Glass microelectrode measurements of sieve tube membrane potentials in internode discs and petiole strips of tomato (Solanum lycopersicum L.)

Summary In tissue slices of tomato (Solanum lycopersicum L.) sieve tube membrane potentials (E_m) were measured by use of glass microelectrodes. In internode discs, the potential differences (pd) of phloem cells near the cut surface fell into two distinct categories with average values of –66 and –109 mV. More distant from the cut surface the values decreased to averages of –71 and –140 mV. These pds were associated with phloem parenchyma cells and sieve tube/companion cell complexes, respectively. In petiole strips, pds were recorded from cells which were identified by iontophoretic injection of fluorescent dye. Averages in two different bathing media, were –140/–146mV, –149/–152mV, and –70/–68mV for sieve tubes, companion cells, and phloem parenchyma cells, respectively. The membrane potentials recorded from sieve tubes were transiently reduced upon sucrose addition. Reduction by CCCP and KCN was more permanent. Sieve tube E_ms recovered more slowly from potassium than from sucrose-induced depolarizations. Light/ dark (L/D) responses were minute (±3 mV). The limitations of the present experimentation are evaluated with special reference to the question as whether the recorded E_ms represent sieve tube membrane potentials occurring in the intact plant.Abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - D dark(ness) - E_m membrane potential - L light - LYCH Lucifer yellow CH - pd potential difference - SE standard error     

Membrane potentials as an indication for plant cell penetration by aphid stylets

Electrical penetration graphs (EPGs) of aphids on plants demonstrate distinct periods of lowered potential level: the potential drop (pd). Such pds are produced frequently during the stylet pathway to the phloem. Experimental evidence supports the hypothesis that the pd corresponds with a stylet puncture of a (epidermal or mesophyll) cell membrane. The intracellular potentials of-100 to-180 mV are thought to be measured thereby with the stylets acting as microelectrodes. The short pd of 5–15 s can be described as a sequence of three distinct phases. Besides the short pd, occurring during pattern B and C, long pds are produced during the complete pattern D+E. Pattern D+E may occur on the potential level of the pd (abbr.: D+E (pd)) and on the potential level of the preceding pattern C (abbr.: D+E(c)). Pattern D+E(pd) seems to be related to sieve element penetration, at least in a number of cases. Both pds, short and long, are produced on host and non-host plants, on susceptible and resistant cultivars, using several aphid species.

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Potentiels membraneux comme indication pour des pénétrations intracellulaires des plantes par les stylets des aphides

Résumé L'enrégistrement électrique de la pénétration du stylet des aphides dans les tissus de la plante se caracterise par des périodes distinctes de chute de potentiel (c.p.). Ces c.p. existent fréquemment au cours du trajet du stylet vers le phloème. Une évidence expérimentale soutient l'hypothèse de la correspondance entre la c.p. et la piqûre de la membrane des cellules épidermales et parenchymateuses. Les potentiels intracellulaires de-100 à-180 mV pourraient être mesurés par l'intermédiaire des stylets comme microélectrodes. Les. c.p. brèves, de 5 à 15 s peuvent être décrites comme des séquences composées de 3 phases distinctes. Outre les c.p. brèves pendant les ondes B et C, les pucerons produisent des c.p. durables qui se maintiennent au cours des ondes D+E. Ou bien l'onde D+E peuve apparaite au niveau de potentiel de c.p. (D+E(cp)), ou au niveau de potentiel de l'onde C précédente (D+E(c)). L'onde D+E(cp) semble être reliée à la pénétration des vaisseaux conducteurs, du moins dans certain cas. Les c.p. brêves et durables sont produits sur les plantes hôtes et non-hôtes chez plusiers espèces d'aphids.

