Figure 8.

A proposed working model. Previous studies show that PIF1, PIF3, and COP1 are negatively regulated by phytochromes and cryptochromes (Lau and Deng, 2010). PIF1 and PIF3 transcription factors directly activate HY5 and BBX23 expression, while COP1 negatively controls their protein stability. HY5 binds to the specific cis-regulatory regions of cell elongation-related and photosynthesis-related genes. BBX23 is recruited to the target genes through interacting with HY5. HY5 and BBX23 thus function synergistically to inhibit cell elongation-related gene expression but activate photosynthesis-related gene transcription, both leading to the promotion of photomorphogenesis.