Transgenic plant

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Transgenic plants possess a gene or genes that have been transfered from a different species.

The most efficient route for gene movement between plant species is cross-pollination to form plant inter-species hybrids. Such hybrids are often new plant species, but they can also form a "bridge" for introgression of genes from one species to another. Non-standard fertilization with more than one pollen grain has also been suggested for transfer of a nuclear genome located gene from Poa grass genus into the distantly related sheep's fescue, Festuca ovina[1].

In addition to the carriage of genes in pollen, there are several other potential natural processes for gene movement between a plant and other species. One that is well documented is gene transfer between plant epiphytes (like mosses) and parasitic plants (like dodder) and the host plants they colonise (see Horizontal gene transfer). Extensive comparative studies of gene content of different genomes provides strong evidence that natural horizontal gene transfer does occur is some plants at a frequency that is significant over evolutionary time scales. Plant mitochondria are a stopping point over evolutionary time for genes that may enter the nuclear genome from other species. ( See Horizontal gene transfer.)

Another example of a mechanism for horizontal gene transfer is Agrobacterium tumefaciens, a bacterium that injects DNA into plant cells. Biotechnology laboratories exploit this bacterium to make artificial transgenic plants with small segments of added DNA inserted in the host cell chromosomes. Others may include plant sucking insects, mites, and possibly viruses.

Over evolutionary time scales, there is evidence for interspecies movement of much mobile DNA ( such as MULE DNA transposons) between different species, plants included.

Such mobile DNA movement between rice and millet plants has been recently documented is a paper discussed in detail below [2].


Hybrid formation in flowering plants and its role in "introgression' or gene movement between species

Hybrid formation between two species by pollination joins two sets of chromosomes together, one from each parent, is a common event in flowering plant evolution, and the main way new plant species are formed. Interestingly, in many cases hybrids are formed by adding two copies of each chromosome from each parent, forming a allotetraploid that is reproductively isolated from both parents - and a new species[3].

Wild emmer wheat is an example of a species formed by hybridization between two diploid wild grasses, Triticum urartu and a wild goatgrass such as Aegilops searsii or Ae. speltoides four sets of chomosomes (is a tetraploid. Triticale (Triticosecale) is crop cultivated today mostly for forage and animal feed which is an artificial hybrid between rye and wheat, first bred during the late 19th century.

A surprising number of plants show evidence of being formed by such processes by which chromosome sets are added : bread wheat ( an allohexaploid having three component genomes) , and cotton are two other examples [4].

Hybrids can occur in the intermediate geographical zone between two species and provide a "bridge" for genes to "introgress" (or move) from one species to another [5] [6].

Natural movements of genes between species by other routes than pollen

It is becoming clear that natural rearrangments of DNA and generation of transgenes play a pervasive role in natural evolution, and plants are no exception. Mobile DNA such as transposons are the most easily detent horizontally transfered genes. A good example of such movemt that is well documented is the movement of MULE mobile DNA between rice and millet.

Movement of DNA between plant epiphytes and the hosts they colonize , and bewteen parasitic plants and their host is also well documented. This is made possibel by close contact via hausoria between parasite and host.

See Horizontal gene transfer,Horizontal gene transfer in plants

Transgenic plants and crop improvement

Production of transgenic plants in wide-crosses by plant breeders has been a vital aspect of conventional plant breeding for a century or so. 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 interpecies transgenic cereal hybrid was actually between wheat and rye (Wilson, 1876).

Transgenic varieties are frequently created by classical breeders by deliberately and artificially force hybridisation between distinct plant species with the intention of developing disease resistant crop varieties. Classical plant breeders may use use of a number of in vitro techniques such as protoplast fusion, embryo rescue or mutagenisis to generate diversity and produce plants that would not exist in nature (see also Plant breeding, Heterosis, New Rice for Africa). Chromosomal rearrangements and translocations occurring in these crosses help limit the amount of new DNA appearing in the final cultivated variety to a fraction of a chromosome, but still comprise substantial numbers of novel genes introduced into food.

These "classical" techniques (used extensively 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 transgenic wheat variety bred by E. S. McFadden with a transgene from a wild grass. Hope saved American wheat growers from devastating stem rust outbreaks in the 1930s.

Introduction of alien germplasm into common foods has repeatedly achieved novel genetic rearrangements of plant chromosomes, such as insertion of large blocks of rye (Secale) genes into wheat chromosomes ('translocations')[7].

The advent of drug colchicine in the late 1930s helped overcome fertility barriers in inter-specific crosses by stimulating doubling of chromosome numbers per cell, and after 1930 perennial wild-grasses were being frequently hybridized with wheat and other cereals with the aim of transferring disease resistance and perenniality into annual crops. Large-scale practical use of hybrids became well established, leading on to development of numerous Triticosecale (Triticale) varieties and other new transgenic cereal crops.

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

Disease resistance to 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

Disease resistance to Stem rust

  • Sr2 T. turgidum ("Hope" ) [8]
  • Sr22 Triticum monococcum
  • Sr36 Triticum timopheevii

Stripe rust

  • Yr15 Triticum dicoccoides

Powdery mildew

  • Pm12 Aegilops speltoides
  • Pm21 Haynaldia villosa
  • Pm25 T. monococcum

Wheat streak mosaic virus

  • Wsm1 Ag. elongatum

Pest resistance

  • Hessian fly
    • H21 S. cereale H23,
    • H24 T. tauschii
    • H27 Aegilops ventricosa
  • Cereal cyst nematode
    • Cre3 (Ccn-D1) T. tauschii

The intentional creation of transgenic plants by laboratory based recombinant DNA methods is more recent (from the mid-1980s on) and has been a controversial development opposed vigourously by many NGOs, and several governments, particularly within the European Community. These transgenic recombinant plants (= biotech crops, modern transgenics) are transforming agricultural productivity in those regions that have allowed farmers to adopt them, and the area sown to these crops has continued to grow globally in each of the ten years since their first introduction in 1996.

