Abstract
Main conclusion
Hydrothermal pretreatment initially removed the lignin-free xylan from the middle layer of secondary wall, followed by the lignin-bound xylan, but the cellulose-bound xylan was seldom removed by this pretreatment.
An in-depth understanding of the mechanism of xylan removal during hydrothermal pretreatment (HTP) of wood is critical for cost-effective conversion of lignocellulosic biomass to biofuels. Several studies demonstrated the kinetics and mechanism of xylan removal during HTP on molecular scale, but the dissolution mechanism of xylan during HTP remains unclear at ultra-structural level. Our study investigated changes in the micro-distribution of xylan in poplar fiber cell walls during HTP by transmission electron microscopy (TEM) in combination with immunogold labeling. The study revealed that HTP caused greater decline in the density of xylan labeling in the S2 layer of fiber wall than in the S1 layer. There was a greater loss in the density of xylan labeling during HTP in the delignified and enzymatically treated fibers compared to untreated fibers. We propose that in the initial stages of HTP lignin-free xylan in the S2 layer was more readily hydrolyzed than in the S1 layer by hydronium ions. With increasing pretreatment time, the xylan covalently bound to lignin was also removed from the S2 layer due to the dissolution of lignin. The xylan tightly bound to cellulose was seldom removed during HTP, but was hydrolyzed in subsequent enzymatic treatment. This TEM-immunolabeling investigation reveals the manner in which different xylan fractions are removed from fiber cell wall during HTP, and we expect the information to be helpful in developing processes tailored for more effective conversion of cellulosic biomass into fermentable sugars.
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Abbreviations
- CML:
-
Compound middle lamella
- D+HTP:
-
Hydrothermal pretreatment after delignification
- HTP:
-
Hydrothermal pretreatment
- HTP+E:
-
Hydrothermal pretreatment followed by enzymatic treatment
- iS2 layer:
-
Inner part of S2 layer
- mS2 layer:
-
Middle part of S2 layer
- oS2 layer:
-
Outer part of S2 layer
- S layer:
-
Secondary cell wall
- S1 layer:
-
Outer layer of secondary cell wall
- S2 layer:
-
Middle layer of secondary cell wall
- TB:
-
Toluidine blue
- TEM:
-
Transmission electron microscopy
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Acknowledgments
The authors are very grateful for the financial support from National Science Fund for Distinguished Young Scholars (31225005), Chinese Ministry of Education (113014A) and National Science and Technology Program of the Twelfth Five-Year Plan Period (2012BAD32B06).
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The authors declare that they have no conflict of interest.
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Ma, J., Ji, Z., Chen, J.C. et al. The mechanism of xylans removal during hydrothermal pretreatment of poplar fibers investigated by immunogold labeling. Planta 242, 327–337 (2015). https://doi.org/10.1007/s00425-015-2313-5
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DOI: https://doi.org/10.1007/s00425-015-2313-5