White rust (Albugo candida) resistance loci on three Arabidopsis chromosomes are closely linked to downy mildew (Peronospora parasitica) resistance loci.

Summary Two accessions of Arabidopsis thaliana (Ksk-1 and Ksk-2) were used to identify and map three loci (RAC1, RAC2 and RAC3) of genes that confer Resistance to Albugo candida (white rust). The phenotypes associated with these genes were classified as either FN (necrotic flecks on upper surface of cotyledons and no blisters) for RAC2 and RAC3, or FYN (flecks surrounded by yellowing and no blisters) for RAC1. Both phenotypes exhibited rapid death of host cells penetrated by the parasite (hypersensitive response), with callose deposition commonly encasing the haustorium. F(6) recombinant inbred lines were produced specifically for the purpose of mapping each RAC locus relative to molecular markers. Dominant resistance at the locus RAC1 in Ksk-1 was previously mapped to chromosome 1 between RFLP markers m253 and m254, and co-segregating with a downy mildew resistance specificity RPP9 in the accession Wei-0. We report here a fine-scale map interval and co-segregating markers for this locus, which in turn enabled mapping of a previously unnoticed source of resistance in Ksk-1 designated RAC3 that exhibits an FN phenotype hyperstatic to the FYN phenotype of RAC1. RAC3 is closely linked to the RPP8/HRT on chromosome 5, a locus which contains specificities for resistance to downy mildew and turnip crinkle virus. Recombinant inbreds also enabled mapping of recessive resistance at RAC2 in Ksk-2 to the bottom arm of chromosome 3, in the 6 cM interval between two downy mildew resistance loci (RPP1 and RPP13).