bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-temperature orientation-selective growth and ultraviolet emission of single-crystal ZnO nanowires
Authors: Kim Tae-Won , Kawazoe Tadashi , Yamazaki Shunsuke , Ohtsu Motoichi , Sekiguchi Takashi
Published: April 2004

We fabricated single-crystal ZnO nanowires at a low temperature of 500 °C without using any metal catalysts via the simple thermal oxidation of metallic Zn precursors. Scanning electron microscopy and high-resolution transmission electron microscopy (HRTEM) studies showed that the ZnO nanowires with orientation-selectivity grew laterally along the sides of the hexagonal-shaped ZnO matrix transformed from the metallic Zn precursors. It was found that the nanowires grew along the 〈112̄0〉 direction and were single crystals by HRTEM study. Typically, the lengths of the ZnO nanowires ranged from 0.5 to 3 μm and their mean diameter was 20±5 nm. Photoluminescence and cathodoluminescence measurements at room temperature showed a strong ultraviolet emission peak with negligibly weak green emissions, confirming that the individual ZnO nanowire was of excellent optical quality. Based on these results, we conclude that the simple thermal oxidation of metal Zn precursors is a useful, feasible approach for fabricating high-quality one-dimensional ZnO nanostructures for integration into nanophotonic integrated circuits.
High-resolution imaging of fast magnetization dynamics in magnetic nanostructures
Authors: Stoll Hermann , Puzic Aleksander , van Waeyenberge Bartel , Fischer Peter , Raabe Joerg , Buess Matthias , Haug Thomas , Höllinger Rainer , Back Christian , Weiss Dieter , Denbeaux Gregory
Published: April 2004

By combining magnetic transmission x-ray microscopy with a stroboscopic pump and probe technique using synchrotron radiation we are able to image the magnetization dynamics in micron sized magnetic particles on a sub-100 ps time scale with a lateral spatial resolution down to 21 nm. We report first observations in squared elements indicating locally varying precessional frequencies which are in agreement with micromagnetic simulations. The experiment opens a route towards a high spatiotemporal resolution of spin patterns which is needed to understand the microscopic origin of magnetization reversal of micron sized and nano-sized magnetic particles.
Full silicidation process for making CoSi2 on SiO2
Authors: Zhao Q. T. , Rije E. , Lenk St. , Bay H. , Mantl S.
Published: April 2004

A silicidation process was developed to produce high-quality CoSi2 directly on SiO2, which can be used for metal gates of metal–oxide-semiconductor field-effect transistors (MOSFETs). Normally, the formation of a CoSi2 layer on SiO2 is very difficult because of the requirement for an exact Co/Si thickness ratio. In our process, an additional Si layer was deposited after the first rapid thermal processing (RTP) at 500 °C and selective etching of the unreacted Co. The additional Si layer provided a Si supply for the second RTP at a higher temperature. This method allows the Co layer thickness to be varied over a fairly large range, and in addition, the microstructure of the silicide layer and the CoSi2/SiO2 interface were substantially improved.
Evolution of dislocation arrays in epitaxial BaTiO3 thin films grown on (100) SrTiO3
Authors: Sun H. P. , Tian W. , Pan X. Q. , Haeni J. H. , Schlom D. G.
Published: April 2004

Dislocation arrays and dislocation half-loops in BaTiO3 thin films were characterized using transmission electron microscopy (TEM). BaTiO3 films with thicknesses ranging from 2 to 20 nm were grown on (100) SrTiO3 by reactive molecular beam epitaxy (MBE). The critical thickness for dislocations to occur in this system was found to lie between 2 and 4 nm. The misfit dislocations are mainly 〈100〉 type. The average spacing between the dislocations in the array becomes smaller when the film is thicker, which indicates gradual relaxation of mismatch strain with increasing film thickness.
Characterization of InGaN/GaN multi-quantum-well blue-light-emitting diodes grown by metal organic chemical vapor deposition
Authors: Ramaiah K. S. , Su Y. K. , Chang S. J. , Kerr B. , Liu H. P. , Chen I. G.
Published: April 2004

The structural, surface morphology, and the temperature dependence photoluminescence of InGaN(3 nm)/GaN(7 nm) 5 period multi-quantum-well blue-light-emitting diode (LED) structures grown by metal organic chemical vapor deposition (MOCVD) have been studied. Quantum dot-like structures and strain contrast evident by black lumps were observed in the quantum wells using high-resolution transmission electron microscopy (HRTEM) analysis. Double-crystal high-resolution x-ray diffraction (HRXRD) spectra of blue LED were simulated using kinematical theory method, to obtain composition, and period thickness of well and barrier. The “S” shape character shift as red–blue–redshift of the quantum-well emission line, i.e., blue emission peak 2.667 eV at 10 K, was observed with variation of temperature in the photoluminescence (PL) spectra. The shift is assigned to the potential fluctuations due to alloy inhomogeneous distribution in the quantum wells. The In composition in the quantum wells obtained by two independent techniques, namely HRXRD and PL, was 8% and 19%, respectively. The reason for this large difference in composition is explained in this letter.
Wavelength tunability of second-harmonic generation from two-dimensional χ(2) nonlinear photonic crystals with a tetragonal lattice structure
Authors: Peng L.-H. , Hsu C.-C. , Ng Jimmy , Kung A. H.
Published: April 2004

