MOLECULAR MAPPING OF THE ARABIDOPSIS LOCUS RPP11 WHICH CONDITIONS ISOLATE-SPECIFIC HYPERSENSITIVE RESISTANCE AGAINST DOWNY MILDEW IN ECOTYPE RLD

Isolate WELA of the plant pathogenic oomycete fungus Peronospora paras itica causes downy mildew in the Arabidopsis thaliana ecotypes Weining en (Wei-O) and La-er, whereas ecotypes RLD and Col-O are resistant. Ge netic crosses between resistant RLD and susceptible Wei-O showed that resistance was inherited in a simple Mendelian fashion as a monogenic dominant trait. The interactions between different isolates of P. para sitica and ecotypes of A. thaliana show race-specific variation and fi t a gene-for-gene relationship. The RPP11 resistance gene was mapped b y following the co-segregation of the resistance phenotype with RFLP m arkers in a mapping population of 254 F-3 families derived from RLD x Wei-O F-2 individuals. Linkage analysis using version 1.9 of the MAPMA KER program placed the RPP11 resistance locus on chromosome III betwee n marker m249 (two recombinants) and marker g2534 (six recombinants). Markers g2534 and g4117 are on YAC EG7H1. Marker g4117 and one end pro be (N5) generated from YAC EG7H1 showed no recombinants. The YAC end p robe N5, which was generated by plasmid rescue, was used to screen clo nes in the Eric Ward YAC library and a YAC was fished (EW19B12) which also hybridised with m249. Thus, a YAC contig has been established ove r the region where the resistance locus maps. Because the YACs were ma de with ecotype Columbia DNA it is necessary to isolate the equivalent region from RLD in order to clone the resistance locus. To this end a phage lambda-DASH(TM) genomic library was prepared from RLD and a con tig covering the relevant region of the YACs is currently under constr uction.