Abstract
BackgroundSecond-generation biofuels are generally produced from the polysaccharides in the lignocellulosic plant biomass, mainly cellulose. However, because cellulose is embedded in a matrix of other polysaccharides and lignin, its hydrolysis into the fermentable glucose is hampered. The senesced inflorescence stems of a set of 20 Arabidopsis thaliana mutants in 10 different genes of the lignin biosynthetic pathway were analyzed for cell wall composition and saccharification yield. Saccharification models were built to elucidate which cell wall parameters played a role in cell wall recalcitrance.ResultsAlthough lignin is a key polymer providing the strength necessary for the plant’s ability to grow upward, a reduction in lignin content down to 64% of the wild-type level in Arabidopsis was tolerated without any obvious growth penalty. In contrast to common perception, we found that a reduction in lignin was not compensated for by an increase in cellulose, but rather by an increase in matrix polysaccharides. In most lignin mutants, the saccharification yield was improved by up to 88% cellulose conversion for the cinnamoyl-coenzyme A reductase1 mutants under pretreatment conditions, whereas the wild-type cellulose conversion only reached 18%. The saccharification models and Pearson correlation matrix revealed that the lignin content was the main factor determining the saccharification yield. However, also lignin composition, matrix polysaccharide content and composition, and, especially, the xylose, galactose, and arabinose contents influenced the saccharification yield. Strikingly, cellulose content did not significantly affect saccharification yield.ConclusionsAlthough the lignin content had the main effect on saccharification, also other cell wall factors could be engineered to potentially increase the cell wall processability, such as the galactose content. Our results contribute to a better understanding of the effect of lignin perturbations on plant cell wall composition and its influence on saccharification yield, and provide new potential targets for genetic improvement.
Highlights
Second-generation biofuels are generally produced from the polysaccharides in the lignocellulosic plant biomass, mainly cellulose
Because perturbations in the lignin biosynthesis often affected plant growth, we first compared the final height and weight of the senesced inflorescence stems of the mutants with those of the wild-type (Table 1)
The mutants c4h-2, ccr1-3, and ccr1-6, with the largest reduction in lignin content, had the highest saccharification yields and an almost complete cellulose conversion that resulted in a stem structure disintegration
Summary
Second-generation biofuels are generally produced from the polysaccharides in the lignocellulosic plant biomass, mainly cellulose. The major component of the secondary cell wall is cellulose, a polymer of 1,4-linked β-D-glucose units, of which the largest fraction is organized into microfibrils through inter- and intramolecular hydrogen bonds and van der Waals forces. Lignin is mainly made from the monolignols coniferyl alcohol and sinapyl alcohol and traces of p-coumaryl alcohol that give rise to guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units. Most of these units are linked via ether bonds (in so-called β–O–4-structures) and carbon-carbon bonds [in resinol (β–β), and phenylcoumaran (β–5) structures] [7,8]. After the monolignols are transported to the cell wall, they are oxidized by laccases and peroxidases to monolignol radicals that couple in a combinatorial fashion, generating the lignin polymer
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