This Week in Science

Science  26 Oct 2007:
Vol. 318, Issue 5850, pp. 529
  1. The Makings of Self-Pollination


    Plants that produce both male and female gametes use various strategies to determine whether they will predominantly self-fertilize or predominantly use pollen from other flowers. Although compatibility of cell surface receptors is part of the story, the geometry of flower parts also affects what pollen lands on the stigma. In the flower of the domesticated tomato, the stigma is buried within the anthers, which makes self-fertilization more likely. In wild relatives of the tomato, however, the stigma projects beyond the anthers and favors cross-fertilization. Chen et al. (p. 643) now identify the gene that regulates the length of the style, and thus regulates the relative geometry of stigma and anthers in the tomato.

  2. A Pocketful of Sugar

    Selective binding of distinct sugars in water is a challenge for molecular recognition because the abundant OH substituents must be differentiated from one another, as well as from the markedly similar surrounding solvent. Ferrand et al. (p. 619) have prepared an organic receptor that achieves the task for certain disaccharides with an efficacy approaching that of the much more structurally complex lectin proteins, and so holds promise for biochemical applications. The receptor binds cellobiose and related compounds, in which all OH groups are equatorially oriented, with an association constant of ∼600 inverse molar; the affinity drops more than 10-fold for substrates with an axial OH group. Nuclear magnetic resonance spectroscopy confirms a binding motif in which polar walls in the receptor interact favorably with the hydroxyls while aromatics at the top and bottom straddle the alkyl portions of the guest.

  3. Mapping Mantle Heterogeneity

    Earth's upper mantle shows a series of seismic discontinuities that have been linked to phase changes in mineralogy. Although the discontinuities occur at similar depths, there are local variations. Schmerr and Garnero (p. 623) investigate mantle structures beneath South America and adjacent oceans by stacking up weak precursors to SS seismic waves to map the discontinuities at depths of approximately 410- and 660-kilometers (km). On the down-dip side of subduction zones, they see that the 410-km discontinuity occurs tens of kilometers deeper, counter to expectations for cold slab material. Explaining the features seen requires both chemical and thermal contributions to the phase changes and confirms the influence of deep tectonic processes in generating mantle heterogeneity.

  4. A Stress on the Insulating Phase


    The band theory for conductivity of solids predicts that materials with an odd number of electrons in their unit cell would behave as metals, but some materials, such as NiO, are insulators at low temperatures because of the dominating effect of Coulomb interactions and electronic correlations. These insulators can undergo a transition to a metallic phase (by increasing temperature), but typically these metal-to-insulator transitions (MITs) are accompanied by structural phase transitions. Moore et al. (p. 615) show that in the layered perovskite Ca1.9Sr0.1RuO4, the surface undergoes its MIT at 130 Kelvin, well below the bulk transition temperature of 154 Kelvin that is accompanied by a structural phase transition. The authors argue that surface stresses that allow that Ca and Sr ions to be pulled into the bulk stabilize a surface phase that favors the Mott insulator ground state relative to the bulk structure. Thus, the MIT can occur before the phase transition.

  5. Inevitable Uncertainty

    Climate sensitivity is defined by the change in global average temperature that would result from changes in radiative forcing equal to that which would be caused by a doubling from preindustrial levels of the atmospheric concentration of CO2. Past work has shown that while the most likely value of climate sensitivity is between 2.0° and 4.5°C, there is a small probability that the increase could be much higher—8°C or even more. This persistent, high-temperature tail of low probability has been one impediment to political action, as policy-makers have been reluctant to formulate policies to address climate change when the range of uncertainty is so large. Roe and Baker (p. 629; see the Perspective by Allen and Frame) assert that this tail of low probability is an intrinsic feature of the climate system, not a result of inadequate data or models, and that this tail will persist even in the face of more observations and more advanced modeling. They conclude that putting off decisions about climate policy until greater certainty is achieved is futile.

  6. Thawing and Warming

    Around 18,000 years ago, near the beginning of the last deglaciation, the atmospheric concentration of the greenhouse gas methane began to rise rapidly, but the origin of this increase is still uncertain. Walter et al. (p. 633) add another potential methane source to the mix: the frozen areas of northern Asia and North America that were not covered by ice sheets. As the climate began to warm, these grounds thawed and lakes formed that would have emitted large volumes of methane bubbling up from their organic-rich sediments. Such a process could have supplied as much as 85% of the methane that Arctic and boreal regions are understood to have contributed during deglaciation. This methane emission would have contributed significantly to the warming that occurred at the Pleistocene-Holocene transition.

