Mycorrhizae are mutual associations between fungi and plant roots. Fungal hyphae penetrate the roots to a certain degree (either entering into the spaces between cells, or into the cells themselves) and allow for the plant to have greater access to nutrients (especially phosphorus, which is crucial for metabolism and cell function) and water. The fungi can often protect roots from pathogens by forming an impermeable sheath around the plant tissues. This helps to increase root longevity and productivity. In exchange, the fungus receives shelter and nutrients such as sucrose. Two main types of mycorrhizae have been described.
Ectomycorrhizae: Ectomycorrhizae are associations in which the fungus penetrates the roots but does not enter the individual cell walls. Instead, hyphae grow to form a Hartig net in the matrix of space between cells. A thick and protective sheath of mycelia develops around the outside of the plant roots, and extramatrical hyphae extend out into the soil in search of nutrients. This greatly increases the surface area available to plant roots, and the fungus can grow much more rapidly than plant tissues. This type of mycorrhizae is formed by some species in the Zygomycota, Ascomycota, and Basidiomycota.
Endomycorrhizae: In this type of mycorrhizae, hyphae penetrate the cell walls of individual cells within the roots. Although there is still growth in the extracellular matrix, no well-developed Hartig net is formed. The sheath is also limited or non-existant. Instead, intracellular hyphae form structures within the root cells. The hyphae puncture the cell wall, but not the cell membranes, instead invaginating the the membrane to form their own structures.
Subtype— Vesicular-Arbuscular Mycorrhizae: The vesicular-arbuscular mycorrhizae (VAM) are a subtype of the endomycorrhizae. This type is categorized by the formation of two types of structures within the plant cells. The first, vesicles, are round cavities that are used for storage. Arbuscules are branched hyphae that look like trees or the alveoli of the lungs. Due to their high surface area, these structures are useful for nutrient exchange. This type of mycorrhizal association is found in the Glomeromycota, the next phylum to be discussed in this short course.
Prevalence: Of the studied plant species, 80% were found to form associations with fungi. Many mycorrhizae-forming fungi, however, are obligate symbionts, meaning that they can only survive if associated with their plant partner. This makes culturing of many species on agar plates difficult or impossible. Fossilized VAM have been found with the earliest plants, suggesting that fungi-plant associations were important or crucial in the development of land plants. Species with ectomycorrhizae or with no associations with fungi at all are believed to have evolved away from those plants who still form VAM associations. One major family that does not form mycorrhizal association is Brassicaceae, the cabbage/mustard family. This family includes vegetables such as broccoli, cabbage, turnip, various mustards, radish, and rapeseed (canola).