Microtubule dynamics and the role of molecular motors in Neurospora crassa

Soft but Not Too Soft-How a Rigid Tube Expands without Breaking

Fungi grow by apical extension of their hyphae. The continuous growth requires constant delivery of vesicles, which fuse with the membrane and secrete cell wall biosynthesis enzymes. The growth mechanism requires the fungal cytoskeleton and turgor pressure. In a recent study by Fukuda et al. (mBio 12:e03196-20, 2021, https://doi.org/10.1128/mBio.03196-20), hyphal growth was studied in microfluidic devices with channels smaller than the hyphal diameter. The authors discovered that fast-growing fungi like Neurospora crassa enter the channels, but hyphal tips become fragile and rupture frequently, whereas slower-growing fungi like Aspergillus nidulans adapt their hyphal diameter and grow without problems through the channels. This study suggests two different growth mechanisms and a tradeoff between hyphal plasticity and growth speed.

Subcellular structure and behaviour in fungal hyphae

This work briefly surveys the diversity of selected subcellular characteristics in hyphal tip cells of the fungal kingdom (Mycota). Hyphae are filamentous cells that grow by tip extension. It is a highly polarised mechanism that requires a robust secretory system for the delivery of materials (e.g. membrane, proteins, cell wall materials) to sites of cell growth. These events result it the self-assembly of a Spitzenkörper (Spk), found most often in the Basidiomycota, Ascomycota, and Blastocladiomycota, or an apical vesicle crescent (AVC), present in the most Mucoromycota and Zoopagomycota. The Spk is a complex apical body composed of secretory vesicles, cytoskeletal elements, and signaling proteins. The AVC appears less complex, though little is known of its composition other than secretory vesicles. Both bodies influence hyphal growth and morphogenesis. Other factors such as cytoskeletal functions, endocytosis, cytoplasmic flow, and turgor pressure are also important in sustaining hyphal growth. Clarifying subcellular structures, functions, and behaviours through bioimagining analysis are providing a better understanding of the cell biology and phylogenetic relationships of fungi. LAY DESCRIPTION: Fungi are most familiar to the public as yeast, molds, and mushrooms. They are eukaryotic organisms that inhabit diverse ecological niches around the world and are critical to the health of ecosystems performing roles in decomposition of organic matter and nutrient recycling (Heath, 1990). Fungi are heterotrophs, unlike plants, and comprise the most successful and diverse phyla of eukaryotic microbes, interacting with all other forms of life in associations that range from beneficial (e.g., mycorrhizae) to antagonistic (e.g., pathogens). Some fungi can be parasitic or pathogenic on plants (e.g., Cryphonectria parasitica, Magnaporthe grisea), insects (e.g., Beauveria bassiana, Cordyceps sp.), invertebrates (e.g., Drechslerella anchonia), vertebrates (e.g., Coccidioides immitis, Candia albicans) and other fungi (e.g., Trichoderma viride, Ampelomyces quisqualis). The majority of fungi, however, are saprophytes, obtaining nutrition through the brake down of non-living organic matter.

Intracellular mechanisms of fungal space searching in microenvironments

Many filamentous fungi colonizing animal or plant tissue, waste matter, or soil must find optimal paths through the constraining geometries of their microenvironment. Imaging of live fungal growth in custom-built microfluidics structures revealed the intracellular mechanisms responsible for this remarkable efficiency. In meandering channels, the Spitzenkörper (an assembly of vesicles at the filament tip) acted like a natural gyroscope, conserving the directional memory of growth, while the fungal cytoskeleton organized along the shortest growth path. However, if an obstacle could not be negotiated, the directional memory was lost due to the disappearance of the Spitzenkörper gyroscope. This study can impact diverse environmental, industrial, and medical applications, from fungal pathogenicity in plants and animals to biology-inspired computation.

