Alterations in the Biosynthesis of Lysine, Threonine, and Methionine by Tissue Culture Approaches in Maize

Abstract

Monogastric animals, including humans, must have several essential amino acids such as lysine provided in their diet. Consequently, the nutritional value of protein in most edible crops has been determined and, for some crops, concerted efforts have been expended to reduce deficiencies in specific amino acids found in the grain. In maize, most commercially grown varieties contain about 8 to 11% protein on a kernel dry weight basis. In a long-term selection program, maize protein concentration was increased significantly to about 27%, but an accompanying reduction in kernel weight resulted in only a slight increase in amount of protein per kernel (Dudley 1974). The high-protein line had a higher proportion of protein in the low-quality, prolamin (zein) fraction of endosperm proteins, indicating that selection for extreme increases in protein concentration most likely would not improve the nutritional quality of maize. The nutritional quality of maize protein is poor because the zein fraction, which contributes about half of the total protein, is seriously deficient in lysine and tryptophan when maize is used for monogastric animal feed or for human food (also see S. Bajaj et al. Chap. VIL 1, this Vol.).