1.1.16
EVOLUTION OF PLANT DISEASE RESISTANCE LOCI IN ARABIDOPSIS

E SINAPIDOU, K WILLIAMS, N GUNN, E HOLUB and J BEYNON

HRI-Wellesbourne, Wellesbourne, Warwick, CV35 9EF, UK

Background and objectives
We have been studying the interaction between the downy mildew parasite Peronospora parasitica and Arabidopsis thaliana. We have mapped several disease-resistance genes within the Arabidopsis genome that interact with a range of Peronospora isolates. These resistance genes are distributed among all five chromosomes of Arabidopsis. One of these genes, RPP2, identified in Arabidopsis accession Columbia, maps to chromosome 4 and interacts with Peronospora isolate Cala2. The host in this case produces a localized fleck at the site of infection and the parasite is prevented from sporulating. A candidate gene for RPP2 has been identified by map-based cloning. This gene is present as a single copy in Col-5 and, when used as a probe, failed to hybridize to DNA from the susceptible accession, Nd-1. The objective of this study was to determine the distribution of functional RPP2 alleles (based on the Col-5 sequence) amongst a wide range of wild Arabidopsis accessions and to make comparisons with alleles at the same locus from Cala2-compatible accessions.

Results and conclusions
DNA from more than 80 different accessions of Arabidopsis from the UK was probed with the Col-5 gene. Four classes were revealed: (i) resistant accessions that showed hybridization; (ii) resistant accessions that did not show hybridization; (iii) susceptible accessions that did not show hybridization; (iv) susceptible accessions that showed hybridization. There was not a clear relationship between resistance to Cala2 and the presence of the Col-5 hybridizing band. Crosses are being carried out to confirm the location of the functional resistance genes at the RPP2 locus. A variety of alleles, from both resistant and susceptible accessions, are being sequenced to analyse variation at this locus. A similar analysis of a more complex region, the RPP1 locus, which contains a multi-gene family in accession Nd-1, is also being carried out. Comparisons between these loci will be presented.

References
1. Tör M, Holub EB, Brose E et al., 1994. Molecular Plant-Microbe Interactions 7, 214-222.