Filamentous fungi that colonize microenvironments, such as animal or plant tissue or soil, must find optimal paths through their habitat, but the biological basis for negotiating growth in constrained environments is unknown. We used time-lapse live-cell imaging of Neurospora crassa in microfluidic environments to show how constraining geometries determine the intracellular processes responsible for fungal growth. We found that, if a hypha made contact with obstacles at acute angles, the Spitzenkörper (an assembly of vesicles) moved from the center of the apical dome closer to the obstacle, thus functioning as an internal gyroscope, which preserved the information regarding the initial growth direction. Additionally, the off-axis trajectory of the Spitzenkörper was tracked by microtubules exhibiting “cutting corner” patterns. By contrast, if a hypha made contact with an obstacle at near-orthogonal incidence, the directional memory was lost, due to the temporary collapse of the Spitzenkörper–microtubule system, followed by the formation of two “daughter” hyphae growing in opposite directions along the contour of the obstacle. Finally, a hypha passing a lateral opening in constraining channels continued to grow unperturbed, but a daughter hypha gradually branched into the opening and formed its own Spitzenkörper–microtubule system. These observations suggest that the Spitzenkörper–microtubule system is responsible for efficient space partitioning in microenvironments, but, in its absence during constraint-induced apical splitting and lateral branching, the directional memory is lost, and growth is driven solely by the isotropic turgor pressure. These results further our understanding of fungal growth in microenvironments relevant to environmental, industrial, and medical applications.

Kinesin-binding-triggered conformation switching of microtubules contributes to polarized transport

Kinesin-1, the founding member of the kinesin superfamily of proteins, is known to use only a subset of microtubules for transport in living cells. This biased use of microtubules is proposed as the guidance cue for polarized transport in neurons, but the underlying mechanisms are still poorly understood. Here, we report that kinesin-1 binding changes the microtubule lattice and promotes further kinesin-1 binding. This high-affinity state requires the binding of kinesin-1 in the nucleotide-free state. Microtubules return to the initial low-affinity state by washing out the binding kinesin-1 or by the binding of non-hydrolyzable ATP analogue AMPPNP to kinesin-1. X-ray fiber diffraction, fluorescence speckle microscopy, and second-harmonic generation microscopy, as well as cryo-EM, collectively demonstrated that the binding of nucleotide-free kinesin-1 to GDP microtubules changes the conformation of the GDP microtubule to a conformation resembling the GTP microtubule.

Hyphal ontogeny in Neurospora crassa: a model organism for all seasons

Filamentous fungi have proven to be a better-suited model system than unicellular yeasts in analyses of cellular processes such as polarized growth, exocytosis, endocytosis, and cytoskeleton-based organelle traffic. For example, the filamentous fungus Neurospora crassa develops a variety of cellular forms. Studying the molecular basis of these forms has led to a better, yet incipient, understanding of polarized growth. Polarity factors as well as Rho GTPases, septins, and a localized delivery of vesicles are the central elements described so far that participate in the shift from isotropic to polarized growth. The growth of the cell wall by apical biosynthesis and remodeling of polysaccharide components is a key process in hyphal morphogenesis. The coordinated action of motor proteins and Rab GTPases mediates the vesicular journey along the hyphae toward the apex, where the exocyst mediates vesicle fusion with the plasma membrane. Cytoplasmic microtubules and actin microfilaments serve as tracks for the transport of vesicular carriers as well as organelles in the tubular cell, contributing to polarization. In addition to exocytosis, endocytosis is required to set and maintain the apical polarity of the cell. Here, we summarize some of the most recent breakthroughs in hyphal morphogenesis and apical growth in N. crassa and the emerging questions that we believe should be addressed.