Transgenic recombinant plants are now generally produced in a laboratory by adding one or more genes to a plant's genome,and the techniques frequently called transformation. Transformation is usually acheived using gold particle bombardment or 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 transgenic crop approved for sale in the US, in 1994, was the FlavrSavr tomato, which was intended to have 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 itself, which is a natural transgenic plant derived from at least three different parenteral species.

Commercial factors, especially high regulatory and research costs, have so far restricted modern transgenic criop 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 [9], and insect protected Brinjal eggplant for India [10].

Plant transformation with foreign DNA

Modern biology can now be used to manipulate plant genomes and introduce short regions of foreign DNA into a plant by the process of plant transformation. This is the most common way transgenic plants are created in the laboratory.

One way this can be done is by exploiting one of the natural mechanisms for the relatively rare movement of DNA between species. The bacterium Agrobacterium tumefaciens has a natural mechanism called conjugation to inject small segments of DNA (T-DNA) into a plant cell. The T-DNA integrates randomly into the plant chromosomes and once inserted can function as a new gene. In the laboratory this mechanism is exploited to insert desired genes into the cells of plant callus tissue culture, which can then be regenerated into a full plant.

The preliminary step to using Agrobacterium for plant transformation is to carry out genetic engineering, using recombinant DNA techniques, to create T-DNA plasmid vectors that carrying the desired foreign DNA. The recombinant T-DNA plasmids are then used to replace the natural plasmids in living Agrobacterium cells which can then do the job of conjugating with plant callus tissue.

An alternative route to getting foreign DNA into plant cells is called biolistics. In this methods genetically manipulated DNA is coated onto small (gold) particles and these are fired into plant cells by a small gun-like device.

Current global picture of modern transgenic crops

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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

  • EPA - evaluates potential environmental impacts, especially for genes which produce pesticides
  • DHHS, Food and Drug Administration (FDA) - evaluates human health risk if the plant is intended for human consumption

Ecological risks

The potential impact on nearby ecosystems is one of the greatest concerns associated with transgenic plants but most domesticated plants mate with wild relative a some location where they are grown, and gene flow from domesticated crops (irrespective of whether they transgenic or non-transgenic) can the have potentially harmful consequences of 1. evolution of increased weediness; 2. increased likihood of extinction of wild-relatives. Weediness of hybrids created with domesticated crops is quite common. For instance in California, cultivated rye hybridises with the wild Secale montanum to produce a weed, and this has led many Californian farmers to abandon rye as a crop. [4]

Transgenes (and traits present in domesticated crop created by conventional breeding) have the potential for significant ecological impact if the plants can increase in frequency and persist in natural populations. This can occur:

  • if transgenic plants "escape" from cultivated to uncultivated areas.
  •  if transgenic plants mate with similar wild plants, the transgene could be incorporated into the offspring. 
  • if these new transgene plants become weedy or invasive, which could reduce
  • if the transgenic crop trait confers a selective advantage in natural environments

Gene flow may affect biodiversity and might affect entire ecosystems.

Pollen flow from conventional crop plants to native species also poses gene-flow derived ecological risks, as crop plants are not selected to have optimal selective advantages in natural environments, and farm fields are different to natural ecosystems. Conventional varieties also posses new traits such as pest resistance that have been deliberately transferred into the crop variety from other species.

There are at least three possible avenues of hybridization leading to escape of a transgene:

  1. Hybridization with non-transgenic crop plants of the same species and variety.
  2. Hybridization with wild plants of the same species.
  3. 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.
  • The hybrid offspring must be viable, and fertile.
  • The hybrid offspring 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.

References

  1. ↑ Ghatnekar L, Jaarola M, Bengtsson BO.(2006) The introgression of a functional nuclear gene from Poa to Festuca ovina.Proc Biol Sci. 2006 Feb 22;273(1585):395-9.
  2. ↑ Jumping Genes Cross Plant Species Boundaries. PLoS Biol 4(1): e35 DOI:10.1371/journal.pbio.0040035 Published: December 20, 2005
  3. ↑ Ramsey, J. and Schemske, D.W. (1998) Pathways, mechanisms, and rates of polyploid formation in flowering plants. Annual Review of Ecology and Systematics. 29, 467-501.
  4. ↑ J. A. Udall and J. F. Wendel (2006) Polyploidy and Crop Improvement. Crop Sci. 46, S-3-S-14
  5. ↑ Rieseberg, L.H. and Wendel, J. (1993). Introgression and its consequences in plants. In Hybrid Zones and the Evolutionary Process. (ed. J. Harrison) p 70-109, Oxford University Press, New York.
  6. ↑ Rieseberg, L.H. and Ellstrand, N.C. (1993) What can molecular and morphological markers tell us about plant hybridization/ Critical Reviews of Plant Science. 12 p213-241.
  7. ↑ [1]
  8. ↑ McFadden, E. S. (1930) J. Am. Soc. Agron. 22, 1020-1031.
  9. ↑ [2]
  10. ↑ [3]

Further reading

See Also

External links

fa:گیاهان تراریخته it:Piante transgeniche fi:Geenimuunneltu elintarvike

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