bluuebunny/crossref_metadata_2025 huggingface dataset:
The Cyanobacterial Thioredoxin Gene Is Required for Both Photoautotrophic and Heterotrophic Growth
Authors: Navarro F. , Florencio F. J.
Published: July 2002

The gene encoding thioredoxin in the facultative heterotrophic cyanobacterium Synechocystis sp. PCC 6803 (trxA) has been cloned by heterologous hybridization using the corresponding gene trxM from the cyanobacterium Anacystis nidulans as a probe. The deduced amino acid sequence of trxA predicts a protein of relative molecular weight of 11,750 and has strong identity with its cyanobacterial counterparts and other m-type thioredoxins of photosynthetic eukaryotes. The trxA gene has been expressed in Escherichia coli as a functional protein of 12 kD and has been shown by western blot analysis to be the same size as in Synechocystis. The trxA gene is transcribed in Synechocystis as a single transcript of 450 nucleotides and accumulates to the same level under photo synthetic and heterotrophic growth conditions. The trxA gene was inactivated with a kanamycin-resistant cassette, and total segregation of the disrupted trxA gene was obtained only when the trxM gene from A. nidulans (E.D.G. Muller, B.B. Buchanan [1989] J Biol Chem 264: 4008–4014) was simultaneously expressed in Synechocystis. Our results suggest an essential role of thioredoxin not only when cells grow photosynthetically but also under heterotrphic conditions.
Composition of Photosystem II Antenna in Light-Harvesting Complex II Antisense Tobacco Plants at Varying Irradiances
Authors: Flachmann R.
Published: July 2002

Plants with genes coding for chlorophyll a/b-binding proteins of light-harvesting complex II (LHCII) in antisense orientation (Lhcb) that are characterized by severely reduced Lhcb transcript levels (below 10% of wild type) do not show a bleached phenotype due to a specific loss of the polypeptide. To produce such a phenotype, a conceptually different antisense approach was tested with a dual-functional transcript encoding the gene for hygromycin phosphotransferase and the transit sequence of Lhcb1–2 in the antisense orientation. Using increasing concentrations of hygromycin, transformants with Lhcb steady-state levels as low as 9% of wild type were regenerated and grown in a growth chamber. Together with Lhcb antisense plants obtained in an earlier study, these antisense plants were analyzed biochemically for their photosystem II (PSII) antenna composition under varying light conditions. All antisense plants showed a characteristic low-irradiance-induced increase of their PSII antenna size as determined by higher chlorophyll concentrations, an increased content of LHCII, and a constant chlorophyll b-to-lutein ratio in comparison with control plants. One to 5% of the total Lhcb transcript amount was sufficient to allow unrestricted formation of the PSII antenna at low irradiance, suggesting that LHCII biogenesis is not controlled primarily by transcription.
Negative Regulation in the Expression of a Sugar-Inducible Gene in Arabidopsis thaliana (A Recessive Mutation Causing Enhanced Expression of a Gene for β-Amylase)
Authors: Mita S. , Hirano H. , Nakamura K.
Published: July 2002

Expression of a β-amylase gene of Arabidopsis thaliana (ATβ-Amy)is regulated by sugars. We identified a mutant, hba1, in which the level of expression of ATβ-Amy in leaves of plants that had been grown in a medium with 2% sucrose was significantly higher than that in wild-type plants. Higher than wild-type levels of β-amylase in hba1 plants depended on the presence of 1 to 2% sucrose or 1% glucose in the medium, whereas leaves of mutant plants grown with higher levels of sugars had β-amylase activities similar to those in leaves of wild-type plants. The hba1 phenotype was recessive and did not affect levels of sugars and starch in leaves. It is proposed that expression of ATβ-Amy is regulated by a combination of both positive and negative factors, dependent on the level of sugars, and that HBA1 might function to maintain low-level expression of ATβ-Amy until the level of sugars reaches some high level. Results of crosses of hba1 plants with transgenic plants that harbored an ATβ-Amy:GUS transgene with 1587 bp of the 5[prime]-upstream region suggested that HBA1 affects expression of ATβ-Amy in trans. The hba1 plants also had growth defects and elevated levels of anthocyanin in their petioles. However, sugar-regulated changes in levels of several mRNAs other than β-amylase mRNA were unaffected in hba1 plants, suggesting that only a subset of sugar-regulated genes is under the control of HBA1.
The Distal Part of the Transition Zone Is the Most Aluminum-Sensitive Apical Root Zone of Maize1
Authors: Sivaguru Mayandi , Horst Walter J.
Published: July 2002

For a better understanding of Al inhibition of root elongation, knowledge of the morphological and functional organization of the root apex is a prerequisite. We developed a polyvinyl chloride-block technique to supply Al (90 μm monomeric Al) in a medium containing agarose to individual 1-mm root zones of intact seedlings of maize (Zea mays L. cv Lixis). Root elongation was measured during a period of 5 h. After Al treatment, callose (5 h) and Al (1 h) contents of individual 1-mm apical root segments were determined. For comparison, callose and Al levels were also measured in root segments after uniform Al supply in agarose blocks to the 10-mm root apex. Only applying Al to the three apical 1-mm root zones inhibited root elongation after 1 h. The order of sensitivity was 1 to 2 > 0 to 1 > 2 to 3 mm. In the 1- to 2-mm root zone high levels of Al-induced callose formation and accumulation of Al was found, independently of whether Al was applied to individual apical root zones or uniformly to the whole-root apex. We conclude from these results that the distal part of the transition zone of the root apex, where the cells are undergoing a preparatory phase for rapid elongation (F. Baluška, D. Volkmann, P.W. Barlow [1996] Plant Physiol 112: 3–4), is the primary target of Al in this Al-sensitive maize cultivar.
Fast Induction of High-Affinity HCO3− Transport in Cyanobacteria1
Authors: Sültemeyer Dieter , Klughammer Barbara , Badger Murray R. , Dean Price G.
Published: July 2002

