Plant Storage Lipids

Abstract

Plant storage lipids, normally in the form of intracellular triacylglycerol‐rich droplets, are important sources of nutrition for people and livestock; besides, they supply a vast range of renewable industrial products from oleochemicals and bioplastics to paints and biofuels. Storage lipids are mainly found in plant propagules such as seeds and pollen grains, where they form an energy source for post‐germinative growth. The main commercial sources of plant storage lipids are oilseed crops such as soybean, rapeseed and maize or oil‐rich fruits such as olive or oil palm. Triacylglycerols also have several additional nonstorage functions in processes including host–pathogen interactions and abiotic stress responses. Improved knowledge of storage lipid metabolism is being used to create new oil crop varieties and to domesticate new species to supply the ever‐increasing demand for plant oils.

Key Concepts
How plants store energy reserves in the form of lipids.
What are the key storage lipids in plants.
How are plant storage lipids synthesised.
Where are store lipids made in plant cells.
What are the biotechnological uses of plant storage lipids.
How is storage lipid metabolism regulated and how can it be manipulated.

Full text

Plant Storage Lipids
Denis J Murphy, University of South Wales, Pontypridd, Wales, UK
Based in part on the previous versions of this eLS article ‘Plant Storage
Lipids’ (2001, 2008).
Advanced article
Article Contents
•Introduction
•Diversity of Storage Lipid Profiles
•Biosynthesis and Roles in Plants
•Dietary Roles
•Nonfood Utilisation of Plant Storage Lipids
•Breeding for Modified Storage Lipids
Online posting date: 15th February 2016
Plant storage lipids, normally in the form of intra-
cellular triacylglycerol-rich droplets, are impor-
tant sources of nutrition for people and livestock;
besides, they supply a vast range of renewable
industrial products from oleochemicals and bio-
plastics to paints and biofuels. Storage lipids are
mainly found in plant propagules such as seeds and
pollen grains, where they form an energy source
for post-germinative growth. The main commer-
cial sources of plant storage lipids are oilseed crops
such as soybean, rapeseed and maize or oil-rich
fruits such as olive or oil palm. Triacylglycerols
also have several additional nonstorage functions
in processes including host–pathogen interactions
and abiotic stress responses. Improved knowledge
of storage lipid metabolism is being used to cre-
ate new oil crop varieties and to domesticate new
species to supply the ever-increasing demand for
plant oils.
Introduction
Acyl lipids are one of the most common means of energy stor-
age in plants and are particularly abundant in some oil-rich seeds,
which can accumulate as much as 75% of their fresh weight
as triacylglycerols. Other plant tissues that are highly enriched
in storage lipids include the mesocarp of oil-rich fruits such
as avocado, olive or oil palm and the pollen grains of many
entomophilous species such as rapeseed and olive. Storage tri-
acylglycerols are extremely diverse in their fatty acid contents
and there is much interest in harnessing them as renewable,
carbon-neutral and environmentally friendly raw materials for
many industrial applications from bioplastics to biofuels. Plant
lipids are also the second most important (after starch) source
of edible calories for human societies; they also provide several
eLS subject area: Plant Science
How to cite:
Murphy, Denis J (February 2016) Plant Storage Lipids. In: eLS.
John Wiley & Sons, Ltd: Chichester.
DOI: 10.1002/9780470015902.a0001918.pub3
key dietary components, including essential fatty acids and the
lipophilic vitamins A and E.
In 2015, the estimated annual global production of traded veg-
etable oils was 130 MT (million tonnes), worth over $90 bil-
lion. About 80% of these oils are used for edible purposes, most
notably in the production of cooking oils, margarines and in
processed food products ranging from biscuits and cakes to ice
creams and ready meals. In addition to their role as long term
stores of carbon and energy in specic oil-rich tissues, much
smaller amounts of triacylglycerols are present in virtually all
cells in the form of lipid droplets. These spherical droplets act as
dynamic short-term stores of the precursors that supply the lipids
that are involved in many cellular processes including membrane
trafcking. These short-term stores also supply precursors used to
generate many signalling and defence-related lipid intermediates
in plants that are analogous to animal hormones and pheromones.
