A complex relationship between hypersensitive response and pathogenicity genes, avirulence genes, and plant resistance genes gives rise to the virulence of bacterial pathogens. Generally, a single avirulence gene (in the bacterium) corresponds to a single resistance gene (in the plant host), giving rise to the concept of a gene-for-gene response. The avirulence gene leads to the pathogen being avirulent, or unable to induce disease on a specific variety of plant hosts, those being the ones carrying the corresponding resistance genes. Soybean cultivars that have been bread to contain resistance genes Rpg1, Rpg2, Rpg3, and Rpg4 have been shown to be resistant to Pseudomonas syringae pv. glycinea race 4. The corresponding avirulence genes in the bacterium are avrB, avrA, avrC, and avrD. A different race of Pseudomonas syringae pv. glycinea may still be able to elicit disease in these soybean cultivars as it may carry different avirulence genes. Researchers have shown that the exchange of avirulence genes can switch a race from being virulent to avirulent and vice versa. Thus, evolution, mutation, and cases of horizontal gene transfer can make it difficult to breed longterm resistance into soybean cultivars.
