Figure 10.

Model depicting the proposed role of TOPP4 in red light-induced PIF5 degradation. In darkness, phyB is inactive and localized to the cytoplasm. When exposed to red light, light-activated phyB (Pfr) translocates to the nucleus and induces PIF5 phosphorylation, ubiquitination, and subsequent degradation by the 26S proteasome, while TOPP4 dephosphorylates the phosphorylation state of PIF5 and leads to PIF5 accumulation. These modifications result in PIF5 levels in a dynamic balance. Therefore, TOPP4 and phyB regulate photomorphogenesis in an antagonistic way (adapted from Castillon et al., 2007; Shen et al., 2007).