Assessment of glycinebetaine and proline compartmentation by analysis of isolated beet vacuoles

Dynamic responses of Haloxylon ammodendron to various degrees of simulated drought stress

Haloxylon ammodendron, a C4 perennial, succulent and xero-halophytic shrub, is highly resistant to harsh environments, therefore, exploring the stress resistance mechanism will be beneficial for the use of xerophytes to prevent desertification. To determine osmotic adjustment (OA) and antioxidase functions under simulated drought stress, 8-week-old seedlings were treated with sorbitol solutions to maintain osmotic potentials (Ψs) at a control and -0.5 and -1.0 MPa. Under -0.5 MPa osmotic stress, H. ammodendron stably maintained the water content of assimilating branches, a result that was not significantly different from the result of the control group. Moreover, the Ψs decreased significantly, which helped plants absorb water efficiently from the environment, as H. ammodendron accumulated massive osmotic regulators in its assimilating branches to adjust shoot Ψs. Specifically, the contribution of Na⁺ to shoot Ψs was up to 45%, and Na⁺ became the main osmotic regulator of OA. During the treatments, the content and contribution of K⁺ remained stable. However, the total contribution of three organic osmotic regulators (free proline, betaine and soluble sugar) was only 20%, and betaine was the main organic osmotic regulator, accounting for approximately 15% of the 20% contribution. Moreover, H. ammodendron seedlings presented strong antioxidases, especially when there was a high activity level of superoxide dismutase, and with an increase in treatment time and degree of osmotic stress, the activity of peroxidase and catalase increased significantly. Substantial accumulation of osmotic adjustment substances was an important strategy for H. ammodendron to cope with simulated drought stress, in particular, H. ammodendron absorbed much Na⁺ and transported Na⁺ into the assimilating branch for OA. The scavenging of reactive oxygen species by antioxidases was another adaptation strategy for H. ammodendron to adapt to simulated drought stress.

Choline but not its derivative betaine blocks slow vacuolar channels in the halophyte Chenopodium quinoa: implications for salinity stress responses

Activity of tonoplast slow vacuolar (SV, or TPC1) channels has to be under a tight control, to avoid undesirable leak of cations stored in the vacuole. This is particularly important for salt-grown plants, to ensure efficient vacuolar Na(+) sequestration. In this study we show that choline, a cationic precursor of glycine betaine, efficiently blocks SV channels in leaf and root vacuoles of the two chenopods, Chenopodium quinoa (halophyte) and Beta vulgaris (glycophyte). At the same time, betaine and proline, two major cytosolic organic osmolytes, have no significant effect on SV channel activity. Physiological implications of these findings are discussed.

The SbASR-1 gene cloned from an extreme halophyte Salicornia brachiata enhances salt tolerance in transgenic tobacco

Salinity severely affects plant growth and development. Plants evolved various mechanisms to cope up stress both at molecular and cellular levels. Halophytes have developed better mechanism to alleviate the salt stress than glycophytes, and therefore, it is advantageous to study the role of different genes from halophytes. Salicornia brachiata is an extreme halophyte, which grows luxuriantly in the salty marshes in the coastal areas. Earlier, we have isolated SbASR-1 (abscisic acid stress ripening-1) gene from S. brachiata using cDNA subtractive hybridisation library. ASR-1 genes are abscisic acid (ABA) responsive, whose expression level increases under abiotic stresses, injury, during fruit ripening and in pollen grains. The SbASR-1 transcript showed up-regulation under salt stress conditions. The SbASR-1 protein contains 202 amino acids of 21.01-kDa molecular mass and has 79 amino acid long signatures of ABA/WDS gene family. It has a maximum identity (73 %) with Solanum chilense ASR-1 protein. The SbASR-1 has a large number of disorder-promoting amino acids, which make it an intrinsically disordered protein. The SbASR-1 gene was over-expressed under CaMV 35S promoter in tobacco plant to study its physiological functions under salt stress. T(0) transgenic tobacco seeds showed better germination and seedling growth as compared to wild type (Wt) in a salt stress condition. In the leaf tissues of transgenic lines, Na(+) and proline contents were significantly lower, as compared to Wt plant, under salt treatment, suggesting that transgenic plants are better adapted to salt stress.

