Movement and compartmentation of abscisic acid in guard cells of Valerianella locusta: Effects of osmotic stress, external H(+)-concentration and fusicoccin

The apoplast and its significance for plant mineral nutrition

It has only recently become apparent that the apoplast plays a major role in a diverse range of processes, including intercellular signalling, plant-microbe interactions and both water and nutrient transport. Broadly defined, the apoplast constitutes all compartments beyond the plasmalemma - the interfibrillar and intermicellar space of the cell walls, and the xylem, including its gas- and water-filled intercellular space - extending to the rhizoplane and cuticle of the outer plant surface. The physico-chemical properties of cell walls influence plant mineral nutrition, as nutrients do not simply pass through the apoplast to the plasmalemma but can also be adsorbed or fixed to cell-wall components. Here, current progress in understanding the significance of the apoplast in plant mineral nutrition is reviewed. The contribution of the root apoplast to short-distance transport and nutrient uptakes is examined particularly in relation to Na⁺ toxicity and Al³⁺ tolerance. The review extends to long-distance transport and the role of the apoplast as a habitat for microorganisms. In the leaf, the apoplast might have benefits over the vacuole as a site for short-term nutrient storage and solute exchange with the atmosphere. Contents Summary 167 I. Introduction 168 II. The properties of the apoplast and its implication for solute movement 168 1. The middle lamella 168 2. The primary wall 168 3. The secondary cell wall 169 III. The root apoplast - nutrient uptake and short-distance transport 170 IV. The apoplast as a compartment for long distance transport 174 V. The apoplast - habitat for microorganisms 175 VI. The apoplast of leaves - a compartment of storage and of reactions 177 1. Transport routes in the leaf apoplast 177 2. Methods of studying apoplastic solutes 177 3. Solute relations in the leaf apoplast 178 4. Concentration gradients in the leaf apoplast 179 5. Ion relations in the leaf apoplast and symptoms of deficiency and toxicity 179 6. Ion relations in the leaf apoplast - influence of nutrient supply 180 7. The leaf apoplast - compartment for transient ion storage 180 8. Ion fluxes between apoplast and symplast 181 9. Apoplastic ion balance 181 10. Leaf apoplast - interaction with the atmosphere 183 VII. Conclusions 183 Acknowledgements 183 References 183.

Induction of RAB18 gene expression and activation of K+ outward rectifying channels depend on an extracellular perception of ABA in Arabidopsis thaliana suspension cells

Important progress has been made regarding the characterization of the ABA signalling components using genetic and molecular approaches (Leung and Giraudat, 1998). However, we do not yet know the mechanism of ABA perception. Conflicting results concerning the site of ABA perception have been published. The prevailing view is that since ABA controls many responses, different sites of perception for ABA might exist. In order to establish the cellular localisation of the ABA receptors in Arabidopsis thaliana suspension cells, we developed two physiological tests based upon the capacity of impermeant ABA-BSA conjugate to mimic permeant free ABA effects. We show that purified ABA-BSA conjugate is able to trigger RAB18 gene expression and that this response is strictly due to the natural (+)-ABA enantiomer. The rate of RAB18 gene expression was independent of the level of ABA uptake by the cells. Using the voltage-clamp technique we show that ABA-BSA, similarly to ABA, evokes a membrane depolarization and activates time- and voltage-dependent outward rectifying currents (ORC). We demonstrate that these ORC are due to a K+ efflux as assessed by tail currents and specific inhibition by both tetraethylammonium (TEA) and Ba2+. These observations provide evidence in favour of an extracellular site for ABA perception.

