A PATHOGENICITY LOCUS FROM XANTHOMONAS-CITRI ENABLES STRAINS FROM SEVERAL PATHOVARS OF XANTHOMONAS-CAMPESTRIS TO ELICIT CANKERLIKE LESIONS ON CITRUS

A virulence enhancement approach was used to clone a pathogenicity (pth) locus from a highly virulent pathogen by assaying the library in a second, less virulent strain that was compatible with the same host. A genomic library of the virulent Asiatic canker pathogen Xanthomonas citri was conjugally transferred to the opportunistic pathogen, X. campestris citrumelo, and the transconjugants were screened on Citrus paradisi 'Duncan' (grapefruit) leaves. Transconjugants able to induce host cell proliferation and raised, Asiatic cankerlike lesions were identified, and clone pSS10.35 was found to carry the gene(s) responsible. This clone was transferred to other Xanthomonas strains, including two that are weakly pathogenic to citrus in greenhouse tests (members of X. c. alfalfae and X. c. cyamopsidis) and two that are avirulent on citrus (X. phaseoli and X. c. malvacearum). Transconjugants of the two weakly pathogenic Xanthomonas strains induced cankerlike lesions when inoculated on citrus; these same strains became avirulent on their homologous host plants. Transconjugants of X. phaseoli and X. c. malvacearum strains remained unaltered in phenotype on citrus. A 3.7-kb region of pSS10.35 carrying the pthA locus was identified by subcloning and Tn5-gusA mutagenesis. Marker-exchange mutagenesis of X. citri using Tn5-gusA insertions in the 3.7-kb region resulted in a complete loss of virulence (disease symptoms and growth in planta) on citrus and loss of the hypersensitive response on heterologous hosts (i.e., an Hrp- phenotype). The Hrp- phenotype, but not growth in planta, of the marker-exchanged mutants was restored by subclones of pSS10.35 containing the 3.7-kb region.