Nonhost resistance describes the immunity of an entire plant species against nonadapted pathogen species. We report that Arabidopsis PEN2 restricts pathogen entry of two ascomycete powdery mildew fungi that in nature colonize grass and pea species. The PEN2 glycosyl hydrolase localizes to peroxisomes and acts as a component of an inducible preinvasion resistance mechanism. Postinvasion fungal growth is blocked by a separate resistance layer requiring the EDS1-PAD4-SAG101 signaling complex, which is known to function in basal and resistance (R) gene-triggered immunity. Concurrent impairment of pre- and postinvasion resistance renders Arabidopsis a host for both nonadapted fungi. Lipka Volker Scheel Dierk 2005 Nov Bednarek Pawel 1180-3 Rosahl Sabine 5751 Dittgen Jan Llorente Francisco Parker Jane Bhat Riyaz 16293760 Molina Antonio Landtag Jörn Stein Monica Schulze-Lefert Paul Wiermer Marcel Brandt Wolfgang Lipka Volker et al. 2005 Nov. Science 310(5751):1180-3. 310 Somerville Shauna Pre- and postinvasion defenses both contribute to nonhost resistance in Arabidopsis.