8 F.-Y. Bouget et al.
systems to study circadian regulation of cell cycle in plants, however, the mo-
lecular mechanisms of the circadian clock are not known inChlamydomonas.
Except for cryptochromes and casein kinase II, the molecular actors of the
circadian clock are not conserved between kingdoms suggesting that clocks
have emerged independently during evolution of plant, animals and fungi
(Gardner et al. 2006). Nevertheless, the principles of the clock organization is
highly conserved between organisms. Coupled oscillators, which rely on in-
terconnected feedback loops produce robust oscillations. These oscillators are
entrained by environmental cycles such as light cycles but also gate input to
the clock by regulating the expression of photoreceptors (Fig. 2). Most of our
knowledge about the plant circadian oscillator comes fromArabidopsis.This
pacemaker consists of two interlocked oscillators. The first transcriptional
feedback loop consists of the pseudo-response regulator TOC1 (Time of CAB
expression1) and the two MYB transcription factors of the REVEILLE family,
CCA1 (Circadian Clock Associated 1)/LHY (Late Elongated Hypocotyl) (Al-
abadi et al. 2001). The transcription of CCA1/LHY is induced by light in the
morning. CCA1/LHY bind to an evening element (EE) found in the promoter
of TOC1 and other genes expressed in the evening (Harmer et al. 2000), re-
Fig. 2A speculative model of CDC control by the circadian clock in plants. The clock con-
sists of two interconnected loops.CCA1/LHYandGIare transcribed in response to light
in the morning. CCA1/LHY and GI have opposite inhibiting and activating roles in the
regulation ofTOC1expression. CCA1/LHY may regulate the transcription ofCDCgenes
such as cyclins, CDK or CDK inhibitors (KRP) through a binding to specific circadian
elements in their promoters