Abstract
FLOWERING PROMOTING FACTOR1 (FPF1), a small protein without any known domains, promotes flowering in several plants; however, its functional mechanism remains unknown. Here, we characterized 2 FPF1-like proteins, FPL1 and FPL7, which, in contrast, function as flowering repressors in Brachypodium distachyon. FPL1 and FPL7 interact with the components of the florigen activation complex (FAC) and inhibit FAC activity to restrict expression of its critical target, VERNALIZATION1 (VRN1), in leaves, thereby preventing overaccumulation of FLOWERING LOCUS T1 (FT1) at the juvenile stage. Further, VRN1 can directly bind to the FPL1 promoter and repress FPL1 expression; hence, as VRN1 gradually accumulates during the late vegetative stage, FAC is released. This accurate feedback regulation of FPL1 by VRN1 allows proper FT1 expression in leaves and ensures sufficient FAC formation in shoot apical meristems to trigger timely flowering. Overall, we define a sophisticated modulatory loop for flowering initiation in a temperate grass, providing insights toward resolving the molecular basis underlying fine-tuning flowering time in plants.