Molecular mechanisms that trigger flowering...( ucleus allowing it to activate the exp...)

ucleus, allowing it to activate the expression of other genes that trigger flowering. However, in darkness these photoreceptors are not activated, and CONSTANS protein is attached to a small protein called ubiquitin, which marks it for degradation by the proteasome. Thus in short days, although CONSTANS mRNA is present, the protein is absent. The expression of CONSTANS mRNA therefore represents a light sensitive rhythm, similar to that proposed by Bnning, which triggers flowering only under long days, when the plant is exposed to light more than 12 hours after dawn.

The analysis of CONSTANS has also demonstrated distinct layers of regulation that were not foreseen by the physiological experiments carried out by Bnning and others. In addition to stabilising CONSTANS protein at the end of the day, the Cologne group showed that light detected by another photoreceptor, phytochrome B, targets CONSTANS for degradation at the beginning of the day. Therefore, CONSTANS activity at the end of the day depends both on circadian clock regulated transcription and antagonism between distinct photoreceptors, some of which target the protein for degradation in the morning and others that stabilise it in the evening.

The significance of these observations extends beyond Arabidopsis. Recently, Japanese researchers demonstrated that CONSTANS and related flowering proteins are present in rice, whose last common ancestor with Arabidopsis lived around 150 million years ago. CONSTANS also controls photoperiodic flowering in this important crop species. Therefore the work described by the Cologne group in Science is likely to have broad significance in flowering plants, and as well as explaining why plants flower in Spring may help in increasing the yield of important crops.

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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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