The wavelength tunability of second-harmonic generation (SHG) in a two-dimensional nonlinear photonic crystal (2D NPC) on lithium niobate is examined by using a 1.55 μm-band optical parametric oscillator. We observed SHG signals that were generated off-axis from the fundamental pump beam and a multiple number of quasi-phase-matchable (QPM) wavelengths for a given crystal setting. The spacing between the phase-matched wavelengths increases with the azimuthal rotation angle (φ) in the x–y plane of the crystal. The peak intensity and propagation direction of the SHG signals are found to vary with the pump wavelength and crystal rotation angle. These observations are ascribed to a 2D distribution of high-order reciprocal lattice vectors Gmn(φ) and the corresponding χ(2)(Gmn) nonlinearities in 2D NPC having a tetragonal structural symmetry.
Performance of a negative index of refraction lens
Authors: Parazzoli C. G. , Greegor R. B. , Nielsen J. A. , Thompson M. A. , Li K. , Vetter A. M. , Tanielian M. H. , Vier D. C.
Published: April 2004

A plano-concave lens with negative index of refraction has been designed and fabricated. Such lenses have been postulated for many years, but only recently has their realization been made possible through improved simulation and fabrication procedures. We report here the simulation, fabrication, and performance of such a lens. The lens images the source field and reproduces the results of standard Gaussian optics. The curved lens with negative index of refraction in the microwave frequency region of the electromagnetic spectrum has been compared to a plano-convex Macor positive index of refraction lens having the same radius of curvature.
Nonlinear optical input–output characteristics of 1.55 μm injection-locked vertical-cavity surface-emitting lasers
Authors: Onishi Y. , Koyama F. , Nishiyama N. , Caneau C. , Zah C. E.
Published: April 2004

The optical input–output characteristics of a 1.55 μm vertical-cavity surface-emitting laser (VCSEL) with external light injection to its fundamental mode were investigated. Appropriate wavelength detuning of external light enables abrupt modal switching in an injection-locked VCSEL. We found that the obtained nonlinear transfer function is strongly dependent on the distance between a VCSEL and an input lensed fiber. As the distance was changed, different optical input–output characteristics were observed, demonstrating that such a VCSEL can be employed in various optical signal-processing schemes, for example as an optical regenerator or an optical inverter.
P-type conductivity in bulk AlxGa1−xN and AlxGa1−xN/AlyGa1−yN superlattices with average Al mole fraction >20%
Authors: Kim J. K. , Waldron E. L. , Li Y.-L. , Gessmann Th. , Schubert E. F. , Jang H. W. , Lee J.-L.
Published: April 2004

Synchrotron radiation photoemission spectroscopy reveals enhanced oxygen incorporation in AlxGa1−xN as the Al mole fraction increases. It is shown that the increased oxygen donor incorporation can result in a conductivity-type change from p-type to n-type in Mg-doped AlxGa1−xN. Consistent with the conductivity-type change, epitaxial Al0.20Ga0.80N films exhibit n-type conductivity despite heavy Mg doping. The p-type conductivity of bulk AlxGa1−xN with a high Al mole fraction can be improved by employing AlxGa1−xN/AlyGa1−yN superlattices (SLs). At 300 K, Mg-doped Al0.17Ga0.83N/Al0.36Ga0.64N SLs (average Al mole fraction of 23%) exhibit strong p-type conductivity with a specific resistance of 4.6 Ω cm, a hole mobility of 18.8 cm2/Vs, and an acceptor activation energy of 195 meV.
Random laser action in ZnO nanorod arrays embedded in ZnO epilayers
Authors: Yu S. F. , Yuen Clement , Lau S. P. , Park W. I. , Yi Gyu-Chul
Published: April 2004

Random laser action with coherent feedback has been observed in ZnO nanorod arrays embedded in ZnO epilayers. The sample was fabricated by depositing a MgO buffer layer and followed by a layer of ZnO thin film onto a vertically well-aligned ZnO nanorod arrays grown on sapphire substrate. Under 355 nm optical excitation at room temperature, sharp lasing peaks emit at around 390 nm with a linewidth less than 0.4 nm has been observed in all directions. In addition, the dependence of the lasing threshold intensity on the excitation area is shown in good agreement with the random laser theory. Hence, it is demonstrated that random laser action can also be supported in ZnO nanorod arrays.
Optical evidence of an electrothermal degradation of InGaN-based light-emitting diodes during electrical stress
Authors: Pavesi M. , Manfredi M. , Salviati G. , Armani N. , Rossi F. , Meneghesso G. , Levada S. , Zanoni E. , Du S. , Eliashevich I.
Published: April 2004

The optical properties of blue InGaN-based light-emitting diodes aged at high current levels have been studied by electroluminescence and cathodoluminescence. The onset of a broad optical band peaked at about 3.1 eV in devices aged without a heat sink (junction temperature higher than 300 °C) has been correlated to an electrothermal threshold effect. The band is attributed to the dissociation of Mg–H complexes inside the p-type layers and to the consequent formation of Mg-related metastable complexes acting as acceptors. Subsequent electron-beam irradiation determines the almost complete quenching of the band.
Optical cavity enhancement of light–sound interaction in acoustic phonon cavities
Authors: Lacharmoise P. , Fainstein A. , Jusserand B. , Thierry-Mieg V.
Published: April 2004

We describe a device that has a resonant cavity for acoustic phonons embedded inside an optical cavity. This double cavity structure is a resonator for acoustical phonons and enhances the interaction between sound and light. We discuss the design and material parameters relevant for the optimization of the acoustic phonon cavities, and we present Raman scattering experiments on GaAs/AlAs structures designed to confine from one to three acoustical phonon modes. We quantitatively study the amplification of the photon–phonon interaction in these devices reporting an enhancement factor of more than five orders of magnitude.
Detection of misfit dislocations at interface of strained Si/Si0.8Ge0.2 by electron-beam-induced current technique
Authors: Yuan X. L. , Sekiguchi T. , Ri S. G. , Ito S.
Published: April 2004

