251 research outputs found
Functional Analysis of the SPA Gene Family in Arabidopsis thaliana
Ambient light conditions affect development throughout the plant life cycle, including seed germination, seedling development and the induction of flowering. In the model plant Arabidopsis, the COP1-SPA ubiquitin ligase complex plays a central role in suppressing light signaling in darkness. The COP1-SPA complex targets positively acting factors like HY5, a protein necessary for normal seedling development in the light, several photoreceptors and the flowering time regulator CONSTANS for degradation via the 26S proteasome. Therefore, one of the major functions of the light signal transduction pathways is the inactivation of the COP1-SPA complex. While COP1 is a single copy gene, the SPA proteins are encoded by four different loci (SPA1-SPA4). All SPA proteins have redundant, but also distinct functions in regulating plant development. SPA1 and SPA2 are the key regulators that suppress photomorphogenesis in dark-grown seedlings. Over-stimulation in light-grown seedlings is primarily prevented by SPA1, and to a minor extent, also by SPA3 and SPA4. SPA2, in contrast has only negligible function in the light. SPA1 is sufficient for repressing flowering under non-inductive short-day conditions. Here, I show that distinct functions of the SPA genes partially correlate with their distinct gene expression patterns. RNA gel blot-analysis revealed that the expression of SPA1, SPA3 and SPA4, but not that of SPA2, is positively influenced by light of different wavelengths. All main photoreceptors contribute to the up-regulation of these SPA transcripts, implying that photoreceptors initiate a negative feedback regulation, which might protect plants from over-stimulation by light. GUS reporter gene experiments show that SPA genes exhibit somewhat distinct tissue-specific expression patterns, which might be important for tissue specific regulation of COP1-SPA targets. However, differences in SPA gene expression cannot account for all distinct SPA gene functions. Promoter-swap experiments with SPA1, SPA2 and SPA4 show that all SPA proteins are potent repressors in dark-grown seedlings. SPA1 and SPA4 also act as repressor in the light. SPA2, however, can never act as a repressor in the light, not even when it is expressed from the strong light-induced SPA1 promoter. These results show that SPA proteins themselves feature properties that determine characteristic SPA protein functions. All SPA proteins feature a characteristic domain structure with a C-terminal WD-repeat, a central coiled-coil domain and a less well-conserved N-terminus that includes a kinase-like motif. The WD-repeat domain and the coiled-coil domain are essential for formation of the COP1-SPA complex as well as interactions with various ubiquitination targets. In contrast, the function of the N-terminal domain is unknown. Aiming to determine the importance of the N-terminal domain of SPA1, I conducted a structure-function analysis. While the N-terminal domain of SPA1 is dispensable for SPA1 function in the seedling stage, this domain is required for SPA1-mediated repression of flowering in non-inductive short-day conditions. These results indicate, that the SPA1 N-terminal domain can full-fill an essential function
A Light-Independent Magnetic Compass in the Leatherback Sea Turtle
Diverse animals can orient to the earth's magnetic field (1-6), but the mechanism or mechanisms undrlying magnetic field detection have not been determined. Behavioral (7-9) amd neurophysiological (10-12) results suggest that the transduction process underlying magnetic compass orientation in vertebrates is light-dependent, a finding consistent with theoretical models proposing that magnetoreception involves a modulation of the response of retinal photoreceptors to light (13, 14). We report, however, that leatherback sea turtle (Dermochelys coriacea) hatchlings orient to the geomagnetic field in complete darkness. Thus, light-dependence is not a universal feature of vertebrate magnetic compasses
Orientation to oceanic waves by green turtle hatchlings
Minutes after emerging from underground nests, hatchling green turtles
(Chelonia my das L.) enter the sea and begin a migration towards the open ocean.
To test the hypothesis that migrating hatchlings use wave cues to maintain their
seaward headings, we released turtles offshore during unusual weather conditions
when waves moved in atypical directions. Hatchlings swam into approaching
waves in all experiments, even when doing so resulted in orientation back towards
land. These data suggest that green turtle hatchlings normally maintain seaward
headings early in the offshore migration by using wave propagation direction as an
orientation cue. Because waves and swells reliably move towards shore in shallow
coastal areas, swimming into waves usually results in movement towards the open
sea.
The physiological mechanisms that underlie wave detection by sea turtle
hatchlings are not known. Calculations indicate that, at the depth at which
hatchlings swim, accelerations produced beneath typical waves and swells along
the Florida coast are sufficient to be detected by the vertebrate inner ear. We
therefore hypothesize that hatchlings determine wave direction while under water
by monitoring the sequence of horizontal and vertical accelerations that occur as
waves pass above
Correlated electron emission in laser-induced nonsequence double ionization of Helium
In this paper, we have investigated the correlated electron emission of the
nonsequence double ionization (NSDI) in an intense linearly polarized field.
The theoretical model we employed is the semiclassical rescattering model, the
model atom we used is the helium. We find a significant correlation between
magnitude and direction of the momentum of two emission electrons, and give a
good explanation for this striking phenomenon by observing the classical
collisional trajectories. We argue that this correlation phenomenon is
universal in NSDI process, as revealed by the recent experiment on the argon.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev.
