The molecular mechanism whereby a light signal causes phase shifting is not yet known, but studies in Arabidopsis have identified some of the key elements of the circadian oscillator and its inputs and outputs (see Chapter 17). The low levels and specific wavelengths of light that can induce phase shifting indicate that the light response must be mediated by specific photoreceptors rather than rates of photosynthesis. For example, the red-light entrainment of rhythmic nyctonastic leaf movements in Samanea, a semi-tropical leguminous tree, is a low-fluence response mediated by phytochrome (see Chapter 17).
Arabidopsis has five phytochromes, and all but one of them (phytochrome C) have been implicated in clock entrainment. Each phytochrome acts as a specific photoreceptor for red, far-red, or blue light. In addition, the CRY1 and CRY2 proteins participate in blue-light entrainment of the clock, as they do in insects and mammals (Devlin and Kay 2000). Surprisingly, CRY proteins also appear to be required for normal entrainment by red light. Since these proteins do not absorb red light, this requirement suggests that CRY1 and CRY2 may act as intermediates in phytochrome signaling during entrainment of the clock (Yanovsky and Kay 2001).
In Drosophila, CRY proteins interact physically with clock components and thus constitute part of the oscillator mechanism (Devlin and Kay 2000). However, this does not appear to be the case in Arabidopsis, in which cry1/cry2 double mutants have normal circadian rhythms. Precisely how Arabidopsis CRY proteins interact with the endogenous oscillator mechanism to induce phase shifting remains to be elucidated (Yanovsky et al. 2001).
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