A mutation in the Neurospora crassa actin gene results in multiple defects in tip growth and branching

Effect of pyriofenone on the infection processes and cytological features of Blumeria graminis on wheat leaves

Pyriofenone demonstrates outstanding efficacy in controlling powdery mildew. We investigated the impact of pyriofenone on the infection processes and cytological features of Blumeria graminis f. sp. tritici on wheat leaves. The preventive application of pyriofenone before inoculation did not inhibit conidial germination but effectively suppressed both appressorial and haustorial formation. Notably, haustorial formation was effectively inhibited, resulting in the complete suppression of successive lesion development and sporulation. Curative application of pyriofenone after inoculation also inhibited lesion expansion and sporulation. Furthermore, it had considerable impact on the morphogenesis of powdery mildew fungus. We observed multi-formed secondary appressoria, shrunken or bifurcated hyphae, abnormal conidiophores, and clubbed conidia-like structures. Subsequently, we employed a histochemical approach to analyze the localization of essential components for the polar growth of fungal hyphae. Pyriofenone induced mislocalization of the actin cytoskeleton, β-glucan and cytoplasmic vesicles, although it did not affect tubulin orientation.

The CHY-type zinc finger protein MoChy1 is involved in polarized growth, conidiation, autophagy and pathogenicity of Magnaporthe oryzae

Polarized growth is critical for the development of filamentous phytopathogens, and the CHY-type zinc finger protein Chy1 regulates microtubule assembly to influence polarized growth and thereby affect plant infections. However, the biological role of a Chy1 homolog MoChy1 remains unknown in Magnaporthe oryzae. We found here that the MoChy1-GFP was distributed in the cytoplasm outside the vacuole in hyphae and localized mainly to the vacuole compartments as the appressorium matured. The Mochy1 mutants showed an extremely slow growth rate, curved and branched mycelium, reduced conidiation, and a smaller size in the appressorium. Meanwhile, the Mochy1 mutants showed increased sensitivity to benomyl, damaged microtubule cytoskeleton, and mislocalized polarisome protein MoSpa2 and chitin synthase MoChs6 in hyphae. Compared to Guy11, the Mochy1 mutants exhibited increased sensitivity to H2O2, impaired ability to eliminate host-derived ROS and reduced penetration into host plants, resulting in a strong reduction in pathogenicity of Mochy1 mutants. Furthermore, the Mochy1 mutants also exhibited defects in chitin distribution, osmotic stress tolerance, and septin ring organization during appressorium differentiation and fungal development. Nonselective autophagy was negatively regulated in Mochy1 mutants compared to Guy11. In summary, MoChy1 plays multiple roles in fungal polar growth and full virulence of M. oryzae.

Chaetoglobosin A Contributes to the Antagonistic Action of Chaetomium globosum Strain 61239 Toward the Apple Valsa Canker Pathogen Cytospora mali

Apple Valsa canker (AVC) weakens apple trees and significantly reduces apple production in China and other East Asian countries. Thus far, very few AVC-targeting biocontrol resources have been described. Here, we present a thorough description of a fungal isolate (Chaetomium globosum, 61239) that has strong antagonistic action toward the AVC causal agent Cytospora mali. Potato dextrose broth culture filtrate of strain 61239 completely suppressed the mycelial growth of C. mali on potato dextrose agar, and strongly constrained the development of AVC lesions in in vitro infection assays. ultra-performance liquid chromatography (UPLC) and HPLC-MS/MS investigations supported the conclusion that strain 61239 produces chaetoglobosin A, an antimicrobial metabolite that inhibits C. mali. Using genome sequencing, we discovered a gene cluster in strain 61239 that may be responsible for chaetoglobosin A production. Two of the cluster's genes-cheA, a PKS-NRPS hybrid enzyme, and cheB, an enoyl reductase-were individually silenced, which significantly decreased chaetoglobosin A accumulation as well as the strain's antagonistic activity against C. mali. Together, the findings of our investigation illustrate the potential use of Chaetomium globosum for the management of AVC disease and emphasize the significant contribution of chaetoglobosin A to the antagonistic action of strain 61239.

Obligate sexual reproduction of a homothallic fungus closely related to the Cryptococcus pathogenic species complex

eLife digest

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Fungi are enigmatic organisms that flourish in soil, on decaying plants, or during infection of animals or plants. Growing in myriad forms, from single-celled yeast to multicellular molds and mushrooms, fungi have also evolved a variety of strategies to reproduce. Normally, fungi reproduce in one of two ways: either they reproduce asexually, with one individual producing a new individual identical to itself, or they reproduce sexually, with two individuals of different 'mating types' contributing to produce a new individual. However, individuals of some species exhibit 'homothallism' or self-fertility: these individuals can produce reproductive cells that are universally compatible, and therefore can reproduce sexually with themselves or with any other cell in the population.

Homothallism has evolved multiple times throughout the fungal kingdom, suggesting it confers advantage when population numbers are low or mates are hard to find. Yet some homothallic fungi been overlooked compared to heterothallic species, whose mating types have been well characterised. Understanding the genetic basis of homothallism and how it evolved in different species can provide insights into pathogenic species that cause fungal disease.

With that in mind, Passer, Clancey et al. explored the genetic basis of homothallism in Cryptococcus depauperatus, a close relative of C. neoformans, a species that causes fungal infections in humans. A combination of genetic sequencing techniques and experiments were applied to analyse, compare, and manipulate C. depauperatus' genome to see how this species evolved self-fertility.

Passer, Clancey et al. showed that C. depauperatus evolved the ability to reproduce sexually by itself via a unique evolutionary pathway. The result is a form of homothallism never reported in fungi before. C. depauperatus lost some of the genes that control mating in other species of fungi, and acquired genes from the opposing mating types of a heterothallic ancestor to become self-fertile.

Passer, Clancey et al. also found that, unlike other Cryptococcus species that switch between asexual and sexual reproduction, C. depauperatus grows only as long, branching filaments called hyphae, a sexual form. The species reproduces sexually with itself throughout its life cycle and is unable to produce a yeast (asexual) form, in contrast to other closely related species.

