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Introduction to mycorrhizal symbioses

What is the relationship between mycorrhizal fungi and plant roots?

Mycorrhizal fungi live in plant roots and provide plants with water and nutrients extracted from the soil. In exchange, plants give their fungal partners sugar they have produced from sunlight. This relationship is called a mutualistic symbiosis and is of great benefit to plants and fungi. Without this partnership, most plants and mycorrhizal fungi would not be able to survive in nature.12

diagram of mycorrhiza
Overview of the mycorrhizal symbiosis.
IMAGE CREDIT AND USE
Overview of mycorrhizal symbiosis by Sabrina Setaro is licensed under a CC BY 4.0 license.

What do mycorrhizal fungi look like?

There are many types of mycorrhizal fungi and some such as fly agarics, porcini mushrooms and truffles may already be familiar to you. Others are only visible with the aid of a microscope and are more difficult to identify. Four of the most prevalent mycorrhizal types that can be distinguished are: arbuscular mycorrhiza, ectomycorrhiza, orchid mycorrhiza and ericoid mycorrhiza2. Other mycorrhizal types do occur but have only been found in relatively few plant species.



Arbuscular mycorrhiza

Arbuscular mycorrhiza is the most common mycorrhizal type. Trees, shrubs, herbs, ferns and liverworts all form arbuscular mycorrhiza. Some plants though have associations with other mycorrhizal fungi or none at all 3. Arbuscular mycorrhiza is also the oldest mycorrhizal type and this relationship had already been formed by the earliest land plants 4.
Fungi forming this type of mycorrhiza belong to the same kinship group 5, which means they can be grouped together based on their phylogenetic relationships. The kinship group of arbuscular mycorrhizal fungi is the fungal phylum Glomeromycota. These fungi do not form fruiting bodies such as e.g. fly agarics and can mainly only be observed in roots or in the soil.

Images of arbuscular mycorrhiza or plants with arbuscular mycorrhizae
1) Arbuscular mycorrhiza. Arrow points to an arbuscle (a characteristic structure of arbuscular mycorrhizal fungi). Scale bar: 100ɥm. 2 - 4) Selected plants and vegetation types where arbuscular mycorrhizal fungi occur: 2. Plants in the savanna. 3. Understory plants and trees in temperate and tropical forests. 4. Liverworts.
IMAGE CREDIT AND USE

1) by Sabrina Setaro is licensed under a CC BY-SA 4.0 license. Permission beyond the scope of this license may be available upon request.
2) by Woodlouse is licensed under a CC BY-SA 2.0 license.
3) by Sabrina Setaro is licensed under a CC BY 4.0 license.
4) by Bushman.K is licensed under a CC BY-NC 2.0 license.


Ectomycorrhiza

This mycorrhizal type is the most conspicuous one as it can be easily distinguished from non-mycorrhizal roots. Ectomycorrhizae can be observed with the naked eye or with the help of a magnifying glass. Many trees of temperate forests such as oaks, pine, spruce and beech form ectomycorrhizae. Also, many edible mushrooms (e.g. porcini mushroom and truffles) are ectomycorrhizal fungi. The reason why these fungi are relatively expensive is because they can only grow in association with their host plants and therefore cannot be cultivated in mushroom farms. Fungi forming ectomycorrhiza do not form a kinship group, instead they belong to different fungal phyla (mainly Basidiomycota and Ascomycota) 2. The term ectomycorrhizal fungi only describes a form group including all fungi that form ectomycorrhizae with plants, but this is not a phylogenetic group.

Pictures of ectomycorrhiza and plants, fungi or vegetation types dominated by ectomycorrhizae
1) Fly agaric is a fungus that forms ectomycorrhizae. 2) Ectomycorrhiza formed by a fly agaric. Notice that roots are interwoven with hyphae (whitish hair-like structures). 3) Pine trees form ectomycorrhizae and deer eat the fungal fruiting bodies. 4) Beech trees form ectomycorrhizae and are the dominant trees of many central European forests.
IMAGE CREDIT AND USE

1) by R Henrik Nilsson et al. is licensed under a CC BY 2.0 license.
2) by R Henrik Nilsson et al. is licensed under a CC BY 2.0 license.
3) by Sabrina Setaro is licensed under a CC BY 2.0 license.
4) by Martin Liebermann is licensed under a CC BY-NC-SA 2.0 license.

  • This wikipedia page (accessed Dec. 2013) also gives good information about ectomycorrhizae.


