We studied the impact of glyphosate tolerance on weed control and tree growth in field-grown transgenic poplars. Using Agrobacterium-mediated transformation, we produced 94 transgenic transformation events in four hybrid genotypes (three Populus trichocarpa 9 P. deltoides and one of P. trichocarpa 9 P. nigra). These lines were screened for high levels of tolerance in two plantations in Oregon. Based on screening results, we propagated four lines from two hybrid genotypes to study their value for weed control and productivity in a 2-year management trial in eastern Oregon, comparing conventional weed control at the time of the study to methods that included over-the-top applications of glyphosate during the growing season. Herbicide tolerance was stable in all of the trees over the 2-year period. Weed control, based on weed abundance, was substantially improved in the over-the-top application. Growth of the trees, as measured by stem volume index, was correspondingly improved; transgenic trees grew approximately 20 % faster than the transgenic and non-transgenic control trees. An exploratory life-cycle analysis of the embodied greenhouse-gas benefits for a coppice bioenergy plantation 123New Forests (2016) 47:653-667 DOI 10.1007 suggested that over a 6-year rotation with three coppice cycles, the growth improvement could provide an *8 % savings in greenhouse gas emissions per unit of wood produced. Despite the potential benefits, adoption of this technology will depend on compatibility with management regimes, regulatory and market acceptance, and probably also the development of a robust transgene containment system.
Background Several enzymes and cofactors have been identified as contributing to the slow utilization of xylose by xylosefermenting strains of Saccharomyces cerevisiae. However, there has been no consensus on which of these possible bottlenecks are the most important to address. A previous strain characterization study from our lab suggested that insufficient NAD+ limits fermentation and may be the most important bottleneck affecting utilization of xylose for the production of ethanol. The development and validation of a genome scale dynamic flux balance model would help to verify the existence and extent of this and other metabolic bottlenecks and suggest solutions to guide future strain development thereby minimizing bottleneck impact on process economics. Results A dynamic flux balance model was developed to identify bottlenecks in several strains of S. cerevisiae, both with wild-type pentose phosphate pathway expression and with the pathway over expressed. ZWF1 was found to be limiting in the oxidative portion of the pentose phosphate pathway under oxygen replete conditions. This pathway is used to regenerate NADPH. Under oxygen limiting conditions, respiration of xylose was limited by the lack of oxygen as a terminal electron acceptor. Ethanol production was also limited under these conditions due to the inability to balance NAD+/NADH. The model suggests the use of the anaplerotic glyoxylate pathway to improve NAD+/NADH balance, increasing ethanol production by 50% while producing succinate as a coproduct at upwards of 20 g/l. Conclusion In the production of high value chemicals from biomass, the use of the respiratory metabolism is a waste of feedstock carbon. Bottlenecks previously identified in the oxidative pentose phosphate pathway are currently only relevant under oxygen-replete conditions and cannot impact the partitioning of carbon between the respiratory and fermentative pathways. Focusing future efforts on the non-respiratory balancing of NAD+/NADH, perhaps through the glyoxylate pathway, would improve the economics of ethanol production both directly and through coproduct formation.
Lands producing mixed lignocellulosic ethanol feedstocks may be able to produce more biomass with fewer resources than conventional monoculture crops, but lignocellulosic ethanol production processes and efficiencies can be highly dependent on feedstock composition. In this study, plants were collected from areas planted to simulate conservation buffers alongside stream channels within three common resource areas the interior Pacific Northwest. Two grasses (tall wheatgrass and alfalfa) and seven forb species (fiddleneck tarweed, dog fennel, kochia, downey brome, tall annual willowherb, prickly lettuce, and tumble mustard) commonly found in these buffers were examined to determine their chemical composition, potential bioethanol yields, and difficulties that may arise if they were to be harvested and processed in a single facility. Potential ethanol yields calculated on the basis of sugar monomer composition in the biomass ranged from 181.5 to 316.5 l/dry ton of biomass. Significant differences were noted in terms of structural sugars (cellulose 19%-33% w/w; hemicellulose 14%-26% w/w), lignin (10%-18% w/w), extractives (20%-40% w/w), and ash content (4.0%-13.8% w/w). These composition variations could vary the processing efficiency in terms of sugar recovery and eventual ethanol production yield. V
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