bluuebunny/crossref_metadata_2025 huggingface dataset:
Tris Is a Competitive Inhibitor of K+ Activation of the Vacuolar H+-Pumping Pyrophosphatase
Authors: Gordon-Weeks R. , Koren'kov V. D. , Steele S. H. , Leigh R. A.
Published: October 2016

The effects of a range of commonly used pH buffers on the hydrolytic activity of the plant vacuolar H+-transporting inorganic pyrophosphatase (V-PPase) from mung bean (Vigna radiata L.) hypocotyls were tested. All of the buffers inhibited K+ stimulation of the V-PPase, and the degree of inhibition was dependent on the concentrations of both the buffer and K+. The effects were dependent on the organic cation used in the buffers, and those tested inhibited in the order: Tris > Bis-Tris-propane > Bicine = Tricine > imidazole. Detailed studies revealed that a model in which Tris affects both the Km and Vmax for K+ stimulation provided an accurate description of the observed kinetics. The ability of different cations to stimulate the V-PPase was measured with a noncompeting buffer (5 mM imidazole-HCl) and the order of effectiveness was K+ = Rb+ > NH4+ >> Cs+ > Na+ > Li+, with the Km for K+ stimulation being about 1 to 2 mM. Published experiments performed in the presence of Tris were re-evaluated and all could be fitted to mixed inhibition kinetics, with kinetic parameters similar to those measured for the mung bean V-PPase. It is concluded that the variations in the published Km for K+ stimulation of the V-PPase are probably due to the effects of pH buffer cations and that the real value for this parameter is in the low millimolar range. The implications of this for regulation of the V-PPase by K+ in vivo and for the role of the enzyme in K+ transport into the vacuole are discussed.
Al Inhibits Both Shoot Development and Root Growth in als3, an Al-Sensitive Arabidopsis Mutant
Authors: Larsen P. B. , Kochian L. V. , Howell S. H.
Published: October 2016

In als3, an Al-sensitive Arabidopsis mutant, shoot development and root growth are sensitive to Al. Mutant als3 seedlings grown in an Al-containing medium exhibit severely inhibited leaf expansion and root growth. In the presence of Al, unexpanded leaves accumulate callose, an indicator of Al damage in roots. The possibility that the inhibition of shoot development in als3 is due to the hyperaccumulation of Al in this tissue was examined. However, it was found that the levels of Al that accumulated in shoots of als3 are not different from the wild type. The inhibition of shoot development in als3 is not a consequence of nonspecific damage to roots, because other metals (e.g. LaCl3 or CuSO4) that strongly inhibit root growth did not block shoot development in als3 seedlings. Al did not block leaf development in excised als3 shoots grown in an Al-containing medium, demonstrating that the Al-induced damage in als3 shoots was dependent on the presence of roots. This suggests that Al inhibition of als3 shoot development may be a delocalized response to Al-induced stresses in roots following Al exposure.
Inorganic Carbon Accumulation Stimulates Linear Electron Flow to Artificial Electron Acceptors of Photosystem I in Air-Grown Cells of the Cyanobacterium Synechococcus UTEX 625
Authors: Li Q. , Canvin D. T.
Published: October 2016

The effect of inorganic carbon (Ci) transport and accumulation on photosynthetic electron transport was studied in air-grown cells of the cyanobacterium Synechococcus UTEX 625. When the cells were depleted of Ci, linear photosynthetic electron flow was almost completely inhibited in the presence of the photosystem I (PSI) acceptor N,N-dimethyl-p-nitrosoaniline (PNDA). The addition of Ci to these cells, in which CO2 fixation was inhibited with glycolaldehyde, greatly stimulated linear electron flow and resulted in increased levels of photochemical quenching and O2 evolution. In aerobic conditions substantial quenching resulted from methyl viologen (MV) addition and further quenching was not observed upon the addition of Ci. In anaerobic conditions MV addition did not result in quenching until Ci was added. Intracellular Ci pools were formed when MV was present in aerobic or anaerobic conditions or PNDA was present in aerobic conditions. There was no inhibitory effect of Ci depletion on electron flow to 2,6-dimethylbenzoquinone and oxidized diaminodurene, which accept electrons from photosystem II. The degree of stimulation of PNDA-dependent O2 evolution varied with the Ci concentration. The extracellular Ci, concentration required for a half-maximum rate (K1/2) was 3.8 [mu]M and the intracellular K1/2 was 1.4 mM for the stimulation of PNDA reduction. These values agreed closely with the K1/2 values of extracellular and intracellular Ci for O2 photoreduction. Linear electron flow to artificial electron acceptors of PSI was enhanced by intracellular Ci, which appeared to exert an effect on PSI or on the intersystem electron transport chain.
Leaf-Atmosphere NH3 Exchange in Barley Mutants with Reduced Activities of Glutamine Synthetase
Authors: Mattsson M. , Hausler R. E. , Leegood R. C. , Lea P. J. , Schjoerring J. K.
Published: October 2016