The pH gradients in the root system and the abscisic acid concentration in xylem and apoplastic saps

Abscisic acid (ABA) is a stress signal that is transported from the root system to leaves, and induces stomatal closure before water relations of the leaves are affected by soil drying. Xylem vessels are in direct contact with the leaf apoplasm, the only leaf compartment that is directly connected with the primary site of ABA action, the outer surface of the guard cell plasma membrane (Hartung 1983). ABA distributes among the leaf compartments according to the anion trap concept and the Henderson-Hasselbalch equation, with the free acid as the permeating and the anion as the nearly non-permeating molecular species. Applying this concept, a flattening of the intracellular pH gradients increases the apoplastic ABA concentration. Indeed, stress increases the apoplastic pH (Hartung et al. 1988) and decreases slightly the cytosolic pH . The validity of this concept has been shown repeatedly and was confirmed by a mathematical leaf model (Slovik et al. 1992). It is appropriate to ask whether these mechanisms also contribute to ABA compartmentation and redistribution in the root system. Therefore, we have incorporated compartmental pH values of unstressed and stressed root cells, the permeability coefficients of root membranes for ABA and anatomical data into a mathematical model, similar to that of Slovik et al. (1992). The simulation shows that ABA redistribution in roots caused by changing pH gradients can account for up to a 2 to 3-fold accumulation of ABA in the xylem sap of stressed plants. The model also predicts that the pH gradient across the cortical plasma membrane has the most distinct effects on redistribution of ABA into the xylem sap of stressed plants and, additionally, that the ABA concentration in the rhizospheric aqueous solution can play an im portant role in root-to-shoot signalling.

Abscisic acid-lipid interactions: a phospholipid monolayer study

Lipid monolayer studies were performed on a Langmuir trough in the absence and in the presence of the plant hormone abscisic acid (ABA). The ABA-induced effects on the lipid monolayers can be summarized as follows: (i) ABA as the free acid (pH below 5.3) increased the molecular area and slightly decreased the surface pressure in the collapse points of monolayers made of saturated, unsaturated and of mixed lipids; ABA as the anion showed only minor effects. (ii) The ABA-induced area increase of the lipid monolayers decreased when the surface pressure increased, but some ABA remained in the monolayers made of unsaturated phospholipids even at collapse pressure. (iii) The incorporation of ABA into the monolayers could be inhibited by adding the plant sterol beta-sitosterol to the monolayer forming phospholipids. (iv) There was no substantial difference of ABA action on plant phospholipids as compared with other phospholipids. (v) ABA had a much stronger influence on unsaturated phospholipids than on saturated ones. (vi) ABA decreased the phase-transition temperature of saturated phospholipids. These results, which agree with those obtained from phospholipid vesicle studies, indicate that the physical state of the lipid is important for the ability of ABA penetrating into the lipid monolayer. Finally, a possible relevance of these results is discussed in terms of the action of ABA on guard cell membranes of plants.

Compartmental distribution and redistribution of abscisic acid in intact leaves : III. Analysis of the stress-signal chain

