Plant Genetic Engineering for Biofuel Production: Towards Affordable Cellulosic Ethanol
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FOCUS ON GLOBAL CHALLREVIEWSENGes RETRACTED Plant genetic engineering for biofuel production: towards affordable cellulosic ethanol Mariam B. Sticklen Abstract | Biofuels provide a potential route to avoiding the global political instability and environmental issues that arise from reliance on petroleum. Currently, most biofuel is in the form of ethanol generated from starch or sugar, but this can meet only a limited fraction of global fuel requirements. Conversion of cellulosic biomass, which is both abundant and renewable, is a promising alternative. However, the cellulases and pretreatment processes involved are very expensive. Genetically engineering plants to produce cellulases and hemicellulases, and to reduce the need for pretreatment processes through lignin modification, are promising paths to solving this problem, together with other strategies, such as increasing plant polysaccharide content and overall biomass. Finite petroleum reserves and the increasing demands Starch- and sugar-derived ethanol already make a for energy in industrial countries have created inter- relatively small but significant contribution to global national unease. For example, the dependence of the energy supplies. In particular, Brazil produces relatively United States on foreign petroleum both undermines its cheap ethanol from the fermentation of sugarcane sugar economic strength and threatens its national security1. to supply one quarter of its ground transportation fuel. In As highly populated countries such as China and India addition, the United States produces ethanol from corn become more industrialized, they too might face similar grain. However, even if all the corn grain produced in problems. It is also clear that no country in the world the United States were converted into ethanol, this could is untouched by the negative environmental effects of only supply about 15% of that country’s transportation petroleum extraction, refining, transportation and use. fuels. Meeting US fuel requirements using starch would For these reasons, governments around the world are mean that corn grain production must be increased or increasingly turning their attention to biofuels as an corn grain be diverted from other uses. For example, alternative source of energy. 50.8% of total US corn grain production is currently The biofuel that is expected to be most widely used used for livestock feed3, and the conversion of corn grain around the globe is ethanol, which can be produced into ethanol has already increased the prices of meat and from abundant supplies of biomass from all land plants dairy products. and plant-derived materials, including animal manure, The future production and use of ethanol that is starch, sugar and oil crops that are already used for obtained from cellulosic matter, supplemented by grain food and energy. In addition, ethanol has a low toxic- ethanol, has been predicted to decrease the need for ity, is readily biodegradable and its use produces fewer petroleum fuel1. The main advantages of using cellu- air-borne pollutants than petroleum fuel. The growth losic matter over starch and sugar for ethanol include of feedstock crops for bioethanol production also the abundant supply of cellulosic biomass as compared reduces greenhouse gas levels, mainly because of the with the limited supplies of grain and sugar. In addi- use of atmospheric carbon dioxide in photosynthesis. tion, starch and sugar that are used for the production Although the conversion of biomass to ethanol and the of ethanol compete with food supplies. Therefore, it is Department of Crop and burning of ethanol produce emissions, the net effect advantageous to use non-food crops and the waste from Soil Sciences, Michigan State can be a large reduction in greenhouse gas emissions food crops for bioethanol production. Furthermore, the University, East Lansing, Michigan 48824, USA. compared with petroleum fuel, meaning that the use use of cellulosic biomass allows bioethanol production e-mail: [email protected] of bioethanol does not contribute to an increase in net in countries with climates that are unsuitable for crops doi:10.1038/nrg2336 atmospheric carbon dioxide2. such as sugarcane or corn. For example, the use of rice NatuRE REVIEWS | GENETICS VOLumE 9 | JUNE 2008 | 433 © 2008 Nature Publishing Group REVIEWS straw for the production of ethanol is an attractive goal lignocellulosic matter to break it down into intermedi- given that it comprisesRETRACTED 50% of the word’s agronomic ates and remove the lignin to allow the access of cellu- biomass. lases to biomass cellulose. These two costs together make Serious efforts to produce cellulosic ethanol on an the price of cellulosic