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Hidden Partners: Mycorrhizal Fungi and Plants
NYBG Herbarium Intern Matthew Pace

What are Mycorrhizae?

If you have ever enjoyed the shade of an oak, decorated a pine, spruce or Douglas fir as a Christmas tree, admired the beauty of an orchid, or eaten a blueberry or chanterelle mushroom, you have benefited from the hidden world of mycorrhizal fungi, a world which makes the survival of most of earth's land plants possible.

Mycorrhizae are symbiotic relationships that form between fungi and plants. The fungi colonize the root system of a host plant, providing increased water and nutrient absorption capabilities while the plant provides the fungus with carbohydrates formed from photosynthesis. Mycorrhizae also offer the host plant increased protection against certain pathogens.

Approximately 90% of all vascular land plants live in some association with mycorrhizal fungi. Mycorrhizal associations are seen in the fossil record and are believed to be one of the contributing factors that allowed early land plants, including Aglaophyton major (one of the first land plants), to conquer the land.

Mycorrhizal fungi encompass many major groups of the fungus Kingdom and in the past were divided into two non-evolutionarily related groups: the ectotrophic and endotrophic based upon the position of the fungal hyphae in relation to the tissues of a plants roots. Ectomycorrhizal fungi ensheath the root cells but usually do not penetrate them (extracellular). Endomycorrhizal fungi penetrate and enter the cells of a plant root (intracellular).

Modern research has lead to the recognition of seven types of mycorrhizal fungi, subdividing the old, traditional groups. The new nomenclature is often more precise and specific to the associated plant taxa. The relatively homogenous ectomycorrhizal group largely remains with only the addition of the subgroup ectendomycorrhizas. The endomycorrhizal group has been dismantled, but specific types are now recognized: Vescicular-Arbuscular Mycorrhizas, the Orchid mycorrihzas, and those which associate with the Ericaceae (Blueberry family): the Arbutoid, Monotropoid and Ericoid mycorrhizas.

What Benefits do Mycorrhizal Fungi Offer to Plants?

Fungi are heterotropic organisms, and must absorb their food. Fungi also have the ability to easily absorb elements such a phosphorus and nitrogen which are essential for life. Plants are autotropic, producing their food in the form of carbohydrates through the process of photosynthesis. However, plants often have difficulty obtaining and absorbing many of the essential nutrients needed for life, specifically nitrogen and phosphorus.

In order to maximize both organisms abilities to thrive most plants allow, and indeed require, mycorrhizal fungi to colonize their roots. In this symbiotic and intimate relationship the hyphae of the fungus greatly increases the surface area that is open to nutrient and water absorption, maximizing the plants access to these essential compounds and elements. In return, the plant supplies the fungus with carbohydrates for use as energy.

This system of interdependence has evolved into many forms and now encompasses most land plants and fungal groups. Each group of mycorrhizal fungi interacts and colonizes its botanical host in a slightly different way. These systems of energy and nutrient exchange are often very complicated and very important ecologically, and have only recently been heavily researched. As a result they are often poorly understood.

For the purpose of simplicity, the ectomycorrhizae shall be discussed in detail throughout the rest of the site.

Page 2: What are Ectomycorrhizae?
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