Mapping fungal ion channel locations

Sensorimotor control of robots mediated by electrophysiological measurements of fungal mycelia

Living tissues are still far from being used as practical components in biohybrid robots because of limitations in life span, sensitivity to environmental factors, and stringent culture procedures. Here, we introduce fungal mycelia as an easy-to-use and robust living component in biohybrid robots. We constructed two biohybrid robots that use the electrophysiological activity of living mycelia to control their artificial actuators. The mycelia sense their environment and issue action potential-like spiking voltages as control signals to the motors and valves of the robots that we designed and built. The paper highlights two key innovations: first, a vibration- and electromagnetic interference-shielded mycelium electrical interface that allows for stable, long-term electrophysiological bioelectric recordings during untethered, mobile operation; second, a control architecture for robots inspired by neural central pattern generators, incorporating rhythmic patterns of positive and negative spikes from the living mycelia. We used these signals to control a walking soft robot as well as a wheeled hard one. We also demonstrated the use of mycelia to respond to environmental cues by using ultraviolet light stimulation to augment the robots' gaits.

Nanoscale chemical mapping of exometabolites at fungal-mineral interfaces

Mineral-associated organic matter is an integral part of soil carbon pool. Biological processes contribute to the formation of such organo-mineral complexes when soil microbes, and in particular soil fungi, deposit a suite of extracellular metabolic compounds and their necromass on the mineral surfaces. While studied in bulk, micro- to nanoscale fungal-mineral interactions remain elusive. Of particular interest are the mutual effects at the interface between the fungal exometabolites and proximal mineral particles. In this work, we have grown saprotrophic and symbiotic fungi in contact with two soil minerals with contrasting properties: quartz and goethite, on top of X-ray transparent silicon nitride membrane windows and analyzed fungal hyphae by synchrotron-based scanning transmission X-ray microscopy in combination with near edge X-ray fine structure spectroscopy at C(K) and Fe(L) absorption edges. In the resultant chemical maps, we were able to visualize and differentiate organic compounds constituting the fungal cells, their extracellular metabolites, and the exometabolites adsorbing on the minerals. We found that the composition of the exometabolites differed between the fungal functional guilds, particularly, in their sugar to protein ratio and potassium concentration. In samples with quartz and goethite, we observed adsorption of the exometabolic compounds on the mineral surfaces with variations in their chemical composition around the particles. Although we did not observe clear alteration in the exometabolite chemistry upon mineral encounters, we show that fungal-mineral interaction result in reduction of Fe(III) in goethite. This process has been demonstrated for bulk systems, but, to our knowledge, this is the first observation on a single hypha scale offering insight into its underlying biological mechanisms. This demonstrates the link between processes initiated at the single-cell level to macroscale phenomena. Thus, spatially resolved chemical characterization of the microbial-mineral interfaces is crucial for an increased understanding of overall carbon cycling in soil.

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.

Dissection of heat-induced systemic signals: superiority of ion fluxes to voltage changes in substomatal cavities

Using non-invasive ion-selective microprobes, that were placed in substomatal cavities, long-distance signalling has been investigated in intact Hordeum vulgare and Vicia faba seedlings. Heat (flame), applied to one leaf (S-leaf), triggers apoplastic ion activity (pH, pCa, pCl) transients in a distant leaf (T-leaf), all largely independent of simultaneously occurring action potential-like voltage changes. While apoplastic pCa and pH increase (Ca(2+)-, H(+)-activities decrease), pCl decreases (Cl(-)-activity increases). As the signal transfer from the S- to the T-leaf is too fast to account for mass flow, the heat-induced pressure change is primarily responsible for changes in voltage (H(+) pump deactivation) as well as for the ion fluxes. The pCa transient precedes the pCl- and pH responses, but not the voltage change. Since the apoplastic pCl decrease (Cl(-) increase) occurs after the pCa increase (Ca(2+) decrease) and after the depolarization, we argue that the Cl(-) efflux is a consequence of the Ca(2+) response, but has no part in the depolarization. Kinetic analysis reveals that pH- and pCl changes are interrelated, indicated by the action of the anion channel antagonist NPPB, which inhibits both pCl- and pH changes. It is suggested that efflux of organic anions into the apoplast causes the pH increase rather than the deactivation of the plasma membrane H(+) pump. Since there is considerably more information in ion activity changes than in a single action- or variation potential and heat-induced ion fluxes occur more reliably than voltage changes, released by milder stimuli, they are considered systemic signalling components superior to voltage.

