The peptide CmGAST1 integrates calcium and gibberellin signaling to regulate flowering in chrysanthemum

The decision to flower in chrysanthemum (Chrysanthemum morifolium) is controlled by the photoperiod imposed by the outside environment along with endogenous gibberellin levels. Small peptides have broad and critical functions throughout the plant life cycle, but whether and how small peptides are involved in photoperiod- and gibberellin-dependent regulation of flowering remain unclear. Here, we demonstrate that a GIBBERELLIC ACID-STIMULATED TRANSCRIPT (GAST) peptide family member, CmGAST1, promotes flowering in chrysanthemum by interacting with SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 9 (SPL9), a key regulator of flowering in the age-dependent pathway. CmGAST1 expression was induced under short-day photoperiods and by gibberellin treatment. In addition, we show that a negative regulator of GA signaling GIBBERELLIC ACID INSENSITIVE (GAI) interacts with CALMODULIN 7 (CAM7), a key factor in calcium signaling, and the resulting CmCAM7-GAI complex directly suppresses CmGAST1 expression. Notably, short-day photoperiods induce the accumulation of bioactive gibberellins and Ca2+ in the shoot apex, thereby inhibiting CmGAI and CmCAM7, respectively, and releasing their inhibition of CmGAST1 expression. We propose that the peptide CmGAST1 integrates gibberellin and calcium signals, coordinating the photoperiod and aging pathways to accelerate chrysanthemum maturation and flowering.