Nonhost resistance is the prevalent type of pathogen resistance of plants in nature and describes the immunity observed when all members of a plant species exhibit resistance to all members of a given pathogen species. The cytological analysis of the interaction between the non-adapted (nonhost) pathogen Phytophthora infestans, the causative agent for late blight disease of potato, and Arabidopsis thaliana revealed that inducible defence reactions comprise the basis to avoid pathogen entry at the cell periphery. As an important component in the cell periphery-associated penetration resistance in Arabidopsis against barley powdery mildew, Blumeria graminis f. sp. hordei, PEN2 (PENETRATION2) has been identified. The analysis of different Arabidopsis signal transduction mutants showed that the combined loss of PEN2 as a part of the preinvasive resistance as well as PAD4 (PHYTOALEXIN DEFICIENT4) and EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1), respectively, as parts of the postinvasive resistance is sufficient to overcome nonhost resistance against pea powdery mildew Erysiphe pisi. The analysis of the A. thaliana mutant pen2-1 (present studies) suggests, that nonhost resistance against P. infestans is also affected. In order to identify further components of the nonhost resistance in A. thaliana, a remutagenized pen2-1-population was analyzed after P. infestans-challenge. For that purpose, an investigation of the necrosis and autofluorescence phenotype (hypersensitive reaction (HR)-phenotype) after P. infestans-infection of approx. 70.000 M2-plants was done. Altogether, 26 putative mutants (13 M1-pools) with an altered nonhost resistance phenotype compared to pen2-1 could be isolated. The mutants 190, 202, 222, 242, 266, 272 and 303 exhibit an increased cell death frequency as well as enhanced accumulation of autofluorescence compounds and H2O2 after infection with P. infestans in comparison to pen2-1. Furthermore, the introduction of the secondary mutations 202 and 222, respectively, in the pen2-1-background leads to an additional increase of the penetration frequency after P. infestans-infection compared to pen2-1. The broader analysis of pathogen resistance in the different isolated mutants indicates a loss of resistance against the necrotrophic pathogen Alternaria brassicicola in the mutants 242 and 272, respectively. In addition, the investigation of a set of Arabidopsis signal transduction mutants provides the evidence for involvement of PAD4 and EDR1 (ENHANCED DISEASE RESISTANCE1), respectively, in the nonhost resistance in Arabidopsis.