bluuebunny/crossref_metadata_2025 huggingface dataset:
Metabolism of hydrogen peroxide by the scavenging system in Chlamydomonas reinhardtii
Authors: Takeda Toru , Ishikawa Takahiro , Shigeoka Shigeru
Published: May 2006

The metabolism of hydrogen peroxide by the scavenging system was studied in Chlamydomonas grown in a selenium‐lacking and a selenium‐containing medium. In cells of the former, 40% of external hydrogen peroxide (H2O2) was scavenged by ascorbate peroxidase (AsAP; EC 1.11.1.11) and the residual H2O2 by catalase (EC 1.11.1.6). The enzymes involved in the ascorbate‐glutathione cycle including AsAP. were localized in the chloroplast. In cells of the latter, glutathione peroxidase (GSHP; EC 1.11.1.9) functioned primarily in the removal of external H2O2. GSHP was located solely in the cytosol. The Chlamydomonas AsAP was relatively stable in ascorbate‐depleted medium as compared with chloroplast AsAP of higher plants. No inactivation of the enzyme was found upon its incubation with hydroxyurea, an inhibitor of the chloroplast enzyme of higher plants. The enzyme showed higher specificity with pyrogallol than with ascorbate. The amino acid sequences in the N‐terminal region of Chlamvdomonas AsAP showed no significant similarity to any other AsAP from higher plants and Euglena. The enzyme had a molecular mass of 34 kDa. The Km values of the enzyme for ascorbate and H2O2 were 5.2±0.3 and 25±3.4 μM, respectively. Hydrogen peroxide was generated at a rate of 6.1±0.8 μmol mg‐1 chlorophyll h‐1 in intact chloroplasts isolated from Chlamydomonas cells grown in the presence of Na‐selenite, and it diffused from the organelles into the medium.
Photosynthesis and inorganic carbon acquisition in the cyanobacterium Chlorogloeopsis sp. ATCC 27193
Authors: Skleryk Robert S. , Tyrrell Pascal N. , Espie George S.
Published: May 2006

The ability of the morphologically complex cyanobacterium Chlorogloeopsis sp. ATCC 27193 to actively transport and accumulate inorganic carbon (C1= CO2+ HCO3−+ CO32−) for photosynthetic CO2 fixation was investigated. Mass‐spectrometric assays revealed that Chlorogloeopsis cells grown under C1 limitation rapidly took up CO2 from the medium in a light‐dependent reaction which was independent of CO2 fixation. Ethoxyzolamide, a carbonic anhydrase (CA) inhibitor, inhibited CO2 transport. Since electrometric and mass‐spectrometric assays did not detect the presence of a periplasmic CA, it is suggested that CO2 transport was mediated by a CA‐like activity which converted CO2 to HCO3− during passage across the membrane. Radiochemical assays, using H14CO3 as substrate, showed that C3‐limited cells also had a high affinity (K0.5 HCO3−= 37 μM), Na+‐independent HCO3− uptake mechanism. HCO3−uptake was light dependent and occurred against its electrochemical potential indicating a carrier‐mediated, active transport process. The rate of Na+‐independent HCO3− transport was sufficient to account for the steady state rate of CO2 fixation. Although not absolutely required. Na+ did specifically enhance the rate of HCO3− transport by up to 2‐fold, but had no effect on the apparent affinity of the transport system for HCO3− Combined CO2 and HCO3− transport resulted in C1 accumulation as high as 25 mM and in excess of 300 times the external concentration. The C1 pool was the source of CO2 for photo‐synthetic fixation and was generated, presumably, by the dehydration of HCO3− catalyzed by an intracellular CA. The collective evidence indicates that Chlorogloeopsis has a physiologically functional CO2‐concentrating mechanism which is essential for photosynthesis.
Mechanistic aspects of xanthophyll cycle‐dependent photoprotection in higher plant chloroplasts and leaves
Authors: Gilmore Adam M.
Published: May 2006

Higher plants must dissipate absorbed light energy that exceeds the photosynthetic capacity to avoid molecular damage to the pigments and proteins that comprise the photosynthetic apparatus. Described in this minireview is a current view of the biochemical, biophysical and bioenergetic aspects of the primary photoprotective mechanism responsible for dissipating excess excitation energy as heat from photosystem II (PSII). The photoprotective heat dissipation is measured as nonphotochemical quenching (NPQ) of the PSII chlorophyll a (Chl a) fluorescence. The NPQ mechanism is controlled by the trans‐thylakoid membrane pH gradient (ΔpH) and the special xanthophyll cycle pigments. In the NPQ mechanism, the de‐epoxidized endgroup moieties and the trans‐thylakoid membrane orientations of antheraxanthin (A) and zeaxanthin (Z) strongly affect their interactions with protonated chlorophyll binding proteins (CPs) of the PSII inner antenna. The CP protonation sites and steps are influenced by proton domains sequestered within the proteo‐lipid core of the thylakoid membrane. Xanthophyll cycle enrichment around the CPs may explain why changes in the peripheral PSII antenna size do not necessarily affect either the concentration of the xanthophyll cycle pigments on a per PSII unit basis or the NPQ mechanism. Recent time‐resolved PSII Chi a fluorescence studies suggest the NPQ mechanism switches PSII units to an increased rate constant of heat dissipation in a series of steps that include xanthophyll de‐epoxidation, CP‐protonation and binding of the xanthophylls to the protonated CPs; the concerted process can be described with a simple two‐step, pH‐activation model. The xanthophyll cycle‐dependent NPQ mechanism is profoundly influenced by temperatures suboptimal for photosynthesis via their effects on the trans‐thylakoid membrane energy coupling system. Further, low temperature effects can be grouped into either short term (minutes to hours) or long term (days to seasonal) series of changes in the content and composition of the PSII pigment‐proteins. This minireview concludes by briefly highlighting primary areas of future research interest regarding the NPQ mechanism.
Calcium ions as second messengers in guard cell signal transduction
Authors: McAinsh Martin R. , Brownlee Colin , Hetherington Alistair M.
Published: May 2006

