Biomedical Engineering Reference
In-Depth Information
at places other than the carboxyl group. Another case would be the change of a crop
like linseed from high linolenic to high linoleic acid content in order to use the oil
for culinary purposes. 56
In the two specific cases for control of the degree of unsaturation mentioned
above, sunflower and linseed, the desired goals have been reached by mutation
breeding. One may ask the question whether genetic engineering could have done
the job as well or better. A decade ago using genetic engineering to create oil crops
with dramatically altered lipid composition was a target almost impossible to reach.
Various fundamental biological observations have to be taken into account and were
not so clear at that time. First, the percentage of plant seeds' three major storage
products (oil, starch, and protein) differs considerably between and within species.
Pea primarily stores carbohydrate with little oil reserve, whereas in rapeseed the
opposite occurs. 54 Second, there are major differences in the types of fatty acids
which can be stored as a major product in seeds. There is a striking difference in
the oil palm fatty acid composition: the major stored triglycerides are lauric acid
(C 12 :0) in the kernel, and palmitic acid (C 16 :0) in the mesocarp. Third, lipids are
known to have a number of different roles in plants and the membrane lipid fatty
acid composition generally is highly conserved. Because storage could be held to
be a largely luxury function, it is more amenable to manipulation in levels and
content. To alter fatty acid composition, a maximal metabolic alteration of storage
lipid fatty acids combined with a minimal perturbation to membrane lipids should
occur. The fourth, and last, of these basic biological observations is that the timing
and tissue specific deposition of storage triglycerides is strictly controlled. 54
It is estimated that three requirements are needed to successfully utilize recom-
binant DNA technology to generate crops with new oils: (1) isolation of genes of
interest, (2) a transformation system, and (3) a regeneration system. For the isolation
of genes of interest, basically two broad classes of sequences need to be identified
and isolated: (a) the promoters, which will show highly selective tissue specific and
temporal expression, and (b) the structural genes of the enzymes which control the
production of the desired product. For the isolation of promoters it is important to
consider that the deposition of storage lipids in plants occurs within a strictly defined
time frame. The expression of the biosynthetic enzymes also is controlled in a
temporal manner. Figure 10.4 shows the expression of a fatty acid synthetase con-
-keto reductase during oilseed embryogenesis. 54 Step B should be selected
to isolate the genes before the synthesis has stopped and the product has been stored.
Promoters covering the time phase B are required. There are several lipid synthesis
genes cloned to date ( Table 10.5 ). 5 4 Another important drawback is that transforma-
tion systems do not exist for all oilseed crops, being only well established for
rapeseed and soybean. Studies on more high-yielding crops, such as oil palm, are
rapidly advancing. Regeneration systems are available for several species, including
rapeseed, corn, and soybean. Again as with palm transformation, there is still much
work to be done in this topic. 54
High-laurate canola became the first transgenic oilseed crop whose DNA code
has been altered by biotechnologists and the first one to be planted as a commercial
crop. Regular canola does not produce lauric fatty acids, but the genetically engi-
neered seed could yield about 40% lauric fatty acids. 54 This oil could be used in the
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