     

Intracellular punctures by the adult whitefly Bemisia argentifolii on DC and AC electronic feeding monitors

One of the most biologically important electrical penetration graph (EPG) waveforms recorded from aphids on DC EPG systems is the potential drop (pd), which is correlated with intracellular punctures by the stylet tips. In this study, pds of the adult female Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae), recorded on a DC EPG, are characterized and compared to pds of aphids. Whitefly pds consisted of 3 phases similar to those recorded from probing aphids. The major difference between aphid pds and whitefly pds was that whitefly pds lacked any observable subphases within the second phase of the pd. In addition, whitefly pds differed from aphid pds in that they: (1) did not occur frequently during stylet penetration, (2) did not occur early within probes, (3) did not occur during brief probes (<1 min). Pds produced by probing whiteflies always were preceded by a variant of waveform C which we named the pre-pd. The differences between pds of aphids and whiteflies are discussed in terms of their implications for virus transmission and host selection. Using a technique where EPG recordings can be switched back and forth between DC and AC systems, we demonstrated that the AC EPG pseudotransition waveform (Pt) was equivalent to the DC pd, and thus was correlated with intracellular punctures. Previously, intracellular punctures by whiteflies had not been detectable on AC EPG systems. The AC Pt consisted of three distinct phases (Pt1, Pt2, and Pt3) and our observations suggest that AC Pt1 correlates with the pre-pd waveform in DC EPGs and that AC Pt 2 and 3 correlate with the intracellular phase of the DC pd. AC Pts (n=47) and DC pds (n=43) were recorded on three separate plant species and were similar on all plant species.     

Technical advance: near-infrared femtosecond laser pulses as a novel non-invasive means for dye-permeation and 3D imaging of localised dye-coupling in the Arabidopsis root meristem

We have used near-infrared femtosecond Titanium: Sapphire laser pulses as novel non-invasive means for dye loading into various cell types of the Arabidopsis root meristem, and by 3D imaging have assessed the extent of dye coupling between the meristematic cells. The post-embryonic primary root of Arabidopsis thaliana has an invariant ontogeny and fixed cellular organisation which makes it an attractive model system to study developmental events involving cell fate determination, cellular differentiation and pattern formation. Local intercellular communication and local transmission of positional signals are likely to play a pivotal role in cell proliferation and regulation of differentiation. We have therefore examined the extent to which the constituent cells in the root meristem are symplastically coupled. Following laser-assisted loading of membrane impermeate fluorescent dye propidium iodide (PI) in single cells, we show by time-lapse and 3D imaging that in the root tip all undifferentiated cells are dye-coupled. When PI is permeated into the central cells, it rapidly moved into the adjacent initials of the columella, cortex, pericycle and stele. Interestingly, when only either of the initials were loaded with the dye, it never moved into any of the central cells. Amongst the epidermal cells, the differentiated hair cells are symplastically isolated. Our data provide evidence (1) for differential dye-coupling behaviour between quiescent centre cells and the neighbouring initials; (2) that cells in the root are coupled during stages at which the cell-lineage pattern is formed and that it becomes progressively secluded as they differentiate and the pattern is fixed. Taken together, our NIR-laser mediated approach is highly efficient and has numerous potential applications for non-invasive permeation of dyes in different cell types.     

Movement of Lucifer Yellow CH in potato tuber storage tissues: A comparison of symplastic and apoplastic transport

The fluorescent dye Lucifer Yellow CH (LYCH) was introduced directly into the symplast of potato (Solanum tuberosum L.) tuber storage parenchyma by microinjection and also into the apoplast through cuts made in the stolon cortex. Microinjected LYCH moved away rapidly from a single storage cell and spread radially via the symplast. When the microinjected tissue was subsequently fixed in glutaraldehyde and sectioned the dye was seen clearly to be localised in the cytoplasm but not in the vacuole. In comparison, when LYCH was introduced into cuts made in the stolon cortex the dye entered the tuber by the xylem and subsequently spread apoplastically. No movement of dye was observed in the phloem. In glutaraldehyde-fixed tissues, in which LYCH was introduced to the apoplast, the dye was found within xylem vessels, in the cell walls and in intercellular spaces. Wall regions, possibly associated with plasmodesmata, became stained by the dye as it moved through the apoplast. Three hours after introduction of the dye to the stolon, intense deposits of LYCH were found in the vacuoles of all cells in the tuber, many aligned along the tonoplast. Differentiating vascular parenchyma elements contained large amounts of dye within enlarging vacuoles. However, with the exception of plasmolysed and-or damaged cells, LYCH was absent from the cytoplasm following its introduction to the plasmalemma it is suggested that the most likely pathway from the cell wall to the vacuole was by endocytosis, the dye being transported across the cytoplasm in membrane-bound vesicles. Clathrin-coated vesicles were abundant in the storage cells, providing a possible endocytotic pathway for dye movement. The significance of these observations is discussed in relation to the movement of LYCH in plant tissues and to the movement of solutes within and between storage cells of the tuber.Abbreviation LYCH Lucifer Yellow CH     

