Divergent roles of FT-like 9 in flowering transition under different day lengths in Brachypodium distachyon

Timing of reproductive transition is precisely modulated by environmental cues in flowering plants. Facultative long-day plants, including Arabidopsis and temperate grasses, trigger rapid flowering in long-day conditions (LDs) and delay flowering under short-day conditions (SDs). Here, we characterize a SD-induced FLOWERING LOCUS T ortholog, FT-like 9 (FTL9), that promotes flowering in SDs but inhibits flowering in LDs in Brachypodium distachyon. Mechanistically, like photoperiod-inductive FT1, FTL9 can interact with FD1 to form a flowering activation complex (FAC), but the floral initiation efficiency of FTL9-FAC is much lower than that of FT1-FAC, thereby resulting in a positive role for FTL9 in promoting floral transition when FT1 is not expressed, but a dominant-negative role when FT1 accumulates significantly. We also find that CONSTANS 1 (CO1) can suppress FTL9 in addition to stimulate FT1 to enhance accelerated flowering under LDs. Our findings on the antagonistic functions of FTL9 under different day-length environments will contribute to understanding the multifaceted roles of FT in fine-tune modulation of photoperiodic flowering in plants.