ORNL/Sub/90-90OR21954/1

Genetic Improvement of Switchgrass and Other Herbaceous Plants for Use as Biomass Fuel Feedstock

Kenneth P. Vogel
USDA-ARS University of Nebraska, 344 Keim Hall, Lincoln, Nebraska

Hans-Joachim G. Jung
USDA-ARS, University of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, Minnesota

Date Published: September 2000

Prepared for the U.S. Department of Energy, Office of Fuels Development, Activity No. EB 52 03 00 0 Prepared by Bioenergy Feedstock Development Program, Environmental Sciences Division, OAK RIDGE NATIONAL LABORATORY, Oak Ridge Tennessee 37831

managed by UNIVERSITY OF TENNESSEE-BATTELLE, LLC for the U.S. DEPARTMENT OF ENERGY under contract DE-AC05-00OR22725

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EXECUTIVE SUMMARY

Switchgrass, a perennial warm-season grass native to the prairies of North America, has been identified by the U.S. Department of Energy (DOE) as its main species of emphasis for development into a herbaceous biomass fuel crop. Switchgrass and other herbaceous plant biomass will be a raw agricultural commodity that must be converted into a liquid fuel, probably ethanol, via conversion technology still under development. If feedstock quality can be genetically improved, the economics and efficiency of the conversion process could be significantly enhanced.

Genetically improving an agricultural product for improved end product use requires knowledge of desired quality attributes; the relative economic value of the quality parameters in relation to yield, genetic variation for the desired traits, or for molecular breeding; knowledge of genes to suppress or add; and knowledge of any associated negative consequences of biomass quality manipulation. Because technology is still under development for conversion of herbaceous biomass to liquid fuels, desirable plant feedstock characteristics have not been completely delineated. Some traits such as cellulose and lignin concentration will undoubtably be important.

Much of the research on the genetic modification of herbaceous plant cell walls has been conducted to improve the utilization of forages by ruminant livestock including beef and dairy cattle and sheep. The rumen of these animals is basically an anaerobic fermentation vat in which the microflora break down the complex polysaccharides of plant cell walls into simpler compounds that can be further digested and absorbed by other organs in the digestive system. Research on improving the forage digestibility of switchgrass and other herbaceous species has demonstrated that genetic improvements can be made in forage quality that can have significant economic value.

It should be highly feasible to genetically modify the feedstock quality of switchgrass and other herbaceous plants using both conventional and molecular breeding techniques. The use of molecular markers and transformation technology will greatly enhance the capability of breeders to modify the morphologic structure and cell walls of herbaceous species. It will be necessary to monitor gene flow to remnant wild populations of biomass plants and have strategies available to curtail gene flow if it becomes a potential problem. It also will be necessary to monitor plant survival and long term productivity as affected by genetic changes that improve biomass quality.

In addition to improving feedstock quality genetically, management procedures to optimize biomass quality in field production, harvesting, storage, and transportation components of the overall production system will be needed. A feedstock quality assessment program to rapidly determine feedstock quality at the farm or factory gate will be needed to ensure that feedstock of desired quality is used in conversion plants. It also would enable conversion plants to pay producers for both quality and tonnage.