Microautoradiographic studies of the role of mesophyll and bundle tissues of the Ricinus cotyledon in sucrose uptake

The aim of the study was to show which tissues and cell types of the cotyledon of Ricinus communis L. are responsible for uptake of sucrose by H⁺-sucrose symport. The cotyledons were incubated in labelled sucrose for up to 20 min and then the amount of radioactivity in each cell type of the cotyledon was assessed by microautoradiography. It was found that 50% of the label was present in the spongy mesophyll, and 10–15% was in the bundles, the epidermal layers and the palisade parenchyma. The sieve tubes contained only 2–3% of the label. The addition of sucrose to cotyledons depolarized the membrane of spongy-mesophyll cells by 33 mV. Therefore, it was concluded that the previously found H⁺-sucrose symport is at least partly located at the spongy mesophyll. No precursor-like behaviour of the label in mesophyll or bundle-sheath cells was observed in pulse-chase experiments, which indicates a direct uptake of sucrose by the sieve tube-companion cell complex from the apoplast.This work was funded by Deutsche Forschungsgemeinschaft. The valuable help by Ina Möller, Elke Schmidt, Christian Schobert (all from Bayreuth, FRG), Dr. Dieter Gradmann (Göttingen, FRG), Dr. Jörg Tittor (MPI Biochemie München, FRG), Dr. Pavlovkin (Plant Pathology, Bratislava), Dr. K. Köhler (Botany Department Würzburg, FRG) and the intense discussions with Dr. Enno Brinkmann (Bayreuth) are gratefully acknowledged. The technical assistance by Beatrix Tannhäuser-Hofmann and Hildegard Stork was of great help for this work.     

Symplast domains in extrastelar tissues of Egeria densa Planch.

A set of hydrophilic fluorescent dyes of known molecular weight has been used to determine the molecular exclusion limit and the extent of apical, epidermal and cortical symplasts in the root, stem and leaf of Egeria densa. These dyes are unable to pass the plasmalemma, so that any cell-to-cell movement of injected dye must occur via the symplast. The shoot-apex symplast has a high molecular exclusion limit, excluding dyes with a molecular weight of 749 dalton (fluorescein hexaglycine) and greater but allowing dyes of up to 665 dalton (fluorescein diglutamic acid) to pass. The leaf epidermal symplast is similar to that in the apex: fluorescein pentaglycine (674 dalton) moves to a limited extent, but fluorescein hexaglycine is immobile. Stem and root epidermal cells have a lower molecular exclusion limit, only the dye 6-carboxyfluorescein (376 dalton) is able to move from cell-to-cell. Cortical and epidermal tissues in both the stem and the root have similar symplast permeabilities. However, a barrier to dye (6-carboxyfluorescein) movement is found between the epidermis and the cortex in both organs. Barriers are also found at the nodes between expanded internodes. The stem barriers are not found in the unexpanded nodes near the shoot tip; apparently they are formed early during internode expansion. In the root tip, a barrier to the movement of dye is found between the root cap and the remainder of the root. Plasmodesmata are found linking all cell types studied, even cells where barriers to dye movement occur. Thus, the plant, far from being one uniform symplast, consists of a large number of symplast domains, which may or may not differ in molecular exclusion limit.Abbreviations F fluorescein isothiocyanate isomer I - Glu l-glutamic acid - (Glu)₂ l-glutamylglutamic acid - (Gly)₅ l-pentaglycine - (Gly)₆ l-hexaglycine     

