Auxin- and pH-induced guttation in Phycomyces sporangiophores: relation between guttation and diminished elongation growth.

Guttation, the formation of exudation water, is widespread among plants and fungi, yet the underlying mechanisms remain largely unknown. We describe the conditions for inducing guttation in sporangiophores of the mucoracean fungus, Phycomyces blakesleeanus. Cultivation on peptone-enriched potato dextrose agar elicits vigorous guttation mainly below the apical growing zone, while sporangiophores raised on a glucose-mineral medium manifest only moderate guttation. Mycelia do not guttate irrespective of the employed media. The topology of guttation droplets allows identifying the non-growing part of the sporangiophore as a guttation zone, which responds to humidity and medium composition in ways that become relevant for turgor homeostasis and thus the sensor physiology of the growing zone. Apparently, the entire sporangiophore, rather than exclusively the growing zone, participates in signal reception and integration to generate a common growth output. Exogenous auxin applied to the growing zones elicits two correlated responses: (i) formation of guttation droplets in the growing and transition zones below the sporangium and (ii) a diminution of the growth rate. In sporangiophore populations, guttation-induction by exogenous control buffer occurs at low frequencies; the bias for guttation increases with increasing auxin concentration. Synthetic auxins and the transport inhibitor NPA suppress guttation completely, but leave growth rates largely unaffected. Mutants C2 carA and C148 carA madC display higher sensitivities for auxin-induced guttation compared to wild type. A working model for guttation includes aquaporins and mechanosensitive ion channels that we identified in Phycomyces by sequence domain searches.

Functions of polyamines in growth and development of Phycomyces blakesleeanus wild-type and mutant strains.

Polyamines are ubiquitous polycationic molecules with multiple effects. Spermidine was present in all the life stages of Phycomyces blakesleeanus, fulfilled the physiological requirement for polyamines during germination, and became most abundant at the emergence of germinating tubes. Putrescine was not found in resting spores or in stationary cultures, but was synthesized during apical growth and greatly exceeded spermidine in fast-growing stages of the vegetative and sexual life cycles. Changes in the polyamines did not correlate with the various stages of sporulation. Ornithine decarboxylase was so strongly inhibited in vitro by its product, putrescine, that it would completely block the enzyme if not compartmentalized away. 1,4-Diamino-2-butanone inhibited mycelial growth throughout the vegetative cycle without killing the cells. The inhibition was counteracted very effectively by putrescine, which acts as a close analog of the inhibitor, and very little by spermidine. Four independent spe mutants were obtained by a procedure that selected for resistance to diaminobutanone among functionally-uninucleate spores that survived exposure to N-methyl-N'-nitro-N-nitrosoguanidine. The stability of the enzyme, in vivo and in vitro, and its inhibition by diaminobutanone in vitro were the same in the wild type and in the mutants. Two of these were hypomorph mutants, with lower affinity of their ornithine decarboxylase for its substrate, ornithine, and lower maximal velocity. The other two were hypermorph transport mutants; we propose that they are affected in a protein that binds putrescine and its analogs for transport across the plasmalemma and sequestration away from the active enzyme. The transport mutants concentrated the exogenous diaminobutanone and the endogenous putrescine in inactive compartments; the highest enzyme activity was reached when the plasmalemma of the mutants was permeabilized with diethylaminoethyl dextran.

Biotransformation of selenium in the mycelium of the fungus Phycomyces blakesleeanus.

