Hypocotyl development in Arabidopsis thaliana is regulated by light and endogenous hormonal cues,making it an ideal model to study the interplay between light and endogenous growth regulators.BBX21,a B-box (BBX)-like zinc-finger transcription factor,integrates light and abscisic acid signals to regulate hypocotyl elongation in Arabidopsis.Heterotrimeric G-proteins are pivotal regulators of plant development.The short hypocotyl phenotype of the G-protein β-subunit (AGB1) mutant (agb1-2) has been previously identified,but the precise role of AGB1 in hypocotyl elongation remains enigmatic.Here,we show that AGB1 directly interacts with BBX21,and the short hypocotyl phenotype of agb1-2 is partially suppressed in agb1-2bbx21-1 double mutant.BBX21 functions in the downstream of AGB1 and overexpression of BBX21 in agb1-2 causes a more pronounced reduction in hypocotyl length,indicating that AGB1 plays an oppositional role in relation to BBX21 during hypocotyl development.Furthermore,we demonstrate that the C-terminal region of BBX21 is important for both its intracellular localization and its transcriptional activation activity that is inhibited by interaction with AGB1.ChIP assays showed that BBX21 specifically associates with its own promoter and with those of BBX22,HY5,and GA2ox1.which is not altered in agb1-2.These data suggest that the AGB1-BBX21 interaction only affects the transcriptional activation activity of BBX21 but has no effect on its DNA binding ability.Taken together,our data demonstrate that AGB1 positively promotes hypocotyl elongation through repressing BBX21 activity.