The Arabidopsis Circadian Clock Circadian clocks have evolved to
provide an adaptive advantage to most organisms that live under
alternating cycles of light and darkness (Ouyang et al., 1998;
Woelfle et al., 2004; Dodd et al., 2005). The clock provides a
mechanism whereby physiological and behavioral processes can be
performed at the appropriate times of day or night. Furthermore,
the evolution of a true oscillator, rather than just a light-
responsive sand-timer, enables organisms to predict the
alterations that occur in relative proportion to day and night
throughout the year in most clines. In mammals, many aspects of
behavior and physiology are regulated by endogenous circadian
clocks and are subject to daily oscillations (Hastings et al.,
2003). In higher plants, the circadian clock regulates many key
physiological processes, ranging from flowering time (Yanovsky
and Kay, 2003; Imaizumi and Kay, 2006) and growth (Dowson-Day
and Millar, 1999) to stomatal opening and CO2 assimilation
(Hennessey and Field, 1