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Transgenic plant

 
Genetics Encyclopedia: Transgenic Plants
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Transgenic plants are plants that have been genetically modified by inserting genes directly into a single plant cell. Transgenic crop plants modified for improved flavor, pest resistance, or some other useful property are being used increasingly.

Transgenic plants are unique in that they develop from only one plant cell. In normal sexual reproduction, plant offspring are created when a pollen cell and an ovule fuse. In a similar laboratory procedure, two plant cells that have had their cell walls removed can be fused to create an offspring.

Genetic Engineering Techniques

There are three general approaches that can be used to insert the DNA into a plant cell: vector-mediated transformation, particle-mediated transformation, and direct DNA absorption. With vector-mediated transformation, a plant cell is infected with a virus or bacterium that, as part of the infection process, inserts the DNA. The most commonly used vector is the crown-gall bacterium, Agrobacterium tumefaciens. With particle-mediated transformation (particle bombardment), using a tool referred to as a "gene gun," the DNA is carried into the cell by metal particles that have been accelerated, or "shot," into the cell. The particles are usually very fine gold pellets onto which the DNA has been stuck. With direct DNA absorption, a cell is bathed in the DNA, and an electric shock usually is applied ("electroporation") to the cell to stimulate DNA uptake.

No matter what gene insertion method is used, a series of events must occur to allow a whole genetically modified plant to be recovered from the genetically modified cell: The cell must incorporate the new DNA into its own chromosomes, the transformed cell must initiate division, the new cells need to organize themselves into all the tissues and organs of a normal plant ("regeneration"), and finally, the inserted gene must continue to work properly ("gene expression") in the regenerated plant.

To help ensure all this occurs, a "cassette" of genes is inserted during the initial transformation. In addition to the gene coding for the desired trait, other genes are added. Some of these genes promote the growth of only those plant cells that have successfully incorporated the inserted DNA. It might do this by providing the transformed cells with resistance to a normally toxic antibiotic that is added to the growth medium, for example. Other genes ("promoters") may be added to control the functioning of the trait gene by directing when and where in the transformed plant it will operate.

The genes put into plants using genetic engineering can come from any organism. Most genes used in the genetic engineering of plants have come from bacteria. However, as scientists learn more about the genetic makeup of plants ("plant genomics"), more plant-derived genes will be used.

Agricultural Applications

Inserted genes can be classified into three groups based on their use: those that protect a crop, those that improve the quality of a harvested product, and those that let the plant perform some new function.

Genes That Protect a Crop

The major use of plant genetic engineering has been to make crops easier to grow by decreasing the impact of pests. Insect resistance has been achieved by transforming a crop using a Bt gene. Bt genes were isolated from Bacillus thuringiensis, a common soil bacterium. They code for proteins that severely disrupt the digestive system of insects. Thus an insect eating the leaf of a plant expressing a Bt gene stops eating and dies of starvation. There are many Bt genes, each of which targets a particular group of insects. Some Bt genes, for example, target caterpillars. Others target beetles.

Genetic engineering also has been used in the battle against weeds. Bacterial genes allow crops to either degrade herbicides or be resistant to them. The herbicides that are used are generally very effective, killing most plants. They are considered environmentally benign, degrading rapidly in the soil and having little impact on humans or other organisms. Thus a whole field of transgenic crops can be sprayed with broad-spectrum herbicides, killing all plants except the crops. Corn, soybeans, canola, and cotton that have been engineered to withstand either insects or herbicides, or both, are widely planted in some countries, including the United States. In addition, other crops, including potatoes, tomatoes, tropical fruits, and melons, have been engineered for resistance to viral diseases.

Genes That Improve Crop Quality

An emerging major use of genetic engineering for crops is to alter the quality of the crop. Fresh fruits and vegetables begin to deteriorate immediately after being harvested. Delaying or preventing this deterioration not only preserves a produce's flavor, and appearance, but maintains the nutritional value of the produce. Genes that change the hormonal status of the harvested crops are the major targets for genetic engineering toward longer shelf-life.

