Fungus

Read about Fungus in the Standard Cyclopedia of Horticulture

Fungi are plants. They differ from other plants chiefly in their lack of chlorophyll, the green coloring matter of green plants, and in the character of the substance of which their cell-walls are composed. This is sometimes spoken of as fungous cellulose, and has characters both of the cellulose of other plants and the chitin of insects. There are thousands of species of fungi, varying greatly in form and structure. Some forms are more or less familiar to everyone; for example, mushrooms, or toadstools, molds, mildews smuts and rusts. Other groups of plants often included under the term fungi are the slime-molds or myxomycetes and bacteria. While they have certain characters in common with fungi, they are sufficiently distinct to be considered separately. The fungus plant consists of a vegetative feeding portion, the mycelium, which, in a way, corresponds to the roots of higher plants, and the fruiting structure, the sporophore. The latter bears the reproductive bodies, the spores, which, while much simpler in structure, function in the same way as do the seeds of higher plants (Fig. 1607). The sporophore is the part most often observed by the layman. The mushroom or toadstool, the puffball, the smut boil on corn, the white powdery mildew on the grape or rose, or the blue mold on stale bread or cheese, are almost entirely the sporophores and spore masses. The mycelium is usually buried in the substratum from which the food is derived and is thus not often observed. In fact it is often too minute and colorless to be seen with the naked eye. It may be observed as a white branching weft in the dung of mushroom beds or in the leaf-mold in the forest. This form is commonly spoken of as spawn. It may also be seen as a white weft-like growth between the bark and wood of rotting logs or dead trees, or as brown leathery sheets in the cracks of rotting logs. It sometimes appears as brown or black shreds or strands under the bark of dying trees. This form of mycelium strand or rhizomorph is characteristic of the often very destructive mushroom parasite of trees, Armillaria mellea. The spores of fungi are minute microscopic bodies cut off from the sporophores for the purpose of reproducing the plant. They are usually one- or two-celled, though often many-celled (Fig. 1608). They are often colorless, though they may be variously tinted or colored, greenish, brown, black, and so on. When placed in sufficient moisture, and given the proper temperature, they usually will germinate quickly, either sending out a sprout-like germ-tube (Fig. 1609, b) which on finding sufficient nourishment grows into mycelium, or the protoplasmic contents of the spore-cell may escape through an opening formed in the cell-wall,as one or more actively swimming and naked protoplasmic masses, called swarm - spores (Fig. 1609, a). These swarm- spores swim about in the water for a time, (usually less than an hour),(Fig. 1609, a'). This latter is the method of germination of the potato- blight fungus, Phytophthora infestans.—A fungus often produces two kinds of spores, the vegetative spores, conidia (Fig. 1608. j), produced usually in great numbers and repeatedly during the season for the purpose of multiplying the form, and the sexual, or resting- spores (Fig. 1608, a, b, c, d, K), adapted primarily to carry the fungus through periods unfavorable to growth, as dry seasons, winter and the like. Either form may. however, function as the other. They are disseminated by wind, water, insects, or by man himself. Because of their lack of chlorophyll, fungi cannot assimilate their carbon directly from the carbon dioxide of the air as can the green plants. They must make use of the food substances already manufactured or elaborated by other plants or animals. With respect to the nature of the substratum from which fungi obtain their food-supply, they are of two general types, saprophytes, those that can feed and develop on non-living organic substances (chiefly dead parts of plants and animals); and parasites, those that may grow upon and take food from living organisms. A true or obligate saprophyte