What is a Mycorrhiza?
The term mycorrhiza refers to a symbiotic relationship that can occur between a fungus and the roots of a plant. This is usually a mutualistic relationship, meaning that it has a positive effect on both the plant and the fungus. In some uncommon cases, however, the relationship can have mild detrimental effects on the plant and shift the balance of the relationship in favor of the fungus. In most cases of mycorrhiza, nutrients are shared between the plant and the fungus to the benefit of both parties. It is important to note that this exchange of nutrients is bi-directional; both the plant and the fungus receive nutrients and essential inorganic minerals from each other.
Mycorrhiza can form in many different forms with varying effects. Sometimes the fungus grows around and between the roots and only superficial contact and sharing can occur. In other cases, the symbiosis occurs at a cellular level and a much deeper and more detailed level of material sharing is possible. In some cases, the fungus is largely restricted to the area directly around and between the plant's roots, while in other cases, the fungus branches out and extends deeper into the soil. The types of relationships possible tend to depend on the soil quality and on the types of fungi and plants present in a given area.
Plants can respond in several different ways to mycorrhiza. Generally, it is beneficial to the plant as it can aid the plant in growing or reproducing more abundantly. In some cases, however, it can depress the plant's growth. This occurs especially in cases in which the fungus is completely dependent on the plant for some kind of nutrient. The response of the plant mostly depends on the nutrients needed by the plant and the fungus; significant overlap can lead to one of the two lacking the nutrients needed to flourish.
There are three primary classifications for mycorrhiza. Endomycorrhiza is a broad class that generally describes a relationship in which parts of the fungal cells penetrate the cellular membranes of some of the plant cells. In ectomycorrhiza, fungal cells form a structure that resembles a net over the root tips on the plant. Ericoid mycorrhiza, the third major classification for mycorrhiza, describes a relationship in which fungal cells coil in and around the other layer of cells in the plant's root. Generally, the fungi involved in ericoid mycorrhiza do not extend very far into the surrounding soil.
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Discussion Comments
@titans62 - You are right that some species of legume are able to fix atmospheric nitrogen into a form usable by the plants. The process isn't associated with mycorrhizal fungi, though. Legumes still have substantial interactions with mycorrhizae, but the nitrogen fixation actually happens with the help of a special bacteria that is found in the soil.
When plants do interact with mycorrhizae, the fungi can extend very far out into the soil. I don't remember the exact numbers, but I think in some cases, the fungi can bring nutrients to a tree from hundreds of feet away.
For a plant, especially a tree, that is extremely important because a lot of resources go into creating a large root system, not to mention a tree needs a lot more nutrients than what is available in the immediate area.
I'm not sure about whether diseases can get transferred between plants. I've never really thought about that, but it would be interesting to know.
Is anyone familiar with legumes and how they use mycorrhizae? I know legumes can fix nitrogen in their roots. I would assume that is done with the help of fungi. What are the reactions that take place to make it happen?
As far as the mycorrhiza and root relationship goes, how much farther out into the soil does the fungus reach? Is it a matter of feet or can it be more? I'm curious, too, can the fungi connect the roots of different trees and pass nutrients, or even diseases, that way?
@cardsfan27 - That's a good question about whether the whether the fungi can die in the soil. I'd be interested to know if anyone has ever heard of that happening.
As far as specific mycorrhizae go, I know there are specific fungi for different plants. The one I have always heard about is the association between orchids and a special fungus.
I don't remember all of the specifics, but I know that somehow orchid roots don't expand very well, and they can't reach a lot of nutrients. There is a special fungus that has to associate with the roots to help it get nutrients. Without that fungus, the plant will die. I know that even if you are trying to grow orchids, you have to make sure that fungus is in the soil first.
I would assume most plants are generalists and associate with a bunch of different mycorrhizal fungi, but like the orchid, I'm sure some species have specific interactions.
I knew fungi were involved in moving nutrients, but I didn't know there were so many different ways it could happen.
Are the mycorrhizal fungi common species that most people would recognize, or are they just special below ground fungi? Also, are different types of fungus associated with different species of plants, or do they all sort of overlap?
The other thing I was wondering is, can the soil ever lose its mycorrhizae? Do the fungi ever just die and stop helping the plants get nutrients?
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