This work offers new insights into how different modes of sexual reproduction have evolved in fungi. It also provides another interesting case of how genome plasticity and evolutionary pressures can produce similar outcomes, homothallism, via different evolutionary paths. Lastly, assembling the complete genome of C. depauperatus will foster comparative studies between pathogenic and non-pathogenic Cryptococcus species.

Disruption of the Trichoderma reesei gul1 gene stimulates hyphal branching and reduces broth viscosity in cellulase production

Hyphal morphology is considered to have a close relationship with the production level of secreted proteins by filamentous fungi. In this study, the gul1 gene, which encodes a putative mRNA-binding protein, was disrupted in cellulase-producing fungus Trichoderma reesei. The hyphae of Δgul1 strain produced more lateral branches than the parent strain. Under the condition for cellulase production, disruption of gul1 resulted in smaller mycelial clumps and significantly lower viscosity of fermentation broth. In addition, cellulase production was improved by 22% relative to the parent strain. Transcriptome analysis revealed that a set of genes encoding cell wall remodeling enzymes as well as hydrophobins were differentially expressed in the Δgul1 strain. The results suggest that the regulatory role of gul1 in cell morphogenesis is likely conserved in filamentous fungi. To our knowledge, this is the first report on the engineering of gul1 in an industrially important fungus.

Actin guides filamentous rhizoid growth and morphogenesis in the zoosporic fungus Chytriomyces hyalinus

The advantage of filamentous growth to the fungal lifestyle is so great that it arose multiple times. Most zoosporic fungi from phylum Chytridiomycota exhibit a monocentric thallus form consisting of anucleate filamentous rhizoids that anchor reproductive sporangia to substrata and absorb nutrients. Actin function during polarized growth and cytokinesis is well documented across eukaryotes, but its role in sculpting nonhyphal, nonyeast fungal cells is unknown. We sought to provide a basis for comparing actin organization among major fungal lineages and to investigate the effects of actin disruption on morphogenesis in a monocentric thallus. Using fluorescence microscopy, we observed fixed, rhodamine phalloidin-stained actin in chemically fixed Chytriomyces hyalinus, exemplifying monocentric thallus development within the diverse, zoosporic phylum Chytridiomycota. We also compared rhizoid lengths and rhizoid branching of thalli incubated with the actin inhibitor latrunculin B to determine the effects of actin disruption on morphology. Actin was concentrated at the tips of growing rhizoids. Actin cables typically formed cortical, parallel arrays in hyphae, but in mature sporangia they were concentrated in a funnel-shaped array in the central region. Thalli treated with latrunculin B had shorter rhizoids with fewer branches than controls. In both hyphae and monocentric thalli, actin localization coincides with active, polarized growth and cytokinesis. Specific actin localization patterns are largely shared between monocentric species but differ significantly from patterns observed in hyphae. Actin integrity is critical for sustaining filamentous growth in all fungi.

Deletion of the small GTPase rac1 in Trichoderma reesei provokes hyperbranching and impacts growth and cellulase production

Background

Trichoderma reesei is widely known for its enormous protein secretion capacity and as an industrially relevant producer of cellulases and hemicellulases. Over the last decades, rational strain engineering was applied to further enhance homologous and heterologous enzyme yields. The introduction of hyperbranching is believed to increase protein secretion, since most exocytosis is located at the hyphal apical tip. There are several genetic modifications which can cause hyperbranching, for example the deletion of the small Rho GTPase rac. Rac plays a crucial role in actin dynamics and is involved in polarisation of the cell during germination and apical extension of the hyphae.

Results

We deleted rac1 in a T. reesei strain with an ectopically overexpressed endoglucanase, CEL12A, under Pcdna1 control. This deletion provoked a hyperbranching phenotype and strong apolar growth during germination and in mature hyphae. The strains displayed dichotomous branching and shorter total mycelium length with a larger hyphal diameter. Δrac1 strains exhibited a decreased radial growth on solid media. Biomass formation in liquid cultures was carbon source dependent; similar to the reference strain during growth on lactose, increased on d-glucose and slightly enhanced on cellulose. While extracellular cellulase activities remained at parental strain levels on d-glucose and cellulose, the specific activity on lactose cultures was increased up to three times at 72 h accompanied by an upregulation of transcription of the main cellulases. Although the morphology of the Δrac1 strains was considerably altered, the viscosity of the culture broth in fed-batch cultivations were not significantly different in comparison to the parental strain.

Conclusions

Deletion of the small Rho GTPase rac1 changes the morphology of the hyphae and provokes hyperbranching without affecting viscosity, independent of the carbon source. In contrast, biomass formation and cellulase production are altered in a carbon source dependent manner in the Δrac1 strains.

The actin motor MYO-5 effect in the intracellular organization of Neurospora crassa

In filamentous fungi, polarized growth is the result of vesicle secretion at the hyphal apex. Motor proteins mediate vesicle transport to target destinations on the plasma membrane via actin and microtubule cytoskeletons. Myosins are motor proteins associated with actin filaments. Specifically, class V myosins are responsible for cargo transport in eukaryotes. We studied the dynamics and localization of myosin V in wild type hyphae of Neurospora crassa and in hyphae that lacked MYO-5. In wild type hyphae, MYO-5-GFP was localized concentrated in the hyphal apex and colocalized with Spitzenkörper. Photobleaching studies showed that MYO-5-GFP was transported to the apex from subapical hyphal regions. The deletion of the class V myosin resulted in a reduced rate of hyphal growth, apical hyperbranching, and intermittent loss of hyphal polarity. MYO-5 did not participate in breaking the symmetrical growth during germination but contributed in the apical organization upon establishment of polarized growth. In the Δmyo-5 mutant, actin was organized into thick cables in the apical and subapical hyphal regions, and the number of endocytic patches was reduced. The microvesicles-chitosomes observed with CHS-1-GFP were distributed as a cloud occupying the apical dome and not in the Spitzenkörper as the WT strain. The mitochondrial movement was not associated with MYO-5, but tubular vacuole position is MYO-5-dependent. These results suggest that MYO-5 plays a role in maintaining apical organization and the integrity of the Spitzenkörper and is required for normal hyphal growth, polarity, septation, conidiation, and proper conidial germination.

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkörper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkörper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.