Lipids are also the major component of the extracellular matrix,
or cuticle, on the outer layer of the plant epidermis. These lipids
provide the interface between plants and their environment and
have important roles in preventing water loss and the entry of
pathogens into the body of the plant. See also:Fatty Acids:
Structures and Properties;Lipids
Diversity of Storage Lipid Profiles
Plant storage lipids, normally in the form of triacylglycerols,
are typically accumulated in the cotyledon or endosperm tis-
sues of seeds. In major oilseed crops, such as rapeseed or sun-
ower, storage lipids can constitute 40–50% of total seed weight.
In plants with relatively large oil-rich seeds, such as nuts like
cashew and hazel, storage lipids can account for as much as
75% of total seed weight. Storage lipids are also found in abun-
dance in certain oil-rich fruits, such as in the mesocarp tissue
of olives, avocado, coconut and oil palm. The majority of com-
monly grown oil crops accumulate mainly C16 or C18 saturated
and unsaturated fatty acids in their storage lipids and hence have
relatively restricted acyl compositions. However, surveys of wild
species have revealed an astonishing natural diversity of seed
oils, many of which have potential value, either for nutritional
or for industrial use (Gunstone et al., 2007; Gunstone, 2011).
Some measure of this diversity can be seen in Tab le 1.This
shows that different plant species can accumulate high levels of
a vast range of fatty acids with chain lengths from C8 to C24.
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Plant Storage Lipids
Tabl e 1 Diversity of fatty acids in plant storage lipids
Fatty acid Amount in oil (%) Plant species Uses
Chain length/functionality Common name
8:0aCaprylic 94 Cuphea avigera Fuel, food
10:0 Capric 95 Cuphea koehneana Detergents, food
12:0 Lauric 94 Betel nut laurel Litsea stocksii Detergents, food
14:0 Myristic 92 Knema globularia Soaps, cosmetics
16:0 Palmitic 62 Chinese Tallow Triadica sebifera Food, soaps
18:0 Stearic 65 Garcinia cornea Food, confectionery
16:1 Palmitoleic 40 Sea Buckthorn Hippophae rhamnoides Cosmetics
18:1 Δ9 Oleic 78 OlivebOlea europea Food, lubricants
18:1 Δ6 Petroselinic 76 CorianderbCoriandrum sativum Nylons, detergents
18:2 Linoleic 75 SunowerbHelianthus annuus Food, coatings
α-18:3 α-Linolenic 60 LinseedbLinum usitatissimum Paints, varnishes
γ-18:3 γ-Linolenic 25 BoragebBorago ofcianalis Therapeutic products
OH-18:1 Ricinoleic 90 CastorbRicinus communis Plasticisers, lubricants
Epoxy-18:1 Vernolic 60 Ironweed Vernonia galamensis Resins, coatings
Triple-18:1 Crepenynic 70 Crepis alpinebCoatings, lubricants
Oxo-18:3 – 78 Oiticica Licania rigida Paints, inks
Conj-18:3 – 70 Tung Vernicia fordii Enamels, varnishes
20:0 Arachidic 35 Rambutan Nephelium lappaceumbLubricants
20:1 Eicosenoic 67 MeadowfoambLimnanthes alba Polymers, cosmetics
22:0 Behenic 48 Brassica tournefortii Lubricants
22:1 Erucic 56 CrambebCrambe abyssinica Polymers, inks
Wax 20:1/22:1 – 95 JojobabSimmondsia chinensis Cosmetics, lubricants
24:1 Nervonic 24 HonestybLunaria biennis –
aNumbers refer to number of carbon atoms:number of double bonds.
bGenes have been isolated for synthesis of these novel fatty acids.
In addition, numerous functionalities can occur in these fatty
acids, including conjugated and nonconjugated double bonds,
triple bonds, hydroxy, epoxy and oxo groups.
