The CmABI5-CmKN1 module regulates seasonal growth and flowering in response to chilling in chrysanthemum

Perennial plants have evolved complex regulatory networks that allow them to perceive environmental changes and trigger adaptive growth responses. Many perennial herbs exhibit a rosette growth habit during winter, and then undergo extension growth/stem elongation growth and flowering in the following spring. However, the mechanisms underlying this seasonal transition are still not well understood. Here, using the perennial herb chrysanthemum (Chrysanthemum morifolium), we identified CmKN1, a class I KNOTTED (KN1)-like homeobox transcription factor, that mediates extension growth and flowering competence in response to prolonged chilling exposure. The rosette growth of chrysanthemum rhizomes in late autumn coincides with downregulation of CmKN1 expression. Overexpression of CmKN1 promotes premature rhizome extension growth and flowering, whereas the kn1 heterozygous mutant shows suppression of these processes. Furthermore, seasonal growth regulation involves the abscisic acid (ABA) signaling pathway. Shoot apices in the rosette state accumulate higher levels of ABA than those in the extension growth state. Notably, in response to ABA, CmKN1 expression is downregulated by CmABI5. Our results demonstrated that the CmABI5-CmKN1 module mediates extension growth and flowering competence in response to chilling, thereby facilitating the perennial growth habit of chrysanthemum.