Cytology and molecular phylogenetics of Monoblepharidomycetes provide evidence for multiple independent origins of the hyphal habit in the Fungi

The evolution of filamentous hyphae underlies an astounding diversity of fungal form and function. We studied the cellular structure and evolutionary origins of the filamentous form in the Monoblepharidomycetes (Chytridiomycota), an early-diverging fungal lineage that displays an exceptional range of body types, from crescent-shaped single cells to sprawling hyphae. To do so, we combined light and transmission electron microscopic analyses of hyphal cytoplasm with molecular phylogenetic reconstructions. Hyphae of Monoblepharidomycetes lack a complex aggregation of secretory vesicles at the hyphal apex (i.e. Spitzenkörper), have centrosomes as primary microtubule organizing centers and have stacked Golgi cisternae instead of tubular/fenestrated Golgi equivalents. The cytoplasmic distribution of actin in Monoblepharidomycetes is comparable to the arrangement observed previously in other filamentous fungi. To discern the origins of Monoblepharidomycetes hyphae, we inferred a phylogeny of the fungi based on 18S and 28S ribosomal DNA sequence data with maximum likelihood and Bayesian inference methods. We focused sampling on Monoblepharidomycetes to infer intergeneric relationships within the class and determined 78 new sequences. Analyses showed class Monoblepharidomycetes to be monophyletic and nested within Chytridiomycota. Hyphal Monoblepharidomycetes formed a clade sister to the genera without hyphae, Harpochytrium and Oedogoniomyces. A likelihood ancestral state reconstruction indicated that hyphae arose independently within the Monoblepharidomycetes lineage and in at least two other lineages. Cytological differences among monoblepharidalean and other fungal hyphae are consistent with these convergent origins.

Ascomycete fungal actins differentially support plant spatial cell and organ development

Actin interacts with a wide variety of cytoplasmic and nuclear proteins to support spatial development in nearly all eukaryotes. Null mutations in plant vegetative actins produce dramatically altered cell, tissue, and organ morphologies. Animal cytoplasmic actins (e.g., human HsACTB, HsACTG1) and some ancestral protist actins fully suppress these mutant phenotypes suggesting that some animal, plant, and protist actins share functional competence for spatial development. Considering that fungi have a phylogenetic origin closer to animals than plants, we were interested to explore whether the fungal actins may have this same capacity to function in plants and support development. We ectopically expressed actins from four highly divergent ascomycete fungi in two different Arabidopsis double vegetative actin null mutants. We found that expression of actin from the earliest diverging ascomycete subphyla, the archiascomycete Schizosaccharomyces pombe, qualitatively and quantitatively suppressed the root cell polarity and root organ developmental defects of act8/act7 mutants and the root-hairless cell elongation phenotype of act2/act8 mutants. Interestingly, the actin from the pyrenomycete Neurospora crassa was modestly effective in the suppression of vegetative actin mutant phenotypes. In contrast, actins from the saccharomycetes Saccharomyces cerevisiae and Candida albicans were unable to support any aspect of plant development, and moreover induced severe dwarfism and sterility. These data imply that basal fungi inherited an actin with full competence for spatial development from their protist ancestor and maintained it via non-progressive sequence evolution, while the later more derived fungal species lost these activities.

Actin cytoskeleton and organelle movement in the sporangiophore of the zygomycete Phycomyces blakesleeanus

Growth, photo- and gravitropism of sporangiophores of the zygomycete Phycomyces blakesleeanus occur within the apical growing zone, a cylindrical structure (diameter about 100 μm) that reaches about 1.5-2.5 mm below the tip and has growth rates up to 50 μm·min(-1) . To better understand morphogenesis and growth of the giant aerial hypha, we investigated with confocal microscopy and inhibitors the actin cytoskeleton and by in-vivo particle tracking the associated organelle movement. We found stage-1 sporangiophores (without sporangium) possess an actin cytoskeleton with polar zonation. (i) In the apex, abundant microfilaments without preferential orientation entangled numerous nuclei as well as a conspicious complex of some 200 lipid globules. Microfilament patches (≈ 1.6-μm diameter) are clustered in the tip and were found in the apical cortex, whereas short, curved microfilament bundles (≈ 2.3-μm long) prevailed in the subapex. (ii) In a transition zone downwards to the shaft, the microfilaments rearranged into a dense mat of longitudinal microfilaments that was parallel close to the periphery but more random towards the cell centre. Numerous microfilament patches were found near the cortex (≈ 10/100 μm(2) ); their number decreased rapidly in the subcortex. In contrast, the short, curved microfilament bundles were found only in the subcortex. (iii) The basal shaft segment of the sporangiophore (with central vacuole) exhibited bidirectional particle movement over long distances (velocity ≈ 2 μm·s(-1) ) along massive longitudinal, subcortical microfilament cables. The zonation of the cytoskeleton density correlated well with the local growth rates at the tip of the sporangiophore, and appears thus as a structural prerequisite for growth and bending.

The transcriptomic signature of RacA activation and inactivation provides new insights into the morphogenetic network of Aspergillus niger

RacA is the main Rho GTPase in Aspergillus niger regulating polarity maintenance via controlling actin dynamics. Both deletion and dominant activation of RacA (Rac(G18V)) provoke an actin localization defect and thereby loss of polarized tip extension, resulting in frequent dichotomous branching in the ΔracA strain and an apolar growing phenotype for Rac(G18V). In the current study the transcriptomics and physiological consequences of these morphological changes were investigated and compared with the data of the morphogenetic network model for the dichotomous branching mutant ramosa-1. This integrated approach revealed that polar tip growth is most likely orchestrated by the concerted activities of phospholipid signaling, sphingolipid signaling, TORC2 signaling, calcium signaling and CWI signaling pathways. The transcriptomic signatures and the reconstructed network model for all three morphology mutants (ΔracA, Rac(G18V), ramosa-1) imply that these pathways become integrated to bring about different physiological adaptations including changes in sterol, zinc and amino acid metabolism and changes in ion transport and protein trafficking. Finally, the fate of exocytotic (SncA) and endocytotic (AbpA, SlaB) markers in the dichotomous branching mutant ΔracA was followed, demonstrating that hyperbranching does not per se result in increased protein secretion.