The induction of a high-affinity state of the CO2-concentration mechanism was investigated in two cyanobacterial species, Synechococcus sp. strain PCC7002 and Synechococcus sp. strain PCC7942. Cells grown at high CO2 concentrations were resuspended in low-CO2 buffer and illuminated in the presence of carbonic anhydrase for 4 to 10 min until the inorganic C compensation point was reached. Thereafter, more than 95% of a high-affinity CO2-concentration mechanism was induced in both species. Mass-spectrometric analysis of CO2 and HCO3− fluxes indicated that only the affinity of HCO3− transport increased during the fast-induction period, whereas maximum transport activities were not affected. The kinetic characteristics of CO2uptake remained unchanged. Fast induction of high-affinity HCO3− transport was not inhibited by chloramphenicol, cantharidin, or okadaic acid. In contrast, fast induction of high-affinity HCO3−transport did not occur in the presence of K252a, staurosporine, or genistein, which are known inhibitors of protein kinases. These results show that induction of high-affinity HCO3− transport can occur within minutes of exposure to low-inorganic-C conditions and that fast induction may involve posttranslational phosphorylation of existing proteins rather than de novo synthesis of new protein components.
Relationship between CO2 Assimilation, Photosynthetic Electron Transport, and Active O2 Metabolism in Leaves of Maize in the Field during Periods of Low Temperature1
Authors: Fryer Michael J. , Andrews James R. , Oxborough Kevin , Blowers David A. , Baker Neil R.
Published: July 2002

Measurements of the quantum efficiencies of photosynthetic electron transport through photosystem II (φPSII) and CO2 assimilation (φCO2) were made simultaneously on leaves of maize (Zea mays) crops in the United Kingdom during the early growing season, when chilling conditions were experienced. The activities of a range of enzymes involved with scavenging active O2 species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures during development, the ratio of φPSII/φCO2 was elevated, indicating the operation of an alternative sink to CO2 for photosynthetic reducing equivalents. The activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and superoxide dismutase and the levels of ascorbate and α-tocopherol were also elevated during chilling periods. This supports the hypothesis that the relative flux of photosynthetic reducing equivalents to O2 via the Mehler reaction is higher when leaves develop under chilling conditions. Lipoxygenase activity and lipid peroxidation were also increased during low temperatures, suggesting that lipoxygenase-mediated peroxidation of membrane lipids contributes to the oxidative damage occurring in chill-stressed leaves.
Molecular Cloning of Vacuolar H+-Pyrophosphatase and Its Developmental Expression in Growing Hypocotyl of Mung Bean1
Authors: Nakanishi Yoichi , Maeshima Masayoshi
Published: July 2002

Vacuolar proton-translocating inorganic pyrophosphatase and H+-ATPase acidify the vacuoles and power the vacuolar secondary active transport systems in plants. Developmental changes in the transcription of the pyrophosphatase in growing hypocotyls of mung bean (Vigna radiata) were investigated. The cDNA clone for the mung bean enzyme contains an uninterrupted open reading frame of 2298 bp, coding for a polypeptide of 766 amino acids. Hypocotyls were divided into elongating and mature regions. RNA analysis revealed that the transcript level of the pyrophosphatase was high in the elongating region of the 3-d-old hypocotyl but was extremely low in the mature region of the 5-d-old hypocotyl. The level of transcript of the 68-kD subunit of H+-ATPase also decreased after cell maturation. In the elongating region, the proton-pumping activity of pyrophosphatase on the basis of membrane protein was 3 times higher than that of H+-ATPase. After cell maturation, the pyrophosphatase activity decreased to 30% of that in the elongating region. The decline in the pyrophosphatase activity was in parallel with a decrease in the enzyme protein content. These findings indicate that the level of the pyrophosphatase, a main vacuolar proton pump in growing cells, is negatively regulated after cell maturation at the transcriptional level.
Tomato Flower Abnormalities Induced by Low Temperatures Are Associated with Changes of Expression of MADS-Box Genes1
Authors: Lozano Rafael , Angosto Trinidad , Gómez Pedro , Payán Carmen , Capel Juan , Huijser Peter , Salinas Julio , Martı́nez-Zapater José M.
Published: July 2002

Flower and fruit development in tomato (Lycopersicon esculentum Mill.) were severely affected when plants were grown at low temperatures, displaying homeotic and meristic transformations and alterations in the fusion pattern of the organs. Most of these homeotic transformations modified the identity of stamens and carpels, giving rise to intermediate organs. Complete homeotic transformations were rarely found and always affected organs of the reproductive whorls. Meristic transformations were also commonly observed in the reproductive whorls, which developed with an excessive number of organs. Scanning electron microscopy revealed that meristic transformations take place very early in the development of the flower and are related to a significant increase in the floral meristem size. However, homeotic transformations should occur later during the development of the organ primordia. Steady-state levels of transcripts corresponding to tomato MADS-box genesTM4, TM5, TM6, and TAG1 were greatly increased by low temperatures and could be related to these flower abnormalities. Moreover, in situ hybridization analyses showed that low temperatures also altered the stage-specific expression ofTM4.
Biochemical Analysis of Plant Protection Afforded by a Nonpathogenic Endophytic Mutant of Colletotrichum magna1
Authors: Redman Regina S. , Freeman Stanley , Clifton David R. , Morrel Jed , Brown Gayle , Rodriguez Rusty J.
Published: July 2002