Multimodal wide-field two-photon excitation imaging: characterization of the technique for in vivo applications
We report fast, non-scanning, wide-field two-photon fluorescence excitation with spectral and lifetime detection for in vivo biomedical applications. We determined the optical characteristics of the technique, developed a Gaussian flat-field correction method to reduce artifacts resulting from non-uniform excitation such that contrast is enhanced, and showed that it can be used for ex vivo and in vivo cellular-level imaging. Two applications were demonstrated: (i) ex vivo measurements of beta-amyloid plaques in retinas of transgenic mice, and (ii) in vivo imaging of sulfonated gallium(III) corroles injected into tumors. We demonstrate that wide-field two photon fluorescence excitation with flat-field correction provides more penetration depth as well as better contrast and axial resolution than the corresponding one-photon wide field excitation for the same dye. Importantly, when this technique is used together with spectral and fluorescence lifetime detection modules, it offers improved discrimination between fluorescence from molecules of interest and autofluorescence, with higher sensitivity and specificity for in vivo applications
Shot noise limited characterization of femtosecond light pulses
Probing the evolution of physical systems at the femto- or attosecond
timescale with light requires accurate characterization of ultrashort optical
pulses. The time profiles of such pulses are usually retrieved by methods
utilizing optical nonlinearities, which require significant signal powers and
operate in a limited spectral
range\cite{Trebino_Review_of_Scientific_Instruments97,Walmsley_Review_09}. We
present a linear self-referencing characterization technique based on time
domain localization of the pulse spectral components, operated in the
single-photon regime. Accurate timing of the spectral slices is achieved with
standard single photon detectors, rendering the technique applicable in any
spectral range from near infrared to deep UV. Using detection electronics with
about ps response, we retrieve the temporal profile of a picowatt pulse
train with fs resolution, setting a new scale of sensitivity in
ultrashort pulse characterization.Comment: Supplementary information contained in raw dat
Strong field double ionization of H2 : Insights from nonlinear dynamics
The uncorrelated (``sequential'') and correlated (``nonsequential'') double
ionization of the H2 molecule in strong laser pulses is investigated using the
tools of nonlinear dynamics. We focus on the phase-space dynamics of this
system, specifically by finding the dynamical structures that regulate these
ionization processes. The emerging picture complements the recollision scenario
by clarifying the distinct roles played by the recolliding and core electrons.
Our analysis leads to verifiable predictions of the intensities where
qualitative changes in ionization occur. We also show how these findings depend
on the internuclear distance
Correlation dynamics between electrons and ions in the fragmentation of D molecules by short laser pulses
We studied the recollision dynamics between the electrons and D ions
following the tunneling ionization of D molecules in an intense short pulse
laser field. The returning electron collisionally excites the D ion to
excited electronic states from there D can dissociate or be further
ionized by the laser field, resulting in D + D or D + D,
respectively. We modeled the fragmentation dynamics and calculated the
resulting kinetic energy spectrum of D to compare with recent experiments.
Since the recollision time is locked to the tunneling ionization time which
occurs only within fraction of an optical cycle, the peaks in the D kinetic
energy spectra provides a measure of the time when the recollision occurs. This
collision dynamics forms the basis of the molecular clock where the clock can
be read with attosecond precision, as first proposed by Corkum and coworkers.
By analyzing each of the elementary processes leading to the fragmentation
quantitatively, we identified how the molecular clock is to be read from the
measured kinetic energy spectra of D and what laser parameters be used in
order to measure the clock more accurately.Comment: 13 pages with 14 figure
Coherent control using adaptive learning algorithms
We have constructed an automated learning apparatus to control quantum
systems. By directing intense shaped ultrafast laser pulses into a variety of
samples and using a measurement of the system as a feedback signal, we are able
to reshape the laser pulses to direct the system into a desired state. The
feedback signal is the input to an adaptive learning algorithm. This algorithm
programs a computer-controlled, acousto-optic modulator pulse shaper. The
learning algorithm generates new shaped laser pulses based on the success of
previous pulses in achieving a predetermined goal.Comment: 19 pages (including 14 figures), REVTeX 3.1, updated conten
A two-dimensional, two-electron model atom in a laser pulse: exact treatment, single active electron-analysis, time-dependent density functional theory, classical calculations, and non-sequential ionization
Owing to its numerical simplicity, a two-dimensional two-electron model atom,
with each electron moving in one direction, is an ideal system to study
non-perturbatively a fully correlated atom exposed to a laser field. Frequently
made assumptions, such as the ``single active electron''- approach and
calculational approximations, e.g. time dependent density functional theory or
(semi-) classical techniques, can be tested. In this paper we examine the
multiphoton short pulse-regime. We observe ``non-sequential'' ionization, i.e.\
double ionization at lower field strengths as expected from a sequential,
single active electron-point of view. Since we find non-sequential ionization
also in purely classical simulations, we are able to clarify the mechanism
behind this effect in terms of single particle trajectories. PACS Number(s):
32.80.RmComment: 10 pages, 16 figures (gzipped postscript), see also
http://www.physik.tu-darmstadt.de/tqe
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