175
Production of novel oils in plants
Denis J Murphy
We have now isolated the great majority of genes encoding
enzymes of storage oil biosynthesis in plants. In the past two
years, particular progress has been made with acyltransferases,
ketoacyl-acyl carrier protein synthetases and with desaturases
and their relatives. In some cases, these enzymes have been reengineered to create novel products. Nevertheless, the single or
multiple insertion of such transgenes into oil crops has not always
led to the desired phenotype. We are only now beginning to
appreciate some of the complexities of storage and membrane
lipid formation, such as acyl group remodelling and the turnover
of unusual fatty acids. This understanding will be vital for future
attempts at the rational engineering of transgenic oil crops. In
parallel with this, the domestication of plants already synthesising
useful fatty acids should be considered as a real alternative to the
transgenic approach to producing novel oil crops.
unexpected ways. We have seen the isolation of some
potentially key genes that contribute both to the quantity
and quality of seed storage oils. There has also been an
increasing appreciation of the importance of fatty acids,
not only as storage or structural components, but also acting as, or giving rise to, important signalling molecules that
regulate many aspects of plant development [1,2]. This
illustrates the importance of ensuring that novel fatty acids
in transgenic oil crops are correctly targeted to the storage
oil and are hence unable to adversely affect membrane or
signalling functions. The purpose of this article is to
review some of the recent progress in understanding the
mechanism and regulation of storage oil formation in
plants, and how this may impact on its biotechnological
manipulation.
Addresses
Brassica and Oilseeds Research Department, John Innes Centre,
Norwich Research Park, Norwich NR4 7UH, UK;
e-mail: denis.murphy@bbsrc.ac.uk
Industrial and edible oils
Current Opinion in Biotechnology 1999, 10:175–180
http://biomednet.com/elecref/0958166901000175
© Elsevier Science Ltd ISSN 0958-1669
Abbreviations
ACP
acyl carrier protein
DGAT
diacylglycerol acyltransferase
DHA
docosahexenoic acid
EPA
eicosapentenoic acid
TAG
triacylglycerol
Introduction
The manipulation of seed oil content via transgene insertion was one of the early successful applications of modern
biotechnology in agriculture. Indeed, the first transgenic
crop with a modified seed composition to be approved for
unrestrictive commercial cultivation in the USA was a lauric oil rapeseed grown in 1995. There are two major reasons
for this. Firstly, rapeseed, Brassica napus, is a species that is
relatively amenable to transformation and regeneration,
whereas many other major crops have proved more recalcitrant. Secondly, the metabolic pathways involved in
storage oil biosynthesis appeared at first to be well defined
and potentially straightforward to manipulate via single
gene insertions.
Nevertheless, much of the early optimism for producing
designer oilseeds has, over recent years, been tempered by
setbacks in obtaining high yields of specific novel fatty
acids in transgenic oilseed crops. During the past two
years, there has been an increasing recognition of the complexity of the metabolic pathways involved in seed oil
biosynthesis and several new enzymes have been discovered that contribute to these processes in quite
Before reviewing some of the recent technical developments, it may be useful to consider what we are trying to
achieve in modifying seed oils and why we are doing it. At
present, over 80% of the 75 million tonnes of globally traded seed oils are used for edible purposes, most notably in
the production of cooking oils, margarines and processed
foods [3]. Global production of plant oils for industrial use
(i.e. oleochemicals) is only about 15 million tonnes per year
with a value of about $400–800 per tonne. This contrasts
with the pre-tax price of refined petroleum products which
are produced in the hundreds of millions of tonnes at a
price of only $100–300 per tonne. These simple economic
factors mean that oleochemicals from transgenic oil crops
can only realistically succeed at present either as highvalue niche products for very specific applications (e.g.
8-linolenic acid as a therapeutic agent) or by competing on
a larger scale with petrochemicals by virtue of higher purity, better performance and/or environmental benefits. Of
course, oleochemicals will eventually become more competitive with petrochemicals as global reserves of
fossil-derived hydrocarbons (oil, coal and gas) begin to run
out in the coming century [4•].
