CmBES1 is a regulator of boundary formation in chrysanthemum ray florets.

Chrysanthemum (Chrysanthemum morifolium) is an ideal model species for studying petal morphogenesis because of the diversity in the flower form across varieties; however, the molecular mechanisms underlying petal development are poorly understood. Here, we show that the brassinosteroid transcription factor BRI1-EMS-SUPPRESSOR 1 (CmBES1) in chrysanthemum (C. morifolium cv. Jinba) is important for organ boundary formation because it represses organ boundary identity genes. Chrysanthemum plants overexpressing CmBES1 displayed increased fusion of the outermost ray florets due to the loss of differentiation of the two dorsal petals, which developed simultaneously with the ventral petals. RNA-seq analysis of the overexpression lines revealed potential genes and pathways involved in petal development, such as CUP-SHAPED COTYLEDON (CUC2), CYCLOIDEA 4 (CYC4), genes encoding MADS-box transcription factors and homeodomain-leucine zippers (HD-Zips) and auxin pathway-related genes. This study characterizes the role of CmBES1 in ray floret development by its modulation of flower development and boundary identity genes in chrysanthemum.