Storage Proteins in Cereals

The seed protein of cereals comprises many different proteins that are synthesized and accumulated in the endosperm during seed development. The total amount of protein produced depends heavily on the availability of nitrogen. High-yielding cereals are generally very effective in their utilization of the available nitrogen for protein production, and the prospect of genetic improvements in this respect seems limited. Traditionally, the protein of the cereal endosperm is classified into albumin, globulin, prolamin, and glutelin. The most typical and best-known storage protein is found in the prolamin fraction, which is present in high or medium amounts in maize, sorghum, wheat, and barley, whereas the prolamin content is low in rice and oats. Prolamin is deposited in protein bodies in the starchy endosperm, and it has a low content of the amino acids that are essential for man and nonruminant animals. It has been shown in maize, sorghum, and barley that prolamin synthesis is controlled by a limited number of genes and that an inactivation of these genes is not lethal for the plant. Genotypes with reduced prolamin synthesis have a substantially increased concentration of lysine and other essential amino acids in the seed protein, because these amino acids are much more abundant in the other endosperm proteins. The low-prolamin genotypes, normally termed high-lysine types, offer great possibilities for improving the nutritional value of cereal protein. However, the somewhat reduced grain yield of most high-lysine types indicates that an inactivation of the prolamin synthesis impairs the accumulation of carbohydrates in the endosperm.