bluuebunny/crossref_metadata_2025 huggingface dataset:
Transduction of an Ethylene Signal Is Required for Cell Death and Lysis in the Root Cortex of Maize during Aerenchyma Formation Induced by Hypoxia
Authors: He C. J. , Morgan P. W. , Drew M. C.
Published: October 2016

Ethylene has been implicated in signaling cell death in the lysigenous formation of gas spaces (aerenchyma) in the cortex of adventitious roots of maize (Zea mays) subjected to hypoxia. Various antagonists that are known to modify particular steps in signal transduction in other plant systems were applied at low concentrations to normoxic and hypoxic roots of maize, and the effect on cell death (aerenchyma formation) and the increase in cellulase activity that precedes the appearance of cell degeneration were measured. Both cellulase activity and cell death were inhibited in hypoxic roots in the presence of antagonists of inositol phospholipids, Ca2+- calmodulin, and protein kinases. By contrast, there was a parallel promotion of cellulase activity and cell death in hypoxic and normoxic roots by contact with reagents that activate G-proteins, increase cytosolic Ca2+, or inhibit protein phosphatases. Most of these reagents had no effect on ethylene biosynthesis and did not arrest root extension. These results indicate that the transduction of an ethylene signal leading to an increase in intracellular Ca2+ is necessary for cell death and the resulting aerenchyma development in roots of maize subjected to hypoxia.
The Tomato E8 Gene Influences Ethylene Biosynthesis in Fruit but Not in Flowers
Authors: Kneissl M. L. , Deikman J.
Published: October 2016

We investigated the function of the tomato (Lycopersicon esculentum) E8 gene. Previous experiments in which antisense suppression of E8 was used suggested that the E8 protein has a negative effect on ethylene evolution in fruit. E8 is expressed in flowers as well as in fruit, and its expression is high in anthers. We introduced a cauliflower mosaic virus 35S-E8 gene into tomato plants and obtained plants with overexpression of E8 and plants in which E8 expression was suppressed due to co-suppression. Overexpression of E8 in unripe fruit did not affect the level of ethylene evolution during fruit ripening; however, reduction of E8 protein by cosuppression did lead to elevated levels during ripening. Levels for ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC), and ACC oxidase mRNA were increased approximately 7-fold in fruit of plants with reduced E8 protein. Levels of ACC synthase 2 mRNA were increased 2.5-fold, and ACC synthase 4 mRNA was not affected. Reduction of E8 protein in anthers did not affect the accumulation of ACC or of mRNAs encoding enzymes involved in ethylene biosynthesis. Our results suggest that the product of the E8 reaction participates in feedback regulation of ethylene biosynthesis during fruit ripening.
Posttranscriptional Regulation of the Sesbania rostrata Early Nodulin Gene SrEnod2 by Cytokinin
Authors: Silver D. L. , Pinaev A. , Chen R. , de Bruijn F. J.
Published: October 2016

The mRNA from the Sesbania rostrata early nodulin gene SrEnod2 accumulates in response to cytokinin application. Nuclear run-on assays using isolated root nuclei have shown that this accumulation occurs posttranscriptionally, and northern blot analysis of nuclear and total RNA levels revealed that it occurs primarily in the cytoplasm and not in the nucleus. After cytokinin enhancement of SrEnod2 mRNA accumulation and the subsequent removal of cytokinin, the levels of SrEnod2 mRNA did not return to basal levels, but oscillated over a 36-h time course. Application of the translational inhibitor cycloheximide was found to inhibit the enhancement of SrEnod2 mRNA accumulation by cytokinin and to cause its rapid decay. Okadaic acid and staurosporine, inhibitors of protein phosphatases and kinases, respectively, also inhibited cytokinin enhancement of SrEnod2 mRNA accumulation. In addition, okadaic acid was found to cause a decrease in SrEnod2 mRNA levels. These results provide evidence for a posttranscriptional mechanism of cytokinin enhancement of SrEnod2 mRNA accumulation, which appears to require concurrent protein synthesis, to involve protein phosphatases and kinases, and to occur primarily in the cytoplasm of the plant cell.
Cool Temperature-Induced Chlorosis in Rice Plants (II. Effects of Cool Temperature on the Expression of Plastid-Encoded Genes during Shoot Growth in Darkness)
Authors: Yoshida R. , Kanno A. , Kameya T.
Published: October 2016

It has been proposed that cool temperature-induced chlorosis (CTIC) in Indica cultivars of rice (Oryza sativa L.) is caused by cell growth and plastid development being impeded at cool temperatures. Since it is well known that the overall rate of transcription of plastid-encoded genes changes dramatically during the early phases of plastid development, in this study we focused on the patterns of expression of these genes. Northern blot analysis revealed that the level of 16S rRNA is decreased in a CTIC-sensitive rice cultivar grown at a cool temperature. The expression of the gene for the [beta] subunit of plasmid RNA polymerase (rpoB) was shown to be somewhat disturbed, particularly in terms of its resuppression under cool conditions. The level of transcripts or proteins of plastid-encoded photosynthetic genes was also decreased in a CTIC-sensitive cultivar at a cool temperature. These results suggest that the temperature-dependent inhibition of the onset of gene expression encoding the transcription/translation apparatus may be primarily involved in the mechanism causing CTIC.
Transport of Salicylic Acid in Tobacco Necrosis Virus-Infected Cucumber Plants
Authors: Molders W. , Buchala A. , Metraux J. P.
Published: October 2016

