Lignocellulosic feedstocks: research progress and challenges in optimizing biomass quality and yield

DEDICATED ENERGY CROPS AND MODELS Lignocellulosic biomass derived from energy crops and agricultural residues is a promising renewable source for the production of transportation fuels and bio-based materials. Plants exhibiting C4 photosynthesis are amongst the most promising dedicated energy crops as they possess tremendous intrinsic efficiency in converting solar energy to biomass. Van der Weijde et al. (2013) provide an excellent overview of the potential of five C4 grasses from the Panicoideae clade (maize, Miscanthus, sorghum, sugarcane, and switchgrass) as lignocellulosic feedstock for the production of biofuels. The authors discuss yield potential, biomass quality and genetic improvement of dual-purpose food and energy cultivars and dedicated energy cultivars through plant breeding and also highlight several research needs. Perennial growth habit provides a number of environmental advantages over annuals as bioenergy crops, including the requirement of less fertilizer, reduced soil erosion, and even the potential for soil carbon sequestration. Schwartz and Amasino (2013) review the importance of the ability of perennial crops to recycle nitrogen, reducing the need for energy intensive N fertilizer, and the subsequent production of potent NOx greenhouse gases. An interesting focus is on the importance of photoperiodic flowering and dormancy in switchgrass and the relevance of these traits to N recycling and genetic variation that contributes to these dynamics. Wang et al. (2013a) review carbon partitioning in sugarcane, which has a source-sink system that creates high concentrations of easily extracted and economically valuable stem sucrose. Nonetheless, the majority of carbohydrate in sugarcane is lignocellulose. A detailed understanding of the molecular, and physiological processes underlying the partitioning of carbon assimilates will provide targets to manipulate the balance between sucrose and lignocellulosic biomass as carbon sinks. These include sugar transport and localization and the engineering of novel sugar sinks. This classic example of a possible dual-purpose crop can also be improved through breeding and genetic engineering of cell wall properties to further optimize biofuel production. Slavov et al. (2013) review the current knowledge about cell wall genetics, chemistry and structure in Miscanthus. This includes the prospects of developing detailed molecular genetic and biochemical models of pathways relevant to biomass conversion efficiency. The life history and genome complexities exhibited by the Miscanthus species in question dictate that genome wide association studies are a necessary approach toward the genetic dissection of these traits. Potential targets for biomass improvement include cell wall regulatory genes, intercellular trafficking, and microtubule organization. Opportunities exist to functionally test gene-trait associations for cell wall quality in this bioenergy crop, short-term progress toward understanding of the molecular underpinnings of cell wall quality traits in Miscanthus will be driven by research in model grasses. Setaria viridis is a rapid cycling C4 panicoid grass with several attributes that make it an excellent model for bioenergy grasses. Petti et al. (2013) describe the composition and saccharification dynamics of S. viridis aboveground biomass as similar to sorghum, maize, and switchgrass, confirming its potential as model species for panicoid translational genomics. Another grass proposed as a model for energy grasses, forage grasses and cereals is Brachypodium distachyon. Rancour et al. (2012) present chemical composition data of cell walls from distinct organs and developmental stages. Results indicate similar cell wall composition to those previously determined for a diverse set of C3 forage grasses and cereals, highlighting the usefulness of B. distachyon as a model for temperate grasses. As with S. virdis, the authors report some differences for particular wall traits.