Membrane potential changes during transport of hexoses in Lemna gibba G1

The membrane potential (pd) of duck weed (Lemna gibba G1) proved to be energy dependent. At high internal ATP levels of 74 to 105 nmol ATP g^-1 FW, pd was between -175 and -265 mV. At low ATP levels of 23 to 46 nmol ATP g^-1 FW, pd was low, about -90 to -120 mV at pH 5.7, but -180 mV at pH 8. Upon addition of glucose in the dark or by light energy the low pd recovered to the high values. The active component of the pd was depolarized by the addition of hexoses in the dark and in the light. Hexose-dependent depolarization of the pd (= pd) followed a saturation curve similar to active hexose influx kinetics. Depolarization of the pd recovered in the dark even in the presence of the hexoses and with a 10fold enhancement in the light. Depolarization and recovery could be repeated several times with the same cell. Glucose uptake caused a maximum depolarization of 133 mV, fructose uptake half that amount, sucrose had the same effect as glucose. During 3-O-methylglucose and 2-deoxyglucose uptake the depolarizing effect was only slightly lower. The pd remained unchanged in the presence of mannitol. The glucose dependent pd and especially the rate of pd recovery proved to be pH-dependent between pH 4 and pH 8. It was independent of the presence of 1 mM KCl. Although no pH could be measured in the incubation medium, these results can be best explained by a H⁺-hexose cotransport mechanism powered by active H⁺ extrusion at the plasmalemma.Abbreviations LD longday - SD shortday - pd membrane electropotential difference - pd maximum membrane potential depolarization - L light - D dark - FW fresh weight - d days of culture of Lemna gibba - 1X perfusing solution without sugar, see methods     

Dye-coupling among frog (Rana catesbeiana) taste disk cells

• 1.1. Dye-coupling among taste disk cells in the bullfrog fungiform papillae was examined histologically by injecting a fluorescent dye (Lucifer yellow) into the cell, and the effects of the dye-coupling on depolarizing responses induced by taste stimuli were studied electrophysiologically.
• 2.2. With dye injection into a taste cell, dye-coupling was found between taste cells (23%) or between taste cell and supporting cell (28%). With dye injection into a supporting cell, dye-coupling was found between supporting cells (34%) or between supporting cell and taste cell (27%).
• 3.3. Depolarizing responses recorded from either a taste cell or a supporting cell to stimulation with 0.5 M NaCl or 10 mM quinine-HCl were the same in amplitude whether the dye-coupling to another cell was present or not. On the other hand, depolarizing responses recorded from a taste cell for 0.5 mM acetic acid became significantly larger when dye-coupled to a supporting cell.
• 4.4. It is concluded that gustatory transduction for acid stimuli is influenced by supporting cells coupled to taste cells.

     

Characterization of theEgeria densa leaf symplast: Response to plasmolysis,deplasmolysis and to aromatic amino acids

Summary The fluorescent dyes 6-carboxyfluorescein and fluorescein glutamylglutamic acid, which move freely in theEgeria densa leaf symplast, fail to move from cells subjected to plasmolysis, demonstrating that plasmolysis disrupts symplastic continuity. Dye movements begins again within 10 minutes of removal of the osmoticum and becomes more extensive with increasing recovery time. The re-established symplastic links show a number of distinctive features compared to untreated leaves: dyes of up to 1678 dalton can pass, compared to the normal limit of 665 dalton; and Ca²⁺ ions, which completely inhibit dye movement in untreated cells, only reduce the extent of dye movement. Aromatic amino acids and their fluorescein conjugates prevent intercellular movement in untreated cells. In deplasmolysed cells the aromatic conjugates move freely. The increased symplast permeability persists for at least 20 hours. Thus, after plasmolysis followed by deplasmolysis, the symplast shows a marked increase in permeability associated with an increased molecular exclusion limit, indicating an increase in pore size, and symplast permeability becomes relatively insensitive to Ca²⁺ ions or to the aromatic conjugates.     

