Alfalfa root nodule invasion efficiency is dependent on Sinorhizobium meliloti polysaccharides

The soil bacterium Sinorhizobium meliloti is capable of entering into a nit rogen-fixing symbiosis with Medicago sativa (alfalfa). particular low-molec ular-weight forms of certain polysaccharides produced by S. meliloti are cr ucial for establishing this symbiosis. Alfalfa nodule invasion by S. melilo ti can be mediated by any one of three symbiotically important polysacchari des: succinoglycan, EPS II, or K antigen (also referred to as KPS). Using g reen fluorescent protein-labeled S. meliloti cells, we have shown that ther e are significant differences in the details and efficiencies of nodule inv asion mediated by these polysaccharides. Succinoglycan is highly efficient in mediating both infection thread initiation and extension. However, EPS I I is significantly less efficient than succinoglycan at mediating both inva sion steps, and K antigen is significantly less efficient than succinoglyca n at mediating infection thread extension. In the case of EPS II-mediated s ymbioses, the reduction in invasion efficiency results in stunted host plan t growth relative to plants inoculated with succinoglycan or K-antigen-prod ucing strains. Additionally, EPS II- and K-antigen-mediated infection threa ds are 8 to 10 times more likely to have aberrant morphologies than those m ediated by succinoglycan. These data have important implications for unders tanding how S. meliloti polysaccharides are functioning in the plant-bacter ium interaction, and models are discussed.