Abstract
Organisms which rely solely on innate defense systems must combat a large number of antagonists with a comparatively low number of defense effector molecules. As one solution of this problem, these organisms have evolved effector molecules targeting epitopes that are conserved between different antagonists of a specific taxon or, if possible, even of different taxa. In order to restrict the activity of the defense effector molecules to physiologically relevant taxa, these target epitopes should, on the other hand, be taxon-specific and easily accessible. Glycans fulfill all these requirements and are therefore a preferred target of defense effector molecules, in particular defense proteins. Here, we review this defense strategy using the example of the defense system of multicellular (filamentous) fungi against microbial competitors and animal predators.
Highlights
Introduction and Scope of This ReviewFilamentous fungi are among the simplest eukaryotic organisms with true multicellularity and tissue differentiation [1]
Not all of the glycoepitopes targeted by above mentioned fungal defense effector proteins (Table 1) have been identified unequivocally, at least two targeting strategies emerge: (1) The fungal defense effector proteins target highly conserved glycoepitopes of the antagonist exemplified by the intermediates of the biosynthetic pathways of cell wall polysaccharides in fungi or bacteria
Studies on the toxicity of fruiting body lectins in the model nematode C. elegans have shown that a loss-of-function mutation in a single gene, e.g., coding for a glycosyltransferase involved in the biosynthesis of the targeted glycoepitope, can lead to complete resistance of the nematode to such a lectin [18]
Summary
Filamentous fungi are among the simplest eukaryotic organisms with true multicellularity and tissue differentiation [1]. Molecules 2015, 20 cells in linear arrays, referred to as hyphae or filaments, that are able to branch and fuse, and penetrate the substrate as a loose, three-dimensional network, referred to as mycelium In addition to this long-lived and constantly renewed vegetative mycelium, hyphae of dikaryotic fungi can form compact, short-lived and spore-producing structures, referred to as fruiting bodies, during their sexual reproduction cycle. The main defense strategy of fungi is chemical defense i.e., the production of molecules impairing the development, growth or viability of the antagonists, by the fungus [4] These defense effector molecules include small molecules (secondary metabolites) [5], peptides (ribosomally or non-ribosomally synthesized) [6,7,8] and proteins [9], and act by binding to specific target molecules in the antagonists. Glycan-targeting killer toxins, produced by some yeast strains, were omitted from this review because they are encoded by viruses [21] and not regarded as intrinsic part of the fungal defense system
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