Evaluation of engineered low-lignin poplar for conversion into advanced bioproducts

Abstract

Background Lignocellulosic resources are promising feedstocks for the manufacture of bio-based products and bioenergy. However, the inherent recalcitrance of biomass to conversion into simple sugars currently hinders the deployment of advanced bioproducts at large scale. Lignin is a primary contributor to biomass recalcitrance as it protects cell wall polysaccharides from degradation and can inhibit hydrolytic enzymes via non-productive adsorption. Several engineering strategies have been designed to reduce lignin or modify its monomeric composition. For example, expression of bacterial 3-dehydroshikimate dehydratase (QsuB) in poplar trees resulted in a reduction in lignin due to redirection of metabolic flux toward 3,4-dihydroxybenzoate at the expense of lignin. This reduction was accompanied with remarkable changes in the pools of aromatic compounds that accumulate in the biomass. Results The impact of these modifications on downstream biomass deconstruction and conversion into advanced bioproducts was evaluated in the current study. Using ionic liquid pretreatment followed by enzymatic saccharification, biomass from engineered trees released more glucose and xylose compared to wild-type control trees under optimum conditions. Fermentation of the resulting hydrolysates using Rhodosporidium toruloides strains engineered to produce α-bisabolene, epi-isozizaene, and fatty alcohols showed no negative impact on cell growth and yielded higher titers of bioproducts (as much as + 58%) in the case of QsuB transgenics trees. Conclusion Our data show that low-recalcitrant poplar biomass obtained with the QsuB technology has the potential to improve the production of advanced bioproducts.

Ectopic Production of 3,4-Dihydroxybenzoate in Planta Affects Cellulose Structure and Organization

“…The transgenic poplar lines were generated by the introduction of 3-dehydroshikimate dehydratase (QsuB), as described previously. , The level of QsuB expression in the different transgenic lines followed the order: QsuB1 > QsuB15 > QsuB5 > WT (which was zero, as the gene is not native to plants). The DHBA content in the trees followed the same order while lignin content followed the reverse order as described below.…”

“…Our overall aim was to investigate a structural basis for the observed increase in saccharification of poplar QsuB transgenic trees. , These lines are characterized by a reduction in lignin deposition that is correlated with increased production of DHBA . Additionally, a concomitant increase in xylan, albeit at lower amounts, was also observed in the transgenic plants …”

“…Wood from three different poplar transgenic lines (QsuB1, QsuB5, and QsuB15) with reduced lignin content and wild type (WT) were grown in controlled conditions at the Joint BioEnergy Institute (JBEI), Emeryville, CA, as previously described . The harvested stems were debarked and air-dried (see Figure S1).…”

“…Wood from three different poplar transgenic lines (QsuB1, QsuB5, and QsuB15) with reduced lignin content and wild type (WT) were grown in controlled conditions at the Joint BioEnergy Institute (JBEI), Emeryville, CA, as previously described. 37 The harvested stems were debarked and air-dried (see Figure S1). The stems were cut into eight pieces approximately 10 cm in length (see Figure S1C); the bottom 0.5 cm (#1) was used for SANS and WAXS evaluation.…”

“…This study investigates the structural changes in the cell walls of transgenic QsuB hybrid poplar using small-angle neutron scattering (SANS) and wide-angle X-ray scattering (WAXS) to understand previously reported increased saccharification in these transgenic plants. , SANS measures the structural changes in cellulose microfibril dimensions, microfibril packing, and the structure of the copolymer matrix on length scales between ∼1 and 600 nm. In contrast, WAXS measures the structure of the cellulose microfibrils themselves and can provide information about the orientation of the cellulose chains, cellulose crystallinity, and cellulose crystal dimensions.…”