PhyB induces intron retention and uORF-mediated translational inhibition of PIF3

The phytochrome B (phyB) photoreceptor stimulates light responses in plants in part by inactivating repressors of light responses such as phytochrome-interacting factor 3 (PIF3). It has been established that activated phyB inhibits PIF3 by rapid protein degradation and decreased transcription. PIF3 protein degradation has been shown to be mediated by EIN3-BINDING F-BOX PROTEIN (EBF) and LIGHT-RESPONSE BTB (LRB) E3 ligases, the latter simultaneously targeting phyB for degradation. In this study, we show that PIF3 level is additionally regulated by alternative splicing and protein translation. Overaccumulation of photo-activated phyB, which occur in the mutant defective for LRB genes under continuous red light (Rc), induces a specific alternative splicing of PIF3 that results in retention of an intron in the 5UTR of PIF3 mRNA. In turn, the upstream opening reading frames (uORF) contained within this intron inhibit PIF3 protein synthesis. The phyB-dependent alternative splicing of PIF3 is diurnally regulated under the short-day light cycle. We hypothesize that this reversible regulatory mechanism may be utilized to fine-tune the level of PIF3 protein in light-grown plants, and may contribute to the oscillation of PIF3 protein abundance under the short-day environment.\n\nOne Sentence SummaryLight down-regulates PIF3 by multiple mechanisms. We show that phyB induces an alternative splicing event that inhibits PIF3 protein translation, and that is regulated by short-day diurnal cycle.