Lipopolysaccharides from Burkholderia cepacia contribute to an enhanced defensive capacity and the induction of pathogenesis-related proteins in Nicotianae tabacum

Lipopolysaccharides (LPS) from the outer cell wall of Gram-negative bacteria can influence the outcome of certain plant-pathogen interactions. LPS from an endophytic strain of Burkholderia cepacia, were purified and characterized by denaturing electrophoresis. A protective effect of LPS from Burholderia cepacia on the Nicotianae tabacum-Phytophthora nicotianae interaction was found when plants were infected with zoospores of the pathogen. Progressive development of Black-shank disease symptoms occurred in the control plants while plants pre-treated with 100 μg ml^-1 LPS remained unaffected. The LPS were found to possess activity as elicitors of plant defense responses in tobacco where the induction of PR-proteins was investigated by selective low pH extraction and electrophoretic analyses. Membrane permeability studies showed a dose dependent increase in permeability and of loss of cell viability due to the increasing toxic effect of higher concentrations (200-1000 μg ml^-1) of LPS. The optimum concentration for PR-protein induction was found to be 75-100 μg ml^-1, where the effect on cell permeability was minimal but induction was optimal. Time studies of 0-4 days, with 100 μg ml^-1 LPS added to cell suspensions and leaf discs, showed an increase in intensity of protein bands with M_rs of 6·5, 15, 17, 23, 33 and 35. These values correspond to PR-proteins from classes VI, IV, I, V, III and II, respectively. Experiments were extended to include whole plant and leaves to compare the results obtained from the cell suspension and leaf discs and were found to be similar with regard to the time and dose-dependent induction of PR-proteins. PR-proteins extracted from the leaves following bacterial inoculation of the roots indicated a systemic response which was also observed in upper leaves following treatment of lower leaves. The results are indicative of an enhanced defensive capacity due to pre-conditioning by the bio-active LPS.