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  1. Abstract

    In preparation for a major GWAS (Genome Wide Association Study) of plant regeneration and transformation, a large number of factors were examined for their effects on indirect regeneration rate in diverse wild genotypes—seeking a high rate of regeneration, but also highly genetically variable and heritable treatments. Many of the factors examined have never before been reported on for their effects on callus, shoot, or root organogenesis in poplar (Populus). Stems had the highest regeneration potential, followed by petioles and leaves, with greenhouse grown explant sources superior toin vitrogrowth explant sources. Changes of ± 50% to Murashige and Skoog (MS) basal medium salts and micronutrients had a minor effect on regeneration. Many popular treatments that were evaluated also had little to no useful effect at the levels studied, including activated charcoal, ascorbic acid, silver nitrate, melatonin, serotonin, sucrose concentration, and lipoic acid. As a result of this wide exploration, treatment combinations that substantially elevated regeneration in diverse genotypes were identified, enabling GWAS.

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  2. null (Ed.)
  3. Hybrid-poplar tree plantations provide a source for biofuel and biomass, but they also increase forest isoprene emissions. The consequences of increased isoprene emissions include higher rates of tropospheric ozone production, increases in the lifetime of methane, and increases in atmospheric aerosol production, all of which affect the global energy budget and/or lead to the degradation of air quality. Using RNA interference (RNAi) to suppress isoprene emission, we show that this trait, which is thought to be required for the tolerance of abiotic stress, is not required for high rates of photosynthesis and woody biomass production in the agroforest plantation environment, even in areas with high levels of climatic stress. Biomass production over 4 y in plantations in Arizona and Oregon was similar among genetic lines that emitted or did not emit significant amounts of isoprene. Lines that had substantially reduced isoprene emission rates also showed decreases in flavonol pigments, which reduce oxidative damage during extremes of abiotic stress, a pattern that would be expected to amplify metabolic dysfunction in the absence of isoprene production in stress-prone climate regimes. However, compensatory increases in the expression of other proteomic components, especially those associated with the production of protective compounds, such as carotenoids and terpenoids, and the fact that most biomass is produced prior to the hottest and driest part of the growing season explain the observed pattern of high biomass production with low isoprene emission. Our results show that it is possible to reduce the deleterious influences of isoprene on the atmosphere, while sustaining woody biomass production in temperate agroforest plantations.

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  4. Summary

    Plants transition through juvenile and adult phases of vegetative development in a process known as vegetative phase change (VPC). In poplars (genusPopulus) the differences between these stages are subtle, making it difficult to determine when this transition occurs. Previous studies of VPC in poplars have relied on plants propagatedin vitro, leaving the natural progression of this process unknown.

    We examined developmental morphology of seed‐grown andin vitroderivedPopulus tremula × alba(clone 717‐1B4), and compared the phenotype of these to transgenics with manipulated miR156 expression, the master regulator of VPC.

    In seed‐grown plants, most traits changed from node‐to‐node during the first 3 months of development but remained constant after node 25. Many traits remained unchanged in clones over‐expressing miR156, or were enhanced when miR156 was lowered, demonstrating their natural progression is regulated by the miR156/SPL pathway. The characteristic leaf fluttering ofPopulusis one of these miR156‐regulated traits.

    Vegetative development in plants grown from culture mirrored that of seed‐grown plants, allowing direct comparison between plants often used in research and those found in nature. These results provide a foundation for further research on the role of VPC in the ecology and evolution of this economically important genus.

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  5. Abstract

    Populus tremuloidesis the widest‐ranging tree species in North America and an ecologically important component of mesic forest ecosystems displaced by the Pleistocene glaciations. Using phylogeographic analyses of genome‐wide SNPs (34,796 SNPs, 183 individuals) and ecological niche modeling, we inferred population structure, ploidy levels, admixture, and Pleistocene range dynamics ofP. tremuloides, and tested several historical biogeographical hypotheses. We found three genetic lineages located mainly in coastal–Cascades (cluster 1), east‐slope Cascades–Sierra Nevadas–Northern Rockies (cluster 2), and U.S. Rocky Mountains through southern Canadian (cluster 3) regions of theP. tremuloidesrange, with tree graph relationships of the form ((cluster 1, cluster 2), cluster 3). Populations consisted mainly of diploids (86%) but also small numbers of triploids (12%) and tetraploids (1%), and ploidy did not adversely affect our genetic inferences. The main vector of admixture was from cluster 3 into cluster 2, with the admixture zone trending northwest through the Rocky Mountains along a recognized phenotypic cline (Utah to Idaho). Clusters 1 and 2 provided strong support for the “stable‐edge hypothesis” that unglaciated southwestern populations persisted in situ since the last glaciation. By contrast, despite a lack of clinal genetic variation, cluster 3 exhibited “trailing‐edge” dynamics from niche suitability predictions signifying complete northward postglacial expansion. Results were also consistent with the “inland dispersal hypothesis” predicting postglacial assembly of Pacific Northwestern forest ecosystems, but rejected the hypothesis that Pacific‐coastal populations were colonized during outburst flooding from glacial Lake Missoula. Overall, congruent patterns between our phylogeographic and ecological niche modeling results and fossil pollen data demonstrate complex mixtures of stable‐edge, refugial locations, and postglacial expansion withinP. tremuloides. These findings confirm and refine previous genetic studies, while strongly supporting a distinct Pacific‐coastal genetic lineage of quaking aspen.

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  6. Abstract

    Faux‐hawk fullerenes are promising candidates for high‐performance organic field‐effect transistors (OFETs). They show dense molecular packing and high thermal stability. Furthermore, in contrast to most other C60derivates, functionalization of the fullerene core by the fluorinated group C6F4CF2does not increase their lowest unoccupied orbital position, which allows the use of air‐stable molecular n‐dopants to optimize their performance. The influence of n‐doping on the performance of OFETs based on the faux‐hawk fullerene 1,9‐C60(cyclo‐CF2(2‐C6F4)) (C60FHF) is studied. An analytic model for n‐doped transistors is presented and used to clarify the origin of the increase in the subthreshold swing usually observed in doped OFETs. It is shown that the increase in subthreshold swing can be minimized by using a bulk dopant layer at the gate dielectric/C60FHF layer instead of a mixed host:dopant layer. Following an optimization of the OFETs, an average electron mobility of 0.34 cm2 V−1 s−1, a subthreshold swing below 400 mV dec−1for doped transistors, and a contact resistance of 10 kΩ cm is obtained, which is among the best performance for fullerene based n‐type semiconductors.

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