PROGRESS IN THE GENETIC MANIPULATION OF CROPS AIMED AT CHANGING STARCH STRUCTURE AND INCREASING STARCH ACCUMULATION

The starch content and its composition have important consequences for the yield of the harvested crop and the materials extracted from it. The functional properties of the foods or other processed materials de rived from these crops are also affected by the structure and composit ion of the starch. Recently, genetic engineering has been used to prod uce plants with an elevated starch content, achieved by transforming t he plant with a mutated bacterial gene coding for an ADPglucose pyroph osphorylase that is active in the presence of metabolites which inhibi t the plant enzyme. Besides the practical implications of these result s, this experiment provided direct evidence for the regulatory role of the ADPglucose pyrophosphorylase in starch synthesis. Other bacterial enzymes, such as glycogen synthase and branching enzyme, could be int roduced in order to modify starch structure, However, a more elegant ( but longer-term) approach would be to learn enough about the structure -function relationships of the plant enzymes so that the product of th eir action could be changed. To achieve this objective, much more will have to be learned about the enzymes involved in the biosynthesis of starch than is presently known. Here, the basic properties of starch a nd the current research approaches to understanding its biosynthesis a re described, together with a perspective of how genetic manipulation of starch structure may be achieved.