can feed only upon nonliving organic substances. There are great numbers of such species, attacking dead and fallen trees, stems and leaves of plants or the dead bodies of animals, infesting dung and other debris, breaking up the complex organic substances into simpler form, and deriving therefrom the food and energy for their development. Most mushrooms, toadstools, molds and the like, are obligate saprophytes, playing the role of disintegrators in the ever-changing cycle of nature. An obligate parasite, on the other hand is, in nature at least, compelled to derive its nutrition through direct attack on the living tissues of other plants or of animals. Of such fungi, the rust- and smut- producing parasites, the leaf-curl fungus of the peach, and the potato-blight organism are good examples. Between these extremes are to be found very many forms which, during a part of their active development, live as parasites, and during the remainder as saprophytes. The apple-scab fungus is a good example. It passes the summer as an active parasite upon the leaves and fruit of the apple, but in the autumn and spring continues its growth and development in the fallen leaves, producing the sexually formed ascospores which in the spring infect the next crop. Other forms, which usually lead a saprophytic existence on the dead and fallen parts of plants, may, under special conditions, take on a parasitic habit. A good example is a common saprophyte, a species of Botrytis, common in greenhouses. When there is an excess of moisture or the plants are in any way weakened, this fungus finds it easy to pass from a saprophytic life on the dead leaves, to that of active and destructive parasitism on the living leaves. It is sometimes destructive to lettuce. Fungi are in general favored by abundance of moisture. For this reason in a wet season mushrooms appear in great profusion, and epidemics of plant- disease-producing fungi often occur over wide areas, causing great losses to the agriculturist. The loss from potato- blight in New York state alone often amounts in wet seasons to over $10,000,000. Warm weather is generally favorable to fungus growth, but there are some forms, like the potato-blight fungus, which nourish only during relatively cool periods. This parasite occurs only in temperate regions, being unknown in the hot low lands of tropical and subtropical regions. The peach leaf-curl fungus is apparently favored as much by the low temperature as by the rains of a wet spring. Other forms seem to thrive best in dry climates, as for example the powdery mildew of grapes. While many fungi are destructive agents of the crops of the agriculturist, causing him heavy losses, most fungi are active co-laborers with him, bringing about, as has been seen, the disintegration of compost, on which the farmer depends so largely for increased crop- production. Other fungi, like the yeasts and certain molds, are necessary agents in the arts and manufactures, as for example, the use of yeast in bread-, beer- and wine-production, molds in cheese-ripening, and so on. The value of these fungi lies chiefly in their ability to produce fermentations of various sorts or to give flavors to the products. Many fungi are edible, as for example the large fruit bodies of mushrooms, puffballs and truffles. While their value as food is perhaps often overestimated, they are valuable and form no unimportant part of the food of many people, especially in Europe. They are to be regarded chiefly as delicacies. The truffles and the cultivated mushroom, Agaricus campestris, are perhaps the best known. A delicacy known to relatively few is the large smut boils occurring on Zizania latifolia. Some fungi are poisonous, as for example the deadly Amanita, the fly-agaric among mushrooms, and the ergot, a fungous parasite of rye and other grasses. Fortunately the number of poisonous species is relatively small. H. H. Whetzel. The above text is from the Standard Cyclopedia of Horticulture. It may be out of date, but still contains valuable and interesting information which can be incorporated into the remainder of the article. Click on "Collapse" in the header to hide this text.

