Mycorrhiza

Source: Wikipedia, the free encyclopedia.
fly agaric (upper left) form ectomycorrhiza (upper right) with tree rootlets. Arbuscular mycorrhiza (lower left) are very common in plants, including crop species such as wheat
(lower right)

A mycorrhiza (from

soil biology, and soil chemistry
.

In a mycorrhizal association, the fungus colonizes the host plant's root tissues, either

extracellularly as in ectomycorrhizal fungi.[3] The association is normally mutualistic. In particular species, or in particular circumstances, mycorrhizae may have a parasitic association with host plants.[4]

Definition

A mycorrhiza is a symbiotic association between a green plant and a fungus. The plant makes organic molecules by

Chenopodiaceae cannot. Different forms for the association are detailed in the next section. The most common is the arbuscular type that is present in 70% of plant species, including many crop plants such as cereals and legumes.[7]

Evolution

Fossil and genetic evidence indicate that mycorrhizae are ancient, potentially as old as the

nitrogen-fixing bacteria is an extension of mycorrhizal symbiosis.[11] The modern distribution of mycorrhizal fungi appears to reflect an increasing complexity and competition in root morphology associated with the dominance of angiosperms in the Cenozoic Era, characterized by complex ecological dynamics between species.[12]

Types

Mycorrhizas are commonly divided into ectomycorrhizas and endomycorrhizas. The two types are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate individual cells within the root, while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane.[13][14] Endomycorrhiza includes arbuscular, ericoid, and orchid mycorrhiza, while arbutoid mycorrhizas can be classified as ectoendomycorrhizas. Monotropoid mycorrhizas form a special category.

Ectomycorrhiza

Beech is ectomycorrhizal
Leccinum aurantiacum, an ectomycorrhizal fungus
Main article: Ectomycorrhiza

Ectomycorrhizas, or EcM, are symbiotic associations between the roots of around 10% of plant families, mostly woody plants including the

orchids,[16] and fungi belonging to the Basidiomycota, Ascomycota, and Zygomycota. Some EcM fungi, such as many Leccinum and Suillus, are symbiotic with only one particular genus of plant, while other fungi, such as the Amanita, are generalists that form mycorrhizas with many different plants.[17] An individual tree may have 15 or more different fungal EcM partners at one time.[18] Thousands of ectomycorrhizal fungal species exist, hosted in over 200 genera. A recent study has conservatively estimated global ectomycorrhizal fungal species richness at approximately 7750 species, although, on the basis of estimates of knowns and unknowns in macromycete diversity, a final estimate of ECM species richness would probably be between 20,000 and 25,000.[19]

Ectomycorrhizas consist of a hyphal sheath, or mantle, covering the root tip and a

leaf litter
.

Nutrients can be shown to move between different plants through the fungal network. Carbon has been shown to move from

Eastern White Pine inoculated with L. bicolor was able to derive up to 25% of its nitrogen from springtails.[22][23] When compared with non-mycorrhizal fine roots, ectomycorrhizae may contain very high concentrations of trace elements, including toxic metals (cadmium, silver) or chlorine.[24]

The first genomic sequence for a representative of symbiotic fungi, the ectomycorrhizal basidiomycete L. bicolor, was published in 2008.

biotrophic lifestyle of the mycorrhizal fungus that enables it to grow within both soil and living plant roots. Since then, the genomes of many other ectomycorrhizal fungal species have been sequenced further expanding the study of gene families and evolution in these organisms.[26]

Arbutoid mycorrhiza

This type of mycorrhiza involves plants of the Ericaceae subfamily Arbutoideae. It is however different from ericoid mycorrhiza and resembles ectomycorrhiza, both functionally and in terms of the fungi involved.[27] It differs from ectomycorrhiza in that some hyphae actually penetrate into the root cells, making this type of mycorrhiza an ectendomycorrhiza.[28]

Endomycorrhiza

Endomycorrhizas are variable and have been further classified as arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizas.[29]

Arbuscular mycorrhiza

Main article: Arbuscular mycorrhiza
Wheat has arbuscular mycorrhiza.

