**1. Introduction**

Mycoparasitic relationships, where a predatory fungal species gains nutrients on the expense of a host fungus, are widespread within the fungal kingdom. By the modalities of this nonmutual relationship, biotrophic and necrotrophic mycoparasitic fungi with different gradations within this classification (contact, invasive or intracellular necrotrophic; haustorial or fusion biotrophic) can be distinguished [1]. Biotrophic mycoparasites co-exist and nourish on their living host in a balanced way and are specifically adapted to one or few host species. In contrast, necrotrophic fungi destructively invade and kill a broad range of hosts to gain

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

nutrients from the remains of their prey [2]. Mycoparasitic fungi are prolific producers of a plethora of volatile and non-volatile secondary metabolites, favoring their ecological fitness and survival under certain environmental conditions. For example, the excretion of siderophores – affecting high affinity iron chelation – is strongly up-regulated under iron-limiting conditions [3] and several antimicrobial metabolites empower the successful perseverance within the ecological niche [4]. The mycoparasitic lifestyle obviously substantiates the overrepresentation of secondary metabolism-associated genes and the extensive excretion of a variety of secondary metabolites [2] enabling the fungus´ successful access to its prey as well as its thriving persistence in or assassination of the host. Furthermore, selected fungal secondary metabolites are known to exhibit beneficial effects on plants: They may promote vitality and growth of roots and shoots, enhance the resilience against abiotic stress factors and prime the plants immune system (induced systemic resistance; ISR) thereby enhancing its resistance and survival in case of prospective infections with pathogens [5]. In recent times, evidence accumulated that some secondary metabolites also act as communication molecules over species boundaries [6, 7].

pathogenic *Rhizoctonia solani* [17] as well as *Coniothyrium minitans* and *Microsphaeropsis ochracea* co-culture antagonizing the plant pathogenic *Sclerotinia sclerotiorum* [18].

Secondary Metabolites of Mycoparasitic Fungi http://dx.doi.org/10.5772/intechopen.75133 39

A characteristic trait of filamentous fungi is that their secondary metabolism-associated genes are mostly situated within subtelomeric regions of the chromosomes in large biosynthetic gene clusters present in the genomes in significantly greater numbers than secondary metabolites currently identified [19]. The unique and often uncommon biosynthetic pathways are mostly characterized by signature enzymes, often also transcription factors and transporters present in the respective gene clusters, which enable the secondary metabolite synthesis starting from simple precursors gained from primary metabolism like amino acids and acetyl-CoA [20]. Most common core enzymes are non-ribosomal peptide synthases (NRPSs), polyketide synthases (PKSs) and terpene-synthases or -cyclases [4]. In necrotrophic mycoparasitic species like *T. atroviride* and *T. virens,* genes for the biosynthesis of secondary metabolites are enriched compared to the only weakly mycoparasitic relative *Trichoderma reesei*. Within the two mentioned strong mycoparasites nearly half of all secondary metabolism-associated genes are positioned on non-syntenic regions in the genome and do not exhibit orthologs in the respective other species [21]. Depending on the species, environmental conditions, and even the strain, a plethora of different compounds is derived by the genus *Trichoderma* [2, 4], awakening hope on the detection of new substances. Since resistance development to the currently applied substances constitutes an increasing problem in agriculture and medicine, the need for environmentally sustainable biological control of pathogens and the discovery of novel antagonistic substances is essential. Secondary metabolites of mycoparasitic fungi could contribute to the solution.

Non-ribosomal peptides (NRPs) are synthesized by NRPSs, enzymes that characteristically consist of multiple domains synthesizing the peptide in one by one steps. Characteristic for NRPSs are the core domains for adenylation, thiolation and condensation. The generated NRPs are very diverse: they mostly comprise of proteinogenic and non-proteinogenic amino acids, can be linear or branched to cyclic with a varying length. After their synthesis outside of the ribosome, they frequently pass extensive secondary modifications. Many fungal NRPs have high economic and/or ecologic value like β-lactam antibiotics, the immunosuppressant

The occurrence of NRPS genes is enriched within the genome of mycoparasitic *T. atroviride*  for 60% and in *T. virens* for 180% to the wood-degrading *T. reesei* [21]. Further, the functional involvement of NRPS and PKS in the mycoparasitic interaction was supported by deletion experiments of the *T. virens* 4-phosphopanteteinyl transferase-encoding gene (*ppt*) – which is essential for NRPs and PKSs activation – resulting in mutant strains defective in mycoparasitism and induction of systemic resistance (ISR) in plants [22]. The main NRPSs derived metabolites in *Trichoderma* species are peptaibiotics, epipolythiodioxypiperazines

**2. Non-ribosomal peptides**

and siderophores.

cyclosporine A but also mycotoxins like gliotoxin.