Research Recommendations

1. Determine herbaceous feedstock characteristics that affect conversion efficiency and rate their relative economic value. Conversion research facilities should routinely use switchgrass biomass samples obtained as per Recommendation 2 in their research. Rapid, inexpensive methods of assaying for the principal traits (cellulose, lignin, conversion efficiency) such as near infrared reflectance spectroscopy (NIRS) need to be evaluated, modified, or developed to facilitate biomass quality evaluation for the thousands of plants that are examined in genetic improvement and evaluation programs.
2. Biomass samples for use in conversion research need to be collected at various stages of development from an array of characterized environments and storage conditions for target herbaceous species such as switchgrass and from genotypes within species known to differ genetically for biomass quality characteristics. A laboratory with extensive prior experience in forage quality research should lead this effort and serve as the primary interface with conversion laboratories such as National Research Energy Laboratory (NREL). Feedstock development research projects would provide samples as per specific protocols. The lead laboratory would characterize samples by NIRS analysis and other emerging rapid, automated analytical technologies and develop calibrations and procedures that could be used in breeding and genetic studies and also for eventual use in buying feedstocks on a quality basis.
3. Conventional breeding and genetic studies to date on forage or biomass quality have used the detergent system of analysis to determine lignin, cellulose, and hemicellulose concentrations. Recent research has clearly shown that acid detergent lignin underestimates lignin concentration which results in an over-estimation of cellulose and hemicellulose concentrations. Genetic studies using both divergent selection and parent-progeny analysis need to be conducted on target species to determine the genetic variation for Klason lignin and other biomass energy specific traits. Because of the large numbers of plants that will need to be analyzed, NIRS calibrations developed via Recommendations 1 and 2 will be needed for plant analysis. Breeding work would then need to be initiated for those traits for which genetic variation indicates genetic gains are feasible.
4. Molecular genetics technology should be used in several thrust areas. Molecular marker maps of switchgrass and other principal herbaceous species need to be developed to enable genetic differences identified via Research Recommendation 3 to be linked to specific genetic markers and utilized in marker assisted selection. The genetic markers will be needed to monitor potential gene flow from biomass crops with modified cell-wall traits to wild populations. Research on developing robust genetic maps and associated sets of markers are being initiated and should continue to receive support.
5. Lignin and cellulose synthase genes are being mapped and cloned by various research groups in an array of species. The impact of the use of these genes in candidate perennial biomass crops such as switchgrass and alfalfa need to be tested and evaluated. The research to date with herbaceous transgenic plants has been primarily on annuals. It should be feasible to determine the effect of the genes on feedstock quality, plant yield, and other traits of transformed plants by using vegetative ramets or clonal propagules of the transformed plants. Genes with desirable characteristics could then be evaluated for any potential deleterious environmental effects and if none, then used directly in genetic improvement programs.
6. To date, only seven genes (five for monomer synthesis, two for polymer assembly) have been cloned for cell-wall polysaccharide synthesis. Only one screen in a mutagenized Arabidopsis population has been done for cell-wall monosaccharide composition. Systematic screening for cell-wall mutants has not been done in grasses other than for brown mid-rib genes. Additional genes, including genes that control cell-wall composition as well as timing, site and extent of deposition of polysaccharides and lignin in cell walls very likely exist. Screening research for cell-wall mutants in C₃ and C₄ grasses and legumes should be initiated. Emphasis should be on diploid, perennial species because any genes identified would already be capable of being used in a perennial such as switchgrass and because finding mutants in plants with high ploidy levels is extremely difficult. As an example, Medicago truncatula, a close diploid relative of alfalfa, could be used to identify cell-wall mutants because mutagenized populations are already available and large-scale gene sequencing is under way.
7. Research on harvesting and storage methods for maintaining and preserving quality needs to be initiated in all potential production areas. Affects of storage conditions on feed stock quality using the quality parameters identified in Research Recommendation 1 needs to be clearly delineated.
8. Economic analyses will need to be conducted to determine the relative economic value of improved biomass quality vs yield because improving biomass quality could limit the rate of genetic improvement in biomass yield currently being achieved by plant breeders and could also limit maximum yields. Breeders, geneticists, and production agronomists need to be able to apply economic weights to yield and quality factors in order to optimize biofuel yield both per acre and ton of feedstock.
9. Herbaceous plants are likely going to be modified to produce bio-products such as biodegradable plastics and vaccines. These products will likely have higher market value than liquid fuels from biomass. It would be extremely difficult to genetically modify plants for two or more different end uses. Rather that attempt to develop biomass plants with specific co-products, the biofuels program should instead concentrate on determining the bioenergy value of the residues derived from processing plants developed for other bioproducts. A notable exception is that attention could be focused on developing transgenic plants to produce the enzymes (cellulases and xylanases) needed to produce liquid fuels from biomass. These enzyme producing biomass plants must be capable of high levels of protein production, such as alfalfa, and could be directly combined with specialized biofuels species, such as switchgrass, during conversion or used as a source for enzyme extraction.