Food Lipids: Chemistry, Nutrition, and Biotechnology

develop and apply methods, evaluate transgenically produced potential products, and
scale-up for production may discourage any commercial projects. Like the projects
described above that do not offer large profit margin opportunities to reward the risk
takers, genetic engineering of the lipids in major oil crops like oil palm and coconut
may be taken up primarily by public sector research units, which do not need to
satisfy short-term investment return expectations.
Another consideration in the modification of seed oil content of specific crops
may be the ability to properly express transgenes in specific tissues of the plant. That
is, seed lipid biosynthesis tends to occur during a very specific phase of seed development.
To be able to modify that biosynthetic pathway to alter the character of
the vegetable oil, transgenes must exert their effects in the correct tissue at the correct
time. Moreover, in some cases, it may be deleterious to have a transgene expressed
in the wrong tissue or at the wrong time. Ricinoleic acid naturally appears only in
the castor bean endosperm and not in leaves and other tissues, where it presumably
has no function and might interfere with the synthesis of necessary structural
molecules.
Technology to obtain the correct transgene expression for seed storage lipid
modification has been well demonstrated in canola and soybean. One can expect the
same principles to apply for peanut, sunflower, and other crops, although fine-tuning
of the technology may be needed for maximum benefit in some cases. This potential
requirement should be taken into account in considerations of transgenic approaches
to oil modification in crops where there is less experience.
D. Target Traits
1. Short Chain Saturates
The first genetically engineered vegetable oil, Calgene’s Laurical, is already a commercial
product. Researchers studying the seed embryos of Umbellularia californica
(California bay tree) identified an enzyme known as a lauroyl–ACP thioesterase that
appeared to account for the 60% laurate content in the California bay seed oil (Fig.
1). When the cDNA corresponding to the mRNA for the bay tree lauroyl–ACP
thioesterase was successfully cloned, adapted to canola gene expression controlling
elements, and transferred into canola, the resulting oil from transgenic seeds contained
lauric acid in amounts ranging from less than 1% to more than 45%. From
these original plants, referred to as transgenic ‘‘events,’’ lines were developed that
produced genetically uniform seed that reliably contained an average 38–42% lauric
acid in the oil.
When the lauric acid rich oil from transgenic canola seed was examined more
closely, it was observed that very little of the lauric acid was in the second position
of the triglyceride molecules. The practical implications of this result are discussed
below. From a scientific point of view, it was assumed that the canola enzyme that
converts lysophosphatidic acid to phosphatidic acid discriminated against lauroyl-
CoA as a substrate (Fig. 2). Indeed, it had been reported that in canola, this enzyme,
known as lysophosphatidic acid acyltransferase (LPAT) discriminates in vitro against
saturated acyl CoAs as well as substrates with monounsaturated acyl groups exceeding
18 carbons in length.
Subsequently, it was shown that coconut endosperm, which contains a triacylglycerol
oil with high levels of laurate in the second position, also contains an LPAT
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.