Acts as negative regulator of seedling photomorphogenesis and light-regulated inhibition of hypocotyl elongation (PubMed:17605755, PubMed:18540109, PubMed:21685177). BBX24/STO and BBX25/STH function as transcriptional corepressors of HY5 activity, leading to the down-regulation of BBX22 expression. BBX24/STO acts additively with BBX25/STH during de-etiolation and the hypocotyl shade avoidance response (PubMed:23624715). Functions as negative regulator of photomorphogenic UV-B responses by interacting with both COP1 and HY5 (PubMed:22410790). May act as a transcription factor in the salt-stress response (PubMed:12909688). Interacts with COP1 WD40 domain (PubMed:11226162, PubMed:21685177, PubMed:22410790). Interacts with HY5 (PubMed:22410790, PubMed:23733077) and HYH (PubMed:23733077). Interacts with RCD1 and TRP4 (PubMed:11018516, PubMed:12909688). A number of isoforms are produced. According to EST sequences. High expression in leaves and lower in roots and flowers. Circadian regulation with a peak before dawn. COP1-mediated ubiquitination and subsequent proteasomal degradation of BBX24/STO occurs in the dark. No visible phenotype under normal growth conditions, but mutant seedlings show reduction of hypocotyls length when grown under continuous blue light and a short primary root phenotype under UV-B radiation. BBX24/STO expression is not changed in plants treated with increasing salt concentrations.