GCB Bioenergy (2009) 1, 251–255, doi: 10.1111/j.1757-1707.2009.01016.x
OPINION
Improving sugarcane for biofuel: engineering for an even
better feedstock
E R I C L A M *, J A M E S S H I N E J R w , J O R G E D A S I L VA z, M I C H A E L L A W T O N *, S T A C Y B O N O S § ,
M A R T I N C A L V I N O } , H E L A I N E C A R R E R k, M A R C I O C . S I L VA - F I L H O **, N E I L G L Y N N w w ,
Z A N E H E L S E L § , J I O N G M A *, zz E D W A R D R I C H A R D J R § § , G L A U C I A M E N D E S S O U Z A } }
and R AY M I N G kk
*Biotechnology Center for Agriculture and the Environment, Rutgers University, New Brunswick, NJ 08901, USA, wSugarcane
Growers Cooperative of Florida, Belle Glade, FL 33430, USA, zDepartment of Soil and Crop Sciences, Texas A&M University,
Weslaco, TX 78596, USA, §Department of Plant Biology and Pathology, Rutgers University, New Brunswick, NJ 08901, USA,
}Waksman Institute of Microbiology, Rutgers University, Piscataway, NJ 08854, USA, kDepartamento de Ciências Biológicas,
Escola Superior de Agricultura Luiz de Queiroz, Universidade de São Paulo, Av. Pádua Dias, 11, Piracicaba-SP 13418-900, Brazil,
**Departamento de Genética, Escola Superior de Agricultura Luiz de Queiroz, Universidade de São Paulo, 13400-970, Piracicaba,
SP, Brazil, wwUSDA-ARS, Sugarcane Field Station, Canal Point, FL 33438, USA, zz106 Euclid Ave, Ardsley, NY 10502, USA,
§§USDA-ARS, Sugarcane Research Unit, Houma, LA 70360, USA, }}Departamento de Bioquı́mica, Instituto de Quı́mica,
Universidade de São Paulo, 05508-900, São Paulo-SP, Brazil, kkDepartment of Plant Biology, University of Illinois at UrbanaChampaign, Urbana, IL 61801, USA
Abstract
Sugarcane is a proven biofuel feedstock and accounts for about 40% of the biofuel
production worldwide. It has a more favorable energy input/output ratio than that of
corn, the other major biofuel feedstock. The rich resource of genetic diversity and the
plasticity of autopolyploid genomes offer a wealth of opportunities for the application of
genomics and technologies to address fundamental questions in sugarcane towards
maximizing biomass production. In a workshop on sugarcane engineering held at
Rutgers University, we identified research areas and emerging technologies that could
have significant impact on sugarcane improvement. Traditional plant physiological
studies and standardized phenotypic characterization of sugarcane are essential for
dissecting the developmental processes and patterns of gene expression in this complex
polyploid species. Breeder friendly DNA markers associated with target traits will
enhance selection efficiency and shorten the long breeding cycles. Integration of cold
tolerance from Saccharum spontaneum and Miscanthus has the potential to expand the
geographical range of sugarcane production from tropical and subtropical regions to
temperate zones. The Flex-stock and mix-stock concepts could be solutions for sustaining
local biorefineries where no single biofuel feedstock could provide consistent year-round
supplies. The ever increasing capacities of genomics and biotechnologies pave the way
for fully exploring these potentials to optimize sugarcane for biofuel production.
Keywords: Genetic engineering, Flex-stock, Mix-stock, Sugarcane improvement
Received 5 March 2009 and accepted 3 April 2009
It is inevitable that fossil fuel will be replaced by renewable biofuels and other alternative energy sources. Global demand for biofuel as a clean renewable energy
source is rising rapidly. By 2017, the US alone will need
135 billion liters of renewable fuels as a goal set by the 20
Correspondence: Eric Lam, e-mail: eric189@hotmail.com;
Ray Ming, e-mail: rming@life.illinois.edu
r 2009 Blackwell Publishing Ltd
in 10 program (reduce gasoline usage by 20% in 10
years) in 2007. The current total global production of
renewable fuels is 50 billion liters a year, about 40% of
which comes from sugarcane that is mostly produced by
Brazil. Recent investments from public and the private
sectors worldwide in biofuel research have brought
sugarcane (Saccharum spp.) to the forefront as the most
productive first generation energy crop. However, there
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is enormous potential to further improve this energy
crop, in spite of its complex genome, by utilizing tools
from the rapid advances in genomics and molecular
biology coupled with conventional breeding.
