Joint BioEnergy Institute researchers engineer plant cell walls to boost sugar yields and reduce cell wall recalcitrance for biofuels
|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. (Image courtesy of JBEI.) Click to enlarge.|
Researchers at the US Department of Energy’s (DOE’s) Joint BioEnergy Institute (JBEI) have developed a new approach to decrease lignin content in biomass while preventing vessel collapse and have devised a new strategy to boost transcription factor expression in native tissues. A paper describing their work recently was published in Plant Biotechnology Journal.
Abundant lignocellulosic biomass could potentially supply the sugars needed to produce advanced biofuels that can supplement or replace fossil fuels, providing several key technical challenges are met. One of these challenges is finding ways to more cost-effectively extract those sugars.
Dominique Loqué, who directs the cell wall engineering program for JBEI’s Feedstocks Division 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.
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, Loqué notes.
Lignin has also long posed problems for pulping and animal feed. To overcome the lignin problem, Loqué 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.
Lignocellulosic biomass was used for thousands of years as animal feed and is now considered a great sugar source for biofuels production. It is composed mostly of secondary cell walls built with polysaccharide polymers that are embedded in lignin to reinforce the cell wall structure and maintain its integrity. Lignin is the primary material responsible for biomass recalcitrance to enzymatic hydrolysis. During plant development, deep reductions of lignin cause growth defects and often correlate with the loss of vessel integrity that adversely affects water and nutrient transport in plants. The work presented here describes a new approach to decrease lignin content while preventing vessel collapse and introduces a new strategy to boost transcription factor expression in native tissues.
We used synthetic biology tools in Arabidopsis to rewire the secondary cell network by changing promoter-coding sequence associations. The result was a reduction in lignin and an increase in polysaccharide depositions in fibre cells. The promoter of a key lignin gene, C4H, was replaced by the vessel-specific promoter of transcription factor VND6. This rewired lignin biosynthesis specifically for vessel formation while disconnecting C4H expression from the fibre regulatory network.
Secondly, the promoter of the IRX8 gene, secondary cell wall glycosyltransferase, was used to express a new copy of the fibre transcription factor NST1, and as the IRX8 promoter is induced by NST1, this also created an artificial positive feedback loop (APFL). The combination of strategies—lignin rewiring with APFL insertion—enhances polysaccharide deposition in stems without over-lignifying them, resulting in higher sugar yields after enzymatic hydrolysis.—Yang et al.
Loqué 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 transferrable to other plant species. They are currently developing new and better versions of these strategies.
This research was supported by the DOE Office of Science.
JBEI is one of three Bioenergy Research Centers established by the DOE’s Office of Science in 2007. It is a scientific partnership led by Berkeley Lab and includes the Sandia National Laboratories, the University of California campuses of Berkeley and Davis, the Carnegie Institution for Science, the Pacific Northwest National Laboratory, and the Lawrence Livermore National Laboratory. DOE’s Bioenergy Research Centers support multidisciplinary, multi-institutional research teams pursuing the fundamental scientific breakthroughs needed to make production of cellulosic biofuels, or biofuels from nonfood plant fiber, cost-effective on a national scale.
Yang, F., Mitra, P., Zhang, L., Prak, L., Verhertbruggen, Y., Kim, J.-S., Sun, L., Zheng, K., Tang, K., Auer, M., Scheller, H. V. and Loqué, D. (2013). Engineering secondary cell wall deposition in plants. Plant Biotechnology Journal, 11: 325–335. doi: 10.1111/pbi.12016