The transport of salicylic acid (SA) was studied in cucumber (Cucumis sativus L.) using 14C-labeled benzoic acid that was injected in the cotyledons at the time of inoculation. Primary inoculation with tobacco necrosis virus (TNV) on the cotyledons led to an induction of systemic resistance of the first primary leaf above the cotyledon against Colletotrichum lagenarium as early as 3 d after inoculation. [14C]SA was detected in the phloem or in the first leaf 2 d after TNV inoculation, whereas [14C]benzoic acid was not detected in the phloem during the first 3 d after TNV inoculation of the cotyledons, indicating phloem transport of [14C]SA from cotyledon. In leaf 1, the specific activity of [14C]SA decreased between 1.7 and 8.6 times compared with the cotyledons, indicating that, in addition to transport, leaf 1 also produced more SA. The amount of SA transported after TNV infection of the cotyledon was 9 to 160 times higher than in uninfected control plants. Thus, SA can be transported to leaf 1 before the development of systemic acquired resistance, and SA accumulation in leaf 1 results both from transport from the cotyledon and from synthesis in leaf 1.
Synthesis of Glutathione in Leaves of Transgenic Poplar Overexpressing [gamma]-Glutamylcysteine Synthetase
Authors: Noctor G. , Strohm M. , Jouanin L. , Kunert K. J. , Foyer C. H. , Rennenberg H.
Published: October 2016

Internode stem fragments of the poplar hybrid Populus tremula x Populus alba were transformed with a bacterial gene (gshl) for [gamma]-glutamylcysteine synthetase ([gamma]-ECS) targeted to the cytosol. Lines overexpressing [gamma]-ECS were identified by northern analysis, and the transformant with the highest enzyme activity was used to investigate the control of glutathione synthesis. Whereas foliar [gamma]-ECS activity was below the limit of detection in untransformed plants, activities of up to 8.7 nmol mg-1 protein min-1 were found in the transformant, in which the foliar contents of [gamma]-glutamylcysteine ([gamma]-EC) and glutathione were increased approximately 10- and 3-fold, respectively, without affecting either the reduction state of the glutathione pool or the foliar cysteine content. A supply of exogenous cysteine to leaf discs increased the glutathione content from both transformed and untransformed poplars, and caused the [gamma]-EC content of the transformant discs to increase still further. The following conclusions are drawn: (a) the native [gamma]-ECS of untransformed poplars exists in quantities that are limiting for foliar glutathione synthesis; (b) foliar glutathione synthesis in untransformed poplars is limited by cysteine availability; (c) in the transformant interactions between glutathione synthesis and cysteine synthesis operate to sustain the increased formation of [gamma]-EC and glutathione; and (d) the foliar glutathione content of the transformant is restricted by cysteine availability and by the activity of glutathione synthetase.
Induction of a Pea Cell-Wall Invertase Gene by Wounding and Its Localized Expression in Phloem
Authors: Zhang L. , Cohn N. S. , Mitchell J. P.
Published: October 2016

A full-length cell-wall invertase cDNA obtained from pea (Pisum sativum L.) seedlings was cloned previously. The gene exhibits tissue-specific expression, and both its transcript and enzyme activities are abundant only in root tissues. Mechanical wounding dramatically induced the accumulation of the cell-wall invertase mRNA in detached or intact leaves, stems, and roots. In both detached and intact tissues mRNA started to accumulate 3 h after wounding and in detached tissues (except root tissues) was much stronger and lasted longer compared with that in intact pea plants. The induction of cell-wall invertase by wounding was not systemic, since no significant increase of transcript was found in the unwounded tissues remote from the site of wounding. Accumulation of this cell-wall invertase was induced by abscisic or jasmonic acid, and in situ hybridization studies show that this invertase mRNA is differentially localized in wounded plant tissue, being most abundant in the phloem. mRNA accumulation was limited mainly to the wounded area; no significant increase was detected in the unwounded portions of the wounded stem segments. The results suggest that, as part of the wounding response, this cell-wall invertase may provide energy through hexose availability to companion cells in the phloem.
Root Growth Inhibition in Boron-Deficient or Aluminum-Stressed Squash May Be a Result of Impaired Ascorbate Metabolism
Authors: Lukaszewski K. M. , Blevins D. G.
Published: October 2016

Although cessation of growth is the most apparent symptom of boron deficiency, the biochemical function of boron in growth processes is not well understood. We propose that the action of boron in root meristems is associated with ascorbate metabolism. Total inhibition of root growth in squash (Cucurbita pepo L.) plants transferred to boron-free medium coincided with a major decrease (up to 98%) in the ascorbate concentration of root apices. Under low-boron conditions, in which root growth was partially inhibited, ascorbate concentration declined in proportion to growth rate. The decline in ascorbate concentration in boron-deficient root tips was not related to ascorbate oxidation. Ascorbate added to the medium improved root growth in plants supplied with insufficient boron. Increasing concentrations of aluminum in the nutrient medium caused progressive inhibition of root growth and a parallel reduction in ascorbate concentration of root apices. Elevated boron levels improved root growth under toxic aluminum conditions and produced root apices with higher ascorbate concentrations. To our knowledge, this is the first report of a correlation between boron nutrition, ascorbate concentration in root apices, and growth. These findings show that root growth inhibition resulting from either boron deficiency or aluminum toxicity may be a consequence of disrupted ascorbate metabolism.
Effect of High Temperature on Photosynthesis in Beans (I. Oxygen Evolution and Chlorophyll Fluorescence)
Authors: Pastenes C. , Horton P.
Published: October 2016

