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From small-scale to large-scale releases of transgenic crops: are there
unresolved issues regarding impacts on the ecosystem?
Philip J Dale
John Innes Centre, Colney Lane, Norwich NR4 7UH, United Kingdom
During the history of crop improvement, plant breeders have become familiar
with the nature and extent of the genetic variation released by
sexual recombination. Where potentially undesirable or harmful plant
products are obtained, procedures have been adopted to eliminate them
from the breeding programme. Within the past decade there have been
significant advances in the way we can genetically modify crop
plants. The essential feature of these advances for biosafety, is
that we can now incorporate genes from microbes, unrelated plants,
animals, humans, and even those synthesised in the laboratory. It is
likely, therefore, that plant phenotypes will be produced that are
outside the experience of conventional plant breeding. Some of the
genes introduced, and recent examples include the ability to produce
pharmaceutical substances (Goddijn and Pen, 1995; Mason and Arntzen,
1995), will provide particular challenges for biosafety assessment.
Other modifications may produce phenotypes similar to those in
conventional breeding, such as virus resistance or pest resistance,
but use novel genetic mechanisms (Shah et al., 1995).
Many thousands of transgenic plants have been obtained in a wide range of
crop species. There have also been many hundreds of field releases
worldwide. In a recent review (Dale, 1995), with data up to the end
of 1993, 38 crop plant species had been tested in field releases in
31 different countries. During the past 2 years, several transgenic
crop lines have been approved for widespread unrestricted use, and
several other crop/transgene combinations are being considered for
similar approval. Even though there is growing experience in
assessing transgenic crops under field conditions, it is sometimes
argued that because the majority of releases to date have been small
scale and confined, we have learned little that is relevant to
assessing the potential impact when those transgenic plants are grown
extensively in agriculture.
The objective in this paper is to summarise what I believe we have
learned about transgenic plants over the past decade, from
laboratory, glasshouse and field studies. I will then discuss some
of the questions and uncertainties I believe need to be considered in
assessing the possible impact of transgenic plants when used widely
What have we learned about transgenic
plants that is relevant to biosafety assessment?
Variation in transgene expression and stability.
There is often substantial variation between independently transformed plants in
transgene expression levels and tissue specificity. Transgene
expression can become switched off during plant development and over
sexual generations. Transgenes can be unstable and become
structurally modified over time. Some of this variation is
influenced by the number of transgene constructs inserted, and the
position of insertion of the transgene in the plant genome (Meyer,
Environment is known to influence transgene expression and stability
Transgenes can influence the expression of genes that would not be expected
to be associated with the action of a
specific transgene (Dale and McPartlan, 1992).
Background genotype effects.
The background genotype of a plant affects transgene expression and stability.
Transgenes moved to different background genotypes by sexual hybridization
have displayed variation in expression and stability (Irwin et al., 1995).
Independently transformed plants can display variation originating from the
plant tissue culture process used during plant transformation
(Dale and McPartlan, 1992).
Co-suppression and transgene interaction.
Transgenes have been observed to interfere with the expression of endogenous
genes, and transgenes can sometimes modify the expression of genes present
on other transgene constructs in the same plant (Meyer, 1995).
Characteristics of specific genes.
Companies have carried out extensive studies on the characteristics of the
transgenes they are incorporating into plant varieties for commercialisation.
This has included assessments on allergenicity, toxicity, non-target
organisms, and on the general growth and physiology of the plant
(Fuchs et al., 1992).
Characteristics of transgene dissemination by pollen.
There have been various studies to assess the distance transgenes are
transported by pollen, and the likelihood of cross pollination with related
sexually compatible species. This information is being used by
Regulatory Authorities to specify isolation distances for the field
evaluation of novel transgenic plants, and in assessing the
likelihood and possible impact of gene transfer to related plants in
wild or weedy plant populations (Scheffler et al., 1993; McPartlan
and Dale, 1994; Scheffler and Dale, 1994).
Many of the characteristics of transgenes and transgenic plants could have
been predicted from our knowledge of conventional genetics. What is
perhaps surprising is the extent of the variation in expression and
transgene instability between different plants transformed with the
same gene construct. There are also very specific interactions
between different transgenic constructs introduced into the same
plant, and interactions between introduced genes and endogenous
genes. It is reasonable to assume that these phenomena reflect
genetic principles that are normal features of interactions between
endogenous genes, but the use of transgenes enables them to be
studied with a degree of precision that was difficult before the
advent of transformation.
Information obtained on transgenic plants over the past 13 years makes
biosafety assessment more informed, and helps with determining whether the
introduced transgene changes the crop plant in ways that affect its
biosafety and its possible impact on the ecosystem.
Are there unresolved issues relevant to assessing the impact of the
widespread use of transgenic crops in agriculture?
