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 beta-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.

Figure 1

The chemical structures of the rhizobial exopolysaccharide (EPS) repeating units. A)-B) S. meliloti, C) Rhizobium leguminosarum bv. trifolii, D) R. leguminosarum bv. trifolii 4S, E) R. leguminosarum bv. viciae 248. Abbreviations: Glc, glucose, GlcA, glucuronic acid, Gal, galactose, Succ, succinate, and Ac, acetyl.

Figure 2

Pathway for the assembly of the repeating unit of EPS I in S. meliloti based on [5, 39, 40, 48, 49, 59]. The synthesis involves three groups of proteins: a) the proteins involved in the biosynthesis of nucleotide sugar precursors (not shown); b) the sugar transferases, engaged in the transfer of precursors onto the lipid carrier (proteins shown in orange and blue); c) modifying enzymes, decorating the unit with non-sugar moieties (green); d) proteins involved in EPS assembly and export (yellow). Abbreviations used: Glc, glucose; Gal, galactose; Ac, acetate; Pyr, pyruvate; Succ, succinate.

Figure 3

Schematic representation of EPS repeating unit biosynthesis and polymerization in R. leguminosarum. Based on 8, 36, 79–82, 84–87, 89–91. Question marks indicate putative, based on the results of homology searches, function of PssR, PssM and other Pss/Exo proteins. Colour marks of the proteins respond to those in Fig. 2. Abbreviations used: Glc, glucose; GlcA, glucuronic acid; Gal, galactose; Ac, acetate; Pyr, pyruvate; IM, inner membrane.

Figure 4

Genetic organization of the pss gene clusters of R. leguminosarum based on GenBank accesions: AF028810, X98117, Y12758, X99850, AF040104, AY237541, AF014054, X98117, AF067140 and AF402596. Colour marks of the pss genes respond to colours of the proteins shown in Fig. 3

Figure 5

Model of the regulation of succinoglycan (EPS I) and galactoglucan (EPS II) production in S. meliloti based on [3, 5, 103, 105, 106].

Figure 6

Model of the regulation of EPS production in R. leguminosarum based on [79, 81, 100, 114, 116, 119, 123].