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mycorrhiza

mycorrhiza The mutually beneficial association (see mutualism) formed between fungi and the roots of plants. This is a very common form of mutualism; the absorption of mineral ions by the plant roots is enhanced by the presence of the fungus, which benefits by obtaining soluble organic nutrients from the root cells. Ectotrophic mycorrhizas form a network of hyphae around the root and grow into the air spaces between the cells of the root. The hyphae of endotrophic mycorrhizas enter the cortical cells of the host roots.

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mycorrhiza

mycorrhiza A close physical association between a fungus and the roots of a plant, from which both fungus and plant appear to benefit; a mycorrhizal root takes up nutrients more efficiently than does an uninfected root. A very wide range of plants can form mycorrhizas of one form or another, and some plants (e.g. some orchids and some species of Pinus) appear incapable of normal development in the absence of their mycorrhizal fungi. See also ECTOTROPHIC MYCORRHIZA and ENDOTROPHIC MYCORRHIZA.

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mycorrhiza

mycorrhiza A close physical association between a fungus and the roots of a plant, from which both fungus and plant appear to benefit; a mycorrhizal root takes up nutrients more efficiently than does an uninfected root. A very wide range of plants can form mycorrhizas of one form or another and some plants (e.g. some orchids and some species of Pinus) appear incapable of normal development in the absence of their mycorrhizal fungi. See also ectomycorrhiza and endomycorrhiza.

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Mycorrhiza

Mycorrhiza

Biology of mycorrhizae

Importance of mycorrhizae

Resources

A fungus root or mycorrhiza (plural: mycorrhizae) is a fungus living in a mutually beneficial symbiosis (or mutualism) with the roots of a vascular plant. In this intimate relationship, the fungus benefits from access to energy-containing carbohydrates, proteins, and other organic nutrients excreted by, or contained in, the roots while the host plant benefits from an enhanced supply of inorganic nutrients, especially phosphorus.

The fungi carry out this function largely by increasing the rate of decomposition of organic matter in the immediate vicinity of the plant root, and by efficiently absorbing the inorganic nutrients that are liberated by this process. From the perspective of the plant, the most important of the mineral nutrients supplied by the fungus are compounds of phosphorus, and to a lesser degree, of nitrogen.

Mycorrhizae are a common type of mutualism; about 90% of the families of vascular plants live in this sort of beneficial relationship with fungi. Only a few economically important plant families do not develop mycorrhizae, among them the mustards (family Brassicaceae) and knotweeds (Polygonaceae).

Biology of mycorrhizae

A mycorrhiza is an intimate, biological relationship in which fungal hyphae integrate closely with the root tissues of a vascular plant. Plant roots that have a mycorrhizal fungus tend not to develop root hairs, relying heavily on the fungus to absorb nutrients and moisture from the soil. The fungal hyphae (root like projections) are associated with the cortical cells of the root, and not with vascular tissues or the actively growing cells of the meristematic tissue at the tip of the root.

There are two basic types of mycorrhizaeendomycorrhizae and ectomycorrhizae. Endomycorrhizae are more common than ectomycorrhizae, although the latter type is more frequent in the economically important species of trees of temperate forests.

In ectomycorrhizae, the fungal hyphae form a diffuse veil or mantle around the outside of the plant root. The fungal biomass of a ectomycorrhiza typically comprises about 40% of the dry weight of the structure. The fungal hyphae penetrate the root to some degree, but they only occur in the spaces between the cortical and epidermal cells. Ectomycorrhizae occur in association with the roots of such tree species as pines (Pinus spp.), spruces (Picea spp.), willows (Salix spp.), oaks (Quercus spp.), and birches (Betula spp.). Under the influence of hormones secreted by the fungus, the plant roots tend to grow in a stubby, thickened, and much-branched fashion. Most of the species of fungi that are involved in ectomycorrhizae are Basidiomycetes, and a smaller number are Ascomycetes. Most forest mushrooms are the fruiting bodies of Basidiomycete fungi that are mycorrhizal with tree species.

In endomycorrhizae, the fungal hyphae mostly grow inside the plant root, and they penetrate and grow inside the cortical cells of the root. Many economically important species of plants depend on endomycorrhizal fungi, including apples (Pyrus malus ), strawberries (Fragaria vesca ), tomatoes (Lycopersicon esculentum ), orchids (family Orchidaceae), and grasses (Poaceae). In fact, most of the important agricultural plants used by humans for food are endomycorrhizal. Although most conifers are ectomycorrhizal, a few species are endomycorrhizal, notably the redwoods (Sequoia sempervirens ) and junipers (Juniperus spp.).

