Biologists identify genes controlling rhythmic plant growth

...then fade as the genes turn off in a daily cycle. Click here for more information.

How plants grow to maximize their survival in different environments has long fascinated biologists. In 1880, Charles Darwin published The Power of Movement in Plants, one of his lesser-known books in which he describes his studies on the manner by which different types of plants grow and move in response to various stimuli.

While most people might assume that plants grow at a slow and steady rate throughout the day and night, Darwin and others found that they grow in regular nightly spurts, with plant stems elongating fastest in the hours just before dawn. See video of soybean sprout growth at: http://www.biology.ucsd.edu/scicomm/video/bigbeansprout.mov

"Plants actually grow rhythmically," said Kay. "Some plants, like sorghum, have the ability to elongate a centimeter or more each night."

Why plants have evolved mechanisms to grow rhythmically at night or in the hours just before dawn is a mystery. But a similar interplay of light sensing, plant hormones and circadian rhythms that leads to a pronounced rhythmic growth by plants during certain seasons and when shaded by other plants has a clear survival value.

"Any plant that is growing is in a situation in which it has to compete with the plants growing around it, so it has to develop ways in which it can measure its environment to enable it to compete," said Kay. "Plant cells have phytochromes, which are essentially shade detectors that measure the ratio of different colors of light that can tell a plant whether it's a cloudy day or whether it's being shaded by another plant. And being shaded by another plant is bad news, because that plant is devouring all of the right color of light for photosynthesis. If plants detect they're shaded, they elicit growth hormones to elongate. You can see that in the extreme when you leave something on your lawn and the grass around it has grown taller."

To determine which genes control these rhythmic patterns of growth, the research team turned to Arabidopsis thaliana, a tiny mustard plant used as a laboratory model by plant geneticists. Because Arabidopsis, like many other plants, grows fastest in the hours before dawn when exposed to day and night cycles of light, the scientists sought to determine which of its genes were being turned on during that period. Using DNA microarray chips, they were able to test thousands of genes at a time to determine which ones were active during that period.