Biologists identify genes controlling rhythmic plant growth

"We did many hundreds of thousands of measurements," said Kay, "and then asked what genes are rhythmically being turned on and are correlated with this rhythmic growth pattern just prior to dawn? What we found was that a whole bunch of genes all scattered around the Arabidopsis genome that deal with hormone biosynthesis, hormone signaling and hormone metabolism are all tightly correlated with rhythmic plant growth. This told us that this set of genes could be the actual molecular signature that defines plant growth at the molecular level."

The scientists said these disparate genes act together to regulate rhythmic plant growth much like a gate with its hinges controlled by photoreceptors and the biological clock—opening in the predawn hours to allow a wave of multiple plant growth hormones to act within the cells, then closing the gate to put the brakes on plant growth until the next 24-hour cycle.

"This temporal integration of hormone pathways allows plants to fine tune phytohormone responses for seasonal and shade-appropriate growth regulation," they write in their paper. "Many different plant hormone genes, including genes for hormones that promote and antagonize growth, are co-expressed at the time of day that plants grow," said Chory. "That such an extensive gene regulatory module exists was quite a surprise."

To illustrate their model, the scientists attached a glowing enzyme, luciferase, to the genes they identified as responsible for rhythmic growth in Arabidopsis plants. As the plants go through their rhythmic growth phase, the Arabidopsis plants glow on and off as genes that regulate the opening and closing of the gate to plant hormones are activated, then deactivated. See video with narration at: http://www.biology.ucsd.edu/scicomm/video/sprouts.mov

The scientists also discovered that most of the genes involved in this rhythmic predawn growth have a DNA sequence in common, a master controller that they dubbed the HUD element—for "Hormone Up at Dawn." This HUD element, they noted, must have a protein that attaches to it that regulates its function.

"We don't know what that is, because we haven't found it yet," said Kay. "Identifying that protein regulator is going to be a key goal for the future because that protein is going to be very, very important for controlling plant growth and yield."

"It's a very exciting time for biologists," Chory added, "because the tools now exist to answer questions about complex processes, such as how plants grow or how human metabolism goes awry."

The study was supported by grants from the Howard Hughes Medical Institute, National Institutes of Health and National Science Foundation.