Importance of mycorrhizae

There is a great deal of evidence that clearly demonstrates the great importance of mycorrhizae to plant nutrition, especially in nutrient-poor soils. For example, many species of trees and shrubs can be grown in the greenhouse, but if these are then transplanted into an abandoned pasture, prairie, or some other non-forested habitat, they commonly fail to survive or grow well, and will exhibit signs of nutritional distress. This happens because the soils of those habitats do not have populations of appropriate species of mycorrhizal fungi to colonize the roots of the tree seedlings. If the seedlings are to do well in such habitats, they must be deliberately inoculated with an appropriate mycorrhizal fungus.

On the other hand, if the seedlings are transplanted into a recent clear-cut that was previously a forest dominated by the same or closely related species of trees (for example during post-harvest regeneration of the stand), they will generally do well. This happens because the clear-cut still has a population of mycorrhizal fungi that are suitable to inoculate the tree seedlings.

In some cases, plants will not do well in the absence of a mycorrhiza, even when they are growing in apparently fertile soil. In such a case, the mycorrhizal mutualism is said to be obligate. In most cases, however, plant species can survive in fertile soil in the absence of a mycorrhizal relationship with a fungus. In less fertile soil, however, these species may do very poorly without a mycorrhiza. In this case, the mutualism is said to be facultative.

When natural ecosystems are being converted into some sort of system that is managed for the benefits of people, it may be critically important to consider the mycorrhizal fungi on the site. In tropical forests, for example, almost all of the tree species depend on mycorrhizae to supply them with nutrients from the typically infertile soils on which these ecosystems develop. Moreover, the mycorrhizal fungi are critical to retaining nutrients within the forest biomass, and in preventing these chemicals from being washed away by the abundant tropical rains. If the forest is cleared and burned to develop new agricultural lands, much of the nutrient capital of the land is lost due to the reduction of growing biomass, including the destruction of much of the mycorrhizal fungi by the disturbance.

Moreover, the fungi that do manage to survive are not necessarily appropriate symbionts for the species of grasses and other crops that farmers attempt to grow on the cleared land. Therefore, the conversion of the tropical forest, with its efficient recycling and use of the scarce nutrient resources, into a more open agricultural system with large losses of nutrient capital, often leads to a rapid degradation of the fertility of the site. This is a major reason why so many conversions of tropical forests into agriculture prove to be unsustainable.

Many mycorrhizal fungi in the Basiodiomycete group develop edible mushrooms, and these are gathered by many people for use in gourmet cooking. Perhaps the most famous of these mushrooms are the truffles, such as Tuber melanosporum, which is commonly mycorrhizal on species of oaks (Quercus spp.). The spore-bearing bodies of the truffle fungi develop underground, and must be found using specially trained, truffle-sniffing pigs or dogs. Mushroom collectors must be careful, however, because some

Key Terms

Cortex The root cortex is a relatively soft tissue that occurs between the epidermis and the internal, vascular tissues. Functions primarily in storage and in movement of water into the vascular cylinder.

Hypha (plural, hyphae) Cellular unit of a fungus, typically a branched and tubular filament. Many strands (hyphae) together are called mycelium.

Root hair These are tiny, tubular outgrowths of the root epidermis, mostly occurring behind the actively growing root tip, and used to increase surface area of the root to enhance the absorption of minerals and water from the soil.

mycorrhizal fungi develop fruiting bodies that are deadly poisonous, as is the case of the death or destroying angel (Amanita virosa ).

Orchids (family Orchidaceae) are examples of plants that can only grow if they have an endomycorrhizal mutualism. Orchid seeds are tiny and dust like, and they have virtually no stored energy to support the seedling when it germinates. The tiny orchid seedlings will only grow if they manage to become inoculated by an appropriate endomycorrhizal fungus. Until this fact was discovered by horticulturalists, orchids were extremely difficult to propagate and grow in greenhouses. Now, orchids are relatively easy to breed and cultivate.

Some species of vascular plants do not contain chlorophyll, and are incapable of photosynthesis. The whitish shoots of species such as the Indian pipe (Monotropa uniflora ) depend entirely on their mycorrhizal fungus to supply the plant with organic nutrients needed for growth and reproduction. This is an unusual case in which the balanced, reciprocal dependence of the plant and fungal symbionts of a mycorrhiza have become unbalanced, to the degree that the plant is now parasitic on the fungus.

