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).