Inhibition of blue light-dependent H+ pumping by abscisic acid through hydrogen peroxide-induced dephosphorylation of the plasma membrane H+-ATPase in guard cell protoplasts

Blue light (BL)-dependent H+ pumping by guard cells, which drives stomatal opening, is inhibited by abscisic acid (ABA). We investigated this response with respect to the activity of plasma membrane H+-ATPase using Vicia guard cell protoplasts. ATP hydrolysis by the plasma membrane H+-ATPase, phosphorylation of the H+-ATPase, and the binding of 14-3-3 protein to the H+-ATPase stimulated by BL were inhibited by ABA at 10 microm. All of these responses were similarly inhibited by hydrogen peroxide (H2O2) at 1 mm. The ABA-induced inhibitions of BL-dependent H+ pumping and phosphorylation of the H+-ATPase were partially restored by ascorbate, an intracellular H2O2 scavenger. A single-cell analysis of the cytosolic H2O2 using 2',7'-dichlorofluorescin revealed that H2O2 was generated by ABA in guard cell protoplasts. We also indicated that H+ pumping induced by fusicoccin and the binding of 14-3-3 protein to the H+-ATPase were inhibited slightly (approximately 20%) by both ABA and H2O2. By contrast, H2O2 at 1 mm did not affect H+ pumping by the H+-ATPase in microsomal membranes. From these results, we concluded that inhibition of BL-dependent H+ pumping by ABA was due to a decrease in the phosphorylation levels of H+-ATPase and that H2O2 might be involved in this response. Moreover, there are at least two inhibition sites by ABA in the BL signaling pathway of guard cells.     

Properties of Proton Pumping in Response to Blue Light and Fusicoccin in Guard Cell Protoplasts Isolated from Adaxial Epidermis of Vicia Leaves

Guard cell protoplasts (GCPs) were isolated from the adaxial epidermis of Vicia leaves. The properties of isolated adaxial GCPs (ad GCPs) were compared with those of abaxial GCPs (ab GCPs) with respect to H+-pumping activity. A saturating pulse of blue light (200 [mu]mol m-2 s-1, 30 s) induced H+ pumping in both ad GCPs and ab GCPs under red light. The maximum rate of blue-light-dependent H+ pumping was slightly higher in ad GCPs than in ab GCPs, but the magnitude of H+ pumping in ad GCPs was 68% of that in ab GCPs. H+ pumping was responsive to the second pulse, and the rate and magnitude of the pumping increased with the time between two pulses. The periods required to achieve 50% of the maximum rate were 12 and 22 min for ad GCPs and ab GCPs, respectively. The rates of blue-light-dependent H+ pumping were saturable, with half-saturation at 630 [mu]mol m-2 (21 [mu]mol m-2 s-1, 30 s) for ad GCPs and 105 [mu]mol m-2 (3.5 [mu]mol m-2 s-1, 30s) for ab GCPs. In contrast, fusicoccin, an activator of the plasma membrane H+- ATPase, induced H+ pumping with a slightly higher rate in ad GCPs than in ab GCPs. Both types of protoplast swelled similarly in response to fusicoccin. These results suggest that ad GCPs have almost the same activity for H+ pumping as ab GCPs, whereas ad GCPs require a larger number of photons to activate the H+ pump than ab GCPs.     

Cytosolic Concentration of Ca2+ Regulates the Plasma Membrane H+-ATPase in Guard Cells of Fava Bean

Opening of the stomata is driven by the light-activated plasma membrane proton pumping ATPase, although the activation and inactivation mechanism of the enzyme is not known. In this study, we show that the H+-ATPase in guard cells is reversibly inhibited by Ca2+ at physiological concentrations. Isolated microsomal membranes of guard cell protoplasts from fava bean exhibited vanadate-sensitive, ATP-dependent proton pumping. The activity was inhibited almost completely by 1 [mu]M Ca2+ with a half-inhibitory concentration at 0.3 [mu]M and was restored immediately by the addition of 1,2-bis(2-aminophenoxy)ethane N,N,N[prime],N[prime]-tetraacetic acid, a calcium chelating reagent. Similar reversible inhibition by Ca2+ was shown by the generation of electrical potential in the membranes. Activity of ATP hydrolysis was inhibited similarly by Ca2+ in the same membrane preparations. The addition of 1,2-bis(2-aminophenoxy)ethane N,N,N[prime],N[prime]-tetraacetic acid and EGTA, Ca2+ chelators, to epidermal peels of fava bean induced stomatal opening in the dark, and the opening was suppressed by vanadate. This suggests that the lowered cytosolic Ca2+ activated the proton pump in vivo and that the activated pump elicited stomatal opening. Inhibition of H+-ATPase by Ca2+ may depolarize the membrane potential and could be a key step in the process of stomatal closing through activation of the anion channels. Furthermore, similar inhibition of the proton pumping and ATP hydrolysis by Ca2+ was found in isolated plasma membranes of mesophyll cells of fava bean. These results suggest that Ca2+ regulates the activity of plasma membrane H+-ATPases in higher plant cells, thereby modulating stomatal movement and other cellular processes in plants.     