Resources

BOOKS

Judd, Walter S., Christopher Campbell, Elizabeth A. Kellogg, Michael J. Donoghue, and Peter Stevens. Plant Systematics: A Phylogenetic Approach. 2nd ed. with CD-ROM. Suderland, MD: Sinauer, 2002.

Bill Freedman

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Mycorrhiza

Mycorrhiza

Refers to a close, symbiotic relationship between a fungus and the roots of a higher plant. Mycorrhiza (from the Greek myketos meaning fungus and rhiza meaning root) are common among trees in temperate and tropical forests. There are generally two formsectomycorrhiza, where the fungus forms a sheath around the plant roots, and endomycorrhiza, where the fungus penetrates into the cells of the plant roots. In both cases, the fungus acts as extended roots for the plant and therefore increase its total surface area. This allows for greater adsorption of water and nutrients vital to growth. Mycorrhiza even allow plants to utilize nutrients bound up in silicate minerals and phosphate-containing rocks that are normally unavailable to plant roots. They also can stimulate the plants to produce chemicals that hinder invading pathogens in the soil . In addition to the physical support, the mycorrhiza obtain carbohydrates from the higher, photosynthetic plant. This obligate relationship between fungi and plant roots is especially important in nutrient-impoverished soils. In fact, many trees will not grow without mycorrhiza.

See also Symbiosis; Temperate rain forest; Tropical rain forest

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Mycorrhiza

Mycorrhiza

A "fungus root" or mycorrhiza (plural: mycorrhizae) is a fungus living in a mutually beneficial symbiosis (or mutualism ) with the roots of a vascular plant . In this intimate relationship, the fungus benefits from access to energy-containing carbohydrates, proteins , and other organic nutrients excreted by, or contained in, the roots while the host plant benefits from an enhanced supply of inorganic nutrients, especially phosphorus .

The fungi carry out this function largely by increasing the rate of decomposition of organic matter in the immediate vicinity of the plant root, and by efficiently absorbing the inorganic nutrients that are liberated by this process. From the perspective of the plant, the most important of the mineral nutrients supplied by the fungus are compounds of phosphorus, and to a lesser degree, of nitrogen .

Mycorrhizae are a common type of mutualism; about 90% of the families of vascular plants live in this sort of beneficial relationship with fungi. Only a few economically important plant families do not develop mycorrhizae, among them the mustards (family Brassicaceae) and knotweeds (Polygonaceae).


Biology of mycorrhizae

A mycorrhiza is an intimate, biological relationship in which fungal hyphae integrate closely with the root tissues of a vascular plant. Plant roots that have a mycorrhizal fungus tend not to develop root hairs, relying heavily on the fungus to absorb nutrients and moisture from the soil . The fungal hyphae (root-like projections) are associated with the cortical cells of the root, and not with vascular tissues or the actively growing cells of the meristematic tissue at the tip of the root.

There are two basic types of mycorrhizae—endomycorrhizae and ectomycorrhizae. Endomycorrhizae are more common than ectomycorrhizae, although the latter type is more frequent in the economically important species of trees of temperate forests .

In ectomycorrhizae, the fungal hyphae form a diffuse veil or mantle around the outside of the plant root. The fungal biomass of a ectomycorrhiza typically comprises about 40% of the dry weight of the structure. The fungal hyphae penetrate the root to some degree, but they only occur in the spaces between the cortical and epidermal cells. Ectomycorrhizae occur in association with the roots of such tree species as pines (Pinus spp.), spruces (Picea spp.), willows (Salix spp.), oaks (Quercus spp.), and birches (Betula spp.). Under the influence of hormones secreted by the fungus, the plant roots tend to grow in a stubby, thickened, and much-branched fashion. Most of the species of fungi that are involved in ectomycorrhizae are Basidiomycetes, and a smaller number are Ascomycetes. Most forest mushrooms are the fruiting bodies of Basidiomycete fungi that are mycorrhizal with tree species.

In endomycorrhizae, the fungal hyphae mostly grow inside the plant root, and they penetrate and grow inside the cortical cells of the root. Many economically important species of plants depend on endomycorrhizal fungi, including apples (Pyrus malus), strawberries (Fragaria vesca), tomatoes (Lycopersicon esculentum), orchids (family Orchidaceae), and grasses (Poaceae). In fact, most of the important agricultural plants used by humans for food are endomycorrhizal. Although most conifers are ectomycorrhizal, a few species are endomycorrhizal, notably the redwoods (Sequoia sempervirens) and junipers (Juniperus spp.).


