A downy mildew effector evades recognition by polymorphism of expression and subcellular localization.

Pathogen co-evolution with plants involves selection for evasion of host surveillance systems. The oomycete Hyaloperonospora arabidopsidis (Hpa) causes downy mildew on Arabidopsis, and race-specific interactions between Arabidopsis accessions and Hpa isolates fit the gene-for-gene model in which host resistance or susceptibility are determined by matching pairs of plant Resistance (R) genes and pathogen Avirulence (AVR) genes. Arabidopsis Col-0 carries R gene RPP4 that confers resistance to Hpa isolates Emoy2 and Emwa1, but its cognate recognized effector(s) were unknown. We report here the identification of the Emoy2 AVR effector gene recognized by RPP4 and show resistance-breaking isolates of Hpa on RPP4-containing Arabidopsis carry the alleles that either are not expressed, or show cytoplasmic instead of nuclear subcellular localization.