Intellectual Merits: The phytochrome (phy) family of sensory photoreceptors in Arabidopsis consists of five members (phyA to phyE) that control many developmental responses to the ambient light environment. Significant progress has been made on characterization of photosensory and biochemical functions and in the identification and functional characterization of signaling intermediates. However, the biochemical mechanism of phy signaling is still not clear. To examine early phy signaling events, phy interacting factors such as PIF1 have been isolated and characterized. PIF1 is a basic helix-loop-helix (bHLH) transcription factor that interacts strongly with the biologically active form of phyA and phyB, as well as another bHLH factor, PIF3. PIF1 binds to a G-box (CACGTG) DNA sequence motif commonly found in many light regulated promoters. PIF1 has also been shown to activate transcription in a transient assay, and the transcriptional activation activity was downregulated by far-red and red light in a phyA- and phyB-dependent manner, respectively. PIF1 overexpressors and pif1 T-DNA insertional mutants showed defective seedling de-etiolation, including aberrant hypocotyl elongation and developmentally regulated loss of greening. These results suggest that PIF1 is a key regulator of the phy-mediated control of the seedling de-etiolation process. The proposed study will investigate the molecular mechanism by which PIF1 controls seedling de-etiolation using molecular genetic, biochemical and functional genomic approaches. Specifically, the PIF1 target genes will be identified and functionally characterized, and the molecular function of PIF1 will be investigated to examine whether it acts as a transcriptional activator or repressor from a native target gene promoter. In addition, the biological significance of physical interactions between PIF1-phy and PIF1-PIF3 will be investigated in vivo. Since PIF1 is a primary phy-signaling partner whose activity is regulated by phy, the results from this study will provide fundamental information on the mechanism of phy signaling. Expected Broader Impacts: Light is arguably one of the most important environmental factors controlling plant growth and development, including many agronomically important traits such as seed germination, seedling establishment and survival, plant height, leaf area, flowering time and plantation density. Understanding the mechanism of light-regulated plant growth and development will provide excellent tools to apply in agricultural biotechnology for increased crop production using reduced amount of fertilizers. This proposal is also ideally suited for training of the next generation of scientists, including undergraduate, graduate and postdoctoral students. Their training in light signaling will give them a unique opportunity to make significant contributions to the field of photobiology and biotechnology.
