Transcription factor that promotes photomorphogenesis in light. Acts downstream of the light receptor network and directly affects transcription of light-induced genes. Specifically involved in the blue light specific pathway, suggesting that it participates in transmission of cryptochromes (CRY1 and CRY2) signals to downstream responses. In darkness, its degradation prevents the activation of light-induced genes (Probable). Involved in responses to cold conditions probably by modulating the expression of several genes and triggering anthocyanin biosynthesis (PubMed:28412546). Acts coordinately with SPL7 to regulate the microRNA miR408 and its target genes in response to changes in light and copper conditions (PubMed:25516599). Regulates the abscisic acid (ABA) signaling pathway. Also involved in root gravitropism (PubMed:26474641). Involved in the repression of hypocotyl cell elongation to promote photomorphogenesis (PubMed:31061103). Recruits the histone deacetylase HDA15 to the promoters of a subset of cell wall organization and auxin signaling-related genes (PubMed:31061103). HDA15 represses their transcription by decreasing the levels of histone H4 acetylation in a light-dependent manner (PubMed:31061103). Homodimer; homodimerizes via the leucine-zipper domains (PubMed:17261584). Heterodimer; heterodimerizes with HYH via the leucine-zipper domains (PubMed:12023303). Interacts with COP1 WD40 domain (PubMed:11226162). Interacts with BBX21 (PubMed:21632973), BBX24/STO (PubMed:23733077) and BBX25/STH (PubMed:23624715). Interacts with SPL7 (PubMed:25516599). Binds to SHW1 in the nucleus (PubMed:26474641). Interacts with HDA15 in the nucleus (PubMed:31061103). Interacts with PFD4 in the nucleus and at low temperature (e.g. at 4 degrees Celsius) (PubMed:28412546). Expressed in root, hypocotyl, cotyledon, leaf, stem and floral organs. Accumulates transiently within 2 days in response to cold; the proteasome-mediated degradation observed after accumulation is triggered by prefoldin co-chaperone complex (e.g. PFD3, PFD4 and PFD5). Phosphorylated by CK2. Shows a stronger interaction with COP1 when unphosphorylated. However, phosphorylation does not affect its susceptibility to be ubiquitinated. Ubiquitinated by COP1 and/or CIP8. Ubiquitination takes place in darkness and leads to its subsequent degradation, thereby preventing activation of photomorphogenesis signals (PubMed:10839542, PubMed:12028569). Ubiquitination and subsequent COP1-mediated degradation is triggered by SHW1 in the darkness during seedling development (PubMed:26474641). Ubiquinated and degraded in response to cold when reaching an accumulation peak after 2 days; this proteasome-mediated decay is promoted by prefoldin co-chaperone complex (e.g. PFD4) (PubMed:28412546). Reduced sensitivity to abscisic acid (ABA) leading to impaired ABA-mediated reduction of seed germination (PubMed:26474641). Abrogated induction of DHU1 in response to UV-B (PubMed:28735869). The double mutant shw1 hy5 has altered root growth, hypocotyl length and hook angle similar to the single mutant shw1 in the darkness and far red light (FR), but shorter hypocotyl in WL, red light (RL) and blue light (BL) (PubMed:26474641). In addition, shw1 hy5 is recued for gravitropic root growth defect observed in hy5 single mutant (PubMed:26474641). The double mutant dhu1-1 hy5-215 phenotype resemble that of the single mutant hy5-215 (PubMed:28735869). Belongs to the bZIP family.