Ca2+ is a ubiquitous second messenger in plant cell signalling. In this review we consider the role of Ca2+‐based signal transduction in stomatal guard cells focusing on three important areas: (1) the regulation of guard cell turgor relations and the control of gene expression in guard cells, (2) the control of specificity in Ca2+ signalling, (3) emerging technologies and new approaches for studying intracellular signalling. Stomatal apertures alter in response to a wide array of environmental stimuli as a result of changes in guard cell turgor. For example, the plant hormone abscisic acid (ABA) stimulates a reduction in stomatal aperture through a decrease in guard cell turgor. Furthermore, guard cells have been shown to be competent to relay an ABA signal from its site of perception to the nucleus. An increase in the concentration of cytosolic free Ca2+ ([Ca2+]1) is central to the mechanisms underlying ABA‐induced changes in guard cell turgor. We describe a possible model of Ca2+‐based ABA signal transduction during stomatal closure and discuss recent evidence which suggests that Ca2+ is also involved in ABA nuclear signal transduction. Many other environmental stimuli which affect stomatal apertures, in addition to ABA, induce an increase in guard cell [Ca2+]1) This raises questions regarding how increases in [Ca2+]1) can be a common component in the signal transduction pathways by which stimuli cause both stomatal opening and closure. We discuss several mechanisms of increasing the amount of information contained within the Ca2+ signal, including encoding information in a stimulus‐specific Ca2+ signal or Ca2+ signature', the concept of the ‘physiological address’ of the cell, and the use of other second messengers. We conclude by addressing the emerging technologies and new approaches which can be used in conjunction with guard cells to dissect further the molecular mechanisms of Ca2+‐mediated signalling in plants.
Blue light‐dependent monovalent anion uptake
Authors: Quiñones Miguel A. , Giráldez Nuria , Witt Federico G. , Aparicio Pedro J.
Published: May 2006

Blue light is one of the most important environmental signals regulating monovalent anion transport in plant cells. In the unicellular freshwater chlorophyte Monoraphidium braunii, blue light is essential for the activation of HCO3−, NO3−, NO22 and Cl− transport systems. These anions are taken up when blue light is present but the uptake ceases when this radiation is suppressed, indicating that blue light is a switch signal for the monovalent anion transport system(s) of this alga. Similar results have been obtained in other green algae and higher plants. The action spectra for the uptake of NO3− and Cl− in M. braunii are very similar and resemble the absorption spectra of flavins or a combination of flavins and pterins. It is proposed that both anions share the same transport system(s). The uptake of monovalent anions consists of a cotransport with H+, thus producing alkalinization of the external medium. The time between the onset of blue light and the beginning of alkalinization can be as short as 2 s. Taken together, the results suggest that the photoreceptor mediating the blue light activation of monovalent anion uptake in this green alga is a plasma membrane‐bound flavoprotein.
Lipid‐transfer proteins: Tools for manipulating membrane lipids
Authors: Bourgis Fabienne , Kader Jean‐Claude
Published: May 2006

Like other eukaryotic cells, plant cells contain proteins able to bind or to transfer lipids. Since they are able to facilitate movements of various phospholipids between membranes and are also capable of binding fatty acids or acyl‐CoAs, they have been termed lipid‐transfer proteins (LTP). LTPs are basic proteins containing 90 to 95 residues (molecular mass 9 kDa), eight of them being cysteines found in conserved locations. These proteins have been used to manipulate in vitro the lipid composition of isolated membranes either from plant or mammalian sources. In addition to purified LTPs, recombinant LTPs produced by genes expressed in microorganisms can be used for this purpose. Several genes coding for these proteins have been characterized in various plants with different patterns of expression. However, it remains to be investigated whether these recombinant proteins behave functionally as LTPs. The use of purified or recombinant LTPs is promising for the study of the effect of lipid composition on membrane functional properties.
The oxidation of cytosolic NAD(P)H by external NAD(P)H dehydrogenases in the respiratory chain of plant mitochondria
Authors: Møller Ian M.
Published: May 2006

The respiratory chain of plant mitochondria differs from that in mammalian mitochondria by containing several rotenone‐insensitive NAD(P)H dehydrogenases. Two of these are located on the outer, cytosolic surface of the inner membrane. One is specific for NADH, the other for NADPH. Only the latter is inhibited by diphenyleneiodonium (DPI). Both of these enzymes are normally dependent upon Ca2+ for activity and this constitutes a potentially important mechanism by which the cell can regulate the oxidation of cytosolic NAD(P)H via the concentration of free Ca2+. This and other potential regulatory mechanisms such as the substrate concentration and polyamines are discussed.
Tonoplast H+ pumps are activated during callus formation of tuber tissues of Jerusalem artichoke (Helianthus tuberosus)
Authors: Ueoka Hanayo , Hase Akira
Published: May 2006

