Heat but no Panic

If I would show you this picture and tell you that it’s an image of a hot spring from the Yellowstone National Park what would be you first though?

Hot spring

Well at least for me it went something like this:
Ah… Yellowstone… the place I must must must go some day…
Hot spring you say? How hot is it? I know they have lots of extremophiles in them… I must must must go there some day…

Ok, so yes, Yellowstone National Park is one of my dream places to go to one day. It is really an incredible place (it is actually on top of a supervolcano) with lots of incredible flora and fauna and as I’ve already mentioned it’s teeming with extremophiles.

Now, what are extremophiles? I guess many have already heard of them but if definition is needed then extremophiles are organisms that live in conditions that us, humans, would consider to be inhabitable. The conditions can vary from things like extreme temperatures to highly acidic water and accordingly the extremophiles are grouped into “something”-loving groups, such as halophile (salt-loving), thermophile (heat-loving), acidophile (acid-loving), etc. To many people the first thought when someone mentions extremophiles is about these bacteria-like microorganisms called Archeaea (they are actually more closely related to us then the “real” bacteria are, but that’s for another post) but an extremophile can be any organism, e.g. even a shrimp living in Mariana Trench → I’m sure it’s not the most comfortable place to live under 100MPa pressure. Having said that, one must remember that although to us the conditions seem to be extreme the fact is that to the organism itself anything else is extreme, therefore, it is so difficult to study extremophiles in the lab because you have to recreate their natural habitat in the lab which is neither easy nor very simple to work with if, say, you have to handle things that need to be kept at 100Co.

Ok, but I went a bit off topic here. The picture, remember?

So what about it? If you google Yellowstone I’m sure you will find much more awesome looking hot springs compared to this one but what is so special about this image is actually the tiny grass plant that you can see in it. Look at it again, and prepare to be amazed as I tell you what’s so special and peculiar about it ☺ .

The plant itself does not look anything special but looks can be misleading and they certainly are in this case! The plant is a panic grass called Dichanthelium lanuginosum, and the first thing to notice is simply the fact that it grows almost in the hot spring! In fact, the soil it grows in is at least 65 °C, which is about the temperature that denaturates proteins (think of what happens to the egg white as you boil it). Naturally, the question then is how does it do it, or can all plants do that? Well, no, normally too much heat (more than 37 °C) will have significant effects on the plant, such as wilting, chlorosis and death, and it is very rarely that plants can withstand such high temperatures.
So here is a point when a second player comes into the scene and its name is Curvularia protuberata, a fungus, which lives in the tissues of the panic grass. Okay so now we also have a fungus that can also live in high temperatures? Well, not exactly, see if you take panic grass and fungus and grow them together they both live happily at high temperatures, however, if you separate them neither can grow at higher than 38°C. So there has to be something else going on in here and sure enough when help is needed we call a microbe and not just any microbe but my favorite one- a virus. It turns out that the C. protuberate fungus has a virus (which now has been aptly named a Curvularia thermal tolerance virus (CThTV)) living inside it and CThTV seems to be the key to thermal resistance of both plant and the fungus. The following experiment proves the point:

Take the wild type (Wt) panic grass (i.e. the one with fungus and the virus inside it) and it grows happily at plant pots that are heated to 65 °C.
Take the panic grass that has no fungi (NS), grow it in 65 °C soil and the plant dies.
Take the panic grass that has the fungus inside but the fungus is virus-free (Vf) and the plant dies.
Take the panic grass and grow it in hot soil with the fungus that has been re-infected with CThTV (An) and the plant shows almost no signs of dead tissue.
The beautiful phenotypes are illustrated below.


And voila! And just to finish fulfilling the Koch’s postulates if you take a tomato plant and infect it with the virus-containing fungus the tomato also becomes much more tolerant to high soil temperature (which it normally is not).

So now we know that the panic grass that grows in the Yellowstone National Park can withstands high soil temperatures because it has a virus-containing fungus growing in it!!! And I wish I could tell you what exactly does the virus do to make the fungus and the plant heat-tolerant, however, here I cannot do much, as it hasn’t yet been fully understood… The initial comparisons of the mRNAs made in fungus with and without CThTV seem to suggest that synthesis of a sugar called trehalose and a pigment called melanin might be involved (the former is known to provide heat and drought tolerance to various fungi and the later has multiple function including abiotic-stress tolerance) but not much more is yet known.

Three-way symbiosis

These are the kind of stories that I love science for. What are the chances of such relationships? I’m sure there are many more that we have missed but this also leaves much to be discovered! In the end it is all about evolution, over millions of years these relations have come to be, as Darwin wrote: “from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

Márquez, Luis M., et al. “A virus in a fungus in a plant: three-way symbiosis required for thermal tolerance.” science 315.5811 (2007): 513-515.

Roossinck, Marilyn J. “The good viruses: viral mutualistic symbioses.” Nature Reviews Microbiology 9.2 (2011): 99-108.

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