Mutants of barley (Hordeum vulgare L. cv Maris Mink) with 47 or 66% of the glutamine synthetase (GS) activity of the wild type were used for studies of NH3 exchange with the atmosphere. Under normal light and temperature conditions, tissue NH4+ concentrations were higher in the two mutants compared with wild-type plants, and this was accompanied by higher NH3 emission from the leaves. The emission of NH3 increased with increasing leaf temperatures in both wild-type and mutant plants, but the increase was much more pronounced in the mutants. Similar results were found when the light intensity (photosynthetic photon flux density) was increased. Compensation points for NH3 were estimated by exposing intact shoots to 10 nmol NH3 mol-1 air under conditions with increasing temperatures until the plants started to emit NH3. Referenced to 25[deg]C, the compensation points were 5.0 nmol mol-1 for wild-type plants, 8.3 nmol mol-1 for 47% GS mutants, and 11.8 nmol mol-1 for 66% GS mutants. Compensation points for NH3 in single, nonsenescent leaves were estimated on the basis of apoplastic pH and NH4+ concentrations. These values were 0.75, 3.46, and 7.72 nmol mol-1 for wild type, 47% GS mutants, and 66% GS mutants, respectively. The 66% GS mutant always showed higher tissue NH4+ concentrations, NH3 emission rates, and NH3 compensation points compared with the 47% GS mutant, indicating that NH4+ release was curtailed by some kind of compensatory mechanism in plants with only 47% GS activity.
The Formation of Nitrogen-Fixing Bacteroids Is Delayed but Not Abolished in Soybean Infected by an α--Ketoglutarate Dehydrogenase-Deficient Mutant of Bradyrhizobium japonicum
Authors: Green L. S. , Emerich D. W.
Published: October 2016

A mutant strain of Bradyrhizobium japonicum USDA 110 devoid of α--ketoglutarate dehydrogenase activity (LSG184) was used to test whether this tricarboxylic acid cycle enzyme is necessary to support nitrogen fixation during symbiosis with soybean (Glycine max). LSG184 formed nodules about 5 d later than the wild-type strain, and the nodules, although otherwise normal in structure, contained many fewer infected host cells than is typical. At 19 d after inoculation cells infected with the mutant strain were only partially filled with bacteroids and showed large accumulations of starch, but by 32 d after inoculation the host cells infected with the mutant appeared normal. The onset of nitrogen fixation was delayed about 15 d for plants inoculated with LSG184, and the rate, on a per nodule fresh weight basis, reached only about 20% of normal. However, because nodules formed by LSG184 contained only about 20% of the normal number of bacteroids, it could be inferred that the mutant, on an individual bacteroid basis, was fixing nitrogen at near wild-type rates. Therefore, the loss of α--ketoglutarate dehydrogenase in B. japonicum does not prevent the formation or the functioning of nitrogen-fixing bacteroids in soybean.
Temperature-Sensitive Plant Cells with Shunted Indole-3-Acetic Acid Conjugation
Authors: Oetiker J. H. , Aeschbacher G.
Published: October 2016

Cells of henbane (Hyoscyamus muticus L.) grow indefinitely in culture without exogenous auxin. Cells of its temperature-sensitive variant XIIB2 grow like the wild type at 26[deg]C but die rapidly at 33[deg]C unless auxin is added to the medium. Despite this temperature-sensitive auxin auxotrophy, XIIB2 produces wild-type amounts of indole-3-acetic acid (IAA). IAA is the predominant auxin and is important for plant growth and development. Since the IAA production of the variant is functional, we investigated whether the synthesis or degradation of IAA metabolites, possibly active auxins themselves, is altered. The IAA metabolites were IAA-aspartate (IAAsp) and IAA-glucose. The wild type converted IAA mainly to IAAsp, whereas the variant produced mainly IAA-glucose. Exogenous auxin corrected the shunted IAA metabolism of the variant. The half-life of labeled IAAsp in the variant was reduced 21-fold, but in the presence of exogenous auxin it was not different from the wild type. The temperature sensitivity of XIIB2 was also corrected by supplying IAAsp. Pulse-chase experiments revealed that henbane rapidly metabolizes IAAsp to compounds not identical to IAA. The data show that the variant XIIB2 is a useful tool to study the function of IAA conjugates to challenge the popular hypothesis that IAA conjugates are merely slow-release storage forms of IAA.
Nitrogenase Activity Is Affected by Reduced Partial Pressures of N2 and NO3- 1
Authors: Blumenthal J. M. , Russelle M. P. , Vance C. P.
Published: October 2016

Optimal use of legumes in cropping systems requires a thorough understanding of the interaction between inorganic N nutrition and symbiotic N2 fixation. Our objective was to test the hypothesis that increased NO3- uptake by alfalfa (Medicago sativa L.) would compensate for lower N2 fixation caused by low partial pressure of N2. Root systems of hydroponically grown alfalfa at 2 mg L-1 NO3--N were exposed to (a) 80% N2, (b) 7% N2, (c) 2% N2, or (d) 0% N2. Exposure to reduced partial pressures of N2 reduced total nitrogenase activity (TNA, measured as H2 production in 20% O2 and 80% Ar) by 40% within less than 30 min, followed by a recovery period over the next 30 min to initial activity. Five hours after treatments began, the TNA of plants exposed to 7 and 2% N2 was substantially higher than pretreatment activities, whereas the TNA of plants exposed either to 0 or 80% N2 did not differ from pretreatment values. The decline in TNA due to NO3- exposure over 4 d was not affected by reduced partial pressure of N2. During the 1st h the proportion of electrons used for the reduction of N2 fell from 0.52 to 0.23 for plants exposed to 7% N2, and to 0.09 for plants exposed to 2% N2, and remained unchanged for the rest of the experiment. Although the hypothesis that alfalfa compensated with increased NO3- uptake for lower N2 fixation was not validated by our results, we unexpectedly demonstrated that the decline in TNA upon exposure to NO3- was independent of the N2-fixing efficiency (i.e. the amount of N2 reduced by nitrogenase) of the symbiosis.
Effect of Cold Acclimation on the Lipid Composition of the Inner and Outer Membrane of the Chloroplast Envelope Isolated from Rye Leaves
Authors: Uemura M. , Steponkus P. L.
Published: October 2016

