Light and the heterotrimeric G-protein are known to antagonistically regulate photomorphogenesis in Arabidopsis.However,whether light and G-protein coordinate the regulation of photomorphogenesis is largely unknown.Here we show that the blue light photoreceptor cryptochrome 1 (CRY1) physically interacts with the G-protein β subunit,AGB1,in a blue light-dependent manner.We also show that AGB1 directly interacts with HY5,a basic leucine zipper transcriptional factor that acts as a critical positive regulator of photomorphogenesis,to inhibit its DNA-binding activity.Genetic studies suggest that CRY1 acts partially through AGB1,and AGB1 acts partially through HY5 to regulate photomorphogenesis.Moreover,we demonstrate that blue light-triggered interaction of CRY1 with AGB1 promotes the dissociation of HY5 from AGB1.Our results suggest that the CRY1 signaling mechanism involves positive regulation of the DNA-binding activity of HY5 mediated by the CRY1-AGB1 interaction,which inhibits the association of AGB1 with HY5.We propose that the antagonistic regulation of HY5 DNA-binding activity by CRY1 and AGB1 may allow plants to balance light and G-protein signaling and optimize photomorphogenesis.