Martin-Luther-Universität Halle-Wittenberg

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Dr. Ralf Horbach

Telefon: +49 (0)345 55-22646
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Institut für Agrar- und Ernährungswissenschaften, Phytopathologie und Pflanzenschutz
Betty-Heimann-Straße 3
06120 Halle

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Nachwuchsgruppe "Pilzlicher Sekundärmetabolismus und Pathogenität"

Dr. Ralf Horbach

Fungal Secondary Metabolism and Pathogenicity

Plant pathogenic fungi cause severe damage on a broad range of crop and ornamental plants, leading to significant yield and quality reduction. A steadily growing demand for food and biofuels requires both increased production of agricultural goods and efforts to reduce yield losses. Improved pest management strategies may substantially contribute to an increased efficiency in agriculture, however, they require detailed knowledge of the infection biology of phytopathogens. The DFG-funded project „Functional analysis of secondary metabolites produced by the maize anthracnose pathogen Colletotrichum graminicola” aims at understanding the role of secreted fungal metabolites that are secreted during the infection process.

Colletotrichum graminicola, the causative agent of maize anthracnose, produces several secondary metabolites some of which have been previously isolated. Structure elucidation and activity assays revealed hitherto unknown polyketides with antimicrobial and cytotoxic properties. In our present work, we want to study the importance of these compounds for a successful plant infection. The following questions are addressed in our current project: What kind of fungal polyketides can be found in infected host cells particularly during the transition from the biotrophic to the destructive necrotrophic phase? What are the corresponding polyketide synthases (PKSs) and how does the deletion of these genes influence virulence or the ability to compete with other microorganisms. Are there significant differences in the transcription profiles of PKS genes at different time points of the maize infection? Is there an epigenetic component in the control of PKSs expression?

Close examination of the C. graminicola genome revealed the presence of 32 PKS genes and 8 PKS-nonribosomal peptide synthase (NRPS) hybrids. Thus far, targeted gene deletion was confirmed for 25 PKSs and 3 accessory genes belonging to a PKS gene cluster. Corresponding metabolites could be assigned to 3 PKS. Detached leaf assays showed that one of the mutants lost the ability to infect host plants completely. Further infection assays with cultivars varying in their susceptibility will be conducted in order to determine putative virulence defects of the other PKS deletion strains.

The growing pool of mutants with single PKS deletions will be analyzed by means of metabolic fingerprinting in cooperation with Prof. Petr Karlovsky (Georg-August-University, Göttingen).

In the course of our investigations we identified two adjacent PKSs in the genome of C. graminicola that collectively synthesize the resorcinol lactone monorden as proven by targeted gene deletion. The monorden synthesis cluster contains enzymes, which function in modification and transport of the PKS product, i. e. genes encoding a halogenase, a cytochrome P450 reductase to form the epoxide ring and a MFS-transporter which seems to be responsible for carrying monorden across the fungal cell membran. Monorden is known as an effective inhibitor of heat shock protein 90 (hsp90) which, in turn, is a chaperone for plant resistance proteins. This particular function raises questions about the in planta production of monorden and the ability of C. graminicola to inhibit maize hsp90, thereby suppressing plant defense to facilitate host infection.

In a previous study, the sfp-phosphopantetheinyl transferase CgPPT1 has been identified as a central activator of all PKS, NRPS and the α-aminoadipate reductase (AAR) required for lysine biosynthesis. Since CgPPT1 is indispensable for fungal pathogenicity the enzyme may represent an excellent target for novel fungicides. In order to enable the screening of compound libraries for specific PPTase inhibitors we developed a robust and inexpensive microplate assay. The assay principle is based on the transfer of a fluorophore-labeled phosphopantetheine residue that is covalently attached to a conserved serine of the 100 amino acid acyl carrier protein domain of CgPKS1.

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