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| Young mushrooms of fly agaric, Amanita muscaria, in a northern Minnesota mixed coniferous-deciduous forest.The caps of the mushrooms will flatten and expand up to 10 inches wide. |
Chances are, you recognize this mushroom. Fly agaric has long been known for its colorful cap, its hallucinogenic effects, and its supposed ability to attract and kill flies. It has also found its way into popular literature and media; It’s the Alice in Wonderland mushroom, the place where Smurfs live, and the Super Mushroom in Super Mario Bros. games. Unfortunately for some, it’s also poisonous, even deadly (1, 2).
Less known but much more significant is what fly agaric does below ground. The mushroom is just the visible part, the body that produces spores for reproduction. Underneath it is an extensive network of thread-like strands called hyphae (HY-fee), together called the mycelium (my-SEE-lee-um). The hyphae are like hunter-gatherers; they run through the soil and absorb water and nutrients to support the rest of the fungus.
Fly agaric also needs organic (carbon-containing) molecules,
such as sugars and amino acids, the building blocks of proteins. Many fungi
obtain these by breaking down dead organic matter in the soil. Fly agaric,
though, goes about it differently: It barters with living plants.
The one above likely does business with white pine (Pinus
strobus), red pine (Pinus resinosa), paper birch (Betula
papyrifera) or quaking aspen (Populus tremuloides) in the northern
Minnesota forest where it grew (3). Beneath the surface, its mycelium wraps
around the tips of the trees’ roots, forming a sheath. Some of the hyphae in
the sheath grow into the root and form a net around some of the cells. That’s
where the exchange occurs: The fungus gives water and nutrients to the plant, while
the plant gives organic molecules to the fungus.
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| Illustration of an ectomycorrhiza, which forms a sheath around root tips. Hyphae cross the epidermis and enter the root cortex. There they form a net, called a Hartig net, between the cells. The unit of measurement in the upper right scale is 100 microns. One hundred microns is the approximate width of a human hair. The photographs are colorized images produced by a scanning electron microscope, or SEM. Illustration by Atrebe10, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license, via Wikimedia. |
This symbiotic relationship is called a mycorrhiza (MY-co-RY-za), literally “fungus root.” The sheathing type, called an ectomycorrhiza, is relatively new, about 200 million years old give or take a few million years. An older type, roughly 400 million years old, doesn’t sheath a plant’s roots but instead grows inside the root cells themselves, where the exchange of nutrients, water and organic compounds occurs. This type is called an endomycorrhiza or arbuscular mycorrhiza, the latter named for the tree-like growth, called an arbuscule, the fungus forms inside the cells.
Development of mycorrhizae (plural, ending in -zee) was critical to the transition of plants from water to land hundreds of millions of years ago (4, 5 7). These early land plants had no roots or vascular (conducting) tissues. Those that eventually developed symbiotic associations with fungi had an advantage, because mycorrhizae could extend the plant's reach into the soil to obtain water and nutrients. Mycorrhizae were so beneficial that they developed independently many times over, involving different plants and fungi (6).
Given that far-reaching history, it’s no surprise that there are more types of mycorrhizae than the two described above. Ecto- and endomycorrhizae are the most common, but there are at least two more types (6). Whatever the type, in most cases both partners benefit and often are dependent on each other for survival. Without their association, neither partner would thrive. Their ecosystems wouldn’t, either.
That’s because approximately 80% of modern terrestrial
plants are mycorrhizal, although the percentage of such plants in a community
can vary (7). Prairies, deciduous forests, coniferous forests, and other
terrestrial ecosystems are generally dominated by mycorrhizal plants. They are
the foundation of these systems, supporting all the trophic (feeding) levels
above them. A mycorrhizal white pine, for example, grows the cones that hold
the seeds that feed a variety of birds and mammals, which in turn have their
own roles in their communities.
Mycorrhizae are so important to ecosystems that restoring
highly disturbed places, such as mines and abandoned agricultural fields, can be
helped by adding mycorrhizal fungi to soils or seeds when restoration begins. Mycorrhizae have also been suggested as possible remedies for high carbon dioxide levels in our atmosphere, because they can help move carbon below ground. These practices will be the subjects of the next post.
References
1. Two cases of severe Amanita muscaria poisoning including a fatality. Ethan M. Meisel, MD, and others. Wilderness and Environmental Medicine, Vol. 33, No. 4, 2022.
2. Notes from the field: Acute intoxications from consumption of Amanita muscaria mushrooms — Minnesota, 2018. Joanne Taylor and others. MMWR Morb Mortal Wkly Rep 2019, Vol. 68: pp. 483–484, 2019.
3. Fungus associates of ectotrophic mycorrhizae. James M. Trappe. Botanical Review, Vol. 28, No. 4: pp. 538-606, 1962. Available for reading with a free JSTOR account at https://www.jstor.org/stable/4353659.
4. The origin and evolution of mycorrhizal symbioses: from palaeomycology to phylogenomics. Christine Strullu-Derrien and others. New Phytologist, Vol. 220, No. 4: pp.1012-1030, 2018.
5. History of mycorrhizae. Jake Sun, Lindenwood University. The Confluence, Vol. 1, No. 2: Art. 2, 2022.
6. Mycorrhizal Fungi. Society for the Protection of Underground Networks. Website accessed February 3, 2025.
7. Coevolution of roots and mycorrhizas of land plants. Mark C. Brundrett. New Phytologist, Vol. 154, No. 2: pp. 275-304, 2002.
