Polarization of Fucoid Eggs by a Calcium Ionophore Gradient

Genome-wide identification of polar auxin transporter gene families reveals a possible new polar auxin flow in inverted cuttings of Populus yunnanensis

Inverted cuttings of Populus yunnanensis are characterized by enlarged stems and dwarfed new shoots, and phytohormones play a crucial role in the response to inversion. The polar auxin transport (PAT) system is distinct from the transport systems of other hormones and is controlled by three major transporter gene families: pin-formed (PIN), auxin-resistant/like aux (AUX/LAX) and ATP-binding cassette transporters of the B class (ABCB). Here, we identified these three families in P. trichocarpa, P. euphratica and P. yunnanensis through a genome-wide analysis. The Populus PIN, AUX/LAX and ABCB gene families comprised 15, 8 and 31 members, respectively. Most PAT genes in Populus and Arabidopsis were identified as clear sister pairs, and some had unique motifs. Transcriptome profiling revealed that the expression of most PAT genes was unrelated to cutting inversion and that only several genes showed altered expression when cuttings were inverted. The auxin content difference at positions was opposite in upright and inverted cutting bodies during rooting, which obeyed the original plant polarity. However, during plant growth, the two direction types exhibited similar auxin movements in the cutting bodies, and the opposite auxin changes were observed in new shoots. Four PAT genes with a positive response to cutting inversion, PyuPIN10, PyuPIN11, PyuLAX6 and PyuABCB27, showed diverse expression patterns between upright and inverted cuttings during rooting and plant growth. Furthermore, PAT gene expression retained its polarity, which differs from the results found for auxin flow during plant growth. The inconformity indicated that a new downward auxin flow in addition to the old upward flow might be established during the growth of inverted cuttings. Some highly polar PAT genes were involved in the maintenance of original auxin polarity, which might cause the enlarged stems of inverted cuttings. This work lays a foundation for understanding the roles of auxin transport in plant responses to inversion.

Physiological Analysis and Transcriptome Profiling of Inverted Cuttings of Populus yunnanensis Reveal That Cell Wall Metabolism Plays a Crucial Role in Responding to Inversion

Inverted cuttings of Populus yunnanensis remain alive by rooting from the original morphological apex and sprouting from the base, but the lateral branches exhibit less vigorous growth than those of the upright plant. In this study, we examined the changes in hormone contents, oxidase activities, and transcriptome profiles between upright and inverted cuttings of P. yunnanensis. The results showed that the indole-3-acetic acid (IAA) and gibberellic acid (GA₃) contents were significantly lower in inverted cuttings than in upright cuttings only in the late growth period (September and October), while the abscisic acid (ABA) level was always similar between the two direction types. The biosynthesis of these hormones was surprisingly unrelated to the inversion of P. yunnanensis during the vegetative growth stage (July and August). Increased levels of peroxidases (PODs) encoded by 13 differentially expressed genes (DEGs) served as lignification promoters that protected plants against oxidative stress. Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis showed that most DEGs (107) were related to carbohydrate metabolism. Furthermore, altered activities of uridine diphosphate (UDP)-sugar pyrophosphorylase (USP, 15 DEGs) for nucleotide sugars, pectin methylesterase (PME, 7 DEGs) for pectin, and POD (13 DEGs) for lignin were important factors in the response of the trees to inversion, and these enzymes are all involved cell wall metabolism.

Nature's Electric Potential: A Systematic Review of the Role of Bioelectricity in Wound Healing and Regenerative Processes in Animals, Humans, and Plants

Natural endogenous voltage gradients not only predict and correlate with growth and development but also drive wound healing and regeneration processes. This review summarizes the existing literature for the nature, sources, and transmission of information-bearing bioelectric signals involved in controlling wound healing and regeneration in animals, humans, and plants. It emerges that some bioelectric characteristics occur ubiquitously in a range of animal and plant species. However, the limits of similarities are probed to give a realistic assessment of future areas to be explored. Major gaps remain in our knowledge of the mechanistic basis for these processes, on which regenerative therapies ultimately depend. In relation to this, it is concluded that the mapping of voltage patterns and the processes generating them is a promising future research focus, to probe three aspects: the role of wound/regeneration currents in relation to morphology; the role of endogenous flux changes in driving wound healing and regeneration; and the mapping of patterns in organisms of extreme longevity, in contrast with the aberrant voltage patterns underlying impaired healing, to inform interventions aimed at restoring them.

Brown algae as a model for plant organogenesis

Brown algae are an extremely interesting, but surprisingly poorly explored, group of organisms. They are one of only five eukaryotic lineages to have independently evolved complex multicellularity, which they express through a wide variety of morphologies ranging from uniseriate branched filaments to complex parenchymatous thalli with multiple cell types. Despite their very distinct evolutionary history, brown algae and land plants share a striking amount of developmental features. This has led to an interest in several aspects of brown algal development, including embryogenesis, polarity, cell cycle, asymmetric cell division and a putative role for plant hormone signalling. This review describes how investigations using brown algal models have helped to increase our understanding of the processes controlling early embryo development, in particular polarization, axis formation and asymmetric cell division. Additionally, the diversity of life cycles in the brown lineage and the emergence of Ectocarpus as a powerful model organism, are affording interesting insights on the molecular mechanisms underlying haploid-diploid life cycles. The use of these and other emerging brown algal models will undoubtedly add to our knowledge on the mechanisms that regulate development in multicellular photosynthetic organisms.

