Plants and bioenergy

As the world's emerging economies increase their petroleum and coal use the inevitable repercussions of fossil fuel dependency will continue to escalate. Among these are eventual restrictions in supply and spiralling costs of energy as easily accessed reserves are depleted. Regardless of supply is an unequivocal imperative to take immediate and aggressive measures to reduce net greenhouse gas emissions by decreasing fossil-fuel consumption and increasing our use of carbon-neutral or carbon-negative fuels. Economic growth and development worldwide depend increasingly on secure supplies of reliable, affordable, clean energy, and climate stability and environmental security require carbon neutral alternatives. Alternative sources of energy are thus a priority for modern society, and chief among these is the only proven mechanism to convert solar energy into marketable fuels, this being photosynthesis in plants and algae. The five leading oil-producing countries in the Americas produce a little more than 23 million barrels of oil per day (bbl d–1) of the world total of 85 m bbl d–1 (US Energy Information Administration, 2015). The marked increase in US output in recent years has made it the world's leading producer at 12 m bbl d–1, yet its imports total almost 19 m bbl d–1. Brazil is energy-independent today because of its forward-thinking energy strategy over the past two decades, its oil production and consumption are balanced, and their aggressive drive to produce bio-based fuels make them the world leader in the production of ethanol. Both Brazil and the United States committed funds and implemented strong research efforts in renewable fuels over the past decade. In response to this growing research effort, leading plant biologists in the Americas came together in a spirit of cooperation to develop a conference for discussing scientific efforts using plants and algae to improve energy security for the western hemisphere. In 2008, in association with the American Society of Plant Biologists, the 1st Pan-American Congress on Plants and BioEnergy was convened in Mérida, Mexico. The first congress encompassed the broad issues of bioenergy capabilities and implementation. Senior governmental officers of Mexico, Brazil, and the US spoke on co-operative research initiatives for plant and microbial organisms to produce novel forms of bioenergy and strategies for the introduction of bioenergy crops into the agricultural landscape. Other talks emphasized how vast improvements in biomass quantity and quality were needed to reduce the agronomic footprint of agroenergy production while optimizing energy output: input for alternative fuels. Microbiologists and chemical engineers laid the groundwork for second- and third-generation biofuels. Other talks discussed the economics of bioenergy, and the governmental policies that would be needed to promote the agro-energy economy. Venture capitalists offered their perspectives on how the new agro-energy initiatives in the commercial sector could be funded. By the acclamation of the colleagues in attendance, it was agreed to meet biennially, rotating between the US, Brazil, and Canada. Subsequent congresses focused on scientific progress. In 2014, the 4th Pan-American Congress on Plants and BioEnergy was convened at the University of Guelph, Ontario, Canada, covering a range of disciplines, including algal and plant systems for bioenergy, plant genetics and genomics, gene discovery for the improvement of bioenergy production and quality, regulatory mechanisms of synthesis and degradation, strategies for third generation biofuel production and the promise of synthetic biology in the production of biofuels and bio-based products, the development of cropping systems for biomass production, and mitigation of environmental impacts of bioenergy production. This special issue ‘Plants and Bioenergy’ is a collection of invited reviews and research articles inspired by the presentations at the Guelph congress and key research leaders of the Americas. Simas-Rodrigues et al. (2015) begin this edition with an overview for how microalgae, typically considered as sources of oil, can also be excellent sources of starch and biomass for ethanol production. Kim et al. (2015) describe how transcriptomic data mining and combinatorial gene stacking has been used for metabolic engineering of Camelina to produce oils containing medium-chain length fatty acids that mimic jet fuel hydrocarbon components. Tsogtbaatar et al. (2015) employed metabolomics as a powerful tool for both quantifying intracellular compounds and qualitatively assessing biochemical pathways and metabolic routes involved in the synthesis of biofuel-relevant oils. Maloney et al. (2015) show that sucrose-phosphate synthase and sucrose-phosphate phosphatase interact to form a metabolic channel that impacts sucrose metabolism and manifests in enhanced plant growth. Biomass recalcitrance is attributed to lignin or lignin–carbohydrate complexes within plant cell walls. McCann and Carpita (2015) present a novel perspective on recalcitrance as new conversion technologies reveal additional determinants that comprise a broad range of molecular, nanoscale, and macroscale factors. They redefine recalcitrance as an emergent property of the interactions of biomass and catalysts during conversion processes. Tavares et al. (2015) describe how understanding the endogenous decomposition mechanisms of plants can be designed to enhance the saccharification efficiency for bioethanol. Understanding endogenous plant cell-wall degradation in processes such as fruit ripening, storage mobilization, abscission, and aerenchyma formation may be used in the future to re-engineer plants for bioenergy production. In this regard, De Souza et al. (2015) report on the manner in which lignin is linked