Pathogen-associated molecular pattern-triggered immunity: veni, vidi...?

For many years, plant pathology was divided into two schools of thought. It was clear that purified molecules or crude extracts from microbes or plants (referred to as general elicitors) could induce activation of general defense responses (Boller, 1995). Geneticists instead were studying plant resistance triggered by the recognition of a given pathogen Avirulence gene product by the corresponding plant Resistance (R) gene product (Dangl and Jones, 2001). This resistance followed the gene-for-gene hypothesis, was often associated with a hypersensitive response, and was widely used in breeding programs. It was however confusing why a pathogen would produce Avirulence products that would cause its recognition and subsequent host resistance.