Lateral Root Regulation
Control of cell division patterns is essential for normal development. De Smet et al. (p. 594) looked in living tissues at the early stages of the formation of lateral roots in the model plant, Arabidopsis thaliana. The cells that give rise to lateral roots responded to a key receptor-like kinase, ACR4, which was also required to maintain stem cells at the root tip meristem. Although sharing a kinase, these processes remain distinct because, while the root tip meristem is a permanent organ, lateral roots are generated only when required during development.
Ring Out, Bright Star
Helioseismology is the study of the oscillations and surface convection patterns, or granulation, of the Sun. From these measurements, internal structure can be inferred. Michel et al. (p. 558; see the Perspective by Montgomery; see the cover) used data gathered by the CoRoT satellite from three other stars in the same class as the Sun. These suns were hotter and had much finer granulation than ours and, in addition, the amplitudes of their global oscillations were about 1.5-fold greater, although less than theoretical estimates. Thus, helioseismology may represent a promising approach to learning about the structure and evolution of more distant suns, as well as our own.
Chlorine Gets Grounded
Reactions of halide atoms with hydrogen are simple to probe with extreme precision and have thus become test cases for understanding the implications of quantum mechanics to chemical reactions. Wang et al. (p. 573) performed highly controlled collision studies between crossed beams of H2 and Cl in the gas phase and found that electronic excitation plays only a minor role in the reaction forming HCl and H. This finding resolves a long-standing controversy about the general applicability of the Born-Oppenheimer approximation that separates nuclear from electronic rearrangements.
Mixing It Up
The transition of matter between an ordered state, such as found in a crystal, and a disordered state, such as found in a liquid, is fairly easy to visualize and is a common occurrence. Transitions between two disordered states are much less common, and it can be hard to understand how two liquids of the same material can both be disordered but nevertheless have a different internal structure. Greaves et al. (p. 566) have now been able to observe a first-order liquid-liquid transition in an yttrium oxide-aluminum oxide melt. By estimating changes in density and entropy, and observing structural changes at the atomic level, the transition between one liquid state and the other was found to coincide with the atomic packing of its components. This approach can be applied to the search for phase transitions in systems ranging from semiconductors to folding proteins.
Titania's Face in Profile
The surface of titanium oxide, unlike the bulk material, can undergo substantial reconstruction. In many of its uses, titania is slightly reduced, and the cations must accommodate this change. Using high-resolution transmission electron microscopy images taken from different planes, Shibata et al. (p. 570) showed that shifts in the relative positions of interstitial titanium atoms and their surrounding oxygen atoms offer a structural explanation for the complexity of the oxide surface. This approach can be applied to the investigation of many complex surface structures.
Modeling the Midbody
During cytokinesis, the midbody forms the final tether between two daughter cells before cleavage of the plasma membrane. To initiate cleavage, the midbody protein, CEP55, recruits the ESCRT-1 complex and an associated protein, ALIX, but Lee et al. (p. 576) have discovered that ALIX and ESCRT-1 compete for binding to the same region of CEP55. This region forms a noncanonical coiled-coil with bulky and charged residues at the interface that create a single binding site for ALIX or for ESCRT. These data, together with related crystallographic and electron microscopic analyses, complete the picture of the structure of the midbody's organization.
Tropical Forest Species Dynamics
The maintenance of the extraordinary levels of diversity seen in tropical forests has long intrigued ecologists. Kraft et al. (p. 580) combined functional traits, especially leaf structure and physiology, with census data on the spatial location of over 150,000 trees in a highly diverse 25-hectare plot in Ecuadorian Amazonia, and found that subtle but pervasive habitat specialization and functional differentiation contribute to species coexistence.
Where Fat Cells Are Born
The intertwined epidemics of obesity and diabetes have heightened interest in the developmental origin of adipocytes (fat cells). The identity and precise anatomic location of the progenitor cells that give rise to adipocytes are unknown. Using mice expressing marker genes that allow cell lineage tracing, Tang et al. (p. 583, published online 18 September; see the Perspective by Kahn) now characterize these long-sought progenitor cells and show, surprisingly, that they reside within the walls, or mural cell compartment, of the blood vessels that feed adipose tissue. In mice, these progenitor cells appear to commit to the adipocyte lineage either prenatally or shortly after birth. Thus the adipose vasculature functions as a progenitor niche and may provide signals for adipocyte development, which may help to explain why drugs that inhibit blood vessel growth can cause fat loss in mice.
Blood Pressure Control: It's (Another) Gas!