? Fungi Fossil range: Early Silurian - Recent
Clockwise from top left: Amanita muscaria, a basidiomycete; Sarcoscypha coccinea, an ascomycete; black bread mold, a zygomycete; a chytrid; a Penicillium conidiophore.
Plant Info
Scientific classification
Domain: Eukaryota Kingdom: Fungi L., 1753
Divisions
Chytridiomycota Zygomycota Glomeromycota Ascomycota Basidiomycota Deuteromycota

Fungi (singular fungus) are a kingdom of eukaryotic organisms. They are heterotrophic and digest their food externally, absorbing nutrient molecules into their cells. Yeasts, molds, and mushrooms are examples of fungi. The branch of biology involving the study of fungi is known as mycology.

Fungi often have important symbiotic relationships with other organisms. Mycorrhizal symbiosis between plants and fungi is particularly important; over 90% of all plant species engage in some kind of mycorrhizal relationship with fungi and are dependent upon this relationship for survival. Fungi are also used extensively by humans: yeasts are responsible for fermentation of beer and bread, and mushroom farming and gathering is a large industry in many countries.

Fungi and bacteria are the primary decomposers of organic matter in most terrestrial ecosystems. There are an estimated 1.5 million species of fungi with around 70,000 of them having been described.

Phylogeny and classification of fungi

Fungi were originally classified as plants, however they have since been separated as they are heterotrophs. This means they do not fix their own carbon through photosynthesis, but use carbon fixed by other organisms for metabolism. Fungi are now thought to be more closely related to animals than to plants, and are placed with animals in the monophyletic group of opisthokonts. For much of the Paleozoic Era, the fungi appear to have been aquatic. The first land fungi probably appeared in the Silurian, right after the first land plants appeared even though their fossils are fragmentary. For some time after the Permian-Triassic extinction event, the fungi went into a fungal spike because they were the dominant life forms - nearly 100% of the available record. Fungi absorb their food while animals ingest it; also unlike animals, the cells of fungi have cell walls. For these reasons, these organisms are placed in their own kingdom, Fungi, or Eumycota.

The Fungi are a monophyletic group, meaning all varieties of fungi come from a common ancestor. The monophyly of the fungi has been confirmed through repeated tests of molecular phylogenetics; shared ancestral traits include chitinous cell walls and heterotrophy by absorption, along with other shared characteristics.

The taxonomy of the Fungi is in a state of rapid flux at present, especially due to recent papers based on DNA comparisons, which often overturn the assumptions of the older systems of classification.

There is no unique generally accepted system at the higher taxonomic levels and there are constant name changes at every level, from species upwards. Fungal species can also have multiple scientific names depending on its life cycle. Web sites such as Index Fungorum, ITIS and Wikispecies define preferred up-to-date names (with cross-references to older synonyms), but do not always agree with each other or with names in Wikipedia in its various language variants.

Types of fungi

The major divisions (phyla) of fungi are mainly classified based on their sexual reproductive structures. Currently, five divisions are recognized:

Arbuscular mycorrhiza seen under microscope. Flax root cortical cells containing paired arbuscules.

Conidiophores of molds of the genus Aspergillus, an ascomycete, seen under microscope.

• The Chytridiomycota are commonly known as chytrids. These fungi produce zoospores that are capable of moving on their own through liquid menstrua by simple flagella.
• The Zygomycota are known as zygomycetes and reproduce sexually with meiospores called zygospores and asexually with sporangiospores. Black bread mold (Rhizopus stolonifer) is a common species that belongs to this group; another is Pilobolus, which shoots specialized structures through the air for several meters. Medically relevant genera include Mucor, Rhizomucor, and Rhizopus. Molecular phylogenetic investigation has shown the zygomycota to be a polyphyletic group.
• Members of the Glomeromycota are also known as the arbuscular mycorrhizal fungi. Only one species has been observed forming zygospores; all other species only reproduce asexually. This is an ancient association, with evidence dating to 400 million years ago.
• The Ascomycota, commonly known as sac fungi or ascomycetes, form meiotic spores called ascospores, which are enclosed in a special sac-like structure called an ascus. This division includes morels, some mushrooms and truffles, as well as single-celled yeasts and many species that have only been observed undergoing asexual reproduction. Because the products of meiosis are retained within the sac-like ascus, several ascomyctes have been used for elucidating principles of genetics and heredity (e.g. Neurospora crassa).
• Members of the Basidiomycota, commonly known as the club fungi or basidiomycetes, produce meiospores called basidiospores on club-like stalks

called basidia. Most common mushrooms belong to this group, as well as rust (fungus) and smut fungi, which are major pathogens of grains.

Although the water molds and slime molds have traditionally been placed in the kingdom Fungi and those who study them are still called mycologists, they are not true fungi. Unlike true fungi, the water molds and slime molds do not have cell walls made of chitin. In the 5-kingdom system, they are currently placed in the kingdom Protista. Water moulds are descended from algae, and are placed within the phylum Oomycota, within the Kingdom Protista.

Morphology

Mold covering a decaying peach over a period of six days. The frames were taken approximately 12 hours apart.