Involvement of intracellular Ca2+ in blue light-dependent proton pumping in guard cell protoplasts from Vicia faba

Recent studies have suggested that Ca²⁺/calmodulin (CaM) or CaM-like proteins may be involved in blue light (BL)-dependent proton pumping in guard cells. As the increase in cytosolic concentration of Ca²⁺ is required for the activation of CaM and CaM-like proteins, the origin of the Ca²⁺ was investigated by measuring BL-dependent proton pumping with various treatments using guard cell protoplasts (GCPs) from Vicia faba . BL-dependent proton pumping was affected neither by Ca²⁺ channel blockers nor by changes of Ca²⁺ concentration in the medium used for the GCPs. Addition of Ca²⁺ ionophores and an agonist to GCPs did not induce proton pumping. However, BL-dependent proton pumping was inhibited by 10 m M caffeine, which releases Ca²⁺ from the intracellular stores, and by 10 μ M 2,5-di-( tert -butyl)-1,4-benzohydroquinone (BHQ) and 10 μ M cyclopiazonic acid (CPA), inhibitors of Ca²⁺-ATPase in the sarcoplasmic and endoplasmic reticulum (ER). By contrast, the inhibitions were not observed by 10 μ M thapsigargin, an inhibitor of animal ER-type Ca²⁺-ATPase. The inhibitions by caffeine and BHQ were reversible. Light-dependent stomatal opening in the epidermis of Vicia was inhibited by caffeine, BHQ, and CPA. From these results, we conclude that the Ca²⁺ thought to be required for BL-dependent proton pumping may originate from intracellular Ca²⁺ stores, most likely from ER in guard cells, and that this origin of Ca²⁺ may generate a stimulus-specific Ca²⁺ signal for stomatal opening.     

Characterization of the Red Beet Plasma Membrane H+-ATPase Reconstituted in a Planar Bilayer System

The transport activity of the red beet (Beta vulgaris L.) plasma membrane H+-ATPase was examined following reconstitution into a planar bilayer membrane. Fusion of partially purified plasma membrane H+-ATPase with the bilayer membrane was accomplished by perfusion of proteoliposomes against the bilayer under hypoosmotic conditions. Following incorporation into the bilayer, an ATP-dependent current was measured that demonstrated properties consistent with those of the plasma membrane H+-ATPase. Current production was substrate specific for ATP, inhibited by orthovanadate, and insensitive to 200 nM erythrosin B but inhibited by 100 [mu]M erythrosin B. When current production was measured as a function of Mg:ATP concentration, a simple Michaelis-Menten relationship was observed and a Km of 0.62 mM was estimated. Current-voltage analysis of ATP-dependent current in the presence of 0.5 mM ATP, 20 mM ADP, 40 mM orthophosphate, and an opposing 2.5-unit [delta]pH revealed a reversal potential of about -149 mV. Based on the free energy available from ATP hydrolysis, this reversal potential is consistent with an H+/ATP stoichiometry of 1. This study demonstrates the usefulness of a planar bilayer system for investigation of energy coupling to H+ transport by the plasma membrane H+-ATPase.     

