Ectomycorrhiza
[4] Well known EcM fungal fruiting bodies include the economically important and edible truffle (Tuber) and the deadly death caps and destroying angels (Amanita).
These ectomycorrhizal fossils show clear evidence of a Hartig net, mantle and hyphae, demonstrating well-established EcM associations at least 50 million years ago.
In Eucalyptus and Alnus the Hartig net is confined to the epidermis, whereas in most gymnosperms the hyphae penetrate more deeply, into the cortical cells or the endodermis.
Most cortical type Hartig nets do not show this elongation, suggesting different strategies for increasing surface contact among species.
[15] The mantles of different EcM pairs often display characteristic traits such as color, extent of branching, and degree of complexity which are used to help identify the fungus, often in tandem with molecular analyses.
Experiments and field studies show that this can lead to the formation of common mycorrhizal networks (CMNs) that allow sharing of carbon and nutrients among the connected host plants.
[22] One study observed a bidirectional carbon transfer between Betula papyrifera and Pseudotsuga menziesii, primarily through the hyphae of the ectomycorrhiza.
[22] The shared nutrient connection through CMNs has been suggested to be involved with other ecological processes such as seedling establishment, forest succession and other plant-plant interactions.
Most of these produce microscopic propagules of about 10 μm that can disperse over large distances by way of various vectors, ranging from wind to mycophagous animals.
[27] It has been suggested that animals are drawn to hypogeous fruiting bodies because they are rich in nutrients such as nitrogen, phosphorus, minerals and vitamins.
Then they must envelope and penetrate the root cap cells and infect them, allowing the symbiotic Hartig net and associated structures to form.
The plant hosts release metabolites into the rhizosphere that can trigger basidiospore germination, growth of hyphae towards the root, and the early steps of EcM formation.
Some host-released metabolites have been shown to stimulate fungal growth in Pisolithus, modify the branching angle of hyphae, and cause other changes in the fungus.
[31] The Hartig net initially forms from the fully differentiated inner layer of the mantle, and penetration occurs in a broad front oriented at right angles to the root axis,[14] digesting through the apoplastic space.
Some plant cells respond by producing stress- and defense-related proteins including chitinases and peroxidases that could inhibit Hartig net formation.
[37][38] This can pose problems for the fungus, which may be unable to produce fruiting bodies,[37] and over the long term can cause changes in the types of fungal species present in the soil.
[41] In many ectomycorrhizas the Hartig net hyphae lack internal divisions, creating a multinuclear transfer cell-like structure that facilitates interhyphal transport.
[36] The hyphae have a high concentration of organelles responsible for energy and protein production (mitochondria and rough endoplasmic reticulum) at their tips.
[44][45] The hyphal sheath enveloping the root tips also acts as a physical barrier shielding plant tissues from pathogens and predators.
[15][46] Many studies also show that EcM fungi allow plants to tolerate soils with high concentrations of heavy metals,[47][48][49] salts,[50][51] radionuclides and organic pollutants.
Many species of ectomycorrhizal fungi can function either as ectomycorrhizas or in the penetrative mode typical of arbuscular mycorrhizas, depending on the host.
[67] Other indirect factors can also play a role in the EcM fungal community, such as leaf fall and litter quality, which affect calcium levels and soil pH.
[70] Pines were difficult to establish in the southern hemisphere for this reason,[71] and many Eucalyptus plantations required inoculation by EcM fungi from their native landscape.
Invasive garlic mustard, Alliaria petiolata, and its allelochemical benzyl isothiocyanate were shown to inhibit the growth of three species of EcM fungi grown on white pine seedlings.
[26] Other fruiting bodies are eaten by invertebrates such as mollusks and fly larvae, some of which are even tolerant to the toxic α-amanitin found in death caps.
[15] The ectomycorrhizal fungus Laccaria bicolor has been found to lure and kill springtails to obtain nitrogen, some of which may then be transferred to the host plant.
It is possible that agriculture indirectly affects nearby ectomycorrhizal species and habitats; for example, increased fertilization decreases sporocarp production.
[98] Genetic differences between populations growing in toxic versus non-toxic habitats have rarely been reported, indicating that metal tolerance is widespread.
[88][118] It has been argued that conservation of ectomycorrhizas requires protection of species across their entire host range and habitat,[88] to ensure that all types of EcM communities are preserved.
In one study concerning Douglas fir seedlings, removal of forest floor debris and soil compaction decreased EcM fungal diversity and abundance by 60%.
