title: poc1: an Arabidopsis mutant perturbed in phytochrome signaling because of a T DNA insertion in the promoter of PIF3, a gene encoding a phytochrome-interacting bHLH protein.
abstract: The phytochrome family of informational photoreceptors has a central role in regulating light-responsive gene expression, but the mechanism of intracellular signal transduction has remained elusive. In a genetic screen for T DNA-tagged Arabidopsis mutants affected in early signaling intermediates, we identified poc1 (photocurrent 1), which exhibits enhanced responsiveness to red light. This phenotype is absent in a phyB (phytochrome B) null mutant background, indicating that the poc1 mutation enhances phyB signal transduction. The T DNA insertion in poc1 was found to be located in the promoter region of PIF3, a gene encoding a basic helix-loop-helix protein. The mutant phenotype seems to result from insertion-induced overexpression of this gene in red-light-grown seedlings, consistent with PIF3 functioning as a positively acting signaling intermediate. These findings, combined with data from a separate yeast two-hybrid screen that identified PIF3 as a phytochrome-interacting factor necessary for normal signaling, provide evidence that phytochrome signal transduction may include a direct pathway to photoresponsive nuclear genes via physical interaction of the photoreceptor molecules with the potential transcriptional regulator PIF3.
mesh_terms: D017360:Arabidopsis / Q000235:genetics*; D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000235:genetics*; D018506:Gene Expression Regulation, Plant / Q000235:genetics; D017343:Genes, Plant; D018257:Helix-Loop-Helix Motifs; D018546:Hypocotyl / Q000378:metabolism; D008027:Light; D016254:Mutagenesis, Insertional; D009154:Mutation; D010641:Phenotype; D010786:Photoreceptor Cells; D010834:Phytochrome / Q000235:genetics*; D051939:Phytochrome B; D012150:Polymorphism, Restriction Fragment Length; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000378:metabolism; D015398:Signal Transduction; D014157:Transcription Factors