For example, the plant hormone ethylene is associated with accelerated ripening, as well as leaf and flower deterioration, in fruits that are injured or harvested. Scientists insert genes that interfere with a plant's ability to synthesize or respond to ethylene, thereby extending postharvest quality for many fresh products, including tomatoes, lettuce, and cut flowers. Scientists are also using gene insertion to improve a plant's nutritional value and color.

Genes That Introduce New Traits

One approach to improving the economic value of crops is to give them traits that are completely new for that plant. Some crops, including potatoes, tomatoes, and bananas, have been engineered with genes from pathogenic organisms. This is done to make animals, including humans, that eat the crops immune to the diseases caused by the pathogens. The genes code for proteins that act as antigens to induce immunity. Thus edible parts of plants are engineered to act as oral vaccines. This approach may be particularly effective for pathogens, such as those causing diarrhea and other gastrointestinal disorders, that enter the body through mucous membranes. This is because the "medicine" in the food comes into direct contact with these membranes and does not have to be absorbed into the blood stream. Genes have also been engineered into crop plants to direct the plants to produce industrial enzymes used in the manufacture of paper. Other genes direct crops to produce small polymers useful in the manufacture of plastics. This general approach is being termed "plant molecular farming."

Rice is another plant that has been engineered for a new trait. During commercial processing, a substantial part of the white rice grains are removed, leaving very little vitamin A. Vitamin A deficiency is a significant health problem in regions dependent on rice as a dietary staple. Scientists engineered a certain form of rice, known as "golden rice" because it has a yellow tinge, to express three introduced genes. These genes let the plant produce the precursor of vitamin A in the portion of the grain that remains after processing, thereby providing a dietary source of the vitamin.

—Brent McCown

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Wikipedia: Transgenic plant
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Kenyans examining insect-resistant transgenic Bt corn.

Transgenic plants possess a gene or genes that have been transferred from a different species. Although DNA of another species can be integrated in a plant genome by natural processes, the term "transgenic plants" refers to plants created in a laboratory using recombinant DNA technology. The aim is to design plants with specific characteristics by artificial insertion of genes from other species or sometimes entirely different kingdoms. See also Genetics, List of genetic engineering topics.

Varieties containing genes of two distinct plant species are frequently created by classical breeders who deliberately force hybridization between distinct plant species when carrying out interspecific or intergeneric wide crosses with the intention of developing disease resistant crop varieties. Classical plant breeders use a number of in vitro techniques such as protoplast fusion, embryo rescue or mutagenesis to generate diversity and produce plants that would not exist in nature (see also Plant breeding, Heterosis, New Rice for Africa).

Such traditional techniques (used since about 1930 on) have never been controversial, or been given wide publicity except among professional biologists, and have allowed crop breeders to develop varieties of basic food crop, wheat in particular, which resist devastating plant diseases such as rusts. Hope is one such wheat variety bred by E. S. McFadden with a gene from a wild grass. Hope saved American wheat growers from devastating stem rust outbreaks in the 1930s.

Methods used in traditional breeding that generate plants with DNA from two species by non-recombinant methods are widely familiar to professional plant scientists, and serve important roles in securing a sustainable future for agriculture by protecting crops from pests and helping land and water to be used more efficiently.[citation needed] (see also Food security, International Fund for Agricultural Development, International development)

Contents

Natural movements of genes between species

Natural movement of genes between species, often called horizontal gene transfer or lateral gene transfer, can occur because of gene transfer mediated by natural processes.

This natural gene movement between species has been widely detected during genetic investigation of various natural mobile genetic elements, such as transposons, and retrotransposons that naturally translocate to new sites in a genome, and often move to new species over an evolutionary time scale. There are many types of natural mobile DNAs, and they have been detected abundantly in food crops such as rice [1].

These various mobile genes play a major role in dynamic changes to chromosomes during evolution [2], [3], and have often been given whimsical names, such as Mariner, Hobo, Trans-Siberian Express (Transib), Osmar, Helitron, Sleeping Princess, MITE and MULE, to emphasize their mobile and transient behavior.

