The synthesis of periplasmic cyclic ,P-1,2-glucans is a property unique to species of the family Rhizobiaceae.For this reason, it is generally believed that these molecules may play an important role in the plant infection process. In the present study, we determined that the cyclic 13-1,2-glucans produced by Rhizobium meliloti 1021 were predominantly anionic in character and contained both phosphoglycerol and succinic acid substituents. In addition, we demonstrated that phosphatidylglycerol was the source of the phosphoglycerol substituents present on these oligosaccharides and that greater than 60% of the total phospholipid turnover in this organism involved this substitution reaction.The interaction of gram-negative bacteria of the genus Rhizobium with the roots of certain leguminous plants leads to the development of nitrogen-fixing nodules (5,15,28,37). This interaction has been shown to be species specific and is believed to be mediated through a "signal exchange" mechanism between the two organisms (see reference 15 for a review). Because of their possible role as signal molecules in nodulation, the cell envelope carbohydrates of members of the genus Rhizobium have been the subject of much study.Compelling evidence for a role of cell surface carbohydrates in nodulation has come primarily from studies of rhizobial mutants that are defective in the synthesis of exopolysaccharides (8,29,30,34,36). Recently, however, attention has focused on the possible role of the periplasmic cyclic ,-1,2-glucans in legume nodulation (1, 7, 12-15, 18, 35, 42). These compounds were first identified in Agrobacterium tumefaciens by McIntire and co-workers (31) in 1942. Since that time, there have been several analyses of the structure and distribution of these compounds (2, 19-21, 26, 27, 3841). With regard to distribution, the cyclic P-1,2-glucans appear to be unique to species of the family Rhizobiaceae. With regard to structure, it has been demonstrated that the cyclic glucans consist of 17 to 24 glucose residues linked solely by P-1,2 glycosidic bonds. Until 1987, it was believed that the cyclic ,B-1,2-glucans were strictly unsubstituted and neutral in character. However, most recently, three laboratories have reported the discovery of anionic, substituted forms of these molecules (4, 21, 33).Initial evidence for a possible role of the cyclic glucans in rhizobial nodulation was reported by Abe et al. (1) and Higashi and Abe (18), who observed that the addition of cyclic P-1,2-glucans to white clover seedlings resulted in the enhancement of infection thread formation and nodule number by Rhizobium trifolii. A second line of evidence has come from recent studies by Nester and co-workers (7,12,13,35) and Ugalde and co-workers (14, 42) of mutants of Agrobacterium tumefaciens and Rhizobium meliloti found to be defective in the synthesis of cyclic P-1,2-glucans. The cyclic 3-1,2-glucan-deficient mutants of A. tumefaciens are avirulent and attachment defective (7,35), and the mutants of R. meliloti form ineffective nodules (13,...
We report the initial characterization of the cell-associated oligosaccharides produced by four Bradyrhizobium strains: Bradyrhizobiumjaponicum USDA 110, USDA 94, and ATCC 10324 and Bradyrhizobium sp. strain 32H1. The cell-associated oligosaccharides of these strains were found to be composed solely of glucose and were predominantly smaller than the cyclic beta-1,2-glucans produced by Agrobacterium and Rhizobium species. Linkage studies and nuclear magnetic resonance analyses demonstrated that the bradyrhizobial glucans are linked primarily by beta-1,6 and beta-1,3 glycosidic bonds. Thus, the bradyrhizobia appear to synthesize cell-associated oligosaccharides of structural character substantially different from that of the cyclic beta-1,2-glucans produced by Agrobacterium and Rhizobium species.Bacterial genera in the family Rhizobiaceae are distinguished by their ability to infect higher plants. In the case of Rhizobium and Bradyrhizobium species, this infection process leads to a beneficial symbiotic relationship in which nitrogen-fixing nodules develop on the roots of leguminous plants. Plant infection by Agrobacterium species, however, results in the production of tumors on susceptible plant hosts. The cell surface carbohydrates of all three genera are believed to play important roles in the plant infection process. These cell surface carbohydrates include extracellular polysaccharides, capsular polysaccharides, lipopolysaccharides, and periplasmic glucans. Recent studies have demonstrated that Agrobacterium and Rhizobium species synthesize neutral and anionic periplasmic glucans of similar structure (2,5,16,17,20,21,23). In both genera, these periplasmic glucans are composed of a cyclic beta-1,2-glucan backbone containing 17 to 24 glucose residues. In Agrobacterium tumefaciens, approximately 50% of the total periplasmic cyclic beta-1,2-glucans are present as neutral, unsubstituted molecules (22). The remaining molecules are substituted with one or more phosphoglycerol moieties (23). In Rhizobium meliloti, as much as 90% of the periplasmic cyclic beta-1,2-glucans may be substituted with anionic moieties (21). As in the cyclic glucans of A. tumefaciens, the predominant anionic substituent present on the cyclic beta-1,2-glucans of R. meliloti 1021 is phosphoglycerol (21).Recently, studies by Nester and co-workers (9, 26) and Geremia and co-workers (11) have provided evidence for a role for cyclic beta-1,2-glucans in the plant infection process. Specifically, cyclic beta-1,2-glucans have been implicated in the attachment of the bacterial cell to the plant host. Although there have been previous reports that at least two strains of Bradyrhizobium japonicum are capable of synthesizing neutral beta-1,2-linked glucans (1, 4), very little characterization of the cell-associated oligosaccharides of Bradyrhizobium species has been performed to date. We now report the results of our analyses of the cell-associated oligosaccharides of four Bradyrhizobium strains.(A preliminary report of this work has appeared previ-* Cor...
We have previously reported that free-living cultures of Bradyrhizobium species produce nove1 oligosaccharides that are cyclic, contain between 10 and 13 glucose residues, and are linked by 8-
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