We studied the effect of increasing temperature on photosynthesis in two bean (Phaseolus vulgaris L.) varieties known to differ in their resistance to extreme high temperatures, Blue Lake (BL), commercially available in the United Kingdom, and Barbucho (BA), noncommercially bred in Chile. We paid particular attention to the energy-transducing mechanisms and structural responses inferred from fluorescence kinetics. The study was conducted in non-photorespiratory conditions. Increases in temperature resulted in changes in the fluorescence parameters nonphotochemical quenching (qN) and photochemical quenching (qP) in both varieties, but to a different extent. In BL and BA the increase in qP and the decrease in qN were either completed at 30[deg]C or slightly changed following increases from 30 to 35[deg]C. No indication of photoinhibition was detected at any temperature, and the ratio of the quantum efficiencies of photosystem II (PSII) and O2 evolution remained constant from 20 to 35[deg]C. Measurements of 77-K fluorescence showed an increase in the photosystem I (PSI)/PSII ratio with temperature, suggesting an increase in the state transitions. In addition, measurements of fast-induction fluorescence revealed that the proportion of PSII[beta] centers increased with increasing temperatures. The extent of both changes were maximum at 30 to 35[deg]C, coinciding with the ratio of rates at temperatures differing by 10[deg]C for oxygen evolution.
Heterogeneity of Arabinogalactan-Proteins on the Plasma Membrane of Rose Cells
Authors: Serpe M. D. , Nothnagel E. A.
Published: October 2016

Arabinogalactan-proteins (AGPs) have been purified from the plasma membrane of suspension-cultured Paul's Scarlet rose (Rosa sp.) cells. The two most abundant and homogeneous plasma membrane AGP fractions were named plasma membrane AGP1 (PM-AGP1) and plasma membrane AGP2 (PM-AGP2) and had apparent molecular masses of 140 and 217 kD, respectively. Both PM-AGP1 and PM-AGP2 had β-(1–3)-, β-(1,6)-, and β-(1,3,6)-galactopyranosyl residues, predominantly terminal α--arabinofuranosyl residues, and (1,4)- and terminal glucuronopyranosyl residues. The protein moieties of PM-AGP1 and PM-AGP2 were both rich in hydroxyproline, alanine, and serine, but differed in the abundance of hydroxyproline, which was 1.6 times higher in PM-AGP2 than in PM-AGP1. Another difference was the overall protein content, which was 3.7% (w/w) in PM-AGP1 and 15% in PM-AGP2. As judged by their behavior on reverse-phase chromatography, PM-AGP1 and PM-AGP2 were not more hydrophobic than AGPs from the cell wall or culture medium. In contrast, a minor plasma membrane AGP fraction eluted later on reverse-phase chromatography and was more negatively charged at pH 5 than either PM-AGP1 or PM-AGP2. The more negatively charged fraction contained molecules with a glycosyl composition characteristic of AGPs and included at least two different macromolecules. The results of this investigation indicate that Rosa plasma membrane contains at least four distinct AGPs or AGP-like molecules. These molecules differed from each other in size, charge, hydrophobicity, amino-acyl composition, and/or protein content.
The Role of Ligand Exchange in the Uptake of Iron from Microbial Siderophores by Gramineous Plants
Authors: Yehuda Z. , Shenker M. , Romheld V. , Marschner H. , Hadar Y. , Chen Y.
Published: October 2016

The siderophore rhizoferrin, produced by the fungus Rhizopus arrhizus, was previously found to be as an efficient Fe source as Fe-ethylenediamine-di(o-hydroxphenylacetic acid) to strategy I plants. The role of this microbial siderophore in Fe uptake by strategy II plants is the focus of this research. Fe-rhizoferrin was found to be an efficient Fe source for barley (Hordeum vulgare L.) and corn (Zea mays L.). The mechanisms by which these Gramineae utilize Fe from Fe-rhizoferrin and from other chelators were studied. Fe uptake from 59Fe-rhizoferrin, 59Fe-ferrioxamine B, 59Fe-ethylenediaminetetraacetic acid, and 59Fe-2[prime]-deoxymugineic acid by barley plants grown in nutrient solution at pH 6.0 was examined during periods of high (morning) and low (evening) phytosiderophore release. Uptake and translocation rates from Fe chelates paralleled the diurnal rhythm of phytosiderophore release. In corn, however, similar uptake and translocation rates were observed both in the morning and in the evening. A constant rate of the phytosiderophore's release during 14 h of light was found in the corn cv Alice. The results presented support the hypothesis that Fe from Fe-rhizoferrin is taken up by strategy II plants via an indirect mechanism that involves ligand exchange between the ferrated microbial siderophore and phytosiderophores, which are then taken up by the plant. This hypothesis was verified by in vitro ligand-exchange experiments.
Oxidative Stress Results in Increased Sinks for Metabolic Energy during Aging and Sprouting of Potato Seed-Tubers
Authors: Kumar GNM. , Knowles N. R.
Published: October 2016

