OVATE family proteins (OFPs) constitute a class of transcription factors regulating various developmental processes in plants. Nevertheless, their precise regulatory functions in melon (Cucumis melo L.) fruit development remain elusive. In this study, we identified expression profiling of melon OFP genes and revealed the molecular function of CmOFP6-19b gene mediating fruit size variation. Quantitative analysis revealed predominant CmOFP expression in reproductive organs (female/male flowers and ovaries), with distinct differential expression patterns observed among paralogs. Through melon genetic transformation, we revealed that CmOFP6-19b gene functions as a negative regulator in fruit enlargement. Overexpression of the CmOFP6-19b 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 CmKNOX16 in melon produced smaller fruits, phenocopying the CmOFP6-19b-Oe lines. Quantitative real-time PCR (RT-qPCR) analysis showed negative correlation between CmOFP6-19b/CmKNOX16 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 CmOFP6-19b and CmKNOX16 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 CmOFP 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.

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