Transcription factor. Involved in responses to transient and long-term shade. Required for the light-mediated inhibition of hypocotyl elongation. Necessary for rapid light-induced expression of the photomorphogenesis- and circadian-related gene APRR9. Seems to play a role in multiple PHYB responses, such as flowering transition and petiole elongation. Homodimer (Probable). Interacts with APRR1/TOC1. Etiolated seedlings. Expressed with a circadian rhythm showing peaks during the light period. Up-regulated by simulated shade in light-grown plants, in a phytochrome-dependent manner; low red/far-red ratio (R/FR) light, but repressed by a high R/FR light. Rapidly down-regulated after seedling deetiolation.