The lipid composition of the inner and outer membranes of the chloroplast envelope isolated from winter rye (Secale cereale L. cv Puma) leaves was characterized before and after cold acclimation. In nonacclimated leaves the inner membrane contained high proportions of monogalactosyldiacylglycerols (MGDG, 47.9 mol% of the total lipids) and digalactosyldiacylglycerols (DGDG, 31.1 mol%) and a low proportion of phosphatidylcholine (PC, 8.1 mol%). The outer membrane contained a similar proportion of DGDG (30.0 mol%); however, the proportion of MGDG was much lower (20.1 mol%) and the proportion of PC was much higher (31.5 mol%). After 4 weeks of cold acclimation, the proportions of these lipid classes were significantly altered in both of the inner and outer membranes. In the inner membrane the proportion of MGDG decreased (from 47.9 to 38.4 mol%) and the proportion of DGDG increased (from 31.1 to 39.3 mol%), with only a slight change in the proportion of PC (from 8.1 to 8.8 mol%). In the outer membrane MGDG decreased from 20.1 to 14.8 mol%, DGDG increased from 30.0 to 39.9 mol%, and PC decreased from 31.5 to 25.4 mol%. Thus, both before and after cold acclimation, the proportion of MGDG was much higher in the inner membrane than in the outer membrane. In contrast, the proportion of PC was higher in the outer membrane than in the inner membrane. The relationship between the lipid composition of the inner and outer membranes of the chloroplast envelope and freeze-induced membrane lesions is discussed.
Cold-Resistant and Cold-Sensitive Maize Lines Differ in the Phosphorylation of the Photosystem II Subunit, CP29
Authors: Mauro S. , Dainese P. , Lannoye R. , Bassi R.
Published: October 2016

The effects of low temperature on the relative contributions of the reaction center and the antenna activities to photosystem II (PSII) electron transport were estimated by chlorophyll fluorescence. The inhibition of PSII photochemistry resulted from photo-damage to the reaction center and/or a reduced probability of excitation energy trapping by the reaction center. Although chill treatment did not modify the proportion of the dimeric to monomeric PSII, it destabilized its main light-harvesting complex. Full protection of the reaction center was achieved only in the presence of the phosphorylated PSII subunit, CP29. In a nonphosphorylating genotype the chill treatment led to photoinhibitory damage. The phosphorylation of CP29 modified neither its binding to the PSII core nor its pigment content. Phosphorylated CP29 was isolated by flat-bed isoelectric focusing. Its spectral characteristics indicated a depletion of the chlorophyll spectral forms with the highest excitation transfer efficiency to the reaction center. It is suggested that phosphorylated CP29 performs its regulatory function by an yet undescribed mechanism based on a shift of the equilibrium for the excitation energy toward the antenna.
Pollination-Induced Ethylene in Carnation (Role of Stylar Ethylene in Corolla Senescence)
Authors: Jones M. L. , Woodson W. R.
Published: October 2016

In carnation (Dianthus caryophyllus L. cv White Sim) cell to cell communication between the pollen and pistil induces ovary development and corolla senescence. The production of elevated ethylene by the style is the first measurable postpollination response. This is followed by a wave of ethylene production from the other floral organs. To investigate the regulation of ethylene biosynthesis in pollinated flowers we measured ethylene production and the expression of 1-aminocyclopropane-1-carboxylate synthase and 1-aminocyclopropane-1-carboxylate oxidase transcripts in individual floral organs after pollination. Ethylene production by pollinated styles can be defined temporally by three distinct peaks. By pollinating a single style from a multistyle gynoecium, it was determined that the unpollinated style produces ethylene that corresponds to the first and third peaks observed from a pollinated style. Inhibition of ethylene action in the pollinated style by diazocyclopentadiene treatment prevented both pollination-induced corolla senescence and ethylene production from the ovaries and petals. Treatment with diazocyclopentadiene decreased stylar ethylene production during the second peak and completely inhibited the third peak of ethylene in both pollinated and unpollinated styles. This later auto-catalytic ethylene in styles is likely responsible for pollination-induced corolla senescence and ovary development.
Effects of Prolonged Washing on Primary and Secondary Transport Processes at the Plasma Membrane in Red Beet Storage Tissue
Authors: Marvier A. C. , Williams L. E. , Leigh R. A. , Hall J. L.
Published: October 2016

Changing patterns of enzyme activity and solute transport in response to washing were investigated in red beet (Beta vulgaris L.) storage tissue. Washing had a pronounced effect on the plasma membrane (PM) H+-ATPase with an increase in both hydrolytic and proton-pumping activities. Immunoblotting indicated that this may be due, in part, to a higher amount of this enzyme in the PM of washed tissue. Activities of the tonoplast (V)H+-ATPase and pyrophosphatase fluctuated during a 4-d washing period, but overall showed no marked change in activity. In tissue discs sucrose (Suc), glucose (Glc), and fructose uptakes increased significantly in response to washing. Cycloheximide, cordycepin, and tunicamycin inhibited both Glc- and Suc-inducible uptake. Monensin also strongly inhibited inducible Glc uptake, but the effect on Suc was less marked. N-Ethylmaleimide inhibited both Suc and Glc uptake, with its effects being more pronounced in fresh tissue. Other protein-modifying reagents showed no significant difference in their level of inhibition between fresh and washed tissue. Transport studies, carried out using energized PM vesicles from fresh and washed tissue, indicated that there was no rise in Suc and Glc uptake rates in response to washing. Results with a range of inhibitors indicated that there was no marked change in transporter sensitivity in vesicles isolated from fresh and washed tissue. The results indicate that the well-described enhancement of solute transport in washed storage tissue may be due to an increased PM H+-ATPase activity rather than to changes in PM carrier activity or to changes in metabolism such as invertase activity.
Increases in Cytosolic Ca2+ in Parsley Mesophyll Cells Correlate with Leaf Senescence
Authors: Huang F. Y. , Philosoph-Hadas S. , Meir S. , Callaham D. A. , Sabato R. , Zelcer A. , Hepler P. K.
Published: October 2016

