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Hidden Partners: Mycorrhizal Fungi and Plants

What are Ectomycorrhizae?

The ectomycorrhizas are composed of at least 65 fungal genera, most of which are in the fungal class Basidiomycetes, in the phylum Basidiomycota, although some are in the Ascomycota. Well known genera in the Basidiomycetes include Amanita and in the Ascomycetes, Tuber (truffle).

What separates ectomycorrhizae from the other groups of mycorrhizal fungi is the presence of a fungal sheath around the plant roots, a hartig net which forms between the cortical root cells, and the absence of fungal hyphae within the plant cells.

Ectomycorrhizal fungi usually form large amounts of mycelia that grow from the tip of the plant roots (the fungal sheath) out into the soil in search of nutrients. Large groupings of mycelia are also found at a distance from the host plant. These additional mycelia are connected to the fungal sheath by structures called rhizomorphs.

Ectomycorrhizas colonize approximately 3% of all seed plants (Gymnosperms and Angiosperms), or about 140 genera including, but not limited to, pine (Pinus), spruce (Picea), eucalyptus (Eucalyptus), beech (Fagus), birch (Betula), and oak (Quercus). The ectomycorrhizas display varying degrees of host specificity, with some fungal species being able to colonize many plant species and genera in a certain biogeographical area, and with many fungal species even colonizing the same individual tree. There is some circumstantial evidence that non-native ectomycorrhizal fungi are "invasive" and are becoming associated with plants with which they have never had an association in the past. An example is Amanita muscaria associating with Nothofagus.

Two ectomycorrhizal fungi that are very important ecologically are Hymenogaster and Melanogaster. These genera are often found living with the roots of oak and eucalyptus. They are important members of forest ecology in the Western United States and Australia. Hymenogaster and Melanogaster are subterranean fungi. They form fruiting bodies which resemble small truffles or golf balls. Inside, the basidiospores mature in a tissue called the gleba. The subclass of Basidiomycetes to which Hymenogaster and Melanogaster belong, the Gasteromycetes, do not forcibly discharge their spores and so rely upon mammals and other vectors for spore dispersal. Mammals are of particular importance to Hymenogaster and Melanogaster as their fruiting boides are often completely submerged beneath the soil surface.
Page 3: The role of the Hartig net and fungal sheath in the Ectomycorrhizae
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