Biotransformation of toxic selenium ions to non-toxic species has been mainly focused on biofortification of microorganisms and production of selenium nanoparticles (SeNPs), while far less attention is paid to the mechanisms of transformation. In this study, we applied a combination of analytical techniques with the aim of characterizing the SeNPs themselves as well as monitoring the course of selenium transformation in the mycelium of the fungus Phycomyces blakesleeanus. Red coloration and pungent odor that appeared after only a few hours of incubation with 10 mM Se+4 indicate the formation of SeNPs and volatile methylated selenium compounds. SEM-EDS confirmed pure selenium NPs with an average diameter of 57 nm, which indicates potentially very good medical, optical, and photoelectric characteristics. XANES of mycelium revealed concentration-dependent mechanisms of reduction, where 0.5 mM Se+4 led to the predominant formation of Se-S-containing organic molecules, while 10 mM Se+4 induced production of biomethylated selenide (Se-2) in the form of volatile dimethylselenide (DMSe) and selenium nanoparticles (SeNPs), with the SeNPs/DMSe ratio rising with incubation time. Several structural forms of elemental selenium, predominantly monoclinic Se8 chains, together with trigonal Se polymer chain, Se8 and Se6 ring structures, were detected by Raman spectroscopy.

Identification of the ergC gene involved in polyene drug sensitivity in the Mucorales species Phycomyces blakesleeanus.

BACKGROUND: A strain of Phycomyces blakesleeanus (Mucorales, Mucoromycota) that was previously isolated after ultraviolet mutagenesis has altered responses to polyene antifungal drugs, sterol profiles, and phototropism of its sporangia. In this study, the genetic basis for these changes was sought. METHODS AND RESULTS: Two base pair substitutions were identified in the mutant within a P. blakelesleeanus gene that is homologous to others characterized from fungi, such as the Saccharomyces cerevisiae ERG3 gene, encoding sterol Δ5,6-desaturase. The polyene resistance and growth reduction phenotypes co-segregated with mutations in the gene in genetic crosses. The P. blakelesleeanus wild type ergC gene complemented a S. cerevisiae deletion strain of ERG3. CONCLUSIONS: This gene discovery may contribute towards better antifungal use in treating mucormycoses diseases caused by related species in the order Mucorales.

Signal transduction in Phycomyces sporangiophores: columella as a novel sensory organelle mediating auxin-modulated growth rate and membrane potential.

The growing zone (GZ) of the unicellular coenocytic sporangiophore of Phycomyces blakesleeanus represents the site of stimulus reception (light, gravity, gas) and stimulus response, i.e., local modulations of the elongation growth, which may result, in dependence of the stimulus direction, in tropic bending. Until now, evidence for a possible participation of the columella in sensory reception is absent. We confirm with light microscopy earlier studies that show that the GZ and the columella are not separated by a membrane or cell wall, but rather form a spatial continuum that allows free exchange of cytoplasm and organelle transport. Evidence is presented that the columella is responsive to external stimuli. Columellae, from which spores and sporangial cell wall had been removed, respond to exogenous auxin with a local depolarization of the membrane potential and an increased growth rate of the GZ. In contrast, auxin applied to the GZ causes a decrease of the growth rate irrespective of the presence or absence of sporangia. The response pattern is specific and relevant for the sensory reception of Phycomyces, because the light-insensitive mutant C148carAmadC, which lacks the RAS-GAP protein MADC, displays abnormal IAA sensitivity and membrane depolarization. We argue that the traditional concept of the GZ as the only stimulus-sensitive zone should be abandoned in favor of a model in which GZ and columella operate as a single entity capable to orchestrate a multitude of stimulus inputs, including auxin, to modulate the membrane potential and elongation growth of the GZ.

Tangent algorithm for photogravitropic balance in plants and Phycomyces blakesleeanus: Roles for EHB1 and NPH3 of Arabidopsis thaliana.