What I have included here as unresolved issues will rarely have a
significant impact on the biosafety of transgenic crop plants.
However, I raise them because I believe they are questions that need
to be considered and debated as we attempt to assess the possible
impacts of specific transgenic plants in agricultural production.
The use of transgenic plants as food was the subject of the 1994
Basel Forum on Biosafety, and is outside the scope of this
Subtle or accumulative effects.
Companies developing transgenic plant varieties usually carry out extensive
tests, on non-target organisms, on toxicity and on allergenicity. These are
essentially short term and small scale tests. With some of the characters like
Bt (Bacillus thuringiensis) insect resistance, there has already been
several decades of applying the Bt insecticidal protein in sprays of
the Bacillus thuringiensis bacterial suspension. However it is
important that long term impact assessments consider subtle
accumulative or less obvious effects on the environment from plants
able to produce the transgene protein, throughout most of their
lifetime, and in the majority of their cells.
There are now many different herbicide tolerance genes available for
incorporation by transformation, and there have
been field trials with transgenic plants with at least 9 different
herbicide tolerance genes (Dale, 1995). There is now the potential
for plant breeders to produce different varieties of a crop carrying
a range of different individual herbicide tolerance genes. This
could, potentially, make it difficult to control ground keeper weeds
of that crop in set-aside land, or in subsequent cropping, or lead to
the production of multiply resistant weed species that receive the
transgenes by cross pollination. Although there has been some debate
on this subject, the results have generally been inconclusive. The
options appear to be:
- to leave the future use of herbicide tolerance genes
to market forces,
- to develop integrated weed management
strategies for different crops and
- to restrict the use of herbicide resistance genes.
It is reasonable to assume that it would not be in the interest of plant
biotechnology companies to introduce a herbicide tolerance transgene
that would be unsuccessful in a new plant variety or would jeopardise
the long term usefulness or commercial future of a particular
herbicide. This is a complex issue, however, which many regulatory
authorities find challenging. Part of the problem is that the use of
herbicide tolerance may well impact more on agricultural strategies,
rather than biosafety or the ecosystem, and that orchestrating
agricultural strategies for the future are not usually the
responsibility of authorities regulating the release and
commercialisation of transgenic crops. However, if the use of
herbicide tolerance genes results in difficulties with weed control
or the requirement to use less environmentally friendly herbicides,
then this could well impact on the agricultural, and possibly the
Disease and pest resistance.
One of the attractive features of modifying plants by transformation is that
many novel pest and disease strategies become available. There is the
opportunity to be much more imaginative in designing resistance mechanisms,
than have hitherto been possible from within the sexual gene-pool
available to the conventional plant breeder. A possible disadvantage
is that the same, or very similar resistance mechanisms, can
potentially be used in a wide range of crops plants. For example Bt
insecticidal proteins can be used in many important crop plant
species (Shah et al., 1995). It is important to develop intelligent
strategies for the use of particular resistance mechanisms, because
their widespread use is likely to result in very intense selection
pressure on pests and pathogens to overcome the resistance mechanism.
There are already indications of insects becoming multiply resistant
to Bt proteins (Gould, 1991), and the Bt working group in the USA is
considering long term strategies for the use of Bt in agriculture (Kidd, 1994).
The incorporation of part of the viral genome to
confer resistance to certain viral diseases is the subject of some
biosafety debate, especially with regard to transcapsidation and
template switching. Virus resistance is the subject of another
presentation at this meeting, but there is a growing belief that
these phenomena probably do not present a new biosafety hazard.
Mixed infections with different viruses are believed to give
opportunities for these two phenomena to occur in conventionally bred
plants. There is still uncertainty, however, about whether mixed
infections with different viral strains provide the same opportunity
for these phenomena as do transgenic plants containing viral DNA
sequences in every cell of the plant (Dale, 1995).
Transgene instability and loss of expression is often regarded as of little
significance for biosafety assessment because the silencing of a transgene will
make the plant like the non-transgenic crop genotype it was derived from.
There are some instances, however, where gene silencing may need to be
considered carefully in biosafety assessments. Transgenes are sometimes used
to down-regulate undesirable proteins eg allergenic proteins in rice
(Tada et al., 1995). Silencing of the transgene in this case, even
if only a rare event, could result in expression of the undesirable
allergenic protein .
Different transgene constructs present in the same plant are found to interact
with one another. When there are homologous sequences in the two constructs,
transgene expression can be modified. This modification appears to be influenced
by the position of the transgene within the plant genome (Delannay et al.,
1989; Matzke et al., 1993). This phenomena may have consequences for
biosafety in two respects:
- when plant breeders begin to combine different
transgenes into the same variety and
- different transgenes may be transferred to wild
species by cross pollination.
