Reduced external calcium or sodium stimulates calcium influx in Pelvetia eggs

Competition of energy between active transport and vesicle fusion at the origin of intracellular gradient fields

It has been reported that the ionic patterns of hyphal growth can be explained by a weakening of the active transport at the tip at the expense of other biosynthesis processes, from which results energy transport from the proximal cells to the apical ones (Potapova et al. 1988). We present here a theory to support this hypothesis, whose extent is much more general than the initial frame where it has been formulated. It can be summarized in two basics mechanisms, one coupling active transport of the plasma membrane, electric potential and vesicle fusion, the other coupling the Ca²⁺-ATPase of the endoplasmic reticulum and vesicle fusion. For some values of parameters introduced in the theory, the uniform state of the cell becomes unstable, at the origin of intracellular gradient fields. Theoretical ionic patterns are spontaneously produced, which can be satisfactorily compared to several observed in and around tip-growing cells.

A Rho family GTPase controls actin dynamics and tip growth via two counteracting downstream pathways in pollen tubes

Tip growth in neuronal cells, plant cells, and fungal hyphae is known to require tip-localized Rho GTPase, calcium, and filamentous actin (F-actin), but how they interact with each other is unclear. The pollen tube is an exciting model to study spatiotemporal regulation of tip growth and F-actin dynamics. An Arabidopsis thaliana Rho family GTPase, ROP1, controls pollen tube growth by regulating apical F-actin dynamics. This paper shows that ROP1 activates two counteracting pathways involving the direct targets of tip-localized ROP1: RIC3 and RIC4. RIC4 promotes F-actin assembly, whereas RIC3 activates Ca(2+) signaling that leads to F-actin disassembly. Overproduction or depletion of either RIC4 or RIC3 causes tip growth defects that are rescued by overproduction or depletion of RIC3 or RIC4, respectively. Thus, ROP1 controls actin dynamics and tip growth through a check and balance between the two pathways. The dual and antagonistic roles of this GTPase may provide a unifying mechanism by which Rho modulates various processes dependent on actin dynamics in eukaryotic cells.

Ionic and osmotic disruptions of the lily pollen tube oscillator: testing proposed models

Two mechanisms have been proposed as the primary control of oscillating tip growth in Lilium longiflorum Thunb. pollen tubes: changes in cell wall strength (Holdaway-Clarke et al. 1997) or alternatively, changes in turgor pressure (Messerli et al. 2000). Here we have modified the ionic and osmotic concentrations of the growth medium to test predictions derived from both models. Raising the [Ca2+]o tenfold above normal reduced the amplitude of the [Ca2+]i oscillations and growth oscillations while it raised the basal [Ca2+]i and growth rate such that the average growth rate did not change. Raising the [H+] of the growth medium tenfold reversibly decreased and sometimes eliminated the [Ca2+]i and growth oscillations without changing the average growth rate. Lowering the [H+] tenfold led to irregular frequency and amplitude [Ca2+]i oscillations, reduced the average growth rate of tubes and led to cell bursting in 33% of tubes. Addition of 50 mM H+ buffer, MES, to prevent pH changes in the cell wall increased the period, amplitude and duration of both [Ca2+]i and growth oscillations. Changing the [K+]o did not markedly effect [Ca2+]i oscillations. Reducing the osmolarity of the medium led to transient large-amplitude [Ca2+]i and growth oscillations while reducing large-amplitude oscillations over long periods. In many different conditions under which growth still occurs, lily pollen tubes maintain growth oscillations, albeit with modified frequency, amplitude and duration. We conclude that modifications to both proposed models are necessary to explain oscillating growth in this system.

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.

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.

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.