Coronin is a component of the endocytic collar of hyphae of Neurospora crassa and is necessary for normal growth and morphogenesis

Coronin plays a major role in the organization and dynamics of actin in yeast. To investigate the role of coronin in a filamentous fungus (Neurospora crassa), we examined its subcellular localization using fluorescent proteins and the phenotypic consequences of coronin gene (crn-1) deletion in hyphal morphogenesis, Spitzenkörper behavior and endocytosis. Coronin-GFP was localized in patches, forming a subapical collar near the hyphal apex; significantly, it was absent from the apex. The subapical patches of coronin colocalized with fimbrin, Arp2/3 complex, and actin, altogether comprising the endocytic collar. Deletion of crn-1 resulted in reduced hyphal growth rates, distorted hyphal morphology, uneven wall thickness, and delayed establishment of polarity during germination; it also affected growth directionality and increased branching. The Spitzenkörper of Δcrn-1 mutant was unstable; it appeared and disappeared intermittently giving rise to periods of hyphoid-like and isotropic growth respectively. Uptake of FM4-64 in Δcrn-1 mutant indicated a partial disruption in endocytosis. These observations underscore coronin as an important component of F-actin remodeling in N. crassa. Although coronin is not essential in this fungus, its deletion influenced negatively the operation of the actin cytoskeleton involved in the orderly deployment of the apical growth apparatus, thus preventing normal hyphal growth and morphogenesis.

Proteomic analysis of Trichoderma atroviride reveals independent roles for transcription factors BLR-1 and BLR-2 in light and darkness

The genus Trichoderma is one of the most widely used biological control agents of plant-pathogenic fungi. The main mechanism for survival and dispersal of Trichoderma is through the production of asexual spores (conidia). The transition from filamentous growth to conidiation can be triggered by light, nutrient deprivation, and mechanical damage of the mycelium. We conducted proteomic profiling analyses of Trichoderma atroviride after a blue light pulse. The use of two-dimensional electrophoresis (2-DE) and mass spectrometry (MS) analysis allowed us to identify 72 proteins whose expression was affected by blue light. Functional category analysis showed that the various proteins are involved in metabolism, cell rescue, and protein synthesis. We determined the relationship between mRNA levels of selected genes 30 min after a light pulse and protein expression levels at different times after the pulse and found this correlation to be very weak. The correlation was highest when protein and mRNA levels were compared for the same time point. The transcription factors BLR-1 and BLR-2 are vital to the photoconidiation process; here we demonstrate that both BLR proteins are active in darkness and affect several elements at both the transcript and protein levels. Unexpectedly, in darkness, downregulation of proteins prevailed in the Δblr-1 mutant, while upregulation of proteins predominated in the Δblr-2 mutant. Our data demonstrate that the BLR proteins play roles individually and as a complex.

Actin organization and dynamics in filamentous fungi

Growth and morphogenesis of filamentous fungi is underpinned by dynamic reorganization and polarization of the actin cytoskeleton. Actin has crucial roles in exocytosis, endocytosis, organelle movement and cytokinesis in fungi, and these processes are coupled to the production of distinct higher-order structures (actin patches, cables and rings) that generate forces or serve as tracks for intracellular transport. New approaches for imaging actin in living cells are revealing important similarities and differences in actin architecture and organization within the fungal kingdom, and have yielded key insights into cell polarity, tip growth and long-distance intracellular transport. In this Review, we discuss the contribution that recent live-cell imaging and mutational studies have made to our understanding of the dynamics and regulation of actin in filamentous fungi.

Meiotic silencing in the homothallic fungus Gibberella zeae

The homothallic ascomycete fungus Gibberella zeae is an important pathogen on major cereal crops. The objective of this study was to determine whether meiotic silencing occurs in G. zeae. Cytological studies demonstrated that GFP and RFP-fusion proteins were not detected during meiosis, both in heterozygous outcrosses and homozygous selfings. The deletion of rsp-1, a homologue used for studies on meiotic silencing of Neurospora crassa, triggered abnormal ascospores from selfing, but outcrosses between the mutant and wild-type strain resulted in some ascospores with mutant phenotype (low occurrence of ascus dominance). When the ectopic mutants that carried an additional copy of rsp-1 were selfed, they primarily produced ascospores with normal shape but a few ascospores (0.23 %) were abnormal, in which both endogenous and ectopically integrated genes contained numerous point mutations. The ectopic mutants showed low occurrence of ascus dominance in outcrosses with strains that carried the wild-type allele. Approximately 10 % of ascospores were abnormal but all of the single-ascospore isolates produced normal-shaped ascospores from selfing. However, no ascus dominance was observed when the mutants were outcrossed with a sad-1 deletion mutant, which lacks the putative RNA-dependent RNA polymerase essential for meiotic silencing in N. crassa. All results were consistent with those generated from an additional gene, roa, required for ascospore morphogenesis. This study demonstrated that G. zeae possesses a functional meiotic silencing mechanism which is triggered by unpaired DNA, as in N. crassa.

Mitochondrial dysfunction resulting from the absence of mitochondrial porin in Neurospora crassa

Porin, the voltage-dependent anion-selective channel (VDAC) in the mitochondrial outer membrane, contributes to metabolism and apoptosis. VDAC function was investigated in Neurospora, an obligate aerobe with a single porin. Porinless strains are viable, with cold-sensitive growth, cytochrome deficiencies and overexpression of alternative oxidase. iTRAQ labeling of mitochondria from a porinless strain and its progenitor revealed a small group of proteins with altered expression levels in the mutant organelles. Porinless Neurospora appears to compensate not by inducing alternative pores, but by altering electron flow and nucleotide metabolism. Transcriptional and post-transcriptional mechanisms contribute to the response, reflecting the extent of porin influence.

