Avirulence Genes

Abstract In gene‐for‐gene systems, resistance of a plant to a pathogen is due to the ‘recognition’ by plant surveillance system (i.e. plant resistance gene product) of a pathogen avirulence determinant encoded by an avirulence gene to eventually result in triggering of plant immunity. Avirulence proteins (or products) actually are effectors involved in pathogenicity. In viruses, virtually all the proteins can behave as avirulence determinants. In all other cases, avirulence genes are extremely diverse, being often species or isolate/strain‐specific and rarely have matches in sequence databases. They often are specifically expressed or strongly over‐expressed during plant–pathogen interaction and the encoded proteins show the presence of secretion signals and translocation signals (e.g. the Type III secretion signal for bacteria). Avirulence genes seem to be submitted to high‐speed diversifying selection allowing the pathogen to diversify its effector repertoire and rapidly escape recognition by the plant resistance gene. Key concepts Effectors of plant pathogens are pathogen molecules that manipulate host cell structure and function thereby facilitating infection. Effectors often contribute quantitatively to pathogen aggressiveness and are dispensable for the pathogen life cycle. In the course of plant–pathogen co‐evolution, plants have evolved receptors that detect pathogen effectors (or the effect of the effectors on a plant target) and activate defence responses. The effectors specifically recognized by ‘matching’ resistance proteins (termed R proteins) are termed avirulence (AVR) proteins . This forms the basis of the gene‐for‐gene concept, genetically demonstrated by Flor between 1942 and 1955 and stating that plants producing a specific R gene product are resistant towards a pathogen that produces the corresponding Avr gene product whereas whenever one of the genes or both are lacking the plant is susceptible and disease ensues. In wild plant and pathogen populations, avirulence and resistance are discontinuous traits, only present in part of the population. Thus resistance will be applied against only part of the pathogen population (harbouring the corresponding Avr allele) and pathogen will only be avirulent towards part of the plant population (harbouring the matching resistance gene). All plant pathogens from viruses to nematodes, but also some pests like aphids produce avirulence proteins. When submitted to resistant populations of plants, pathogens can alter or delete their avirulence proteins to avoid defence elicitation, at risk of a fitness cost associated with loss‐of‐function of those effectors.