A nonpathogenic mutant ofColletotrichum magna (path-1) was previously shown to protect watermelon (Citrullus lanatus) and cucumber (Cucumis sativus) seedlings from anthracnose disease elicited by wild-type C. magna. Disease protection was observed in stems of path-1-colonized cucurbits but not in cotyledons, indicating that path-1 conferred tissue-specific and/or localized protection. Plant biochemical indicators of a localized and systemic (peroxidase, phenylalanine ammonia-lyase, lignin, and salicylic acid) “plant-defense” response were investigated in anthracnose-resistant and -susceptible cultivars of cucurbit seedlings exposed to four treatments: (1) water (control), (2) path-1 conidia, (3) wild-type conidia, and (4) challenge conditions (inoculation into path-1 conidia for 48 h and then exposure to wild-type conidia). Collectively, these analyses indicated that disease protection in path-1-colonized plants was correlated with the ability of these plants to mount a defense response more rapidly and to equal or greater levels than plants exposed to wild-type C. magna alone. Watermelon plants colonized with path-1 were also protected against disease caused by Colletotrichumorbiculare and Fusarium oxysporum. A model based on the kinetics of plant-defense activation is presented to explain the mechanism of path-1-conferred disease protection.
Arabidopsis Sec21p and Sec23p Homologs. Probable Coat Proteins of Plant COP-Coated Vesicles1
Authors: Movafeghi Ali , Happel Nicole , Pimpl Peter , Tai Gui-Hua , Robinson David G.
Published: July 2002

Intracellular protein transport between the endoplasmic reticulum (ER) and the Golgi apparatus and within the Golgi apparatus is facilitated by COP (coatprotein)-coated vesicles. Their existence in plant cells has not yet been demonstrated, although the GTP-binding proteins required for coat formation have been identified. We have generated antisera against glutathione-S-transferase-fusion proteins prepared with cDNAs encoding the Arabidopsis Sec21p and Sec23p homologs (AtSec21p and AtSec23p, respectively). The former is a constituent of the COPI vesicle coatomer, and the latter is part of the Sec23/24p dimeric complex of the COPII vesicle coat. Cauliflower (Brassica oleracea) inflorescence homogenates were probed with these antibodies and demonstrated the presence of AtSec21p and AtSec23p antigens in both the cytosol and membrane fractions of the cell. The membrane-associated forms of both antigens can be solubilized by treatments typical for extrinsic proteins. The amounts of the cytosolic antigens relative to the membrane-bound forms increase after cold treatment, and the two antigens belong to different protein complexes with molecular sizes comparable to the corresponding nonplant coat proteins. Sucrose-density-gradient centrifugation of microsomal cell membranes from cauliflower suggests that, although AtSec23p seems to be preferentially associated with ER membranes, AtSec21p appears to be bound to both the ER and the Golgi membranes. This could be in agreement with the notion that COPII vesicles are formed at the ER, whereas COPI vesicles can be made by both Golgi and ER membranes. Both AtSec21p and AtSec23p antigens were detected on membranes equilibrating at sucrose densities equivalent to those typical for in vitro-induced COP vesicles from animal and yeast systems. Therefore, a further purification of the putative plant COP vesicles was undertaken.
Purification and Partial Characterization of a Dehydrin Involved in Chilling Tolerance during Seedling Emergence of Cowpea1
Authors: Ismail Abdelbagi M. , Hall Anthony E. , Close Timothy J.
Published: July 2002

Dehydrins are a family of proteins (LEA [late-embryogenesisabundant] D11) commonly induced by environmental stresses associated with low temperature or dehydration and during seed maturation drying. Our previous genetic studies suggested an association of an approximately 35-kD protein (by immunological evidence a dehydrin) with chilling tolerance during emergence of seedlings of cowpea (Vigna unguiculata) line 1393-2-11. In the present study we found that the accumulation of this protein in developing cowpea seeds is coordinated with the start of the dehydration phase of embryo development. We purified this protein from dry seeds of cowpea line 1393-2-11 by using the characteristic high-temperature solubility of dehydrins as an initial enrichment step, which was followed by three chromatography steps involving cation exchange, hydrophobic interaction, and anion exchange. Various characteristics of this protein confirmed that indeed it is a dehydrin, including total amino acid composition, partial amino acid sequencing, and the adoption of α-helical structure in the presence of sodium dodecyl sulfate. The propensity of dehydrins to adopt α-helical structure in the presence of sodium dodecyl sulfate, together with the apparent polypeptide adhesion property of this cowpea dehydrin, suggests a role in stabilizing other proteins or membranes. Taken together, the genetic, physiological, and physicochemical data are at this stage consistent with a cause-and-effect relationship between the presence in mature seeds of the approximately 35-kD dehydrin, which is the product of a single member of a multigene family, and an increment of chilling tolerance during emergence of cowpea seedlings.
Does Free-Air Carbon Dioxide Enrichment Affect Photochemical Energy Use by Evergreen Trees in Different Seasons? A Chlorophyll Fluorescence Study of Mature Loblolly Pine1
Authors: Hymus Graham J. , Ellsworth David S. , Baker Neil R. , Long Stephen P.
Published: July 2002

