The RPP5 gene confers resistance to the fungus Peronospora parasitica in Arabidopsis and has recently been isolated. However, the biochemical role of RPP5 in disease resistance remains unknown. The homology between RPP5 and other resistance genes indicate that they function through similar signal perception and transduction pathways. Therefore, the RPP5 gene provides an opportunity to analyse genetically in Arabidopsis the genes required for the function of a very general class of resistance genes. Characterization of the mode of action of RPP5 will be carried out by complementary and parallel approaches: (1) Chemical and transposon mutational analysis of Arabidopsis genes required for the function of RPP5, (2) Interaction trap cloning of genes encoding proteins that bind to RPP5 from Arabidopsis cDNA expression libraries. Cloning these genes is a first step in elucidating the role of RPP5 in host defence.