The ability to maintain the cytoplasmic Ca2+ concentration ([Ca2+]cyt) at homeostatic levels has been examined during leaf senescence in detached parsley (Petroselinum crispum) leaves. Fluorescence ratiometric imaging of mesophyll cells isolated from parsley leaves at various senescence stages and loaded with the Ca2+ indicator fura-2 has revealed a distinct elevation of [Ca2+]cyt, which was positively correlated with the progress of leaf senescence. This initial increase of [Ca2+]cyt, which was first observed in cells isolated from 3-d-senescent leaves, occurred 1 d before or in parallel with changes in two established senescence parameters, chlorophyll loss and lipid peroxidation. However, the [Ca2+]cyt elevation followed by 2 d the initial increase in the senescence-associated proteolysis. Whereas the [Ca2+]cyt of nonsenescent cells remained at the basal level, the elevated [Ca2+]cyt of the senescent cells was a long-lasting effect. Experimental retardation of senescence processes, achieved by pretreatment of detached leaves with the cytokinin benzyladenine, resulted in maintenance of homeostatic levels of [Ca2+]cyt in cells isolated from 3-d-senescent leaves. These observations demonstrate for the first time to our knowledge a correlation between elevated [Ca2+]cyt and the process of senescence in parsley leaves. Such senescence-associated elevation of [Ca2+]cyt, which presumably results from a loss of the cell's capability to extrude Ca2+, may serve as a signal inducing subsequent deteriorative processes.
Formation of Di-Isodityrosine and Loss of Isodityrosine in the Cell Walls of Tomato Cell-Suspension Cultures Treated with Fungal Elicitors or H2O2
Authors: Brady J. D. , Fry S. C.
Published: October 2016

About 84% of the hydroxyproline residues in a cell culture of tomato (Lycopersicon esculentum x Lycopersicon peruvianum) were present in phenol-inextractable (i.e. covalently wall-bound) material. Treatment of the cells with any of three fungal elicitors (wall fragments from Phytophthora megasperma and Pythium aphanidermatum and xylanase from Aureobasidium pullulans) or with 1 mM H2O2 had little effect on the quantity of phenolinextractable hydroxyproline per milligram of freeze-dried cells. However, each treatment induced a decrease in the content of phenol-inextractable isodityrosine (Idt) residues. Each treatment, except with the P. megasperma fragments, also induced an increase in phenol-inextractable di- (Di-Idt). The increase in Di-Idt partly accounted for the loss of Idt. We conclude that the elicitors and H2O2 acted to reinforce the existing cross-linking of cell wall (glyco)proteins by evoking oxidative coupling reactions to convert Idt to Di-Idt plus unidentified products. The promotion of cross-linking by elicitor treatment is proposed to be a defensive response that restricts the penetration of pathogens.
Actin Filaments Modulate Both Stomatal Opening and Inward K+-Channel Activities in Guard Cells of Vicia faba L
Authors: Hwang J. U. , Suh S. , Yi H. , Kim J. , Lee Y.
Published: October 2016

Actin antagonists have previously been shown to alter responses of Commelina communis stomata to physiological stimuli, implicating actin filaments in the control of guard cell volume changes (M. Kim, P.K. Hepler, S.-O. Eun, K.S. Ha, Y. Lee [1995] Plant Physiol 109: 1077–1084). Since K+ channels in the guard cell play an important role in stomatal movements, we examined the possible regulation of K+-channel activities by the state of actin polymerization. Agents affecting actin polymerization altered light-induced stomatal opening and inward K+-channel activities measured by patch clamping in Vicia faba. Cytochalasin D, which induces depolymerization of actin filaments, promoted light-induced stomatal opening and potentiated the inward K+ current in guard cell protoplasts. Phalloidin, a stabilizer of filamentous actin, inhibited both light-induced stomatal opening and inward K+ current. Inward K+-channel activities in outside-out membrane patches showed responses to these agents that support results at the whole-cell current level, suggesting that cytochalasin D facilitates and phalloidin inhibits K+ influx in intact guard cells, thus resulting in enhancement and inhibition of stomatal opening, respectively. To our knowledge, this is the first report that provides evidence that actin filaments may regulate an important physiological process by modulating the activities of ion channels in plant cells.
Sym2 of Pea Is Involved in a Nodulation Factor-Perception Mechanism That Controls the Infection Process in the Epidermis
Authors: Geurts R. , Heidstra R. , Hadri A. E. , Downie J. A. , Franssen H. , van Kammen A. , Bisseling T.
Published: October 2016

In pea (Pisum sativum) up to 50 nodulation mutants are known, several of which are affected in the early steps of the symbiotic interaction with Rhizobium sp. bacteria. Here we describe the role of the sym2 gene in nodulation (Nod) factor perception. Our experiments show that the sym2A allele from the wild pea variety Afghanistan confers an arrest in infection-thread growth if the Rhizobium leguminosarum bv viciae strain does not produce Nod factors with a NodX-mediated acetylation at their reducing end. Since the induction of the early nodulin gene ENOD12 in the epidermis and the formation of a nodule primordium in the inner cortex were not affected, we conclude that more than one Nod factor-perception mechanism is active. Furthermore, we show that sym2A-mediated control of infection-thread growth was affected by the bacterial nodulation gene nodO.
Tomato Polyphenol Oxidase (Differential Response of the Polyphenol Oxidase F Promoter to Injuries and Wound Signals)
Authors: Thipyapong P. , Steffens J. C.
Published: October 2016

