Comparative analysis of Ca2+ and H+ flux magnitude and location along growing hyphae of Saprolegnia ferax and Neurospora crassa

Methods for the detection of intracellular calcium in filamentous fungi

Calcium (Ca2+), a critical secondary messenger, is also known as the molecule of life and death. The cell responds to a minute change in Ca2+ concentration and tightly maintains Ca2+ homeostasis. Therefore, determining the cell Ca2+ level is critical to understand Ca2+ distribution in the cell and various cell processes. Many techniques have been developed to measure Ca2+ in the cell. We review here different methods used to detect and measure Ca2+ in filamentous fungi. Ca2+-sensitive fluorescent chlortetracycline hydrochloride (CTC), Ca2+-selective microelectrode, Ca2+ isotopes, aequorins, and RGECOs are commonly used to measure the Ca2+ level in filamentous fungi. The use of CTC was one of the earliest methods, developed in 1988, to measure the Ca2+ gradient in the filamentous fungus Neurospora crassa. Subsequently, Ca2+-specific microelectrodes were developed later in the 1990s to identify Ca2+ ion flux variations, and to measure Ca2+ concentration. Another method for quantifying Ca2+ is by using radio-labeled Ca2+ as a tracer. The usage of 45Ca to measure Ca2+ in Saccharomyces cerevisiae was reported previously and the same methodology was also used to detect Ca2+ in N. crassa recently. Subsequently, genetically engineered Ca2+ indicators (GECIs) like aequorins and RGECOs have been developed as Ca2+ indicators to detect and visualize Ca2+ inside the cell. In this review, we summarize various methodologies used to detect and measure Ca2+ in filamentous fungi with their advantages and limitations. •Chlortetracycline (CTC) fluorescence assay is used for visualizing Ca2+ level, whereas microelectrodes technique is used to determine Ca2+ flux in the cell.•Radioactive 45Ca is useful for quantification of Ca2+ in the cellular compartments.•Genetically modified calcium indicators (GECIs) are used to study Ca2+ dynamics in the cell.

Apical but not sub-apical hyphal compartments are self-sustaining in growth

It was recently demonstrated that apical compartments of Aspergillus niger hyphae are self-sustaining in growth. This was shown by assessing the growth rate of individual hyphae before and after dissection of the second compartment. Using the same methodology, it is here demonstrated that single apical compartments of the septate fungi Penicillium chrysogenum and Schizophyllum commune as well as the 500-µm-apical region of the non-septate fungus Rhizopus stolonifer are also self-sustaining in growth. In contrast, single 2nd compartments (obtained by dissection of the first and third compartment) of the septate fungi or the region between 500 and 1000 µm from tips of R. stolonifer were severely impacted in their growth rate. In addition, it is shown that existing or newly formed branches originating from the 2nd compartments function as a backup system for hyphal growth when the apical part of the hypha of the three studied fungi is damaged. Together, it is concluded that the apical compartments/zones of the studied fungi are self-sustaining in growth. In contrast, the subapical region is not self-sustaining but functions as a backup once the apical zone is damaged. This back up system is relevant in nature because the apices of hyphae are the first to be exposed to (a)biotic stress conditions when entering an unexplored substrate.

Mutual independence of alkaline- and calcium-mediated signalling in Aspergillus fumigatus refutes the existence of a conserved druggable signalling nexus

Functional coupling of calcium‐ and alkaline responsive signalling occurs in multiple fungi to afford efficient cation homeostasis. Host microenvironments exert alkaline stress and potentially toxic concentrations of Ca²⁺, such that highly conserved regulators of both calcium‐ (Crz) and pH‐ (PacC/Rim101) responsive signalling are crucial for fungal pathogenicity. Drugs targeting calcineurin are potent antifungal agents but also perturb human immunity thereby negating their use as anti‐infectives, abrogation of alkaline signalling has, therefore, been postulated as an adjunctive antifungal strategy. We examined the interdependency of pH‐ and calcium‐mediated signalling in Aspergillus fumigatus and found that calcium chelation severely impedes hyphal growth indicating a critical requirement for this ion independently of ambient pH. Transcriptomic responses to alkaline pH or calcium excess exhibited minimal similarity. Mutants lacking calcineurin, or its client CrzA, displayed normal alkaline tolerance and nuclear translocation of CrzA was unaffected by ambient pH. Expression of a highly conserved, alkaline‐regulated, sodium ATPase was tolerant of genetic or chemical perturbations of calcium‐mediated signalling, but abolished in null mutants of the pH‐responsive transcription factor PacC, and PacC proteolytic processing occurred normally during calcium excess. Taken together our data demonstrate that in A. fumigatus the regulatory hierarchy governing alkaline tolerance circumvents calcineurin signalling.

