Additional Viral
Recombination and Other Risks in Transgenic Crops
( The address of this page
is www.btinternet.com/~nlpwessex/Documents/gmviral.htm )
In addition to the common use of the CaMV viral promoter in the majority of transgenic plants, some GM plants are also modified with the intention that they gain protection from attack from viruses in the environment. A number of such crops are already being released into the environment in some parts of the globe.
However, the scientific literature reveals that such crops can
also be susceptible to recombination
and other events which may enable the
creation of new viral diseases. The literature confirms
that it may be very difficult to prevent such phenomena when GM
crops of this type are released into the environment.
The following extracts from Hull R.
(1998) Detection of risks associated with coat protein
transgenics. In: 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, form part of an assessment of the
nature of these risks, pointing out that compared to laboratory
situations monitoring and detection of such phenomena in the case
of large-scale field-releases "will be
much more difficult, if not impossible."
The author of the paper is Professor Roger Hull, of the
Department of Virus Research at the John Innes Centre, UK.
[Extracts from paper]
1. Introduction
"Recent advances in the understanding of the molecular
mechanisms of how viruses function and how they interact with
plants have led to the development of various nonconventional
approaches to protection of plants against viruses. Many of these
approaches involve the introduction of viral or virus-based
sequences into the plant's genome....
Many of these transgenic plant lines... have reached the stage of
field testing for the efficacy of protection, and are even being
more generally field-released. This raises the question of
possible risks that could arise on general field release, a topic
which has previously been discussed by Hull (5-7), Hull and Davis
(12), de Zoeten (17), and Tepfer (18). Depsite these discussions, the issue has not been
fully resolved, and various other aspects are being
raised.....
1.1 Potential Risks on Field Release
".......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........
1.3. Recombination
Three sorts of recombination have been recognized (20):
homologous with crossovers between related RNAs at precisely
matched sites, aberrant homologous with crossovers between
related RNAs not at corresponding sites, and nonhomologous with
crossovers between unrelated RNAs at noncorresponding sites.
There is considerable evidence for extensive recombination in RNA
viruses (see refs. 20 and 21 for details), and probably all three
mechanisms have been involved at one time or another. 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.....
1.4. Heteroencapsidation
This involves the superinfection of a plant expressing the CP
of a virus, say virus A, by an unrelated virus B.
Heteroencapsidation is the encapsidation of the genome of virus B
by the CP of A, thereby conferring on virus B properties of virus
A. 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.).
The discussion of heteroencapsidation has focused on
superinfecting viruses. However, there is the possibility that
heteroencapsidation of retrotransposons could present a problem.
Retrotransposons are a major class of transposable elements whose
structure resembles the integrated copies of retroviruses, and
which are considered to be important in evolution (see ref. 30).
The Tyl-copia group of retrotransposons is widespread in plant
genomes (31—33), and it has been suggested that there might
be horizontal transmission between species (31). Sequencing has
shown that most copies of the Tyl-copia retrotransposons in
plants are mutated, so they would not be active. However, several
active ones capable of retrotransposition have been described
(30,34—37) and presumably replicate, as do all
retrotransposable elements, via RNA. 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."
1.5 Risk Reduction and Control
"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 [to control heteroencapsidation].......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 personel to identify any unusual event that might be associated with transgenic plants. This will be the challenge for the future."
Evidence (1999) that a plant virus switched hosts
to infect a vertebrate and then recombined with a
vertebrate-infecting virus
Additional
risks associated with crops incorporating viral transgenes
identified by Monsanto and USDA Scientists
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What leading scientist have said
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