| Plant-microbe interactions often involve the exchange of specific molecular signals. In this study, the possibility that rhizobial polysaccharides may act as specific factors in rhizobia-legume symbiosis was addressed. An examination of lipopolysaccharide (LPS) function in rhizobia-soybean interaction led to the finding that Rhizobium fredii USDA205 produces previously unreported, cell-associated polysaccharides. One of the newly identified polysaccharides consists of 3-deoxy- scD-manno-2-octulosonic acid (Kdo) and galactose: {dollar}lbrackto 3) alpha{dollar}- scD-Galp (1 {dollar}to{dollar} 5) {dollar}beta{dollar}- scD-Kdop (2{dollar}torbracksb{lcub}rm n{rcub}{dollar}. Prior to this, Kdo had only been found in rhizobial LPSs. The regulation of polysaccharide production was also examined: R. fredii cells were cultured in the presence of apigenin, a plant-derived inducer of symbiosis-related R. fredii genes, and host-plant root extract; subsequent analyses demonstrated that these compounds elicited specific modifications in the production of the Kdo-rich polysaccharides. R. meliloti, which is an important subject of rhizobia-legume research and closely related to R. fredii, was also studied, and it was found that R. meliloti AK631 also produces Kdo-rich polysaccharides that are distinct from the LPS. The novel polysaccharides of R. fredii and R. meliloti are structurally analogous to the capsular polysaccharides (K-antigens) of Escherichia coli, which are important bacterial surface factors in the infection of animals. The rhizobial polysaccharides have also been shown to be surface components, and a correlation has been established between the production of the Kdo-rich polysaccharide and the ability of R. meliloti AK631 to infect the host plant. In addition, K-antigen production by strain AK631 was specifically modulated by host root extract. These results suggest a specific molecular exchange between the microsymbiont and the host plant in AK631-alfalfa interaction. |
