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Abstract:
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Plants sense light intensity , quality and direction through a group of photoreceptors to modulate their growth and development . One family of photoreceptor is called phytochromes (phys ) that perceives red and far red light . Phys transduce light signals via a sub -family of the basic Helix -Loop -Helix (bHLH ) transcription factors called Phytochrome Interacting Factors (PIFs ) . PIFs function as negative regulators in the phy -mediated light signaling pathways . In darkness , PIFs regulate downstream gene expressions to inhibit photomorphogenesis . Upon light exposure , PIFs are phosphorylated and poly -ubiquitylated prior to their rapid degradation through the 26S proteasome pathway . One of the PIFs , PIF1 , has the highest affinity for both phyA and phyB and also displayed the fastest degradation kinetics under both red and far red light . Here we showed that PIF1 directly and indirectly regulates key genes involved in chlorophyll biosynthesis to optimize the greening process in Arabidopsis . PIF1 binds to a G -box (CACGTG ) DNA sequence element present in its direct target genes (e .g . , protochlorophyllide oxidoreductase C , PORC ) in darkness and regulates their expression . Structure -function studies revealed two separate regions called APB and APA necessary for binding to phyB and phyA , respectively , located at the amino -terminus and a novel phosphorylation site at the carboxy -terminus of PIF1 . Both amino - and carboxy -terminal regions are necessary for the light -induced degradation of PIF1 . However , the DNA binding is not necessary for the light -induced degradation of PIF1 . Using a targeted systems biology approach , we identified new factors , HECATE proteins that promote photomorphogenesis by negatively regulating the function of PIF1 . Moreover , we employed an unbiased genetic screening using luciferase imaging system to identify new mutants defective in the light -induced degradation of PIF1 . The cloning and characterization of these mutants will help identify the factors , such as the kinase and E3 ligase , responsible for the light -induced degradation of PIF1 . Taken together , these data revealed detail mechanisms of how PIF1 negatively regulates photomorphogenesis and how light induces rapid degradation of PIF1 to promote photomorphogenesis . |