IntroductionGenetic modification has the potential to improve both agriculture and health. Scientists suggest that because genetic mutations can cause diseases, genetic modifications may be able to reverse them. Yet genetic modifications are controversial. Critics suggest that genetic modification in agriculture increases toxins, decreases nutrient value and poses great risks to people and plants. They also suggest that part of the problem with genetically modified organisms is that organisms are altered in ways that do not occur naturally. Yet some methods of genetic modification are more natural than others.
Agrobacterium, for instance, evokes natural genetic modification in sweet potatoes. Scientists have mimicked this approach to modify organisms to give them more desirable traits. This method of genetic modification is certainly more natural than the other primary method of gene transfers through plants – which involves the use of a “gene gun”, which fires gold particles, which bear new DNA into the cells of plants, into the nucleus of plant cells. Nevertheless, despite the fact that agrobacterium mediated genetic transfers are more organic, and, in that sense, more natural, critics suggest that transfers of foreign DNA from one plant to the next are unsafe and unhealthy. This study explores whether “natural” genetic modification through agrobacterium mediation offers significant advantages over other forms of modification or the status quo.
Advantages over the Status Quo
Certain conditions may modification desirable. Among these are: drought, pestilence, fuel shortages, vaccine production and hunger. Modification can make crops drought and pest resistant. It can boost the size and number of crops grown to combat hunger and bio-fuel shortages. Vaccines can be introduced to bananas to combat diarrhea. Each of these benefits offers an advantage over the status quo.
Possible Disadvantages versus the Status Quo
Critics of genetic modification make several arguments that apply to plant transfers mediated by agrobacterium. First, critics argue that because genetically modified crops may contain more mutations than non-modified alternatives, the re-arrangements which come about may create crops which lack nutrient value or include higher levels of toxins or allergens. Genetically modified crops currently must go through rigorous testing which should prevent toxic crops from landing on shelves; however, this testing can be expensive and can make genetically modified crop production more costly. Some critics voice concern that because genetically modified crops often contain traits that lead to antibiotic resistance, antibiotic resistant genes may be transferred to humans during digestion.
Answers to Objections
Although some may worry that genetically modified foods might lead to increased antibiotic resistance in humans, van den Eed et al., suggest that there is little reason to worry. They write the following:
Therefore, it can be concluded that the calculated amount of recombinant DNA consumed per capita (0.049 mg/day for maize and 0.011 mg/day for soy), such as published by Jonas et al. (2001) based on the data of Herbel and Montag (1987) and Lassek and Montag (1990) and on the amount of maize and potato products consumed per capita in Austria (Jonas et al., 2001), is probably overestimated with respect to the availability for gene transfer, because most of these plant food materials are processed before eating.
Furthermore, due to the DNA degradative activities in the intestinal tract, only a small percentage of the calculated amounts will finally reach the colon, which contains high numbers of bacteria. Since only linear DNA fragments are available from the food, genes can only be acquired by transformation via homologous recombination to resident prokaryotic genome. This implies that the acquisition of new genes, such as antibiotic resistance genes from food, is—most probably—a rare event.
They do, however, suggest that such an occurrence is a possibility.
Agrobacterium vs Particle Bombardment
Some findings suggest that particle blasting, the second-most popular method of genetic modification, is more disorganized that agrobacterium mediated genetic transfers and that this “increases the risk of insertion into critical endogenous sequences.” Furthermore, Gao and Nielsen find that Agrobacterium’s ability to “generate single-copy insertions compared with the prevalence of multicopy insertions and broken transgene integration when using particle bombardment” make it an attractive option. Agrobacterium-mediated transfers, they suggest, have a limited host range, and cannot affect some species of plants, and therefore particle bombardment is the only option for genetic modification in these plants. Furthermore, they say that when scientists use agrobacterium-mediated methods, they must place the transgene between T-DNA repeats and that, therefore, the T-DNA is “naturally excised from the vector during the transformation process.” This frequently leads to the integration of vector backbone sequences into the plant genome. This result is seen as “messy”.
Because researchers have developed a “clean gene” DNA strategy which removes all vectors before firing, particle bombardment is free of this concern. However, Agrobacterium transfers were clearly better when it came to producing transgenic plants and “predictable transgene transcription”. According to Gao and Nielsen, when Agrobacterium was used for genetic transfers to tall fescue plants, only 20% of the resulting plants’ leaves GUS (beta-glucuronidase enzyme) activity however, when particle bombardment was used, 53% of these leaves developed this activity. Some critics have voiced concern over the safety of the GUS gene in genetically modified foods, since GUS activity is found mainly in bacteria. Gliseen et al. suggest, however, that because the type of GUS that occurs in transgenic foods comes from the “enterobacterial species Escherichia coli” which is common in the intestines of vertebrates, it should have little to no effect on those who consume it. Gao and Nielsen ultimately concluded, that Agrobacterium-mediated transfers showed significant advantages over particle bombardment – at least in cases such as these.
Travella et al. who compared barley lines produced by Agrobacterium-mediated methods and by particle bombardment similarly found that Agrobacterium-mediated transformations were twice as efficient as particle bombardment lines. They also reported that while the Agrobacterium lines they studied each integrated fewer than three copies of the transgene, particle bombardment lines generated upwards of eight 60% of the time. Travella et al., also indicated that they found greater instances of stability in the T-DNA created by Agrobacterium transfers than they did through particle bombardment methods. “In most of the Agrobacterium-derived lines, the integrated T-DNA was stable and was inherited as a simple Mendelian trait,” they reported. However, in the particle bombardment lines, they observed Transgene silencing frequently.
Conclusion
Although Agrobacterium-mediated gene transfers are among the most natural methods of genetic modification available, these transfers do come with certain risks. They may introduce foreign DNA to crops and may introduce unexpected mutations to plants or transfer unwanted contents such as antibiotic resistant genes to humans. Studies suggest, however, that the risk of these occurrences is minimal. Meanwhile, the benefits to be reaped from such modification are many. Improvements in crop size and durability, decreased pestilence, destruction during drought, hunger and increased spread of immunizations are among the advantages genetic modification offers. Because the advantages are so great and the proven disadvantages are so minimal, the use of efficient methods of genetic transfers should be encouraged. In most cases, Agrobacterium-mediated genetic transfers appear to be the most efficient. Studies suggest that Agrobacterium-mediated transfers offer advantages in efficiency, stability and safety.
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- Travella, S, et al. “A comparison of transgenic barley lines produced by particle bombardment.” Plant Cell Reproduction 23 (2005): 780-789. Online.
- van den Eede, G, et al. “The relevance of gene transfer to the safety of food and feed derived from genetically modified (GM) plants.” Food and Chemical Toxicology (2004): 1127–1156. Online.
