Mechanism of the Decline in Vacuolar H -ATPase Activity in Mung Bean Hypocotyls during Chilling

Vacuolar membrane H+-ATPase c`` subunit gene (ThVHAc``1) from Tamarix hispida Willd improves salt stress tolerance

The plant vacuolar H⁺-ATPase (V-ATPase) is a multisubunit complex. In addition to performing basic housekeeping functions, this complex is also involved in abiotic stress resistance in plants. In this study, a V-ATPase c`` subunit gene (ThVHAc``1) from Tamarix hispida Willd was cloned with a 534-bp ORF. Sequence analysis showed that the ThVHAc``1 protein contains four transmembrane helices and lacks a signal peptide. qRT-PCR results showed that ThVHAc``1 was primarily induced by treatments of NaCl, NaHCO₃, PEG₆₀₀₀, CdCl₂ or ABA in roots, stems and leaves of T. hispida. The expression pattern of ThVHAc``1 was significantly different from that of ThVHAc1 (a V-ATPase c subunit in T. hispida). Furthermore, the cell survival rates and density (OD<sub>600</sub>) results showed that the transgenic yeast overexpressing ThVHAc``1 exhibited increased tolerance to the above-mentioned abiotic stresses. In addition, the overexpression of ThVHAc``1 confers salt tolerance to transgenic Arabidopsis plants by improving the ROS content and decreasing the accumulation of O<sup>2-</sup> and H<sub>2</sub>O<sub>2</sub>. Similarly, the homologous transformation of the ThVHAc``1 gene into T. hispida also improved salt tolerance. Our results suggest that the ThVHAc``1 gene plays an important role in plant stress tolerance.

Functionally redundant LNG3 and LNG4 genes regulate turgor-driven polar cell elongation through activation of XTH17 and XTH24

In this work, we genetically characterized the function of Arabidopsis thaliana, LONGIFOLIA (LNG1), LNG2, LNG3, LNG4, their contribution to regulate vegetative architecture in plant. We used molecular and biophysical approaches to elucidate a gene function that regulates vegetative architecture, as revealed by the leaf phenotype and later effects on flowering patterns in Arabidopsis loss-of-function mutants. As a result, LNG genes play an important role in polar cell elongation by turgor pressure controlling the activation of XTH17 and XTH24. Plant vegetative architecture is related to important traits that later influence the floral architecture involved in seed production. Leaf morphology is the primary key trait to compose plant vegetative architecture. However, molecular mechanism on leaf shape determination is not fully understood even in the model plant A. thaliana. We previously showed that LONGIFOLIA (LNG1) and LONGIFOLIA2 (LNG2) genes regulate leaf morphology by promoting longitudinal cell elongation in Arabidopsis. In this study, we further characterized two homologs of LNG1, LNG3, and LNG4, using genetic, biophysical, and molecular approaches. Single loss-of-function mutants, lng3 and lng4, do not show any phenotypic difference, but mutants of lng quadruple (lngq), and lng1/2/3 and lng1/2/4 triples, display reduced leaf length, compared to wild type. Using the paradermal analysis, we conclude that the reduced leaf size of lngq is due to decreased cell elongation in the direction of longitudinal leaf growth, and not decreased cell proliferation. This data indicate that LNG1/2/3/4 are functionally redundant, and are involved in polar cell elongation in Arabidopsis leaf. Using a biophysical approach, we show that the LNGs contribute to maintain high turgor pressure, thus regulating turgor pressure-dependent polar cell elongation. In addition, gene expression analysis showed that LNGs positively regulate the expression of the cell wall modifying enzyme encoded by a multi-gene family, xyloglucan endotransglucosylase/hydrolase (XTH). Taking all of these together, we propose that LNG related genes play an important role in polar cell elongation by changing turgor pressure and controlling the activation of XTH17 and XTH24.

Ascorbate-glutathione cycle of mitochondria in osmoprimed soybean cotyledons in response to imbibitional chilling injury

Osmopriming treatment of chilling-sensitive soybean (Glycine max L. cv. Zhonghuang-22) seeds for 72h at 25°C with polyethylene glycol (PEG8000) solution at -1.5MPa strongly improves chilling resistance. The aim of the present work was to investigate whether the beneficial effect of osmopriming is associated with restoration of the ascorbate-glutathione (ASC-GSH) cycle of mitochondria in soybean seeds. Compared with the control, both H(2)O(2) and malondialdehyde (MDA) contents in mitochondria of osmoprimed seeds decreased after chilling treatment, and these changes were associated with increased activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and L-galactono-γ-lactone dehydrogenase (GLDH). However, the activity of dehydroascorbate reductase (DHAR) showed no obvious change during osmopriming treatment. Increased ASC and GSH contents accompanied prolonged osmopriming, and the reduced/oxidized ratios of ASC and GSH increased differently during osmopriming. These results indicate that osmopriming treatment enhances activity of the ASC-GSH cycle of mitochondria, which raises the chilling tolerance in soybean seeds and protects against H(2)O(2) that is generated in mitochondria during imbibition at low temperature.

