Genetic Manipulation - Environmental Biotechnology: Theory and Application - page 226

genes may be cut out and replaced by DNA containing the gene of choice to
be introduced into the plant. There are many additional elements which may be
included in the construct. For example, if the aim is to express the gene, it is pre-
ceded by a strong promoter, most commonly the '35S' promoter of Cauliflower
Mosaic Virus (CaMV).
In addition to the above, it is important to know if the 'foreign' gene is being
expressed and so frequently a 'reporter' gene described in the section above is
also included located close to the gene of interest. Recombination is not 100%
efficient, and so a method of selection is required such that only plants con-
taining the novel DNA grow. This is frequently a gene coding for resistance
to weedkiller or antibiotic. On the grounds of size, this is usually introduced
more successfully on a second Ti plasmid during a co-infection with Agrobac-
terium carrying the plasmid containing the gene of interest. The experiment
can become somewhat complicated at this stage, as other selector genes are
introduced into the plasmids to ensure that growth is only possible if all the
desired elements are present in the plant cell. This can involve infection with
two or three cultures of Agrobacterium each containing its own engineered Ti
plasmid. A very detailed description of the Ti plasmid is published elsewhere
(Hughes 1996).
Examples of developments in plant GE
The purpose of these examples is to illustrate the potential plant genetic engi-
neering could bring to future practical applications in the field of environmental
biotechnology. In some cases the intention is to reduce the amount of herbicide
and pesticides, or other agricultural chemicals required to produce a given crop
yield, in others it is to improve tolerance of harsh conditions or to protect the
plants from attack thus reducing wastage. The intention is to note the technical
details here, while the effects such developments may have on the environment
as a whole, feature elsewhere throughout this topic.
Broad range protection
A general strategy to protect plants from various viruses, fungi and oxidative
damage by a range of agents, has been proposed using tobacco plants as a model.
The transgenics express the iron-binding protein, ferritin, in their cells which
appears to afford them far-ranging protection (Deak et al . 1999).
Resistance to herbicides
'Glyphosate', one of the most widely used herbicides, is an analogue of phos-
phoenol pyruvate and shows herbicidal activity because it inhibits the enzyme
5-enolpyruvylshikimate-3-phosphate synthase . The gene coding for this enzyme
has been identified, isolated and inserted into a number of plants including