Plant organs that are exposed to continuous unilateral light reach in the steady-state a photogravitropic bending angle that results from the mutual antagonism between the photo- and gravitropic responses. To characterize the interaction between the two tropisms and their quantitative relationship we irradiated seedlings of Arabidopsis thaliana that were inclined at various angles and determined the fluence rates of unilateral blue light required to compensate the gravitropism of the inclined hypocotyls. We found the compensating fluence rates to increase with the tangent of the inclination angles (0° < γ < 90° or max. 120°) and decrease with the cotangent (90°< γ < 180° or max. 120°of the inclination angles. The tangent dependence became also evident from analysis of previous data obtained with Avena sativa and the phycomycete fungus, Phycomyces blakesleeanus. By using loss-of function mutant lines of Arabidopsis, we identified EHB1 (enhanced bending 1) as an essential element for the generation of the tangent and cotangent relationships. Because EHB1 possesses a C2-domain with two putative calcium binding sites, we propose that the ubiquitous calcium dependence of gravi- and phototropism is in part mediated by Ca2+-bound EHB1. Based on a yeast-two-hybrid analysis we found evidence that EHB1 does physically interact with the ARF-GAP protein AGD12. Both proteins were reported to affect gravi- and phototropism antagonistically. We further showed that only AGD12, but not EHB1, interacts with its corresponding ARF-protein. Evidence is provided that AGD12 is able to form homodimers as well as heterodimers with EHB1. On the basis of these data we present a model for a mechanism of early tropism events, in which Ca2+-activated EHB1 emerges as the central processor-like element that links the gravi- and phototropic transduction chains and that generates in coordination with NPH3 and AGD12 the tangent / cotangent algorithm governing photogravitropic equilibrium.

Helical growth during the phototropic response, avoidance response, and in stiff mutants of Phycomyces blakesleeanus.

The sporangiophores of Phycomyces blakesleeanus have been used as a model system to study sensory transduction, helical growth, and to establish global biophysical equations for expansive growth of walled cells. More recently, local statistical biophysical models of the cell wall are being constructed to better understand the molecular underpinnings of helical growth and its behavior during the many growth responses of the sporangiophores to sensory stimuli. Previous experimental and theoretical findings guide the development of these local models. Future development requires an investigation of explicit and implicit assumptions made in the prior research. Here, experiments are conducted to test three assumptions made in prior research, that (a) elongation rate, (b) rotation rate, and (c) helical growth steepness, R, of the sporangiophore remain constant during the phototropic response (bending toward unilateral light) and the avoidance response (bending away from solid barriers). The experimental results reveal that all three assumptions are incorrect for the phototropic response and probably incorrect for the avoidance response but the results are less conclusive. Generally, the experimental results indicate that the elongation and rotation rates increase during these responses, as does R, indicating that the helical growth steepness become flatter. The implications of these findings on prior research, the "fibril reorientation and slippage" hypothesis, global biophysical equations, and local statistical biophysical models are discussed.

Light regulates a Phycomyces blakesleeanus gene family similar to the carotenogenic repressor gene of Mucor circinelloides.

The transcription of about 5-10 % of the genes in Phycomyces blakesleeanus is regulated by light. Among the most up-regulated, we have identified four genes, crgA-D, with similarity to crgA of Mucor circinelloides, a gene encoding a repressor of light-inducible carotenogenesis. The four proteins have the same structure with two RING RING Finger domains and a LON domain, suggesting that they could act as ubiquitin ligases, as their M. circinelloides homolog. The expression of these genes is induced by light with different thresholds as in other Mucoromycotina fungi like Blakeslea trispora and M. circinelloides. Only the P. blakesleeanus crgD gene could restore the wild type phenotype in a M. circinelloides null crgA mutant suggesting that P. blakesleeanus crgD is the functional homolog of crgA in M. circinelloides. Despite their sequence similarity it is possible that the P. blakesleeanus Crg proteins do not participate in the regulation of beta-carotene biosynthesis since none of the carotene-overproducing mutants of P. blakesleeanus had mutations in any of the crg genes. Our results provide further support of the differences in the regulation of the biosynthesis of beta-carotene in these two Mucoromycotina fungi.

Measurement of Phototropism of the Sporangiophore of Phycomyces blakesleeanus.

The giant sporangiophore, fruiting body, of the fungus Phycomyces blakesleeanus is a single cell that grows guided by several environmental signals, including light. The phototropic response has been investigated in detail. Three proteins, the components of a photoreceptor and transcription factor complex and a regulator of the signal transduction protein Ras, participate in the signal transduction pathway. We describe the basic methods for characterizing phototropic bending and the correlated elongation and rotation responses of the sporangiophore.

