Genetically modified plants are plants whose DNA is modified using genetic engineering techniques. In most cases the aim is to introduce a new trait to the plant which does not occur naturally in this species. Examples include resistance to certain pests, diseases or environmental conditions, or the production of a certain nutrient or pharmaceutical agent.
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Some degree of natural flow of genes, often called horizontal gene transfer or lateral gene transfer, occurs between plant species.[1] This is facilitated by transposons, retrotransposons, proviruses and other mobile genetic elements that naturally translocate to new sites in a genome.[2][3] They often move to new species over an evolutionary time scale[4] and play a major role in dynamic changes to chromosomes during evolution.[5][6]
The introduction of foreign germplasm into common foods has been achieved by traditional crop breeders by artificially overcoming fertility barriers. A hybrid cereal was created in 1875, by crossing wheat and rye.[7] Since then important traits have been introduced into wheat, including dwarfing genes and rust resistance.[8] Plant tissue culture and the induction of mutations have also enabled humans to artificially alter the makeup of plant genomes.[9][10]
The first field trials of genetically engineered plants occurred in France and the USA in 1986, when tobacco plants were engineered to be resistant to herbicides.[11] In 1987, Plant Genetic Systems (Ghent, Belgium), founded by Marc Van Montagu and Jeff Schell, was the first company to develop genetically engineered (tobacco) plants with insect tolerance by expressing genes encoding for insecticidal proteins from Bacillus thuringiensis (Bt).[12] The People’s Republic of China was the first country to allow commercialized transgenic plants, introducing a virus-resistant tobacco in 1992.[13] The first genetically modified crop approved for sale in the U.S., in 1994, was the FlavrSavr tomato, which had a longer shelf life.[14] In 1994, the European Union approved tobacco engineered to be resistant to the herbicide bromoxynil, making it the first commercially genetically engineered crop marketed in Europe.[15] In 1995, Bt Potato was approved safe by the Environmental Protection Agency, making it the first pesticide producing crop to be approved in the USA.[16] In 2010, 11 different transgenic crops were grown commercially on 365 million acres (148 million hectares) in 29 countries such as the USA, Brazil, Argentina, India, Canada, China, Paraguay, Pakistan, South Africa and Uraguay.[17]
The U.S. has adopted the technology most widely whereas Europe has very little genetically engineered crops[18] with the exception of Spain where one fifth of maize grown is genetically engineered,[19] and smaller amounts in five other countries.[20] The EU had a 'de facto' ban on the approval of new GM crops, from 1999 until 2004;[21] in a controversial move.[22] GM crops are now regulated by the EU.[23] Developing countries grew 48 per cent of genetically engineered crops in 2010.[17]
Genetically engineered plants are generated in a laboratory by altering the genetic makeup, usually by adding one or more genes, of a plant's genome using genetic engineering techniques. Most genetically modified plants are generated by the biolistic method (particle gun) or by Agrobacterium tumefaciens mediated transformation.
In the biolistic method, DNA is bound to tiny particles of gold or tungsten which are subsequently "shot" into plant tissue or single plant cells under high pressure. The accelerated particles penetrate both the cell wall and membranes. The DNA separates from the metal and is integrated into the plant genome inside the nucleus. This method has been applied successfully for many cultivated crops, especially monocots like wheat or maize, for which transformation using Agrobacterium tumefaciens has been less successful.[24] The major disadvantage of this procedure is that serious damage can be done to the cellular tissue.
Agrobacteria are natural plant parasites, and their natural ability to transfer genes is used for the development of genetically engineered plants. To create a suitable environment for themselves, these Agrobacteria insert their genes into plant hosts, resulting in a proliferation of plant cells near the soil level (crown gall). The genetic information for tumour growth is encoded on a mobile, circular DNA fragment (plasmid). When Agrobacterium infects a plant, it transfers this T-DNA to a random site in the plant genome. When used in genetic engineering the bacterial T-DNA is removed from the bacterial plasmid and replaced with the desired foreign gene. The bacterium is a vector, enabling transportation of foreign genes into plants. This method works especially well for dicotyledonous plants like potatoes, tomatoes, and tobacco. Agrobacteria infection is less successful in crops like wheat and maize.
