Plant Proteins: Engineering for Improved Quality

Abstract

Plant protein is the primary source of all dietary protein for humans and livestock. In 1992, world protein production was 137 million metric tons, of which 65% was obtained from plants directly and 35% through animal protein production.' In comparison to meat protein, plant protein is more economic to produce and convenient to transport and store. However, when used as a source of dietary protein for human and monogastric animals, most plant proteins are nutritionally incomplete because of their deficiency in certain essential amino acids. In general, cereal proteins are low in lysine (Lys) and tryptophan (Trp), and legume and most vegetable proteins are deficient in the sulfur amino acids methionine (Met) and cysteine (CYS).~ Amino acid deficiencies can be corrected in at least three ways, namely, supplementation with synthetic amino acids, supplementation with small amounts of protein which are a rich source of the deficient amino acid(s), or complementation with other protein(s) so that the combined protein sources will balance each other's deficiencies or excesses. The American animal feed industry annually spends 150 and 70 million dollars on synthetic Met and Lys, respectively, to supplement the protein base feeds. Conventional plant breeding methods have made some progress in identifying mutations with improved seed protein quality, such as high-Lys corn and barley.3 However, the undesirable traits associated with these mutations, such as lower yields and greater susceptibility to pests and diseases, have prevented their agronomic uti l i~ation.~ Because of a lack of genetic variability in Met content, efforts to breed legumes with increased levels of Met have not had significant success. Recent advances in biotechnology offer alternative approaches to improving the nutritional quality of plant proteins. These include sequence modifications, synthetic genes, overexpression of homologous genes, and transfer and expression of heterologous genes. Progress in this area of research and development was reviewed r e ~ e n t l y . ~ , ~