Syringomycin is a necrosis-inducing phytotoxin produced by Pseudomonas syringae pv. syringae. To determine whether syringomycin production is a determinant in virulence or pathogenicity, we isolated nontoxigenic (Tox) Tn5-containing mutants and then quantitatively evaluated them for the ability to multiply and cause disease in immature sweet-cherry fruits. Transposon Tn5 was delivered to Tox strain B301D-R by using the suicide vector, pGS9, and the resultant kanamycin-resistant (Km) colonies were screened for changes in syringomycin production by testing for antibiosis against Geotrichum candidum. Southern blot analysis of KpnI-and EcoRI-digested DNA showed that 15 (0.3%) Tox mutants were isolated which had Tn5 inserted into 1 of 14 distinct loci. Phenotypic characterization of the Tox mutants identified three major groups, which were differentiated by pathogenicity and ability to cause a tobacco hypersensitive reaction (HR). The eight strains in group A were pathogenic (Path) in cherry fruit assays, but the disease index was 17 to 66% lower (significant at P = 0.01) than for the parental Tox strain, B301D-R. The population dynamics of group A strains W4S770 and W4S116 in cherry fruits were, however, indistinguishable from that of strain B301D-R. The remaining seven Tox strains were nonpathogenic; group B strain W4S2545 (Path HR) and group C strain W4S468 (Path HR) developed significantly lower populations (10 to 10 CFU per cherry fruit) 3 days after inoculation than strain B301D-R did (nearly 10 CFU per fruit). The data indicate that syringomycin is not essential for pathogenicity, but contributes significantly to virulence.
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Xu et al. (1988) studied this question.