Our research is aimed at understanding light signal transduction, specifically those pathways mediated by the phytochrome (phy) family of sensory photoreceptors that absorb light in the red and far-red region of the spectrum. The phy system, consisting of five members in Arabidopsis (phyA-phyE), controls almost every aspect of the plant life cycle including seed germination, de-etiolation and flowering time. To understand 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. PIF1 overexpression and pif1 mutants showed defective seedling de-etiolation including, aberrant hypocotyl elongation and developmentally regulated loss of greening, suggesting that PIF1 is a key regulator of the phy-mediated control of seedling de-etiolation process. Future projects include investigating the molecular function of PIF1, identifying and functionally characterizing PIF1 interacting proteins and PIF1 target genes using biochemical, genetic and functional genomic approaches.