Editors' Choice

Science  16 Jul 2004:
Vol. 305, Issue 5682, pp. 311

    Limits From Leaf Litter

    1. Andrew M. Sugden

    Macroecology aims to explain the broad-scale geographical patterns exhibited by living organisms, such as the distribution of vegetation types or species richness with latitude. Documentation of the patterns themselves, however, is often far from straightforward, impeding progress to mechanistic understanding. Reich and Oleksyn have completed a global survey of the nitrogen (N) and phosphorus (P) content of leaves of more than 1000 plant species. The availability of these two elements generally set the limits to plant growth, and their availability is thus a key determinant of vegetation structure and ecosystem properties. Even so, the complexity of interactions between temperature, soil nutrient content, soil age, and plant physiology makes it hard to predict what might be the geographical correlates of leaf nutrient stoichiometry. The findings suggest that at lower latitudes, where temperatures are higher, leaf N and P content declines and N:P ratios increase—consistent with the observation that P is the limiting nutrient in older tropical soils. In contrast, in younger temperate soils N is limiting. — AMS

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.0403588101 (2004).


    Epigenetic Origins

    1. Guy Riddihough

    The nature of DNA replication origins in higher eukaryotes is not yet fully understood. Unlike in yeast, in metazoans there does not seem to be a specific replication origin sequence, leading to the suggestion that metazoan origins of replication may be determined epigenetically. Previous studies have also suggested an intimate connection between replication and the transcription apparatus.

    Danis et al. examined the replication of a plasmid containing an inducible promoter injected into Xenopus eggs. In the absence of the corresponding transcription factors, initiation of replication was nonspecific, occurring throughout the plasmid. In the presence of the transcription factors, however, there was a preferential increase in initiation of replication in the region of the promoter, but transcription itself was not required to see this effect. Nor did the presence of the transcription factors bound to DNA increase the binding of the DNA replication origin recognition complex in the vicinity of the promoter. Instead, the local level of histone acetylation was increased—an epigenetic mark implicated in the formation of an open chromatin structure permissive for transcription. Such open chromatin may be critical for facilitating the activation of replication origins within these regions. — GR

    Nature Cell Biol. 10.1038/ncb1149 (2004).


    Beyond Triphenylene

    1. Phil D. Szuromi

    The aromatic molecule triphenylene, (C6H4)3, in which three six-carbon-atom rings fuse to form a common fourth ring, has been known for more than a century. Triphenylene can be described as a member of the triannulenylenes, which result from the fusion of three conjugated rings, each containing a triple bond that forms part of the central fusion ring (triphenylene itself forms from benzyne). However, until recently, triphenylene would have been the only member of this family reported. Kiesewetter et al. have now synthesized the next larger symmetric member of the family, tri-[8]annulenylene. The reaction under high vacuum of C8H7Br with potassium-tert-butoxide and subsequent reduction with potassium metal appears to occur in a stepwise fashion, and thus the [8]annulyne generated attacks the C8H7Br.− radical anion to form a dimer and then finally the trimer. Electron paramagnetic resonance (EPR) analysis shows that the reaction actually goes beyond completion to the radical anion [(C8H6)3.−]. Quenching with I2 in pentane and subsequent distillation allows enough of the fragile, antiaromatic neutral (C8H6)3 species to be isolated so that its nuclear magnetic resonance spectrum can be taken. — PDS

    J. Am. Chem. Soc. 10.1021/ja049020h (2004).


    Viral DNA Relocation

    1. Caroline Ash

    For plant viruses to disseminate within host plants, viral DNA needs to traverse the cellulose cell wall to gain access to new target cells. How viruses can move within an individual infected plant has been controversial for the agronomically important group of plant viruses known as the geminiviruses. Three viral proteins mediate transport: the coat protein (CP), the nuclear shuttle protein (NSP), and the movement protein (MP). All three are implicated in forcing the transit of viral DNA through plasmodesmata— “tunnels” across plant cellulose walls. Hehnle et al. examined the binding characteristics of the three transport proteins to plasmid DNA. The data support a “couple-skating” model, in which NSP and MP cooperate in binding viral DNA. MP appears to insert itself and bind to single-stranded DNA segments within the double-stranded supercoiled DNA molecule. MP is membrane- anchored and thus should be able to cooperate with NSP to drag the viral DNA across the cell wall. MP would also appear to help to spread out the DNA at the cell membrane, which may ease its passage as it threads through the narrow plasmodesmata into the neighboring cell. — CA

    J. Virol. 78, 7698 (2004).


    Finer Optical Ticks

    1. Ian S. Osborne

    In mechanical timepieces, the resolution to which time can be sliced is dependent on how fine the movement is. Typically, this is limited to hundredths of a second, or equivalent to the movement running at several hundred cycles per second. However, the precision to which the time slice can be determined is also dependent on the width of the ticks around the clock face. The development of optical frequency standards allows a much finer movement to be accessed (∼1015 cycles per second), but analogously, the precision to which the time increments can be determined is dependent on the linewidth of the laser light being used. Webster et al. tackle the problem of linewidth broadening by isolating the light frequency, in this case the resonance frequency of a high-finesse Fabry-Perot etalon, from many of the sources of external noise. By reducing external parameters such as temperature and pressure fluctuations, acoustic noise, and expansion due to heating from the light itself, they show that the linewidth of the etalon resonance frequency can be reduced to below 0.5 Hz. The ability to stabilize light to such high resolution is important not only for the further development of precise timekeeping but also for applications in fundamental physics and tests of special relativity. — ISO

    Op. Lett. 29, 1497 (2004).


    Ancient Silver Generations?

    1. Brooks Hanson

    An important factor in societal function and cultural development is the proportion of adults that live long lives. Human societies in which few adults survive for long develop differently from ones with more older individuals, because of the different needs for care of children and for care of the elderly, and because of the different levels of shared knowledge. Caspari and Lee investigate this question over time during human evolution, by examining the available fossil record. They assessed the change in the average longevity of adults in four separate groups of hominids, from Australopithecines to more recent societies that replaced Neanderthals in the Late Pleistocene, using dental wear of adult teeth. Adult longevity increased with human evolution, from a ratio of old to young adults of about 0.12 to 0.4 for Neanderthal fossils, with a particularly dramatic increase in paleolithic societies to more than two older adults for each younger adult. Thus, a significant increase in longevity seems to have arisen late in human evolution. — BH

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.0402857101 (2004).


    Itching for a New Cytokine

    1. Elizabeth M. Adler

    Activation of the immune system can guard against pathogens that cause disease, but when they go awry the same mechanisms can lead to autoimmune diseases or allergies. For instance, atopic dermatitis, an inflammatory skin disease that is frequently associated with hay fever or asthma, appears to involve abnormal cytokine production by T helper type 2 (TH2) cells. Dillon et al. identified a TH2 cell-derived cytokine, interleukin 31 (IL-31), and its receptor IL-31 receptor A (IL-31RA) on lymphoid and epithelial cells. Transgenic mice that overexpressed IL-31 in lymphocytes developed pruritis, skin lesions, and hair loss, as did mice treated directly with IL-31. In a model of airway hyperresponsiveness, exposure of presensitized mice to antigen stimulated IL-31RA expression in the lung. Thus, IL-31 appears to be involved in mediating dermatitis and hyperresponsive airway disease. — EMA

    Nat. Immunol. 5, 752 (2004).