Transcription Factor ClTCP4 Regulates Leaf Size and Wavy Margin in Leaf Morphogenesis of Citrullus lanatus

The formation of leaf morphology is crucial for the normal role of leaves in various growth and development processes of plants. Citrullus lanatus (watermelon), is an important economic crop and its leaf development mechanism is still not well understood. In this study, a CIN-TCP transcription factor, ClTCP4, was identified as participating in the process of watermelon leaf morphogenesis. ClTCP4 knockout lines (CR lines) generated by the CRISPR/Cas9 system show a wavy leaf margin. The levels of auxin, gibberellin, and cytokinin hormones were elevated in CR lines, and transcriptome analysis indicated altered gene expression in the hormone metabolism pathways mentioned above. At the cellular level, the functional loss of ClTCP4 causes smaller cells and an increased cell number in the leaves of CR lines. A negative regulatory relationship between ClTCP4 and ClWOX1 were determined with the tobacco transient overexpression system. Leaf margin regulator "ClWOX1" may inhibit the transcriptional activation effect of ClTCP4 on the leaf margin suppressor "ClNGAL1" in the process of leaf morphology formation. This study revealed the regulatory role of ClTCP4 in leaf morphogenesis in watermelon plants, and provided a theoretical basis and gene resources for improving leaf morphology in watermelon.