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

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

^*Department of Plant and Microbial Biology, University of California, Berkeley, CA 94720; and ^†U.S. Department of Agriculture/Agricultural Research Center, Plant Gene Expression Center, 800 Buchanan Street, Albany, CA 94710

Communicated by Winslow R. Briggs, Carnegie Institution of Washington, Stanford, CA (received for review January 15, 1999)

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.

Footnotes

↵ ‡ Present address: Biology Department, University of Leicester, Leicester LE1 7RH, U.K.
↵ § Present address: Biomarin Pharmaceutical, 11 Pimentel Court, Novato, CA 94949.
↵ ¶ To whom reprint requests should be addressed. e-mail: quail{at}nature.berkeley.edu.
ABBREVIATIONS:

R,

red light;

FR,

far red light;

RT,

reverse transcription;

Rc,

continuous R;

FRc,

continuous FR;

BAC,

bacterial artificial chromosome