Role of the Plasma Membrane H+-ATPase in K+ Transport

The role of the plant plasma membrane H+-ATPase in K+ uptake was examined using red beet (Beta vulgaris L.) plasma membrane vesicles and a partially purified preparation of the red beet plasma membrane H+-ATPase reconstituted in proteoliposomes and planar bilayers. For plasma membrane vesicles, ATP-dependent K+ efflux was only partially inhibited by 100 [mu]M vanadate or 10 [mu]M carbonyl cyanide-p-trifluoromethoxyphenylhydrazone. However, full inhibition of ATP-dependent K+ efflux by these reagents occurred when the red beet plasma membrane H+-ATPase was partially purified and reconstituted in proteoliposomes. When reconstituted in a planar bilayer membrane, the current/voltage relationship for the plasma membrane H+-ATPase showed little effect of K+ gradients imposed across the bilayer membrane. When taken together, the results of this study demonstrate that the plant plasma membrane H+-ATPase does not mediate direct K+ transport chemically linked to ATP hydrolysis. Rather, this enzyme provides a driving force for cellular K+ uptake by secondary mechanisms, such as K+ channels or H+/K+ symporters. Although the presence of a small, protonophore-insensitive component of ATP-dependent K+ transport in a plasma membrane fraction might be mediated by an ATP-activated K+ channel, the possibility of direct K+ transport by other ATPases (i.e. K+-ATPases) associated with either the plasma membrane or other cellular membranes cannot be ruled out.     

A Plasma Membrane-Type Ca2+-ATPase of 120 Kilodaltons on the Endoplasmic Reticulum from Carrot (Daucus carota) Cells (Properties of the Phosphorylated Intermediate)

Cytosolic Ca2+ levels are regulated in part by Ca2+-pumping ATPases that export Ca2+ from the cytoplasm; however, the types and properties of Ca2+ pumps in plants are not well understood. We have characterized the kinetic properties of a 120-kD phosphoenzyme (PE) intermediate formed during the reaction cycle of a Ca2+-ATPase from suspension-cultured carrot (Daucus carota) cells. Only one Ca2+-dependent phosphoprotein was formed when carrot membrane vesicles were incubated with [[gamma]-32P]ATP (W.L. Hsieh, W.S. Pierce, and H. Sze [1991] Plant Physiol 97: 1535-1544). Formation of this 120-kD phosphoprotein was inhibited by vanadate, enhanced by La3+, and decreased by hydroxylamine, confirming its identification as an intermediate of a phosphorylated-type Ca2+-translocating ATPase. The 120-kD Ca2+-ATPase was most abundant in endoplasmic reticulum-enriched fractions, in which the Ca2+-ATPase was estimated to be 0.1% of membrane protein. Direct quantitation of Ca2+-dependent phosphoprotein was used to examine the kinetics of PE formation. PE formation exhibited a Km for Ca2+ of 1 to 2 [mu]M and a Km for ATP of 67 nM. Relative affinities of substrates, determined by competition experiments, were 0.075 [mu]M for ATP, 1 [mu]M for ADP, 100 [mu]M for ITP, and 250 [mu]M for GTP. Thapsigargin and cyclopiazonic acid, specific inhibitors of animal sarcoplasmic/endoplasmic reticulum Ca2+-ATPase, had no effect on PE formation; erythrosin B inhibited with 50% inhibition at <0.1 [mu]M. Calmodulin (1 [mu]M) stimulated PE formation by 25%. The results indicate that the carrot 120-kD Ca2+-ATPase is similar but not identical to animal plasma membrane-type Ca2+- ATPase and yet is located on endomembranes, such as the endoplasmic reticulum. This type of Ca2+ pump may reside on the cortical endoplasmic reticulum, which is thought to play a major role in anchoring the cytoskeleton and in facilitating secretion.     

Immunohistochemical detection of blue light-induced phosphorylation of the plasma membrane H+-ATPase in stomatal guard cells

