Analysis of chlorophyll fluorescence reveals stage specific patterns of chloroplast-containing cells during Arabidopsis embryogenesis

http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0716-97602010000100012&lng=es&nrm=isoThe basic body plan of a plant is established early in embryogenesis when cells differentiate, giving rise to the apical and basal regions of the embryo. Using chlorophyll fluorescence as a marker for chloroplasts, we have detected specific patterns of chloroplast-containing cells at specific stages of embryogenesis. Non-randomly distributed chloroplast-containing cells are seen as early as the globular stage of embryogenesis in Arabidopsis. In the heart stage of embryogenesis, chloroplast containing cells are detected in epidermal cells as well as a central region of the heart stage embryo, forming a triangular septum of chloroplast-containing cells that divides the embryo into three equal sectors. Torpedo stage embryos have chloroplast-containing epidermal cells and a central band of chloroplast-containing cells in the cortex layer, just below the shoot apical meristem. In the walking-stick stage of embryogenesis, chloroplasts are present in the epidermal, cortex and endodermal cells. The chloroplasts appear reduced or absent from the provascular and columella cells of walking-stick stage embryos. These results suggest that there is a tight regulation of plastid differentiation during embryogenesis that generates specific patterns of chloroplast-containing cells in specific cell layers at specific stages of embryogenesis

The PIN-FORMED (PIN) protein family of auxin transporters

The PIN-FORMED (PIN) proteins are secondary transporters acting in the efflux of the plant signal molecule auxin from cells. They are asymmetrically localized within cells and their polarity determines the directionality of intercellular auxin flow. PIN genes are found exclusively in the genomes of multicellular plants and play an important role in regulating asymmetric auxin distribution in multiple developmental processes, including embryogenesis, organogenesis, tissue differentiation and tropic responses. All PIN proteins have a similar structure with amino- and carboxy-terminal hydrophobic, membrane-spanning domains separated by a central hydrophilic domain. The structure of the hydrophobic domains is well conserved. The hydrophilic domain is more divergent and it determines eight groups within the protein family. The activity of PIN proteins is regulated at multiple levels, including transcription, protein stability, subcellular localization and transport activity. Different endogenous and environmental signals can modulate PIN activity and thus modulate auxin-distribution-dependent development. A large group of PIN proteins, including the most ancient members known from mosses, localize to the endoplasmic reticulum and they regulate the subcellular compartmentalization of auxin and thus auxin metabolism. Further work is needed to establish the physiological importance of this unexpected mode of auxin homeostasis regulation. Furthermore, the evolution of PIN-based transport, PIN protein structure and more detailed biochemical characterization of the transport function are important topics for further studies

Valoración de activos y riesgo de liquidez enel mercado de valores chileno

This paper studies whether or not a premium exists for the risk of liquidity in the Chilean stock market. Using the methodology in Fama and French (1993), liquidity risk factors are constructed on the basis of 4 indexes which evaluate various models. The results show the existence of a premium for liquidity risk, but this is captured by more than a liquidity risk factor.Este artículo analiza la existencia o no de una prima por riesgo de liquidez en el mercado de acciones de Chile. En base a la metodología de Fama y French (1993), se construyen factores de riesgo de liquidez basados en cuatro índices, los cuales se valoran en base a varios modelos. Los resultados muestran la existencia de una prima por riesgo de liquidez, la cual se refleja en más de un factor de riesgo

Exposicion economica al tipo de cambio de las sociedades anonimas chilenas

68 p.Las empresas chilenas, en este caso las sociedades anónimas abiertas, independientes de si tienen o no operaciones de comercio exterior tienen de una u otra forma dependencia de las variaciones en el tipo de cambio. Sin embargo existe una limitada gama de estudios relativos al tema en el ámbito local, y la evidencia disponible proviene en su mayoría de economías desarrolladas. En un reciente estudio, Chen et al. (2004) encuentra para Nueva Zelanda, economía pequeña y abierta, evidencia que indicaría que para este país los movimientos del tipo de cambio afectan el valor de las empresas transadas en bolsa. Basándose en la similitud con la economía Neo Zelandesa, nace la inquietud de replicar ese estudio en Chile. Esta investigación se justifica en el hecho de que a entender del autor no existen estudios relacionados con el tema en Chile, siendo relevante por al menos dos razones: la alta volatilidad que ha presentado el tipo de cambio desde que se adoptó la paridad flotante a mediados del año 1999 (ver apéndice 1) y por el poco desarrollo del mercado de instrumentos de cobertura. El propósito fundamental de esta memoria es determinar el nivel de exposición de las sociedades anónimas chilenas a variaciones en el tipo de cambio, y analizar algunos factores potenciales que influyen en la magnitud de esta exposición. Bajo la hipótesis de los mercados eficientes, el efecto del tipo de cambio debería ser reflejado en el precio de acciones. Las empresas que cumplen con los criterios de selección son 105, de las cuales alrededor del 40% presenta un nivel significativo de exposición al riesgo de cambio. El grado de exposición de las empresas en forma global es de carácter inverso, es decir, a medida que aumenta el valor del tipo de cambio, el valor de las empresas disminuye. Cabe destacar que la significancia económica es importante, esto se grafica en los montos que las empresas ganan o pierden de su capitalización bursátil ante una variación del dólar. A modo de ejemplo, la empresa menos afectada perdería aproximadamente 351 millones de pesos ante la subida de un 1% del valor de la divisa

