The following points highlight the three important characteristics of genetically modified crops (GMC). The characteristics are: 1. Increased Nutrition 2. Allergens Modification 3. Improved Functional Properties.
Characteristic # 1. Increased Nutrition:
Using bioengineering, scientists have added or modified nutrients in various crops, and created several nutritionally enhanced products. Although few have reached commercialization, examples include adding iron to rice, or increasing beta-carotene and vitamin E in vegetable oils to boost the nutritional value.
Other genetic modifications have altered the fatty acid composition in oils from soy and canola to create healthier fats.
Plants have also been engineered to increase phytonutrients-substances exclusive of nutrients that have benefits for improving health or preventing disease. These include iso-flavones in soy and lycopene in tomatoes. Two genetic modification strategies have also been devised to increase the iron levels in cereal crops. One is the introduction of the gene that encodes for ferritin, an iron- storage protein.
Over expression of this gene improves the storage capacity of plants by as much as three-fold. Using this and other genetic technologies, rice was engineered to contain beta-carotene, which it normally lacks, and enhanced iron content. This transgenic “golden rice” has yet to be bred into hybrid and native strains, so field testing of modified local varieties, commercial production and acceptance are still years away.
Another method for enhancing iron is reducing phytic acid content, which improves the degree and rate at which iron and other minerals are absorbed. In one experiment, corn genetically modified to be low in phytic acid was processed into tortillas. The iron absorption from these tortillas was 49 per cent greater than from tortillas made with conventional corn.
To further explore the effectiveness of iron absorption by reducing phytic acid, additional iron was added in the form of iron salt supplements and consumed with either strain of corn fed as porridge instead of tortillas. In this case, no absorption effect was observed.
Although it is not clear why the phytic acid level had no effect, it is well known that when dietary iron levels increase, absorption decreases. Other substances in the diet may also have contributed to the reduced absorption.
While plants are the primary dietary source of vitamin E, they contain relatively low concentrations of the vitamin. Recent genetic engineering technology has been able to increase the vitamin E content of oils.
As it happens, many seeds have abundant levels-up to 20-fold more of gamma-tocopherol, the immediate precursor of alpha-tocopherol, the active form of the vitamin. However, little of the gamma form is converted to the active vitamin.
Researchers identified, isolated and cloned the gene responsible for expressing the enzyme that converts gamma-tocopherol to alpha-tocopherol. The gene was transferred to Arabidopsis, which subsequently exhibited a nine-fold increase in vitamin E.
Incorporation of this gene to stimulate similar gamma- tocopherol to alpha-tocopherol conversion into soy, canola and corn is probably not far in the future. Seed oils-particularly mustard and canola-have also been developed to contain carotenoids, especially beta-carotene, a nutrient widely studied for its role in cancer prevention. But this project is still in the testing stage.
Protein (or rather specific amounts of essential amino acids, the building blocks of protein) is needed to fulfil human nutritional requirements for growth, health maintenance and muscle development.
In regions of the world where cereal grains cannot be grown, people often rely upon starchy vegetables (roots, tubers or rhizomes) to supply most of their calories. While such crops often have high yields, the primary disadvantage is their very low protein content, less than one per cent.
Researchers are seeking to improve protein content and quality in vegetable staples such as cassava and plantain through changes in amino acid profiles. For example, a non-allergenic seed albumin gene was introduced into the potato to increase its protein content. Transgenic tubers had 35 to 45 per cent more protein and enhanced levels of essential amino acids.
Moreover, transgenic plants produced more tubers and a yield increase of 3 to 3.5 per cent. Scientists have also altered soybeans for higher protein in tofu. In an attempt to create healthier fats, researchers have modified the fatty acid composition of soy and canola in several ways.
They have produced oils from soy and canola with reduced or zero levels of saturates; canola with medium chain fatty acids; high stearate canola oil free of trans-fatty acids; high oleic acid soybean oil, and canola with the long chain fatty acids gamma linolenic and stearidonic acid.
The latter is of interest as an indirect source of docosahexaenoic acid (DHA), one of two long chain Omega-3 fatty acids shown to be beneficial in protecting against heart attack. DHA is available almost exclusively from seafood, primarily fatty fish. The plant precursor of DHA, linolenic acid, is poorly converted to DHA.
Transgenic high oleic acid soybean oil has 80 per cent more oleic acid, one-third less saturated fatty acid than olive oil, and no trans-fatty acids. Researchers have also modified sunflower oil for high oleic acid content.
Another type of modified soybean oil is low in saturated fatty acids and richer in linoleic acid than commodity soybean oil. Still another has reduced linolenic acid and no trans-fatty acids, increasing its stability for use as an ingredient in processed foods.
Another seed unique for its high level of a single fatty acid is mangosteen. This tropical tree, grown in India, the East Indies and Southeast Asia produces seeds with as much as 56 per cent by weight of stearic acid, a saturated fatty acid widespread in foods.
