Genetic improvement of bioenergy crops

Genetic improvement of bioenergy crops is a comprehensive text-book on different aspects of bioenergy research with a special emphasis on a range of bioenergy crops themselves. The book is split into two main parts. Part 1 deals with an introduction to bioenergy sources, the reasoning behind the usage of bioenergy, an introduction into the chemistry and biology of bioenergy crops, an introduction into molecular marker techniques that can be used for crop improvement, and the technology of screening the bioenergy properties of biomass feedstock lines together with current technologies for fuel-ethanol production from lignocellulosic plant biomass. Part 2 of the book introduces 15 annual and perennial biomass feedstock crops with respect to the state of their genetic resources, their breeding and bioenergy-related properties. The utilization of the genetic potential of these crops will, as the editor states, help to‘ … address many of the concerns raised by critics of bioenergy’. In the first chapter ‘Why bioenergy makes sense’ the editor provides an excellent overview of all major industrial energy sources and their principles of generation and usage. Against this backdrop a rationale is formulated as to why we should work on bioenergy crops, and the major concerns about bioenergy are discussed. Of particular note is the potential of bioenergy production from lignocellulosic biomass and sugars, which is most promising compared to bioenergy production from ethanol (some concerns voiced against the latter are also addressed). Vermerris states that, ‘A lot of misconceptions could be avoided if the origin of biofuels was indicated more clearly … In addition to not having an established processing scheme for lignocellulosic biomass to incorporate into models, the development of biomass crops specifically for bioenergy applications is still in its infancy’. In the US, the major feedstock for ethanol production is corn grain. The contributing authors Nancy Nichols and Rodney Bothast describe state-of-the art procedures and new developments in the production of bioethanol. Areas for improvements include more efficient separation technologies during milling, new enzymes and yeast strains, and new corn hybrids. Furthermore, ethanol generation will become more competitive by improved co-product quality, and utilization and conversion of non-starch polymers. These authors summarize that‘ … starch crops will continue to be a major feedstock for ethanol production … Although corn is the primary feedstock for ethanol production in the US, interest in alternative starch crops and cellulosic biomass is on the rise’. In recognition of the importance of a shift towards lignocellulosic biomass feedstock for further developments (and acceptance) of bioenergy production, a comprehensive chapter by Vermerris is devoted to the composition and biosynthesis of lignocellulosic compounds. Mark Davis and colleagues give an in-depth introduction into the characterization of promising biomass feedstock lines by the use of high-throughput spectrometric and enzymatic assays. The last chapter in Part 1, authored by Yulin Lu and Nathan Mosier, describes in detail current technologies for fuel-ethanol production from lignocellulosic plant biomass. They point out that current technologies are still far from cost-competitive, while many factors that impede a highly efficient lignocellulosic ethanol process are not yet well understood. According to the authors, the ideal solution is to achieve consolidated bioprocessing (CBP), which means that the steps of cellulase production, enzymatic hydrolysis and ethanol fermentation are conjoined in one process stage in a future biorefinery. To achieve CBP, it is necessary to engineer a single micro-organism or a consortium of mixed-culture microbes accordingly. Their final conclusion, leading into Part 2 of the book, is‘ … that a better understanding of the interaction between the plant cell wall structure and processing performance will yield crops that are tailor-made for advanced thermochemical and biological processing technologies to yield fuels and chemicals in a low cost, efficient, and sustainable manner’. Crops for biomass production need to be selected for their climatic and ecological growing optimum for specific regions of the world. Thus, 15 of the most recognized annual and perennial plant species are introduced in Part 2 of this book – but many more suitable plant species might be out there still to be discovered for bioenergy production. Many of the species presented belong to the grass family. As regards woody perennial plants, willow, southern pine and poplar are introduced. Corn breeding efforts are ongoing and have been for a long time, and this plant species is also an ‘excellent model system for studying biomass production potential’, as stated by Natalia de Leon and James Coors, who go on to say that ‘One of the main advantages of using corn stover as feedstock is the possibility of separately commercializing the lignocellulosic material along with grain as a high-value co-product’. Potential for the improvement of this plant species lies in the modification of plant architecture, planting density, drought resistance and nutrient acquisition. An important bioenergy crop of the future for warmer climates is sorghum, as noted in the chapter by Ana Saballos, who summarizes target traits and breeding schemes for this species. Thomas Tew and Robert Cobill present sugarcane as one of the most important bioenergy crops of the southern hemisphere. Sugarcane has a broad genetic pool and diversity, but complicated genetics. Many more advances could be made with this crop species. John Clifton-Brown and co-authors introduce a popular energy crop with potential for different climatic regions, especially for Europe: Miscanthus. This C4 grass is not native to Europe, but has excellent potential to be more widely cultivated in this region of the world. The authors describe the biology of the crop, cultivation and harvest protocols and ongoing breeding efforts. The illustrations in this chapter are very informative, including pictures on the morphology of the crop, on the different phenotypes, and on mechanical planting. Switchgrass is an emerging bioenergy C4 grass, mainly in the US, and a large effort is being put into developing this species into a bioenergy crop, as discussed in the chapter by Joe Bouton. Even if this species has excellent potential, it is still in its infancy in terms of crop development. This chapter gives an excellent introduction into the current state-of-the art and breeding for the improvement of the species towards a bioenergy crop. Other minor perennial forage species for feedstocks for bioenergy are described by William Anderson and co-authors. They provide concise overviews on current progress with reed canary grass, alfalfa, wildrye, big bluestem, bermudagrass, napiergrass and eastern gamagrass. Since these are less well-known species, the inclusion of an illustration page with their morphology would have been desirable. Lawrence Smart and Kimberley Cameron provide a detailed discussion regarding the development of willow as a bioenergy crop. This chapter is well illustrated, explaining processes such as the crossability relationships among the species and practical pollination of willow for crossing. Willow is established as a bioenergy crop most widely in Sweden, but its use is expanding in other parts of Europe and Northern America. The development of genomics resources and mapping populations are ongoing, and comparative genomics between willow and the better-characterized bioenergy crop poplar are described. John Davis has produced a chapter on the genetic improvement of poplar, which is a particularly attractive crop since the processing infrastructure already exists having been established for the conversion of wood into pulp and paper products. The Populus gene pool is wide and allows for many interspecific crosses. The poplar genome has been entirely sequenced, mapping populations for quantitative trait loci (QTL) have been established, and it can be genetically modified by Agrobacterium-mediated transformation. The prospects for a wide deployment of poplar are excellent and will be further enhanced by breeding. The term ‘southern pine’ contains several pine species, which have been described by Gary Peter in the last chapter of the book. Pine plantations comprise about half of the world's industrial forest plantations: ‘Because of the magnitude of operations, the forest products industry is currently the largest producer of bioenergy in the US’. Two main processing strategies exist to use southern pine biomass: (1) as raw material for integrated forest biorefineries that produce bioenergy and biofuels in tandem with cellulose-based products, and (2) as fuel for burning for the generation of heat and electricity through co-firing with coal or wood pellets and biofuels using bioconversion or gasification. This chapter describes the relevant steps in detail: harvesting, processing, and systems of short-rotation plantings. In addition, the biology, breeding and biotechnology of the southern pine species are well illustrated and described. Since a long life cycle is in the nature of southern pine, more rapid advances in breeding of trees with improved bioenergy properties are to be expected by the application of biotechnology approaches. I highly recommend this book as a standard text for every library in universities and research centres that are working on the engineering of bioenergy applications and on the agronomy, breeding and biology of bioenergy crops.