|Bhattacharya, Anjanabha -|
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: March 16, 2011
Publication Date: February 1, 2012
Citation: Bhattacharya, A., Knoll, J.E. 2012. Conventional and molecular breeding for improvement of biofuel crops: past, present and future. In: Kole, C., Joshi, C. P., Shonnard, D. R. (Eds.) Handbook of Bioenergy Crop Plants. CRC Press, Taylor and Francis Group, Boca Raton, FL. pp. 3-20. Interpretive Summary: First-generation biofuels are derived from food and feed crops rich in sugar, starch, or oil, such as sugarcane (Saccharum hyb.), maize (Zea mays), or soybean (Glycine max), as these are easily converted into liquid fuels. Sugar can be directly fermented by yeast to produce ethanol, and starch can be easily converted into fermentable sugars. Many types of fats and oils can be chemically converted into biodiesel. However, these crops alone cannot meet the projected demand for biofuels, and there are concerns about the use of food and feed for fuel production. Second-generation biofuels are derived from non-food sources, such as crop residues or from dedicated energy crops. These materials are more difficult to convert into liquid fuels, but the technologies to do so are being developed. Improvement of biomass yield and convertibility of biofuel crops can be achieved through plant breeding. A combination of traditional plant breeding approaches and emerging molecular marker technologies and genomics tools are being used to improve biofuel crops. The Brazilian sugarcane/ethanol industry has demonstrated the most successful deployment of biofuels. Sugarcane is an economically important food crop in Brazil and other tropical regions, so significant genomics resources have been developed for this crop, despite the high complexity of its genome. Several molecular maps have been created for sugarcane, though none are complete. Still, some molecular markers have been associated with important traits, and these are being used in sugarcane improvement. In the U.S. ethanol is primarily made from maize grain. A long history of maize breeding in the U.S., and in particular the development and use of hybrids, has resulted in the U.S. being the world’s top maize producer. Some research is also being conducted to develop sweet sorghum (Sorghum bicolor) as a source of fermentable sugars for biofuel production. As a model grass crop with a small genome, molecular markers and many linkage maps have been developed for sorghum. Crops such as peanut (Arachis hypogaea) and soybean are a source of oil feedstock to produce biodiesel. Of greatest interest to plant breeders is the oil profile of these crops, as this has major affects on the properties of biodiesel. Jatropha curcas is a relatively undomesticated tree species that is being considered as a dedicated biodiesel crop. Molecular markers have been developed for this species, and a genomic sequence was recently completed. Crop residues, sometimes called stover, are a readily available source of biomass that can be used to produce ethanol. This ‘cellulosic’ ethanol is an example of second-generation biofuel. Increasing the digestibility of crop stover should improve its suitability as a feedstock for biofuel production. Some studies have been conducted to determine if digestibility of maize or sorghum stover can be improved while maintaining grain yields. Perennial grasses have high biomass yield potential, and should require less energy inputs, since they do not need to be replanted each year. Switchgrass (Panicum virgatum) has been widely studied as a biomass crop in much of the U.S. It is a long-lived native prairie grass with a broad range of adaptability. It can be used as forage, so named cultivars have already been developed. Molecular tools are being developed for further switchgrass improvement. Switchgrass is grown from seed, but many perennial grasses are propagated vegetatively. These include energycane (Saccharum hyb.), Miscanthus spp., and napiergrass (Pennisetum purpureum). Energycanes were developed specifically as bioenergy crops by crossing sugarcane with wild Saccharum species. Type I energycanes are used for both sugar and biomass, while Type II energycanes primarily are grown only for biomass. Miscanthus is being widely grown in Europe as a bioener
Technical Abstract: First-generation biofuels are derived from food and feed crops rich in sugar, starch, or oil, such as sugarcane (Saccharum hyb.), maize (Zea mays), or soybean (Glycine max), as these are easily converted into liquid fuels. However, these crops alone cannot meet the projected demand for fuel, so second-generation biofuels are being developed. These are fuels derived from crop residues and dedicated bioenergy crops, thus utilizing the whole plant, and capturing more energy per unit of land area. This chapter addresses the progress made in the development of crop cultivars for both first and second-generation biofuels, with a major emphasis on perennial grasses for second-generation lignocellulosic feedstock, and also considers further improvements that could be made in the future. This chapter describes traditional plant breeding approaches as well as non-transgenic molecular tools available to the plant breeder, including emerging genomics technologies. Molecular markers have been developed for most biofuel crop species, and these are being used to assess germplasm diversity and to identify quantitative trait loci (QTL) that can be used in marker assisted breeding. Several genomic sequences have also been completed for important biofuel crop species.