Blue light (BL) receptor phototropins activate the plasma membrane H(+)-ATPase in guard cells through phosphorylation of a penultimate threonine and subsequent binding of the 14-3-3 protein to the phosphorylated C-terminus of H?-ATPase, mediating stomatal opening. To date, detection of the phosphorylation level of the guard cell H?-ATPase has been performed biochemically using guard cell protoplasts (GCPs). However, preparation of GCPs from Arabidopsis for this purpose requires >5,000 rosette leaves and takes >8 h. Here, we show that BL-induced phosphorylation of guard cell H?-ATPase is detected in the epidermis from a single Arabidopsis rosette leaf via an immunohistochemical method using a specific antibody against the phosphorylated penultimate threonine of H?-ATPase. BL-induced phosphorylation of the H?-ATPase was detected immunohistochemically in the wild type, but not in a phot1-5 phot2-1 double mutant. Moreover, we found that physiological concentrations of the phytohormone ABA completely inhibited BL-induced phosphorylation of guard cell H?-ATPase in the epidermis, and that inhibition by ABA in the epidermis is more sensitive than in GCPs. These results indicate that this immunohistochemical method is very useful for detecting the phosphorylation status of guard cell H?-ATPase. Thus, we applied this technique to ABA-insensitive mutants (abi1-1, abi2-1 and ost1-2) and found that ABA had no effect on BL-induced phosphorylation in these mutants. These results indicate that inhibition of BL-induced phosphorylation of guard cell H?-ATPase by ABA is regulated by ABI1, ABI2 and OST1, which are known to be early ABA signaling components for a wide range of ABA responses in plants.     

Effects of Boron on Proton Transport and Membrane Properties of Sunflower (Helianthus annuus L.) Cell Microsomes

Boron deficiency and toxicity inhibit ATP-dependent H+ pumping and vanadate-sensitive ATPase activity in sunflower roots and cell suspensions. The effects of boron on H+ pumping and on passive H+ conductance, as well as on fluorescence anisotropy in KI-washed microsomes isolated from sunflower (Helianthus annuus L. cv Enano) cell suspensions, have been investigated. Boron deficiency reduced the total and vanadate-sensitive ATPase activities as well as the vanadate-sensitive ATP-dependent H+ pumping without affecting the amount of antigenic ATPase protein as measured by immunoblotting with an Arabidopsis thaliana plasma membrane anti-H+-ATPase polyclonal antibody. Kinetic studies revealed that boron deficiency reduced Vmax of vanadate-sensitive ATPase activity with little change in the apparent Km for Mg2+-ATP. Proton leakage was greater in microsomal vesicles isolated from cells grown without boron and incubated in reaction medium without added boron, and this effect was reversed by addition of boron to the reaction medium. Fluorescence anisotropy indicated that diphenyl hexatriene and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene probes were immobilized to a greater extent in microsomes from cells grown without boron than in those from cells grown with 100 [mu]M H3BO3. The apparent decrease of membrane fluidity in microsomes from cells grown without boron was reversed by the addition of boron to the reaction medium. Taken together these data suggest that inhibition of H+ gradient formation in microsomes from sunflower cells grown in the absence of boron could be due to the combined effects of reduced H+-ATPase activity and increased passive conductance across the membrane, possibly resulting from increased membrane rigidity.     

3H]dizocilpine binding to N-methyl-D-aspartate (NMDA) receptor is modulated by an endogenous Na+, K+-ATPase inhibitor. Comparison with ouabain

An endogenous Na+, K+-ATPase inhibitor termed endobain E has been isolated from rat brain which shares several biological properties with ouabain. This cardiac glycoside possesses neurotoxic properties attributable to Na+, K+-ATPase inhibition, which leads to NMDA receptor activation, thus supporting the concept that Na+/K+ gradient impairment has a critical impact on such receptor function. To evaluate potential direct effects of endobain E and ouabain on NMDA receptors, we assayed [3H]dizocilpine binding employing a system which excludes ionic gradient participation. Brain membranes thoroughly washed and stored as pellets ('non-resuspended' membranes) or after resuspension in sucrose ('resuspended' membranes) were employed. Membrane samples were incubated with 4 or 10 nM ligand with or without added endobain E or ouabain, in the presence of different glutamate plus glycine combinations, with or without spermidine. [3H]dizocilpine basal binding and Na+, K+- and Mg2+-ATPase activities proved very similar in 'non-resuspended' or 'resuspended' membranes. Endobain E decreased [3H]dizocilpine binding to 'resuspended' membranes in a concentration-dependent manner, attaining roughly 50% binding inhibition with the highest endobain E concentration assayed. Among tested conditions, only in 'resuspended' membranes, with 4 nM ligand and with 1x10(-8) M glutamate plus 1x10(-5) M glycine, was [3H]dizocilpine binding enhanced roughly +24% by ouabain (1 mM). After Triton X-100 membrane treatment, which drastically reduces Na+, K+-ATPase activity, the effect of ouabain on binding was lost whereas that of endobain E remained unaltered. Results indicate that not only membrane preparation but also treatment and storage are crucial to observe direct endobain E and ouabain effects on NMDA receptor, which are not attributable to changes in Na+, K+-ATPase activity or to Na+/K+ equilibrium alteration.     

