Mycorrhizae

Mycorrhizae are intimate, mutually beneficial associations between fungi and the roots of plants (mycorrhiza comes from the Greek word meaning "fungus-root"). All gymnosperms and approximately 80 percent of all angiosperms are thought to have naturally occurring mycorrhizal associations. The plant provides the fungus with carbohydrates made in photosynthesis, and the fungus provides the plant with increased amounts of mineral elements and water absorbed from the soil. The fungus also protects the root from pathogens .

There are two major types of mycorrhizae, the ectomycorrhizae (also called ectotropic mycorrhizae; ecto, meaning "outside") and the endomycorrhizae (endotropic mycorrhizae; endo, meaning "inside"), that are distinguished on the basis of whether or not the fungus penetrates the root cells.

Ectomycorrhizae

In ectomycorrhizae the fungal component is usually a basidiomycete or sometimes an ascomycete. Ectomycorrhizae occur on certain groups of temperate shrubs and trees such as beeches, oaks, willows, poplars, cottonwoods, and pines. The associations are most common in vegetation experiencing seasonal growth, where they are thought to extend the growing period. In addition, ectomycorrhizae are common on trees growing in the cold, dry conditions close to the Arctic Circle and high on the slopes of mountains where they make the trees better able to survive in harsh conditions.

In an ectomycorrhizal association, the fungus forms a thick mat, called a mantle, on the outside of the young roots, and it also grows in between epidermal cells and into the cortex of the root interior. Within the root, the fungus never penetrates any of the cells but instead remains confined to the intercellular spaces where it forms a network called a Hartig net. The fungal filaments , called hyphae , also extend outward from the root where they increase the volume of soil available to be "mined" for nutrients. They also increase the surface area for the absorption of water and mineral salts, particularly phosphates but also NH₄, K, Cu²⁺, Zn²⁺, and NO₃^-. Once the root is colonized by the fungus, the production of root hairs slows or even ceases as the absorptive role of the root hairs is taken over by the hyphae of the ectomycorrhizal fungus.

Endomycorrhizae

Far more common are the endomycorrhizae, which have a zygomycete as the fungal component and which actually penetrate the cell walls of the root cortex. Although the hyphae do not enter the cytoplasm of the cortical cells, in most cases they cause the plasma membrane to bulge inward, forming highly branched structures called arbuscules and terminal swellings called vesicles. Thus, this type of endomycorrhizae is referred to as vesicular-arbuscular mycorrhizae, or VAM. The arbuscules are in intimate contact with the cortical cells and provide an increased surface area over which carbohydrates can pass from the plant to the fungus and mineral elements from the fungus to the plant. The vesicles are thought to function as storage compartments for the fungus. As with the ectomycorrhizae, the fungal hyphae extend from the root into the soil and increase the surface area for absorption, but there is no mantle or Hartig net, and root hairs are often present. The VAM are found on almost all herbaceous angiosperms, some gymnosperms, and many ferns and mosses. Endomycorrhizae are particularly important in the tropics where the soils are typically poor in phosphates. Studies have indicated that roots associated with mycorrhizal fungi can take up phosphate four times faster than roots without such fungi. Mycorrhizal fungi are particularly effective in utilizing highly insoluble rock phosphorus, Ca₃(PO₄)₂, that cannot be used by plants. The fungal hyphae make phosphates available to the plant by converting them to a soluble form.

Other Associations

Two other types of mycorrhizae are found in the heather and orchid families. In heather (family Ericaceae), the fungus secretes enzymes into the soil that convert materials, particularly nitrogen-containing compounds , into forms that can be taken up more readily. In orchids (family Orchidaceae), the seeds contain a mycorrhizal fungus that is required for seed germination. Within the seed, the hyphae absorb stored carbohydrates and transfer them to the plant embryo.

Some plants, such as those of the mustard family (Brassicaceae) and the sedge family (Cyperaceae), lack mycorrhizae. In addition, most plants growing in flooded soils (or under hydroponics) do not form mycorrhizae nor do plants grown where conditions are extremely dry or saline. Also, plants growing in very fertile (i.e., nutrient-rich soils) have less-developed mycorrhizae compared to plants growing in nutrient-poor soils.

Ecological Importance of Mycorrhizae

The importance of mycorrhizae in ecosystems became particularly apparent in the 1960s when plants grown in greenhouses were transplanted into areas such as slag heaps, landfills, and strip-mined areas in order to reclaim the land. With few exceptions, such plants did not survive in these infertile areas. Not until later was it realized that greenhouse soil is often sterilized to prevent the growth of pathogens, and the sterilization process killed the mycorrhizal fungi as well. Today, such reclamation attempts are much more successful because mycorrhizal fungi are inoculated with the plants when they are transplanted into the reclaimed areas. Similarly, attempts to grow certain species of European pines in the United States were unsuccessful until mycorrhizal fungi from their native soils were added at the time of transplanting.

Mycorrhizae are thought to have played an important role in the colonization of the land by plants some four hundred million years ago. Studies of fossil plants have shown that endomycorrhizae were prevalent at that time, and such associations may have been crucial in helping plants make the transition from the nutrient-rich sea to the nutrient-poor land.

see also Ecosystem; Fungi; Interactions, Plant-Fungal; Nutrients.

Robert C. Evans

Bibliography

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.

Ricklefs, Robert E. Ecology, 3rd ed. New York: W. H. Freeman and Company, 1990.

Salisbury, Frank B., and Cleon Ross. Plant Physiology, 4th ed. Belmont, CA: Wadsworth, Inc., 1992.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, Inc., 1998.

Mycorrhizae

Symbioses are intimate associations between two unrelated organisms. Mycorrhizae are very common but largely unseen symbioses between plant roots and fungi that are important in plant nutrition, community structure, and nutrient cycling. Throughout the course of their evolution, plants and fungi have formed many different types of mycorrhizal partnerships involving most plant families and thousands of fungal species.

These diverse symbioses have been grouped into general types: arbuscular mycorrhizae, ectomycorrhizae, orchid mycorrhizae, and mycorrhizae in plants in the order Ericales including ericoid, arbutoid, and monotropoid mycorrhizae.

Mycorrhizae are critical for the mineral nutrition of many plants because threadlike fungal hyphae can exploit soil much more extensively than plant roots, and thus mycorrhizal associations greatly increase the absorption of minerals and water. Usually, mycorrhizal fungi supply minerals to their host plants, which reciprocate by supplying carbohydrates to their fungal associates, but there are a few exceptions. In orchids, and some chlorophyll-free plants in the order Ericales, the flow of carbon is reversed, and mycorrhizal fungi supply the plant with organic carbon derived from dead plant matter or from neighboring living plants.

Arbuscular mycorrhizae, the most common type, are associations between most crop plants, grasses, forbs , and many trees and fungi in the division Zygomycota, order Glomales. Both fossil and molecular evidence indicate that the earliest land plants had arbuscular mycorrhizal partnerships 450 million years ago. Ectomycorrhizae are commonly formed by woody shrubs and trees and a diverse array of fungi in the divisions Basidiomycota and Ascomycota. Pines and other forest trees often grow poorly or cannot survive in the absence of ectomycorrhizae.

Taxa of mycorrhizal fungi differ greatly in their effects on plant fitness. Consequently, interactions between communities of mycorrhizal fungi and plants may have strong impacts on the structure and function of communities and ecosystems .

see also Community; Conifers; Fungi; Symbiosis

Nancy Collins Johnson

Bibliography

Smith, Sally E., and David J. Read. Mycorrhizal Symbiosis, 2nd ed. San Diego, CA: Academic Press, 1997.