EFFECTS OF A VIRULENCE LOCUS FROM XANTHOMONAS-CAMPESTRIS 528(T) ON PATHOVAR STATUS AND ABILITY TO ELICIT BLIGHT SYMPTOMS ON CRUCIFERS

All Xanthomonas campestris pv. campestris strains cause systemic black vein and rot of crucifers; and some strains, such as Xcc528(T), addit ionally 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 Xc c528 library clones were individually transferred by conjugation into Xca417, and vice versa. No effects on pathogenicity of Xcc528 were obs erved by any of the clones from Xca417. However, a 5.4-kb DNA fragment from Xcc528 significantly altered the phenotype elicited by Xca417 wh en inoculated onto cabbage. Plasmid pJC41, carrying the 5.4kb Xcc528 D NA 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/pJ C41 transconjugants. This hypersensitive response appeared to be tissu e specific and localized to the vascular region, but the response was insufficient to stop the spread of blight symptoms induced by these tr ansconjugants. Southern hybridization analyses of 16 different Xanthom onas 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 s trains capable of causing blight on crucifers. Since Xca417 causes onl y a leaf spot on crucifers, and since pJC41 conferred distinctive symp toms associated with X. c. campestris strains, the 5.4-kb fragment aff ected the apparent pathovar status of the of the recipient strain.