7,236 research outputs found
Cryptic blues: mechanism in sight!
Several plant responses are mediated by blue-light as also UV-A radiation. Until a few years ago, the identity of the blue/UV-A sensing photoreceptors was equivocal. In the past few years, however, the molecular genetic analysis of Arabidopsis mutants has greatly aided in the identification and characterization of blue/UV-A sensing cryptochrome 1 (cry1), cryptochrome 2 (cry2) and phototropin (JK224/ nph1). The cry1 and cry2 receptors, like DNA photolyases, are dual chromophore proteins and may harbour both a flavin (FAD) and a pterin for primarily regulating hypocotyl growth inhibition, cotyledon expansion and flowering time, besides sensing light for entraining endogenous clocks. In contrast, JK224/nph1 anchors another flavin, FMN, and regulates phototropism. In addition to flavins and pterin, evidence is mounting in favour of a carotenoid, zeaxanthin, for regulating blue-light-induced stomatal opening. There is little information on the signalling components acting downstream to blue sensory receptors, but nph1 has an intrinsic kinase domain that on photoexcitation initiates a phosphorelay through interaction with phosphoproteins like NPH3. Although the primary mechanism of action of cry1 and cry2 is unknown, their localization in the nucleus and protein-protein interaction with phytochromes indicates that they may directly regulate changes in gene expression. The occurrence of cryptochromes in both plants and animals indicates their ubiquitous nature and a prominent role in regulating diverse responses
Floral induction in a photoperiodically insensitive duckweed, Lemna paucicostata LP6
The effects of 20 amino acids and two amides were studied on the flowering of a photoperiodically insensitive duckweed, Lemna paucicostata LP6. Alanine, asparagine, aspartate, cystine, glutamate, glutamine, glycine, lysine, methionine, proline, serine, and threonine induced flowering under a photoperiodic regime of 16 hours light and 8 hours darkness. Among these, glutamate and aspartate were found to be the most effective for flower induction. These acids could initiate flowering even at 5×10-7 molar level, though maximal flowering (about 80%) was obtained at 10-5 molar. Change in the photoperiodic schedule or the pH of the nutrient medium did not influence glutamate- or aspartate-induced flowering. The low concentrations at which glutamate and aspartate are effective suggests that they may have a regulatory role rather than simply acting as metabolites
Inelastic X-ray scattering in correlated (Mott) insulators
We calculate the inelastic light scattering from X-rays, which allows the
photon to transfer both energy and momentum to the strongly correlated charge
excitations. We find that the charge transfer peak and the low energy peak both
broaden and disperse through the Brillouin zone similar to what is seen in
experiments in materials like Ca_2 Cu O_2 Cl_2.Comment: 5 pages Revtex4, 6 figure
Phytochrome modulation of calcium fluxes in wheat (Triticum aestivum L.) protoplasts
Employing the metallochromic dye murexide and by monitoring the uptake of radiolabelled calcium, photoreversible calcium fluxes were measured in wheat leaf protoplast suspensions. Results obtained by both methods were identical - red light promoted and subsequent far-red irradiation reversed an influx of Ca++ ions into the protoplasts. These findings imply phytochrome regulation of Ca++ fluxes across the plasma membrane. The influx of Ca++ stimulated by 2 min red irradiation could be maintained in total darkness for the initial 16-18 min after illumination, after which a 6-8 min efflux process was triggered and the basal Ca++ level restored. Verapamil, a calcium channel blocker, inhibited the red-promoted influx, whereas the far-red mediated efflux could be checked by the use of the ATPase inhibitor vanadate, and also by the calmodulin antagonist chlorpromazine, thus suggesting a role of ion channels and pumps in phytochrome-controlled Ca++ fluxes. The possible involvement of phosphoinositides in phytochrome-modulated calcium fluxes was also investigated
Apomixis: an enigma with potential applications
Apomixis has been the focus of research in plant sciences
in recent years with lot of scope for crop improvement.
It results in clonal progeny without fertilization, having
maternal genetic constitution. The impact of introducing
apomixis in crop plants could be significant mainly for
its use in fixation of hybrid vigour. Because of epigenetic
barriers, introgression of apomixis from a close
relative to a sexual crop plant by conventional plant
breeding methods could not generate expected results.