Stearic acid is noteworthy from a nutritional perspective for its stability and textural properties and because it is one of the few saturated fatty acids that does not appear to raise blood cholesterol levels. Thus, it is useful in fats for manufactured and processed foods.
Enzymes cloned from mangosteen have also been expressed in canola with resulting increased levels of stearic acid. This research demonstrates the potential of the technology and the unusual sources of enzymes to alter fatty acid profiles in popular food oils such as canola.
Plant biotechnology has also aimed at increasing phytonutrients-substances in plants-exclusive of nutrients that have benefits for improving health or preventing disease. For example, new research in nutrition suggests lutein may support multiple lines of defence against eye disease, and that lycopene serves as a powerful antioxidant in cancer prevention.
Also called “accessory health factors” phytonutrients include iso-flavones in soy, lycopene in tomatoes and polyphenols in green tea. In the laboratory, scientists have engineered tomatoes with 2.5 times as much lycopene as traditional tomatoes. At least one company is developing soy with more iso-flavones, and canola with increased antioxidants and beta-carotenes, lutein and lycopene.
There are major constraints on this research, in part because there is still much about phytonutrients that is unknown. For example, some members of a class of phytonutrients may have deleterious effects while others are beneficial, as is the case with various flavonoids, water- soluble plant pigments that, while not considered essential, helps maintain overall health as anti-inflammatory, antihistaminic and antiviral agents.
In addition, scientists do not fully understand the biosynthetic pathways, or the succession of enzyme activities, for many phytonutrients. Another constraint is the limited scientific information about the safety and efficacy of potentially beneficial phytonutrients.
Some plants, especially cereals and legumes, are nutritious foods and feeds but also contain varying amounts of substances that interfere with digestibility and nutrient absorption. In excess, these materials may even be toxic.
Genetic modifications are being explored to reduce these anti-nutritional substances, including phytate in cereals and legumes; glycoalkaloids such as solanine and chaconine in potatoes; tomatine, solanine, lectins and oxalate in tomatoes and eggplant; gossypol in cottonseed; trypsin and other protease inhibitors in soy, and tannins and raffinose in legumes.
Phytate is widely distributed in cereals and legumes and reduces the absorption of iron, zinc, phosphorus and other minerals in humans and other animals. Phytate is indigestible for swine and poultry because their digestive tracts lack the enzyme phytase, which releases phosphorus from phytate.
Studies have shown that including phytase in the food ration improves phosphorus absorption and reduces phosphorus excretion. In the food animal industry, particularly for swine and poultry, high phytate feeds are associated with high levels of phosphorus excretion. Excess phosphorus in animal manures can be washed into streams or leach into ground water and become a serious source of water pollution.
Research has indicated that poultry have substantially reduced phosphorus excretion when fed phytase as a supplement alongside ordinary soybeans or alternatively, genetically transformed soybeans expressing the phytase enzyme. Similarly, swine fed low-phytate corn showed increased phosphorus retention and reduced excretion.
Genetically modified low-phytate corn contains at least five times as much available phosphorus as unmodified corn. Low-phytate corn feed was also associated with improved growth and finishing characteristics.
In wheat engineered to expresses the enzyme phytase, seeds exhibited a two to four-fold increase in phytase activity. This opens the possibility of improving the digestibility of wheat, especially among non-ruminant animals.
Scientists are also seeking ways to reduce toxic substances such as glycoalkaloids. Researchers inserted antisense genes into potatoes to block the activity of the enzyme UDP-glucose glucosyl transferase, key to the production of the glycoalkaloid alpha-chaconine.
This toxic substance can, at high enough levels, cause irritation of the gastrointestinal tract or impairment of the nervous system. Preliminary findings indicated that the transgenic potatoes produced fewer glycoalkaloids.
Characteristic # 2. Allergens Modification:
Food allergies and sensitivities cause a wide variety of conditions, symptoms and diseases, a few of which can be life threatening. A food allergy or hypersensitivity is one that provokes an immune response, while a food intolerance incites an abnormal physiological reaction.
Experts estimate that 2 per cent of adults, and from 2 to 8 per cent of children, are truly allergic to certain foods. Food intolerance is a much more common problem than allergy.
Unlike allergies, intolerances generally intensify with age. The eight most commonly allergenic foods are milk, eggs, peanuts, soybeans, fish, crustaceans, tree nuts and wheat. There are also significant allergies to non-food plants, such as ryegrass and other plants with airborne pollens that may cause hay fever or other seasonal allergic symptoms.
Most known allergens in food are proteins, suggesting the possibility of modifying the structure, or possibly eliminating the allergenic protein from the food. In some cases, traditional plant breeding has identified hypoallergenic strains that are targets for further genetic modification to reduce allergenicity.
Neutralizing the allergens in major food grains would have an enormous impact on millions of families, where one or more members cannot eat these foods that are household staples.
