Transcription factor acting negatively in the phytochrome B signaling pathway to promote the shade-avoidance response. Regulates PHYB abundance at the post-transcriptional level, possibly via the ubiquitin-proteasome pathway. Promotes ethylene activity in the dark. May regulate the expression of a subset of genes by binding to the G-box motif. Might be involved in the integration of light-signals to control both circadian and photomorphogenic processes. Activated by CRY1 and CRY2 in response to low blue light (LBL) by direct binding at chromatin on E-box variant 5'-CA[CT]GTG-3' to stimulate specific gene expression to adapt global physiology (e.g. hypocotyl elongation in low blue light) (PubMed:26724867). Homodimer (Probable). Interacts specifically with the Pfr form of phytochrome B and with TOC1/APRR1. May form a heterodimer with PIF3. Interacts with PHYB, CRY1 and CRY2 in the nucleus in response to low blue light (LBL) (PubMed:26724867). Interacts with TOPP4 (PubMed:26704640). Expressed in roots, leaves, stems, and flowers. Circadian-controlled expression. Follow a free-running robust circadian rhythm, with higher levels during the light phase. Rapidly induced by light in etiolated plants. Up-regulated by white light. Rapid degradation after red light exposure (at protein level). Accumulates to high levels in the dark, is selectively degraded in response to red light and remains at high levels under shade-mimicking conditions. The active phytochrome binding (APB) motif (26-39) is involved in interaction with PHYB and is required for proteasome-mediated degradation. Phosphorylated. Additional phosphorylations induced within 60 seconds following phytochrome B photoactivation. Dephosphorylated by TOPP4 during photomorphogenesis, leading to subsequent degradation of PIF5 by the proteasomal pathway. May be due to a competing acceptor splice site. May be due to a competing acceptor splice site.