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The Fungal Web


The Microbial World:
The Fungal Web

Produced by Jim Deacon
Institute of Cell and Molecular Biology, The University of Edinburgh


This is one of 23 Profiles on fungi and fungus-like organisms. It provides an introduction to fungi and fungal activities.
Other Profiles, listed below, deal with specific aspects of fungi.

For a comprehensive list of fungal resources on the WWW see: http://muse.bio.cornell.edu/~fungi/findex.html

 

Fungi are one of the three major groups of eukaryotic organisms, equivalent in status to plants and animals. They have major environmental and economic significance:
  • Fungi are the major decomposers of organic matter, especially of structural polymers such as cellulose, the most abundant polymer on earth (see wood-decay fungi)
  • Fungi cause about 70% of all the major crop diseases (see Biotrophic plant pathogens)
  • By forming mycorrhizas with most land plants, fungi play a vital role in mineral nutrient uptake and in vegetational dynamics
  • The diverse metabolic pathways of fungi generate many commercial products such as ethanol, organic acids, enzymes, antibiotics, etc., but also some extremely potent toxins that affect human and animal health (Figures B, C)
  • with the advent of transplant surgery and the increase in immunosuppressive conditions, including AIDS, fungi are emerging as a significant group of life-threatening human pathogens (see Yeasts and yeast-like fungi).

These and other aspects of fungal biology are discussed in various Profiles on this site.


The fungal lifestyle

We can think of fungi in a 'narrow' taxonomic sense or in terms of lifestyle. This distinction is necessary because some of the organisms that we regard as fungi - the extremely important potato blight pathogen (Phytophthora infestans) is one example - are quite unrelated to the true fungi and belong to a separate kingdom of organisms (see later).

However, we can define the fungi broadly in terms of lifestyle and accommodate all these organisms.

1. Typically, fungi grow as filaments termed hyphae (singular: hypha), about 5-10 micrometres diameter. Hyphae are surrounded by a wall and extend at their tips, while drawing the protoplasm forwards as they grow. So fungi exhibit apical growth (Figure A). The hyphae branch repeatedly to form the 'body' of a fungus, termed the mycelium. The left-hand image at the top of this page shows several examples of this. The only major departure from the mycelial growth form is found in the unicellular yeasts, but even yeasts can produce hyphae in appropriate conditions.

2. All fungi depend on pre-formed organic nutrients for energy and for synthesis of cellular materials. They obtain these nutrients in a characteristic way - by absorbing simple, soluble nutrients (sugars, amino acids, etc.) through their walls, and by releasing extracellular enzymes to degrade polymers that they cannot absorb.

3. Fungi are dispersed by means of spores of almost infinite variety, produced by either an asexual or a sexual process (see Airborne microorganisms).


Figure A. Tip growth of fungal hyphae. These three images from a videotape sequence were taken over a period of 20 minutes after a glass coverslip was placed on the margin of a colony of Neurospora crassa. The left-hand image is typical of a hypha, with extension growth at the hyphal apex, and branches arising from points behind the main apex. The addition of a coverslip disturbs the normal tip growth, causing several branches to emerge from the existing tips (centre and right-hand images).


Apical growth is the hallmark of fungi (see
Fungal tip growth and hyphal tropisms). It enables them to grow continuously into fresh zones of nutrients and also to penetrate hard surfaces such as plant cell walls, insect cuticle, etc. This is why fungi are so important as plant pathogens and as decomposer organisms.


Fungi produce a wide range of unusual metabolites, termed secondary metabolites because they play no role in the normal, basic metabolic pathways used for growth and energy production, etc. Some of these secondary metabolites have antibiotic properties (see antibiotics). Others are potent toxins that are dangerous when eaten by humans or other animals (see below for one example), and others include pigments inserted into the fungal walls or released into the surrounding environment, or flavour or odour components of toadstools. The range of these metabolites is almost endless, and most of them have no known roles.

Figures B, C show two examples of potent fungal toxins (mycotoxins). B. A sheep suffering from facial eczema, a condition caused by the toxin sporidesmin in spores of the fungus Pithomyces chartarum. The fungus grows on pasture grasses in New Zealand, and sheep that ingest the spores develop liver damage leading to a photosensitive response (see Airborne microorganisms). [Image supplied by Dr Eric McKenzie]. C. Ergot of cereals, caused by the fungus Claviceps purpurea. This fungus infects cereals and grasses, forming large black resting bodies (ergots) in place of some of the grains. The ergots contain extremely toxic alkaloids that can lead to death of people who eat bread made from contaminated flour.

