13.8 Introduction to mycorrhizas

The mycorrhizal mutualistic association between fungi and the roots of plants has contributed, from the earliest times, to the evolution of the Earth’s terrestrial ecosystem. In this relationship the roots of the plant are infected by a fungus, but the rest of the fungal mycelium continues to grow through the soil, digesting and absorbing nutrients and water and sharing these with its plant host. This was discovered by a German botanist called Albert Bernhard Frank in 1885. He began a study of the possibility of cultivating truffle groves in Prussia but his study developed into a revolutionary theory of tree nutrition via symbiosis between fungi and tree roots in a compound structure he called a Wurzelpilze, or root-fungus. In fact, Frank used the words Wurzelsymbiose, Wurzelpilze and Mycorhiza in the titles of three successive papers in 1885 (and mykorhiza and mycorrhiza in titles of two later papers). The spelling has now been standardised to ‘mycorrhiza’, but it still means fungus-root! Later studies have shown that just about all of Frank’s interpretations were correct.

A mycorrhiza is a mutualistic (or symbiotic) interaction between a fungus and the roots of a plant the plant benefits from increased nutrient uptake (particularly phosphate) via the fungal mycelium, while the fungus is supplied with photosynthetic sugars by its host. The arrangement seems to have evolved as soon as plants first colonised the land some 450 to 600 million years ago. Today about 6,000 species of fungi are known to form mycorrhizas with something like 240,000 plant species. Overall, 95% of vascular plants have mycorrhizas associated with their roots, in all habitats, including deserts, lowland tropical rainforests, through high latitudes and altitudes, and including aquatic ecosystems. The few non-mycorrhizal plant families, such as rushes (Juncus), sedges (Carex, Kobesia), Campions  (Silene)  and crop plants such as rapeseed (Brassica napus; one particular group of cultivars of which is known as canola), tend to colonise open habitats, where competition for nutrients is reduced, particularly in habitats where phosphorus availability is likely to be adequate (Brundrett, 2009; Varma et al., 2017). For more information and illustration of mycorrhizas visit Mark Brundrett’s website at http://mycorrhizas.info. Soudzilovskaia et al. (2019) have introduced a database, called FungalRoot, which summarises publicly available data (to April 2019) about mycorrhizal associations published in nine of the most widely-used languages. Genre et al. (2020) review studies of phylogenomics, molecular and cell biology about the current state of knowledge of the origin of mycorrhizal fungi and the evolutionary history of their relationship with land plants.

Mycorrhizal fungi also link plants together into communities that are more resilient to stress and disturbance than single plants. Nutrients can be transferred between two different plants which are connected to the same mycorrhizal system; for example from a well-placed donor plant to a shaded recipient plant. The mycorrhizal interconnections form a network through which plant-to-plant, plant-to-fungus and fungus-to-plant transfers of nutrients and signalling molecules can take place (it's been called the 'Wood-Wide-Web'; check out this clip from the BBC World Service programme How trees secretly talk to each other). In nutrient-poor soils, mycorrhizal fungi can provide nitrogen to their host plant that their mycelia have obtained by saprotrophic digestion of nutrients in the soil. Van der Heijden & Horton (2009) completed a comprehensive review of the importance of mycorrhizal networks in natural ecosystems and state that ‘mycorrhizal networks play a key role in plant communities by facilitating and influencing seedling establishment, by altering plant-plant interactions and by supplying recycled nutrients’.

For plants, the arrangement has become the rule rather than the exception, conversely, although there are common and important examples of mycorrhizas from all the major types of fungi, most fungi are not mycorrhizal. The mycorrhizal fungi you are most likely to meet are the mushrooms common in wooded areas. Names already mentioned, like Amanita and Boletus are mycorrhizal partners with trees and other forest plants, as are chanterelles, and truffles too, although truffles are not mushrooms, of course, but a hypogeous fruit body of the Ascomycota.

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Updated September, 2020