Trichoderma mycoparasitism enters the post-genomic era

The majority of plant diseases are caused by fungi and, in spite of the use of chemical fungicides in pest management, these fungi still are responsible for extensive economical losses. Some species of Trichoderma are able to impede growth of and can even kill these phytopathogenic fungi by mycoparasitism, and therefore are commercially applied as biological control agents against fungal pathogens in agriculture. The mycoparasitic interaction is host-specific and several proteins involved in intracellular signal transduction pathways play prominent roles in the recognition of host-derived ligands and in the activation of the mycoparasitic response. Recent investigations proved the relevance of G protein signalling for host recognition and for regulating mycoparasitism-relevant processes such as the production and secretion of hydrolytic enzymes and antimicrobial metabolites and the formation of mycoparasitism-associated infection structures in Trichoderma atrovirde. To obtain further insights into the molecular processes enabling Trichoderma atroviride to act as mycoparasite, the identification of the involved genes is indispensable. The recent release of the Trichoderma atroviride genome database now allows the generation of genome-wide expression profiles of this fungus under varying cultivation conditions by employing microarray technology. In the frame of this project, respective expression profiles of the wild-type and an avirulent mutant with defects in G protein signalling mediated by the Gpr1 G protein-coupled receptor (GPCR) cultivated in the presence or absence of a living host fungus shall be obtained. By comparing these expression profiles using bioinformatics, differentially expressed mycoparasitism-relevant genes as well as targets of the Gpr1 GPCR can be identified. To prove the functionality and involvement of selected genes in mycoparasitism-related processes, respective recombinant Trichoderma strains shall be generated and subjected to various biocontrol assays. This genome-wide approach for isolating mycoparasitism-relevant genes is highly topical and, in combination with the elucidation of underlying regulatory mechanisms, bears the potential to develop Trichoderma-based products with enhanced performance. As Trichoderma is a prominent producer of enzymes as well as antimicrobial metabolites, the identified genes in addition are putative targets for new products with industrial and medical applications.

The mycoparasitic fungus Trichoderma atroviride is able to antagonize and kill other fungi by direct parasitism. We identified the cell surface receptor Gpr1, a protein of the G protein-coupled receptor (GPCR) family, to govern essential mycoparasitism-related processes. In the absence of Gpr1, T. atroviride is blind, i.e. is unable to recognize and attach to the prey fungus, and impaired in the production of certain molecular weapons such as hydrolytic enzymes and secondary metabolites involved in prey attack and lysis. A screen for proteins interacting with Gpr1 led to the identification of a fungal-specific protein which co-localizes with Gpr1 in the fungal membrane and whose absence also impairs the mycoparasitic activity of the fungus. Based on their antagonistic action against phytopathogenic fungi, mycoparasites are commercially applied as biocontrol agents to guard plants against fungal diseases, thereby representing a promising alternative to chemical fungicides. Sensing and recognizing the prey fungus is an essential step in the mycoparasitic interaction and hence knowledge on the involved receptors and intracellular signalling pathways is indispensable. Analysis of the entity of actively expressed genes (transcriptome) of T. atroviride revealed that the fungus reduces the expression of genes associated with antibiotic production and of enzymes required for metabolism of simple sugars during the early interaction with a prey fungus, whereas several genes involved in attack and defence against the prey are up-regulated. Gpr1 not only affects the expression of mycoparasitism-relevant genes but also of genes involved in defence reactions and other signal transduction pathways and hence seems to have a broad effect on the cellular outcome. Further analyses revealed that Gpr1 resides in the fungal plasma membrane and there probably interacts with a fungal-specific membrane protein. Mutants missing this fungal-specific protein are, similar to gpr1-mutants, impaired in their mycoparasitic activity; however, they heavily overproduce a certain chitinolytic enzyme involved in cell wall remodelling during growth as well as chitin degradation during mycoparasitism. The outcomes of this project resulted in a better understanding of the cellular responses of T. atroviride during prey recognition and the role the Gpr1 receptor plays in this processes which, in a long-term perspective, may contribute to improvement of Trichoderma as biocontrol agent.