Principles of Plant Genetics and Breeding

Table 22.1 Essential amino acids that are low in selected
major world food crops.
Crop Deficient amino acid
Corn Tryptophan
Lysine
Wheat Lysine
Rye Tryptophan
Lysine
Rice (polished) Lysine
Threonine
Millet Lysine
Soybean Methionine
Cystine
Valine
Lima bean Methionine
Cystine
Peanut Lysine
Methionine
Cystine
Threonine
Pigeon pea Tryptophan
Common bean Tryptophan
Potato Methionine
Cystine
in peanut. Protein augmentation is a major breeding
objective in many major world crops.
Brief history of breeding for improved
nutritional quality of crops
Breeding for high protein content in crop plants is perhaps
the highest priority in improving the nutritional
quality of plants because about 70% of the protein supply
of human consumption is of plant origin. Further,
cereals are deficient in some essential amino acids and
low in total protein. Maize was one of the first crops on
which formal nutritional augmentation work was done.
In 1896, C. G. Hopkins initiated a project to breed for
high protein and oil content at the Illinois Agricultural
Experimental Station. Work by T. B. Osborne in the
early 1900s resulted in the fractionation and classification
of proteins according to solubility properties. He
and his colleague discovered zein (the prolamin or
alcohol soluble fraction) as comprising the bulk of the
protein of maize endosperm. Later work in the mid
1900s by K. J. Frey demonstrated that breeding for protein
augmentation primarily increased the zein content.
There was a need to find a way to enhance the useful
part of the protein. E. J. Mertz in 1964 discussed the
BREEDING COMPOSITIONAL TRAITS AND ADDED VALUE 405
nutritional effects of the opaque-2 gene in maize. The
mutant gene increased the lysine content, called high
lysine (discussed below). High lysine research has since
been conducted in sorghum. Another cereal food of
world importance is rice. However, it has significant
nutritional problems, being low in protein as well as
completely lacking vitamin A. Rice nutritional augmentation
was initiated in 1966 at the International Rice
Research Institute (IRRI) in the Philippines. The vitamin
A deficiency is being addressed using genetic engineering
(see below).
Breeding for improved protein content
The key components of food that impact nutrition are
carbohydrates, fats, proteins, minerals, water, vitamins,
and fiber. The first three components provide caloric
energy, while proteins, minerals, and water play a role in
body tissue and structure. The roles of regulation and
utilization are played by proteins, minerals, water, vitamins,
and fiber. After satisfying caloric energy needs,
proteins are the next most important nutritional component
of a diet. Twenty-two amino acids are generally
recognized in human nutrition, of which eight are
essential for monogastric animals (Table 22.2). The
utilization efficiency of the entire protein is diminished
if the diet is deficient in any of the essential amino acids.
Breeding high lysine content of grain
Breeders using conventional methods of ear-to-row
selection were able to increase the total protein content
of corn kernels from 10.9% to 26.6%. Unfortunately,
because the protein of corn is about 80% zein, and hence
nutritionally inadequate, the high increase in total protein
was nutritionally unprofitable to non-ruminant animals.
The zein fraction of the total protein is deficient
in lysine and tryptophan. This deficiency was corrected in
Table 22.2 Important amino acids in animal and human
nutrition; those in bold are essential in human adults.
Isoleucine Alanine Serine
Leucine Arginine Tryosine
Lysine Cysteine Asparagine
Methionine Glutamic acid Glutamine
Phenylalanine Glycine Cystine
Threonine Histidine Hydroxyglutamic acid
Tryptophan Proline Norleucine
Valine