PLASMID DNA FINGERPRINTS DISTINGUISH PATHOTYPES OF XANTHOMONAS-CAMPESTRIS PV CITRI, THE CAUSAL AGENT OF CITRUS BACTERIAL CANKER DISEASE

Plasmid DNA was isolated from 54 strains of Xanthomonas campestris pv. citri, associated with different forms of citrus bacterial canker disease (CBCD). The number of plasmids per strain varied from one to five. A total of 24 plasmid bands with sizes from 7 to 100 kilobases (kb) were identified. Strains that had identical plasmid profiles were generally associated with the same form of CBCD. After digesting the plasmid DNA with each of three restriction endonucleases, 87 fragments with different sizes from about 1 to 30 kb were visualized. Strains belonging to a specific CBCD group shared plasmid DNA fragments of similar sizes. Dendrograms derived from plasmid DNA fingerprint analyses allowed us to clearly distinguish A, B, and C pathotypes of X. c. citri. The strain Xc90, associated with bacteriosis of Mexican lime in Mexico (CBCD-D) was not clearly distinguishable from strains associated with cancrosis B (CBCD-B) from Argentina and Uruguay. Plasmid DNA fragments specifically associated with some groups of strains were identified. A BamHI fragment from a CBCD-A strain was used as a hybridization probe. A strong signal was recorded in all CBCD-A strains studied. Weaker hybridization signals were observed with one or two high molecular weight bands in all CBCD-B strains studied. All three type C strains had a band of slightly smaller size than the probe, but which hybridized only very weakly. Strain Xc70 also had a homologous larger band similar in size to one found in the CBCD-B strains. Hierarchical cluster analysis of the RFLP data from the plasmid DNA revealed phenetic clusters strikingly similar to those obtained previously from analysis of genomic DNA, lending support to the concept of balanced co-evolution of plasmid and chromosomal genomes.