hyphae do not in fact penetrate the protoplast (i.e. the interior of the cell), but invaginate the cell membrane, creating a so-called peri-arbuscular membrane. The structure of the arbuscules greatly increases the contact surface area between the hypha and the host cell cytoplasm to facilitate the transfer of nutrients between them. Arbuscular mycorrhizas are fungi that are obligate biotrophs, meaning that they use the plant host for both growth and reproduction.[30] Twenty percent of the photosynthetic products made by the plant host are consumed by the fungi, the transfer of carbon from the terrestrial host plant is then exchanged by equal amounts of phosphate from the fungi to the plant host.[31]

Arbuscular mycorrhizas are formed only by fungi in the

heterokaryosis).[34]

Ericoid mycorrhiza

Woollsia pungens[35]
Main article: Ericoid mycorrhiza

Ericoid mycorrhizas are the third of the three more ecologically important types. They have a simple intraradical (growth in cells) phase, consisting of dense coils of hyphae in the outermost layer of root cells. There is no periradical phase and the extraradical phase consists of sparse hyphae that don't extend very far into the surrounding soil. They might form sporocarps (probably in the form of small cups), but their reproductive biology is poorly understood.[14]

Ericoid mycorrhizas have also been shown to have considerable

saprotrophic capabilities, which would enable plants to receive nutrients from not-yet-decomposed materials via the decomposing actions of their ericoid partners.[36]

Orchid mycorrhiza

Main article: Orchid mycorrhiza

All

orchids are myco-heterotrophic at some stage during their lifecycle, meaning that they can survive only if they form orchid mycorrhizas with basidiomycete fungi.[citation needed] Their hyphae penetrate into the root cells and form pelotons (coils) for nutrient exchange.[citation needed
]

Monotropoid mycorrhiza

Main article: Myco-heterotrophy

This type of mycorrhiza occurs in the subfamily

mixotrophic and derive their carbon from the fungus partner. This is thus a non-mutualistic, parasitic type of mycorrhizal symbiosis.[citation needed
]

Mutualist dynamics

Nutrient exchanges and communication between a mycorrhizal fungus and plants.

Mycorrhizal fungi form a

Orchidaceae are notorious as a family in which the absence of the correct mycorrhizae is fatal even to germinating seeds.[37]

Recent research into

boreal forests has indicated that mycorrhizal fungi and plants have a relationship that may be more complex than simply mutualistic. This relationship was noted when mycorrhizal fungi were unexpectedly found to be hoarding nitrogen from plant roots in times of nitrogen scarcity. Researchers argue that some mycorrhizae distribute nutrients based upon the environment with surrounding plants and other mycorrhizae. They go on to explain how this updated model could explain why mycorrhizae do not alleviate plant nitrogen limitation, and why plants can switch abruptly from a mixed strategy with both mycorrhizal and nonmycorrhizal roots to a purely mycorrhizal strategy as soil nitrogen availability declines.[38] It has also been suggested that evolutionary and phylogenetic relationships can explain much more variation in the strength of mycorrhizal mutualisms than ecological factors.[39]

Within mycorrhiza, the plant gives carbohydrates (products of photosynthesis) to the fungus, while the fungus gives the plant water and minerals.

Sugar-water/mineral exchange

In this mutualism, fungal hyphae (E) increase the surface area of the root and uptake of key nutrients while the plant supplies the fungi with fixed carbon (A=root cortex, B=root epidermis, C=arbuscle, D=vesicle, F=root hair, G=nuclei).

The mycorrhizal mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose.[40] The carbohydrates are translocated from their source (usually leaves) to root tissue and on to the plant's fungal partners. In return, the plant gains the benefits of the mycelium's higher absorptive capacity for water and mineral nutrients, partly because of the large surface area of fungal hyphae, which are much longer and finer than plant root hairs, and partly because some such fungi can mobilize soil minerals unavailable to the plants' roots. The effect is thus to improve the plant's mineral absorption capabilities.[41]

Unaided plant roots may be unable to take up

slash and burn rainforest destruction,[44] relies upon mycorrhiza within the root system of species of Inga to prevent the rain from washing phosphorus out of the soil.[45]

In some more complex relationships, mycorrhizal fungi do not just collect immobilised soil nutrients, but connect individual plants together by

mycorrhizal networks that transport water, carbon, and other nutrients directly from plant to plant through underground hyphal networks.[46]

lodgepole pine (Pinus contorta var. latifolia). These structures have been shown to host nitrogen fixing bacteria which contribute a significant amount of nitrogen and allow the pines to colonize nutrient-poor sites.[47]

