Coordinated brassinosteroid and auxin signaling modulates ray floret ligule and leaf morphology in <i>Gerbera hybrida</i>

Inflorescences in the Asteraceae plant family contain distinct floret types. The peripheral ray florets typically develop large and showy blade-like ligules that promote the reproductive success of Asteraceae by contributing to the flower-like appearance of the inflorescences. So far, most studies have focused on understanding the regulation of ray floret identity, while the late differentiation of ligules is poorly understood. Here, we show that the coordinated action of 2 phytohormones, brassinosteroids (BRs) and auxin, plays an essential role in defining the final size and shape of the ligules in gerbera (Gerbera hybrida). Epibrassinolide treatment or precocious activation of BR signaling by ectopic or inducible overexpression of gerbera BZR1/BES1 HOMOLOG 2 (GhBEH2) converted the flat ligules into curved structures. The curved phenotype was associated with upregulation of the auxin biosynthetic gene GhYUCCA2a (GhYUC2a) and differential growth of the adaxial and abaxial epidermal cells. A similar curved phenotype was observed in auxin-treated ligules. Moreover, co-silencing of GhBEH1 and GhBEH2 paralogs reduced the ligule size by affecting cell expansion. We further demonstrate that GhBEH2 directly binds the GhYUC2a promoter to activate its transcription. Altogether, our results indicate that ligule morphogenesis requires coordinated action of BR and auxin signaling. Our functional data reveal that this coordination is also necessary for late differentiation of gerbera leaves and petals and leaves of Arabidopsis (Arabidopsis thaliana), suggesting a conserved mechanism in regulating the differentiation of flat organs across distinct plant families.