Genetic Manipulation - Environmental Biotechnology: Theory and Application - page 222

of the reconstructed gene, or group of genes, to the Baculovirus DNA occurs by
recombination in much the same way as described for the formation of recom-
binant yeast. One example of interest to environmental biotechnology is the
replacement of p10, one of the two major Baculovirus proteins, polyhedrin being
the other, by the gene for a scorpion neurotoxin, with a view to improving the
insecticidal qualities of the virus, sketched in Figure 9.3 (Stewart et al ., 1991).
The 'promoters' at the start of the gene, referred to in the figure, are the regions of
RNA which regulate protein synthesis, from none at all, to maximum expression.
Transgenic Plants
Currently, GE in agribiotechnology is focusing on genetic modifications to
improve crop plants with respect to quality, nutritional value and resistance
to damage by pests and diseases. Other avenues under investigation aim to
increase tolerance to extreme environmental conditions, to make plants better
suited for their role in pollutant assimilation, degradation or dispersion by
phytoremediation, or to modify plants to produce materials which lead to
the reduction of environmental pollution. Crop quality improvements such as the
control of fruit ripening (Grierson and Schuch, 1993), an example of which is
the oft quoted, Flavr-Savr tomato, and the production of cereals with improved
nutritional value, are not addressed here because, although of great interest
to the food industry, they are of more peripheral relevance to environmental
biotechnology. Many of the transgenic plants, examples of which are given
later in this chapter, have been produced using the Ti plasmid transfer system
of Agrobacterium tumefaciens and are often used together with the 35S CaMV
promoter. Both of these tools are described from a GE techniques viewpoint in
this chapter and from a biological standpoint in Chapter 10.
Transformation of plants
There are two practical problems associated with GE of plants which make them
more difficult to manipulate than bacteria. Firstly they have rigid cell walls and
secondly they lack the plasmids which simplify so much of GE in prokaryotes.
The first problem is overcome by the use of specialised techniques for transfor-
mation, and the second by performing all the manipulations in bacteria and then
transferring the final product into the plant. The DNA construct contains regions
of DNA which are complementary to the plant DNA to enable the inserted piece
to recombine into the plant genome.
The most popular method of transforming plants is by the Ti plasmid but
there are at least two other methods also in use. The first is a direct method
where DNA is affixed to microscopic bullets which are fired directly into plant
tissue. An example of this technology is the introduction into sugarcane of genes
able to inactivate toxins produced by the bacterium, Xanthomonas albilineans ,
causing leaf scald disease (Zhang, Xu and Birch, 1999). This method of biolistic