Study of electrogenic electron transfer steps in chromatophore membrane of Chromatium vinosum by the response of merocyanin dye

1. Electrogenic steps in photosynthetic cyclic electron transport in chromatophore membrane of Chromatium vinosum were studied by measuring absorption changes of added merocyanin dye and of intrinsic carotenoid.

2. The change in dye absorbance was linear with the membrane potential change induced either by light excitation or by application of diffusion potential by adding valinomycin in the presence of K⁺ concentration gradient.

3. It was estimated that chromatophore membrane became 40–60 mV and 110–170 mV inside positive upon single and multiple excitations with single-turnover flashes, respectively, from the responses of the dye and the carotenoid.

4. Electron transfers between cytochrome c-555 or c-552 and reaction center bacteriochlorophyll dimer (BChl₂) and between BChl₂ and the primary electron acceptor were concluded to be electrogenic from the redox titration of the dye response.

5. No dye response which corresponded to the change of redox level of cytochrome b was observed in the titration curve. Addition of antimycin A slightly decreased the dye response.

6. The dye response was decreased under phosphorylating conditions.

7. From the results obtained localization of the electron transfer components in chromatophore membrane is discussed.     

Transmembrane electrical potentials in growing maize roots

The anti-auxin 4-chlorophenoxyisobutyric acid (PCIB) applied at a concentration of 10^-2 mol m^-3 to maize root segments was found to induce a transmembrane electrical potential of up to-130 mV (pd of 30 mV). The kinetics of this response were comparable to the time scale for PCIB-stimulated H⁺-extrusion. Both effects are eliminated by the addition of p-fluoromethoxycarbonyl cyanide phenylhydrazone (FCCP). Treatment with fusicoccin (FC) and PCIB together does not result in a hyperpolarization greater than with FC alone. Benzoic acid (10^-2 mol m^-3) had no effect on the transmembrane electrical potentials. These results are discussed in relation to a possible electrogenic proton pump which may be regulated by perturbations in the cellular auxin content or activity.Abbreviations ATPase adenosine triphosphatase - FC fusicoccin - FCCP p-fluoromethoxy carbonylcyanide phenylhydrazone - IAA indole-3yl-acetic acid - NAA naphthyl-lylacetic acid - PCIB 4-chlorophenoxyisobutyric acid - PD potential difference     

Isotocinergic neurons in the goldfish hypothalamus: Physiological and morphological studies on chemically identified cells

Summary Isotocinergic (IT) neurons show physiological and morphological characteristics that are similar to those of other preoptic neuroendocrine cells in the goldfish. Preoptic IT cells show resting membrane potentials of 20–55 mV, action potentials of up to 100mV, and physiological evidence of axonal branching. Dye-marked IT cells measure 14–56 m, their dendrites projecting to the ependyma and into the hypothalamic neuropil, their multiple beaded axons projecting to the pituitary. Indirect immunofluorescence identifies these dyemarked cells as IT. By combining electrophysiological, dye-marking and immunocytochemical techniques we can now, for the first time, study single, antidromically-identified peptidergic neurons of a specific type in vertebrate and invertebrate species.Supported by Grants from the USPHS (NS-13411 and NS-05696)The authors wish to thank Ms. S. Curtis for editorial assistance, Ms. D. Cronce for skillful technical assistance, Dr. W.W. Stewart for helpful suggestions and for his generous gift of Lucifer Yellow-CH, Dr. M. Manning for his generous gift of high quality peptides, and Dr. R.R. Dries and J.D. Fernstrom for kindly supplying antisera     

Identification of a secretomotor centre in the brain of Locusta migratoria,controlling the secretory activity of the adipokinetic hormone producing cells of the corpus cardiacum

