Editors' Choice

Science  02 Dec 2005:
Vol. 310, Issue 5753, pp. 1391

    A Hedging Strategy

    Seed dormancy is a common adaptation in annual plants that live in highly seasonal or unpredictable habitats such as deserts. By delaying germination, plants can hope to escape conditions that are likely to be adverse for seedling growth. However, rather than germinating at once in response to a favorable cue such as rainfall, plants hedge their bets by varying the germination rates according to how reliably the cue predicts future conditions. In a study of annuals in the Negev desert, Tielböger and Valleriani show that germination rates are higher for the relatively few seeds produced during dry years than for the large numbers of seeds produced in wet years, regardless of the abiotic cue. It appears that the plants predict the likelihood of future survival according to the density of seeds: a measure of the likely intensity of competition among seedlings. The authors suggest that information about the density of neighbors may be encoded in the seeds via maternal effects from the parent plant. — AMS

    Oikos 111, 235 (2005).


    Gut Reactions

    Celiac disease (CD) is caused by an immunological response to gluten peptides in wheat. This response damages the intestine and can compromise the absorption of essential nutrients. Specific variants of HLA class II genes (which encode proteins that participate in the immune recognition of gluten) confer an elevated risk of CD, but additional genes are likely to contribute to the disorder. Lifelong adherence to gluten-free diets is difficult, and there is interest in devising alternative therapies.

    Promising new leads have emerged from genome-based studies of both the human victims and the plant assailant. In a genetic association analysis of two Dutch populations, Monsuur et al. identified a sequence variant that conferred a twofold greater risk of CD. This variant resides within an intron of the human MYO9B gene, which encodes an unconventional myosin that may play a role in the ability of intestinal epithelial cells to form a tight barrier, and the variant allele may increase the access of gluten peptides to immune cells. Spaenij-Dekking et al. investigated whether different varieties of wheat contain different levels of the gluten peptides that trigger the pathogenic immune reaction. Based on the results of database searches of gluten sequences and in vitro immunological assays, the authors concluded that sufficient genetic variation exists in wheat to warrant consideration of selection strategies that would produce varieties that are better tolerated by celiacs. — PAK

    Nat. Genet. 10.1038/ng1680 (2005); Gastroenterology 129, 797 (2005).


    To Till or Not to Till

    Soils contain approximately twice as much carbon as either land plants or the atmosphere. Because carbon is transferred so easily and quickly between soil and the air, how human activity might affect that transfer has important implications for the atmospheric carbon dioxide budget. Approximately 1.5 billion hectares (11% of the total land area of Earth) is cultivated, making the impact of agriculture on the concentration of atmospheric carbon dioxide potentially significant. A large debate has centered on how agricultural practices—whether the soil is tilled, a practice that accelerates the erosion of organic-rich topsoil, or cultivated using no-till methods—might affect fluxes of carbon between the land and the atmosphere.

    Van Oost et al. use radionuclide and soil organic carbon data to analyze the fate of sediment and soil organic carbon during erosion and deposition in agricultural uplands. They find that, contrary to earlier studies, which did not include depositional processes, agricultural uplands can experience a net gain of carbon by the formation of new soil organic carbon at eroding sites and the burial of eroded soil organic carbon below plough depth. Thus, rather than causing a net carbon loss, tillage might be an important mechanism for carbon sequestration in certain cases. — HJS

    Global Biogeochem. Cycles 19, 10.1029/2005GB002471 (2005).


    Peak Growth

    There is wide interest in fabricating large, defect-free, three-dimensional periodic crystals for use in photonic applications. One simple method involves the growth of colloidal crystals; however, most such methods produce crystals with stacking faults and macroscopic cracks. The defects arise in part because the difference in free energy between the face-centered cubic and hexagonal close-packed structures is small.

    Jin et al. found that by reducing the growth temperature from 65° to 24°C and by decreasing the concentration of particles in solution, they were able to grow crystals with both the (111) and the more desirable but less energetically favorable (100) orientations on a flat substrate. They explored the role of templating the substrate by building pillars of hydrogen silsesquioxane with spacings of 308 to 320 nm, on which they grew crystals with a particle diameter of 299 nm. By vastly slowing down the growth rate and tilting the substrate, they obtained crystals that were free of cracks and faults, although there was the odd defect where differently sized colloidal particles were located. The crack-free nature of the crystals is due to the underlying template, which forces the bottom layer of particles to take on a non-close-packed arrangement, giving the particles a bit of space to move about as the crystal grows and dries. — MSL

    Nano Lett. 10.1021/nl051905j (2005).


    Helpful Helminths

    Pathogens have evolved countless devious means of thriving within their hosts. These range from antigenic escape from the attention of B and T cells to usurping the early detection network of the innate immune system.

    Wilson et al. provide evidence to suggest that the nematode gut parasite Heligmosomoides polygyrus protects itself by suppressing allergic T cell responses in the host. Nematode infection was found to decrease the pulmonary allergic inflammation normally evoked in mice by an allergen from the house dust mite. Tying several lines of evidence together, the effects were narrowed to a population of regulatory CD4+ T cells from gut-associated lymph nodes of infected mice. Smith et al. found that another helminth, the trematode parasite Schistosoma mansoni, produces a chemokine-binding protein (CKBP) to protect itself from the ill effects of host inflammation. CKBP was detected specifically in the egg stage of the parasite and bound CXCL8 (IL-8) and CCL3 (MIP1a). Predominantly through effects on neutrophil activity, CKBP inhibited different forms of experimental inflammation in mice. Both studies reveal a new layer of diversity by which helminths modify their host environment. — SJS

    J. Exp. Med. 202, 1199; 1319 (2005).


    Impedance Matching

    The current vogue for treating metabolic and regulatory pathways as circuits in which parts can be swapped in and out, with sensors at the input side and cellular behavior at the output side, has been driven by the ability to construct sensors by modifying natural ligand-binding receptors and to insert heterologous genetically coded components. Invasin is a cell-surface protein of Yersinia pseudotuberculosis that initiates bacterial uptake by binding to integrin, a protein on the surface of some mammalian cells, and previous work has shown that transferring the inv gene into Escherichia coli is sufficient to enable it to invade integrin-expressing cells. Anderson et al. have engineered E. coli in which inv is under the control of the promoter from fdhF, a gene whose expression is induced by hypoxia (one characteristic of tumor microenvironments). They discovered that in order to dial down the basal level of inv expression in their construct, it was necessary to etiolate the wild-type ribosome-binding site by randomizing flanking bases in a library of 106 members and screening for the handful of clones in which sensor input and behavioral output were matched so as to support a strictly anaerobic-dependent invasion. — GJC

    J. Mol. Biol. 10.1016/j.jmb.2005.10.076 (2005).


    Heat and Meet

    The formation of well-ordered supramolecular arrays on metal surfaces by large molecules is favored by high surface mobility and strong molecular interactions, requirements that work at cross purposes. Stöhr et al. show that a large perylene derivative, DPDI (4,9-diaminoperylenequinone-3,10-diimine), does not form hydrogen bonds at room temperature on an atomically flat Cu(111) surface, but does after annealing at 300°C, which causes the loss of H2 and converts some of the amino groups into hydrogen bond acceptors. Scanning tunneling microscopy (STM) revealed the formation of open honeycomb networks for surface coverages of DPDI between 0.1 and 0.7 monolayer (ML) after high-temperature annealing; above 0.7 ML, the honeycomb structure occupied too much area, and at 0.85 ML, trimers formed instead. Finally, at 1 ML, chained structures that minimize the space between molecules formed. — PDS

    Angew. Chem. Int. Ed. 44, 7394 (2005).