Phytochrome B (phyB)， the primary red light photoreceptor， promotes photomorphogenesis in Arabidopsis by interacting with the basic helix-loop-helix transcriptional factor PIF3 and inducing its phosphorylation and degradation. Heterotrimeric G proteins are known to regulate various developmental processes in plants and animals. In Arabidopsis， the G-protein β subunit AGB1 is known to repress photomorphogenesis. However， whether and how phyB and AGB1 coordinately regulate photomorphogenesis are largely unknown. Here we show that phyB physically interacts with AGB1 in a red light-dependent manner and that AGB1 interacts directly with PIF3. Moreover， we demonstrate that the AGB1-PIF3 interaction inhibits the association of PIF3 with phyB， leading to reduced phosphorylation and degradation of PIF3， whereas the phyB-AGB1 interaction represses the association of PIF3 with AGB1， resulting in enhanced phosphorylation and degradation of PIF3. Our results suggest that phyB and AGB1 antagonistically regulate PIF3 stability by dynamically interacting with each other and PIF3. This dynamic mechanism may allow plants to balance phyB and G-protein signaling to optimize photomorphogenesis.