This Week in Science

Science  17 Sep 1999:
Vol. 285, Issue 5435, pp. 1817
  1. Monitoring Plant Break-Ins

    Fungal pathogens are capable of directly breaking through the tough exterior layer of a plant, the cuticle. Previous studies have suggested that high internal pressures within the appresoria push a structure called the penetration peg into the cuticle. Bechinger et al. (p. 1896; see the Perspective by Talbot) have used optical imaging techniques to quantify this pressure. The deformation of a plant cell surface layer was measured with waveguide optics, and pressures were calibrated with a glass capillary similar in diameter to the penetration plug. Pressures with the appresoria of about 5 megapascals generated forces of 17 micronewtons at the plant cell surface.

  2. The Rise and Fall of Magma

    Observations of surface deformation and seismicity during the recent eruptions at Soufriere Hills volcano, Montserrat, showed oscillations of summit inflation and deflation as well as earthquake swarms that punctuated seismic quiescence on time scales of 3 hours to 3 days. Wylie et al. (p. 1883) modeled the change in pressure, volatile dissolution rate, and magma flow rate in a simplified volcanic conduit, where the lower part of the conduit has elastic walls that can flex with the addition of magma. With this model, they were able to produce oscillatory flow in their simplified conduit that fits with the observations at Montserrat. Their model, which depends on a known or estimated rate of volatile dissolution from the magma in the conduit, can be generalized to describe the instability of any siliceous magmatic eruption and thus provides the potential to assess volcanic hazards in the future.

  3. Curved Cracks on Europa

    Images of the icy surface of Jupiter's moon, Europa, taken by the Voyager and Galileo spacecrafts have shown a widely distributed series of connected bowlike (cycloidal) features whose formation remains a puzzle. Hoppa et al. (p. 1899) developed a model in which the cycloidal features are formed by the propagation of a crack caused by tidal stresses. Because the tidal stresses on Europa vary in orientation and time, the crack is curved and during short periods (about 3 hours) the crack may stop propagating when the stresses are too low to fracture the ice. When high stresses return, a new curved crack begins to form. Repetition of this cycle eventually creates a series of connected bowlike features. This model also places some limits on the thickness of the Europan ice layer (less than 1 kilometer) and requires a global fluid layer (the inferred Europan ocean) beneath the ice to generate enough tidal stress to fracture the icy surface.

  4. Writing Bacterial Histories in Iron

    The processes through which bacteria mineralize iron can lead to characteristic isotopic fractionation in the iron species formed. Two reports discuss how such fractionation could be used to construct records of the distribution of iron-reducing microorganisms or of the temperature of the bacteria's paleoenvironment. Sedimentary rocks, such as banded iron formations, show a small variation in the iron isotopic composition of their iron-bearing minerals that is not seen in igneous rocks. Beard et al. (p. 1889) performed controlled laboratory experiments in which they grew iron-reducing bacterium (Shewanella algae) on ferrihydrite. They found a small change toward lighter iron isotopes in the solution formed on the ferrihydrite that was not observed in their control experiment without the bacterium. They suggest that the iron isotopic variation in some sedimentary rocks is related to microorganisms and can be used to trace the occurrence of these organisms through the entire rock record. The intracellular formation of magnetite (Fe3O4) by magnetotactic bacteria results in a characteristic morphology of long chains of small single domains. Mandernack et al. (p. 1892) studied the fractionation patterns of oxygen and iron in magnetite produced by two different bacterial strains in laboratory-controlled experiments kept in the temperature range from 4° to 35°Celsius. They found no fractionation of the iron isotopes, but the oxygen isotopes showed a temperature-dependent fractionation between the magnetite and water. The correlation between oxygen isotopes and temperature should be useful for determining the temperature of environments from which the bacteria have long since disappeared.

  5. Blue Lasers Warming Up

    The development of semiconductor lasers emitting in the blue is crucial as the demand for reading and writing higher density optical disks grows. The vertical cavity surface-emitting lasers (or VCSELs) have the additional benefit of being readily patterned into an array containing many devices, thus allowing parallel processing. However, if they are to find mass application, the operation temperature must be increased above the cryogenic temperatures currently required. By careful design of the laser cavity, with particular attention being paid to growth of the mirrors, Someya et al. (p. 1905) show lasing operation in VCSELs at room temperature.

  6. Carbon Membranes

    Small molecules such as nitrogen and oxygen can be separated with nanoporous inorganic membranes, but even the smallest cracks can render such materials useless. Shiflett and Foley (p. 1902) show that by carefully controlling the thickness of the deposited precursor films, nanoporous carbon molecular sieves could be made that were thick enough to have high selectivities yet thin enough to remain plastic and thus avoid stress-induced catastrophic cracking. Such membranes could produce a 100% enrichment of oxygen in air in a single pass.

  7. Sending the Right Signals

    Animals often use chemical signals (pheromones) to attract receptive mates, but some species use them to induce receptive responses such as ovulation in a partner they are courting. The male Plethodon jordani salamander produces a pheromone in a specialized gland under his chin. Rollmann et al. (p. 1907) purified and cloned the plethodontid pheromone and showed that when the pheromone is applied to the nares of the female salamander during courtship, female receptivity is affected. This plethodontid receptivity factor is homologous to interleukin-6, which suggests that this increased receptivity may be a specific neurological response induced in the female.

  8. Staying Moist

    Soil arthropods—collectively one of the most abundant terrestrial life forms—are usually considered transitional between aquatic and terrestrial with respect to their water balance. In particular, they have been thought to have no physiological adaptation to water stress because of the high permeability of their skin; during drought, they have been assumed simply to migrate to deeper soil layers. Bayley and Holmstrup (p. 1909) show that, on the contrary, a common soil arthropod increases the osmolarity of its body fluids in response to low humidity and thus increases its ability to absorb water from the surrounding air. This finding requires a re-evaluation of the physiological adaptations of terrestrial arthropods to drought and desiccation.

