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Molecular Plant Pathology

Effects of a Virulence Locus from Xanthomonas campestris 528T on Pathovar Status and Ability to Elicit Blight Symptoms on Crucifers. Jienan Chen,Plant Pathology Department, University of Florida, Gainesville 32611; P. D. Roberts, and D. W. Gabriel. Plant Pathology Department, University of Florida, Gainesville 32611. Phytopathology 84:1458-1465. Accepted for publication 29 August 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-1458.

All Xanthomonas campestris pv. campestris strains cause systemic black vein and rot of crucifers; and some strains, such as Xcc528T, additionally cause blight symptoms. By contrast, Xanthomonas campestris pv. armoraciae strain Xca417 causes a nonsystemic leaf spot of crucifers. Total DNA libraries of Xcc528 and Xca417 were constructed, and the Xcc528 library clones were individually transferred by conjugation into Xca417, and vice versa. No effects on pathogenicity of Xcc528 were observed by any of the clones from Xca417. However, a 5.4-kb DNA fragment from Xcc528 significantly altered the phenotype elicited by Xca417 when inoculated onto cabbage. Plasmid pJC41, carrying the 5.4-kb Xcc528 DNA fragment, conferred to Xca417 the ability to cause blight symptoms in cabbage. A vascular hypersensitive response, characteristic of wild type Xca417, was also observed on crucifers inoculated with Xca417/pJC41 transconjugants. This hypersensitive response appeared to be tissue specific and localized to the vascular region, but the response was insufficient to stop the spread of blight symptoms induced by these transconjugants. Southern hybridization analyses of 16 different Xanthomonas species and pathovars revealed that only X. c. campestris strains capable of causing blight symptoms hybridized with pJC41. The gene(s) conferring blight symptoms may therefore be unique to X. campestris strains capable of causing blight on crucifers. Since Xca417 causes only a leaf spot on crucifers, and since pJC41 conferred distinctive symptoms associated with X. c. campestris strains, the 5.4-kb fragment affected the apparent pathovar status of the of the recipient strain.

Additional keywords: systemic movement.