. In this study, we performed a genome-wide identification of BXL genes in allotetraploid cotton. A total of 25 GhBXL genes were identified and classified into six phylogenetic clades. Gene structure and conserved domain analyses showed that all GhBXL proteins possess the characteristic tripartite GH3 domain architecture. Chromosomal distribution and synteny analyses indicated that the expansion of the cotton BXL family may be associated with whole-genome duplication and allopolyploidization. Promoter cis-element analysis detected stress-responsive regulatory motifs in the GhBXL promoters, including STRE, W-box, DRE core, and as-1 elements. Under PEG-simulated drought and salt stress, expression profiling, independently confirmed by qRT-PCR, showed distinct temporal response patterns among GhBXL members. Weighted gene co-expression network analysis (WGCNA) further identified GhBXL-8, GhBXL-9, and GhBXL-20 as hub genes in stress-responsive modules. Their co-expressed partners were enriched in transcription factors, kinases, and stress-related proteins. These findings provide a systematic foundation for understanding the evolutionary dynamics and functional roles of BXL genes in cotton. They also highlight candidate genes for future functional investigation.