Strain-Ecotype Specificity in Sinorhizobium meliloti - Medicago truncatula Symbiosis Is Correlated to Succinoglycan Oligosaccharide Structure

Abstract

Molecular signals, including Nod factors and succinoglycan, are necessary for the establishment of nitrogenfixing nodules (Fix ؉ ) in Medicago truncatula-Sinorhizobium meliloti symbiosis. This report shows that M. truncatula-S. meliloti interactions involve ecotype-strain specificity, as S. meliloti Rm41 and NRG247 are Fix ؉ (compatible) on M. truncatula A20 and Fix ؊ (incompatible) on M. truncatula A17, the Fix phenotypes are reversed with S. meliloti NRG185 and NRG34, and there is a correlation between the host specificity and succinoglycan oligosaccharide structure. S. meliloti NRG185 produces oligosaccharides that are almost fully succinylated, with two succinate groups per subunit, whereas the oligosaccharides produced by S. meliloti Rm41 include many nonsuccinylated subunits, as well as subunits with a single succinate group and others with malate. The results of this study demonstrated the following: (i) incompatibility is not a consequence of an avirulence factor or lack of Nod factor activity; (ii) the Fix ؉ phenotypes are succinoglycan dependent; (iii) there is structural variability in the succinoglycan oligosaccharide populations between S. meliloti strains; (iv) the structural nature of the succinoglycan oligosaccharides is correlated to compatibility; most importantly, (v) an S. meliloti Rm41 derivative, carrying exo genes from an M. truncatula A17-compatible strain, produced a modified population of succinoglycan oligosaccharides (similar to the donor strain) and was Fix ؉ on A17.Sinorhizobium meliloti and Medicago sativa (alfalfa) enter into a nitrogen-fixing symbiosis, and the initiation of a functional symbiosis involves an exchange of signal molecules. Plant-derived flavonoids activate the transcription of bacterial nodulation (nod) genes, resulting in the biosynthesis of Nacylated chito-oligosaccharides (Nod factors). The Nod factors then elicit root hair curling and nodule initiation in the plant (11,14). The subsequent infection of the nodule by the microsymbiont results in the development of nitrogen-fixing nodules (Fix ϩ ) containing physiologically distinct bacteroids. Sinorhizobium spp. lipopolysaccharides, capsular polysaccharides (K antigens), and exopolysaccharides (succinoglycan or galactoglucan) are involved in the infection process, and specific polysaccharide mutants yield nonfunctional "pseudonodules" and a Fix Ϫ phenotype (9,26,29,36). Succinoglycan (exo) mutants of S. meliloti Rm1021 elicit Fix Ϫ nodules due to a lack of infection thread development (10,31,38). Succinoglycan is composed of octasaccharide repeat units (one galactosyl and seven glucosyl residues) substituted with pyruvyl (1-carboxyethylidene), acetyl, and succinyl groups (2, 23, 24, 43). The functional oligosaccharin, an oligosaccharide with signal activity (1), consists of three repeats (5, 53), and the presence of the succinyl groups is essential for activity (30). S. meliloti also produces K antigens (46), and the K R 5 antigen of S. meliloti Rm41 exoB functionally replaces succinoglycan in the infection of a...

Molecular Determinants of a Symbiotic Chronic Infection

“…Similar to NF, several of these molecules exert their effects on symbiosis in a structurally dependent manner, arguing that they may function as signals between invading bacteria and their host. In fact, recent evidence suggests that the exopolysaccharide succinoglycan may help further define species-specificity in addition to NF (153). A shared and outstanding question regarding these bacterial polysaccharides is their potential role in modulating a plant defense response to bacterial invasion.…”

Host-secreted antimicrobial peptide enforces symbiotic selectivity in Medicago truncatula

“…In extreme cases, such specificity can lead to the formation of infected nodules that are unable to fix nitrogen, due to the failure of intracellular survival of the differentiated bacteroids (13)(14)(15)(16). Despite recent advances in our understanding of the signaling pathways leading to initial recognition and nodule development (3,4), host genetic control of this fixation-level incompatibility is not well understood.…”

“…Results and Discussion S. meliloti strain Rm41 forms Fix + nodules on the M. truncatula accession DZA315.16 (DZA315), but Fix − nodules on Jemalong A17 (A17) (15,16). The Fix − phenotype of A17 results from its incompatibility with Rm41, because it can establish efficient symbioses with other strains (16).…”

“…In this system, nitrogen fixation efficiency is dependent on genome-by-genome interactions between the symbiotic partners, and no single host genotypes or rhizobial strains are consistently associated with the best nitrogen fixation performance (12,16). Frequently, the same bacterial strains can form either functional (Fix + ) or nonfunctional (Fix − ) nodules depending on the host genotype (14)(15)(16). Here, we report the isolation of the M. truncatula NFS2 gene that is involved in regulation of the fixation-level incompatibility with S. meliloti strain Rm41.…”

Genomics of Specificity in the Symbiotic Interaction between Rhizobium leguminosarum and Legumes

“…A similar phenotype was found with ß-glucan exporting mutants (Dylan et al, 1986). Changes in EPS profile are correlated with host range in different systems, for example in the Medicago truncatula -E. meliloti system, strain NRG185 is able to form effective nodules in cultivar A17 but not in A20, while strain NRG247 has the opposite phenotype (Simsek et al, 2007) . Similar phenotypes were found in other systems like with B. japonicum wild type/exoB mutant and Glycine soja/G.…”