Acts as negative regulator of seedling photomorphogenesis (PubMed:18540109). BBX25/STH and BBX24/STO function as transcriptional corepressors of HY5 activity, leading to the down-regulation of BBX22 expression. BBX25/STH acts additively with BBX24/STO during de-etiolation and the hypocotyl shade avoidance response (PubMed:23624715). Interacts with COP1 WD40 domain (PubMed:11226162). Interacts with HY5 (PubMed:23624715) and HYH (PubMed:23733077). COP1-mediated ubiquitination and subsequent proteasomal degradation of BBX25/STH occurs in the dark. Reduced length of hypocotyls.