PHENOTYPIC CHARACTERIZATION AND MOLECULAR MAPPING OF THE ARABIDOPSIS-THALIANA LOCUS RPP5, DETERMINING DISEASE RESISTANCE TO PERONOSPORA-PARASITICA

Peronospora parasitica causes downy mildew on crucifers. An isolate of P. parasitica (denoted NoCO2) was identified that infected Arabidopsis plants of the land race Columbia (Col-0) but not plants of land race Landsberg erecta (La-er). Segregation analysis of F2 plants derived from a La-er x Col-0 cross established that the resistance was inherited as a single locus, denoted RPP5. Macroscopic and microscopic examinations of inoculated La-er and Col-0 cotyledons showed that restriction of fungal growth in La-er was accompanied by massive callose accumulation and death of plant cells in direct contact with points of attempted fungal penetration. La-er x Col-0 F1 plants exhibited an intermediate resistance response in all aspects of fungal development, indicating that RPP5 is semi-dominant in its action. F8 recombinant inbred lines generated between La-er and Col-0 were used to map RPP5 to a narrow interval (<l.l cM) on chromosome 4, utilizing existing restriction fragment length polymorphic (RFLP) markers and newly generated random amplified polymorphic DNA (RAPD) markers. The data provide a basis for the isolation of the RPP5 locus by positional cloning as a first step towards understanding recognitional specificity in plant-pathogen interactions at a molecular level.