Oligosaccharins and Their Regulatory Functions
|An oligosaccharide is a short chain of glycosyl residues interconnected by glycosidic linkages. Some oligosaccharides exert, at low concentrations, signaling effects in plant tissues and cells. Such biologically active oligosaccharides have been termed "oligosaccharins" by Albersheim and Darvill (1, 2).
Most of the known oligosaccharins have been generated in vitro by acid or enzymic catalyzed fragmentation of plant cell wall polysaccharides. Glycoproteins have also been reported to be a source of oligosaccharins. The concept has thus arisen that in vivo polysaccharides and glycoproteins, many of which have structural and/or enzymatic roles, act as latent forms of signaling molecules.
Some oligosaccharins are referred to as "elicitors," since they elicit plant defense responses and may have a role in a plants resistance to pathogens. Elicitors and other oligosaccharins also have effects on plant growth and development that are apparently unrelated to defense responses. The effects of oligosaccharins involved in plant morphogenesis will be described. The effects of defense-related elicitors are described in the section on defense-related oligosaccharins.
The Primary Wall of Growing Plant Cells
The walls of growing higher plant cells are composed predominantly of pectin, hemicellulose, cellulose, and small amounts of glycoproteins, and phenolics. The wall determines the size and shape of individual cells and ultimately plant morphology. Growing plant cells expand by modification of existing wall polymers and insertion of newly synthesized polymers into the wall. The modification of wall polymers may generate oligosaccharins which themselves can influence cell and tissue growth. Many microbial plant pathogens secrete wall-hydrolyzing glycans. These enzymes generate fragments of wall polysaccharides that may induce defense responses. In the following section the structures and biological effects of oligosaccharins derived from pectin, xyloglucan, and glycoproteins, and oligosaccharins (Nod factors) secreted by Rhizobia will be described.
Oligosaccharins Derived from Pectin
Pectin is one of the main structural components of the primary walls of dicots. Treating these walls with homogeneous endo-polygalacturonase solubilizes oligogalacturonides (OGs), rhamnogalacturonan I (RG-I), and rhamnogalacturonan II (RG-II), indicating that pectin is composed of three structurally related polymers-homogalacturonan, RG-I, and RG-II. OGs induce numerous effects in plants (Table I), including defense responses, control of organogenesis, inhibition of auxin-induced elongation and rapid repsonses at the cell surface (3). Many of these effects may be examples of OGs antagonizing the multiple actions of auxin on plant cells and tissue. Nevertheless, the mechanism (s) by which OGs regulate plant growth and development are not understood.
An allelopathic substance (aGalpA-(12)-Rha) secreted from cress roots has been reported to promote, at micromolar concentrations, hypocotyl elongation in Amarenthus (4). The biosynthetic origin of the disaccharide was not determined, although it may have been generated by the action of an endo-lyase that fragments RG-I. RG-I has a backbone composed of the repeating disaccharide 4)-a-GalpA-(12)-Rha-(1.
Xyloglucan (XG) is a structural component of primary cell walls. It has a backbone of 1, 4-linked b-D-glucosyl residues. Up to 75% of these residues may be substituted at O-6 with a-Xyl-, b-Gal-(12)-a-Xyl- , or a-Fuc-(12)-b-Gal-(12)-a-Xyl- . Partial fragmentation of XG with cellulase generates a series of XG oligosaccharides, some of which have biological activity (see xyloglucan oligosaccharins).
GLcN-containing oligosaccharides have been shown to have defense-related effects and activity to promote tomato fruit ripening (5). The biosynthetic origin of the oligomeric N-glycans has not been established, but one strong possibility is that they arise by the partial hydrolysis of apoplastic N-linked glycoproteins.
Symbiotic relationships between Rhizobium and legumes (e.g. soybean) require controlled chemical communication between the bacterium and the plant. Legumes secrete flavonoids and isoflavonoids that specifically activate the biosynthesis of nodulation (Nod) factors. Nod factors are glycolipids. They are composed of a GlcNAc-containing oligosaccharide backbone (DP 3 - 5). Various fatty acids are linked to the non-reducing end. Nod factors induce root hair curling and cell differentiation that is required for the formation of root nodules. The bacteroids within the nodules fix atmospheric nitrogen. Nod factors, at micro to nanomolar concentrations, have also been reported to restore the ability of mutant non-embryogenic carrot cells to undergo somatic embryogenesis (6).
It is likely that many more oligosaccharins will be discovered. Understanding the physiological functions of oligosaccharins at the molecular level will undoubtedly lead to numerous applications for carbohydrates in agriculture and forestry.
|Fig. Structure of oligogalacturonide|
|aDP(degree of polymerization): range of oligogalacturonides that show the designnate biological activity.
bEstimation of the order of magnitude concentration of oligogalacturonides to give half-maximum
biological response. The concentration is listed only when purified oligogalacturonides were assayed.
cNunbers in parentheses are DP of the most active oligogalacturonide.
dND: DP of active oligogalacturonides not determined; therefore, molar concentration not determined.
|Tadashi Ishii (Forestry Forest Products Research Institute)|
|Dec. 15, 1999|