Tomato (Lycopersicon esculentum Mill.) polyphenol oxidases (PPOs) are encoded by a seven-member gene family that exhibits complex patterns of differential expression during growth and differentiation. Antisense down-regulation of constitutive and induced PPO expression results in hypersusceptibility to pathogens, suggesting a critical role for PPO-mediated phenolic oxidation in plant defense. However, the nature and extent of PPO induction and its contribution to resistance are unclear. In this study we examined the inducibility of the tomato PPO gene family. In mature plants PPO transcript levels systemically increased in young leaves (nodes 1–3) when mature leaflets (node 5) were injured. Transcripts hybridizing to PPO E/F-specific probes were the predominant wound-induced PPO mRNAs in young leaves. Analysis of PPO promoter: GUS fusion constructs shows that mechanical wounding and infection by fungal and bacterial pathogens induced transcription of PPO F. Different injuries, salicylic acid, ethylene, and jasmonates elicited distinct, cell-specific and developmental stage-specific patterns of PPO F expression. Whereas jasmonates and mechanical wounding significantly induced PPO F only in young leaves (nodes 1–3), and ethylene induced PPO F only in older leaves (node 7), salicylic acid induced PPO F in stems and foliage at all developmental stages. These results demonstrate that cis-element(s) sufficient for PPO F inducibility reside in the 5[prime] flanking region, and these elements are responsive to a broad range of signals.
Asymmetric, Blue Light-Dependent Phosphorylation of a 116-Kilodalton Plasma Membrane Protein Can Be Correlated with the First- and Second-Positive Phototropic Curvature of Oat Coleoptiles
Authors: Salomon M. , Zacherl M. , Rudiger W.
Published: October 2016

The possible correlation between blue light-dependent phosphorylation of a 116-kD protein and phototropic responses of etiolated oat (Avena sativa L.) seedlings was tested by a micromethod for protein phosphorylation. Quantitation of the basipetal distribution of this protein showed that the in vitro 32P phosphorylation values declined exponentially from tip to node, with more than 50% of the total label being found in the uppermost 5 mm. Nonsaturating preirradiation of the coleoptiles in vivo resulted in partial phosphorylation with endogenous ATP. Subsequent in vitro phosphorylation under saturating irradiation allowed the determination of the degree of in vivo phosphorylation. Unilateral preirradiation resulted in higher in vivo phosphorylation on the irradiated than on the shaded side of the coleoptile. The fluence-response curve for the difference in phosphorylation between both sides of the coleoptile resembles the fluence-response curve for first-positive phototropic curvature, although it is shifted by two orders of magnitude to higher fluences. Possible reasons for this shift are discussed. In the coleoptile base the phosphorylation gradient across the coleoptile becomes larger with increasing time of irradiation at a constant fluence. Thus, phosphorylation of the 116-kD protein, in accordance with second-positive phototropic curvature, does not obey the Bunsen-Roscoe reciprocity law.
Exposure of Oat Seedlings to Blue Light Results in Amplified Phosphorylation of the Putative Photoreceptor for Phototropism and in Higher Sensitivity of the Plants to Phototropic Stimulation
Authors: Salomon M. , Zacherl M. , Luff L. , Rudiger W.
Published: October 2016

Dark recovery of blue light-induced in vitro phosphorylation in oat (Avena sativa L.) seedlings after in vivo preirradiation with blue light revealed different recovery kinetics for the coleoptile base and tip. Although, in both cases, maximum in vitro phosphorylation was observed 90 min after in vivo blue light treatment, the phosphorylation levels for the entire base were about 3-fold higher than those found in nonpreirradiated plants. The tip response only slightly exceeded that of the dark controls. The fluence applied during preirradiation determined the extent of the increase in phosphorylation. Consequently, unilateral irradiation and subsequent dark incubation resulted in a more pronounced increase in phosphorylation in the irradiated than in the shaded side of the coleoptile base. Furthermore, blue light-irradiation conditions, known to induce neither first- nor second-positive curvature in nonpreirradiated plants, stimulated both asymmetric distribution of protein phosphorylation and second-positive phototropic curvature in the coleoptile base when administered to blue light-pretreated plants. Based on these data, we conclude that photosensitivity of the coleoptile base increases upon exposure to blue light in a time-and fluence-dependent manner, providing an excellent explanation of the invalidity of the Bunsen-Roscoe reciprocity law for second-positive phototropism.
Identification of a Transport Mechanism for NH4+ in the Symbiosome Membrane of Pea Root Nodules
Authors: Mouritzen P. , Rosendahl L.
Published: October 2016

Symbiosome membrane vesicles, facing bacteroid-side-out, were purified from pea (Pisum sativum L.) root nodules and used to study NH4+ transport across the membrane by recording vesicle uptake of the NH4+ analog [14C]methylamine (MA). Membrane potentials ([delta][psi]) were imposed on the vesicles using K+ concentration gradients and valinomycin, and the size of the imposed [delta][psi] was determined by measuring vesicle uptake of [14C]tetraphenylphosphonium. Vesicle uptake of MA was driven by a negative [delta][psi] and was stimulated by a low extravesicular pH. Protonophore-induced collapse of the pH gradient indicated that uptake of MA was not related to the presence of a pH gradient. The MA-uptake mechanism appeared to have a large capacity for transport, and saturation was not observed at MA concentrations in the range of 25 [mu]M to 150 mM. MA uptake could be inhibited by NH4+, which indicates that NH4+ and MA compete for the same uptake mechanism. The observed fluxes suggest that voltage-driven channels are operating in the symbiosome membrane and that these are capable of transporting NH4+ at high rates from the bacteroid side of the membrane to the plant cytosol. The pH of the symbiosome space is likely to be involved in regulation of the flux.
Characterization of Water Channels in Wheat Root Membrane Vesicles
Authors: Niemietz C. M. , Tyerman S. D.
Published: October 2016