  9. Wandering Attention

    When looking around, we constantly make saccades with our eyes, jumping from one point of attention in the visual world to the next. How does the central nervous system incorporate information about the visual scene into the neuronal commands that generate saccades? Moore (p. 1914) presents recordings from neurons in area V4 of the visual cortex of monkeys. These cells not only respond to a stimulus of appropriate orientation but also show bursts of activity after a delay just prior to a saccade toward the stimulus. In addition, there is more presaccadic activity in trials when the saccades are effectively guided by orientation information, that is, when the monkey's gaze lands along the axis of an object that is the target for a saccade.

  10. All in the Family

    Mutations in the tumor suppressor protein APC inappropriately activate the transcription factor Tcf4, which in turn activates genes that cause intestinal epithelial cells to become transformed into cancer cells. Roose et al. (p. 1923) show that the gene encoding another member of this transcription factor family, Tcf1, is a direct target of Tcf4. Tcf1 acts as a feedback repressor of Tcf4 by antagonizing its effects on cell transformation. Thus, Tcf1 cooperates with APC in suppressing tumorigenesis.

  11. Multiple Threat

    Bacterial pathogens can inject target cells with virulence factors. Orth et al. (p. 1920) have determined that a virulence factor of Yersinia pseudotuberculosis, the YopJ protein, can bind to several members of the mitogen-activated protein kinase (MAPK) superfamily and prevent their phosphorylation and activation. This binding resulted in a specific block in signaling pathways that would otherwise have led to cytokine synthesis and apoptosis, both of which are involved in host defenses against bacterial infection. Proteins related to YopJ are found in many bacteria that are plant and animal pathogens.

  12. Conformation Matters

    Antithrombin belongs to the serpin family of serine protease inhibitors and functions in the blood clotting cascade. Carboxyl-terminal cleavage induces a conformational change in antithrombin, and O'Reilly et al. (p. 1926; see the Perspective by Carrell) show that this change confers a new biological activity on the protein. Cleaved antithrombin exhibits potent antiangiogenesis and antitumor activity in mouse models. This discovery suggests that clot formation and angiogenesis may be regulated in tandem and raises the possibility that antithrombin, already in clinical use, may find new applications in the treatment of angiogenesis-dependent diseases such as cancer.

  13. Hormones and Blood Pressure

    Women are less susceptible than men to cardiovascular disease and atherosclerosis as long as their blood estrogen levels are elevated. Valverde et al. (p. 1929; see the Perspective by Silberberg and Magleby) show that estrogens can directly bind to a subunit of the maxi-K channel. These calcium-regulated potassium channels are key regulators of vascular smooth muscle cells, which constrict arteries and raise blood pressure. This acute estrogen effect does not require genome activation or generation of other intracellular signals.

  14. How Synapses Grow Stronger

    Synapses, the billions of tiny connections between brain cells, can be induced to work more efficiently than usual. This revving up of synaptic strength underlies the brain's ability to learn. As neurobiologists have tried to understand the mechanisms of this strengthening, they have focused largely on the synapses of a particular cell in the hippocampus (the CA1 pyramidal cell). How this strengthening, called long-term potentiation (LTP), operates has been the subject of controversy for more than a decade, but finally, as Malenka and Nicoll (p. 1870) discuss in their review, the evidence is solidifying into one model. LTP seems to be triggered by a coincident presynaptic and postsynaptic depolarization, which activates NMDA receptors, and then is maintained by the recruitment of previously silent receptors for the neurotransmitter glutamate, AMPA receptors.

  15. Deglaciation, Step by Step

    The climate of the previous interglacial, known as the Eemian, might provide an analog for the climate of the current interglacial, the Holocene. Some records, particularly from Greenland ice cores, have suggested that the climate during the Eemian was quite variable, but detailed records have been hard to obtain and the ice core records may be unreliable. Frogley et al. (p. 1886) now present a detailed climate record of the Eemian from a lake in Greece. The resolution in carbon and oxygen isotopes and pollen abundances of about 100 years shows that the climate at the beginning and end of the Eemian changed in a series of short steps.

  16. Toll and Immunity in Drosophila

    In Drosophila, the Toll signaling pathway regulates the antifungal host defense. In this immune response, when the ligand Spaetzle (Spz) binds to the Toll receptor, the antimicrobial peptide drosomycin is up-regulated. Levashina et al. (p. 1917) now add Spn43Ac, a member of the serpin family of serine protease inhibitors, to the Toll cascade. When Spn43Ac is eliminated, Spz is continuously cleaved, which leads to the constitutive activation of Toll signaling; Spn43Ac negatively regulates Toll. In bacterial infection, the lipopolysaccharide (LPS) component of the Gram-negative bacterial cell wall is sensed by the host cells, which triggers an immune response involving the Toll pathway. Toll has been reported to act as a pattern recognition receptor in mammals for LPS. Because the Spn43Ac is located upstream of Toll in Drosophila, Toll does not appear to be its pattern recognition signal. The actual pattern recognition signal in Drosophila is still unknown.

  17. Working Together

    The induction of long-term changes in synaptic strength, such as long-term facilitation (LTF), at one site can help induce changes in plasticity at another site. Sherff and Carew (p. 1911) show in the sea slug Aplysia that pulses of the neurotransmitter serotonin that would normally be insufficient to create LTF at one site can induce this phenomenon if applied first at the cell bodies of tail sensory neurons and again 15 minutes later at their distal synapses next to tail motor neurons. The narrow time window within which this coincident form of LTF can be induced suggests that subthreshold effects at remote sites are interacting.

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