Genome-wide identification and expression analysis of the citrus calcium-dependent protein kinase (CDPK) genes in response to arbuscular mycorrhizal fungi colonization and drought

The calcium-dependent protein kinases (CDPKs, also termed CPKs) are a large, diverse protein family that play significant roles in biotic and abiotic stress responses in plants. However, CDPKs in citrus are relatively unknown, as are their responses to drought stress (abiotic stress) and arbuscular mycorrhizal fungi (AMF) colonization. In this study, the citrus CDPKs were characterized by analyzing the presence of EF-hand and Pkinase domains and searching the citrus genome. Twenty-nine Citrus sinensis CDPKs (CsCDPKs) were identified on at least nine chromosomes in the citrus genome. Three clusters were found on chromosome 4 with three or two CsCDPKs, and 17 CsCDPKs showed syntenic relationships with Arabidopsis thaliana CPKs (AtCPKs). Twenty-nine CsCDPKs were expressed in the roots, and the regulation of the CsCDPKs was not always consistent with the cis-elements in their promoters. CsCDPK1, 2, 5, 6, 7, 8, 9, 13, 17, 18, 19, 21, 24, 25, 26, 27 and 29 were upregulated by AMF colonization, and CsCDPK3, 7 and 28 were downregulated. Drought-repressed expression of CsCDPK10, 11, 12, 15 and 16 induced CsCDPK20 and 22. Furthermore, AMF colonization and drought stress exerted synergistic effects; the expression levels of CsCDPK20 and CsCDPK22 were repressed in W_AMF treatment (well-watered with AMF) but induced in D_AMF treatment (drought stress with AMF). Together, these results provide a basis for further functional studies of the CsCDPK proteins and their responses to AMF and drought stress.