Phosphoryl Group Exchange between ATP and ADP Catalyzed by H+-ATPase from Oat Roots

ATP-ADP exchange was estimated in the presence of plasma membrane H+-ATPase of oat (Avena sativa) roots partially purified with Triton X-100 by measuring [14C]ATP formation from [14C]ADP. Most studies were done at 0[deg]C. At pH 6.0 the exchange showed: (a) Mg2+ requirement with a biphasic response giving maximal activity at 152 [mu]M and (b) insensitivity to ionic strength, [Na+], and [K+]. ATP and ADP dependence were analyzed with a model in which nucleotide-enzyme interactions are at rapid-random equilibrium, whereas E1ATP [left right arrow] E1P-ADP transitions occur in steady state. The results indicated competition between ADP and ATP for the catalytic site, whereas ATP interaction with the ADP site was extremely weak. At 0[deg]C the exchange showed a 3-fold pH increase, from pH 5.5 to 9.0. At an alkaline pH the reaction was not affected by sodium azide and carbonyl cyanide p-trifluometoxyphenyl-hydrazone, had a biphasic response to Mg2+ (maximal at 513 [mu]m), and was insensitive to ionic strength. At 20[deg]C ATP-ADP exchange was pH insensitive. At both temperatures ATP hydrolysis displayed a bell-shaped response, with a maximum around pH 6.0 to 6.5. Because no adenylate kinase activity was detected under any condition, these results demonstrate the existence of an ATP-ADP exchange reaction catalyzed by the plant H+-ATPase.     

Inhibition of Maize Root H+-ATPase by Fluoride and Fluoroaluminate Complexes

Vesicles derived from maize roots retain a membrane-bound H+-ATPase that is able to pump H+ at the expense of ATP hydrolysis. The H+ pumping and the ATPase activity of these vesicles are inhibited by lithium fluoride and by the complex formed between fluoride and aluminum. The inhibition promoted by lithium fluoride increases as the MgCl2 concentration in the medium is increased from 2 to 20 mM. The inhibitory activity of both lithium fluoride and aluminum fluoride increases as the temperature of the medium is increased from 20 to 35[deg]C. Inorganic phosphate (10-40 mM) inhibits the H+ -ATPase at pH 6.5 but not at pH 7.0, and at both pH values, it antagonizes the inhibition promoted by lithium fluoride and fluoroaluminate complexes.     

Evidence for an Extracellular Reception Site for Abscisic Acid in Commelina Guard Cells

The phytohormone abscisic acid (ABA) triggers stomatal closing as a physiological response to drought stress. Several basic questions limit an understanding of the mechanism of ABA reception in guard cells. Whether primary ABA receptors are located on the extracellular side of the plasma membrane, within the intracellular space of guard cells, or both remains unknown. Furthermore, it is not clear whether ABA must be transported into guard cells to exert control over stomatal movements. In the present study, a combination of microinjection into guard cells and physiological assays of stomatal movements have been performed to determine primary sites of ABA reception in guard cells. Microinjection of ABA into guard cells of Commelina communis L. resulted in injected cytosolic concentrations of 50 to 200 [mu]M ABA and in additional experiments in lower concentrations of approximately 1 [mu]M ABA. Stomata with ABA-loaded guard cells (n > 180) showed opening similar to stomata with uninjected guard cells. The viability of guard cells following ABA injection was demonstrated by neutral red staining as well as monitoring of stomatal opening. Extracellular application of 10 [mu]M ABA inhibited stomatal opening by 98% at pH 6.15 and by 57% at pH 8.0. The pH dependence of extracellular ABA action may suggest a contribution of an intracellular ABA receptor to stomatal regulation. The findings presented here show that intracellular ABA alone does not suffice to inhibit stomatal opening under the imposed conditions. Furthermore, these data provide evidence that a reception site for ABA-mediated inhibition of stomatal opening is on the extracellular side of the plasma membrane of guard cells.     