Electron-beam-induced current (EBIC) has been employed to investigate misfit dislocations (MDs) at the interface of strained Si/Si0.8Ge0.2, which are located within the depletion region of Schottky contact. The MDs are intentionally introduced by growing the strained-Si layer to a thickness larger than the critical thickness. Two orthogonal sets of weak dark lines and some weak dark dots are observed with low electron-beam energy at a low temperature. These dark lines and dark dots correspond to the MDs and threading dislocations (TDs), respectively. The MDs and TDs are found to be nearly electrically inactive at room temperature and increase their activities at lower temperature, indicating that they are accompanied by shallow levels and free from metallic contamination. Comparisons with the chemical etched pattern reveal that each of the EBIC dark lines corresponds to a bundle of MDs.
The linearly graded two-dimensional p–n junction
Authors: Petrosyan S. , Yesayan A. , Reuter D. , Wieck A. D.
Published: April 2004

We study the potential distribution, depletion length, and capacitance–voltage characteristics of two-dimensional (2D) linearly graded p–n junctions. It is shown that the total-depletion length is proportional to the square root of the contact potential difference, in contrast to the linear dependence for an abrupt 2D junction. The junction capacitance varies logarithmically with the applied voltage. A graph of capacitance as a function of logarithm of voltage drop across the depletion layer should be a straight line, whose slope indicates the abruptness of a 2D p–n junction.
Freestanding SiGe/Si/Cr and SiGe/Si/SixNy/Cr microtubes
Authors: Golod S. V. , Prinz V. Ya. , Wägli P. , Zhang L. , Kirfel O. , Deckhardt E. , Glaus F. , David C. , Grützmacher D.
Published: April 2004

We report on hybrid microtubes and rings fabricated from rolled-up strained metal–semiconductor SiGe/Si/Cr and metal–insulator–semiconductor SiGe/Si/SixNy/Cr films. For making suspended microtubes, a method of directional rolling of the patterned films by anisotropic underetching of silicon substrate was introduced. It is shown quantitatively that Cr and SixNy layers are highly strained, the tensile stress being sufficient to cause the rolling-up of the hybrid films into microtubes of preset diameter. The proposed controllable and reproducible technology is promising for fabricating cylindrical-shaped microcapacitors, induction coils, transistors, and building blocks of microelectromechanical devices.
High-efficiency screen-printed solar cell on edge-defined film-fed grown ribbon silicon through optimized rapid belt co-firing of contacts and high-sheet-resistance emitter
Authors: Rohatgi Ajeet , Hilali Mohamed M. , Nakayashiki Kenta
Published: April 2004

High-quality screen-printed contacts were achieved on a high-sheet-resistance emitter (∼100 Ω/sq.) using PV168 Ag paste and rapid co-firing in the belt furnace. The optimized co-firing cycle developed for a 100 Ω/sq. emitter produced 16.1% efficient 4 cm2 planar edge-defined film-fed grown (EFG) ribbon Si cells with a low series-resistance (0.8 Ω cm2), high fill factor of ∼0.77, along with very significant bulk lifetime enhancement from 3 to 100 μs. This represents the highest-efficiency screen-printed EFG Si cells with single-layer antireflection (AR) coating. These cells were fabricated using a simple process involving POCl3 diffusion for a high-sheet-resistance emitter, SiNx AR coating and rapid cofiring of Ag grid and Al-doped back-surface field in a conventional belt furnace. The rapid cofiring process also prevented junction shunting while maintaining very effective SiNx-induced hydrogen passivation of defects, resulting in an average bulk lifetime exceeding 100 μs.
Transgenic down-regulation of caffeic acid O-methyltransferase (COMT) led to improved digestibility in tall fescue (Festuca arundinacea)
Authors: Chen Lei , Auh Chung-Kyoon , Dowling Paul , Bell Jeremey , Lehmann Deane , Wang Zeng-Yu
Published: April 2004

Dry matter digestibility is one of the most important characteristics of forage. The major constraint on ruminant digestion of forage cell walls is lignin. Sequences of cDNA encoding a key lignin biosynthetic enzyme, caffeic acid O-methyltransferase (COMT), was cloned from the widely grown monocot forage species tall fescue (Festuca arundinacea Schreb.). Enzymatic properties of recombinant COMT protein expressed in E. coli were determined using six substrates. The preferred substrates for tall fescue recombinant COMT were 5-hydroxyferulic acid and caffeoyl aldehyde. Transgenic tall fescue plants carrying either sense or antisense COMT gene constructs were obtained by microprojectile bombardment of single-genotype-derived embryogenic suspension cells. Consistent and closely related molecular and biochemical data demonstrated that two co-suppressed transgenic lines were down-regulated in their lignin biosynthesis. These COMT down-regulated transgenic tall fescue plants showed substantially reduced levels of transcripts, significantly reduced enzymatic activities, significantly decreased lignin content, apparently altered lignin composition and significantly increased (9.8-10.8%) digestibility.
Mineralisation of soil orthophosphate monoesters under pine seedlings and ryegrass
Authors: Chen C. R. , Condron L. M. , Turner B. L. , Mahieu N. , Davis M. R. , Xu Z. H. , Sherlock R. R.
Published: April 2004