Glutathione-mediated free-radical-scavenging and plasma membrane ATPase activities increase as sinks for metabolic energy with advancing tuber age. Plasma membrane ATPase activity from 19-month-old tubers was 77% higher than that from 7-month-old tubers throughout sprouting. The higher activity was not attended by an increase in the amount of ATPase per unit plasma membrane protein. Concentrations of oxidized (GSSG) and reduced glutathione more than doubled as tuber age advanced from 6 to 30 months, but the proportion of GSSG to total glutathione remained constant with age. The activity of glutathione transferase, an enzyme that catabolizes lipid-hydroperoxides, increased by 44 and 205% on a fresh weight and protein basis, respectively, as tubers aged from 6 to 30 months. Glutathione reductase activity also increased with advancing age, by 90% on a fresh weight basis and 305% on a protein basis. Older tubers had more glutathione reductase per unit of soluble and mitochondrial protein. The age-induced increase in cytosolic glutathione transferase activity was likely due to increased availability of lipid-hydroperoxides and/or a positive effector. Synthesis of glutathione requires ATP, and the increased reduction of GSSG resulting from catalysis of lipid-hydroperoxides is NADPH-dependent. Thus, increased plasma membrane ATPase and glutathione-mediated free-radical-scavenging activities likely constitute substantial sinks for ATP in older tubers prior to and during sprouting. Increased oxidative stress and loss in membrane integrity and central features of aging that undoubtedly contribute to the enhanced respiration of sprouting older tubers.
Polyamine Accumulation and Near Loss of Morphogenesis in Long-Term Callus Cultures of Rice (Restoration of Plant Regeneration by Manipulation of Cellular Polyamine Levels)
Authors: Bajaj S. , Rajam M. V.
Published: October 2016

We have shown (S. Bajaj and M.V. Rajam [1995] Plant Cell Rep 14: 717–720) that a significant reduction in morphogenetic potential occurs in callus cultures of rice (Oryza sativa L. cv TN-1) (up to 1 year old), and that plant regeneration could be improved in such cultures with spermidine treatment. We now show a near loss in plant regeneration capacity, concomitant with massive polyamine accumulation (primarily the diamine putrescine), due to the increase in arginine decarboxylase activity and an altered putrescine-to-spermidine ratio in 20- and 36-month-old rice callus cultures. The blockage of polyamine accumulation due to the reduction in arginine decarboxylase activity by a putrescine synthesis inhibitor, α--difluoromethylarginine, completely restored plant regeneration capacity in these long-term cultures. Additionally, spermidine treatment of long-term cultures caused an increase in cellular spermidine content and a reduction in putrescine content and arginine decarboxylase activity, leading to an adjustment in putrescine-to-spermidine ratio and the restoration of plant regeneration ability.
Red Xylem and Higher Lignin Extractability by Down-Regulating a Cinnamyl Alcohol Dehydrogenase in Poplar
Authors: Baucher M. , Chabbert B. , Pilate G. , Van Doorsselaere J. , Tollier M. T. , Petit-Conil M. , Cornu D. , Monties B. , Van Montagu M. , Inze D. , Jouanin L. , Boerjan W.
Published: October 2016

Cinnamyl alcohol dehydrogenase (CAD) catalyzes the last step in the biosynthesis of the lignin precursors, the monolignols. We have down-regulated CAD in transgenic poplar (Populus tremula X Populus alba) by both antisense and co-suppression strategies. Several antisense and sense CAD transgenic poplars had an approximately 70% reduced CAD activity that was associated with a red coloration of the xylem tissue. Neither the lignin amount nor the lignin monomeric composition (syringyl/guaiacyl) were significantly modified. However, phloroglucinol-HCl staining was different in the down-regulated CAD plants, suggesting changes in the number of aldehyde units in the lignin. Furthermore, the reactivity of the cell wall toward alkali treatment was altered: a lower amount of lignin was found in the insoluble, saponified residue and more lignin could be precipitated from the soluble alkali fraction. Moreover, large amounts of phenolic compounds, vanillin and especially syringaldehyde, were detected in the soluble alkali fraction of the CAD down-regulated poplars. Alkaline pulping experiments on 3-month-old trees showed a reduction of the kappa number without affecting the degree of cellulose degradation. These results indicate that reducing the CAD activity in trees might be a valuable strategy to optimize certain processes of the wood industry, especially those of the pulp and paper industry.
Acclimation of Foliar Antioxidant Systems to Growth Irradiance in Three Broad-Leaved Evergreen Species
Authors: Grace S. C. , Logan B. A.
Published: October 2016

The protective role of leaf antioxidant systems in the mechanism of plant acclimation to growth irradiance was studied in Vinca major, Schefflera arboricola, and Mahonia repens, which were grown for several months at 20, 100, and 1200 [mu]mol photons m-2 s-1. As growth irradiance increased, several constituents of the “Mehler-peroxidase” pathway also increased: superoxide dismutase, ascorbate peroxidase, glutathione reductase, ascorbate, and glutathione. This occurred concomitantly with increases in the xanthophyll cycle pool size and in the rate of nonphotochemical energy dissipation under steady-state conditions. There was no evidence for photosystem II overreduction in plants grown at high irradiance, although the reduction state of the stromal NADP pool, estimated from measurements of NADP-malate dehydrogenase activity, was greater than 60% in V. major and S. arboricola. Ascorbate, which removes reactive O2 species generated by O2 photoreduction in the chloroplast and serves as a reductant for the conversion of the xanthophyll cycle pigments to the de-epoxidized forms A plus Z, generally exhibited the most dramatic increases in response to growth irradiance. We conclude from these results that O2 photoreduction occurs at higher rates in leaves acclimated to high irradiance, despite increases in xanthophyll cycle-dependent energy dissipation, and that increases in leaf antioxidants protect against this potential oxidative stress.
Ethylene Biosynthesis during Aerenchyma Formation in Roots of Maize Subjected to Mechanical Impedance and Hypoxia
Authors: He Cj. , Finlayson S. A. , Drew M. C. , Jordan W. R. , Morgan P. W.
Published: October 2016