Click here for further images and information on facial eczema.


The major groups of fungi and fungus-like organisms

A 'narrow' definition of fungi would include only the organisms in group 1 below. These are closely related, with a common ancestor, and represent the Kingdom Mycota. However, several other organisms have traditionally been treated as fungi. They are shown in groups 2 and 3 below.

1. True fungi (Kingdom Mycota) with walls typically containing chitin, and with many other characteristic cellular and biochemical features. Five sub-groups are recognised.

A. Chytridiomycota Typically unicellular, or with primitive chains of cells attached to a food base by tapering rhizoids; sexual reproduction is by fusion of motile gametes; asexual reproduction is by cytoplasmic cleavage in a sporangium, producing motile, uniflagellate zoospores.(See Catenaria anguillulae)

B. Zygomycota (Figures D,E). Typically grow as hyphae without cross-walls (aseptate); sexual reproduction is by fusion of sex organs (gametangia) leading to thick-walled resting spores (zygospores); asexual reproduction is by cytoplasmic cleavage in a sporangium, producing non-motile spores.

C. Ascomycota (Figures F, G) Grow as hyphae with cross-walls (septa) or yeasts; sexual reproduction is by fusion of modified hyphae (or yeasts), sometimes by fusion of a "male" spore (spermatium) with a "female" receptive hypha (trichogyne), leading to development of an ascus containing ascospores; asexual reproduction as in the deuteromycota (see below).

D. Deuteromycota (Figures H, I) Grow as hyphae (with septa) or yeasts; sexual reproduction is absent, rare or unknown; asexual spores (conidia) are formed in various ways from hyphae but never by cytoplasmic cleavage in a sporangium.

E. Basidiomycota (Figures J-N) Grow as hyphae or yeasts; asexual spores are relatively rare; sexual reproduction is by fusion of compatible hyphae, leading ultimately to production of basidiospores on basidia, sometimes on or in a fruiting body (e.g. toadstool).

2. Organisms with a fungal lifestyle (Kingdom Stramenopila) but with cellulose walls and with cellular and biochemical features resembling those of plants. For further details, see Zoosporic fungi online (not on this server). The Kingdom Stramenopila contains brown algae, diatoms and some fungus-like organisms, including the Oomycota.

Oomycota (Figures O, P) Grow as aseptate hyphae; asexual reproduction is by formation of motile, biflagellate zoospores in a sporangium; sexual reproduction is by fusion of a "male" sex organ (antheridium) with a "female" sex organ (oogonium), leading to production of thick-walled resting spores (oospores). (see Pythium oligandrum, Fungal zoospores)

3. Organisms with some fungus-like features, but which grow as wall-less protoplasmic stages. There are several unrelated organisms in this group, loosely termed "slime moulds"

A. Acrasids and dictyostelids (cellular slime moulds). These are amoeboid organisms that engulf bacteria and other food particles by phagocytosis; they aggregate to form a fungus-like fruiting body that releases dry, air-borne spores.

B. Myxomycetes (plasmodial slime moulds). Grow as a network of protoplasm (the plasmodium) that engulfs bacteria and other food particles; at the onset of starvation they form fruiting bodies that release dry, air-borne spores. (see Slime moulds)

C. Plasmodiophorids. Obligate intracellular parasites of fungi, algae and higher plants; they grow as naked plasmodia in the host cells. One important example is Plasmodiophora brassicae which causes the damaging clubroot disease of cruciferous plants. See Fungal zoospores: chytrids and plasmodiophorids. Also see Plasmodiophorid home page (not on this server)

Click HERE (not on this server) for further details of fungal groups, including the fossil record and phylogenetic relationships

Figures D-E. Zygomycota. Asexual reproduction (D) is by production of spores in a sporangium on an aerial hypha termed a sporangiophore (see Fungal tip growth). The appearance of these structures leads to the common name "pin moulds". Sexual reproduction (E) is by fusion of two gametangia to produce a black, warty zygospore, with swellings on either side termed suspensors.



Images supplied by Dr Nick Read

Figures F, G. Ascomycota. Sexual reproduction in this group leads to the production of one or more asci, each containing usually 8 ascospores. Figure G shows a cluster of asci of Sordaria macrospora; the mature ascospores are black. Often the asci are contained in a fruiting body which may be flask-shaped (termed a perithecium and shown for S. macrospora in Figure F) or cup-shaped (an apothecium) or closed (a cleistothecium). See Lichens for examples of apothecia; see Thermophilic microorganisms for an example of a cleistothecium.


