Abstract
Phytochrome A (phyA) and phyB are red and far-red photoreceptors that interact with PHYTOCHROME-INTERACTING FACTORs (PIFs) via active phyA-binding (APA) or active phyB-binding (APB) motifs. While APB interacts with the N-terminal photosensory module of phyB (phyBPSM), it remains unclear whether APA interacts with phyAPSM. We report that both phyA and phyB interact with APA through C-terminal output module of phy (phyOPM), while phyB interacts additionally with APB through phyBPSM. Marchantia Mp-phy also interacts with PIFs via the phyOPM-APA interaction. The phyBOPM-APA interaction promotes PIF3 degradation but not mutual phyB destruction. The full-length phy-APA interaction is light-dependent, whereas the underlying phyOPM-APA interaction is not. We show that the Pr form, not the Pfr, of phyPSM competes with APA for phyOPM binding, explaining how the light-dependent phy-APA interaction arises from the light-independent phyOPM-APA interaction. Together, our results suggest that the phyOPM-APA interaction is an ancient feature conserved in both Arabidopsis phyA, phyB and Marchantia Mp-phy.