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Genetic engineering - an overview2. GENETICALLY ENGINEERED FOOD
Food derived from genetically engineered crop plants or their products has already found its way to the market places of North America, Europe, China, Japan and Australia. Examples include oil produced from herbicide-resistant canola, tomatoes engineered to withstand longer postharvest storage, and squash, potatoes, cotton and maize resistant to insect pests or disease. Genetically engineered high-yield rice has been available in Asia for many years. The preferred method of transferring genes into plants involves Agrobacterium tumefaciens, a soil bacterium that can cause 'crown gall' in some plants. This bacterium is a 'natural genetic engineer' that infects plants by inserting a fragment of its own DNA into the plant cell. It was early recognised that, if the disease-causing DNA could be replaced with some other genetic material, the bacterium could be used to insert foreign DNA into plants. The approach is limited by the natural host range of Agrobacterium and has the drawback of not working well for some species. For example, it does not work well with cereal crops such as maize, but it has proved to be very successful in many cases. The Agrobacterium approach was used to cultivate a moth-resistant strain of a favourite New Zealand potato variety, the Ilam Hardy. This potato develops tubers close to the surface of the soil making it an especially easy target for potato tuber moths. Bacillus thuringiensis, or Bt, is a common soil bacterium that is well known for its insecticidal properties, especially against moths and butterflies. Researchers at HortResearch Institute in Auckland developed a Bt gene that was suitable for growing in Agrobacterium culture. They dipped leaves from potato plants obtained from a sterile tissue culture environment into an actively growing Agrobacterium culture that contained the cloned Bt moth resistance gene. The leaves were blotted dry and cultured on a medium supporting the growth of potato cells. Whole potato plants were regenerated over a few months from the cells containing the modified DNA. These plants, grown in a glasshouse, were fed to tuber moth larvae to test their resistance to insect. Promising lines of genetically modified plants were field-tested. Current research projects include developing virus-resistant strains of tamarillo. Most of the testing is done in greenhouses, but some (with fruit gathered before ripening and protected with netting from birds) is done in the field. Apples and kiwifruit are also being researched at the Institute. (b) Risks and benefits: asking the hard questions Food producers look to genetic engineering for a more abundant, cheaper and more nutritious food supply. Genetic improvement of crops via traditional breeding has been successfully practiced for decades to produce new cultivars with improved yields or increased resistance to pests . Such conventional breeding has, over the years, been greatly assisted by the transfer of genes from related wild species, but questions have been raised about the environmental and food safety aspects of 'transgenic' cultivars (i.e., organisms containing introduced, 'foreign' genes, particularly from unrelated species) prepared by GE techniques. On the face of it, genetic engineering provides a number of advantages over the traditional methods of breeding new strains of plants. These include the ability
There have been calls for the rigorous testing of foods engineered in this way. This produces its own problems, because specific tests can be difficult to achieve and interpret. The risks and benefits of these advances are being widely questioned. The process of genetically modifying food does not, in itself, pose a health risk. However, the genes being transferred and-more importantly-their products must be considered. But, because genetic products of most transferred genes are known, their safety and hazard potential can be evaluated. Current evidence suggests that eating genetically engineered food is no more dangerous than eating conventional food bred by traditional methods. However, that said, even food bred by traditional methods has sometimes been withdrawn because of high levels of toxins. With transgenic cultivars there are additional elements, such as secondary effects arising from expression of the new gene and genetic changes resulting from the random insertion of transgenes into the plant genome. In the worst cases, the products of artificially inserted genes or their products may be toxic or allergenic in themselves, or may, through interacting with host genes, trigger the production of such products by the host plant. It is true that, although safety assessment methods used in checking out new drugs are rigorous, serious side effects are sometimes missed by laboratory drug testing. Detecting the possible hazardous effects of a new GM food is probably more difficult than with a new drug. Introducing a gene from, say, brazil nuts into an unrelated food may trigger an allergic reaction