The role of sho1 in polarized growth of Aspergillus fumigatus

Aspergillus fumigatus is an opportunistic pathogen that may cause severe invasive disease in immunocompromised patients. The filamentous fungi undergo polarized growth, searching for nutrients in the environment and causing invasive growth in tissue. Sho1 is a sensor of the high osmolarity glycerol pathway, and the sho1 mutant showed a decrease in growth rate. We found that sho1 is involved in the polarized growth of A. fumigatus. The sho1 mutation resulted in extended isotropic growth of germinating conidia followed by multiple germ tubes and wide hyphae with short intercalary cells by calcofluor white staining. The mechanism by which sho1 gene affected polarized growth is investigated. A reduced number of apical vesicles with greater dispersion were observed by transmission electron microscopy in the Spitzenkörper body of the sho1 mutant. Actin patches were distributed randomly at low density at early stages of mutant strain fungal development and reaggregated to the hyphal tip of later stages when long filamentous fungi formed. Actin patches located at the tip of polarized wild-type cells. RNA levels of polarized growth-related genes Rho GTPases were detected by real-time PCR. The sho1 gene did not affect the RNA expression when strains were cultured at 37°C for 6 h. At 17 h, the RNA expression of rho1, rho3 and CDC42 in the sho1 mutant were 0.18-, 0.18- and 0.33-fold of that in the wild type. The sho1 gene affected the polarized growth through affecting the expression of Rho GTPases, the distribution of actin cytoskeleton, vesicle quantity and distribution.

Traffic of chitin synthase 1 (CHS-1) to the Spitzenkörper and developing septa in hyphae of Neurospora crassa: actin dependence and evidence of distinct microvesicle populations

We describe the subcellular location of chitin synthase 1 (CHS-1), one of seven chitin synthases in Neurospora crassa. Laser scanning confocal microscopy of growing hyphae showed CHS-1-green fluorescent protein (GFP) localized conspicuously in regions of active wall synthesis, namely, the core of the Spitzenkörper (Spk), the apical cell surface, and developing septa. It was also present in numerous fine particles throughout the cytoplasm plus some large vacuoles in distal hyphal regions. Although the same general subcellular distribution was observed previously for CHS-3 and CHS-6, they did not fully colocalize. Dual labeling showed that the three different chitin synthases were contained in different vesicular compartments, suggesting the existence of a different subpopulation of chitosomes for each CHS. CHS-1-GFP persisted in the Spk during hyphal elongation but disappeared from the septum after its development was completed. Wide-field fluorescence microscopy and total internal reflection fluorescence microscopy revealed subapical clouds of particles, suggestive of chitosomes moving continuously toward the Spk. Benomyl had no effect on CHS-1-GFP localization, indicating that microtubules are not strictly required for CHS trafficking to the hyphal apex. Conversely, actin inhibitors caused severe mislocalization of CHS-1-GFP, indicating that actin plays a major role in the orderly traffic and localization of CHS-1 at the apex.

Visualization of F-actin localization and dynamics with live cell markers in Neurospora crassa

Filamentous actin (F-actin) plays essential roles in filamentous fungi, as in all other eukaryotes, in a wide variety of cellular processes including cell growth, intracellular motility, and cytokinesis. We visualized F-actin organization and dynamics in living Neurospora crassa cells via confocal microscopy of growing hyphae expressing GFP fusions with homologues of the actin-binding proteins fimbrin (FIM) and tropomyosin (TPM-1), a subunit of the Arp2/3 complex (ARP-3) and a recently developed live cell F-actin marker, Lifeact (ABP140 of Saccharomyces cerevisiae). FIM-GFP, ARP-3-GFP, and Lifeact-GFP associated with small patches in the cortical cytoplasm that were concentrated in a subapical ring, which appeared similar for all three markers but was broadest in hyphae expressing Lifeact-GFP. These cortical patches were short-lived, and a subset was mobile throughout the hypha, exhibiting both anterograde and retrograde motility. TPM-1-GFP and Lifeact-GFP co-localized within the Spitzenkörper (Spk) core at the hyphal apex, and were also observed in actin cables throughout the hypha. All GFP fusion proteins studied were also transiently localized at septa: Lifeact-GFP first appeared as a broad ring during early stages of contractile ring formation and later coalesced into a sharper ring, TPM-1-GFP was observed in maturing septa, and FIM-GFP/ARP3-GFP-labeled cortical patches formed a double ring flanking the septa. Our observations suggest that each of the N. crassa F-actin-binding proteins analyzed associates with a different subset of F-actin structures, presumably reflecting distinct roles in F-actin organization and dynamics. Moreover, Lifeact-GFP marked the broadest spectrum of F-actin structures; it may serve as a global live cell marker for F-actin in filamentous fungi.

Nuclear dynamics, mitosis, and the cytoskeleton during the early stages of colony initiation in Neurospora crassa

Neurospora crassa macroconidia form germ tubes that are involved in colony establishment and conidial anastomosis tubes (CATs) that fuse to form interconnected networks of conidial germlings. Nuclear and cytoskeletal behaviors were analyzed in macroconidia, germ tubes, and CATs in strains that expressed fluorescently labeled proteins. Heterokaryons formed by CAT fusion provided a rapid method for the imaging of multiple labeled fusion proteins and minimized the potential risk of overexpression artifacts. Mitosis occurred more slowly in nongerminated macroconidia (1.0 to 1.5 h) than in germ tubes (16 to 20 min). The nucleoporin SON-1 was not released from the nuclear envelope during mitosis, which suggests that N. crassa exhibits a form of "closed mitosis." During CAT homing, nuclei did not enter CATs, and mitosis was arrested. Benomyl treatment showed that CAT induction, homing, fusion, as well as nuclear migration through fused CATs do not require microtubules or mitosis. Three ropy mutants (ro-1, ro-3, and ro-11) defective in the dynein/dynactin microtubule motor were impaired in nuclear positioning, but nuclei still migrated through fused CATs. Latrunculin B treatment, imaging of F-actin in living cells using Lifeact-red fluorescent protein (RFP), and analysis of mutants defective in the Arp2/3 complex demonstrated that actin plays important roles in CAT fusion.

