Comprehensive analysis of bZIP transcription factors uncovers their roles during dimorphic floret differentiation and stress response in Cleistogenes songorica

BACKGROUND: Transcription factors act as important regulators of transcription networks. Basic leucine zipper (bZIP) transcription factors have been shown to be involved in multiple biological processes in plants. However, no information is available for the bZIP family in Cleistogenes songorica, which is an important xerophytic and allotetraploid grass in desert grasslands. RESULTS: In this study, 86 CsbZIPs were identified in the allotetraploid C. songorica genome. For location analysis, CsbZIPs were distributed evenly across two subgenomes of C. songorica. Phylogenetic tree analysis among three species indicated that CsbZIPs were evolutionarily more closely related to OsbZIPs than AtbZIPs. Syntenic and phylogenetic analyses confirmed that the CsbZIPs were mainly expanded by whole-genome duplication events. Furthermore, it was determined that rice and C. songorica might have undergone purified selection during their long evolutionary history by calculating the Ks values and Ka/Ks ratios of orthologous gene pairs. By analysing the expression patterns of CsbZIPs in different tissues and under abiotic stresses, 21 CsbZIP genes were differentially expressed between chasmogamous (CH) and cleistogamous (CL) flowers, including two FLOWERING LOCUS D (FD) genes. In shoots and roots, 79.1 and 87.2% of the CsbZIP genes, respectively, displayed transcript changes under at least one stress treatment, such as heat, cold, drought and salt. Strikingly, 17 common CsbZIP genes showed differential expression under stress response and during CL flowering. Co-expression network, GO annotation and real-time quantitative reverse transcription PCR (qRT-PCR) analyses revealed a close relationship between CL flowering-associated genes and abiotic stress-related genes. CONCLUSIONS: BZIP TFs were comprehensively analysed and identified in allotetraploid C. songorica. Our results provide insights into the evolutionary history of the bZIP family in C. songorica and provide abiotic stress-responsive and CL-associated candidate CsbZIP genes for potential applications in the genetic improvement of plants.