Gene tells plants when its time to flower.
Snowdrops push up through the last snows of winter, bluebells bloom in spring, and clematis hang heavy in high summer, all thanks to one gene, researchers at the Scripps Research Institute in San Diego, California, have found. This discovery could help manipulate food crops to better suit the seasons of equatorial regions.
Called CONSTANS, the gene enables plants to measure day length and so to tell when to flower, say Marcelo Yanovsky and Steve Kay1. This is one of the most crucial decisions a plant has to make: too early in the year and the insects needed for pollination may not be around; too late and there won't be enough time to make seeds and fruits before winter comes again.
The amount of protein produced by the CONSTANS gene rises and falls under the control of a plant's 24 hour biological clock2. In a series of elegant experiments on thale cress (Arabidopsis thaliana), Yanovsky and Kay show that a plant starts flowering only if sunlight falls on it when the amount of CONSTANS protein is above a certain threshold, usually reached in the late afternoon.
The quality of the light is also important. Two light receptors must be activated. Called cryptochrome 2 and phytochrome A, they respond to blue and red light, respectively.
When all these criteria are simultaneously fulfilled the CONSTANS protein flicks on the plant's flowering switch, another gene called FLOWERING LOCUS T. This activates a cascade of hundreds of genes to build flowers.
The idea that flowering depends on concurrently detecting daylight and knowing that it is late in the day is not new. It was proposed in the 1930s by one of the pioneers of the science of biological clocks, Erwin Bünning 3.
By understanding its exact molecular mechanism, we can see how the system is adapted for different plants. Those that flower as spring begins have an earlier time window for the light activation, either by having a lower threshold for CONSTANS protein or by increasing its production more quickly; later-flowering plants, have a later one.
An almost identical system of genes operates in plants that, unlike Arabidopsis, flower as days shorten rather than lengthen. These include rice and several other major world food crops. Genetic manipulation may now make it possible to produce varieties for countries whose seasons pass too quickly for their current crops to thrive.
Christopher Surridge is a Senior Biological Sciences Editor at Nature.
Yanovsky, M. J. & Kay, S. A. Molecular basis of seasonal time measurement in Arabidopsis. Nature, 419, 308 - 312, (2002).
Suárez-López, P. CONSTANS mediates between the circadian clock and the control of flowering in Arabidopsis. Nature, 410, 1116 - 1120, (2002).
Bünning, E. Die endogene Tagesrhythmik als Grundlage der photoperiodischen Reaktion. Ber. Dtsch Bot. Ges., 54, 590 - 607, (1936).