Synchrony between flower opening and petal-color change from red to blue in morning glory, Ipomoea tricolor cv. Heavenly Blue

Petal color change in morning glory Ipomoea tricolor cv. Heavenly Blue, from red to blue, during the flower-opening period is due to an unusual increase in vacuolar pH (pHv) from 6.6 to 7.7 in colored epidermal cells. We clarified that this pHv increase is involved in tonoplast-localized Na+/H+ exchanger (NHX). However, the mechanism of pHv increase and the physiological role of NHX1 in petal cells have remained obscure. In this study, synchrony of petal-color change from red to blue, pHv increase, K+ accumulation, and cell expansion growth during flower-opening period were examined with special reference to ItNHX1. We concluded that ItNHX1 exchanges K+, but not Na+, with H+ to accumulate an ionic osmoticum in the vacuole, which is then followed by cell expansion growth. This function may lead to full opening of petals with a characteristic blue color.

Freezing tolerance by vesicle-mediated fructan transport

Fructans are fructose-based polymers associated with freezing tolerance. They might act directly via membrane stabilization or indirectly by stimulating alternative cryoprotectants. Fructans and fructan biosynthetic enzymes, in general, are believed to be present in the vacuole. This paper draws particular attention to the surprising presence of fructans and fructan exohydrolase activity in the apoplast of cold-stressed plants. This observation raises questions concerning the origin of apoplastic fructans and suggests that fructans are transported to the apoplast by post-synthesis mechanisms, perhaps induced by cold. We propose a conceptual vesicle-mediated transport model for the movement of vacuolar fructans to the apoplast, where they could assist in stabilizing the plasma membrane.

Deletion of a histidine-rich loop of AtMTP1, a vacuolar Zn(2+)/H(+) antiporter of Arabidopsis thaliana, stimulates the transport activity

Arabidopsis thaliana AtMTP1 belongs to the cation diffusion facilitator family and is localized on the vacuolar membrane. We investigated the enzymatic kinetics of AtMTP1 by a heterologous expression system in the yeast Saccharomyces cerevisiae, which lacked genes for vacuolar membrane zinc transporters ZRC1 and COT1. The yeast mutant expressing AtMTP1 heterologously was tolerant to 10 mm ZnCl(2). Active transport of zinc into vacuoles of living yeast cells expressing AtMTP1 was confirmed by the fluorescent zinc indicator FuraZin-1. Zinc transport was quantitatively analyzed by using vacuolar membrane vesicles prepared from AtMTP1-expressing yeast cells and radioisotope (65)Zn(2+). Active zinc uptake depended on a pH gradient generated by endogenous vacuolar H(+)-ATPase. The activity was inhibited by bafilomycin A(1), an inhibitor of the H(+)-ATPase. The K(m) for Zn(2+) and V(max) of AtMTP1 were determined to be 0.30 microm and 1.22 nmol/min/mg, respectively. We prepared a mutant AtMTP1 that lacked the major part (32 residues from 185 to 216) of a long histidine-rich hydrophilic loop in the central part of AtMTP1. Yeast cells expressing the mutant became hyperresistant to high concentrations of Zn(2+) and resistant to Co(2+). The K(m) and V(max) values were increased 2-11-fold. These results indicate that AtMTP1 functions as a Zn(2+)/H(+) antiporter in vacuoles and that a histidine-rich region is not essential for zinc transport. We propose that a histidine-rich loop functions as a buffering pocket of Zn(2+) and a sensor of the zinc level at the cytoplasmic surface. This loop may be involved in the maintenance of the level of cytoplasmic Zn(2+).

Inorganic phosphate uptake in intact vacuoles isolated from suspension-cultured cells of Catharanthus roseus (L.) G. Don under varying Pi status

Inorganic phosphate (Pi) uptake across the vacuolar membrane of intact vacuoles isolated from Catharanthus roseus suspension-cultured cells was measured. Under low Pi status, Pi uptake into the vacuole was strongly activated compared to high Pi status. Since Pi uptake across the vacuolar membrane is correlated with H+ pumping, we examined the dependency of H+ pumping on plant Pi status. Both H+ pumping and the activities of the vacuolar H+-pumps, the V-type H+-ATPase and the H+-PPase were enhanced under low Pi status. Despite this increase in H+ pumping, Western blot analysis showed no distinct increase in the amount of proton pump proteins. Possible mechanisms for the activation of Pi uptake into the vacuole under low Pi status are discussed.

