Comparative Analysis of Calcium-Dependent Protein Kinase in Cucurbitaceae and Expression Studies in Watermelon

Both the calcium-dependent protein kinases (CDPKs) and CDPK-related kinases (CRKs) play numerous roles in plant growth, development, and stress response. Despite genome-wide identification of both families in Cucumis, comparative evolutionary and functional analysis of both CDPKs and CRKs in Cucurbitaceae remain unclear. In this study, we identified 128 CDPK and 56 CRK genes in total in six Cucurbitaceae species (C. lanatus, C. sativus, C. moschata, C. maxima, C. pepo, and L. siceraria). Dot plot analysis indicated that self-duplication of conserved domains contributed to the structural variations of two CDPKs (CpCDPK19 and CpCDPK27) in C. pepo. Using watermelon genome as reference, an integrated map containing 25 loci (16 CDPK and nine CRK loci) was obtained, 16 of which (12 CDPK and four CRK) were shared by all seven Cucurbitaceae species. Combined with exon-intron organizations, topological analyses indicated an ancient origination of groups CDPK IV and CRK. Moreover, the evolutionary scenario of seven modern Cucurbitaceae species could also be reflected on the phylogenetic trees. Expression patterns of ClCDPKs and ClCRKs were studied under different abiotic stresses. Some valuable genes were uncovered for future gene function exploration. For instance, both ClCDPK6 and its ortholog CsCDPK14 in cucumber could be induced by salinity, while ClCDPK6 and ClCDPK16, as well as their orthologs in Cucumis, maintained high expression levels in male flowers. Collectively, these results provide insights into the evolutionary history of two gene families in Cucurbitaceae, and indicate a subset of candidate genes for functional characterizations in the future.