A clumpy and anisotropic galaxy halo at z=1 from gravitational-arc tomography

Every star-forming galaxy has a halo of metal-enriched gas extending out to at least 100 kpc, as revealed by the absorption lines this gas imprints on the spectra of background quasars. However, quasars are sparse and typically probe only one narrow pencil beam through the intervening galaxy. Close quasar pairs and gravitationally lensed quasars have been used to circumvent this inherently one-dimensional technique, but these objects are rare and the structure of the circum-galactic medium remains poorly constrained. As a result, our understanding of the physical processes that drive the re-cycling of baryons across the lifetime of a galaxy is limited. Here we report integral-field (tomographic) spectroscopy of an extended background source -a bright giant gravitational arc. We can thus coherently map the spatial and kinematic distribution of Mg II absorption -a standard tracer of enriched gas- in an intervening galaxy system at redshift 0.98 (i.e., ~8 Gyr ago). Our gravitational-arc tomography unveils a clumpy medium in which the absorption-strength decreases with increasing impact parameter, in good agreement with the statistics towards quasars; furthermore, we find strong evidence that the gas is not distributed isotropically. Interestingly, we detect little kinematic variation over a projected area of ~600 kpc squared, with all line-of-sight velocities confined to within a few tens of km/s of each other. These results suggest that the detected absorption originates from entrained recycled material, rather than in a galactic outflow.Comment: Published online in Nature on 31 January 201

On the Incidence of C IV Absorbers Along the Sightlines to Gamma-Ray Bursts

We report on the statistics of strong (W_r > 0.15 A) C IV absorbers at z=1.5-3.5 toward high-redshift gamma-ray bursts (GRBs). In contrast with a recent survey for strong Mg II absorption systems at z < 2, we find that the number of C IV absorbers per unit redshift dN/dz does not show a significant deviation from previous surveys using quasi-stellar objects (QSOs) as background sources. We find that the number density of C IV toward GRBs is dN/dz(z~1.5)= 2.2 +2.8/-1.4, dN/dz(z~2.5)= 2.3 +1.8/-1.1 and dN/dz(z~3.5)= 1.1 +2.6/-0.9. These numbers are consistent with previous C IV surveys using QSO spectra. Binning the entire dataset, we set a 95% c.l. upper limit to the excess of C IV absorbers along GRB sightlines at twice the incidence observed along QSO sightlines. Furthermore, the distribution of equivalent widths of the GRB and QSO samples are consistent with being drawn from the same parent population. Although the results for Mg II and C IV absorbers along GRB sightlines appear to contradict one another, we note that the surveys are nearly disjoint: the C IV survey corresponds to higher redshift and more highly ionized gas than the Mg II survey. Nevertheless, analysis on larger statistical samples may constrain properties of the galaxies hosting these metals (e.g. mass, dust content) and/or the coherence-length of the gas giving rise to the metal-line absorption.Comment: Accepted version (for publication in ApJ), results unchanged, 18 pages, 3 tables, 5 figure

Towards the statistical detection of the warm-hot intergalactic medium in inter-cluster filaments of the cosmic web