Changes in tonoplast H+‐ATPase (EC 3.6.1.3) and H+–PPase (EC 3.6.1.1) activities were examined during the early period of callus formation in tuber tissues of Jerusalem artichoke (Helianthus tuberosus L.). In callus‐forming tissues cultured on a medium containing 2,4‐D, the ATP‐dependent H+‐translocation activity of tonoplast vesicles increased 3‐fold after a 2‐day lag phase, while the ATP‐hydrolytic activity and amount of tonoplast H+‐ATPase protein were relatively constant after the lag phase. In the control tissue disks cultured on a medium free of 2,4‐D, large declines in ATP‐hydrolytic and ATP‐dependent H+‐translocation activities were observed. By contrast, the PP‐dependent H+‐translocation activity of tonoplast vesicles increased about 8‐fold during the first 3 days of culture without any lag phase, and regardless of the presence of 2,4‐D in the culture medium. However, the PP‐hydrolytic activity and amount of H+‐PPase protein did not change during the culture period, independently of callus formation. Transfer of the control tissue disks to the 2,4‐D‐containing medium, however, resulted in a further rapid stimulation of PP‐dependent H+‐translocation as well as an activation of ATP‐dependent H+‐translocation. These results suggest that both tonoplast H+ pumps are involved in callus formation of tuber tissues of Jerusalem artichoke.
The role of temperature in determining the stimulation of CO2 assimilation at elevated carbon dioxide concentration in soybean seedlings
Authors: Ziska Lewis H. , Bunce James A.
Published: May 2006

Soybean (Glycine max cv. Clark) was grown at both ambient (ca 350 μmol mol−1) and elevated (ca 700 μmol mol−1) CO2 concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 35 and 40°C) for 17–22 days after sowing to determine the interaction between temperature and CO2 concentration on photosynthesis (measured as A, the rate of CO2 assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demonstrated increasingly greater stimulation of A at elevated CO2 as temperature increased from 25 to 35°C (i.e. optimal growth rates). At 40°C, primary leaves failed to develop and plants eventually died. In contrast, for both whole plant A and total biomass production, increasing temperature resulted in less stimulation by elevated CO2 concentration. For whole plants, increased CO2 stimulated leaf area more as growth temperature increased. Differences between the response of A to elevated CO2 for single leaves and whole plants may be related to increased self‐shading experienced by whole plants at elevated CO2 as temperature increased. Results from the present study suggest that self‐shading could limit the response of CO2 assimilation rate and the growth response of soybean plants if temperature and CO2 increase concurrently, and illustrate that light may be an important consideration in predicting the relative stimulation of photosynthesis by elevated CO2 at the whole plant level.
A sunflower leaf antifungal peptide active against Sclerotinia sclerotiorum
Authors: Regente Mariana C. , Oliva Claudia R. , Feldman Mariana L. , Castagnaro Atilio P. , de la Canal Laura
Published: May 2006

A 5‐kDa antifungal peptide (APS) was isolated from Helianthus annum L. (line HA89) leaves infected with a virulent isolate of Sclerotinia sclerotiorum (Lib.) de Bary. AP5 was purified by gel filtration, cation exchange chromatography and reverse phase FPLC and HPLC. This peptide in vitro inhibits ascospores germination of the fungal pathogen S. sclerotiorum or produces mycelial growth inhibition, depending on its concentration. The effective concentration of AP5 giving 50% growth inhibition (IC50) against S. sclerotiorum was 0.4 μM. The antifungal efficacy of AP5 is higher than that of other antimicrobial proteins already described that have no appreciable effect on S. sclemtiorum below 4 μM. The relevance of this finding with regard to the function of AP5 in sunflower resistance to pathogens is discussed.
Photoinactivation and photoprotection of photosystem II in nature
Authors: Anderson Jan M. , Park Y.‐I. , Chow W. S.
Published: May 2006

Photosystem II plays a central role not only in energy transduction, but also in monitoring the molecular redox mechanisms involved in signal transduction for acclimation to environmental stresses. Central to the regulation of photosystem II (PSII) function as a light‐driven molecular machine in higher plant leaves, is an inevitable photo‐inactivation of one PSII after 106–107 photons have been delivered to the leaf, although the act of photoinactivation per se requires only one photon. PSII function in acclimated pea leaves shows a reciprocity between irradiance and the time of illumination, demonstrating that the photoinactivation of PSII is a light dosage effect, depending on the number of photons absorbed rather than the rate of photon absorption. Hence, PSII photoinactivation will occur at low as well as high irradiance. There is a heterogeneity of PSII functional stability, possibly with less stable PSII monomers being located in grana margins and more stable PSII dimers in appressed granal domains. Matching the inevitable photoinactivation of PSII, green plants have an intrinsic capacity for D1 protein synthesis to restore PSII function which is saturated at very low light. Photoinhibition of PSII in vivo is often a photoprotective strategy rather than a damaging process.
The role of glycine in determining the rate of glutathione synthesis in poplar. Possible implications for glutathione production during stress
Authors: Noctor Graham , Arisi Ana‐Carolina M. , Jouanin Lise , Valadier Marie‐Hélène , Roux Yvette , Foyer Christine H.
Published: May 2006