Ca2+ influx and phosphoinositide signalling are essential for the establishment and maintenance of cell polarity in monospores from the red alga Porphyra yezoensis

The asymmetrical distribution of F-actin directed by cell polarity has been observed during the migration of monospores from the red alga Porphyra yezoensis. The significance of Ca2+ influx and phosphoinositide signalling during the formation of cell polarity in migrating monospores was analysed pharmacologically. The results indicate that the inhibition of the establishment of cell polarity, as judged by the ability of F-actin to localize asymmetrically, cell wall synthesis, and development into germlings, occurred when monospores were treated with inhibitors of the Ca2+ permeable channel, phospholipase C (PLC), diacylglycerol kinase, and inositol-1,4,5-trisphosphate receptor. Moreover, it was also found that light triggered the establishment of cell polarity via photosynthetic activity but not its direction, indicating that the Ca2+ influx and PLC activation required for the establishment of cell polarity are light dependent. By contrast, inhibition of phospholipase D (PLD) prevented the migration of monospores but not the asymmetrical localization of F-actin. Taken together, these findings suggest that there is functional diversity between the PLC and PLD signalling systems in terms of the formation of cell polarity; the former being critical for the light-dependent establishment of cell polarity and the latter playing a role in the maintenance of established cell polarity.

Pattern formation of stationary transcellular ionic currents in Fucus

Stationary and nonstationary spatiotemporal pattern formations emerging from the cellular electric activity are a common feature of biological cells and tissues. The nonstationary ones are well explained in the framework of the cable model. Inversely, the formation of the widespread self-organized stationary patterns of transcellular ionic currents remains elusive, despite their importance in cell polarization, apical growth, and morphogenesis. For example, the nature of the breaking symmetry in the Fucus zygote, a model organism for the experimental investigation of embryonic pattern formation, is still an open question. Using an electrodiffusive model, we report here an unexpected property of the cellular electric activity: a phase-space domain that gives rise to stationary patterns of transcellular ionic currents at finite wavelength. The cable model cannot predict this instability. In agreement with experiments, the characteristic time is an ionic diffusive one (<2 min). The critical radius is of the same order of magnitude as the cell radius (30 microm). The generic salient features are a global positive differential conductance, a negative differential conductance for one ion, and a difference between the diffusive coefficients. Although different, this mechanism is reminiscent of Turing instability.

Cytoskeletal and Ca2+ regulation of hyphal tip growth and initiation

Hyphal tip growth is a complex process involving finely regulated interactions between the synthesis and expansion of cell wall and plasma membrane, diverse intracellular movements, and turgor regulation. F-actin is a major regulator and integrator of these processes. It directly contributes to (a) tip morphogenesis, most likely by participation in an apical membrane skeleton that reinforces the apical plasma membrane, (b) the transport and exocytosis of vesicles that contribute plasma membrane and cell wall material to the hyphal tips, (c) the localization of plasma membrane proteins in the tips, and (d) cytoplasmic and organelle migration and positioning. The pattern of reorganization of F-actin prior to formation of new tips during branch initiation also indicates a critical role in early stages of assembly of the tip apparatus. One of the universal characteristics of all critically examined tip-growing cells, including fungal hyphae, is the obligatory presence of a tip-high gradient of cytoplasmic Ca2+ that probably regulates both actin and nonactin components of the apparatus, and the formation of which may also initiate new tips. This review discusses the diversity of evidence behind these concepts.

Pattern formation by local self-activation and lateral inhibition

In 1972, we proposed a theory of biological pattern formation in which concentration maxima of pattern forming substances are generated through local self-enhancement in conjunction with long range inhibition. Since then, much evidence in various developmental systems has confirmed the importance of autocatalytic feedback loops combined with inhibitory interaction. Examples are found in the formation of embryonal organizing regions, in segmentation, in the polarization of individual cells, and in gene activation. By computer simulations, we have shown that the theory accounts for much of the regulatory phenomena observed, including signalling to regenerate removed parts. These self-regulatory features contribute to making development robust and error-tolerant. Furthermore, the resulting pattern is, to a large extent, independent of the details provided by initial conditions and inducing signals.

Cortical actin filaments form rapidly during photopolarization and are required for the development of calcium gradients in Pelvetia compressa zygotes

Previous research has shown that cortical gradients of cytosolic Ca(2+) are formed during the photopolarization of Pelvetia compressa zygotes, with elevated Ca(2+) on the shaded hemisphere that will become the site of rhizoid germination. We report here that the marine sponge toxin, latrunculin B, which blocks photopolarization at nanomolar concentrations, inhibited the formation of the light-driven Ca(2+) gradients. Using low concentrations of microinjected fluorescent phalloidin as a tracer for actin filaments, we found that exposure to light induced a striking increase in actin filaments in the cells as indicated by an increase in fluorescence. The increase was quantified in the cortex, where it was most apparent, and the fluorescence there was found to increase by about a factor of 3. This increase in cortical phalloidin fluorescence was inhibited by latrunculin B at the same concentration required to inhibit Ca(2+) gradient formation and photopolarization. The distribution of the increasing phalloidin fluorescence was uniform with respect to the developing rhizoid-thallus axis during the formation of the axis, and no intense patches of fluorescence were observed. After germination, fluorescence suggestive of an apical ring of actin filaments was seen near the rhizoid tip. Finally, inhibitor studies indicated that myosin may be involved in the photopolarization process.