The extreme diversity of naturally occurring storage lipid pro-
les in such storage tissues, sometimes with dramatic variations
in fatty acid composition even between genetic variants of the
same species, indicates that their major role is as straightfor-
ward energy and/or carbon sources. In contrast, lipids involved
in membrane structure or cell signalling processes have much
more limited fatty acid compositions. While mutations that sig-
nicantly alter proportions of the major membrane (e.g. galac-
tolipids) or signalling (e.g. oxylipins) lipids are normally strongly
selected against, there appears to be little or no reduction in
plant tness when storage lipid proles are altered. Many of the
enzymes that generate ‘unusual’ fatty acids are mutated forms
of enzymes such as desaturases or elongases that give rise to
‘normal’ membrane-located fatty acids. For example, the hydrox-
ylases and epoxidases (responsible, respectively, for formation of
hydroxy and epoxy fatty acids) found in plants such as castor bean
or Crepis spp. are variants of oleate desaturases that normally give
rise to linoleic acid (Murphy and Piffanelli, 1998).
The properties and hence the potential end uses of a given
plant oil depend largely on the chemical structures of its fatty
acid constituents. This means that the wide range of naturally
occurring fatty acids can potentially give rise to an equally broad
spectrum of potential end uses. It is noteworthy that fewer than
0.1% of known plant species are grown in organised agriculture
and fewer than 10% of known plant species have ever been
surveyed for their oil content. It is certain, therefore, that there
is an even greater natural range of oils in plants than is reected
in Tabl e 1.
Biosynthesis and Roles in Plants
The rst committed step of fatty acid biosynthesis is the car-
boxylation of acetyl-CoA to form malonyl-CoA (Figure 1). The
pathway for acetyl-CoA formation is different in photosynthetic
and nonphotosynthetic tissues. In green photosynthesising tis-
sues, such as leaves, carbon dioxide is xed to triosephosphates
in the chloroplast stroma. Triosephosphates are then converted
to pyruvate, and hence to acetyl-CoA, by glycolytic enzymes. In
largely nonphotosynthetic tissues, such as developing fruits and
seeds, the carbon source for fatty acid biosynthesis is sucrose,
which is imported from photosynthetic organs. Sucrose cannot
enter plastids and is therefore converted to hexosephosphates,
such as glucose 6-phosphate, in the cytosol of these nongreen
tissues. In developing oilseeds that synthesise large amounts of
storage lipids, these cytosolic hexosephosphates are converted
into intermediates such as malate or phosphoenolpyruvate, which
are then transported into the plastids, via specic carriers, to serve
as fatty acid precursors via acetyl-CoA (Kubis et al., 2004). One
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Plant Storage Lipids
Cytosol
Sucrose
Plastid
SOL-DES
C14–C18
monounsaturates
C8–C18
saturates
Acyl-CoA
pool
Fatty acid
synthetase
Fatty acid
biosynthesis
Import of carbon
precursors
Malonyl-CoA
Acetyl-CoA
Pyruvate
Phosphoenol
pyruvate
Phosphoenol
pyruvate
Triose-P
Triose-P
Glucose 6-P
Glucose 6-P
GPT
TPT
PPT
Acyl-CoA
pool
ACT
PC18:1
Modified
acyl-CoAs
Assembly of
triacylglycerols
Fatty acid
modification
LysoPC
PDAT
DAG PA Lyso PA G3P
DAG
MAG
TAG
Oleosin
Endoplasmic reticulum
DGTA
DGAT
Acyl-CoA
ER
DES
PC18:2
18:3
Epoxy
hydroxy
Storage
oil
body
Figure 1 Biosynthesis of storage lipids in plants: Import of carbon precursors. Sucrose is transported from photosynthetic tissues into developing seeds
where it is converted in the cytosol of embryo and/or endosperm cells into precursors, such as glucose 6-phosphate and phosphoenolpyruvate, for onward