F-actin dynamics in Neurospora crassa

This study demonstrates the utility of Lifeact for the investigation of actin dynamics in Neurospora crassa and also represents the first report of simultaneous live-cell imaging of the actin and microtubule cytoskeletons in filamentous fungi. Lifeact is a 17-amino-acid peptide derived from the nonessential Saccharomyces cerevisiae actin-binding protein Abp140p. Fused to green fluorescent protein (GFP) or red fluorescent protein (TagRFP), Lifeact allowed live-cell imaging of actin patches, cables, and rings in N. crassa without interfering with cellular functions. Actin cables and patches localized to sites of active growth during the establishment and maintenance of cell polarity in germ tubes and conidial anastomosis tubes (CATs). Recurrent phases of formation and retrograde movement of complex arrays of actin cables were observed at growing tips of germ tubes and CATs. Two populations of actin patches exhibiting slow and fast movement were distinguished, and rapid (1.2 microm/s) saltatory transport of patches along cables was observed. Actin cables accumulated and subsequently condensed into actin rings associated with septum formation. F-actin organization was markedly different in the tip regions of mature hyphae and in germ tubes. Only mature hyphae displayed a subapical collar of actin patches and a concentration of F-actin within the core of the Spitzenkörper. Coexpression of Lifeact-TagRFP and beta-tubulin-GFP revealed distinct but interrelated localization patterns of F-actin and microtubules during the initiation and maintenance of tip growth.

Reconstruction of signaling networks regulating fungal morphogenesis by transcriptomics

Coordinated control of hyphal elongation and branching is essential for sustaining mycelial growth of filamentous fungi. In order to study the molecular machinery ensuring polarity control in the industrial fungus Aspergillus niger, we took advantage of the temperature-sensitive (ts) apical-branching ramosa-1 mutant. We show here that this strain serves as an excellent model system to study critical steps of polar growth control during mycelial development and report for the first time a transcriptomic fingerprint of apical branching for a filamentous fungus. This fingerprint indicates that several signal transduction pathways, including TORC2, phospholipid, calcium, and cell wall integrity signaling, concertedly act to control apical branching. We furthermore identified the genetic locus affected in the ramosa-1 mutant by complementation of the ts phenotype. Sequence analyses demonstrated that a single amino acid exchange in the RmsA protein is responsible for induced apical branching of the ramosa-1 mutant. Deletion experiments showed that the corresponding rmsA gene is essential for the growth of A. niger, and complementation analyses with Saccharomyces cerevisiae evidenced that RmsA serves as a functional equivalent of the TORC2 component Avo1p. TORC2 signaling is required for actin polarization and cell wall integrity in S. cerevisiae. Congruently, our microscopic investigations showed that polarized actin organization and chitin deposition are disturbed in the ramosa-1 mutant. The integration of the transcriptomic, genetic, and phenotypic data obtained in this study allowed us to reconstruct a model for cellular events involved in apical branching.

Cytoplasmic bulk flow propels nuclei in mature hyphae of Neurospora crassa

We used confocal microscopy to evaluate nuclear dynamics in mature, growing hyphae of Neurospora crassa whose nuclei expressed histone H1-tagged green fluorescent protein (GFP). In addition to the H1-GFP wild-type (WT) strain, we examined nuclear displacement (passive transport) in four mutants deficient in microtubule-related motor proteins (ro-1, ro-3, kin-1, and a ro-1 kin-1 double mutant). We also treated the WT strain with benomyl and cytochalasin A to disrupt microtubules and actin microfilaments, respectively. We found that the degree of nuclear displacement in the subapical regions of all strains correlated with hyphal elongation rate. The WT strain and that the ro-1 kin-1 double mutant showed the highest correlation between nuclear movement and hyphal elongation. Although most nuclei seemed to move forward passively, presumably carried by the cytoplasmic bulk flow, a small proportion of the movement detected was either retrograde or accelerated anterograde. The absence of a specific microtubule motor in the mutants ro-1, ro-3, or kin-1 did not prevent the anterograde and retrograde migration of nuclei; however, in the ro-1 kin-1 double mutant retrograde migration was absent. In the WT strain, almost all nuclei were elongated, whereas in all other strains a majority of nuclei were nearly spherical. With only one exception, a sizable exclusion zone was maintained between the apex and the leading nucleus. The ro-1 mutant showed the largest nucleus exclusion zone; only the treatment with cytochalasin A abolished the exclusion zone. In conclusion, the movement and distribution of nuclei in mature hyphae appear to be determined by a combination of forces, with cytoplasmic bulk flow being a major determinant. Motor proteins probably play an active role in powering the retrograde or accelerated anterograde migrations of nuclei and may also contribute to passive anterograde displacement by binding nuclei to microtubules.

The machinery for cell polarity, cell morphogenesis, and the cytoskeleton in the Basidiomycete fungus Ustilago maydis-a survey of the genome sequence

Ustilago maydis, a Basidiomycete fungus that infects maize, exhibits two basic morphologies, a yeast-like and a filamentous form. The yeast-like cell is elongated, divides by budding, and the bud grows by tip extension. The filamentous form divides at the apical cell and grows by tip extension. The repertoire of morphologies is increased during interaction with its host, suggesting that plant signals play an important role in generation of additional morphologies. We have used Saccharomyces cerevisiae and Schizosaccharomyces pombe genes known to play a role in cell polarity and morphogenesis, and in the cytoskeleton as probes to survey the U. maydis genome. We have found that most of the yeast machinery is conserved in U. maydis, albeit the degree of similarity varies from strong to weak. The U. maydis genome contains the machinery for recognition and interpretation of the budding yeast axial and bipolar landmarks; however, genes coding for some of the landmark proteins are absent. Genes coding for cell polarity establishment, exocytosis, actin and microtubule organization, microtubule plus-end associated proteins, kinesins, and myosins are also present. Genes not present in S. cerevisiae and S. pombe include a homolog of mammalian Rac, a hybrid myosin-chitin synthase, and several kinesins that exhibit more similarity to their mammalian counterparts. We also used the U. maydis genes identified in this analysis to search other fungal and other eukaryotic genomes to identify the closest homologs. In most cases, not surprisingly, the closest homolog is among filamentous fungi, not the yeasts, and in some cases it is among mammals.

Polarized growth in fungi--interplay between the cytoskeleton, positional markers and membrane domains

One kind of the most extremely polarized cells in nature are the indefinitely growing hyphae of filamentous fungi. A continuous flow of secretion vesicles from the hyphal cell body to the growing hyphal tip is essential for cell wall and membrane extension. Because microtubules (MT) and actin, together with their corresponding motor proteins, are involved in the process, the arrangement of the cytoskeleton is a crucial step to establish and maintain polarity. In Saccharomyces cerevisiae and Schizosaccharomyces pombe, actin-mediated vesicle transportation is sufficient for polar cell extension, but in S. pombe, MTs are in addition required for the establishment of polarity. The MT cytoskeleton delivers the so-called cell-end marker proteins to the cell pole, which in turn polarize the actin cytoskeleton. Latest results suggest that this scenario may principally be conserved from S. pombe to filamentous fungi. In addition, in filamentous fungi, MTs could provide the tracks for long-distance vesicle movement. In this review, we will compare the interaction of the MT and the actin cytoskeleton and their relation to the cortex between yeasts and filamentous fungi. In addition, we will discuss the role of sterol-rich membrane domains in combination with cell-end marker proteins for polarity establishment.

