Page 248 - Plant-Based Remediation Processes

12 Transgenic Approaches to Enhance Phytoremediation of Heavy Metal-Polluted Soils 241

12.2 Risks Associated with the Use of Transgenic Plants
and Risk Mitigation Strategies

The impact that the introduction of transgenic phytoremediation plants into the
outdoor environment would have on biodiversity and general safety should be
carefully evaluated and weighted against known disadvantages of conventional
remediation techniques or risks of having the recalcitrant heavy metal species in
our environment. Since the introduction of the first genetically modified crops, a
large number of safety-assessment studies have been devoted to the potential and
imagined risks of transgenic plants for agricultural use (Singh et al. 2006; Kok et al.
2008; Kwit et al. 2011). Unlike with genetically modified crops, the issues such as
food safety or allergenicity are not relevant when transgenic plants are considered
for use in phytoremediation. Some risk assessment methods suggest that the danger
of entry of metals to food chains through genetically modified accumulator would
be low in most cases, because such plants would be in isolated industrial districts.
However, an improved tolerance to toxic metals implemented through genetic
engineering would provide modified plants with a selective advantage at
contaminated sites, for example, with acquired metallotolerance (Linacre et al.
2003; Davison 2005). Potential changes in biological diversity due to invasion of
privileged transgenic plants and the effects of they may exert on related soil
microorganisms, herbivores and other organisms along the food chain must be
also taken into account.
The main risk concerns the gene flow from cultivated plants to wild relatives via
cross-pollination. The threat of uncontrolled pollination and crossing with the
relatives and spreading of seeds can obviously be often avoided by harvest before
flowering. In addition, various genetic methods are available that would restrict
transgenic flow in a self-maintaining manner and facilitate the use of transgenic
plants also in remediation of contaminated countryside farmlands. Most of these
strategies were developed to target pre-hybridization steps; far fewer target post-
hybridization events. One approach is targeting the heterologous gene into
chloroplasts. Chloroplast DNA is maternally inherited and its transmission via
pollen occurs rarely, though a danger of transfer of a functional heterologous
gene nuclear DNA and thus to the nucleus of the next generation pollen exists
(Kwit et al. 2011). A suitable technique restricting the spread of transgenes by seeds
is based on poison/antidote idea and employs lethal ribonuclease barnase of Bacil-
lus amyloliquefaciens as poison and protein barstar as antidote (Kuvshinov et al.
2001). To implement poison/antidote pathway, the plant is also transformed with
the barnase and barstar genes. The barnase gene is controlled by the promoter,
which is only active at the time of seed-pod development. Expression of barstar
gene is regulated by heat-shock promoter. Correct seed development and germina-
tion is possible only when the barstar is produced due to the controlled heating of
developing seeds to 40 C. Such conditions are unlikely in the field, making the

germination of progeny likely to fail there. A strategy highly appropriate in
phytoremediation crops, whose primary purpose is not tied to fruit or seed