Exopolysaccharide from Rhizobia: Production and Role in Symbiosis

Abstract

Symbiotic nitrogen fixation (SNF) is the signature feature of legumes in which the microsymbiont collectively called as rhizobia can reduce atmospheric nitrogen (N2) into ammonia; otherwise, N2 is metabolically unavailable to higher plants. The fixed nitrogen is generally used for plant growth or the excess of fixed nitrogen is released into the rhizosphere for improving soil fertility. Hence SNF have a significant impact on sustainable agriculture. The rhizobial diversity is enormous due to their wide geographical distribution, diverse hosts, and niches they occupy all over the globe. Rhizobia are Gram-negative bacteria belonging to class alpha-, beta-, and gamma-proteobacteria, including species of the Rhizobiaceae, Phyllobacteriaceae, Methylobacteriaceae, Brucellaceae, Hyphomicrobiaceae, Bradyrhizobiaceae, Burkholderiaceae, and Pseudomonadaceae families. Host specificity exists in the process of SNF. The specificity of rhizobium for a legume host plant is determined by the exchange of molecules between both symbiotic partners. Each step of establishment of symbiosis is tightly controlled through a complex network of signaling cascades. Among them, plants liberate flavonoids into the rhizospheric region that upregulate rhizobial genes responsible for nodule formation. Rhizobia produce a variety of extracellular polymeric substances (EPSs), from simple glycans to complex heteropolymers. The secretion of EPS by rhizobia is associated with the invasion process and bacteroid and nodule development, as well as being a response to environmental stresses. There are different types of surface polysaccharides such as lipopolysaccharides, capsular polysaccharides, and neutral and acidic polysaccharides found in rhizobia. The production of symbiotically active polysaccharides may also provide stress adaptability to rhizobial strains against changing environmental conditions. This chapter focuses on different kinds of polysaccharides produced by rhizobia, their genetics and biosynthesis, as well as their biological role on effective symbiosis.