International transfer of transgenic plant material.
Most regulatory authorities concentrate their biosafety assessment on the
release of a transgenic crop plant within their own country. When transgenic
plants are in widespread agricultural production, seeds will be
transported deliberately or unintentionally from one continent to
another. National regulatory authorities should take this
possibility into account in their biosafety assessments, and consider
wider ecosystem impacts, including the possibility that the modified
plant may be transferred to the centre of origin or diversity where
it may hybridize freely with plants in natural populations.
Monitoring after commercialisation
Although detailed monitoring of transgenic crops is often required for
small-scale releases, there is frequently no statutory requirement to
monitor crops once they have been approved for widespread commercial
use. To bridge the gap between small-scale releases and the large
scale commercial use of transgenic crop plants, it makes sense to
search for answers to some of the questions raised above, by a
monitoring programme. There are a number of difficulties with
monitoring, eg. who should do it, who should fund it, what should be
measured, is the data meaningful for assessing biosafety? It is
important that any monitoring has a strong scientific base, but
because of the nature of the work, it is often not very satisfying
scientifically. The question of monitoring needs to be considered
further to determine what is possible, sensible and relevant to long
term assessments of the impact of transgenic crops on biosafety and
thank the Biotechnology and Biological Sciences Research Council, the
Ministry of Agriculture Fisheries and Food and the European Union,
for financial support.
Dale, P.J. (1995). R & D regulation and field trialling of transgenic
crops. Trends in Biotechnology 13, 398-403.
Dale, P.J. and McPartlan, H.C. (1992). Field performance of transgenic
potato plants compared with controls regenerated from tuber discs and
shoot cuttings. Theoretical and Applied Genetics 84, 585-591.
Delannay, X., LaVallee, B.J., Proksch, R.K., Fuchs, R.L., Sims, S.R.,
Greenplate, J.T., Marrone, P.G., Dodson, R.B., Augustine, J.J.,
Layton, J.G., and Fischhof, D.A. (1989). Field performance of
transgenic tomato plants expressing the Bacillus thuringiensis var.
Kurstaki insect control protein. Bio/technology 7, 1265-1269.
Fuchs, R.L., Berberich, S.A., and Serdy, F.S. (1992). The
biosafety aspects of commercialization: insect resistant cotton as a
case study. In The Biosafety Results of Field Tests of Genetically
Modified Plants and Microorganisms. R. Casper and J. Landsmann, eds.
(Braunschweig, Germany: Biologische Bundesanstalt fur
Land- und Forstwirtschaft), pp. 171-178.
Goddijn, O.J.M. and Pen, J. (1995). Plants as bioreactors. Trends in
Biotechnology 13, 379-387.
Gould, F. (1991). The evolutionary potential of crop pests. American
Scientist. 79, 496-507.
Irwin, J.A., Dale, P.J., and Owen, N.E. (1995). Possible factors influencing
transgene stability in Brassica napus. In: Proceedings of a Workshop
on Brassica napus. Roskilde, (Eucarpia Section on Oil and Protein
Crops), pp. 1-5.
Kidd, G. (1994). The Bt working group really does work. Bio/technology
Mason, H.S. and Arntzen, C.J. (1995). Transgenic plants as vaccine
production systems.Trends in Biotechnology 13, 388-392.
Matzke, .A., Neuhuber, F., and Matske, A.J.M. (1993). A variety of epistatic
interactions can occur between partially homologous transgene loci
brought together by sexual crossing. Molecular and General Genetics
McPartlan, C. and Dale, P.J. (1994). The transfer of introduced genes from
field grown transgenic potatoes to non-transgenic potatoes and
related solanaceous species. Transgenic Research 3, 216-225.
Meyer, P. (1995). Understanding and controlling transgene expression. Trends
in Biotechnology 13, 332-337.
Scheffler, J.A., Parkinson, R., and Dale, P.J. (1993). Frequency and distance of
pollen dispersal from transgenic oilseed rape (Brassica napus).
Transgenic Research 2, 356-364.
Scheffler, J.A. and Dale, P.J. (1994). Opportunities for gene transfer from
transgenic oilseed rape (Brassica napus) to related species.
Transgenic Research 3, 263-278.
Shah, D.M., Rommens, C.M.T., and Beachy, R.N. (1995). Resistance to
diseases and insects in transgenic plants: progress and applications
to agriculture. Trends in Biotechnology 13, 362-368.
Tada, Y., Shimada, H., and Fujimura, T. (1995). Reduction of allergenic
protein in rice grain. In: The Biosafety Results of Field Tests of
Genetically Modified Plants and Microorganisms. D.D. Jones, ed.
(Oakland, California: University of California), pp. 290.