The functional significance of water channels in wheat (Triticum aestivum L.) root membranes was assessed using light scattering to measure vesicle shrinking in response to osmotic gradients rapidly imposed in a stopped flow apparatus. Vesicles were obtained from both a plasma membrane fraction and a plasma membrane-depleted endomembrane fraction including tonoplast vesicles. Osmotic water permeability (Pos) in the endomembrane fraction was high (Pos= 86.0 [mu]m s-1) with a low activation energy (EA= 23.32 kJ mol-1 [plus or minus] 3.88 SE), and was inhibited by mercurials (K1= 40 [mu]M HgCl2, where K1 is the inhibition constant for half-maximal inhibition), suggesting participation of water channels. A high ratio of osmotic to diffusional permeability (Pd) (using D2O as a tracer, Pos/Pd = 7 [plus or minus] 0.5 SE) also supported this view. For the endomembrane fraction there was a marked decrease in Pos with increasing osmotic gradient that was not observed in the plasma membrane fraction. Osmotic water permeability in the plasma membrane fraction was lower (Pos= 12.5 [mu]m s-1) with a high activation energy (EA= 48.07 kJ mol-1 [plus or minus] 3.63 SE) and no mercury inhibition. Nevertheless, Pos/Pd was found to be substantially higher than one (Pos= 3 [plus or minus] 0.2 SE), indicating that water channels mediated water flow in this fraction, too. Possible distortion of the Pos/Pd value by unstirred layer effects was shown to be unlikely.
A Radial Concentration Gradient of Indole-3-Acetic Acid Is Related to Secondary Xylem Development in Hybrid Aspen
Authors: Tuominen H. , Puech L. , Fink S. , Sundberg B.
Published: October 2016

The radial distribution pattern of indole-3-acetic acid (IAA) was determined across the developing tissues of the cambial region in the stem of hybrid aspen (Populus tremula L. x Populus tremuloides Michx). IAA content was measured in consecutive tangential cryo-sections using a microscale mass spectrometry technique. Analysis was performed with wild-type and transgenic trees with an ectopic expression of Agrobacterium tumefaciens IAA-biosynthetic genes. In all tested trees IAA was distributed as a steep concentration gradient across the developing tissues of the cambial region. The peak level of IAA was within the cambial zone, where cell division takes place. Low levels were reached in the region where secondary wall formation was initiated. The transgenic trees displayed a lower peak level and a wider radial gradient of IAA compared with the wild type. This alteration was related to a lower rate of cambial cell division and a longer duration of xylem cell expansion in the transgenic trees, resulting in a decreased xylem production and a larger fiber lumen area. The results indicate that IAA has a role in regulating not only the rate of physiological processes such as cell division, but also the duration of developmental processes such as xylem fiber expansion, suggesting that IAA functions as a morphogen, conveying positional information during xylem development.
Molecular Rigidity in Dry and Hydrated Onion Cell Walls
Authors: Ha M. A. , Apperley D. C. , Jarvis M. C.
Published: October 2016

Solid-state nuclear magnetic resonance relaxation experiments can provide information on the rigidity of individual molecules within a complex structure such as a cell wall, and thus show how each polymer can potentially contribute to the rigidity of the whole structure. We measured the proton magnetic relaxation parameters T2 (spin-spin) and T1p (spin-lattice) through the 13C-nuclear magnetic resonance spectra of dry and hydrated cell walls from onion (Allium cepa L.) bulbs. Dry cell walls behaved as rigid solids. The form of their T2 decay curves varied on a continuum between Gaussian, as in crystalline solids, and exponential, as in more mobile materials. The degree of molecular mobility that could be inferred from the T2 and T1p decay patterns was consistent with a crystalline state for cellulose and a glassy state for dry pectins. The theory of composite materials may be applied to explain the rigidity of dry onion cell walls in terms of their components. Hydration made little difference to the rigidity of cellulose and most of the xyloglucan shared this rigidity, but the pectic fraction became much more mobile. Therefore, the cellulose/xyloglucan microfibrils behaved as solid rods, and the most significant physical distinction within the hydrated cell wall was between the microfibrils and the predominantly pectic matrix. A minor xyloglucan fraction was much more mobile than the microfibrils and probably corresponded to cross-links between them. Away from the microfibrils, pectins expanded upon hydration into a nonhomogeneous, but much softer, almost-liquid gel. These data are consistent with a model for the stress-bearing hydrated cell wall in which pectins provide limited stiffness across the thickness of the wall, whereas the cross-linked microfibril network provides much greater rigidity in other directions.
Abolition of Posttranscriptional Regulation of Nitrate Reductase Partially Prevents the Decrease in Leaf NO3- Reduction when Photosynthesis Is Inhibited by CO2 Deprivation, but Not in Darkness
Authors: Lejay L. , Quillere I. , Roux Y. , Tillard P. , Cliquet J. B. , Meyer C. , Morot-Gaudry J. F. , Gojon A.
Published: October 2016

The activity of nitrate reductase (NR) in leaves is regulated by light and photosynthesis at transcriptional and posttranscriptional levels. To understand the physiological role of these controls, we have investigated the effects of light and CO2 on in vivo NO3- reduction in transgenic plants of Nicotiana plumbaginifolia lacking either transcriptional regulation alone or transcriptional and posttranscriptional regulation of NR. The abolition of both levels of NR regulation did not modify the light/dark changes in exogenous 15NO3- reduction in either intact plants or detached leaves. The same result was obtained for 15N incorporation into free amino acids in leaves after 15NO3- was supplied to the roots, and for reduction of endogenous NO3- after transfer of the plants to an N-deprived solution. In the light, however, deregulation of NR at the posttranscriptional level partially prevented the inhibition of leaf 15NO3- reduction resulting from the removal of CO2 from the atmosphere We concluded from these observations that in our conditions deregulation of NR in the transformants investigated had little impact on the adverse effect of darkness on leaf NO3- reduction, and that posttranscriptional regulation of NR is one of the mechanisms responsible for the short-term coupling between photosynthesis and leaf NO3- reduction in the light.
Possible Involvement of Al-Induced Electrical Signals in Al Tolerance in Wheat
Authors: Papernik L. A. , Kochian L. V.
Published: October 2016

