MicroRNA319-regulated TCPs interact with FBHs and PFT1 to activate CO transcription and control flowering time in Arabidopsis

The transcription factor CONSTANS (CO) is a central component that promotes Arabidopsis flowering under long-day conditions (LDs). Here, we show that the microRNA319-regulated TEOSINTE BRANCHED/CYCLOIDEA/PCF (TCP) transcription factors promote photoperiodic flowering through binding to the CO promoter and activating its transcription. Meanwhile, these TCPs directly interact with the flowering activators FLOWERING BHLH (FBHs), but not the flowering repressors CYCLING DOF FACTORs (CDFs), to additively activate CO expression. Furthermore, both the TCPs and FBHs physically interact with the flowering time regulator PHYTOCHROME AND FLOWERING TIME 1 (PFT1) to facilitate CO transcription. Our findings provide evidence that a set of transcriptional activators act directly and additively at the CO promoter to promote CO transcription, and establish a molecular mechanism underlying the regulation of photoperiodic flowering time in Arabidopsis.