Restriction fragment length polymorphism evidence for genetic homology within a pathovar of Pseudomonas syringae

Pseudomonas syringae pv. phaseolicola NPS3121 hrp sequences were used as hybridization probes in a restriction fragment length polymorphism (RFLP) analysis of 24 P. syringae pv. tabaci strains as a means to evaluate the genetic and taxonomic relationship of pathovars of P. syringae. Southern blot analyses of genomic restriction digests, with hrpA-S sequences as hybridization probes, and restriction analyses of PCR-amplified DNA of regions within hrpD were conducted. The resulting RFLP patterns were uniform for 23 of the 24 isolates tested, with strain BR2R having a unique pattern. BR2R is a pathogen of bean which was classified as pathovar tabaci because of its ability to produce tabtoxin, but unlike the other 23 tabaci strains in this study, it does not incite disease symptoms on tobacco. When a DNA fragment containing hrpM sequences was used as a hybridization probe, the tabaci isolates could be divided into three groups on the basis of the RFLP patterns : BR2R, Pt11528R and Pt113R, and the remaining strains. For all of the above analyses, BR2R shared identical RFLP patterns with P. syringae pv. phaseolicola NPS3121, also a bean pathogen which does not cause disease on tobacco. However, BR2R AND NPS3121 could be differentiated from each other on the basis of the RFLP patterns from restriction analysis of PCR-amplified DNA of argF, while the remaining tabaci strains had a third pattern. These studies indicate that hrp genes and argF are conserved in strains of P. syringae pathogenic to tobacco, suggesting that P. syringae strains pathogenic to specific hosts may have a high level of genetic similarity. We believe that these analyses have shown that distinct identifiable genetic differences may be correlated with host range and suggest that such information may be useful for assigning pathovar designations.