transport into plastids for production of fatty acids. Import into plastids occurs via specific carriers such as GPT, glucose 6-phosphate transporter; TPT,
triose phosphate transporter and PPT, phosphoenolpyruvate transporter. Fatty acid biosynthesis de novo: Acetyl-CoA and malonyl-CoA are the precursors
for assembly of C8–C18 saturated fatty acyl-ACPs on a plastidial multienzyme fatty acid synthetase complex. Plastids are also the site of the insertion of
the first double bond by a soluble desaturase (SOL-DES) to produce fatty acid monounsaturates. Both unsaturates and monounsaturates are exported via
an acyl-CoA transporter (ACT) from plastids to the endoplasmic reticulum for further processing. Fatty acid modification: Plastid-derived acyl-CoAs can be
modified in the endoplasmic reticulum by a huge variety of enzymes to produce some of the hundreds of different fatty acids found in naturally occurring
seed oils. However, as not all of these enzymes are present in any given plant species, nontransgenic oilseeds normally accumulate a relatively restricted
range of fatty acids. Most fatty acid modification reactions occur via membrane-bound phosphatidylcholine (PC) specific ER desaturases or desaturase-like
enzymes (ER-DES) such as hydroxylases or epoxidases. Acyl-CoAs are then assembled into complex lipids on the endoplasmic reticulum. Similar ER-located
pathways produce the various membrane lipids, storage lipids and also some signalling lipids although recent evidence suggests that these pathways are
spatially separated in discrete ER domains. Storage oil bodies can accumulate virtually any type of fatty acid, whereas the biological functions of membrane
and signalling lipids require that they only contain a small range of C16 and C18 fatty acids. One of the challenges to producing oilseeds with novel acyl
compositions is therefore to maintain the segregation of exotic fatty acids away from pools of membrane or signalling lipids. Assembly of triacylglycerols:
Triacylglycerols are assembled via a complex process involving sequential acylation of a glycerol moiety (the traditional Kennedy pathway) plus extensive acyl
editing via phosphatidylcholine-dependent desaturases or desaturase-like enzymes (see earlier). The final conversion of DAG into TAG can occur via at least
three enzymes: DGAT, acyl-CoA dependent diacylglycerol acyltransferase; PDAT, phosphatidylcholine-dependent acyltransferase or DGTA, diacylglycerol
transacylase. Nascent TAG droplets are coated with a phospholipid monolayer into which is embedded an annulus of specific proteins, such as oleosins
and caleosins, hence forming the mature storage oil bodies that are finally released into the cytosol. DAG, diacylglycerol; G3P, glycerol 3-phosphate; MAG,
monoacylglycerol; PA, phosphatidic acid and TAG, triacylglycerol.
of the major mechanisms for the regulation of carbon ux to fatty
acids, and thence to triacylglycerols is via transcription factors,
most notably WRI1, which is produced by the WRINKLED1 gene
as discussed below (Ma et al., 2013; Maeo et al., 2009;Toet al.,
2012). See also:Fatty Acid Biosynthesis
The vast majority of fatty acid biosynthesis de novo in
plants occurs in the plastids. Acetyl-CoA is carboxylated to
malonyl-CoA, which serves as the building block for the assem-
bly of fatty acids on the multienzyme fatty acid synthetase
complex. This enzyme complex can produce fatty acids ranging
in length from C8 to C18 in plastids. Plastids are also the site of
the initial desaturation reactions, which convert saturated fatty
acids to monounsaturates. The resulting C8–C18 saturated and
monounsaturated fatty acids are then exported from the plas-
tids as acyl-CoAs for further modication on the endoplasmic
reticulum. The endoplasmic reticulum is the site for the various
fatty acid modication reactions found in different plant species.
