Structures of rhizobial lipopolysaccharides and their role in symbiosis process

Lipopolysaccharides synthesized by rhizobia have a various structure. Differences are observed in lipid A (considered as the most conservative part of LPS), in the core region, and in the O-specific polysaccharide. Lipids A may have different compositions of the sugar backbone and the acylation pattern. The core region of rhizobia mainly consists of hexoses, uronic acids, N-acetylquinohozamine, and Kdo, but has no heptose region typical for enterobacteria. The O-PSs may have a different structure even among strains of the same species. They are built of various monosaccharides and are often hydrophobic. An appropriate structure of LPS domains is required for establishment of an effective symbiosis between bacteria and their plant host. Changes in the structure of LPS (most often caused by mutations) resulted in a decrease in efficiency or failure of atmospheric nitrogen fixation. Complete LPS protects symbiotic bacteria penetrating plant cells and determines the proper organization and maturation of symbiosomes.