The Nature of Seeds
Seeds provide a mechanism by which many types of plants propagate, and they are an important food source for many animals, including humans. The seed contains a dormant embryo and a mixture of stored metabolites (protein, starch, and lipid) that support its germination and prephotosynthetic growth. The storage proteins are a source of nitrogen and sulfur for the synthesis of new enzymes in the germinating seedling, while the starch and lipid initially provide the energy and carbon skeletons for making a variety of organic molecules. In angiosperms, which include most seed crops of agricultural importance, these storage compounds are deposited in one or more specialized tissues in the seed: the endosperm (especially in the cereals), the cotyledons of the embryo (particularly in legumes), or more rarely, the maternal perisperm tissue, as in the case of beet (Bewley and Black, 1995).
Metabolites Stored in Seeds and Their Uses
The storage proteins, carbohydrates, and lipids of particular seed crops have unique chemistries that are responsible for the physical and functional characteristics of the foods created from them. For example, the storage proteins in wheat, corn, and soybeans are responsible for the bread-making (Shewry et al., 2003a), tortilla-making (Hamaker and Larkins, 2000), and tofu-making (Saio et al., 1969) characteristics of their respective flours. The structure of starch, which can be altered by various mutations, allows creation of candies, sauces, or puddings with unique gelling characteristics (Orthoefer, 1987). The high contents of monounsaturated fatty acids found in olives, nuts, and rape seeds (Canola) produce the healthiest types of cooking oils (Taubes, 2001).
The nature of storage proteins, starches, and oils in seeds is subject to genetic variation and through selection, plant breeders have been able to create varieties of crop plants with unique compositions of these compounds that make them suitable for particular uses. However, there are limits to the natural qualitative and quantitative variation of these molecules, and this places restrictions on what breeders can accomplish with conventional methods of crop improvement. Furthermore, domestication and breeding of wild species for use as seed crops occurred through selective pressure for a limited number of traits, most notably improved yield. In some cases, this led to selection for one particular attribute at the expense of others. For example, the sulfur amino acid content of modern domestic corn appears to be much lower than that of its wild ancestors (Swarup et al., 1995). Conventional plant breeding is sometimes analogized to working in a ‘‘black box’’ because it is possible to monitor only a limited number of traits during this process.
With the advent of plant genetic engineering technology, it became possible to consider novel ways of altering and enhancing seed storage metabolites. Indeed, biotechnology is currently being used to modify a number of crop traits, including the nature of the protein, starch, and lipid in seeds. In this chapter, we consider research that is being done to improve the nutritional quality and functional characteristics of seed storage proteins. Before describing this research and its potential in detail, we first provide some background information regarding the nature of seed storage proteins, how they are synthesized in seeds, and how they influence the nutritional value and the functional properties of our food and livestock feed.
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