Researchers have used this approach in rice, the first food crop with reduced allergenicity to be created through genetic engineering. Further testing and development work continues to assure that people with known allergies to rice products can consume this genetically engineered food with-out developing their typical allergic reaction.
In foods such as peanuts, however, which are highly allergenic to some sensitive individuals, the allergenic proteins constitute the majority of the plant’s protein, so that elimination may not be possible.
Another example where genetic modification may be used to reduce allergenicity is in wheat, one of the “big eight” allergenic foods. Although not yet commercially available, scientists have genetically engineered wheat to overexpress the gene responsible for the synthesis of thioredoxin, an enzyme that catalyses the reduction of disulfide bonds within protein molecules, thus reducing the protein’s allergenic properties.
When expressed in wheat, the enzyme reduced the bonds in the major allergenic proteins the gliadens and glutenins and to a lesser extent the minor ones, too, making them markedly less allergenic. At the same time, the functional characteristics of the wheat were not impaired.
Scientists are also exploring the potential of recombinant DNA technology to reduce the allergenicity of non-food allergens. For example, ryegrass is a dominant source of airborne pollen in temperate climates, and using antisense technology, scientists engineered ryegrass with reduced Lol p 5 protein levels.
As this is the major allergen in ryegrass, the modification reduced the plant’s allergenicity. Although genetic engineering has the potential to reduce allergenicity of foods, it also has the potential for unintentionally introducing new allergens.
Characteristic # 3. Improved Functional Properties:
Researchers are in search of enhanced functional properties for specific purposes, such as firmer tomatoes for canning, or beans with less breakage. One of the first applications to reach the market was the highly publicised Flavr Savr tomato, which was genetically engineered for delayed ripening.
While the transformation process did delay ripening and extend shelf life, the product was expensive to produce and purchase and some consumers did not like the taste. This led to its withdrawal from the market. Other work aims to create a tomato that ripens on the vine but remains firm during harvest, handling and shipping.
Firm tomatoes are preferable for canning, which consumes the largest share of tomato production. Using antisense technology, researchers have created tomatoes that are 40 percent firmer than their conventional counterparts and stay firm for at least two weeks.
Scientists have also engineered beans for desirable canning characteristics such as firm texture and seed coats that do not split. Several experiments are being conducted with soybeans. One would diminish that undesirable byproduct of bean consumption, flatulence, by creating high sucrose soybeans through reduction of the carbohydrate raffinose.
Another seeks to modify soybean oil to reduce the linoleic acid content so that it is more stable for industrial applications. Presently, barley is an unsuitable feed for poultry because poultry lack the enzyme to break down β-glucan, the predominant polysaccharide (a type of carbohydrate) in endosperm cell wails. Scientists have created transgenic malt that can depolymerise β-glucan.
Adding transgenic malt to barley-based poultry feed enabled poultry to metabolize barely, grow as well as poultry fed a corn-soybean diet, and produce more hygienic droppings. The digestibility of feeds can also be improved with modification of starch levels in different crops. For example, cattle can more readily digest amylose- free wheat in feed.
Extensive research has been directed toward altering the properties, quantity and distribution of starch in many plants, for a variety of purposes. The principal forms of starch are either linear (amylose) or branched polymers amylopectin.
Using genetic engineering technology to influence the amount and length of chain branching and polymerisation increases the availability of starches with different properties. It also enables the development of novel starches.
However, plants differ widely in where they store different types of starch; thus modification of starch production must be tailored to the particular plant. What works in the potato, for example, may not work in wheat or rice. Moreover, results in one variety of a crop may not be obtained in another.
A well-known example of the modulation of starch synthesis has been the development of transgenic potatoes engineered to contain a gene for an enzyme affecting starch synthesis. The transgenic potatoes had up to 60 per cent more starch than non-engineered strains. The increased starch content made the potatoes take up less fat during frying, resulting in a lower-fat product.
About 40 per cent of tapioca starch is used for the production of modified starch, sweeteners and the flavour enhancer monosodium glutamate. In processing tapioca, a significant amount of starch remains in the waste material and wastewater. It is estimated that even after extraction, the waste still contains 50 per cent starch.
Bioengineered improvements in tapioca, such as reduction in water content and higher starch concentration, may increase the ease of processing the plant material into a finished starch product.
Further, raising the efficiency of starch utilisation in the processing of sweeteners reduces the amount of starch reaching the waste stream. The possibility of converting the starch content of wastewater to energy, using high rate anaerobic digestion, is promising.
However, a number of factors remain to be overcome, including the effect of environmental sulfates in the waste stream and the efficiency of energy production. The use of transgenic organisms offers potential solutions. When the enzyme thioredoxin is overexpressed in barley endosperm, the activity of the enzyme pullulanase, a rate-limiting enzyme in breaking down starch, increases four-fold.
Breaking down starch is a key part of the barley malting process, and tests with this engineered variety showed that the time required could be reduced by up to a day. Over expression of thioredoxin also hastened barley germination, of special interest to growers of this normally slow-germinating grain.