Abstract
A representative strain of Pseudomonas syringae pv. eriobotryae, the causal agent of stem canker of loquat, was studied to determine whether plasmids are involved in its virulence. The strain NAE6 harbored three plasmids of approximately 25, 52, and 60 megadaltons (Mdal). The derivative strains which were cured of the 52 Mdal plasmid by culturing at a maximum growth temperature of 32°C were uniformly avirulent. The 52 Mdal plasmid DNA, which was isolated from agarose gels using agarase, was digested with BamHI, ligated into the BamHI cloning site of the broad host range cosmid pLAFR3, packaged into λ phage particles, and transduced into Escherichia coli HB101. By use of helper plasmid pRK2013, twenty-five recombinant plasmids were mobilized into an avirulent recipient P. s. pv. eriobotryae strain, which was cured of three plasmids. The transconjugants which received the recombinant plasmid pVIR6 containing ca. 23kb insert DNA regained virulence. Southern hybridization analysis indicated that the 23kb insert DNA originated from the 52 Mdal plasmid. These data demonstrate that the 52 Mdal plasmid of P. s. pv. eriobotryae is associated with virulence.