Germinated maize (Zea mays L.) seedlings were enclosed in modified triaxial cells in an artificial substrate and exposed to oxygen deficiency stress (4% oxygen, hypoxia) or to mechanical resistance to elongation growth (mechanical impedance) achieved by external pressure on the artificial substrate, or to both hypoxia and impedance simultaneously. Compared with controls, seedlings that received either hypoxia or mechanical impedance exhibited increased rates of ethylene evolution, greater activities of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, ACC oxidase, and cellulase, and more cell death and aerenchyma formation in the root cortex. Effects of hypoxia plus mechanical impedance were strongly synergistic on ethylene evolution and ACC synthase activity; cellulase activity, ACC oxidase activity, or aerenchyma formation did not exhibit this synergism. In addition, the lag between the onset of stress and increases in both ACC synthase activity and ethylene production was shortened by 2 to 3 h when mechanical impedance or impedance plus hypoxia was applied compared with hypoxia alone. The synergistic effects of hypoxia and mechanical impedance and the earlier responses to mechanical impedance than to hypoxia suggest that different mechanisms are involved in the promotive effects of these stresses on maize root ethylene biosynthesis.
The Propagation of Slow Wave Potentials in Pea Epicotyls
Authors: Stahlberg R. , Cosgrove D. J.
Published: October 2016

Slow wave potentials are considered to be electric long-distance signals specific for plants, although there are conflicting ideas about a chemical, electrical, or hydraulic mode of propagation. These ideas were tested by comparing the propagation of hydraulic and electric signals in epicotyls of pea (Pisum sativum L). A hydraulic signal in the form of a defined step increase in xylem pressure (Px) was applied to the root of intact seedlings and propagated nearly instantly through the epicotyl axis while its amplitude decreased with distance from the pressure chamber. This decremental propagation was caused by a leaky xylem and created an axial Px gradient in the epicotyl. Simultaneously along the epicotyl surface, depolarizations appeared with lag times that increased acropetally with distance from the pressure chamber from 5 s to 3 min. When measured at a constant distance, the lag times increased as the size of the applied pressure steps decreased. We conclude that the Px gradient in the epicotyl caused local depolarizations with acropetally increasing lag times, which have the appearance of an electric signal propagating with a rate of 20 to 30 mm min-1. This static description of the slow wave potentials challenges its traditional classification as a propagating electric signal.
Responses of Antioxidants to Paraquat in Pea Leaves (Relationships to Resistance)
Authors: Donahue J. L. , Okpodu C. M. , Cramer C. L. , Grabau E. A. , Alscher R. G.
Published: October 2016

Differnential sensitivity to the oxidant paraquat was observed in pea (Pisum sativum L.) based on cultivar and leaf age. To assess contributions of inductive responses of the antioxidant enzymes in short-term resistance to oxidative damage, activities of glutathione reductase (GR), superoxide dismutase (SOD), and ascorbate peroxidase (APX) and transcript levels for plastidic GR, Cu,Zn SOD, and cytosolic APX were determined. Responses to paraquat exposure from three different leaf age classes of pea were studied. Resistance was correlated with leaf age, photosynthetic rates, enzyme activities, and pretreatment levels of plastid GR and plastid Cu,Zn SOD transcripts. In response to paraquat, small increases in activities of GR and APX were observed in the more resistant leaves. These changes were not reflected at the mRNA level for the plastidic GR or Cu,Zn SOD. Paraquat-mediated increases in cytosolic APX mRNA occurred in all leaf types, irrespective of resistance. Developmentally controlled mechanisms determining basal antioxidant enzyme activities, and not inductive responses, appear to be critical factors mediating short-term oxidative stress resistance.
Phloem Glutamine and the Regulation of O2 Diffusion in Legume Nodules
Authors: Neo H. H. , Layzell D. B.
Published: October 2016

The aim of the present study was to test the hypothesis that the N content or the composition of the phloem sap that supplies nodulated roots may play a role in the feedback regulation of nitrogenase activity by increasing nodule resistance to O2 diffusion. Treating shoots of lupin (Lupinus albus cv Manitoba) or soybean (Glycine max L. Merr. cv Maple Arrow) with 100 [mu]L L-1 NH3 caused a 1.3-fold (lupin) and 2.6-fold (soybean) increase in the total N content of phloem sap without altering its C content. The increase in phloem N was due primarily to a 4.8-fold (lupin) and 10.5-fold (soybean) increase in the concentration of glutamine N. In addition, there was a decline in both the apparent nitrogenase activity and total nitrogenase activity that began within 4 h and reached about 54% of its initial activity within 6 h of the start of the NH3 treatment. However, the potential nitrogenase activity values in the treated plants were not significantly different from those of the control plants. These results provide evidence that changes in the N composition of the phloem sap, particularly the glutamine content, may increase nodule resistance to O2 diffusion and, thereby, down-regulate nodule metabolism and nitrogenase activity by controlling the supply of O2 to the bacteria-infected cells.
Polyamines and Pectins (I. Ion Exchange and Selectivity)
Authors: Messiaen J. , Cambier P. , Van Cutsem P.
Published: October 2016

The ion-binding and -exchange properties of putrescine, spermidine, and spermine on purified walls of carrot (Daucus carota L.) cell suspensions were investigated by producing ion-exchange isotherms and comparing them with the behavior of Na+, Mg2+, and Ca2+. The cation exchange capacity of the carrot cell walls was 0.8 equivalent kg-1 dry matter, and the ionic selectivity sequence of the walls for polyamines followed the sequence spermine4+ > spermidine3+ [almost equal to] Ca2+ > putrescine2+. The polyamines were subjected to only electroselectivity and probably did not induce any favorable supramolecular conformation of pectin like the one induced by Ca2+. Triangular ion exchanges were also performed with three diamines: ethanediamine, butanediamine, and octanediamine. The shorter the diamine, the higher the total adsorption and selectivity of the exchange. The lower selectivity of the cell wall for putrescine was partly attributed to its inability to access and displace Ca2+ from higher affinity sites within dimerized pectic sequences. The polyamine adsorption and exchange on pectic sequences could result in pectic signal modulation in pathogenesis and in differentiation.
Natural Senescence of Pea Leaves (An Activated Oxygen-Mediated Function for Peroxisomes)
Authors: Pastori G. M. , del Rio L. A.
Published: October 2016