Orientation of chemotactic cells and growth cones: models and mechanisms

A model is proposed for an amplification step in chemotactically sensitive cells or growth cones that accounts for their extraordinary directional sensitivity. It is assumed that cells have an intrinsic pattern forming system that generates the signals for extension of filopods and lamellipods. An external signal such as a graded cue is assumed to impose some directional preference onto the pattern formed. According to the model, a saturating, self-enhancing reaction is coupled with two antagonistic reactions. One antagonist equilibrates rapidly over the whole cell, causing competition between different surface elements of the cell cortex for activation. It will be won by those cortical regions of the cell that are exposed to the highest concentrations of the external graded cues. The second antagonistic reaction is assumed to act more locally and has a longer time constant. It causes a destabilization of peaks after they have formed. While the total activated area on the cell surface is maintained, the disappearance of some hot spots allows the formation of new ones, preferentially at positions specified by the actual external guiding signal. Computer simulations show that the model accounts for the highly dynamic behaviour of chemotactic cells and growth cones. In the absence of external signals, maxima of the internal signals emerge at random positions and disappear after some time. Travelling waves or oscillations in counter phase can emerge on the cell cortex, in agreement with observations reported in the literature. In other ranges of parameters, the model accounts for the generation of a stable cell polarity.

Symmetry breaking in the zygotes of the fucoid algae: controversies and recent progress

Despite its many advantages as an experimental system for the study of the epigenesis of polarity, it is obvious that the fucoid zygote also presents many problems. The development of polarity proceeds largely independently of direct gene action and thus may be considered a problem in cellular physiology. Ca2+ appears to play an important role in the process, but the optical properties of the zygotes (opacity and autofluorescence) hamper the use of modern methods of visualizing the distribution of Ca2+ and other ions. Likewise, other approaches, such as injection of fluorescent-labeled G-actin, in order to study the dynamics of actin filaments, are subject to the same limitations. It may be that the application of two-photon microscopy will enable experimenters to avoid some of these problems. This technique uses excitation wavelengths that are twice the wavelength of maximum absorption by fluorophores, and sufficient photon density for absorption is achieved only in a thin section. The fucoid zygotes are considerably more transparent to longer wavelengths, so attenuation of the exciting light and autofluorescence should be significantly reduced. Perhaps we will then be able to see further into these opaque cells. Another problem concerns the use of different species and genera. This may be unavoidable; for example, those of us who are land-locked tend to rely on Pelvetia, as it travels and stores better than the various species of Fucus and is less seasonal. Our colleagues fortunate enough to work near the ocean prefer to use the species that are locally available. Nevertheless, it is important to be careful about cross-genus and cross-species generalizations. While it is unlikely, based on what we know, that there are fundamental differences in physiological mechanisms among species, there may be small but still important differences in details. Obviously, investigators should directly compare results in more than one species whenever possible. The area of greatest disagreement, perhaps, concerns the mechanism of polarity formation, as opposed to its overt manifestation, germination. Are Ca2+ and actin involved or not? Assuming Ca2+ is involved, is the source internal or external? One basis for the different findings may be the differences in the strength of the polarizing signal provided to the zygotes. Clearly, the cells have powerful mechanisms for amplifying a faint asymmetry and developing an axis in response to an external signal. Furthermore, the fucoids generally develop in the intertidal zone and thus must be adapted to meeting the challenge of a widely varying external environment. They may have alternate mechanisms for responding to unilateral light. We have adopted the approach of presenting the cells with a fairly weak light signal--the minimum required to induce a considerable degree of organization of a population of zygotes. We then determine the effects of various inhibitors on photopolarization. One advantage of this approach is that it has allowed us to find treatments that increase the sensitivity of the zygotes to light, something that would not be possible if the untreated controls were fully polarized. Some of the differences between our results and those of others may be related to their use of a stronger light stimulus. It may be that if given a strong stimulus, a sufficient trace is left in the cells so that they can organize an axis when an inhibitor is removed. Careful consideration of this point may help to reconcile apparently contradictory findings. Despite these difficulties, the fucoid zygotes are likely to continue to be an important experimental system. Technology, including the development of more specific inhibitory reagents, may allow some of the shortcomings of the system to be overcome, and careful consideration of experimental conditions may resolve some of the points of disagreement.

Cytoplasmic calcium gradients and calmodulin in the early development of the fucoid alga Pelvetia compressa

