ABSTRACT
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Fungi play a central role in many soil microbiological processes, influencing
soil fertility decomposition, the cycling of minerals and organic matter, plant
health, and nutrition. They also influence the structure and functioning of plant
communities and soil ecosystems. Fungi are immensely diverse, both
structurally and functionally, and have adopted different trophic strategies,
occurring as saprotrophs, symbionts and pathogens. Although this chapter is
about “soil” fungi, the filamentous growth habit of many fungi, coupled with
their different trophic strategies, implies that individual fungi can often
simultaneously colonize different substrates, such as living or dead plant
tissues, coarse woody debris, soil animals and mineral substrates-in addition
to the soil itself. Together with animals and plants, fungi represent one of three
major evolutionary branches of multicellular organisms, and their uniqueness
is reflected in the fact that they have the traclitional status of a kingdom
(theMycota). The diversity of fungi is high and, although only 75,000 species
have been described so far, it is likely that this represents only 5% of the true
number of fungal species, which is estimated to be about 1.5 million [1].
Despite this structural and functional diversity, all fungi share certain common
features, such as the fact that they are eukaryotic, contain a range of
membrane-bound organelles such as mitochondria and vesicles, and possess
membrane-bound nuclei containing several chromosomes. Fungi are hetero-
trophs, requiring external carbon sources. They also typically display
filamentous growth as a result of their hyphae, which exhibit apical growth;
thus, they are able to colonize new substrates, forming a mycelium by repeated
branching. Some fungi, such as single-celled yeasts, exhibit dimorphic growth
and can reproduce in liquids by budding as well as by colonizing other
substrates through mycelial growth. Unlike other eukaryotes, fungi typically
have haploid nuclei; however, the hyphae may have several nuclei in each
compartment. Moreover, many budding yeasts are diploid. Fungi charac-
teristically produce spores and many can reproduce both sexually and
asexually. They have cell walls composed of polymers of glucose, such as
chitins and glucans, and they secrete enzymes that degrade complex polymers
at their hyphal tips, allowing them to take up smaller molecules. Being
heterotrophic, fungi require external carbon sources for energy and cellular
be distinguished in accordance with the trophic strategies adopted to acquire
the organic compounds. Apart from their major impact on natural terrestrial
ecosystems, soil fungi have important, and still largely unexploited, appli-
cations in the biological control of pathogens, the bioremediation of polluting
compounds, and the biofertilization of soil. Further, soil fungi produce a large
range of secondary metabolites with potential for medical, biocontrol, or
environmental applications, including antibiotics such as penicillins and
cephalosporins, immunosuppressants, mycotoxins and aflatoxins. Other
compounds, such as organic acids and siderophores, may be involved in the
release and sequestration of mineral nutrients as well as in antagonistic
interactions with other organisms. In addition, fungi secrete a wide variety
of enzymes used in either pathogenic interactions or in the degradation of
plant litter and woody substrates. Some of these enzymes find important
applications in bioremediation of organic pollutants. Symbiotic mycorrhizal
fungi produce mycelia by growing from the roots of their host plants into the
surrounding soil. This connects them to the heterogeneously distributed
nutrients required for their growth, enabling the flow of energy-rich
compounds required for nutrient mobilization while simultaneously providing
conduits for the translocation of the mobilized products back to their hosts.
