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Mycorrhizae: What are they and what do they do?
The success of any plant species such as chestnut is dependent upon environmental conditions such as soil pH, soil porosity, water availability, light availability, and so on. Also important are the mycorrhizal fungi present in the soil. "Myco" means fungus and "rhiza" means root, so mycorrhiza literally means "fungus-root." The mycorrhizal association was first recognized and described by Albert Bernhard Frank in 1885, although even Theophrastus of ancient Greece reported seeing this unique relationship. Mycorrhizal fungi form mutualistically beneficial relationships with the roots of 90% of all plants by providing increased uptake of nutrients such as nitrogen, potassium and phosphorus. In return, the fungi receive sugars from photosynthesis of the plant host (symbiont). In most cases, plants are dependent on mycorrhizae for survival.
Figure 1: Mycorrhizal fungi come in many shapes, colors, and sizes. Above (A) the chanterelle (Cantharellus cibarius) and below (B) the death angel (Amanita bisporigera) are examples ectomycorrhizal fungi. [click on image to see full sized photo]
In a basic sense, mycorrhizal fungi wrap themselves around the roots of a suitable plant host and their hyphae (filamentous vegetative cells of fungi) emanate out into the surrounding soil. This effectively increases the surface area of the root system, allowing for the uptake of water and essential nutrients. There are seven different kinds of mycorrhizal associations recognized, but the two most common types are endomycorrhizae and ectomycorrhizae. As with other members of the family Fagaceae (including oak and beech), chestnut is known to form ectomycorrhizae with members of the Basidiomycota and Ascomycota (these are the mushroom forming fungi). Other forest trees, such as maple and walnut, form endomycorrhizae with microscopic members of the Zygomycota (Glomeromycota).
Ectomycorrhizal fungi include many of the edible "gourmet mushrooms" such as the chanterelle, Cantharellus cibarius (Figure 1a), while others are deadly poisonous such as the Death Angel, Amanita bisporigera (Figure 1b). Fruiting bodies (mushrooms) of ectomycorrhizal fungi are incredibly diverse, ranging in color, shape (as you can see in Figure 1), to tiny inconspicuous species that are often overlooked. Ectomycorrhizal root tips tend to grow short and thick, branch dichotomously, and may have vegetative fungal elements (hyphae) wrapped around the root (Figure 2 c, d). Root tips also display diverse characteristics, similar to their above-ground counterparts.
Figure 2: Above (C) and below (D) are ecotomycorrhizal root tips that also display various shapes, colors, and sizes. [click on image to see full sized photo]
In terms of distribution, fungi follow basic ecological principles just like plants and animals; different species inhabit different ecological niches spatially and temporally (through space and time). In general, mycorrhizal fungal diversity in a community parallels the diversity of the plant community in the same ecosystem. So if the plant community is more diverse, typically the mycorrhizal community is also very diverse and vice versa. Because mycorrhizae involve intimate contact (some actually form structures inside the plant root), there is a degree of specificity between fungal species and plant hosts. Some mycorrhizal plant species are incredibly specific – only forming mycorrhizae with the 'correct' fungal partner. For example, mycorrhizal fungi that are found with Douglas fir may not be able to form mycorrhizae with chestnut (of course, there are always exceptions). It is also important to note that there is a succession of mycorrhizal fungi that are associated with plants throughout their life spans. For example, fungi that benefit a seedling may not be the same fungi that would benefit a mature tree. As you can imagine, the microbial interactions that occur below ground are extremely complex.
The American Chestnut and Mycorrhizae
The simplest answer to the question, "What do we know about mycorrhizae and the American chestnut tree?" would be ... Not Much. Because most mycorrhizal fungi are obligate mutualists (they cannot be cultured on laboratory media) and the American chestnut has been decimated in its native range for more than 50 years, studying chestnut mycorrhizae is very difficult. Relatively recent advances in molecular biology have opened the door (however slight it may be) to studying these important interactions in nature. Using DNA extraction and sequencing techniques we are able to identify which species of fungi are associated with which plant hosts in their natural ecosystems. Two recent studies have examined different aspects of chestnut mycorrhizae. (1) Kris Dulmer and Thomas Horton (State University of New York - College of Environmental Science and Forestry in Syracuse, NY) have shown that American chestnut seedlings can tap into existing ectomycorrhizal networks. Chestnut seedlings were planted in several locations that were dominated by oak species (mostly Quercus rubra), and their results indicate that the seedlings were able to form mycorrhizae with fungi that were also colonizing roots of the oak species. (2) Another study was done by myself, Daniel Lindner (USDA Forest Service – Center for Forest Mycology Research), and Thomas Volk (University of Wisconsin-La Crosse). We looked at the mycorrhizal community of a disjunct stand of American chestnut in West Salem, in western Wisconsin. Through a combination of morphological observations (identifying above-ground fruiting bodies) and molecular techniques (DNA based identifications of mycorrhizae on root tips) we were able to identify putative mycorrhizal associates of a mature stand of American chestnut. This work is currently in preparation for submission, and I've created a web site that includes species names and pictures as well as DNA sequences: www.chestnutfungi.com. It is our hope that these data will be useful for the reforestation of the once dominant American chestnut. By understanding which mycorrhizal associates are beneficial for a successful chestnut stand, future research can focus on how these mycorrhizae can be utilized and applied to facilitate reforestation of this once majestic tree.
Graduate Assistant – Nancy Keller Lab
B70 Russell Labs
UW-Madison, Department of Plant Pathology
1630 Linden Dr., Madison, WI 53706