Induced defense responses in plants by bacterial lipopolysaccharides

M.Sc.Plant disease can be naturally suppressed by plant growth promoting rhizobacteria and endophytic / endorhizosphere bacteria. Apart from direct antagonism against pathogenic organisms, these plant growth promoting bacteria and endophytes can induce a form of systemic resistance (ISR) in plants. The main bacterial inducing component has been suggested to be the outer membrane lipopolysaccharides (LPS), found in the cell walls of Gramnegative bacteria. Burkholderia cepacia (Pseudomonas cepacia) is a bacterial endophyte that has potential as a biocontrol agent. Although a few studies have indicated that LPS from, certain Pseudorrionads has a protective effect in plants against disease, a controlled investigation has not been attempted previously with a purified preparation of LPS. LPS was isolated from the bacterial cell wall, prepared and characterized by denaturing electrophoresis. Characterization of the LPS also included the determination of 2-keto-3-deoxyoctonate, carbohydrate —, as well as the protein content. The purified LPS was found to possess activity as an elicitor of plant defence responses in tobacco where the induction of pathogenesisrelated (PR) proteins were investigated and electrophoretically analysed. An optimum LPS concentration range of 50-150 14/m1 was determined by studying cell death using the Evans blue procedure. Time and concentration ranges for LPS induced responses were established in cell suspensions, leaf discs, whole leaves and whole plants. It was determined that the PR-protein response could be optimally induced after four days following elicitation with 100 fag/ml LPS. Systemic induction of resistance was tested by treatment of the lower leaves and following the response in the upper leaves; as well as bacterial inoculation of the plant roots followed by PR-protein extraction of the leaves. Treatment of tobacco plants with LPS protected the plants against subsequent infection by the pathogen Phytophthora nicotianae, thereby suggesting a role for LPS as activators of systemic acquired resistance (SAR). It can be concluded from this study that the lipopolysaccharides from Burkholderia cepacia, that were used in this study, are effective local as well as systemic inducers of the defense PR-proteins in Nicotianae tabacum cv Samsun NN. The fact that protection is associated with PR-protein induction distinguishes it from the protection induced by rhizobacteria