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Upon exposure to light,developing seedlings undergo photomorphogenesis,as illustrated by inhibition of hypocotyl elongation,cotyledon opening,and leaf greening.During hypocotyl photomorphogenesis,light signals are sensed by multiple photoreceptors,among which the red/far-red light-sensing phytochromes have been extensively studied.However,it is not fully understood how the phytochromes modulate hypocotyl growth.Here,we demonstrated that HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES 1 (HOS1),which is known to either act as E3 ubiquitin ligase or affect chromatin organization,inhibits the transcriptional activation activity of PHYTOCHROME INTERACTING FACTOR 4 (PIF4),a key transcription factor that promotes hypocotyl growth.Consistent with the negative regulatory role of HOS1 in hypocotyl growth,HOS1-defective mutants exhibited elongated hypocotyls in the light.Notably,phyB induces HOS1 activity in inhibiting PIF4 function.Taken together,these observations provide a molecular basis for the phyB-mediated suppression of hypocotyl growth in Arabidopsis.