Signal transduction in host sensing of T. harzianum

Trichoderma harzianum has been shown to act as a mycoparasite and is therefore commercially applied as biological control agent against a number of plant pathogenic fungi. The mycoparasitic interaction is host-specific and not merely a contact response. It is thus likely that signals from the host fungus are recognised by Trichoderma and provoke antifungal activities which are accompanied by morphological changes (e.g. appressorium formation) and the secretion of hydrolytic enzymes (e.g. chitinases) and antibiotics. In contrast to the current knowledge about these factors acting in mycoparasitism, little is known on the signaling pathway(s) except that unidentified receptors appear to recognise lectins or other ligands from the host. As for some plant pathogenic fungi signal transduction cascades are already quite well characterised it is interesting to observe that several morphological changes are highly conserved within the strategy of pathogenicity. Especially the formation of appressoria but also the secretion of hydrolytic enzymes appears to be a general mechanism of virulence in plant pathogens as well as mycoparasites. Therefore, it is planned to use this particular knowledge as basis for identifying key components of the cAMP and MAP kinase signaling pathways involved in virulence and to isolate the initiation point of the involved cascades, either a G protein-coupled receptor or receptor-independent activators of G protein signaling pathways. To prove their functionality and involvement in mycoparasitism, a set of disruptant strains of T. harzianum shall be generated and their biocontrol activities examined by applying various in vitro and in vivo approaches. Furthermore, respective mutants of the plant pathogenic fungus Ustilago maydis shall be complemented to investigate a general exchangeability of pathogenicity effectors in plant and fungal pathogenic fungi.

In the frame of this project, signal transduction pathways involved in host sensing and the mycoparasitic response during Trichoderma biocontrol were investigated. Trichoderma species are commercially applied as biocontrol agents against a number of phytopathogenic fungi due to their mycoparasitic abilities. The mycoparasitic interaction is host specific and includes recognition, attack and subsequent penetration and killing of the host involving infection structure formation and secretion of hydrolytic enzymes (e.g. chitinases) and antifungal metabolites (antibiotics, toxins) by the mycoparasite. Trichoderma specifically recognizes and transduces host-derived signals to their respective intracellular regulatory targets, thereby activating the processes involved in the mycoparasitic attack. To elucidate the underlying signal transduction pathways involved, several genes encoding key components of the cAMP (cyclic adenosine monophosphate) and MAPK (mitogen-activated protein kinase) signaling pathways, as alpha and beta subunits of heterotrimeric G proteins, the regulatory subunit of cAMP-dependent protein kinase, adenylate cyclase, and three MAP kinases were isolated from Trichoderma atroviride in the frame of the project. Analysis of mutants bearing deletions of single genes encoding the above mentioned signaling compounds revealed that at least two alpha-subunits of heterotrimeric G proteins (Tga1 and Tga3) as well as a MAP kinase (Tmk1) are participating in mycoparasitism-related signal transduction. Whereas G protein signalling was found to be involved in all three mycoparasitism-related processes namely infection structure formation, regulation of chitinase gene transcription and production of antifungal metabolites (e.g. 6-pentyl-alpha-pyrone and of substances with sesquiterpene structure), signalling via the Tmk1 MAP kinase was shown to affect the production of enzymes required for lysis of the host cell wall. The results on the regulatory mechanisms involved in Trichoderma biocontrol obtained in the frame of this project help to understand the mode of action of this fungus and can lead to improvement of Trichoderma as biocontrol agent.