Three-Way Symbiosis

Three-Way Symbiosis Photo Credit: James St. John (CC BY 2.0) via Flickr. https://www.flickr.com/photos/jsjgeology/20797185616/

You may have studied symbiotic, or mutually beneficial, relationships between two different species. A recent study discovered a triangular version.


How a virus helps a fungus help a plant. I'm Bob Hirshon and this is Science Update.

There's a kind of grass in Yellowstone National Park that thrives in hot geothermal soils, which can reach 120 degrees Fahrenheit. It was recently discovered that a fungus on its roots makes this possible. But in a strange twist, researchers have now found that the fungus confers this heat-resisting power only when it's infected with a virus. Team leader Marilyn Roossinck of the Samuel Roberts Noble Foundation in Oklahoma says this unlikely threesome is of more than just academic interest.

Marilyn Roossinck:

With the global climate changes that we are all facing now, we're going to see a lot more extreme environments on the planet. So we need to understand how plants normally tolerate natural extreme environments.

That may help us grow crops if environments that seem extreme today become more normal. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

A symbiotic relationship is a mutually beneficial relationship between two different species—kind of an "I'll scratch your back, you scratch mine" arrangement. Among the most famous is the relationship between a bird called the Spur-winged plover and the Nile crocodile. Amazingly, the normally ferocious crocodile will open its mouth wide and allow a plover to stand in it, unharmed, picking bits of food from between the crocodile's teeth. The plover gets a meal, and the crocodile gets free dental care.

Most well-known cases of symbiosis involve two parties. This one, however, is three-dimensional. Scientists had only recently discovered the relationship between the plant and the fungus: grasses found in these searingly hot soils always have the fungus on their roots, and lab experiments showed that neither the virus nor the fungus could withstand these temperatures alone.

Roossinck and her colleagues noticed that the fungi living in the soils carried a viral infection. So they decided to cure some fungus samples of the virus, reintroduce them to some grasses, and test both of them against the geothermal soil temperatures. Even together, these grasses and virus-free fungi shriveled and died, just as they had done in the earlier experiments when they were separated. Just to be sure, the researchers then cured more fungus samples of the virus, then re-infected them, and found that the re-infected fungi were once again fully heat-resistant, when paired with the grasses, of course.

It's not yet known exactly how this all works. But Roossinck's team tried colonizing tomato plants with virus-infected fungi. When they succeeded, the tomato plants became heat-resistant as well. As Roossinck suggests, that trick may come in handy in the future, since global warming may jack up the temperature of the world's soils, making it tougher to grow crops that currently thrive in them.

Now try and answer these questions:

  1. Describe the three-way symbiotic relationship that these researchers discovered.
  2. How did the researchers prove that the relationship was beneficial to all three organisms?
  3. In one of the experiments you heard about, the researchers cured some fungus samples of the virus, and then re-infected them. Suppose these re-infected fungi were not heat-resistant, even when reintroduced to the grasses. What might this suggest?
  4. Can you think of three-way symbiotic relationships in human society?

You may want to check out the March 30, 2007, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: deciphering the calls of blue whales, genetic tests for mental conditions, a three-way symbiotic relationship, studying tear film, and the truth about tanning beds.

For Educators

The Ecology of Your Skin 1: Bacteria That Live on the Skin and The Ecology of Your Skin 2: The Microbial World Is an Olfactory World both deal with humans' bacterial symbionts.

Endosymbiosis: A Friend Within, explores the symbiotic relationship between termites and microorganisms in their guts called flagellates.

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