This Week in Science

Science  12 Aug 2005:
Vol. 309, Issue 5737, pp. 985
  1. A Time and a Place for a Flower


    Early plant responses to spring environmental signals, such as day length or vernalization, occur in the leaves, but the flowers form at a meristem that shifts from vegetative to floral growth. The mysterious mobile mediator between these two sites has been called “florigen.” Wigge et al. (p. 1056) and Abe et al. (p. 1052) investigated the integration of floral initiation signals in Arabidopsis (see the Perspective by Blázquez). Flowering locus T (FT) is expressed in leaves in response to environmental changes conducive to flowering. Meanwhile, the meristem is primed to be ready for floral production by expression of a transcription factor FD. Together, FT and FD activate floral identity genes. Whether FT actually is the mobile mediator remains unclear, but these two genes together do integrate the “when” and the “where” of the flowering response.

  2. Dopant Distribution and Superconductivity

    On the microscopic scale, the electronic structure of the high-temperature (high-Tc) superconducting cuprates is inhomogeneous, and this nanoscale electronic disorder could be caused by a random distribution of dopant atoms. However, the identification of the dopant atoms in the material, and quantifying what influence they have on the electronic structure has been difficult to realize. McElroy et al. (p. 1048; see the news story by Cho) used scanning probe microscopy to image the dopant atom locations and probe the atomic-scale electronic structure simultaneously in the high-Tc superconductor Bi2Sr2CaCu2O8+x. Correlating the dopant atom distribution with the electronic properties may provide a clearer understanding of not only the cuprates but also other doped complex materials.

  3. More Is…Less?

    The abundance of photosynthetic organisms in much of the upper ocean is thought to be limited by the amount of the essential nutrient phosphorus (P) that is available. Thus, the number of primary producers in a P-depleted ocean region would increase if this nutrient was added. Thingstad et al. (p. 1068) performed such an experiment on a 16-square-kilometer area of the Mediterranean Sea, where productivity is very low and P is the limiting nutrient. The chlorophyll content of the waters actually decreased, opposite what would be expected, and the abundance of copepod eggs, ciliate biomass, and bacterial production all increased. The authors discuss several possible reasons for this response and how the effects of P limitation might differ with respect to season and to groups of organisms.

  4. Sum Information Recorded

    The Inka Empire was the largest pre-Columbian empire in the New World, yet apparently it lacked a written language. What is preserved are khipu, groups of intricately knotted colored strings that are thought to be used for accounting or record keeping, but deciphering their meaning, relationship, or significance has been problematic. Urton and Brezine (p. 1065; see the news story by Mann), working with seven khipu that do have some contextual information so that they can be treated as a group, show that successive khipu record summations of other ones. It appears that khipu were used to pass accounting information upward through the Inka bureaucracy.

  5. Oscillating Flashes

    Proteins sample an enormous conformational space as they fold and unfold, and traditional measurements reveal limited details of the process because each molecule may follow a slightly different path at a slightly different time. Fluorescence measurements with single-molecule resolution can overcome this blurring effect and trace individual pathways. Baldini et al. (p. 1096) examined a green fluorescent protein mutant suspended in a gel during unfolding. Just prior to unfolding, the protein chromophore oscillates with remarkable regularity between two states, a blue-fluorescing neutral state and green-fluorescing anionic state. The oscillatory frequencies fall in the 400- to 1000-Hertz range, and the process can be driven by applied resonant electric or acoustic fields. The molecular mechanism underlying this process remains elusive.

  6. Choosing Your Fate

    During differentiation, cell lineages must choose between different alternate fates. Hong et al. (p. 1074) provide evidence that a protein known as TAZ (for transcriptional coactivator with PDZ-binding motif) is a key regulator that helps determine the fate of mesenchymal stem cells that can differentiate into osteoblasts or adipocytes. TAZ contains a protein interaction domain that binds to Pro-Pro-X-Tyr motifs (where X represents any amino acid). Two transcription factors that control differentiation of mesencymal stem cells, Runx2 and PPARγ, contain such a motif in their activation domains. In tissue culture and in zebrafish embryos, TAZ promotes formation of osteoblasts (by cooperating with Runx2) and inhibits differentiation of adipocytes (by antagonizing the effects of PPARγ).

  7. Bend and Snap


    Iridium is the only face-centered cubic metal that undergoes brittle cleavage after deformation. Using simulations, Cawkwell et al. (p. 1059) account for this behavior as arising from the interchange of two dislocation types within the material. The cores of screw dislocations in iridium can be either planar (glissile, and moving readily) or be distributed on two planes, causing them to be nonplanar (sessile, and moving only under high stress). The athermal transformation of one defect type into the other leads to a rapid increase in the dislocation density. This process in turn leads to fast strain hardening, which eventually causes the flow stress to rise fast enough to cause brittle failure.

