The current understanding of carbohydrate metabolism in plants has benefited greatly from availability of naturally occurring mutants. Transgenic plants offer the potential to add to this knowledge by altering metabolism in specific tissues, targeting precise steps in biosynthetic pathways and diverting metabolites from endogenous pathways. Transgenic plants may also assist in understanding the factors, which regulate partitioning of carbohydrate into various forms of storage reserve at the cellular and whole plant level. Reserves may be in the form of carbohydrate polymers consisting of glucose (starch) or fructose (fructan). The purpose of this chapter is to consider only a small number of the substantial differences that exist between plants producing starch or fructan. It is also directed towards exploring the possibility of transforming a starch-storing plant into one that accumulates at least a modest amount of fructan. One reason for considering such a proposal is to determine whether transgenic plants represent a renewable source of novel polymers that may be used as alternatives to starch.
Starch is a polymer of repeating glucose residues connected by a-1,4 or a-1,6 linkages. It is the most common form of non-structural carbohydrate polymers found among higher plant species. The role of starch is both as a transient and long-term storage polysaccharide. Alternative forms of storage polysaccharides include polymers of fructose, which were identified in plants nearly 200 years ago . Polymers of fructose are now known to accumulate in more than one out of every seven higher plant species . Polymers containing primarily (3-2,1 linkages are often referred to as inulin and those containing mostly [5-2,6 linked fructose are known as levan. Fructan is a general term for all fructose polymers, regardless of linkage type. Fructans are also synthesized by several bacterial species and may be distinguished from those produced in plants by the difference in polymer size. Plant fructans are of low molecular weight, often containing between 10 to 20 fructose residues. Bacterial polymers are much larger, containing well over 1000 fructose residues .
The commercial use of carbohydrate polymers in food and industrial applications is extensive. Native starch is widely used, but may not perform adequately in every application. Chemical modification of starch is necessary to improve its functional properties . Alternative carbohydrate polymers may be more appropriate for specific end-uses. Fructan, for example, is regarded as an excellent replacement for starch in several food and non-food applications . Despite unique functional properties, the commercial potential of fructan remains unrealized, principally due to an inadequate supply of low cost material. Most fructan-producing plants are not traditional crop species and the few that are those accumulate very low levels of polymer in easily harvested parts of the plant, such as seeds. The fundamental barriers to commercializing fructan are matters of agronomic deficiencies, which make expression of a gene with fructose polymerizing activity (fructosyltransferase) in transgenic crops an attractive goal. Competition with current low cost material strongly suggests that transformation of a traditional crop species is necessary , Maize is well suited
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