JBEI and USDA researchers boost switchgrass biofuels potential by adding a maize gene; more starch, easier to extract
Researchers with the US Department of Energy (DOE)’s Joint BioEnergy Institute (JBEI) and the US Department of Agriculture’s Agricultural Research Service (ARS), have demonstrated that introducing a maize (corn) gene into switchgrass (Panicum virgatum), a potential feedstock for advanced biofuels, more than doubles (250%) the amount of starch in the plant’s cell walls, resulting in higher glucose release for fermentation with or without biomass pretreatment.
The gene, a variant of the maize gene known as Corngrass1 (Cg1), holds the switchgrass in the juvenile phase of development, preventing it from advancing to the adult phase. The results of this research are described in an open access paper published in the Proceedings of the National Academy of Sciences (PNAS).
Plant biomass can be broken down to monosaccharides (saccharification) and converted to fuels. Plant cell walls are composed of cellulose microfibrils embedded in a cross-linked network of matrix polysaccharides and copolymerized with lignin. This complex structure inhibits the saccharification of cell wall polysaccharides by cell wall degrading enzymes. Furthermore, byproducts of the harsh pretreatments necessary to enable saccharification inhibit growth of microorganisms used to produce biofuels. Therefore, improving saccharification efficiency is one of the major goals in developing an efficient, cost-effective biofuel industry.
...Plants go through a series of development stages over time in response to a variety of stimuli, both external and internal. Each phase displays unique morphological and physiological characteristics that change when the plant undergoes a transition to the next phase. One such developmental transition is the switch from the juvenile to adult phase of development. In general, juvenile plant material is less lignified and displays differences in biomass accumulation and character. These juvenile traits may reduce the recalcitrance of biomass to conversion into fermentable sugars. By controlling the genes that regulate the juvenile to adult phase transition in plants, it may be possible to modify or enhance the biomass properties of a wide range of bioenergy feedstocks.
The dominant maize Corngrass1 (Cg1) mutant fixes plant development in the juvenile phase and affects both biomass accumulation and saccharification efficiency. Cg1 mutants increase biomass due to continuous initiation of axillary branches (tillers) and leaves. The resulting biomass has reduced adult characteristics and ectopic juvenile cell identities. In addition, Cg1 mutant leaves contain decreased amounts of lignin and increased levels of glucose and other sugars compared with wild type, which could provide an improved substrate for saccharification.—Chuck et al.
Lignocellulosic biomass is the most abundant organic material on earth. Studies have consistently shown that biofuels derived from lignocellulosic biomass could be produced in the United States in a sustainable fashion and could replace today’s gasoline, diesel and jet fuels on a gallon-for-gallon basis. Unlike ethanol made from grains, such fuels could be used in today’s engines and infrastructures and would be carbon-neutral, meaning the use of these fuels would not exacerbate global climate change.
Among potential crop feedstocks for advanced biofuels, switchgrass offers a number of advantages. As a perennial grass that is both salt- and drought-tolerant, switchgrass can flourish on marginal cropland, does not compete with food crops, and requires little fertilization.
The original Cg1 was isolated in maize about 80 years ago. We cloned the gene in 2007 and engineered it into other plants, including switchgrass, so that these plants would replicate what was found in maize. The natural function of Cg1 is to hold pants in the juvenile phase of development for a short time to induce more branching. Our Cg1 variant is special because it is always turned on, which means the plants always think they are juveniles.—George Chuck, lead author, with joint appointments at ARS and UC Berkeley
Chuck and his colleague Sarah Hake, another co-author of the PNAS paper and director of the Plant Gene Expression Center, proposed that since juvenile biomass is less lignified, it should be easier to break down into fermentable sugars. Also, since juvenile plants don’t make seed, more starch should be available for making biofuels. To test this hypothesis, they collaborated with Simmons and his colleagues at JBEI to determine the impact of introducing the Cg1 gene into switchgrass.
In addition to reducing the lignin and boosting the amount of starch in the switchgrass, the introduction and overexpression of the maize Cg1 gene also prevented the switchgrass from flowering even after more than two years of growth, an unexpected but advantageous result. The lack of flowering limits the risk of the genetically modified switchgrass from spreading genes into the wild population, says Chuck.
The results of this research offer a promising new approach for the improvement of dedicated bioenergy crops, but there are questions to be answered. For example, the Cg1 switchgrass biomass still required a pre-treatment to efficiently liberate fermentable sugars.
The alteration of the switchgrass does allow us to use less energy in our pre-treatments to achieve high sugar yields as compared to the energy required to convert the wild type plants. The results of this research set the stage for an expanded suite of pretreatment and saccharification approaches at JBEI and elsewhere that will be used to generate hydrolysates for characterization and fuel production.—Blake Simmons, head of JBEI’s Deconstruction Division
Another question to be answered pertains to the mechanism by which Cg1 is able to keep switchgrass and other plants in the juvenile phase.
Co-authoring the PNAS paper with Chuck and Simmons were Christian Tobias, Lan Sun, Florian Kraemer, Chenlin Li, Dean Dibble, Rohit Arora, Jennifer Bragg, John Vogel, Seema Singh, Markus Pauly and Sarah Hake.
This research was supported in part by DOE’s Office of Science and by the USDA-ARS. 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, and the Lawrence Livermore National Laboratory.
George S. Chuck, Christian Tobias, Lan Sun, Florian Kraemer, Chenlin Li, Dean Dibble, Rohit Arora, Jennifer N. Bragg, John P. Vogel, Seema Singh, Blake A. Simmons, Markus Pauly, and Sarah Hake (2011) Overexpression of the maize Corngrass1 microRNA prevents flowering, improves digestibility, and increases starch content of switchgrass. PNAS 108 (42) 17550-17555 doi: 10.1073/pnas.1113971108