  8. Containing a Potential Pandemic

    In the 20th century, there were three influenza pandemics. Currently, the world is threatened by avian influenza in Southeast Asia and may be only a reassortment or mutation event away from another pandemic. However, there is a good chance of preventing the spread of any emergent influenza strain at the source through good surveillance and the aggressive use of influenza antiviral agents, quarantine, and vaccines. In a detailed epidemic simulation model for a Southeast Asian population, Longini et al. (p. 1083, published online 4 August 2005) analyze possible strategies for containing a newly emergent influenza strain and show that such a strain should be containable at the source under a broad set of potential conditions.

  9. How Rice Releases Methane

    Rice agriculture is possibly the biggest source of anthropogenic methane: Rice paddies cover about 130 million hectares of the earth's surface, of which almost 90% are in Asia, and emit 50 to 100 million metric tons of methane a year. Most of this methane is derived from rice photosynthates excreted into the rhizosphere. Lu and Conrad (p. 1088) used pulse-labeling of rice plants with 13CO2 followed by in situ stable isotope probing of rhizospheric archaeal RNA to show that a group of methanogenic archaea, the so-called Rice Cluster I, of which no isolates exist so far, is responsible for this methane production from the degradation of photosynthates.

  10. Scrutinizing Cholera Toxin


    Cholera toxin catalyzes reactions that lead to the devastating diarrhea characteristic of the disease. The toxin is activated by a family of human G proteins, adenosine diphosphate-ribosylation factors (ARFs), which normally act as molecular switches through binding effector proteins in eukaryotic cells. O'Neal et al. (p. 1093) now report high-resolution structures of the catalytic cholera toxin A1 subunit (CTA1) bound to ARF-guanosine triphosphate (GTP), with and without substrate bound. Although cholera toxin is not structurally similar to human protein partners of ARF, the toxin:ARF-GTP interface mimics ARFGTP recognition of human effector proteins. The binding causes conformational changes that open the CTA1 active site to substrate access.

  11. Planktonic Pathogen Genome

    Given the incredible number of viruses present in the ocean, there are surprisingly few marine viral genomes known. Wilson et al. (p. 1090) provide a complete genome sequence, annotated via a microarray analysis, of a Coccolithovirus pathogen of the ubiquitous and globally important phytoplankton Emiliania huxleyi. This huge viral genome contains a family of noncoding repeats and a viral RNA polymerase gene that might function together as transcription machinery. The genome also appears to contain an apoptosis activation system, which may be pivotal in understanding the bloom behavior of the host alga. The majority of the genes in the virus are transcribed and the virus can act as a vehicle for horizontal gene transfer within this species of coccolithophorids; indeed, E. huxleyi is one of the fastest evolving species of phytoplankton known.

  12. Network Analysis of Cell Regulation

    Fuller understanding of cellular regulation requires analysis of the interactions of multiple signaling pathways in the complex networks that control cellular functions. Ma'ayan et al. (p. 1078) analyze the network properties of 545 components that undergo 1259 interactions underlying signaling and cell regulation in hippocampal neurons. The presence of regulatory motifs and other characteristics begin to reveal how cells may process information to allow, for example, transient or stable changes in cell function.

  13. Meteoritic Sulfur Isotope Anomalies

    Most isotope fractionation during chemical reactions depends on the mass of the isotopes. However, mass independence has been seen for sulfur and oxygen isotope systems; distinct oxygen isotope variations are present in many primitive meteorites, and both sulfur and oxygen isotope variations occur in ultraviolet-driven reactions in Earth's upper atmosphere. Similar reactions have also been identified on Mars. Rai et al. (p. 1062) now show that similar sulfur isotope anomalies are present in sulfur minerals in a class of meteorites, the achnodrites, representing early planetesimals. This signature likely originated in the solar nebula and was somehow preserved during the accretion of these protoplanets. A similar process may explain the origin of the enigmatic oxygen isotope signature.

  14. Nutrients and the Germ Line

    In Drosophila, growth in the germ line and its surrounding somatic follicle cells responds to the level of available nutrients. Insulin-like peptides produced in the brain are involved in this ovarian response to nutrition, but the mechanism of action is unknown. Using methods for the induction of insulin receptor mutant clones and analyzing mosaic ovarioles, LaFever and Drummond-Barbosa (p. 1071) show that these insulin-like peptides directly affect the germline stem cells by regulating their rate of division and the rate of cyst growth. In contrast, follicle cells do not respond directly to insulin signals but instead receive a signal from the germ cells for coordinated growth.