Summary After retrograde filling of axons terminating in the glandular lobe of the corpus cardiacum (CC) of Locusta migratoria with cobalt chloride, a paired group of about 15 cobalt containing cells was demonstrated in the lateral area of the protocerebrum. The axons of these cells run via the NCC II into the glandular lobe of the CC. These small neurons have the characteristics of secretory cells; they contain secretory granules of about 1000 Å in diameter. The axon terminals in the glandular lobe, making synaptic contacts with the glandular cells, contain secretory granules of the same size. It is therefore concluded that the cell groups in the protocerebrum control the activity of the glandular cells which produce an adipokinetic hormone. Arborizations of fibers of the lateral secretomotor cells are present in the dorsal neuropile of the protocerebrum, ventral of the mushroom bodies and along the tracts of the NCC I within the brain. It is proposed that these arborizations are sites of synaptic input. It is discussed that the axons of these cells might receive additional synaptic input in the storage lobe of the CC.The localization of cell bodies, the axons of which enter the storage part of the CC is described. The course of the axon tracts of the various cell groups in the protocerebrum and their connections with the NCC I and NCC II are demonstrated.Supported by the Foundation for Fundamental Biological Research (BION) which is subsidized by the Netherlands Organization for the Advancement of Pure Research (ZWO). The electron microscopical investigations were performed at the EM-unit of the Faculty of Biology, State University of Utrecht (Director: Prof. Dr. J.C. van de Kamer)The author is greatly indebted to Dr. A.M.Th. Beenakkers and Dr. H.H. Boer for their active interest and helpful advise. Thanks are also due to Mr. H. van Kooten and his staff for making the macro- and microphotographs, to Mr. L.W. van Veenendaal for preparing the electron micrographs and final assistance in the preparation of the photo pages and to Mr. D. Smit, who made the drawings     

Uptake of Lucifer Yellow CH into intact barley roots: Evidence for fluid-phase endocytosis

Intact barley (Hordeum vulgare L.) roots have been shown to take up the highly fluorescent dye Lucifer Yellow CH (LYCH) into their cell vacuoles. In the apical 1 cm of root tip, differentiating and dividing cells showed a prolific uptake of LYCH into their provacuoles. The LYCH was retained during fixation, apparently becoming bound to electron-dense material in the vacuoles. The dye freely entered the apoplast of roots in which the Casparian band was not developed, being taken up into the vacuoles of cells in both the cortex and stele. However, when LYCH was applied to a 1-cm zone approx. 6 cm behind the root tip the Casparian band on the radial walls of the endodermis completely prevented the dye from entering the cells of the stele, only the cell walls and vacuoles of the cortical cells taking up the dye. The inability of LYCH to cross the plasmalemma of the endodermal cells and enter the stele via the symplast substantiates previous claims that the dye is unable to cross the plasmalemma of plant cells. The results are discussed in the light of recent demonstrations that LYCH is a particularly effective marker for fluid-phase endocytosis in animal and yeast cells. A calculation of the energetic requirements for LYCH uptake into barley roots supports the contention that LYCH is taken up into the vacuoles of plant cells by fluid-phase endocytosis.Abbreviation LYCH Lucifer Yellow CH     

Gap junctions in ovarian follicles of Drosophila melanogaster: inhibition and promotion of dye-coupling between oocyte and follicle cells

The analysis of chimeras has shown that communication between germ-line and soma cells plays an important role during Drosophila oogenesis. We have therefore investigated the intercellular exchange of the fluorescent tracer molecule, Lucifer yellow, pressure-injected into the oocyte of vitellogenic follicles of Drosophila. The dye reached the nurse cells via cytoplasmic bridges and entered, via gap junctions, the somatic follicle cells covering the oocyte. The percentage of follicles showing dye-coupling between oocyte and follicle cells was found to increase with the developmental stage up to stage 11, but depended also on the status of oogenesis, i.e., the stage-spectrum, in the respective ovary. During late stage 10B and stage 11, dye-coupling was restricted to the follicle cells covering the anterior pole of the oocyte. No dye-coupling was observed from stage 12 onwards. During prolonged incubation in vitro, the dye was found to move from the follicle cells back into the oocyte; this process was suppressable with dinitrophenol. Dyecoupling was inhibited when prolonged in vitro incubation preceded the dye-injection. Moreover, dye-coupling was inhibited with acidic pH, low [K⁺], high intracellular [Ca²⁺], octanol, dinitrophenol, and NaN₃, but not with retinoic acid, basic pH, or high extracellular [Ca²⁺]. Dyecoupling was stimulated with a juvenile hormone analogue and with 20-hydroxyecdysone. Thus, gap junctions between oocyte and follicle cells may play an important role in intercellular communication during oogenesis. We discuss the significance of our findings with regard to the electrophysiological properties of the follicles, and to the coordinated activities of the different cell types during follicle development and during the establishment of polarity in the follicle.     

