In this relationship, the fungus invades a plant’s roots and takes carbon-based nutrients from the plant. At first glance, you might think the fungus is a parasite that infects the plant and takes nutrients from it. If you look at the picture above, for example, you might be inclined to think that the root is infected with a fungal parasite. That’s not the case, however, because while the fungus does, indeed, take nutrients from the plant, it also supplies the plant with critical nitrogen- and phosphorus-based chemicals that the plant has a hard time extracting from the soil. Thus, this is a mutually-beneficial relationship, which is often called a mutualistic relationship.
Because mycorrhizae are common throughout creation, there are many species of plants and fungi that participate in them. Nevertheless, the details of how mycorrhizae work are poorly understood. A new study has started to unravel those details, and the results are truly fascinating.
In the study, the scientists gave a plant’s root two different fungi. Each fungus was introduced to a different portion of the root, and each portion of the root was given a separate compartment of soil. The researchers then varied the amount of phosphorus in the soil compartment, which varied how much phosphorus-based chemicals each fungus could give to the plant. They found that when each fungus could give the plant more phosphorus-based chemicals, the plant gave the fungus more nutrients. However, when each fungus could not give the plant many phosphorus-based chemicals, the plant did not give the fungus many nutrients, even if it had a plentiful supply that it could give.2
Interestingly enough, they also manipulated the root to control how many nutrients it gave to the fungus. They found that when the root gave each fungus a lot of nutrients, the fungus gave the plant a lot of phosphorus-based chemicals. However, when the plant gave each fungus only a few nutrients, the fungus gave the plant only a few phosphorus-based chemicals, regardless of how much phosphorus was available to the fungus.
In the end, then, in a mycorrhizae, there is a real relationship between the plant and the fungus. If the fungus “treats the plant nicely,” the plant will treat the fungus nicely in return. However, if the plant doesn’t think the fungus is “doing its fair share,” the plant will “punish” the fungus accordingly. Of course, this “give and take” happens both ways, because the fungus will also “punish” or “reward” the plant, depending on what the plant does.
In the end, the authors see this as an example of a nonhuman “market economy,” where fungi and plants try to do the best job that they can do in order to get the best reward. Here is how they put it:
…in the mycorrhizal mutualism, both sides interact with multiple partners, so that neither partner can be “enslaved.” Cooperation is only stable because both partners are able to preferentially reward the other. This provides a clear, nonhuman example of how cooperation can be stabilized in a form analogous to a market economy, where there are competitive partners on both sides of the interaction and higher quality services are remunerated in both directions.
Humans think we are so clever because we have a market economy that rewards industrious behavior and punishes sloth. Well, it looks like plants and fungi have a very similar system. We know that our market economy is the result of a lot of forethought and design. How did the plant/fungus market economy come into being? Most likely, it was also the result of a lot of forethought and design.
REFERENCES
1. Gopi K. Podila and Ajit Varma, Basic Research and Applications of Mycorrhizae, I K International Publishing House, 2004, p. 87
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2. E. Toby Kiers, et. al., “Reciprocal Rewards Stabilize Cooperation in the Mycorrhizal Symbiosis,” Science 333:880-882, 2011
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I had been under the impression that the market economy sort of fell out naturally via Adam Smith’s invisible hand. Dr. Wile, would you elaborate more fully on why this form of mutualism and the market economy require forethought? Thanks!
Actually, WSH, I didn’t say that this form of mutualism required forethought. I just think it is most likely that it came about as the result of forethought. After all, Adam Smith’s invisible hand notwithstanding, the market economy we have in the US is definitely the result of forethought and design. Thus, it makes sense that similar structures are also the result of forethought and design.
Fascinating! I learned a bit about mychorrizal relationships from your exploring creation with biology textbook, but I had no idea it was this complex.
Neither did I, Evan. The more we study God’s creation, the more marvelously complex it appears.
Love this kind of stuff. You might also be interested in the signaling compounds that are apparently involved in establishing these relationships — strigolactones. Strigolactones are a newly recognized class of plant growth hormone that appear to be involved in branching responses. When excreted by plant roots however, the strigos stimulate branching in mycorrhizal fungi. This branching of the fungi increases the likelyhood that roots and fungus will ‘find’ one another in the soil environment.
Another interesting bit: Strigolactones stimulate germination of parasitic plant seeds and actually derive their name from genus of Witchweed (Striga). The mechanism is similar: Striga seeds lie dormant until they sense the strigolactones being excreted by a nearby plant. Then they germinate, find their host, etc. Striga species are still a major problem limiting production of corn/sorghum in some developing countries.
So it’s interesting to see how one class of compounds can be used by plants for benefits (plant/fungi), but can also be exploited by damaging parasites.
Wow Greg! Thanks so much for the information. It’s time for me to spend some time learning about strigolactones!
I think the fact that they can be used for beneficial and parasitic purposes is a great illustration of the effects of the Fall. A chemical the plant used solely for benefit was co-opted after the Fall by a mutated plant to help it cause damage.
Really neat post Dr. Wile.
Thanks, Pyrodin!