MAP POSITIONS OF 3 LOCI IN ARABIDOPSIS-THALIANA ASSOCIATED WITH ISOLATE-SPECIFIC RECOGNITION OF PERONOSPORA-PARASITICA (DOWNY MILDEW)

Our research is aimed at understanding the molecular basis for gene-fo r-gene interactions between plant parasites and their hosts. As a prel ude to cloning, the positions in the Arabidopsis thaliana genome were investigated for four of the 10 loci (RPP1, RPP2, RPP4, and RPP7) that have been identified as associated with the genotype-specific recogni tion of the biotrophic oomycete Peronospora parasitica (downy mildew). A single cross between accessions Col-5 and Nd-1 was primarily used t o map their chromosomal locations. RPP1 from Nd-1 was characterized by the absence of asexual sporulation and the occurrence of necrotic pit s, visible macroscopically on cotyledons 3 days after inoculation with isolates Emoy2 or Hiks1; this locus was mapped to chromosome 3 in the interval between gl1 and m249 (6.6 cM above m249 and 3.9 cM below a R APD marker OPC12(1250)). RPP2 and RPP7 from accession Col-5(gl(1)) wer e characterized by the absence of asexual sporulation and the occurren ce of necrotic flecks visible 7 days after inoculation,vith isolates C ala2 and Hiks1, respectively. RPP2 was located between ag and B9 on ch romosome 4. RPP7 maps within 13 cM of m422, but linkage with other mar kers on chromosome 5 was not confirmed. RPP4 from accession Col-5 was characterized by the occurrence of necrotic flecks and delayed, light sporulation 7 days after inoculation with isolates Emoy2 and Emwal; th is locus also maps to chromosome 4, 14.8 cM above RPP2.