Modern analyses of structure formation predict a universe tangled in a 'cosmic web' of dark matter and diffuse baryons. These theories further predict that at low-z, a significant fraction of the baryons will be shock-heated to $T \sim 10^{5}-10^{7}$K yielding a warm-hot intergalactic medium (WHIM), but whose actual existence has eluded a firm observational confirmation. We present a novel experiment to detect the WHIM, by targeting the putative filaments connecting galaxy clusters. We use HST/COS to observe a remarkable QSO sightline that passes within $\Delta d = 3$ Mpc from the 7 inter-cluster axes connecting 7 independent cluster-pairs at redshifts $0.1 \le z \le 0.5$. We find tentative excesses of total HI, narrow HI (NLA; Doppler parameters $b<50$ km/s), broad HI (BLA; $b \ge 50$ km/s) and OVI absorption lines within rest-frame velocities of $\Delta v \lesssim 1000$ km/s from the cluster-pairs redshifts, corresponding to $\sim 2$, $\sim 1.7$, $\sim 6$ and $\sim 4$ times their field expectations, respectively. Although the excess of OVI likely comes from gas close to individual galaxies, we conclude that most of the excesses of NLAs and BLAs are truly intergalactic. We find that the covering fractions, $f_c$, of BLAs close to cluster-pairs are $\sim 4-7$ times higher than the random expectation (at the $\sim 2 \sigma$ c.l.), whereas the $f_c$ of NLAs and OVI are not significantly enhanced. We argue that a larger relative excess of BLAs compared to those of NLAs close to cluster-pairs may be a signature of the WHIM in inter-cluster filaments. By extending the present analysis to tens of sightlines our experiment offers a promising route to detect the WHIM.Comment: MNRAS in press; 21 pages, 10 figures, 5 tables and 15-page appendix with additional 5 figures and 4 table

Galaxy Clusters in the Line of Sight to Background Quasars: I. Survey Design and Incidence of MgII Absorbers at Cluster Redshifts

We describe the first optical survey of absorption systems associated with galaxy clusters at z= 0.3-0.9. We have cross-correlated SDSS DR3 quasars with high-redshift cluster/group candidates from the Red-Sequence Cluster Survey. We have found 442 quasar-cluster pairs for which the MgII doublet might be detected at a transverse (physical) distance d<2 Mpc from the cluster centers. To investigate the incidence (dN/dz) and equivalent-width distribution n(W) of MgII systems at cluster redshifts, two statistical samples were drawn out of these pairs: one made of high-resolution spectroscopic quasar observations (46 pairs), and one made of quasars used in MgII searches found in the literature (375 pairs). The results are: (1) the population of strong MgII systems (W_0>2.0 Ang.) near cluster redshifts shows a significant (>3 sigma) overabundance (up to a factor of 15) when compared with the 'field' population; (2) the overabundance is more evident at smaller distances (d<1 Mpc) than larger distances (d<2 Mpc) from the cluster center; and, (3) the population of weak MgII systems (W_0<0.3 Ang.) near cluster redshifts conform to the field statistics. Unlike in the field, this dichotomy makes n(W) in clusters appear flat and well fitted by a power-law in the entire W-range. A sub-sample of the most massive clusters yields a stronger and still significant signal. Since either the absorber number density or filling-factor/cross-section affects the absorber statistics, an interesting possibility is that we have detected the signature of truncated halos due to environmental effects. Thus, we argue that the excess of strong systems is due to a population of absorbers in an overdense galaxy environment, and the lack of weak systems to a different population, that got destroyed in the cluster environment. (Abridged)Comment: Accepted for publication in the Astrophysical Journa

Non variability of intervening absorbers observed in the UVES spectra of the "naked-eye" GRB080319

The aim of this paper is to investigate the properties of the intervening absorbers lying along the line of sight of Gamma-Ray Burst (GRB) 080319B through the analysis of its optical absorption features. To this purpose, we analyze a multi-epoch, high resolution spectroscopic observations (R=40000, corresponding to 7.5 km/s) of the optical afterglow of GRB080319B (z=0.937), taken with UVES at the VLT. Thanks to the rapid response mode (RRM), we observed the afterglow just 8m:30s after the GRB onset when the magnitude was R ~ 12. This allowed us to obtain the best signal-to-noise, high resolution spectrum of a GRB afterglow ever (S/N per resolution element ~ 50). Two further RRM and target of opportunity observations were obtained starting 1.0 and 2.4 hours after the event, respectively. Four MgII absorption systems lying along the line of sight to the afterglow have been detected in the redshift range 0.5 < z < 0.8, most of them showing a complex structure featuring several components. Absorptions due to FeII, MgI and MnII are also present; they appear in four, two and one intervening absorbers, respectively. One out of four systems show a MgII2796 rest frame equivalent width larger than 1A. This confirms the excess of strong MgII absorbers compared to quasars, with dn/dz = 0.9, ~ 4 times larger than the one observed along quasar lines of sight. In addition, the analysis of multi-epoch, high-resolution spectra allowed us to exclude a significant variability in the column density of the single components of each absorber. Combining this result with estimates of the size of the emitting region, we can reject the hypothesis that the difference between GRB and QSO MgII absorbers is due to a different size of the emitting regions.Comment: 10 pages, 15 ps figures, submitted to MNRA