Characterization, Phylogeny, and Genome Analyses of Nonpathogenic Strains Isolated from Brassica Seeds.

Xanthomonads were detected by using the Xan-D(CCF) medium from the brassica seeds, and their pathogenicity was determined by plant inoculation tests. It was found that some seed lots were infested with pv. , some with pv. , and some with nonpathogenic xanthomonads. The nonpathogenic xanthomonad strains were identified as , and the multilocus sequence analysis showed that the nonpathogenic strains were grouped together with pathogenic , but not with nonpathogenic strains of In addition, all isolated pv. and pv. strains were positive in the -PCR, but the nonpathogenic strains were negative. It was further found that nonpathogenic strain nE1 does not contain the entire pathogenicity island ( gene cluster; type III secretion system) and all type III effector protein genes based on the whole genome sequence analyses. The nonpathogenic strain nE1 could acquire the entire pathogenicity island from the endemic pv. and pv. strains by conjugation, but type III effector genes were not cotransferred. The studies showed that the nonpathogenic strains indeed exist on the brassica seeds, but it could be differentiated by the PCR assays on the and type III effector genes. Nevertheless, the nonpathogenic strains cannot be ignored because they may be potential gene resources to increase genetic diversity in the endemic pathogenic pv. and pv. strains.