The effects of radiata pine (Pinus radiata D. Don) seedlings and ryegrass (Lolium perenne L.) on the mineralisation of orthophosphate monoesters in 7 grassland soils were assessed in a 10-month pot trial using NaOH–EDTA extraction and solution 31P NMR spectroscopy. Extraction with NaOH–EDTA recovered 46–86% of the total soil P, and NaOH–EDTA-extractable organic P determined by molybdate colourimetry ranged between 194 and 715 mg/kg soil, representing 34–85% of the total soil organic P. Orthophosphate monoesters were the predominant species of the extracted organic P in all soils, with much smaller concentrations of orthophosphate diesters, and traces of phosphonates. Concentrations of orthophosphate monoesters were consistently lower in soils under pine (103–480 mg P/kg soil) compared with the initial soils (142–598 mg P/kg soil) and most soils under grass (122–679 mg/kg soil). Mineralisation of myo-inositol hexakisphosphate accounted for 18–100% of the total mineralisation of orthophosphate monoesters in most soils under radiata pine. This suggests that supposedly recalcitrant inositol phosphates are available for uptake by radiata pine, although the extent of this varies among soils.
Wettability of soil aggregates from cultivated and uncultivated Ustolls and Usterts
Authors: Eynard Anna , Schumacher Thomas E. , Lindstrom Michael J. , Malo Douglas D. , Kohl Robert A.
Published: April 2004

Soil organic matter can modify the interaction of clay minerals with water, limiting the rate of water intake of swelling clays and stabilising soil aggregates. Soil structural stability and organic C content usually decrease with cultivation. Faster wetting increases stresses on aggregates and decreases stability. Aggregate wettabilities of prairie soils under 3 different management systems (grassland, no-till, and conventional-till) were compared in the Northern Great Plains of the USA. Six Ustolls and 2 Usterts were selected as replications along the Missouri River. Wettability was measured as water drop penetration time (WDPT) and as rate of water intake under 30 and 300 mm tension. At low tension, aggregates from both cultivated fields and uncultivated grasslands showed similar wettability. Water intake in grass aggregates was attributed to a greater amount of stable pores relative to cultivated aggregates. In cultivated aggregates, slaking created planes of failure that allowed rapid water entry. Differences of wettability between management systems at 300 mm tension (in Ustolls, grasslands had greater wettability than cultivated soils, 0.24 v. 0.17 g water/h.g dry soil) and between soil orders (Usterts had longer WDPT than Ustolls, 2.9 v. 1.7 s) were explained by both clay and organic C contents. Simple measurements of aggregate wettability may be effectively used for soil quality characterisation. Aggregate wettability is a desirable property for agricultural soils when it is related to stable porosity, as may be found in high organic matter soils (e.g. grasslands). Wettability is excessive when fast aggregate wetting results in aggregate destruction as observed in low organic matter cultivated soils.
The role of N transformations in the formation of acidic subsurface layers in stock urine patches
Authors: Condon J. R. , Black A. S. , Conyers M. K.
Published: April 2004

This study examines the role of nitrogen transformations in the acidification of soil under stock urine patches, specifically the formation of acidic subsurface layers. These are horizontal planes of acidity several centimetres below the soil surface. Glasshouse studies were conducted to relate nitrogen transformations to measured pH changes in soil treated with urine or urea solution (simulated urine). Acidic subsurface layers formed in both urine- and simulated urine-treated soil. With the development of a H+ balance model, the contribution of nitrogen transformations to changes in the H+ concentrations in simulated urine patches was determined.During the first 9 days following treatment, urea hydrolysis and NH3 volatilisation dominated changes in H+ concentration. After that, net immobilisation contributed to H+ changes; however, nitrification was the dominant process occurring. Downward movement of NH4+ originating from urea hydrolysis allowed more nitrification to occur in lower soil layers. The net result of these processes was net acidification of the 4–6, 6–8, and 8–10 cm layers by approximately 0.7, 0.6, and 0.3 pH units, respectively. Thus nitrogen transformations were responsible for the formation of acidic subsurface layers in simulated stock urine patches within 6 weeks of application.
Extracellular acidosis increases neuronal cell calcium by activating acid-sensing ion channel 1a
Authors: Yermolaieva Olena , Leonard A. Soren , Schnizler Mikael K. , Abboud Francois M. , Welsh Michael J.
Published: April 2004

Acid-sensing ion channel (ASIC) 1a subunit is expressed in synapses of central neurons where it contributes to synaptic plasticity. However, whether these channels can conduct Ca
2+
and thereby raise the cytosolic Ca
2+
concentration, [Ca
2+
]
c
, and possibly alter neuronal physiology has been uncertain. We found that extracellular acidosis opened ASIC1a channels, which provided a pathway for Ca
2+
entry and elevated [Ca
2+
]
c
in wild-type, but not ASIC1
-/-
, hippocampal neurons. Acid application also raised [Ca
2+
]
c
and evoked Ca
2+
currents in heterologous cells expressing ASIC1a. Although ASIC2a is also expressed in central neurons, neither ASIC2a homomultimeric channels nor ASIC1a/2a heteromultimers showed H
+
-activated [Ca
2+
]
c
elevation or Ca
2+
currents. Because extracellular acidosis accompanying cerebral ischemia contributes to neuronal injury, we tested the effect of acidosis on cell death measured as lactate dehydrogenase release. Eliminating ASIC1a from neurons or treating ASIC1a-expressing cells with the ASIC blocker amiloride attenuated acidosis-induced cell injury. These results indicate that ASIC1a provides a non-voltage-gated pathway for Ca
2+
to enter neurons. Thus, it may provide a target for modulation of [Ca
2+
]
c
.
The Immunophilin-Interacting Protein AtFIP37 from Arabidopsis Is Essential for Plant Development and Is Involved in Trichome Endoreduplication
Authors: Vespa Laurent , Vachon Gilles , Berger Frédéric , Perazza Daniel , Faure Jean-Denis , Herzog Michel
Published: April 2004