These include acyl-chain elongation up to C24 and numerous
additional desaturase and desaturase-like reactions that can result
in the formation of hydroxylated, conjugated and nonconjugated
polyunsaturated fatty acids. See also:Plant Chloroplasts and
Other Plastids
Storage lipids in plants typically accumulate as uid cytoso-
lic triacylglycerol or wax ester droplets. These are surrounded
by a phospholipid monolayer and often by an additional annulus
of specic proteins including oleosins and caleosins. The nal
assembly of these complex macromolecular structures occurs on
the endoplasmic reticulum. Acyl-CoAs produced by the various
fatty acid modication enzymes described earlier are transferred
to a glycerol backbone, resulting in the progressive formation
of mono-, di- and triacylglycerol derivatives. The three acylation
reactions are catalysed by different acyltransferases. The differ-
ing specicities of the acyltransferases for different acyl-CoAs
mean that these enzymes play an important role in regulating the
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Plant Storage Lipids
nal acyl composition of the storage lipids (Bates and Browse,
2012). The mechanism of storage lipid assembly in plants is
similar in many respects to that in animals and microbes (Mur-
phy and Vance, 1999). It is also now apparent that storage lipids
in all organisms, including plants, are far from being the inert
end products of metabolism, as was once thought (Martin and
Parton, 2006; Murphy, 2012). For example, leaves and meristem-
atic tissues contain triacylglycerol-rich lipid droplets that have
roles in both long-term storage and in more short-term dynamic
metabolic processes (Lersten et al., 2006; Slocombe et al., 2009).
The size of the cytosolic lipid droplets in which most plant stor-
age lipids accumulate can vary from <0.5 μmtowellover30μm.
Smaller lipid droplets of 0.5–2.0 μm tend to be found in seeds
that undergo desiccation as a normal part of their development.
This includes the vast majority of temperate species where the
seeds can lie dormant for many years or even decades. The lipid
droplets are invariably surrounded by a continuous protein layer,
principally oleosins. These relatively small amphipathic proteins
(15–24 kDa) are uniquely associated with plant lipid droplets.
The ratio of oleosin to triacylglycerol determines the nal size
of the storage lipid droplet. Oleosins appear to play a critical role
in maintaining lipid droplet stability during the extreme hydra-
tion stress associated with seed imbibition. Many tropical oil
seeds, which do not normally undergo desiccation, contain rel-
atively large lipid droplets of 5–15 μm and few, if any, detectable
oleosins. Such seeds are unable to survive dehydration and rehy-
dration, because of a catastrophic coalescence of the lipid droplets
that causes a phase inversion of the cytosolic contents, leading to
cell death (Leprince et al., 1998).
Biosynthesis of storage lipids in specic plant tissues, such
as seeds and fruit mesocarp, is regulated at the genetic level
by transcription factor genes (Baud and Lepiniec, 2010). These
genes encode DNA (deoxyribonucleic acid)-binding proteins that
are able to up- or downregulate other gene-encoding enzymes
of lipid biosynthetic pathways. The best-characterised group of
such transcription factor genes is the WRI1 (WRINKLED1)fam-
ily (Ma et al., 2013; Tajima et al., 2013;Toet al., 2012), which
are downstream components that are themselves the targets of
master regulator genes such as LEC1 (LEAFY COTYLEDON1),
PKL (PICKLE)andFUS3 (FUSCA3) domain genes (Peng and
Weselake, 2013; Swaminathan et al., 2008). WRI1 directly regu-
lates expression of genes involved in glycolysis and FA biosyn-
thesis (Maeo et al., 2009;Quet al., 2012) and may also be
involved in TAG biosynthesis and assembly into lipid droplets
(Santos-Mendoza et al., 2008; Baud and Lepiniec, 2010).
Dietary Roles
Plant lipids, and especially storage lipids, are essential dietary
components for most animals (Gurr et al., 2016). This is because
acyl lipids in animal membranes require polyunsaturated fatty
acids for their optimal function. Mammals lack the desaturases
required to synthesise polyunsaturated fatty acids de novo and
must therefore obtain them from their diet. Hence, linoleic and
α-linolenic acids, in particular, are regarded as ‘essential fatty
acids’ and must be present in the mammalian diet. Deciencies in
the dietary intake of polyunsaturated fatty acids lead to membrane
and skin abnormalities, hormonal imbalances and other serious
and sometimes fatal symptoms. In addition to these ‘essential
fatty acids’, several other polyunsaturated fatty acids are gener-
ally regarded as being useful dietary supplements. Most notable
amongst these is γ-linolenic acid, which is often derived from
evening primrose or borage (also called starower) oils. Although
this fatty acid can be synthesised by most healthy people, it is
possible that it may become decient due to poor diet, stress
or disease. While conclusive epidemiological data are lacking,
dietary γ-linolenic acid has nevertheless come to be regarded as
a panacea for ailments ranging from eczema and diabetes to pre-
menstrual tension and even some cancers.
