Toxin production by pathovars ofPseudomonas syringae and their antagonistic activities against epiphytic microorganisms

Abstract

75 strains of 21 various Pseudomonas syringae (P.) pathovars were investigated in different tests for their toxin production. Data from literature about the production of the known phytotoxins phaseolotoxin (pv. phaseolicola), tabtoxin (pv. coronafaciens, pv. tabaci), coronatine (pv. atropurpurea, pv. glycinea, pv. maculicola, pv. morsprunorum, pv. tomato), and toxins of the lipodepsipeptide group (pv. aptata, pv. atrofaciens, pv. syringae) could be confirmed. Besides, a production of the phytohormone ethylene was detected for P. phaseolicola isolates from kudzu (Pueraria lobata) and for all tested P. glycinea and P. cannabina strains. Strains of P. apii, P. aptata, P. atrofaciens, and P. tomato produced antimetabolic toxins which could be detected with an agar diffusion assay with Escherichia coli as indicator strain. These antimetabolites inhibit a step in the arginine/ornithine biosynthesis. P. maculicola strains caused inhibition zones in this assay which could not be reversed by the tested amino acids. All strains with inhibitory effect against E. coli and Geotrichum candidum were also proved for their antagonistic activity against a selection of typical phyllosphere microorganisms. Most of the phytotoxins possess antimicrobial activity with different spectrum and efficiency. Only the lipodepsipeptide-producers showed antifungal activities. Our results show that the production of toxins is a widespread property among Pseudomonas syringae pathovars, and that some pathovars can produce more than one toxin. This characteristic and the antimicrobial activity of most toxins could be of advantage for the toxin-producing bacteria to adapt to different habitats.