Abstract
BackgroundThe expression of biomass-degrading enzymes (such as cellobiohydrolases) in transgenic plants has the potential to reduce the costs of biomass saccharification by providing a source of enzymes to supplement commercial cellulase mixtures. Cellobiohydrolases are the main enzymes in commercial cellulase mixtures. In the present study, a cellobiohydrolase was expressed in transgenic corn stover leaf and assessed as an additive for two commercial cellulase mixtures for the saccharification of pretreated sugar cane bagasse obtained by different processes.ResultsRecombinant cellobiohydrolase in the senescent leaves of transgenic corn was extracted using a simple buffer with no concentration step. The extract significantly enhanced the performance of Celluclast 1.5 L (a commercial cellulase mixture) by up to fourfold on sugar cane bagasse pretreated at the pilot scale using a dilute sulfuric acid steam explosion process compared to the commercial cellulase mixture on its own. Also, the extracts were able to enhance the performance of Cellic CTec2 (a commercial cellulase mixture) up to fourfold on a range of residues from sugar cane bagasse pretreated at the laboratory (using acidified ethylene carbonate/ethylene glycol, 1-butyl-3-methylimidazolium chloride, and ball-milling) and pilot (dilute sodium hydroxide and glycerol/hydrochloric acid steam explosion) scales. We have demonstrated using tap water as a solvent (under conditions that mimic an industrial process) extraction of about 90% recombinant cellobiohydrolase from senescent, transgenic corn stover leaf that had minimal tissue disruption.ConclusionsThe accumulation of recombinant cellobiohydrolase in senescent, transgenic corn stover leaf is a viable strategy to reduce the saccharification cost associated with the production of fermentable sugars from pretreated biomass. We envisage an industrial-scale process in which transgenic plants provide both fibre and biomass-degrading enzymes for pretreatment and enzymatic hydrolysis, respectively.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-014-0131-9) contains supplementary material, which is available to authorized users.
Highlights
The expression of biomass-degrading enzymes in transgenic plants has the potential to reduce the costs of biomass saccharification by providing a source of enzymes to supplement commercial cellulase mixtures
Ongoing technology improvements have substantially reduced the cost of biomass pretreatment and enzymatic hydrolysis to fermentable sugars for bioethanol production; recent estimates still indicate that these processes account for about 30% of the total process cost [8]
Expression of recombinant CBH transgenic corn The CBH utilised in this study (Additional file 1: Figure S1) is a proprietary biomolecule obtained from Verenium Corporation [32] that shares 96% amino acid sequence identity with CBH I from Penicillium occitanis [33] and 61% amino acid sequence identity with CBH I (Cel7A) from Trichoderma reesei [34]
Summary
The expression of biomass-degrading enzymes (such as cellobiohydrolases) in transgenic plants has the potential to reduce the costs of biomass saccharification by providing a source of enzymes to supplement commercial cellulase mixtures. Plant biomass is the primary source of simple sugars, and bacterial, fungal, and algal fermentation enable the transformation of these sugars into a wide range of renewable platform chemicals, fuels, and value-added products. Most accessing these sugars is not simple, because the plant cell wall is a resilient, intractable barrier composed of complex polysaccharides (cellulose and hemicellulose), lignin, and proteins. The cost of enzymatic hydrolysis mainly depends on the enzyme dosage and the hydrolysis rate, which in turn depends on cellulose accessibility to enzymes
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