Glycinebetaine protects plants against abiotic stress: mechanisms and biotechnological applications

Various compatible solutes enable plants to tolerate abiotic stress, and glycinebetaine (GB) is one of the most-studied among such solutes. Early research on GB focused on the maintenance of cellular osmotic potential in plant cells. Subsequent genetically engineered synthesis of GB-biosynthetic enzymes and studies of transgenic plants demonstrated that accumulation of GB increases tolerance of plants to various abiotic stresses at all stages of their life cycle. Such GB-accumulating plants exhibit various advantageous traits, such as enlarged fruits and flowers and/or increased seed number under non-stress conditions. However, levels of GB in transgenic GB-accumulating plants are relatively low being, generally, in the millimolar range. Nonetheless, these low levels of GB confer considerable tolerance to various stresses, without necessarily contributing significantly to cellular osmotic potential. Moreover, low levels of GB, applied exogenously or generated by transgenes for GB biosynthesis, can induce the expression of certain stress-responsive genes, including those for enzymes that scavenge reactive oxygen species. Thus, transgenic approaches that increase tolerance to abiotic stress have enhanced our understanding of mechanisms that protect plants against such stress.

Quantification of water stress-induced osmotic adjustment and proline accumulation for Arabidopsis thaliana molecular genetic studies

For the genetic potential of model systems such as Arabidopsis thaliana to be most effectively used to understand drought resistance, reliable and rapid protocols are needed for laboratory study of phenotypes relevant to stress responses in the field. Osmotic adjustment, the amount of additional solutes accumulated by plants under water stress, is often measured in drought physiology studies and requires quantification of both relative water content and solute content (osmotic potential) of the plant tissue. Water stress also elicits high levels of proline accumulation. Protocols are presented here to measure both of these parameters in Arabidopsis seedlings that have been exposed to controlled water stress treatments using polyethylene glycol-agar plates. For the ninhydrin-based assay of proline, a protocol for performing the assay in 96-well format to increase sample throughput is presented.

Shared and novel molecular responses of mandarin to drought

Drought is the most important stress experienced by citrus crops. A citrus cDNA microarray of about 6.000 genes has been utilized to identify transcriptomic responses of mandarin to water stress. As observed in other plant species challenged with drought stress, key genes for lysine catabolism, proline and raffinose synthesis, hydrogen peroxide reduction, vacuolar malate transport, RCI2 proteolipids and defence proteins such as osmotin, dehydrins and heat-shock proteins are induced in mandarin. Also, some aquaporin genes are repressed. The osmolyte raffinose could be detected in stressed roots while the dehydrin COR15 protein only accumulated in stressed leaves but not in roots. Novel drought responses in mandarin include the induction of genes encoding a new miraculin isoform, chloroplast beta-carotene hydroxylase, oleoyl desaturase, ribosomal protein RPS13A and protein kinase CTR1. These results suggest that drought tolerance in citrus may benefit from inhibition of proteolysis, activation of zeaxanthin and linolenoyl synthesis, reinforcement of ribosomal structure and down-regulation of the ethylene response.

