GIGANTEA antagonizes SWI/SNF protein BAF60 to regulate flowering in Chrysanthemum morifolium

Flowering represents a critical transition from vegetative to reproductive growth in plants. For the ornamental species chrysanthemum (Chrysanthemum morifolium), the precise regulation of its flowering time is of particular importance. GIGANTEA (GI) is a conserved plant-specific protein whose expression is photoperiod-regulated and which plays a key role in flowering time control. However, its function in short-day plants such as chrysanthemum remains poorly understood. This study demonstrates that CmGI functions as a flowering repressor in chrysanthemum and exhibits rhythmic expression in the shoot apex. CmGI binds directly to the promoter of the floral integrator SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (CmSOC1) and represses its expression. We further found that CmGI can interact with BAF60, a subunit of the SWI/SNF chromatin remodeling complex. Overexpression of CmBAF60 resulted in early flowering, whereas knockdown lines showed delayed flowering. Genetic evidence demonstrated that CmGI antagonizes CmBAF60 to regulate chrysanthemum flowering through CmSOC1. Additionally, CmBAF60 increased H3K4me3 deposition and chromatin accessibility at the CmSOC1 promoter, whereas CmGI reduced them. Therefore, our work reveals that CmGI fine-tunes the flowering process in chrysanthemum by antagonizing CmBAF60 to co-regulate H3K4me3 deposition and chromatin accessibility at the CmSOC1 promoter.