Exopolysaccharides produced by the symbiotic nitrogen-fixing bacteria of leguminosae

Abstract

SUMMARYThe process of biological nitrogen fixation (BNF), performed by symbiotic nitrogen fixing bacteria with legume species, commonly known as α and β rhizobia, provides high sustainability for the ecosystems. Its management as a biotechnology is well succeeded for improving crop yields. A remarkable example of this success is the inoculation of Brazilian soybeans with Bradyrhizobium strains. Rhizobia produce a wide diversity of chemical structures of exopolysaccharides (EPS). Although the role of EPS is relatively well studied in the process of BNF, their economic and environmental potential is not yet explored. These EPS are mostly species-specific heteropolysaccharides, which can vary according to the composition of sugars, their linkages in a single subunit, the repeating unit size and the degree of polymerization. Studies have showed that the EPS produced by rhizobia play an important role in the invasion process, infection threads formation, bacteroid and nodule development and plant defense response. These EPS also confer protection to these bacteria when exposed to environmental stresses. In general, strains of rhizobia that produce greater amounts of EPS are more tolerant to adverse conditions when compared with strains that produce less. Moreover, it is known that the EPS produced by microorganisms are widely used in various industrial activities. These compounds, also called biopolymers, provide a valid alternative for the commonly used in food industry through the development of products with identical properties or with better rheological characteristics, which can be used for new applications. The microbial EPS are also able to increase the adhesion of soil particles favoring the mechanical stability of aggregates, increasing levels of water retention and air flows in this environment. Due to the importance of EPS, in this review we discuss the role of these compounds in the process of BNF, in the adaptation of rhizobia to environmental stresses and in the process of soil aggregation. The possible applications of these biopolymers in industry are also discussed.Index terms: Exopolysaccharides, nodules, environmental stress, soil aggregation, rhizobia. RESUMO: EXOPOLISSACARÍDEOS PRODUZIDOS POR BACTÉRIAS FIXADORAS DE NITROGÊNIO SIMBIÓTICAS DE LEGUMINOSAE O processo de fixação biológica de nitrogênio (FBN), realizado por bactérias fixadoras de

Characterization of Exopolysaccharides Produced by Rhizobia Species

“…In the anomeric region, the absorption band around 796 cm -1 that revealed the possible presence of alpha glycosidic linkages is not observed. Most documented polysaccharides from rhizobia are linked in alpha or beta conformations through several possible glycosidic linkages (Chouly et al, 1995;Fraysse et al, 2003;Bomfeti et al, 2011). However, the difference in chemical composition, type of glycosidic linkage, and degree of polysaccharide branching influence the secondary and tertiary structures of the single chains and their macromolecular assembly, determining the physical properties of the polysaccharide, which are related to the polysaccharides structural or physiological functions (Sutherland, 2001).…”

“…However, unlike those from other species, the EPS composition reported here for rhizobia has not previously been reported. One research group only noted the appearance of colonies characterized by high gum production in a solid medium, and the precipitation of EPS from strains of M. plurifarium BR3804 and R. tropici CIAT899 T , which are recommended by MAPA (Ministério da Agricultura, Pecuária e Abastecimento) of Brazil as inoculants for the species Chamaecrista ensiformis and phaseolus vulgaris, respectively, demonstrating the high potential of these strains for production of these biopolymers (Bomfeti et al, 2011).…”

“…Exopolysaccharides (EPS) play important roles in various biological mechanisms, such as adhesion and infection, especially important for the initial stages of the symbiotic invasion that leads to the formation of indeterminate-type nodules on legume plants (Fabaceae family) such as pea and alfalfa (Skorupska et al, 2006). Exopolysaccharides are long-chain polysaccharides consisting of branched, repeating units of sugars or sugar derivatives and non-carbohydrate components (Whitfield, 1988;Sutherland, 2001;Bomfeti et al, 2011).…”

“…Knowledge of the eps composition now facilitates further investigations relating polysaccharide structure and dynamics to rheological properties. compounds are important in soil aggregation (Bomfeti et al, 2011). Rhizobial EPS can increase the adhesion of soil particles to plant roots and the mechanical stability of rhizospherical soils, and can also increase the level of water retention in this environment (Bomfeti et al, 2011;Donot et al, 2012).…”

“…The functional significance of some EPS structures in nodule invasion motivated an investigation of the details of their chemistry (Fraysse et al, 2003;Downie, 2010;Bomfeti et al, 2011). Since there are no studies on the commercial production of gum by rhizobia, they can be considered unexplored sources of microbial polysaccharides, highly promising for industrial applications.…”