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.

Unstirred water layers in rabbit intestine: effects of pectin.

Pectins have been shown to affect the absorption of several different nutrients in clinical studies; however, the mechanisms for decreased absorption have not been defined. A possibility not studied with regards to pectin, but previously demonstrated to be important in absorption, is the effect of change in the unstirred water layer. As the unstirred water layer increases in thickness, the rate of absorption decreases for certain nutrients. The effect of pectin on the unstirred water layer in the lumen of rabbit jejunum was examined by previously described techniques. It was observed that: (1) increases in pectin concentration resulted in an increased thickness of the unstirred water layer; (2) for any stir rate, the addition of pectin increased the thickness of the unstirred water layer; and (3) stir rate is inversely related to the thickness of the unstirred water layer. It was concluded from these results that pectin increases the thickness of the unstirred water layer in rabbit jejunum. This mechanism may explain, in part, the reduction of the rate of absorption of certain nutrients seen following pectin ingestion.