Cytosolic free calcium dynamics as related to hyphal and colony growth in the filamentous fungal pathogen Colletotrichum graminicola

Tip growth of pollen tubes and root hairs of plants is oscillatory and orchestrated by tip-focussed variations of cytosolic free calcium ([Ca(2+)]cyt). Hyphae of filamentous fungi are also tubular tip-growing cells, and components of the Ca(2+) signalling machinery, such as Ca(2+) channels and Ca(2+) sensors, are known to be important for fungal growth. In this study, we addressed the questions if tip-focussed [Ca(2+)]cyt transients govern hyphal and whole-colony growth in the maize pathogen Colletotrichum graminicola, and whether colony-wide [Ca(2+)]cyt dynamics rely on external Ca(2+) or internal Ca(2+) stores. Ratiometric fluorescence microscopy of individual hyphae expressing the Ca(2+) reporter Yellow Cameleon 3.6 revealed that Ca(2+) spikes in hyphal tips precede the re-initiation of growth after wounding. Tip-focussed [Ca(2+)]cyt spikes were also observed in undisturbed growing hyphae. They occurred not regularly and at a higher rate in hyphae growing at a medium-glass interface than in those growing on an agar surface. Hyphal tip growth was non-pulsatile, and growth speed was not correlated with the rate of spike occurrence. A possible relationship of [Ca(2+)]cyt spike generation and growth of whole colonies was assessed by using a codon-optimized version of the luminescent Ca(2+) reporter Aequorin. Depletion of extracellular free Ca(2+) abolished [Ca(2+)]cyt spikes nearly completely, but had only a modest effect on colony growth. In a pharmacological survey, some inhibitors targeting Ca(2+) influx or release from internal stores repressed growth strongly. However, although some of those inhibitors also affected [Ca(2+)]cyt spike generation, the effects on both parameters were not correlated. Collectively, the results indicate that tip growth of C. graminicola is non-pulsatile and not mechanistically linked to tip-focused or global [Ca(2+)]cyt spikes, which are likely a response to micro-environmental parameters, such as the physical properties of the growth surface.

Calcium Signaling in Oomycetes: An Evolutionary Perspective

Oomycetes are a family of eukaryotic microbes that superficially resemble fungi, but which are phylogenetically distinct from them. These organisms cause major global economic losses to agriculture and fisheries, with representative pathogens being Phytophthora infestans, the cause of late potato blight and Saprolegnia diclina, the instigator of “cotton molds” in fish. As in all eukaryotes, cytoplasmic Ca²⁺ is a key second messenger in oomycetes, regulating life-cycle transitions, controlling motility and chemotaxis and, in excess, leading to cell-death. Despite this, little is known about the molecular mechanisms regulating cytoplasmic Ca²⁺ concentrations in these organisms. Consequently, this review analyzed the presence of candidate calcium channels encoded within the nine oomycete genomes that are currently available. This revealed key differences between oomycetes and other eukaryotes, in particular the expansion and loss of different channel families, and the presence of a phylum-specific group of proteins, termed the polycystic kidney disease tandem ryanodine receptor domain (PKDRR) channels.

Structural and functional organization of growing tips of Neurospora crassa Hyphae

Data are presented on a variety of intracellular structures of the vegetative hyphae of the filamentous fungus Neurospora crassa and the involvement of these structures in the tip growth of the hyphae. Current ideas on the molecular and genetic mechanisms of tip growth and regulation of this process are considered. On the basis of comparison of data on behaviors of mitochondria and microtubules and data on the electrical heterogeneity of the hyphal apex, a hypothesis is proposed about a possible supervisory role of the longitudinal electric field in the structural and functional organization of growing tips of the N. crassa hyphae.

Activation of a TRP-like channel and intracellular Ca2+ dynamics during phospholipase-C-mediated cell death

The model organism Neurospora crassa undergoes programmed cell death when exposed to staurosporine. Here, we show that staurosporine causes defined changes in cytosolic free Ca²⁺ ([Ca²⁺]_c) dynamics and a distinct Ca²⁺ signature that involves Ca²⁺ influx from the external medium and internal Ca²⁺ stores. We investigated the molecular basis of this Ca²⁺ response by using [Ca²⁺]_c measurements combined with pharmacological and genetic approaches. Phospholipase C was identified as a pivotal player during cell death, because modulation of the phospholipase C signaling pathway and deletion of PLC-2, which we show to be involved in hyphal development, results in an inability to trigger the characteristic staurosporine-induced Ca²⁺ signature. Using Δcch-1, Δfig-1 and Δyvc-1 mutants and a range of inhibitors, we show that extracellular Ca²⁺ entry does not occur through the hitherto described high- and low-affinity Ca²⁺ uptake systems, but through the opening of plasma membrane channels with properties resembling the transient receptor potential (TRP) family. Partial blockage of the response to staurosporine after inhibition of a putative inositol-1,4,5-trisphosphate (IP₃) receptor suggests that Ca²⁺ release from internal stores following IP₃ formation combines with the extracellular Ca²⁺ influx.

