The role of Arabidopsis transcription factors WRKY18 and WRKY40 in plant immunity

Two related Arabidopsis thaliana transcription factors, WRKY18 and WRKY40, are induced upon infection with the obligate biotrophic powdery mildew, Golovinomyces orontii (G. orontii), during early stages of infection. WRKY18 and WRKY40 negatively regulate host resistance as wrky18wrky40 double mutants are resistant towards this fungus. Differential expression of hormone biosynthesis and response genes between susceptible wildtype and resistant wrky18wrky40 plants suggested a crucial role of hormone signaling during G. orontii infection. Investigating the potential contribution of hormonal changes to resistance during this plant-pathogen-interaction is one focus of this thesis. Although hormone measurements did not reveal major differences between susceptible wildtype and resistant wrky18wrky40 plants, genetic studies demonstrated that SA biosynthesis is indispensable for resistance. Besides hormone-dependent defense responses, secondary metabolites, such as the indol-glucosinolate 4MI3G (4-Methoxyindol-3-ylmethylglucosinolat), have been shown to contribute to antifungal defense. Elevated levels of 4MI3G in infected wrky18wrky40 plants indicate a potential role of this compound in resistance towards G. orontii. Whereas WRKY18 and WRKY40 are negative regulators of resistance towards G. orontii, this was not the case for other powdery mildews. Hence, wrky18wrky40 mutants do not exhibit a broad-spectrum but rather specific resistance towards G. orontii infection. Furthermore, comprehensive wrky18wrky40 infection studies including different biotrophic, hemi-biotrophic and necrotrophic pathogens revealed a positive role of WRKY18 and WRKY40 in effector-triggered resistance towards avirulent Pseudomonas syringae DC3000 expressing the AvrRPS4 effector gene. This response appears to be highly specific since it was not observed with bacteria expressing other tested Avr genes. To further dissect roles of WRKY18 and WRKY40 in plant immunity and to uncover potential direct target genes of these transcription factors, global expression analyses of wrky18 and wrky40 single mutants upon G. orontii were performed. Overall, WRKY18 and WRKY40 function partly redundantly, but regulate highly diverse sets of genes. Direct binding of potential direct target genes will be analyzed by ChIP-PCR employing the newly generated WRKY18-HA complementation line. First results demonstrated WRKY18 feedback regulation on its own gene and the WRKY40 gene during G. orontii infection. In addition, a yeast 2-hybrid screen against a pathogen-induced cDNA-library revealed potential interaction partners of WRKY18 and WRKY40 that co-localize to the plant cell nucleus. In conclusion, this thesis contributes to further understanding the roles of WRKY18 and WRKY40 in plant immunity