HARNESSING PLANT BIOMASS FOR BIOFUELS AND BIOMATERIALS High-value oils from plants John M. Dyer 1,* , Sten Stymne 2 , Allan G. Green 3 and Anders S. Carlsson 2 1 United States Department of Agriculture, Agricultural Research Service, US Arid-Land Agricultural Research Center, Maricopa, AZ 85238, USA, 2 Department of Plant Breeding and Biotechnology, Swedish University of Agricultural Sciences, SE-230 53, Alnarp, Sweden, and 3 CSIRO Plant Industry, Canberra, ACT 2601, Australia Received 15 November 2007; revised 20 December 2007; accepted 4 January 2008. * For correspondence (fax +1 520 316 6330; e-mail [email protected]). Summary The seed oils of domesticated oilseed crops are major agricultural commodities that are used primarily for nutritional applications, but in recent years there has been increasing use of these oils for production of biofuels and chemical feedstocks. This is being driven in part by the rapidly rising costs of petroleum, increased concern about the environmental impact of using fossil oil, and the need to develop renewable domestic sources of fuel and industrial raw materials. There is also a need to develop sustainable sources of nutritionally important fatty acids such as those that are typically derived from fish oil. Plant oils can provide renewable sources of high-value fatty acids for both the chemical and health-related industries. The value and application of an oil are determined largely by its fatty acid composition, and while most vegetable oils contain just five basic fatty acid structures, there is a rich diversity of fatty acids present in nature, many of which have potential usage in industry. In this review, we describe several areas where plant oils can have a significant impact on the emerging bioeconomy and the types of fatty acids that are required in these various applications. We also outline the current understanding of the underlying biochemical and molecular mechanisms of seed oil production, and the challenges and potential in translating this knowledge into the rational design and engineering of crop plants to produce high-value oils in plant seeds. Keywords: biorefining, industrial oils, lipids, seed storage oils, triacylglycerols, unusual fatty acids. Introduction Current world production and use of vegetable oil in food and non-food applications Plant oils represent an important renewable resource from nature. With few exceptions, such as the waxes of jojoba oil, plant oils consist almost entirely of triacylglycerol (TAG) esters containing three fatty acids (FAs) with chain lengths of C8–C24, with C16 and C18 being the most common. Plant oils are used primarily for food and feed purposes, although the oils are increasingly being utilized as renewable sources of industrial feedstocks and fuel. The world production of plant oils amounted to 127 million tonnes in 2006, which represents an increase of about 50 million tonnes compared to only 10 years before (FAOSTAT, 2007). As a point of ref- erence, the annual production of animal fats (tallow, lard and butter) is approximately 22 million tonnes, while fish oils represent about 1 million tonnes (Gunstone and Harwood, 2006). Together with the plant oils, these oils represent the world’s natural oil supply. The majority of vegetable oils are produced from just four crops, namely oil palm, soybeans, rapeseed and sunflower, which together account for approximately 79% of the total production. It is estimated that about 14% of the fats and oils are used chemically and 6% as feed material (Patel et al., 2006). Food and feed The largest proportion of plant oils is consumed as food and feed, and the oils used in these markets contain various proportions of the five common, nutritionally 640 Journal compilation ª 2008 Blackwell Publishing Ltd No claim to original US government works The Plant Journal (2008) 54, 640–655 doi: 10.1111/j.1365-313X.2008.03430.x
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important FAs (Table 1). These five fatty acids are palmitic
(16:0), stearic (18:0), oleic (18:1D9), linoleic (18:2D9,12) and a-
linolenic (18:3D9,12,15) acids.1 The properties of oils depend
greatly on their fatty acid composition, and certain com-
positions are desirable for specific end uses. For example,
cooking oils generally contain a higher proportion of
mono-unsaturated FAs (such as oleic acid), which are
more stable under high temperature, while margarines and
spreads are often rich in saturated fatty acids (e.g. palmitic
and stearic acids). Other oils, such as salad oils, contain
more polyunsaturated FAs (e.g. linoleic and a-linolenic
acids). Traditionally, the production of oils for specific
applications has been achieved by mixing of various plant
oils (Sakurai and Pokorny, 2003) or by partial hydrogena-
tion, whereby double bonds of fatty acids are removed to
make the oil more saturated. However, hydrogenation also
introduces unwanted trans FAs into the oil, which has
undesirable effects on human health and nutrition
(Ascherio, 2006).