Sugarcane is one of the world’s most efficient crops in
converting solar energy into chemical energy. It is among
the crops having the most favorable total energy output
per unit of energy required to produce the crop, the
output/input ratio (Heichel, 1974). An energy balance of
3 U output per 1 U input (3 : 1 ratio) was estimated for
sugarcane grown in Hawaii (Tew, 1980) while ratios as
high as 8 : 1 have been reported in Brazil (Bourne, Jr.,
2007), which starkly contrasts with the 1.25 : 1 output/
input ratio for corn (Hill et al., 2006). It should be noted
that sugarcane has longer growing seasons in the tropics.
Sugarcane is an important crop for many tropical and
subtropical regions worldwide, including four states
in the United States: Florida, Texas, Louisiana, and
Hawaii. In addition to crop production for sugar as a
sweetener and juice as a feedstock for ethanol production, the large amount of biomass has been used as a
fuel for generation of electricity and is expected to
eventually be used as an additional source for ethanol
production. These current and future uses have generated increased interest among sugarcane producers as
the societal demand for alternative fuels dramatically
increases. As the production cost for ethanol from
sugarcane steadily drops while the price of crude oil
and gasoline increases as a long-term trend, the demand
for ethanol as an alternative fuel is expected to increase
on a global basis. For example, Japan could require 2–6
billion liters of ethanol per year, depending on the
required ethanol mix that is between 3% and 10%.
Moreover, correlation between the increase in atmospheric greenhouse gases such as carbon dioxide from
petroleum use and global warming has created an
urgent need to develop and optimize ‘green fuels’ that
will have carbon neutral or even carbon negative capabilities (Schiermeier et al., 2008). Biomass feedstocks
such as sugarcane and corn are currently the major
operating biofuel systems in large production scales in
the world.
Corn has been the chief source for ethanol production
in the United States for several years. Corn and rice
could serve as model species for improving biofuel
production in the grass family as extensive genomic
resources already developed have potential for modifying these crops for increased biofuel capacity (Bush &
Leach, 2007; Lawrence & Walbot, 2007). However, the
ethanol production capacity of corn on an area basis is
far below that of sugarcane (495 vs. 2105 gallons/acre).
Other energy crops in the grass family include sorghum
and millet, with the sorghum genome having been
sequenced recently (Paterson et al., 2009). The sorghum
genome can now be used as a template for resequencing
sweet sorghum varieties for marker discovery and
functional genomic analyses. Sweet sorghum has greater capacity for biofuel production since more of the
plant’s biomass can be readily used for fermentation
instead of only the starch from the grains. It can also be
used as a template for assembling the autopolyploid
genome of sugarcane since they diverged as recently as
8–9 million years ago (Janoo et al., 2007), while maize
and rice diverged from sorghum about 12 and 45
million years ago, respectively (Gaut & Doebley, 1997;
R. Ming, unpublished results). Within the Saccharum
complex, a term used to describe a group of closely
related genera (Sreenivasan et al., 1987), sugarcane and
Miscanthus are biofuel feedstocks, and sugarcane could
serve as a genomic model for Miscanthus, which has
comparatively less genetic information and fewer genomic resources available.
Brazil has been an exemplary model in developing
and commercializing use of biofuels in its bid to minimize dependency on foreign oil while decreasing
hydrocarbon air pollution. At the forefront of these
activities, sugarcane-derived ethanol has played an
indispensable role. Brazil’s successful implementation,
as the largest economy in South America, of the transition to a largely ethanol-based fuel consumption model
serves as a significant verification of the feasibility to
replace fossil fuels with renewable biofuels. However,
for sustainability and to provide adequate fuel for
consumption in a high energy demand country such
as the US, biomass production in feedstock crops such
as sugarcane needs to be further optimized and enhanced. Specifically, this means increasing sugarcane
yields in countries that already grow it, such as Brazil
and the United States. In addition, we need to adapt
the crop for sustainable production and more reliable
delivery of the needed biomass in more temperate
climates that include much of the United States, as well
as in other countries. With these objectives in mind, a
2-day workshop on sugarcane engineering was held on
October 8th and 9th of 2008 at Rutgers University.