Surface tip-to-base Ca2+ and H+ ionic fluxes are involved in apical growth and graviperception of the Phycomyces stage I sporangiophore

Net fluxes of Ca(2+) and H(+) ions were measured non-invasively close to the surface of Phycomyces blakesleeanus sporangiophores stage I using ion-selective vibrating microelectrodes. The measurements were performed on a wild type (Wt) and a gravitropic mutant A909 kept in either vertical or tilted orientation. Microelectrodes were positioned 4 μm from the surface of sporangiophore, and ion fluxes were recorded from the apical (0-20 μm) and subapical (50-100 μm) regions. The magnitude and direction of ionic fluxes measured were dependent on the distance from the tip along the growing zone of sporangiophore. Vertically oriented sporangiophores displayed characteristic tip-to-base ion fluxes patterns. Ca(2+) and H(+) fluxes recorded from apical region of Wt sporangiophores were inward-directed, while ion fluxes from subapical locations occurred in both directions. In contrast to Wt, mutant A909 showed opposite (outward) direction of Ca(2+) fluxes and reduced H(+) influxes in the apical region. Following gravistimulation, the magnitude and direction of ionic fluxes were altered. Wt sporangiophore exhibited oppositely directed fluxes on the lower (influx) and the upper (efflux) sides of the cell, while mutant A909 did not show such patterns. A variable elongation growth in vertical position and reduced growth rate upon gravistimulation were observed in both strains. The data show that tip-growing sporangiophores exhibit a tip-to-base ion flux pattern which changes characteristically upon gravistimulation in Wt in contrast to the mutant A909 with a strongly reduced gravitropic response.

Expression of the Cameleon calcium biosensor in fungi reveals distinct Ca(2+) signatures associated with polarized growth, development, and pathogenesis

Calcium is a universal messenger that translates diverse environmental stimuli and developmental cues into specific cellular and developmental responses. While individual fungal species have evolved complex and often unique biochemical and structural mechanisms to exploit specific ecological niches and to adjust growth and development in response to external stimuli, one universal feature to all is that Ca(2+)-mediated signaling is involved. The lack of a robust method for imaging spatial and temporal dynamics of subcellular Ca(2+) (i.e., "Ca(2+) signature"), readily available in the plant and animal systems, has severely limited studies on how this signaling pathway controls fungal growth, development, and pathogenesis. Here, we report the first successful expression of a FRET (Förster Resonance Energy Transfer)-based Ca(2+) biosensor in fungi. Time-lapse imaging of Magnaporthe oryzae, Fusarium oxysporum, and Fusarium graminearum expressing this sensor showed that instead of a continuous gradient, the cytoplasmic Ca(2+) ([Ca(2+)](c)) change occurred in a pulsatile manner with no discernable gradient between pulses, and each species exhibited a distinct Ca(2+) signature. Furthermore, occurrence of pulsatile Ca(2+) signatures was age and development dependent, and major [Ca(2+)](c) transients were observed during hyphal branching, septum formation, differentiation into specialized plant infection structures, cell-cell contact and in planta growth. In combination with the sequenced genomes and ease of targeted gene manipulation of these and many other fungal species, the data, materials and methods developed here will help understand the mechanism underpinning Ca(2+)-mediated control of cellular and developmental changes, its role in polarized growth forms and the evolution of Ca(2+) signaling across eukaryotic kingdoms.

Electrical phenotypes of calcium transport mutant strains of a filamentous fungus, Neurospora crassa

We characterized the electrical phenotypes of mutants with mutations in genes encoding calcium transporters-a mechanosensitive channel homolog (MscS), a Ca(2+)/H(+) exchange protein (cax), and Ca(2+)-ATPases (nca-1, nca-2, nca-3)-as well as those of double mutants (the nca-2 cax, nca-2 nca-3, and nca-3 cax mutants). The electrical characterization used dual impalements to obtain cable-corrected current-voltage measurements. Only two types of mutants (the MscS mutant; the nca-2 mutant and nca-2-containing double mutants) exhibited lower resting potentials. For the nca-2 mutant, on the basis of unchanged conductance and cyanide-induced depolarization of the potential, the cause is attenuated H(+)-ATPase activity. The growth of the nca-2 mutant-containing strains was inhibited by elevated extracellular Ca(2+) levels, indicative of lesions in Ca(2+) homeostasis. However, the net Ca(2+) effluxes of the nca-2 mutant, measured noninvasively with a self-referencing Ca(2+)-selective microelectrode, were similar to those of the wild type. All of the mutants exhibited osmosensitivity similar to that of the wild type (the turgor of the nca-2 mutant was also similar to that of the wild type), suggesting that Ca(2+) signaling does not play a role in osmoregulation. The hyphal tip morphology and tip-localized mitochondria of the nca-2 mutant were similar to those of the wild type, even when the external [Ca(2+)] was elevated. Thus, although Ca(2+) homeostasis is perturbed in the nca-2 mutant (B. J. Bowman et al., Eukaryot. Cell 10:654-661, 2011), the phenotype does not extend to tip growth or to osmoregulation but is revealed by lower H(+)-ATPase activity.

