Abstract
BackgroundLignocellulose from fast growing hardwood species is a preferred source of polysaccharides for advanced biofuels and “green” chemicals. However, the extensive acetylation of hardwood xylan hinders lignocellulose saccharification by obstructing enzymatic xylan hydrolysis and causing inhibitory acetic acid concentrations during microbial sugar fermentation. To optimize lignocellulose for cost-effective saccharification and biofuel production, an acetyl xylan esterase AnAXE1 from Aspergillus niger was introduced into aspen and targeted to cell walls.ResultsAnAXE1-expressing plants exhibited reduced xylan acetylation and grew normally. Without pretreatment, their lignocellulose yielded over 25% more glucose per unit mass of wood (dry weight) than wild-type plants. Glucose yields were less improved (+7%) after acid pretreatment, which hydrolyses xylan. The results indicate that AnAXE1 expression also reduced the molecular weight of xylan, and xylan–lignin complexes and/or lignin co-extracted with xylan, increased cellulose crystallinity, altered the lignin composition, reducing its syringyl to guaiacyl ratio, and increased lignin solubility in dioxane and hot water. Lignin-associated carbohydrates became enriched in xylose residues, indicating a higher content of xylo-oligosaccharides.ConclusionsThis work revealed several changes in plant cell walls caused by deacetylation of xylan. We propose that deacetylated xylan is partially hydrolyzed in the cell walls, liberating xylo-oligosaccharides and their associated lignin oligomers from the cell wall network. Deacetylating xylan thus not only increases its susceptibility to hydrolytic enzymes during saccharification but also changes the cell wall architecture, increasing the extractability of lignin and xylan and facilitating saccharification.
Highlights
Lignocellulose from fast growing hardwood species is a preferred source of polysaccharides for advanced biofuels and “green” chemicals
The acetyl esterase activity of wall-bound protein extracts from the transgenic plants’ developing wood tissues was 45–65% higher than that of the wild type (WT) when tested against the synthetic substrate para-nitrophenyl acetate, but soluble protein extracts from transgenic and WT plants exhibited similar levels of esterase activity (Fig. 1c)
AnAXE1‐expressing trees exhibited reduced xylem O‐acetylation and reduced xylan acetylation at the C‐2 position To determine whether the cell wall acetyl content was reduced in transgenic plants, the acetic acid released from their wood after saponification was quantified, revealing a reduction of 13–16% (Fig. 1d)
Summary
Lignocellulose from fast growing hardwood species is a preferred source of polysaccharides for advanced biofuels and “green” chemicals. Saccharification of Arabidopsis mutants with reduced glucuronoxylan acetylation has not improved sugar yields [14,15,16] Many of these mutants exhibited dwarfism, mechanically weak stems, and collapsed xylem vessels [14,15,16,17,18], indicating that glucuronoxylan acetylation has important but poorly understood biological functions. Lignocellulose from Arabidopsis plants expressing Aspergillus niger acetyl xylan esterase 1 Encouraged by these results, we have generated aspens expressing AnAXE1 to study its effects on lignocellulose properties relevant to saccharification. These findings will be relevant for all applications of lignocellulose involving lignin extraction
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