Structure of Extracellular Polysaccharides (EPS) Produced by Rhizobia and their Functions in Legume–Bacteria Symbiosis: — A Review

The symbiotic diazotrophs comprise with a very diverse group of Gram negative soil bacteria, collectively called as rhizobia found in nodule of legume plant. Rhizobia adopt themselves in different environment including soil, rhizosphere and grown within legume roots, where they fix nitrogen. The establishment of symbiosis is a very complicated process involving a coordinated exchange of signal between legume plants and the symbionts. The nodule development requires synthesis of signal molecules such as Nod factors that are important for induction of nodule development. There are different types of surface polysaccharides such as lipopolysaccharides, capsular polysaccharides, neutral and acidic polysaccharides found in rhizobia. The production of symbiotically active polysaccharides may allow rhizobial strains to adapt themselves to changing environmental conditions and interact efficiently with legume plants. Despite extensive research, the actual molecular function of the surface polysaccharides of rhizobia in symbiosis remains unclear. This review emphasized on the structural composition of extracellular polysaccharide of different rhizobia isolated from different legume plants. The compositions of extracellular polysaccharides are different in different rhizobia. The various compositions of extracellular polysaccharides produced by the symbionts are considered as the signaling molecules essential for determining host plant specificity. The present status of the biological functions of the exo-polysaccharide in symbiosis such as host specificity, successful invasion, formation of infection thread and induction of nodule formation in legume plants is also summarized here.