Symplastic isolation of the sieve element-companion cell complex in the phloem of Ricinus communis and Salix alba stems

The anatomical and physiological isolation of the sieve element-companion cell complex (se-cc complex) was investigated in stems of Ricinus communis L. and Salix alba L. In Ricinus, the plasmodesmatal frequencies were in the proportions 8∶1∶2∶30, in the order given, at the interfaces between sieve tube-companion cell, sieve tube-phloem parenchyma cell, companion cellphloem parenchyma cell, and phloem parenchyma cellphloem parenchyma cell. The membrane potentials of the se-cc complex and the surrounding phloem-parenchyma cells sharply contrasted: the membrane potential of the se-cc complex was about twice as negative as that of the phloem parenchyma. Lucifer Yellow CH injected into the sieve element or into the companion cell remained within the se-cc complex. Dye introduced into phloem parenchyma only moved (mostly poorly) to other phloem-parenchyma cells. The distribution of the plasmodesmatal frequencies, the differential dye-coupling and the sharp discontinuities in membrane potentials indicate that the se-cc complexes constitute symplast domains in the stem phloem. Symplastic autonomy is discussed as a basic necessity for the functioning of the se-cc complex in the stem.     

Na+ as coupling ion in energy transduction in extremophilic Bacteria and Archaea

For microorganisms to live under extreme physical conditions requires important adaptations of the cells. In many organisms the use of Na⁺ instead of protons as coupling ion in energy transduction is associated with such adaptation. This review focuses on the enzymes that are responsible for the generation and utilization of Na⁺ gradients in extremophilic microorganisms. Aspects that are dealt with include: bioenergetics and ion homeostasis in extremophilic Bacteria and Archaea; the molecular mechanism of Na⁺ translocation; and (dis)advantages of Na⁺ as coupling ion in energy transduction.G. Speelmans was and B. Poolman and W.N. Konings are with the Department of Microbiology, Biology Centre University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands; G. Speelmans is now with the Department of Biochemistry of Membranes, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands.     

Deposition and reorientation of cellulose microfibrils in elongating cells of Petunia stylar tissue

According to Roelofsen and Houwink's (1953, Acta Bot. Neerl. 2, 218–225) multinet growth hypothesis, microfibrils originally deposited transversely in the cell wall become gradually reoriented towards more axial orientations during cell elongation. To establish the extent of reorientation, microfibrils were studied during their deposition and elongation, using stylar parenchyma and transmitting tissue cells of Petunia hybrida L. At the inner surface of very young cells, microfibrils were deposited in alternating Z- and S-helical orientations. The following sequence in deposition, from the exterior to the interior side of the wall, could be inferred: Axial: 150°–180° (Z-helical), 0°–30° (S-helical); oblique: 110°–150° (Z-helical), 30°–70° (S-helical); transverse: 90°–110° (Z-helical), 70°–90° (S-helical). With the increasing pitch, the density of the deposited microfibrils increased as well, giving rise to an alternating helical texture. During elongation, only transversely S- and Z-helically oriented microfibrils were deposited and all microfibrils underwent a certain reorientation as described in the multinet growth hypothesis. The texture resembled that of young cells and the wall maintained its thickness. The extent of passive reorientation was in agreement with the theoretical calculations made by Preston.Dedicated to Professor Dr. A.B. Wardrop, Melbourne, on the occasion of his 70th birthday