to polysaccharides and discuss how polysaccharide interactions within cell walls of Miscanthus sinensis are associated with recalcitrance to hydrolytic processes. Li and colleagues (2015) investigated the relationship between cell-wall properties in diverse maize lines that contribute to the differences in enzymatic hydrolysis yields for both untreated and mild NaOH-pre-treated biomass. Zhong and Ye (2015) review the remarkable progress that has been made in defining the transcriptional networks involved in the vascular development of lignocellulosic biomass, which has provided a rich context for the application of genome-wide association studies for improvement of bioenergy crops. Clark et al. (2015) and Nagano et al. (2015) used high-density genetic markers to identify gene flow from diploid Miscanthus sinensis to tetraploid M. sacchariflorus in Japan, contrasting the allelic distribution of Co/Hd1 homologues in M. sinensis ecotypes. Metcalfe et al. (2015) report that retrotransposon-insertion polymorphisms can be used as markers for sugarcane selection. Carpita and McCann (2015) describe how advanced analytical techniques have been used in mutant analysis to uncover genes encoding the protein machinery responsible for the synthesis and assembly of the plant cell wall. Three research articles contribute new technologies to identify important traits for biomass improvement. Kapp et al. (2015) describe how the rapid visualization and relative quantification of lignin staining in stems of Brachypodium distachyon using the fluorogenic dye Basic Fuchsin illuminates spatio-temporal patterns and subcellular relationships of lignification in grasses. Pattathil and colleagues (2015) used glycome profiling and AFEX pre-treatment to reveal differences in cell-wall architecture among different plant taxa and implicate subclasses of polysaccharides whose loosening reduces recalcitrance of cell walls to degradation. Von Forell et al. (2015) proposed structural models of maize stalks that suggest a new strategy for developing bioenergy varieties in which tissue weaknesses are counterbalanced by small increases in stalk diameter that reduce structural weaknesses. Shyu and Brutnell (2015) describe new insights on how jasmonates regulate diverse signalling in grasses and discuss how this understanding will enhance the potential to uncouple growth and defence responses in bionenergy crops. Cass et al. (2015) show how the suppression of phenylalanine lyase lowers lignin content which increases susceptibility to fungal attack but minimally affects insect herbivory or abiotic stress. Low lignin content is preferred in bioenergy crops as this enhances the ability to degrade tissues and extract fermentable carbohydrates in cellulose and other wall compounds. Lara-Chavez et al. (2015) explore genotypic differences in expression between switchgrass cultivars following inoculation with a bacterial pathogen. Expanding the range of bioenergy crops is the subject of two reviews. Cushman et al. (2015) review how Agave spp. and Opuntia spp. crassulacean acid metabolism plants achieve high water-use efficiencies while supporting biomass production comparable to C3 and C4 bioenergy crops. If C4 perennial grasses were tolerant of cold extremes, they could be more productive than C3 plants in cool climates. Sage et al. (2015) review the cold tolerance of the leading C4 perennial grass candidates for bioenergy production at higher latitude. Freisen et al. (2015) found that Spartina pectinata (prairie cordgrass) had superior rhizome freezing tolerance, spring leaf frost and freezing tolerance, and greater first year establishment compared with Miscanthus×giganteus at a cool temperate site. Peixoto et al. (2015) found that LT50 values in diploid rhizomes of Miscanthus indicated that they retain mechanisms that confer a higher tolerance of lower temperatures than occurs in the more-productive polyploid lines. Along the lines of ecological diversity and response to abiotic stress, Attia et al. (2015) show that isohydric poplars have high water-use efficiency, while anisohydric poplars show faster growth under a variable water supply. Their studies provide predictive models for performance of the different genotypes for woody biomass production. The chronosequence field experiments of Boersma et al. (2015) found unexpected differences in leaf senescence symptoms between different aged Miscanthus×giganteus stands, potentially indicating differential senescence with plant age and nutrient status. In closing, we are grateful to the editors of the Journal of Experimental Botany for providing us the opportunity to prepare a focused issue to highlight some of the best bioenergy research in the western hemisphere. Taken together, the issue covers a broad spectrum of the leading topics being addressed in plant research pertaining to agroenergy. Bioenergy research, as it pertains to plants, is a highly collaborative and integrative field, as developments at one end of the research spectrum (for example, agronomic production) have to occur in tandem with developments at the other end (for example, cost-effective fuel production from plant products). Otherwise, gaps in the supply chain will delay economically competitive development of the agroenergy sector. We encourage readers to read the range of articles in the issue, both to appreciate the scope of the work, and to develop an understanding of the advances being made towards viable alternatives to fossil fuels. The organizers are grateful for support by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (BES), and the Office of Biological and Environmental Research (BER), DE-FOA-0000995, which generated 30 travel awards for young scientists to attend the 4th Pan-American Congress on Plants and Bioenergy.

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