ethanol about two to three-fold industrial scale are already underway. Notably, in 2006, higher than the price of corn grain ethanol. Plant genetic US president George W. Bush announced the goal of engineering technology offers great potential to reduce reducing 30% of foreign oil requirements by 2030 by the costs of producing cellulosic ethanol. First, all neces- using crop biomass for biofuel production. As a result, sary cell-wall-degrading enzymes such as cellulases and the Department of Energy announced the funding of hemicellulases could be produced within the crop bio- three major biofuel centres and the establishment of six mass so there would be no need, or only minimal need, cellulosic ethanol refineries, which, when fully opera- for producing these enzymes in bioreactors. Second, tional, are expected to produce more than 130 million plant genetic engineering technology could be used to gallons of cellulosic ethanol per year3,4, Other than the modify lignin amount and/or configuration in order Canadian Iorgen plant, no commercial cellulosic ethanol to reduce the needs for expensive pretreatment proc- plant is yet in operation or under construction. However, esses. Finally, future research on the upregulation of cel- research in this area is underway and funding is becom- lulose and hemicellulose biosynthesis pathway enzymes ing available around the world for this purpose, from for increased polysaccharides will also have the potential both governmental and commercial sources. For exam- to increase cellulosic biofuel production. ple, British Petroleum have donated half a billion dol- In this Review, I first provide an overview of the lars to US institutions to develop new sources of energy process of cellulosic ethanol production, including a — primarily biofuel crops. brief description of the nature of the plant cell wall as Presently, several problems face the potential com- a source of biomass, and the enzymes that are used in mercial production of cellulosic ethanol. First, the high the cellulosic conversion process. I then focus on the costs of production of cellulases in microbial bioreactors. potential for plant genetic engineering to overcome Second, and most important, are the costs of pretreating the challenges described above. The basics of cellulosic ethanol production Feedstock crops and lignocellulosic biomass. The factors a that affect the suitability of potential new feedstock crops around the globe for bioethanol production are complex, and relate to country- and region-specific agricultural Pectin practices, market forces, and political as well as biologi- Middle cal issues. These factors include land availability, locally lamella Cellulose accepted cropping systems, and types and forms of microfibril transportation fuel. In addition, the current status of a particular species in terms of its development as a crop Primary wall (for example, the development of breeding strategies) is another important issue; in terms of biology, the feedstock crops that have so far been recommended for Hemicellulose Plasma conversion to cellulosic ethanol have a high amount of membrane cellulosic biomass. These include corn, rice, sugarcane, Soluble protein fast-growing perennial grasses such as switchgrass and b giant miscanthus, and woody crops such as fast-growing Plasma poplar and shrub willow5,6. Depending on where they membrane Secondary wall c Secondary are planted, the ideal characteristics of non-food cel- wall (S3) lulosic crops are: use of the C4 photosynthetic pathway; Secondary long canopy duration; perennial growth; rapid growth wall (S2) in spring (to out-compete weeds); high water-usage Rosette Secondary efficiency; and possibly partitioning of nutrients to wall (S1) Lignin subterranean storage organs in the autumn. Celluose Primary The source of lignocellulosic biomass is the plant wall cell wall (FIG. 1), which has important roles in determin- Hemicellulose Middle ing the structural integrity of the plant, and in defence Protein lamella against pathogens and insects7. The structure, configura- tion and composition of cell walls vary depending on Middle lamella plant taxa, tissue, age and cell type, and also within each Primary wall cell wall layer8,9. The basic structure of the primary cell Figure 1 | Plant plasma membrane and cell-wall structure. a | Cell wall containing wall is a scaffold of cellulose with crosslinking glycans, cellulose microfibrils, hemicellulose, pectin, lignin and soluble proteins. b | Cellulose and there are two types of primary cell wall, which are synthase enzymes are in the form of rosette complexes, which float in the plasma classified according to the type of crosslinks. Type I walls Nature Reviews | Genetics membrane. c | Lignification occurs in the S1, S2 and S3 layers of the cell wall. are present in dicotyledonous plants and