Rhizobial exopolysaccharides: genetic control and symbiotic functions

Abstract

Specific complex interactions between soil bacteria belonging to Rhizobium, Sinorhizobium, Mesorhizobium, Phylorhizobium, Bradyrhizobium and Azorhizobium commonly known as rhizobia, and their host leguminous plants result in development of root nodules. Nodules are new organs that consist mainly of plant cells infected with bacteroids that provide the host plant with fixed nitrogen. Proper nodule development requires the synthesis and perception of signal molecules such as lipochitooligosaccharides, called Nod factors that are important for induction of nodule development. Bacterial surface polysaccharides are also crucial for establishment of successful symbiosis with legumes. Sugar polymers of rhizobia are composed of a number of different polysaccharides, such as lipopolysaccharides (LPS), capsular polysaccharides (CPS or K-antigens), neutral β-1, 2-glucans and acidic extracellular polysaccharides (EPS). Despite extensive research, the molecular function of the surface polysaccharides in symbiosis remains unclear.This review focuses on exopolysaccharides that are especially important for the invasion that leads to formation of indetermined (with persistent meristem) type of nodules on legumes such as clover, vetch, peas or alfalfa. The significance of EPS synthesis in symbiotic interactions of Rhizobium leguminosarum with clover is especially noticed. Accumulating data suggest that exopolysaccharides may be involved in invasion and nodule development, bacterial release from infection threads, bacteroid development, suppression of plant defense response and protection against plant antimicrobial compounds. Rhizobial exopolysaccharides are species-specific heteropolysaccharide polymers composed of common sugars that are substituted with non-carbohydrate residues. Synthesis of repeating units of exopolysaccharide, their modification, polymerization and export to the cell surface is controlled by clusters of genes, named exo/ exs, exp or pss that are localized on rhizobial megaplasmids or chromosome. The function of these genes was identified by isolation and characterization of several mutants disabled in exopolysaccharide synthesis. The effect of exopolysaccharide deficiency on nodule development has been extensively studied. Production of exopolysaccharides is influenced by a complex network of environmental factors such as phosphate, nitrogen or sulphur. There is a strong suggestion that production of a variety of symbiotically active polysaccharides may allow rhizobial strains to adapt to changing environmental conditions and interact efficiently with legumes.

RNA Sequencing Analysis of the Broad-Host-Range Strain Sinorhizobium fredii NGR234 Identifies a Large Set of Genes Linked to Quorum Sensing-Dependent Regulation in the Background of a traI and ngrI Deletion Mutant

“…With respect to biosynthesis and regulation of EPS, S. meliloti appears to be one of the best-studied model organisms (65). In this strain, the ExpR/Sin QS system controls the expression of the succinoglucan biosynthesis genes (16,70).…”

Lipoprotein PssN of Rhizobium leguminosarum bv. trifolii: Subcellular Localization and Possible Involvement in Exopolysaccharide Export

“…They have been shown to be critical factors in symbiotic interactions of rhizobia with leguminous plants that form the indeterminate type of nodules (2). Accumulating data suggest that exopolysaccharides may be involved in several steps of nodule development, such as invasion, bacterial release from infection threads, bacteroid development, suppression of plant defense response, and protection against plant antimicrobial compounds (15,67).…”

Legumes in the reclamation of marginal soils, from cultivar and inoculant selection to transgenic approaches

“…As explained above, the physiology of the bacterial surface may also be a competitive factor, and the competitive capacity of rhizobial mutants with altered lipopolysaccharides or exopolysaccharides may be altered (Fraysse et al 2003;Ormeño-Orrillo 2005). For example, the production of exopolysaccharides is influenced by a complex network of environmental factors such as phosphate, nitrogen, or sulphur (Skorupska et al 2006). Finally, genetic exchange among rhizobia in the soil and the often reported genome instability of Rhizobium may affect competitiveness (Ding and Hynes 2009;OrozcoMosqueda et al 2009).…”