2. Genetically Engineered Plants
Mutations occur randomly in any genome, they are either without effect or are most likely to be disadvantageous to the organism. As a consequence they will be discarded by natural selection or, in the case of wheat, by the wheat breeder.
To increase the mutation rate in plant genomes with the aim of increasing the frequency of advantageous mutations, the induction of mutations with chemicals or ionizing radiation and subsequent selection was quite popular in breeding in the 1980s and 1990s.
Mutagenesis breeding is still carried out, but to a much lesser degree. In contrast to mutagenesis breeding, genetic engineering is highly specific since defined DNA fragments that encode a well characterized protein are transferred.
Only single genes or a few genes are integrated into the genome of the transgenic plant, the genetically modified organism (GMO), whereas in breeding whole genomes are "shuffled" to combine desirable traits.
In the case of mutagenesis breeding random, mostly detrimental, mutations occur somewhere in the genome.
In the following the term "gene" is used to describe a DNA fragment that is transcribed into mRNA and subsequently translated into a protein.
Either alone or through interaction with other proteins (or DNA) the protein finally confers a certain phenotype.
The term "expression" of a gene of interest in this context can equally mean transcription or translation.
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| Fig. 219 : From the DNA or gene via mRNA to the protein |
Fig. 219 illustrates the expression of a gene into a protein from DNA via mRNA as it occurs in all living cells with a few exceptions (i.e. some viruses).
Results of genetic engineering first became widely noticed in 1994 when the FlavrSavr® tomato from Calgene Inc. became the first transgenic food that was granted a licence for human consumption by the U.S. Food and Drug Administration (FDA).
A decade of testing at a cost of 525 mio USD had been invested to prove that the tomatoes were as safe as traditionally bred varieties.
Although the FlavrSavr® tomatoes were more resistant to rotting while maintaining their taste, consumer reluctance and their relatively high price meant that production ceased only a few years later.
By 2002 transgenic crops had gained greater acceptance, at least in the USA, Argentina, Canada and China. Together these countries made up 99% of the worldwide area under transgenic crop cultivation.
In total, 16 countries were growing genetically modified (GM) crops in 2002 (James, 2002). The traits that have been introduced into GM plants are mainly various tolerances to herbicides and resistances to insects, alone or in combination.
The current global status of all approved GM plants is summarized in Tab. 136, in alphabetical order (agbios, 2005).
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| Tab. 136 : Global status of all approved genetically modified plants to date (June 2005) |
To obtain the certification "approved genetically modified plant" in the USA and Canada, a risk assessment process lasting 7 to 10 years from the stage of discovery via confined field trials up to a food, feed and environmental assessment has to be conducted.
The transgenic plants are examined with respect to the same types of risk as traditionally generated plants, e.g. toxins, potential allergens, weediness and pest potential.
In the case of wheat, a herbicide-tolerant variety is the only GM variety that has been approved for release to date.
Considering the many acknowledged field trials as part of the risk assessment process for GM plants the number of transgenic wheat varieties finally released is expected to increase drastically in the coming years.
In the USA alone, as at April 2005, about 350 field trials have been acknowledged just for wheat. Nearly half of these field experiments investigate herbicide tolerance.
The second-largest group of field trials (24%) assesses transgenic wheat with potentially improved resistance to fungal diseases caused by Fusarium ssp., powdery mildew and Septoria leaf blotch or viral infections (BYDV, WSMV).
Further trials have verified the performance of genetically modified wheat in the field with respect to higher yields, altered starch or seed protein composition and drought tolerance.
For more details of field trials classified according to individual countries we recommend the web link http://www.nbiap.vt.edu/cfdocs/globalfieldtests.cfm.
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