A reassessment of the function of the so-called compatible solutes in the halophytic plumbaginaceae Limonium latifolium

The compatible solute hypothesis posits that maintaining osmotic equilibrium under conditions of high salinity requires synthesis of organic compounds, uptake of potassium ions, and partial exclusion of NaCl. To assess whether osmotic adaptation in Limonium latifolium proceeds according to this hypothesis, a comprehensive analysis of solute accumulation during NaCl treatments was conducted. Determination of prevailing inorganic ions and establishment of the metabolic profiles for low M(r) organic substances revealed that contrary to the mentioned hypothesis the major contributors to osmolarity were constituted by inorganic solutes. Independent of salinity, only 25% of this osmolarity resulted from organic solutes such as Suc and hexoses. Proline (Pro), beta-alanine betaine, and choline-O-sulfate were minor contributors to osmolarity. Compatible inositols also occurred, especially chiro-inositol, characterized for the first time in this species, to our knowledge. Principal component analysis showed that only a limited number of metabolic reconfigurations occurred in response to dynamic changes in salinity. Under such conditions only sugars, chiro-inositol, and Pro behave as active osmobalancers. Analysis of metabolic profiles during acclimatization to either mild salinity or nonsaline conditions showed that organic solute accumulation is predominantly controlled by constitutive developmental programs, some of which might be slightly modulated by salinity. Osmolarity provided under such conditions can be sufficient to maintain turgor in salinized seedlings. Compartmental analysis of Pro and beta-alanine betaine in leaf tissues demonstrated that these solutes, mainly located in vacuoles under nonsaline conditions, could be partly directed to the cytosol in response to salinization. Thus they did not conform with the predictions of the compatible solute hypothesis.

Effect of water deficits on the activity of anti-oxidative enzymes and osmoregulation among three different genotypes of Radix Astragali at seeding stage

Water is a key factor influencing the yield and quality of crops. Plants mainly adapt to water deficits by biochemical changes and osmotic adjustment (OA). Research on drought tolerance of field crops has been done intensively, but there is little work to be done in medical plants. Traditional Chinese medicine (TCM) has a long history of several thousand years. TCM is playing an important role in daily life in China and applied widely in clinical experience on the globe. More and more wild medical plants are cultivated and introduced. It is known that ecological and environmental conditions are vital to cultivation and efficient component accumulation of medical plants. This study is concerned about biochemical changes of three genotypes of Radix Astragali during water deficient periods and we evaluated the relative ability of their drought tolerance on the above basis. We analyzed the effect of soil water deficits on antioxidant enzymes activity and osmoregulation substances in R. Astragali leaves of three genotypes collected on day 0, 5, 10, 15, 20 and 25 after onset of water deprivation. Under water deficient conditions, biochemical changes include protecting enzyme system, for instance superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD). Osmoregulation matters include proline (Pro) and soluble sugar. Antioxidant enzyme activities and Pro, and soluble sugar content correlated between water deficient degree and time course. Antioxidant enzyme activities increased before 20 days, then decreased at the end of experiment. Proline content increased gradually, and soluble sugar content reached the highest on day 20. The order of the ability of drought tolerance in three genotypes of R. Astragali is Mongolia>Wild>Hebei by using index of drought tolerance. The research results are instructive for cultivation and introduction of R. Astragali under different conditions of water status.

Exogenously Supplied Compatible Solutes Rapidly Ameliorate NaCl-induced Potassium Efflux from Barley Roots

It has been suggested that the role of compatible solutes in plant stress responses is not limited to conventional osmotic adjustment, but also includes some other regulatory or osmoprotective functions. In this study, we hypothesized that one such function is in maintaining cytosolic K+ homeostasis by preventing NaCl-induced K+ leakage from the cell, a feature that may confer salt tolerance in many species, particularly in barley. This hypothesis was investigated using the non-invasive microelectrode ion flux (MIFE) measuring technique. We show that low (0.5-5 mM) concentrations of exogenously supplied proline or betaine significantly reduced NaCl-induced K+ efflux from barley roots in a dose-response manner. This effect was instantaneous, implying that large intracellular concentrations of compatible solutes are not required for an amelioratory role. Exogenously supplied betaine also significantly enhanced NaCl-induced H+ efflux, but only in pre-incubated roots, implying some alternative mechanism of regulation. Sap K+ and Na+ analysis and membrane potential measurements are also consistent with the model that one function of compatible solutes is in maintaining cytosolic K+ homeostasis by preventing NaCl-induced K+ leakage from the cell, possibly through the enhanced activity of H+-ATPase, controlling voltage-dependent outward-rectifying K+ channels and creating the electrochemical gradient necessary for secondary ion transport processes. These data provide the first direct evidence for regulation of ion fluxes across the plasma membrane by physiologically relevant low concentrations of compatible solutes.