Though fungi are part of the opisthokont clade, all phyla except for the chytrids have lost their posterior flagella. Fungi are unusual among the eukaryotes in having a cell wall of chitin. All fungi are made up of many thin thread-like structures called hyphae. These hyphae can be one of two types: septate, or coenocytic. Septate hyphae have "walls" between their cells, called septa, though these septa have holes that allow cytoplasm, organelles, and sometimes nuclei to pass through. Coenocytic hyphae have no such marked divisions between cells. Coenocytic hyphae are essentially multinucleate supercells. Parasitic fungi have special structures on their hyphae called haustoria, which penetrate directly into a host organism's cells, allowing nutrients to be taken by the fungus. All of a fungus's hyphae form a structure called the mycelium. In mushroom forming fungi, the mycelium is normally underground. In molds, the mycelium forms directly on the food source. The only fungi that do not form hyphae or mycleia are yeasts, which are unicellular.

Fungi, unlike animals and vascular plants, do not spend the majority of their life cycle in a diploid condition. When a spore begins to grow into a mycelium, the organism is haploid. The haploid mycelium may or may not produce haploid spores asexually. When one haploid organism encounters another, through growth of the mycelium, since fungi are not motile, the two may merge, in a process called plasmogamy. The fungi then enter a heterokaryotic, or multinucleate stage. Usually, one nucleus from one parent fungus will pair off with one nucleus from the other parent. Some fungi spend most of their life cycle in this stage. At a given time, the paired off nuclei will merge, in a process called karyogamy, producing a diploid nucleus. This will normally happen in a separate reproductive structure; in basidiomycetes and ascomycetes, the mushroom. The diploid nucleus will then undergo meiosis to produce haploid nuclei, which are then released as spores to start the cycle once again.

Reproduction

Fungi on a fence post near Orosí, Costa Rica.

Fungi may reproduce sexually or asexually. In asexual reproduction, the offspring are genetically identical to the “parent” organism (they are clones). During sexual reproduction, a mixing of genetic material occurs so that the offspring exhibit traits of both parents. Many species can use both strategies at different times, while others are apparently strictly sexual or strictly asexual. Sexual reproduction has not been observed in some fungi of the Glomeromycota and Ascomycota. These are commonly referred to as Fungi imperfecti or Deuteromycota.

Yeasts and other unicellular fungi can reproduce simply by budding, or “pinching off” a new cell. Many multicellular species produce a variety of different asexual spores tha t are easily dispersed and resistant to harsh environmental conditions. When the conditions are right, these spores will germinate and colonize new habitats.

Sexual reproduction in fungi is somewhat different from that of animals or plants, and each fungal division reproduces using different strategies. Fungi that are known to reproduce sexually all have a haploid stage and a diploid stage in their life cycles. Ascomycetes and basidiomycetes also go through a dikaryotic stage, in which the nuclei inherited by the two parents do not fuse right away, but remain separate in the hyphal cells (see heterokaryosis).

In zygomycetes, the haploid hyphae of two compatible individuals fuse, forming a zygote, which becomes a resistant zygospore. When this zygospore germinates, it quickly undergoes meiosis, generating new haploid hyphae and asexual sporangiospores. These sporangiospores may then be distributed and germinate into new genetically-identical individuals, each producing their own haploid hyphae. When the hyphae of two compatible individuals come into contact with one another, they will fuse and generate new zygospores, thus completing the cycle.

In ascomycetes, when compatible haploid hyphae fuse with one another, their nuclei do not immediately fuse. The dikaryotic hyphae form structures called asci (sing. ascus), in which karyogamy (nuclear fusion) occurs. These asci are embedded in an ascocarp, or fruiting body, of the fungus. Karyogamy in the asci is followed immediately by meiosis and the production of ascospores. The ascospores are disseminated and germinate to form new haploid mycelium. Asexual conidia may be produced by the haploid mycelium. Many ascomycetes appear to have lost the ability to reproduce sexually and reproduce only via conidia.