The discovery in the 1980s that the gaseous signaling molecule nitric oxide regulates blood vessel dilation and blood flow revolutionized biomedical research, leading most famously to new drugs for erectile dysfunction such as Viagra. Yang et al. (p. 587) provide evidence that vascular function is also controlled by hydrogen sulfide (H2S), the same gas that is responsible for the smell of rotten eggs and that recently has been shown to induce a hibernation-like state in animals. Mice genetically deficient in cystathionine γ-lyase, an enzyme that produces H2S, developed age-related hypertension and their blood vessels showed an impaired response to treatments that promote vasorelaxation. Thus, like nitric oxide, H2S regulates blood pressure—a finding that could pave the way toward new treatments for vascular disorders.
RNA Polymerase Caught in the Act
After binding to the promoter to form an initiation complex, RNA polymerase (RNAP) synthesizes short transcripts in a process known as abortive synthesis before forming the elongation complex that produces a full-length RNA transcript. In the bacteriophage T7 RNAP system, large conformational differences occur when the promoter is released and initiation shifts into elongation, but how these changes happen is unclear. Durniak et al. (p. 553) describe structures of an intermediate state: T7 RNAP bound to promoter DNA with a seven- or eight-nucleotide RNA transcript. The structures reveal the rotation that allows the polymerase to accommodate the growing transcript during abortive synthesis while still remaining bound to the promoter. Similar rearrangements are likely to occur during transcription initiation in multisubunit RNA polymerases found in bacteria and eukaryotes.
Chloride Channel Clinched
The origin of calcium-dependent chloride currents, required for cell excitability and fluid secretion, has been confusing. Taking an unusual approach, Caputo et al. (p. 590, published online 4 September; see the Perspective by Hartzell) identified the transmembrane protein TMEM16A as a key component. This chloride current is up-regulated when bronchial epithelial cells are treated with interleukin-4 (IL-4). By inhibiting each messenger RNA up-regulated by IL-4 using specific small interfering RNAs in bronchial epithelial cells, the authors were able to identify the gene responsible for the chloride current. Because defects in chloride transport underlie the pathology of cystic fibrosis, this discovery may offer leads into new treatments.
DNA Repair at the Edge
The nucleus of a cell can be divided into functional compartments, based on transcriptional activity and DNA replication. Nagai et al. (p. 597) reveal a physical connection between the nuclear periphery and processes of DNA damage and repair in budding yeast. Damaged DNA, specifically collapsed replication forks and persistent double-stranded breaks, are transferred to the nuclear pores, where nuclear pore components and other proteins facilitate recombinational repair.
Warm Drink--Warm Thoughts
The priming of attitudes, beliefs, or behaviors has become an active area of research, both for the remarkable range of situations under which it can be demonstrated and for the insights into unconscious cognitive processes. In many cases, the unconscious cognitive system is activated via semantic, stemword completion tasks, with a subliminally introduced belief (for example, feeling old) buried among a variety of other, unrelated characteristics. Williams and Bargh (p. 606) describe a situation in which a physical stimulus influenced people's judgments about interpersonal warmth, to the extent that subjects holding a warm cup of coffee would unwittingly make more prosocial (as opposed to selfish) choices.
The Sun's in Fine Shape
Knowing the shape of the Sun is fundamental to measurements of relativity and gravity. Its shape reflects its internal processes, as well as magnetism and rotation, but, until now, its exact shape has been difficult to determine. Fivian et al. (p. 560, published online 2 October; see the Perspective by Chapman) have used data from the RHESSI satellite and find that the Sun is more oblate than expected. This excess girth is attributed to enhanced magnetic activity, and removing this effect leads to a shape more consistent with solar rotation.
Characterizing Quantum Components
Building and testing a quantum information processing system also requires its components to be characterized. Characterization of such components is not easy. Lobino et al. (p. 563, published online 25 September) have developed a new method for quantum process tomography that uses a laser, can be readily scaled up, and applied to the development of quantum computers and communication networks.
Interference, Processing, and Transcription
RNA interference (RNAi) silences gene expression and is required for the formation of centromeric heterochromatin in fission yeast. Bayne et al. (p. 602) have characterized genes involved in RNA-imediated gene silencing in centromeric heterochromatin. All were messenger RNA splicing factors and were required for the processing and amplification of centromeric transcripts into small interfering RNAs. The association of these factors with components of the RNAi machinery and centromeric heterochromatin, indicate that transcription, RNA processing, and RNAi at the centromere are linked.