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Tension wood holds clues to higher fuel yields from biomass crops


Poplar stems (left) respond to bending stress by producing tension wood, which has characteristics desirable in a bioenergy feedstock. Electron micrographs from a comprehensive BESC study reveal how tension wood (bottom right) develops a secondary cell wall layer, in contrast to normal wood (top right).

The properties that enable trees to flex in the wind might also prove use­ful in designing better biomass crops. Researchers at the BioEnergy Science Center (BESC) at Oak Ridge National Laboratory (ORNL) are studying those prop­erties to understand how they affect the release of plant sugars that become biofuels.

Tension wood forms in broadleaf woody plants in response to bending stresses. It has several features desirable in biomass, including higher amounts of fiber, less lignin, thicker cell walls, and more crystalline forms of cellulose, the sugar that ferments into ethanol and other transportation fuels. Previous stud­ies have focused on individual features of tension wood, but the BESC team is the first to characterize comprehensively the gene expression and protein networks that come into play during tension wood formation.

 “Tension wood in poplar trees has a special type of cell wall that is of interest because it is composed of more than 90% cellulose, whereas typical woody biomass is composed of 40 to 55% cellulose,” said team mem­ber Udaya Kalluri of the ORNL’s Biosciences Division. “If you increase the cellulose in your feedstock material, then you can potentially extract more sugars.” The team is extending its study of ten­sion wood to the molecular level and hopes to reveal the genetic basis of the desirable physical features.

The work is supported by the DOE Office of Science. The team also in­cludes Sara Jawdy and Gerald Tuskan of the Biosciences Division; Marcus Foston, Chris Hubbell, Reichel Samuel, Seokwon Jung, Arthur J. Ragauskas, and Hu Fan of Georgia Institute of Technology; and Robert Sykes, Shi-You Ding, Yining Zeng, Erica Gjersing, and Mark Davis of the National Renewable Energy Laboratory.

Reference: Foston, M., et al. 2011. “Chemical, Ultrastructural and Supramolecular Analysis of Tension Wood in Populus tremula x alba as a Model Substrate for Reduced Recalcitrance,” Energy & Environmental Science 4, 4962–71.

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