The CmYAB1-CmCUC2 transcriptional module shapes the iconic ray florets in Chrysanthemum morifolium.

Sympetaly (fused petals) is a major innovation driving floral diversity, yet its genetic basis in the highly successful Asteraceae family is elusive. We therefore investigated the molecular mechanisms controlling this trait in Chrysanthemum morifolium. We conducted functional analyses of candidate genes, examined their spatiotemporal expression patterns, and performed biochemical assays to investigate the interaction between the transcription factors CmYAB1 and CmCUC2. Abaxially expressed CmYAB1 regulates floret fusion: its suppression enhanced fusion, while its overexpression caused petal splitting and curling. CmCUC2, a NAM/ATAF1/2/CUC2 (NAC) transcription factor and homolog of the Arabidopsis boundary-specific genes CUC1/2, was downregulated in CmYAB1-silenced lines. Interestingly, its expression shifts dynamically from early floral organ boundaries to later abaxial petal domains, overlapping CmYAB1. CmCUC2 knockdown also increased fusion, while its overexpression led to unfused, curved petals. CmYAB1 directly activates CmCUC2 by binding its promoter, and transient CmCUC2 knockdown in CmYAB1-overexpressing plants rescued the unfused phenotype. Furthermore, silencing either gene upregulated and sustained CmCYC2c expression in dorsal petals, correlating with their elongation. This study delineates the CmYAB1-CmCUC2 regulatory pathway, thereby revealing a novel module of polarity and boundary genes that controls sympetalous development in Asteraceae, advancing our understanding of floral diversity, and enabling the targeted engineering of ornamental traits.