Using a computer model written for whole leaves (Slovik et al. 1992, Planta 187, 14-25) we present in this paper calculations of abscisic acid (ABA) redistribution among different leaf tissues and their compartments in relation to stomatal regulation under drought stress. The model calculations are based on experimental data and biophysical laws. They yield the following results and postulates: (i) Under stress, compartmental pH-shifts come about as a consequence of the inhibition of the pH component of proton-motive forces at the plasmalemma. There is a decrease of net proton fluxes by about 8.6 nmol · s(-1) · m(-2). (ii) Using stress-induced pH-shifts we demonstrate how 'stress intensities' can be quantified on a molecular basis. (iii) As the weak acid ABA is the only phytohormone which behaves in vivo and in vitro ideally according to the Henderson-Hasselbalch equation, pH-shifts induce a complicated redistribution amongst compartments in the model leaf. (iv) The final accumulation of ABA in guard-cell walls is intensive: up to 16.1-fold compared with only up to 3.4-fold in the guard-cell cytosol. We propose that the binding site of the guard-cell ABA receptor faces the apoplasm. (v) A twoto three-fold ABA accumulation in guard-cell walls is sufficient to induce closure of stomata. (vi) The minimum time lag until stomata start to close is 1-5 min; it depends on the stress intensity and on the guard-cell sensitivity to ABA: the more moderate the stress is, the later stomata start to close or they do not close at all. (vii) In the short term, there is almost no influence of the velocity of pH-shifts on the velocity of the ABA redistribution, (viii) Six hours after the termination of stress there is still an ABA concentration 1.4-fold the initial level in the guard-cell cytosol (delay of ABA relaxation, 'aftereffect'), (ix) The observed 'induction' of net ABA synthesis after onset of stress may be explained by a decrease in cytosolic ABA degradation. About 1 h after onset of stress the model leaf would start to synthesise ABA (and its conjugates) automatically, (x) This ABA net synthesis serves to 'inform roots' via an increased ABA concentration in the phloem sap. The stress-induced ABA redistribution is per se not sufficient to feed the ploem sap with ABA. (xi) The primary target membrane of 'stress' is the plasmalemma, not thylakoids. (xii) The effective 'stress sensor', which induces the proposed signal chain finally leading to stomatal closure, is located in epidermal cells. Mesophyll cells are not capable of creating a significant ABA signal to guard cells if the epidermal plasmalemma conductance to undissociated molecular species of ABA (HABA) is indeed higher than the plasmalemma conductance of the mesophyll (plasmodesmata open), (xiii) All model conclusions which can be compared with independent experimental data quantitatively fit to them. We conclude that the basic experimental data of the model are consistent. A stress-induced ABA redistribution in the leaf lamina elicits stomatal closure.

Compartmental distribution and redistribution of abscisic acid in intact leaves : II. Model analysis

A computer model written for whole leaves and described in the preceding publication (Slovik et al. 1992, this volume) has been developed for calculating the distribution and fluxes of weak acids or bases amongst different leaf tissues and their compartments, considering membrane transport, transpiration-driven mass transport, symplasmic and apoplasmic diffusion, and metabolic turnover rates in specified compartments. The model is used to analyse flux equilibria and the transport behaviour of the phytohormone abscisic acid (ABA) in unstressed and stressed leaves. We compare experimental data of unstressed Valerianella locusta L. leaves and expectations based on the detailed analysis of the data. (i) The mean daily influx of ABA into the leaf lamina via the xylem sap is about 10 nmol · m(-2) · day(-1). It is balanced by the sum of an export of ABA via the phloem sap (0.7%), possibly also by a basipetal ABA transport in the petiole parenchyma of young leaves (up to 18%), by an irreversible conjugation of ABA (0.4-4%) and by net degradation of ABA in the leaf lamina (80-95%). (ii) The estimated kinetic parameters of this net degradation are for the mesophyll apoplasm: apparent K m = 3.7 nM and V max = 12.9 nmol · m(-3) · s(-1), or for the mesophyll cytosol: apparent K m = 8.1 nM and V max = 32.3 nmol · m(-3) · s(-1). (iii) The dynamic ABA concentration in the phloem sap of Valerianella is 2.8 nM. This is only 5.5% of the static ABA equilibrium concentration in excised leaves or 70% of the ABA concentration in the mesophyll apoplasm, and it equilibrates within a few hours after source concentrations in the mesophyll apoplasm are changed under stress. Thus, the phloem sap is a flexible medium for transporting 'new phytohormone information' from the lamina to the shoot and roots, (iv) Measured compartmental ABA concentrations are close to calculated equilibrium concentrations in unstressed leaves. We conclude that model calculations are close to reality, (v) pH gradients within the apoplasm influence the apoplasmic distribution of ABA. Its concentration is maximally about twofold higher in guard-cell walls relative to the mesophyll apoplasm. (vi) Unexpectedly, all compartmental equilibrium concentrations of ABA in the leaf lamina depend on plasmalemma conductances for undissociated ABA and on the transport properties of the plasmodesmata. This is a consequence of the cyclic diffusion pathway: mesophyll cytosol - mesophyll plasmalemma - mesophyll apoplasm - epidermal apoplasm - epidermal plasmalemma - epidermal cytosol - plasmodesmata - mesophyll cytosol (in this direction), if there are different apoplasmic or cytosolic pH values in both tissues. The cyclisation rate is 42 fmol · s(-1) · m(-2) leaf area, which corresponds to a turnover time = 11.0 h for the total ABA content within the leaf lamina. A decrease of the epidermal plasmalemma conductance by 90% yields a threefold ABA concentration in the guard-cell free space, (vii) Compartmental relaxation-time coefficients are estimated and summarised for all leaf tissues and its major compartments. They range from 1.5 min for chloroplasts up to 3.3 d for mesophyll vacuoles, (viii) The highest ABA concentration, which can be expected in any leaf compartment, is 7 mM in the guard-cell cytoplasm of certain plant species, (ix) We employed circadian changes (equal day + night, 12 h each = equinoctium) of the stromal pH ± 0.3 in C(3) plants, and for Crassulacean acid metabolism (CAM) plants, additionally, vacuolar pH ± 2.5 changes, and calculated the consequences for ABA redistribution within the lamina. In plants of both photosynthesis types, the ABA concentration in guard-cell walls is only 1.5 times higher in the night relative to the day. We conclude that stomata may not be regulated by ABA in a night-day regime. The influence of the extreme vacuolar pH changes on ABA distribution is small in CAM plants for two reasons: the ABA content in CAM mesophyll vacuoles is low (maximum 2.7% of the total ABA mass per unit leaf area) and there is only a 6.5-fold increase of the mole fraction of undissociated ABA when the the vacuolar pH is lowered from 5.5 to 3.0 (importance of the absolute pKa = 4.75 of ABA).