in individuals allergic to brazil nuts. These people would have no reason to suspect the presence of such material in the non-related food they have just eaten. The careful and accurate labeling of GM foods is a basic issue in the safety debate. The situation is complicated by the fact that much of the so-called 'GM' food consists of a secondary product of a genetically-manipulated material-for example, honey from pollen gathered from a GM blossom or oil derived from a GM plant. The key to building consumer confidence is the provision of accessible information in the marketing of GM foods, giving the consumers choice. (d) Can genetically engineered plants get out of control and harm our environment? The short answer is yes, this could happen. Aware of the risk, scientists are looking at ways to minimise it by, for example, developing sterile pollen on GM plants. In Scotland and Denmark, the herbicide-resistant properties of a GM rape seed have crossed over into related wild species, conferring the same resistance. This does not mean that the wild cousins have become a 'superweed' that will resist all herbicides. They simply need to be sprayed with a different herbicide. While of concern to farmers, the prospect of 'superweeds' is not, in itself, a health risk. There is also no real risk that resistant plants will cause an ecological imbalance by, for example, wiping out a specific breed of pest or disease, causing a chain reaction and an imbalance in natural food chains. Farmers have released new, traditionally bred, cultivars of plants for a hundred years without such a catastrophe eventuating. The development of 'superpests', resistant to all currently known methods of control, has been put forward as a risk. The ecological 'ripple effects' that can potentially result from the expression of a gene targeting a particular pest or pathogen are difficult to predict and need to be studied. Research in this field is already going on, with a programme investigating the ecological effects of GMOs. In New Zealand before 1998, two committees, the Advisory Committee on Novel Genetic Techniques (ACNGT) and the Interim Assessment Group (IAG), oversaw the research, development and use of GMOs. These committees were superseded by the Environmental Risk Management Authority (ERMA), which began work on 1 July 1998. These organisations have applied rigorous standards of testing, containing and using GMOs in this country for many years. (e) How can genetic engineering help health and the environment? Genes conferring improvements in crop performance have been integrated into all major crops-maize, wheat, rice, soybean, potato, canola and cotton. The immediate transfer of new genes to elite lines in plant breeding programmes allows the efficient development of new lines without many generations of hybrids and selective breeding. The nature of the transferred DNA can also be controlled in a very precise manner. It is expected that pest and disease-free plants will allow less chemical pest control. This will reduce environmental hazards and make transgenic cultivars part of the integrated pest management system of the 21st century. While developing GM insect-resistant potatoes and cotton, researchers found that less insecticide was needed. This encouraged the return of many useful invertebrates, which helped control other minor pests and diseases. Developing herbicide-resistant crops generally involves genes conferring resistance to herbicides such as glyphosate (Roundup), sulphonyl ureas, and glufosinate ammonium. These are all more environmentally friendly than many other commonly used herbicides. This new technology may do away with some of the stronger agricultural chemicals which contaminate the environment for long periods. Many new generation herbicides are used at substantially lower doses, are much less toxic to animals, and disappear faster from the environment. (f) What are the economic aspects of GE food? The value of the global market in GM crops was estimated to be between two and three billion US dollars in 2000 and expected to double by 2005. However, critics have proposed that it would be to New Zealand's economic advantage to retaining its current GM-free farming methods - that growing any GM food will tarnish the country's image. They suggest that keeping New Zealand GM-free is equivalent to the anti-nuclear stance the country took some years back and would reinforce the market of ecotourism. This opportunity is essentially unique to New Zealand because of its geographical isolation. The recent Royal Commission on Genetic Manipulation recommended that the current practice whereby ERMA continues to deal with applications for field trials and general release on a case-by-case basis. However, no general release of a GMO in New Zealand has yet been approved and the Government has ruled that ERMA should not be permitted to consider any application for release until late 2002, to give time for further regulatory and consulting procedures to be put in place. There is no way of knowing where this technology is headed, but