Abstract
This chapter discusses an overview of avirulence genes. Avirulence ( avr ) genes are defined as genes of the pathogen that govern its specific recognition by particular plant genotypes. Recognition depends upon the presence of a pair of matching genes, an avr gene in the pathogen and a resistance (R) gene in the plant. The specific recognition of the pathogen results in the induction of the plant's defense response, often a hypersensitive reaction (HR). Further, the chapter discusses identification of bacterial avirulence genes. The prerequisite for identifying and genetically analyzing avr/R gene pairs is the availability of at least two genetically different bacterial strains and plant lines. In Gram-negative plant-pathogenic bacteria, avr genes are identified only in pathovars of Pseudomonas syringae and Xanthornonas spp. In addition, isolation of avirulence genes is discussed.
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