Antagonistic effects of Pseudomonas jessenii against Pythium aphanidermatum: morphological, ultrastructural and cytochemical aspects

Pseudomonas jessenii isolate EC-S101, an antagonistic rhizobacterium, induces morphological abnormalities such as topical swelling and excessive lateral branching in phytopathogenic Peronosporomycetes Pythium aphanidermatum hyphae as a result of radial growth inhibition in a dual culture assay. Rhodamine-phalloidin staining revealed that these abnormalities were associated with disorganization of actin cytoskeleton. Both the morphological forms of actin, filaments and plaques, were affected progressively. At early stage of interaction (in less affected hyphae), the filaments were either eliminated or disarrayed. At advance stage of interaction (in severely affected hyphae), even the plaques population was decreased or disappeared. The effects of P. jessenii on actin architecture of Py. aphanidermatum were comparable to latrunculin B, a known actin assembly inhibitor. In addition, at early stage of interaction, the quantities of nuclei, lipid bodies and mitochondria became higher than those in control. At advance stage of interaction, the quantities of these organelles were almost similar, higher and lower, respectively, compare to those in control. Scanning electron microscopy exhibited cell wall disruption and accumulation of extracellular material associated with severely affected hyphae. Ultrastructural observations of the affected hyphae displayed additional features of considerable thickening of cell wall, enlargement of vacuoles, sinking of redundant lipid bodies into the enlarged vacuoles and wall appositions. We conclude that in addition to interference in morphogenesis and growth of Py. aphanidermatum, P. jessenii suppresses the pathogen through sub-cellular disorganization, specifically the actin architecture. This is the first report on disruption of cytoskeleton in a eukaryotic phytopathogen by an antagonistic rhizobacterium.

The role of actin, fimbrin and endocytosis in growth of hyphae in Aspergillus nidulans

Filamentous fungi are ideal systems to study the process of polarized growth, as their life cycle is dominated by hyphal growth exclusively at the cell apex. The actin cytoskeleton plays an important role in this growth. Until now, there have been no tools to visualize actin or the actin-binding protein fimbrin in live cells of a filamentous fungus. We investigated the roles of actin (ActA) and fimbrin (FimA) in hyphal growth in Aspergillus nidulans. We examined the localization of ActA::GFP and FimA::GFP in live cells, and each displayed a similar localization pattern. In actively growing hyphae, cortical ActA::GFP and FimA::GFP patches were highly mobile throughout the hypha and were concentrated near hyphal apices. A patch-depleted zone occupied the apical 0.5 microm of growing hypha. Both FimA::GFP and Act::GFP also localize transiently to septa. Movement and later localization of both was compromised after cytochalasin treatment. Disruption of fimA resulted in delayed polarity establishment during conidium germination, abnormal hyphal growth and endocytosis defects in apolar cells. Endocytosis was severely impaired in apolar fimA disruption cells. Our data support a novel apical recycling model which indicates a critical role for actin patch-mediated endocytosis to maintain polarized growth at the apex.

An F-actin-depleted zone is present at the hyphal tip of invasive hyphae of Neurospora crassa

The distribution of filamentous actin (F-actin) in invasive and noninvasive hyphae of the ascomycete Neurospora crassa was investigated. Eighty six percent of noninvasive hyphae had F-actin in the tip region compared to only 9% of invasive hyphae. The remaining 91% of the invasive hyphae had no obvious tip high concentration of F-actin staining; instead they had an F-actin-depleted zone in this region, although some F-actin, possibly associated with the Spitzenkörper, remained at the tip. The size of the F-actin-depleted zone in invasive hyphae increased with an increase in agar concentration. The membrane stain FM 4-64 reveals a slightly larger accumulation of vesicles at the tips of invasive hyphae relative to noninvasive hyphae, although this difference is unlikely to be sufficient to account for the exclusion of F-actin from the depleted zone. Antibodies raised against the actin filament-severing protein cofilin from both yeast and human cells localize to the tips of invasive hyphae. The human cofilin antibody shows a more random distribution in noninvasive hyphae locating primarily at the hyphal periphery but with some diffuse cytoplasmic staining. This antibody also identifies a single band at 21 kDa in immunoblots of whole hyphal fractions. These data suggest that a protein with epitopic similarity to cofilin may function in F-actin dynamics that underlie invasive growth. The F-actin-depleted zone may play a role in the regulation of tip yielding to turgor pressure, thus increasing the protrusive force necessary for invasive growth.

Regulation of hyphal morphogenesis by cdc42 and rac1 homologues in Aspergillus nidulans

The ability of filamentous fungi to form hyphae requires the establishment and maintenance of a stable polarity axis. Based on studies in yeasts and animals, the GTPases Cdc42 and Rac1 are presumed to play a central role in organizing the morphogenetic machinery to enable axis formation and stabilization. Here, we report that Cdc42 (ModA) and Rac1 (RacA) share an overlapping function required for polarity establishment in Aspergillus nidulans. Nevertheless, Cdc42 appears to have a more important role in hyphal morphogenesis in that it alone is required for the timely formation of lateral branches. In addition, we provide genetic evidence suggesting that the polarisome components SepA and SpaA function downstream of Cdc42 in a pathway that may regulate microfilament formation. Finally, we show that microtubules become essential for the establishment of hyphal polarity when the function of either Cdc42 or SepA is compromised. Our results are consistent with the action of parallel Cdc42 and microtubule-based pathways in regulating the formation of a stable axis of hyphal polarity in A. nidulans.

