Molecular mechanisms that trigger flowering...( hat the circadian clock may also contro...)

hat the circadian clock may also control photoperiodism. He suggested that a rhythm generated by the circadian clock controls flowering, but that one stage of this rhythm is sensitive to light. In this way, flowering would occur under long days and not under short days, because the light sensitive stage of the rhythm would occur in day light during long days but not short days. This mechanism relies upon an external signal, light, coinciding with an internal rhythm, and therefore became known as the external coincidence model.

Bnning's proposal was extended in the last few years by the isolation of genes that control flowering. Arabidopsis thaliana, thale cress, is the model plant most widely used for genetic experiments, and its genome has been completely sequenced, generating a catalogue of the 25,000 genes required for plant life. In Nature, Arabidopsis flowers in Spring in response to longer days. Inactivation of any of several genes prevents Arabidopsis from distinguishing between long and short days. One of these genes, CONSTANS, is controlled by the circadian clock, so that the abundance of its mRNA rises around 12 hours after dawn and stays high through the night. This characteristic expression pattern causes the gene to be expressed when the plant is exposed to light under long days, but under short days it is only expressed in the dark. Therefore, if CONSTANS protein somehow triggers flowering only when plants are exposed to light, then this would explain how it activates flowering under long and not short days.

Researchers at the Max Planck Institute for Plant Breeding Research have now demonstrated a major route through which light regulates CONSTANS protein. Specific plant proteins that detect blue and far-red light, the photoreceptors cryptochrome and phytochrome A, are required to activate CONSTANS. When exposed to these wavelengths of light at the end of a long day, the photoreceptors stabilise the CONSTANS protein in the n
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Contact: Prof. George Coupland
coupland@mpiz-koeln.mpg.de
49-221-5062-205
Max-Planck-Gesellschaft
13-Feb-2004

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