Two B-box proteins orchestrate vegetative and reproductive growth in summer chrysanthemum

Floral transition, the switch from vegetative to reproductive growth, is extremely important for the growth and development of flowering plants. In the summer chrysanthemum, CmBBX8, a member of the subgroup II B-box (BBX) family, positively regulates the transition by physically interacting with CmERF3 to inhibit CmFTL1 expression. In this study, we show that CmBBX5, a B-box subgroup I member comprising two B-boxes and a CCT domain, interacts with CmBBX8. This interaction suppresses the recruitment of CmBBX8 to the CmFTL1 locus without affecting its transcriptional activation activity. CmBBX5 overexpression led to delayed flowering under both LD (long-day) and SD (short-day) conditions, while lines expressing the chimeric repressor gene-silencing (CmBBX5-SRDX) exhibited the opposite phenotype. Subsequent genetic evidence indicated that in regulating flowering, CmBBX5 is partially dependent on CmBBX8. Moreover, during the vegetative growth period, levels of CmBBX5 expression were found to exceed those of CmBBX8. Collectively, our findings indicate that both CmERF3 and CmBBX5 interact with CmBBX8 to dampen the regulation of CmFTL1 via distinct mechanisms, which contribute to preventing the premature flowering of summer chrysanthemum.