Light-induced phosphorylation and degradation of the negative regulator PHYTOCHROME-INTERACTING FACTOR1 from Arabidopsis depend upon its direct physical interactions with photoactivated phytochromes.

The phytochrome (phy) family of photoreceptors regulates changes in gene expression in response to red/far-red light signals in part by physically interacting with constitutively nucleus-localized phy-interacting basic helix-loop-helix transcription factors (PIFs). Here, we show that PIF1, the member with the highest affinity for phys, is strongly sensitive to the quality and quantity of light. phyA plays a dominant role in regulating the degradation of PIF1 following initial light exposure, while phyB and phyD and possibly other phys also influence PIF1 degradation after prolonged illumination. PIF1 is rapidly phosphorylated and ubiquitinated under red and far-red light before being degraded with a half-life of approximately 1 to 2 min under red light. Although PIF1 interacts with phyB through a conserved active phyB binding motif, it interacts with phyA through a novel active phyA binding motif. phy interaction is necessary but not sufficient for the light-induced phosphorylation and degradation of PIF1. Domain-mapping studies reveal that the phy interaction, light-induced degradation, and transcriptional activation domains are located at the N-terminal 150-amino acid region of PIF1. Unlike PIF3, PIF1 does not interact with the two halves of either phyA or phyB separately. Moreover, overexpression of a light-stable truncated form of PIF1 causes constitutively photomorphogenic phenotypes in the dark. Taken together, these data suggest that removal of the negative regulators (e.g., PIFs) by light-induced proteolytic degradation might be sufficient to promote photomorphogenesis.