Growing to their own drummers

Plants' internal rhythms tune into latitudes of origin

Published November 17, 2003; Category: ARTS & SCIENCES

Dartmouth researchers have learned that natural variations in a plant's circadian clock correlate with the plant's latitude.

C. Robertson McClung, Professor of Biological Sciences and an author of a new journal article, and his team studied circadian behaviors of Arabidopsis plants obtained from around the world: the Arctic Circle and middle Europe, Japan and the Cape Verde Islands.

"Clocks are adaptive; they contribute to the fitness of an organism in any particular environment," McClung said. "We found that in species like Arabidopsis, which cover a huge environmental range, there are underlying variations in their circadian clocks to subtly optimize their ability to function in a particular environment."

Published in the Nov. 7 issue of Science, the study examines the clock rhythms in Arabidopsis from different parts of the earth. The researchers analyzed leaf movements and measured the period of time it took the leaves to complete one circadian cycle, noted the time of day when the leaves were pointing straight upward, and calculated the distance the leaves moved during a cycle. All three measurements showed considerable differences in the plants from different areas.

"When we determined day length in each of the latitudes for the plants, we found the correlation was highly significant between the circadian variations in period length and the latitudes of origin of the different plants," McClung said.

A second component of the study provides evidence suggesting that a large number of genes contribute to fine-tuning the Arabidopsis clock.

By crossing Arabidopsis from two different geographic areas, and allowing them to segregate for a number of generations, the researchers identified five chromosomal regions, called quantitative trait loci (QTL), which significantly contribute to either period length or phase or amplitude of the rhythms. Each of these QTL regions includes many genes, at least one of which contributes to the plant's circadian rhythm. McClung's group chose one candidate gene, called APRR7, which was in the one of the chromosomal regions. When it was knocked out, it affected period length. The researchers then examined the additional members of APRR7's gene family. They knocked out each of them, which all impacted normal clock function. The results indicate that this gene family works together to control different parts of the clock mechanism.

"This illustrates the point of the QTL analysis that some genes can contribute incrementally to clock behaviors," says McClung. "When we saw huge clock variations in the offspring, more than in the parents, it's commonly interpreted to mean that the behaviors in question are the products of interaction of multiple genes. Our study doesn't negate that there are a few major genes that work the clock, but it does demonstrate that there are a lot of genes that contribute incrementally to that. It argues for a much more complicated clock structure."

Other authors on the paper include: Todd Michael, a recent Dartmouth graduate with a doctorate in biological sciences; Patrice Salom��, a Dartmouth doctoral candidate in biological sciences; Hannah Yu '02, Taylor Spencer '03, and Emily Sharp '05, all Dartmouth undergraduate students; Mark McPeek, Professor of Biological Sciences at Dartmouth; Jos�� Alonso, Assistant Professor of Genetics at North Carolina State University; and Joseph Ecker, Professor in the Plant Molecular and Cellular Biology Laboratory at the Salk Institute for Biological Studies in San Diego.

This work was funded by grants from the National Science Foundation to Ecker, McPeek and McClung. Yu and Spencer were supported by Richter Undergraduate Research Fellowships, and Sharp was supported by a Women in Science Project Internship, all administered through Dartmouth.

By SUSAN KNAPP