Identification of a Gene in Arabidopsis thaliana Controlling Resistance to Clubroot (Plasmodiophora brassicae) and Characterization of the Resistance Response. Harald Fuchs. Freie Universitat Berlin, Institut fur Angewandte Genetik, Albrecht-Thaer-Weg 6, D-14195 Berlin, Germany. Maria Dolores Sacristan Freie Universitat Berlin, Institut fur Angewandte Genetik, Albrecht-Thaer-Weg 6, D-14195 Berlin, Germany. MPMI 9:091-097. Accepted 10 October 1995, Copyright 1996 The American Phytopathological Society.

Plasmodiophora brassicae, the causal agent of clubroot, is known to be pathogenic on Arabidopsis thaliana. We identified for the first time pathotype-specific resistance to clubroot in Arabidopsis. Ecotypes Tsu-0 and Ze-0 showed incompatible interactions to one isolate (isolate e) of the pathogen, but not three others tested. The resistance reaction is characterized by the lack of typical clubroot swellings after infection. Microscopical investigations revealed that resistance is accompanied by a hypersensitive reaction (HR). Cell wall alterations associated with the HR were detectable as autofluorescent substances. Positive phloroglucinoI-HCl staining suggests a nullification of cell walls involved in defense reaction. By genetic analysis, we found that resistance to P. brassicae isolate e is conferred by a dominant allele of a single nuclear gene that we designated RPBI. With the aid of morphological markers this locus was mapped near the marker dis2 to chromosome 1.