Cereals constitute a major portion of human nutrition because of the polysaccharides the plants produce. Annually, over one billion tons of cereal grains are harvested, and half the calories consumed by humans are from rice and wheat alone. In addition, grazing animals consume vast amounts of grasses. Although cellulose is the primary polysaccharide of plants, plant cell walls also contain hemicelluloses and pectins (Carpita (1996) Annu. Rev. Plant Physiol. Plant Mol. Biol. 47:445–476).
Plant growth is determined by concerted synthesis of cell wall polymers, such as hemicelluloses and pectins. Thus, increased synthesis of one cell wall polymer is expected to cause an increase in the synthesis of the other polymers as well. Increased production of a plant polysaccharide generally accelerates plant growth. Conversely, decreased production of a plant polysaccharide generally inhibits the synthesis of other cell wall polymers and slows plant growth.
Mature plant cells generally contain about 30–40% hemicellulose. In monocot species, arabinoxylan (also referred to as glucurono-arabinoxylans or pentosan) is the main component of hemicellulose in the cell wall. In contrast, dicot cell walls contain xyloglucan as the primary hemicellulosic polymer (Carpita (1996) Annu. Rev. Plant Physiol. Plant Mol. Biol. 47:445–476).
Arabinoxylans are anti-nutritional components of animal feed, yet these polymers constitute 45–65% of the plant cell wall. Arabinoxylans absorb large amounts of water thus increasing the viscosity of the chyme and sequestering other digestible nutrients away from the digestive enzymes (WO 99/67404). In addition, the increased viscosity of the chyme results in sticky feces that contribute to animal hygiene and enteric disturbance problems for the livestock producer (Selinger et al. (1996) Anaerobe 2:263–284). Therefore, in certain circumstances, it would be desirable to lower the concentration of arabinoxylans in plants.
However, dietary fiber, particularly arabinoxylan, reduces cholesterol and low density lipoprotein levels in humans (WO 99/67,404). In breadmaking, bread quality depends heavily on the consistency of the dough. Dough that lacks viscosity alters the crumb structure of the bread and decreases the volume of bread produced. Arabinoxylan provides the viscous properties of dough (Girhammar et al. (1995) Food Hydrocolloids 9:133–140). Additionally, industries use isolated arabinoxylan preparations as thickeners, emulsifiers, or stabilizers in food, cosmetics, and pharmaceuticals. Therefore, in certain circumstances, it would be desirable to increase the concentration of arabinoxylans in plant.
The modulation of hemicellulose content can also be utilized to control plant growth. For example, plant growth is determined by concerted synthesis of cell wall polymers. It is expected that increased synthesis of one of the cell wall polymers, such as hemicellulose, will cause an increase in the synthesis of the rest of the polymers as well. It is expected that increased production of arabinoxylan or xyloglucan in vegetative tissue will accelerate plant growth. In contrast, it is expected that decreased production of arabinoxylan will slow plant growth. Additionally, tissue-specific control of hemicellulose productivity is used to modify plant organ growth and development. Early flowering, larger fruit size, or stronger stalk or stem quality is achieved by operably linking a tissue specific promoter to a gene which when expressed increases hemicellulose biosynthesis (U.S. Pat. No. 6,194,638). In view of the foregoing, it would be desirable to modulate the arabinoxylan and xyloglucan concentration in crop plants.
Clearly, modulating the concentrations of polysaccharides in various crops is a desirable goal. However, a direct approach using the enzymes that synthesize polysaccharides has been obscured for some time due to difficulties in isolating and cloning any of the plant polysaccharide synthase genes. Polysaccharide synthase enzymes for the common polysaccharides are estimated to number in the hundreds. Recently, several cellulose-synthase genes have been identified. The cellulose synthase genes share regions of homology that allow the identification of novel genes that participate in polysaccharide synthesis (Cutler et al. (1997) Current Biology 7: R108–R111).
Compositions and methodologies useful in the modulation of polysaccharide levels in plants are needed.