Transgenic plant

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

The most efficient route for gene movement between species is cross-pollination, which generates hybrids.

For gene movement to be successful the added genes must replicate themselves successfully in in some way, or be inserted into existing chromosomes which are able to replicate succesfully. The cells carrying the added genes must also produce the normal reproductive process for the plant, that is produce seed formation.

These events are most likely if cross-pollination is carried out between closely related plant species. Distantly related species generally fail at some point, for instance by being infertile.

There are however natural process for gene transfer in addition to pollen.

One example is Agrobacterium tumefaciens, which is a bacterium that injects DNA into plant cells. Biotechnology laboratories exploit this bacterium to make artificial transgenic plants with small segment of added DNA inserted in the host cell chromosomes.

But there are other routes for gene movement that occur but which are ill-characterised. This is shown by the widespread movemment of mobile DNA like transposons such as Mariner and its relatives between different species. It is also known that mobile DNA can move from site to site within a genome, so the possibilities for horizontal gene transfer could be a combination of different mechanisms, including movement of mobile DNA.

Hybrid formation in flowering plants

Hybrid formation which adds sets of chromosomes which is a common event in flowering plant evolution, and the main way new plant species are formed.

Wild emmer wheat is an example of a species 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 tetrapoid. Triticale 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.

Transgenic plants and crop improvement

Prior to the current era of Molecular genetics starting around 1975, transgenic plants including cereal crops were (since the mid 1930s) were part of conventional Plant breeding.

Transgenic varieties are frequently created by classical breeders who deliberately force hybridisation between distinct plant species when carrying out interspecific or intergeneric wide crosses with the intention of developing disease resistant crop varieties. Classical plant breeder 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).

These "classical" 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 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.

Methods used in traditional breeding that generate transgenic plants 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 pest and helping land and water to be used more efficiently. (see also Food security, International Fund for Agricultural Development, International development)

Natural movements of genes between species.

Natural movement of genes between species, often called Horizontal gene transfer or lateral gene transfer, can also because of gene transfer mediated by natural agents such as microrganisms, viruses or mites. Such transfers occur at a frequency that is low compared with the hybridization that occurs during natural pollination, but can be frequent enough to be a significant factor in genetic change of a chromosome on evolutionary time scales, Syvanen, M. and Kado, C. I. Horizontal Gene Transfer. Second Edition. Academic Press 2002.

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 transfer to new locations in a Genome, and often move to new species host 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 DNA-binding specificity of rice mariner-like transposases and interactions with Stowaway MITEs.

These various mobile genes play a major role in dynamic changes to chromosomes during evolution [1], [2], and have often been given whimsical nanes, such as Mariner, Hobo, Trans-Siberian Express (Transib), Osmar, Helitron, Sleeping Princess, MITE and MULE, to emphasise their mobile and transient behaviour.

Such genetically mobile DNA contitutute 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 large and growing scientific literature about natural transgenic events in plants, such as the creation of shibra millet in Africa, and movement of natural mobile DNAs called MULEs between rice and millet [3].

It is becomming clear that natural rearrangments of DNA and generation of transgenes 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 transgenic varieties in traditional breeding.

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 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).

Introduction of alien germplasm into common foods was repeatedly achieved by traditional crop breeders by artificially overcoming fertility barriers throughout the last century, and novel genetic rearrangements of plant chromosomes, such as insertion of large blocks of rye (Secale) genes into wheat chromosomes ('translocations'), have also been exploited widely for many decades [4].

By the late 1930s with the advent of drug 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.

Important transgenic pathogen and parasite resistance traits 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" ) McFadden, E. S. (1930) J. Am. Soc. Agron. 22, 1020-1031 .
• 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-80s 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 [5], and insect protected Brinjal eggplant for India [6].

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.

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:

• USDA Animal and Plant Health Inspection Service - who state that

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. [7]

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

• Syvanen, M. and Kado, C. I. Horizontal Gene Transfer. Second Edition. Academic Press 2002.
• Chrispeels, M.J. and Sadova, D.E. Plants, Genes, and Crop Biotechnology. Second Edition. James and Bartlett 2003.
• Plant genetic resources: What can they contribute toward increased crop productivity? David Hoisington*, Mireille Khairallah, Timothy Reeves, Jean-Marcel Ribaut, Bent Skovmand, Suketoshi Taba, and Marilyn Warburton, 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).
• U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA-APHIS) Publications Biotechnology.
• Biotechnology, Federal Regulation, and the U.S. Department of Agriculture, February 2006, USDA-APHIS Fact Sheet
• Biotechnology Regulatory Services, Coordinated Framework for the Regulation of Biotechnology, USDA-APHIS Outreach Material
• Questions and Answers About Biotechnology and the USDA, August 2006, USDA-APHIS Fact Sheet
• Permitting Genetically Engineered Plants That Produce Pharmaceutical Compounds, February 2006, USDA-APHIS Fact Sheet

See Also

• Plant breeding
• Food security
• Transposon
• Mobile genetic elements
• Transposons as a genetic tool
• Genome
• Arabidopsis thaliana
• Food security
• United States Department of Agriculture
• Foreign Agricultural Service
• Food and Agriculture Organization
• Developing country
• International development
• International Fund for Agricultural Development
• Hunger

External links

• Food Security and Ag-Biotech News — balanced news on the debate over transgenic crops
• Plant genetic resources and transgenics contributions towards increased crop productivity — Context of transgenics for food security
• Information Systems for Biotechnology (ISB) based at Virginia Tech — Authoritative information in the form of readable up-to-date articles to support the environmentally responsible use of agricultural biotechnology products, including transgenic plants.
• Foundation for Biotechnology Awareness and Education
• PG Economics - Reports on agricultural economic benefits.
• AgBioForum Journal- Professional economics papers on crop biotechnology
• Internation Food Policy Research Institute The food security context.
• Truth About Trade- Forthright support of agricultural technology for farmers and the value of free trade.
• AgbioWorld- Comprehensive scientifically sound resource that supports the use on transgenic crops and provides news from around the world on agriculture.