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Some Pathovars of Pseudomonas syringae Producing Syringomycin-Like Toxin

The majority of virulent strains of Pseudomonas syringae pv. syringae, the wide-host-range plant pathogenic bacterium, produces one of phytotoxins termed syringomycin1,7), syringotoxin4) or syringostatin5) depending on their original host, and these toxins have been reported to have a role in symptom development of the diseases caused by various ecotypes of this bacterium6,9). The toxins show not only necrosis-inducing activity to host plants, but also antimicrobial activity especially against Geotrichum candidum. Therefore the assay of growth inhibitory activity against this fungus has been used for the detection of syringomycin productivity. Recently we found that the causal bacterium of flower rot of kiwifruit also shows the growth inhibitory activity to the fungus8). This bacterium has been classified as P. syringae group, and has been thought to be distinguishable from P. syringae pv. syringae on the basis of the host range and some bacteriological properties. This finding suggested that any bacterial strains other than P. syringae pv. syringae may also produce syringomycin or its related toxins. This paper reports that some pathovars of P. syringae other than P. syringae pv. syringae also produce syringomycin-like toxin as one of virulence factors. To test whether other bacterial pathovars excepting P. syringae pv. syringae also produce syringomycin-like toxin, growth inhibitory activity against G. candidum was examined using a lot of strains of plant pathogenic bacteria. Bacterial cells grown on King's B medium for 24hr were spotted onto CPDA medium plate (potato decoction 1 liter, casamino acid (Difco) 4g, glucose 15g, agar 15g, pH 6.8), incubated for 24hr at 22•Ž, and the plate was sprayed with spore suspension of G. candidum (ca. 106 spore/ml). Then the plate was incubated for 24hr at 28•Ž, and growth inhibition zone formed around the bacterial colony was measured. Among total 70 strains belonging to 19 species of 5 genera, the growth inhibitory activity was observed in the following strains: P. syringae pv. aceris Kaede 1-1, pv. aptata ISB 8661, MAFF301008, pv. japonica BPST802, MAFF301072, pv. pisi Pea8506 (these are the pathogens of maple tree, beet, wheat and pea, respectively), P. syringae isolates from kiwifruit flower rot (TMR275280, Kw710) and from onion leaf spot disease (TMR231) (these pathovar names have not been determined) and P. viridiflava TVP1, LTPV2, MAFF301343 (a pathogen of chinese cabbage) (Table 1, Plate I-1). An exception was that MAFF301208, one strain of P. syringae pv. pisi did not show the inhibitory activity. Bacterial strains showing the growth inhibitory activity against G. candidum were incubated on King's B agar slant for 48hr at 28•Ž and suspended in 5ml of distilled water at the concentration of ca. 108cfu/ml. One liter of CPD medium in 3 liter flask was inoculated with the suspension. After that, the mixture was incubated under still condition for 6 days at 22•Ž, and antimicrobial substance from the culture broth was extracted by the procedures of Gross and DeVay3) with a slight modification. Lilac isolate TMR124 of P. syringae pv. syringae which was thought to be a syringostatin producer was used as a positive control strain.