Previous studies of the effects of growth at elevated CO2 on energy partitioning in the photosynthetic apparatus have produced conflicting results. The hypothesis was developed and tested that elevated CO2increases photochemical energy use when there is a high demand for assimilates and decreases usage when demand is low. Modulated chlorophyll a fluorescence and leaf gas exchange were measured on needles at the top of a mature, 12-m loblolly pine (Pinus taeda L.) forest. Trees were exposed to ambient CO2 or ambient plus 20 Pa CO2 using free-air CO2 enrichment. During April and August, periods of shoot growth, light-saturated photosynthesis and linear electron transport were increased by elevated CO2. In November, when growth had ceased but temperatures were still moderate, CO2treatment had no significant effect on linear electron transport. In February, when low temperatures were likely to inhibit translocation, CO2 treatment caused a significant decrease in linear electron transport. This coincided with a slower recovery of the maximum photosystem II efficiency on transfer of needles to the shade, indicating that growth in elevated CO2 induced a more persistent photoinhibition. Both the summer increase and the winter decrease in linear electron transport in elevated CO2resulted from a change in photochemical quenching, not in the efficiency of energy transfer within the photosystem II antenna. There was no evidence of any effect of CO2 on photochemical energy sinks other than carbon metabolism. Our results suggest that elevated CO2 may increase the effects of winter stress on evergreen foliage.
Changes in Cytosolic pH within Arabidopsis Root Columella Cells Play a Key Role in the Early Signaling Pathway for Root Gravitropism
Authors: Scott Amie Caroline , Allen Nina Strömgren
Published: July 2002

Ratiometric wide-field fluorescence microscopy with 1′,7′- bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF)-dextran demonstrated that gravistimulation leads to rapid changes in cytoplasmic pH (pHc) in columella cells of Arabidopsis roots. The pHc of unstimulated columella cells in tiers 2 and 3, known sites of graviperception (E.B. Blancaflor, J.B. Fasano, S. Gilroy [1998] Plant Physiol 116: 213–222), was 7.22 ± 0.02 pH units. Following gravistimulation, the magnitude and direction of pHc changes in these cells depended on their location in the columella. Cells in the lower side of tier 2 became more alkaline by 0.4 unit within 55 s of gravistimulation, whereas alkalinization of the cells on the upper side was slower (100 s). In contrast, all cells in tier 3 acidified by 0.4 pH unit within 480 s after gravistimulation. Disrupting these pHc changes in the columella cells using pHc modifiers at concentrations that do not affect root growth altered the gravitropic response. Acidifying agents, including bafilomycin A1, enhanced curvature, whereas alkalinizing agents disrupted gravitropic bending. These results imply that pHc changes in the gravisensing cells and the resultant pH gradients across the root cap are important at an early stage in the signal cascade leading to the gravitropic response.
Abscisic Acid Inhibition of Radicle Emergence But Not Seedling Growth Is Suppressed by Sugars
Authors: Finkelstein Ruth R. , Lynch Tim J.
Published: July 2002

Low concentrations of sugars altered the sensitivity of seed germination to inhibition by exogenous abscisic acid (ABA). Germination of wild-type and ABA-insensitive (abi) Arabidopsis seeds was assayed on media containing ABA and a variety of sugars and sugar alcohols. The inhibitory effects of ABA were strongly repressed in the presence of 15 to 90 mm glucose (Glc), sucrose, or fructose, but not by comparable concentrations of sorbitol or mannitol. Several features of the response to Glc are inconsistent with a purely nutritional effect: The optimal sugar concentration is low and differs between the wild type and the abi mutants. Furthermore, Glc suppression of ABA inhibition is light dependent and limited to the process of radicle emergence.
A Stress-Inducible Gene for 9-cis-Epoxycarotenoid Dioxygenase Involved in Abscisic Acid Biosynthesis under Water Stress in Drought-Tolerant Cowpea
Authors: Iuchi Satoshi , Kobayashi Masatomo , Yamaguchi-Shinozaki Kazuko , Shinozaki Kazuo
Published: July 2002

Four cDNA clones named CPRD (cowpea responsive to dehydration) corresponding to genes that are responsive to dehydration were isolated using differential screening of a cDNA library prepared from 10-h dehydrated drought-tolerant cowpea (Vigna unguiculata) plants. One of the cDNA clones has a homology to 9-cis-epoxycarotenoid dioxygenase (named VuNCED1), which is supposed to be involved in abscisic acid (ABA) biosynthesis. The GST (glutathioneS-transferase)-fused protein indicates a 9-cis-epoxycarotenoid dioxygenase activity, which catalyzes the cleavage of 9-cis-epoxycarotenoid. The N-terminal region of the VuNCED1 protein directed the fused sGFP (synthetic green-fluorescent protein) into the plastids of the protoplasts, indicating that the N-terminal sequence acts as a transit peptide. Both the accumulation of ABA and expression of VuNCED1 were strongly induced by drought stress in the 8-d-old cowpea plant, whereas drought stress did not trigger the expression of VuABA1 (accession no.AB030295) gene that encodes zeaxanthin epoxidase. These results indicate that the VuNCED1 cDNA encodes a 9-cis-epoxycarotenoid dioxygenase and that its product has a key role in the synthesis of ABA under drought stress.
Alteration of the Adenine Nucleotide Response and Increased Rubisco Activation Activity of Arabidopsis Rubisco Activase by Site-Directed Mutagenesis
Authors: Kallis Russell P. , Ewy Robert G. , Portis Archie R.
Published: July 2002

