Genetically Engineered Crops and Soil FertilityContents
(Below we are using the word "transgene" which is the same as "genetically engineered gene") One gram of agricultural soil contains millions of living microorganisms, including about:
In addition earthworms and other macroorganisms contribute to the condition of the soil. The number of different soil organism species found in one gram may be over one thousand. All these are mutually interdependent and create together the necessary conditions for soil fertility. Research has shown that deviations in the numbers and kinds of soil organisms may influence the fertility considerably for example by decreasing the ability to retain water and nutrients. A proposed mechanism for disruption of soil ecology It has been well established that flow of genes between organisms of different species occurs in nature. This is called horizontal transfer. Normally, special protection mechanisms prevent foreign genes, after entering a cell, to mix with its DNA. However, special genetic elements, so called vectors, are used in genetic engineering to overcome these so called species barrier mechanisms. Mae-Wan Ho et al (1). have pointed out that this vector DNA may leak out into the soil and from there it may be transferred to bacteria that are pathogenic to humans. They warn that it may promote the emergence of new and dangerous human pathogens. This scenario might apply as well to soil ecology. If so, the spread of vectors to soil microbes might result in the emergence of altered soil microorganisms that might disturb soil ecology, e.g. by not functioning properly or because or by disturbing other organisms. As the soil ecosystem is of crucial importance for soil fertility, this might have serious agricultural consequences especially as such damage may be impossible to repair as the released genes cannot be recalled. It is impossible to judge to what extent this might happen without experimental studies. This possibility is presented in a scientific article, "Genetically Engineered Crops and Soil Fertility" [AL] proposing the hypothesis: "Horizontal transfer of genes between soil micro-organisms may be facilitated by vector DNA from the GE plants resulting in such changes or disturbances in the functioning of the micro-organisms that soil ecology and fertility may be affected." About transfer of genes between different species – "horizontal transfer" Horizontal transfer is the transfer of genetic material between different organisms. It may occur through three mechanisms: 1. Uptake of naked genes from the environment (transformation). 2. By virus infection. Viruses are packages of genes wrapped in a protein shell. In addition to their own genes they can carry with them other genes (transduction) 3. Mating, which may occur between unrelated species (conjugation). Genetic engineering uses genes that greatly promote sexual transfer of genes between unrelated species. Such transfer has been experimentally demonstrated between plants and bacteria and between bacteria and fungi as well as between unrelated bacterial species. In addition, transformation, the uptake of naked genes from the soil into bacteria has been experimentally demonstrated. Transfer through viruses (transduction) is known to occur mainly in water environment. Conclusion: It has been experimentally demonstrated that transfer of genes is possible from genetically engineered crops to soil microorganisms. It has likewise been demonstrated that genes may be transferred between different species of soil microorganisms. Naked DNA can survive long enough for direct uptake into soil microorganisms Transfer through sexual transmission was shown above to be possible between unrelated microorganisms. But another important avenue for transfer of GE genes is transformation, the direct uptake of naked DNA from the surroundings. There are good preconditions for such uptake as naked DNA can survive for many hours in the soil, which is long enough to become accessible to soil microorganisms. Such naked DNA is secreted by the plants into the environment. Also it will survive plant death and can be taken up by scavenging microorganisms. One ordinary field of GE maize for example may contain about 50.000 plants. Each maize plant contains about 1 billion cells. Each cell contains genetically altered DNA. This means that thousands of billions of transgenic DNA reaches the soil after harvest of one field only when the debris is ploughed back into the soil. In addition, DNA is continuously being secreted by the live plants into the soil. DNA can survive in the soil long enough to be taken up by soil microorganisms. It cannot be excluded that the numbers of these genes may increase significantly as the organisms that have taken up transgenic DNA multiply. The vectors may promote gene transfer between unrelated species The persistence of different species is upheld by mechanisms that prevent foreign genes that may enter the cell from being incorporated into its DNA. This is called the species barrier. To make insertion of foreign genes possible, genetic engineers have developed gene packages that help overcome this barrier. They are called vectors and are commonly artificial combinations (chimera) of parts of genes from for example pathogenic bacteria (plasmid genes), viruses and so-called transposons (jumping genes). They have a considerable ability to make horizontal transfer possible between widely unrelated species. In experiments with horizontal transfer it has been directly demonstrated that vectors of the kind that are used in genetic engineering can mediate gene transfer between soil, freshwater and marine bacteria. In wastewater treatment ponds horizontal transfer has occurred of the antibiotic resistance genes found in several varieties of GE crops. Conclusion: Experimental findings confirm that vectors can promote transfer of genes between soil microorganisms that normally don't exchange genes or do so only rarely. Other above mentioned findings show that vectors can be transferred from GE plants to soil microorganisms. Taken together, this means that there are reasons to suspect that the cultivation of genetically engineered plants may lead to transfer of genetic material between unrelated soil microorganisms. Such transfer may give rise to micro-organisms with altered properties. Potential agricultural consequences If the proposed mechanism works, there may occur a gradual accumulation of a significant number of altered soil microorganisms that are unable to uphold the functions required for good soil productivity. The diversity of soil microorgansisms would decrease progressively. This would damage soil fertility as diversity is crucial for good soil fertility. Such damage to soil fertility would probably not become obvious before an extensive disturbance has occurred, which might take several years. A very serious problem is that such abnormal organsims would probably be impossible to eradicate. The genes might even multiply and cause cumulative deterioriation long after the cultivation of transgenic crops has ceased. If so, in a region where transgenic crops are cultivated, this might perhaps ultimately lead to widespread and irreparable changes resulting in permanent impairment of soil fertility. There is no scientific basis for excluding this possibilty until careful experimental studies have been done. : It appears that there is enough evidence to justify a careful investigation whether the cultivation of genetically engineered plants may lead to the transfer of GE genes including vector DNA into soil microorganisms. The vector DNA might enable gene transfer between unrelated speicies. If this would occur to a significant extent, it would lead to changes in soil organsims with an increasing loss of diversity over time. This would lead to a progressive loss of soil fertility as diversity is crucial for good fertility. Such altered microorgansims would most probably be impossible to eradicate completely. They might spread uncontrollably through various pathways. It is not possible to judge on theoretical grounds what is the risk for the proposed mechanism to produce altered soil microorganisms to a significant extent. Only careful experimental investigation can provide the answer. Considering that none of present GE crops are of significant value to mankind, PSRAST finds it unjustified and potentially dangerous to permit the cultivation of any genetically engineered plants until this possibility has been carefully elucidated, which will take extensive long term horizontal transfer and soil ecology research that must be done under strictly contained laboratory conditions. If this mechanism works in reality, it would be an important argument against any release in the future of transgenic crops and other organisms whose DNA may end up in the soil. Reference: (1) Ho, M-W, Traavik, Olsvik et al, "Gene Technology and Gene Ecology of Infectious Diseases" Microbial Ecology in Health and Disease vol. 10, 33-39m 1998Jaan Suurkula M.D. Published June 28, 1999. Last updated on January 04, 2004 Related articles
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