The FKBP12 (FK506-binding protein 12 kD) immunophilin interacts with several protein partners in mammals and is a physiological regulator of the cell cycle. In Arabidopsis, only one specific partner of AtFKBP12, namely AtFIP37 (FKBP12 interacting protein 37 kD), has been identified but its function in plant development is not known. We present here the functional analysis of AtFIP37 in Arabidopsis. Knockout mutants of AtFIP37 show an embryo-lethal phenotype that is caused by a strong delay in endosperm development and embryo arrest. AtFIP37 promoter::β-glucuronidase reporter gene constructs show that the gene is expressed during embryogenesis and throughout plant development, in undifferentiating cells such as meristem or embryonic cells as well as highly differentiating cells such as trichomes. A translational fusion with the enhanced yellow fluorescent protein indicates that AtFIP37 is a nuclear protein localized in multiple subnuclear foci that show a speckled distribution pattern. Overexpression of AtFIP37 in transgenic lines induces the formation of large trichome cells with up to six branches. These large trichomes have a DNA content up to 256C, implying that these cells have undergone extra rounds of endoreduplication. Altogether, these data show that AtFIP37 is critical for life in Arabidopsis and implies a role for AtFIP37 in the regulation of the cell cycle as shown for FKBP12 and TOR (target of rapamycin) in mammals.
Immunophilins and Parvulins. Superfamily of Peptidyl Prolyl Isomerases in Arabidopsis
Authors: He Zengyong , Li Legong , Luan Sheng
Published: April 2004

Immunophilins are defined as receptors for immunosuppressive drugs including cyclosporin A, FK506, and rapamycin. The cyclosporin A receptors are referred to as cyclophilins (CYPs) and FK506- and rapamycin-binding proteins are abbreviated as FKBPs. These two groups of proteins (collectively called immunophilins) share little sequence homology, but both have peptidyl prolyl cis/trans isomerase (PPIase) activity that is involved in protein folding processes. Studies have identified immunophilins in all organisms examined including bacteria, fungi, animals, and plants. Nevertheless, the physiological function of immunophilins is poorly understood in any organism. In this study, we have surveyed the genes encoding immunophilins in Arabidopsis genome. A total of 52 genes have been found to encode putative immunophilins, among which 23 are putative FKBPs and 29 are putative CYPs. This is by far the largest immunophilin family identified in any organism. Both FKBPs and CYPs can be classified into single domain and multiple domain members. The single domain members contain a basic catalytic domain and some of them have signal sequences for targeting to a specific organelle. The multiple domain members contain not only the catalytic domain but also defined modules that are involved in protein-protein interaction or other functions. A striking feature of immunophilins in Arabidopsis is that a large fraction of FKBPs and CYPs are localized in the chloroplast, a possible explanation for why plants have a larger immunophilin family than animals. Parvulins represent another family of PPIases that are unrelated to immunophilins in protein sequences and drug binding properties. Three parvulin genes were found in Arabidopsis genome. The expression of many immunophilin and parvulin genes is ubiquitous except for those encoding chloroplast members that are often detected only in the green tissues. The large number of genes and diversity of structure domains and cellular localization make PPIases a versatile superfamily of proteins that clearly function in many cellular processes in plants.
Transient Exposure to Ethylene Stimulates Cell Division and Alters the Fate and Polarity of Hypocotyl Epidermal Cells
Authors: Kazama Haruko , Dan Haruka , Imaseki Hidemasa , Wasteneys Geoffrey O.
Published: April 2004

After transient exposure to the gaseous hormone ethylene, dark-grown cucumber (Cucumis sativus) hypocotyls developed unusual features. Upon ethylene's removal, the developing epidermis showed significant increases in cell division rates, producing an abundance of guard cells and trichomes. These responses to ethylene depended on the stage of development at the time of ethylene exposure. In the upper region of the hypocotyl, where cells were least differentiated at the onset of ethylene treatment, complex, multicellular protuberances formed. Further down the hypocotyl, where stomata and trichomes were beginning to develop at the onset of ethylene exposure, an increase in the number of stomata and trichomes was observed. Stomatal complexes developing after the ethylene treatment had a significant increase in the number of stomatal subsidiary cells and the number of cells per trichome increased. Analysis of division patterns in stomatal complexes indicated that exposure to ethylene either suspended or altered cell fate. Ethylene also altered cell division polarity, resulting in aberrant stomatal complexes and branched trichomes. To our knowledge, the results of this study demonstrate for the first time that transient treatment with physiological concentrations of ethylene can alter cell fate and increase the propensity of cells to divide.
Hydraulic Analysis of Water Flow through Leaves of Sugar Maple and Red Oak
Authors: Sack Lawren , Streeter Christopher M. , Holbrook N. Michele
Published: April 2004

Leaves constitute a substantial fraction of the total resistance to water flow through plants. A key question is how hydraulic resistance within the leaf is distributed among petiole, major veins, minor veins, and the pathways downstream of the veins. We partitioned the leaf hydraulic resistance (R  leaf) for sugar maple (Acer saccharum) and red oak (Quercus rubra) by measuring the resistance to water flow through leaves before and after cutting specific vein orders. Simulations using an electronic circuit analog with resistors arranged in a hierarchical reticulate network justified the partitioning of total R  leaf into component additive resistances. On average 64% and 74% of the R  leaf was situated within the leaf xylem for sugar maple and red oak, respectively. Substantial resistance—32% and 49%— was in the minor venation, 18% and 21% in the major venation, and 14% and 4% in the petiole. The large number of parallel paths (i.e. a large transfer surface) for water leaving the minor veins through the bundle sheath and out of the leaf resulted in the pathways outside the venation comprising only 36% and 26% of R  leaf. Changing leaf temperature during measurement of R  leaf for intact leaves resulted in a temperature response beyond that expected from changes in viscosity. The extra response was not found for leaves with veins cut, indicating that water crosses cell membranes after it leaves the xylem. The large proportion of resistance in the venation can explain why stomata respond to leaf xylem damage and cavitation. The hydraulic importance of the leaf vein system suggests that the diversity of vein system architectures observed in angiosperms may reflect variation in whole-leaf hydraulic capacity.
The CPH1 Gene of Chlamydomonas reinhardtii Encodes Two Forms of Cryptochrome Whose Levels Are Controlled by Light-Induced Proteolysis
Authors: Reisdorph Nichole A. , Small Gary D.
Published: April 2004

