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 F 2 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 F 1 plants exhibited an intermediate resistance response in all aspects of fungal development, indicating that RPP5 is semi‐dominant in its action. F 8 recombinant inbred lines generated between La‐ er and Col‐0 were used to map RPP5 to a narrow interval (<1.1 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.
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Parker et al. (1993) studied this question.