Functions of Ericoid Mycorrhizal Fungi Information

Ericoid Mycorrhizal Fungi (ERF) are a group of fungi that associate with plants in the Ericaceae family. These plants are found in boreal and mediterranean forests as well as some heathlands. ^[1] Their symbiosis is particularly important because ericaceous plants are able to grow in polluted areas and in high latitude areas where the soil is acidic. A major benefit of the fungal infection to host plants is ERF's ability to utilize enzymes to degrade organic matter. ^[2] The outermost cell layer, the epidermis, provides both the interface with the soil and it is these cells that are colonized by ERF. ^[3] The symbiotic epidermal tissue becomes colonized by fungal endophytes, and their volume becomes fully covered by hyphal complexes. ^[3] The epidermal tissue lasts a few weeks, which is then overtaken by root thickening and root recolonization of ERM occurs. ^[4]

Interactions with host and non-host plants

It is clear that the interaction between ERF and ericaceous plants has a selective advantage to both the plant and the fungus. ^[5] This interaction allows for the availability of nutrients that plants otherwise would not get. Electron micrographs show a clear pattern between the plant host and symbiont membranes because the surrounding plant cytoplasm is enhanced in mitochondria and rough endoplasmic reticulum. ^[3] This physiology means that it is likely to be associated with uptake of fungus acquired nutrients. ^[5]

A study used pot cultures to show that ERM can help increase the fitness of ericaceous plants and help them out-compete other plant species within different nutrient conditions. Calluna vulgaris, a host plant of the fungi, was observed to form pure plant colonies in northwestern Europe. ^[5]

In another study, an experiment was done in vitro of ERF interactions with a non-host plant such as clover, and the results showed that the fungi became parasitic to the plant. ^[6] This is due to the ERF's ability to produce and maintain high concentrations of a pectin degrading enzyme polygalacturonase in the non-host roots, whereas it is inhibited by host plants in a mutualistic interaction and it may be either be due different growth rates or different mechanisms in the induction of these enzymes. ^[6] Polygalacturonase also come in play during ERF root colonization to penetrate the cell wall of host plant and begin initial symbiosis. Once established, plant inhibits further release of the enzyme. ^[6] Some ERF, however, have been found to form ectomycorrhizal associations with other types of plants. ^[7] Further studies need to be done to determine under what circumstances they establish symbiosis with non-ericaceous plants.

Nutrient supply

ERF can provide nutrients to their host plant because they contain enzymes that acquire nitrogen and phosphorous, and since their mycelia are very fine, they have increased surface area for the absorption of these nutrients. ^{[3] [8] [9]} Nitrogen and phosphorous are acquired via breaking down organic residue of animals and other plants and transporting them to host plant. ^{[3] [10]} Their symbiosis with plants such as dwarf shrubs are important in alpine and arctic regions because ERF has the capacity to decompose organic matter through the secretion of hydrolytic enzymes. ^{[6] [10]} Direct evidence for ERF has shown that they use oxidation of polyphenols as a way to release and accumulate nitrogen for host plants. ^[11] A study showed that a species of ERF fungi, Hymenoscyphus ericae, produce two types of polyphenol oxidases that are important for breaking down tannic acid and make nitrogen available to plants. ^[2] They have saprotrophic potentials and ability to degrade complex organic polymers in soil which they transfer to the plant root. ^{[3] [12]} Low concentrations of polygalacturonase plays an important role because ERF secrete them in order to decompose organic matter in the surrounding soil. Decomposition takes longer in these regions where there is humus soils and most of the nitrogen is in forms unavailable to plants, thus the plants relies on the fungi to provide nitrogen so that they can grow and survive. ^[13] In heathlands, soils are very acidic and nutrient mineralization is so low that it can lead to impoverishment in ericaceous plants. ^[14] In these types of acidic soils there are high levels of nitrogen available, however low levels of nitrogen in the form of ammonium or nitrate. These fungi attack and break down organic materials and obtain ammonium and nitrate, a usable form of nitrogen for plants to uptake. ^{[15] [8]}

Without ERF functioning as saprotrophs or mutualists, plants would not have access to the otherwise unavailable nitrogen and phosphorous. ^[6] In return, host plants transfer carbon to ERF which they utilize as their source of energy to grow and multiply. ^[16]

Protection against harsh environments

ERF, along with ectomycorrhizal fungi, have the ability to catch base cations and restrict losses of nutrients by filtering and have the ability to release those nutrients through weathering mineral surfaces. Their ability to do this is what makes them critical in acidic soils. ^[17] ERF have evolved two mechanisms that provide either protection against high metal concentrations in soil or aid in the uptake when metal concentrations such as iron are low. They either operate for specific metals at relatively low concentrations or for non-specific metals at high concentration in surrounding soil. ^[18] For instance when there is low iron concentrations, ERF produce and release siderophores such as ferricrocin to convert iron into a mineral form for uptake. However, at high concentrations they inhibit the production of siderophores and have mechanisms to protect themselves and host plant by either preventing uptake or storing metals in storage organs. ^{[18] [19]} ERF protect host plants against soils that contain high concentrations of metals such as aluminum, manganese and iron, which can become toxic to plants by. ^[19]

ERF reduce metal concentrations within host shoots to protect the plant in two known mechanisms: either with metal sequestration or via an exclusion mechanism in the infected root system. ^[19] Restricting the movement of these metal ions can be due to the ERF's ability to produce mucilaginous slime around the plant root protecting both the plant and itself. ^[19]

ERF are capable of protecting themselves against heavy metal toxicity by having high levels of resistance. ^[20] Avoidance mechanisms restricting metal ions to enter the cytoplasm is one way, and because the fungal cell wall is a binding site for the metals, this suggests that the fungi may be reducing the metal supply to the host.  ^[5] One study suggested a potential mechanism could be that acid phosphatase produced by these fungi aid in resistance of high aluminum and iron concentrations. ^[20]

Oh yeah, and double check if all your sources are good, i looked through a couple of them and it seems good so far, but just double check!

References