Similar Regulation Patterns of Choline Monooxygenase, Phosphoethanolamine N-Methyltransferase and S-Adenosyl-l-Methionine Synthetase in Leaves of the Halophyte Atriplex nummularia L

Glycinebetaine (betaine) highly accumulates as a compatible solute in certain plants and has been considered to play a role in the protection from salt stress. The betaine biosynthesis pathway of betaine-accumulating plants involves choline monooxygenase (CMO) as the key enzyme and phosphoethanolamine N-methyltransferase (PEAMT), which require S-adenosyl-L-methionine (SAM) as a methyl donor. SAM is synthesized by SAM synthetase (SAMS), and is needed not only for betaine synthesis but also for the synthesis of other compounds, especially lignin. We cloned CMO, PEAMT and SAMS isogenes from a halophyte Atriplex nummularia L. (Chenopodiaceous). The transcript and protein levels of CMO were much higher in leaves and stems than in roots, suggesting that betaine is synthesized mainly in the shoot. The regulation patterns of transcripts for SAMS and PEAMT highly resembled that of CMO in the leaves during and after relief from salt stress, and on a diurnal rhythm. In the leaves, the betaine content was increased but the lignin content was not changed by salt stress. These results suggest that the transcript levels of SAMS are co-regulated with those of PEAMT and CMO to supply SAM for betaine synthesis in the leaves.

LWR1 and LWR2 are required for osmoregulation and osmotic adjustment in Arabidopsis

With the goal of identifying molecular components of the low-water-potential response, we have carried out a two-part selection and screening strategy to identify new Arabidopsis mutants. Using a system of polyethylene glycol-infused agar plates to impose a constant low-water-potential stress, putative mutants impaired in low-water-potential induction of the tomato (Lycopersicon esculentum) le25 promoter were selected. These lines were then screened for altered accumulation of free Pro. The seedlings of 22 mutant lines had either higher or lower Pro content than wild type when exposed to low water potential. Two mutants, designated low-water-potential response1 (lwr1) and lwr2, were characterized in detail. In addition to higher Pro accumulation, lwr1 seedlings had higher total solute content, greater osmotic adjustment at low water potential, altered abscisic acid content, and increased sensitivity to applied abscisic acid with respect to Pro content. lwr1 also had altered growth and morphology. lwr2, in contrast, had lower Pro content and less osmotic adjustment leading to greater water loss at low water potential. Both lwr1 and lwr2 also had altered leaf solute content and water relations in unstressed soil-grown plants. In both mutants, the effects on solute content were too large to be explained by the changes in Pro content alone, indicating that LWR1 and LWR2 affect multiple aspects of cellular osmoregulation.

Induction of salt and osmotic stress tolerance by overexpression of an intracellular vesicle trafficking protein AtRab7 (AtRabG3e)

Adaptation to stress requires removal of existing molecules from various cellular compartments and replacing them with new ones. The transport of materials to and from the specific compartments involved in the recycling and deposition of macromolecules is carried out by an intracellular vesicle trafficking system. Here, we report the isolation of a vesicle trafficking-regulating gene, AtRabG3e (formerly AtRab7), from Arabidopsis. The gene was induced during programmed cell death after treatment of intact leaves with superoxide and salicylic acid or infection with necrogenic pathogens. Transgenic plants that expressed the AtRabG3e gene under the constitutive 35S promoter from cauliflower mosaic virus exhibited accelerated endocytosis in roots, leaves, and protoplasts. The transgenic plants accumulated sodium in the vacuoles and had higher amounts of sodium in the shoots. The transgenic plants also showed increased tolerance to salt and osmotic stresses and reduced accumulation of reactive oxygen species during salt stress. These results imply that vesicle trafficking plays an important role in plant adaptation to stress, beyond the housekeeping function in intracellular vesicle trafficking.

