Transcription activator. Regulates negatively chlorophyll biosynthesis and seed germination in the dark, and lightinduced degradation of PIF1 relieves this negative regulation to promote photomorphogenesis. Binds to the G-box motif (5'-CACGTG-3') found in many light-regulated promoters. Promotes the expression of SOM, and thus modulates responses to abscisic acid (ABA) and gibberellic acid (GA). Homodimer (Probable). Interacts with the photoactivated conformer (Pfr) of phytochromes A and B, PHYA and PHYB. Interacts also with APRR1/TOC1. Binds to RGL2, RGA and FHY3 (via N-terminus). Expressed constitutively in roots, leaves, stems, and flowers. Repressed by red (R) and far red (FR) light treatments in a phyB- and phyA-dependent manner. Phosphorylated at Thr-10, Thr-197, Ser-202, Ser-464, Ser-465, Ser-466 and Ser-469 by CK2 (PubMed:21330376). Phosphorylated and ubiquitinated after an exposure to light (especially red and far-red), in a phytochrome-dependent manner. Modified proteins undergo a proteasome-dependent degradation. Its stability and degradation plays a central role in photomorphogenesis of seedlings. Plants overaccumulate free protochlorophyllide in the darkness and exhibit photooxidative damage (bleaching) in subsequent light, probably caused by the photosensitizing activity of this tetrapyrrole intermediate. May be due to a competing acceptor splice site.