PHENOTYPIC AND GENOTYPIC CHARACTERIZATION OF INTERACTIONS BETWEEN ISOLATES OF PERONOSPORA-PARASITICA AND ACCESSIONS OF ARABIDOPSIS-THALIANA

The parasitic symbiosis between the wild crucifer Arabidopsis thaliana and the biotrophic oomycete Peronospora parasitica (downy mildew) provides an excellent opportunity to explore the evolution and molecular basis of plant defense. By describing the phenotypic and genotypic variation observed in this symbiosis, a foundation has been laid for investigating the process of genotype-specific recognition. Interaction phenotypes, incorporating aspects of both host response and parasite development, are described in this paper for combinations of 11 host accessions and seven parasite isolates. The timing and degree of asexual reproduction by the parasite varied among the combinations and provided an indirect assessment of colonization. Necrosis of host cells ranged from minute flecks visible macroscopically 7 days after inoculation (dai) to more extensive pits clearly visible 3 dai and often expanding until the entire cotyledon was necrotic 7 dai. Twelve host loci associated with different interaction phenotypes were postulated in part by. studying segregation among F-2 progeny from crosses of a half-diallel among nine A. thaliana accessions; advanced generations including recombinant inbred lines were used to confirm the identity of loci. A single cross between two host accessions can be used to characterize and map several loci associated with isolate-specific recognition of P. parasitica and distinct interaction phenotypes. Partial dominance and genetic epistasis are postulated as being common features of this parasitic symbiosis.