Chrysanthemum lavandulifolium homolog ClCYCA2;1 modulates cell division in ray florets

Morphology of ray florets in chrysanthemums is tightly associated with cell division and cell expansion, both of which require proper cell cycle progression. Here we identified a Chrysanthemum lavandulifolium homolog ClCYCA2;1, whose expression in ray florets is negatively correlated with petal width in C. lavandulifolium. Two TCP transcription factors in CYCLOIDEA2 (CYC2) family, ClCYC2a interacts with and stabilizes ClCYC2b and the latter can bind to the promoter of ClCYCA2;1 to activate its transcription. Overexpression of ClCYCA2;1 in C. lavandulifolium reduces the size of capitula and ray florets. Cytological analysis reveals that ClCYCA2;1 overexpression inhibits both cell division and cell expansion via repressing mitotic cell cycle in ray florets whose latitudinal development was more negatively influenced leading to increased ratios of petal length to width at later developmental stages. Yeast two hybrid library screening reveals multiple ClCYCA2;1 interacting proteins including ARP7, and silencing ClARP7 inhibits the development of ray florets. Co-immunoprecipitation assays confirm that ClCYCA2;1 can induce the degradation of ClARP7 to inhibit the development of ray florets. Taken together, our study constitutes a regulatory network containing ClCYC2b-ClCYCA2;1-ClARP7 in ray floret development via governing mitosis, which may facilitate breeding efforts targeted for novel ornamental traits of chrysanthemums.