Grafting as a Tool for Study of Phototropism in Phycomyces.

Physiological responses, such as phototropism and carotenogenesis, are usually exhibited through a specific cascade composed of several gene functions. The number of such gene functions can be determined by means of complementation analysis. For this purpose, a procedure is needed to produce heterokaryons easily and with a high success rate. Here, we present a method of grafting sporangiophores from different mutants to obtain heterokaryotic regenerates at the graft union, based on the large size of the sporangiophore and its remarkable regeneration capability.

A Statistical Model of Expansive Growth in Plant and Fungal Cells: The Case of Phycomyces.

Expansive growth is a process by which walled cells of plants, algae, and fungi use turgor pressure to mediate irreversible wall deformation and regulate their shape and volume. The molecular structure of the primary cell wall must therefore perform multiple functions simultaneously, including providing structural support by combining elastic and irreversible deformation and facilitating the deposition of new material during growth. This is accomplished by a network of microfibrils and tethers composed of complex polysaccharides and proteins that can dynamically mediate the network topology via periodic detachment and reattachment events. Lockhart and Ortega have provided crucial macroscopic understanding of the expansive growth process through global biophysical models, but these models lack the connection to molecular processes that trigger network rearrangements in the wall. Interestingly, the helical growth of the fungal sporangiophores of Phycomyces blakesleeanus is attributed to a limited region (called the growth zone) where microfibrils are deposited, followed by reorientation and slip. Based on past evidence of dominant shear strain between microfibrils (slippage), we propose a mechanistic model of a network of sliding fibrils connected by tethers. A statistical approach is introduced to describe the population behavior of tethers that have elastic properties and the ability to break and reform in time. These properties are responsible for global cell wall mechanics such as creep and stress relaxation. Model predictions are compared with experiments from literature on stress relaxation and turgor pressure step up for the growing cells of P. blakesleeanus, which are later extended to incised pea (Pisum sativus L.) and the algae Chara corallina using the unique dimensionless number Πpe for each species. To our knowledge, this research is the first attempt to use a statistical approach to model the cell wall during expansive growth, and we believe it provides critical insights on cell wall dynamics at a molecular level.

Mystique of Phycomyces blakesleeanus is a peculiar mitochondrial genetic element that is highly variable in DNA sequence while subjected to strong negative selection.

A DNA region in the mitochondrial genome of the fungus Phycomyces blakesleeanus (Mucorales, Mucoromycota) was characterized in a population of wild-type strains. The region encodes a predicted protein similar to the reverse transcriptases encoded by mitochondrial retroplasmids of Neurospora species and other Sordariomycetes (Ascomycota), but is uncommon in other fungi. DNA sequences of this element, named mystique, are highly variable between the strains, having greater than 2.5% divergence, yet most of the nucleotide differences fall in codon positions that do not change the amino acid sequence. The high proportion of polymorphisms coupled to the rarity of nonsynonymous changes suggests that mystique is subject to counteracting forces of hypermutation and purifying selection. However, while evidence for negative selection may infer that the element provides a fitness benefit, some strains of P. blakesleeanus do not have the element and grow equivalently well as those strains with it. A mechanism to explain the variability between the mystique alleles is proposed, of error-prone replication through an RNA intermediate, reverse transcription and reintegration of the element into the mitochondrial genome.

The effect of auxin (indole-3-acetic acid) on the growth rate and tropism of the sporangiophore of Phycomyces blakesleeanus and identification of auxin-related genes.