Interaction with mycorrhiza helper bacterium Streptomyces sp. AcH 505 modifies organisation of actin cytoskeleton in the ectomycorrhizal fungus Amanita muscaria (fly agaric)

The actin cytoskeleton (AC) of fungal hyphae is a major determinant of hyphal shape and morphogenesis, implicated in controlling tip structure and secretory vesicle delivery. Hyphal growth of the ectomycorrhizal fungus Amanita muscaria and symbiosis formation with spruce are promoted by the mycorrhiza helper bacterium Streptomyces sp. AcH 505 (AcH 505). To investigate structural requirements of growth promotion, the effect of AcH 505 on A. muscaria hyphal morphology, AC and actin gene expression were studied. Hyphal diameter and mycelial density decreased during dual culture (DC), and indirect immunofluorescence microscopy revealed that the dense and polarised actin cap in hyphal tips of axenic A. muscaria changes to a loosened and dispersed structure in DC. Supplementation of growth medium with cell-free bacterial supernatant confirmed that reduction in hyphal diameter and AC changes occurred at the same stage of growth. Transcript levels of both actin genes isolated from A. muscaria remained unaltered, indicating that AC changes are regulated by reorganisation of the existing actin pool. In conclusion, the AC reorganisation appears to result in altered hyphal morphology and faster apical extension. The thus improved spreading of hyphae and increased probability to encounter plant roots highlights a mechanism behind the mycorrhiza helper effect.

Localization of RHO-4 indicates differential regulation of conidial versus vegetative septation in the filamentous fungus Neurospora crassa

Rho-4 mutants of the filamentous fungus Neurospora crassa lack septa and asexual spores (conidia) and grow slowly. In this report, localization of green fluorescent protein-tagged RHO-4 is used to elucidate the differences in factors controlling RHO-4 localization during vegetative growth versus asexual development. RHO-4 forms a ring at incipient vegetative septation sites that constricts with the formation of the septum toward the septal pore; RHO-4 persists around the septal pore after septum completion. During the formation of conidia, RHO-4 localizes to the primary septum but subsequently is relocalized to the cytoplasm after the placement of the secondary septum. Cytoplasmic localization and inactivation of RHO-4 are mediated by a direct physical interaction with RDI-1, a RHO guanosine nucleotide dissociation inhibitor. Inappropriate activation of the cyclic AMP-dependent protein kinase A pathway during vegetative growth causes mislocalization of RHO-4 away from septa to the cytoplasm, a process which was dependent upon RDI-1. An adenylate cyclase cr-1 mutant partially suppresses the aconidial defect of rho-4 mutants but only rarely suppresses the vegetative septation defect, indicating that conidial septation is negatively regulated by CR-1. These data highlight the differences in the regulation of septation during conidiation versus vegetative septation in filamentous fungi.

The actin gene in Paracoccidioides brasiliensis: organization, expression and phylogenetic analyses

PbrACT1, the gene responsible for the synthesis of actin in Paracoccidioides brasiliensis, was found as a single copy, organized into six exons and five introns. Its open reading frame (ORF) codes for a putative protein of 375 amino acids, with a molecular mass of 41.5 kDa and an isoelectric point of 5.6. Analysis of the nucleotide sequence revealed a high homology to other fungal actins, the presence of characteristic fungal actin sequences, and heat shock elements at the 5' untranslated region (UTR). Phylogenetic analyses with deduced amino acid sequences of fungal actins grouped P. brasiliensis within the phylum Ascomycota, order Onygenales, in concordance with a few previous reports. Patterns of expression through the temperature-induced morphological transitions from mycelial to yeast-like shapes and reverse, suggests that PbrACT1 is regulated in this process. The PbrACT1 gene sequence is available at the GenBank database under accession number AY383732.

Metrafenone: studies on the mode of action of a novel cereal powdery mildew fungicide

Powdery mildew fungi are among the major pathogens causing diseases of cereals in the world. The mode of action of a novel systemic benzophenone fungicide, metrafenone, which is based on a precursor that is discussed in the preceding paper, has been analysed on the powdery mildew fungi of barley (Blumeria graminis Speer f. sp. hordei Marchal) and wheat (Blumeria graminis Speer f. sp. tritici Marchal). Preventive treatments reduced germination and blocked development beyond formation of appressoria, which penetrated less often. Moreover, metrafenone turned out to be an efficient curative fungicide, which rapidly affected fungal survival at low concentrations. The fungicide induced swelling, bursting and collapse of hyphal tips, resulting in the release of globules of cytoplasm. Bifurcation of hyphal tips, secondary appressoria and hyperbranching were also frequently observed. A histochemical analysis showed that metrafenone caused disruption of the apical actin cap and apical vesicle transport as well as weakening of the cell wall at hyphal tips. Finally, metrafenone strongly reduced sporulation. Reduced sporulation was associated with malformation of conidiophores that showed irregular septation, multinucleate cells and delocalisation of actin. Microtubules appeared to be only secondarily affected in metrafenone-treated B. graminis. The results suggest that the mode of action of metrafenone interferes with hyphal morphogenesis, polarised hyphal growth and the establishment and maintenance of cell polarity. Metrafenone likely disturbs a pathway regulating organisation of the actin cytoskeleton.

The Spitzenkörper: a molecular perspective

The Spitzenkörper is a dynamic structure present at the tips of hyphal cells with a single highly polarized growth site. It is closely connected with cell morphogenesis and polar growth, and is only present at actively growing sites. Morphogenesis of such highly polarized cells is complex, and requires the coordinated action of multiple protein complexes. We discuss the relevance of these complexes for the structure and function of the Spitzenkörper.

Cell polarity in filamentous fungi: shaping the mold

The formation of highly polarized hyphae that grow by apical extension is a defining feature of the filamentous fungi. High-resolution microscopy and mathematical modeling have revealed the importance of the cytoskeleton and the Spitzenkorper (an apical vesicle cluster) in hyphal morphogenesis. However, the underlying molecular mechanisms remain poorly characterized. In this review, the pathways and functions known to be involved in polarized hyphal growth are summarized. A central theme is the notion that the polarized growth of hyphae is more complex than in yeast, though similar sets of core pathways are likely utilized. In addition, a model for the establishment and maintenance of hyphal polarity is presented. Key features of the model include the idea that polarity establishment is a stochastic process that occurs independent of internal landmarks. Moreover, the stabilization of nascent polarity axes may be the critical step that permits the emergence of a new hypha.