A plant pan-genome immunity landscape Plant pathogens elicit an immune response through effector proteins. In turn, plant genomes encode genes that determine species-specific recognition of these effectors by a process known collectively as effector-triggered immunity (ETI). By examining a range of strains of the pathogen Pseudomonas syringae that infect the model plant Arabidopsis thaliana , Laflamme et al. generated a P. syringae Type III Effector Compendium (PsyTEC) and in turn identified the genes responsible for ETI in Arabidopsis. This pan-genome analysis revealed that relatively few A. thaliana genes are responsible for recognizing the majority of P. syringae effectors. These results provide insight into why most pathogenic microbes only infect specific plant species. Science , this issue p. 763