Mechanisms

The mechanisms by which mycorrhizae increase absorption include some that are physical and some that are chemical. Physically, most mycorrhizal mycelia are much smaller in diameter than the smallest root or root hair, and thus can explore soil material that roots and root hairs cannot reach, and provide a larger surface area for absorption. Chemically, the cell membrane chemistry of fungi differs from that of plants. For example, they may secrete organic acids that dissolve or chelate many ions, or release them from minerals by ion exchange.[48] Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils.[49]

Disease, drought and salinity resistance and its correlation to mycorrhizae

Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens. These associations have been found to assist in plant defense both above and belowground. Mycorrhizas have been found to excrete enzymes that are toxic to soil borne organisms such as nematodes.[50] More recent studies have shown that mycorrhizal associations result in a priming effect of plants that essentially acts as a primary immune response. When this association is formed a defense response is activated similarly to the response that occurs when the plant is under attack. As a result of this inoculation, defense responses are stronger in plants with mycorrhizal associations.[51]

Ecosystem services provided by mycorrhizal fungi may depend on the soil microbiome.[52] Furthermore, mycorrhizal fungi was significantly correlated with soil physical variable, but only with water level and not with aggregate stability[53][54] and can lead also to more resistant to the effects of drought.[55][56][57] Moreover, the significance of mycorrhizal fungi also includes alleviation of salt stress and its beneficial effects on plant growth and productivity. Although salinity can negatively affect mycorrhizal fungi, many reports show improved growth and performance of mycorrhizal plants under salt stress conditions.[58]

Resistance to insects

Plants connected by mycorrhizal fungi in mycorrhizal networks can use these underground connections to communicate warning signals.[59][60] For example, when a host plant is attacked by an aphid, the plant signals surrounding connected plants of its condition. Both the host plant and those connected to it release volatile organic compounds that repel aphids and attract parasitoid wasps, predators of aphids.[59] This assists the mycorrhizal fungi by conserving its food supply.[59]

Colonization of barren soil

Plants grown in sterile soils and growth media often perform poorly without the addition of spores or hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients.[61] The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.[62] The introduction of alien mycorrhizal plants to nutrient-deficient ecosystems puts indigenous non-mycorrhizal plants at a competitive disadvantage.[63] This aptitude to colonize barren soil is defined by the category Oligotroph.

Resistance to toxicity

Fungi have a protective role for plants rooted in soils with high metal concentrations, such as

Pisolithus tinctorius planted in several contaminated sites displayed high tolerance to the prevailing contaminant, survivorship and growth.[64] One study discovered the existence of Suillus luteus strains with varying tolerance of zinc. Another study discovered that zinc-tolerant strains of Suillus bovinus conferred resistance to plants of Pinus sylvestris. This was probably due to binding of the metal to the extramatricial mycelium of the fungus, without affecting the exchange of beneficial substances.[63]

Occurrence of mycorrhizal associations

Mycorrhizas are present in 92% of plant families studied (80% of species),[15] with arbuscular mycorrhizas being the ancestral and predominant form,[15] and the most prevalent symbiotic association found in the plant kingdom.[40] The structure of arbuscular mycorrhizas has been highly conserved since their first appearance in the fossil record,[6] with both the development of ectomycorrhizas, and the loss of mycorrhizas, evolving convergently on multiple occasions.[15]

Associations of fungi with the roots of plants have been known since at least the mid-19th century. However early observers simply recorded the fact without investigating the relationships between the two organisms.[65] This symbiosis was studied and described by Franciszek Kamieński in 1879–1882.[66][67]

Climate change

CO2 released by human activities is causing

enzyme activity of ectomycorrhizal roots."[71]

Conservation and mapping

In 2021 the Society for the Protection of Underground Networks was launched. SPUN is a science-based initiative to map and protect the mycorrhizal networks that regulate the Earth’s climate and ecosystems. The stated goals of SPUN are mapping, protecting, and harnessing mycorrhizal fungi.

See also

References

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External links

Wikisource has the text of the 1920 Encyclopedia Americana article Mycorriza.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Mycorrhiza&oldid=1214685528"