Host specificity in Medicago-Sinorhizobium interactions: Structural characterization of symbiotically significant LMW-EPS from Sinorhizobium meliloti

Rhizobium, Bradyrhizobium, and Sinorhizobium (rhizobia) may bring about the formation of root nodules on leguminous host plants, in which they reduce dinitrogen to ammonia in a symbiotic relationship with specific legume plants and molecular signals are included in the establishment of nitrogen-fixing nodules on the legume roots. For example, bacterial exopolysaccharides (EPS) and K antigens promote the infection of alfalfa by Sinorhizobium meliloti. This study focused on ecotype-strain specificity in Medicago truncatula-S. meliloti interactions: Specific strains of S. meliloti infect M. truncatula ecotype A17 (compatible), but fail to establish nitrogen-fixing nodules on ecotype A20 at 28 days post infection (incompatible). Importantly, the phenotypes are reversed with other S. meliloti strains. The first part of the study demonstrated the following: incompatibility is not a consequence of an avirulence factor or Nod factor activity; there is structural variability in the succinoglycan oligosaccharide populations between S. meliloti strains; the structural nature of the succinoglycan oligosaccharides is correlated to compatibility; and most importantly, that an S. meliloti Rm41 derivative, carrying exo genes from an M. truncatula A17-compatible strain, produced a modified population of succinoglycan oligosaccharides and conferred A17-compatibility to strain Rm41. Thus, a host-plant structural requirement for succinoglycan activity determines compatibility in M. truncatula-S. meliloti interactions. The second part of the study involved the analysis of succinoglycan oligosaccharide production by each strain to determine if the biologically active oligosaccharides result from specific biosynthesis rather than the random assembly of available precursors. The analysis of the S. meliloti NRG247 oligosaccharides showed that the biosynthesis of the oligosaccharides is not a random polymerization of the monomer population. The last part of the study examined whether there is an altered pattern of oligosaccharide substitution or size range upon exposure of S. meliloti NRG185 to a plant signal, apigenin. There was a significant decrease in the production of the active trimer within the trimer population. This would effectively turn off this signal system after infection has begun