Abstract
M.Sc.
Lipopolysaccharides (LPS) are ubiquitous, indispensable components of the cell surface of Gram-negative bacteria that have diverse roles in bacterial pathogenesis of plants. LPS as pathogen-associated molecular pattern (PAMP) molecules can be recognized by plants to directly trigger some defense—related responses. LPS can also alter the response of plants to subsequent bacterial inoculation; these delayed effects include alterations in the expression patterns of genes coding for some pathogenesis related (PR) proteins, promotion of the synthesis of the antimicrobial conjugates, and prevention of the hypersensitive reaction caused by avirulent bacteria. Prevention of the response may allow expression of resistance in the absence of catastrophic tissue damage. LPS from Burkholderia cepacia (LPSB. cep.) have been found to trigger a strong response in plants resulting in the activation of genes coding for some pathogenesis related proteins, receptor-like kinases and resistance (R) proteins.
LPS are tripartite amphipathic molecules, consisting of a Lipid A moiety that is embedded in the outer leaflet of the phospholipids/protein bilayer, a core oligosaccharide, and a polysaccharide consisting of repeating units (0-Antigen/O-side chain). Typically the Lipid A consists of a bisphosphorylated glucosamine disaccharide which is substituted by amide- and ester-bound fatty acids and / or acyloxyacyl groups. The core region, a non-repetitive oligosaccharide, is usually connected to the Lipid A part via one 3-deoxy-D-manno-oct-2-ulosonic (Kdo) residue. The core is attached in turn to the 0- Antigen that consists in most cases of a repetitive polysaccharide and that represents the major part of LPS. The bond between the Lipid A section and the Kdo residue of the core is labile under mild acid hydroysis conditions; and this allows for the fractionation of the LPS molecule into a Lipid A part and an 0-Antigen part, attached to the core.
Thus far the eliciting (active) parts of LPSB. cep. have not yet been identified. In general, it is known that the Lipid A is more conserved from one organism to another as compared to the 0-Antigen. In animals, Lipid A is believed to be the active part as it was found to elicit some defense-related responses. In plants, Lipid A was also found to trigger defense responses. Several structures of the 0-Antigens from different bacteria have been characterised, but their biological activities have not yet been investigated in detail.