Sinorhizobium meliloti polysaccharides play an important role in the establishment of a functional symbiosis with its host plant alfalfa. Three different polysaccharides (succinoglycan, EPS II and K-antigen) are each capable of promoting infection thread initiation and extension. Though the three polysaccharides mediated infection thread growth, EPS II and K-antigen does less efficiently than succinoglycan. EPS II is a polymer of a glucose-galactose disaccharide unit modified with one acetyl and one pyruvyl substituent per repeating unit. As the case for succinoglycan, low molecular weight (LMW) forms of EPS II are the active forms in symbiosis. Small amounts of LMW EPS II, as low as 7 pmol, allow nodule invasion when added to non-infective strains defective in exopolysaccharide production. This strongly suggests that exopolysaccharides may act as a signal during the infection process, necessary for correct infection thread development.
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