A great diversity of mycoparasitic species exists in the fungal kingdom, especially within the order *Hypocreales* [8]. In this aspect *Trichoderma* (teleomorph *Hypocrea*), a worldwide abundant, diverse fungal genus, is one of the best-studied examples [2]. *Trichoderma* comprises necrotrophic mycoparasitic species like *Trichoderma atroviride* or *Trichoderma virens*, which are successfully applied in agriculture as biocontrol agents against plant pathogenic fungi of crop plants. Further they are reported to promote plant growth, vitality and systemic resistance via priming the plants' immune system. The genomes of several *Trichoderma* species have been sequenced and analyzed revealing the ancestral mycoparasitic lifestyle of these fungi [9, 10]. The second largest lineage of mycorparasites within the *Hypocreales* is the genus *Tolypocladium*. *Tolypocladium* comprises, besides of some entomopathogenic species like *Tolypocladium inflatum,* mostly mycoparasitic species like the widespread on northern hemisphere *Tolypocladium ophioglossoides* which mycoparasitizes with a narrow host range on truffles of the genus *Elaphomyces* [11, 12]. In contrast to *Trichoderma*, the genus *Tolypocladium* exhibits an ancestral entomopathogenic lifestyle and developed to a mycoparasite by host jumping. The genomes of *Tolypocladium* species are rich in secondary metabolite gene clusters of which some, like the clusters for the production of peptaibiotics, seem to be exclusive to mycoparisitic lineages [12]. A further well-investigated mycoparasitic fungus is *Escovopsis weberi*. *E. weberi* is a contact necrotrophic mycoparasite on *Leucoagaricus* sp. in leaf-cutting ant agriculture [13]. As foraging for leaves causes considerable economic damages in neotropic agriculture by defoliation of a wide variety of crop plants, *E. weberi* would be a suitable biocontrol agent as it causes a breakdown of the fungal feeding structures, thereby starving out the ant colony leading to a collapse of the whole system [14]. Like other very specialized mycoparasites, the *E. weberi* genome exhibits a reduced size and content, but very unique secondary metabolite clusters for host attack, facilitating the excretion of fungicidal substances which can break down the host mycelia even without contact [15, 16]. Further examples of secondary metabolite analysis on mycoparasitic species include *Stachybotrys elegans,* a potential biocontrol agent against plant pathogenic *Rhizoctonia solani* [17] as well as *Coniothyrium minitans* and *Microsphaeropsis ochracea* co-culture antagonizing the plant pathogenic *Sclerotinia sclerotiorum* [18].

A characteristic trait of filamentous fungi is that their secondary metabolism-associated genes are mostly situated within subtelomeric regions of the chromosomes in large biosynthetic gene clusters present in the genomes in significantly greater numbers than secondary metabolites currently identified [19]. The unique and often uncommon biosynthetic pathways are mostly characterized by signature enzymes, often also transcription factors and transporters present in the respective gene clusters, which enable the secondary metabolite synthesis starting from simple precursors gained from primary metabolism like amino acids and acetyl-CoA [20]. Most common core enzymes are non-ribosomal peptide synthases (NRPSs), polyketide synthases (PKSs) and terpene-synthases or -cyclases [4]. In necrotrophic mycoparasitic species like *T. atroviride* and *T. virens,* genes for the biosynthesis of secondary metabolites are enriched compared to the only weakly mycoparasitic relative *Trichoderma reesei*. Within the two mentioned strong mycoparasites nearly half of all secondary metabolism-associated genes are positioned on non-syntenic regions in the genome and do not exhibit orthologs in the respective other species [21]. Depending on the species, environmental conditions, and even the strain, a plethora of different compounds is derived by the genus *Trichoderma* [2, 4], awakening hope on the detection of new substances. Since resistance development to the currently applied substances constitutes an increasing problem in agriculture and medicine, the need for environmentally sustainable biological control of pathogens and the discovery of novel antagonistic substances is essential. Secondary metabolites of mycoparasitic fungi could contribute to the solution.
