224 Environmental Biotechnology
Transgenic Plants
Currently, genetic engineering in agribiotechnology is focusing on genetic mod-
ifications 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, 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 ofAgrobacterium tume-
faciensand often, together with the 35S CaMV promoter. Both of these tools are
described from a GE technique viewpoint in this chapter and from a biological
standpoint in Chapter 10.
Transformation of plants
There are two practical problems associated with genetic engineering 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
genetic engineering in prokaryotes. The first problem is overcome by the use of
specialised techniques for transformation, 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
bombardment, may increase in popularity in line with improvements to plastid
transformation. It is now possible to produce fertile transgenics expressing foreign
proteins in their edible fruit (Rufet al. 2001).
The second is by protoplast fusion which is a process whereby the plant cell
wall is removed leaving the cell surrounded only by the much more fragile
membrane. This is made permeable to small fragments of DNA and then the
cells allowed to recover and grow into plants. These methods can be unsuccessful
due to difficulties in recovery of the cells from the rather traumatic treatments
and also because the DNA introduced, has a tendency to be inserted randomly