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.
Institute for Life Sciences,
Edmond J. Safra Campus, Givat Ram,
Hebrew University,
Jerusalem,
Israel.