Responses and adaptations of root growth and metabolism to low temperature

A comparative study of the carbohydrate metabolism of roots of pea (Pisum sativum var. meteor) and maize (Zea mays var kelvedon glory) seedlings was undertaken at low temperatures (2-14°C) with the aim of demonstrating differences between these species which may be associated with the differing growth capacities of their roots mot this temperature range. Pea roots displayed linear growth rates at all temperatures tested whereas maize roots ceased growth over five days at temperatures below 6°C At the respective temperatures which were minimal for root growth of the two species, roots behaved similarly with regards to soluble sugar content; firstly, total content was maintained in the roots at the initial level, and secondly, sucrose content was at its highest value and glucose content at its lowest. With rise in temperature sucrose content declined while glucose content increased. In maize roots kept at those temperatures where growth was not sustained this relationship broke down. Total sugar content of the roots was not maintained, glucose content was abnormally high and sucrose content very low. Similarly, respiration rate of maize roots at 2°C was abnormally low. When seedings were grown with roots bathed in an external solution of glucose at 2°C (or of glucose or of sucrose at 6°C the disturbances to sugar metabolism and respiration rate of maize roots were partially alleviated and this was associated with a greater amount of growth made by the roots. Examination of the activity and Km of acid invertase extracted from the roots and partially purified, showed that the sucrose levels in roots of both species were inversely related to invertase activity. However in pea, but not in maize roots, Km values for invertase showed a lowering in value after growth of seedlings at 2°C compared with 20°C. Furthermore, in pea, after growth of seedlings at 14 or 2°C Km determined at 2°C was significantly lower than when determined at 14°C. These properties are of adaptive significance at low temperatures since they will act to maintain an appreciable reaction rate. Shifts in Km of a homeostatic nature with respect to temperature were not recorded for invertase from maize roots and in this species the failure to control invertase activity at low temperatures with consequent depletion of sucrose may be associated with the inability of this species to show sustained growth at 2°C. Examination of the Km of MDH likewise revealed a shift in Km value tending to buffer the effect of temperature on reaction rate for MDH from pea but not from maize roots

A learning rule for place fields in a cortical model: theta phase precession as a network effect

We show that a model of the hippocampus introduced recently by Scarpetta, Zhaoping & Hertz ([2002] Neural Computation 14(10):2371-96), explains the theta phase precession phenomena. In our model, the theta phase precession comes out as a consequence of the associative-memory-like network dynamics, i.e. the network's ability to imprint and recall oscillatory patterns, coded both by phases and amplitudes of oscillation. The learning rule used to imprint the oscillatory states is a natural generalization of that used for static patterns in the Hopfield model, and is based on the spike time dependent synaptic plasticity (STDP), experimentally observed. In agreement with experimental findings, the place cell's activity appears at consistently earlier phases of subsequent cycles of the ongoing theta rhythm during a pass through the place field, while the oscillation amplitude of the place cell's firing rate increases as the animal approaches the center of the place field and decreases as the animal leaves the center. The total phase precession of the place cell is lower than 360 degrees, in agreement with experiments. As the animal enters a receptive field the place cell's activity comes slightly less than 180 degrees after the phase of maximal pyramidal cell population activity, in agreement with the findings of Skaggs et al (1996). Our model predicts that the theta phase is much better correlated with location than with time spent in the receptive field. Finally, in agreement with the recent experimental findings of Zugaro et al (2005), our model predicts that theta phase precession persists after transient intra-hippocampal perturbation.Comment: 10 pages, 7 figures, to be published in Hippocampu

Gamma oscillatory firing reveals distinct populations of pyramidal cells in the CA1 region of the hippocampus

Hippocampal place cells that fire together within the same cycle of theta oscillations represent the sequence of positions (movement trajectory) that a rat traverses on a linear track. Furthermore, it has been suggested that the encoding of these and other types of temporal memory sequences is organized by gamma oscillations nested within theta oscillations. Here, we examined whether gamma-related firing of place cells permits such discrete temporal coding. We found that gamma-modulated CA1 pyramidal cells separated into two classes on the basis of gamma firing phases during waking theta periods. These groups also differed in terms of their spike waveforms, firing rates, and burst firing tendency. During gamma oscillations one group's firing became restricted to theta phases associated with the highest gamma power. Consequently, on the linear track, cells in this group often failed to fire early in theta-phase precession (as the rat entered the place field) if gamma oscillations were present. The second group fired throughout the theta cycle during gamma oscillations, and maintained gamma-modulated firing at different stages of theta-phase precession. Our results suggest that the two different pyramidal cell classes may support different types of population codes within a theta cycle: one in which spike sequences representing movement trajectories occur across subsequent gamma cycles nested within each theta cycle, and another in which firing in synchronized gamma discharges without temporal sequences encode a representation of location. We propose that gamma oscillations during theta-phase precession organize the mnemonic recall of population patterns representing places and movement paths

