Workshop on Potential environmental impacts associated with virus-resistant transgenic plants
Date and Location
Versailles, France, 5-6 June 2001
Report
The workshop comprised 23 invited speakers, each presenting different aspects of the programme and was divided into four sessions corresponding to different themes:
- Gene silencing and resistance breakdown
- Recombination and virus evolution
- Synergy and heterologous encapsidation
- Gene flow
Each session was followed by an active group discussion. The conference was terminated by an open discussion on "emerging issues, research needs, potential conclusions". A summary of this final discussion is given below.
Heterologous encapsidation
The question was raised as to whether the problem of heterologous encapsidation in transgenic virus-resistant (coat protein producing) plants, poses any new biosafety issues (i. e. different from those already posed in nature during double infection, by different viruses, of non-transgenic plants). It was noted that this is an area in which considerable progress has been made in eliminating possible sources of risk, by expressing genes encoding a modified coat protein, which confers resistance but does not interact with vector organisms. It was pointed out that this has been accomplished for potyviruses and luteoviruses, but that we lack the knowledge of the exact sites of interaction between the coat protein and the vector to do this for most other virus groups. This gap in current knowledge needs to be addressed. A general theme that appeared here and repeatedly in the following discussion was that any studies in this field must be compared to the natural base-line (absence of transgenic plants). Knowledge in this field is still presently insufficient.
Recombination between plant viruses
Plant viruses may recombine to produce new strains that may express novel phenotypes. For the present discussion there are two types of pertinent situations:
- recombination between similar viruses co-infecting non-transgenic plants
- recombination between an infecting virus and a transgenic plant expressing mRNA bearing sequences from the target virus.
As with heterologous encapsidation, the additional biosafety issues posed by recombination in transgenic plants (b) must be compared to the base-line natural situation (a). These problems are actively being investigated by certain participants of the workshop, who have formed a consortium and received a Quality of Life grant from the European Commission. This EC-supported project will address these questions with potyviruses and cucumoviruses; it remains to be determined if these results can or should be used to extrapolate to other virus groups. There was a clear agreement here on the overall strategies to be employed.
Transmission by pollen of virus resistance transgenes
The question of the potential impact of plant to plant transmission of a virus resistance transgene (either to identical but non-transgenic cultivated species or to non-identical but related non-transgenic species, including weeds) was actively discussed. Extensive field experimentation is necessary and is actively being undertaken by certain participants of the workshop. Such studies involving gene flow from cultivated to wild Brassica and Beta species are included in the QoL project mentioned in the preceding paragraph. However, it was pointed out that it would be important to carry out similar studies on plant species, such as pasture and turf grasses, in which the cultivated forms are nearly identical to the wild ones. Progress in this field is currently hampered by political decisions and indecisions on the cultivation of transgenic plants in open fields.
Post transcriptional gene silencing (PTGS)
It has been known for several years that expression of a coat protein gene can confer resistance either through accumulation of the coat protein, or through effects induced by the transgene’s mRNA itself. In the latter case, resistance is mediated by mechanisms related to post-transcriptional gene silencing (PTGS). There was considerable discussion as to the relative merits of these two types of resistance. Opinions on this issue were strong and varied. No consensus agreement was reached, indicating perhaps that the state of the art is premature or that different situations apply in different virus/plant situations. PTGS-mediated resistance has potential advantages both from the point of view of efficacy and of biosafety. In most cases, this type of resistance is expressed as immunity, and since little or no protein or RNA is accumulated, essentially no problems related to heterologous encapsidation would be anticipated, and one would predict that recombination between transgene-encoded mRNA and viral RNA would be unlikely. However, since plant viruses have proteins that interfere with PTGS, it is also quite possible that infection with a non-target virus could lead to resistance breakdown. This possibility was addressed in three presentations at the workshop, and clearly requires further investigation. Another area that requires clarification is the minimum length of sequence identity required to induce PTGS-based resistance. It was proposed that production of homologous RNA of only 25 identical nucleotides could be sufficient. Other opinions raised this figure to >100bp identical nucleotides. The subject of potential effects on virus evolution is intimately related to these possible mechanisms. For example, if a 25-mer of exact sequence identity is sufficient to provide plant protection, then the possibility should be considered that PTGS-insensitive virus variants may evolve via nucleotide changes in this small region. It was also proposed that an mRNA that can fold into a hairpin structure could be a more effective inducer of resistance, which could also be less sensitive to breakdown due to interference with PTGS by non-target viruses.
Conclusions
The workshop was considered to be successful on several grounds. As mentioned above, this was the first meeting ever of the entire European research community in this field. As a result, many new contacts were made, and new, often as yet unpublished results were presented. One of the hopes in organizing this workshop was that it would be an occasion to create new groupings of research laboratories that could be translated into consortia for EC grant proposals. To our knowledge, at least two new future consortia did in fact emerge following the workshop. One concerns the extremely new field of viral genomes that are integrated in plant genomes, and the role that they may play in the epidemiology of certain viral diseases. The other focuses on a comparison of protein-mediated and PTGS-mediated virus resistance, in order to assess which type of resistance has the characteristics most favourable for integration into sustainable agricultural practices.
Workshop organiser
- MarkTepferE-Mail
- INRADépartement de Biologie CellulaireVersaillesFrance
