Genetic manipulation of lignin reduces recalcitrance and improves ethanol production from switchgrass
- aForage Improvement Division and
- dPlant Biology Division, The Samuel Roberts Noble Foundation, Ardmore, OK 73401;
- bBiosciences Division and
- cBioEnergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN, 37831; and
- eSchool of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332
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Contributed by Richard A. Dixon, January 8, 2011 (sent for review November 3, 2010)
↵1C.F. and J.R.M. contributed equally to this work.

Abstract
Switchgrass is a leading dedicated bioenergy feedstock in the United States because it is a native, high-yielding, perennial prairie grass with a broad cultivation range and low agronomic input requirements. Biomass conversion research has developed processes for production of ethanol and other biofuels, but they remain costly primarily because of the intrinsic recalcitrance of biomass. We show here that genetic modification of switchgrass can produce phenotypically normal plants that have reduced thermal-chemical (≤180 °C), enzymatic, and microbial recalcitrance. Down-regulation of the switchgrass caffeic acid O-methyltransferase gene decreases lignin content modestly, reduces the syringyl:guaiacyl lignin monomer ratio, improves forage quality, and, most importantly, increases the ethanol yield by up to 38% using conventional biomass fermentation processes. The down-regulated lines require less severe pretreatment and 300–400% lower cellulase dosages for equivalent product yields using simultaneous saccharification and fermentation with yeast. Furthermore, fermentation of diluted acid-pretreated transgenic switchgrass using Clostridium thermocellum with no added enzymes showed better product yields than obtained with unmodified switchgrass. Therefore, this apparent reduction in the recalcitrance of transgenic switchgrass has the potential to lower processing costs for biomass fermentation-derived fuels and chemicals significantly. Alternatively, such modified transgenic switchgrass lines should yield significantly more fermentation chemicals per hectare under identical process conditions.
Footnotes
- 2To whom correspondence may be addressed. E-mail: radixon{at}noble.org or zywang{at}noble.org.
Author contributions: J.R.M., J.B., R.A.D., and Z.-Y.W. designed research; C.F., J.R.M., X.X., Y.G., C.Y.H., M.R., F.C., M.F., and A.R. performed research; C.F., J.R.M., and Z.-Y.W. analyzed data; and C.F., J.R.M., R.A.D., and Z.-Y.W. wrote the paper.
The authors declare no conflict of interest.
Data deposition: The switchgrass COMT coding sequence in this paper has been deposited in GenBank with the (accession no. HQ645965).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1100310108/-/DCSupplemental.
Freely available online through the PNAS open access option.