CmYAB2 transcriptional module regulates fruit morphogenesis in Cucumis Melo

Melon (Cucumis melo L.) is a globally important horticultural crop valued for its favorable qualities and economic value. The YABBY transcription factor family, known for dual activator/repressor functions in regulating secondary metabolites and developmental processes, remains functionally uncharacterized in melon. Here, we identified seven CmYAB proteins containing conserved C2C2 zinc-finger and YABBY domains. Transcriptome profiling and RT-qPCR results revealed specific expression of CmYAB2 in the melon bisexual flowers and developing fruits. We performed melon genetic transformation and found that CmYAB2 overexpression significantly increased fruit vertical and horizonal diameters. Transcriptome sequencing of transgenic melons suggested that CmYAB2 affects melon fruit size by mediating cell division/expansion related genes and hormone pathway regulators. Performing yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays, we revealed the direct protein interaction between CmYAB2 and CmYAB5b transcription factors in plant nucleus. Then, we also found that CmYAB2 protein can directly bind to the promoter of a melon fruit size regulator gene CmSUN25-26-27c and inhibits its expression. This work elucidates the role of CmYAB2 transcriptional module in regulating melon fruit development and provides critical genetic resources for further molecular breeding.