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Abstract
BackgroundA key barrier that limits the full potential of biological processes to create new, sustainable materials and fuels from plant fibre is limited enzyme accessibility to polysaccharides and lignin that characterize lignocellulose networks. Moreover, the heterogeneity of lignocellulosic substrates means that different enzyme combinations might be required for efficient transformation of different plant resources. Analytical techniques with high chemical sensitivity and spatial resolution that permit direct characterization of solid samples could help overcome these challenges by allowing direct visualization of enzyme action within plant fibre, thereby identify barriers to enzyme action.ResultsIn the current study, the high spatial resolution (about 30 nm) of scanning transmission X-ray microscopy (STXM), and the detection sensitivity (ppm) of time-of-flight secondary ion mass spectrometry (ToF-SIMS), were harnessed for the first time to investigate the progression of laccase, cellulase and xylanase activities through wood samples, and to evaluate complementary action between lignin-modifying and polysaccharide-degrading enzymes. In particular, complementary insights from the STXM and ToF-SIMS analyses revealed the key role of laccase in promoting xylanase activity throughout and between plant cell walls.ConclusionsThe spatial resolution of STXM clearly revealed time-dependent progression and spatial distribution of laccase and xylanase activities, whereas ToF-SIMS analyses confirmed that laccase promoted protein penetration into fibre samples, leading to an overall increase in polysaccharide degradation. Spectromicroscopic visualizations of plant cell wall chemistry allowed simultaneous tracking of changes to lignin and polysaccharide contents, which provides new possibilities for investigating the complementary roles of lignin-modifying and carbohydrate-active enzymes.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-014-0176-9) contains supplementary material, which is available to authorized users.
Highlights
A key barrier that limits the full potential of biological processes to create new, sustainable materials and fuels from plant fibre is limited enzyme accessibility to polysaccharides and lignin that characterize lignocellulose networks
The current analyses suggest that breaking through lignin and hemicellulose networks, either through laccase-mediated modification of lignin or xylanase-mediated solubilization of xylan and associate lignin structures, is critical to the further penetration of applied enzymes through plant cell walls
Whereas the spatial resolution of scanning transmission X-ray microscopy (STXM) clearly revealed timedependent as well as spatial progression of lignin-modifying and subsequent polysaccharide-degrading activities, ToFSIMS analyses confirmed that samples treated with a high laccase dose promoted protein penetration into fibre samples, leading to an overall increase in polysaccharide degradation
Summary
A key barrier that limits the full potential of biological processes to create new, sustainable materials and fuels from plant fibre is limited enzyme accessibility to polysaccharides and lignin that characterize lignocellulose networks. Once purified, many carbohydrate- and ligninactive enzymes are characterized using model substrates that facilitate product detection but only partially reflect plant polysaccharide or lignin chemistry. By consequence, this approach does not capture the significance of composite lignocellulose architecture and heterogeneous distribution of lignocellulose components on enzyme efficiency (reviewed in [4] and [5]). By integrating several complementary imaging techniques including real-time visualizations of enzyme action, Bayer and co-workers observed that the detrimental effect of native lignin on cellulase activity largely results from the physical impedance of enzyme penetration through plant cell walls [11]
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