The manipulation of oils for new types of edible, rather than
industrial, use appears much more restricted as the vast
majority of plant lipids are already regarded as desirable
dietary components, with better nutritional qualities than
animal fats. There are, however, two obvious targets for modified edible oils in seeds. Firstly, to manipulate the ratio of
saturated to polyunsaturated fatty acids in order to avoid the
need for chemical hydrogenation, which produces the high
levels of trans-fatty acids that many believe to be undesirable
in the diet [5]. Secondly, there is increasing interest in producing very long-chain polyunsaturates, such as
docosahexenoic acid (DHA) and eicosapentenoic acid (EPA),
which are nutritionally beneficial as precursors for certain
prostaglandins and as cholesterol-lowering agents [6]. These
176
Plant biotechnology
fatty acids are particularly enriched in fish oils but are now in
increasingly short supply due to the depletion of the world’s
fish stocks. A recent advance here is the isolation of a gene
encoding a ∆5 desaturase from the yeast Mortierella aplina [7].
This enzyme is responsible for the conversion of di-homo-γlinolenic acid to arachidonic acid which is the central
precursor for the production of eicosonoids, such as
prostaglandins, leukotrienes and thromboxanes.
While the manipulation of oil crops for human consumption
has many attractions, this is an acutely sensitive topic of
public concern at present, particularly in the case of new
food products. For example, petroselinic acid, which can
serve as a useful industrial raw material for polymer and
detergent manufacture, has also been proposed as a hardening agent for margarines [8]. Dietary studies in rats,
however, indicate that petroselinic acid ingestion is associated with liver abnormalities and inhibition of arachidonic
acid biosynthesis [9,10]. This case illustrates the unforeseen
difficulties that may arise from the introduction of novel
(particularly transgenic-derived) oils into the diet and indicates that such transgenic oil crops may be better targeted
initially to produce industrial, rather than edible, products.
Non-oil products
In addition to producing seed oils with novel fatty acid compositions, there are numerous other actual or potential
applications of transgenic oil crops. For example, as recently reviewed [11], following the insertion of a relatively small
number of genes from certain bacteria, such as Alcaligenes
spp, carbon can be diverted from oil synthesis towards the
accumulation of polyhydroxyalkanoates. These polyesters
are biodegradable thermoplastics. Their use is currently limited by their high price (up to tenfold higher than
conventional plastics) due to the high cost of their manufacture via bacterial fermentation. Significant reductions in the
price of such biodegradable polyesters could be expected if
they were produced instead via the large-scale cultivation of
transgenic oil crops. This would probably result in a considerable increase in their share of the enormous global market
for plastics. It is interesting that the major US agribusiness
company Monsanto has now acquired from Zeneca/ICI the
rights to polyhydroxyalkanoate production in plants.
In a separate development, rapeseed oil has been used as a
‘carrier’ in order to facilitate the purification and large-scale
production of pharmaceutical peptides and other high-value
proteins. This is via a recombinant oleosin-fusion protein
technology developed by the Canadian biotech company
SemBioSys [P1]. The level of interest in this technology is
illustrated by the recent investment by Dow Elanco of $17
million for its commercialisation via the cultivation of transgenic rapeseed plants in western Canada [12].
Engineering fatty acid desaturases
An overview of the major pathways involved in storage
lipid metabolism is shown in Figure 1. Over the past few
years there has been considerable progress in isolating
genes encoding the vast majority of these enzymes. Some
of the most significant developments have taken place in
characterising desaturases and related diiron-oxo proteins
in plants, as recently reviewed in detail [13•].
It now appears that plants contain two major families of
diiron-oxo enzymes. Firstly, there are the soluble plastidlocalised acyl-acyl carrier protein (ACP) desaturases that
typically work on C14, C16 and C18 saturated acyl-ACP
substrates. These plant desaturases have similar tertiary
structures and ligand-binding sites to microbial methane
monooxygenases and ribonucleotide reductases and fall
into class II of the diiron-oxo enzymes [14]. One of these
plant enzymes, ∆-9 stearoyl-ACP desaturase, was the first
desaturase from any organism for which a high resolution
crystal structure (down to 2.4 Å) was obtained [15]. This
knowledge has considerably assisted efforts to engineer
novel positional and chain-length specificities into desaturases, for example, via site-directed mutagenesis. For
example, using information from the crystal structure, a
∆-9 stearoyl-ACP desaturase was converted into an
enzyme with a substrate preference for palmitoyl-ACP by
the replacement of two residues (Leu118→Phe and
Pro179→Ile) [16••]. In a parallel study, the single sitedirected mutagenesis of residue Leu118→Trp resulted in
the conversion of a stearoyl-ACP desaturase to an enzyme
with an 80-fold increase in specificity for palmitoyl-ACP
[17]. This represents one of the first successful attempts at
the rational modification of an enzyme of lipid biosynthesis. In the future, this approach holds great promise for the
re-engineering of desaturases and other enzymes for the
production of novel fatty acids in transgenic oil crops.