Sexual reproduction in basidiomycetes is similar to that of ascomycetes. Sexually compatible haploid hyphae fuse to produce a dikaryotic mycelium. This leads to the production of a basidiocarp. The most commonly-known basidiocarps are mushrooms, but they may also take many other forms. Club-like structures known as basidia generate haploid basidiospores following karyogamy and meiosis. These basidiospores then germinate to produce new haploid mycelia.

Ecological role

Polypores growing on a tree in Borneo

Although often inconspicuous, fungi occur in every environment on Earth and play very important roles in most ecosystems. Along with bacteria, fungi are the major decomposers in most terrestrial (and some aquatic) ecosystems, and therefore play a critical role in biogeochemical cycles and in many food webs.

Many fungi are important as partners in symbiotic relationships with other organisms, as mutualists, parasites, or commensalists, as well as in symbiotic relationships that do not fall neatly into any of these categories. One of the most important of these relationships are various types of mycorrhiza, which is a kind of mutualistic relationship between fungi and plants, in which the plant's roots are closely associated with fungal hyphae and other structures. The plant donates to the fungus sugars and other carbohydrates that it manufactures from photosynthesis, while the fungus donates water and mineral nutrients that the hyphal network is able to find much more efficiently than the plant roots alone can, particularly phosphorus. The fungi also protect against diseases and pathogens and provide other benefits to the plant. Recently, plants have been found to use mycorrhizas to deliver carbohydrates and other nutrients to other plants in the same community and in some cases can make plant species that would normally exclude each other able to coexist in the same plant community. Such mycor rhizal communities are called "common mycorrhizal networks". Over 90% of the plant species on Earth are dependent on mycorrhizae of one type or another in order to survive, and it is hypothesized that the presence of terrestrial fungi may have been necessary in order for the first plants to colonize land. Research in 2005 showed that mycorrhizal fungi facilitate significant nitrogen transfer to their plant hosts.

Lichens are formed by a symbiotic relationship between algae or cyanobacteria (referred to in lichens as "photobionts") and fungi (mostly ascomycetes of various kinds and a few basidiomycetes), in which individual photobiont cells are embedded in a complex of fungal tissue. As in mycorrhizas, the photobiont provides sugars and other carbohydrates while the fungus provides minerals and water. The functions of both symbiotic organisms are so closely intertwined that they function almost as a single organism.

Certain insects also engage in mutualistic relationships with various types of fungi. Several groups of ants cultivate various fungi in the Agaricales as their primary food source, while ambrosia beetles cultivate various kinds of fungi in the bark of trees that they infest.

Some fungi are parasites on plants, animals (including humans), and even other fungi. Pathogenic fungi are responsible for numerous diseases, such as athlete’s foot and ringworm in humans and Dutch elm disease in plants. Some fungi are predators of nematodes, which they capture using an array of devices such as constricting rings or adhesive nets.

Human uses of fungi

Sacharomyces cerevisiae cells in DIC microscopy.

The study of the historical uses and sociological impact of fungi is known as ethnomycology.

Fungi have a long history of use by humans. Many types of mushrooms and other fungi are eaten, including button mushrooms, shiitake mushrooms, and oyster mushrooms.

Many species of mushrooms are poisonous and are responsible for numerous cases of sickness and death every year. In the US the most common cause of deadly mushroom poisoning is the Amanita phalloides or death cap mushroom.

A type of single-celled yeast fungus called Saccharomyces cerevisiae yeast is used in baking bread. Yeast is also used to create alcoholic beverages through fermentation. Mycelial fungus is used to make Shoyu (soy sauce) and tempeh. Fungi are also used to produce industrial chemicals like lactic acid, antibiotics and even to make stonewashed jeans. Some types of fungi are ingested for their psychedelic properties, both recreationally and religiously (see main article, Psychedelic mushroom).

Edible and poisonous fungi

Black Périgord Truffle (Tuber melanosporum), cut in half.

Stilton cheese veined with Penicillium roqueforti.

