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 Arabidopsi s plants of the land race Columbia (Col-0) but not plants of land race Landsberg erecta (La-er). Segregation analysis of F2 plants derived f rom a La-er x Col-0 cross established that the resistance was inherite d as a single locus, denoted RPP5. Macroscopic and microscopic examina tions of inoculated La-er and Col-0 cotyledons showed that restriction of fungal growth in La-er was accompanied by massive callose accumula tion and death of plant cells in direct contact with points of attempt ed fungal penetration. La-er x Col-0 F1 plants exhibited an intermedia te resistance response in all aspects of fungal development, indicatin g that RPP5 is semi-dominant in its action. F8 recombinant inbred line s 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 fra gment length polymorphic (RFLP) markers and newly generated random amp lified polymorphic DNA (RAPD) markers. The data provide a basis for th e isolation of the RPP5 locus by positional cloning as a first step to wards understanding recognitional specificity in plant-pathogen intera ctions at a molecular level.