Transgenic plant

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See also: horizontal gene transfer in plants

Transgenic plants possess a gene or genes that have been transferred from a different species such as another plant, or a microorganism, or other source. They are created in nature during horizontal gene transfer They can also be created during plant breeding, and especially in recent years through the use of the plant DNA transformation technique in biotechnology. (See Plant breeding, Biotechnology and plant breeding).

Introgression

The most efficient natural route for gene movement between plant species is by cross-pollination to form fertile plant inter-species hybrids. Such hybrids are sometimes new plant species, but they can also form a gene transmission "bridge" in the hybrid-zone between two distinct plant populations, and thus facilitate introgression, which is movement of genes from one distinct species or population to another.

More distant natural transfers

The best documented route for natural formation of transgenic plants is gene transfer between a plant epiphyte (such as [[moss]es]), or a parasitic plant (like dodder) and the host plant it colonizes.

Non-standard fertilization of 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].

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

Recent comparative studies of gene content of different genomes provides strong circumstantial evidence that natural horizontal gene transfer does occur in plants at a frequency that is significant over evolutionary time scales. Over the evolutionary time-scales plant mitochondria are a stopping point for genes that may enter the nuclear genome from other species, and can in some cases be very active in inter-species gene-traffic (See Horizontal gene transfer).

The natural DNA transfer of mobile DNA between rice and millet is well documented [2], but genes involved in metabolic process and bacterial genes can also take part in natural genes transfers (See horizontal gene transfer in plants).

Hybrid formation in flowering plants and its role in introgression of genes between species

Cross-pollination between plant species generates interspecies hybrids occurs widely in nature and has been exploited in plant breding for more than 100 years to create artificial transgenic plants ( see Plant breeding).

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[3] [4].

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 an allotetraploid that is reproductively isolated from both parents - and a new species[5].

Wild emmer wheat is an example of a species formed by hybridization between two diploid wild grasses, Triticum urartu (AA) and a wild goatgrass Ae. speltoides (BB) to form hybrid new species with four sets of chomosomes (AABB) which is a tetraploid. Triticale (Triticosecale) is a 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 [6].

Gene transfer could occur by wide cross-pollination even if foreign chromosomes are lost. It has been suggested that 'wide crosses' are a possible mechanism of horizontal transfer of mobile DNA in plants, and that these might transfer only mobile DNAs, due to one of the participating sets of chromosomes being lost [7][8]

Natural movements of genes between species by other routes than pollen

See Horizontal gene transfer in plants, Horizontal gene transfer

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')[9].

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 are [10]:

Disease resistance to Leaf rust Disease resistance to powdery mildew
Gene Source Gene Source
Lr9 Aegilops umbellulata Pm12 Aegilops speltoides
Lr18 Triticum timopheevi Pm21 Haynaldia villosa
Lr19 Thinopyrum Pm25 T. monococcum
Lr23 T. turgidum
Lr24 Ag. elongatum Disease resistance to wheat streak mosaic virus
Lr25 Secale cereale Wsm1 Ag. elongatum
Lr29 Ag. elongatum
Lr32 T. tauschii Pest resistance
Hessian fly
Disease resistance to stem rust H21 Secale cereale
Sr2 T. turgidum ("Hope" [11]) H23 Secale cereale
Sr22 Triticum monococcum H24 T. tauschii
Sr36 Triticum timopheevii H27 Aegilops ventricosa
Disease resistance to stripe rust Cereal cyst nematode
Yr15 Triticum dicoccoides Cre3 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. In those regions that have allowed farmers to adopt them these transgenic recombinant plants (= biotech crops, modern transgenics) are transforming agricultural productivity, 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 achieved 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 [12], and insect protected Brinjal eggplant for India [13].

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. In addition, transgenic plants have been created by adding DNA to protoplasts, inducing them to take it up, and then selecting and regenerating plants from those cells.

Current global picture of modern transgenic crops

A good source of information is the ISAAA [5]

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 expressed about transgenic plants. 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 [14].

The main concerns are of 1. evolution of increased weediness; 2. increased likihood of extinction of wild-relatives. There are known instances of unwanted weediness of hybrids created by unintended gene flow from domesticated crops to wild-relatives. 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. [15]

Transgenes and other new traits such as mutation to herbicide tolerance present in domesticated crop created by conventional breeding have the potential for significant ecological impact if the plants receiving the trait ccan 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. 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.
  4. 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.
  5. 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.
  6. J. A. Udall and J. F. Wendel (2006) Polyploidy and Crop Improvement. Crop Sci. 46, S-3-S-14
  7. Ananiev EV, Riera-Lizarazu O, Rines HW, Phillips RL (1997) Oat maize chromosome addition lines: a new system for mapping the maize genome. Proc Natl Acad Sci USA 94: 3524–3528.
  8. Bennetzen, J. L., (2000) Transposable element contributions to plant gene and genome evolution. Plant Molecular Biology 42: 251–269, 2000.
  9. [1]
  10. Plant genetic resources: What can they contribute toward increased crop productivity? Hoisington, D. and others (1999) Proc. Natl. Acad Sci USA. Vol. 96, Issue 11, 5937-5943, May 25, 1999. (This paper was presented at the National Academy of Sciences colloquium "Plants and Population: Is There Time?" held December 5-6, 1998, at the Arnold and Mabel Beckman Center in Irvine, CA).
  11. McFadden, E. S. (1930) J. Am. Soc. Agron. 22, 1020-1031.
  12. [2]
  13. [3]
  14. Morris S.H. (2006) EU biotech crop regulations and environmental risk: a case of the emperor's new clothes? Trends Biotechnol. 2006 Nov 17; [Epub ahead of print]
  15. [4]