Fungi

Fungi

Mycology is the study of fungi (mykes, Greek for "fungi," and ology, meaning "study of"). Most contemporary mycologists consider the fungi to be in two kingdoms: kingdom Fungi with five phyla and kingdom Stramenopila with three phyla. The total number of fungi in the world is estimated to be over 1.5 million with less than 5 percent of the species described. Some mycologists believe that the total number of fungi may be more than 2 million. Two other kingdoms are sometimes mistaken for fungi: the slime molds (kingdom Myxomycota), which have a creeping plasmodium, and the bacteria and actinomycetes (kingdom Monera).

Structure and Life Cycle

Fungi are nonphotosynthetic, lacking the chlorophyll of higher plants and algae, and are recognized by their fruiting bodies, which is the visible part of the fungus. Examples include mushrooms, puffballs, molds, cup fungi, and morels. The vegetative structure consists of minute filamentous cells called hyphae, which are microscopic in size, usually from 1 micron to 10 microns in diameter. An aggregate of hyphae is called a mycelium, which is the thallus or vegetative part of the fungus plant known as spawn in the mushroom industry. In the kingdom Fungi, the mycelium has one haploid nucleus per cell (only one set of chromosomes) or is dikaryotic (two haploid nuclei per cell). In contrast, in the kingdom Stramenopila, mycelium has diploid nuclei (one nucleus with chromosomes from both parents). In both kingdoms, the mycelium has rigid cell walls usually composed of chitin (a complex carbon compound ), although it is infrequently made up of cellulose in kingdom Fungi.

In both kingdoms, fungi obtain their nutrition by excreting enzymes into the host or any organic material, which is then broken down and absorbed into the hyphal cell to provide the nutrition necessary for growth. Fungi function in the ecosystem as saprophytes, or decomposers. They break down dead organic matter as parasites by attacking living hosts or host cells, and as mycorrhizae (mycor, meaning "fungi," and rhizae, meaning "root") by forming jointly beneficial unions with the roots of higher plants. Fungi and algae combine to form a plant called a lichen. Only fungi and bacteria decompose various kinds of organic matter and change complex organic structures, such as plant cell walls containing lignin or the chitinous exoskeletons of insects, into simple carbohydrates that can then be assimilated by a wide variety of organisms.

The hyphae grow until they form an extensive mycelium of fungal tissue. At this point a young fruiting body initial (or button) begins to form and develops into a mature fruiting body. In some phyla fruiting bodies are large and variously recognized as mushrooms, boletes, puffballs, conks, cup fungi, morels, false morels, truffles, and witches' butter, to mention only a few. However, many of the aquatic fungi, molds, and other fungi (such as the yeasts) form minute fruiting structures that can only be seen with the aid of a magnifying glass or a microscope.

The function of the fruiting body is to form a tissue in or on which the spore-bearing surface is formed. The spore-bearing surface covers the gills of a mushroom, is inside the tubes of the bolete, or forms a spore mass inside the puffball and truffle. The spore of the fungus serves the same purpose as the seed of the green plants, but the spore is composed of only one or several simple cells. The spore forms following meiosis in sexual cells located in the spore-bearing surface. In the mushrooms, boletes, cup fungi, and morels, for example, the nearly mature spores are forcibly discharged at maturity from the spore-bearing surface. If one blows over the surface of a cup fungus at maturity, a small cloud (the puffing or a discharge of the spores) can be seen. However, in other fungi such as the puffballs, stinkhorns, and truffles, no forcible discharge occurs. The powdery spore mass of the puffball is often discharged through a pore in the top that forms at maturity. The greenish-gray spore mass of the stinkhorn emits a strong odor, which attracts insects that eat, contact, and spread the spores. The truffle, which is found at the surface of or beneath the soil, gradually matures and produces strong smells that attract small rodents that dig up and eat the fruiting bodies and distribute the spores.

Molds, such as Penicillium, produce microscopic asexual fruiting bodies that in turn produce asexual spores called conidia on structures known as conidiophores. Some yeast cells bud and reproduce asexually. Other fungi, such as the bread mold Rhizopus, produce asexual fruiting structures known as sporangiophores that support sacs called sporangia in which asexual spores are produced. Aquatic fungi also produce a variety of asexual spores, some of which are motile (called zoospores ). These spores swim to a potential host, retract their flagella , and enter the host producing an oval fruiting body with a feeding tube or minute root-like rhizoids. The zoospores of the kingdom Fungi have one whiplash flagellum, while in the kingdom Stramenopila the zoospores have two flagella, one whiplash and one tinsel type, that move rapidly to propel the zoospore. Spores, either sexual or asexual, motile or nonmotile, usually germinate to form thin cylindric hyphal cells that rapidly elongate and branch to form the mycelium of the new fungus plant.

Nutrition

The fungus cell must grow into the host plant or a bit of organic material in order to gain nutrition from it. This is achieved by discharging enzymes (called exoenzymes) from the cells. Complex carbohydrates and proteins are broken down by this process and then are absorbed by the hyphae. The nutrients can then be translocated from one cell to another. The growth of most fungi is indeterminate (that is, it never stops) because the fungus must continue to grow into new areas to seek new sources of food. The typical fairy ring represents a visible bright green grass ring where the active mycelium is, and it is along this ring that the mushrooms will fruit. Each year the diameter of the ring will increase while the mycelium dies out in the middle because the food base is exhausted.

