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 pho- tosynthesis, and sunlight is not available continu- ously. Therefore, plants are stuck with a day/night cycle of light and temperature, with the possible exceptions of buried, germinating seedlings and po- lar inhabitants. Each day's solar energy propels their metabolism into a spate of carbon fixation, which must end at nightfall. Locomotion would not allevi- ate 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 metabo- lism, physiology, and behavior (Harmer et al., 2001; Young and Kay, 2001). The hallmarks of circadian regulation are very sim- ilar in all organisms, most obviously the persistence of biological rhythms even under constant environ- mental conditions. The rhythms are all reset by light and/or temperature signals in a characteristic fash- ion that synchronizes the clock with the environ- ment. This process of "entrainment" is crucial to ensure that rhythmic processes occur at an appropri- ate time of day (circadian phase), particularly be- cause the period of circadian clocks in the absence of entraining signals often differs from 24 h. Plant cir- cadian 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 con- tributes to plant physiology by regulating the phase of entrained rhythms, and natural selection acts pri- marily on phase, not on period. The period of the clock that we measure in constant conditions will nonetheless affect the phase of en- trainment, all else being equal, so a rhythm with a longer period under constant conditions will have a later phase under entrainment. This relationship can be used experimentally to alter the phase of entrain- ment (see below in the discussion of photoperiodic regulation; Yanovsky and Kay, 2002). The converse relationship does not necessarily hold: A rhythm with an early phase can arise without a change in period (for example, in the phyB mutant; Hall et al., 2002).