Biotechnology of Grain Quality Traits
Alan L. Kriz
The value of grain crops accrues from the types of compounds that normally accumulate during the important stage of seed development commonly referred to as grain-filling. During this period, compounds such as protein, starch, and oil accumulate to high levels as part of the natural seed growth process. Over the past century, plant breeders have developed new varieties of seed crop plants that allow for increased production of grain components. Advances have been made not only with respect to increasing the starch, oil, and protein content in the seed but, in some cases, to the quality of these components as well. For example, naturally occurring genetic variation has allowed maize varieties to develop with modified starch content and increased or altered oil content.
Alan Kriz examines laboratory corn plants.
An important aspect of grain quality that is being studied at the University of Illinois Department of Agronomy involves the nutritional characteristics of seed proteins. Seed proteins of any given crop usually do not contain sufficient quantities of certain amino acids that are essential in the diet of humans and other monogastric animals. Because cereals are usually deficient in lysine and tryptophan, and legumes are deficient in the sulfur amino acids methionine and cystine, a mixture of cereals and legumes is used to provide a balance of amino acids in the diet.
Nutritionists have specified those aspects of grain protein quality that are most desirable, but to date the development of corn or soybeans containing a satisfactory balance of amino acids has not been possible. Plant breeders have had only limited success in improving the nutritional quality of seed proteins, primarily because genes encoding the seed storage proteins with high levels of essential amino acids do not normally exist in any given species. Modifying the genes that encode seed proteins by genetic engineering may therefore be an ideal solution to the problem of nutritional quality.
A general approach to enhancing grain nutritional quality involves altering the proteins in the grain that normally contain the bulk of the amino acids. Specific genes from plants are readily isolated. Using routine laboratory techniques, the DNA sequence, or program, of those genes is then altered to create a new gene sequence that encodes a protein of desirable nutritional quality. For example, over a few months, we have been able to increase the lysine content of a specific corn grain protein by 50 percent, and the tryptophan content of that same protein by 700 percent. The extent to which the seed tolerates such altered genes remains to be seen.
However straightforward the genetic engineering approach, certain problems must be overcome to apply successfully this technology to the particular problem of improving seed protein quality. Recombinant DNA clones are routinely isolated that correspond to genes that encode specific proteins. These clones may be easily manipulated so that they contain appropriate instructions for desired amino acids. The major limitation in implementing this technology now is the lack of efficient genetic transformation systems for corn and soybeans. But recent advances in this area indicate that it will soon be possible to use biotechnology to genetically manipulate grain protein quality in these major crop species.
Alan L. Kriz, assistant professor of crop molecular genetics, Department of Agronomy
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