The arbuscular mycorrhizal (AM) fungi, long considered to belong in the Zygomycota, are now recognized as comprising a distinct phylum, Glomeromycota (Shüβler et al. 2001). This is an ancient group of fungi, recognizable in the fossil record dating back at least 400 million years. The AM fungi form obligate, mutualistic associations, called endomycorrhizae, with the roots of most (~80%) vascular plants. Only a small number (~160) of species is recognized in the phylum. One of the most distinctive features of these fungi is the highly branched arbuscules formed inside the cortical cells of host roots; arbuscules are the point of exchange between fungus and plant, where carbohydrates produced by the plant are acquired by the fungus, and where nitrogen, phosphorous, and other minerals acquired by the mycelium of the fungus are transferred to the plant. The fungus acquires as much as 20-40% of the photosynthate produced by the plant. Some AM fungi also produce storage structures inside plant roots called vesicles. Endomycorrhizal fungi produce an extensive network of hyphae outside the roots (extraradical hyphae). The extraradical hyphae act like an extension of the plant roots, increasing the plant’s access to water and soil minerals, particularly phosphorous and nitrogen. The fungus is also able to access phosphate not otherwise available to plants, for example from organic matter by production of acid phosphatases. Reproduction of AM fungi is by thick-walled spores ranging in size from 40–800 µm in diameter, each of which can contain hundreds or thousands of nuclei. Spores may be formed singly or in clusters, and the mycelium of AM fungi is coenocytic. Sexual reproduction is not known to occur in this phylum.
“Chytrids” are a small group of fungi with approximately 900 identified species occurring in a wide range of aquatic and terrestrial habitats around the world. The feature that is shared by all members of this phylum is the formation of zoospores with one posteriorly directed, whiplash flagellum. A few chytrids are economically important plant pathogens, e.g., Synchytrium endobioticum, which causes the black wart disease of potato, others are vectors of plant viruses (Olpidium), but most are saprotrophs using substrates such as cellulose, chitin, and keratin as a food source. As previously noted, the frog chytrid, Batrachochytrium dendrobatidis, has been implicated as a major factor in population declines of frogs and other amphibians around the world (Berger et al. 1998; Wake and Vredenburg 2008).
This phylum contains approximately 900 identified species divided amongst two ecologically distinct classes, Zygomycetes and Trichomycetes (White et al. 2006). Hibbett et al. (2007) indicate the phylum is polyphyletic and further work is needed to clarify relationships of fungi traditionally considered in Zygomycota. The most commonly encountered Zygomycetes are members of orders Mortierellales and Mucorales. Many members of these two orders are saprotrophs with rapidly growing, coenocytic mycelium. The sexual reproductive state is the zygospore (Fig. 57 – general life cycle), but many of these fungi produce a large number of readily dispersed asexual spores called sporangiospores. Members of order Mucorales, commonly called mucoraceous fungi, are common in soil, dung, plant material, and other types of organic matter. Some mucoraceous fungi are plant or animal pathogens, and others are used in the production of Asian foods such as tempeh. Species of Mucor and Rhizopus (Fig. 58) are commonly isolated from decaying organic matter and can cause decay diseases of fleshy fruits, vegetables, and sunflower peduncles.
Species of Pilobolus (Figs. 59 & 60) are among the first fungi observed growing on herbivore dung incubated in moist chambers. Other Zygomycetes are associated with animals. For example, some species of Rhizopus and Mucor cause zygomycosis in immunocompromised humans. Entomophthorales, as the name suggests, include parasites of insects and other animals. Members of class Trichomycetes live in the guts of insects, millipedes, and crustaceans, but cause little or no harm to their hosts.
