Figure 10.

Simplified Model of the Negative Feedback Loop between PIFs and HECs Shows Fine-Tuning of Photomorphogenesis.

In the dark (left panel), phytochromes are present in the cytosol as Pr form, while PIFs are constitutively localized to the nucleus. PIFs activate their target genes, including HEC1 and HEC2 expression, in the dark. HECs in turn repress the DNA binding and transcriptional activation activity of PIFs reducing their own expression as well as other PIF target genes. Phytochromes perceive R light signal, undergo allosteric changes in conformation into the Pfr form, and migrate into the nucleus. Within the nucleus, phys interact with PIFs and induce rapid degradation of PIFs while stabilizing HEC2. PIFs repress photomorphogenesis, while HECs promote photomorphogenesis. However, the balance between PIF:HEC stoichiometry determines the level of photomorphogenesis in a dynamic environmental light condition.