MdCDPK24 Encoding Calcium-Dependent Protein Kinase Enhances Apple Resistance to Colletotrichum gloeosporioides

Calcium-dependent protein kinases (CDPKs) are unique serine/threonine kinases that play significant roles in response to environmental stresses in plants. In this study, we comprehensively characterized the CDPK gene family in the apple cultivar ‘Hanfu’ at the genome-wide level, and 38 MdCDPKs were identified. They were unevenly distributed across 14 chromosomes. Based on phylogenetic analysis, the MdCDPKs were classified into four subfamilies. Conserved domain analysis indicated that MdCDPKs contain the catalytic kinase domain and the Ca2+ binding domain. During Colletotrichum gloeosporioides infection, the expression level of MdCDPK24 was significantly upregulated. Subsequently, MdCDPK24 was fused to GFP to generate the MdCDPK24-GFP construct, and confocal microscopy imaging confirmed its cytoplasmic localization in Nicotiana benthamiana leaves. Using agrobacterium-mediated transformation, we generated the overexpression of MdCDPK24 transgenic calli. MdCDPK24-overexpressing calli demonstrated significantly reduced disease severity against C. gloeosporioides infection, indicating its positive role in apple bitter rot resistance. The analysis of the CDPK gene family in the apple cultivar ‘Hanfu’ provides a new insight into the identification of CDPK genes involved in biotic stress. MdCDPK24 represents a promising candidate for genetic manipulation to enhance apple bitter rot resistance.