All for One, and One for All!

by Mark Martin

Panic grass growing well at a soil temperature
of over 50 °C (123 °F). Source.

When I recently attended the Sixth International Symbiosis Society Congress in Madison, Wisconsin, I was awed by the fascinating forms that symbiotic relationships take among diverse organisms. One talk that particularly intrigued me was from the laboratory of Marilyn Roossinck of the Samuel Roberts Nobel Foundation in Oklahoma, which described the mutualistic relationship between a virus, an endophytic fungus, a monocot, and elevated temperatures in geothermal soils. It also made me consider how readily we seem to associate the word "virus" with pathogenic associations, when nature is often far more subtle when it comes to mutualistic partnerships.

The story began in 2002 when it was found that a type of grass growing in the geothermal zones of Yellowstone National Park—panic grass, Dichanthelium lanuginosum—was able to survive intermittent high temperatures in geothermal soils (up to 65 °C.) due to its association with an endophytic fungus, Curvularia protuberata. The fungus is essential to the plant's ability to tolerate temperatures that are lethal to the non-colonized plant. Panic grass, incidentally, has nothing to do with botanical phobias; instead, the name derives from the Latin panicum, referring to foxtail millet.

The thermal areas of Yellowstone National
Park are home to thermotolerant microbes,
and even a few “macrobes”! Source.

Endophytic fungi are quite common among plants and have been implicated in a number of mutualistic associations that display enhanced stress tolerance. But the story was far stranger than this. It turns out that the "thermal tolerance" trait conferred by the endophytic fungus is actually due to a specific RNA virus onboard. (See the 2007 report in Science.) This dsRNA virus is aptly named "Curvularia thermal tolerance virus" (CThTV). Infected fungal mycelia contain two viral dsRNA molecules: a 2.2 kb dsRNA molecule that encodes two ORFs related to viral replication and a 1.8 kb dsRNA molecule with two ORFs with no similarity to any protein of known function.

It has long been known that viruses can modulate the ability of a fungus to interact pathogenically with a host organism, but this is the first report showing a clear-cut mutualistic interplay. Isolates of C. protuberata "cured" of CThTV by freeze-thaw and desiccation cycles conferred no thermal tolerance on the host plant; re-infection of the virus-free fungus with CThTV restored its ability to confer thermal tolerance to the panic grass.

The Roossinck group went on to show that C. protuberata carrying CThTV could colonize eudicots such as tomato, and even provide some thermal tolerance to the new host plant. However, colonization was not as extensive and the thermal tolerance was not as pronounced as with panic grass. As one might guess, work is continuing to determine the mechanism by which the uncharacterized ORFs within the 1.8 dsRNA of CThTV confer the thermal tolerance in this fungal-plant mutualism.

Panic grass phenotype after laboratory testing of thermotolerance (soil temperature cycling between 65 °C for ten hours a day and 37 °C for fourteen hours a day, for two weeks). Legend: Wt = plant infected with virus-bearing fungi. An = plant infected with previously “cured” fungi carrying reintroduced virus. VF = plant infected with virus-free fungi. NS = plant not infected by fungi. Source.

This kind of fungal-conferred stress tolerance may be much more that a rare oddity. In fact, there is some evidence that the ability of fungi (with or without viral modification) to increase stress tolerance is fairly common, and evolutionarily ancient. The existence of multiple "partners" involved in a mutualism has been described for ants, leeches, and many other associations. This relationship between Dichanthelium lanuginosum, Curvularia protuberata, and CThTV is reminiscent of the interrelationships between bacteriophage, Wolbachia, and insects described here.

Clearly, in what appears to be a warming world, understanding how plants can tolerate and prosper at elevated temperatures is an intriguing topic. To a "symbiophile" like myself, learning that the plant has a fungal partner, and that the fungal partner has a viral passenger—and all are working together in a finely tuned mutualistic dance—is a hot topic indeed!

Mark is associate professor in the Department of Biology, University of Puget Sound, an Associate Blogger for STC, and a passionate advocate for the Small Things.