Cryptochromes are proteins related to DNA photolyases and have been shown to function as blue-light photoreceptors and to play important roles in circadian rhythms in both plants and animals. The CPH1 gene from Chlamydomonas reinhardtii was originally predicted to encode a putative cryptochrome protein of 867 amino acids with a predicted molecular mass of 91 kD (Small et al., 1995). However, western blotting with antibodies specific to the CPH1 protein revealed the presence of two proteins that migrate at apparent molecular mass of approximately 126 and 143 kD. A reexamination of the assigned intron-exon boundaries has shown that the previously assigned intron 7 is in fact part of exon 7 which leads to a predicted protein of 1,007 amino acids corresponding to a size of 104.6 kD. The two forms of CPH1 that migrate slower on SDS-PAGE presumably result from unknown posttranslational modifications. In C. reinhardtii cells synchronized by light to dark cycles, the two slow migrating forms of CPH1 protein accumulate in the dark and disappear rapidly in the light. Both red and blue light are effective at inducing the degradation of the CPH1 proteins. Proteasomes are implicated because degradation is inhibited by MG132, a proteasome inhibitor. Studies with deletion mutants indicate that the C-terminal region is important for both the posttranslational modification and the protein's stability under both light and dark conditions.
Regulation of Flower Development in Arabidopsis by SCF Complexes
Authors: Ni Weimin , Xie Daoxin , Hobbie Lawrence , Feng Baomin , Zhao Dazhong , Akkara Joseph , Ma Hong
Published: April 2004

SCF complexes are the largest and best studied family of E3 ubiquitin protein ligases that facilitate the ubiquitylation of proteins targeted for degradation. The SCF core components Skp1, Cul1, and Rbx1 serve in multiple SCF complexes involving different substrate-specific F-box proteins that are involved in diverse processes including cell cycle and development. In Arabidopsis, mutations in the F-box gene UNUSUAL FLORAL ORGANS (UFO) result in a number of defects in flower development. However, functions of the core components Cul1 and Rbx1 in flower development are poorly understood. In this study we analyzed floral phenotypes caused by altering function of Cul1 or Rbx1, as well as the effects of mutations in ASK1 and ASK2. Plants homozygous for a point mutation in the AtCUL1 gene showed reduced floral organ number and several defects in each of the four whorls. Similarly, plants with reduced AtRbx1 expression due to RNA interference also exhibited floral morphological defects. In addition, compared to the ask1 mutant, plants homozygous for ask1 and heterozygous for ask2 displayed enhanced reduction of B function, as well as other novel defects of flower development, including carpelloid sepals and an inhibition of petal development. Genetic analyses demonstrate that AGAMOUS (AG) is required for the novel phenotypes observed in the first and second whorls. Furthermore, the genetic interaction between UFO and AtCUL1 supports the idea that UFO regulates multiple aspects of flower development as a part of SCF complexes. These results suggest that SCF complexes regulate several aspects of floral development in Arabidopsis.
The Timing of Senescence and Response to Pathogens Is Altered in the Ascorbate-Deficient Arabidopsis Mutant vitamin c-1
Authors: Barth Carina , Moeder Wolfgang , Klessig Daniel F. , Conklin Patricia L.
Published: April 2004

The ozone-sensitive Arabidopsis mutant vitamin c-1 (vtc1) is deficient in l-ascorbic acid (AsA) due to a mutation in GDP-Man pyrophosphorylase (Conklin et al., 1999), an enzyme involved in the AsA biosynthetic pathway (Smirnoff et al., 2001). In this study, the physiology of this AsA deficiency was initially investigated in response to biotic (virulent pathogens) stress and subsequently with regards to the onset of senescence. Infection with either virulent Pseudomonas syringae or Peronospora parasitica resulted in largely reduced bacterial and hyphal growth in the vtc1 mutant in comparison to the wild type. When vitamin c-2 (vtc2), another AsA-deficient mutant, was challenged with P. parasitica, growth of the fungus was also reduced, indicating that the two AsA-deficient mutants are more resistant to these pathogens. Induction of pathogenesis-related proteins PR-1 and PR-5 is significantly higher in vtc1 than in the wild type when challenged with virulent P. syringae. In addition, the vtc1 mutant exhibits elevated levels of some senescence-associated gene (SAG) transcripts as well as heightened salicylic acid levels. Presumably, therefore, low AsA is causing vtc1 to enter at least some stage(s) of senescence prematurely with an accompanying increase in salicylic acid levels that results in a faster induction of defense responses.
Cytoplasmic Alkalization Precedes Reactive Oxygen Species Production during Methyl Jasmonate- and Abscisic Acid-Induced Stomatal Closure
Authors: Suhita Dontamala , Raghavendra Agepati S. , Kwak June M. , Vavasseur Alain
Published: April 2004

