Die Rolle von programmiertem Zelltod in der epidermalen Resistenz von Gerste

Ustilago hordei is the causative agent of covered smut on barley and oat. This basidiomycetous fungus establishes a biotrophic interaction with its host plant and therefore relies on a great variety of secreted effector proteins in order to cope with plant defense responses. This particularly implies the avoidance of host cell death reactions. Deletion mutants of the secreted effector Pep1 are blocked in epidermal penetration and elicit various plant defense responses, eventually leading to death of the invaded cell. The virulence factor Pep1 is functionally conserved in the corn smut pathogen Ustilago maydis and was recently shown to directly interact with apoplastic peroxidases, thus inhibiting the PAMP-triggered oxidative burst. Barley infections with the incompatible non-host pathogen U. maydis also result in epidermal cell death, which is similar to the hypersensitive response (HR). This study concentrates on molecular and cellular processes determining compatibility and incompatibility in different Ustilago/barley-interactions. Based on transcriptional analyses of U. hordei genes involved in compatible and incompatible plant interactions, 18 candidate genes were selected and further analyzed for a potential role during host colonization. So far, preliminary results point towards a virulenz function of five candidates. In the second part of this study cellular responses of barley were investigated, comparing the compatible U. hordei-interaction with responses during incompatibility caused by the non-host pathogen U. maydis as well as U. hordei and U. maydis pep1 mutants. Although incompatibility leads to programmed cell death respectively, a combination of live cell imaging techniques, an enzymatic activity assay and infections of transgenic plants overexpressing Bax Inhibitor-1 (BI-1) enabled the differentiation of two distinct pathways. While cell death responses towards pep1 mutants involve autophagy, non-host reactions after U. maydis infection trigger BI-1-dependent apoptosis-like cell death. These data sets were supported by transmission electron microscopy. Additionally, by evaluating H2O2 contents in apoplastic fluid and the transcriptional regulation of typical HR-associated marker genes, it became apparent that a HR is only caused by U. maydis infections, whereas infections by pep1 mutants result in basal defense and PAMP-tiggered immunity. Therefore it was shown, that different cell death mechanisms determine resistance of barley in host and non-host interactions.