The predicted existence of cytoplasmic Ca2+ gradients during the photopolarization of the zygotes of the brown algae, Pelvetia and Fucus, has proved to be difficult to establish, and the downstream targets of the putative gradients are not known. We have used quantitative microinjection of the long excitation wavelength Ca2+ indicator, Calcium Crimson, and of antibodies against calmodulin to investigate these matters in the zygotes and early embryos of Pelvetia. We found that there is a window of cytoplasmic Calcium Crimson concentration that gives an adequate signal above autofluorescence yet allows normal development of the zygotes. As Calcium Crimson is not a ratiometric indicator, we injected other zygotes with a Ca2+-insensitive dye, rhodamine B, and imaged the cells at the same time that Calcium Crimson-injected cells were imaged. Ratios were calculated by dividing the averaged pixel values of Calcium Crimson images by the averaged pixel values of corresponding rhodamine B images. By this method, we observed the formation of a cytoplasmic Ca2+ gradient within one hour of the exposure of the cells to unilateral blue light during the photosensitive period. The region of high Ca2+ was localized to and predictive of the site of future rhizoid formation. We validated this somewhat indirect method by applying it to the growing rhizoid, where the existence of a tip-localized Ca2+ gradient is well established. The method clearly revealed the known gradient. The injection of ungerminated zygotes with antibodies made against Dictyostelium calmodulin inhibited germination, and this inhibition was abolished if the calmodulin antibodies were coinjected with an excess of purified maize calmodulin. Likewise, the growth of the rhizoids was inhibited by calmodulin antibody injections. The fungus-derived calmodulin antagonist, ophiobolin A, which has previously been shown to be a potent inhibitor of germination, also inhibited rhizoidal growth. Our results provide evidence that a cytoplasmic Ca2+ gradient is present during photopolarization and that calmodulin acts as a mediator of Ca2+ gradients throughout the early developmental processes of germination and rhizoidal growth in Pelvetia compressa.

Retinal identification in Pelvetia fastigiata

Unidirectional blue light directs the rhizoid-thallus axis in the apolar zygote of the brown alga Pelvetia fastigiata. This effect is mediated by an increase in the intracellular concentration of cGMP. Here, we show the extraction, purification and identification of 1 microgram of all-trans retinal from 1.2 x 10(6) Pelvetia zygotes. The number of retinal molecules per cell was about 4 x 10(9). Since retinal, wherever present, is exclusively associated with an opsin to form a light sensitive complex (rhodopsin-like proteins), and since the physiological response originated by this protein produces a variation of cGMP concentration, this new finding suggests that a rhodopsin-like protein could be the photoreceptor in this brown alga.

The coupling of cyclic GMP and photopolarization of Pelvetia zygotes

Unidirectional blue light directs the rhizoid-thallus axis in the apolar zygotes of Fucus and Pelvetia. Here, it is shown that blue light (but not red light) increased cyclic GMP levels of Pelvetia zygotes by about a factor of 2. When the increase in cyclic GMP was blocked by a guanylyl cyclase inhibitor, photopolarization was also blocked. Bathing the cells in a permeant cyclic GMP analog, which should tend to collapse intracellular cyclic GMP gradients, reduced the degree of photopolarization. Growing the cells in the dark in a gradient of the analog caused the rhizoids to tend to form on the low concentration side. It appears that the stimulation of the blue light photoreceptors on the side nearer the light activates guanylyl cyclase and results in a transcytoplasmic cyclic GMP gradient that is necessary for polarization.

Calcium and related channels in fertilization and early development ofFucus

Unfertilized eggs ofFucus serratusare primed to respond rapidly to the fertilizing sperm. The unfertilized egg plasma membrane is excitable due to the presence of voltage-regulated Ca2+and K+channels. Sperm-egg interaction elicits a fertilization potential as the first observable fertilization event. It is speculated that sperm-gated Na+channels are responsible for the initial depolarization phase, leading to opening of Ca2+channels, allowing Ca2+influx and further depolarizing the membrane to the threshold for outward K+channels. K+efflux repolarizes the membrane and the zygote plasmalemma quickly becomes dominated by a large K+conductance. The involvement of Ca2+in axis formation and fixation is not clear. Ca2+carries a proportion of the inward current at the future rhizoid pole and asymmetric45Ca influx has been detected in polarizing zygotes. However, there is no requirement for external Ca2+in axis fixation. In contrast, Ca2+influx is required for expression of polarity and rhizoid growth. New developments in patch clamping can now enable localized areas of the plasma membrane in polarized cells to be studied. So far, both inward and outward single channel currents have been observed in the growing rhizoid tip, most probably carrying Cl-and K+respectively. These channels can be related to the currents identified by previous studies using the extracellular vibrating probe.

Ratio confocal imaging of free cytoplasmic calcium gradients in polarising and polarised Fucus zygotes

In the marine brown alga, Fucus, two poles are differentiated before cell division determining the future rhizoid or thallus. We have used a combination of the Ca(2+)-sensitive dye Calcium Green and the pH-sensitive dye SNARF monitored at pH-insensitive wavelengths to obtain confocal ratio images of free cytoplasmic calcium distribution at different stages in polarising Fucus zygotes. These dyes have the advantage that they can be used in most confocal microscopes and their longer excitation wavelengths greatly reduce autofluorescence problems. Dyes of varying molecular weights (free acid form, 10,000 mol.wt or 70,000 mol.wt dextran-conjugated) were pressure microinjected into early zygotes which were allowed to polarise in unidirectional light. Dextran-conjugated dyes remained non-compartmentalised and fluorescence could be monitored for up to 3 days following microinjection. Currently we have been able to detect Ca2+ gradients at the tip of the rhizoid, confirming earlier results. Localised Ca2+ elevations have also been observed at the rhizoid pole of the polarising zygote before the onset of rhizoid germination. Limitations of this technique and the significance of these Ca2+ gradients are discussed.

