Negative regulator of photomorphogenesis modulating both light and abscisic acid (ABA) signaling pathways (PubMed:19704523, PubMed:18375596, PubMed:26474641). Regulates negatively the light-mediated inhibition of hypocotyl elongation, probably in a PHYB-mediated signaling pathway, but promotes flowering time (especially in long days) and lateral root formation (PubMed:18375596, PubMed:19704523). Enhances light-regulated gene expression (PubMed:18375596). Promotes COP1-mediated degradation of HY5 during seedling development (e.g. hypocotyl growth) through enhanced ubiquitination in the darkness. Also involved in root gravitropism (PubMed:26474641, PubMed:18375596). Interacts with HY5 and COP1 in the nucleus. Constitutively localized in the nucleus of hypocotyl cells. Additional isoforms seem to exist. Expressed in young seedlings (e.g. hypocotyl and cotyledons) and in green tissues (e.g. leaves, stems, sepals, and young siliques). In seedlings, expressed in hypocotyl and cotyledons up to the fifth day after germination. Levels decrease gradually in hypocotyl to become undetectable in 8 days old seedlings. In cotyledons, repartition becomes later patchy. In adult plants, observed in green tissues, with a gradual decrease during aging leading to patchy distribution. Also detected in seeds still partly green but disappear in dry seeds. Induced by white light (WL). Barely detectable in dark and in various wavelengths of light such as red light (RL), far red light (FR), and blue light (BL). Short hypocotyl in dark with drastic reduction in apical hook curvature. Enhanced inhibition in hypocotyl elongation in white light (WL), especially at lower fluence rates (PubMed:18375596, PubMed:26474641). Delayed flowering under long-day conditions (PubMed:18375596). Reduced lateral roots formation (PubMed:18375596, PubMed:26474641). Reduced chlorophyll accumulation and expression of light-regulated genes. Increase in the anthocyanin level in the dark (PubMed:18375596). Reduced sensitivity to abscisic acid (ABA) leading to impaired ABA-mediated reduction of seed germination (PubMed:19704523, PubMed:26474641). The double mutant shw1 phyB exhibits a strongly reduced hypocotyl length in WL (PubMed:19704523). The double mutant shw1 cop1 displays an enhanced photomorphogenic growth in the darkness as well as abnormal accumulation of HY5 (PubMed:26474641, PubMed:18375596). 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). In addition, shw1 hy5 is recued for gravitropic root growth defect observed in hy5 single mutant (PubMed:26474641).