Fungal-like Organisms Studied by Plant Pathologists and Mycologists
Oomycetes are fungal-like organisms that form zoospores with two flagella—a whiplash flagellum that is directed backwards and propels the zoospore, and a tinsel flagellum adorned with hairs that is directed forward, pulling the zoospore. The cell walls of Oomycetes contain cellulose, rather than chitin, and glucans. (Kirk et al., 2008). Another characteristic of Oomycetes is the formation of an oospore, a thick-walled, resistant propagule which is the result of sexual reproduction. Oomycetes belong in Kingdom Straminipila, also known as Chromista. In addition to Oomycetes, this kingdom includes diatoms, golden and brown algae, a type of algae called cryptomonads, and two other groups of organisms studied by mycologists in phyla Labyrinthulomycota and Hyphochytriomycota. The tinsel type flagellum is a characteristic of all members of Kingdom Straminipila, hence it is also called a straminipilous flagellum.
Oomycetes include some of our most devastating plant pathogens. These fungal-like organisms have changed the course of history. Consider 19th century Ireland; life was hard for the millions of Irish in the 1840s who relied almost entirely upon the "lumper" potatoes they grew on leased quarter-acre plots for food and rent. It’s said that the stomachs of these "cottiers" were distended from eating up to fourteen pounds of potatoes each day (Large 1940). Then, in 1845, the potatoes began to rot from a malady known as “Potato Murrain,” what we now call late blight of potato. Without potatoes, 4.5 million Irish faced starvation. Over the next 15 years, one million Irish died from the famine, and one and a half times that number fled Ireland. Late blight is caused by Phytophthora infestans, and this oomycete continues to be a major pathogen in potato production, although we now have the ability to control it through the application of fungicides.
A number of other important plant pathogens are found among the Oomycetes, but only a few will be mentioned here. Phytophthora ramorum causes sudden oak death and ramorum blight. Pythium species cause damping-off diseases under extremely wet conditions. Seedlings are particularly vulnerable to attack by damping-off pathogens because their tissues are soft and easily invaded. Seedlings may be killed before or after they emerge from the soil. Downy mildews (Peronosporales) are biotrophic Oomycetes that are characterized by the formation of white, downy sporangiophores on the surface of infected hosts. The white rusts (Albugo spp.) produce chains of sporangia that erupt through the cuticle of infected hosts. Albugo species parasitize crucifers and produce blister-like pustules filled with sporangia, which will germinate to produce motile zoospores. White rusts can also cause infected stems to grow in a contorted or twisted manner.
Members of Saprolegniales are called water molds. Many of these produce fast-growing, robust hyphae on organic matter in aquatic environments, but some species of Saprolegnia are parasitic on fish and fish eggs. For more information on Oomycetes, see Introduction to Oomycetes (Fry & Grünwald 2010).
Labyrinthulids (phylum Labyrinthulomycota), a small group of Straminipila, include organisms that cause rapid blight of turf grass and the wasting disease of eelgrass. Labyrinthulids have a unique manner of movement—their microscopic, football-shaped cells produce an ectoplasmic net through which the cells glide. The slow, gliding movement of the cells within the ectoplasmic net can be observed under the microscope.
Hyphochytrids (phylum Hyphochytridomycota, Kingdom Straminipila) are similar to chytrids in appearance, as their name suggests, and produce zoospores with a single anterior tinsel flagellum. Hyphochytrids are one of the smallest groups of fungal-like organisms, both in size and in number of species with only 23 known. Some hyphochytrids are known to parasitize algae, spores of AM fungi, and oospores of Oomycetes.
Slime molds are organisms that have a trophic (feeding) stage in their life cycle that lacks a cell wall, either uninucleate (amoeba) or multinucleate (plasmodium). The lack of a cell wall facilitates engulfment of food, in contrast to true fungi that must absorb their nutrients through a cell wall. The slime molds are now included in the Amoebozoa (Adl et al. 2005).