Compartmental distribution and redistribution of abscisic acid in intact leaves : I. Mathematical formulation

Using experimental information obtained in earlier studies on the permeabilities of mesophyll and guard-cell membranes to abscisic acid (ABA), and on stress-induced pH shifts in the apoplasm and in symplasmic compartments (Hartung et al., 1988, Plant Physiol. 86, 908-913; Hartung et al. 1990, BPGRG Monogr. 215-235), a mathematical model is presented which will permit computer analysis of the stress-induced redistribution of ABA amongst different leaf cell types (mesophyll, epidermis, guard cells, phloem cells) and their compartments (cell wall, cytosol, chloroplast stroma, vacuole). Metabolism and conjugation of ABA and its transport in the xylem and the phloem are also taken into consideration. We ask whether the stressinduced redistribution of ABA is fast and intensive enough to induce stomatal closure within a few minutes. The model can be adapted to any other weak acid or base, e.g. to other phytohormones (auxins, gibberellins), which differ from ABA, e.g. by their membrane conductances, anion permeabilities and pKa values. Our wholeleaf model can predict the time course and the compartmentation of, for example, phytohormone concentrations as a function of changing source-sink patterns (e.g. by compartmental pH shifts in the leaf lamina). An analysis of the present knowledge of the ABA physiology of leaves and studies on stress effects are presented in subsequent publications. In this communication we describe the whole-leaf model and present and discuss all necessary morphological (volumes, surfaces etc.) and physiological (pH, membrane conductances etc.) parameters of an unstressed leaf of Valerianella locusta L. We draw fundamental conclusions by comparing determined and calculated ABA concentrations in the leaf-cell compartments. We found that the model predictions are close to measured data, and we conclude that in unstressed leaves ABA is close to flux equilibrium amongst the different tissues and compartments.

A biophysical study of abscisic acid interaction with membrane phospholipid components

Molecular area/surface pressure Langmuir isotherms of amphiphilic dipalmitoylphosphatidylcholine (DPPC) monolayers indicated that abscisic acid (ABA) has a marked rigidifying effect, expressed as reduction of molecular area and increase of monolayer collapse point. Moreover, ABA markedly increased aqueous droplet hydrophobicity, as indicated by a concentration-dependent increase of contact angle when placed on a hydrocarbon chain surface; no such effects were obtained on either amphiphilic or octadecyltrichlorosilane surfaces. A combination of TLC and mass spectometry revealed the presence of DPPC in Vicia faba and Commelina communis guard-cell protoplast membranes. ABA also increased plasma membrane rigidity as evidenced by probing with lipid specific membrane probes, namely diphenylhexatriene and its trimethyl derivative. Regarded together the results suggest a specific site of ABA binding to DPPC. The linkage between senescence and stomatal closure is discussed in the light of the new data presented here. It is suggested that DPPC in guard-cell membranes may have a physical role in preventing collapse and/or bursting. In this connection an analogy is drawn with pulmonary mechanisms.

