Alteration of the Fatty Acid Composition of Vegetable Oils
Considerable progress has been made in the genetic alteration of the relative
amounts of palmitic, stearic, oleic, linoleic, and α-linolenic acids in seed oils of
crop plants. These modifications have been achieved primarily by either up- or
downregulating expression of genes for fatty acid desaturases or acyl-ACP thioesterases.
The production of seed oils with high levels of oleic acid represents the
most significant commercial achievement to date in the metabolic engineering of
seed oil composition. This oil modification was achieved in numerous crop
species by blocking expression of
FAD2 genes, through both transgenic and
chemical mutagenic approaches. The major remaining target is development of
temperate crops with seed oils that are solid at room temperature and, therefore,
do not require hydrogenation for use in margarine production. Achieving this
target will require the engineering of seeds to produce high levels of saturated
fatty acids that are sequestered in TAG, but not accumulated in membrane lipids
such as PC. The enrichment of saturated fatty acids in PC and other phospholipids
likely compromises membrane integrity, especially when seeds are subjected to
low germination temperatures (A&a0cute; lvarez-Ortega
et al., 1997; Knutzon
et al., 1992;
Liu
et al., 2002).
Metabolic engineering of novel fatty acid synthesis and accumulation in seeds
of transgenic plants has met with limited success. cDNAs for numerous divergent
fatty acid modification and biosynthetic enzymes have been identified. These
include variant forms of acyl-ACP desaturases, acyl-ACP thioesterases, acyl-CoA
desaturases, Δ
12-oleic acid desaturases, fatty acid elongases, cytochrome P450s,
and cytochrome b
5-fusion desaturases (Voelker and Kinney, 2001). The availability
of these cDNAs offers numerous possibilities for the metabolic engineering of
seeds with enhanced nutritional, industrial, and animal feed properties. The
development of canola seeds with high lauric acid content for detergent applications
and the development of canola and soybean seeds with high GLA content for nutraceutical applications are perhaps the most notable technical successes in this
research area (Del Vecchio, 1996; Sato
et al., 2004).
Despite these accomplishments, most metabolic engineering efforts have
resulted in the development of seeds with only low to moderate levels of unusual
fatty acids. In general terms, a major limitation on unusual fatty acid accumulation
in transgenic seeds appears to be the inefficient flux from the site of synthesis
to the final deposition in oil bodies as a component of TAG (Cahoon
et al., 2007). In
the case of divergent forms of
FAD2, such as epoxygenases, conjugases, acetylenases,
and hydroxylases, the modification reaction occurs while the fatty acid
substrate is bound to the membrane lipid PC. The unusual fatty acid product then
must be efficiently removed from PC and mobilized onto glycerol backbones for
storage as TAG in lipid bodies. This movement or channeling of novel fatty acids
between PC and TAG likely involves specialized forms of metabolic enzymes
such as acyltransferases and phospholipases that are absent from seeds of transgenic
plants. The lack of these specialized enzymes could result in the aberrant
accumulation of novel fatty acids in membrane phospholipids in seeds of transgenic
plants, as has been observed for the production of acetylenic and conjugated
fatty acids (Thomaeus
et al., 2001; Cahoon
et al., 2006). The accumulation of
medium-chain length fatty acids also appears to be limited by inefficient incorporation
onto glycerol backbones for TAG production in seeds of transgenic
plants.
In this case, the synthesis of decanoic and lauric acids in plastids of
transgenic B.
napus seeds by the activity of divergent acyl-ACP thioesterases
resulted in the enrichment of decanoyl-CoA and lauroyl-CoA, relative to CoA
esters of common fatty acids, in acyl-CoA pools (Larson
et al., 2002). A similar
enrichment was not observed in seeds of
Cuphea hookeriana, which naturally
accumulate high levels of these fatty acids. In addition, seeds of developing
B.
napus that have been engineered to produce decanoic (10:0) and lauric (12:0)
acids contained elevated amounts of these fatty acids in phospholipids, relative
to seeds that naturally accumulate these fatty acids (Wiberg
et al., 2000). These
results suggest that specialized forms of metabolic enzymes such as acyltransferases
are also important for the storage of unusual medium-chain length fatty
acids generated in seed plastids. Inefficient incorporation of novel fatty acids
into TAG, as evidenced by their enrichment in acyl-CoA pools, may ultimately
induce b-oxidation for the breakdown of these fatty acids. Such a phenomenon
has been observed in seeds that have been engineered to produce mediumchain-
length fatty acids and epoxy-fatty acids (Eccleston and Ohlrogge, 1998;
Moire
et al., 2004).
Also poorly characterized is the intracellular organization of enzymes
associated with the synthesis and metabolism of unusual fatty acids. These
enzymes might be closely associated in specific physical or metabolic domains.
The synthesis of petroselinic acid, for example, appears to involve the association
of at least three enzymes in a biosynthetic complex or metabolon, and it cannot be
ruled out that specialized plastids have evolved for the synthesis of this fatty acid
(Cahoon and Ohlrogge, 1994; Schultz
et al., 1996). In addition, it is possible that
variant
FAD2s (e.g., hydroxylases, epoxygenases, conjugases) (Fig. 7.4 and
Table 7.5) naturally function in discrete domains of the ER that contain specialized forms of acyltransferases and phospholipases that efficiently metabolize novel
fatty acids following their synthesis on PC.
In addition, aspects of gene expression and protein accumulation have not
been well characterized in most attempts to produce unusual fatty acids in seeds
of transgenic plants. Promoters for seed storage protein genes are typically used to
mediate the expression of transgenes in these experiments. Such promoters might
not provide the proper timing or levels of gene expression in the engineered seeds,
particularly when compared to seeds that naturally accumulate large amounts
of unusual fatty acids. It is also possible that certain enzymes associated with
unusual fatty acid synthesis and metabolism are prone to high rates of turnover in
transgenic plants, which may affect levels of unusual fatty acid accumulation.
Clearly, a basic understanding of the underlying enzymology, cell biology, and
gene expression associated with unusual fatty acid synthesis and metabolism is
essential in facilitating efforts to produce novel vegetable oils in existing crop
plants, while maintaining the agronomic viability of the engineered seeds.