Identifying citrus CBL and CIPK gene families and their expressions in response to drought and arbuscular mycorrhizal fungi colonization

The calcineurin B-like protein (CBL)-interacting protein kinase (CIPK) complex is an essential calcium sensor and contributes to biotic and abiotic stress responses. However, citrus CBL and CIPK gene family members and their underlying roles during drought and arbuscular mycorrhizal fungi (AMF) colonization remain relatively unknown. In the present study, CBLs and CIPKs were characterized in Citrus sinensis by analyzing the presence of specific domains such as the elongation factor (EF)-hand motif in CBLs, and a protein kinase and an Asn-Ala-Phe domain in CIPKs. After mining the C. sinensis genome, we identified 8 CsCBLs and 17 CsCIPKs. Among these genes, three CsCBLs and nine CsCIPKs showed syntenic relationships with the Arabidopsis thaliana homologs AtCBLs and AtCIPKs, respectively. According to gene expression and cis-acting element analysis, all 8 CsCBLs and 16 CsCIPKs were expressed in the roots, where the regulation of expression was not consistent with their promoter cis-elements. Drought treatment remarkably downregulated the expression of CsCBL8 and upregulated CsCBL7, CsCIPK4, and CsCIPK7 expressions. The AMF colonization induced CsCBL4, 5, 6, and 7 as well as CsCIPK2, 4, 10, 11, 13, 14, and 16 expressions and repressed CsCBL1, 2, 3, and 8 and CsCIPK1, 3, 6, 8, 9, and 12 expressions. Based on the expression data and co-expression analysis, CsCBL1-CsCIPK1, CsCBL1-CsCIPK3, CsCBL1-CsCIPK6, and CsCBL1-CsCIPK9 showed the significant positive correlations to drought and AMF responses.