This which a plant is fertilized and watered.

This lesson explores common types of mycorrhizae and describes their function. Additionally, this lesson examines their ecological and economic importance.

What Are Mycorrhizae?

Mycorrhizae are a symbiosis, or close, long-term relationship between two or more species. In this case, the species are the roots of a host plant and a mycorrhizal fungus. The word mycorrhizal means ‘fungus root’ in Greek.

Approximately, 95% of vascular plants belong to genera that form mycorrhizal relationships, and it is likely that 80% of plant species associate with mycorrhizal fungi. Plant families that typically do not associate with mycorrhizal fungi are often weedy or early colonizers, and include Amaranthaceae, the pigweed family, and Brassicaceae, the mustard family.

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How Do Mycorrhizae Work?

In most cases, the symbiotic relationship between host plants and the mycorrhizal fungus is mutualistic, or mutually beneficial. Mycorrhizal fungi come into direct contact with plant roots and with the soil, adding to the plants ability to gather nutrients and water from the soil through the fungus. In exchange, the plant feeds the fungus sugars it produces during photosynthesis. Thus, the relationship is beneficial to both.

The plant receives more water and nutrients than it could on its own, and the fungus gets the food it needs for energy.Sometimes, however, this relationship can change. Consider a situation in which a plant is fertilized and watered.

It doesn’t need assistance from a fungus to gather nutrients and water, so it is feeding the fungus for nothing. In this case, the relationship isn’t mutualistic, only the fungus is benefiting, so the relationship could be considered mildly parasitic. Often, under these circumstances, a plant will reduce its associations with mycorrhizal fungi.

Mycorrhizal Fungi & Their Hosts

There are three major types of mycorrhizal fungi, each associating with different types of plants.

  • Ectomycorrhizal fungi (or EM fungi) form a dense sleeve around the root and a net between and around cells of roots.

    However, they don’t enter the actual plant cells. To remember this, recall that ‘ecto-‘ means ‘outside’. EM fungi are most common in temperate regions on the roots of trees and shrubs, especially conifers. These fungi are important in phosphorous and often nitrogen nutrition for the trees and shrubs with which they associate. Many mushrooms of forest floors are the reproductive organs of EM fungi. We only see a very small portion of the total fungus.

  • Arbuscular mycorrhizal fungi (or AM fungi) are very common and associate with a variety of plant species from crops and grasses to sunflowers and legumes. AM fungi actually penetrate the cells of roots and build special organs referred to as arbuscules. It is thought that arbuscules, which resemble trees, are a place for exchange of nutrients between the fungus and plant. AM fungi explore the soil with many hyphae, which are very much like branches or plant roots, and are important for phosphorous and water acquisition for the plants with which they associate.
  • Ericoid mycorrhizal fungi associate with a particular group of plants referred to as ericaceous, in the family Ericaceae. These plants inhabit acidic soils that are typically very low in nitrogen, and include alpine species, heathers and common crop species, like cranberries and blueberries.

    Ericoid mycorrhizal fungi form a loose net of hyphae around the root and penetrate root cells.

There are a few other, more specialized types of mycorrhizal fungi, including those that assist parasitic plants, orchids, and others.

Ecological ; Economic Importance

Associations with mycorrhizal fungi can give plants an edge in certain situations, and in some cases, may harm their competitive ability. The way a plant community looks aboveground is often directly affected by mycorrhizal associations since half or more of the plant exists belowground.

Mycorrhizae can sometimes allow for the ‘stealing’ of nutrients by one plant from another plant, but they can also provide protection from plant predators and pathogens. Many crop plants benefit from mycorrhizae, which becomes a direct benefit to our dinner tables.Associations between plants and fungi as well as other organisms in the soil are complex, and mycorrhizal fungi are typically involved in some way. Their importance ecologically and economically is vast, but in some ways, immeasurable.

One could say that mycorrhizeae are priceless!

Lesson Summary

Mycorrhizae are a symbiotic association between a plant host root and a mycorrhizal fungus. Typically, the symbiosis is mutualistic, but can sometimes be parasitic. There are several types of mycorrhizal fungi, with the three most common being ectomycorrhizal fungi, arbuscular mycorrhizal fungi and ericacoid mycorrhizal fungi. They are important ecologically as mitigators of plant relationships and economically as helpers of many crop plants. Mycorrhizae are a fascinating component of the biology of soil and the belowground life of plants.


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