Seed Protein Fractions of Maize, Sorghum, and Related Cereals

Abstract

More and more studies are being made on seeds as the final storage site for the products of photosynthesis and nitrogen metabolism. The cereals are especially noted for storing starch, yet cereal proteins make up a large proportion of the protein in the diets of many societies around the world. Maize is the highest yielding cereal, while sorghum and other more distant maize relatives are either high-yielding species or capable of relatively high yields in harsh environments. This may be related to their possession of the C4 photosynthetic system. Unfortunately, the major storage proteins in most cereals are prolamins which are low or lacking in the essential amino acids lysine and tryptophan. Table 1 shows that the average content of essential amino acids found in 379 species of plants is adequate, except for the sulfur amino acids, and that the non-zein proteins of maize endosperm are also adequate. Zein, however, possesses little or no lysine and tryptophan, less than adequate levels of threonine and valine, and has an imbalance in the leucine/isoleucine ratio which is also deleterious. Zein makes up half or more of the total endosperm protein in normal genotypes, thus producing the well-known amino acid deficiency in maize protein. Mutants have been found in maize, barley, and sorghum which contain elevated levels of lysine, or more precisely, lowered levels of prolamins. This has stimulated much research on the proteins of these species and the search has intensified for other mutants in these and other species with the desired properties of high-lysine, high-protein, and high-yield.