Funktionelle Charakterisierung eines LysM-Proteins von Ustilago maydis

Ustilago maydis, the causal agent of corn smut, depends on the secretion of various effector proteins in order to guarantee the establishment of a biotrophic interaction with its host plant Zea mays. Secreted LysM proteins in phytopathogenic fungi have been shown to play an essential role as virulence factors, since they are able to sequester chito-oligosaccarides that were released by plant chitinases, thereby preventing PAMP-triggered immunity. In that respect, one gene (um11464) encoding for a LysM-containing protein has been identified in the genome of U. maydis. In this study it was shown that the deletion of um11464 resulted in a hypervirulent phenotype in planta. A detailed phenotypical characterization of an um11464 deletion strain revealed that this strain shows a morphological phenotype in axenic culture as well as enhanced sensitivity towards cell wall stressors. Furthermore, the deletion of um11464 was shown to positively influence filament and appressoria formation, although the penetration efficiency of SG200∆um11464 was not impaired. Secretion assays revealed that the signal peptide of Um11464 is functionally active. However, Um11464 could never be detected in the culture supernatant. Using confocal microscopy and immuno-labelling, Um11464 could be localized within the fungal cell wall. It could be shown that the protein binds non-covalently to the cell wall. However, polysaccharide binding assays revealed, that the attachment might not be achieved through binding to chitin or any other cell wall component. The positive effect of the um11464 deletion on differentiation and virulence could indicate that Um11464 might change plant defense responses. Transcriptional profiling of infected maize plants revealed that several plant genes involved in defense responses were indeed significantly induced in response to the um11464 deletion strain. This suggests that plant defense responses are actively suppressed after um11464 deletion.