Light Response of the Circadian Waves of the APRR1/TOC1 Quintet: When Does the Quintet Start Singing Rhythmically in Arabidopsis?

We previously identified a novel class of proteins, named A:rabidopsis pseudo-response regulators (APRRs), each of which (APRR1/TOC1, APRR3, APRR5, APRR7, APRR9) has an intriguing structural design containing an N-terminal pseudo-receiver domain and a C-terminal CONSTANS motif. Expression of these APRR1/TOC1 family members is under the control of a coordinate circadian rhythm at the level of transcription such that the APRR-mRNAs start accumulating sequentially after dawn with 2 to 3 h intervals in the order of APRR9-->APRR7-->APRR5-->APRR3-->APRR1/TOC1 in a given 24 h photo-period. Based on these data, we previously proposed that these sequential and rhythmic events of transcription, termed 'circadian waves of APRR1/TOC1 quintet', may be a basis of a presumed Arabidopsis biological clock (named 'bar code clock') [Matsushika et al. (2000) Plant and Cell Physiol. 41: 1002]. Here we further characterized the event of circadian waves, by demonstrating that certain light stimuli are crucial determinants to induce the robust circadian waves, and accordingly, the first-boosted and light-induced APRR9 appears to be primarily responsible for this light response of the circadian waves. Also, as such a light stimulus, a red light pulse that is presumably perceived by phytochromes appears to be sufficient to induce (or synchronize) the APRR1/TOC1 circadian waves.