Ion and oxygen fluxes in the unicellular alga Eremosphaera viridis

Plasma membrane fluxes of the large unicellular model algal cell Eremosphaera viridis (De Bary) were measured under various light regimes to explore the role of plasma membrane fluxes during photosynthesis and high light-induced chloroplast translocation. Plasma membrane fluxes were measured directly and non-invasively with self-referencing ion-selective (H(+), Ca(2+), K(+) and Cl(-)) potentiometric microelectrodes and oxygen amperometric microelectrodes. At light irradiances high enough to induce chloroplast migration from the cell periphery to its center, oxygen evolution declined to respiratory net O(2) uptake prior to any significant chloroplast translocation, while net K(+) and Cl(-) influx increased during the decline in photosynthetic activity (and the membrane potential depolarized). The results suggest that chloroplast translocation is not the cause of the cessation of O(2) evolution at high irradiance. Rather, the chloroplast translocation may play a protective role: shielding the centrally located nucleus from damaging light intensities. At both high and low light intensities (similar to ambient growth conditions), there was a strong inverse correlation between H(+) net fluxes and respiratory and photosynthetic net O(2) fluxes. A similar inverse relationship was also observed for Ca(2+) net fluxes, but only at higher light intensities. The net H(+) fluxes are small relative to the buffering capacity of the cell, but are clearly related to both photosynthetic and respiratory activity.

Turgor and net ion flux responses to activation of the osmotic MAP kinase cascade by fludioxonil in the filamentous fungus Neurospora crassa

The internal hydrostatic pressure (turgor) of the filamentous fungus Neurospora crassa is regulated at about 400-500 kiloPascals, primarily by an osmotic MAP kinase cascade which activates ion uptake from the extracellular medium and glycerol synthesis. In the absence of hyperosmotic stress, the phenylpyrrole fungicide fludioxonil activates the osmotic MAP kinase cascade, resulting in cell death. Turgor, the electrical potential and net ion fluxes were measured after treatment with fludioxonil. In wildtype, fludioxonil causes a hyperpolarization of the plasma membrane and net H(+) efflux from the cell, consistent with activation of the H(+)-ATPase. At the same time, net K(+) uptake occurs, and turgor increases (about 2-fold above normal levels). None of these changes are observed in the os-2 mutant (which lacks a functional MAP kinase, the last of the three kinases in the osmotic MAP kinase cascade). Tip growth ceases as hyperpolarization, net ion flux changes, and turgor increases begin. The inappropriate turgor increase is the probable cause of eventual lysis and death. The results corroborate a multi-pathway response to hyperosmotic stress that includes activation of plasma membrane transport. The relation to cell expansion (tip growth) is not direct. Increases in turgor due to ion transport might be expected to increase growth rate, but this does not occur. Instead, there must be a complex regulatory interplay between the growth and the turgor driving force, possibly mediated by regulation of cell wall extensibility.

Structure and distribution of organelles and cellular location of calcium transporters in Neurospora crassa

We wanted to examine the cellular locations of four Neurospora crassa proteins that transport calcium. However, the structure and distribution of organelles in live hyphae of N. crassa have not been comprehensively described. Therefore, we made recombinant genes that generate translational fusions of putative organellar marker proteins with green or red fluorescent protein. We observed putative endoplasmic reticulum proteins, encoded by grp-78 and dpm, in the nuclear envelope and associated membranes. Proteins of the vacuolar membrane, encoded by vam-3 and vma-1, were in an interconnected network of small tubules and vesicles near the hyphal tip, while in more distal regions they were in large and small spherical vacuoles. Mitochondria, visualized with tagged ARG-4, were abundant in all regions of the hyphae. Similarly, we tagged the four N. crassa proteins that transport calcium with green or red fluorescent protein to examine their cellular locations. NCA-1 protein, a homolog of the SERCA-type Ca(2+)-ATPase of animal cells, colocalized with the endoplasmic reticulum markers. The NCA-2 and NCA-3 proteins are homologs of Ca(2+)-ATPases in the vacuolar membrane in yeast or in the plasma membrane in animal cells. They colocalized with markers in the vacuolar membrane, and they also occurred in the plasma membrane in regions of the hyphae more than 1 mm from the tip. The cax gene encodes a Ca(2+)/H(+) exchange protein found in vacuoles. As expected, the CAX protein localized to the vacuolar compartment. We observed, approximately 50 to 100 mum from the tip, a few spherical organelles that had high amounts of tagged CAX protein and tagged subunits of the vacuolar ATPase (VMA-1 and VMA-5). We suggest that this organelle, not described previously in N. crassa, may have a role in sequestering calcium.

