Rhizobium-legume symbiosis offers a model system for the examination of plant microbe interaction and plant developmental regulation. Many bacterial polysaccharides have been shown to be important factors in successful nodulation of the host plant, particularly LPS and EPS. This project is aimed at furthering an understanding of the infection and nodulation process. R. meliloti Rm1021 EPS- mutants are unable to infect the normal host plant; however strain AK631 (EPS-) produces a non-LPS, 3-deoxy-D-manno-2- octulosonic acid (Kdo) containing polysaccharide that appears to substitute for the EPS in the nodulation of alfalfa. The Kdo-rich polysaccharide is novel to rhizobia and is analogous to a class of K-antigens from E. coli. A form of this polysaccharide has been found in all R. meliloti strains examined to date. Preliminary data indicate that the lpsZ gene and the fix-23 gene region are involved in the synthesis and modification of this polysaccharide; mutation of these genes result in strains that are unable to nodulate the host plant and are defective in the production of this polysaccharide. In addition, the incorporation of the lpsZ gene into R. meliloti Rm1021 EPS- mutants results in the modification of the Kdo polysaccharide and the restoration of the Fix+ phenotype. The objectives of this project are: 1) the structural characterization of the wild type polysaccharide from R. meliloti AK631; 2) the characterization of the polysaccharides from fix-23 and lpsZ mutants of R. meliloti AK 631; 3) the purification of bacterial surface components for use in biological activity studies; and 4) the determination of the biochemical effect of lpsZ in Rm 1021. Biological nitrogen fixation offers a cost effective and environmentally sound alternative to chemical fertilizer, where applicable. This research constitutes a significant contribution to the broad base of scientific knowledge required for advancements in the areas of plant-microbe interaction, plant developmental regulation, and biological nitrogen fixation in agriculture.