The second type of plant desaturase falls into class III of
the diiron-oxo enzymes; all such proteins are membranebound and utilise either complex lipid or acyl-CoA
substrates. It has recently been demonstrated that several
hydroxylases [18,19], epoxidases and acetylinases [20••]
are also members of this family of enzymes, and that all
such proteins contain a similar ligand-binding site involving three separate histidine clusters. All of the class III
diiron-oxo proteins probably have similar tertiary structures involving four transmembrane domains, although
otherwise their amino acid sequences can be quite divergent. The isolation of the above desaturase-like enzymes
raises the exciting possibility of the rational design of both
membrane-bound and soluble desaturases in order to carry
out a wide range of chemical reactions, including the stereospecific insertion of conjugated double and triple bonds
and expoxy or hydroxy groups in almost any position in an
alkyl chain. This may allow us in the future to produce a
whole host of novel fatty acid derivatives, many of which
are difficult, or even impossible, to synthesise by conventional chemical methods. Efforts are now underway to
obtain high-resolution structural information about these
membrane-bound desaturases and desaturase-related proteins [21•], which will be important for their future
re-engineering to produce commercial products.
Production of novel oils in plants Murphy
177
Figure 1
Plastid
Sucrose
G6P
Pyr
Endoplasmic reticulum
Other C14–C18
monounsaturates
C14–C18
monounsaturates
18:1∆6
16:1
∆6
G6P
Pyr
ACP–
DES
Acetyl-CoA
ACC
ACS
20:1 HYD
KAS
acyl-CoAs
ACBP?
ϖ3DES
18:1 epoxy
ACT
18:2 acetylinic
γ18:3
18:2
C8–C18
saturates
LPA
18:1-OH
EPX
18:1∆9
ϖ3DES
KAS
FAS
DGAT
KAS
∆6DES
Malonyl-CoA
G3P
22:1
18:1∆9
14:1
∆9
24:1
KAS
LPAT
acyl-CoA
pool
DAG
TAG
C8–C18
saturates
TA
DGAT
α18:3
TE
Signalling &
membrane
lipids
PA
Storage
oil
body
OLN
(i) Fatty acid
biosynthesis
(ii) Fatty acid
modification
(iv) Removal of unusual
faty acids
Acetyl-CoA
Sucrose
(iii) Assembly of
complex lipids
Unusual
fatty acids
β-oxidation
Glyoxylate
cycle
Current Opinion in Biotechnology
Storage lipid metabolism in plant tissues. Fatty acid precursors, such
as pyruvate (Pyr) [39] and malate [40], are imported into plastids for
conversion to acetyl-CoA. The nature of the imported precursor may
be a major determinant of whether carbon is channelled to fatty
acids, and hence oil, or to starch, for example, as in cereals [41]. The
fatty acid synthetase (FAS) complex then converts acetyl-CoA and
malonyl-CoA units into C8–C18 saturated acyl-ACPs, whose final
chain length is regulated by β-ketoacyl-ACP synthetases (KAS)
[24,25•,26•] and thioesterases (TE) [23]. Depending on the plant
species, C14–C18 saturates may be desaturated by a variety of
soluble acyl-ACP desaturases (ACP-DES) [12]. The acyl-ACPs are
then converted to acyl-CoAs [42,43] and exported to the
endoplasmic reticulum (ER), possibly with involvement of an acylCoA binding protein (ACBP) [44,45]. On the ER membrane, oleate
is a central metabolite that can be subject to a variety of
modifications by various desaturases (DES) [12,21•,22], acetylinases
(ACT) [19], epoxidases (EPX) [19], hydroxylases (HYD) [17,18] and
β-ketoacyl-ACP synthetase-dependent elongases (KAS) [26•]. All of
these modified fatty acids, together with the plastid-derived saturates
of monounsaturates, comprise the acyl-CoA pool of the ER. This pool
is utilised by acyltransferases (glycerol-3-phosphate acyltransferase
[GPAT], lysophosphatidate acyltransferase [LPAT], and diacylglycerol
acyltransferase [DGAT]) for the synthesis of storage triacylglycerols
(TAGS) [27•,28,29•], although some fatty acids may also be
channelled to signalling or membrane lipid formation. In some
transgenic plants, the accumulation of unusual fatty acids (possibly
on membrane lipids) induces acyl breakdown via the β-oxidation and
glyoxysomal pathways [35,36•]. Finally, storage oil bodies normally
bounded by an oleosin annulus bud off from the ER, although even
here the triacyloglycerol may still be available for further metabolism,
for example, via transacylases (TA) [30••,31•,32•]. ACS, acyl-CoA
synthetase; DAG, diacylglycerol; G3P, glycerol 3-phosphate; G6P,
glucose 6-phosphate; LPA, lysophosphatidic acid; OLN, oleosin;
PA, phosphatidic acid.