Establishment and expression of cellular polarity in fucoid zygotes

Zygotes of fucoid algae have long been studied as a paradigm for cell polarity. Polarity is established early in the first cell cycle and is then expressed as localized growth and invariant cell division. The fertilized egg is a spherical cell and, by all accounts, bears little or no asymmetry. Polarity is acquired epigenetically a few hours later in the form of a rhizoid/thallus axis. The initial stage of polarization is axis selection, during which zygotes monitor environment gradients to determine the appropriate direction for rhizoid formation. In their natural setting in the intertidal zone, sunlight is probably the most important polarizing vector; rhizoids form away from the light. The mechanism by which zygotes perceive environmental gradients and transduce that information into an intracellular signal is unknown but may involve a phosphatidylinositol cycle. Once positional information has been recorded, the cytoplasm and membrane are reorganized in accordance with the vectorial information. The earliest detectable asymmetries in the polarizing zygote are localized secretion and generation of a transcellular electric current. Vesicle secretion and the inward limb of the current are localized at the presumptive rhizoid. The transcellular current may establish a cytoplasmic Ca2+ gradient constituting a morphogenetic field, but this remains controversial. Localized secretion and establishment of transcellular current are sensitive to treatment with cytochalasins, indicating that cytoplasmic reorganization is dependent on the actin cytoskeleton. The nascent axis at first is labile and susceptible to reorientation by subsequent environmental vectors but soon becomes irreversibly fixed in its orientation. Locking the axis in place requires both cell wall and F-actin and is postulated to involve an indirect transmembrane bridge linking cortical actin to cell wall. This bridge anchors relevant structures at the presumptive rhizoid and thereby stabilizes the axis. Approximately halfway through the first cell cycle, the latent polarity is expressed morphologically in the form of rhizoid growth. Elongation is by tip growth and does not appear to be fundamentally different from tip growth in other organisms. The zygote always divides perpendicular to the growth axis, and this is controlled by the microtubule cytoskeleton. Two microtubule-organizing centers on the nuclear envelope rotate such that they align with the growth axis. They then serve as spindle poles during mitosis. Cytokinesis bisects the axial spindle, resulting in a transverse crosswall. Although the chronology of cellular events associated with polarity is by now rather detailed, causal mechanisms remain obscure.

To shape a cell: an inquiry into the causes of morphogenesis of microorganisms

We recognize organisms first and foremost by their forms, but how they grow and shape themselves still largely passes understanding. The objective of this article is to survey what has been learned of morphogenesis of walled eucaryotic microorganisms as a set of problems in cellular heredity, biochemistry, physiology, and organization. Despite the diversity of microbial forms and habits, some common principles can be discerned. (i) That the form of each organism represents the expression of a genetic program is almost universally taken for granted. However, reflection on the findings with morphologically aberrant mutants suggests that the metaphor of a genetic program is misleading. Cellular form is generated by a web of interacting chemical and physical processes, whose every strand is woven of multiple gene products. The relationship between genes and form is indirect and cumulative; therefore, morphogenesis must be addressed as a problem not of molecular genetics but of cellular physiology. (ii) The shape of walled cells is determined by the manner in which the wall is laid down during growth and development. Turgor pressure commonly, perhaps always, supplies the driving force for surface enlargement. Cells yield to this scalar force by localized, controlled wall synthesis; their forms represent variations on the theme of local compliance with global force. (iii) Growth and division in bacteria display most immediately the interplay of hydrostatic pressure, localized wall synthesis, and structural constraints. Koch's surface stress theory provides a comprehensive and quantitative framework for understanding bacterial shapes. (iv) In the larger and more versatile eucaryotic cells, expansion is mediated by the secretion of vesicles. Secretion and ancillary processes, such as cytoplasmic transport, are spatially organized on the micrometer scale. The diversity of vectorial physiology and of the forms it generates is illustrated by examples: apical growth of fungal hyphae, bud formation in yeasts, germination of fucoid zygotes, and development of cells of Nitella, Closterium, and other unicellular algae. (v) Unicellular organisms, no less than embryos, have a remarkable capacity to impose spatial order upon themselves with or without the help of directional cues. Self-organization is reviewed here from two perspectives: the theoretical exploration of morphogens, gradients, and fields, and experimental study of polarization in Fucus cells, extension of hyphal tips, and pattern formation in ciliates. Here is the heart of the matter, yet self-organization remains nearly as mysterious as it was a century ago, a subject in search of a paradigm.

Calcium buffer injections block fucoid egg development by facilitating calcium diffusion

The polarity of fucoid eggs is fixed either when tip growth starts or a bit earlier. A steady flow of calcium ions into the incipient tip is thought to establish a high calcium zone that is needed for its localization and formation. To test this hypothesis, we have injected seven different 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-type calcium buffers into Pelvetia eggs many hours before tip growth normally starts. Critical final cell concentrations of each buffer prove to block outgrowth (as well as cell division) for up to 2 weeks. This critical inhibitory concentration is lowest for two buffers with dissociation constants or Kd values of 4-5 x 10(-6) M and increases steadily as the buffers' Kd values shift either below or above this optimal value to ones as low as 4 x 10(-7) M or as high as 9.4 x 10(-5) M. To analyze these results, we have derived an equation (based on the concept of facilitated diffusion) for the effects of diffusable calcium buffers on steady-state calcium gradients. The data fit this equation quite well if it is assumed that cytosolic free calcium at the incipient tip is normally kept at about 7 microM and, thus, far above the general cytosolic level.

