Ethylene restrains brassinosteroid-driven petal growth through RhMPK7-mediated phosphorylation of RhBZR1 in rose

Precise control of cell expansion kinetics is fundamental to plant organogenesis. This process requires the dynamic integration of growth-promoting and growth-inhibiting signals to maintain morphological homeostasis. While brassinosteroids (BRs) promote growth via the transcription factor BRASSINAZOLE RESISTANT 1 (BZR1), the mechanisms that prevent deregulated overgrowth during rapid expansion remain elusive. We find that RhBZR1 promotes rose (Rosa hybrida) petal enlargement by specifically accelerating the rate, rather than prolonging the duration, of cell expansion. Conversely, RhMPK7 functions as a key repressor of petal enlargement. RhMPK7 phosphorylates RhBZR1 at Ser-173, impairing its DNA-binding affinity and transcriptional activity; consequently, plants expressing the non-phosphorylatable RhBZR1^S173A variant display significantly enlarged petals. We further show that ethylene simultaneously activates RhMPK7 and stabilizes the RhMPK7-RhBZR1 interaction. Genetic analyses confirm that RhMPK7 restricts petal expansion specifically by antagonizing RhBZR1, and this module is essential for ethylene-mediated growth suppression. These results indicate a phosphorylation-based 'speed governor' that dynamically restrains hormone-induced rapid cell expansion, offering a new paradigm for the posttranslational gating of organ size.