Importance of mycorrhizae

There is a great deal of evidence that clearly demonstrates the great importance of mycorrhizae to plant nutrition , especially in nutrient-poor soils. For example, many species of trees and shrubs can be grown in the greenhouse, but if these are then transplanted into an abandoned pasture, prairie , or some other non-forested habitat , they commonly fail to survive or grow well, and will exhibit signs of nutritional distress. This happens because the soils of those habitats do not have populations of appropriate species of mycorrhizal fungi to colonize the roots of the tree seedlings. If the seedlings are to do well in such habitats, they must be deliberately inoculated with an appropriate mycorrhizal fungus.

On the other hand, if the seedlings are transplanted into a recent clear-cut that was previously a forest dominated by the same or closely related species of trees (for example during post-harvest regeneration of the stand), they will generally do well. This happens because the clear-cut still has a population of mycorrhizal fungi that are suitable to inoculate the tree seedlings.

In some cases, plants will not do well in the absence of a mycorrhiza, even when they are growing in apparently fertile soil. In such a case, the mycorrhizal mutualism is said to be obligate. In most cases, however, plant species can survive in fertile soil in the absence of a mycorrhizal relationship with a fungus. In less fertile soil, however, these species may do very poorly without a mycorrhiza. In this case, the mutualism is said to be facultative.

When natural ecosystems are being converted into some sort of system that is managed for the benefits of people, it may be critically important to consider the mycorrhizal fungi on the site. In tropical forests, for example, almost all of the tree species depend on mycorrhizae to supply them with nutrients from the typically infertile soils on which these ecosystems develop. Moreover, the mycorrhizal fungi are critical to retaining nutrients within the forest biomass, and in preventing these chemicals from being washed away by the abundant tropical rains. If the forest is cleared and burned to develop new agricultural lands, much of the nutrient capital of the land is lost due to the reduction of growing biomass, including the destruction of much of the mycorrhizal fungi by the disturbance. Moreover, the fungi that do manage to survive are not necessarily appropriate symbionts for the species of grasses and other crops that farmers attempt to grow on the cleared land. Therefore, the conversion of the tropical forest, with its efficient recycling and use of the scarce nutrient resources, into a more open agricultural system with large losses of nutrient capital, often leads to a rapid degradation of the fertility of the site. This is a major reason why so many conversions of tropical forests into agriculture prove to be unsustainable.

Many mycorrhizal fungi in the Basiodiomycete group develop edible mushrooms, and these are gathered by many people for use in gourmet cooking. Perhaps the most famous of these mushrooms are the truffles, such as Tuber melanosporum, which is commonly mycorrhizal on species of oaks (Quercus spp.). The spore-bearing bodies of the truffle fungi develop underground, and must be found using specially trained, truffle-sniffing pigs or dogs. Mushroom collectors must be careful, however, because some mycorrhizal fungi develop fruiting bodies that are deadly poisonous, as is the case of the death or destroying angel ( Amanita virosa).

Orchids (family Orchidaceae) are examples of plants that can only grow if they have an endomycorrhizal mutualism. Orchid seeds are tiny and dust-like, and they have virtually no stored energy to support the seedling when it germinates. The tiny orchid seedlings will only grow if they manage to become inoculated by an appropriate endomycorrhizal fungus. Until this fact was discovered by horticulturalists, orchids were extremely difficult to propagate and grow in greenhouses. Now, orchids are relatively easy to breed and cultivate.

Some species of vascular plants do not contain chlorophyll , and are incapable of photosynthesis . The whitish shoots of species such as the Indian pipe (Monotropa uniflora) depend entirely on their mycorrhizal fungus to supply the plant with organic nutrients needed for growth and reproduction. This is an unusual case in which the balanced, reciprocal dependence of the plant and fungal symbionts of a mycorrhiza have become unbalanced, to the degree that the plant is now parasitic on the fungus.


Resources

books

Atlas, R.M. and R. Bartha. Microbial Ecology. Menlo Park, CA: Benjamin-Cummings Pub. Co., 1987.

Raven, Peter, R.F. Evert, and Susan Eichhorn. Biology ofPlants. 6th ed. New York: Worth Publishers Inc., 1998.


Bill Freedman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cortex

—The root cortex is a relatively soft tissue that occurs between the epidermis and the internal, vascular tissues. Functions primarily in storage and in movement of water into the vascular cylinder.

Hypha (plural, hyphae)

—Cellular unit of a fungus, typically a branched and tubular filament. Many strands (hyphae) together are called mycelium.

Root hair

—These are tiny, tubular outgrowths of the root epidermis, mostly occurring behind the actively growing root tip, and used to increase surface area of the root to enhance the absorption of minerals and water from the soil.

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