Plant sterol inhibition of abscisic acid-induced perturbations in phospholipid bilayers

Abscisic acid (ABA)-induced phospholipid bilayer perturbations (permeability and lipid vesicle aggregation) are shown to be reversed by incorporation of a commercially available mixture of plant sterols (60% beta-sitosterol, 27% campesterol and 13% dihydrobrassicasterol) into the membranes. As little and 5 membrane mol% plant sterol inhibits ABA-stimulated permeability of both saturated and unsaturated mixed phosphatidylcholine/phosphatidylethanolamine bilayers to the fluorescent anion carboxyfluorescein by more than 50%. The same conclusion was reached by an osmotic swelling technique for the uncharged permeant solute erythritol. Hormone-induced carboxyfluorescein permeability to mixed acyl chain phosphatidylcholine bilayers was similarly inhibited by the sterols, but only if the membranes were tested at a temperature where liquid crystal and gel states coexist. The plant sterols were also shown to prevent the ABA-induced fusion of mixed phosphatidylcholine/phosphatidylethanolamine bilayers. The ABA effect on membranes is inhibited equally by plant sterols as well as cholesterol. From these experiments a possible role is suggested for plant sterols in controlling the mode of action of ABA.

Potassium channel currents in intact stomatal guard cells: rapid enhancement by abscisic acid

Evidence of a role for abscisic acid (ABA) in signalling conditions of water stress and promoting stomatal closure is convincing, but past studies have left few clues as to its molecular mechanism(s) of action; arguments centred on changes in H(+)-pump activity and membrane potential, especially, remain ambiguous without the fundamental support of a rigorous electrophysiological analysis. The present study explores the response to ABA of K(+) channels at the membrane of intact guard cells of Vicia faba L. Membrane potentials were recorded before and during exposures to ABA, and whole-cell currents were measured at intervals throughout to quantitate the steady-state and time-dependent characteristics of the K(+) channels. On adding 10 μM ABA in the presence of 0.1, 3 or 10 mM extracellular K(+), the free-running membrane potential (V m) shifted negative-going (-)4-7 mV in the first 5 min of exposure, with no consistent effect thereafter. Voltage-clamp measurements, however, revealed that the K(+)-channel current rose to between 1.84- and 3.41-fold of the controls in the steady-state with a mean halftime of 1.1 ± 0.1 min. Comparable changes in current return via the leak were also evident and accounted for the minimal response in V m. Calculated at V m, the K(+) currents translated to an average 2.65-fold rise in K(+) efflux with ABA. Abscisic acid was not observed to alter either K(+)-current activation or deactivation.These results are consistent with an ABA-evoked mobilization of K(+) channels or channel conductance, rather than a direct effect of the phytohormone on K(+)-channel gating. The data discount notions that large swings in membrane voltage are a prerequisite to controlling guard-cell K(+) flux. Instead, thev highlight a rise in membrane capacity for K(+) flux, dependent on concerted modulations of K(+)-channel and leak currents, and sufficiently rapid to account generally for the onset of K(+) loss from guard cells and stomatal closure in ABA.

