Date Published: September 6, 2018
Publisher: Public Library of Science
Author(s): Markus Künzler, Deborah A. Hogan.
Filamentous fungi arrange their cells in linear, coenocytic arrays, referred to as hyphae, that extend at their tips and are able to branch and fuse, leading to a loose, three-dimensional network referred to as mycelium . This architecture represents an optimal adaptation to the osmotrophic lifestyle of fungi in that it maximizes the surface for nutrient absorption and enables the fungus to efficiently reach and colonize its substrates. Some hyphae of the long-lived and constantly renewed vegetative mycelium may differentiate in other, more compact tissues, e.g., the (usually) short-lived and spore-producing fruiting bodies formed by dikaryotic fungi during their sexual reproduction. The different fungal tissues are exposed to different types of antagonists dependent on the ecological niche of the fungus. The vegetative mycelium of a saprophytic fungus, e.g., is exposed to other microorganisms that compete for the same nutrients and may feed on the degradation products released by the action of the hydrolytic enzymes secreted by the fungus. Accordingly, nutrient-rich substrates, such as the dung of herbivores, are battlefields of competing saprophytic bacteria and fungi . On the other hand, the lack of motility and high content of nutrients make both the fungal vegetative mycelium and the fruiting bodies attractive dietary resources for animal predators. Accordingly, soil-inhabiting fungi are an important dietary resource for soil arthropods and nematodes .
Fungi have evolved different strategies to increase their competitiveness for nutrient acquisition toward other microorganisms and to protect themselves from predation by animals. Similar to plants, the main defense strategy of fungi is chemical defense, i.e., the production of toxins impairing the growth, development, or viability of the antagonists by the fungus . These defense effectors include secondary metabolites , peptides (ribosomally or nonribosomally synthesized) [6, 7], and proteins  and usually act by binding to specific target molecules of the antagonists (Table 1). It has been hypothesized that effectors against microbial competitors are secreted, whereas effectors against metazoan predators are usually stored within the fungal cells and are taken up during predation (Fig 1) . Examples of fungal defense effectors in accordance with this hypothesis are the β-lactam antibiotic penicillin produced by some Penicillium species , the antifungal lipopeptide pneumocandin B0 produced by Glarea lozoyensis , and the cytotoxic, ribosomally synthesized octapeptide α-amanitin produced by some Amanita, Galerina, Conocybe, and Lepiota species . Penicillin is secreted and binds and inhibits extracellular enzymes involved in peptidoglycan biosynthesis, an essential and conserved process in all bacteria . Similarly, pneumocandin B0 is secreted and inhibits 1,3-β-D-glucan synthase, one of the main enzymes involved in fungal cell wall biosynthesis and is therefore called “penicillin of the antifungals” . In contrast, α-amanitin is taken up from the fungal cell upon predation and enters epithelial cells of the digestive tract of animal predators where it binds and inactivates the essential and conserved nuclear enzyme RNA polymerase II . Exceptions to the hypothesis are a number of secreted insecticidal and nematicidal secondary metabolites . In addition to the action of toxins, fungi have more subtle ways of chemical defense, e.g., by the production of molecules interfering with bacterial and animal communication. Examples are intracellular lactonases of the coprophilous ink cap mushroom Coprinopsis cinerea acting as a sink for quorum sensing signals of gram-negative bacteria  and the production of insect juvenile hormones by the mold Aspergillus nidulans .
The biosynthesis of chemical defense effectors is usually tightly regulated because these molecules are not essential for the viability of an organism, and their biosynthesis requires resources that may be limited . This regulation can be autonomous, i.e., independent of the antagonist and/or antagonist-dependent. Accordingly, it has been shown that the regulation of secondary metabolism and sexual development are coordinated in A. nidulans ; some of the secondary metabolites, whose biosynthesis is restricted to the fruiting body, exert toxicity toward arthropods suggesting that these organs are prey of and therefore require protection from animal predators . Similarly, the concentration of amatoxins of the mushroom Amanita phalloides, including α-amanitin, is lowest in the vegetative mycelium and highest in the fruiting body . Analogously, genome-wide gene expression analysis of the vegetative mycelium and young fruiting bodies of the model mushroom C. cinerea revealed that the secreted antibacterial peptide Copsin is almost exclusively produced in the vegetative mycelium, whereas most of the C. cinerea genes coding for intracellular insecticidal and nematicidal lectins are specifically expressed in the fruiting body (Fig 1) . This spatiotemporal, autonomous regulation results in an efficient constitutive protection of specific fungal tissues against the most relevant antagonists because some of the defense effectors are already in place when the antagonist attacks the fungus. On the other hand, at least some of the lectin-encoding genes directed against animal predators were induced in the C. cinerea vegetative mycelium when this tissue was challenged with a fungivorous nematode . Similarly, challenge of the vegetative mycelium of the basidiomycete Oudemansiella murata with two different Penicillium spp. induced the production of the antifungal strobilurin A , and challenge of various ascomycetous molds with bacteria and arthropods led to the induction of various gene clusters coding for the biosynthetic machineries of antimicrobial and cytotoxic secondary metabolites, respectively [25–29]. These results suggest that fungi possess, in addition to an autonomous, tissue-specific defense, also an inducible defense (Fig 1). This type of regulation is also known from the innate defense systems of plants and animals .
The presence of innate defense systems in multicellular fungi, plants, and animals suggests that such systems are a universal requirement of multicellular organisms. In order to clarify whether these defense systems are the result of divergent or convergent evolution, the fungal defense system has to be better characterized with regard to three key issues of innate defense.
Future in-depth characterization of the fungal innate defense against microbial competitors and animal predators is not only important in terms of basic research, e.g., the evolution of innate defense in eukaryotes, but also in terms of applied research. Fungi are a rich source of chemically diverse natural products, many of which are used for antagonistic interactions. These compounds have a high potential to be used as drugs in management of relevant human or animal diseases and pests. As many of these compounds are produced only in response to an antagonist, investigations of fungal antagonistic interactions are key to exploitation of this “treasure of nature” [63, 64].