Editors' Choice

Science  30 Aug 2013:
Vol. 341, Issue 6149, pp. 938
  1. Ecology

    About FACE

    1. Andrew M. Sugden

    Globally, forests and their soils account for about twice as much carbon as is present in the atmosphere, and hence the response of forests to increasing atmospheric CO2 is expected to be an important component of the global carbon cycle. However, compared to what is known from studies of small trees and herbaceous vegetation, in which elevated CO2 has been shown to have a fertilizing effect on growth, there has been relatively little experimental research on mature forest stands. Using a 45-m canopy crane in a 100-year-old mixed deciduous forest in Switzerland, Bader et al. exposed tree crowns to 550 ppm CO2 over 8 years and compared the ecosystem's response to that of control trees not exposed to elevated CO2. Key plant traits, including litter production, radial stem growth, leaf biomass, and foliar nitrogen, were unaffected. However, there were several indicators of altered below-ground processes, including enhanced nitrification and nitrate leaching and reduced water uptake by roots. Hence, it appears that—at least for the trees in this mature forest stand—elevated CO2 does not translate into increased carbon sequestration by the vegetation and that the consequences are more likely to manifest themselves in alterations in soil processes and nutrient cycling.

    J. Ecol. 101, 10.1111/1365-2745.12149 (2013).

  2. Biomedicine

    Model Therapies

    1. L. Bryan Ray

    If cellular signaling pathways were discrete and linear, controlling signals gone awry—like those from growth-promoting receptor tyrosine kinases often linked to cancer—would be straightforward. But these pathways form entangled and dynamic networks, and inhibiting signal transmission at one node, although successful in the short term, is often thwarted by regulatory mechanisms that keep cells healthy by rendering them robust to perturbations. Two groups have used a combination of mathematical modeling and experiments to identify strategies that may more effectively fight excess signaling by the ErbB family of receptors, which is associated with breast cancer. Kirouac et al. used their model to search for combinations of two or three inhibitors that would overcome adaptive feedback and validated these effects in cell and animal models of cancer. Meyer et al. used a model, data from public databases, and their own experiments to identify a second receptor, AXL, which allowed cancer cells to resist the effects of ErbB receptor inhibitors. In this scenario, ligand-independent activating interactions between receptors of the ErbB family and AXL appeared to be crucial, suggesting that reducing receptor number or activity is more likely to be effective than treatments that target ligand-induced activation of the receptors.

    Sci. Signal. 6, ra68; ra66 (2013).

  3. Molecular Biology

    Noncoding mRNAs

    1. Guy Riddihough

    Chromatin insulators are protein complexes that act to insulate portions of the genome from the powerful and potentially disruptive effects of transcriptional enhancers, which can activate the expression of genes across vast stretches of DNA. This barrier function of insulators allows them to demarcate transcriptional domains in chromatin. The Drosophila gypsy insulator complex consists of three core proteins: suppressor of Hairy wing [Su(Hw)], modifier of mdg4 [Mod(mdg4)], and centrosomal protein 190 (CP190). RNA has also been implicated in gypsy insulator function, and Matzat et al. find that nine messenger RNAs (mRNAs) are specifically associated with the gypsy complex. Curiously, two of these interacting mRNAs code for the Su(Hw) and CP190 proteins themselves, a phenomenon not seen with other chromatin-associated proteins and their mRNAs. The expression of untranslatable versions of the Su(Hw) and CP190 mRNAs in larvae, which were already expressing wild-type Su(Hw) and CP190 mRNAs, resulted in the formation of supernumerary insulator bodies in the nucleus. They also increased the gypsy insulator enhancer-blocking activity in two marker genes that affect wing notching and eye color in adult flies. The insulator-targeted mRNAs may bind to the gypsy core complex through adaptor proteins.

    EMBO Rep. 14, 10.1038/embor.2013.118 (2013).

