Animals express an innate immune system against pathogens through receptor-mediated recognition of conserved microbial structures called pathogen-associated molecular patterns (PAMPs). In plants; resistance to invading microorganisms is often governed by specific recognition between plant and pathogen proteins. Perception of more broadly conserved 'general' pathogen elicitors constitutes another layer of plant resistance and prompts questions of where, mechanistically and evolutionarily, this mode of non-self discrimination fits within known systems of microbial surveillance in animals and plants.