Science Articles

Genetically Modified (GM) Plants

Wed, 21st Sep 2005

What's it all about ?

Mark Tester

DNA fragmentGenetic modification (GM) is the heritable alteration of the genetic make-up of an organism. As a natural process, genetic modification (GM) is as old as genes themselves, and has been utilised by human beings since the beginnings of agriculture. Recently, the term has come to apply specifically to newly developed DNA technologies, where the genome of an organism is modified using artificial techniques. These rely on the ability to cut DNA precisely, isolate desired fragments and insert them into a single cell of another organism. From this transformed cell a new multicellular organism can be regenerated. There is a wide range of applications of the new GM technology, from employing micro-organisms to synthesise recombinant human insulin or growth hormone, to making crops resistant to pest and diseases. However, it has also attracted much opposition. It has been criticised for being unnatural, for posing an unassessable risk to the environment and to human health, and for providing an instrument for the manipulation of human genetic make-up that might invite serious abuse.

What is genetic modification? Genetic modification (GM) is the modification of genetic make-up such that the modification is passed on to the organism's descendants. Strictly, it is a general term that covers many processes - some of these are new, some have been occurring since life began, and some have been strategically used for 10,000 years, since agriculture began at the end of the last ice age. However, the term 'genetic modification' has more recently come to be used for the process of 'genetic engineering', where newly developed processes of molecular biotechnology are employed to insert relatively few genes into an organism's genome. Other terms that have been used to describe this technology include 'recombinant DNA technology' and 'genetic manipulation'. In this article, GM using the new technologies is distinguished from GM using traditional techniques by referring to the former as 'new GM'.

Methods of GM (Genetic modification) : There are at least three traditional methods of genetic modification: a) Selecting for variability within existing populations arising from genetic recombinations resulting from sexual reproduction. It is noteworthy that most modern crops have been so altered using this technique that it is difficult to identify their wild progenitors. Variability in dog breeds provides another example of the results of this type of GM. b) Crossing closely related species. For example, modern bread wheat has arisen from two sequential crossings of, in total, three species. c) Isolating mutants. For example, herbicide resistant oilseed rape has been developed from plants that appeared spontaneously in Canadian fields.

In addition to these traditional approaches, there is new GM, involving the modification of specific genes in single cells using recently developed biotechnologies. Essentially, this process involves: 1) The identification and isolation of a linear polymer of nucleotides that will either direct the synthesis of a protein with desired characteristics (a gene), or alter the level of protein synthesis directed by another gene. The process of isolation of specific nucleotide sequences requires in particular the action of bacterial enzymes that cut nucleotide polymers in precise locations identified by their nucleotide sequence. These are called restriction enzymes, the first of which was isolated in 1970. (More recently, another technique for the rapid in vitro replication of DNA sequences, termed the polymerase chain reaction or PCR, has become an increasingly central tool to aid the isolation of desired DNA fragments.) 2) The movement of the desired gene(s) into another organism. Genes are usually moved into other organisms by exploiting natural pathogens whose mode of infection involves the injection into the host of genetic material. Herbert Boyer and Stanley Cohen performed the first successful gene transfer in 1973. 3) This gene transfer occurs into a single cell. When genetically modifying plants or animals, an entire organism must then be regenerated from this single cell. Thus, transfer into single-celled embryos is technically desirable, as these are programmed for growth into a multicellular organism. Plants, however, have the useful trait of totipotency, whereby cells from the adult plant have the ability to regenerate into new adults - thus a range of cell types can be used for gene manipulations. The recent cloning of mammals from adult cells, most famously 'Dolly' the sheep, opens the door for this technology being transferred to mammals.

Traditional techniques for gene modification limited modifications to those occurring between closely related organisms. New GM can be used for similar types of gene modifications, but it also enables the transfer of genes between any two organisms. Thus, although new GM enables the addition to an organism's genome of just one gene, with a specific trait (unlike traditional breeding programmes, where thousands of genes are transferred at a time), the one gene could come from any organism, or even be created de novo in the laboratory. Overall, new GM tends to bring in fewer genes, but potentially from 'further away', than old GM.