Signaling events during abscisic acid (ABA) or methyl jasmonate (MJ)-induced stomatal closure were examined in Arabidopsis wild type, ABA-insensitive (ost1-2), and MJ-insensitive mutants (jar1-1) in order to examine a crosstalk between ABA and MJ signal transduction. Some of the experiments were performed on epidermal strips of Pisum sativum. Stomata of jar1-1 mutant plants are insensitive to MJ but are able to close in response to ABA. However, their sensitivity to ABA is less than that of wild-type plants. Reciprocally, the stomata of ost1-2 are insensitive to ABA but are able to close in response to MJ to a lesser extent compared to wild-type plants. Both MJ and ABA promote H2O2 production in wild-type guard cells, while exogenous application of diphenylene iodonium (DPI) chloride, an inhibitor of NAD(P)H oxidases, results in the suppression of ABA- and MJ-induced stomatal closure. ABA elevates H2O2 production in wild-type and jar1-1 guard cells but not in ost1-2, whereas MJ induces H2O2 production in both wild-type and ost1-2 guard cells, but not in jar1-1. MJ-induced stomatal closing is suppressed in the NAD(P)H oxidase double mutant atrbohD/F and in the outward potassium channel mutant gork1. Furthermore, MJ induces alkalization in guard cell cytosol, and MJ-induced stomatal closing is inhibited by butyrate. Analyses of the kinetics of cytosolic pH changes and reactive oxygen species (ROS) production show that the alkalization of cytoplasm precedes ROS production during the stomatal response to both ABA and MJ. Our results further indicate that JAR1, as OST1, functions upstream of ROS produced by NAD(P)H oxidases and that the cytoplasmic alkalization precedes ROS production during MJ or ABA signal transduction in guard cells.
Intercellular Distribution of Glutathione Synthesis in Maize Leaves and Its Response to Short-Term Chilling
Authors: Gómez Leonardo D. , Vanacker Hélène , Buchner Peter , Noctor Graham , Foyer Christine H.
Published: April 2004

To investigate the intercellular control of glutathione synthesis and its influence on leaf redox state in response to short-term chilling, genes encoding γ-glutamylcysteine synthetase (γ-ECS) and glutathione synthetase (GSH-S) were cloned from maize (Zea mays) and specific antibodies produced. These tools were used to provide the first information on the intercellular distribution of γ-ECS and GSH-S transcript and protein in maize leaves, in both optimal conditions and chilling stress. A 2-d exposure to low growth temperatures (chill) had no effect on leaf phenotype, whereas return to optimal temperatures (recovery) caused extensive leaf bleaching. The chill did not affect total leaf GSH-S transcripts but strongly induced γ-ECS mRNA, an effect reversed during recovery. The chilling-induced increase in γ-ECS transcripts was not accompanied by enhanced total leaf γ-ECS protein or extractable activity. In situ hybridization and immunolocalization of leaf sections showed that γ-ECS and GSH-S transcripts and proteins were found in both the bundle sheath (BS) and the mesophyll cells under optimal conditions. Chilling increased γ-ECS transcript and protein in the BS but not in the mesophyll cells. Increased BS γ-ECS was correlated with a 2-fold increase in both leaf Cys and γ-glutamylcysteine, but leaf total glutathione significantly increased only in the recovery period, when the reduced glutathione to glutathione disulfide ratio decreased 3-fold. Thus, while there was a specific increase in the potential contribution of the BS cells to glutathione synthesis during chilling, it did not result in enhanced leaf glutathione accumulation at low temperatures. Return to optimal temperatures allowed glutathione to increase, particularly glutathione disulfide, and this was associated with leaf chlorosis.
Dehydroascorbate Influences the Plant Cell Cycle through a Glutathione-Independent Reduction Mechanism
Authors: Potters Geert , Horemans Nele , Bellone Silvia , Caubergs Roland J. , Trost Paolo , Guisez Yves , Asard Han
Published: April 2004

Glutathione is generally accepted as the principal electron donor for dehydroascorbate (DHA) reduction. Moreover, both glutathione and DHA affect cell cycle progression in plant cells. But other mechanisms for DHA reduction have been proposed. To investigate the connection between DHA and glutathione, we have evaluated cellular ascorbate and glutathione concentrations and their redox status after addition of dehydroascorbate to medium of tobacco (Nicotiana tabacum) L. cv Bright Yellow-2 (BY-2) cells. Addition of 1 mm DHA did not change the endogenous glutathione concentration. Total glutathione depletion of BY-2 cells was achieved after 24-h incubation with 1 mm of the glutathione biosynthesis inhibitor l-buthionine sulfoximine. Even in these cells devoid of glutathione, complete uptake and internal reduction of 1 mm DHA was observed within 6 h, although the initial reduction rate was slower. Addition of DHA to a synchronized BY-2 culture, or depleting its glutathione content, had a synergistic effect on cell cycle progression. Moreover, increased intracellular glutathione concentrations did not prevent exogenous DHA from inducing a cell cycle shift. It is therefore concluded that, together with a glutathione-driven DHA reduction, a glutathione-independent pathway for DHA reduction exists in vivo, and that both compounds act independently in growth control.
A Novel Role of Water-Soluble Chlorophyll Proteins in the Transitory Storage of Chorophyllide
Authors: Reinbothe Christiane , Satoh Hiroyuki , Alcaraz Jean-Pierre , Reinbothe Steffen
Published: April 2004

