Biotechnology of Seed Crops: Genetic Engineering of Seed Storage Proteins

Abstract

Table 1. Amino acid composition of total maize and soybean seed protein and purified storage protein fractions.z During their development, seeds accumulate large amounts of storage, or reserve, products that are metabolized during germination and the early stages of seedling growth. Energy reserves include complex carbohydrates (starch) and lipids, while reserves of N, C, and S are accumulated in specific storage proteins. The uses of seed products by man are innumerable, ranging from fermentation of maize starch for alcohol production to refinement of soybean protein for use in infant formula. Agronomic crops, such as maize, wheat, oats, and soybeans, are grown because they generate large amounts of seed reserves per unit of cultivated land. A major concern of the agricultural industry is increasing yields of seed components to enhance the value of these crops. During the past century, plant breeders have been quite successful in developing new cultivars of seed crop plants that give higher yields of seed components than older ones. Advances have been made not only with respect to increasing the content of starch, oil, and protein in the seed, but also, in some cases, in altering the quality of these components. For example, naturally occurring genetic variation has been exploited in the development of maize cultivars with modified starch content (Nelson, 1979; Shannon and Garwood, 1984) and increased or altered oil content (Alexander 1988, 1989; Weber, 1987). In a similar manner, the composition of soybean oil has been altered through selection to decrease the content of linolenic acid, resulting in a product with a more desirable flavor (Fehr, 1989; Hymowitz, 1987). The synthesis of carbohydrates and lipids involve complex metabolic processes, and although a great deal is known about these pathways, it remains to be determined how recent advances in genetic engineering technology will aid in the modification of these processes to enhance seed quality. For example, alteration of the activities of one or two enzymes in the starch biosynthetic pathway might allow for the accumulation of desirable intermediates of starch biosynthesis. Such a situation may, however, result in a serious imbalance in the overall carbon flow in the seed, which could affect factors such as grain yield and seed viability. Conventional selection and breeding strategies may, therefore, be more efficient than the use of genetic engineering in the development of crop cultivars possessing seed carbohydrates and lipids of desirable composition (Alexander, 1988). An important aspect of seed quality that is directly amenable to manipulation through genetic engineering involves the nutritional characteristics of seed proteins. In most cases, seed proteins of any one crop do not contain sufficient quantities of certain amino acids that are essential in the diet of humans and other monogastric animals (Nelson, 1969). Generally, cereals are most limiting in lysine and tryptophan, and legumes are deficient in the sulfur amino acids methionine and cysteine (Table 1). The economic advantages for increasing the nutritional quality of seed proteins are enormous: estimates of total protein production for each of two major seed crops in the United States, maize and soybeans, exceed 16 million tons annually (Wilson, 1987). Development of nutritionally balanced seed proteins in high-production crops like maize and soybean would benefit all areas of the agricultural industry concerned with grain production for human and livestock consumption. Plant breeders have had only limited success in improving the nutritional quality of seed proteins, primarily because genes encod-