Genome-wide identification, characterization, and expression pattern of the CsBAG family in Citrus sinensis (L.) Osbeck under pathogen and hormone stress

B-cell lymphoma 2 (Bcl-2)-associated athanogene (BAG) proteins constitute a multifunctional family of co-chaperone regulators. They play pivotal roles in modulating various biological processes, encompassing plant growth and development and response to biotic and abiotic stresses. However, despite advancements in our understanding of plant BAGs, the underlying mechanisms remain unelucidated. Therefore, this study aimed to systematically investigate citrus BAG and their responses to hormone treatment and Huanglongbing (HLB) infection. In this study, we conducted a genome-wide in silico analysis of the CsBAG gene family in a globally significant citrus crop to explore its potential roles in sweet orange (Citrus sinensis). We identified and characterized eight CsBAG genes encoding ten transcripts, and these genes were distributed across five of the nine citrus chromosomes. All ten encoded proteins exhibited a characteristic BAG domain. CsBAG2, 4, 5, and CsBAG8 possessed an additional ubiquitin-like domain, while CsBAG1 and CsBAG6 feature a calmodulin-binding motif (IQ motif). Most CsBAGs were predicted to be localized in the nucleus, mitochondria, or chloroplasts. Phylogenetic analysis revealed four major clusters, further categorized into G1–G4 groups. Cis-regulatory elements within all CsBAG promoters were identified and categorized, and the associated transcription factors were predicted. The findings suggest the potential involvement of these genes in defense against biotic and abiotic stresses, photoperiodic regulation, hormonal responses, growth, and development. This notion was further supported by gene expression analysis, revealing varying degrees of responsiveness to treatment with plant hormones (GA₃ and ABA) and infection with the citrus HLB pathogen Candidatus Liberibacter asiaticus (CLas). Segmental duplications contributed to the expansion of the CsBAG gene family in citrus. Our findings suggest that certain members of the CsBAG gene family may play roles in stress response and pathogen immunity in citrus. This study could help to comprehensively analyze the characteristics of the citrus BAG gene family, and the results will offer additional target genes for molecular disease resistance breeding against citrus HLB, laying a theoretical and practical foundation for the future rational utilization of BAG genes.