Despite their often quite significant sequence differences,
virtually all plant membrane-bound desaturases fall into a
single recognisable grouping [13•]. These enzymes use
complex lipid substrates, such as phosphatidylcholine or
monogalactosyl diacylglycerol, and are localised mainly on
the endoplasmic reticulum and plastid envelope membranes. A new subclass of membrane-bound desaturases
with similarity to animal and yeast acyl-CoA-dependent
and cyanobacterial acyl lipid-dependent desaturases has
recently been identified in plants, although their substrate
specificities, biological function and possible biotechnological applications remain to be determined [22].
Another interesting development has been the isolation of
a class of plant desaturases containing an amino-terminal
cytochrome b5 domain [23]. At present, desaturase–
cytochrome b5 fusions are only found in ‘front end’ desaturases, that is, enzymes that introduce a double bond into
an acyl chain between an existing double bond and the carboxy terminal. As this type of desaturase is involved in the
178
Plant biotechnology
synthesis of medically important fatty acids, such as
γ-linolenic acid, EPA and DHA, this discovery has implications for ongoing efforts to engineer transgenic oil crops
with high levels of such products.
Other key enzymes of fatty acid modification
One of the earliest successes in producing transgenic plants
with modified storage oil was the addition of a California
Bay thioesterase gene to rapeseed, resulting in the accumulation of ~40% lauric acid in its seed triacylglycerol (TAG).
The accumulation of higher levels (50–60%) of this C12
fatty acid required the additional transfer of a coconut sn-2
acyltransferase gene [3]. The important contribution of
thioestereases to oil quality has also been shown by the
accumulation of ~20% stearic acid in transgenic rapeseed
containing a thioesterase gene from the tropical tree mangosteen [24]. Such an oil could have uses in the production
of margarines and other spreads. There are also several
reports, however, that demonstrate the importance of
β-ketoacyl-ACP synthases in regulating the accumulation of
both short- [25•,26•] and long-chain [27•] fatty acids in storage oils. These studies indicate that it may well be possible,
in principle, to use β-ketoacyl-ACP synthase genes as part of
a strategy to engineer oils with fatty acid chain lengths from
C8 to at least C24. In order to achieve the high levels of the
desired fatty acid that are often required by industry, however, it may be necessary to transfer at least two additional
genes (thioesterase and sn-2 acyltransferase), and possibly
several more, into the oil crop of interest. In addition to
increasing the development time and financial costs, the
presence of multiple transgenes can sometimes lead to
instability of gene expression (e.g. co-suppression effects).
insect cells, produced a protein with DGAT activity in vitro
[30••]. Very recently, a homologue of this gene has been isolated from Arabidopsis and the derived protein has been
shown to have DGAT activity when expressed in insect cells
(MJ Hills, personal communication). The isolation of this
key gene may allow for more radical manipulation of seed oil
yield in transgenic crops. It also holds out the prospect of
engineering high levels of storage oil accumulation in other
sink tissues, such as tubers and fruits. As the biomass of the
latter is normally much higher than that of most seeds, this
could both increase yields and cut the costs of vegetable oils
to the extent that they may eventually compete economically with petroleum as bulk industrial raw materials.