In addition to their role in the function of cell membranes,
polyunsaturated fatty acids are also precursors for very long chain
ω-3 and 𝜔-6 polyunsaturated fatty acids (VLCPUFAs). There
have been numerous reports concerning the importance of dietary
supplementation with these fatty acids for human health and
well-being. For example, dietary VLCPUFAs have been shown
to confer protection against common chronic diseases such as
cardiovascular disease, metabolic syndrome and inammatory
disorders, as well as enhancing the performance of the eyes,
brain and nervous system (Benatti et al., 2004). Although plant
oils are the most important dietary sources of the C18 linoleic
and α-linolenic acids, the richest sources of very long chain (i.e.
>C20) 𝜔-3 fatty acids are sh and seafood, particularly fatty
species such as mackerel, herring and sardines. Concerns that
dwindling sh stocks around the world may no longer provide an
adequate source of long-chain 𝜔-3 fatty acids have led to efforts
to develop new transgenic oil crops able to produce VLCPUFAs
(Haslam et al., 2013).
Plants are also sources of many lipid-soluble antioxidant vita-
mins, such as the vitamin E group, which includes α-tocopherol,
tocotrienols and tocotrienes that are often mainly found in unre-
ned seed or fruit oils. The process of rening most vegetable
oils for human consumption removes or destroys vitamin E com-
pounds, so food manufacturers sometimes add them back to prod-
ucts such as milk, margarines and cooking oils. In addition to
their nutritional benets, vitamin E group compounds are used in
cosmetic formulations such as antiageing creams and sunscreens.
The vitamin A group includes β-carotene, a deciency of which
can lead to night blindness and xerophthalmia. Lycopenes are the
related red pigments that are prominent in tomatoes, cayenne and
bell peppers, red grapefruit and some major vegetable oils includ-
ing virgin palm oil. Like carotenes, lycopenes are antioxidants
and are believed to have many benecial properties ranging from
the alleviation of ageing-related conditions to the prevention of
certain forms of cancer. Although both carotenoids and lycopenes
are available as vitamin supplements, by far the most reliable way
to ensure their efcient uptake in the body is by consuming them
as part of an original food product, such as fruits or unprocessed
plant oils, rather than in isolated capsule form. See also:Vitamin
E Deciency
Edible vegetable oils are often partially hydrogenated, both to
extend their shelf life and to make them more solid, for example,
for manufacture of margarines and spreads. This can result in the
conversion of normal cis fatty acid double bonds to their trans
isomers. In some margarines, over half of all cis double bonds
have been converted to the trans isomer. Recent concerns about
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Plant Storage Lipids
the possible adverse nutritional effects of trans fatty acids have
led to efforts to reduce or eliminate them from many brands of
margarine and shortening products.
Nonfood Utilisation of Plant
Storage Lipids
Global production of plant oils for industrial use, that is,
oleochemicals, is >20 MT year−1, with a value of US$500 to
US$1200 per tonne. Some of the more common uses of individ-
ual plant oils are shown in Tab le 1. The major industrial products
derived from plant oils are soaps and detergents, plastics, per-
sonal care products, coatings, resins and lubricants. With the
development of novel oils, either from transgenic or from newly
domesticated crops, both the number and the pattern of end-use
applications for plant lipids is likely to change substantially
in the future. A major development in the early twenty-rst
century has been the emergence of biofuels, some of which are
derived from plant oils and used as transport fuel in the form of
biodiesel. Biodiesel is derived from the methyl esters of plant
storage lipids. In terms of suitability as biofuel feedstocks, oil
crops have an advantage over carbohydrate crops in that their
major products, long-chain triacylglycerols, are chemically much
closer to hydrocarbon fuels and hence require less elaborate and
costly processing. Typical C16 and C18 triacylglycerol oils from
such crops can be efciently transesteried to methyl esters for
use in virtually all engine types (Murphy, 2012).