Turgor regulation in the osmosensitive cut mutant of Neurospora crassa

The internal hydrostatic pressure (turgor) of fungal cells is maintained at 400-500 kPa. The turgor is regulated by changes in ion flux and by production of the osmotically active metabolite glycerol. In Neurospora crassa, there are at least two genetically distinct pathways that function in adaptation to hyperosmotic shock. One involves a mitogen-activated protein (MAP) kinase cascade (kinases OS-4, OS-5 and OS-2 downstream of the osmosensing OS-1); the other is less understood, but involves the cut gene, which encodes a putative phosphatase. This study examined turgor regulation, electrical responses, ion fluxes and glycerol accumulation in the cut mutant. Turgor recovery after hyperosmotic treatment was similar to that in the wild-type, for both time-course ( approximately 40 min) and magnitude. Prior to turgor recovery, the hyperosmotic shock caused a rapid transient depolarization of the membrane potential, followed by a sustained hyperpolarization that occurred concomitant with increased H(+) efflux, indicating that the plasma membrane H(+)-ATPase was being activated. These changes also occurred in the wild-type. Net fluxes of Ca(2+) and Cl(-) during turgor recovery were similar to those in the wild-type, but K(+) influx was attenuated in the cut mutant. The similar turgor recovery can be explained by the ion uptake, since glycerol did not accumulate in the cut mutant within the time frame of turgor recovery (but did accumulate in the wild-type). The results suggest that turgor regulation involves multi-faceted coordination of both ion flux and glycerol accumulation. Ion uptake is activated by a MAP kinase cascade, while CUT is required for glycerol accumulation.

Non-invasive microelectrode ion flux measurements to study adaptive responses of microorganisms to the environment

The regulation of membrane-transport activity is crucial for intracellular pH homeostasis, maintenance of cell osmotic potential, nutrient acquisition, signalling, and adaptation of bacterial cells. The non-invasive microelectrode ion flux estimation (MIFE) technique is a powerful tool for kinetic studies of membrane-transport processes across cellular membranes. Since 2001, when this technique was first applied to the study of membrane-transport processes in bacterial cells (J Microbiol Methods 46, 119-129), a large amount of information has been accumulated. This review summarizes some of these findings and discusses the advantages and applicability of this technique in studying bacterial adaptive responses to adverse environmental conditions. First, various methodological aspects of the application of this novel technique in microbiology are discussed. Then, several practical examples ('case studies') are described. The latter include changes in membrane-transport activity in response to various stresses (acidic, osmotic, and temperature stresses) as well as flux changes as a function of bacterial growth stage and nutrient availability. It is shown that non-invasive ion flux measurements may provide a significant conceptual advance in our understanding of adaptive responses in bacteria, fungi and biofilms to a variety of environmental conditions. The technique can also be used for the rapid assessment of food-processing treatments aimed at reducing bacterial contamination of food and for the development of strategies to assess the resistance of organisms to antimicrobial agents.

The role of tip-localized mitochondria in hyphal growth

Hyphal tip-growing organisms have a high density of tip-localized mitochondria which maintain a membrane potential based on Rhodamine 123 fluorescence, but do not produce ATP based on the absence of significant oxygen consumption. Two possible roles of these mitochondria in tip growth were examined: Calcium sequestration and biogenesis, because tip-high cytoplasmic calcium gradients are a common feature of tip-growing organisms, and the volume expansion as the tip extends would require a continuous supply of additional mitochondria. Co-localization of calcium-sensitive fluorescent dye and mitochondria-specific fluorescent dyes showed that the tip-localized mitochondria do contain calcium, and therefore, may function in calcium clearance from the cytoplasm. Short-term inhibition of DNA synthesis or mitochondrial protein synthesis did not affect either tip growth, or mitochondrial shape or distribution. Therefore, mitochondrial biogenesis may not occur from the tip-localized mitochondria in hyphal organisms.

