The research, which appears in the current issue of Science, focuses on enhancing poplar trees so they can break down easier and thus improving their viability as a biofuel. The long-term efforts and teamwork involved to find this solution can be described as a rare, top-down approach to engineering plants for digestibility, said Curtis Wilkerson, Michigan State University plant biologist and the lead author.
"By designing poplars for deconstruction, we can improve the degradability of a very useful biomass product," said Wilkerson, Great Lakes Bioenergy Research Center scientist. "Poplars are dense, easy to store and they flourish on marginal lands not suitable for food crops, making them a non-competing and sustainable source of biofuel."
The idea to engineer biomass for easier degradation first took shape in the mid-1990s in the lab of John Ralph, University of Wisconsin-Madison professor and GLBRC plants leader. Ralph's group was looking to reduce energy usage in the paper pulping process by more efficiently removing lignin – the polymer that gives plant cell walls their sturdiness – from trees. If weak bonds could be introduced into lignin, this hardy material could be "unzipped," making it much easier for chemical processes to break it down.
Ralph's approach had clear benefits for the biofuels industry as well. The difficulty in removing and processing lignin remains a major obstacle to accessing the valuable sugars contained within biomass, adding energy and cost to the production of biofuels. Seeing an opportunity to carry out Ralph's concept in poplar, GLBRC researchers pooled their expertise.
To produce the enhanced poplars, Wilkerson identified and isolated a gene capable of making monomers – molecular glue of sorts – with bonds that are easier to break apart. Next, Shawn Mansfield with the University of British Columbia successfully put that gene into poplars.
The team then determined that the plants not only created the monomers but also incorporated them into the lignin polymer. This introduced weak links into the lignin backbone and transformed the poplars' natural lignin into a more easily degradable version.
"We can now move beyond tinkering with the known genes in the lignin pathway to using exotic genes to alter the lignin polymer in predesigned but plant-compatible ways," Ralph said. "This approach should pave the way to generating more valuable biomass that can be processed in a more energy efficient manner for biofuels and paper products."
The research also is noteworthy for being the direct result of a collaboration funded by the GLBRC, funded by the U.S. Department of Energy and created to make transformational breakthroughs in new cellulosic biofuels technology. Realizing the collaborative project called for a wide array of expertise, from finding the gene and introducing it into the plants, to proving, via newly designed analyses, that the plant was utilizing the new monomers in making its lignin.
"I guarantee that John Ralph and I would never have met without the GLBRC," Wilkerson said. "When I first met him at a group retreat, I knew very little about lignin. But I ended up sharing some techniques I'd been using for totally different projects that I thought might be useful for his ‘zip-lignin' research. The collaboration really grew from there."
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