Applications of new GM The first major product of new GM was developed in 1982, for the production of human insulin by bacteria for the treatment of diabetes. In 1990, the first GM food product, an enzyme employed in cheese making, was approved for use in the USA. In 1994 the first food product was sold commercially, the so-called FlavrSavr tomato, that had reduced activity of a gene essential for ripening. The development of GM animals with disrupted gene function is providing numerous insights into the molecular basis of disease, and there is the distinct possibility of modifying pigs to provide organs for human transplants.

Recent commercial applications of new GM include the introduction of herbicide tolerance into crops such as soya bean and oilseed rape, and the ability to synthesise insecticidal proteins in cotton and maize. Many other applications of new GM are being developed, including conferral of the ability to make antibodies in fruits and the ability to decontaminate polluted land by degrading organic pollution. New GM also provides opportunities to alter the composition of food to increase its nutritive value, such as increasing the mineral and vitamin content of grain (e.g. 'golden rice'). Increases in food production are also possible, by improving overall plant qualities (e.g. dwarfing rice) and by increasing tolerance to biotic stresses (pests and diseases) and abiotic stresses (e.g. low temperature, drought or salinity).

Problems of application GM has been subject to various types of objection. It has been pronounced unnatural, held to pose an unjustifiable risk to the environment and to human health, and felt to bring us unacceptably close to being able to manipulate human genetic make-up. Only the last of these is specific to GM, and then not to new GM - concern about eugenic policies antedates modern biotechnology.

The charge of being unnatural has been levelled at a host of targets. Those who bring it in this case still have to explain what is specifically unnatural about new GM that it does not share with many long accepted procedures.

More serious is the contention that new GM threatens unintended, undesirable and perhaps also unforeseeable environmental and medical consequences. It brings the risk of the escape of organisms, or at least their genes, into wild populations. For example, the spread of insecticidal proteins into wild plants could confer a competitive advantage on those plants, disrupting semi-natural systems. Likewise, effects on insect populations could be significant. Although having herbicide-resistant crops will decrease herbicide use, it will increase the effectiveness of applications, reducing weed densities and thus continuing a decline of wildlife that has been going on since agriculture began.

Exposure of human populations to large amounts of novel proteins that have never previously been in the human food chain could cause unpredictable problems. In particular, allergenicity could cause problems that would be difficult to detect, as symptoms can take a long time to develop.

Issues involving new GM and animals raise a further range of ethical questions. They also suggest the potential for the transfer of GM technology to humans and its use as an instrument for manipulation of human genetics - giving rise to the fear of its use for objectionable ends, or of damage to human life as an unforeseen consequence of well-meaning actions. The thought here may be that there are limits to the powers with which human beings are good enough, or clever enough, to be trusted.

These issues, however, concern only particular applications of new GM, not the technology per se. Since it has so many applications, generalisations about the whole technology are difficult. Most features of most applications of new GM are not profoundly different from the processes that have been performed on plants and animals for the past 10,000 years, so objections to new GM need to be carefully formulated to address its unique features.

Problems of public acceptance Recent concerns with genetically modified (GM) plants have not arisen in a significant way with GM bacteria and GM yeast, despite the widespread commercial application of new GM techniques to these organisms for many years. Diabetics have used insulin from a synthetic, human-like gene inserted into bacteria and yeast, for well over two decades, with no loud public protest. Another example of widespread application of new GM is vegetarian cheese - people have been consuming genetically engineered rennet protein for many years.

However, current traits conferred on crops reduce application of insecticides and herbicides, and although benefiting producers offer no evident benefits to consumers. For those who experience no benefit but sense a possible risk, the natural reaction is one of rejection. Furthermore, allowing the patenting of the technological processes of GM (in contrast to plant variety rights that protect the output of traditional breeding) places new GM in a different position to traditional GM, a situation related to the fact that this second 'green revolution' is privately funded, in contrast to the publicly funded first Green Revolution. This comes in the context of a general distrust of science, fed by the historically recent misuse of science (e.g. for the development of bio-weapons) and mistakes by scientists (e.g. BSE).


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