Structural analyses of two arabinose containing oligosaccharides derived from olive fruit xyloglucan: XXSG and XLSG.

Xyloglucan oligosaccharides were prepared by endo-(1-->4)-beta-D-glucanase digestion of alkali-extractable xyloglucan from olive fruit and purified by a combination of gel-permeation (Bio-Gel P-2) chromatography and high-performance anion-exchange chromatography. The two most abundant oligosaccharides were converted to the corresponding oligoglycosyl alditols by borohydride reduction and structurally characterised by NMR spectroscopy and post-source decay (PSD) fragment analysis of matrix-assisted laserinduced desorption/ionisation time-of-flight (MALDI-TOF) mass spectra. The results revealed that olive fruit xyloglucan is mainly built from two novel oligosaccharides: XXSG and XLSG. The structure of the oligosaccharides confirmed the presence of a specific xyloglucan in olive fruit with alpha-L-Araf-(1-->2)-alpha-D-Xylp sidechains as was suggested previously. The presence of such sidechains is a common feature of xyloglucans with an XXGG core produced by solanaceous plants but has not been demonstrated for other dicotyledonous plants, which have in general an XXXG core. Direct treatment of cell wall material from olive fruit with pectin degrading enzymes in combination with endo-(1-->4)-beta-D-glucanase revealed that some of the arabinose residues of the oligosaccharides XXSG and XLSG are substituted with either 1 or 2 O-acetyl groups.

Structural characterization of the pectic polysaccharide rhamnogalacturonan II: evidence for the backbone location of the aceric acid-containing oligoglycosyl side chain.

Monomeric rhamnogalacturonan II (mRG-II) was isolated from red wine and the reducing-end galacturonic acid of the backbone converted to L-galactonic acid by treatment with NaBH4. The resulting product (mRG-II'ol) was treated with a cell-free extract from Penicillium daleae, a fungus that has been shown to produce RG-II-fragmenting glycanases. The enzymatically generated products were fractionated by size-exclusion and anion-exchange chromatographies and the quantitatively major oligosaccharide fraction isolated. This fraction contained structurally related oligosaccharides that differed only in the presence or absence of a single Kdo residue. The Kdo residue was removed by acid hydrolysis and the resulting oligosaccharide then characterized by 1- and 2D 1H NMR spectroscopy, ESMS, and by glycosyl-residue and glycosyl-linkage composition analyses. The results of these analyses provide evidence for the presence of at least two structurally related oligosaccharides in the ratio approximately 6:1. The backbone of these oligosaccharides is composed of five (1-->4)-linked alpha-D-GalpA residues and a (1-->3)-linked L-galactonate. The (1-->4)-linked GalpA residue adjacent to the terminal non-reducing GalpA residue of the backbone is substituted at O-2 with an apiosyl-containing side chain. Beta3-L-Araf-(1-->5)-beta-D-DhapA is likely to be linked to O-3 of the GalpA residue at the non-reducing end of the backbone in the quantitatively major oligosaccharide and to O-3 of a (1-->4)-linked GalpA residue in the backbone of the minor oligosaccharide. Furthermore, the results of our studies have shown that the enzymically generated aceryl acid-containing oligosaccharide contains an alpha-linked aceryl acid residue and a beta-linked galactosyl residue. Thus, the anomeric linkages of these residues in RG-II should be revised.

A novel highly unsaturated fatty acid moiety of lipo-oligosaccharide signals determines host specificity of Rhizobium.

In Rhizobium leguminosarum biovar viciae, the nodABC and nodFEL operons are involved in the production of lipo-oligosaccharide signals which mediate host specificity. The structure of these metabolites and those produced in nod mutants links the nodE and nodL genes to specific chemical features of the signal molecules. A nodE-determined, highly unsaturated fatty acid and a nodL-determined O-acetyl substituent are essential for the ability of the signals to induce nodule meristems on the host plant Vicia sativa.

Structure and function of plant cell wall polysaccharides.

Studies of the primary structures of polysaccharides of growing plant cell walls have shown that these structures are far more complex than was anticipated just a few years ago. This complexity can best be appreciated by considering xyloglucan, a hemicellulose present in the cell wall of both monocots and dicots, and rhamnogalacturonan II (RG-II) and rhamnogalacturonan I (RG-I), two structurally unrelated pectic polysaccharides. This realization led us to postulate that cell wall polysaccharides have functions beyond determining the size, shape and strength of plants. Some years ago we demonstrated that oligosaccharide fragments of a branched beta-linked glucan of fungal cell walls can elicit the production of phytoalexins (antibiotics) in plants by inducing the formation of the enzymes responsible for synthesis of the phytoalexins. It has now been ascertained and confirmed by synthesis that the elicitor activity resides in a very specific hepta-beta-D-glucoside. The heptaglucoside has been shown to elicit phytoalexins by activating the expression of specific genes, that is, by causing the synthesis of the mRNAs that encode the enzymes that synthesize phytoalexins. In other words, complex carbohydrates can be regulatory molecules. Further experiments established that oligosaccharide fragments of polysaccharides, produced by acid or base hydrolysis or by enzymolysis of primary cell walls of plants, also evoked defence responses in plants. Subsequently, we learned that defined fragments of polysaccharides, released from covalent attachment within plant cell walls, can function as regulators of various physiological processes such as morphogenesis, rate of cell growth and time of flowering and rooting, in addition to activating mechanisms for resisting potential pathogens. Examples of plant oligosaccharides with regulatory properties (called oligosaccharins) will be described.