An auxin-induced transcriptional cascade CmBES1-CmSAUR66 orchestrates the ray floret development in Chrysanthemum...

Sympetaly (fused petals) is a key morphological innovation that has arisen multiple times during evolution. It occurs via one of two developmental pathways: late sympetaly (from initially separate petal primordia that later fuse) or early sympetaly (directly from a shared ring-like primordium). Nevertheless, the genetic basis of corolla tube formation in early-sympetalous lineages remains largely unknown. In Chrysanthemum morifolium, an early-sympetalous species, ray floret corolla tube formation depends specifically on dorsal petal elongation. Through integrated genome-wide association studies and RNA-seq analyses, we identified CmSAUR66 as a negative regulator of corolla tube fusion. Functional studies showed that CmSAUR66 knockdown enhances corolla tube fusion by promoting dorsal petal elongation. Mechanistically, CmSAUR66 acts by modulating the spatiotemporal expression of the dorsoventral asymmetry gene CmCYC2c; knocking down CmCYC2c in the amiR-CmSAUR66 background rescues the fusion phenotype. We found that sustained auxin treatment promotes corolla tube fusion while suppressing CmSAUR66 expression, and knocking down this gene alters auxin sensitivity. Importantly, we found that the auxin-induced transcription factor CmBES1 directly represses CmSAUR66 expression. Consistent with this, transient overexpression of CmSAUR66 in OX-CmBES1 plants significantly rescued the fusion phenotype. Our study thus unveils the CmBES1-CmSAUR66 regulatory module as a central component of auxin-mediated sympetalous development, providing insights into the hormonal control of floral morphological evolution.