Transgenic plants: Wikis

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Kenyan farmers examining transgenic Bt corn

Transgenic plants are plants possessing a single or multiple genes, transferred from a different species. Though DNA from another species can be integrated into a plants' genome via natural processes, the term "transgenic plants" refers to plants created in a laboratory using recombinant DNA technology.

The aim of creating transgenic plants is to design plants with specific characteristics through artificial insertion of genes from other species (or taxonomically up to different kingdoms).

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 ordinarily exist in nature (see also Plant breeding, Heterosis, New Rice for Africa).

Such traditional techniques (used from ca 1930) 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. Hope is one such wheat variety bred by E. S. McFadden with a gene from a wild grass to have rust resistance. 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. (see also Food security, International Fund for Agricultural Development, International development)

Contents

Natural gene flow between species

Natural flow of genes between bacterial 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 bacteria 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

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. 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

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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

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 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 U.S. has adopted the technology most widely whereas Europe has almost no genetically engineered crops.[7] The EU had a formal ban on GM crops, until it was overturned in 2006[8 ]; in a controversial move[9 ] GM crops are now regulated by the EU[10 ].
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, (e.g. drought or nitrogen starvation), and nutritional improvement (see Golden rice).

The first modern recombinant crop approved for sale in the U.S., 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 (triticale) in 1876 (Wilson, 1876). The first transgenic cereal may have been wheat, which itself is a natural transgenic plant derived from at least three parental species.

Genetically modified organisms came before 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 (e.g. GMO-soy, GMO-corn, etc.) in the US were being grown outdoors on large scales.

Commercial factors, including high regulatory and research costs, have so far restricted modern transgenic crop varieties to major traded commodity crops. Recently, R&D has targeted enhancement of crops that are locally important in developing countries, such as insect-resistant cow-pea for Africa [11] and insect-resistant Brinjal eggplant for India. [12]

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[13].

Cisgenics

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.[14]

Regulation of transgenic plants

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

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. ^ "EU GMO ban was illegal, WTO rules". Euractiv.com. Friday 12 May 2006. http://www.euractiv.com/en/trade/eu-gmo-ban-illegal-wto-rules/article-155197. Retrieved 5 January 2010.  
  9. ^ "GMO UPDATE: US-EU BIOTECH DISPUTE; EU REGULATIONS; THAILAND". International Centre for Trade and Sustainable Development. http://ictsd.org/i/news/biores/9482/. Retrieved 5 January 2010.  
  10. ^ "Genetically Modified Food and Feed". http://ec.europa.eu/food/food/biotechnology/index_en.htm. Retrieved 5 January 2010.  
  11. ^ http://www.pi.csiro.au/enewsletter/PDF/PI_info_Cowpeas.pdf
  12. ^ http://www.fbae.org/Channels/Views/indian_bt_brinjal_in_public.htm
  13. ^ 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.  
  14. ^ 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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