Submitted to: BioResources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2016
Publication Date: 4/13/2016
Citation: Fleming, M.B., Decker, S.R., Bedinger, P.A. 2016. Investigating the role of extensin proteins in poplar biomass recalcitrance. BioResources. 11(2):4727-4744.

Interpretive Summary: Plants are a source of renewable energy – the energy contained within their cell walls as cellulose can be converted into glucose and then into biofuel. However, other cell wall components, particularly hemicelluloses and lignin, are known to block and reduce the efficiency of the conversion process. This study examined whether a different cell wall component, extensin proteins, might also block the conversion process in poplar wood. Genes encoding extensin proteins were identified in the poplar genome, and extensin proteins were found to be present in poplar wood. Thermochemical pretreatments, which are used to remove hemicelluloses and lignin before beginning the conversion process, reduced but did not completely remove extensin proteins from poplar wood. Protease treatment, which removes proteins, further reduced the amount of extensin proteins in poplar wood, and also further improved the conversion of cellulose into glucose. These results suggest that extensin proteins do block the conversion process in poplar wood. Incorporating extensin protein removal during pretreatment may help increase the efficiency and cost-effectiveness of biofuel production.

Technical Abstract: The biological conversion of cellulosic biomass to biofuel is hindered by cell wall recalcitrance, which can limit the ability of cellulases to access and break down cellulose. The purpose of this study was to investigate whether hydroxyproline-rich cell wall proteins (extensins) are present in poplar stem biomass, and whether these proteins may contribute to recalcitrance. Three classical extensin genes were identified in Populus trichocarpa through bioinformatic analysis of poplar genome sequences, with the following proposed names: PtEXTENSIN1 (Potri.001G019700); PtEXTENSIN2 (Potri.001G020100); PtEXTENSIN3 (Potri.018G050100). Tissue print immunoblots localized the extensin proteins in poplar stems to regions near the vascular cambium. Different thermochemical pretreatments reduced but did not eliminate hydroxyproline (Hyp, a proxy for extensins) from the biomass. Protease treatment of liquid hot water-pretreated poplar biomass reduced Hyp content by a further 16% and increased subsequent glucose yield by 20%. These data suggest that extensins may contribute to recalcitrance in pretreated poplar biomass, and that incorporating protease treatment into pretreatment protocols could result in a small but significant increase in the yield of fermentable glucose.