Phytochromes and blue-light receptors are known to affect the period of the biological clock, with the different pigments reporting on different wavelengths and intensities of light. This diversity of photoreceptors could be an adaptation to the changes in the light environment that a plant experiences in the course of a day or a season. How do these photoreceptors interact with a plant's biological clock?
The biological clock of Arabidopsis is based on the activities of at least three "clock genes." The clock genes encode regulatory proteins that interact to produce a circadian oscillation. How does this oscillating clock interact with photore-ceptors and the environment?
Arabidopsis is an LDP. Its clock controls the activity of CONSTANS (a gene that is not part of the clock mechanism) in such a way that the CONSTANS product, CO protein, accumulates in one phase of the clock's cycle—the phase in which night falls. Under long nights (short days), CO protein is found at night. Under short nights (long days), CO is also relatively abundant at dawn and dusk. When CO protein levels are high, light absorbed by phytochrome A and the blue-light receptor cryptochrome 2 leads to flowering (Figure 39.14). Thus, Arabidopsis flowering results from the coincidence of light (detected by the two photoreceptors) with a clock-determined phase of the circadian oscillation.
Where is this coincidence-based photoperiodic mechanism located in relation to where flowering occurs? Is the timing device for flowering located in a particular plant part, or are all parts able to sense the length of the night? This question was resolved by "blindfold" experiments, as described next.
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