Food Lipids: Chemistry, Nutrition, and Biotechnology

(viz., seed, fruit) genes, and this is a pivotal issue regarding the commercial exploitation
of specific traits by genetic manipulation (Sec. VI and Chapter 28). This is
because the objective of genetic transformation is to manipulate the nature of lipid
accumulation in specific (storage) organs without compromising the vitality of the
whole plant.
Many enzymes of FAS are known to be subject to developmental control. KAS
I, II, and III are induced, in a coordination fashion, prior to lipid accumulation in
developing rape seeds (42,127,128). Similarly, both �9DES isoforms accumulate in
developing sesame seeds (97). The TE isoforms selective for medium chain acyl-
ACPs inCuphea palustris seeds are seed-specific and are developmentally regulated
to coincide with seed maturation (129). BC and ACCase expression are well coordinated
in developing castor seed, and it is the MS-ACCase (and not MF-ACCase)
that is expressed in a manner paralleling seed development and lipid deposition
(110,115). EAR is developmentally regulated and coordinated with seed embryo
development and oil deposition such that activity in developing oilseeds is 15–30
times that in leaf tissues (130–132). Both EAR isoforms are highly conserved and
are present in seed and leaf tissue; there is no specific seed gene for EAR (133,134).
The importance of organ-specific gene expression is exemplified by experiments
with rape transformed with a medium chain–specific TE from California bay
(135). Chloroplasts from both leaf and seed tissue of transformants produced 12:0
as an end product of FAS, indicating that the TE was expressed in both organs.
However, in intact leaf tissue no 12:0 accumulated, indicating that newly synthesized
laurate was immediately exported to the peroxisomes for � oxidation to prevent
incorporation of this fatty acid in functional glycerolipids. Although this illustrates
an important survival mechanisms for the plant, such a futile cycle represents a waste
of cellular energy.
4. Presence of Isoforms of Units of FAS in Plants
As pointed out in the preceding sections (III.A.3 and III.B), many of the component
enzymes/proteins of FAS systems in plants have multiple isoforms. Substrate selectivity
features are known for some of these isoforms; however, generally little is
known about their influence on regulatory control of fatty acid biosynthesis.
Enzymes involved in the reduction cycle have not been considered much in
this context. However, now that the gene for the minor KAR isoform has been
isolated from Cuphea lanceolata (136), an understanding of a possible role for this
isoform in control of fatty acid biosynthesis may develop in the next several years.
MCAT exists as at least two isoforms, and it exhibits considerable variability
among plants in molecular size and kinetics (74,80), indicating some degree of regulatory
control or species-specific adaptation. The presence of two isoforms each of
�9DES (97) and a medium chain length-specific TE (129) leads us to infer some
regulatory function of these specific enzymes.
Perhaps the one clear role of isoforms of FAS units resides with the function
of ACP. ACP was once characterized as the heat-stable protein factor required by
FAS (36,65). It exists in as many as three isoforms (137), although two isoforms
appear to be dominant in plant tissues (74,138,139). Surprisingly (65), only one
isoform appears to exist in seed tissue (where a greater demand for functional diversity
might be expected), whereas both major isoforms are found in leaf tissue
(140). ACP levels in rape and soybean (Glycine max) seed increase just prior to
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.