Digestibility data on various mild alkaline–pretreated transgenic poplar lines compared to wild type. Error bars indicate SD from the mean of triplicate determinations; *P < 0.01; **P < 0.005. Wilkerson et al. Click to enlarge.

By designing poplars for deconstruction, we can improve the degradability of a very useful biomass product. 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.

—Curtis Wilkerson, lead author, MSU and GLBRC

The idea to engineer biomass for easier degradation first took shape in the lab of University of Wisconsin-Madison professor and Great Lakes Bioenergy Research Center (GLBRC) Plants Leader John Ralph, who was then working at the US Dairy Forage Research Center. In the mid-1990s, Ralph’s group was looking for ways to reduce energy usage in the paper pulping process by more efficiently removing lignin from trees. The group surmised that if they could introduce weak bonds into lignin, they could simply “unzip” the material, making it much easier for chemical processes to break it down.

Studies of natural plant tissues, along with those from mutants and transgenics with misregulated monolignol biosynthetic genes, have led to some remarkable discoveries, including plants that produce homopolymers from a range of traditional as well as nontraditional monomers. These observations illustrate the inherent pliability of the lignification process. Therefore, the formal design of an improved polymer using unconventional monomers seems to be a feasible path to tailor plants with superior processing properties for both paper and biofuels production.

To that end, the introduction of monolignol ferulate conjugates … into the lignin monomer pool appears to be one of the most promising. These exotic conjugates have been shown, but to date only in in vitro model systems, to be capable of introducing readily cleavable ester bonds into the lignin backbone, permitting easier depolymerization.

—Wilkerson et al.

Ralph’s approach had clear benefits for the biofuels industry as well, where 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 successfully engineer poplars highly amenable to degradation and, by extension, to industrial processing.

To produce the poplars, Wilkerson identified and isolated a gene capable of making monomers with bonds that are easier to break apart. Next, University of British Columbia professor Shawn Mansfield successfully put that gene into the poplar. The group then determined that the plants not only created the monomers but also incorporated them into the lignin polymer, thereby introducing the weak links into the lignin backbone and transforming 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, essentially designing lignin for (chemical) deconstruction. 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.

—John Ralph

The research is the direct result of a collaboration funded by the Great Lakes Bioenergy Research Center, one of three US Department of Energy-funded Bioenergy Research Centers 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 (Wilkerson) and introducing it into the plants (Mansfield), to proving, via newly designed analyses, that the plant was utilizing the new monomers in making its lignin (Fachuang Lu, Ralph).

The technology is available for licensing.

Resources

• C. G. Wilkerson, S. D. Mansfield, F. Lu, S. Withers, J.-Y. Park, S. D. Karlen, E. Gonzales-Vigil, D. Padmakshan, F. Unda, J. Rencoret, and J. Ralph (2014) “Monolignol Ferulate Transferase Introduces Chemically Labile Linkages into the Lignin Backbone,” Science 344 (6179), 90-93 doi: 10.1126/science.1250161