Increased Production of the Exopolysaccharide Succinoglycan Enhances Sinorhizobium meliloti 1021 Symbiosis with the Host Plant Medicago truncatula

The nitrogen-fixing rhizobial symbiontSinorhizobium meliloti 1021 produces acidic symbiotic exopolysaccharides that enable it to initiate and maintain infection thread formation on host legume plants. The exopolysaccharide that is most efficient in mediating this process is succinoglycan (exopolysaccharide I [EPSI]), a polysaccharide composed of octasaccharide repeating units of 1 galactose and 7 glucose residues, modified with succinyl, acetyl, and pyruvyl substituents. Previous studies had shown thatS. meliloti 1021 mutants that produce increased levels of succinoglycan, such asexoR mutants, are defective in symbiosis with host plants, leading to the hypothesis that high levels of succinoglycan production might be detrimental to symbiotic development. This study demonstrates that increased succinoglycan production itself is not detrimental to symbiotic development and, in fact, enhances the symbiotic productivity ofS. meliloti 1021 with the host plantMedicago truncatula cv. Jemalong A17. Increased succinoglycan production was engineered by overexpression of theexoY gene, which encodes the enzyme responsible for the first step in succinoglycan biosynthesis. These results suggest that the level of symbiotic exopolysaccharide produced by a rhizobial species is one of the factors involved in optimizing the interaction with plant hosts.