Agriculture Reference
In-Depth Information
genetic material during evolution than we expected.
Also, we are being reminded of the existence of par-
allel natural processes for much of what we regard as
novel. For example, bacteria, viruses and phages already
have successfully evolved mechanisms to transfer genes
just in the way we regard as being so alien! But clearly,
the new techniques are allowing modern plant breed-
ers to create new variability beyond that existing in the
currently available germplasm on a different scale and
in a different time frame from that which was possible
previously.
Although plant transformation has added (and some
say dramatically) to the tools available to the breeder for
genetic manipulation, it does (as with all techniques)
have limitations. Some of the limitations will reduce
with increased development of methodologies, others
are those that are inherent to the basic approach.
At present, recombinant DNA techniques can gener-
ally only transfer rather limited lengths of DNA and so
tend to be restricted to the transfer of single genes. This
means that they are very effective where the trait can be
substantially affected by one, or a few, gene(s) of large
effect, but will be dependent on how much of the varia-
tion that is important in many agronomic traits showing
continuous variation is actually controlled by a few loci
showing rather large effects and how much by a myriad
of ones with much lesser effects. So, for example, it is
not clear how much yield itself, which could be argued
is one of the most important characters of interest, can
be manipulated by discrete steps of individual trans-
formation events. Interestingly, however, recent reports
do indicate the potential to transform with a number
of genes (constructs) in one go with a reasonably high
level of co-transformation.
It may seem obvious, but another restriction cur-
rently that is imposed, is that the techniques are only
readily applied to genes that have been identified and
cloned. The number of such desirable genes is still mod-
est, but increasing rapidly. What is becoming clear is
a deficiency in the knowledge of the underlying bio-
chemistry or physiology of most traits. Another feature,
which has recently provided at least a temporary limit
to the technique, has been the identification of suitable
promoters for the genes that are to be introduced. The
inappropriate expression of a transgene, in the develop-
ment of the plant, or particular organ, or in its timing
has now been fully recognized and so the search for
promoters now equals that for the genes themselves.
In addition, it has been recognized that because of the
uncontrolled nature of the incorporation of transgenes
into the host's genome a large number of transformed
plants need to be produced in order to allow the selec-
tion of the few that have the desired expression of the
transgene without any detrimental alteration of all the
characters of the host.
Some applications of genetic engineering to
plant breeding
Already there is a growing list of dicot crop species that
have proved successful hosts for transformation includ-
ing: alfalfa, apple, carrot, cauliflower, celery, cotton,
cucumber, flax, horseradish, lettuce, potato, rape-
seed, rice, rye, sugarbeet, soybean, sunflower, tomato,
tobacco and walnut. In monocots, maize is leading the
way, but is being followed by wheat, barley and rice.
Initial cultivar development using recombinant
DNA techniques has focused on modifying or enhanc-
ing traits that relate directly to the traditional role of
farming. These have included the control of insects,
weeds and plant diseases. The first genetically engi-
neered crops have now been released into large-scale
agriculture (including, maize, tomato, canola, squash,
potato, soybean and cotton) and other species are
already 'in the pipeline'. More recently work has focused
on altering end-use quality (especially oil fatty acid,
starch and vitamin precursors).
Engineering herbicide tolerance into crops represents
a new alternative for conferring selectivity of specific
herbicides. Two general approaches have been taken in
engineering herbicide tolerance:
•
Altering the level and sensitivity of the target enzyme
for the herbicide
•
Incorporating a gene that will detoxify the herbicide
As an example of the first approach, glyphosate,
the active ingredient of herbicides such as 'Roundup',
acts by specifically inhibiting the enzyme
5-enolpyruvylshimate-3-phosphate synthase (EPSPS).
Tolerance to glyphosate has been engineered into vari-
ous crops by introducing genetic constructions for the
over-production of EPSPS.
The production of plants that are resistant to
insect attack has been another application of genetic
