Risk Management


Mitigation and Termination Strategies
Response Infrastructure


An important part of risk assessment is to determine the extent to which it is possible to monitor transgenes after the release and the efficiency with which it is possible to destroy plant material if it becomes necessary. Efficient methods of identifying transgenic plants or transgenes in species they may have transferred to may be necessary. This may be by a visual marker (e.g. beta-glucuronidase), a selectable marker (e.g. antibiotic resistance) or by molecular analysis e.g. PCR and Southern hybridization.

Once plants are released from containment and particularly if allowed to flower and set seeds there is the possibility of plants, seeds, pollen carrying the transgenes being transferred out of the immediate release environment. There are ways of minimizing genetic exchange which might be considered. For field trials, it may also be appropriate to describe ways in which plant material can be destroyed at the end of the release experiment or if considered to be necessary during the course of the experiment.

Examples of General Issues

Gene transfer: One of the major concerns associated with the release of modified organisms is that the inserted information may be transferred to wild populations. The process of introgression is of concern to many authors as a mechanism which may lead to undesirable traits being transferred from modified organisms. Inter-specific hybridisation is a ubiquitous process, notwithstanding the barriers that exist to cross-breeding, but most hybrids are rare and the majority are sterile. 'Gene Flow' is believed to be highly restricted , but there is some evidence that this may be misleading. Modified plants could, in theory, become weeds difficult to control, possibly in contexts other than their normal agricultural environment.

A transgene may escape from a crop if the transgene is transferred to another crop or to another, wild, related plant and the plant containing it persists after the crop on the agricultural land or if the plant invades semi-natural habitats.

Gene transfer and non-target organisms: Herbicide resistance was one of the first traits subject to genetic modification as the mechanisms of resistance had been characterized. A range of herbicide tolerance genes have been introduced into various crop plants. In general the resistance is a dominant single gene trait. One of the principal attractions for this application is that it provides a means of providing selectivity for herbicides that are quickly degraded in the environment. An environmental risk that needs to be considered is whether the transgenic crop plant that is herbicide tolerant may become a weed that is then difficult to control. Another factor that needs to be considered is the likelihood of the herbicide resistance genes becoming established in weed populations by hybridisation between crop and weeds. If the hybrids are fertile, they may be difficult to control in an agricultural system which depends for weed control on the same herbicide, or in adjacent crops that depend on the use of the herbicide for weed control. If there are adjacent populations of the same crop which have been modified to be tolerant to different herbicides, would a crop plant resistant to multiple herbicides pose extra problems within either the agricultural or natural environment? Were the herbicide tolerance to be transferred to non-managed, non-agricultural species within the 'wild' environment, would there be cause for concern? Such environments are not normally subject to herbicide treatment and the presence of wild relatives of crop species displaying resistance may be of little significance. The risk assessment should attempt to identify the possible consequences of such a transfer.

Resistance: Another aspect that needs to be considered is the effect of the resistance genes on pest and pathogen populations. If the transgene provides a very efficient defense it is possible that the pest or pathogen will rapidly become resistant. This is a phenomenon that is well known in conventional plant breeding and is arguably more about devising a sound agricultural strategy than assessing risk, but the possibility of using the same resistance gene in a range of different crops by transformation means that this prospect has to be taken seriously. One of the most important uses of this technology for the insertion of genes leading to pest-resistance has been the use of Bt toxins. A problem that may be associated with the use of Bt toxins is the evolution of resistance in the target pests. This resistance is due to reduced affinity for the toxin to a mutant membrane receptor. Transgenic crops containing proteinase inhibitors may pose similar problems, and their use should be carefully monitored to avoid the evolution of resistant pests. Resitance must be monitored and strategies for minimizing it instituted.

Microbial Issues

The establishment of monitoring procedures for persistence and spread of an engineered organism is required as part of the information package for approval of the release. In general the procedures involve development or application of already existing techniques for identifying the organism and enumerating in environmental samples. These procedures have been developed and are, in most cases, well accepted. A problem exists with bacteria which may convert to a Viable, non Culturable (VNC) phase in nature. VNC microbes have lost the ability to multiply in conventional laboratory situations. Use of standard laboratory media will indicate no microorganisms present, while extremely time consuming, logistically difficult tests demonstrate that some viable microorganisms are present. It has been shown that the VNC organisms are capable of reversion to normal state when exposed to the appropriate conditions.

For example, Linder and Oliver demonstrated that conversion of Vibrio vulnificus to VNC occurred after 24 days in a microcosm. They studied the occurrence of VNC in V. vulnificus and, for comparison, that of Escherichia coli in artificial-seawater microcosms at 5 C. They reported that while total counts remained constant, comparison to plate counts suggested nonculturability by day 24. In contrast, direct viable counts indicated that the cells remained viable throughout 32 days of incubation.

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Last Modified: June 12, 2001
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