Glycine betaine accumulation, ionic and water relations of red-beet at contrasting levels of sodium supply

Exposure of plants to sodium (Na) and salinity may increase glycine betaine accumulation in tissues. To study this, red-beet cvs. Scarlet Supreme and Ruby Queen, were grown for 42 days in a growth chamber using a re-circulating nutrient film technique with 0.25 mmol/L K and either 4.75 mmol/L (control) or 54.75 mmol/L (saline) Na (as NaCl). Plants were harvested at weekly intervals and measurements were taken on leaf water relations, leaf photosynthetic rates, chlorophyll fluorescence, chlorophyll levels, glycine betaine levels, and tissue elemental composition. Glycine betaine accumulation increased under salinity and this accumulation correlated with higher tissue levels of Na in both cultivars. Na accounted for 80 to 90% of the total cation uptake under the saline treatment. At final harvest (42 days), K concentrations in laminae ranged from approximately 65-95 micromoles g-1 dry matter (DM), whereas Na in shoot tissue ranged from approximately 3000-4000 micromoles g-1. Leaf sap osmotic potential at full turgor [psi(s100)] increased as lamina Na content increased. Glycine betaine levels of leaf laminae showed a linear relationship with leaf sap [psi(s100)]. Chlorophyll levels, leaf photosynthetic rates, and chlorophyll fluorescence were not affected by Na levels. These results suggest that the metabolic tolerance to high levels of tissue Na in red-beet could be due to its ability to synthesize and regulate glycine betaine production, and to control partitioning of Na and glycine betaine between the vacuole and the cytoplasm.

Aquaporin Nt-TIPa can account for the high permeability of tobacco cell vacuolar membrane to small neutral solutes

Members of the major intrinsic protein (MIP) family, described in plants as water-selective channels (aquaporins), can also transport small neutral solutes in other organisms. In the present work, we characterize the permeability of plant vacuolar membrane (tonoplast; TP) and plasma membrane (PM) to non-electrolytes and evaluate the contribution of MIP homologues to such transport. PM and TP vesicles were purified from tobacco suspension cells by free-flow electrophoresis, and membrane permeabilities for a wide range of neutral solutes including urea, polyols of different molecular size, and amino acids were investigated by stopped-flow spectrofluorimetry. For all solutes tested, TP vesicles were found to be more permeable than their PM counterparts, with for instance urea permeabilities from influx experiments of 74.9 +/- 9.6 x 10(-6) and 1.0 +/- 0.3 x 10(-6) cm sec-1, respectively. Glycerol and urea transport in TP vesicles exhibited features of a facilitated diffusion process. This and the high channel-mediated permeability of the same TP vesicles to water suggested a common role for MIP proteins in water and solute transport. A cDNA encoding a novel tonoplast intrinsic protein (TIP) homologue named Nicotiana tabacum TIPa (Nt-TIPa) was isolated from tobacco cells. Immunodetection of Nt-TIPa in purified membrane fractions confirmed that the protein is localized in the TP. Functional expression of Nt-TIPa in Xenopus oocytes showed this protein to be permeable to water and solutes such as urea and glycerol. These features could account for the transport selectivity profile determined in purified TP vesicles. These results support the idea that plant aquaporins have a dual function in water and solute transport. Because Nt-TIPa diverges in sequence from solute permeable aquaporins characterized in other organisms, its identification also provides a novel tool for investigating the molecular determinants of aquaporin transport selectivity.