All chlorophyll (Chl)-binding proteins involved in photosynthesis of higher plants are hydrophobic membrane proteins integrated into the thylakoids. However, a different category of Chl-binding proteins, the so-called water-soluble Chl proteins (WSCPs), was found in members of the Brassicaceae, Polygonaceae, Chenopodiaceae, and Amaranthaceae families. WSCPs from different plant species bind Chl a and Chl b in different ratios. Some members of the WSCP family are induced after drought and heat stress as well as leaf detachment. It has been proposed that this group of proteins might have a physiological function in the Chl degradation pathway. We demonstrate here that a protein that shared sequence homology to WSCPs accumulated in etiolated barley (Hordeum vulgare) seedlings exposed to light for 2 h. The novel 22-kD protein was attached to the outer envelope of barley etiochloroplasts, and import of the 27-kD precursor was light dependent and induced after feeding the isolated plastids the tetrapyrrole precursor 5-aminolevulinic acid. HPLC analyses and spectroscopic pigment measurements of acetone-extracted pigments showed that the 22-kD protein is complexed with chlorophyllide. We propose a novel role of WSCPs as pigment carriers operating during light-induced chloroplast development.
High-Resolution Physical Mapping in Pennisetum squamulatum Reveals Extensive Chromosomal Heteromorphism of the Genomic Region Associated with Apomixis
Authors: Akiyama Yukio , Conner Joann A. , Goel Shailendra , Morishige Daryl T. , Mullet John E. , Hanna Wayne W. , Ozias-Akins Peggy
Published: April 2004

Gametophytic apomixis is asexual reproduction as a consequence of parthenogenetic development of a chromosomally unreduced egg. The trait leads to the production of embryos with a maternal genotype, i.e. progeny are clones of the maternal plant. The application of the trait in agriculture could be a tremendous tool for crop improvement through conventional and nonconventional breeding methods. Unfortunately, there are no major crops that reproduce by apomixis, and interspecific hybridization with wild relatives has not yet resulted in commercially viable germplasm. Pennisetum squamulatum is an aposporous apomict from which the gene(s) for apomixis has been transferred to sexual pearl millet by backcrossing. Twelve molecular markers that are linked with apomixis coexist in a tight linkage block called the apospory-specific genomic region (ASGR), and several of these markers have been shown to be hemizygous in the polyploid genome of P. squamulatum. High resolution genetic mapping of these markers has not been possible because of low recombination in this region of the genome. We now show the physical arrangement of bacterial artificial chromosomes containing apomixis-linked molecular markers by high resolution fluorescence in situ hybridization on pachytene chromosomes. The size of the ASGR, currently defined as the entire hemizygous region that hybridizes with apomixis-linked bacterial artificial chromosomes, was estimated on pachytene and mitotic chromosomes to be approximately 50 Mbp (a quarter of the chromosome). The ASGR includes highly repetitive sequences from an Opie-2-like retrotransposon family that are particularly abundant in this region of the genome.
Differential Expression of Vacuolar H+-ATPase Subunit c Genes in Tissues Active in Membrane Trafficking and Their Roles in Plant Growth as Revealed by RNAi
Authors: Padmanaban Senthilkumar , Lin Xiaoying , Perera Imara , Kawamura Yukio , Sze Heven
Published: April 2004

Acidification of intracellular compartments by the vacuolar-type H+-ATPases (VHA) is known to energize ion and metabolite transport, though cellular processes influenced by this activity are poorly understood. At least 26 VHA genes encode 12 subunits of the V1Vo-ATPase complex in Arabidopsis, and how the expression, assembly, and activity of the pump are integrated into signaling networks that govern growth and adaptation are largely unknown. The role of multiple VHA-c genes encoding the 16-kD subunit of the membrane Vo sector was investigated. Expression of VHA-c1, monitored by promoter-driven β-glucuronidase (GUS) activity was responsive to light or dark in an organ-specific manner. VHA-c1 expression in expanding cotyledons, hypocotyls of etiolated seedlings, and elongation zone of roots supported a role for V-ATPase in cell enlargement. Mutants reduced in VHA-c1 transcript using dsRNA-mediated interference showed reduction in root growth relative to wild-type seedlings. In contrast, VHA-c3 promoter::GUS expression was undetectable in most organs of seedlings, but strong in the root cap. Interestingly, dsRNA-mediated mutants of vha-c3 also showed reduced root length and decreased tolerance to moderate salt stress. The results suggest that V-ATPase functions in the root cap influenced root growth. Expression of VHA-c1 and VHA-c3 in tissues with active membrane flow, including root cap, vascular strands, and floral style would support a model for participation of the Vo sector and V1Vo-ATPase in membrane trafficking and fusion. Two VHA-c genes are thus differentially expressed to support growth in expanding cells and to supply increased demand for V-ATPase in cells with active exocytosis.
The Crystal Structures of Zea mays and Arabidopsis 4-Hydroxyphenylpyruvate Dioxygenase
Authors: Fritze Iris M. , Linden Lars , Freigang Jörg , Auerbach Günter , Huber Robert , Steinbacher Stefan
Published: April 2004

The transformation of 4-hydroxyphenylpyruvate to homogentisate, catalyzed by 4-hydroxyphenylpyruvate dioxygenase (HPPD), plays an important role in degrading aromatic amino acids. As the reaction product homogentisate serves as aromatic precursor for prenylquinone synthesis in plants, the enzyme is an interesting target for herbicides. In this study we report the first x-ray structures of the plant HPPDs of Zea mays and Arabidopsis in their substrate-free form at 2.0 Å and 3.0 Å resolution, respectively. Previous biochemical characterizations have demonstrated that eukaryotic enzymes behave as homodimers in contrast to prokaryotic HPPDs, which are homotetramers. Plant and bacterial enzymes share the overall fold but use orthogonal surfaces for oligomerization. In addition, comparison of both structures provides direct evidence that the C-terminal helix gates substrate access to the active site around a nonheme ferrous iron center. In the Z. mays HPPD structure this helix packs into the active site, sequestering it completely from the solvent. In contrast, in the Arabidopsis structure this helix tilted by about 60° into the solvent and leaves the active site fully accessible. By elucidating the structure of plant HPPD enzymes we aim to provide a structural basis for the development of new herbicides.