Modification of the Pseudomonas syringae pv. tabaci -tobacco leaf interaction by bacterial oligosaccharides

Cell-associated oligosaccharides were extracted with trichloroacetic acid treatment from Pseudomonas syringae pv. aptata, purified by column chromatography, analysed by GC-MS and fast atom bombardment-mass spectrometry, and bioassayed on tobacco leaves. The material was shown to be a mixture of neutral oligosaccharides, with a mol. wt of 800-1800, degree of polymerization 5-11, and linear glucose and mannose chains. Local infiltrations of the mixture, in concentrations of 1 mg, 100, 10 and 1 μg ml-1, did not cause tobacco tissue necrosis within 1 week. When the mixture was infiltrated in the interveinal leaf panels, in concentrations of 1 μg, 100, 10 and 1 ng ml-1, 48 h before challenge inoculation, it delayed or prevented normosensitive necrosis by P. syringae pv. tabaci but not hypersensitive necrosis by P. syringae pv. aptata. Five days after inoculation with P. syringae pv. tabaci, inhibition of normosensitive necrosis, as compared with the control tissue, was 80·55, 77·21, 56·36 and 45·46 % for the four concentrations. In the bioassay with P. syringae pv. tabaci the efficacy threshold was assessed at approx. 1 ng ml-1. Pre-treatment with oligosaccharides (1 μg ml-1) significantly inhibited the growth of P. syringae pv. tabaci between 3 and 5 days after inoculation and, between day 6 and 8, it delayed the death of bacteria when the tissue was not subject to necrosis. An interval of 6 h was necessary between the infiltration of oligosaccharides (1 μg ml-1) and inoculation of P, syringae pv. tabaci for a clear inhibition of normosensitive necrosis. These oligosaccharides acted as signal molecules by modifying, at nanomolar concentrations, the P. syringae pv. tabaci-tobacco leaf interaction, but they were not elicitors of the hypersensitive reaction. © 1994 Academic Press, Inc.