Genetically modified plants have been developed commercially to improve shelf life, disease resistance, herbicide resistance and pest resistance. Plants engineered to tolerate non-biological stresses like drought,[25][26] frost[27][28][29] and nitrogen starvation[30] or with increased nutritional value (e.g. Golden rice[31]) were in development in 2011. Future generations of GM plants are intended to be suitable for harsh environments, produce increased amounts of nutrients or even pharmaceutical agents, or are improved for the production of bioenergy and biofuels. Due to high regulatory and research costs, the majority of genetically modified crops in agriculture consist of commodity crops, such as soybean, maize, cotton and rapeseed.[32][33] However, commercial growing was reported in 2009, of smaller amounts of genetically modified sugar beet, papayas, squash (zucchini), sweet pepper, tomatoes, petunias, carnations, roses and poplars.[33] Recently, some research and development has been targeted to enhancement of crops that are locally important in developing countries, such as insect-resistant cowpea for Africa[34] and insect-resistant brinjal (eggplant) for India.[35]
In research tobacco and Arabidopsis thaliana are the most genetically modified plants, due to well developed transformation methods, easy propagation and well studied genomes.[36] They serve as model organisms for other plant species. Genetically modified plants have also been used for bioremediation of contaminated soils. Mercury, selenium and organic pollutants such as polychlorinated biphenyls (PCBs) have been removed from soils by transgenic plants containing genes for bacterial enzymes.[37]
Transgenic plants have genes inserted into them that are derived from another species. The inserted genes can come from species within the same kingdom (plant to plant) or between kingdoms (bacteria to plant). In many cases the inserted DNA has to be modified slightly in order to correctly and efficiently express in the host organism. Transgenic plants are used to express proteins like the cry toxins from Bacillus thuringiensis, herbicide resistant genes and antigens for vaccinations[38]
Cisgenic plants are made using genes found within the same species or a closely related one, where conventional plant breeding can occur. Some breeders and scientists argue that cisgenic modification is useful for plants that are difficult to crossbreed by conventional means (such as potatoes), and that plants in the cisgenic category should not require the same level of legal regulation as other genetically modified organisms.[39]
In research plants are engineered to help discover the functions of certain genes. One way to do this is to knock out the gene of interest and see what phenotype develops. Another strategy is to attach the gene to a strong promoter and see what happens when it is over expressed. A common technique used to find out where the gene is expressed is to attach it to GUS or a similar reporter gene that allows visualisation of the location.[40]
The first commercialised genetically modified plants (Flavr Savr tomatoes) used RNAi technology, where the inserted DNA matched an endogenous gene already in the plant. When the inserted gene is expressed it can repress the translation of the endogenous protein. Host delivered RNAi systems are being developed, where the plant will express RNA that will interfere with insects, nematodes and other parasites protein synthesis.[41] This may provide a novel way of protecting plants from pests.
In the USA genetically modified organisms are assessed by the US Department of Agriculture (USDA), the Food and Drug Administration (FDA) and the Environmental protection agency (EPA). The USDA evaluate the plants potential to become weeds, the FDA review plants that could enter or alter the food supply and the EPA regulate the genetically modified plants with pesticide properties. Most developed genetically modified plants are reviewed by at least two of the agencies, with many subject to all three.[42] Final approval can still be denied by individual counties within each state. In 2004, Mendocino County, California became the first and only county to impose a ban on the "Propagation, Cultivation, Raising, and Growing of Genetically Modified Organisms", the measure passing with a 57% majority.[43]
Allergic proteins were detected during testing of a transgenic soybean inserted with a gene from the Brazil nut. The inserted gene did not translate into a known allergen, the allergenic nature of the protein was discovered when tested with serum from people allergic to Brazil nut.[44] Four federal district court suits have been brought against Animal and Plant Health Inspection Service, the agency within USDA that regulates genetically modified plants. Two involved field trials (herbicide-tolerant turfgrass in Oregon; pharmaceutical-producing corn and sugar in Hawaii) and two the deregulation of GM alfalfa.[42] and GM sugar beet.[45] Initially APHIS lost all four cases, with the judges ruling they failed to diligently follow the guidelines set out in the National Environmental Policy Act. In 2010 the US Supreme Court nullified the district court ban on growing GM alfalfa.[46] and in 2011 an appeal court allowed the partial deregulation of GM sugar beet.[47]
The European Union (EU) has possibly the most stringent regulations for genetically modified plants in the world.[48] All genetically modified plants are considered "new food" and subject to extensive, case-by-case, science based food evaluation by the European Food Safety Authority (EFSA). The European Commission (EC) then draft a proposal that is submitted to the Section on GM Food and Feed of the Standing Committee on the Food Chain and Animal Health, where if accepted it will be adopted by the EC or passed on to the Council of Agricultural Ministers.[48] There is also a safeguard clause that any Member State can invoke to restrict or prohibit the use or sale of a GM plant within their territory if they have a justifiable reasons to consider that the approved GM plant constitutes a risk to human health or the environment.[49]
Currently (2010) the only genetically modified plants approved for cultivation in Europe are MON810, a Bt expressing maize conferring resistance to the European corn borer and a potato called Amflora, approved only for industrial applications.[50] The EC has issued guidelines to allow the co-existence of GM and non-GM crops through buffer zones (where no GM crops are grown).[51] These are regulated by individual countries and vary from 15 meters in Sweden to 800 meters in Luxembourg.[48] All food (including processed food) or feed which contains greater than 0.9% of approved GMOs must be labelled.
