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  Section: Molecular Biology of Plant Pathways » Genetic Engineering of Seed Storage Proteins
 
 
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Summary and Future Prospects

 
     
 
In 2001, more than 65% of soybean acres and more than 20% of corn acres in the United States were planted with GM varieties (Lusk and Sullivan, 2002), indicating that, at least in the United States, crop biotechnology has largely been accepted at the farm level. This is partly due to the fact that most US cultivation of corn and soybean is for livestock feed, so the issue of consumer acceptance has not been a problem. Furthermore, the cost and labor savings resulting from reduced pesticide or herbicide use made possible by transgenic traits is directly realized by the farmer. Improving grain nutritional quality can reduce costs for the livestock farmer and will become more important as the practice of lowering the amount of protein in livestock rations to reduce nitrogen levels in manure becomes more widely adopted (Johnson et al., 2001). Corn with improved nutritional characteristics can lower the costs for the livestock producer by reducing feed supplements, assuming that the modified grain is available at a competitive price (Johnson et al., 2001). Feed cost savings resulting from a variety of possible nutritional modifications to corn seed have been estimated (Johnson et al., 2001). For example, lysine is the first limiting amino acid in pigs receiving corn– soybean meal diets. If the lysine level in corn were to be doubled, it was calculated that feed cost savings would range from $4.65 to $6.89 per ton in 2001 (Johnson et al., 2001).

Considering all that has been learned about storage protein structure and gene expression, it is somewhat surprising that there are currently no GM seed storage protein products on the market. However, the development of such crops to the point where they are commercially viable is a long and expensive process. Success depends on the product providing significant value relative to its cost, and this must be carefully projected before embarking on product development. Consideration must be given to questions such as whether the cost of creating and managing a high-methionine maize feedstock that does not require amino acid supplementation would allow the grain to be grown, marketed, and distributed at a competitive price. This chapter has described preliminary research using an array of ingenious approaches for improving protein quality by genetic engineering, and in many cases, limitations to transgene expression remain to be resolved. A few types of storage proteins make up the bulk of seed proteins, and their amino acid compositions determine the protein quality of the seed. In order to improve essential amino acid balances, the transgenic proteins must be accumulated at very high levels. Even using strong, seed-specific promoters, proteins encoded by low copy number transgenes generally accumulate to less than 5% of the total seed protein, and this is usually insufficient to produce the required improvements in protein quality. In cases such as BNA expression, where high-level transgenic protein accumulation was achieved, this often resulted in changes of endogenous proteins, so that the gain in protein quality was less significant than expected.

The use of genetic engineering for the modification of grain processing characteristics in crops, such as wheat, may ultimately be useful. Presently, transgenic research is providing an increased understanding of the roles of various HMWGSs in gluten properties. However, given the complex nature and incomplete understanding of HMW-GS interactions, identifying modifications that will have value will require more research.

One promising application of GM technology in the near term is in the reduction or removal of antinutritional components and allergens from seeds. Perhaps the most time-consuming step here is determining the identity and epitopic composition of allergenic proteins. Food hypersensitivity in children and adults is the most common type of allergy (Chandra, 2002). Furthermore, it is increasing in prevalence (Maleki and Hurlburt, 2002) and the list of foods known to elicit allergic reactions is growing. In the future it will be possible to modify allergenic domains of essential endogenous proteins or remove them completely using gene silencing. Indeed, this technique can be used to downregulate entire gene families encoding allergenic proteins. The availability of genomic, transcriptomic, and proteomic data for crops such as rice, corn, and soybean should help in identifying these proteins and the gene families that encode them.

Early research on the genetic modification of storage proteins in crop plants was initiated in the absence of knowledge of many technical constraints, such as limitations to sulfur amino acid availability. Also influencing the consummation of this research are the contentious issues of consumer perception and acceptance of GM crops. To date, the most successful GM traits in crop plants, herbicide and insect resistance, allow decreased introduction of chemicals into the environment. Some people consider these traits to have benefited the producer more than the consumer. Although the potential grain nutritional improvements described here provide the most direct benefits to the livestock producer, they would reduce food costs and improve protein nutrition for people who consume the grain directly. Unfortunately, there are limited research resources in the developing countries where the immediate benefits of grain nutritional improvements for human consumption could be realized. At present, there is little incentive for biotechnology companies to invest heavily in the development of products for primary use in developing countries, despite the humanitarian value.

Some consumers remain skeptical about GM products due to negative perceptions of the agricultural biotechnology industry and perceived environmental or personal risks. However, consumers are benefiting from the environmental effects of reduced chemical use and the more cost-effective production of commodities. The development of products with improved nutritional value, enhanced taste and appearance, and increased shelf life will surely increase consumer appreciation of the value of GM crops.

In the past, information regarding the benefits of GM technology has not been effectively communicated to the general public. In a study, it was found that consumers reading about the benefits of GM soybeans were significantly more comfortable eating them than those reading about GM soybeans with no explanation of their benefit (Brown and Ping, 2003). However, the groups did not differ in their desire for labeling foods made with these soybeans (Brown and Ping, 2003). In the United States, most consumers are not aware of the extent that GM foods have entered the marketplace. In the United Kingdom, all products containing GM ingredients must be labeled as such, but in most cases this has discouraged consumers from buying them. For example, GM tomato products were sold by several UK supermarket chains in the nineties but were withdrawn due to poor sales following anti-GMO campaigns. Information regarding the nature of the transgenic modification and its potential for flavor improvement was not readily available to the consumer. Perhaps another reason for consumer skepticism, especially in Europe, is that while GM crops are frequently cited as a vital component in sustaining the growing human population, past research is perceived to have been shrouded in secrecy and the products thought to benefit only the large agricultural biotechnology companies. It is thus becoming increasingly clear that the scientific community must place a priority on educating the public about the immediate and future benefits as well as the safety of GM crops, if their potentials are to be realized.
 
     
 
 
     



     
 
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