The terminal step of glutathione (GSH) synthesis is the condensation of γ‐glutamyl‐cysteine (γ‐EC) with glycine. Relatively little information exists concerning the importance of photorespiratory glycine in determining the rate of conversion of γ‐EC to GSH. Consequently, the effect of exogenous glycine and of illumination on foliar contents of γ‐EC and GSH was studied in excised leaves and leaf discs from untransformed poplar (Populus tremula×P. alba) and poplar overexpressing γ‐glutamylcysteine synthetase (γ‐ECS; EC 6.3.2.2). Poplars strongly overexpressing γ‐ECS (ggs28) had enhanced levels of γ‐EC and GSH compared to untransformed poplars. The relationship between γ‐EC and GSH contents in ggs28 was light dependent. In illuminated leaves, GSH contents were up to 50‐fold higher than γ‐EC. On darkening, γ‐EC accumulated markedly and GSH declined, so that the GSH:γ‐EC ratio was close to 1. These dark‐induced changes were prevented by supplying glycine through the petiole or by incubation of leaf discs on glycine. Dark accumulation of γ‐EC in leaf discs from untransformed poplar was also prevented by supplying glycine. Supplying cysteine in the dark to discs from untransformed poplar and ggs28 increased γ‐EC levels markedly but GSH levels only slightly. Subsequent illumination caused γ‐EC to decrease and GSH to increase. Supplying glycine in concert with cysteine had similar effects to illumination. The data suggest that photorespiratory glycine is essential for GSH synthesis, especially under stress conditions, where increased amounts of GSH are required.
Ozone‐induced oxidative stress: Mechanisms of action and reaction
Authors: Pell ‐Eva J. , Schlagnhaufer Carl D. , Arteca Richard N.
Published: May 2006

In this review we explore several models which might explain ozone (O3)‐induced injury to plant foliage. Ozone enters the cell through the wall and plasma membrane where active oxygen species are generated. If the concentration of O3 is very high, unregulated cell death will occur. Alternatively, the active oxygen species, or succeeding reaction products, may serve as elicitors of regulated plant responses. These regulated responses include the induction of ethylene which could serve as a primary signal for—or a facilitator of—subsequent responses. The role of regulated suppression of photosynthetic genes and induction of chitinases and β‐1,3‐glucanase in programmed cell death is explored. Induction of antioxidants, enzymes of lignification and glutathione‐S‐transferase are discussed in the context of O3‐induced cell repair or cell protection. A second model is postulated to explain induction of accelerated foliar senescence by low levels of O3. The notion that O3‐induced elicitation of responses in the nucleus might lead to increased oxidative stress in the chloroplast is considered as a mechanism for accelerating the rate of degradation of ribulose‐1,5‐bisphosphate car‐boxylase/oxygenase (Rubisco). The mechanisms by which O3 induces loss of Rubisco, and the relationship to accelerated foliar senescence are discussed.
Multiple signals and mechanisms that regulate leaf growth and stomatal behaviour during water deficit
Authors: Thompson David Stuart , Wilkinson Sally , Bacon Mark A. , Davies William J.
Published: May 2006

We highlight the novel observation that the responses of maize leaf growth to abscisic acid (ABA) signals can be amplified both by mild water deficits and by nutrient stress. Under our experimental conditions these stresses alone had no effect on leaf growth rate. In most cases leaf growth responses were not attributable to changes in the turgors of growing cells, focusing attention on a regulatory role for cell wall biochemistry. Roles for xyloglucan endotransglycosylase (XET), expansins and peroxidases are discussed. An effect of drought on the activity of expansins seems particularly attractive if xylem sap pH is considered as a chemical signal. We show how changes in xylem sap pH can also modify local accumulation of ABA and thereby modify the apparent sensitivity of guard cells to ABA signals.
Induction of pathogen resistance and pathogenesis‐related genes in tobacco by a heat‐stable Trichoderma mycelial extract and plant signal messengers
Authors: Chang Pi‐Fang Linda , Xu Yi , Narasimhan Meena L. , Cheah Kheng T. , D'Urzo Matilde Paino , Damsz Barbara , Kononowicz Andrzej K. , Abad Laura , Hasegawa Paul M. , Bressan Ray A.
Published: May 2006

Heat‐stable mycelial extracts of the nonpathogenic fungus Trichoderma longibrachiatum induced resistance in tobacco seedlings (Nicotiana tabacum L. cv. Wisconsin 38) to the pathogen Phytophthora parasitica var. nicotianae (race 0), which did not involve a hypersensitive response. Resistance could not be induced with mycelial extract prepared in the same manner from P. parasitica. The nonpathogenic mycelial extract induced expression of PR‐1b and osmotin (PR‐5) genes to a higher level than did mycelial extract from the pathogenic fungus. The tissue‐specific pattern of PR gene induction by the nonpathogenic mycelial extract was different from that of the pathogenic mycelial extract and was consistent with the ability of the former to cause disease resistance. The expression patterns of these two PR genes and the accumulations of their encoded proteins also were affected by salicylic acid (SA), methyl jasmonate (MeJA), ethylene (E) and combinations of these plant signal messengers. However, only combined SA and MeJA treatment mimicked the pattern of PR gene mRNA and protein accumulation induced by the nonpathogenic mycelial extract. E inhibitors blocked both mycelial extract‐induced and SA/MeJA‐induced PR gene expression, and the cis pattern of responsiveness on the osmotin promoter was the same for the mycelial extract, SA, E, or E/MeJA. Seedlings treated with P. parasitica spores in the presence of SA/MeJA were protected from pathogen colonization. However, these seedlings exhibited symptoms of cell death (disease symptoms) both in the absence and presence of P. parasitica spores, in contrast to seedlings treated with nonpathogenic mycelial extract, which remained healthy. These results suggest that the signal transduction pathways for elicitation of defense responses by exogenously applied heat‐stable nonpathogenic mycelial extract and SA/MeJA overlap at the point of PR protein induction but are not identical.
Strategies of ultraviolet‐B protection in microscopic algae
Authors: Xiong Fusheng , Komenda Josef , Kopecký Jiři , Nedbal Ladislav
Published: May 2006

