Many proteins from plant pathogens affecting the interaction with the host plant have dual functions: they promote virulence on the host species and they function as avirulence determinants by eliciting defense reactions in host cultivars expressing the appropriate resistance genes. In viruses all proteins encoded by the small genomes can be expected to be essential for viral development in the host. However, in different plants surveillance systems have evolved that are able to recognize most of these proteins. Bacteria and fungi have specialized pathogenicity and virulence genes. Many of the latter were originally identified through the resistance gene-dependent elicitor activity of their products. Their role in virulence only became apparent when they were inactivated or transferred to different microbes or after their ectopic expression in host plants. Many microbes appear to maintain these genes despite their disadvantageous effect, introducing only few mutations to abolish the interaction of their products with the plant recognition system. This has been interpreted as been indicative of a virulence function of the gene products that is not impaired by the mutations. Alternatively, in particular in bacteria there is now evidence that pathogenicity was acquired through horizontal gene transfer. Genes supporting virulence in the donor organism's original host appear to have traveled along. Being gratuitous in the new situation, they may have been inactivated without loss of any beneficial function for the pathogen.

The secreted effector protein NIP1 from the barley pathogen Rhynchosporium secalis is a specific elicitor of defense reactions in host plants carrying the resistance gene Rrs1. In addition, it has activities associated with fungal virulence; independent of the plant genotype it stimulates the plant plasma membrane H+-ATPase and induces leaf necrosis. Four NIP1 isoforms differing in single amino acid residues were isolated from various naturally occurring fungal strains. All three activities of the protein (race specificity, H+-ATPase stimulation, necrosis induction) were affected by the amino acid alterations in a similar way suggesting that they are mediated through a single plant receptor.

Being surrounded by putatively hostile microorganisms, but immobile and hence unable to escape, plants constantly need to be prepared for defensive battle. During their coevolution with heterotrophic parasites they have therefore acquired efficient passive, preformed barriers that provide protection against the majority of aggressors. In addition, however, plants have an arsenal of offensive weapons for counterattack at their disposal once the passive bulwark has failed. Usually, this weaponry is launched rapidly and decisively, thus negating any further progression of the invader in order to maintain the plant’s structural and functional integrity.