In flowering plants, light is one of the major environmental stimuli that determine the timing of the transition from the vegetative to reproductive phase. In Arabidopsis, phytochrome B (phyB); phyA; cryptochrome 2; and FLAVIN-BINDING, KELCH REPEAT, F-BOX 1 are major photoreceptors that regulate flowering. Unlike phyA; cryptochrome 2; and FLAVIN-BINDING, KELCH REPEAT, F-BOX 1, phyB delays flowering mainly by destabilizing the CONSTANS (CO) protein, whose reduction leads to decreased expression of a florigen gene, FLOWERING LOCUS T. However, it remains unclear how the phyB-mediated CO destabilization is mechanistically regulated. Here, we identify a unique PHYTOCHROME-DEPENDENT LATE-FLOWERING (PHL) gene, which is mainly involved in the phyBdependent regulation of flowering. Plants with mutant phl exhibited a late-flowering phenotype, especially under long-day conditions. The late-flowering phenotype of the phl mutant was completely overridden by a phyB mutation, indicating that PHL normally accelerates flowering by countering the inhibitory effect of phyB on flowering. Accordingly, PHL physically interacted with phyB both in vitro and in vivo in a red light-dependent manner. Furthermore, in the presence of phyB under red light, PHL interacted with CO as well. Taken together, we propose that PHL regulates photoperiodic flowering by forming a phyB-PHL-CO tripartite complex.light signaling | FT