4 ARABIDOPSIS RPP LOCI CONTROLLING RESISTANCE TO THE NOCO2 ISOLATE OFPERONOSPORA-PARASITICA MAP TO REGIONS KNOWN TO CONTAIN OTHER RPP RECOGNITION SPECIFICITIES

Interactions between Arabidopsis thaliana and the downy mildew fungus Peronospora parasitica provide a model system to study the genetic and molecular basis of plant-pathogen recognition. With the use of the No co2 isolate of P. parasitica, the reaction phenotypes of 46 accessions of Arabidopsis were examined and 31 accessions exhibited resistance. Resistance phenotypes examined ranged from distinct necrotic pits or f lecks to a weak necrosis accompanied by late and sparse fungal sporula tion. Segregating populations generated from crosses between the susce ptible accession Col-0 and the resistant accessions Ws-0, Pr-0, Oy-0, Po-1, Bch-1, Ge-1, Di-1, Ji-1, and Te-0 were also screened with Noco2. The genetic data were consistent with the presence of single resistan ce (RPP) loci in all of these accessions except Oy-0, in which resista nce was inherited as a digenic trait. As a first step to molecular clo ning, the map positions of four resistance loci were determined. These have been designated RPP14.1 from Ws-0, RPP14.2 from Pr-0, and RPP14. 3 and RPP5.2 from Oy-0. RPP14.1 was mapped to a 3.2-cM interval on chr omosome 3 that is linked to a region between the markers Gl-1 and m249 known to contain other P. parasitica resistance specificities. RPP14. 2 from Pr-0 and RPP14.3 from Oy-0 were also positioned in this interva l. Moreover, RPP14.1 and RPP14.2 showed linkage of <0.05 cM, suggestin g possible allelism. The second RPP locus from Oy-0, RPP5.2 was locate d on chromosome 4 and exhibited strong linkage (<2 cM) to RRP5.1, a lo cus previously identified in the Arabidopsis accession Landsberg-erect a. The results reinforce evidence for RPP gene clustering in the Arabi dopsis genome and provide new targets for cloning and examination of R PP gene structure, function, allelic variation, and organization withi n defined loci.