Arabidopsis Rubisco was activated in vitro at rates 2- to 3-fold greater by recombinant Arabidopsis 43-kD Rubisco activase with the amino acid replacements Q111E and Q111D in a phosphate-binding loop, G-G-K-G-Q-G-K-S. However, these two mutant enzymes had only slightly greater rates of ATP hydrolysis. Activities of the Q111D enzyme were much less sensitive and those of Q111E were somewhat less sensitive to inhibition by ADP. Both mutant enzymes exhibited higher Rubisco activation activities over the physiological range of ADP to ATP ratios. Enzymes with non-polar, polar, and basic residues substituted at position Gln-111 exhibited rates of Rubisco activation less than the wild-type enzyme. Estimates of the relative affinity of the wild type and the Q111D, Q111E, and Q111S enzymes for adenosine nucleotides by a variety of methods revealed that the nucleotide affinities were the most diminished in the Q111D enzyme. The temperature stability of the Q111D and Q111E enzymes did not differ markedly from that of the 43-kD recombinant wild-type enzyme, which is somewhat thermolabile. The Q111D and Q111E enzymes, expressed in planta, may provide a means to better define the role of the ADP to ATP ratio in the regulation of Rubisco activation and photosynthesis rate.
Post-Transcriptional Maturation of the S Receptor Kinase ofBrassicaCorrelates with Co-Expression of theS-Locus Glycoprotein in the Stigmas of TwoBrassicaStrains and in Transgenic Tobacco Plants
Authors: Dixit Ram , Nasrallah Mikhail E. , Nasrallah June B.
Published: July 2002

The S-locus-encoded S receptor kinase (SRK) is an intrinsic plasma membrane protein that is viewed as the primary stigma determinant of specificity in the self-incompatibility response ofBrassica spp. We analyzed two self-compatible mutant strains that express low levels of the S-locus glycoprotein (SLG), a cell wall-localized protein also encoded at theS locus that is coordinately expressed with SRK. We found that mutant stigmas synthesized wild-type levels ofSRK transcripts but failed to produce SRK protein at any of the developmental stages analyzed. Furthermore, SRK was shown to form aberrant high-molecular mass aggregates when expressed alone in transgenic tobacco (Nicotiana tabacum) plants. This aggregation was prevented in tobacco plants that co-expressed SRK and SLG, but not in tobacco plants that co-expressed SRK and SLR1, an SLG-related secreted protein not encoded at the S locus. In analyses of protein extracts under reducing and non-reducing conditions, evidence of intermolecular association was obtained only for SLG, a fraction of which formed disulfide-linked oligomers and was membrane associated. The data indicate that, at least in plants carrying the S haplotypes we analyzed, SRK is an inherently unstable protein and that SLG facilitates its accumulation to physiologically relevant levels in Brassicastigmas.
Arabidopsis Species Hybrids in the Study of Species Differences and Evolution of Amphiploidy in Plants
Authors: Nasrallah Mikhail E. , Yogeeswaran Krithika , Snyder Stephen , Nasrallah June B.
Published: July 2002

It is estimated that 5 million years of evolution separateArabidopsis thaliana from its close relativeArabidopsis lyrata. The two taxa differ by many characteristics, and together they exemplify the differentiation of angiosperms into self-fertilizing and cross-fertilizing species as well as annual and perennial species. Despite their disparate life histories, the two species can be crossed to produce viable and vigorous hybrids exhibiting heterotic effects. Although pollen sterile, the hybrids produce viable ovules and were used as female parent in backcrosses to both parental species. The resulting backcross plants exhibited transgressive variation for a number of interesting developmental and growth traits as well as negative nuclear/cytoplasmic interactions. Moreover, the genesis of a fertile amphidiploid neospecies, apparently by spontaneous somatic doubling in an interspecific hybrid, was observed in the laboratory. The mechanisms responsible for the generation of amphiploids and the subsequent evolution of amphiploid genomes can now be studied through direct observation using the large arsenal of molecular tools available for Arabidopsis.
Regulation of Two Carotenoid Biosynthesis Genes Coding for Phytoene Synthase and Carotenoid Hydroxylase during Stress-Induced Astaxanthin Formation in the Green Alga Haematococcus pluvialis
Authors: Steinbrenner Jens , Linden Hartmut
Published: July 2002

Astaxanthin is a high-value carotenoid used as a pigmentation source in fish aquaculture. In addition, a beneficial role of astaxanthin as a food supplement for humans is becoming evident. The unicellular green alga Haematococcus pluvialis seems to be a suitable source for natural astaxanthin. Astaxanthin accumulation in H. pluvialis occurs in response to environmental stress such as high light and salt stress. Here, the isolation of the H. pluvialis carotenoid biosynthesis gene phytoene synthase is reported. Furthermore, the expression of phytoene synthase and carotenoid hydroxylase, two key enzymes in astaxanthin biosynthesis, was investigated at the transcriptional level. The application of environmental stress resulted in increased steady-state mRNA levels of both genes. High-light intensity led to a transient increase in carotenoid hydroxylase mRNA followed by moderate astaxanthin accumulation. In contrast, salt stress in combination with high light resulted in a sustained increase in both transcripts. The addition of compounds inducing reactive oxygen species did not influence transcript levels of phytoene synthase and carotenoid hydroxylase. The application of an inhibitor of photosynthesis, 3-(3, 4-dichlorophenyl)-1,1-dimethylurea, indicated that the light-induced expression of these carotenoid biosynthesis genes may be under photosynthetic control.
Advanced solution scattering data analysis methods and their applications
Authors: Svergun D. I.
Published: July 2002

