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 parasitica causes downy mildew in the Arabidopsis thaliana ecotypes Weiningen (Wei-0) and La-er, whereas ecotypes RLD and Col-0 are resistant. Genetic crosses between resistant RLD and susceptible Wei-0 showed that resistance was inherited in a simple Mendelian fashion as a monogenic dominant trait. The interactions between different isolates of P. parasitica and ecotypes of A. thaliana show race-specific variation and fit a gene-for-gene relationship. The RPP11 resistance gene was mapped by following the co-segregation of the resistance phenotype with RFLP markers in a mapping population of 254 F₃ families derived from RLD x Wei-0 F₂ individuals. Linkage analysis using version 1.9 of the MAPMAKER program placed the RPP11 resistance locus on chromosome III between marker m249 (two recombinants) and marker g2534 (six recombinants). Markers g2534 and g4117 are on YAC EG7H1. Marker g4117 and one end probe (N5) generated from YAC EG7H1 showed no recombinants. The YAC end probe N5, which was generated by plasmid rescue, was used to screen clones in the Eric Ward YAC library and a YAC was fished (EW19B12) which also hybridised with m249. Thus, a YAC contig has been established over the region where the resistance locus maps. Because the YACs were made 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 -DASH genomic library was prepared from RLD and a contig covering the relevant region of the YACs is currently under construction.