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Early spring. Arabidopsis plants grown under 16 hours of light per day (left) start flowering, whereas plants of the same age that see only 10 hours of light do not.

How Flowers Know It's Spring

During the dark months of winter, plants keep busy monitoring the skies for signs of spring. Only when the days are long enough, a surefire indicator, do they begin to flower. Now researchers have figured out how a kind of molecular hourglass, filled and emptied each day, tells the plant that it's the season to blossom.

Many plants monitor and respond to day length. In Arabidopsis thaliana (thale cress), a popular experimental plant, the hours are tracked by a gene called CONSTANS. Levels of CONSTANS mRNA start rising about 12 hours after dawn, remain high throughout the night, then drop during the day. That means that during the short days of winter, it's expressed only when it's dark. On longer days, CONSTANS starts to crank up in the waning hours of daylight. Scientists have long suspected that those hours of light must do something to the CONSTANS protein to somehow let the plant know it's spring, but they didn't know how.

To unravel the mystery, George Coupland and colleagues at the Max Planck Institute for Plant Breeding Research in Cologne, Germany, decided to study the effect of light on the protein. They bred Arabidopsis plants engineered to keep CONSTANS mRNA levels high and found more CONSTANS protein during the light phase of long days than short days. That suggests that light helps preserve the CONSTANS protein and dark destroys it.

The next step was to examine the plants' photoreceptors, molecules that detect light of particular wavelengths. By creating plants that lacked various kinds of photoreceptors, then exposing them to light of a range of wavelengths, the team discovered what was going on. Photoreceptors sensitive to blue and far-red light--both components of sunlight--inhibit breakdown of the CONSTANS protein. In the reddish light of morning, however, another photoreceptor triggers the breakdown of the protein. That means only long days will allow the CONSTANS protein to accumulate to a high enough level to trigger other flowering genes, explains Coupland.

Although researchers knew that CONSTANS production levels varied, the new study shows that the protein is subject to "a whole other level of regulation ... that's controlled by these photoreceptors," says molecular biologist Mark Doyle of the University of Wisconsin, Madison. But he points out that as most of the experiments were done under monochromatic light, it's difficult to predict what the full suite of photoreceptors do in natural daylight.

Related sites
Home page of George Coupland's group.