Oilseed crops, such as rapeseed, sunower and soybean are
sources of high-quality edible vegetable oils but can also be
used for biodiesel production. Compared to tropical oil crops,
temperate oilseeds have two major drawbacks as biodiesel pro-
duction platforms. First, they have comparatively low yields of
about 1 tonne ha−1of oil; and, second, they can only be harvested
once a year, which entails relatively inefcient batch processing,
the storage of large quantities of seeds or fuel and the annual
replanting of the entire crop. The major European biodiesel crop
is rapeseed, with Germany and France the most important produc-
ers. Rapeseed biodiesel is most commonly used as a vehicle fuel
and is widely promoted for its environmental benets. However,
more studies imply that oilseed biofuels may not be so unequiv-
ocally ‘green’ as often claimed, especially in the context of a full
life cycle analysis (Righelato and Spracklen, 2007); and biodiesel
production in Europe is unlikely to increase in the foreseeable
future (Long et al., 2015).
Tropical oil crops such as oil palm and coconut produce high
yields of oil in the fruits as well as in their seeds. The economics
and logistics of tropical oil crops, with their higher yields, con-
tinuous harvesting (on a regional or large plantation scale) and
replanting only every 25 years or so, are much more favourable
than those of temperate oilseeds. By far the most important
tropical oil crop is the oil palm, which can produce up to 5–8
tonnes ha−1of oil for transesterication to methyl esters. Interest
in biofuels has led to an expansion of oil crop production and
the diversion of edible oil to fuel use. Another tropical oil crop
being developed in countries such as India is Jatropha curcas,
whose oil-rich nuts can potentially yield 1.5–2.5 tonnes ha−1
biodiesel. However, the long-term sustainability of the so-called
rst-generation biofuels, such as oil crops, is increasingly being
questioned.
Looking further to the future and possible ‘second-generation’
biofuel crops, one of the best potential sources are lipogenic
microalgae, some of which can accumulate as much as 50%
of their mass as storage lipid, either in cytosolic oil bodies
or in plastoglobuli. Examples include many species of green
algae, diatoms and cyanobacteria. Most lipogenic microalgae
grow considerably faster than land plants and, providing high-oil
varieties can be identied and cultured on a sufciently large
scale, it is possible that they could generate very large quantities
of biodiesel (Hu et al., 2008; Long et al., 2015). For example,
it has been estimated that 1 billion tonnes of biodiesel could
be produced from microalgae grown in ponds over an area of
0.2 million ha. Hence, sufcient biodiesel to replace all petroleum
transport fuels in the United States could be produced from
3.8 million ha. Although this is a large land area, many microalgae
will grow well in sunny, warm habitats such as shallow ponds
located in arid areas like the Colorado Desert, and therefore will
not need to take up any arable land required for food production
(Murphy, 2008).
Breeding for Modified Storage
Lipids
Conventional breeding
During the past 50 years, plant breeders have achieved an impres-
sive record of manipulating the storage lipid content of many
of the major crop plants. One of the best examples is that of
rapeseed, which is the third most important global oilseed crop.