Genetically mobile DNA constitutes a major fraction of the DNA of many plants, and the natural dynamic changes to crop plant chromosomes caused by this natural transgenic DNA mimics many of the features of plant genetic engineering currently pursued in the laboratory, such as using transposons as a genetic tool, and molecular cloning. See also transposon, retrotransposon, integron, provirus, endogenous retrovirus, heterosis, Gene duplication and exon shuffling by helitron-like transposons generate intraspecies diversity in maize.

There is new scientific literature about natural transgenic events in plants, through movement of natural mobile DNAs called MULEs between rice and Setaria millet [4].

It is becoming clear that natural rearrangements of DNA and horizontal gene transfer play a pervasive role in natural evolution. Importantly many, if not most, flowering plants evolved by transgenesis - that is, the creation of natural interspecies hybrids in which chromosome sets from different plant species were added together. There is also the long and rich history of interspecies cross-breeding with traditional methods.

Deliberate creation of transgenic plants during breeding

Production of transgenic plants in wide-crosses by plant breeders has been a vital aspect of conventional plant breeding for about a century. Without it, security of our food supply against losses caused by crop pests such as rusts and mildews would be severely compromised[citation needed]. The first historically recorded interspecies transgenic cereal hybrid was actually between wheat and rye (Wilson, 1876).

In the 20th century, the introduction of alien germplasm into common foods was repeatedly achieved by traditional crop breeders by artificially overcoming fertility barriers. Novel genetic rearrangements of plant chromosomes, such as insertion of large blocks of rye (Secale) genes into wheat chromosomes ('translocations'), has also been exploited widely for many decades [5].

By the late 1930s with the introduction of colchicine, perennial grasses were being hybridized with wheat with the aim of transferring disease resistance and perenniality into annual crops, and large-scale practical use of hybrids was well established, leading on to development of Triticosecale and other new transgenic cereal crops. In 1985 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). [6]

Transgenic resistance traits in bread wheat varieties

Important transgenic pathogen and parasite resistance traits in current bread wheat varieties (gene, eg "Lr9" followed by the source species) are:

Disease resistance

  • Leaf rust
    • Lr9 (from Aegilops umbellulata)
    • Lr18 Triticum timopheevi
    • Lr19 Thinopyrum
    • Lr23 T. turgidum
    • Lr24 Ag. elongatum
    • Lr25 Secale cereale
    • Lr29 Ag. elongatum
    • Lr32 T. tauschii
  • Stem rust
    • Sr2 T. turgidum ("Hope" ) McFadden, E. S. (1930) J. Am. Soc. Agron. 22, 1020-1031 .
    • Sr22 Triticum monococcum
    • Sr36 Triticum timopheevii
  • Stripe rust
    • Yr15 Triticum dicoccoides
  • Powdery mildew
  • Wheat streak mosaic virus
    • Wsm1 Ag. elongatum

Pest resistance

Genetically engineered plants

Plums that have been genetically engineered to be resistant to the plum pox virus
Common white rice and golden rice

The intentional creation of transgenic plants by laboratory based recombinant DNA methods is more recent (from the mid-70s on) and has been a controversial development in the field of biotechnology opposed vigorously by many NGOs, and several governments, particularly in Europe. These transgenic recombinant plants (biotech crops, modern transgenics) are transforming agriculture in those regions that have allowed farmers to adopt them, and the area sown to these crops has continued to grow globally in every years since their first introduction in 1996. As of 2006 there were around 250 million acres of genetically engineered crops being grown commercially in 22 countries. The USA has adopted the technology most widely whereas Europe has almost no genetically engineered crops.[7]

Transgenic recombinant plants are generated in a laboratory by adding one or more genes to a plant's genome,and the techniques frequently called transformation. Transformation is usually achieved using gold particle bombardment or through the process of Horizontal gene transfer using a soil bacterium, Agrobacterium tumefaciens, carrying an engineered plasmid vector, or carrier of selected extra genes.