Effects of ABA, auxin, and gibberellin on the expression of genes for vacuolar H+ -inorganic pyrophosphatase, H+ -ATPase subunit A, and Na+/H+ antiporter in barley

We analyzed the effects of ABA, auxin, and gibberellin on the expression of two genes (HVP1 and HVP10) for vacuolar H(+)-inorganic pyrophosphatase (EC 3.6.1.1) and one (HvVHA-A) for the catalytic subunit (subunit A) of vacuolar H(+)-ATPase (EC 3.6.1.3) by quantification of the transcript levels, to identify the hormones responsible for regulating the expression of these genes in barley (Hordeum vulgare L.) in response to environmental changes such as salt stress. ABA markedly induced the expression of HVP1 and slightly increased the expression of HVP10 and HvVHA-A. In contrast, 2,4-D only increased the expression of HVP1, and GA(3) had no significant effects on any gene. The maximum level of HVP1 transcripts in response to these hormones was also much higher than the levels of HVP10 and HvVHA-A transcripts. In addition, we also analyzed the expression of one gene (HvNHX1) for vacuolar Na(+)/H(+) antiporter, and HvNHX1 expression changed in a pattern similar to that of the HVP1 expression. Furthermore, treatment with ABA and 2,4-D increased Na(+)/H(+) antiport activity and proton-translocating activities by H(+)-PPase and H(+)-ATPase in tonoplast vesicles, and treatment with ABA also increased the amount of V-PPase protein of tonoplast vesicles. These results suggest that the hormones ABA and 2,4-D regulate the expression of the H(+)-pump and Na(+)/H(+) antiporter genes and are thus important effectors that regulate the expression of HVP1and HvNHX1.

Differential response of vacuolar proton pumps to osmotica

The vacuole is a fundamental and dominant organelle and occupies a large part of the total cell volume in most mature plant cells. The higher-plant vacuole contains two types of proton-translocating pumps, H⁺-ATPase (EC 3.6.1.3) and H⁺-pyrophosphatase (EC 3.6.1.1), residing on the same membrane. These two enzymes generate roughly equal proton gradients across the vacuolar membrane for the secondary transport of ions and metabolites. However, the pumps respond differentially to stress in order to maintain critical functions of the vacuole. In this work, tonoplasts from etiolated mung bean seedlings (Vigna radiata L.) were used to investigate the function of these two enzymes under high osmotic pressure. At high concentrations of sucrose or sorbitol, the light scattering and volume of isolated vesicles were progressively changed. Concomitantly, enzymatic activities, proton translocation, and coupling efficiencies of these two proton-pumping enzymes were inhibited to various extents under high osmotic pressure. No significant change in enzymatic activities of purified vacuolar H⁺-PPase and H⁺-ATPase under similar conditions was observed. We thus believe that the membrane structure is an important determinant for proper function of proton pumping systems of plant vacuoles. Furthermore, kinetic analysis shows different variation in apparent V_max but not in K_M values of vacuolar H⁺-PPase and H⁺-ATPase at high osmolarity of sucrose and sorbitol, respectively, suggesting probable alterations in substrate hydrolysis reactions but not substrate-binding affinity of the enzymes. A working model is proposed to interpret supplemental roles of vacuolar H⁺-PPase and H⁺-ATPase to maintain appropriate functions of plant tonoplasts.

Subcellular localization of Strboh proteins and NADPH-dependent O2(-)-generating activity in potato tuber tissues

Rapid generation of reactive oxygen species (ROS) at the cell surface has been implicated in plant defence responses. Genetic evidence indicates that a plant NADPH oxidase (Rboh; respiratory burst oxidase homologue) is associated with oxidative burst. However, there is not enough physiological evidence of Rboh localization available yet. Isozyme-specific antibodies against potato StrbohA and StrbohB (St; Solanum tuberosum) were prepared to investigate the localization of these proteins. Immunoblot analyses using potato microsomal proteins revealed that StrbohA was expressed constitutively at a low level, whereas the accumulation of StrbohB protein was induced by the cell wall elicitor of the potato pathogen Phytophthora infestans. It is demonstrated here that StrbohA and StrbohB are distributed in plasma membrane fractions which have been separated by sucrose density-gradient centrifugation using their specific antibodies. Green fluorescent protein-tagged Strboh proteins were also located on the plasma membrane by transient expression assay in onion epidermal cells. Additionally, NADPH-dependent O2(-)-generating activities in plasma membrane fractions were diphenylene iodonium-sensitive and NaN3-insensitive. These data suggest that StrbohA and StrbohB are predominantly localized on the plasma membrane and regulate ROS production in defence signalling.

Salt stress in Thellungiella halophila activates Na+ transport mechanisms required for salinity tolerance

Salinity is considered one of the major limiting factors for plant growth and agricultural productivity. We are using salt cress (Thellungiella halophila) to identify biochemical mechanisms that enable plants to grow in saline conditions. Under salt stress, the major site of Na+ accumulation occurred in old leaves, followed by young leaves and taproots, with the least accumulation occurring in lateral roots. Salt treatment increased both the H+ transport and hydrolytic activity of salt cress tonoplast (TP) and plasma membrane (PM) H(+)-ATPases from leaves and roots. TP Na(+)/H+ exchange was greatly stimulated by growth of the plants in NaCl, both in leaves and roots. Expression of the PM H(+)-ATPase isoform AHA3, the Na+ transporter HKT1, and the Na(+)/H+ exchanger SOS1 were examined in PMs isolated from control and salt-treated salt cress roots and leaves. An increased expression of SOS1, but no changes in levels of AHA3 and HKT1, was observed. NHX1 was only detected in PM fractions of roots, and a salt-induced increase in protein expression was observed. Analysis of the levels of expression of vacuolar H(+)-translocating ATPase subunits showed no major changes in protein expression of subunits VHA-A or VHA-B with salt treatment; however, VHA-E showed an increased expression in leaf tissue, but not in roots, when the plants were treated with NaCl. Salt cress plants were able to distribute and store Na+ by a very strict control of ion movement across both the TP and PM.