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.

TOK homologue in Neurospora crassa: first cloning and functional characterization of an ion channel in a filamentous fungus

In contrast to animal and plant cells, very little is known of ion channel function in fungal physiology. The life cycle of most fungi depends on the "filamentous" polarized growth of hyphal cells; however, no ion channels have been cloned from filamentous fungi and comparatively few preliminary recordings of ion channel activity have been made. In an attempt to gain an insight into the role of ion channels in fungal hyphal physiology, a homolog of the yeast K(+) channel (ScTOK1) was cloned from the filamentous fungus, Neurospora crassa. The patch clamp technique was used to investigate the biophysical properties of the N. crassa K(+) channel (NcTOKA) after heterologous expression of NcTOKA in yeast. NcTOKA mediated mainly time-dependent outward whole-cell currents, and the reversal potential of these currents indicated that it conducted K(+) efflux. NcTOKA channel gating was sensitive to extracellular K(+) such that channel activation was dependent on the reversal potential for K(+). However, expression of NcTOKA was able to overcome the K(+) auxotrophy of a yeast mutant missing the K(+) uptake transporters TRK1 and TRK2, suggesting that NcTOKA also mediated K(+) influx. Consistent with this, close inspection of NcTOKA-mediated currents revealed small inward K(+) currents at potentials negative of E(K). NcTOKA single-channel activity was characterized by rapid flickering between the open and closed states with a unitary conductance of 16 pS. NcTOKA was effectively blocked by extracellular Ca(2+), verapamil, quinine, and TEA(+) but was insensitive to Cs(+), 4-aminopyridine, and glibenclamide. The physiological significance of NcTOKA is discussed in the context of its biophysical properties.

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.

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

Calcium and proton ion fluxes were mapped at the growing apices of two hyphal organisms, the oomycete Saprolegnia ferax and the ascomycete Neurospora crassa and pseudohyphal Saccharomyces cerevisiae using self-referencing ion-selective probes. S. ferax exhibited well-defined transport zones absent in N. crassa. Ca2+ fluxes were located within 8 microm of the growing hyphal tip; the net Ca2+ flux was either inward (75% of all experiments) or outward. The inward component of the net flux was inhibited by Gd3+, known to inhibit Ca2+ permeable stretch-activated channels. Because the Ca2+ flux is located at the region of maximal hyphal expansion, exocytosis may contribute to Ca2+ efflux, in addition to the stretch-activated channel mediated influx. Maximal inward H+ flux was observed 10-30 microm behind the hyphal tip where peak mitochondria densities taper off at the onset of a vacuolation zone, presumably due to highly localized H+ cotransporter activity. By contrast, N. crassa exhibited no net Ca2+ flux and a consistently inward H+ flux (93% of all experiments) that was homogeneously distributed up to 60 microm behind the hyphal apex. Both hyphal organisms have similar tip morphology and growth rates, and are reported to have tip-high cytosolic Ca2+ gradients associated with growth. Only S. ferax exhibited tip-localized Ca2+ fluxes and a well defined H+ influx zone just behind the tip. Differences in ecological habitats and cytology--S. ferax is an aquatic organism that grows as a migrating plug of cytoplasm while N. crassa is normally terrestrial with a cytoplasm-rich mycelium and highly active cytoplasmic streaming behind the growing margin--may account for the differences in the 'architecture' of ion transport occurring during the process of tip growth. Net Ca2+ efflux and H+ influx of growing S. cerevisiae pseudohyphae were also measured but localization was not possible due to small cell size.