Genetic Modification of Food

I

Introduction

Genetic Modification of Food, the alteration of the genome of plants grown for food in order to produce crops with specific advantages such as improved yield or resistance to herbicides and insect pests. We have been modifying the genetic material of crops since the dawn of agriculture, by selection of seeds from better yielding plants, then later by deliberate cross-pollination to select desirable characteristics. The difference with modern genetic modification (recombinant DNA technology) is that we can now introduce specific genes, for a defined purpose, not only from another strain of the same plant, but from a totally different species (see Genetic Engineering); genetically modified (GM) organisms are sometimes known as transgenic. The gene donor may be another plant, an animal, or a micro-organism. This raises potential problems of both the safety of consuming food from the modified crop, and possible environmental hazards. There are also ethical questions to be considered and wider political and economic issues related to the control of modified crops and seeds by powerful corporations, and their control over small farmers, particularly in developing countries. As more information on the function of plant genes becomes available, so more precise genetic modification to introduce desirable characteristics will be possible. The complete genome of Arabidopsis thaliana (thale cress) was elucidated in December 2000, providing a model for dicotyledonous plants (see Dicots). Rice is the model for monocotyledonous plants (see Monocots); it has the smallest genome of the major grain crops, and provides the dietary staple of half the world's population. The rice genome sequence was completed in 2001.

Most GM crops are grown in Argentina, Canada, China, and the United States; GM cotton is also grown in Australia and South Africa. Some 45 per cent of all soya beans grown worldwide, and almost 70 per cent of those grown in the US, are GM varieties. Similarly, 20 per cent of cotton grown worldwide (and 70 per cent of that grown in the US) is genetically modified to be resistant to the boll weevil, and about 10 per cent of maize worldwide (and 23 per cent of that in the US) in genetically modified. Within the European Union (EU) there is little commercial cultivation of GM crops, pending the results of trials to evaluate environmental hazards.

The first GM crop to be grown commercially was the FlavrSavr® tomato, which was introduced in 1994. It was not a commercial success because, despite having improved keeping qualities, and hence a longer shelf-life, it gave poor yields, and has been withdrawn from commercial cultivation.

II

Advantages of Genetic Modification of Crops

There are potential advantages of genetically modified crops for three groups of people:

(i) Growers, who will benefit from the resistance of crops to insect pests, viruses, and fungi, by the introduction of natural insecticides or fungicides from other species, so reducing the need for application of agricultural chemicals, hence also an environmental gain; resistance to herbicides, so that a field of the crop can be sprayed with a weedkiller without damaging the crop, so reducing the amount of work required; increased yields or specific tolerance to cold, salinity, or drought, so permitting crops to be grown in regions that at present have low agricultural productivity. (ii) Food processors and manufacturers, who will benefit from produce with a longer shelf-life, and better properties for processing and manufacture. (iii) Consumers, who will have cheaper and more plentiful food as a result of the advantages to growers and processors, as well as possibly better flavour, colour, and texture in the food, and possibly also increased nutritional value or other health benefits. GM varieties of canola (oil-seed rape) have been designed to modify the proportions of different polyunsaturated fatty acids, and so improve the nutritional quality of the oil.

To date the major GM crops in commercial cultivation are maize, soya bean, and canola, which are resistant to specific herbicides, so permitting easier weed control, and maize, cotton, and potatoes, which express the insecticidal protein from Bacillus thuringiensis (hence they are known as known as Bt varieties), so minimizing losses from insect damage.

Concerns have been raised about the business ethics that may drive the application of these powerful technologies, especially since the products concerned are often staple foods. Many people are unhappy about a herbicide-resistant crop being developed and marketed by a company that also manufactures the herbicide, raising the suspicion that the main beneficiary will be the company concerned rather than the consumer.

Equally, many people are extremely unhappy about the development of the so-called “terminator gene”, which causes seeds from genetically modified crops to be sterile. This would force farmers to buy fresh seed from the supplier each year, instead of keeping a proportion of the harvest for the next season’s seed. The major plant biotechnology companies have (at least at present) undertaken not to pursue further development of terminator genes, although a number have been planted.

Despite widespread food shortage and impending famine in southern Africa in 2002-2003, a number of countries refused to accept GM maize as food aid, because of fears about both the environmental impact and the safety for people whose diet is very largely based on maize, and who therefore consume relatively large amounts. Some countries were willing to accept flour milled from GM maize, but not intact grain.

III

Marker Genes

Once the gene for the desired characteristic has been identified and isolated, it has to be introduced into the plant that is to be modified, together with a control (promoter) region that will ensure the new gene is expressed in the plant. There are three ways in which the new DNA containing a gene can be introduced into plant tissue in culture:

(i) by inserting the new DNA into a virus that will infect the host plant and insert its DNA (and that introduced in the laboratory) into the plant’s chromosomal DNA.

(ii) by inserting the DNA into a plasmid (a small circular piece of DNA) that can be introduced into the cells of the plant. A plasmid is not inserted into the chromosomal DNA of the host, but remains in the cytoplasm, where it replicates, so being propagated as the cells divide.