  7. Pack Mentality?

    Explaining the origin of altruism and parochialism has posed a challenge for theoretical biologists. Choi and Bowles (p. 636; see the Perspective by Arrow) describe a simulation based on interactions between groups in a game-theory framework in which hostile intergroup interactions lead to war and nonhostile interactions lead to trade. Groups composed of individuals who are both altruistic and parochial—who favor members of their own group and disfavor outsiders—are more successful than groups that are either only altruistic or only parochial.

  8. Regulating Nuclear Transport


    In eukaryotic cells, the nuclear pore complex (NPC) acts selectively to gate entry and exit of macromolecules into and out of the nucleus. Lim et al. (p. 640; published online 4 October) studied the biophysical response of the phenylalanine-glycine (FG)-domain of one of the nuclear pore proteins to biochemical interactions that govern nucleocytoplasmic transport. The FG-domains collapse into more compact structures upon binding to the nuclear transport receptor karyopherin-β. This collapse is reversed by Ran guanosine triphosphate, a known regulator of nuclear transport. The reversible collapse of the FG-domains may represent the underlying mechanism that regulates passage through the NPC.

  9. Plant-Pathogen Arms Race

    Plants recognize pathogens through immune-like receptors, which activate a resistance response. In turn, pathogens have evolved means to modify plant signaling pathways to avoid triggering the resistance response. Kay et al. (p. 648) and Römer et al. (p. 645) tackle the molecular mechanisms underlying this evolutionary arms race between plants and pathogens. The bacterial type III effector protein, AvrBs3, functions as a pathogenicity factor in susceptible host plants, which lack a resistance gene known as Bs3, by acting as a transcriptional activator. In contrast, plants carrying Bs3 recognize the AvrBs3 protein and activate the resistance gene Bs3, which simulates the plant resistance pathway.

  10. Extracellular Death Factor

    Programmed cell death (PCD) has traditionally been considered to be restricted to eukaryotic multicellular organisms; however, several genetic modules in prokaryotes are known to mediate a type of programmed cell death. In Escherichia coli, mazF encodes a stable toxin, and mazE encodes a labile antitoxin, that prevents the lethal effect of MazF. Kolodkin-Gal et al. (p. 652; see the Perspective by Kolter) now show that E. coli mazEF-mediated cell death is a population phenomenon that requires a quorum-sensing signal molecule, extracellular death factor (EDF). EDF is a symmetric, linear pentapeptide whose amino acid sequence is Asn-Asn-Trp-Asn-Asn. In synthetic peptides, the symmetrical arrangement of each of the five amino acids of EDF was important for mazEF-mediated killing activity.

  11. Drug Craving, Malaise, and the Insula

    An important factor that contributes to drug-seeking in addicted individuals is the negative feelings that result from abstinence. Such mood states are monitored by the interoceptive sensory system, and particularly by a brain area called the insular cortex, known to process emotional information. Contreras et al. (p. 655) observed that inactivation of the rat posterior granular insula reversibly disrupts the craving for amphetamine in animals repeatedly injected with amphetamine, as well as the behavioral signs of malaise induced by lithium administration. Thus, therapeutic interventions in the insula may help to alleviate drug cravings.

  12. Out They Go

    The solar nebula contained a variety of materials born in hot-to-cold regions that are now mixed across the solar system, as was evident in the comet samples brought back to Earth by the Stardust mission. Similar mixing is seen in protoplanetary disks around young stars. However, in theoretical models of disks, it has proven problematic to move grains from the inner to outer regions, as most forces act to pull material in toward the star. By applying two-dimensional models, Ciesla (p. 613) shows that outward transport is more efficient than was thought at the midplane of the viscous disk. Counter-intuitively, larger grains are transported outward more because they settle into the midplane and so are less susceptible to inward flows. High-temperature grains can be transported into the edges of the planetary system where they can be incorporated into comets.

  13. Agricultural Resolution

    Agricultural soil erosion is thought to perturb the carbon cycle, but estimates of its effect do not even agree on its sign of this effect—magnitude estimates for both sink and source, range up to about 1 billion tons of carbon per year. Van Oost et al. (p. 626) measured carbon inventories and soil erosion rates from a wide range of sites in Europe and the United States in order to calculate how much carbon is moved between soils and the atmosphere. They found that agricultural erosion creates only a small sink of carbon on a global scale and sequesters an amount approximately 2% as large as fossil fuel emissions alone.