Perennial mouse-ear cress

What's mouse-ear cress, and why should you care whether it's perennial?

Mouse-ear cress (better known to scientists as Arabidopsis thaliana) is a little weed in the mustard family that has become the most studied plant in the world in the last thirty years. Its genome has been fully sequenced, and a tremendous amount is known about the underlying molecular mechanisms that are responsible for variation in many of its traits. It's become a "model organism" for many studies in plant genetics, plant molecular biology, plant physiology, plant ... anything. Why?

photo by W.S. Justice, courtesy Smithsonian Institution

Well, Arabidopsis thaliana is small, easy to grow, and it has a small genome. It can complete its entire life cycle in six weeks, so scientists can get through eight generations a year, which is very important if you're doing genetics experiments that require several generations to complete. In the wild, it's an annual (occasionally a biennial, depending on when it flowers and when seeds germinate). And because it's so small you can grow hundreds in a growth chamber in the corner of a lab, keeping the growing conditions quite uniform and growing a lot of individuals. For these reasons and more this little plant has an entire web site of its own. The Arabidopsis Information Resource provides a database of genetic and molecular biology data, as well as links to a variety of other Arabidopsis resources and information about the Arabidopsis research community. In short, by working on Arabidopsis, plant biologists and plant geneticists can take advantage of a lot of work that other people have already done to ask very sophisticated and complicated problems using the most powerful tools of genetics and molecular biology. Which brings us finally to the topic of this post, perennial mouse-ear cress.

Wait! Perennial mouse-ear cress? Didn't I tell you a few sentences ago that it's an annual, or sometimes a biennial?

"Perennial" mouse-ear cress. It isn't a true perennial, but these plants differ from the typical lab strains at only two loci, and the result is a long lived bushy plant (d) with woody stems (e). In (f) you see the difference between the mutant strains and the wild-type. (From Melzer et al.)

True enough. It normally is. But in a recent paper,¹ Siegbert Melzer and colleagues show that changes in just three genes can change the wimpy little mouse-cress pictured above into a robust, woody (or at least semi-woody) perennial. The precise mechanisms responsible for the phenotype aren't clear, but two of the loci involved SOC1 and FUL are involved in the transition from vegetative growth to flower production at the tip of growing stems. The third locus, FT, enhances the effects seen in the figure to the left making the triple mutants look even more like a typical perennial.

There's a long way to go before claiming that changes in these three genes are primarily responsible for annual-perennial life-history transitions, but results in this paper illustrate a very important point. Dramatic life-history and morphological changes may be influenced by a small number of genes. Understanding the ecological conditions that determine when such mutations are favored isn't easy, and unraveling the full evolutionary story of the transition will be even more difficult. And there's no assurance that the same story will apply to all transitions from annual to perennial or the reverse. In fact, my bet is that there will be at least several different stories associated with several different mechanisms.

But results in this paper remind us that major life-history transitions and morphological changes are possible with relatively simple genetic changes. Perennial mouse-ear cress provides yet another example of how biologists experimentally investigate the evolution of new features.²

¹Siegbert Melzer, Frederic Lens, Jerôme Gennen, Steffen Vanneste, Antje Rohde, Tom Beeckman (2008). Flowering-time genes modulate meristem determinacy and growth form in Arabidopsis thaliana Nature Genetics, 40 (12), 1489-1492 DOI: 10.1038/ng.253
²When was the last time you saw a creationist or intelligent design proponent do that?

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