Different species of microalgae show a wide range of susceptibility to ultraviolet‐B (UV‐B) radiation. To identify factors responsible for the UV‐B tolerance of some of the algae, we compared 8 species that are highly tolerant to UV‐B to 8 species that are highly susceptible. The tolerant species contained substantial amounts of an acetolysis‐resistant residue. The residue consists of sporopollenin, a biopolymer of variable chemical composition that occurs in the algal cell walls and absorbs UV‐B radiation. The susceptible species contained little or no sporopollenin. We propose that sporopollenin provides protection to the tolerant species by screening the incident UV‐B radiation. Previous studies showed that the mycosporine‐like amino acids (MAA) also act as effective UV‐B screens. Our data indicate that sporopollenin provides a constant protection while MAA are induced by radiation stress and occur with some delay. The tolerant species also differ from the susceptible species in their capacity to repair the reaction centers damaged by UV‐B. The tolerant algae became vulnerable to UV‐B when protein synthesis needed for repair was blocked by streptomycin. In the susceptible species, streptomycin had no effect during the UV‐B stress. The repair deficiency in the susceptible species can be explained either by relatively less effective protein synthesis or by an inhibition of the protein synthesis by UV‐B. In the tolerant species, the structures needed for protein synthesis are protected by UV‐B screening of sporopollenin and MAA.
BT1, a possible adenylate translocator, is developmentally expressed in maize endosperm but not detected in starchy tissues from several other species
Authors: Cao Heping , Shannon Jack C.
Published: May 2006

Brittle 1 protein (BT1), the major protein in amyloplast envelope membranes of wild‐type maize endosperm, is required for normal levels of starch accumulation in maize kernels. To gain information about theBt1locus in control of starch synthesis in plants, we examinedBt1expression in kernels from different ages and from different genotypes of maize as well as in starchy tissues from different species. Both BT1 protein andBt1transcripts were detected in kernels from about 10 days post‐pollination (DPP) and increased up to 14 DPP. BT1 protein level was similar in kernels after 14 DPP. Furthermore, membranes from amyloplasts containing larger starch granules contained more BT1 protein than those containing smaller starch granules from the same age of kernels. These results indicate thatBt1gene expression was developmentally regulated and that BT1 protein level was correlated with starch accumulation in normal maize endosperm. Unexpectedly, except for a weak band at a higher molecular mass in microsomal proteins extracted from spring barley grain which reacted with maize BT1 antibody, no BT1 protein was observed in starchy tissues from several other species, including grains of sorghum, wheat, winter barley and oats, potato tubers, pea roots and cauliflower buds. Thus either the BT1 antibody raised against a fusion protein including the 56 amino acid residues in the carboxyl terminal sequence of maize endosperm BT1 protein does not recognize BT1 homologues in amyloplasts from other tissues or species or amyloplasts from different tissues or species have alternative membrane transporters.
Flavonoid‐like compounds from seeds of red alder (Alnus rubra) influence host nodulation by Frankia (Actinomycetales)
Authors: Benoit Larry F. , Berry Alison M.
Published: May 2006

Nitrogen‐fixing root nodules are formed by Frankia spp. (Actinomycetales) on dicotyledonous hosts such as alders (Alnus spp.). Flavonoid‐containing preparations from seed washes of red alder (Alnus rubra Bong.), and individual compounds isolated from such preparations, influenced nodulation of A. rubra by Frankia. Nodulation was enhanced by one flavonoid‐like compound, and apparently inhibited by two other such compounds. Four flavonoid‐like compounds had no significant effect on nodulation. The seven individual compounds purified from the seed washes were characterized spectrally as possible flavanones and isoflavones. Both the enhancer and the inhibitors appeared to be possible flavanones.
Cloning of a full‐length symbiotic hemoglobin cDNA and in situ localization of the corresponding mRNA in Casuarina glauca root nodule
Authors: Gherbi Hassen , Duhoux Emile , Franche Claudine , Pawlowski Katharina , Nassar Atef , Berry Alison M. , Bogusz Didier
Published: May 2006

We have characterized a full‐length cDNA (hb‐Cg1F) that represents symbiotic mRNA hemoglobin (hb) from Casuarina glauca root nodules. In situ hybridization was used to examine the correlation between hb‐Cg1F mRNA and the state of the Frankia infection process. The efficiency of in situ hybridization using DIG‐labeled vs [35S]‐labeled probes was compared. The expression of hb‐Cg1F gene is induced in young infected host cells prior to the detection of Frankia nifH mRNA. Since Frankia does not form vesicles in C. glauca nodules, it is proposed that Hb is necessary to reduce the O2 concentration in the cytoplasm of the host cells before the nif genes are expressed.
C2H2 and Ar induce rapid declines in nitrogenase activity and CO2 evolution in nodules of Datisca glomerata
Authors: Tjepkema John D.
Published: May 2006

Preliminary studies have indicated that after addition of C2H2 there is a rapid decline in nitrogenase activity in the nodules of Datisca glomerata. The present work was undertaken to determine whether (1) there is also a decline in respiration and (2) the decline is associated with the cessation of ammonia production. The rates of C2H4 and CO2 evolution by nodulated root systems of Datisca were measured as a function of time after exposure to C2H2. The peak rate of C2H4 evolution occurred at 30 s after C2H2 exposure, while the rate of CO2 evolution started to decline at 60 s after exposure to C2H2. Incubation of nodules in a gas mixture containing Ar also caused a decline in CO2 evolution. Further, pretreatment with Ar eliminated most of the C2H2‐induced decline in nitrogenase activity and CO2 evolution. These C2H2‐ and Ar‐induced declines in Datisca nodules are more rapid than those reported in any other nodules. They are evidence that continued ammonia formation is essential for maintenance of normal nitrogenase activity in Datisca nodules.
The capacity of Jamaican mine spoils, agricultural and forest soils to nodulateMyrica cerifera, Leucaena leucocephalaandCasuarina cunninghamiana
Authors: Zimpfer Jeff F. , Smyth Charles A. , Dawson Jeffrey O.
Published: May 2006

