Gene Cloning

370 Gene Cloning
that has been used to produce glyphosate-resistant plants was to clone the
plant’s own EPSPS gene, mutate it so that the enzyme it produced was now
no longer inhibited by glyphosate, and to return it into the plant. The construction
of these plants is described in more detail in Section 12.3.
In the case above, protection from the herbicide is produced by introducing
a gene encoding a protein which is resistant to the herbicide’s toxic
effect. An alternative way of engineering herbicide resistance is to engineer
plants to produce a protein that can break down the herbicide, and this
approach has indeed been used for producing transgenic herbicide resistant
plants. For example, the herbicide glufosinate (which sounds confusingly
like glyphosate, but which is not related to it by either structure or
mechanism of action) acts by inhibiting the enzyme glutamine synthetase
(GS). GS uses ammonia in the synthesis of the amino acid glutamine, and
when GS is inhibited, ammonia accumulates and the plants die. The active
ingredient in glufosinate (a compound called phosphinothricin) has a
structure that resembles glutamine, and it is able to bind to and inhibit GS.
To make plants resistant to glufosinate, a gene was identified from a bacterium
that encodes a protein which inactivates phosphinothricin by
acetylation. The gene, referred to as the pat gene as it produces the enzyme
phosphinothricin acetyl transferase, was cloned from bacteria, modified
for plant expression, and introduced into plants, rendering them glufosinate
resistant. Both these modes of resistance are illustrated in Figure 12.4.
Q12.2. Summarize the essential difference between glyphosate and glufosinate
resistance.Would you expect either of them to be achievable in plants
without the use of genetic modification techniques? Explain your answer.
There are many examples of traits which have been engineered into
plants which are to do with the potential uses of the plants in agriculture.
Herbicide resistance is one. Engineering resistance to pests is another
common one: transgenic plants have been constructed which are resistant
to attack by specific viruses, or which produce their own insecticide. Plants
are also being developed which are more resistant to various stresses such
as high temperature, freezing and high salinity, although none of these are
yet grown commercially. More ambitious plans exist for plants with altered
properties which enhance their value as food, for example by changing the
balance of fatty acids present in the oil that they produce to a more healthy
one. Perhaps one of the most celebrated, albeit controversial, examples of
a transgenic plant developed to enhance its value as a food is so-called
Golden Rice, developed by Ingo Potrykus and Peter Beyer, which has high
levels of vitamin A, and which has been touted as part of a solution to the
problem of vitamin A deficiency in developing countries, where it causes
much preventable blindness. There are of course numerous social,