Avirulence (Avr) genes interact in a gene-for-gene relation with resistance genes. Viral Avr genes encode for coat proteins, replicase or movement proteins. (Hammond-Kosack KE, Jones JD. PLANT DISEASE RESISTANCE GENES. Annu Rev Plant Physiol Plant Mol Biol. 1997 Jun;48:575-607 PMID:15012275).
The term "avirulence genes" has been coined by H.H. Flor, as a result of the observation of the response of R-genes to direct o indirect products of pathogen genes, so named "avirulence genes". Avr genes act in a dominant fashion and the response of the host occurs through the interaction with R-genes. Avr genes can codify for a ligand recognised by a plant cell receptor which, in turn, can induce the defense response.
In this model, virulence is recessive and might arise from a loss of function mutation in the Avr gene or from a gain of function mutation in plant genome. Most plant genomes contain different resistance genes, which confer them resistance to the attack of variable pathogens. The evolution of R-genes is presmuably due to plant genome rearrangements. (Richter TE, Ronald PC. The evolution of disease resistance genes.Plant Mol Biol. 2000 Jan;42(1):195-204. PMID: 10688137).
Since the discovery of Avr genes, molecular genetics reseaches have provided data about Avr genes that promote virulence in specific hosts not expressing the correspondeing R-gene (Figure, a). Evolutionary developments can lead to the production of a resistance protein that recognizes directly or indirectly the Avr gene product and promotes a defense response (Figure, b and c). (McDowell JM, Woffenden BJ.Plant disease resistance genes: recent insights and potential applications.Trends Biotechnol. 2003 Apr;21(4):178-83. PMID:12679066).