Transgenic recombinant plants are identified as a class of genetically modified organism(GMO); usually only transgenic plants created by direct DNA manipulation are given much attention in public discussions.

Transgenic plants have been deliberately developed for a variety of reasons: longer shelf life, disease resistance, herbicide resistance, pest resistance, non-biological stress resistances, such as to drought or nitrogen starvation, and nutritional improvement (see Golden rice). The first modern recombinant crop approved for sale in the US, in 1994, was the FlavrSavr tomato, which had a longer shelf life. The first conventional transgenic cereal created by scientific breeders was actually a hybrid between wheat and rye in 1876 (Wilson, 1876). The first transgenic cereal may have been wheat, which itself is a natural transgenic plant derived from at least three different parenteral species.

Genetically modified organisms were prior to the coming of the commercially viable crops as the FlavrSavr tomato, only strictly grown indoors (in laboratories). However, after the introduction of the Flavr Savr tomato, certain GMO-crops as GMO-soy and GMO-corn where in the USA being grown outdoors on large scales.

Commercial factors, especially high regulatory and research costs, have so far restricted modern transgenic crop varieties to major traded commodity crops, but recently R&D projects to enhance crops that are locally important in developing counties are being pursued, such as insect protected cow-pea for Africa [8] and insect protected Brinjal eggplant for India. [9]

Transgenic plants have 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[10].

The term cisgenic is being used by some plant breeders and scientists to refer to artificial genetic transfers that could theoretically have been produced by conventional plant breeding methods. Breeders and scientists argue that "cisgenically" produced organisms do not have the same degree of novelty as transgenic organisms, and involve no environmental issues that are not already present in conventional crossbreeding. It is argued that cisgenic modification is useful for plants that are difficult to crossbreed predictably 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.[11]

Regulation of transgenic plants

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In the United States the Coordinated Framework for Regulation of Biotechnology governs the regulation of transgenic organisms, including plants. The three agencies involved are:

The Biotechnology Regulatory Services (BRS) program of the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) is responsible for regulating the introduction (importation, interstate movement, and field release) of genetically engineered (GE) organisms that may pose a plant pest risk. BRS exercises this authority through APHIS regulations in Title 7, Code of Federal Regulations, Part 340 under the Plant Protection Act of 2000. APHIS protects agriculture and the environment by ensuring that biotechnology is developed and used in a safe manner. Through a strong regulatory framework, BRS ensures the safe and confined introduction of new GE plants with significant safeguards to prevent the accidental release of any GE material. APHIS has regulated the biotechnology industry since 1987 and has authorized more than 10,000 field tests of GE organisms. In order to emphasize the importance of the program, APHIS established BRS in August 2002 by combining units within the agency that dealt with the regulation of biotechnology. Biotechnology, Federal Regulation, and the U.S. Department of Agriculture, February 2006, USDA-APHIS Fact Sheet

Ecological risks

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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:

  • Is the transgenic plant capable of growing outside a cultivated area?
  • Can the transgenic plant pass its genes to a local wild species, and are the offspring also fertile?
  • Does the introduction of the transgene confer a selective advantage to the plant or to hybrids in the wild?

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:

  • Hybridization with non-transgenic crop plants of the same species and variety.
  • Hybridization with wild plants of the same species.
  • Hybridization with wild plants of closely related species, usually of the same genus.

However, there are a number of factors which must be present for hybrids to be created.

  • The transgenic plants must be close enough to the wild species for the pollen to reach the wild plants.
  • The wild and transgenic plants must flower at the same time.
  • The wild and transgenic plants must be genetically compatible.

In order to persist, these hybrid offspring:

  • Must be viable, and fertile.
  • Must carry the transgene.

Studies suggest that a possible escape route for transgenic plants will be through hybridization with wild plants of related species.