Integrative Genomics Reveals the Genetics and Evolution of the Honey Bee’s Social Immune System

Social organisms combat pathogens through individual innate immune responses or through social immunity—behaviors among individuals that limit pathogen transmission within groups. Although we have a relatively detailed understanding of the genetics and evolution of the innate immune system of animals, we know little about social immunity. Addressing this knowledge gap is crucial for understanding how life-history traits influence immunity, and identifying if trade-offs exist between innate and social immunity. Hygienic behavior in the Western honey bee, Apis mellifera, provides an excellent model for investigating the genetics and evolution of social immunity in animals. This heritable, colony-level behavior is performed by nurse bees when they detect and remove infected or dead brood from the colony. We sequenced 125 haploid genomes from two artificially selected highly hygienic populations and a baseline unselected population. Genomic contrasts allowed us to identify a minimum of 73 genes tentatively associated with hygienic behavior. Many genes were within previously discovered QTLs associated with hygienic behavior and were predictive of hygienic behavior within the unselected population. These genes were often involved in neuronal development and sensory perception in solitary insects. We found that genes associated with hygienic behavior have evidence of positive selection within honey bees (Apis), supporting the hypothesis that social immunity contributes to fitness. Our results indicate that genes influencing developmental neurobiology and behavior in solitary insects may have been co-opted to give rise to a novel and adaptive social immune phenotype in honey bees.York University Librarie

Diamond surface engineering for molecular sensing with nitrogen-vacancy centers

Quantum sensing using optically addressable atomic-scale defects, such as the nitrogen--vacancy (NV) center in diamond, provides new opportunities for sensitive and highly localized characterization of chemical functionality. Notably, near-surface defects facilitate detection of the minute magnetic fields generated by nuclear or electron spins outside of the diamond crystal, such as those in chemisorbed and physisorbed molecules. However, the promise of NV centers is hindered by a severe degradation of critical sensor properties, namely charge stability and spin coherence, near surfaces (< ca. 10 nm deep). Moreover, applications in the chemical sciences require methods for covalent bonding of target molecules to diamond with robust control over density, orientation, and binding configuration. This forward-looking Review provides a survey of the rapidly converging fields of diamond surface science and NV-center physics, highlighting their combined potential for quantum sensing of molecules. We outline the diamond surface properties that are advantageous for NV-sensing applications, and discuss strategies to mitigate deleterious effects while simultaneously providing avenues for chemical attachment. Finally, we present an outlook on emerging applications in which the unprecedented sensitivity and spatial resolution of NV-based sensing could provide unique insight into chemically functionalized surfaces at the single-molecule level.Comment: Review paper, 36 page

Earth-Abundant Tin Sulfide-Based Photocathodes for Solar Hydrogen Production.

Tin-based chalcogenide semiconductors, though attractive materials for photovoltaics, have to date exhibited poor performance and stability for photoelectrochemical applications. Here, a novel strategy is reported to improve performance and stability of tin monosulfide (SnS) nanoplatelet thin films for H2 production in acidic media without any use of sacrificial reagent. P-type SnS nanoplatelet films are coated with the n-CdS buffer layer and the TiO2 passivation layer to form type II heterojunction photocathodes. These photocathodes with subsequent deposition of Pt nanoparticles generate a photovoltage of 300 mV and a photocurrent density of 2.4 mA cm-2 at 0 V versus reversible hydrogen electrode (RHE) for water splitting under simulated visible-light illumination (λ &gt; 500 nm, Pin = 80 mW cm-2). The incident photon-to-current efficiency at 0 V versus RHE for H2 production reach a maximum of 12.7% at 575 nm with internal quantum efficiency of 13.8%. The faradaic efficiency for hydrogen evolution remains close to unity after 6000 s of illumination, confirming the robustness of the heterojunction for solar H2 production

Vibrational and electronic ultrafast relaxation of the nitrogen-vacancy centers in diamond

Abstract. Two dimensional electronic spectroscopy and transient grating measurements were performed, for the first time, on nitrogen-vacancy centers in diamond. These measurements reveal energy transfer and vibrational pathways with consequences for spin coherence