Figure 8.

A Model Depicting How LUH Inhibits Light-Dependent Seed Germination.

In the dark, when phytochromes are in the cytosol in their inactive Pr form, PIF1 binds various promoters and recruits LUH in a context-dependent manner to either activate or repress its target genes in imbibed seeds. This increases ABA signaling, decreases GA signaling, and decreases cell wall loosening, ultimately inhibiting seed germination. Although showing a slight bias toward corepression, LUH is capable of both coactivating PIF1-activated targets and corepressing PIF1-repressed targets. This model also suggests the existence of one or more unidentified factors (X and Y) that permit the context-dependent switching of the PIF1-LUH complex from an activator to a repressor. Genes independently regulated by either PIF1 or LUH are not included in the model. In the light, phytochromes in their active Pfr form enter the nucleus and inhibit PIF1 both by dissociating it from DNA and promoting its degradation.