Genetically Modified Plants And Selective Breeding Biology Essay

Published: Last Edited:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

The words "genetically modified" constitute a serious misnomer. Humans have bred plants and animals for thousands of years. In selective breeding, we have built into animals and plants gene combinations that are not normally found in nature and that probably would not survive without human intervention. The term "genetically modified" is currently applied to plants and animals that results from adding genes, in particular genes from completely unrelated organisms, to preexisting plants or animals. Genetically modified plants are commonly grown today in the United States. Most of the corn and soyabean-and conala derived products sold in this country are the result of plants engineered with recombinant genes. The most common genetic modifications are those that confer resistance to certain insect pests and those conferring resistance to certain herbicides. Let us first see how genetic modification of plants is achieved.

What follows is a brief potted account of techniques of genetic engineering of plant varieties and their degree of commercialization. This is penned, not from the standpoint of trying to satisfy an introduction to microbiology, but to give some idea of the technical background that is relevant to understanding environmental debates about GM crops.

The first step is to extract the piece of DNA that has the desired characteristic. The two most widely utilized (in commercial terms) genetically 'transplanted' characteristics have been herbicide tolerance and insect resistance. (Anderson, L, 1999)

Herbicide tolerance means that the GM crop can be sprayed with a broad range herbicide that will kill most types of weeds. The main types of this are ammonium glyphosate, marketed as 'roundup' by Monsanto and ammonium gluphosinate used by Bayer (which incorporated Aventis). Both are regarded by US and European regulators as being reasonably benign compared to other herbicides, although, of course, for fans of organic food, there is no such thing as a benign herbicide. Many types of herbicide tolerant crop are available including corn (maize), soya, canola (oil seed rape) and sugar beet. Herbicide tolerant soya has been the most successful, being grown by the USA and Argentina, the first and third biggest global soya producers. Brazil, the second biggest, is still non-GM as are other important soya producers such as China and India. Herbicide tolerant canola (oil seed rape) is grown in the USA and Canada. (Anderson, L, 1999)

GM insect resistant crops exude a toxin in their pollen that kills insects which would otherwise eat the crop. The insect resistant crops are actually referred to by the name of the bacteria, bacillus thuringiensis. The most popular Bt crop is cotton, grown in the USA, South Africa, India, China and Australia. Its relatively rapid spread to these countries must be explained partly by the fact that it is not a food crop. This makes the product much less controversial than GM food crops. Bt versions of food crops such as corn (maize) and potato are also available and are widely grown in the USA and Argentina. (Anderson, L, 1999)

Despite the take-up of GM food crops in the USA, Canada and Argentina, the expected spread of GM food technology has not (yet) occurred. People often mistake reports of 'trials' for commercialization. In China, for example, there have been a large number of trials of a number of GM crops (Huang et al. 2002:675), but, apart from the case of Bt cotton, the trials do not seem to have resulted in widespread commercialization. Trials do not automatically lead to commercial planting since GM plants which are the subject of trials may not be suitable for commercialization, they may not be licensed, and they may simply not be grown (as in Europe) because there is no market for their products.

Both herbicide tolerant and insect resistant characteristics are associated with bacteria which can be found in the soil. The required DNA sections are extracted from the chromosomal string of these bacteria by so-called 'restriction enzymes', which are used by micro-organisms to resist attack from viruses. The enzymes act as biochemical 'scissors'. Having separated the vital genes these genes are then spliced onto a 'plasmid'. Plasmids are self-contained bundles of genetic information which supplement the principal chromosomal bundles held in bacteria, and the desired characteristic is added to the DNA chain in the plasmid by the help of 'ligase enzymes'. These enzymes are normally used in organisms to repair accidental breaks in DNA strands. The assembly of these plasmids are key events, for the plasmids are the agents which carry the required genetic information for implantation into the target plant cells.

