Essential Differences Between GM and non-GM Plants
Paper Extracts From Scientists
At The John Innes Centre, Norwich, UK
www.btinternet.com/~nlpwessex/Documents/jicgmcharacteristics.htm

Note:
'Transgenic' = Genetically modified/engineered
'Transgene' = Gene introduced through genetic modification/engineering

Click here for definitions of some of the additional technical terms used below as defined in: 'A History of Genetically Modified Plants', Paul.F. Lurquin, Columbia University Press 2001 or Henderson's Dictionary of Biological Terms

"Because we rely largely on our knowledge of classical genetics to inform biosafety assessment, it is important to determine whether transgenes are the same as resident genes in every respect. As there are some differences, it is important to consider the consequences of those differences for influencing biosafety assessments.... The essential difference between conventionally bred varieties and transgenic varieties is that the former are produced by cross pollination between varieties of the same species or between closely related species, while the latter can use genes from any class of organism.  The consequence of this is that conventional breeding is limited to the genes (the gene pool) from the same species or closely related species; whereas for genetic modification, genes can be taken from viruses, bacteria, unrelated plants, animals (including humans) and made synthetically......By [GM recombinant DNA] transformation it is possible to introduce genes into plants from any class of organism and therefore it is considered potentially possible to produce a plant phenotype of which there is little or no experience ....For biosafety assessment purposes the essential differences between conventional breeding and transgenic modification are summarised in Table 17.1... most [recombinant DNA] gene constructs contain some sequences derived from plant pathogens.... [With conventionally bred plants] phenotypic variation usually falls within a familiar range...[With GM transgenic plants] there is the potential to change plants fundamentally.... A comparison of the characteristics of resident genes and of transgenes is summarised in Table 17.3. The essential differences between transgenes and resident genes are outlined... During evolution it is reasonable to assume that genomes have evolved a degree of genetic balance so that the position of genes within the genome has some adaptive significance.  With transformation, it is possible to introduce alien gene sequences into random locations.... transgenes do often display unusually high levels of expression and structural instability... The plant may ...... treat DNA inserted during the transformation process in a similar way to invading pathogenic nucleic acids.  The difficulties that have been experienced in developing transformation methods in many species over the past decade of research, illustrate that plants do not readily take up and incorporate 'invading DNA' into the genome..... comparison of the expression of single copy transgenes from different transformation events (ie in different locations in the genome), frequently shows considerable variation in transgene expression.... Very high levels of transgene instability can be observed, especially among primary transformed plants, and early sexual generations....Instability can be of various kinds: transgenes or parts of them can be lost; expression can be silenced; transgenes can display abnormal inheritance; transgenes can show abnormal tissue specificity and expression can vary through plant development and inheritance....Areas of sequence homology in transgene constructs are known to interact with one another in some plants.  An effect of this can be that one transgene construct may inactivate the expression of another construct in the same plant.... It is possible that a rare event may have insignificant consequences when transgenic crops are grown on a small scale, but become important when transgenic crops are grown over thousands of hectares...."
Dale (John Innes Centre) et al
Transgenic Plant Research 1998, 277 - 285
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"In a recent survey of at least thirty companies developing transgenic plants for use in agriculture, all companies observed some transgene instability (Finnegan, et al., 1994). Many research laboratories have also reported difficulties in obtaining sufficiently high expression of certain transgenes or in stabilising transgene expression of an introduced gene. In a recent study in our laboratory, one hundred Brassica napus [rape seed] transgenic lines were produced and half of them displayed unstable or unusual transgene behaviour.... Any areas of internal homology or of inverted DNA sequence repeats may potentially increase the likelihood of instability and unusual transgene expression....It is common to observe substantial variation in transgene expression between independently transformed plants.  The source of this variation is often attributed to transgene position or copy number effects. Structural rearrangements associated with the transformation process, including deletions, duplications and repeated areas of homology are likely to influence transgene stability and expression.... Evidence from our laboratory, following the transfer of a construct into a range of different spring and winter lines of Brassica napus has shown that in some cases the transgenes behaved normally, and in other cases abnormally, with evidence of loss of particular transgenes... The precise cause of the interaction between transgenes and the background genotype is not known.... it was found that one transgene could switch off the expression of another transgene. When the transgenes were separated sexually, the previously silenced transgene was able to express. When the same two constructs were combined again sexually, the silenced phenotype was re-imposed... There are a number of cases where environmental stimuli affect transgene stability....Recent research in our laboratory with Brassica napus plants containing the 35S promoter from the mosaic virus (CaMV) has shown that upon infection with the CaMV the driven transgene is silenced (Al-Kaff et al unpublished). Intensive research at present is directed towards understanding this silencing mechanism and its significance. As the 35S promoter is widely used to regulate transgenics in brassicas, it is important that we strive to obtain a clear understanding of the mechanisms of this silencing and its significance. This is important for two reasons, for assessing the use of the 35S promoter in agriculture and also for assessing the significance of this effect for biosafety. ... Many factors influence the ways in which transgenes express, but a factor of crucial importance is the effect of DNA sequences that are homologous to areas of transgene constructs. .....the take home message from this paper is 'watch out for homology'..."