Sixteen participants from Rutgers and other US institutions, as well as colleagues from Brazil who are involved
or interested in biofuel feedstock research attended to
present their research and discuss areas of importance
for biofuel feedstock improvement, especially in reference to further optimization of sugarcane as an energy
crop. From the roundtable discussion that took place in
the second day of this Workshop, three categories of
recommendations were synthesized from collective discussion by the group. We believe these are useful ideas to
share with the plant biology community since the biofuel
problem presents an important opportunity that can
have great impact for our society’s wellbeing.
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IMPROVING SUGARCANE FOR BIOFUEL
Critical areas of activity to further optimize
sugarcane as an energy crop
Physiological knowledge and education
Currently there are not enough educational programs
supporting plant and/or crop physiological research in
sugarcane. Traditional plant physiology studies combined with molecular techniques are needed to develop
whole system models for a better understanding of
plant development and gene expression. Sucrose synthesis and translocation pathways, cell wall composition
and pathways for lignin synthesis are examples of
relevant biological problems that need to be more
thoroughly understood before biotechnology advances
can be fully utilized to improve regulation of these and
other pathways.
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301N and 301S latitude. As a perennial crop, sugarcane
offers many advantages compared with annual crops as
a feedstock for bioenergy production. In addition to its
more efficient solar-energy conversion, sugarcane can
be harvested annually for a number of years without
replanting. For these reasons sugarcane represents an
attractive component of our global bioenergy future.
One of the biggest technical challenges for energy
production from sugarcane biomass in the United States
is to expand its adaptability, including drought and cold
tolerance. Comparative analyses between sugarcane
and other potential energy grasses that have more
tolerance to cold and drought, such as its close relatives
Miscanthus and sorghum, are thus important for future
engineering and improvement of sugarcane.
Broaden sugarcane germplasm with Miscanthus
Phenotype characterization
Sugarcane researchers need to reach a consensus on
standardized protocols to phenotypically characterize
and document germplasm for traits such as drought
and flood tolerance, cold tolerance, photosynthetic efficiency and nutrient use efficiency in particular environments. These are complex traits that depend on the
functions of multiple biochemical and physiological
processes. Plant physiology research will be needed to
determine appropriate metrics to characterize these
traits in a high throughput manner and in consistent
ways by different research programs.
Miscanthus spp. is a member of the saccharum complex,
and produces viable hybrids when crossed with sugarcane. Above freezing low temperatures during growth
has little effect on Miscanthus photosynthesis, as opposed to a decrease of 80% in corn (Naidu et al., 2003). If
cold tolerance genes from Miscanthus could be identified and isolated, and then transferred into tropical
energy crops such as sugarcane, the enhanced lines
might achieve high biomass production under temperate conditions. Each crop has its relative strengths and
weaknesses, and interspecific or intergeneric hybridization may thus be a method to transfer traits of value
from one species to another or to create a unique new
species, specifically developed for biofuel production.
Breeder friendly markers
The identification of markers associated with stress
tolerance traits is needed for screening germplasm
and progeny derived from crosses. This quest should
be aided by the rapidly advancing genomic technologies such as NextGen Sequencing methods, but will
require more concerted efforts in the community to
develop user-friendly bioinformatics support and programs. High throughput genotyping platforms need to
be developed to overcome the relatively small effects
that may be contributed by individual quantitative trait
loci.
Resequencing of sweet sorghum varieties
Potential impact of resequencing sweet sorghum resides on the efficient manipulation of physiological
parameters with a view to increase biomass yield and
the recovery of biofuel from biomass with less than
optimum water availability. Comparative analysis of
sweet sorghum and field sorghum genomes might
reveal genes or regulatory elements responsible for
increased sugar production. Genetic mapping could
identify genes or quantitative trait loci controlling
drought tolerance. These genes are desirable for sugarcane improvement via genetic transformation.