Soils from seven sites on the island of Jamaica were assayed for the symbiotic diazotrophsFrankiaandRhizobiumusing serial dilutions. Most probable number and least squares regression methods were used to estimate each soil's capacity to nodulate nativeMyrica cerifera, exoticLeucaena leucocephalaand exoticCasuarina cunninghamiana. The sample sites included a montane forest, a slash‐and‐burn agricultural site, reclaimed bauxite mining areas, abandoned sugar cane fields, and a garden plot. None of the host plants used in the bioassay were present on the sites sampled except for scatteredL.leucocephalaon one site.Frankiacapable of nodulatingM. cerifera, which is native to Jamaican highlands, occurred at all sites sampled. NoC. cunninghamiana‐infective Frankiawas detected in soils sampled. Only soils from one site on the tropical coastal plain harbored rhizobia able to nodulateL. leucocephala(37 nodulation units cm−3 of soil). A subset of nodulatedM. ceriferaandL. leucocephalareduced acetylene to ethylene indicating nitrogenase activity. The slash‐and‐burn agricultural site, which was situated at an elevation of 200 m and possessed both high natural fertility and high soil moisture‐supplying capacity, had significantly greaterMyricainfectious capacity (1 000 nodulation units cm−3of soil) than the other sites (7−207 nodulation units cm−3of soil). A planned, paired comparison revealed that a recently cultivated sugar cane field and a recently reclaimed bauxite mining site together had significantly lessMyrica‐infectiveFrankia(4 nodulation units cm−3of soil) than a corresponding pair of sites consisting of a sugar cane field abandoned for 25 years and a bauxite mining site reclaimed 20 years before sampling (118 nodulation units cm−3of soil). Results indicate that Myrica‐infectiveFrankiais widespread in Jamaica, that the number ofMyrica‐infective Frankiaunits vary from site to site in accordance with soil type and soil history, that Jamaican sites sampled lack soilFrankiapopulations capable of nodulating a casuarina host, that rhizobial symbionts capable of nodulatingL. leucocephalamay be geographically restricted to lowlands in Jamaica, and that the occurrence ofFrankiain these soils is independent of host plant presence.
The use of PCR‐based typing methods to assess the diversity of Frankia nodule endophytes of the actinorhizal shrub Ceanothus
Authors: Murry Marcia A. , Konopka Anastasia S. , Pratt S. Diane , Vandergon Thomas L.
Published: May 2006

Our understanding of the actinorhizal symbiosis, in particular of the Frankia‐Ceanothus association, has been hampered by the failure to isolate infective strains in pure culture. Recently, the polymerase chain reaction (PCR) has been utilized to amplify regions of the Frankia genome, allowing analysis of the microsymbiont genome without first isolating the microbe in pure culture. Root nodules were collected from six Ceanothus spp. common to the coastal regions of the Santa Monica Mountains of southern California. Individual lobes were surface‐sterilized, total DNA was extracted and amplified using prokaryotic‐specific primers. To assess the genetic diversity of Frankia endophytes in the population studied, the BOX primer was used to generate genomic fingerprints of prokaryotic nodule inhabitants using rep‐PCR. Fingerprint patterns fell into twelve distinct groups indicating the occurrence of genetic diversity of Frankia in the nodules sampled. DNA extracts of individual lobes that gave distinct BOX‐PCR fingerprints were also amplified by PCR using primers directed against conserved regions of the 16S ribosomal RNA gene. The nucleotide sequences of the PCR products were determined and aligned with the corresponding region from other taxa for phylogenetic analysis. The sequences from Ceanothus nodules share a common ancestor to that of the Elaeagnus–infective strains.
Distribution and N2‐fixing activity of Frankia strains in relation to soil depth
Authors: Nalin Renaud , Normand Philippe , Domenach Anne‐Marie
Published: May 2006

The qualitative and quantitative distribution of Frankia strains infective on Elaeagnus angustifolia L. from different depths in the same soil (10–20 cm; 30–40 cm; 50–60 cm) were compared. The soil samples collected from a natural stand devoid of Elaeagnaceae were planted with Elaeagnus angustifolia L. seedlings. All plants became nodulated, demonstrating that Frankia strains were present at least down to 60 cm in the soil. The decreased density of Frankia strains was correlated to the decline of soil organic matter content with soil depth. DNA extracted from nodules produced on Elaeagnus angustifolia L. seedlings were polymerase chain reaction (PCR) characterized by amplifying the nifD‐K intergenic spacer (IGS), using the polymerase chain reaction (PCR) followed by restriction fragment length polymorphism (RFLP) analysis. This showed the presence of seven nif‐HaeIII profiles within this Frankia community. Diversity of Frankia strains was maintained throughout the soil column, but the relative distribution of strains varied. Some nif‐HaeIII profiles were only found in the deeper soil, suggesting different selective advantages to withstand the constraints of soil depth. 15N2 experiments indicated that all the strains tested had N2‐fixing activity. However, efficiency was not significantly different among nodules of different nif‐HaeIII profiles. Therefore, N2‐fixing activity does not seem to be the main factor responsible for the different distribution of Frankia strains at different soil depths.
Does plasma membrane H+‐ATPase activation by fusicoccin involve protein kinase?
Authors: Trofimova Marina S. , Smolenskaya Irina N. , Drabkin Artem V. , Galkin Alexander V. , Babakov Alexey V.
Published: May 2006