(iii) by a “shotgun” technique, in which DNA is coated on to minute particles of colloidal gold that are then fired into the target cells. Some of the particles will hit DNA, causing breaks that are repaired by the plant’s DNA repair mechanism. In the process of repair, the new DNA is incorporated into the plant’s chromosomes.

These techniques for introduction of DNA into cells are far from perfect, and there has to be a simple way of detecting those cells in which the gene transplant has been successful, so that these can be grown up, and those in which the procedure has failed can be discarded. If the introduced gene confers a property such as herbicide resistance, then it is easy to eliminate those cells in which the procedure has failed. All that is needed is to culture the plant tissue in the presence of the herbicide; only cells in which the gene has been introduced successfully will grow.

In other cases a marker gene is introduced together with the desired gene. This is commonly a gene conferring antibiotic resistance, so that growth in the presence of the antibiotic will eliminate those cells in which the procedure has failed. This means that a gene for antibiotic resistance is introduced into a new organism, raising fears that disease-causing organisms may acquire resistance to clinically useful antibiotics (see Environmental Hazards of Genetically Modified Crops below). Increasingly, other marker genes, such as those conferring the ability to grow on unusual sugars, are being used. In this case, selection of those cells in which the gene has been introduced successfully is by culturing the cells on a medium providing only the unusual sugar, so as to eliminate those that cannot use this substrate.

IV

Safety and Hazards of Genetically Modified Crops

The introduced gene itself does not present any health hazard, since chemically all DNA is essentially the same, and will be hydrolysed to its constituent nucleic acid bases in the gut. Although some GM plants contain viral DNA, raising the fear that humans might be infected by the virus used, this is extremely unlikely, since viruses are highly specific for the hosts they will infect, and the viruses used to introduce novel genes into plants are, of necessity, plant viruses.

The product of the introduced gene, that is the protein coded for by the novel gene, may be harmful. The main concern is about allergic reactions and toxicity. Where the introduced gene has been derived from a known source of allergens, then there is an obvious need for intensive testing of the novel food in people who are known to be allergic to the donor organism. Indeed, it has been argued that known sources of food allergens should not be used as gene donors at all. In most cases an evaluation of the likely hazard can be made by comparing the chemistry of the novel protein with known allergens—for example, whether it is resistant to digestion (as are most food allergens), whether it has a similar structure to known food allergens, and so on. To date there is no evidence from public health authorities (most notably the Center for Disease Control in the US, where GM foods are widely consumed) that there is any greater incidence of allergic reactions to GM foods than to conventional varieties. However, there is no information on possible allergic reactions to inhaled pollen from GM plants, which would cause increased incidence of hay fever, nor indeed from inhaled dust from GM crops, which might affect workers in the milling and food processing industries.

The effects of the introduced gene on metabolic processes in the plant itself is another concern. This may lead to increased production of toxins, reduced nutritional value, and so on. This problem is not restricted to genetically modified organisms, but may also arise as a result of conventional selective breeding. For example, a novel variety of celery that was resistant to wilting after harvest was found to be the cause of skin rashes in people handling it in packing sheds. The new variety contained increased amounts of a natural product, a psoralen, which both confers resistance to wilting and also causes skin irritation.

Transfer of an antibiotic resistance marker gene from genetically modified crops to intestinal bacteria is a potential worry, but is unlikely to occur. The DNA containing the gene for antibiotic resistance would have to survive breakdown in the gut, then enter a bacterium, and fuse with its DNA. Even if a few bacteria did acquire a gene for antibiotic resistance, there would have to be some kind of selective pressure, such as chronic consumption of the antibiotic, for the small number of resistant organisms to multiply and become a significant population. Nevertheless, it remains possible that such a gene could be transferred to bacteria in the wild, so the antibiotics chosen should not be those that are important in treating disease in humans or farm animals. However, it is noteworthy that the main cause of spreading antibiotic resistance among disease-causing organisms is the widespread indiscriminate use of antibiotics in treating infections for which they are inappropriate (for example, viral infections) and as growth promoters in farm animals.

United Nations expert committees and national and international authorities have proposed clear guidelines for testing for possible hazards, while noting the considerable potential advantages in terms of increasing world food supplies to feed the ever-growing population of the world. In response to consumers’ concerns about genetically modified crops, a number of major food retailers have announced that they will not include any in their own-brand products. In advance of legal requirement many instituted a policy of voluntary labelling of foods containing ingredients from genetically modified organisms, so as to permit consumers to make a conscious choice. EU legislation introduced in 2003 requires that all foods made from or containing more than 0.9 per cent GM material must be so labelled; this includes highly refined oils from GM plants, even though they contain neither DNA nor protein. Foods made using (as opposed to containing) GM organisms do not have to be labelled as such—this includes, for example, cheese made using “vegetarian rennet”, an enzyme derived from GM bacteria that does not remain in the cheese. There remains the problem that GM and conventional crops may be mixed before coming to market, so that it may not be possible to ensure that foods are indeed produced only from conventional crops.