The Circadian Clock. A Plant's Best Friend in a Spinning World

The circadian clock is an intricate, even delicate, regulator of plant physiology, yet at least one of the selective pressures that drove its evolution is brutally simple. Plants must be exposed to sunlight for photosynthesis, and sunlight is not available continuously. Therefore, plants are stuck with a day/night cycle of light and temperature, with the possible exceptions of buried, germinating seedlings and polar inhabitants. Each day's solar energy propels their metabolism into a spate of carbon fixation, which must end at nightfall. Locomotion would not alleviate the problem. Plants, like other eukaryotes and some prokaryotes, have adapted to the day/night cycle by evolving the circadian system, which drives matching rhythms in very many aspects of metabolism, physiology, and behavior (Harmer et al., 2001; Young and Kay, 2001). The hallmarks of circadian regulation are very similar in all organisms, most obviously the persistence of biological rhythms even under constant environmental conditions. The rhythms are all reset by light and/or temperature signals in a characteristic fashion that synchronizes the clock with the environment. This process of “entrainment” is crucial to ensure that rhythmic processes occur at an appropriate time of day (circadian phase), particularly because the period of circadian clocks in the absence of entraining signals often differs from 24 h. Plant circadian rhythms in nature are always entrained to 24 h by the day/night cycle; the non-24 h period is expressed only in exceptional circumstances (or in the laboratory). Therefore, the circadian clock contributes to plant physiology by regulating the phase of entrained rhythms, and natural selection acts primarily on phase, not on period.