Substrate specifity of the hexose carrier in the plasmalemma of Chenopodium suspension cells probed by transmembrane exchange diffusion

Substrate specifity of the proton-driven hexose cotransport carrier in the plasmalemma of photoautotrophic suspension cells of Chenopodium rubrum L. has been studies through the short-term perturbation of (14)C-labelled efflux of 3-O-methyl-D-glucose. Efflux, occurring exclusively via carrier-mediated exchange diffusion, is trans-stimulated by the substrate and trans-inhibited by the glucose-transport inhibitors phlorizin (K 1/2=7.9 mM) and its aglucon phloretin (K 1/2=84 μM); with both inhibitors, 3-O-methyl-D-glucose efflux may be blocked completely. Trans-stimulation of efflux (up to fourfold) by a variety of the D-enantiomers of neutral hexoses, including glucose (K 1/2=48 μM), 3-O-methyl-D-glucose (K 1/2=139 μM), and fructose (K 1/2=730 μM), but not by, for instance, D-allose, and L-sorbose, shows that carrier-substrate interaction critically involves the axial position at C-1 and C-3, respectively. We suggest that substrate binding by the Chenopodium hexose carrier involves both hydrophobic interaction with the pyran-ring and hydrogen-ion bonding at C-1 and C-3 of the D-glucose conformation.

Abscisic-acid contents and concentrations in protoplasts from guard cells and mesophyll cells ofVicia faba L

The abscisic-acid (ABA) contents of isolated guard-cell protoplasts and mesophyll-cell protoplasts fromVicia faba were determined by high-pressure liquid chromatography followed by gas chromatography. The amounts of ABA found immediately after preparation of the protoplasts varied from 90 to 570 amol per guard-cell protoplast, and from 75 to 100 amol per mesophyll-cell protoplast. These contents correspond to concentrations between 36 and 230 μmol per liter in guard-cell protoplasts and between 2.7 and 3.3 μmol per liter in mesophyll-cell protoplasts. During exposure of protoplasts to betaine concentrations of 0.3, 0.5, and 0.8 mol·l(-1) at 0° and 20°C for 30 min, ABA contents as well as the fractions of ABA that leaked into the medium remained constant for both protoplast types. There was no evidence for net production of ABA in isolated protoplasts subjected to osmotic stress.

Abscisic acid enhances aggregation and fusion of phospholipid vesicles

The plant hormone abscisic acid (ABA) is shown to enhance the aggregation and fusion of small unilamellar lipid vesicles composed of 80 mol% dimyristoylphosphatidylcholine (DMPC) and 20 mol% dimyristoylphosphatidylcholine (DMPE). Aggregation and fusion did not occur with single component (100 mol%) DMPC vesicles. Fusion was followed by two fundamentally different techniques, fluorescence resonance energy transfer which monitors intermixing of bilayers and ANTS-DPX which monitors intermixing of the sequestered aqueous interiors. It is suggested that a previously unreported role of ABA may be as a membrane fusagen.

Guard cells of Commelina communis L. do not respond metabolically to osmotic stress in isolated epidermis: Implications for stomatal responses to drought and humidity

We investigated the hypothesis that stomatal aperture is regulated by epidermal water status. Detached epidermal peels of Commelina communis L. or leaf disks with epidermis attached were incubated in graded solutions of mannitol (0-1.2 M) containing KCl. In isolated epidermis, guard-cell solute content of open stomata did not decrease in response to desiccation. Guard cells of closed stomata accumulated solutes to the same extent in all levels of mannitol tested. There was no evidence of stress-induced hydroactive closure nor of inhibition of hydroactive opening, even when guard cells of closed stomata were initially plasmolyzed. Hydropassive, osmometer-like, changes in stomatal aperture in the isolated epidermis were induced by addition or removal of mannitol, but these did not involve changes in guard-cell solute content. In leaf disks, stomata exhibited clear hydroactive stomatal responses. Steady-state guard-cell solute content of initially open and initially closed stomata decreased substantially with increasing mannitol. Stomata were completely closed above approx. 0.4 M mannitol, near the turgor-loss point for the bulk leaf tissue. Stomata of Commelina did not exhibit direct hydroactive responses to environmental or epidermal water status. Stomatal responses to water deficit and low humidity may be indirect, mediated by abscisic acid or other signal metabolite(s) from the mesophyll.