26.7 Fungal Symbioses with Plant Roots
Fungi form many types symbioses with plants. These also would fit into the concept
of cheating to gain evolutionary benefit against ones competitors.
Bonfante and Genre (2010) very nicely summarized the mechanisms and benefits
of mycorrhizal symbioses including an explanation of the differences between
ectomycorrhiza and arbuscular endomycorrhiza. The word ‘mycorrhiza’ means
‘mushroom root’ in its original interpretation, and that term was created by Bernhard
Frank (Frank 1885).
26.7.1 Ectomycorrhizae
Ectomycorrhizal fungi produce an externally visible symbiosis with the roots of their
host plants. Figure 26.31 {Ectomycorrhiza of Amanita} shows the appearance of
plant root tips that are covered with a white layer of hyphae. That external layer is
termed a fungal sheath or mantle. That fungal growth typically, although not always,
extends inside of the root with fungal hyphae invading the intercellular spaces of the
epidermis and cortex to produce what is described as a Hartig net. This net is an
interface which facilitates exchange between the two symbionts. Hyphae also extend
outward from the mantle into the surrounding soil. Presumably, the fungus helps the
plant to take up nutrients and minerals from the surrounding soil and that action
could include the fungus helping to increase availability of nitrogenous compounds.
In return, the fungus may receive carbohydrate products from the plant. An individual plant may have ectomycorrhizal symbioses with more than a single species of
fungus. Some of the ectomycorrhizal fungi are generalists that can form symbiotic
associations with many different host plant species. Other fungi seem to enter
ectomycorrhizal associations with only a single species of host plant.
The fruiting bodies of some ectomycorrhizal fungi, such as the Amanita shown in
Fig. 26.31, emerge above the ground. Other ectomycorrhizal fungi produce fruiting
bodies that must be excavated from the soil and then ingested by symbiotic animals
in order to achieve dispersal of the fungal spores. Examples of the latter category are
the fungal fruiting bodies know as truffles.
Bernhard Frank (Frank 1885) was the first person to describe ectomycorrhiza. He
was, at that time, trying to develop practical methods for truffle cultivation. Frank
believed that the organic connection between the root and the fungal mycelium
formed a morphologically independent organ, with their being an interdependence of
the growth of both parts which revealed a close relationship of physiological
functions existing between the plant and fungus. By my translation, I would quote
Frank as stating “The whole body is therefore neither tree root nor fungus alone, but
similar to the lichen thallus, a union of two different beings into a single morphological organ, which can perhaps be aptly referred to as a mushroom root, a
‘mycorhiza’”.
26 The Game of Evolution Is Won by Competitive Cheating
581
Fungi form many types symbioses with plants. These also would fit into the concept
of cheating to gain evolutionary benefit against ones competitors.
Bonfante and Genre (2010) very nicely summarized the mechanisms and benefits
of mycorrhizal symbioses including an explanation of the differences between
ectomycorrhiza and arbuscular endomycorrhiza. The word ‘mycorrhiza’ means
‘mushroom root’ in its original interpretation, and that term was created by Bernhard
Frank (Frank 1885).
26.7.1 Ectomycorrhizae
Ectomycorrhizal fungi produce an externally visible symbiosis with the roots of their
host plants. Figure 26.31 {Ectomycorrhiza of Amanita} shows the appearance of
plant root tips that are covered with a white layer of hyphae. That external layer is
termed a fungal sheath or mantle. That fungal growth typically, although not always,
extends inside of the root with fungal hyphae invading the intercellular spaces of the
epidermis and cortex to produce what is described as a Hartig net. This net is an
interface which facilitates exchange between the two symbionts. Hyphae also extend
outward from the mantle into the surrounding soil. Presumably, the fungus helps the
plant to take up nutrients and minerals from the surrounding soil and that action
could include the fungus helping to increase availability of nitrogenous compounds.
In return, the fungus may receive carbohydrate products from the plant. An individual plant may have ectomycorrhizal symbioses with more than a single species of
fungus. Some of the ectomycorrhizal fungi are generalists that can form symbiotic
associations with many different host plant species. Other fungi seem to enter
ectomycorrhizal associations with only a single species of host plant.
The fruiting bodies of some ectomycorrhizal fungi, such as the Amanita shown in
Fig. 26.31, emerge above the ground. Other ectomycorrhizal fungi produce fruiting
bodies that must be excavated from the soil and then ingested by symbiotic animals
in order to achieve dispersal of the fungal spores. Examples of the latter category are
the fungal fruiting bodies know as truffles.
Bernhard Frank (Frank 1885) was the first person to describe ectomycorrhiza. He
was, at that time, trying to develop practical methods for truffle cultivation. Frank
believed that the organic connection between the root and the fungal mycelium
formed a morphologically independent organ, with their being an interdependence of
the growth of both parts which revealed a close relationship of physiological
functions existing between the plant and fungus. By my translation, I would quote
Frank as stating “The whole body is therefore neither tree root nor fungus alone, but
similar to the lichen thallus, a union of two different beings into a single morphological organ, which can perhaps be aptly referred to as a mushroom root, a
‘mycorhiza’”.
26 The Game of Evolution Is Won by Competitive Cheating
581