Abstract
Plant seedlings emerging from darkness into the light environment undergo photomorphogenesis, which enables autotrophic growth with optimized morphology and physiology. During this transition, plants must rapidly remove photomorphogenic repressors accumulated in the dark. Among them is PHYTOCHROME-INTERACTING FACTOR 3 (PIF3), a key transcription factor promoting hypocotyl growth. Here we report that, in response to light activation of phytochrome photoreceptors, EIN3-BINDING F BOX PROTEINs (EBFs) 1 and 2 mediate PIF3 protein degradation in a manner dependent on light-induced phosphorylation of PIF3. Whereas PIF3 binds EBFs independent of light, the recruitment of PIF3-EBFs to the core SKP1-CUL1-F box protein (SCF) scaffold is facilitated by light signals or PIF3 phosphorylation. We also found that previously identified LIGHT-RESPONSE BRIC-A-BRACK/TRAMTRACK/BROAD (LRB) E3 ubiquitin ligases target phytochrome B (phyB) and PIF3 primarily under high-light conditions, whereas EBF1/2 vigorously target PIF3 degradation under wide ranges of light intensity without affecting the abundance of phyB. Both genetic and molecular data support that SCF(EBF1/2) function as photomorphogenic E3s during seedling development.