Why do we need to improve crops?

The so-called 'traditional' methods of crop improvement are limited by the sexual compatibility of the plants involved. The most significant advance in crop improvement to arise from genetic modification is that, theoretically, a characteristic from any organism, of any species, can be introduced into a plant. Plant breeders therefore have access to a much wider gene pool than they have using crossing methods. For example, one of the genes introduced into plants to provide resistance to the herbicide RoundUp™ was originally isolated from bacteria. An insecticidal toxin used as a crop spray was also first identified in bacteria. Genetically modified cotton and maize expressing this protein is currently being grown in a number of countries around the world, including South Africa (see http://www.isaaa.org/Resources/Publications/briefs/35/executivesummary/pdf/Brief%2035%20-%20Executive%20Summary%20-%20English.pdf).

Another major difference between 'old' and 'new' methods of crop improvement is the number of genes transferred to the offspring in each case. On average, plants contain approximately 80 000 genes which recombine during sexual hybridisation. The offspring may therefore inherit around 1 000 new genes as a result of this recombination. This is equivalent to a 0.0125% change in the genome. By contrast, only one or two new genes are transferred during plant transformation. As this represents a 0.0025% change in the genetic information of the plant, it is argued that plant transformation provides a more precise approach to crop improvement than sexual hybridisation.

It has been suggested that genetic modification could cause harmful toxins to be made by transformed plants. However, where this has been reported, it is unclear whether it is due to the technique itself, or to the nature of the foreign gene. For example, the introduction of a gene that is known to encode a toxin in one organism will almost certainly have a similar effect when introduced into a different organism. It was, therefore, not surprising that the highly publicised transgenic soybean containing a gene from Brazil nuts also elicited an allergic reaction among sensitive patients. The gene from Brazil nut had been well characterised and its product known to be allergenic, hence the extensive laboratory tests. This illustrates why rigorous characterisation of a gene is required before permitting its introduction into a novel species.

It is also important to look at potentially toxic products that have arisen in plants as a result of 'traditional' methods of crop improvement. For example, potato varieties with increased pest resistance have continually been selected as giving higher crop yields. These varieties contain high levels of natural pesticides, called glucoalkaloids. However, these compounds are toxic to animals, so could have harmful effects when eaten. This demonstrates that the nature of the novel feature should be open to question, rather than the method by which it is introduced.

Practically speaking, one big difference between genetic modification and traditional plant breeding is that, while extensive restrictions are in place to limit the development and release of genetically modified varieties, those developed by sexual hybridisation and mutagenesis are under no restrictions.

A major concern surrounding the cultivation of genetically modified crops is the possibility of cross pollination between transgenic and related crops. While this is clearly possible for some species, it is important to remember that not all crop species have native wild relatives with which they are sexually compatible, meaning that the generation of 'superweeds' may not be as likely as is often suggested. Also, many plants, such as carrots, are only allowed to flower for seed production, which means that of cross-pollination during normal commercial cultivation procedures is unlikely.

Many ethical objections have been raised to plant transformation. One of the most common is that it is unnatural, and those that carry it out are 'playing God'. However, some supporters argue that it is the intent to make changes, rather than the method by which they are brought about, that is the real subject of the ethical debate. From this point of view, humans have been manipulating the 'natural' world for millennia.

Other concerns are based on the potential problems that could arise from introducing genes that are 'foreign' (i.e. from other species) into plants. Critics argue that these genes could not have become naturally incorporated into the plant genome, and could therefore be potentially hazardous. However, what makes one species a 'foreigner' to another? The DNA from all organisms on the planet is chemically identical, it is the sequence of the genetic information that differs. Even so, there are often very close similarities between the gene sequences of totally unrelated species from different kingdoms. It might therefore be argued that all organisms descend from a common ancestor and, as such, are distantly related. This idea is particularly important when considering ethical or religious objections about the introduction of genes from certain animals into plants.