Food Lipids: Chemistry, Nutrition, and Biotechnology

in 16:1 �9, 16:2 �9,12, and 18:2 �9,12 (235). These transformations also enhanced the levels
of pleasant character impact flavoring compounds [which are mediated by a
lipoxygenase/hydroperoxide lyase/isomerase/reductase enzyme system (211)] in
freshly macerated fruit tissues. Since tomato fruit has virtually no storage lipid, this
study represents an approach to the modification of functional lipids in a manner
designed to enhance their capacity as a reservoir for flavoring precursors.
The strategic manipulation of an assembly of lipids in the tomato fruit requires
an understanding of polar glycerolipid assembly, which was beyond the scope of this
chapter. (But the initial steps involve some of the glycerolipid assembly reactions in
and beyond Fig. 3, and the Kennedy pathway in Fig. 5; functional glycerolipid
assembly is covered in some detail in Refs. 15, 106, and 108.) Another potential
extension of transgenic modification of membrane lipids is to enhance the postharvest
storage life of plants tissues, particularly edible organs. Chilling sensitivity in plants
and their organs is correlative to the proportion of phosphatidylglycerol (PG) that
exists as species containing two saturated fatty acids (236). When plastidic GPAT
from a chilling-insensitive species (spinach) and very chilling-sensitive species
(squash, Curcurbita moschata) was cloned into a moderately chilling-sensitive species
(tobacco, Nicotiana tabacum), tobacco constructs with the spinach GPAT had
improved chilling resistance, whereas tobacco constructs with the squash GPAT had
greater chilling sensitivity than did the wild-type tobacco plants (237). Relative chilling
sensitivity of the transgenic plants was correlated to the proportion of disaturated
PG. Thus, the variable reaction selectivities of plastidic GPAT among plants may
have a central role in conferring chilling sensitivity by influencing the fatty acyl
composition of polar glycerolipids assembled in the plastid. It should be noted that
an alternative explanation for the aforementioned observations is that more global
changes in lipid metabolism may account for the changes in sensitivity of transgenic
plants to cool climates (16,238).
A more mainstream chemical functionality problem that may be ameliorated
by genetic manipulation of storage lipid assembly is that of oxidative stability of
plant oils. The most simplistic approach here would be to enhance the levels of
monounsaturated fatty acids (18:1 �9) and, concomitantly, to diminish the levels of
polyunsaturated fatty acids (18:2 �9,12 and/or 18:3 �9,12,15) (ignoring for the moment the
issue of essential fatty acids). Applications in this area were initiated with chemical
mutagenesis in sunflower lines (239,240). High-18:1 �9 sunflower oils have potential
for use as oil sprays, dairy product substitutes, salad oils, and frying oils. Indeed,
high-18:1 �9 sunflower oil possesses greater thermal stability as a frying oil that does
conventional sunflower oil (241).
Reductions in polyunsaturated fatty acid levels have also been achieved by
genetic manipulation. In soybean and canola transformants, 18:2 �9,12 has been reduced
from 55% to 2.5% and from 19% to 6.3%, respectively, with a corresponding
increase in 18:1 �9 content (218). In this application, use was made of antisense
technology toward genes encoding for �12DES (218,222). This approach was also
extended to using antisense technology toward �15DES to reduce 18:3 �9,12,15 content
from 6.9% to 1.4% in transgenic canola (218).
A more complex and challenging, but still feasible, approach to engineeringimproved
oxidative stability relies on a thorough understanding of the influence of
specific molecular species of triacylglycerols on the progress of lipid oxidation. The
positional distribution of fatty acids (both saturated and unsaturated) along the glyc-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.