Fuel/energy
Plant oils have been used to generate heat and light since
ancient times. Vegetable oils have a higher energy content
than other bioenergy resources such as ethanol, have 90%
of the heat content of petroleum-derived diesel, and a
favorable energy input/output ratio of about 1:2 to 1:4 for
unirrigated crops (i.e. the amount of energy required to
produce the crop compared to the amount of energy
obtained from the seed oil) (Agarwal, 2007). In light of
rising petroleum prices and environmental concerns, the
use of plant oils as liquid fuel has seen a strong increase,
especially in Europe where biodiesel is already a major
fuel derived from oils such as rapeseed, sunflower or
palm. For more information on this subject, see the review
by Durrett et al. (2008).
Industrial feedstocks
As shown in Table 2, there are numerous uses and
applications for plant-derived industrial feedstocks.
Table 1 Fatty acid composition of oils from major and minor oil crops and in wild species
aGunstone et al., 2006; bOgunniyi, 2006; cRamadan and Morsel, 2002; dDehesh, 2001; eBaye et al., 2005; fArmougom et al., 1998; gAckman, 1990;hLazzeri et al., 1997.iThe properties of unusual fatty acids are provided in parentheses (see main text for additional details).
1A note regarding nomenclature: the number before the colon designates the
total number of carbons in the fatty acid chain, the number after the colon
represents the number of double bonds, and the number after D indicates the
position of the double bond with respect to the carboxyl end of the fatty acid
structure. For example, oleic acid (18:1D9) is an 18-carbon long fatty acid with
a single double bond at the D9 position. All double bonds are in the cis
configuration unless otherwise specified.
High-value oils from plants 641
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Processing of plant oils is achieved through conversion
reactions, which result in the production of derivatives of
the original TAG molecules (Metzger and Bornscheuer,
2006). The composition of fatty acids in the oil, with
regard to fatty acid composition, chain length or the
presence of additional functional groups, influences the
characteristics and end uses of the derived oleochemicals.
For example, medium-chain length fatty acids such as
lauric acid (12:0), derived primarily from palm kernel and
coconut oils, are excellent surfactants that are used
extensively for the production of soaps and detergents.
The market for lauric acid alone is estimated to be worth
more than $1.4 billion annually. Another example of an
industrially important fatty acid derived from plants is
erucic acid (22:1D13), which is a very-long-chain fatty acid
derived from oilseed rape. Erucic acid is used to produce
erucamide, which is used as a slipping agent for pro-
duction of extruded polyethylene and propylene films
such as shopping or refuse bags (Friedt and Luhs, 1998;
Wang et al., 2003). Global demand for erucic acid and the
related behenic acid (22:0) is expected to continue to
increase, rising from 18 and 15 million tonnes in 1990 to
35 and 46 million tonnes, respectively, by 2010 (Jadhav
et al., 2005).
The double bonds present in fatty acids also represent
excellent starting points for modification of the hydrocar-
bon chain for the production of new types of feedstocks.
For example, plant oils can be treated with a variety of
chemicals to convert the double bonds of fatty acids into
hydroxyl groups, and the resulting ‘polyols’ can be mixed
with compounds such as isocyanate to form polyure-
thanes. These environmentally friendly, renewable alter-
natives to petroleum-derived polyurethane have excellent
physical characteristics and perform well in a variety of
applications, such as spray insulating foams, rigid foams,
flexible foams such as interior car parts, coatings, sealants,
adhesives and elastomers (Mielewski et al., 2005). These
and other types of chemical modifications to plant oils
can create new and novel compounds with interesting
properties, including medium- and long-chain diacids,
x-hydroxy fatty acids and x-unsaturated fatty acids (Bier-
mann et al., 2000; Metzger and Bornscheuer, 2006; Wagner
et al., 2001). The treatment of plant oils with chemicals,
however, adds to the overall cost and environmental
footprint of utilizing these oils for industrial purposes.
