Phylogenetic analysis of cyclin-dependent kinase (CDK) genes and the role of PbCDK9, PbCDK17, and PbCDK18 in cell proliferation in pear fruit

Cyclin-dependent kinases (CDKs) critical regulators of cell proliferation; however, their role in fruit size remains largely unexplored. This study identified 35 CDK genes from the pear genome, which were categorized into eight groups. The expression levels of 15 CDK genes exhibited a correlation with cell number variations during pear fruit development. Combining with the gene expression levels, PbCDK9, PbCDK17, and PbCDK18 were selected for over-expression analysis in pear fruit callus. Transgenic fruit calli displayed a greater number of cells compared to control calli, suggesting the involvement of PbCDK9, PbCDK17, and PbCDK18 in cell proliferation. Moreover, four concentrations of exogenous forchlorfenuron (CPPU), a synthetic cytokinin, were applied to fruitlets on the tree, resulting in a significant increase in fruit size at 1 and 25 mg/L CPPU. Notably, the expression levels of PbCDK17 and PbCDK18 were increased in CPPU-treated fruits compared to untreated fruits. These results suggest that exogenous CPPU treatment promotes cell proliferation by enhancing the expression of PbCDK17 and PbCDK18, leading to increased fruit size. The findings provide a foundational understanding of the regulatory mechanisms underlying fruit size in agricultural production.