Calmodulin-binding proteins are developmentally regulated in gametes and embryos of fucoid algae

Calcium-binding proteins and calmodulin-binding proteins were identified in gametes and zygotes of the marine brown algae Fucus vesiculosus, Fucus distichus, and Pelvetia fastigiata using gel (SDS-PAGE) overlay techniques. A calcium current appears to be important during cell polarization in fucoid zygotes (K.R. Robinson and L.F. Jaffe, 1975, Science 187, 70-72; K.R. Robinson and R. Cone, 1980, Science 207, 77-78), but there are no biochemical data on calcium-binding proteins in these algae. By using a sensitive 45Ca2+ overlay method designed to detect high-affinity calcium-binding proteins, at least 9-11 polypeptides were detected in extracts of fucoid gametes and zygotes. All samples had calcium-binding proteins with apparent molecular weights of about 17 and 30 kDa. A 17-kDa calcium-binding protein was purified by calcium-dependent hydrophobic chromatography and was identified as calmodulin by immunological and enzyme activator criteria. A 125I-calmodulin overlay assay was used to identify potential targets of calmodulin action. Sperm contained one major calmodulin-binding protein of about 45 kDa. Eggs lacked major calmodulin-binding activity. A 72-kDa calmodulin-binding protein was prominent in zygotes from 1-65 hr postfertilization. Both calmodulin-binding proteins showed calcium-dependent binding activity. Overall, the data suggest that the appearance and distribution of certain calcium-binding and calmodulin-binding proteins are under developmental regulation, and may reflect the different roles of calcium during fertilization and early embryogenesis.

Spatial memory during the tropism of maize (Zea mays L.) coleoptiles

Photo- or gravitropic stimulation of graminean coleoptiles involves the formation of putative tropistic transverse polarities. It had been postulated that these polarities can be extended by stabilization to developmentally active polarities. Such polarities are known from unicellular spores and zygotes of lower plants and regeneration experiments in dicotyledonous plants. In coleoptiles, photo- or gravitropic stimulation results in stability to counterstimulation of equal strength (with only transient bending in the direction of the second stimulus), as a result of a directional memory, if the time interval between both stimuli exceeds 90 min. This directional memory develops from a labile precursor, which is present from at least 20 min after induction. Once it is stable, spatial memory is conserved for many hours. The formation of spatial memory involves at least one step not present in the common tropistic transduction chain. The spatial expression of memory as curvature is restricted to three distinct responses: (i) curving in the direction of the first stimulus (for time intervals exceeding 90 min); (ii) curving in the direction of the second stimulus (for time intervals shorter than 65 min); and (iii) zero-curvature (for time intervals between 65 and 90 min). This can be interpreted in terms of a stable transverse polarity, which is not identical with the putative tropistic transverse polarity, but might be an extension of it.

Ionic currents in morphogenesis

Morphogenetic fields must be generated by mechanisms based on known physical forces which include gravitational forces, mechanical forces, electrical forces, or some combination of these. While it is unrealistic to expect a single force, such as a voltage gradient, to be the sole cause of a morphogenetic event, spatial and temporal information about the electrical fields and ion concentration gradients in and around a cell or embryo undergoing morphogenesis can take us one step further toward understanding the entire morphogenetic mechanism. This is especially true because one of the handful of identified morphogens is Ca2+, an ion that will not only generate a current as it moves, but which is known to directly influence the plasma membrane's permeability to other ions, leading to other transcellular currents. It would be expected that movements of this morphogen across the plasma membrane might generate ionic currents and gradients of both electrical potential and intracellular concentration. Such ionic currents have been found to be integral components of the morphogenetic mechanism in some cases and only secondary components in other cases. My goal in this review is to discuss examples of both of these levels of involvement that have resulted from investigations conducted during the past several years, and to point to areas that are ripe for future investigation. This will include the history and theory of ionic current measurements, and a discussion of examples in both plant and animal systems in which ionic currents and intracellular concentration gradients are integral components of morphogenesis as well as cases in which they play only a secondary role. By far the strongest cases for a direct role of ionic currents in morphogenesis is the polarizing fucoid egg where the current is carried in part by Ca2+ and generates an intracellular concentration gradient of this ion that orients the outgrowth, and the insect follicle in which an intracellular voltage gradient is responsible for the polarized transport from nurse cell to oocyte. However, in most of the systems studied, the experiments to determine if the observed ionic currents are directly involved in the morphogenetic mechanism are yet to be done. Our experience with the fucoid egg and the fungal hypha of Achlya suggest that it is the change in the intracellular ion concentration resulting from the ionic current that is critical for morphogenesis.

Neurotransmitters in the regulation of neuronal cytoarchitecture

Recent experiments in isolated neurons in cell culture have demonstrated that neurotransmitters and associated electrical activity can directly affect neurite outgrowth. The results indicate that neurotransmitters have considerable potential to control the development of the neuronal circuits in which they participate in information coding in the adult. Cellular mechanisms regulating growth cone motility have been found to be similar to those regulating neurotransmitter release at the synapse and involve electrical activity, calcium and other second messengers. These similarities suggest that the morphological changes in connections observed in adult plasticity may involve the transition of synaptic terminals back to a growth mode. Excitatory and inhibitory neurotransmitters can interact to yield a net effect on neuronal morphology. In the intact nervous system a balance between these neurotransmitter inputs is probably important in maintaining circuits. Studies of neurotransmitter involvement in learning and memory processes indicate that brain function can alter brain structure and that neurotransmitters may control these structural changes. The hippocampus is one brain region in which we are beginning to define roles for neurotransmitters as sculptors of neuronal cytoarchitecture. The neurotransmitter glutamate was found to specifically affect the cytoarchitecture of hippocampal pyramidal neuron dendrites in a graded manner which suggests that glutamate may be involved in: establishing hippocampal circuitry during brain development; maintaining and modifying circuitry in the adult; and inducing neurodegeneration in several disorders including epilepsy, Alzheimer's disease, and stroke. Therapeutic approaches to disorders which affect brain cytoarchitecture may now be devised based upon knowledge of the neurotransmitters and their cellular mechanisms in the pertinent brain region.