The Involvement of Tonoplast Proton Pumps and Na+(K+)/H+ Exchangers in the Change of Petal Color During Flower Opening of Morning Glory, Ipomoea tricolor cv. Heavenly Blue

The petal color of morning glory, Ipomoea tricolor cv. Heavenly Blue, changes from purplish red to blue during flower opening. This color change is caused by an unusual increase in vacuolar pH from 6.6 to 7.7 in the colored adaxial and abaxial cells. To clarify the mechanism underlying the alkalization of epidermal vacuoles in the open petals, we focused on vacuolar H+-ATPase (V-ATPase), H+-pyrophosphatase (V-PPase) and an isoform of Na+/H+ exchanger (NHX1). We isolated red and blue protoplasts from the petals in bud and fully open flower, respectively, and purified vacuolar membranes. The membranes contained V-ATPase, V-PPase and NHX1, which were immunochemically detected, with relatively high transport activity. NHX1 could be detected only in the vacuolar membranes prepared from flower petals and its protein level was the highest in the colored petal epidermis of the open flower. These results suggest that the increase of vacuolar pH in the petals during flower opening is due to active transport of Na+ and/or K+ from the cytosol into vacuoles through a sodium- or potassium-driven Na+(K+)/H+ exchanger NXH1 and that V-PPase and V-ATPase may prevent the over-alkalization. This systematic ion transport maintains the weakly alkaline vacuolar pH, producing the sky-blue petals.

Isolation of intact vacuoles and proteomic analysis of tonoplast from suspension-cultured cells of Arabidopsis thaliana

A large number of proteins in the tonoplast, including pumps, carriers, ion channels and receptors support the various functions of the plant vacuole. To date, few proteins involved in these activities have been identified at the molecular level. In this study, proteomic analysis was used to identify new tonoplast proteins. A primary requirement of any organelle analysis by proteomics is that the purity of the isolated organelle needs to be high. Using suspension-cultured Arabidopsis cells (Arabidopsis Col-0 cell suspension), a method was developed for the isolation of intact highly purified vacuoles. No plasma membrane proteins were detected in Western blots of the isolated vacuole fraction, and only a few proteins from the Golgi and endoplasmic reticulum. The proteomic analysis of the purified tonoplast involved fractionation of the proteins by SDS-PAGE and analysis by LC-MS/MS. Using this approach, it was possible to identify 163 proteins. These included well-characterized tonoplast proteins such as V-type H+ -ATPases and V-type H+ -PPases, and others with functions reasonably expected to be related to the tonoplast. There were also a number of proteins for which a function has not yet been deduced.

Modulation of vacuolar H+ -pumps and aquaporin by phytohormones in rice seedling leaf sheaths

The effects of exogenously applied gibberellic acid (GA(3)) or brassinolide (BL) on the H(+)-pumps and aquaporin in the vacuolar membrane of rice seedling leaf sheath were investigated. Antibodies against mung bean H(+)-PPase, the B subunit of V-ATPase, and radish tonoplast intrinsic protein (TIP) cross-reacted with the vacuolar membrane proteins of rice seedling leaf sheath. The results of immunoblot analysis showed that the amounts of H(+)-PPase and V-ATPase were retained at a high level for two days in the presence of GA(3), although the level gradually decreased without phytohormones, indicating that GA(3) has a promotive effect on the activities of vacuolar H(+)-pumps. However, the levels of V-ATPase and H(+)-PPase were increased on day 2 and then decreased when treated with BL. There were no visible differences in the level of TIP upon treatment with GA(3) and BL. In comparison with the water control, the activity of H(+)-PPase treated with BL was also retained at a relatively high level, suggesting that BL has a stimulative effect on the activities of H(+)-PPase. These results indicate that GA(3) and BL might be involved in the regulation of the quantity and activities of plant vacuolar H(+)-pumps.

Distinct localization of two closely related Ypt3/Rab11 proteins on the trafficking pathway in higher plants

Ypt/Rab proteins are Ras-related small GTPases that act on the intracellular membrane through the trafficking pathway, and their function depends on their localization. Approximately 25 genes encoding Ypt3/Rab11-related proteins exist in Arabidopsis, but the reason for the presence of many genes in plants remains unclear. Pea Pra2 and Pra3, members of Ypt3/Rab11, are closely related proteins. Because possible orthologs are conserved among dicots, they can be studied to determine their possible localization. Biochemical analysis revealed that these proteins were localized on distinct membranes in pea. Furthermore, using green fluorescent protein-Pra2 and green fluorescent protein-Pra3 fusion proteins, we demonstrated that these proteins are distinctively localized on the trafficking pathway in tobacco Bright Yellow 2 cells. Pra2 was predominantly localized on Golgi stacks and endosomes, which did not support the localization of Pra2 on the endoplasmic reticulum (Kang, J. G., Yun, J., Kim, D. H., Chung, K. S., Fujioka, S., Kim, J. I., Dae, H. W., Yoshida, S., Takatsuto, S., Song, P. S., and Park, C. M. (2001) Cell 105, 625--636). In contrast, Pra3 was likely to be localized on the trans-Golgi network and/or the prevacuolar compartment. We concluded that Pra2 and Pra3 proteins are distinctively localized on the trafficking pathway. This finding suggests that functional diversification takes place in the plant Ypt3/Rab11 family.