Until recently, the TAG produced in storage tissues was
regarded as an end product which remained metabolically
inert until its mobilisation following seed germination. This
view has now been questioned by several studies that
demonstrate the accessibility of storage TAG to further
metabolism, for example, by desaturases [31•]. The concept
of TAG remodelling has received further support from studies in developing safflower seeds showing transacylase
activities capable of exchanging acyl groups between mono, di- and tri-acylglycerols [32•]. It is already known that the
three acyltransferases of the TAG biosynthetic pathway can,
by virtue of their substrate specificities, play important roles
in regulating the fatty acid composition of storage oils [3].
The additional discovery of transacylases in safflower and
castor bean [33] raises the question of whether such
enzymes are distributed more widely and whether they too
play a role in the regulation of oil quality in plants.
Fatty acid segregation and recycling
Clearly the primary enzymes of fatty acid biosynthesis and
modification are essential to lipid accumulation. There is
now a growing recognition, however, that enzymes further
downsteam in the metabolic pathways also play key roles
in regulating both the channelling of fatty acids to storage
(rather than membrane) lipids and in determining their
overall yield in the seed or fruit [28]. For example, the
expression of a yeast sn-2 acyltransferase gene in transgenic Arabidopsis and rapeseed has been reported to result
in substantial (8–48%) increases in seed oil content [29•].
This result was unexpected as the sn-2 acyltransferase was
not regarded to be a rate-limiting step in triacylglycerol formation. It is possible that this is partially due to the use of
the highly active CaMV 35S promoter to drive expression
of the transgene but, if confirmed, this finding also illustrates how little we know about the regulation of carbon
flux to oil in plant storage tissues.
The only enzyme that is unique to storage TAG formation is
diacylglycerol acyltransferase (DGAT) — all of the other
enzymes can and do also contribute to membrane lipid
biosynthesis. The isolation of a DGAT gene has for long
been a ‘Holy Grail’ of researchers in both animal and plant
lipid metabolism. It was, therefore, interesting to learn of the
cloning of a mouse cDNA which, when expressed in H5
An important challenge facing biotechnologists is to develop transgenic oil crops, such as rapeseed, with high levels
of useful fatty acids, many of which are not normally produced by such species [3]. To date, most transgenic lines
have been reported to accumulate relatively low (typically
1–40%) levels of the new fatty acids, such as ricinoleic
[18,19], stearic [24], or γ-linolenic [34]. One explanation for
this is that rapeseed appears to be less efficient at segregating exotic fatty acids away from accumulation in
membrane lipids than are the species that originally produced such fatty acids [35].
Accumulation of some fatty acids can lead to membrane
instability and may trigger protective mechanisms leading
to the removal of these fatty acids. For example, the presence in transgenic rapeseed of exotic fatty acids, such as
lauric [36•] and petroselinic [37], can induce the pathways
for β-oxidation and the glyoxylate cycle leading to the
selective breakdown of the novel fatty acids. In some
cases, this breakdown is compensated for by an upregulation of fatty acid biosynthesis [36•] but in transgenic
rapeseed lines producing petroselinic acid, we observed a
dramatic and specific breakdown of this fatty acid during
seed development [37]. Clearly, it will be necessary in
future to elucidate the mechanisms involved in channeling
Production of novel oils in plants Murphy
unusual fatty acids away from membrane lipids and ensuring that such protective catabolic pathways are not
induced. This will be an important objective if we are to
realise the goal of producing transgenic plants with high
yields of novel valuable fatty acids.
Conclusions
Although nearly all of the genes encoding enzymes of storage lipid biosynthesis have now been cloned, there have
been many surprising results when these genes are
expressed in transgenic plants. This highlights our relative
ignorance of the interactions between the components of
this and other metabolic pathways in vivo. We also know
very little about the mechanisms regulating the partitioning
of carbon to storage products in sink tissues such as oilseeds.
A very promising recent approach is to attempt to identify
and map quantitative trait loci (QTL) that contribute to
characteristics such as oil yield or fatty acid composition.
