Genome-Wide Characterization of bZIP Transcription Factors and Their Drought-Responsive Expression in Astragalus...

Astragalus membranaceus is an important medicinal plant with considerable pharmacological and economic value; however, its growth and productivity are frequently threatened by drought stress. Basic leucine zipper (bZIP) transcription factors play crucial roles in plant growth, development, and abiotic stress responses, yet a comprehensive investigation of the bZIP gene family in A. membranaceus remains unavailable. In this study, 74 bZIP genes (AmbZIPs) were identified in the A. membranaceus genome and classified into 12 subfamilies based on phylogenetic relationships with Arabidopsis thaliana. Analyses of gene structure, conserved motifs, chromosomal distribution, and duplication events revealed high conservation within subfamilies and indicated that segmental duplication was the major driver of AmbZIP family expansion. Codon usage analysis showed that AmbZIP genes exhibited relatively weak codon usage bias, with codon preference predominantly shaped by natural selection rather than mutation pressure. A total of 23 optimal codons were identified, of which 91.3% were A/T-ending codons. Codon adaptability analysis further demonstrated that tobacco possessed the highest codon compatibility among five tested hosts, whereas Escherichia coli exhibited the lowest adaptability, suggesting that plant expression systems may be more suitable for functional studies of AmbZIP genes. Promoter analysis identified numerous cis-acting elements associated with phytohormone signaling and abiotic stress responses, particularly those related to abscisic acid, methyl jasmonate, salicylic acid, and drought responsiveness. Transcriptome analysis and quantitative real-time polymerase chain reaction (qRT-PCR) validation revealed that several AmbZIP genes were significantly induced under drought stress. Among them, AmbZIP46 displayed strong drought-responsive expression, transcriptional activation activity, and exclusive nuclear localization. These findings provide the first comprehensive characterization of the bZIP gene family in A. membranaceus and establish a valuable foundation for elucidating drought-tolerance mechanisms and facilitating molecular breeding in this medicinal plant.