Seed storage proteins: structures and biosynthesis

The plant seed is not only an organ of propagation and dispersal but also the major plant tissue harvested by humankind.The amount of protein present in seeds varies from -10% (in cereals) to -40% (in certain legumes and oilseeds) of the dry weight, forming a major source of dietary protein. Although the vast majority of the individual proteins present in mature seeds have either metabolic or structural roles, a11 seeds also contain one or more groups of proteins that are present in high amounts and that serve to provide a store of amino acids for use during germination and seedling growth. These storage proteins are of particular importance because they determine not only the total protein content of the seed but also its quality for various end uses. For example, the low content of lysine, threonine, and tryptophan in various cereal seeds and of cysteine and methionine in legume seeds is due to the low proportions of these amino acids in the major storage proteins and may limit the nutritional quality of the seeds for monogastric animals. In the case of wheat, the storage proteins form the gluten fraction, whose properties are largely responsible for the ability to use wheat flour to make bread, other baked goods, and pasta. These properties are not shared by the storage proteins of other cereals. In this article, we provide a broad overview of the structures and properties of the seed storage proteins of the major crop plants, emphasizing their biological roles, their evolutionary origins, and their modes of synthesis and deposition. Although some storage proteins may also play roles in defense or metabolism, we focus on those that function solely for storage.