genetic engineering is creating the same sort of revolution as that precipitated by the silicon chip at the dawn of the cyber age. The genie is out of the bottle and there is really no prospect of it being shut down or contained again. The keys to understanding and adapting to this new technology are research and communication. Unless one takes the stance that GMOs should be banned outright, the future of GE food in this country comes down to two factors: clear labeling of foods and public education about the science behind the labels. Foods, including ingredients from GE crops or those produced by GE microbes, are assessed for safety, but there is not yet any ongoing monitoring. Food Standards Australia New Zealand is responsible for assessing the safety of foods, including those with ingredients from GE crops or produced by GE microbes. The authority has largely relied on tests carried out overseas, mostly by the U.S. Food and Drug Administration. Increasing public concern has questioned whether it is appropriate for New Zealand to follow overseas guidelines on GE foods. New Zealand law now requires shops selling foods with GE ingredients to be labelled - though some food applications are exempt. An ESR laboratory in Christchurch recently received the first accreditation in Australasia to test foods for such GE ingredients. |
METHODS FOR TRANSFERRING GENES FROM ONE SPECIES TO ANOTHER Bacteria provide the tools for most GE experiments. Many of these simple microbes contain 'restriction enzymes' or ‘chemical scissors’ that can cut DNA into short lengths. Many bacteria also contain small circles of DNA, called ‘plasmids’, that they pass on to their progeny when they divide (plants and animals lack these plasmids). Laboratory scientists used restriction enzymes to cut off pieces of DNA from a ‘foreign’ cell. They then spliced those pieces of DNA into plasmids and returned them to their host bacteria. They did this in the hope that the plasmid plus foreign DNA would be transferred from the bacterial cell to its progeny. An antibiotic resistance gene was also spliced into each plasmid. This provided an easy tool to weed out all bacteria that did not contain a plasmid for, if the broth in which the bacteria are growing is spiked with antibiotic, only cells with the plasmid—and thus the foreign gene—will be able to grow. This GE process had two important outcomes that have revolutionised our understanding of genes and how they work. (i) The process allowed scientists to prepare unlimited quantities of rare DNA fragments for further study (e.g. by direct sequence analysis or for use as 'probes'). Once enough bacterial cells containing the plasmid had been grown, it was a simple matter to reverse the process that was used to put the foreign DNA in the cell in the first place and isolate and purify the, now much greater, amount of the original DNA fragment involved. This allowed biologists to study the chemical detail of fragments of larger DNA molecules. GE led to methods for isolating and identifying whole genes from an organism. It also led to new methods for reading off the billions of DNA letters (sequencing the nucleotides) in plants, animals and people over recent years. (ii) Under appropriate conditions, the gene could often be expressed. The protein corresponding to it would be synthesised in the bacterial cells and could be purified from them by standard methods. In this way, the protein products of rare gene sequences could also be prepared in quantity. In the last quarter of the twentieth century, these two applications of GE were used to elucidate many of the details of the ways in which living cells control replication and genetic expression. We now know that, in addition to coding directly for the structures of proteins, parts of the DNA code act to control regions that decide when and where specific genes will be 'turned on' and the genetic information expressed. Specific controlling sequences must be included in any plasmid that is to be used for the successful expression of foreign genetic material in a cell. Furthermore, these studies showed that there is a significant difference between the ways in which bacterial genes and genes from plants and animal are constructed and their genetic information processed. These differences must be taken into account if plant or animal genes are to be successfully put to work in bacterial cells. |
Although the first successful uses of GE
were in making purified proteins for use in human health, it was not long before
the technology was used to transfer genes from one species to another in order
to alter food crops. In this way, the plants or animals could be altered to
improve their nutritive value, production, processing, storage properties and
resistance to pests and disease. Commercial examples include the use of genes
from cold-water fish to make strawberries resist the cold and to produce 'super
carrots', richer in carotene (vitamin A). In Australia, genetically engineered
pork rich in protein has been produced and bumper sized salmon are under development
in Canada. 