A method for ab initio low-resolution shape and internal structure retrieval from contrast variation solution scattering data is described. The method uses a multiphase model of a particle build from densely packed dummy atoms and employs simulated annealing to find a compact interconnected configuration of phases that fits the available experimental data. In the particular case of a single phase particle (shape determination) the method is compared to another ab initio method using low resolution envelope functions. Examples of the shape determination of several proteins from experimental X-ray scattering data are presented.
Novel detector systems for time resolved SAXS experiments
Authors: Menk R.H. , Sarvestani A. , Amenitsch H. , Bernstorff S. , Besch H.J. , Orthen A. , Pavel N. , Rappolt M. , Sauer N. , Walenta A.H.
Published: July 2002

Two novel detector concepts will be presented which together satisfy, in principle, most of the requirements of modern diffraction experiments with synchrotron radiation. One is based on a gaseous single photon counter with asynchronous read out and interpolating position encoding, combining the advantages of a pure pixel read out (high local and global rate capability) and of a projecting read out (small number of channels). In order to demonstrate the suitability of this detector for X-ray diffraction applications, measurements at the synchrotron radiation source Elettra (Trieste, Italy) have been performed with the prototype (140 x 140 pixels) recording diffraction patterns from different biological samples (a phospholipid and a protein crystal). These measurements have proven the good spatial resolution, the high intensity precision and the high local rate capability. Moreover, the single photon read out was utilized in order to perform highly time-resolved measurements in case of SAXS studies and to apply fine phi-slicing in case of protein crystallography. The other detector system is a highly segmented one-dimensional prototype ionization chamber with an active area of 5 * 30 mm2. Fast recording sequences in the order of 200 µs are ensured by a shielding grid, which is based on the principle of the recently invented MicroCAT structure. The grid enables new modes of operation such as gas amplification in combination with integration. In this fashion imaging on a sub photon noise level with respect to the integration time is possible. A continuous transition from integrating mode to single photon counting mode results in a huge dynamic range that covers at least 8 orders of magnitude. Preliminary experiments on biological samples will be presented.
Global least-squares determination of Eulerian angles from single electron diffraction patterns of tilted crystals
Authors: Dimmeler Eva , Schröder Rasmus R.
Published: July 2002

Three-dimensional structure determination using electron diffraction of crystalline samples necessitates the determination of the Eulerian angles of tilted samples. For experimental tilt series, even with approximately known tilt, the resolution of the final three-dimensional reconstructions is reduced as a result of the large errors of the refined tilt angles and crystal axes positions. The presented new least-squares procedure determines the orientation of the crystal with very high accuracy from a single electron diffraction pattern. Instead of evaluating the averaged pattern geometry, each diffraction spot position is individually included in an analytical non-linear fit. This procedure is very stable against potential experimental errors, as demonstrated by Monte Carlo simulations. As a test sample, a three-dimensional microcrystal of an organic crystal compound was used. Contrary to the conventional method, which produced erroneous Miller indices for some reflections, the indexing obtained with the new algorithm was more consistent for each individual pattern. Preliminary data from frozen hydrated protein crystals, the samples of which are beam sensitive and for which only a few patterns can be recorded from a single crystal, indicate that the new angle determination promises to be particularly beneficial under such conditions.
Time-of-flight neutron transmission diffraction
Authors: Santisteban J. R. , Edwards L. , Steuwer A. , Withers P. J.
Published: July 2002

The positions of Bragg edges in neutron transmission experiments can be defined with high accuracy using the time-of-flight (TOF) technique on pulsed neutron sources. A new dedicated transmission instrument has been developed at ISIS, the UK spallation source, which provides a precision of Δd/d≃ 10−5in the determination of interplanar distances. This is achieved by fitting a theoretical three-parameter expression to the normalized Bragg edges appearing in the TOF transmission spectra. The technique is demonstrated by experiments performed on iron, niobium and nickel powders. The applicability of using the instrument for the determination of lattice strains in materials has been investigated using a simplein situloading experiment. Details of the calibration process are presented and the dependence of the resolution and the experimental times required by the transmission geometry on the instrumental variables are studied. Finally, the requirements for a Rietveld-type refinement of transmission data and the advantages and limitations over traditional neutron diffraction peak analysis are discussed.
Determination of γ′ solution temperature in Re-rich Ni-base superalloy by small-angle neutron scattering
Authors: Strunz Pavel , Mukherji Debashis , Gilles Ralph , Wiedenmann Albrecht , Fuess Hartmut
Published: July 2002

A harmful segregation of heavy elements (e.g.W, Mo, Re) during solidification of Ni-base superalloys can only be eliminated by using a homogenizing heat treatment, which needs to be carried out in the single-phase (γ) field above the γ′ solvus temperature but below the solidus temperature. Small-angle neutron scattering (SANS) was employed forin situobservation of the dissolution of precipitates in an Re-rich superalloy. The temperature dependence of the relative volume fraction and the size distribution of smaller γ′ precipitates, and the specific surface of large inhomogeneities as well as some other parameters were determined from the two-dimensional scattering curves measured for as-cast and heat-treated samples. Overlap of the incipient melting region with the region where a certain amount of precipitates remained undissolved was observed, thus complicating a determination of the temperature at which all γ′ precipitates are already dissolved. Nevertheless, conclusions about the temperature at which the precipitates dissolve and about the temperature at which the incipient melting starts could be formulated. The total scattering probability is suggested as the measure of the overall homogeneity of the distribution of elements in the sample. The temperature dependence of this parameter indicates the optimum solution procedure.
Application of the reverse Monte Carlo method to crystalline materials
Authors: Tucker Matthew G. , Dove Martin T. , Keen David A.
Published: July 2002