Membrane-bound Ca2+ distribution visualized by chlorotetracycline fluorescence during morphogenesis of soredia in a lichen

In the lichen Parmelia caperata (L.) Ach. the distribution pattern of membrane-bound Ca2+ is investigated in the symbionts by chlorotetracycline (CTC)-induced fluorescence during the development of propagative structures, the soredia. The results demonstrate that Ca2+ accumulation in the alga and the fungus is associated with this morphogenetic process; particularly, polarized hyphal growth involves a tip-to-base Ca2+ gradient. CTC fluorescence distribution is coincident with that of cholinesterase (ChE) activity during morphogenesis of soredia. A comparison is suggested with 'embryonic ChE' of animal cells, where developmental events are regulated by a cholinergic mechanism that also modulates Ca2+ levels.

Localization of membrane-associated calcium during development of fucoid algae using chlorotetracycline

During the first day of development, fertilized eggs of fucoid algae generate an embryonic axis and commence rhizoid growth at one pole. Using Fucus distichus (L.) Powell, F. vesiculosus L. and Pelvetia fastigiata (J.Ag.) DeTony we have investigated the role of calcium in axis formation and fixation as well as in tip growth. The intracellular distribution of membrane-associated calcium was visualized with the fluorescent calcium probe chlorotetracycline (CTC). Punctate fluorescence associated with organelle-like structures was found in conjunction with diffuse staining at all developmental stages. This membrane-associated calcium remained uniformly distributed throughout the cortical cytoplasm while the axis was established, but increased in the rhizoid protuberance at germination. In subsequent development, fluorescence was restricted to the cortical cytoplasm at the elongating tip and at sites of crosswall biosynthesis.The requirement for Ca(2+) uptake during development was investigated through inhibition studies; influx was impaired with transport antagonists or by removal of extracellular calcium. Both treatments curtailed germination and tip elongation but had little effect on axis polarization. Reductions in external calcium that interfered with elongation also markedly reduced the apical CTC fluorescencence, indicating that calcium uptake and localization are prerequisites for tip growth. This apical Ca(2+) is probably involved in the secretory process that sustains tip elongation. By contrast, calcium was not implicated in the generation of an embryonic axis.

Nonsudorific skin potential level: current hypothesis and psychophysiological significance

Skin potential level has two mechanisms of generation: that due to sweat gland activity (sudorific) and that due to other causes (nonsudorific). These mechanisms have, in the past, been frequently confounded. This paper addresses the importance of discriminating between the two mechanisms and describes two kinds of nonsudorific skin potentials. There is reason to except nonsudorific skin potential level to be an important factor in embryogenesis, tissue regeneration and atypical growth. Early work that associated these potentials with changes in consciousness (e.g., sleep and hypnosis) is inconclusive. The literature concerning physiological and psychological correlates of skin potential levels that have been attributed to nonsudorific mechanisms is reviewed, and the methodological problem of measuring these potentials is considered.

Stimulation of direct embryogenesis from mesophyll protoplasts of Medicago sativa

Mesophyll protoplasts of Medicago sativa were exposed to low voltage electrical fields immediately following isolation. Several exposure times and voltages were utilized. At the lower doses, protoplast aggregation and subsequent embryogenesis were stimulated. A clone of 'Rangelander', which was directly-embryogenic (i.e. embryos were derived from single mesophyll protoplasts without an intervening callus phase), was induced to form embryos in all samples exposed to the lowest level electrical fields, while unexposed controls formed few or no embryos. A clone of 'Regen S', which was previously not directly-embryogenic, was induced to follow the 'Rangelander' pattern of development and to produce early globular embryos.

Cytochalasin treatment disrupts the endogenous currents associated with cell polarization in fucoid zygotes: studies of the role of F-actin in embryogenesis

We determined the distribution of F-actin in fucoid (Pelvetia, Fucus) embryos with nitrobenzoxadiazole-phallacidin, and studied the effect of cytochalasin upon the endogenous currents associated with cell polarization by using the vibrating probe. F-actin is not localized at the presumptive rhizoid immediately after experimental induction of the polar axis with a light gradient; however, a preferential distribution of F-actin develops at the presumptive rhizoid by the time the position of the polar axis is fixed. F-actin continues to be localized at the tip of the rhizoid after germination, except during cytokinesis, when the furrow is the only brightly staining region of the embryo. Incubation with cytochalasin can result in either an enhanced or a diminished pool of F-actin in the embryonic cortex (see Results). Cytochalasin D (100 micrograms/ml) significantly reduces the inward current at the rhizoid pole (n = 11) after a 2.5-h incubation. This drop is concentration dependent and occurs within approximately 30 min at 100 micrograms/ml and approximately 60 min at 10 micrograms/ml. Cytochalasin treatment eliminates the pulsatile component of the current. Preliminary results suggest that 100 micrograms/ml cytochalasin D prevents development of inward current at the presumptive rhizoid but does not completely delocalize this locus if added after photopolarization. We conclude that microfilaments are required for the establishment and maintenance of the pattern of endogenous currents observed during early embryogenesis. This suggests a new model for axis formation and fixation.