[[Image:Amanita phalloid es 1.JPG|thumb|200px|right|"Death cap", Amanita phalloides.]] Some of the most well-known types of fungi are the edible and poisonous mushrooms. Many species are commercially raised, but others must be harvested from the wild. Button mushrooms (Agaricus bisporus) are the most commonly eaten species, used in salads, soups, and many other dishes. Portobello mushrooms are the same species, but are allowed to grow to a much larger size. Other commercially-grown mushrooms that have gained in popularity in the West and are often available fresh in grocery stores include straw mushrooms (Volvariella volvacea), oyster mushrooms (Pleurotus ostreatus), shiitakes (Lentinula edodes), and enokitake (Flammulina spp.).

There are many more mushroom species that are harvested from the wild for personal consumption or commercial sale. Milk mushrooms, morels, chanterelles, truffles, black trumpets, and porcini mushrooms (also known as king boletes) all command a high price on the market. They are often used in gourmet dishes.

It is also a common practice to permit the growth of specific species of mold in certain types of cheeses that give them their unique flavor. This mold is non-toxic and is safe for human consumption. This accounts for the blue colour in cheeses such as Stilton or Roquefort which is created using Penicillium roqueforti spores.

Hundreds of mushroom species are toxic to humans, causing anything from upset stomachs to hallucinations to death. Some of the most deadly belong to the genus Amanita, including A. virosa (the "destroying angel") and A. phalloides (the "death cap"). Stomach cramps, vomiting, and diarrhea usually occur within 6-24 hours after ingestion of these mushrooms, followed by a brief period of remission (usually 1-2 days). Patients often fail to present themselves for treatment at this time, assuming that they have recovered. However, within 2-4 weeks liver and kidney failure leads to death if untreated. There is no antidote for the toxins in these mushrooms, but kidney dialysis and administration of corticosteroids may help. In severe cases, a liver transplant may be necessary (Kaminstein 2002). It is difficult to identify a "safe" mushroom without proper training and knowledge, thus it is often advised to assume that a mushroom in the wild is poisonous and leave it alone.

Fly agaric mushrooms (A. muscaria) are also responsible for a large number of poisonings, but these cases rarely result in death. The most common symptoms are nausea and vomiting, drowsiness, and hallucinations. In fact, this species is used ritually and recreationally for its hallucinogenic properties. Historically Fly agaric was used by Celtic Druids in Northern Europe and the Koryak people of north-eastern Siberia for religious or shamanic purposes.

Fungi in the biological control of pests

Many fungi compete with other organisms, or directly infect them. Some of these fungi are considered beneficial because they can restrict, and sometimes eliminate, the populations of noxious organisms like pest insects, mites, weeds, nematodes and other fungi, such as those that kill plants. There is much interest on the manipulation of these beneficial fungi for the biological control of pests. Some of these fungi can be used as biopesticides, like the ones that kill insects (entomopathogenic fungi). Specific examples of fungi that have been developed as bioinsecticides are Beauveria bassiana, Metarhizium anisopliae, Hirsutella, Paecilomyces fumosoroseus, and Verticillium lecanii.

See also

• Carnivorous fungus
• Wood-decay fungus
• MycoBank
• List of fungal orders

References

• Deacon JW. (2005). Fungal Biology (4th ed). Malden, MA: Blackwell Publishers. ISBN 1-4051-3066-0.
• Kaminstein D. (2002). Mushroom poisoning.

External links

• The WWW Virtual Library: Mycology

• MykoWeb: Mycological Links
• MushroomExpert.com
• MykoWeb
• Tom Volk's Fungi
• Illinois Mycological Association Mycological Glossary
• "Introduction to the Fungi", University of California Museum of Paleontology.
• Fungal Biology, University of Sydney, School of Biological Sciences, June, 2004. – Online textbook
• The Fifth Kingdom – Online textbook
• Australian National Botanic Gardens Fungi Web Site – Online textbook
• Pacific Northwest Fungi, Online Journal
• North American Mycological Association
• Mycological Society of America
• British Mycological Society
• Sydney Fungal Studies Group
• fungi as mycobionts in lichens
• CABI Bioscience Databases - Includes Index Fungorum genus and species names and top-down hierarchy
• Mushroom Observer - A site for recording observations of mushrooms and getting help identifying unknown collections