An implementation of the reverse Monte Carlo (RMC) method for the study of crystalline materials from polycrystalline neutron total scattering data is presented. The new feature is that explicit account is taken of the intensities of Bragg peaks, which are extracted from the data using the Pawley method. The use of Bragg peaks ensures that the RMC models reproduce both the long-range and the short-range order reflected in the experimental data. The relative effects of different contributions to the data sets in the RMC method are assessed and successful applications are illustrated using the quartz and cristobalite polymorphs of silica as examples.
Three-dimensional imaging of crystal defects by `topo-tomography'
Authors: Ludwig W. , Cloetens P. , Härtwig J. , Baruchel J. , Hamelin B. , Bastie P.
Published: July 2002

A new technique, `topo-tomography', suitable for the characterization of the three-dimensional defect structure in bulk crystals, is proposed. The technique consists of the combination of diffraction topography and microtomography. It is applicable to high-quality single crystals, provided that the `direct image' is the dominant contrast mechanism. In this case, crystal defects give rise to locally enhanced intensity compared with the perfect-crystal matrix. The additional intensities sum along the diffracted-beam direction and yield projections of the local Bragg reflectivity. Like in the case of absorption tomography, the three-dimensional distribution of this reflectivity can be reconstructed from a large number of projections. The first experimental results of this technique, applied to a synthetic diamond crystal, are presented.
A new method for analysing peak broadening caused by compositional fluctuation in X-ray diffraction measurements
Authors: Nakashima Kiichi , Kawaguchi Yoshihiro
Published: July 2002

A new method for analysing X-ray peak broadening caused by compositional fluctuation is proposed. The method is applicable to epitaxial layers with diamond or zinc-blende structure on (001) substrates. In the new method, a rescaling procedure with a difference variable ΔAis applied to measured X-ray profiles and the dependence of the profiles on various reflection indiceshklis analysed. The theoretical formula reveals that X-ray peak profiles become independent ofhklafter the rescaling. A new criterion is proposed; an experimental examination based on the criterion makes it possible simply to judge whether or not X-ray peak broadening is caused by compositional fluctuation. The method is verified experimentally and demonstrated by applying it to an InGaAs multilayer sample having artificial compositional fluctuation.
Strain inhomogeneity mapping in single-crystal layers and membranes by X-ray diffractometry
Authors: Zsoldos Lehel , Gröger Viktor , Haugeneder Ernst
Published: July 2002

For the measurement of the strain distribution in single-crystalline thin layers, it is in principle enough to measure the shift of the angular position of a single Bragg reflection. This is usually disturbed by the curvature of the specimen. Because of the strain sensitivity of different reflections, it is possible to choose two appropriate Bragg peaks which allow the elimination of the effect of any macroscopic curvature by comparing their shifts. The strain sensitivity of a strained isotropic layer under biaxial stress is presented, together with a few examples of choosing a pair of reflections. The success of the method is demonstrated by the results of a one-dimensional scan of a strained Si membrane.
Waveguide-enhanced scattering from thin biomolecular films
Authors: Pfeiffer F. , Mennicke U. , Salditt T.
Published: July 2002

An X-ray diffraction experiment on multilamellar membranes incorporated into an X-ray waveguide structure is reported. In the device, the lipid bilayers are confined to one side by the silicon substrate and to the other side by an evaporated thin metal cap layer. Shining a highly brilliant X-ray beam onto the system, resonantly enhanced, precisely defined and clearly distinguishable standing-wavefield distributions (modes) are excited. The in-plane structure of the acyl chain ordering is then studied by grazing incidence diffraction under simultaneously excited modes. A significant gain in signal-to-noise ratio as well as enhanced spatial resolution can be obtained with such a setup.
The application of structure envelopes in structure determination from powder diffraction data
Authors: Brenner Simon , McCusker Lynne B. , Baerlocher Christian
Published: July 2002

A structure envelope is a special type of periodic nodal surface that separates regions of high electron density from those of low electron density. Once such a surface has been generated, it can be used in combination with direct-space methods to facilitate structure solution from powder data. To generate an informative structure envelope, the phases of the structure factors of a few strong low-order reflections must be determined; an algorithm has been developed for this purpose. The programSayPermcombines (a) the use of error-correcting codes (e.c.c.'s) to sample phase space efficiently, (b) a pseudo-atom approximation of structure fragments to simulate atomic resolution atca2.5 Å, and (c) phase extension and phase set ranking using the Sayre equation. The effect of using a structure envelope in structure solution was first tested in combination with a subroutine for finding zeolite topologies in the programFOCUS. Then extension to molecular structures in combination with a simulated-annealing program was explored. This resulted in the development of the programSafeand the subsequent determination of the structure of a tri-β-peptide (C32N3O6H53) with 17 variable torsion angles.
A study of the ordered structures of the Au–Mn system by high-voltage–high-resolution electron microscopy. II. Two-dimensional antiphase structures based on the DO
22 structure
Authors: Terasaki O. , Watanabe D. , Hiraga K. , Shindo D. , Hirabayashi M.
Published: July 2002