Massive actin bundle couples macrocilia to muscles in the ctenophore Beroë

Macrocilia are thick compound ciliary organelles arising individually from elongated epithelial cells on the lips of beroid ctenophores. A giant wedge-shaped bundle of microfilaments extends 25-30 microns from the base of each macrocilium to the lower end of the cell, terminating at a junction with an underlying smooth muscle cell. The broad end of the microfilament bundle is anchored to the macrocilium by striated rootlet fibers that extend from the basal bodies into the bundle and are linked to the microfilaments by periodic bridges. Fluorescence microscopy of rhodamine-phalloidin stained intact tissue, dissociated macrociliary cells, and Triton/glycerol-isolated bundles shows that the microfilaments contain actin. The microfilaments run generally parallel to the long axis of the bundle but are not highly ordered. Filaments decorated with myosin S1 show a uniform polarity with arrowheads pointing away from the tapered membrane-associated end of the bundle. No variations in bundle length (nor changes in rootlet periodicity) were observed in tissue fixed under conditions of calcium activation. Isolated bundles did not contract in Mg-ATP, even though detached macrocilia underwent reactivated beating and sliding disintegration. Macrocilia are used to bite through food organisms or transport prey into the stomach. The actin filament bundles probably play a supporting role as a structural linker between macrocilia and subepithelial muscle fibers and may serve as intracellular tendons to mechanically coordinate the motor activities of macrocilia and muscles during prey ingestion.

Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L. : IV. Calcium as affected by its external concentration

From compartmental analysis of radioisotope elutin measurements, fluxes of Ca(2+) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to complete nutrient solutions containing a range of calcium concentrations ([Ca0]) from 2 μeq l(-1) to 20 meq l(-1), increasing in 10-fold steps for Ca(2+). Except for the calcium counter-ion (usually NO 3 (-) , sometimes Cl(-) at the highest [Ca0]), the composition of the nutrient solution was other-wise the same at all calcium concentrations. Compartmental analysis indicated that the cytoplasm had a high content of exchangeable Ca(2+) but, in the light of evidence from animal studies, ionic activity of calcium in the cytoplasm was assumed to be no greater than 0.002 μeq ml(-1). With the Ussing-Teorell flux equation as the criterion, it was concluded that at all values of [Ca0] tested, Ca(2+) entered the cytoplasm passively and was actively pumped back into the external solution. Entry of calcium to the vacuole from the cytoplasm was active in all cases. The conclusions regarding the character of ion transport across the plasmalemma were the same as when the whole calcium content of the cytoplasm was taken to contribute to the ionic activity. However, the electrochemical activity gradient was very much steeper than formerly estimated. Calcium was transported to the stele in proportion to the calcium content of the cytoplasm and moved in the xylem almost exclusively in the basipetal direction.

Polarization of Fucoid Eggs by a Calcium Ionophore Gradient

When the eggs of the brown alga Pelvetia were grown in a gradient of the calcium ionophore A23187, they tended to form their rhizoidal outgrowths on the sides that were exposed to the higher concentration of ionophore. This result supports the hypothesis that the formation of an intracellular calcium gradient is an essential step in the polarization of these eggs; the rhizoid forms at the pole that has the higher concentration of calcium.

Effects of membrane potential on calcium fluxes of Pelvetia eggs

(45)Ca(2+) fluxes across the plasma membrane of zygotes of the fucoid alga, Pelvetia fastagiata (J. Ag.) De Toni, were studied in artificial sea waters of various potassium concentrations. Except for two cases, hyperpolarization of the cell membrane (with low [K(+)]) increases, and depolarization (with high [K(+)]) decreases the influx of Ca(2+) over the range of [K(+)] studied (1-100 mM). The fractional increases of influx during hyperpolarization are close to the fractional increases in membrane potential but the decreases during depolarization are much smaller than those in membrane potential. In two anomalous cases, the influxes of (45)Ca(2+) at a potassium concentration of 30 mM were about 20% higher than the control value instead of being 10% lower.The effluxes of (45)Ca(2+) are increased by both hyperpolarization and by depolarization. On balance (and excepting the two anomalous cases) the net result of hyperpolarization should be to increase and that of depolarization to decrease intracellular [Ca(2+)].