Salinity promotes accumulation of 3-dimethylsulfoniopropionate and its precursor S-methylmethionine in chloroplasts

Wollastonia biflora (L.) DC. plants accumulate the osmoprotectant 3-dimethylsulfoniopropionate (DMSP), particularly when salinized. DMSP is known to be synthesized in the chloroplast from S-methylmethionine (SMM) imported from the cytosol, but the sizes of the chloroplastic and extrachloroplastic pools of these compounds are unknown. We therefore determined DMSP and SMM in mesophyll protoplasts and chloroplasts. Salinization with 30% (v/v) artificial seawater increased protoplast DMSP levels from 4.6 to 6.0 mumol mg-1 chlorophyll (Chl), and chloroplast levels from 0.9 to 1.9 mumol mg-1 Chl. The latter are minimum values because intact chloroplasts leaked DMSP during isolation. Correcting for this leakage, it was estimated that in vivo about one-half of the DMSP is chloroplastic and that stromal DMSP concentrations in control and salinized plants are about 60 and 130 mM, respectively. Such concentrations would contribute significantly to chloroplast osmoregulation and could protect photosynthetic processes from stress injury. SMM levels were measured using a novel mass-spectrometric method. About 40% of the SMM was located in the chloroplast in unsalinized W. biflora plants, as was about 80% in salinized plants; the chloroplastic pool in both cases was approximately 0.1 mumol mg-1 Chl. In contrast, > or = 85% of the SMM was extrachloroplastic in pea (Pisum sativum L.) and spinach (Spinacia oleracea L.), which lack DMSP. DMSP synthesis may be associated with enhanced accumulation of SMM in the chloroplasm.

Chloride transport in stomatal guard cells

Stomatal guard cells regulate the size of the stomatal pore by the changes in their shape and volume, which are associated with changes in their turgor. Accumulation of potassium salts plays a major role in this process, and frequently chloride, if available, provides the major balancing anion. Measurements have been made of two-way ion fluxes in guard cells, in epidermal strips Commelina communis L. , after treatment at low pH to kill all cells except the guard cells. In such material, opening depends on the ion concentration in the bathing solution, and for this purpose the three salts KC1, KBr and RbCl seem to be equivalent. 82 Br - and 86 Rb+ fluxes have been measured in a range of steady states, with different apertures, and in the transitions between one steady state and another. Analysis of the kinetics of tracer efflux in the steady states allows calculation of the cytoplasmic and vacuolar contents, and their changes with aperture, with wider opening produced by increasing concentration in the bathing solution, or by light incubation compared with dark incubation. The results show that the increases with aperture of the cytoplasmic salt concentration are comparable with the osmotic changes required, but the changes in vacuolar concentration are much less than those required osmotically. Opening must therefore be associated with vacuolar accumulation not only of salt, but also of some other solute. The decrease in aperture on addition of 2 x 10 -5 M abscisic acid to the solution bathing ‘isolated’ guard cells, or on their transfer from light to dark, is achieved by marked transient increases in ion efflux, with little change in influx. There are also changes in tonoplast fluxes. The aperture is determined by the level of vacuolar solute accumulation, and thus the results show that this responds to environmental signals by control of plasmalemma efflux rather than influx, and by control of tonoplast fluxes. The ability to transfer salt from cytoplasm to vacuole may be critical for the maintenance of turgor and aperture.

Salt tolerance in plants and microorganisms: toxicity targets and defense responses

Salt tolerance of crops could be improved by genetic engineering if basic questions on mechanisms of salt toxicity and defense responses could be solved at the molecular level. Mutant plants accumulating proline and transgenic plants engineered to accumulate mannitol or fructans exhibit improved salt tolerance. A target of salt toxicity has been identified in Saccharomyces cerevisiae: it is a sodium-sensitive nucleotidase involved in sulfate activation and encoded by the HAL2 gene. The major sodium-extrusion system of S. cerevisiae is a P-ATPase encoded by the ENA1 gene. The regulatory system of ENA1 expression includes the protein phosphatase calcineurin and the product of the HAL3 gene. In Escherichia coli, the Na(+)-H+ antiporter encoded by the nhaA gene is essential for salt tolerance. No sodium transport system has been identified at the molecular level in plants. Ion transport at the vacuole is of crucial importance for salt accumulation in this compartment, a conspicuous feature of halophytic plants. The primary sensors of osmotic stress have been identified only in E. coli. In S. cerevisiae, a protein kinase cascade (the HOG pathway) mediates the osmotic induction of many, but not all, stress-responsive genes. In plants, the hormone abscisic acid mediates many stress responses and both a protein phosphatase and a transcription factor (encoded by the ABI1 and ABI3 genes, respectively) participate in its action.