Sugar beet (Beta vulgaris L.) root suspension‐cultured cells were converted to protoplasts which responded to fusicoccin (FC) by a rise in cytoplasmic pH (pHcyt) averaging 0.25 units in the fluorimetric assay. This effect was blocked by erythrosin B, a specific inhibitor of the plasma membrane H+‐ATPase. A protein kinase inhibitor, staurosporine also caused cytosolic alkalinization that was sensitive to H+‐ATPase inhibitors. Most strikingly, the effect of staurosporine was suppressed by fusicoccin and vice versa. Addition of okadaic acid, entailing overall protein phosphorylation, also led to H+‐ATPase activation, whereupon fusicoccin lost its effect on proton transport. In parallel, kinetic and inhibitor analyses demonstrated that FC binding to the protoplast plasma membrane involved two sites with dissociation constants of 1 nM and 0.2 μM and was indifferent to phosphorylation and dephosphorylation inhibitors. Thus, it could be concluded that (1) the effect of FC on cytoplasmic pH probably depends on the phosphorylation state of plasma membrane proteins and may have either sign; (2) the activation of H+‐ATPase by FC most likely proceeds directly through conformational receptor‐enzyme interaction.
Evidence for vacuolar sequestration of paraquat in roots of a paraquat‐resistant Hordeum glaucum biotype
Authors: Lasat Mitch M. , DiTomaso Joseph M. , Hart Jonathan J. , Kochian Leon V.
Published: May 2006

Paraquat resistance in the grass weed Hordeum glaucum Steud. has been proposed to result from herbicide sequestration away from the growing points. In the present study, we used roots as a model system to investigate cellular transport of paraquat in resistant (R) and susceptible (S) H. glaucum biotypes. Both time‐ and concentration‐dependent kinetics of paraquat influx across the root cell plasma membrane were similar in the S and R biotype. However, compartmentation analysis indicated greater herbicide accumulation in root vacuoles of the R seedlings. In contrast, the amount of paraquat accumulated in the cytoplasm of S was double that found in R biotype. While paraquat efflux from the cytoplasm back into the external solution was similar in the two biotypes, efflux across the tonoplast from the vacuole back into the cytoplasm was 5 times slower in the R than in the S biotype. At the end of a 48‐h efflux period, nearly 7‐fold more herbicide was retained in the roots of the R compared with those of the S biotype. These results suggest that paraquat resistance in H. glaucum may be due to the herbicide sequestration in the vacuole.
Sodium‐calcium interactions in two wheat species differing in salinity tolerance
Authors: Davenport R. J. , Reid R. J. , Smith F. A.
Published: May 2006

Sodium/calcium interactions were compared in two species of wheat known to differ in sodium accumulation in the shoot and biomass reduction under saline conditions. Unidirectional uptake of 22Na to the roots was measured in 5, 50, 100 and 150 mM NaCl with four levels of calcium activity. Translocation of sodium to the shoot was measured as 22Na uptake over 7 days. The rate of sodium uptake into the roots did not vary between the salt‐sensitive Triticum turgidum cv. Modoc and the tolerant Triticum aestivum cv. Kharchia. except at low calcium, when the rate of uptake was higher in Kharchia, and more sensitive to inhibition by additional calcium. The rate of translocation to the shoot was similar in both species, however the shootroot ratio was higher in Kharchia and so the concentration of sodium, on a shoot fresh weight basis, was more dilute in Kharchia. There was no evidence for differences in sodium transport processes between the species which might confer differential salinity tolerance. Translocation of calcium to the shoot was measured as 45Ca uptake in 5 and 150 mM NaCl with 4 levels of calcium activity. Translocation of calcium was more inhibited by sodium in the salt‐sensitive Modoc, and this may reflect differences in calcium transport processes, which could account in part for the greater salt sensitivity of this species.
Na+/H+ antiport activity in tonoplast vesicles isolated from sunflower roots induced by NaCl stress
Authors: Ballesteros Emilia , Blumwald Eduardo , Donaire Juan Pedro , Belver Andrés
Published: May 2006

Na+ transport across the tonoplast and its accumulation in the vacuoles is of crucial importance for plant adaptation to salinity. Mild and severe salt stress increased both ATP‐ and PPi‐dependent H+ transport in tonoplast vesicles from sunflower seedling roots, suggesting the possibility that a Na+/H+ antiport system could be operating in such vesicles under salt conditions (E. Ballesteros et al. 1996. Physiol. Plant. 97: 259–268). During a mild salt stress, Na+ was mainly accumulated in the roots. Under a more severe salt treatment, Na+ was equally distributed in shoots and roots. In contrast to what was observed with Na+, all the salt treatments reduced the shoot K+ content. Dissipation by Na+ of the H+ gradient generated by the tonoplast H+‐ATPase, monitored as fluorescence quenching of acridine orange, was used to measure Na+/H+ exchange across tonoplast‐enriched vesicles isolated by sucrose gradient centrifugation from sunflower (Helianthus annuus L.) roots treated for 3 days with different NaCl regimes. Salt treatments induced a Na+/H+ exchange activity, which displayed saturation kinetics for Na+ added to the assay medium. This activity was partially inhibited by 125 μM amiloride, a competitive inhibitor of Na+/H+ antiports. No Na+/H+ exchange was detected in vesicles from control roots. The activity was specific for Na+. since K+ added to the assay medium slightly dissipated H+ gradients and displayed non‐saturating kinetics for all salt treatments. Apparent Km for Na+/H+ exchange in tonoplast vesicles from 150 mM NaCl‐treated roots was lower than that of 75 mM NaCl‐treated roots, Vmax remaining unchanged. The results suggest that the existence of a specific Na+/H+ exchange activity in tonoplast‐enriched vesicle fractions, induced by salt stress, could represent an adaptative response in sunflower plants, moderately tolerant to salinity.
Stress‐induced formation of haploid plants through anther culture in cork oak (Quercus suber)
Authors: Bueno Maria Angeles , Gómez Arancha , Boscaiu Monica , Manzanera José Antonio , Vicente Oscar
Published: May 2006

