Functional investigation of transactivating effectors in plant-pathogenic fungi

• Applicant: Professor Dr. Gunther Döhlemann
• Subject Area: Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 564415201

Ustilago maydis is a fungal pathogen that causes smut in corn. The disease is characterized by plant tumors that develop locally at the sites of infection. Several studies by us and others have shown that the virulence of U. maydis is largely determined by its arsenal of secreted effector proteins. However, little is known about the molecular mechanisms underlying effector-driven tumorigenesis. The starting point of this project was the discovery that U. maydis expresses its effector genes in an organ-specific manner, which led to the identification of a number of organ-specific virulence factors. Subsequent functional characterization of leaf-specific effectors identified See1, which modulates host DNA synthesis and cell cycle, and Erc1, a fungal glucanase with a highly specific virulence function in the host bundle sheath. The concept of organ specificity was extended to the level of cell type-specific regulation of effector genes. Based on our observation of specific processes in bundle sheath and mesophyll cells, we hypothesized that organ- or cell-type-specific effectors have specific functions in tumorigenesis and/or suppression of organ-specific immunity. We functionally investigated the virulence function of several organ/cell type-specific effectors. We identified the cell type-specific effectors Hap1/Hap2/Hap3 that modulate the sugar metabolism of the host plant. Furthermore, we showed that the effector Tip6 modulates host gene expression by interfering with maize topless proteins. Finally, we identified Sts2, which itself acts as a transcriptional activator and thus represents a new class of fungal effector proteins: the transcriptional activator effectors (TAEs). Sts2 activates key factors of cell development in maize to induce leaf tumor formation. Our latest preliminary work also shows that there are other TAEs besides Sts2, not only in U. maydis but also in other plant pathogenic fungi. The present project proposal aims to 1) comprehensively elucidate the underlying mechanism by which Sts2 modulates host gene expression and the evolutionary adaptation of Sts2 orthologs in smut fungi, and 2) identify novel TAEs in various fungal pathogens and investigate their virulence function. The proposed work plan aims to investigate the biological and molecular functions of fungal transactivating effectors (TAEs). A major goal is to gain deeper insight into the mechanism of transcriptional activation by Sts2 and to elucidate the evolution of this effector in smut fungi. In parallel, we plan to characterize novel TAEs in other fungal plant pathogens, using the U. maydis maize pathosystem to study the function of TAEs in maize leaf pathogens. 

• DFG Programme: Research Grants