At this point it is necessary to mark the bundles of DNA material which are being transferred to the plant in such a way that you can isolate the plant cells that contain the desired new DNA recombination (hence the term 'recombinant DNA'). Herein lies a particularly acute controversy because it was for a long time the usual practice, because of the criteria of sheer convenience, for antibiotic resistant marker genes to be spliced into the plasmids containing the genes being transferred. Genes which confer antibiotic resistance are especially convenient for the purpose of marking the plasmids that contain the DNA that is to be transferred because it is a very simple procedure to isolate the plant cells containing the recombinant by dousing the cell culture with that antibiotic. The cells that do not carry the antibiotic resistant genes die. This appeared to the GM pioneers, to be an elegant way of isolating the cells containing the recombinant DNA. Unfortunately many years later people started to worry that if antibiotic resistant genes were carried into the guts of animals or humans then the DNA might pass into the genes of pathogens. This might promote the proliferation of diseases that were resistant to those antibiotics. (Pusztai, A, 2002)

The practice of using antibiotic resistant marker genes in GM plants became a major item of controversy when it involved an antibiotic that was still in use, that is ampicillin, that was used in a variety of Bt corn. In fact other types of marker gene are available today, including genes which allow the cells which have combined with the desired genetic characteristics to grow in the presence of mannose, a type of sugar. In this case the cells that grow are taken for further processing and the unrecombined genes are discarded.

Before this happens the plasmids containing the genes with the new characteristics and the marker genes have to be transferred to the target plant cells. There are different ways of doing this. One way, suitable for 'broad leaved plants (such as sugar beet, soybean and oilseed rape) involves a bacterium agrobacterium tumifaciens which is used to transfer the DNA' (Mayer 2000:97). This bacteria causes crown gall normally, and it is thus quite effective at penetrating plant cells. This is good news for genetic engineers who can place the specially prepared plasmid in an agrobacterium tumifaciens bacterium that has been neutered to stop it spreading and it will easily infect the required plant with itself, and with it, the required genes. The cells which have been recombined with the new DNA will be separated from the rest and they can then be grown into full sized plants. Then seed can be produced and the product can be tested and, much later, marketed.

Of course a lot of crops are not susceptible to infection by this bacterium, and these include rice and maize. Instead, genetic engineers take recourse to firing gold particles, coated with the plasmids, directly into the cells. This is, as with using crown gall bacteria, a hit and miss affair, but the recombinant genes can be isolated by utilizing the properties of the marker genes.

Judging from current trials going on in the USA, future developments are likely to occur in four directions. First, development of more crop strains that have advantages for farmers such as crop varieties which are resistant to various types of diseases or which are suitable for currently inhospitable conditions like high salinity, low rainfall or excessive soil aluminium. Biotechnologists point to the adoption of virus resistant GM technology which they say has saved the papaya industry in Hawaii. Second, crops which deliver 'quality' attributes to the consumer like the high betacarotine, Vitamin A inducing, (and much talked up) 'golden' rice being developed in India. Third there are 'plant based pharmaceutical crops' (or biopharming) that can be used to grow medical products like blood thinners, clotting agents, anti-arthritis, contraceptive products or even anti-cancer drugs. Fourth are animals modified either to deliver larger amounts of product, like the large salmon that are already being considered for commercial approval in the USA or animals that have been genetically modified to produce drugs for humans. For example, the widely used enzyme-drug trypsin can be produced from genetically engineered animals.

Certainly the downbeat assessments made by many 'establishment' as well as radical environmentalists in Europe provide a stark contrast to the rallying cries for the cause of biotechnology made by the scientific elite in the USA. As a keynote paper at a National Academy of Sciences colloquim put it: 'Widespread adaptation of biotech-derived products of agriculture should lay the foundation for transformation of our society from a production-driven system to a quality and utility-enhanced system' (Kishaw and Shewmaker 1999:5968).

Theoretically the biotechnologists will be on relatively safe ground with products where there is no fear about the consequences of 'genetic pollution'. For example, will there be any tears if the trait giving higher levels of beta-carotene spreads to other types of rice? Perhaps not, but many anti-GM groups are still opposed to this innovation arguing that as yet unknown genetic modifications will enter ecosystems.

However, as far as future development is concerned, the biggest problem faced by agricultural biotechnology is consumer resistance. Unfortunately for the biotechnology industry, European consumers may prove not to be idiosyncratic in their skepticism about GM food. In India, where the dominant Hindu religion favors vegetarianism and where many are worried about animal genes being spliced onto vegetables, sale of GM food is still, at the time of writing, actually illegal.

Biotechnologists have long held hopes that China would prove more receptive to GM food technology, but commercialization of GM food technology seems to have stalled. Increasingly farmers around the world are becoming unwilling to grow GM crops lest their produce becomes unsaleable in a rising tide of consumer resistance to GM food. In short, the USA may win some sort of victory at the WTO, but the key battle is in the food markets, and here it is heading for defeat.