A more immediate result could be achieved by genetic
manipulation, at least in oil seeds. To pin-point genes worth
considering for manipulation, one needs to have an understanding of the control of carbon flux into TAG. There are
several ways in which the regulation of oil accumulation can
be studied (Table 1, page 82), and all have provided useful data
over the last four decades. However, quantitative information
on control is best obtained by flux studies. Two aspects of these
are metabolic flux analysis (MFA) and metabolic control analysis (MCA). Detailed descriptions of these methods are unnecessary here, but readers can learn more from the references in
the further reading list on page 80.
We have used MCA to indicate which parts of the oil accumulation pathway are worth manipulating in a range of different
crops. In three crops (oil palm, olive, and soybean), the synthesis of fatty acids exerts more control than TAG assembly.
Oilseed rape (canola), on the other hand, shows the reverse
with TAG assembly holding a greater degree of control. Previous studies had already suggested that the final enzyme of TAG
biosynthesis—diacylglycerol acyltransferase (DGAT)—was an
important control point in oilseed rape, and when the DGAT1
isoform was over-expressed, it led to increased TAG production
under both greenhouse and field conditions.
Over the years, manipulation of several enzymes has led
to increased seed oil content in the model plant Arabidopsis,
its close relative oilseed rape, and other oil crops. However,
the nature of flux control means that increasing the activity of
one partly-limiting enzyme will immediately shift the control
of regulation elsewhere, (just as flow on a motorway may be
slowed by a traffic accident, but once that is cleared, the jam will
occur somewhere else on the road). With this realization, recent
attempts at increasing yields have concentrated on “push/pull”
strategies often involving the use of multigene cassettes.
For push and pull approaches, increased entry of carbon
into the start of the biosynthetic pathway is combined with
elevated enzyme activity near the end of the pathway (usually
DGAT). As mentioned in the previous section, the transcription factor called WRI1 was found to increase the activity of
several enzymes which are used to generate acetyl-CoA as well
as many of the enzymes of the fatty acid synthase complex.
Thus, WRI1 increases fatty acid production. When this transcription factor is up-regulated in combination with DGAT
there is in an additive (or even synergistic) effect.
The story does not end once TAG is formed. In seeds, TAG
is packaged in oil droplets surrounded by a half-unit membrane
that typically contains specialized proteins such as oleosins.
Increased oleosin production has been beneficial in efforts to
increase the lipid content of leaves and hence raise the nutritional value of such vegetation. Furthermore, oil droplets may
be subject to some degradation (even in maturing seeds) and
this can be prevented by suppression of a TAG lipase. With all
of these approaches, scientists are gradually able to boost the
efficiency of overall oil production.
MoDIFIcAtIon oF tHe FAtty AcID
coMPosItIon oF oIl
The functional properties of a plant oil are dependent on its
fatty acid composition. For example, seed oils containing high
proportions of saturated and monounsaturated fatty acids are
more oxidatively stable and thus more suitable for applications
such as frying. In contrast, oils rich in PUFA are subject to rapid
oxidation and have low shelf life. Inclusion of PUFA, such as
eicospentaenoic acid (EPA; 20:5n-3) and docosahexaenoic
acid (DHA; 22:5n-3), in the diet, however, have been shown to
provide a number of health benefits, including decreased risk of
cardiovascular disease and anti-inflammatory effects.
Over time, conventional breeding has had a large impact
on the fatty acid profile of crop oils. One dramatic example
where plant breeding affected both fatty acid composition and
the economy was the near elimination of erucic acid in oilseed
rape contributing to the generation of modern day canola in
Canada. Through a process of selection, oilseed rape breed-
ers identified lines which were incapable of elongating oleic
acid to erucic acid. Canola is also characterized by decreased
glucosinolate content in the seeds. A mutated elongase gene
FIg. 3. The four reactions of the Kennedy Pathway. This is
the main way to produce TAG but is also used to synthesize
membrane lipids, such as the glycosylglycerides and phos-
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