This can be combined with map-based cloning of the major
genes involved and hence the elucidation of their function
[38•,39]. Such a ‘top down’ genetics approach may allow for
the isolation of higher level regulatory components (e.g.
transcription factors), that have already been shown to be
important in the control of other metabolic pathways, such
as anthacyanin biosynthesis [38•]. It is important that this is
combined with the ‘bottom up’ approaches, via biochemistry and analysis of individual genes and enzymes, in order
to understand fully and hence be able to modify the complex processes of oil accumulation in plants.
179
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hydroxylase of Pseudomonas oleovorans, the structural information is likely to
be relevant to the plant membrane-bound desaturases and related enzymes.
33. Mancha M, Stymne S: Remodelling of triacylglycerols in
microsomal preparations from developing castor bean (Ricinus
communis L.) endosperm. Planta 1997, 203:51-57.
22. Fukuchi-Mizutani M, Tasaka Y, Tanaka Y, Ashikari T, Kusumi T,
Murata N: Characterization of delta-9 acyl-lipid desaturase
homologues from Arabidopsis thaliana. Plant Cell Physiol 1998,
39:247-253.
23. Napier JA, Sayanova O, Stobart AK, Shewry PR: A new class of
cytochrome b5 fusion proteins. Biochem J 1997, 328:717-718.
24. Hawkins DJ, Kridl JC: Characterization of acyl-ACP thioesterases of
mangosteen (Garcinia manostana) seed and high levels of
stearate production in transgenic canola. Plant J 1998, 13:743-752.
25. Slabaugh MB, Leonard JM, Knapp SJ: Condensing enzymes from
•
Cuphea wrightii associated with medium chain fatty acid
biosynthesis. Plant J 1998, 13:611-620.
This report and the accompanying paper [26•] describe the importance of βketoacyl-ACP synthases in the determination of short-medium chain lengths
in seed oils. This is in addition to the long recognised role of thioesterases in
specifying chain length and confirms that this is a complex trait likely to be
regulated by at least three sets of genes (including acyltransferases) in plants.
26. Leonard JM, Knapp SJ, Slabaugh MB: A Cuphea b-ketoacyl-ACP
•
synthase shifts the synthesis of fatty acids towards shorter
chains in Arabidopsis seeds expressing Cuphea FatB
thioesterases. Plant J 1998, 13:621-628.
See annotation to [25•].
27.
•
Millar AA, Kunst L: Very-long-chain fatty acid biosynthesis is
controlled through the expression and specificity of the
condensing enzyme. Plant J 1997, 12:121-131.
An important study showing that it is the β-ketoacyl-ACP synthase (KAS)
that is the component of the fatty acid elongase which determines acyl chain
length in both plants and yeast. Hence, the same class of enzyme is a key
regulator of both short, medium [25•,26•] and very long chain fatty acid
accumulation in oilseeds.
28. Kinney AJ: Manipulating flux through plant metabolic pathways.
Cur Opin Plant Biol 1998, 1:173-178.
29. Zou J, Katavic V, Giblin EM, Barton DL, MacKenzie SL, Keller WA,
•
Hu X, Taylor DC: Modification of seed oil content and acyl
composition in the Brassicaceae by expression of a yeast sn-2
acyltransferase gene. Plant Cell 1997, 9:909-923.
In this unexpected finding, the authors report that the expression of a yeast
sn-2 acyltransferase gene in transgenic rapeseed and Arabidopsis resulted
in significant (<48%) increases in seed oil yield. If confirmed, this indicates
that apparently non rate-limiting enzymes may still exert considerable control
over carbon flux in oilseeds.
30. Cases S, Smith SJ, Zheng Y, Myers HM, Sande ER, Novak S,
•• Lear SR, Erickson SK, Farese RV: Cloning and expression of a
candidate gene for diacylglycerol acyltransferase. FASEB J 1998,
12:A814.
This is the first report of the isolation of a diacylglycerol acyltransferase
(DGAT) from any organism. The enzyme catalyses the last step in triacylglycerol biosynthesis and is the only activity not shared with membrane lipid
formation. As such, DGAT is a key target for attempts to manipulate oil yield
in seeds and fruits, or even to redirect oil accumulation to other tissues, such
as tubers. Doubtless, the sequence data from this murine DGAT will be useful to isolate homologs from plants.
