Plants are well suited as producers of modified starch and novel carbohydrates. Photosynthesis supplies a sucrose feedstock to carbohydrate storage organs, and the many intermediate steps in the conversion of sucrose to starch represent potential branch points at which the sugar may be shunted to new products. Storage starch, being insoluble, is not physiologically active, and the many mutations affecting starch synthesis demonstrate that a wide variety of structures can be tolerated by the plant. Tubers and storage roots (beets) are not required for propagation, so the storage starch in these organs need not be accessible to the plant for turnover. Cereal grain starch is, however, important in germination; therefore, modification or replacement of this carbon source must take physiological needs into account.
To date, the main efforts in carbohydrate engineering in plants have been directed to alterations in starch yield, to increasing or decreasing the effective amylose content, and to changing the degree of branching in amy-lopectin. Antisense approaches have been highly effective in bringing about qualitative changes in starch, although the type of starch produced has not been fully predictable. Great progress has been made over the last several years in understanding amylopectin biosynthesis. However, the intricacies of the interactions between the various isoforms of starch synthase, starch branching enzyme, and debranching enzymes continue to dog attempts at rational starch design based on the functional properties desired in the final product. Part of the difficulty lies as well in the elaborate nature of the starch structure itself, our limited ability to determine the structure fully as can be done for proteins or nucleic acids, and the complex relationship between gelatinization and retrogradation thermodynamics and rheology and starch structure. The most success in producing novel carbohydrates has been achieved by the transfer of fructan biosynthesis from plants where it is common to other crops, particularly sugar beet and potato, where these carbohydrates are not normally found. Fructans attract increasing interest as functional foods.
Ultimately, the position of starch and other plant carbohydrates as "green," greenhouse-neutral replacements for petrochemicals offers great potential for the farming of crops containing specialized storage products. In addition to nonfood uses, applications ranging from fat substitutes to fiber (as "resistant starch") in novel foods promise to create new markets for plant carbohydrates and new demand for their creation. At present, however, the rejection by the public of genetic engineering in general, widespread in Europe and growing in North America and elsewhere despite the environmental benefits it can bring to agriculture, is discouraging growth in the production of transgenic carbohydrates. It remains to be seen whether modified starch and carbohydrates produced in transgenic plants but destined for nonfood products can escape such pressures.
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