An auxin-induced transcriptional cascade CmBES1-CmSAUR66 orchestrates the ray floret development in <i>Chrysanthemum morifolium</i>.

Abstract

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 <i>Chrysanthemum morifolium</i>, 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 <i>CmSAUR66</i> as a negative regulator of corolla tube fusion. Functional studies showed that <i>CmSAUR66</i> knockdown enhances corolla tube fusion by promoting dorsal petal elongation. Mechanistically, <i>CmSAUR66</i> acts by modulating the spatiotemporal expression of the dorsoventral asymmetry gene <i>CmCYC2c</i>; knocking down <i>CmCYC2c</i> in the amiR-<i>CmSAUR66</i> background rescues the fusion phenotype. We found that sustained auxin treatment promotes corolla tube fusion while suppressing <i>CmSAUR66</i> expression, and knocking down this gene alters auxin sensitivity. Importantly, we found that the auxin-induced transcription factor CmBES1 directly represses <i>CmSAUR66</i> expression. Consistent with this, transient overexpression of <i>CmSAUR66</i> in OX-<i>CmBES1</i> 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.