Avirulence

Avirulence refers to the property of certain pathogen genes, called avirulence (Avr) genes, whose products are detected by the host plant's corresponding resistance (R) genes. The interaction between an Avr gene in a pathogen and its plant host R gene follows the gene-for-gene model proposed by Flor in the 1940s, wherein recognition triggers a strong defense, often including a localized hypersensitive response, and blocks pathogen replication at the site of infection. In this framework, a pathogen strain carrying a functional Avr allele is avirulent on a plant line that has the matching R gene, whereas strains lacking the Avr allele or harboring altered Avr alleles can evade recognition and exhibit virulence on that host.

Avr genes generally encode secreted effector proteins that manipulate host processes. Plant recognition can be direct, through physical interaction between R proteins and Avr effectors, or indirect, via the guard or decoy models, in which R proteins monitor host targets affected by Avr effectors and trigger immunity upon disruption. The broader plant immune response triggered by Avr-R interactions is often called effector-triggered immunity (ETI).

Evolutionary dynamics of avirulence involve mutations, gene loss, or silencing of Avr alleles, driven by selection pressure from resistant hosts. This can lead to durable virulence on resistant varieties unless multiple R genes or diverse resistance strategies are deployed. In plant breeding, Avr genes help identify R genes and guide the deployment of resistance. Caution: the term avirulence is host-genotype specific; a pathogen may be avirulent on one cultivar but virulent on another.