  1. It is known that some crop plants have been found to hybridize with wild counterparts.
  2. It is understood, as a basic part of population genetics, that the spread of a transgene in a wild population will be directly related to the fitness effects of the gene in addition to the rate of influx of the gene to the population.  Advantageous genes will spread rapidly, neutral genes will spread with genetic drift, and disadvantageous genes will only spread if there is a constant influx.
  3. The ecological effects of transgenes are not known, but it is generally accepted that only genes which improve fitness in relation to abiotic factors would give hybrid plants sufficient advantages to become weedy or invasive.  Abiotic factors are parts of the ecosystem which are not alive, such as climate, salt and mineral content, and temperature. Genes improving fitness in relation to biotic factors could disturb the (sometimes fragile) balance of an ecosystem. For instance, a wild plant receiving a pest resistance gene from a transgenic plant might become resistant to one of its natural pests, say, a beetle. This could allow the plant to increase in frequency, while at the same time animals higher up in the food chain, which are at least partly dependent on that beetle as food source, might decrease in abundance. However, the exact consequences of a transgene with a selective advantage in the natural environment are almost impossible to predict reliably.

It is also important to refer to the demanding actions that government of developing countries had been building up among the last decades.

Agricultural impact of transgenic plants

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.

References

  1. ^ DNA-binding specificity of rice mariner-like transposases and interactions with Stowaway MITEs, C. Feschotte et al., Nucleic Acids Research 2005 33(7):2153-2165;[1]
  2. ^ Birth of a chimeric primate gene by capture of the transposase gene from a mobile element — PNAS:1. Richard Cordaux*,2. Swalpa Udit†,3. Mark A. Batzer*, and 4. Cédric Feschotte†,‡(+Author Affiliations)1.*Department of Biological Sciences, Biological Computation and Visualization Center, Center for BioModular Multi-Scale Systems, Louisiana State University, 202 Life Sciences Building, Baton Rouge, LA 70803; and 2.†Department of Biology, University of Texas, Arlington, TX 76019 1. Edited by Susan R. Wessler, University of Georgia, Athens, GA, and approved March 27, 2006 (received for review February 10, 2006)
  3. ^ November 2003 Vol 4 No 11 Nature Reviews Genetics 4, 865-875 (2003); doi:10.1038/nrg1204 THE ORIGIN OF NEW GENES: GLIMPSES FROM THE YOUNG AND OLD (By Manyuan Long, Esther Betrán, Kevin Thornton & Wen Wang
  4. ^ PLoS Biology - (2006) Jumping Genes Cross Plant Species Boundaries. PLoS Biol 4(1): e35 doi:10.1371/journal.pbio.0040035 Published: December 20, 2005 Copyright: © 2005 Public Library of Science.
  5. ^ Plant genetic resources: What can they contribute toward increased crop productivity? — PNAS: 1. David Hoisington*, 2. Mireille Khairallah, 3. Timothy Reeves, 4. Jean-Marcel Ribaut, 5. Bent Skovmand, 6. Suketoshi Taba, and 7. Marilyn Warburton
  6. ^ Vaeck, M., A. Reynaerts, H. Hofte, S. Jansens, M. De Beuckeleer, C. Dean, M. Zabeau, M. Van Montagu & J. Leemans. 1987, Transgenic plants protected from insect attack, Nature 328: 33-37.
  7. ^ Lemaux, Peggy (February 19, 2008). "Genetically Engineered Plants and Foods: A Scientist's Analysis of the Issues (Part I)". Annual Review of Plant Biology 59: 771-812. http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.arplant.58.032806.103840. Retrieved 9 May 2009. 
  8. ^ http://www.pi.csiro.au/enewsletter/PDF/PI_info_Cowpeas.pdf
  9. ^ http://www.fbae.org/Channels/Views/indian_bt_brinjal_in_public.htm
  10. ^ Meagher, RB (2000). "Phytoremediation of toxic elemental and organic pollutants". Current Opinion In Plant Biology 3 (2): 153–162. doi:10.1016/S1369-5266(99)00054-0. PMID 10712958. 
  11. ^ http://www.newscientist.com/article/mg19926671.600-how-the-humble-potato-could-feed-the-world.html Deborah MacKenzie, "How the humble potato could feed the world" (cover story) New Scientist No2667 2 August 2008 30-33
Notes

See also

External links


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