Effects of Acetyl-Coenzyme A Carboxylase Inhibitors on Root Cell Transmembrane Electric Potentials in Graminicide-Tolerant and -Susceptible Corn (Zea mays L.)

Herbicidal activity of aryloxyphenoxypropionate and cyclohexanedione herbicides (graminicides) has been proposed to involve two mechanisms: inhibition of acetyl-coenzyme A carboxylase (ACCase) and depolarization of cell membrane potential. We examined the effect of aryloxyphenoxypropionates (diclofop and haloxyfop) and cyclohexanediones (sethoxydim and clethodim) on root cortical cell membrane potential of graminicide-susceptible and -tolerant corn (Zea mays L.) lines. The graminicide-tolerant corn line contained a herbicide-insensitive form of ACCase. The effect of the herbicides on membrane potential was similar in both corn lines. At a concentration of 50 [mu]M, the cyclohexanediones had little or no effect on the membrane potential of root cells. At pH 6, 50 [mu]M diclofop, but not haloxyfop, depolarized membrane potential, whereas both herbicides (50 [mu]M) dramatically depolarized membrane potential at pH 5. Repolarization of membrane potential after removal of haloxyfop and diclofop from the treatment solution was incomplete at pH 5. However, at pH 6 nearly complete repolarization of membrane potential occurred after removal of diclofop. In graminicide-susceptible corn, root growth was significantly inhibited by a 24-h exposure to 1 [mu]M haloxyfop or sethoxydim, but cell membrane potential was unaffected. In gramincide-tolerant corn, sethoxydim treatment (1 [mu]M, 48 h) had no effect on root growth, whereas haloxyfop (1 [mu]M, 48 h) inhibited root growth by 78%. However, membrane potential was the same in roots treated with 1 [mu]M haloxyfop or sethoxydim. The results of this study indicate that graminicide tolerance in the corn line used in this investigation is not related to an altered response at the cell membrane level as has been demonstrated with other resistant species.     

The Two Km's for ATP of Corn-Root H+-ATPase and the Use of Glucose-6-Phosphate and Hexokinase as an ATP-Regenerating System

Plasma membrane vesicles derived from corn (Zea mays L.) roots retain a membrane-bound H+-ATPase that is able to form a H+ gradient across the vesicle membranes. The activity of this ATPase is enhanced 2- to 3-fold when Triton X-100 or lysophosphatidylcholine is added to the medium at a protein:detergent ratio of 2:1 (w/w). In the absence of detergent, the ATPase exhibits only one Km for ATP (0.1-0.2 mM), which is the same as for the pumping of H+. After the addition of either Triton X-100 or lysophosphatidylcholine, two Km's for ATP are detected, one in the range of 1 to 3 [mu]M and a second in the range of 0.1 to 0.2 mM. The Vmax of the second Km for ATP increases as the temperature of the assay medium is raised from 15[deg]C to 38[deg]C. The Arrhenius plot reveals a single break at 30[deg]C, both in the absence and in the presence of detergents. In the presence of Triton X-100 the H+-ATPase catalyzes the cleavage of glucose-6-phosphate when both hexokinase and ADP are included in the assay medium. There is no measurable cleavage when the apparent affinity for ATP of the H+-ATPase is not enhanced by Triton X-100 or when 1 mM glucose is included in the assay medium. These data indicate that when the high-affinity Km for ATP is unmasked with the use of detergent, the ATPase can use glucose-6-phosphate and hexokinase as an ATP-regenerating system.     

Response of Cultured Maize Cells to (+)-Abscisic Acid, (-)-Abscisic Acid,and Their Metabolites