Ca2+and H+Ion Fluxes near the Surface of Gravitropically StimulatedPhycomycesSporangiophore

The single-celled fungus Phycomyces blakesleeanus forms vertically oriented sporangiophores from hyphae, which display a negative gravitropic response. Longitudinal growth and gravitropic bending of these sporangiophores were measured with noninvasive H(+) and Ca(2+) ion-selective microelectrode measurements. The directions of H(+) and Ca(2+) fluxes, recorded at different locations of sporangiophores, were opposite when the sporangiophores were kept in vertical position. Ca(2+) fluxes were in most experiments positive (efflux), while H(+) fluxes were negative (influx). The direction of ion fluxes depended on developmental stage of sporangiophores and changed with gravistimulation.

Mass flow and pressure-driven hyphal extension in Neurospora crassa

Mass flow of cytoplasm inNeurospora crassatrunk hyphae was directly confirmed by injecting oil droplets into the hyphae. The droplets move in a manner similar to cytoplasmic particles and vacuoles within the hyphae. The direction of mass flow is towards the growing hyphal tips at the colony edge. Based on flow velocities (about 5 μm s−1), hyphal radius and estimates of cytoplasm viscosity, the Reynolds number is about 10−4, indicating that mass flow is laminar. Therefore, the Poiseulle equation can be used to calculate the pressure gradient required for mass flow: 0·0005–0·1 bar cm−1(depending on the values used for septal pore radius and cytoplasmic viscosity). These values are very small compared to the normal hydrostatic pressure of the hyphae (4–5 bar). Mass flow stops after respiratory inhibition with cyanide, or creation of an extracellular osmotic gradient. The flow is probably caused by internal osmotic gradients created by differential ion transport along the hyphae. Apical cytoplasm migrates at the same rate as tip extension, as do oil droplets injected near the tip. Thus, in addition to organelle positioning mediated by molecular motors, pressure-driven mass flow may be an integral part of hyphal extension.

Quantification of substratum contact required for initiation of Colletotrichum graminicola appressoria

Colletotrichum graminicola, like many plant pathogenic fungi develop appressoria on germling apices, to facilitate penetration of their host. Induction of these structures occurs after contact with the host surface has been established by the germling. Surface contact and subsequent development of appressoria by germlings of C. graminicola was assessed using interference-reflection microscopy (IRM) and microfabricated pillared silicon substrata. Observations with IRM revealed that under low nutrient conditions, 90% of the germlings developed appressoria once they established 4.5 microm of continuous contact with the substratum. Substrata bearing pillars < or =5 microm in width supported < or =10% appressoria; however, as pillar width was increased the percentage of appressoria formed increased in a sigmoid fashion to a maximum of 80%. The percentage of appressoria produced experimentally on these surfaces was compared to data sets generated from a model designed to calculate the probability of appressorium development on similar pillar arrays at various germ tube contact lengths. These results indicate that germ tubes of C. graminicola require more than 4microm of continuous contact with a hydrophobic substratum for induction of appressoria.

Oxygen flux magnitude and location along growing hyphae ofNeurospora crassa

Oxygen fluxes were mapped at the growing apices and along mycelial hyphal segments of the ascomycete Neurospora crassa. High spatial resolution was obtained using micro-oxygen probes (2-3 microm tip diameters) and the self-referencing technique to maximize the sensitivity of oxygen flux measurements. As expected, oxygen influx was inhibited by cyanide, although oxygen influx (and hyphal growth) resumed with the induction of an alternate oxidase activity. Along hyphal segments, variations in oxygen influx were not correlated with location, near or far from septa, and varied over time along the same hyphal segment. Growing hyphae had a region of maximal oxygen influx greater than 10 microm behind the hyphal tip, the oxygen influx was correlated with hyphal growth rate. The region of maximal oxygen influx did not correspond with mitochondrial density, which is maximal (about 30% of hyphal volume) 5-10 microm behind the tip. Therefore, tip-localized mitochondria do not contribute to the respiratory requirements of the growing hypha. The tip-localized mitochondria may function in clearing calcium from the cytoplasm, although a decline in chlortetracycline fluorescence after cyanide inhibition could also be due to ATP-depletion due to inhibition of actively respiring mitochondria. Alternatively, the growing tip may be the site of mitochondrial biogenesis.

Possible role of ionic gradients in the apical growth of Neurospora crassa

The effects of the Ca2+/H+ exchanger A23187 and the K+/H+ exchanger nigericin on the growth of Neurospora crassa were analyzed. Both ionophores had the same effects on the fungus. They both inhibited growth in liquid media, apical extension being more affected than protein synthesis. A sudden challenge to either ionophore on solid media rapidly stopped hyphal extension. Additionally, both ionophores induced profuse mycelium branching and upward hyphal growth. Hyphae growing on nigericin-containing media also burst at the apex. Both ionophores caused a rapid inhibition in the apically-occurring synthesis of structural wall polysaccharides, but they did not affect mitochondrial energy conservation. With the use of DiBAC, a membrane-potential sensitive fluorophore, it was excluded that their effects were due to depletion of the plasma membrane potential. Considering that both ionophores exchange H+ for different metallic ions, we concluded that their effect was due to dissipation of a proton gradient, which is directly or indirectly involved in the apical growth of the fungus.

