Molecular Mechanisms that Trigger Flowering in Spring
Max Planck scientists have discovered how plants initiate the formation of flowers depending on the length of day and time of year
The appearance of flowers in spring is one of the surest signs that winter is over, but how do plants follow the changing seasons, and use this information to trigger the formation of flowers? That plants contain internal clocks enabling them to measure day length was proposed 80 years ago and was initially controversial, but now the mechanisms by which plants measure time are being explained by the isolation of genes and proteins that play central roles in this process. In the recent issue of Science (Science,13th February 2004) a group at the Max Planck Institute for Züchtungsforschung in Cologne describes how a molecular circuit around the protein CONSTANS induces flowering. This protein accumulates in the nuclei of plants exposed to long days of spring, but is rapidly degraded if the plant is exposed to short winter days. This molecular circuit is widely conserved in the plant Kingdom and knowledge of how it initiates the formation of flowers in spring could help increase the yield of crop plants.
Around 80 years ago, tobacco plants were shown to distinguish summer and winter by measuring the length of the day and night. This process, called photoperiodism, was later shown to be widespread in the plant Kingdom, and to occur in mammals, insects and birds. In addition to flowering, other seasonal responses in plants are controlled by day length, including the formation of potato tubers and the dormancy of buds in trees.
The first breakthrough in explaining the mechanism of photoperiodism was proposed in the 1930s by Erwin Bünning (1906 - 1990), who worked in Jena, Koenigsberg, and since 1946 Tuebingen. Many aspects of plant behaviour show daily rhythms, such as the movement of leaves to optimise exposure to sun light, or the opening and closing of pores on the leaves to reduce water loss during the day. These behaviours are controlled by the circadian clock, an internal timer that takes 24 hours to complete one cycle. Bünning proposed that 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.
Bünning’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 nucleus, 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 Bünning, 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 Bünning 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.