The metabolism and effects of (+)-S- and (-)-R-abscisic acid (ABA) and some metabolites were studied in maize (Zea mays L. cv Black Mexican Sweet) suspension-cultured cells. Time-course studies of metabolite formation were performed in both cells and medium via analytical high-performance liquid chromatography. Metabolites were isolated and identified using physical and chemical methods. At 10 [mu]M concentration and 28[deg] C, (+)-ABA was metabolized within 24 h, yielding natural (-)-phaseic acid [(-)-PA] as the major product. The unnatural enantiomer (-)-ABA was less than 50% metabolized within 24 h and gave primarily (-)-7[prime]-hydroxyABA [(-)-7[prime]-HOABA], together with (+)-PA and ABA glucose ester. The distribution of metabolites in cells and medium was different, reflecting different sites of metabolism and membrane permeabilities of conjugated and nonconjugated metabolites. The results imply that (+)-ABA was oxidized to (-)-PA inside the cell, whereas (-)-ABA was converted to (-)-7[prime]-HOABA at the cell surface. Growth of maize cells was inhibited by both (+)- and (-)-ABA, with only weak contributions from their metabolites. The concentration of (+)-ABA that caused a 50% inhibition of growth of maize cells was approximately 1 [mu]M, whereas that for its metabolite (-)-PA was approximately 50 [mu]M. (-)-ABA was less active than (+)-ABA, with 50% growth inhibition observed at about 10 [mu]M. (-)-7[prime]-HOABA was only weakly active, with 50% inhibition caused by approximately 500 [mu]M. Time-course studies of medium pH indicated that (+)-ABA caused a transient pH increase (+0.3 units) at 6 h after addition that was not observed in controls or in samples treated with (-)-PA. The effect of (-)-ABA on medium Ph was marginal. No racemization at C-1[prime] of (+)-ABA, (-)-ABA, or metabolites was observed during the studies.     

Aminomethylenediphosphonate: A Potent Type-Specific Inhibitor of Both Plant and Phototrophic Bacterial H+-Pyrophosphatases

The suitability of different pyrophosphate (PPi) analogs as inhibitors of the vacuolar H+-translocating inorganic pyrophosphatase (V-PPase; EC 3.6.1.1) of tonoplast vesicles isolated from etiolated hypocotyls of Vigna radiata was investigated. Five 1,1-diphosphonates and imidodiphosphate were tested for their effects on substrate hydrolysis by the V-PPase at a substrate concentration corresponding to the Km of the enzyme. The order of inhibitory potency (apparent inhibition constants, Kiapp values, [mu]M, in parentheses) of the compounds examined was aminomethylenediphosphonate (1.8) > hydroxymethylenediphosphonate (5.7) [almost equal to] ethane-1-hydroxy-1,1-diphosphonate (6.5) > imidodiphosphate (12) > methylenediphosphonate (68) >> dichloromethylenediphosphonate (>500). The specificity of three of these compounds, aminomethylenediphosphonate, imidodiphosphate, and methylenediphosphonate, was determined by comparing their effects on the V-PPase and vacuolar H+-ATPase from Vigna, plasma membrane H+-ATPase from Beta vulgaris, H+-PPi synthase of chromatophores prepared from Rhodospirillum rubrum, soluble PPase from Saccharomyces cerevisiae, alkaline phosphatase from bovine intestinal mucosa, and nonspecific monophosphoesterase from Vigna at a PPi concentration equivalent to 10 times the Km of the V-PPase. Although all three PPi analogs inhibited the plant V-PPase and bacterial H+-PPi synthase with qualitatively similar kinetics, whether substrate hydrolysis or PPi-dependent H+-translocation was measured, neither the vacuolar H+-ATPase nor plasma membrane H+-ATPase nor any of the non-V-PPase-related PPi hydrolases were markedly inhibited under these conditions. It is concluded that 1, 1-diphosphonates, in general, and aminomethylenediphosphonate, in particular, are potent type-specific inhibitors of the V-PPase and its putative bacterial homolog, the H+-PPi synthase of Rhodospirillum.     

Guard-cell chloroplasts provide ATP required for H(+) pumping in the plasma membrane and stomatal opening

To elucidate the role of guard-cell chloroplasts (GCCs) in stomatal movement, we investigated the effects of oligomycin, an inhibitor of oxidative phosphorylation, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II, on fusicoccin (FC)-induced H(+) pumping and stomatal opening. FC was found to induce H(+ )pumping in guard-cell protoplasts (GCPs) from Vicia faba and stomatal opening in the epidermis of Commelina benghalensis; and, red light (RL) slightly stimulated these responses. Oligomycin strongly inhibited the pumping and stomatal opening in the dark. RL partially reversed the inhibitions, and DCMU decreased the effect of RL. FC activated the plasma membrane H(+)-ATPase (EC 3.6.1.35) in GCPs similarly irrespective of these treatments, indicating that the H(+)-ATPase activity was not the limiting step in H(+) pumping. Oligomycin significantly decreased the ATP content in GCPs in the dark. RL partially reversed this effect, and DCMU eliminated the effect of RL. A significant part of the ATP produced by photophosphorylation to H(+) pumping was indicated under RL. These results suggest that GCCs supply ATP to the cytosol under RL, and that the ATP is utilized by the plasma membrane H(+)-ATPase for H(+) pumping.     