Mycorrhizae

Mycorrhizal fungi invade the healthy outer cells of the tiny rootlets of higher plants. Ectomycorrhizae surround the rootlet with a sheath of fungal cells, and special hyphae penetrate between the cortical cells of the root-let and exchange nutrients with the higher plant, usually a tree or a shrub. Endomycorrhizae called VA (vesicular arbuscular) mycorrhizae form oval storage cells (vesicles) and minute branchlike processes (arbuscules) in the root cells of the host where nutrients are exchanged. Because fungi do not carry out photosynthesis and cannot make their own sugar, the mycorrhizal fungus obtains moisture and carbohydrates from its green plant host and, in return, provides the host with nitrogen, phosphorus, zinc, and other essential compounds. It does this using the miles of tiny mycelium to successfully compete for phosphorus and nitrogen, which extends the root system of the green plant. Most of the woody plants such as the pine, oak, birch, and beech have ectomycorrhizae, and most herbaceous plants such as grass, corn, wheat, and rye have VA endomycorrhizae.

Food, Drugs, and Poisons

Fungi play a major role in the diet of humans. Yeasts (Saccaromyces cerevisiae ) are used in the process of fermentation, in which they break down carbohydrates to liberate carbon dioxide and to produce alcohol. Gin is made when juniper berries are fermented, wine from grapes, beer from grains, bourbon from corn, and scotch from barley. Yeasts are also used in making Limburger cheese, yogurt, and Kombucha tea. Baker's yeast produces a high proportion of CO₂, which causes the dough to rise. Molds, generally species of Penicillium, are used to produce cheese such as blue, Roquefort, and Camembert.

The new age of antibiotics was ushered in with Sir Alexander Fleming's discovery of penicillin in 1929. It was first produced by the blue-green mold Penicillium notatum. Many other antibiotics are produced from Actinomycetes. On the other hand, aflatoxins produced by species of Aspergillus cause food spoilage and are carcinogenic. Mushrooms also produce toxins that only affect humans when they are eaten. Examples of these are the amatoxins and phallotoxins produced by a mushroom, Amanita virosa, that are often fatal to humans; muscarine and muscimol produced by the fly agaric, Amanita muscaria, are usually not fatal. Hallucinogens such as psilocybin and psilocin are produced by several species of mushrooms including Psilocybe cubensis and the protoplasmic poison monomethyhydrozine (MMH) by the false morel Gyromitra esculenta.

Fungal Diseases

Fungi that are parasitic on humans include the common dermatophytes on the skin, hair, and nails, causing such diseases as barber's itch and athlete's foot (Microsporium canis ). More serious diseases, such as Histoplasma capsulatum or histoplasmosis found in warm temperate climates and coccidiomycosis (Coccidioides immitis ) in arid areas, grow in bird dung and soil, producing a respiratory infection in humans that is occasionally fatal. North and South American blastomycosis, sporotrichosis, and other diseases caused by fungi attack tissues and organs within the body and are incapacitating or fatal to their victims.

Diseases that affect major economic plants have historically impacted people. The ergot (Claviceps purpurea ), which infects the grains of rye, produces deadly brown specks in bread and led to deformity and the death of thousands of people in the Middle Ages. The European grape was saved from the grips of the downy mildew (Plasmopara viticola) in the 1800s by the discovery of Bordeaux Mixture (copper sulfate and lime); the discovery gave birth to plant pathology as a science. The European potato famine, caused by the potato blight fungus (Phytophthora infestans ), in the years 1845 to 1847 forced more than a million Irish to flee from Ireland. In the United States, the chestnut blight (Cryphonectria parasitica ) has reduced the tall and highly valued American chestnut from the eastern forests to a rare shrub, while the Dutch elm disease (Ceratocystis ulmi ) threatens to eliminate the American elm. Scientists struggle continually to produce resistant strains of wheat that will not be parasitized by the wheat rust (Phytophthora infestans ) and corn that will be resistant to the corn smut (Ustilago maydis ). Mexicans and Hispanic Americans cook the infected ears in many ways and consider them to be a delicacy.

The shelves of every supermarket have the meadow mushroom (Agaricus bisporus ) and specialty mushrooms like Shiitake (Lentinus edodes ), oyster shell (Pleurotus ostreatus ), and the Portabello (Agaricus sp.) for sale. In fact, the leading agricultural crop in Pennsylvania is mushrooms.

see also Chestnut Blight; Dutch Elm Disease; Interactions, Plant-Fungal; Lichen; Mycorrhizae; Pathogens; Plant Systematics; Potato Blight; Taxonomy; Taxonomy, History of.

Orson K. Miller Jr.

Bibliography

Alexopoulos, C. J., C. W. Mims, and M. Blackwell. Introductory Mycology. New York:John Wiley & Sons, 1996.

Kavaler, L. Mushrooms Molds and Miracles. New York: The New American Library, 1965.

Large, E. C. The Advance of the Fungi. New York: Henry Holt & Co., 1940.

Miller, O. K. Mushrooms of North America. New York: E. P. Dutton Inc., 1973.

Rolfe, R. T. and F. W. Rolfe. The Romance of the Fungus World. New York: Dover Publications Inc., 1925.

Schaechter, E. In the Company of Mushrooms. Cambridge, MA: Harvard University Press, 1997.

Stamets, P. Growing Gourmet & Medicinal Mushrooms. Berkeley, CA: Ten Speed Press,1993.