Translational research between energy grasses and
model plants
Novel technology development
Expand the geographical range for sugarcane production
Flex-stock concept for using different biofuel feedstocks
Traditional breeding and genetic engineering have the
potential to develop sugar/energy cane genotypes with
adaptability beyond its ideal growing region, between
This concept involves constructing refineries for biofuel
production that can use diverse sources of biofuel feedstock, e.g. cane, cassava, corn, sweet sorghum etc. This
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254 E . L A M et al.
approach can alleviate problems of seasonal availability
of a particular feedstock by having a refinery that can
use multiple inputs. Feedstock flexibility would have
economic advantages, as the refinery can then potentially be operated year-round while specific feedstocks
might be seasonal. The ability of a local refinery to use
multiple feedstocks would also encourage the development of minor feedstocks used to fill in the gaps in
production associated with seasonal bulk crops. This
concept depends on the economics and technical feasibility of building such a refinery. It was pointed out that
in China, several ethanol producing plants that were
recently built to use only a single feedstock are consistently working below capacity and thus not economically sustainable. It was suggested that the Flex-Stock
approach might work well for industrial plants built to
produce syngas from biomaterials, as these can readily
use multiple types of feedstock as input. Another FlexStock scenario could be a biorefinery that can handle
both sugar (e.g. sugarcane or sweet sorghum) and
starch (e.g. corn) based feedstocks efficiently. While
the concept was not explored in detail, the fundamental
issue of ensuring a year-round supply of biofuels feedstock was recognized as an important one, especially in
climates that experience more extreme seasonal variations than the traditional sugarcane producing regions.
Mix-Stock concept for the separate production of enzymes
involved in feedstock degradation, transformation and
fermentation
The notion of introducing transgenes into plants that
encode enzymes for degradation of recalcitrant polymers, or that contribute to the conversion and fermentation of starch and sugars to alcohol is well established
and is the goal of some current projects in the Lam lab at
Rutgers University. The Mix-Stock approach attempts to
reduce the cost of adding enzymes from exogenous
sources (that have to be produced, extracted and purified from microbial cultures) by building them into an
enzyme feedstock to supplement the bulk feedstock in
biofuels processing. In this ‘Mix-Stock’ concept, the
degradative enzymes are incorporated into a separate
transgenic plant variety (called the ‘enzyme stock’) that
is then mixed in with the bulk feedstock, such as
sugarcane, cassava, corn or grasses as they are fed into
the refinery. Such ‘enzyme stock’ needs to be developed
in multiple varieties or even species to prevent disruption caused by a crop failure. The advantages of this
approach are that different transgenic enzyme stock
plants can be developed and cultivated independently
of the bulk feedstock. Moreover, some feedstock can be
difficult to transform while there are other cultivars that
can be transformed more efficiently and these would
normally need to be introduced into breeding programs
to facilitate the transformation of the recalcitrant bulk
feedstock. This can add many years to field deployment. In addition, by making the feedstock ‘transgenic’
there is the added burden of obtaining deregulation for
the transgenic feedstock. Separating the production of
bulk biomass from the production of enzymes needed
for lignocellulose breakdown and starch conversion
may minimize or avoid these problems, thus providing
considerable flexibility to the breeders and processors.
Moreover, transgenic production of degradative enzymes can occur in a plant species that is already
approved for field production (e.g. tobacco, tomato)
and helps avoid the need to engineer different feedstock
species. In addition, separating enzyme production
provides flexibility of cultivation (the plants can be
intercropped, or grown in separate fields or even locations), allows for multiple and interspersed (and even
separately grown) crops of both the feedstock and the
enzyme stock species. It also expands the potential
market for enzyme stock plants that can be provided
to producers of biofuels from multiple feedstock
sources. Discussion of this concept among the workshop participants was positive and it was recognized
that this approach can allow rapid improvements in
enzyme performance (e.g. from directed evolution) to
be more quickly incorporated into biofuel production
systems.
In summary, all participants in this Workshop agreed
that further collaborations between investigators from
the United States and Brazil would be highly desirable
and synergistic in order to realize the objective of
optimizing sugarcane and other related crops as biofuel
feedstocks. Research and development of even a subset
of the ideas outlined above could have major impact in
the further implementation of a global transition from
petroleum-based fuels to a renewable and environmentally sustainable biofuel economy. With the urgent
climate issue of global warming and energy security
in mind, we hope our Workshop and the ideas that were
generated will seed new collaborative projects in the
near future.
Acknowledgement
We thank Paul Moore for reviewing and editing the manuscript.
Funding for this workshop was generously provided by the
International Cluster of Sugarcane Engineering of Rutgers University, the School of Environmental and Biological Sciences, the
Biotechnology Center for Agriculture and the Environment, the
Rutgers Energy Institute, the Waksman Institute of Microbiology,
and the Turfgrass Center of the NJ Agricultural Experiment
Station of Rutgers University.
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IMPROVING SUGARCANE FOR BIOFUEL
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