ATTENUATION OF PHYTOCHROME-A AND PHYTOCHROME-B SIGNALING PATHWAYS BY THE ARABIDOPSIS CIRCADIAN CLOCK

In higher plants, environmental cues such as light signals are integra ted with circadian clock signals to control precisely the daily rhythm s observed for many biological functions, We have used a fusion of the promoter of a chlorophyll alb binding protein gene, CABP, with firefl y luciferase (cab2::luc) to monitor the detailed kinetics of transcrip tion in response to photoreceptor activation in Arabidopsis. Using thi s marker in phototransduction and circadian-dysfunctional mutants, we studied how signals from phytochrome and the circadian clock are integ rated for the regulation of CABP transcription, Results from these mut ant studies demonstrate that similar expression features, namely, the acute and circadian responses, are present in both etiolated and green seedlings and that the acute and circadian responses are genetically separable. We also demonstrate that persistent Pfr signaling occurs in red light-pulsed etiolated seedlings, which suggests that the circadi an clock antagonizes Pfr-mediated signal transduction. Based on these genetic studies, we propose a model for the regulation of CABP transcr iption in which individual photoreceptors and phototransduction compon ents have been assigned to specific pathways for the regulation of dis crete kinetic components of the CABP expression pattern.