We studied the activated oxygen metabolism of peroxisomes in naturally and dark-induced senescent leaves of pea (Pisum sativum L.). Peroxisomes were purified from three different types of senescent leaves and the activities of different peroxisomal and glyoxysomal enzymes were measured. The activities of the O2-- and H2O2-producing enzymes were enhanced by natural senescence. Senescence also produced an increase in the generation of active oxygen species (O2- and H2O2) in leaf peroxisomes and in the activities of two glyoxylate-cycle marker enzymes. A new fraction of peroxisomes was detected at an advanced stage of dark-induced senescence. Electron microscopy revealed that this new peroxisomal fraction varied in size and electron density. During senescence, the constitutive Mn-superoxide dismutase (SOD) activity of peroxisomes increased and two new CuZn-SODs were induced, one of which cross-reacted with an antibody against glyoxysomal CuZn- SOD. This fact and the presence of glyoxylate-cycle enzymes support the idea that foliar senescence is associated with the transition of peroxisomes into glyoxysomes. Our results indicate that natural senescence causes the same changes in peroxisome-activated oxygen metabolism as dark-induced senescence, and reinforce the hypothesis of an effective role of peroxisomes and their activated oxygen metabolism in this stage of the life cycle.
Acetaldehyde Is a Causal Agent Responsible for Ethanol-Induced Ripening Inhibition in Tomato Fruit
Authors: Beaulieu J. C. , Peiser G. , Saltveit M. E.
Published: October 2016

Inhibition of tomato (Lycopersicon esculentum Mill.) fruit ripening by exogenously applied ethanol was shown to be caused by elevated endogenous levels of acetaldehyde (AA). Exposure of excised pericarp discs of mature-green tomato fruit to ethanol or AA vapors produced elevated levels of both compounds in the tissue, but only the levels of AA were associated with ripening inhibition. Ripening inhibition was dependent on both the applied concentration and the duration of exposure. Discs treated with inhibitory levels of AA had levels of ethanol that were elevated but below that associated with inhibition of ripening. The in vivo activity of alcohol dehydrogenase was inhibited 40 to 60% by 4-methylpyrazole (4-MP), a competitive inhibitor of this enzyme. The inhibitory effect of ethanol on ripening was reduced by the simultaneous application of 4-MP. Tissue treated with 4-MP plus AA vapors had higher endogenous levels of AA and ripening was inhibited longer than in tissue without 4-MP. The tissue AA level resulting from ethanol or AA application appears to be the critical determinant of ripening inhibition.
A β-Amylase in Potato Tubers Is Induced by Storage at Low Temperature
Authors: Nielsen T. H. , Deiting U. , Stitt M.
Published: October 2016

A new starch-degrading enzyme activity is induced by storage of potato (Solanum tuberosum L.) tubers at low temperatures (L. Hill, R. Reimholz, R. Schroder, T.H. Nielsen, M. Stitt [1996] Plant Cell Environ 14: 1223–1237). The cold-induced activity was separated from other amylolytic activities in zymograms based on iodine staining of polyacrylamide gels containing amylopectin. A similar band of activity was detected at normal growth temperatures in leaves, stems, and growing tubers but was present only at low activity in warm-stored tubers. The cold-induced enzyme was separated by ion-exchange chromatography from other amylolytic activities. It has a broad neutral pH optimum. Characterization of its hydrolytic activity with different substrates showed that the cold-induced activity is a β-amylase present at low activity in tubers stored at 20[deg]C and induced progressively when temperatures are decreased to 5 and 3[deg]C. The first clear induction of β-amylase activity was observed within 3 d of storage at 3[deg]C, and the activity increased 4- to 5-fold within 10 d. The possible involvement of the cold-induced β-amylase in sugar accumulation during cold storage is discussed.
Identification and Characterization of Linoleic Acid as an Endogenous Modulator of in Vitro N-1-Naphthylphthalamic Acid Binding
Authors: Suttle J. C.
Published: October 2016

An endogenous inhibitor of the in vitro binding of the phytotropin N-1-naphthylphthalamic acid to microsomal membranes was detected in extracts prepared from etiolated pea (Pisum sativum L.) epicotyls. Following extensive purification, the inhibitor was identified as linoleic acid. Authentic linoleic acid inhibited N-1-naphthylphthalamic acid binding noncompetitively in a dose-dependent manner, exhibiting a 50% inhibitory concentration of approximately 24 ([mu]M. Using a variety of fatty acids and their derivatives, this inhibition was found to exhibit strict structural requirements, with both linoleic and linolenic acids being the most inhibitory. A variety of membrane-solubilizing detergents elicited no such inhibitory activity when tested at equivalent concentrations. The possible physiological significance of this interaction is discussed and it is proposed that linoleic acid serves as an intracellular modulator of phytotropin binding and therefore polar auxin transport.
Distinction between Endoplasmic Reticulum-Type and Plasma Membrane-Type Ca2+ Pumps (Partial Purification of a 120-Kilodalton Ca2+-ATPase from Endomembranes)
Authors: Hwang I. , Ratterman D. M. , Sze H.
Published: October 2016

