Transcription factor acting in the phytochrome signaling pathway (PubMed:10466729, PubMed:14508006). Activates transcription by binding to the G box (5'-CACGTG-3') (PubMed:10797009). Acts as a negative regulator of phytochrome B signaling (PubMed:14508006). Represses chlorophyll biosynthesis and photosynthesis in the dark (PubMed:23548744). Recruits the histone deacetylase HDA15 to the promoters of chlorophyll biosynthetic and photosynthetic genes (PubMed:23548744). HDA15 represses their transcription by histone deacetylation (PubMed:23548744). Promotes the expression of MDP60 to modulate hypocotyl cell elongation in response to light and ethylene signaling (PubMed:29167353). Required for submergence-induced and ethylene-dependent underwater hypocotyl elongation (PubMed:31638649). Homodimer (Probable). Can form a heterodimer with REP1 and PIF4. Phytochrome B binds specifically to DNA-bound PIF3, but only upon red light induced conversion to the Pfr form (PfrB). Reconversion to Pr form causes rapid dissociation. Interacts with APRR1/TOC1. Binds to PIA2; this interaction may trigger the repression of PHYA-mediated phosphorylation (PubMed:27143545). Interacts with TOPP4 (PubMed:26704640, PubMed:11828023, PubMed:12826627, PubMed:12897250, PubMed:27143545) (Probable). Interacts with FYPP1 AND FYPP3 (PubMed:31527236). Interacts with HDA15 in the dark (PubMed:23548744). By UV treatment. Phosphorylated by PHYA; this phosphorylation is repressed by PIA2. Dephosphorylated by TOPP4 during photomorphogenesis, leading to subsequent degradation of PIF3 by the proteasomal pathway. Inhibited submergence-induced and ethylene-dependent underwater hypocotyl elongation.