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Petal size is a critical factor in plant reproduction and horticulture, and is largely determined by cell expansion. Gerbera hybrida is an important horticultural plant and serves as a model for studying petal organogenesis. We have previously characterized GhWIP2, a Trp-Ile-Pro (WIP)-type zinc protein, that constrains petal size by suppressing cell expansion. However, the underlying molecular mechanism remains largely unclear. Using yeast two-hybrid screening, bimolecular fluorescence complementation, and co-immunoprecipitation, we identified a TEOSINTE BRANCHED1/CYCLOIDEA/PROLIFERATING CELL FACTOR (TCP) family transcription factor, GhTCP7, that interacts with GhWIP2 both in vitro and in vivo. Using reverse genetic approaches, we elucidated the function of the GhTCP7-GhWIP2 complex in controlling petal expansion. GhTCP7 overexpression severely reduced cell expansion and petal size, whereas GhTCP7 silencing resulted in increased cell expansion and petal size. GhTCP7 showed similar expression patterns to GhWIP2 in various types of G. hybrida petals. We further identified GhIAA26 , which encodes an auxin signalling regulator, that is activated by the GhTCP7-GhWIP2 complex, leading to the suppression of petal expansion. Our findings reveal a previously unknown transcriptional regulatory mechanism that involves protein-protein interactions between two different transcription factor families to activate a negative regulator of petal organogenesis. [ABSTRACT FROM AUTHOR]