Two biochemical types of Ca2+-pumping ATPases were distinguished in membranes that were isolated from carrot (Daucus carota) suspension-cultured cells. One type hydrolyzed GTP nearly as well as ATP, was stimulated by calmodulin, and was resistant to cyclopiazonic acid. This plasma membrane (PM)-type pump was associated with PMs and endomembranes, including vacuolar membranes and the endoplasmic reticulum (ER). Another pump (“ER-type”) that was associated mainly with the ER hydrolyzed ATP preferentially, was insensitive to calmodulin, and was inhibited partially by cyclopiazonic acid, a blocker of the animal sarcoplasmic/ER Ca2+ pump. Oxalate stimulation of Ca2+ accumulation by ER-type, but not PM-type, pump(s) indicated a separation of the two types on distinct compartments. An endomembrane 120-kD Ca2+ pump was partially purified by calmodulin-affinity chromatography. The purified polypeptide bound calmodulin reacted with antibodies to a calmodulin-stimulated Ca2+ pump from cauliflower and displayed [32P]phosphoenzyme properties that are characteristic of PM-type Ca2+ pumps. The purified ATPase corresponded to a phosphoenzyme and a 120-kD calmodulin-binding protein on endomembranes. Another PM-type pump was suggested by a 127-kD PM-associated protein that bound calmodulin. Thus, both ER- and PM-type Ca2+ pumps coexist in most plant tissues, and each type can be distinguished from another by a set of traits, even in partially purified membranes.
Characterization of the Glycerolipid Composition and Biosynthetic Capacity of Pea Root Plastids
Authors: Xue L. , McCune L. M. , Kleppinger-Sparace K. F. , Brown M. J. , Pomeroy M. K. , Sparace S. A.
Published: October 2016

The glycerolipid composition of pea (Pisum sativum L.) root plastids and their capacity to synthesize glycerolipids from [UL-14C]glycerol-3-phosphate were determined. Pea root plastids primarily consist of monogalactosyldiacylglycerol, triacylglycerol, phosphatidylcholine, digalactosyldiacylglycerol, and diacylglycerol. Maximum rates of total glycerolipid biosynthesis were obtained in the presence of 2.4 mM glycerol-3-phosphate, 15 mM KHCO3, 0.2 mM sodium-acetate, 0.5 mM each of NADH and NADPH, 0.05 mM coenzyme A, 2 mM MgCl2, 1 mM ATP, 0.1 M Bis-Tris propane (pH 7.5), and 0.31 M sorbitol. Glycerolipid biosynthesis was completely dependent on exogenously supplied ATP, coenzyme A, and a divalent cation, whereas the remaining cofactors improved their activity from 1.3- to 2.4-fold. Radioactivity from glycerol-3-phosphate was recovered predominantly in phosphatidic acid, phosphatidylglycerol, diacylglycerol, and triacylglycerol with lesser amounts in phosphatidylcholine and monoacylglycerol. The proportions of the various radiolabeled lipids that accumulated were dependent on the pH and the concentration of ATP and glycerol-3-phosphate. The data presented indicate that pea root plastids can synthesize almost all of their component glycerolipids and that glycerolipid biosynthesis is tightly coupled to de novo fatty acid biosynthesis. pH and the availability of ATP may have important roles in the regulation of lipid biosynthesis at the levels of phosphatidic acid phosphatase and in the reactions that are involved in phosphatidylglycerol and triacylglycerol biosynthesis.
Hypoosmotic Shock Induces Increases in Cytosolic Ca2+ in Tobacco Suspension-Culture Cells
Authors: Takahashi K. , Isobe M. , Knight M. R. , Trewavas A. J. , Muto S.
Published: October 2016

Hypoosmotic shock treatment increased cytosolic Ca2+ ion concentration ([Ca2+]cyt) in tobacco (Nicotiana tabacum) suspension-culture cells. [Ca2+]cyt measurements were made by genetically transforming these cells to express apoaequorin and by reconstituting the Ca2+-dependent photoprotein, aequorin, in the cytosol by incubation with chemically synthesized coelenterazine. Measurement of Ca2+-dependent luminescence output thus allowed the direct monitoring of [Ca2+]cyt changes. When cells were added to a hypoosmotic medium, a biphasic increase in [Ca2+]cyt was observed; an immediate small elevation (phase 1) was observed first, followed by a rapid, large elevation (phase 2). Phase 1 [Ca2+]cyt was stimulated by the V-type ATPase inhibitor bafilomycin A1. Phase 2 was inhibited by the protein kinase inhibitor K-252a and required the continued presence of the hypoosmotic stimulus to maintain it. Although Ca2+ in the medium was needed to produce phase 2, it was not needed to render the cells competent to the hypoosmotic stimulus. If cells were subject to hypoosmotic shock in Ca2+- depleted medium, increases in luminescence could be induced up to 20 min after the shock by adding Ca2+ to the medium. These data suggest that hypoosmotic shock-induced [Ca2+]cyt elevation results from the activity of a Ca2+ channel in the plasma membrane or associated hypoosmotic sensing components that require Ca2+- independent phosphorylation and a continued stimulus to maintain full activity.
Accumulation of Zeaxanthin in Abscisic Acid-Deficient Mutants of Arabidopsis Does Not Affect Chlorophyll Fluorescence Quenching or Sensitivity to Photoinhibition in Vivo
Authors: Hurry V. , Anderson J. M. , Chow W. S. , Osmond C. B.
Published: October 2016

