Investigations of the central oscillator of the Arabidopsis circadian clock

Circadian rhythms are generated by endogenous central oscillators that respond to input from the environment and regulate rhythmic outputs. In the model plant Arabidopsis, although many components that affect rhythms have been identified and have been used to propose models of the circadian clock, none have been conclusively shown to be part of the central oscillator. A key criterion of a central oscillator component, that a pulse in expression resets all rhythms, has remained untested in plants until the study presented here. The ethanol-inducible gene system is presented as a potentially valuable tool experimentally testing the effect of a pulse of virtually any protein in plants.;The transcription factors CIRCADIAN CLOCK ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) are demonstrated to fulfill the criteria of central oscillator components. In addition to exhibiting properties that have suggested they serve such a role, a pulse of expression of these factors under constant light conditions results in dramatic phase-shifts in several different rhythms of gene expression. In contrast, pulsing TIMING OF CAB EXPRESSION 1 (TOC1) expression, a previously proposed component of the Arabidopsis central oscillator, did not result in such shifts. CCA1 and LHY are proven to function as central oscillator components in plants. TOC1, however, cannot by itself serve such a function.;In addition to its function as an oscillator component, CCA1 interacts with the transcription factor HY5 to modulate output rhythms from the circadian clock. CCA1 and HY5 bind adjacent regions on the CHLOROPHYLL A/B BINDING PROTEIN 2 (CAB2) promoter and act together to regulate the circadian expression of the gene.