Marquez, L., Redman, R., Rodriguez, R., & Roossinck, M. (2007). A Virus in a Fungus in a Plant: Three-Way Symbiosis Required for Thermal Tolerance Science, 315 (5811), 513-515 DOI: 10.1126/science.1136237

Comments

[Roy]

October 22nd, 2009 post.

For your information, Xylaria longipes is known as 'Dead Moll's Fingers, whereas Xylaria polymorpha is known as 'Dead Man's Fingers

[Mark O. Martin]

Nice comments from all! My first attempt to reply was eaten with gusto by my web browser crashing. So here I go again:

1. First, this entire discussion reminds me of the following:

“So naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite ‘em;
And so proceed ad infinitum.”
-Jonathan Swift (“Poetry: A Rhapsody”).

2. Dr. Worthen, I have known Jorg Graf for a number of years, and his passion for leech-microbial associations (and his puckish sense of humor) always lead me to following this topic. He gave a superb seminar at the symbiosis symposium, for example.

3. Apologies for misunderstanding qetzal’s comment. I believe that a “Russian Nesting Dolls” paradigm for thinking about microbial interactions (in some cases, “intra-action”?) is useful and accurate. Merry’s topic is particularly instructive (I came to that line of thinking while considering “R-bodies” and the “kappa” phenotype in the protist Paramecium---look at how much more is known nowadays!).

We tend to think in reductionist terms, looking at simple systems and associations to gain an intellectual foothold on a problem. And there has been much benefit from that approach.

But looking, for example, at gut microbiota creates a different kettle of metaphorical fish, with over a thousand phylotypes to consider. There are many classes of “microbial associate” in that case, clearly, including: residents that use the location as a surface and little more, co-ops that contribute to upkeep or defense, transients that are just passing through (!), and “squatters” that “set up shop” in a disturbed or damaged location, to name a few.

Some of the associations may turn out to be trivial to understand, and others rivaling systems biology style complexity.

This makes me think, in turn, of “group activities” among microbes where “cheating” arises (say, among myxobacters, as studied by Greg Velicer and others). Surely such “cheating” will occur among the many denizens of a complex microbiota…ranging from (again) trivial examples to the very subtle.

I am then reminded of this great quote:
“All organisms are nothing but a bag of other organisms walking around.”
-Thomas Miller (UC Riverside).

Some of those organisms cooperate. Others fight. Still others are oblivious to one another. In a way, we are all ecospheres, writ moderately large. And enormous---whole worlds!--- to the microbes around us!

Thanks for the nice comments.

[PWorthen]

I suspect that the number of 'true' mutualists involved in many of these systems are quite small. I've often wondered if the majority of these microbes aren't just 'hangers-on', forming a sort of prokaryotic entourage that does little to help the host organism, but doesn't do much to hinder it either. It concerns me that researchers may push mutualism because it's sexier than commensalism - ie. it looks more interesting in journal articles and grant applications.

I got a thrill when I saw the link to my paper on leeches (Worthen et al. 2006) I've been out of the research world for a few years now, but it's always nice to see someone's still reading my stuff!

[Elio]

I believe that Quetzals' question has another facet. I would interpret it as asking how many levels of parasitism can coexist in one infected host. Try salmon poisoning in dogs that that get sick from eating salmon that carry a worm that carries a rickettsia. I count four levels. Add an imaginary phage to the rickettsia and you'd have five levels. Merry came up with five already (wasp, aphid, Buchnera, Hamiltonella, and phage) in her story at http://schaechter.asmblog.org/schaechter/2009/09/good-guys-bad-guys.html. but I'm not sure all these operate at different levels.

[Mark O. Martin]

That is a great question, qetzal. There are well over a thousand phylotypes of microbes in the human gut, but how many are truly mutualists remains uncertain.

Or think about the termite hindgut!

I think that this whole concept that eukaryotes are "superorganisms" composed of a collection of vast numbers of different species may have quite a bit of validity. I feel a new T-shirt and button image coming on!

[qetzal]

I wonder what is the record for most number of essential partners in a single mutualistic relationship?