Published online 31 January 2005
Published in Crop Sci 45:435-436 (2005)
© 2005 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
GENETIC AND METABOLIC ENGINEERING FOR VALUE-ADDED TRAITS
Schuyler S. Korban*
Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, IL 61801
* Corresponding author (korban{at}uiuc.edu)
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SYNOPSIS
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THE FIELD of molecular biology has provided scientists with powerful tools for isolating, cloning, and characterizing myriad genes controlling useful traits in all living organisms. In turn, molecular analysis of structural genes and their functional regulation and expression often lead to crucial knowledge of components of metabolic pathways of various economic traits. This ever-exploding information of fundamental genomic biology in combination with the tools of genetic engineering continues to have significant impact on genetic advances and genetic improvements of agricultural crops. Fundamental knowledge of traits inherent in these crops and in various other living organisms now allows for genetic and metabolic engineering of crops for enhanced tolerance to biotic and/or abiotic stress, amelioration of nutritional quality, and enrichment with novel traits.
There are numerous examples of genetic enhancement of crops for biotic stress, i.e., insect and disease resistance. More recently, significant advances in our knowledge of abiotic stress have been made. Among abiotic stresses, salinity is one of the major problems that adversely affect crop productivity and quality. About 20% of irrigated agricultural land is adversely affected by salinity. The problem of soil salinity is further aggravated because of the use of poor quality water for irrigation and poor drainage. Hence, engineering crops with resistance to salinity stress is critical for sustaining food production practices and achieving future food security. Understanding the molecular basis of salt stress signaling and tolerance mechanisms is essential for pursuing genetic engineering efforts for salt tolerance in crops. Therefore, Dr. Jian-Kang Zhu and his team provide a review of the molecular basis of cellular ion homeostasis, osmotic homeostasis, stress damage control and repair under salt stress, and how all this information has been used in genetic engineering of salt tolerant crops.
Agricultural crops are important sources of food and nutrition for human and animal populations. Diverse food crops are important dietary sources of proteins, vitamins, minerals, lipids, and fiber. For humans, plant foods are important sources of folates. Strong epidemiological evidence has indicated that folate intake is suboptimal for most of the world's human population, even in many developed countries. Low folate intake contributes to predisposition to several major diseases such as cancer, cardiovascular disease, and neural tube defects. Although efforts to fortify folate, in the form of synthetic folic acid pill supplements, have effectively decreased the incidence of folate deficiency; however, this approach is difficult to implement in developing countries. An alternative low-cost effective approach is to enhance folate content in crops with the tools of genetic engineering. Therefore, understanding the folate synthesis pathway and its unique transport and catabolism are critical to any genetic engineering efforts to enhance folate content in plants. This field of research is reviewed herein by Dr. Andrew Hanson and his research team.
Other components of food crops that are critical for human diets and animal rations are proteins. In some crops, even those high in protein content, modifying the nutritional composition of particular amino acids in a crop can enhance the nutritional value of these crops. For example, in soybean seeds, both methionine and cysteine contents are rather low, thus limiting the nutritional value of soybean. Efforts to increase the levels of these two essential amino acids are underway by focusing on the sulfur assimilatory pathway in soybeans as adequate supplies of sulfur amino acids in developing seeds may facilitate accumulation of sulfur-rich proteins to a level sufficient to meet the nutritional requirements of livestock and poultry. This work is being pursued in Dr. Hari Krishnan's USDA laboratory at the University of Missouri-Columbia.
Just as efforts are underway to enhance levels of particular nutritional components in food crops, the tools of genetic engineering are also being used to remove antimetabolic compounds or elicitors of food allergies in some crops. For example, soybeans are known to contain various antimetabolic compounds such as phytin, trypsin inhibitors, and oligosacchrides as well as proteins that elicit allergenic responses leading to adverse gastric responses in some segments of the human and animal populations. By using a gene silencing strategy, the accumulation of target immunogenic allergens in transgenic soybeans can be inhibited. This approach has been successfully used in Dr. Eliot Herman's laboratory.
One of the exciting areas of genetic engineering is the introduction of genes that code for immunogenic proteins and/or therapeutic proteins used in the pharmaceutical industry. Essentially, these engineered plants serve as "bioreactors" for production of desired recombinant proteins and subunit vaccines. In recent years, development of plant-based vaccines directed at human and animal diseases has opened up a new and exciting opportunity for added value of agricultural crops, thus increasing the uses and profitability of these agricultural crops. Because of their relative ease of genetic manipulation and rapid growth, genetically engineered bacteria and yeast are the most widely used large-scale production systems for recombinant proteins. However, recombinant proteins over-expressed in these organisms require extensive purification to remove host protein and other compounds prior to use in the health industry. This adds to the cost of using recombinant proteins. Transgenic plants provide an economically superior alternative for scaling-up production for recombinant proteins, simply by planting more acreage of these plants. Interestingly, not only are these plants useful for production of recombinant proteins, but for some crops, e.g., fruit, leaf vegetables, or grains, they can also serve as delivery systems of these high-value proteins to human and animal populations. Thus far, this technology has resulted in a $40 billion industry of new therapeutics and industrial enzymes, and promises to grow much larger. An industrial perspective of the potential of this area of genetic engineering is provided by Dr. John Howard.
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ACKNOWLEDGMENTS
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All symposium papers have been reviewed and edited by Shawn Kaeppler (University of Wisconsin, Madison) and Schuyler Korban. A great thank you is extended to Henry Nguyen (University of Missouri-Columbia), former Chair of C-7 Division, for giving me the opportunity to organize this symposium, and for CSSA for providing funds to support this symposium.
Received for publication December 3, 2003.
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