Sodium, potassium, chloride and proline concentrations of chloroplasts isolated from a halophyte, Mesembryanthemum crystallinum L

Concentrations of four major solutes (Na(+), K(+), Cl(-), proline) were determined in isolated, intact chloroplasts from the halophyte Mesembryanthemum crystallinum L. following long-term exposure of plants to three levels of NaCl salinity in the rooting medium. Chloroplasts were obtained by gentle rupture of leaf protoplasts. There was either no or only small leakage of inorganic ions from the chloroplasts to the medium during three rapidly performed washing steps involving precipitation and re-suspension of chloroplast pellets. Increasing NaCl salinity of the rooting medium resulted in a rise of Na(+) und Cl(-) in the total leaf sap, up to approximately 500 and 400 mM, respectively, for plants grown at 400 mM NaCl. However, chloroplast levels of Na(+) und Cl(-) did not exceed 160-230 and 40-60 mM, respectively, based upon a chloroplast osmotic volume of 20-30 μl per mg chlorophyll. At 20 mM NaCl in the rooting medium, the Na(+)/K(+) ratio of the chloroplasts was about 1; at 400 mM NaCl the ratio was about 5. Growth at 400 mM NaCl led to markedly increased concentrations of proline in the leaf sap (8 mM) compared with the leaf sap of plants grown in culture solution without added NaCl (proline 0.25 mM). Although proline was fivefold more concentrated in the chloroplasts than in the total leaf sap of plants treated with 400 mM NaCl, the overall contribution of proline to the osmotic adjustment of chloroplasts was small. The capacity to limit chloroplast Cl(-) concentrations under conditions of high external salinity was in contrast to an apparent affinity of chloroplasts for Cl(-) under conditions of low Cl(-) availability.

Membrane-potential changes in vacuoles isolated from storage roots of red beet (Beta vulgaris L.)

The membrane potential in vacuoles isolated from storage roots of red beet (Beta vulgaris L.) has been studied by following changes in the fluorescence of the dye 3,3'-diethylthiodicarbocyanine iodide, and by determining the uptake of the lipophilic triphenylmethylphosphonium cation. The vacuoles have a membrane potential, internal negative, which is estimated to be around-60 mV. These potentials become less negative by nearly 10 mV on addition of ATP. This ATP-dependent depolarisation is inhibited by the protonophore carbonylcyanide p-trifluoromethoxyphenylhydrazone and by the ATPase inhibitors, N,N'-dicyclohexylcarbodiimide and trimethyltin chloride, but it is largely insensitive to sodium orthovanadate. Fusicoccin had no significant effect on the isolated vacuoles, but its addition to excised tissue caused a hyperpolarisation of the cells measured using a microelectrode.

An attempt to use isolated vacuoles to determine the distribution of sodium and potassium in cells of storage roots of red beet (Beta vulgaris L.)

