Plants
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.