The roles of fungal auxins in the regulation of elongation growth, photo-, and gravitropism are completely unknown. We analyzed the effects of exogenous IAA (indole-3-acetic acid), various synthetic auxins including 1-NAA (1-naphthaleneacetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid), and the auxin transport inhibitor NPA (N-1-naphtylphtalamic acid) on the growth rate and bending of the unicellular sporangiophore of the zygomycete fungus, Phycomyces blakesleeanus. Sporangiophores that were submerged in an aqueous buffer responded to IAA with a sustained enhancement of the growth rate, while 1-NAA, 2,4-D, and NPA elicited an inhibition. In contrast, sporangiophores kept in air responded to IAA with a 20 to 40% decrease of the growth rate, while 1-NAA and NPA elicited an enhancement. The unilateral and local application of IAA in the growing zone of the sporangiophore elicited in 30 min a moderate negative tropic bending in wild type C2 and mutant C148madC, which was, however, partially masked by a concomitant avoidance response caused by the aqueous buffer. Auxin transport-related genes ubiquitous in plants were found in a BLAST search of the Phycomyces genome. They included members of the AUX1 (auxin influx carrier protein 1), PILS (PIN-LIKES, auxin transport facilitator protein), and ABCB (plant ATP-binding cassette transporter B) families while members of the PIN family were absent. Our observations imply that IAA represents an intrinsic element of the sensory transduction of Phycomyces and that its mode of action must very likely differ in several respects from that operating in plants.

A Ras GTPase associated protein is involved in the phototropic and circadian photobiology responses in fungi.

Light is an environmental signal perceived by most eukaryotic organisms and that can have major impacts on their growth and development. The MadC protein in the fungus Phycomyces blakesleeanus (Mucoromycotina) has been postulated to form part of the photosensory input for phototropism of the fruiting body sporangiophores, but the madC gene has remained unidentified since the 1960s when madC mutants were first isolated. In this study the madC gene was identified by positional cloning. All madC mutant strains contain loss-of-function point mutations within a gene predicted to encode a GTPase activating protein (GAP) for Ras. The madC gene complements the Saccharomyces cerevisiae Ras-GAP ira1 mutant and the encoded MadC protein interacts with P. blakesleeanus Ras homologs in yeast two-hybrid assays, indicating that MadC is a regulator of Ras signaling. Deletion of the homolog in the filamentous ascomycete Neurospora crassa affects the circadian clock output, yielding a pattern of asexual conidiation similar to a ras-1 mutant that is used in circadian studies in N. crassa. Thus, MadC is unlikely to be a photosensor, yet is a fundamental link in the photoresponses from blue light perceived by the conserved White Collar complex with Ras signaling in two distantly-related filamentous fungal species.

The inhibition of mating in Phycomyces blakesleeanus by light is dependent on the MadA-MadB complex that acts in a sex-specific manner.

Light is an environmental signal that influences reproduction in the Mucoromycotina fungi, as it does in many other species of fungi. Mating in Phycomyces blakesleeanus is inhibited by light, but the molecular mechanisms for this inhibition are uncharacterized. In this analysis, the role of the light-sensing MadA-MadB complex in mating was tested. The MadA-MadB complex is homologous to the Neurospora crassa White Collar complex. Three genes required for cell type determination in the sex locus or pheromone biosynthesis are transcriptionally-regulated by light and are controlled by MadA and MadB. This regulation acts through the plus partner, indicating that the inhibitory effect of light on mating is executed through only one of the two sexes. These results are an example whereby the mating types of fungi have acquired sex-specific properties beyond their role in conferring cell-type identity, and provide insight into how sex-determining chromosomal regions can expand the traits they control.

Growth inhibition of fungus Phycomyces blakesleeanus by anion channel inhibitors anthracene-9-carboxylic and niflumic acid attained through decrease in cellular respiration and energy metabolites.