Orchid mycorrhiza

Orchids have a special association with mycorrhizal fungi and are completely dependent on their fungi for germination and early growth. This allows the plants to have seeds without nutrients, which is why orchid have the smallest seeds found among plants. Mycorrhizal fungi feed the orchid embryo until it becomes photosynthetic and able to produce its own nutrients. As soon as the orchid develops green leaves, the symbiosis becomes mutualistic and the fungi get paid back by receiving nutritious sugars 2. Orchid mycorrhizal fungi are a form group - not a kinship group, because they belong to different fungal phyla (Ascomycota and Basidiomycota). The opposite is true for their host plants - only plants of the family Orchidaceae form orchid mycorrhiza, which suggests that this mycorrhizal symbiosis evolved only once in the evolutionary history of plants.

Pictures of orchid and orchid seeds
1) Flowers of an orchid 2) Orchid seeds (arrows) emerging from orchid seed capsule.
IMAGE CREDIT AND USE

1) by Sabrina Setaro is licensed under a CC BY 4.0 license.
2) by douneika is licensed under a CC BY-NC-SA 2.0 license.


Ericoid mycorrhiza

Ericoid mycorrhiza are only found in plants belonging to the heather family (Ericaceae) and in some ferns2. Fungi forming ericoid mycorrhiza belong to the phyla Ascomycota and Basidiomycota. Some of these fungi have extraordinary capabilities of breaking down organic materials and releasing these to their plant hosts 6. Because of their mycorrhizal fungi, Ericaceae are able to grow in harsh environmental conditions with high pH and low nutrients - such as occur in heathlands 7. Ericoid mycorrhizal fungi can even protect their host plants from accumulating toxic substances (heavy metals) that can occurr in soils 8.

Images of Ericaceae and ericoid mycorrhizae
1) Ericoid mycorrhiza characterized by hyphae (arrows) forming coils in the outer root cells. 2) Flowers of Vaccinium gracilis, a blueberry of the heather family.
IMAGE CREDIT AND USE

1) by Sabrina Setaro. With kind permission from Springer Science+Business Media: Mycological Progress, Anatomy and ultrastructure of mycorrhizal associations of neotropical Ericaceae, 5, 2006, 249, Sabrina Setaro, Ingrid Kottke, Franz Oberwinkler, 2b, copyright by German Mycological Society and Springer-Verlag 2006
2) by Sabrina Setaro is licensed under a CC BY 4.0 license.



1. Moore, D., G. D. Robson, and A. P. J. Trinci. 21st Century Guidebook to Fungi. Cambridge University Press, 2011. http://books.google.com/books?id=oEmKmLkFaWwC.
2. Smith, S. E, and D. J Read. Mycorrhizal Symbiosis. Amsterdam; Boston: Academic Press, 2008.
3. Wang, B., and Y.-L. Qiu. “Phylogenetic Distribution and Evolution of Mycorrhizas in Land Plants.” Mycorrhiza 16, no. 5 (May 6, 2006): 299–363. doi:10.1007/s00572-005-0033-6.
4. Redecker, D. “Glomalean Fungi from the Ordovician.” Science 289, no. 5486 (September 15, 2000): 1920–1921. doi:10.1126/science.289.5486.1920.
5. Schüβler, Arthur, Daniel Schwarzott, and Christopher Walker. “A New Fungal Phylum, the Glomeromycota: Phylogeny and Evolution.” Mycological Research 105, no. 12 (2001): 1413–1421. http://www.sciencedirect.com/science/article/pii/S0953756208620262.
6. Stribley, D. P., and D. J. Read. “The Biology of Mycorrhiza in the Ericaceae.” New Phytologist 86, no. 4 (1980): 365–371. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1980.tb01677.x/abstract.
7. Cairney, J. W. G., and A. A. Meharg. “Ericoid Mycorrhiza: a Partnership That Exploits Harsh Edaphic Conditions.” European Journal of Soil Science 54, no. 4 (2003): 735–740. http://onlinelibrary.wiley.com/doi/10.1046/j.1351-0754.2003.0555.x/full.
8. Bradley, R., A. J. Burt, and D. J. Read. “The Biology of Mycorrhiza in the Ericaceae.” New Phytologist 91, no. 2 (1982): 197–209. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1982.tb03306.x/abstract.





Creative Commons License
This text by Sabrina Setaro is licensed under a Creative Commons Attribution 4.0 International License.

Please note: Images have individual licenses. For credit and use see information below each image.


Created by ssetaro. Last Modification: Friday 30 of May, 2014 19:47:11 EDT by ssetaro.