Characterization of three tandem-duplicated calcium binding protein (CaBP) genes and promoters reveals their roles in the phytohormone and wounding responses in citrus

Accumulated evidences have revealed the critical roles of calcium binding protein (CaBP) in growth and stress responses of plants. However, its function in woody plants is poorly understood. In this study, we cloned the CDS, gDNA and promoter sequences of three tandem-duplicated CaBPs (CsCaBP1, CsCaBP2 and CsCaBP3) from Citrus sinensis, analyzed their sequence characteristics, and investigated their gene expression patterns and promoter activities under treatments of CaCl2, several phytohormones and wounding. Results showed that the three CsCaBPs have high sequence similarity. Their expression was strongly induced by CaCl2, ethylene, jasmonic acid, salicylic acid and wounding, and the promoting effect of wounding on their expression was found to be partially ethylene-dependent. Consistently, we identified many phytohormone-related cis-acting elements in their promoters, and their promoter activity could be induced significantly by ethylene, jasmonic acid, salicylic acid and wounding. All the three CsCaBPs can interact with WRKY40, whose encoding gene showed a similar expression pattern to CsCaBPs under phytohormone and wounding treatments. In addition, CsERF14, CsERF21, CsERF3 and CsERF2 could bind to their promoters. The results obtained in this study indicated that the three duplicated CsCaBPs were functionally redundant and played similar roles in the phytohormone and wounding responses of C. sinensis.