Significance of the V-type ATPase for the adaptation to stressful growth conditions and its regulation on the molecular and biochemical level

Two electrogenic H(+)-pumps, the vacuolar type H(+)-ATPase (V-ATPase) and the vacuolar pyrophosphatase, coexist at membranes of the secretory pathway of plants. The V-ATPase is the dominant H(+)-pump at endomembranes of most plant cells, both in terms of protein amount and, frequently, also in activity. The V-ATPase is indispensable for plant growth under normal conditions due to its role in energizing secondary transport, maintenance of solute homeostasis and, possibly, in facilitating vesicle fusion. Under stress conditions such as salinity, drought, cold, acid stress, anoxia, and excess heavy metals in the soil, survival of the cells depends strongly on maintaining or adjusting the activity of the V-ATPase. Regulation of gene expression and activity are involved in adapting the V-ATPase on long- and short-term bases. The mechanisms known to regulate the V-ATPase are summarized in this paper with an emphasis on their implications for growth and development under stress.

Osmoconditioning reduces physiological and biochemical damage induced by chilling in soybean seeds

The aim of the present work was to investigate the effects of osmoconditioning on chilling injury in soybean (Glycine max (L.) Merr.) seeds during imbibition. Soybean seeds germinated readily over a large range of temperatures (10-35 degrees C), the thermal optimum being 25-30 degrees C. Low temperatures reduced the germination rate and no seed germinated at 1 degrees C. Pre-treatment of seeds at 1 degrees C reduced further germination at the optimal temperature (25 degrees C). This deleterious effect of chilling increased with duration of the treatment, and was maximal after 4 days. Osmoconditioning of seeds at 20 degrees C with a polyethylene glycol-8000 solution at -1.5 MPa for at least 24 h followed by drying back the seeds to their initial moisture content reduced their chilling sensitivity and even allowed germination at 1 degrees C. Chilling of control seeds resulted in a sharp decline in in vivo ACC-dependent ethylene production and in an increase in electrolyte leakage in the medium, which indicated deterioration of membrane properties. Osmoconditioned seeds placed at 1 degrees C did not show any reduction in their ability to convert ACC to ethylene nor any strong increase in electrolyte leakage. Imbibition of both control and osmoconditioned seeds at 1 degrees C resulted in a marked increase in ATP level (more than 50% of the total nucleotides) and energy charge; however, the latter cannot be considered as an indicator of chilling since it remained high (0.74-0.88) throughout the cold treatment. Chilling treatment longer than 6 days induced accumulation of malondialdehyde in the embryonic axis, which was more marked in control seeds than in osmoconditioned seeds, suggesting that chilling sensitivity was associated with lipid peroxidation. Imbibition of seeds at 1 degrees C resulted in an increase in superoxide dismutase, catalase and glutathione reductase activity, which was generally higher in osmoconditioned seeds than in control ones. This stimulation of the antioxidant defence systems occurred during the 4 first days of chilling and decreased then in control seeds while it remained high in osmoconditioned ones. Re-warming seeds at 25 degrees C resulted in an increase in all enzyme activity involved in antioxidant defence. However this effect of re-warming decreased in control seeds after 4 days of chilling, whereas it was maintained in osmoconditioned seeds.

Quantification of Ca2+/H+ antiporter VCAX1p in vacuolar membranes and its absence in roots of mung bean

Vacuolar Ca2+/H+ antiporter VCAX1p, which contributes to the Ca2+ accumulation into vacuoles, was quantified by immunochemistry. The antiporter content in vacuolar membranes was 0.14 and 1.1 microg mg(-1) of membrane protein for hypocotyls and epicotyls, respectively. The calculated turnover number was 120 s(-1). Roots lacked the antiporter protein and the transcript.

Mechanism of the chilling-induced decrease in proton pumping across the tonoplast of rice cells

The ATP-generated proton pumping across tonoplast vesicles from chilling-sensitive Boro rice (Oryza sativa L. var. Boro) cultured cells was markedly decreased by chilling at 5 degrees C for 3 d. The membrane fluidity of core hydrophobic and surface hydrophilic regions of the lipid bilayer was measured by steady-state fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene and trimethylammonium 1,6-diphenyl-1,3,5-hexatriene and by electron spin resonance spectroscopy of 16- and 5-doxyl stearic acid, respectively. The fluidity of the surface region of the lipid bilayer of the tonoplast vesicles decreased by chilling. The fluidity of the surface region of the liposomes and the proton pumping across the reconstituted proteoliposomes with tonoplast H+-ATPase decreased with increasing content of the glycolipids. The proton pumping across chimera proteoliposomes was reduced by chilling only when it was reconstituted in the presence of tonoplast glycolipids from chilled Boro cells. These data suggest that the reduction in ATP-generated proton pumping across the tonoplast by chilling is due to the decrease in the fluidity of the surface region of the lipid bilayer of the tonoplast, which is caused by the changes in glycolipids.