The relationship between Al-induced depolarization of root-cell transmembrane electrical potentials (Em) and Al tolerance in wheat (Triticum aestivum L.) was investigated. Al exposure induced depolarizations of Em in the Al-tolerant wheat cultivars Atlas and ET3, but not in the Al-sensitive wheat cultivars Scout and ES3. The depolarizations of Em occured in root cap cells and as far back as 10 mm from the root tip. The depolarization was specific to Al3+; no depolarization was observed when roots were exposed to the rhizotoxic trivalent cation La3+. The Al-induced depolarization occurred in the presence of anion-channel antagonists that blocked the release of malate, indicating that the depolarization is not due to the electrogenic efflux of malate2-. K+-induced depolarizations in the root cap were of the same magnitude as Al-induced depolarizations, but did not trigger malate release, indicating that Al-induced depolarization of root cap cell membrane potentials is probably linked to, but is not sufficient to trigger, malate release.
Ethylene-Mediated Programmed Cell Death during Maize Endosperm Development of Wild-Type and shrunken2 Genotypes
Authors: Young T. E. , Gallie D. R. , DeMason D. A.
Published: October 2016

We characterized the progression of programmed cell death during maize (Zea mays L.) endosperm development of starchy (Su; wild-type) and shrunken2 (sh2) genotypes and tested the involve ment of ethylene in mediating this process. Histological and viability staining demonstrated that endosperm cell death was initiated earlier and progressed more rapidly in sh2 endosperm compared with Su endosperm. Internucleosomal DNA fragmentation accompanied endosperm cell death and occurred more extensively in sh2 endosperm. 1-Aminocyclopropane-1-carboxylic acid levels peaked approximately 16 d after pollination (dap) in Su endosperm and gradually decreased during subsequent development, whereas two large 1-aminocyclopropane-1-carboxylic acid peaks were observed in sh2 endosperm, the first between 16 and 20 dap and the second at 36 dap. Ethylene levels were elevated in sh2 kernels compared with Su kernels, with an initial peak 20 dap approximately 3-fold higher than in Su kernels and a second peak 36 dap approximately 5-fold higher than that in Su kernels. Ethylene treatment of Su kernels resulted in earlier and more extensive endosperm cell death and DNA fragmentation. Aminoethoxyvinylglycine treatment of sh2 kernels reduced the extent of DNA fragmentation. We conclude that ethylene is involved in triggering programmed cell death in developing maize endosperm and is responsible for the aberrant phenotype of sh2 kernels.
Photoregulation of Chloroplast Gene Transcription in the Chromophytic Alga Heterosigma carterae
Authors: Doran E. , Cattolico R. A.
Published: October 2016

Light acts as a complex regulator of cellular development and gene expression in photoautotrophs. Although light signals are highly effective in controlling cellular division and chloroplast biogenesis in the toxic marine alga Heterosigma carterae, their influence on gene expression has not been well characterized. To address this need cultures of H. carterae synchronized by an alternating light-dark regime were sampled through 12 h of light and 12 h of dark to characterize cell division, chloroplast complement, and chloroplast RNA abundance. These studies have identified a unimodal pattern of chloroplast transcriptional activity for a suite of cellular and photosynthetic genes. To determine the alga's response to a change in photoperiod, 12-h light/12-h dark-synchronized cultures were transferred to constant light and then periodically sampled. Although cellular and chloroplast division cycles remained synchronized in constant conditions for 24 h, the transcriptional apparatus responded by increasing ctRNA abundance within 45 min of the change in photoperiod. However, the ability of the alga to mount this rapid transcriptional response was limited to the first 2 h of the putative dark period. Thus, the chloroplast transcriptional apparatus of H. carterae may initiate a rapid, temporally gated response to a change in photoperiod that is independent of ongoing light-entrained cellular and chloroplast division cycles.
Sodium Dodecyl Sulfate-Stable Proteases in Chloroplasts
Authors: Sokolenko A. , Altschmied L. , Herrmann R. G.
Published: October 2016

Chloroplast subfractions were monitored for sodium dodecyl sulfate-stable proteases. Nine distinct activities in the molecular mass range from 14 to 66 kD have been detected. Five of the proteases associated with thylakoid membranes belong to the serine and cysteine types of proteases. These activities could be preserved and purified by a two-step electrophoresis procedure.
Rapid and Transient Activation of a Myelin Basic Protein Kinase in Tobacco Leaves Treated with Harpin from Erwinia amylovora
Authors: Adam A. L. , Pike S. , Hoyos M. E. , Stone J. M. , Walker J. C. , Novacky A.
Published: October 2016