Vacuoles isolated from red beet (Beta vulgaris L.) storage roots contain Na(+) and K(+) but their analysis does not give reliable information about the size of vacuolar pools of these ions in vivo. Analyses of isolated vacuoles indicated that between 53% and 90% of the Na(+) was located in the vacuole and that the vacuolar concentrations of Na(+) ranged between 4 and 45 mol m(-3). Calculated concentrations of K(+) in the vacuoles varied between 32 and 72 mol m(-3) but, in contrast to Na(+), only about 50% of the K(+) was located in the vacuole. Considerations of the likely cytoplasmic concentrations of Na(+) and K(+) suggest that if these results indicate conditions in vivo a large proportion of these ions must be located in the extracellular space, where they would exert considerable osmotic pressure. To test this, the effect of washing on cell turgor (measured directly with a pressure probe) and on loss of Na(+) and K(+) was determined. Washing caused an increase in turgor of 5 bar but losses of Na(+) and K(+) were less than predicted by the experiments with isolated vacuoles. It is concluded that beet vacuoles leak Na(+) and K(+) when isolated resulting in an underestimation of the size of vacuolar pools of these cations in vivo. Nonetheless, the turgor measurements provide evidence for the presence of osmotically active solute in the extracellular space. The possible contribution of extracellular Na(+) and K(+) to the observed turgor reduction is calculated and the physiological importance of the accumulation of extracellular solutes is discussed.

Solute distribution in Suaeda maritima

The distribution of sodium, potassium and glycinebetaine in shoot tissues of salt-treated Suaeda maritima was examined by semi-micro techniques after extraction into toluene-water. Much higher K/Na ratios were observed in the apical regions and in axillary buds than in more mature, fully vacuolated tissues. The younger tissues also contained very high levels of glycinebetaine. Electron-probe X-ray microanalysis of bulkfrozen and fractured preparations showed higher K/Na ratios and higher levels of sulphur and phosphorus in the cytoplasm of leaf primordial cells than in vacuoles of either young or old leaves, although the total counts were higher in the vacuolar samples. The results are discussed in relation to current models of subcellular solute compartmentation and salt tolerance in the Chenopodiaceae.

Glycinebetaine biosynthesis and its control in detached secondary leaves of spinach

In secondary leaves from spinach plants pretreated in vermiculite for 24 h with 300 mM NaCl, glycinebetaine accumulated at a rate of circa 0.16 μmol 100 μg(-1) Chl d(-1) (2 μmol g(-1) FW d(-1)), about three times the rate of control plants. The soluble carbohydrate and free amino acid contents did not increase significantly following salinisation until after 4 d when the relative growth rate also decreased. Leaf proline levels remained very low throughout the experimental period. K(+) on a tissue water basis remained constant at 200 mM while Cl(-) and Na(+) levels increased linearly to reach 175 and 100 mM respectively after 5 d of saline treatment. The osmotic pressure of leaf tissue also increased from 300 to 500 mosmol kg(-1). These experimental conditions were considered suitable to study glycinebetaine biosynthesis and its induction by salinity in the absence of marked growth inhibition or metabolic disturbance. Radioactive labelled [(14)C]serine, ethanolamine and choline (all 1 μmol, 13.3 MBq in 10 μl) were fed to detached secondary leaves via the petiole 24 h after the exposure of plants to salt. The rate of isotope incorporation into water soluble products, lipids and residue was measured over a further 24 h. The major metabolic fate of exogenous [(14)C]choline and [(14)C]ethanolamine was incorporation into glycinebetaine while less (14)C-label was found in phosphatidyl choline and phosphatidyl ethanolamine. Incorporation rates were identical in control and salinised leaves and were adequate to account for observed values of glycinebetaine accumulation previously reported in spinach. In contrast the labelling of glycinebetaine from [(14)C]serine was twice as great in salinated plants as in the controls. These results, together with short term labelling experiment with [(14)C]ethanolamine using leaf slices, were consistent with the formation of glycinebetaine via serine, ethanolamine and its methylated derivatives to choline with some control being exerted at the serine level. However a flux through the phosphorylated intermediates is not excluded.From a consideration of these results and the published data on barley subjected to water stress (Hanson and Scott, 1980 Plant Physiol. 66, 342-348) there appear to be significant differences in the biosynthetic pathways in spinach and barley.