Tissue specificity of E subunit isoforms of plant vacuolar H(+)-ATPase and existence of isotype enzymes

Immunoblot analyses and partial amino acid sequencings revealed that both the 40- (E1) and 37-kDa (E2) subunits of V-ATPase in the pea epicotyl were E subunit isoforms. Similarly, both the 35- (D1) and 29-kDa (D2) subunits were D subunit isoforms, although the similarity of the amino acid sequences is still unknown. In immunoblot analyses, two or three E subunit isoforms with molecular masses ranging from 29 to 40 kDa were detected in other plants. Two isotypes of V-ATPase from the pea epicotyl were separated by ion exchange chromatography and had subunit compositions differing only in the ratio of E1 and E2. There was a difference in the V(max) and K(m) of ATP hydrolysis between the two isotypes. E1 was scarcely detected in crude membrane fractions from the leaf and cotyledon, while E2 was detected in fractions from all of the tissues examined. The compositions of D subunit isoforms in the leaf and epicotyl were different, and the vacuolar membrane in the leaf did not contain D2. The efficiency of H(+) pumping activity in the vacuolar membrane of the leaf was higher than that of the epicotyl. The results suggest that the presence of the isoforms of D and E subunits is characteristic to plants and that the isoforms are closely related to the enzymatic properties.

Vacuolar H(+)-ATPase is expressed in response to gibberellin during tomato seed germination

Completion of germination (radicle emergence) by gibberellin (GA)-deficient (gib-1) mutant tomato (Lycopersicon esculentum Mill.) seeds is dependent upon exogenous GA, because weakening of the endosperm tissue enclosing the radicle tip requires GA. To investigate genes that may be involved in endosperm weakening or embryo growth, differential cDNA display was used to identify mRNAs differentially expressed in gib-1 seeds imbibed in the presence or absence of GA(4+7). Among these was a GA-responsive mRNA encoding the 16-kD hydrophobic subunit c of the V(0) membrane sector of vacuolar H(+)-translocating ATPases (V-ATPase), which we termed LVA-P1. LVA-P1 mRNA expression in gib-1 seeds was dependent on GA and was particularly abundant in the micropylar region prior to radicle emergence. Both GA dependence and tissue localization of LVA-P1 mRNA expression were confirmed directly in individual gib-1 seeds using tissue printing. LVA-P1 mRNA was also expressed in wild-type seeds during development and germination, independent of exogenous GA. Specific antisera detected protein subunits A and B of the cytoplasmic V(1) sector of the V-ATPase holoenzyme complex in gib-1 seeds only in the presence of GA, and expression was localized to the micropylar region. The results suggest that V-ATPase plays a role in GA-regulated germination of tomato seeds.

Characterization of a red beet protein homologous to the essential 36-kilodalton subunit of the yeast V-type ATPase

V-type proton-translocating ATPases (V-ATPases) (EC 3.6.1.3) are electrogenic proton pumps involved in acidification of endomembrane compartments in all eukaryotic cells. V-ATPases from various species consist of 8 to 12 polypeptide subunits arranged into an integral membrane proton pore sector (Vo) and a peripherally associated catalytic sector (V1). Several V-ATPase subunits are functionally and structurally conserved among all species examined. In yeast, a 36-kD peripheral subunit encoded by the yeast (Saccharomyces cerevisiae) VMA6 gene (Vma6p) is required for stable assembly of the Vo sector as well as for V1 attachment. Vma6p has been characterized as a nonintegrally associated Vo subunit. A high degree of sequence similarity among Vma6p homologs from animal and fungal species suggest that this subunit has a conserved role in V-ATPase function. We have characterized a novel Vma6p homolog from red beet (Beta vulgaris) tonoplast membranes. A 44-kD polypeptide cofractionated with V-ATPases upon gel-filtration chromatography of detergent-solubilized tonoplast membranes and was specifically cross-reactive with anti-Vma6p polyclonal antibodies. The 44-kD polypeptide was dissociated from isolated tonoplast preparations by mild chaotropic agents and thus appeared to be nonintegrally associated with the membrane. The putative 44-kD homolog appears to be structurally similar to yeast Vma6p and occupies a similar position within the holoenzyme complex.

