Cell expansion in petals plays a crucial role in flower opening and final size in rose (Rosa hybrida), which largely determines its market value. While cell expansion is known to be closely associated with gibberellins (GAs), the underlying molecular mechanism remains elusive. Here, we measured the levels of GAs during flower opening and demonstrated that GA₃ treatment significantly increases petal size. Moreover, we identified RhMYB70, an R2R3 MYB transcription factor, whose expression was inhibited by GA₃ treatment. RhMYB70 silencing resulted in larger petals and petal cell size than those of TRV control. Through transcriptome analysis and biochemical identification, RhMYB70 could directly bind to the promoter of the cellulose synthase gene RhCESA8 and repress its transcription, thereby resulting in decreased cellulose content of petals and final size. In addition, we also identified the GA biosynthesis gene RhGA3ox3 as an RhMYB70 target and demonstrated that RhMYB70 directly binds to and inhibits the promoter activity of RhGA3ox3, leading to decreased cellulose content of petals and petal size. Besides, knocking down RhMYB70 expression not only resulted in increasing GA₁ and GA₃ levels in petals compared to TRV but also elevated cellulose content. Together, our findings reveal that the feedback regulation of GAs and RhMYB70 signaling fine-tunes cell expansion and petal size by modulating cellulose content of rose petals, providing genetic targets for improving rose flower quality.

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