Calcium gradient dependence of Neurospora crassa hyphal growth

A tip-high cytoplasmic calcium gradient has been identified as a requirement for hyphal growth in the fungus Neurospora crassa. The Ca2+ gradient is less steep compared to wall vesicle, wall incorporation and vesicular Ca2+ gradients, but this can be explained by Ca2+ diffusion. Analysis of the relation between the rate of hyphal growth and the spatial distribution of tip-localized calcium indicates that hyphal growth rates depend upon the tip-localized calcium concentration. It is not the steepness of the calcium gradient, but tip-localized calcium and the difference in tip-localized calcium versus subapical calcium concentration which correlate closely with hyphal growth rate. A minimal concentration difference between the apex and subapical region of 30 nM is required for growth to occur. The calcium concentration dependence of growth may relate directly to biochemical functions of calcium in hyphal extension, such as vesicle fusion and enzyme activation during cellular expansion. Initiation of tip growth may rely upon random Ca2+ motions causing localized regions of elevated calcium. Continued hyphal expansion may activate a stretch-activated phospholipase C which would increase tip-localized inositol 1,4,5-trisphosphate (IP3). Hyphal expansion, induced by mild hypoosmotic treatment, does increase diacylglycerol, the other product of phospholipase C activity. This is consistent with evidence that IP3-activated Ca2+ channels generate and maintain the tip-high calcium gradient.

An IP3-activated Ca2+ channel regulates fungal tip growth

Hyphal extension in fungi requires a tip-high Ca(2+) gradient, which is generated and maintained internally by inositol (1,4,5)-trisphosphate (IP(3))-induced Ca(2+) release from tip-localized vesicles and subapical Ca(2+) sequestration. Using the planar bilayer method we demonstrated the presence of two types of IP(3)-activated Ca(2+) channels in Neurospora crassa membranes with different conductances: one low (13 picosiemens), the other high (77 picosiemens). On sucrose density gradients the low conductance channel co-localized with endoplasmic reticulum and plasma membrane, and the high conductance channel co-localized with vacuolar membranes. We correlated the effect of inhibitors on channel activity with their effect on hyphal growth and Ca(2+) gradients. The inhibitor of IP(3)-induced Ca(2+) release, 2-aminoethoxidiphenylborate (2-APB), inhibits both channels, while heparin, 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate, hydrochloride (TMB-8) and dantrolene inhibit only the large conductance channel. Because 2-APB inhibits hyphal growth and dissipates the tip-high cytosolic [Ca(2+)] gradient, whereas heparin microinjection, TMB-8 and dantrolene treatments do not affect growth, we suggest that the small conductance channel generates the obligatory tip-high Ca(2+) gradient during hyphal growth. Since IP(3) production must be catalyzed by tip-localized phospholipase C, we show that a number of phospholipase C inhibitors [neomycin, 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]- 1H-pyrrole-2,5-dione (U-73122) (but not the inactive pyrrolidine U-73343), 3-nitrocoumarin] inhibit hyphal growth and affect, similarly to 2-APB, the location of vesicular Ca(2+) imaged by chlortetracycline staining.

Gleanings of a chemiosmotic eye

In 1961, an inventive Englishman, named Peter Mitchell, proposed a radically novel hypothesis to explain how energy is conserved during respiration and photosynthesis, and applied to the generation of ATP and other kinds of functional work. The chemiosmotic hypothesis sparked an intense controversy that lasted for 15 years. Today, Mitchell's conception of proton currents and their role in phosphorylation and active transport is generally accepted, and has ramified into many corners of cellular physiology. His most profound contribution may have been to introduce spatial direction into biochemistry, and thereby transform our perception of the relationship between molecules and cells.

Ca(2+) shuttling in vesicles during tip growth in Neurospora crassa

Tip-growing organisms maintain an apparently essential tip-high gradient of cytoplasmic Ca(2+). In the oomycete Saprolegnia ferax, in pollen tubes and root hairs, the gradient is produced by a tip-localized Ca(2+) influx from the external medium. Such a gradient is normally dispensable for Neurospora crassa hyphae, which may maintain their Ca(2+) gradient by some form of internal recycling. We localized Ca(2+) in N. crassa hyphae at the ultrastructural level using two techniques (a) electron spectroscopic imaging of freeze-dried hyphae and (b) pyroantimoniate precipitation. The results of both methods support the presence of Ca(2+) in the wall vesicles and Golgi body equivalents, providing a plausible mechanism for the generation and maintenance of the gradient by Ca(2+) shuttling in vesicles to the apex, without exogenous Ca(2+) influx. Ca(2+) sequestration into the vesicles seems to be dependent on Ca(2+)-ATPases since cyclopiazonic acid, a specific inhibitor of Ca(2+) pumps, eliminated all Ca(2+) deposits from the vesicles of N. crassa.