Genes involved in osmoregulation during turgor-driven cell expansion of developing cotton fibers are differentially regulated

Cotton (Gossypium hirsutum L.) fibers are single-celled trichomes that synchronously undergo a phase of rapid cell expansion, then a phase including secondary cell wall deposition, and finally maturation. To determine if there is coordinated regulation of gene expression during fiber expansion, we analyzed the expression of components involved in turgor regulation and a cytoskeletal protein by measuring levels of mRNA and protein accumulation and enzyme activity. Fragments of the genes for the plasma membrane proton-translocating ATPase, vacuole-ATPase, proton-translocating pyrophosphatase (PPase), phosphoenolpyruvate carboxylase, major intrinsic protein, and alpha-tubulin were amplified by polymerase chain reaction and used as probes in ribonuclease protection assays of RNA from a fiber developmental series, revealing two discrete patterns of mRNA accumulation. Transcripts of all but the PPase accumulated to highest levels during the period of peak expansion (+12-15 d postanthesis [dpa]), then declined with the onset of secondary cell wall synthesis. The PPase was constitutively expressed through fiber development. Activity of the two proton-translocating-ATPases peaked at +15 dpa, whereas PPase activity peaked at +20 dpa, suggesting that all are involved in the process of cell expansion but with varying roles. Patterns of protein accumulation and enzyme activity for some of the proteins examined suggest posttranslational regulation through fiber development.

Molecular cloning of vacuolar H(+)-pyrophosphatase and its developmental expression in growing hypocotyl of mung bean

Vacuolar proton-translocating inorganic pyrophosphatase and H(+)-ATPase acidify the vacuoles and power the vacuolar secondary active transport systems in plants. Developmental changes in the transcription of the pyrophosphatase in growing hypocotyls of mung bean (Vigna radiata) were investigated. The cDNA clone for the mung bean enzyme contains an uninterrupted open reading frame of 2298 bp, coding for a polypeptide of 766 amino acids. Hypocotyls were divided into elongating and mature regions. RNA analysis revealed that the transcript level of the pyrophosphatase was high in the elongating region of the 3-d-old hypocotyl but was extremely low in the mature region of the 5-d-old hypocotyl. The level of transcript of the 68-kD subunit of H(+)-ATPase also decreased after cell maturation. In the elongating region, the proton-pumping activity of pyrophosphatase on the basis of membrane protein was 3 times higher than that of H(+)-ATPase. After cell maturation, the pyrophosphatase activity decreased to 30% of that in the elongating region. The decline in the pyrophosphatase activity was in parallel with a decrease in the enzyme protein content. These findings indicate that the level of the pyrophosphatase, a main vacuolar proton pump in growing cells, is negatively regulated after cell maturation at the transcriptional level.

Al-Induced, 51-Kilodalton, Membrane-Bound Proteins Are Associated with Resistance to Al in a Segregating Population of Wheat

Incorporation of 35S into protein is reduced by exposure to Al in wheat (Triticum aestivum), but the effects are genotype-specific. Exposure to 10 to 75 [mu]M Al had little effect on 35S incorporation into total protein, nuclear and mitochondrial protein, microsomal protein, and cytosolic protein in the Al-resistant cultivar PT741. In contrast, 10 [mu]M Al reduced incorporation by 21 to 38% in the Al-sensitive cultivar Katepwa, with effects becoming more pronounced (31-62%) as concentrations of Al increased. We previously reported that a pair of 51-kD membrane-bound proteins accumulated in root tips of PT741 under conditions of Al stress. We now report that the 51-kD band is labeled with 35S after 24 h of exposure to 75 [mu]M Al. The specific induction of the 51-kD band in PT741 suggested a potential role of one or both of these proteins in mediating resistance to Al. Therefore, we analyzed their expression in single plants from an F2 population arising from a cross between the PT741 and Katepwa cultivars. Accumulation of 1,3-[beta]-glucans (callose) in root tips after 24 h of exposure to 100 [mu]M Al indicated that this population segregated for Al resistance in about a 3:1 ratio. A close correlation between resistance to Al (low callose content of root tips) and accumulation of the 51-kD band was observed, indicating that at least one of these proteins cosegregates with the Al-resistance phenotype. As a first step in identifying a possible function, we have demonstrated that the 51-kD band is most clearly associated with the tonoplast. Whereas Al has been reported to stimulate the activity of the tonoplast H+-ATPase and H+-PPase, antibodies raised against these proteins did not cross-react with the 51-kD band. Efforts are now under way to purify this protein from tonoplast-enriched fractions.

A Vacuolar-Type H+-ATPase in a Nonvacuolar Organelle Is Required for the Sorting of Soluble Vacuolar Protein Precursors in Tobacco Cells

In plant cells, vacuolar matrix proteins are separated from the secretory proteins at the Golgi complex for transport to the vacuoles. To investigate the involvement of vacuolar-type ATPase (V-ATPase) in the vacuolar targeting of soluble proteins, we analyzed the effects of bafilomycin A1 and concanamycin A on the transport of vacuolar protein precursors in tobacco cells. Low concentrations of these inhibitors caused the missorting of several vacuolar protein precursors; sorting was more sensitive to concanamycin A than to bafilomycin A1. Secretion of soluble proteins from tobacco cells was also inhibited by bafilomycin A1 and concanamycin A. We next analyzed the subcellular localization of V-ATPase. V-ATPase was found in a wide variety of endomembrane organelles. Both ATPase activity and ATP-dependent proton-pumping activity in the Golgi-enriched fraction were more sensitive to concanamycin A than to bafilomycin A1, whereas these activities in the tonoplast fraction were almost equally sensitive to both reagents. Our observations indicate that the V-ATPase in the organelle that was recovered in the Golgi-enriched fraction is required for the transport of vacuolar protein precursors and that this V-ATPase is distinguishable from the tonoplast-associated V-ATPase.