31. Sarmiento C, Garces R, Mancha M: Oleate desaturation and acyl
•
turnover in sunflower (Helianthus annuus L.) seed lipids during
rapid temperature adaptation. Planta 1998, 205:595-600.
The latest in a series of reports that triacylglycerols in sunflower seeds are
still available for modification by desaturases, that is, storage oil is not necessarily an inert end-product of metabolism.
34. Sayanova O, Smith MA, Lapinskas P, Stobart AK, Dobson G, Christie
WW, Shewry PR, Napier J: Expression of a borage desaturase
cDNA containing an N-terminal cytochrome b5 domain results in
the accumulation of high levels of delta-6-desaturated fatty acids
in transgenic tobacco. Proc Natl Acad Sci USA 1997,
94:4211-4216.
35. Wiberg E, Banas A, Stymne S: Fatty acid distribution and lipid
metabolism in developing seeds of laurate-producing rape
(Brassica napus L.). Planta 1997, 203:341-348.
36. Eccleston VS, Ohlrogge JB: Expression of lauroyl-acyl carrier
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protein thioesterase in Brassica napus seeds induces pathways
for both fatty acid oxidation and biosynthesis and implies a set
point for triacylglycerol accumulation. Plant Cell 1998,
10:613-621.
The presence of a thioesterase transgene in rapeseed leads to the induction
of β-oxidation and glyoxylate cycle genes. This leads to the breakdown of a
large proportion of newly synthesised lauric acid to acetyl-CoA and sucrose.
Most of this carbon is recovered for oil formation due to an increase in fatty
acid synthetase activity but the result is a wasteful futile cycle of metabolism.
The results indicate that rapeseed is not efficient in channeling novel fatty
acids towards storage oil and that this leads to the induction of mechanisms
to prevent their accumulation in membranes. This illustrates some of the
unexpected pleiotropic consequences of transgene insertions in plants.
37.
Fairbairn DJ, Bowra S, Murphy DJ: Expression of unusual fatty acids
in transgenic rapeseed causes induction of glyoxylate cycle
genes. John Innes Centre Annual Report, 1998-99: in press.
38. Murphy DJ: Impact of genomics on improving the quality of
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agricultural products. In Genomics: Commercial Opportunities from
a Scientific Revolution. Edited by Dixon GK, Copping LG,
Livingstone D. Cambridge: University of Cambridge, Society of
Chemical Industry; 1997:199-210.
This review looks at some of the general issues relating to the use of
genomics for the improvement of quality characters, such as oil yield
in crops.
39. Martin GB: Gene discovery for crop improvement. Curr Opin
Biotechnol 1998, 9:220-226.
40. Eastmond PJ, Rawsthorne S: Developmental changes in substrate
utilization for fatty acid synthesis by plastids isolated from
oilseed rape embryos. In Physiology, Biochemistry and Molecular
Biology of Plant Lipids. Edited Williams JP, Khan MU, Lem NW.
Dordrecht, The Netherlands: Kluwer; 1997:66-68.
41. Eastmond PJ, Dennis DT, Rawsthorne S: Evidence that a
malate/inorganic phosphate exchange translocator imports
carbon across the leucoplast envelope for fatty acid synthesis in
developing castor seed endosperm. Plant Physiol 1997,
114:851-856.
42. Eastmond PJ, Kang F, Rawsthorne S: Carbon flux to fatty acids in
plastids. In Regulation of Primary Metabolic Pathways in Plants.
Edited by Kruger NJ, Hills SA, Ratcliffe RG. Dordrecht, The
Netherlands: Kluwer; 1999:137-157.
43. Fulda M, Heinz E, Wolter FP: Brassica napus cDNAs encoding fatty
acyl-CoA synthetase. Plant Mol Biol 1997, 33:911-922.
44. Chye ML: Arabidopsis cDNA encoding a membrane-associated
protein with an acyl-CoA binding domain. Plant Mol Biol 1998,
38:827-838.
45. Brown AP, Johnson P, Rawsthorne S, Hills MJ: Expression and
properties of acyl-CoA binding protein from Brassica napus. Plant
Physiol Biochem 1998, 36:629-635.
Patent
P1. Moloney M: Oil body proteins as carriers of high value proteins.
Industrial patent application 11 July 1997, WO 96/21 029.