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  • Review Article
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Plant genetic engineering for biofuel production: towards affordable cellulosic ethanol

Nature Reviews Genetics volume 9, pages 433–443 (2008)Cite this article

A Retraction to this article was published on 02 March 2010

Key Points

  • Recently, biofuels have received increasing global attention because of finite reserves of fossil fuels, which particularly affects industrialized nations, and an increased knowledge of the effect of the extraction and use of fossil fuels on the environment. These factors have led to most nations initiating research into the production and use of biofuels as the main source of transportation fuel.

  • At present, the main sources of biofuel ethanol include starch from corn seeds and sugar from sugarcane. However, neither starch nor sugar can be produced at a sufficient level to meet biofuel needs. In addition, increasing the use of these products for biofuel production would push up food prices. Therefore, using cellulosic material for biofuel production is an important area of research.

  • One environmental advantage of biofuel crop production is a reduction of greenhouse gas levels owing to increased photosynthesis. In addition, conversion of biomass to ethanol and the burning of the ethanol fuel reduces greenhouse gas emissions compared with petroleum fuel, so that its use does not contribute to an increase in net carbon dioxide.

  • Important steps that are associated with commercial production of cellulosic ethanol include production, transportation and storage of cellulosic biomass; pretreatment processes to break down the biomass into intermediates and remove its lignin; production of cellulases in microbial bioreactors used to convert biomass into fermentable sugars; and fermentation of sugars into ethanol.

  • Problems presently associated with the commercial production of cellulosic ethanol include the high costs of production of cellulases in microbial bioreactors and the costs of pretreatment processes of lignocellulosic matter, which together bring the price of cellulosic ethanol to about two to three-fold higher than the price of corn grain ethanol.

  • In nature, plant cell walls are deconstructed (decomposed) by hydrolysis enzymes (including cellulases and hemicellulases), which are produced mainly by microorganisms.

  • One important area of research is the genetic modification of relevant feedstock crops to self-produce heterologous cellulases. To date a few non-biofuel model crops (such as Arabidopsis thaliana, tobacco and alfalfa) and feedstock biofuel crops (such as corn and rice) have been developed as biofactories that self-produce microbial cellulases within their biomass.

  • Subcellular targeting is important for the safe accumulation of these heterologous molecules. Microbial cellulases that are produced in the cytoplasm of biomass biofuel crops have been targeted for safe accumulation in subcellular compartments such as the apoplast, chloroplast, mitochondria, endoplasmic reticulum and vacuole — away from cytoplasmic metabolic activities. New crop genotypes that have modified lignin levels and configurations can also reduce pretreatment processes, and have been developed for species including poplar, alfalfa, tobacco and corn.

  • Plant cell walls contain cellulose, hemicellulose, pectin and lignin. A complete understanding of the biosynthetic pathways involved is lacking, and is an important area of ongoing research, with the aim of manipulating these pathways to optimize plant cell-wall composition for cellulosic bioethanol production.

  • Cellulosic biomass can be increased by the transfer of genes that shift energy from the reproductive to the vegetative state, and possibly through overexpressing enzymes that are associated with the biosynthesis of plant cellulose and hemicellulose.

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.

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Figure 1: Plant plasma membrane and cell-wall structure.
Figure 2: Overview of cellulosic ethanol production.
Figure 3: Lignin biosynthesis.

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    Mariam B. Sticklen

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Glossary

Antisense oligonucleotides

Short synthetic pieces of DNA that are designed to bind to their target mRNA through base pairing. As a result, they inhibit the expression of the target mRNA, causing inhibition of translation, splicing or transport of the target mRNA, or degradation of the DNA–RNA hybrid by RNase H.

RNA interference

In RNAi, long double-stranded RNAs (dsRNAs of around >200 nt) are used to silence the expression of specific target genes. Long dsRNAs are first processed into 20–25 nt small interfering RNAs (siRNAs) by the Dicer RNase III-like enzyme. SiRNAs then assemble into endoribonuclease-containing RNA-induced silencing complexes (RISCs), and subsequently guide RISCs to complementary RNA molecules, which they cleave and destroy.

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Sticklen, M. Plant genetic engineering for biofuel production: towards affordable cellulosic ethanol. Nat Rev Genet 9, 433–443 (2008). https://doi.org/10.1038/nrg2336

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