Chill-Induced Changes in the Activity and Abundance of the Vacuolar Proton-Pumping Pyrophosphatase from Mung Bean Hypocotyls

Changes in the properties of extractable vacuolar H+-pumping pyrophosphatase (V-PPase) and vacuolar ATPase activities in chilling-sensitive seedlings of mung bean (Vigna radiata) were investigated. Following chilling at 4[deg]C for 48 h, both hydrolytic and proton-pumping activities of the V-PPase increased 1.5- to 2-fold over controls and remained elevated even after 72 h at low temperatures. Vacuolar ATPase levels did not change significantly throughout the chilling regime. However a large increase in alcohol dehydrogenase activity during chilling suggests a shift toward fermentative metabolism, which can be expected to decrease ATPase activity in situ. Western blotting of vacuolar membrane-enriched fractions from control and treated plants has confirmed that the changes in V-PPase activity are mirrored by increases in the amount of pump protein. Results suggest a specific role for the V-PPase in protecting chill-sensitive plants from the injurious effects of low temperatures via the maintenance of the proton gradient across the vacuolar membrane.

Characterization of a low-temperature-induced cDNA from winter Brassica napus encoding the 70 kDa subunit of tonoplast ATPase

A cDNA clone, pBN59, was isolated by differential screening of a cDNA library of winter Brassica napus during cold acclimation. Nucleotide sequence of BN59 was found to be homologous to that encoding the 70 kDa subunit of the vacuolar H(+)-ATPase in plants. Transcripts hybridizing to BN59 accumulated during exposure to low temperatures and to the exogenous application of abscisic acid (ABA). Western blot analyses also indicated an increase in the 70 kDa subunit during cold acclimation. The accumulation of an endomembrane H(+)-ATPase is consistent with the observation of osmotic adjustment, increases in endogenous ABA and the proliferation of endomembranes during cold acclimation.

Accumulation of Vacuolar H+-Pyrophosphatase and H+-ATPase during Reformation of the Central Vacuole in Germinating Pumpkin Seeds

Protein storage vacuoles were examined for the induction of H+-pyrophosphatase (H+-PPase), H+-ATPase, and a membrane integral protein of 23 kD after seed germination. Membranes of protein storage vacuoles were prepared from dry seeds and etiolated cotyledons of pumpkin (Cucurbita sp.). Membrane vesicles from etiolated cotyledons had ATP- and pyrophosphate-dependent H+-transport activities. H+-ATPase activity was sensitive to nitrate and bafilomycin, and H+-PPase activity was stimulated by potassium ion and inhibited by dicyclohexylcarbodiimide. The activities of both enzymes increased after seed germination. On immunoblot analysis, the 73-kD polypeptide of H+-PPase and the two major subunits, 68 and 57 kD, of vacuolar H+-ATPase were detected in the vacuolar membranes of cotyledons, and the levels of the subunits of enzymes increased parallel to those of enzyme activities. Small amounts of the subunits of the enzymes were detected in dry cotyledons. Immunocytochemical analysis of the cotyledonous cells with anti-H+-PPase showed the close association of H+-PPase to the membranes of protein storage vacuoles. In endosperms of castor bean (Ricinus communis), both enzymes and their subunits increased after germination. Furthermore, the vacuolar membranes from etiolated cotyledons of pumpkin had a polypeptide that cross-reacted with antibody against a 23-kD membrane protein of radish vacuole, VM23, but the membranes of dry cotyledons did not. The results from this study suggest that H+-ATPase, H+-PPase, and VM23 are expressed and accumulated in the membranes of protein storage vacuoles after seed germination. Overall, the findings indicate that the membranes of protein storage vacuoles are transformed into those of central vacuoles during the growth of seedlings.

Presynaptic events involved in neurotransmission

Vacuolar H(+)-ATPase (V-ATPase) plays a key role in neurotransmission. It provides the energy for the uptake and storage of neurotransmitters in synaptic vesicles and granules. It also may play a role in the biogenesis of synaptic vesicles as well as in neurosecretion. This is one of the most conserved fundamental enzymes in nature, but functions in a wide variety or organelles and membranes. Its structure, function, molecular biology and biogenesis is discussed in relation to its role in neurotransmission. Termination of neurotransmission is carried out by neurotransmitter transporters that function in the reuptake of the neurotransmitters into the presynaptic cells. We cloned, sequenced and expressed several cDNAs encoding neurotransmitter transporters. Their specificity and site of synthesis revealed some new aspects of neurotransmission.