Recent developments in plant molecular biology and
biotechnology can help in developing potential strategies.
This article summarizes various aspects of apomixis
research that are being followed in India and abroad
Raman scattering through a metal-insulator transition
The exact solution for nonresonant A1g and B1g Raman scattering is presented
for the simplest model that has a correlated metal-insulator transition--the
Falicov-Kimball model, by employing dynamical mean field theory. In the general
case, the A1g response includes nonresonant, resonant, and mixed contributions,
the B1g response includes nonresonant and resonant contributions (we prove the
Shastry-Shraiman relation for the nonresonant B1g response) while the B2g
response is purely resonant. Three main features are seen in the nonresonant
B1g channel: (i) the rapid appearance of low-energy spectral weight at the
expense of higher-energy weight; (b) the frequency range for this low-energy
spectral weight is much larger than the onset temperature, where the response
first appears; and (iii) the occurrence of an isosbestic point, which is a
characteristic frequency where the Raman response is independent of temperature
for low temperatures. Vertex corrections renormalize away all of these
anomalous features in the nonresonant A1g channel. The calculated results
compare favorably to the Raman response of a number of correlated systems on
the insulating side of the quantum-critical point (ranging from Kondo
insulators, to mixed-valence materials, to underdoped high-temperature
superconductors). We also show why the nonresonant B1g Raman response is
``universal'' on the insulating side of the metal-insulator transition.Comment: 12 pages, 11 figures, ReVTe
Theory of the temperature and doping dependence of the Hall effect in a model with x-ray edge singularities in d=oo
We explain the anomalous features in the Hall data observed experimentally in
the normal state of the high-Tc superconductors. We show that a consistent
treatment of the local spin fluctuations in a model with x-ray edge
singularities in d=oo reproduces the temperature and the doping dependence of
the Hall constant as well as the Hall angle in the normal state. The model has
also been invoked to justify the marginal-Fermi-liquid behavior, and provides a
consistent explanation of the Hall anomalies for a non-Fermi liquid in d=oo.Comment: 5 pages, 4 figures, To appear in Phys. Rev. B, title correcte
Magnetotransport in the doped Mott insulator
We investigate the Hall effect and the magnetoresistance of strongly
correlated electron systems using the dynamical mean-field theory. We treat the
low- and high-temperature limits analytically and explore some aspects of the
intermediate-temperature regime numerically. We observe that a
bipartite-lattice condition is responsible for the high-temperature result
obtained by various authors, whereas the general
behavior is , as for the longitudinal conductivity. We
find that Kohler's rule is neither obeyed at high nor at intermediate
temperatures.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.
Non Fermi Liquid Behaviour near a spin-glass transition
In this paper we study the competition between the Kondo effect and RKKY
interactions near the zero-temperature quantum critical point of an Ising-like
metallic spin-glass. We consider the mean-field behaviour of various physical
quantities. In the `quantum- critical regime' non-analytic corrections to the
Fermi liquid behaviour are found for the specific heat and uniform static
susceptibility, while the resistivity and NMR relaxation rate have a non-Fermi
liquid dependence on temperature.Comment: 15 pages, RevTex 3.0, 1 uuencoded ps. figure at the en
Tuning a Josephson junction through a quantum critical point
We tune the barrier of a Josephson junction through a zero-temperature
metal-insulator transition and study the thermodynamic behavior of the junction
in the proximity of the quantum-critical point. We examine a
short-coherence-length superconductor and a barrier (that is described by a
Falicov-Kimball model) using the local approximation and dynamical mean-field
theory. The inhomogeneous system is self-consistently solved by performing a
Fourier transformation in the planar momentum and exactly inverting the
remaining one-dimensional matrix with the renormalized perturbation expansion.
Our results show a delicate interplay between oscillations on the scale of the
Fermi wavelength and pair-field correlations on the scale of the coherence
length, variations in the current-phase relationship, and dramatic changes in
the characteristic voltage as a function of the barrier thickness or
correlation strength (which can lead to an ``intrinsic'' pinhole effect).Comment: 16 pages, 15 figures, ReVTe
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