Apr

2

If You Can’t Beat the Lignin Change It

April 2, 2013 | Leave a Comment

Lignocellulosic Biomass (LB) is the most abundant organic material on Earth.  LB could supply the sugars needed to produce advanced biofuels that can supplement or replace fossil fuels.  Humans have been using LB as animal feed for thousands of years, and for the past two centuries it’s been the raw material of the paper industry.  We are blessed with an overwhelming abundance of LB.  All most everyone understands it’s generally a good idea to make valuable use of this kind of resource.

The problem is finding ways to more cost-effectively extract the sugars in LB.

Joint BioEnergy Institute at the U.S. Department of Energy (JBEI) is a scientific partnership led by Lawrence Berkeley National Laboratory (Berkeley Lab) whose mission is to advance the development of next generation biofuels where major steps towards achieving a breakthrough are being taken by researchers.

Dominique Loque, who directs the cell wall engineering program for JBEI’s Feedstocks Division has something of an announcement about LB, “Through the tools of synthetic biology, we have engineered healthy plants whose lignocellulosic biomass can more easily be broken down into simple sugars for biofuels. Working with the model plant, Arabidopsis, as a demonstration tool, we have genetically manipulated secondary cell walls to reduce the production of lignin while increasing the yield of fuel sugars.”

Genetically engineered Arabidopsis plants (#89) yielded as much biomass as wild types (WT) but with enhanced polysaccharide deposition in the fibers of their cell walls. Click image for the largest view.

Genetically engineered Arabidopsis plants (#89) yielded as much biomass as wild types (WT) but with enhanced polysaccharide deposition in the fibers of their cell walls. Click image for the largest view.

The team’s paper, titled “Engineering Secondary Cell Wall Deposition in Plants,” describing this research in detail has been published in Plant Biotechnology Journal.

Loque and his research group have focused on reducing the natural recalcitrance of plant cell walls to give up their sugars. Unlike the simple starch-based sugars in corn and other grains, the complex polysaccharide sugars in plant cell walls are locked within a robust aromatic polymer called lignin. Setting these sugars free from their lignin cage has required the use of expensive and environmentally harsh chemicals at high temperatures, a process that helps drive production costs of advance biofuels prohibitively high.

Loque explains the biochemical situation, “By embedding polysaccharide polymers and reducing their extractability and accessibility to hydrolytic enzymes, lignin is the major contributor to cell wall recalcitrance. Unfortunately, most efforts to reduce lignin content during plant development have resulted in severe biomass yield reduction and a loss of integrity in vessels, a key tissue responsible for water and nutrient distribution from roots to the above-ground organs.”

Lignin has also long presented problems for pulping and animal feed. To overcome the lignin problem, Loque and his colleagues rewired the regulation of lignin biosynthesis and created an artificial positive feedback loop (APFL) to enhance secondary cell wall biosynthesis in specific tissue. The idea was to reduce cell wall recalcitrance and boost polysaccharide content without impacting plant development.

Loque said, “When we applied our APFL to Arabidopsis plants engineered so that lignin biosynthesis is disconnected from the fiber secondary cell wall regulatory network, we maintained the integrity of the vessels and were able to produce healthy plants with reduced lignin and enhanced polysaccharide deposition in the cell walls. After various pretreatments, these engineered plants exhibited improved sugar releases from enzymatic hydrolysis as compared to wild type plants. In other words we accumulated the good stuff – polysaccharides – without spoiling it with lignin.”

Loque and his colleagues believe that the APFL strategy they used to enhance polysaccharide deposition in the fibers of their Arabidopsis plants could be rapidly implemented into other vascular plant species as well. This could increase cell wall content to the benefit of the pulping industry and forage production as well as for bioenergy applications. It could also be used to increase the strength of cereal straws, reducing crop lodging and seed losses. Since regulatory networks and other components of secondary cell wall biosynthesis have been highly conserved by evolution, the researchers feel their lignin rewiring strategy should also be readily transferable to other plant species. They are currently developing new and even better versions of these strategies.

“We now know that we can significantly re-engineer plant cell walls as long as we maintain the integrity of vessels and other key tissues,” Loque says.

The research details have been published in Plant Biotechnology Journal. The paper is titled “Engineering secondary cell wall deposition in plants.” Loque is the corresponding author. Co-authors are Fan Yang, Prajakta Mitra, Ling Zhang, Lina Prak, Yves Verhertbruggen, Jin-Sun Kim, Lan Sun, Kejian Zheng, Kexuan Tang, Manfred Auer and Henrik Scheller.

It’s an interesting and innovative take on a problem that bedevils biomass processing.  One expects the team has an answer for the intended consequences.  It’s the unintended consequences that remain to be seen and could take quite a while to be evident.


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