Biomass formation and sugar release efficiency ofPopulusmodified by altered expression of a NAC transcription factor

AbstractWoody biomass is an important feedstock for biofuel production. Manipulation of wood properties that enable efficient conversion of biomass to biofuel reduces cost of biofuel production. Wood cell wall composition is regulated at several levels that involve expression of transcription factors such as wood-/secondary cell wall-associated NAC domains (WND or SND). InArabidopsis thaliana, SND1regulates cell wall composition through activation of its down-stream targets such as MYBs. The functional aspects ofSND1homologs in the woodyPopulushave been studied through transgenic manipulation. In this study, we investigated the role ofPdWND1B, Populus SND1sequence ortholog, in wood formation using transgenic manipulation through over-expression or silencing under the control of a vascular-specific4-coumarate-CoAligase (4CL) promoter. As compared to control plants,PdWND1B-RNAi plants were shorter in height, with significantly reduced stem diameter and dry biomass, whereas there were no significant differences in growth and productivity ofPdWND1Bover-expression plants. Conversely,PdWND1Bover-expression lines showed a significant reduction in cellulose and increase in lignin content, whereas there was no significant impact on lignin content of down-regulated lines. Stem carbohydrate composition analysis revealed a decrease in glucose, mannose, arabinose, and galactose, but an increase in xylose in the over-expression lines. Transcriptome analysis revealed upregulation of several downstream transcription factors and secondary cell wall related structural genes in thePdWND1Bover-expression lines that corresponded to significant phenotypic changes in cell wall chemistry observed inPdWND1Boverexpression lines. Relative to the control, glucose release and ethanol production from stem biomass was significantly reduced in over-expression lines but appeared enhanced in the RNAi lines. Our results show thatPdWND1Bis an important factor determining biomass productivity, cell wall chemistry and its conversion to biofuels inPopulus.