Transgenic plant

Transgenic plants possess a gene or genes that have been transfered from a different species. This can occur by cross pollination between different species, be mediated by vectors such as viruses or by manipulation in a laboratory setting.

The most efficient route for gene movement between plant species is cross-pollination, which generates inter-species hybrids that are usually new plant species. Interspecies hybridization generates polyploidy which is important since all major crops are examples of polyploid plants .

For gene movement to be successful the added genes must replicate themselves and be passed onto the next generation. These conditions are most likely to be met when cross-pollination is carried out between closely related plant species. Distantly related species generally fail to succesfully cross-pollinate at some point, for instance by producing infertile seeds, or by lacking two copies of every chromosome which are needed to participate in sucessful meiosis.

There are several known natural process for gene movement between different species in addition to the carriage of genes in pollen. One example 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.

Other routes for gene movement, such as passage inside virus particles, is suggested by the widespread movement of mobile DNA such transposons and MULE elements between different species. Since transposons are not stable and can move within a genome, they can sometimes mobilize adjacent genes too, increasing the posability for virally induced horizontal gene transfer (HGT). An example of this type of HGT has been documented for the MULE transposon between rice and millet plants.

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.

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 .

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 .

Natural movements of genes between species by other routes than pollen

Natural movement of genes between species 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 pollination, but can be frequent enough to be a significant factor in genetic change of a chromosome on evolutionary time scales

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 .

These various mobile or jumping genes play a major role in dynamic changes to chromosomes during evolution , , and have often been given whimsical names, 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 .

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 . An article about natural MULE gene movement between rice and millet is worth describing fully:

Jumping Genes Cross Plant Species Boundaries

In the early 1950s, legendary plant geneticist Barbara McClintock found the first evidence that genetic material can jump from one place to another within the genome. The variegated kernels of her maize plants, she determined, resulted from mobile elements that had inserted themselves into pigment-coding genes, changing their expression. McClintock's mobile elements, or transposons, moved over generations within a single species. More recently, another form of genetic mobility has been discovered—genetic information can sometimes be transferred between species, a process called horizontal gene transfer. While horizontal genetic transfer occurs most commonly in bacteria, it has been detected in animals as well. Most transfers between higher animals involve the movement of transposons. Horizontal transfer can also occur between the mitochondrial DNA of different plant species. Until now, however, no one had found evidence for horizontal transfer in the nuclear DNA of plants.

In a new report, Xianmin Diao, Michael Freeling, and Damon Lisch studied the genomes of millet and rice, two distantly related grasses that diverged 30–60 million years ago. While the two grasses show significant genetic divergence from accumulating millions of years of mutations, they carry some transposon-related DNA segments that are surprisingly similar. The authors conclude that these sequences were transferred horizontally between the two plants long after they went their separate ways.

Transposons of the class identified by Diao et al. typically consist of a variable length of DNA that codes for one or more enzymes flanked by repeating sequences called terminal inverted repeats (TIRs). These repeats can bind to each other to form a “lollipop” that is easily excised from the DNA strand, carrying the rest of the transposon along with it. Plant genomes are rife with transposons, many of which are relatively passive. Transposons from the “Mutator” family in maize, however, are especially active, frequently causing mutations as they insert themselves into new positions in the genome. They perform this jump with assistance from the two proteins they code for, a transposase and a helper gene.

DNA from many species of plants contains several families of cousins of the Mutator transposons. These “Mutator-like elements,” or MULEs, code for a protein similar to the transposase, as well as the TIR sequences. Diao et al. identified 19 distinct MULEs in the DNA of various species of millet (genus Setaria), and compared these with the rice genome sequence, which was published in 2002. They compared the sequence similarity of these MULEs to that of other proteins that are also conserved in the same species for which sequences are available. Strikingly, they observed much higher sequence similarity between the MULEs from millet and rice than is typical for transposons. The greater similarity of the MULE DNA is easily explained if it jumped somehow, horizontally, between the species, but there could be alternative explanations. The match could have arisen without horizontal transfer, for example, if the MULE DNA had been under positive selection, as typically happens for protein-coding genes that confer some survival or reproductive benefit. In such cases, natural selection tends to preserve the integrity of these sequences.

To test for signs of selection, the researchers looked at regions of the MULE DNA that don't appear to code for protein. The similarity between these noncoding regions in millet and rice MULEs was just as high as for the coding regions, even though selection probably doesn't influence them. Even within the coding sections, “synonymous” mutations—which don't change the protein sequence and so are not prone to selection—showed few differences between these elements.

Another explanation for the low divergence of the rice and millet MULE sequences could be that they occur within a genomic region that, for whatever reason, experienced lower than average mutation rates. If this were the case, sequences adjacent to the elements should also show reduced variation. The authors tested this alternative hypothesis with the help of maize, which has more genomic sequence available than millet, by comparing genes flanking MULE regions in rice with evolutionarily conserved sequences in maize. The sequences did not show the similar degree of reduced variation predicted for below-average mutation rates.

Since neither selection nor low mutation frequency can explain the similar DNA between the grasses, the authors conclude, a transposon must have carried it between millet and rice long after these species diverged. Interestingly, the authors also found similar sequences in bamboo, raising the question of how common horizontal transfer may be between plant species. Given that plant mitochondrial genes appear “particularly prone to horizontal transfer,” the authors note, “it is remarkable that these results represent the first well-documented case of horizontal transfer of nuclear genes between plants.” But as researchers begin to explore the growing databases of plant genomic sequences, they can determine whether this finding constitutes an anomaly—or points to a significant force in plant genome evolution. —Don Monroe

Citation: (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. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

It is thus becoming 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.

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').

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" )
• 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 , and insect protected Brinjal eggplant for India .

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.

Further reading

• Syvanen, M. and Kado, C. I. Horizontal Gene Transfer. Second Edition. Academic Press 2002.
• Syvanen, M. Cross-species gene transfer: a major factor in evolution. Trends in Genetics. 2, page 63-66.
• 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.