Cloned avirulence genes from the tomato pathogen Pseudomonas syringae pv. tomato confer cultivar specificity on...

Three different cosmid clones were isolated from a genomic library of the tomato pathogen Pseudomonas syringae pv. tomato, which, when introduced into the soybean pathogen P. syringae pv. glycinea, caused a defensive hypersensitive response (HR) in certain soybean cultivars. Each clone was distinguished by the specific cultivars that reacted hypersensitively and by the intensity of the HR elicited. Unlike wild-type P. syringae pv. tomato isolates, which elicit the HR on all soybean cultivars, all three clones exhibited cultivar specificities analogous to avirulence genes previously cloned from P. syringae pv. glycinea. However, the collective phenotypes of the three clones accounted for HRs on all tested soybean cultivars. One of the three P. syringae pv. tomato clones contained an avirulence gene homologous to avrA, which was previously cloned from P. syringae pv. glycinea race 6. The other two P. syringae pv. tomato clones expressed unique HR patterns on various soybean cultivars, which were unlike those caused by any known P. syringae pv. glycinea race or previously cloned P. syringae pv. glycinea avr gene. Further characterization of the second P. syringae pv. tomato clone indicated that the avirulence phenotype resided on a 5.6-kilobase HindIII fragment that, in Southern blot analyses, hybridized to an identical-size fragment in various P. syringae pathovars, including all tested glycinea races. These results demonstrate that avirulence genes may be distributed among several P. syringae pathovars but may be modified so that the HR is not elicited in a particular host plant. Furthermore, the data raise the possibility that avirulence genes may function in host-range determination at levels above race-cultivar specificity.