AVRA AND AVRE IN PSEUDOMONAS-SYRINGAE PV TOMATO PT23 PLAY A ROLE IN VIRULENCE ON TOMATO PLANTS

Four avirulence genes (avrA, avrD, avrE, and avrPto) from Pseudomonas syringae pv. tomato strain PT23 function in P. s. pv. glycinea to elicit the hypersensitive response in soybean leaves. To determine whether these genes are important for virulence on the normal host, tomato, or for resistance in nonhost plants, such as soybean, PT23 marker exchange mutants were constructed. All of the mutant strains grew normally in culture media and utilized several carbon sources, as did the wild-type bacterium. Mutations in avrA, avrD, avrE, and avrPto did not affect the ability of PT23 to cause hypersensitive reactions on leaves of tobacco and several soybean cultivars. However, mutation of avrE greatly reduced virulence and bacterial multiplication in tomato leaves. Mutation of avrA also resulted in a small but reproducible reduction in virulence on tomato leaves. These results indicate that the studied PT23 avirulence genes are not involved in nonhost resistance in tobacco and soybean but that avrA and avrE are important for high virulence on the normal host plant, tomato.