Four groups of slime molds are recognized—plasmodial slime molds (Myxomycota), cellular slime molds (Dictyosteliomycota and Acrasiomycota) and endoparasitic slime molds (Plasmodiophoromycota). We will briefly cover plasmodial slime molds and endoparasitic slime molds. For information on cellular slime molds, refer to one of the introductory mycology texts listed below (Recommended Further Reading). The plasmodial slime molds are most commonly found in temperate forests, where they occur on plant litter, tree bark, and other types of plant material. They produce a multinucleate trophic stage lacking a cell wall called a plasmodium (Fig. 61) that moves over and through decaying organic matter, engulfing bacteria, fungi, and other microorganisms (Frederick 1990). The most conspicuous stage of the plasmodial slime mold is the fruiting structures, called sporophores (Alexopoulos et al. 1996), which are often brightly colored and visible to the naked eye (Fig. 62). One of the most common slime molds in temperate regions is Fuligo septica. The sporophores of this slime mold are often found in ornamental bark and mulch, and look more like an animal's vomit than the fruiting structure of a living organism, thus earning the nickname: "dog vomit slime" (Fig. 63). The presence of slime molds in landscaping (Fig. 64) occasionally prompts calls to plant disease clinics, however, none of the plasmodial slime molds are known to be plant or animal parasites, and are of no known economic importance except as model organisms for research.
Members of phylum Plasmodiophoromycota are biotrophic parasites that produce their plasmodial stage inside the cells of plants, algae, diatoms, and Oomycetes. Several members of this phylum are economically important plant parasites, including Plasmodiophora brassicae, which causes clubroot of crucifers (Fig. 65) and Spongospora subterranea, which causes powdery scab of potato (Fig. 66). Polymyxa graminis is a vector for soilborne wheat mosaic virus, an economically important disease of wheat. Members of this phylum produce cysts inside host cells; the cysts are released when the plant tissue breaks down, and germinate to release a zoospore that infects the host by injecting its cytoplasm into a host cell. Infected host tissue may become greatly swollen as in clubroot of crucifers.
A Brief Summary
As we have seen, a fungus is a eukaryotic organism that absorbs nutrients through its cell walls and generally reproduces by spores. True fungi belong to Kingdom Fungi, and other fungal-like organisms are placed in phyla outside the Kingdom Fungi. Most fungi consist of a hyphal thallus that allows these organisms to colonize and exploit many different substrates and fill various ecological niches, as parasites, pathogens, mutualists, saprotrophs and decomposers. Fungi and fungal-like organisms survive and reproduce via a huge diversity of spore types, characteristic of each taxonomic group. This introduction has provided some basic information on reproduction, nutrient acquisition, and roles in the ecosystem, but much more information is available (see Recommended Further Reading and the References). Fungi are fascinating in and of themselves, but they are also critically important to humans in both detrimental and beneficial ways.
Recommended Further Reading
For more information on fungi, popularized accounts by Hudler (1998), Money (2002, 2007), and Moore (2001) are good sources of information written in an engaging manner. Introductory mycology books by Alexopoulos et al. (1996), Deacon (2006), Kendrick (2000), Moore et al. (2011), and Webster and Weber (2007) provide more detailed information on fungi than can be included in this brief introduction. Tales of fungi, folklore, and human affairs can be found in Dugan (2008) and Findlay (1982), and an engaging book on the impact of fungal plant pathogens by Money (2007). A number of books on hallucinogenic/psychoactive mushrooms have been written, some describing the history of these intriguing mushrooms. Gordon Wasson’s (1968) book “Soma” is one of the first ethnomycological treatments of hallucinogenic mushrooms, and a recent book on the history of magic mushrooms by Letcher (2007) relays the story of their use from ancient Aztecs to contemporary society.
We thank APS Press for allowing us to use images from “Fundamental Fungi.” All other images used with permission. We thank a number of colleagues who have kindly provided us with images. PPNS No. 0572, Department of Plant Pathology, College of Agriculture, Human, and Natural Resource Sciences, Agricultural Research Center, Project No WNP0837, Washington State University, Pullman, WA 99164-6430, USA. This paper is Contribution No. 11-288-J from the Kansas Agricultural Experiment Station, Manhattan.
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