Genome-wide identification, Characterization, and Expression Analysis of the CsBAG family in Citrus sinensis (L.) Osbeck

Background : 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 processes, encompassing plant growth and development and response to biotic and abiotic stress. However, despite advancements in our understanding of plant BAGs, the underlying mechanisms remain unelucidated. Therefore, this study aims to systematically examine citrus BAG and their respond to hormonal treatment and Huanglongbing 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 fruit trees. We identified and characterized 10 CsBAGs and eight CsBAGs, revealing their distribution across five of the nine citrus chromosomes. Results: All 10 proteins exhibited a characteristic BAG domain. CsBAG2, 4, 5, and CsBAG8 possess an additional ubiquitin-like domain, while CsBAG1 and CsBAG6 feature a calmodulin-binding motif (IQ motif). Most CsBAGs are 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 involvement of these genes in defense against biotic and abiotic stresses, photoperiodic control, hormonal responses, growth, and development. This notion was further supported by gene expression analysis, revealing varying degrees of responsiveness to treatment with plant hormones (GA ₃ and ABA) and infections with the citrus Huanglongbing (HLB) pathogen Candidatus Liberibacter asiaticus ( C Las). Segmental duplications contributed to the expansion of the CsBAG gene family in citrus. Conclusions : Our findings suggest that certain members of the CsBAG gene family may have roles in stress response and pathogen immunity. 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 of citrus HLB, laying a theoretical and practical foundation for the future rational utilization of BAG genes.