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

The phytochrome family of informational photoreceptors has a central role i n regulating light-responsive gene expression, but the mechanism of intrace llular signal transduction has remained elusive. In a genetic screen for T DNA-tagged Arabidopsis mutants affected in early signaling intermediates, w e identified pod (photocurrent 1), which exhibits enhanced responsiveness t o red light. This phenotype is absent in a phyB (phytochrome B) null mutant background, indicating that the poc1 mutation enhances phyB signal transdu ction, The T DNA insertion in pod was found to be located in the promoter r egion of PIF3, a gene encoding a basic helix-loop-helix protein. The mutant phenotype seems to result from insertion-induced overexpression of this ge ne in red-light-grown seedlings, consistent with PIF3 functioning as a posi tively acting signaling intermediate. These findings, combined with data fr om a separate yeast two-hybrid screen that identified PIF3 as a phytochrome -interacting factor necessary for normal signaling, provide evidence that p hytochrome signal transduction may include a direct pathway to photorespons ive nuclear genes via physical interaction of the photoreceptor molecules w ith the potential transcriptional regulator PIF3.