The daily rhythms of day and night affect nearly all life on Earth. As a result, many organisms, from bacteria to humans, have evolved endogenous biological clocks to measure time and control processes that need to occur at certain times of day. Such processes include a wide range of responses from sleep-wake cycles in mammals to nitrogen-fixation in cyanobacteria, and mating in fruit flies. These rhythms are controlled by a “clock” that has an approximate 24-hour period (hence the term “circadian” clock, from the Latin circa meaning about, and dies meaning a day). In plants, the circadian clock system regulates a diverse range of cellular and physiological events from gene expression and protein phosphorylation to cellular calcium oscillations, hypocotyl (seedling shoot) growth and leaf movements. The circadian system also plays a crucial role in monitoring day-length to regulate photoperiodic (day-length)-dependant flowering. 

Department of Plant and Environmental Sciences,
Institute for Life Sciences,
Edmond J. Safra Campus, Givat Ram,
Hebrew University,
Jerusalem,
Israel.
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