Environmental risks associated with viral recombination in virus resistant transgenic plants - Final report
Consultancy Report by CSIRO
for Environment Australia, June 2002
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In Australia, the commercialisation of transgenic plants containing viral derived sequences for disease resistance promises both economic and environmental benefits through improvements in crop productivity and quality concurrent with a reduction in the use of chemical pesticides and other agricultural inputs. The mass cultivation of these crops, however, also poses potential risks.
One of the more controversial of these risks is the potential for environmental impacts arising as a result of genetic exchange between naturally occurring plant viruses and virus-derived sequences deployed in some GM crop plants. This is because there is a finite probability that virus-virus recombination and virus-transgene recombination could give rise to a new (chimeric) virus capable of spreading as a virulent disease in the Australian environment.
While virus recombination is a natural phenomenon, and has occurred many times in the past, the use of virus transgenes in plants is a very recent development and many people have voiced significant concern about the potential problems these may pose. In April 2001, as part of a new CSIRO program on the scientific assessment of GM technologies in Australia, Environment Australia commissioned a twelve-month consultancy to conduct research and review current scientific understanding on the risk associated with virus recombination as this relates to transgenic plants. This report describes the progress in three parts.
- A 'Hazard Assessment' workshop to identify and prioritise the risks associated with RNA recombination and transgenic plants.
- Design and development of an experimental system to measure the relative frequency of virus recombination in transgenic and non-transgenic plants.
- A review of the scientific literature concerning virus recombination and its relevance to transgenic plants.
The workshop was held in October, 2001. It involved a number of leading Australian scientists in this field, environmental managers and gene technology regulators in discussions on the need for and nature of future research effort in this country. The workshop report is provided in Appendix I. Discussions concluded that while it was not practicable to assess the probability and consequences of virus recombination for all virus-plant-transgene combinations, several key points could be made:
- Very similar genetic sequences are much more likely to recombine than dissimilar sequences to form a viable new virus. The likelihood of successful recombination diminishes rapidly as sequence similarity declines.
- Risk is a product of two components: hazard x frequency. There is no scientific reason to suspect that the nature of the hazard associated with virus recombination (i.e. formation of a new virus) will differ for transgenic and non-transgenic plants. The transgenic plant carrying a virus-derived sequence presents an increase in risk compared to the non-transgenic plant only if the frequency with which viable recombinants are generated in the former is significantly greater.
The workshop recommended that priority in research should be to identify an experimental system (year 1) in order to measure the relative frequency of virus recombination in a transgenic and non-transgenic plant (years 2-3). Two types of experimental system were suggested:
- a transgenic plant containing any one of several virus-derived 'promoter' sequences used for controlling the action of a variety of transgenes;
- a transgenic plant containing a virus-derived transgene for conferring disease resistance (a virus resistant transgenic plant or 'VRTP').
An experimental system based on the second type (i.e. a VRTP) was chosen for development. The plant component was tobacco and the virus component was represented by several closely related strains of potato virus Y (PVY). Potato virus Y is an economically important viral disease in Australian crops. To detect genetic exchange a highly sensitive diagnostic procedure was designed. It was based on a procedure known as the 'polymerase chain reaction' (PCR). In experiments, tobacco plants were deliberately infected with two PVY strains and the PCR diagnostic tool was used to search for chimeric PVY strains (recombinants). In one of these tests a virus recombinant was detected.
This result represents a highly significant achievement and a very successful conclusion to the project aims in year 1:
- the research has demonstrated a system for measuring virus recombination frequency in transgenic plants carrying virus-derived sequences. Notably recombination was detected in the first experiment in the development of this experimental system, in non-transgenic tobacco after only two virus strains had been sequentially passed through five host plant generations. This is equivalent to many millions of virus generations; and
- the project generated valuable baseline data on virus recombination frequency.
However, it cannot yet be concluded from this result that recombination between a transgenic plant and a single virus strain occurs at a lower rate than recombination between two naturally infecting strains. Experiments involving the inoculation, with a single virus strain, of a transgenic tobacco carrying a PVY transgene were completed for two generations of plants, with no recombinants detected. Further tests on transgenic and non-transgenic plants (years 2-3) need to be conducted for a reliable estimation of relative recombination frequency in this system.