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Plant Mutualism: the connection between fungi and plants

May 7, 2012

Plant Mutualism: Mycorrhizae

Fig. 1 -ectomycorrhiza from a fungus in the genus Amanita, common name fly agaric.

What are mycorrhizae? Mycorrhizae (mahy-kuh-rahy-zee), mycorrhiza singular, refers to the symbiotic interaction between plants and fungi and are found widely across derived families of plants. This association occurs between the hyphae of a fungus and the roots of a plant where nutrients are exchanged between the two individuals. This association has a large spectrum from mutualistic to parasitic. Most terrestrial plants form mycorrhizal connections. (Brundrett, M. 2004),(Hoeksema, J.D., Kummel, M. 2003). These plant individuals often form many mycorrhizae at once, and may associate with a very broad range of species. The Pseudotsuga menziesii, or Douglas fir, of the Pacific Northwest has been estimated to associate with near 2,000 different species of fungi in forming mycorrhizae (Hoeksema, J.D., Kummel, M. 2003).

The connections between the hyphae and roots appear in two forms, vesicular-arbuscular mycorrhizae or ectomycorrhizae. Ectomycorrhizal associations form on the epidermis with the body of the fungi as a mantle covering the root structure with hyphae extended into the rhizosphere. In contrast vesicular-arbuscular mycorrhizae push into the root structure but remain outside the cell membranes (Brundrett, M. 2004), (Hoeksema, J.D., Kummel, M. 2003). These two morphologies are species dependent, a given plant species will generally not form both types of association on their roots, and a fungi species will generally not create both although exceptions for both plants and fungi exist.

Fig. 2 No mychorrizal connection (top), Ectomycorrhizal connection (bottom left), Vesicular-Arbuscular mycorrhizal connection (bottom right).

Mycorrhizae symbiosis ranges across a spectrum between parasitism by either plant or fungi, to mutualism between the two individuals associated. Mutualism, in which the relationship is beneficial to both the fungi and plant, is the most common type of mycorrhizal association (Brundrett, M. 2004) (Bronstein, J. L. 2009). Many mycorrhizae forming fungi are also obligate symbionts, meaning they are dependent on the plant species they form with for survival. All Vesicular-Mycorrhizal fungi are obligate, and most ectomycorrhizal fungi species are obligate. (Brundrett, M. 2004) The actual mutualistic benefit is the transfer of nutrients between plants and fungi. Carbon and Nitrogen transfer is important to the plant, and the fungi use nutrients in the root for sustenance. Some plant individuals are linked together via their hyphal networks, some research indicates that this plays an ecologically important role. Carbon in particular appears to move from plant to plant via hyphal networks and actually enter the vascular tissue of the plant. The movement of nutrients may also be bidirectional (Simard, S.W. et al. 1997). Further research has been suggested in order to view just how much carbon is directly transferred via the hyphae as opposed to being lost by the original plant or fungi and then taken up by the plant receiving the transferred carbon. Whether the quantities of carbon transferred make a significant ecological impact is also uncertain, but is important to keep in mind that mycorrhizal networks are all but ubiquitous in the plant world and have a significant ecological impact in direct plant-fungi mutualism (Simard, S.W. et al. 1997) (Robinson, D. Fitter A. 1999).

The mutualistic relationships of mycorrhizae generally increase the fitness of both individuals involved, allowing them to persist and succeed in their given ecosystem. It is important to note though that because an individual plant may have so many mycorrhizae from different species, the different fungi are competing with each other for area and resources. Some fungi are more strongly mutualistic than others. These fungi and those that create parasitic mycorrhizal relationships would be expected to have a competitive edge at beating out strongly mutualistic fungi (Hoeksema, J.D., Kummel, M. 2003). However mutualism is the predominant type of mycorrhizal association and strong mutualism persists. It has been considered that many plants might facilitate the success of strongly mutualistic fungi that form mycorrhizae with them by causing dieback in root tips that are colonized by less mutualistic or parasitic fungi. The environment in which the mycorrhizae are formed effects the degree of mutualism benefit to the plant, therefore plants shouldn’t have evolved to discriminate against the parasitic and weakly mutualistic species specifically as one member of a species may benefit from a fungus that provides no benefit in another habitat. This indicates that a mechanism is in place that is controlled by the nutrient benefits being obtained by the plant (Hoeksema, J.D., Kummel, M. 2003).

Though some of the aspects of competition, exclusion, and directional network nutrient transfer are still not fully understood, it is clear mycorrhizae have become a significant ecological component in the derived evolutionary history of both kingdoms, most often increasing plant and fungi fitness. This symbiosis helps a given plant or fungi better compete for nutrients and further growth against its neighbors. Mycorrhizae also aid a plant or fungi in gathering a nutrient when it is scarce by essentially increasing the area in which it might draw nutrients from. The benefits or costs of these associations become more complex when hyphal networks between plants and the hundreds of mycorrhizae that may be on a single plant are considered. Understanding these associations and how they affect population dynamics is an important aspect for trying to understand the ecology of plants or fungi when any mycorrhizae are present in the community.

Resources

Brundrett, M. (2004) Diversity and classification of mycorrhizal associations. Biol. Rev. 79, pp. 473-495.

Hoeksema, J.D., Kummel, M. (2003) Ecological Persistence of the Plant-Mycorrhizal Mutualism: A Hypothesis from Species Coexistence  Theory. The American Naturalist. Vol. 162, No. S4, pp. S40-S50.

Simard, S.W. et al. (1997) Net carbon transfer between ectomycorrhizal tree species in field. Nature. Vol 388, pp. 579-582.

Bronstein, J. L. (2009) The evolution of facilitation and mutualism. Journal of Ecology, Vol. 97, pp. 1160–1170.

Robinson, D. Fitter A. (1999) The magnitude and control of carbon transfer between plants linked by a common mycorrhizal network. Journal of Experimental Botany, Vol. 50, No. 330, pp. 9-13

Images

Fig. 1 – Nilsson, R. Henrik, et al. The fly agaric: a common mycorrhizal fungus. 2005. Photograph. http://en.wikipedia.org/wiki/Mycorrhiza. Web. 13 Dec. 2005.
Fig. 2 – Hiott, Sally. Three views of a cross section of plant root, illustrating two predominant types of soil mycorrhizae 2009. Drawing. wild ones journal. Mar/Apr. 2009.

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2 Comments leave one →
  1. May 11, 2012 9:50 am

    Does the scientific community know anything about whether different fungi species compete with each other to form mychorrhizal relationships?

    • A Andersen permalink
      May 19, 2012 10:43 am

      Different fungi species do in fact compete for resources and the space on a root in which to uptake those resources. So the many fungi forming a mycorrhiza on one plant are all in competition with each other. The Hoeksema, J.D., Kummel, M reading that is referenced defines the factors of competition between mycorrhizal fungi in several algorithms.

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