Two-dimensional antiphase domain structures existing in the composition range 20–23 at.% Mn were investigated by a high-resolution structure-imaging technique with a 1 MV electron microscope. The structures are based on the DO
22 structure and consist of parallelogram-shaped domains containing 4 × 3 columns of Mn atoms and lozenge-shaped domains with 4 × 4 and 3 × 3 columns, and the domains are separated by two-dimensional antiphase boundaries parallel to the ({\bar 2}40) and (240) planes of the fundamental face-centred structure. The configuration of the domains changes delicately with a slight change of composition or annealing temperature, and the symmetry of the structure is lowered below about 670 K. The ideal structure models have compositions of about 22.7 at.% Mn. The images of about half of the specimen area of the 22.6 at.% Mn alloy annealed at 570 K do not correspond to these new structures, but bear a resemblance to the image expected from the two-dimensional antiphase structure proposed by Watanabe [J. Phys. Soc. Jpn (1960), 15, 1030–1040] for Au3Mn, which is based on the L12 structure and has boundaries parallel to the (100) and (010) planes.
Crystal data for the oxidation product of bis(2,2'-bipyridyl)copper(I) perchlorate in nitroalkanes
Authors: Garland M. T. , Spodine E. , Zamudio W.
Published: July 2002

Single crystals of [Cu(bpy)2NO2]ClO4, [Cu(C10H8N2)2NO2]ClO4, C20H16CuN5O2.ClO4, with space group P21/c, were grown from the reacting mixture of bis(2,2′-bipyridyl)copper(I) perchlorate with molecular oxygen in nitroalkanes. The unit-cell parameters are a = 10.83 (2), b = 12.51 (1), c = 17.04 (1) Å, β = 112.0 (2)°, Z = 4, Dm
= 1.62 (1) Mg m−3.
Calculation of scattering curves for macromolecules in solution and comparison with results of methods using effective atomic scattering factors
Authors: Müller J. J.
Published: July 2002

An improved cube method was developed for the computation of X-ray scattering curves of macromolecules in solution. For double-helical DNA and RNA molecules the efficiency of this method is shown. The results are compared with curves calculated by effective atomic scattering factor methods. In the small-angle and in the wide-angle regions the improved cube method is superior to the effective atomic scattering factor methods. This was proved by the calculation of structure parameters and by a comparison with experimental scattering data. On the basis of the improved cube method, models with a reduced structure resolution are deduced for DNA and RNA molecules. The models consisting of the three scattering centres phosphate, sugar and base per nucleotide are equivalent in scattering to the real structure up to a scattering angle of about 0.15 rad for copper radiation.
On the variation of X-ray diffraction contrast with wavelength: A study with synchrotron radiation
Authors: Lang A. R. , Makepeace A. P. W. , Moore M. , Machado W. C.
Published: July 2002

Diffraction contrast phenomena on X-ray topographs taken with continuous-spectrum synchrotron radiation have been studied at wavelengths of 0.057, 0.064, 0.071, 0.100, 0.154, 0.206 and 0.250 nm. The specimen was a polished plate of natural diamond with surfaces parallel to (110), ½ mm thick. Using the {\bar 1}11 reflection and a stored electron beam energy of 1.8 GeV all topographs (except that taken with λ = 0.25 nm) were harmonic free. The specimen exhibited mixed-habit growth, containing sectors of normal faceted {111} growth and sectors of non-faceted `cuboid' growth in which growth-surface orientation was variable and only approximately parallel to {100}. Prior to X-ray topography the specimen had received localized damage from implantation with fluorine ions of 17 MeV energy. Features whose variation with wavelength was studied included (1) the relative strengths of integrated reflections from {111} and `cuboid' growth sectors, (2) the intensity of `spike' disorder diffuse reflections relative to sharp Bragg reflections, (3) contrast from inclusions, polishing striae and fracture damage and (4) lattice bending and diffraction contrast at the sites of fluorine ion implantation. Theoretical predictions of the wavelength variation of the intensity of the diffuse reflection images and of contrast due to resolved defects showed good agreement with the observations.
Crystal data on some potassium lithium metal sulphides (KLiMS2, M being Zn, Mn or Fe)
Authors: Nicholas D. M. , Whaite M. S. , Ho T. M.
Published: July 2002

Compounds of the formula KLiMS2, where M is one of the divalent transition elements Zn, Mn or Fe, belong to the tetragonal crystal system. They are not, however, isomorphous. KLiZnS2: P, a = 6.600(5), c = 15.570(5) Å; KLiMnS2: P, a = 6.800(5), c = 11.055(5) Å; KLiFeS2: I, a = 3.840(5), c = 13.270(5) Å. X-ray powder diffraction data for all three compounds are given. The JCPDS Diffraction File Nos. for these compounds are: KLiZnS2 33-1494; KLiMnS2 33-1493; KLiFeS2 33-1492.
The estimation of standard deviations in powder diffraction Rietveld refinements
Authors: Scott H. G.
Published: July 2002

It is shown that when the observed and calculated intensities in a powder diffraction pattern differ because of model errors which affect the calculated integrated intensities, in addition to the random counting errors, Rietveld refinement with weights determined solely by counting statistics yields estimated standard deviations for the crystal structure parameters which tend to be measures of the precision rather than the accuracy of these parameters. Under these conditions, e.s.d.'s calculated by Rietveld refinement may be significantly smaller than those obtained by integrated intensity refinement of the same data set. A method of adjusting Rietveld e.s.d.'s, to provide comparability with integrated intensity refinement, is proposed.