Abscisic acid (ABA)-deficient mutants of Arabidopsis do not synthesize the epoxy-xanthophylls antheraxanthin, violaxanthin, or neoxanthin. However, thylakoid membranes from these mutants contain 3-fold more zeaxanthin than wild-type plants. This increase in zeaxanthin occurs as a stoichiometric replacement of the missing violaxanthin and neoxanthin within the pigment-protein complexes of both photosystem I and photosystem II (PSII). The retention of zeaxanthin in the dark by ABA-deficient mutants sensitizes the leaves to the development of nonphotochemical quenching (NPQ) during the first 2 to 4 min following a dark-light transition. However, the increase in pool size does not result in any increase in steady-state NPQ. When we exposed wild-type and ABA-deficient mutants leaves to twice growth irradiance, the mutants developed lower maximal NPQ but suffered similar photoinhibition to wildtype, measured both as a decline in the ratio of variable to maximal fluorescence and as a loss of functional PSII centers from oxygen flash yield measurements. These results suggest that only a few of the zeaxanthin molecules present within the light-harvesting antenna of PSII may be involved in NPQ and neither the accumulation of a large pool of zeaxanthin within the antenna of PSII nor an increase in conversion of violaxanthin to zeaxanthin will necessarily enhance photoprotective energy dissipation.
Molecular Genetics of Crassulacean Acid Metabolism
Authors: Cushman J. C. , Bohnert H. J.
Published: October 2016

Most higher plants assimilate atmospheric CO2 through the C3 pathway of photosynthesis using ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). However, when CO2 availability is reduced by environmental stress conditions, the incomplete discrimination of CO2 over O2 by Rubisco leads to increased photorespiration, a process that reduces the efficiency of C3 photosynthesis. To overcome the wasteful process of photorespiration, approximately 10% of higher plant species have evolved two alternate strategies for photosynthetic CO2 assimilation, C4 photosynthesis and Crassulacean acid metabolism. Both of these biochemical pathways employ a “CO2 pump” to elevate intracellular CO2 concentrations in the vicinity of Rubisco, suppressing photorespiration and therefore improving the competitiveness of these plants under conditions of high light intensity, high temperature, or low water availability. This CO2 pump consists of a primary carboxylating enzyme, phosphoenolpyruvate carboxylase. In C4 plants, this CO2-concentrating mechanism is achieved by the coordination of two carboxylating reactions that are spatially separated into mesophyll and bundle-sheath cell types (for review, see R.T. Furbank, W.C. Taylor [1995] Plant Cell 7: 797–807;M.S.B. Ku, Y. Kano-Murakami, M. Matsuoka [1996] Plant Physiol 111: 949–957). In contrast, Crassulacean acid metabolism plants perform both carboxylation reactions within one cell type, but the two reactions are separated in time. Both pathways involve cell-specific changes in the expression of many genes that are not present in C3 plants.
Enhanced Employment of the Xanthophyll Cycle and Thermal Energy Dissipation in Spinach Exposed to High Light and N Stress
Authors: Verhoeven A. S. , Demmig-Adams B. , Adams III W. W.
Published: October 2016

The involvement of the xanthophyll cycle in photoprotection of N-deficient spinach (Spinacia oleracea L. cv Nobel) was investigated. Spinach plants were fertilized with 14 mM nitrate (control, high N) versus 0.5 mM (low N) fertilizer, and grown under both high- and low-light conditions. Plants were characterized from measurements of photosynthetic oxygen exchange and chlorophyll fluorescence, as well as carotenoid and cholorophyll analysis. Compared with the high-N plants, the low-N plants showed a lower capacity for photosynthesis and a lower chlorophyll content, as well as a lower rate of photosystem II photosynthetic electron transport and a corresponding increase in thermal energy dissipation activity measured as nonphotochemical fluorescence quenching. The low-N plants displayed a greater fraction of the total xanthophyll cycle pool as zeaxanthin and antheraxanthin at midday, and an increase in the ratio of xanthophyll cycle pigments to total chlorophyll. These results indicate that under N limitation both the light-collecting system and the photosynthetic rate decrease. However, the increased dissipation of excess energy shows that there is excess light absorbed at midday. We conclude that spinach responds to N limitation by a combination of decreased light collection and increased thermal dissipation involving the xanthophyll cycle.
Regulation of the Cinnamate 4-Hydroxylase (CYP73A1) in Jerusalem Artichoke Tubers in Response to Wounding and Chemical Treatments
Authors: Batard Y. , Schalk M. , Pierrel M. A. , Zimmerlin A. , Durst F. , Werck-Reichhart D.
Published: October 2016

trans-Cinnamate 4-hydroxylase (C4H) is a plant-specific cytochrome (P450) that is encoded by the gene CYP73A and catalyzes the second step of the multibranched phenylpropanoid pathway. Increases in C4H activity in response to physical and chemical stresses have been well documented, but the mechanism of these increases has never been studied in detail. This paper reports on the regulatory mechanism controlling C4H activity in Jerusalem artichoke (Helianthus tuberosus) tubers in response to wounding and chemical treatments. We compared induction of C4H and other P450-catalyzed activities. C4H was moderately induced by chemicals relative to other P450s. Increases in enzyme activity, C4H protein, and transcripts were quantified and compared in tuber tissue 48 h after wounding and chemical treatments. Our data suggest that induction of the enzyme activity results primarily from gene activation. Time-course experiments were performed after wounding and aminopyrine treatment. Compared with wounded tissues, aminopyrine triggered an additional and delayed peak of transcript accumulation. The timing of the induced changes in activity, protein, and transcripts confirms that C4H induction results primarily from an increase in CYP73A1 mRNA, in both wounded and aminopyrine-treated tissues. However, posttranscriptional mechanisms might also contribute to the regulation of C4H activity.