Effects of Ca2+ Channel Blockers on Ca2+ Translocation Across Synaptosomal Membranes

The binding of [3H]nimodipine to purified synaptic plasma membranes (SPM) isolated from sheep brain cortex was characterized, and the effects of nimodipine, nifedipine, and (+)-verapamil on the [3H]nimodipine binding were compared to the effects on 45Ca2+ translocation under conditions that separate 45Ca2+ fluxes through Ca2+ channels from 45Ca2+ uptake via Na+/Ca2+ exchange. [3H]Nimodipine labels a single class of sites in SPM, with a KD of 0.64 +/- 0.1 nM, a Bmax of 161 +/- 27 fmol X mg-1 protein, and a Hill slope of 1.07, at 25 degrees C. Competition of [3H]nimodipine binding to purified SPM with unlabelled Ca2+ channel blockers shows that: nifedipine and nimodipine are potent competitors, with IC50 values of 4.7 nM and 5.9 nM, respectively; verapamil and (-)-D 600 are partial competitors, with biphasic competition behavior. Thus, (+)-verapamil shows an IC50 of 708 nM for the higher affinity component and the maximal inhibition is 50% of the specific binding, whereas for (-)-verapamil the IC50 is 120 nM, and the maximal inhibition is 30%; (-)-D 600 is even less potent than verapamil in inhibiting [3H]nimodipine binding (IC50 = 430 nM). However, (+)-verapamil, nifedipine, and nimodipine are less potent in inhibiting depolarization-induced 45Ca2+ influx into synaptosomes in the absence of Na+/Ca2+ exchange than in competing for [3H]nimodipine binding. Thus, (+)-verapamil inhibits Ca2+ influx by 50% at about 500 microM, whereas it inhibits 50% of the binding at concentrations 200-fold lower, and the discrepancy is even larger for the dihydropyridines. The Na+/Ca2+ exchange and the ATP-dependent Ca2+ uptake by SPM vesicles are also inhibited by the Ca2+ channel blockers verapamil, nifedipine, and d-cis-diltiazem, with similar IC50 values and in the same concentration range (10(-5)-10(-3) M) at which they inhibit Ca2+ influx through Ca2+ channels. We conclude that high-affinity binding of the Ca2+ blockers by SPM is not correlated with inhibition of the Ca2+ fluxes through channels in synaptosomes under conditions of minimal Na+/Ca2+ exchange. Furthermore, the relatively high concentrations of blockers required to block the channels also inhibit Ca2+ translocation through the Ca2+-ATPase and the Na+/Ca2+ exchanger. In this study, clear differentiation is made of the effects of the Ca2+ channel blockers on these three mechanisms of moving Ca2+ across the synaptosomal membrane, and particular care is taken to separate the contribution of the Na+/Ca2+ exchange from that of the Ca2+ channels under conditions of K+ depolarization.     

ABA、IAA和CaM对发育菜豆子叶质膜H~＋－ATPase活力的效应

以亲水性两相分配法从发育菜豆子叶制备的质膜制剂经冻融循环操作,部分膜微囊可转变成密闭的翻转型。取冻融４次的质膜微囊用于Ｈ＋－ＡＴＰａｓｅ试验表明,ＡＴＰａｓｅ活力为ＡＢＡ和ＣａＭ显著地激活,但受ＩＡＡ显著抑制;质子泵活力被ＡＢＡ显著促进,但为ＣａＭ显著抑制,ＩＡＡ对质子泵活力无显著效应。可以认为：ＡＢＡ促进发育菜豆子叶吸收光合同化物可能是通过促进质膜Ｈ＋－ＡＴＰａｓｅ活力,从而促进质子／蔗糖同向运输而获得;ＩＡＡ则可能对菜豆子叶的质膜Ｈ＋－ＡＴＰａｓｅ无显著效应。在激素信号传导途径中,ＣａＭ对质膜Ｈ＋－ＡＴＰａｓｅ活力可能无直接影响。