Phosphoglycerol substituents present on the cyclic beta-1,2-glucans of Rhizobium meliloti 1021 are derived from phosphatidylglycerol

Abstract

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,...

Response of intracellular carbohydrates to a NaCl shock in Rhizobium leguminosarum biovar trifolii TA-1 and Rhizobium meliloti SU-47

“…has a high osmotic pressure (Kennedy & Rumley, 1988;Miller et al, 1986Miller et al, , 1988Zorreguieta et al, 1990).…”

The exoD gene of Rhizobium meliloti encodes a novel function needed for alfalfa nodule invasion

“…The cyclic 13 (1,2) glucan that is missing in ndvB mutants has been reported to be present in both exo and exoD mutants, however (4,23). More recently, it has been reported that the periplasmic glucans produced by R. meliloti Rm1021 have acidic modifications (34). Conceivably, the glucan in exoD strains could lack some such decoration.…”

A novel cyclic beta-1,2-glucan mutant of Rhizobium meliloti

“…After centrifugation, the supernatant was removed, concentrated under vacuum, and chromatographed on a Sephadex G-50 column by use of conditions described previously (27). Fractions containing cyclic ␤-1,2-glucans were pooled, concentrated, and desalted on a Sephadex G-15 column as described previously (27). Finally, neutral and anionic cyclic ␤-1,2-glucans were separated by DEAE-cellulose column chromatography as described previously (27).…”

“…Cultures were then centrifuged at 4ЊC for 25 min at 20,000 ϫ g. Cell pellets were extracted with 15 ml of 70% (vol/vol) ethanol at 70ЊC for 30 min. After centrifugation, the supernatant was removed, concentrated under vacuum, and chromatographed on a Sephadex G-50 column by use of conditions described previously (27). Fractions containing cyclic ␤-1,2-glucans were pooled, concentrated, and desalted on a Sephadex G-15 column as described previously (27).…”

“…In Agrobacterium and Rhizobium species, these molecules contain glucose residues linked solely by ␤-1,2-glycosidic bonds, and the predominant ring size distribution is between 17 and 25 glucose residues (4). Additionally, these glucans may become highly modified with anionic substituents such as phosphoglycerol and/or succinyl moieties (1,3,27,30). For example, the anionic cyclic ␤-1,2-glucans of Rhizobium meliloti carry, on average, a total net negative charge of approximately 2.5, and some cyclic ␤-1,2-glucan subfractions synthesized by this bacterium contain as many as six anionic substituents (6).…”