The trees that produced the engineered enzyme had slightly less total lignin in their cell walls. But on further analysis, the scientists found that these trees also had dramatically altered lignin structure, with a significant reduction in the level of one of the two major types of lignin components normally found in aspen trees. These findings were further confirmed using two-dimensional nuclear magnetic resonance spectroscopic imaging by a team led by John Ralph of the University of Wisconsin and the Great Lakes Bioenergy Research Center, a DOE Bioenergy Research Center. Specifically, the engineered trees had less "labile" lignin, while the remaining lignin components became structurally more condensed, forming an increased number of cross-linkages among the polymers.
"We expected that this condensed, more cross-linked lignin might make the plants even harder to digest, but found that wood containing these structures released up to 62 percent more simple sugars when treated with digestive enzymes," Liu said. The yield of ethanol from this modified wood was almost 50 percent higher than the ethanol yield of wood derived from untreated control trees.
Interestingly, by imaging aspen wood samples using infrared light at NSLS, the scientists found that their approach for altering lignin content and composition also increased the production of cellulose fibers, the major source of fermentable sugars in the cell wall. This increased cellulose content might partially contribute to the increased release of simple sugars, they said.
Importantly, the changes in lignin and cell wall structures did not affect the growth of the engineered aspens. The wood densities and the biomass yields were comparable to those of the control trees.
"These data suggest that lignin condensation itself is not a critical factor affecting the digestibility of the cell wall," said Liu. "The findings also support the idea that engineering the enzymes that modify lignin precursors represents a useful biotechnological solution for effectively tailoring the digestibility of poplar-family woody biomass to generate feedstocks for biofuel production.
"It's gratifying when fundamental studies of enzyme function, such as the findings that underpin this work, can be translated to contribute to solving real-world problems," he added.Literature"Enhancing digestibility and ethanol yield of Populus wood via expression of an engineered monolignol 4-O-methyltransferase
", DOI: 10.1038/NCOMMS11989Source