|Content of Genetic Engineering of Seed Storage Proteins
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.