OVATE family gene <i>CmOFP6-19b</i> negatively regulates fruit size in melon (<i>Cucumis melo</i> L.)

OVATE family proteins (OFPs) constitute a class of transcription factors regulating various developmental processes in plants. Nevertheless, their precise regulatory functions in melon (<i>Cucumis melo</i> L.) fruit development remain elusive. In this study, we identified expression profiling of melon <i>OFP</i> genes and revealed the molecular function of <i>CmOFP6-19b</i> gene mediating fruit size variation. Quantitative analysis revealed predominant <i>CmOFP</i> expression in reproductive organs (female/male flowers and ovaries), with distinct differential expression patterns observed among paralogs. Through melon genetic transformation, we revealed that <i>CmOFP6-19b</i> gene functions as a negative regulator in fruit enlargement. Overexpression of the <i>CmOFP6-19b</i> gene resulted in reduced fruit size, while its downregulation led to increased fruit size. Bimolecular fluorescence complementation and yeast two-hybrid assays confirmed nuclear-localized physical interaction between CmOFP6-19b and CmKNOX16. Overexpression of <i>CmKNOX16</i> in melon produced smaller fruits, phenocopying the <i>CmOFP6-19b</i>-Oe lines. Quantitative real-time PCR (RT-qPCR) analysis showed negative correlation between <i>CmOFP6-19b</i>/<i>CmKNOX16</i> expression level and fruit size, with peak expression levels observed in a cultivar displaying minimal longitudinal diameter. The results of histological section and expression analysis suggest that <i>CmOFP6-19b</i> and <i>CmKNOX16</i> may affect melon fruit size by regulating genes related to cell division and cell expansion. In conclusion, our findings systematically characterized the phylogenetic architecture and expression divergence of <i>CmOFP</i> genes, and elucidated the function and molecular mechanism of CmOFP6-19b-CmKNOX16 regulatory module in mediating melon fruit development, providing a theoretical foundation for melon breeding.