Regulation of the tip-high [Ca2+] gradient in growing hyphae of the fungus Neurospora crassa

Previous work has shown that hyphal elongation in the fungus Neurospora crassa requires a tip-high cytosolic Ca2+ gradient. The source of the Ca2+ appears to be intracellular stores as there is no net transplasma membrane Ca2+ flux at the elongating hyphal tip and modification of ion fluxes across the plasma membrane using voltage clamp is without effect on tip growth. To decode the internal mechanisms which generate and maintain the tip-high Ca2+ gradient we first identified calcium regulators which affect hyphal growth and morphology, then determined how they modify cytosolic [Ca2+] and the actin cytoskeleton using fluorescent dyes and confocal microscopy. Cyclopiazonic acid (a known inhibitor of the endoplasmic reticulum calcium ATPase) inhibits growth and increases cytoplasmic [Ca2+] in the basal region 10-25 microm behind the hyphal tip. 2-APB (2-aminoethoxydiphenyl borate, an inhibitor of IP3-induced Ca2+ release) inhibits hyphal elongation and dissipates the tip-high Ca2 gradient 0-10 microm from the tip. Microinjections of the IP3 receptor agonists adenophostin A and IP3 (but not control microinjections of the biologically inactive L-IP3) transiently inhibited growth and induced subapical branches. IP3 microinjections, but not L-IP3, lowered tip-localized [Ca2+] and increased basal [Ca2+]. Even though their effect on [Ca2+] gradients was different, both cyclopiazonic acid and 2-APB disrupted similarly the normal actin pattern at the hyphal apex. Conversely, disruption of actin with latrunculin B dissipated tip-localized Ca2+. We conclude that the tip-high Ca2+ gradient is generated internally by Ca2+ sequestration into endoplasmic reticulum behind the tip and Ca2+ release via an IP3 receptor from tip-localized vesicles whose location is maintained by the actin cytoskeleton.

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.

Calcineurin is required for hyphal elongation during mating and haploid fruiting in Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes meningitis in immunocompromised patients. Its growth is sensitive to the immunosuppressants FK506 and cyclosporin, which inhibit the Ca2+- calmodulin-activated protein phosphatase calcineurin. Calcineurin is required for growth at 37 degrees C and virulence of C.neoformans. We found that calcineurin is also required for mating. FK506 blocks mating of C.neoformans via FKBP12-dependent inhibition of calcineurin, and mutants lacking calcineurin are bilaterally sterile. Calcineurin is not essential for the initial fusion event, but is required for hyphal elongation and survival of the heterokaryon produced by cell fusion. It is also required for hyphal elongation in diploid strains and during asexual haploid fruiting of MATalpha cells in response to nitrogen limitation. Because mating and haploid fruiting produce infectious basidiospores, our studies suggest a second link between calcineurin and virulence of C.neoformans. Calcine urin regulates filamentation and 37 degrees C growth via distinct pathways. Together with studies revealing that calcineurin mediates neurite extension and neutrophil migration in mammals, our findings indicate that calcineurin plays a conserved role in the control of cell morphology.

Ion transport in roots: measurement of fluxes using ion-selective microelectrodes to characterize transporter function

The transport of mineral ions into and out of tissues and cells is central to the life of plants. Ion transport and the plasma membrane transporters themselves have been studied using a variety of techniques. In the last 15 years, measurement of specific ion fluxes has contributed to the characterization of transport systems. Progress in molecular genetics is allowing gene identification and controlled expression of transporter molecules. However the molecular expression of transporter gene products must be characterized at the functional level. The ion-selective microelectrode technique to measure specific ion fluxes non-invasively is ideally suited to this purpose. This technique, its theory, its links with others and its application and prospects in plant science, are discussed. Ions studied include hydrogen, potassium, sodium, ammonium, calcium, chloride and nitrate. Applications discussed include: solute ion uptake by roots; gravitropism and other processes in the root cap, meristematic and elongation zones; Nod factor effect on root hairs; osmotic and salt stresses; oscillations; the effects of light and temperature. Studies have included intact roots, leaf mesophyll and other tissues, protoplasts and bacterial biofilms. A multi-ion capability of the technique will greatly assist functional genomics, particularly when coupled with imaging techniques, patch clamping and the use of suitable mutants.