  4. Cell Biology

    Neutrophil Knowhow

    1. Stella M. Hurtley

    Chemotaxis involves the movement of cells along an attractant gradient and is important, for example, in the migration of neutrophils toward sites of infection. Chemotaxis requires both increased motility and sustained directionality. The molecular mechanisms involved in directionality control are largely unknown. Working with mouse neutrophils, Kamakura et al. found that the directional movement of neutrophils was regulated via a heterotrimeric G protein signaling pathway. Chemoattractant binding to G protein–coupled receptors induced accumulation of the GDP-bound Gαi subunit at the front of migrating neutrophils, where it recruited the conserved cell polarity protein mInsc. Neutrophils lacking mInsc, although motile, were unable to stay the course and failed to stabilize pseudopods appropriately at the leading edge.

    Dev. Cell 26, 292 (2013).

  5. Climate Science

    Which Emissions to Reduce?

    1. H. Jesse Smith

    A fierce debate is being waged about the relative merits of controlling the anthropogenic emissions of climate-affecting pollutants with long atmospheric lifetimes, such as carbon dioxide, and those with shorter atmospheric lifetimes, such as methane or black carbon. Both classes of compounds have substantial radiative impacts, and arguments about which is more important to reduce have been made for each. By using a sophisticated global model, Smith and Mizrahi examined the potential climate effects of a strategy to mitigate the short-lived climate-forcing agents methane and black carbon and concluded that the likely benefits of such an approach are much more modest than had been suggested recently. Even assuming the maximally feasible reductions in emissions over the next decades, the rise in global average surface air temperature by the year 2050 would be only about one-half of the previous best estimate. Thus, a policy of focusing on reducing emissions of short-lived climate forcers, attractive to many because of its rapid effect and possibly easier and less costly implementation, may not be the silver bullet some have hoped it might be.

    Proc. Natl. Acad. Sci. U.S.A. 110, 10.1073/pnas.1308470110 (2013).

  6. Chemistry

    Safer Cyanation

    1. Jake Yeston

    Cyanide (typically in an alkali salt) is such a well-known poison that many people may not realize how central it is to the production of pharmaceuticals and fine chemicals more generally. Covalently bound to carbon, the CN group no longer manifests the same sort of toxic properties, but getting it there—particularly onto aromatic rings—can be a fraught process. This is not only on account of its danger in bulk quantities; another problem is its tendency to bind tightly to metal catalysts and deactivate them before the reaction is complete. Senecal et al. present a practical and highly versatile catalytic method for cyanation of aryl chloride and bromides. They combine a previously investigated, nontoxic cyanide source (potassium ferrocyanide) with a particular palladium precursor and phosphine ligand combination that manifests high activity and broad functional group tolerance in a dioxane/water mixture with acetate base. The reactions, which generally proceed within 1 hour at 100°C with <1% catalyst loading, show compatibility with OH and NH groups, esters, aldehydes (at lower temperature to suppress competing benzoin condensation), and sulfur and nitrogen heterocyclic frameworks.

    Angew. Chem. Int. Ed. 52, 10.1002/anie.201304188 (2013).

  7. Materials Science

    Insensitive Superconductors

    1. Jelena Stajic

    Superconductors are materials that perfectly conduct electricity below a transition temperature Tc, which generally depends on the density of the current carriers. The bilayer of the insulating cuprate compound La2CuO4 and the metal La1.65Sr0.45CuO4 is an unusual system, where superconductivity has been shown to occur in a single copper-oxide plane. Wu et al. studied the dependence of Tc on carrier density in a system in which the Sr content of the doped layer La2-xSrxCuO4, labeled x, was varied in tiny steps. The films were grown by molecular beam epitaxy with a small doping gradient, so that a single film, after lithographic patterning, yielded a combinatorial library of chemical compositions; this technique reduced sample-to-sample variations associated with experimental conditions. The authors measured more than 800 different compositions, covering the range of x from 0.15 to 0.47, and found that Tc remained virtually unchanged; in contrast, the Hall resistance, which reflects carrier density, varied by almost an order of magnitude over the same range. This unusual insensitivity of the transition temperature to carrier density presents a challenge to the theories of superconductivity in interfacial systems.

    Nat. Mater. 10.1038/nmat3719 (2013).

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