Figures H, I. Deuteromycota. Deuteromycota produce asexual spores (conidia) in many ways but never within a sporangium. Several examples can be found in Thermophilic microorganisms. The example here is Monilinia fructigena, a common cause of apple rots in orchards. It infects the fruit through wounds caused by birds or wasps, then rots the fruit rapidly and forms many sporing pustules on the surface (Figure H). At high magnification (Figure I) the spores are seen to develop in branched chains by successive budding and swelling of the hyphal tips.

Click here for more information on apple rot fungi


Figures J-N. Basidiomycota. The basidiomycota include the economically important rust fungi of crop plants and the mushroom- and toadstool-producers. Figure J shows toadstools of Coprinus comatus, one of the 'ink-caps'. In this and other Coprinus species the gills are digested progressively and drip down as an inky fluid containing the basidiospores (see Figure K, taken 2 days after Fig. J). Figures L and M show microscopical cross sections of the gills of a typical toadstool. The basidiospores (seen in Fig. M) are produced from basidia that line the gills. Figure N shows a different type of fruiting body - a puffball (Lycoperdon species) which contains many basidiospores. At maturity, the puffball dries to a 'papery' sack with a hole at the top, and the spores are puffed out by falling raindrops (see the book cover at the top of this page).

Click here for more information on basidiomycota


Figures O, P. Oomycota. Sexual reproduction is shown for a species of Saprolegnia in Figure O. An oogonium is fertilised by 'male' hyphal branches that clasp onto it and produce antheridia (a). Then the contents of the oogonium are converted to one or more thick-walled resting spores termed oospores (o). Asexual reproduction is by release of motile zoospores from a sporangium. Figure P shows a single sporangium of Phytophthora palmivora. This fungus is related to P. infestans (the cause of potato blight). The sporangia are wind-dispersed, land on a plant surface and their contents differentiate to produce zoospores. A small papillum at the tip of the sporangium breaks down, and zoospores escape (arrowhead) by swimming through the resulting pore. [Figure P supplied by David Grayson]

Click here for more about Phytophthora infestans and potato blight


Distinctive features of the true fungi (Kingdom Mycota)

The true fungi share several features that clearly distinguish them from all other organisms, showing that they are a 'natural' (monophyletic) group of organisms. These features are important in practice because they can provide targets for the actions of antifungal agents. In the list below we see that some of the most powerful drugs for treatment of fungal infections of humans, and the fungicides used for plant disease control, are targetted at the unique biochemical or structural features of fungi (see Penicillin and other antibiotics).

1. Chitin is a major component of fungal walls (but also found in insects, etc.). The enzyme that synthesizes chitin (chitin synthase) is a target for the polyoxin antibiotics.

2. Fungi are haploid, whereas the other major groups of eukaryotes are diploid.

3. Fungal cell membranes contain ergosterol, whereas animals have cholesterol and plants have sitosterol and other 'phytosterols'. Several antifungal drugs (e.g. ketoconazole) used in human therapy act by blocking ergosterol synthesis. The antifungal antibiotics (e.g. nystatin, amphotericin B) combine with ergosterol in fungal membranes. And several fungicides used for plant disease control act by disrupting specific steps in the ergosterol synthesis pathway.

4. Fungi synthesise the amino acid lysine by a unique pathway, different from that of other organisms.

5. Fungi have characteristic soluble carbohydrates (the disaccharide trehalose and polyhydric alcohols like mannitol and arabitol) and storage compounds (e.g. glycogen), differing from those of most plants and animals.

6. Fungi have several characteristic ultrastructural features, such as plate-like cristae in the mitochondria (like animals), and tubular unstacked Golgi cisternae (unlike animals or plants). See Fungal tip growth.

7. The microtubules of fungi have unique binding affinity for anti-tubulin agents. In particular, fungal tubulins bind to the antibiotic griseofulvin (used to treat some fungal infections of humans) and to the benzimidazole fungicides (used widely for control of fungal pathogens of plants).

8. Finally, fungi differ from other organisms in a range of biochemical and molecular features such as the regulation of some enzymes, some aspects of mitochondrial codon usage, etc.

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This site is no longer maintained and has been left for archival purposes

Text and links may be out of date