Histological and transcriptomic analysis provides new insights into the regulatory mechanism of floral organogenesis in sunflower.

The initiation of floral organ primordia at the peripheral zone of the floral meristem (FM) is a fundamental process in flower development, yet its regulatory mechanisms in sunflower remain unclear. In this study, we characterized a sunflower space-mutagenized mutant, crazy broccoli 1 (cb1), in which FM-like structures fail to generate floral organs and maintain meristematic activity throughout development. This defect leads to floral development malformation, resulting in an indeterminate FM-like structure consisting of successive whorls of bracts and meristems. Through comparative transcriptomics and expression profile analysis of wild type and cb1 plants, we analyzed the expression dynamics of key gene families in regulating flower development, including APETALA2 (AP2), MADS-box, WUSCHEL (WUS), and LEAFY (LFY). We identified LFY, SHORT VEGETATIVE PHASE (SVP), AGAMOUS (AG), and WUS as central regulators of floral organogenesis. In cb1, elevated LFY and SVP-like transcription represses AG-like expression, which in turn relieves the repression on WUS-like. Sustained WUS-like activity prevents FM termination, leading to repeated formation of meristem-bract whorls. Conversely, in wild type plants, low LFY and SVP-like levels permit high AG-like expression, which suppresses WUS-like transcription and ensures timely FM termination and normal floral organogenesis. These findings provide clues for understanding the molecular mechanism underlying FM activity and floral organogenesis in sunflower.