Avirulence genes in plant - pathogenic bacteria : signals or weapons ? Alan Vivian and

Phytopathogenic bacteria generally have limited host ranges, often confined to members of a single plant species or genus. This appears to result from negative factors restricting the host range ra.ther than from positive factors which allow the pathogen to infect its hosts. These negative factors are avirulence genes present in the pathogen, which interact with matching resistance genes in the host. When a pathogen carrying an avirulence gene attacks a plant with the corresponding resistance gene, a cascade of responses (the hypersensitive response, HR ; Klement, 1982) is triggered in the plant and this results in localized host-cell death, preventing spread of the pathogen and the onset o f disease (the resistant response, described as an ‘incompatible’ interaction). In the absence of either one or both of the matching gene pair, the plant fails to recognize the bacterium as a pathogen, the H R is not triggered and disease will usually follow (a susceptible response, resulting from a ‘ compatible:’ interaction).