Regional water changes during oocyte meiotic maturation: evidence of ooplasmic segregation

Cryomicrodissection was used to measure the intraoocytic distribution of water before and during meiotic maturation in Rana pipiens oocytes. Animal ooplasm contained about 10% more water in matured than in ovarian oocytes. The increase was not dependent on the uptake of extracellular water, occurring even when oocytes were matured in a paraffin oil medium. Rather, animal ooplasm hydration appeared to be due to an increase in the volume fraction occupied by cytoplasm (reduced yolk density) through: (1) migration of cytoplasm from the vegetal to animal hemisphere and (2) mixing of ooplasm with nuclear sap during germinal vesicle breakdown (GVBD). Cytoplasmic migration (or ooplasmic segregation) began prior to GVBD, probably within an hour of exposure to progesterone and appeared to continue through the period of GVBD. The volume of cytoplasm that moved significantly reduced water concentrations in vegetal ooplasm at 6 hr postprogesterone and offset any subsequent water gain due to the mixing of nuclear sap and vegetal ooplasm at GVBD. The findings suggest that segregational movements are among the early maturational changes entrained by progesterone. Ooplasmic segregation is considered in the context of theories of cytomatrix movement in which control resides in regional Ca2+ activity gradients. We address the problem of the vegetal----animal directionality of movement and suggest that the annulate lamellae play a role.

The effects of the ionophore A23187 on pattern formation in the alga Micrasterias

We have used the divalent cation ionophore A23187 to investigate the hypothesis that cytoplasmic localization of Ca2+ is responsible for localized growth in the alga Micrasterias. In a preliminary study we found that, of the major salts contained in the cell's medium, only CaCl2 was needed for normal development. In cells developing in the presence of A23187 and extracellular Ca2+, we postulated that the ionophore would induce a spatially uniform influx of Ca2+ that would overwhelm endogenous Ca2+ gradients. When developing cells were treated with A23187 and 2 mM CaCl2, we observed a delocalization of the cell's normal pattern of wall deposition. This effect was less pronounced when cells were exposed to A23187 and 2 mM MgCl2. These results support the hypothesis that localized regions of high Ca2+ concentration normally mediate localized expansion of tip-growing lobes in Micrasterias.

Calcium ionophore polarizes ooplasmic segregation in ascidian eggs

Calcium ionophore A23187 promotes ooplasmic segregation and orange crescent formation in eggs of the ascidian Boltenia villosa. When eggs were exposed to a gradient A23187 the orange crescent was induced to form in the region corresponding to the highest concentration of ionophore. This result is consistent with the hypothesis that a local increase in intracellular calcium polarizes cytoplasmic localization in the ascidian embryo.

Oriented growth of Blastocladiella emersonii in gradients of ionophores and inhibitors

To investigate whether ion currents help to localize growth and development of Blastocladiella emersonii, we grew the organisms in gradients of various ionophores and inhibitors. Gradients were generated by placing into the culture fine glass fibers coated with insoluble inhibitors; in some cases, inhibitors were adsorbed onto beads of ion-exchange resin. Organisms growing in many of these gradients exhibited a striking tendency for the thalli to grow toward the fiber. This proved to be misleading; the cells grew not toward the source of the ionophore but into the unoccupied zone of inhibition adjacent to the fiber. Fibers coated with gramicidin-D induced marked effects on the growth of the rhizoids, which were greatly enlarged and grew toward and onto the fiber. None of the other inhibitors produced such effects, except for beads coated with the proton conductors tetrachlorosalicylanilide and compound 1799. The results suggest that orientation of rhizoid growth results from enhancement of proton flux across the plasma membrane. Growth of the rhizoids was also strongly oriented by gradients of inorganic phosphate and an amino acid mixture; gradients of glucose, K+, Ca2+, and glutamate were ineffective. We propose that a major physiological function of the rhizoid is to transport nutrients to the thallus. Finally, we examined the effects of a series of benzimidazole antitubulins as well as the cytochalasins. These did not orient growth but grossly perturbed the pattern of cellular organization, producing small spherical cells with multiple stunted rhizoids. The findings are interpreted in terms of the interaction of an endogenous transcellular proton current with elements of the cytoskeleton in the determination of form.

Endogenous electrical currents in the water mold Blastocladiella emersonii during growth and sporulation

We have explored the pattern of electrical currents generated by single cells of the water mold Blastocladiella emersonii at several stages of its life cycle. Extracellular currents were measured with a vibrating probe constructed after the design of Jaffe and Nuccitelli [Jaffe, L. F. & Nuccitelli, R. (1974) J. Cell Biol. 63, 614-628]. In growing cells positive current, of the order of 1 microA/cm2, enters the rhizoid and leaves from the thallus; circumstantial evidence suggests that protons carry much of the current. Sporulation is associated with reversal of the current pattern, such that positive current enters the thallus and leaves from the rhizoidal region; the ions that carry the current have not been identified. These current patterns appear to play a role in the spatial localization of fungal growth and development.