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Sugar from Tobacco?

Research Update - Plants and Microbes

Charles Hagedorn, Ph.D., Virginia Tech
"NBIAP News Report." U.S. Department of Agriculture (November 1994)

Fructose is a sweetener used in many food products, primarily as a replacement for sucrose. One reason for its increasing popularity with food manufacturers is the availability of bulk quantities of corn starch, which is enzymatically converted to fructose on an industrial scale. An alternative and cheaper source of fructose could be fructan, a storage carbohydrate in many plants. Fructans are multiple fructose (polyfructose) polymers, which can be enzymatically or chemically hydrolyzed to yield fructose.

Plant fructans are sweet tasting, but in the human digestive tract there are no enzymes that can degrade their glycosidic linkages. Consequently fructans are low caloric food ingredients. This property has been exploited for the production of natural low calorie sweeteners for the health food industry, particularly in Japan, where they are made enzymatically in bioreactors. In addition to food uses of fructans, these molecules have potential in many non-food applications.

For non-food uses, a high degree of polymerization of the fructan molecules greatly increases functionality of the polymers. But in fructan-producing plants, which synthesize polyfructose from sucrose, the polymer consists of only 5-60 fructose units. Efficient synthesis and accumulation of highly polymerized fructans in plants would extend the technological applicability of these plant products.

Many microorganisms are also capable of producing fructans, but unlike those made in plants, bacterial fructans usually possess a high degree of polymerization and consist of over 100,000 fructose units. In Bacilli, Pseudomonads and Streptococci, extracellular enzymes convert sucrose to bacterial fructan, often called levan.

Tobacco plants containing a modified Bacillus subtilis levansucrase gene (SacB) accumulate a stable fructan similar to that produced by the microbe. Although the level of fructan accumulation in the transgenic tobacco plants was impressive and ranged from 3-8 percent of the total plant dry weight, no microbial levansucrase or its messenger RNA could be detected in these plants. Expansion of the work should permit the production of this high molecular weight bacterial fructan in crop plants for use in food and non-food products.

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