Dale (John Innes Centre) et al,
1998 ACTA Horticulturae 459, 167-171
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"Data from the 1998 trial showed that transgenic barley lines performed as well as non-transformed control plants and controls from tissue culture-derived parents for several agronomic traits, including yield. For other traits, a significant difference was seen between transgenic and control lines. The transgenic lines were significantly shorter and also slightly later flowering […]. When we examined the next generation of the same transgenic line in the field during 1999, there was evidence that the transgenic plants were more variable compared to the controls than those in the 1998 field trial. This could be because somaclonal variation, resulting from the tissue culture and [genetic modification] transformation procedures, and was more obvious in later generations. These results show that transgenic lines need to be examined over a number of generations under field conditions to obtain the necessary data on transgene stability and agronomic performance. Further field trials […] combined with detailed molecular and genetic analysis will allow us to increase our understanding of the transformation process so that we are better able to assess the long term effects of genetic modification."
John Innes Centre & Sainsbury Laboratory Annual Report 1999/2000, p.28
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".......The area of concern specific to viral transgenes is the potential risks on any interactions between the viral or virus-related sequences being expressed from the transgene and another virus superinfecting that plant. Three main scenarios are usually considered: synergism, recombination and heteroencapsidation........It is generally considered that recombination plays an important role in the evolution of RNA viruses (see refs. 20—23). Evidence is now forthcoming of recombination between superinfecting viral RNA and RNA expressed from a transgene (24) through the aberrant homologous recombination mechanism. The finding of recognizable host RNA sequences within viral RNAs (25,26) is suggestive of nonhomologous recombination. All the experimentation on recombinants between plant virus sequences has been done in controlled laboratory situations. It is difficult to devise detailed protocols for the detection of recombinants produced in the field..... There are several examples of heteroencapsidation in transgenic plants, both between viruses of the same group (27,28), and between unrelated viruses (29). The main property of CP that is considered is that of vector transmission characteristics. However, there is increasing evidence that CPs are involved in long distance viral movement around infected plants, and heteroencapsidation could enhance the movement of a superinfecting virus that did not normally move systemically (see Subheading 1.2.).....Among the factors that activate plant retrotransposons is tissue-culture, a process involved in transformation (37). This raises the possibility that introduction of the CP transgene could activate retrotransposon RNA, which becomes heteroencapsidated and transmitted horizontally to other species..... The main question to be addressed is whether the risk on field release of the transgenic plant is significantly more than the risk from the nontransgenic plant .... It is likely that it will take some time for a full risk assessment on the viral transgenic plants to be performed and commercial and other pressures will be very strong for field release. There are two approaches to risk reduction and control that can be put into effect relatively soon. One is to use biological containment ...... Much more difficult is to avoid recombination. ......The second approach is to design methods for monitoring the effects of field release. For small-scale releases, it is relatively easy to design monitoring procedures for analyzing pollen flow into related weed species and for detecting heteroencapsidants or recombinants. This will be much more difficult, if not impossible, for large-scale releases, in which the approach should be to educate farmers and extension service personnel to identify any unusual event that might be associated with transgenic plants. This will be the challenge for the future....."
Hull R. (John Innes Centre)
Methods in Molecular Biology: Plant Virology Protocols: from Virus Isolation to Transgenic Resistance (Eds: Foster G.D., Taylor S.C.). New Jersey, Humana Press Inc. 81, 574-555, (1998)
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"At present, the success of transgene expression is variable, and many transformation experiments have to be carried out in order to isolate a small number of useful lines.  Many factors can influence the behaviour of foreign DNA when it integrates into the plant genome.  Such factors include the position of integration, possible rearrangements of the exogenous DNA by recombination, and the activation of endogenous plant defence systems that have evolved to suppress the activity of 'invading' DNA. (Fig.17)... Fig.17 Transgene rearrangements often occur at regions rich in DNA secondary structure, such as the CAMV 35S promoter, which can form the cruciform structure shown above.  This allows recombination to occur, as shown by the green arrow."
John Innes Centre and Sainsbury Laboratory Annual Report 1998/99, p22 -23
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"Analysis of junctions between genomic and transforming DNA, and between individual plasmid molecules at integration sites, demonstrates the predominance of microhomology-mediated illegitimate recombination events involving regions with secondary structure. One such region occurs in the CaMV 35S promoter, widely used to drive transgene expression in plants. The plasmid backbone provides other such regions, including the origin of replication [...]. The influence of transgene rearrangements on expression and silencing has been understated in the past....."
John Innes Centre & Sainsbury Laboratory Annual Report 1999/2000, p.29.
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