Induction of haploid embryos and regeneration of plantlets have been obtained, for the first time, in cork oak (Quercus suber L.) by combining a starvation treatment in anther culture with a mild heat shock at 33°C for 5 days, followed by culture at 25°C in a simple agar medium without growth regulators. The same conditions had been shown previously to be optimal for embryogenic induction in isolated microspore cultures of several model species such as tobacco and wheat. These results support the notion that stress, particularly sucrose starvation, a heat shock or a combination of both treatments could be the major and general signal responsible for the inhibition of normal gametophytic development of the microspores and for the induction of the alternative embryogenic pathway. A similar approach may be used for the production of haploid and doubled haploids for plant breeding in other species that, like most forest trees, are still recalcitrant in anther culture.
Expression of sunflower cytochrome c mRNA is tissue‐specific and controlled by nitrate and light
Authors: Felitti Silvina A. , Chan Raquel L. , Gago Gabriela , Valle Estela M. , Gonzalez Daniel H.
Published: May 2006

A PCR‐based strategy with two degenerate oligonucleotides followed by library screening was used to isolate a 590‐bp cDNA clone encoding a full‐length apocytochrome c polypeptide from a sunflower (Helianthus annuus L. line HA401B) root library. The sequence of the encoded polypeptide (112 amino acids) differs from the previously reported protein sequence (Ramshaw et al. 1970. Biochem. J. 119: 535‐539) at four positions. Southern blot analysis indicates the possible existence of two different cytochrome c genes in sunflower. The isolated cDNA clone was used to perform expression studies of cytochrome c mRNA(s) in different sunflower organs and under different conditions. Higher transcript levels were observed in flowers and roots relative to leaves, stems, hypocotyls and cotyledons. Light promoted an increase in the expression of cytochrome c mRNA in 6‐day‐old seedlings. Nitrate treatment, in tum, reduced expression in roots. It is concluded that the steady‐state mRNA levels of cytochrome c encoding transcripts in sunflower are subject to tissue‐specific regulation and also respond to environmental factors.
Closeness in Young Adult Sibling Relationships: Affective and Physiological Processes
Authors: Shortt Joann Wu , Gottman John M.
Published: May 2006

What aspects of the relationships of young adult siblings are related to closeness or distance? This study takes a process approach to answering this question, contrasting it with the status approach of family structure variables. Close and distant sibling pairs were compared on physiological variables and specific aspects of warmth, con‐flict, and power during two conversations ‐ one on enjoyable topics and one on topics of disagreement. While there was no evidence that sibling closeness was related to family structure variables, differences in closeness were found in affective patterns and physiological responses during sibling interaction. Interaction between close sib‐lings was characterized not only by higher positive affect but also by fewer power struggles and lower heart rate reactivity. Close siblings also had higher scores on emotional empathy and cognitive aspects of empathy such as perspective‐taking than distant siblings. The developmental importance of considering power struggles in young adult sibling relationships was also discussed.
Friendships and Violent Behavior During Adolescence
Authors: Dishion Thomas J. , Eddy J. Mark , Haas Eric , Li Fuzhong , Spracklen Kathleen
Published: May 2006

This study investigated the extent to which interpersonal processes within male friendships are associated with violent behavior patterns during adolescence. At ages 13‐14, 15‐16, and 17‐18, we observed the participants (206 boys) in our laboratory discussing problem solving situations with a close friend. Although the boys typically brought in different friends for each of the three assessments, we found considerable continuity in the boys' behaviors, most notably in the topics discussed. In particular, the tendency of a dyad to engage in deviant and violent talk was uniquely associated with violence in adolescence, controlling for childhood antisocial behavior and coercive discipline practices in the home. These findings suggest that adolescent violence is embedded within enduring social interactional patterns of friendships, where the faces change but the process remains the same.
Prediction of Positive Peer Relations from Observed Parent–Child Interactions
Authors: Leve Leslie D. , Fagot Beverly I.
Published: May 2006

The nature of 7‐year‐old children's (n = 140) positive peer relations was examined using a multi‐method, multi‐agent approach. A confirmatory factor analysis suggested three factors clustered by reporting agent and method: teachers' perceptions of the child's positive attributes, observation of the child‐peer interaction in three play settings, and observation of the child‐peer interaction in a teaching context. Similar pathways were found for boys and girls. Once it was established that stable constructs could be developed to measure positive peer relations, earlier parent‐child characteristics were examined to assess their utility in predicting children's positive friendships. Parental scaffolding and warmth were measured at 18 months and 5 years of age. Structural equation modeling suggests that positive parent interactions were positively related to later peer relations, but this relationship was significant only for boys. Possible explanations for this sex difference are discussed.