Regulation in Australia are provided by the Office of the Gene Technology Regulator and Food Standards Australia New Zealand.[52][53] Each state in Australia individually assess the impact of release on markets and trade and can apply further legislation if they deem it necessary. In 2011 genetically modified plants were grown in all states except South Australia and Tasmania, who have extended their moratoriums until 2019 and 2014.[54][55] Health Canada and the Canadian Food Inspection Agency[56] are responsible for evaluating the safety and nutritional value of genetically modified plants in Canada.[57]
The Argentine government was one of the first to accept genetically modified plants, with assessment provided by the National Agricultural Biotechnology Advisory Committee (for environmental impact), the National Service of Health and Agrifood Quality (for food safety) and the National Agribusiness Direction (for theeffect on trade) The final decision is made by the Secretariat of Agriculture, Livestock, Fishery and Food.[58] In Brazil the 27 member National Biosafety Technical Commission is responsible for assessing environmental impact, food safety and provides guidelines for transport, importation and field experiments. In 2005, the Mexican senate passed a law allowing planting and selling of genetically modified crops.[59]
Genetically modified crops in China go through three phases of field trials (pilot field testing, environmental release testing, and preproduction testing) before they are submitted to the Office of Agricultural Genetic Engineering Biosafety Administration (OAGEBA) for assessment.[60] The 75 member National Biosafety Committee evaluates all applications, although OAGEBA has the final decision. The cost of regulation enforcement in India is generally higher than China, while the enforcement of regulations has proved more effective in China.[61] The National Assembly of Burkina Faso passed a biosafety law in early 2006, which established a National Biosafety Agency that would regulate GM products with the advise of various governmental and non-governmental advisory committees.[62]
Genetically modified plants can spread the trans gene to other plants or – theoretically – even to bacteria. Depending on the transgene, this may pose a threat to the environment by changing the composition of the local ecosystem.[63] Therefore, in most countries environmental studies are required prior to the approval of a GM plant for commercial purposes, and a monitoring plan must be presented to identify potential effects which have not been anticipated prior to the approval.
Little research has been conducted on human and animal health. However, in most countries every GM plant is tested in feeding trials to prove its safety, before it is approved for use and marketing. The project GMO-Safety collects and presents biosafety research on GMOs with more in-depth information on this topic.[1]
The potential impact on nearby ecosystems is one of the greatest concerns associated with transgenic plants.
Transgenes have the potential for significant ecological impact if the plants can increase in frequency and persist in natural populations. These concerns are similar to those surrounding conventionally bred plant breeds. Several risk factors should be considered:[64]
Many domesticated plants can mate and hybridise with wild relatives when they are grown in proximity, and whatever genes the cultivated plant had can then be passed to the hybrid. This applies equally to transgenic plants and conventionally bred plants, as in either case there are advantageous genes that may have negative consequences to an ecosystem upon release. This is normally not a significant concern, despite fears over 'mutant superweeds' overgrowing local wildlife: although hybrid plants are far from uncommon, in most cases these hybrids are not fertile due to polyploidy, and will not multiply or persist long after the original domestic plant is removed from the environment. However, this does not negate the possibility of a negative impact.
In some cases, the pollen from a domestic plant may travel many miles on the wind before fertilising another plant. This can make it difficult to assess the potential harm of crossbreeding; many of the relevant hybrids are far away from the test site. Among the solutions under study for this concern are systems designed to prevent transfer of transgenes, such as Terminator Technology, and the genetic transformation of the chloroplast only, so that only the seed of the transgenic plant would bear the transgene. With regard to the former, there is some controversy that the technologies may be inequitable and might force dependence upon producers for valid seed in the case of poor farmers, whereas the latter has no such concern but has technical constraints that still need to be overcome. Solutions are being developed by EU funded research programmes such as Co-Extra and Transcontainer.
There are at least three possible avenues of hybridization leading to escape of a transgene:
However, there are a number of factors which must be present for hybrids to be created.
In order to persist, these hybrid offspring:
Studies suggest that a possible escape route for transgenic plants will be through hybridization with wild plants of related species.
It is also important to refer to the demanding actions that government of developing countries had been building up among the last decades.
Outcrossing of transgenic plants not only poses potential environmental risks, it may also trouble farmers and food producers. Many countries have different legislations for transgenic and conventional plants as well as the derived food and feed, and consumers demand the freedom of choice to buy GM-derived or conventional products. Therefore, farmers and producers must separate both production chains. This requires coexistence measures on the field level as well as traceability measures throughout the whole food and feed processing chain. Research projects such as Co-Extra, SIGMEA and Transcontainer investigate how farmers can avoid outcrossing and mixing of transgenic and non-transgenic crops, and how processors can ensure and verify the separation of both production chains.
In many countries, and especially in the European Union, consumers demand the choice between foods derived from GM plants and conventionally or organically produced plants. This requires a labelling system as well as the reliable separation of GM and non-GM crops at field level and throughout the whole production chain.
Research has demonstrated, that coexistence can be realised by several agricultural measures, such as isolation distances or biological containment strategies.[2]
For traceability, the OECD has introduced a "unique identifier" which is given to any GMO when it is approved. This unique identifier must be forwarded at every stage of processing.[65]
Many countries have established labelling regulations and guidelines on coexistence and traceability. Research projects like Co-Extra, SIGMEA and Transcontainer are aimed at investigating improved methods for ensuring coexistence and providing stakeholders the tools required for the implementation of coexistence and traceability.
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