The B-box Protein CmBBX8 Recruits Chromatin Modifiers CmFDM2/CmSWI3B to Induce Flowering in Summer Chrysanthemum.

The transition from vegetative to reproductive growth is essential for the flowering process of plants. In summer chrysanthemum, CmBBX8 exploits prominence function in floral transition by activating the expression of CmFTL1. However, how CmBBX8 induces CmFTL1 during the photoperiod inductive cycles remains unknown. Here, we show that CmBBX8 interacts with the SGS3-like protein CmFDM2, and the CmFDM2 overexpression strains presented early flowering, while knockdown strains delayed flowering. Additionally, CmFDM2 could bind to the CmFTL1 promoter and activate the expression of CmFTL1, and associate with chromatin remodeling factor CmSWI3B, and CmBBX8 induces flowering dependent on CmFDM2 and CmSWI3B. CmFDM2 also partially depends on CmSWI3B. The CmSWI3B knockdown strains exhibited a significant late flowering phenotype. Interestingly, CmBBX8 also interacts with CmSWI3B. Moreover, the level of H3K27me3 at the CmFTL1 locus was reduced when CmBBX8 and CmFDM2/CmSWI3B occupied the locus to promote chrysanthemum flowering during the photoperiod inductive cycles, which was accompanied by the increasing level of CmFTL1 transcripts. Thus, our work provides novel insights into the gradually increasing level of CmFTL1 for the floral transition through CmBBX8 recruiting chromatin modifiers CmFDM2/CmSWI3B.