Like other Brassicaceae, rapeseed normally accumulates a seed
oil rich in the C22 monounsaturate, erucic acid. This fatty acid
has numerous industrial applications but is not generally regarded
as having a high nutritional value. Following a breeding pro-
gramme in Canada in the 1960s, new rapeseed varieties were
developed on the basis of naturally occurring mutants that accu-
mulated <2% of erucic acid in the seed oil. These new ‘canola’
varieties instead produced a high-oleic oil with a nutritional value
similar to that of olive oil. Despite these impressive achievements,
however, the range of oils that can be produced by a given crop
species is obviously limited to some extent by its genotype. The
desire to produce a greater range of oils, particularly for indus-
trial purposes, has led to the use of recombinant DNA methods
to produce transgenic oil crops. See also:Plant Breeding and
Crop Improvement
Transgenic oil crops
The manipulation of seed oil content via transgene insertion was
one of the rst successful applications of modern biotechnology
in agriculture. Indeed, the rst transgenic crop with a modied
seed composition to be approved for unrestricted commercial
cultivation in the United States was a lauric acid-rich (C12)
rapeseed grown in 1995. The majority of the genes encoding key
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Plant Storage Lipids
enzymes of fatty acid biosynthesis, modication and storage lipid
assembly, have now been isolated from a wide range of plant
species, as shown in Tab le 1. Several transgenic rapeseed and
soya bean varieties, with modied seed oils, are now available
for commercial cultivation. In principle, biotechnologists have
shown that it is possible to transfer genes from donor species into
major crops, such as soya bean and rapeseed, to produce any fatty
acid from C10 to C24 and with any given functionality, such as
double or triple bonds, or hydroxy or epoxy groups. The current
challenge is to produce these novel fatty acids in transgenic plants
at high enough levels to ensure their commercial viability (Thelen
and Ohlrogge, 2002). See also:Transgenic Plants
At present, most novel fatty acids only accumulate at rela-
tively low levels in transgenic species. Sometimes, levels can
be increased by transferring appropriate acyltransferase genes to
ensure that the novel fatty acids are efciently assembled onto
triacylglycerols. One problem is that in some crop species, such
as rapeseed, not all of the novel fatty acids are necessarily chan-
nelled to storage lipids. It appears that the accumulation of novel
fatty acids in membrane lipids often triggers a regulatory mech-
anism, which results in the removal of the novel fatty acids and
their breakdown via the β-oxidation and glyoxylate cycle path-
ways. This is one reason why some transgenic plants are unable
to accumulate high levels of novel fatty acids. It is also an impor-
tant reminder of the complexity of metabolic regulation in plants
and the difculties of manipulating this process via the insertion
of one or a few transgenes.
For example, the most serious technical challenge to engineer-
ing VLCPUFA production in transgenic plants is the number of
enzymes that are needed for conversion of plant C18 PUFAs, such
as linoleate or linolenate, to the C20 and C22 VLCPUFAs with
up to six double bonds found in the nutritionally desirable 𝜔-3
fatty acids from marine organisms such as sh and algae (Haslam
et al., 2013). There have been several reports that encourage the
view that the economic production of VLCPUFAs in transgenic
plants might one day be possible (Abbadi et al., 2004;Qiet al.,
2004). In one rather heroic experiment, no fewer than nine genes
from various fungi, algae and higher plants were inserted into the
oilseed, Brassica juncea, with the resultant accumulation of as
much as 25% arachidonic acid and 15% eicosapentaenoic acid
(Wu et al., 2005).
As an alternative to modifying existing storage lipids, their
biosynthetic pathways can be diverted to the production of other
useful carbon-based compounds such as biopolymers. There is
great interest in the large-scale production in transgenic crops of
biodegradable polymers such as polyhydroxyalkanoates, which
could potentially substitute for conventional plastics currently
derived from nonrenewable petroleum (van Beilen and Poirier,
2008). There is also interest in trying to increase the levels
of lipidic vitamins in plant oils using a variety of approaches.
For example, transgenic plants accumulating 10- 15-fold higher
levels of vitamin E compounds have been engineered by adding
homogentisic acid geranylgeranyl transferase genes from several
cereals to Arabidopsis plants (Cahoon et al., 2003). Unrened
palm oil also contains signicant amounts of vitamin E group
compounds and breeders have identied several palm varieties
with <2500 ppm tocols and >5000 ppm lycopene/carotene (Han
et al., 2004). This has stimulated interest in the potential of palm
oil in the lucrative health foods market.
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Further Reading
Murphy DJ (2012b) The dynamic roles of intracellular lipid droplets:
from archaea to mammals. Protoplasma 249: 541–585.
Vanhercke T, Wood CC, Stymne S, Singh SP and Green AG (2013)
Metabolic engineering of plant oils and waxes for use as industrial
feedstocks. Plant Biotechnology Journal 11: 196–210.
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