Harpins are bacterial protein elicitors that induce hypersensitive response-like necrosis when infiltrated into nonhost plants such as tobacco (Nicotiana tabacum L.) (Z.-M. Wei, R.J. Laby, C.H. Zumoff, D.W. Bauer, S.Y. He, A. Collmer, S.V. Beer [1992] Science 257: 85–88). Activity of a 49-kD Mg2+-dependent and Ca2+-independent kinase in tobacco leaves increased 50-fold 15 min after infiltration of harpin from Erwinia amylovora (harpinEa). Much less pronounced and more transient activation was detected in water-infiltrated leaves. Biochemical characteristics of the harpinEa-activated protein kinase (HAPK) activity are consistent with those of the mitogen-activated protein kinase family. HAPK is cytosolic and phosphorylates myelin basic protein on serine/threonine residues. Treatment with a protein tyrosine phosphatase completely eliminated HAPK activity, suggesting that tyrosine phosphorylation is required for posttranslational activation. Sustained HAPK activation after cycloheximide treatment implies that HAPK may be negatively regulated by a translation-dependent mechanism. The extracellular Ca2+ chelator EGTA or the protein kinase inhibitor K252a, infiltrated in planta together with harpinEa, partially blocked HAPK activation. The Ca2+-channel blocker La3+ had no effect on HAPK activation, suggesting that phosphorylation events precede and/or do not depend on the entry of extracellular Ca2+ into the cell. These results suggest that early signal transduction events during harpinEa- induced hypersensitive response elicitation depend in part on the activation of HAPK.
S-Methylmethionine Conversion to Dimethylsulfoniopropionate: Evidence for an Unusual Transamination Reaction
Authors: Rhodes D. , Gage D. A. , Cooper AJL. , Hanson A. D.
Published: October 2016

Leaves of Wollastonia biflora (L.) DC. synthesize the osmoprotectant 3-dimethylsulfoniopropionate (DMSP) from methionine via S-methylmethionine (SMM) and 3-dimethylsulfoniopropionaldehyde (DMSP-ald); no other intermediates have been detected. To test whether the amino group of SMM is lost by transamination or deamination, [methyl-2H3,15N]SMM was supplied to leaf discs, and 15N-labeling of amino acids was monitored, along with synthesis of [2H3]DMSP. After short incubations more 15N was incorporated into glutamate than into other amino acids, and the 15N abundance in glutamate exceeded that in the amide group of glutamine (Gln). This is more consistent with transamination than deamination, because deamination would be predicted to give greater labeling of Gln amide N due to reassimilation, via Gln synthetase, of the 15NH4+ released. This prediction was borne out by control experiments with 15NH4Cl. The transamination product of SMM, 4-dimethylsulfonio-2-oxobutyrate (DMSOB), is expected to be extremely unstable. This was confirmed by attempting to synthesize it enzymatically from SMM using L-amino acid oxidase or Gln transaminase K and from 4-methylthio-2-oxobutyrate using methionine S-methyltransferase. In each case, the reaction product decomposed rapidly, releasing dimethylsulfide. The conversion of SMM to DMSP-ald is therefore unlikely to involve a simple transamination that generates free DMSOB. Plausible alternatives are that DMSOB is channeled within a specialized transaminase-decarboxylase complex or that it exists only as the bound intermediate of a single enzyme catalyzing an unusual transamination-decarboxylation reaction.
Regulation of Triacylglucose Fatty Acid Composition (Uridine Diphosphate Glucose:Fatty Acid Glucosyltransferases with Overlapping Chain-Length Specificity)
Authors: Kuai J. P. , Ghangas G. S. , Steffens J. C.
Published: October 2016

UDP-glucose (UDP-Glc):fatty acid glucosyltransferases catalyze the UDP-Glc-dependent activation of fatty acids as 1-O-acyl-β-glucoses. 1-O-Acyl-β-glucoses act as acyl donors in the biosynthesis of 2,3,4-tri-O-acylglucoses secreted by wild tomato (Lycopersicon pennellii) glandular trichomes. The acyl composition of L. pennellii 2,3,4-tri-O-acylglucoses is dominated by branched short-chain acids (4:0 and 5:0; approximately 65%) and straight and branched medium-chain-length fatty acids (10:0 and 12:0; approximately 35%). Two operationally soluble UDP-Glc:fatty acid glucosyltransferases (I and II) were separated and partially purified from L. pennellii (LA1376) leaves by polyethylene glycol precipitation followed by DEAE-Sepharose and Cibacron Blue 3GA-agarose chromatography. Whereas both transferases possessed similar affinity for UDP-Glc, glucosyltransferase I showed higher specificity toward short-chain fatty acids (4:0) and glucosyltransferase II showed higher specificity toward medium-chain fatty acids (8:0 and 12:0). The overlapping specificity of UDP-Glc:fatty acid glucosyltransferases for 4:0 to 12:0 fatty acid chain lengths suggests that the mechanism of 6:0 to 9:0 exclusion from acyl substituents of 2,3,4-tri-O-acylglucoses is unlikely to be controlled at the level of fatty acid activation. UDP-Glc:fatty acid glucosyltransferases are also present in cultivated tomato (Lycopersicon esculentum), and activities toward 4:0, 8:0, and 12:0 fatty acids do not appear to be primarily epidermal when assayed in interspecific periclinal chimeras.
Effect of Nitrogen Nutrition on Remobilization of Protein Sulfur in the Leaves of Vegetative Soybean and Associated Changes in Soluble Sulfur Metabolites
Authors: Sunarpi  , Anderson J. W.
Published: October 2016

The hypothesis that protein S is remobilized from mature leaves in response to N stress but not S stress was examined by transferring vegetative soybean (Glycine max L. Merr) plants grown with adequate sulfate and nitrate to nutrient medium with low sulfate (5 [mu]M) and nitrate at either 15, 7.5, 2, or 0.25 mM. Soluble S decreased to very low levels in mature and maturing leaves, especially in low-N plants. At high [N], insoluble S (protein) in mature leaves remained constant, but at low [N], after the soluble S declined, up to 40% of the insoluble S was exported. The losses were complemented by gains, initially in soluble S, but subsequently in insoluble S, in the expanding leaves and the root. In low-N plants, but not in high-N plants, the decrease in insoluble S in mature leaves was complemented by increases in homoglutathione (hGSH), Cys, and Met. At low [N], but not at high [N], the developing leaf, leaf 5, contained high amounts of soluble S, mostly hGSH. The results suggest that, at low [N], protein S is metabolized to hGSH, which serves as the principal transport compound for the export of organic S.