Increasing resistance of fungal strains to known fungicides has prompted identification of new candidates for fungicides among substances previously used for other purposes. We have tested the effects of known anion channel inhibitors anthracene-9-carboxylic acid (A9C) and niflumic acid (NFA) on growth, energy metabolism and anionic current of mycelium of fungus Phycomyces blakesleeanus. Both inhibitors significantly decreased growth and respiration of mycelium, but complete inhibition was only achieved by 100 and 500 µM NFA for growth and respiration, respectively. A9C had no effect on respiration of human NCI-H460 cell line and very little effect on cucumber root sprout clippings, which nominates this inhibitor for further investigation as a potential new fungicide. Effects of A9C and NFA on respiration of isolated mitochondria of P. blakesleeanus were significantly smaller, which indicates that their inhibitory effect on respiration of mycelium is indirect. NMR spectroscopy showed that both A9C and NFA decrease the levels of ATP and polyphosphates in the mycelium of P. blakesleeanus, but only A9C caused intracellular acidification. Outwardly rectifying, fast inactivating instantaneous anionic current (ORIC) was also reduced to 33±5 and 21±3 % of its pre-treatment size by A9C and NFA, respectively, but only in the absence of ATP. It can be assumed from our results that the regulation of ORIC is tightly linked to cellular energy metabolism in P. blakesleeanus, and the decrease in ATP and polyphosphate levels could be a direct cause of growth inhibition.

The interactions of vanadate monomer with the mycelium of fungus Phycomyces blakesleeanus: reduction or uptake?

The possibility of reduction of vanadate monomer in the mycelium of fungus Phycomyces blakesleeanus was investigated in this study by means of polarography. Control experiments were performed with vanadyl [V(IV)] and vanadate [V(V)] in 10 mM Hepes, pH 7.2. Addition of P. blakesleeanus mycelium resulted in disappearance of all V(IV) polarographic waves recorded in the control. This points to the uptake of all available V(IV) by the mycelium, up to 185 µmol/gFW, and suggests P. blakesleeanus as a potential agent in V(IV) bioremediation. Polarographic measurements of mycelium with low concentrations (0.1-1 mM) of V(V), that only allows the presence of monomer, showed that fungal mycelia removes around 27% of V(V) from the extracellular solution. Uptake was saturated at 104 ± 2 µmol/gFW which indicates excellent bioaccumulation capability of P. blakesleeanus. EPR, 51V NMR and polarographic experiments showed no indications of any measurable extracellular complexation of V(V) monomer with fungal exudates, reduction by the mycelium or adsorption to the cell wall. Therefore, in contrast to vanadium oligomers, vanadate monomer interactions with the mycelium are restricted to its transport into the fungal cell, probably by a phosphate transporter.

Fungi as Architects of the Rimstone Dams in Huanglong, NSD, Sichuan, China.

The Huanglong park area of the Sichuan Province of China is a unique scenic area of the world. It is known for its thousands of aquamarine-colored pools that are formed behind naturally formed rimstone dams of travertine (calcite) along a cold water stream. The travertine, based on its crystalline structural analysis, is of biological origin. This makes sense since the temperature of the waters of Huanglong varies from 5 to 7 °C and thus geochemical crystallization does not occur as it does in other locations around the world possessing thermal pools whose structures are primarily formed through cooling processes. Fungi and bacteria were discovered associated with both leaves associated with the calcite dams as well as in the older parts of well-established dams. Several species of Phytium, a phycomycete and an endophyte, accounted for over 45 % of all of the fungi successfully isolated from the well-established dam samples and at least 85 % in the floating leaf samples. Saprolegnia spp. (Phycomycetes) along with Phoma spp. (Ascomycetes) were noted along with Mortierella sp. as other dam-associated fungi. The fungal hyphae observed on dead leaf material as well as in the calcite dams directly served as nucleation points for the formation of crystalline CaCO3. Eventually, these crystals grow large enough to fuse to make calcite plates which form the main structural feature of all of the travertine dams in this area. Interestingly, each of the individual crystals associated with the dams has an associated hole in its core where a fungal hypha used to reside as observed by scanning electron microscopy. While diatoms were present in the analysis, they too seem to contribute to the structure of the dams but in a minor way. The only bacteria isolated from the older dam of this aquatic environment were Pseudomonas spp. and their role in dam formation is uncertain. Huanglong is a unique and beautiful place, and the water features present in this area can definitely be attributed to those fungal architects that encourage calcite crystal formation.