A wide variety of structurally diverse fatty acids occurs in
the seed oils of wild plant species (Aitzetmuller et al., 2003;
Badami and Patil, 1980; Smith, 1971), and many of these
stocks for industry (Table 1). They include unusual mono-
unsaturated fatty acids, medium, short, or very-long-chain
fatty acids, fatty acids with additional functional groups such
as epoxy and hydroxy groups, or fatty acids with conjugated
or acetylenic bonds, and significant research has been
conducted into their biosynthesis in plant seeds (Figure 1).
Other unusual plant oils are those composed of wax esters
instead of TAGs. Importantly, a single unusual fatty acid may
account for up to 90% of the seed oil composition, which
greatly simplifies downstream processing and purification.
The commercial production of these seed oils, however, is
hampered by the poor agronomic traits of the plants (e.g.
small seeds, limited geographical growing areas), which
significantly increase the costs associated with their
production (Cuperus et al., 1996).
Consequently, if the production of industrially important
fatty acids in high-yielding oil crops can be developed
successfully through genetic engineering approaches, there
is a huge market volume available. Assuming that these
designer oils can compete efficiently with petroleum-based
alternatives with regard to both improved functional qual-
ities and price, a major growth potential for plant-derived
industrial feedstocks will be created.
Lubricants
Lubricants represent a large non-food product area in
which plant oils can be increasingly utilized. In 2003, the
worldwide consumption of lubricants totaled 36 million
Table 2 Derivatives produced from plant oils by various processes, together with possible uses and applications
Derivatives Uses, applications
Fatty acids and derivatives Metallic soaps, detergents, soaps, cosmetics, alkyd resins, paints, textile, leather and paper industries,rubber, lubricants
Fatty acid methyl esters Biodiesel, cosmetics, solvents, intermediates in the production of alcoholsGlycerol and derivatives Cosmetics, toothpaste, pharmaceuticals, food, paints, plastics, synthetic resins, tobacco, explosives,
cellulose processingFatty alcohols and derivatives Detergents, cosmetics, textile, leather, and paper industries, duplicator stencils, petroleum additivesFatty amines and derivatives Surfactants, fabric softeners, mining, road building, biocides, textile and fiber industries,
DGAT2). Although castor bean has not yet been stably
transformed (which would permit the introduction of
affinity-tagged ‘bait’ proteins), new methods have been
developed to assist in membrane protein complex anal-
ysis, including rate-zonal centrifugation of large protein
complexes (Hartman et al., 2007) and ‘top-down’ mass
spectrometry of integral membrane proteins (Whitelegge,
2004; Whitelegge et al., 2006). Collectively, these
approaches will greatly assist in the identification of
proteins involved in castor oil biosynthesis, and should
provide significant insight into the production of industri-
ally important oils in the seeds of plants.
Conclusions
Plant oil (TAG) is the bioproduct that is chemically most
similar to fossil oil, and therefore has the greatest potential
to replace it in the chemical industry. In fact, fossil oil is
believed to be derived from ancient, lipid-rich organic
material, such as spores and planktonic algae, sedimented
and transformed under high pressure and temperature
over millions of years (Hunt et al., 2002). There is
increasing appreciation that the production of TAG in plant
cells is far more complex than the simple linear pathway
represented by the traditional Kennedy pathway (Napier,
2007). Elucidation of enzyme activities responsible for
controlling the flux of fatty acids between phospholipid
and acyl CoA pools, in particular, will improve not only the
transfer of unusual fatty acids into storage oils, but will
also assist in the rational improvement of pathways for
VLC-PUFA production, where enzymes act upon fatty acid
substrates attached either to phospholipids or CoA
(Abbadi et al., 2004). A major challenge that has only just
started to receive attention is the spatial organization
of the lipid biosynthetic machinery within the various
High-value oils from plants 651
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subcellular compartments. Until a better picture of the
metabolic channelling occurring within the cell is obtained,
the full potential of genetic engineering of plant oil quality
cannot be realized.
Acknowledgements
Financial support for this work was provided by the United StatesDepartment of Agriculture, Current Research Information System(CRIS) project number 5347-21000-009-00 to J.M.D., the SwedishResearch Council (VR) to S.S., the Swedish Oil Growers Association(SSO) and the Swedish Research Council for Environment, Agri-cultural Sciences and Spatial Planning (FORMAS) to S.S. and A.S.C.,and the Swedish International Development Cooperation Agency(SIDA) to A.S.C.
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