Microtubules and associated molecular motors in Neurospora crassa

The cytoskeleton provides structure, shape and movement to various cells. Microtubules (MTs) are tubular structures made of α and β-tubulin heterodimers organized in 13 protofilaments, forming a hollow cylinder. A vast group of MT-associated proteins determines the function, behavior and interaction of the MTs with other cellular components. Among these proteins, molecular motors such as the dynein-dynactin complex and kinesin superfamily play roles in MT organization and organelle transport. This article focuses on the MT cytoskeleton and associated molecular motors in the filamentous fungus Neurospora crassa In addition to reviewing current available information for this fungus and contrasting it with knowledge of other fungal species, we present new experimental results that support the role of dynein, dynactin and conventional kinesin in MT organization, dynamics and transport of subcellular structures (nuclei and secretory vesicles). In wild type hyphae of N. crassa, cytoplasmic MTs are arranged longitudinally along hyphae and display a helical curvature. They interlace with one another to form a network throughout the cytoplasm. N. crassa dynein and dynactin mutants have a scant and disorganized MT cytoskeleton, an erratic and reduced Spitzenkörper (Spk) and distorted hyphal morphology. In contrast, hyphae of mutants with defective conventional kinesin exhibit only minor disruptions in MT and Spk organization. Although nuclear positioning is affected in all mutants, the MT-associated motor proteins are not major contributors to nuclear movement during hyphal growth. Cytoplasmic bulk flow is the vehicle for nuclear displacement in growing hyphal regions of N. crassa Motors are involved in nuclei saltatory movements in both retrograde or anterograde direction. In the dynein and kinesin mutants, micro and macrovesicles can reach the Spk, although growth is slightly impaired and the Spk displays an erratic path. Hyphal growth requires MTs, and their associated motors are required for their organization and dynamics and Spk integrity.

Two microtubule-plus-end binding proteins LIS1-1 and LIS1-2, homologues of human LIS1 in Neurospora crassa

LIS1 is a microtubule (Mt) plus-end binding protein that interacts with the dynein/dynactin complex. In humans, LIS1 is required for proper nuclear and organelle migration during cell growth. Although gene duplication is absent from Neurospora crassa, we found two paralogues of human LIS1. We named them LIS1-1 and LIS1-2 and studied their dynamics and function by fluorescent tagging. At the protein level, LIS1-1 and LIS1-2 were very similar. Although, the characteristic coiled-coil motif was not present in LIS1-2. LIS1-1-GFP and LIS1-2-GFP showed the same cellular distribution and dynamics, but LIS1-2-GFP was less abundant. Both LIS1 proteins were found in the subapical region as single fluorescent particles traveling toward the cell apex, they accumulated in the apical dome forming prominent short filament-like structures, some of which traversed the Spitzenkörper (Spk). The fluorescent structures moved exclusively in anterograde fashion along straight paths suggesting they traveled on Mts. There was no effect in the filament behavior of LIS1-1-GFP in the Δlis1-2 mutant but the dynamics of LIS1-2-GFP was affected in the Δlis1-1 mutant. Microtubular integrity and the dynein-dynactin complex were necessary for the formation of filament-like structures of LIS1-1-GFP in the subapical and apical regions; however, conventional kinesin (KIN-1) was not. Deletion mutants showed that the lack of lis1-1 decreased cell growth by ∼75%; however, the lack of lis1-2 had no effect on growth. A Δlis1-1;Δlis1-2 double mutant showed slower growth than either single mutant. Conidia production was reduced but branching rate increased in Δlis1-1 and the Δlis1-1;Δlis1-2 double mutants. The absence of LIS1-1 had a strong effect on Mt organization and dynamics and indirectly affected nuclear and mitochondrial distribution. The absence of LIS1-1 filaments in dynein mutants (ropy mutants) or in benomyl treated hyphae indicates the strong association between this protein and the regulation of the dynein-dynactin complex and Mt organization. LIS1-1 and LIS1-2 had a high amino acid homology, nevertheless, the absence of the coiled-coil motif in LIS1-2 suggests that its function or regulation may be distinct from that of LIS1-1.

MTB-3, a microtubule plus-end tracking protein (+TIP) of Neurospora crassa

The microtubule (MT) “plus end” constitutes the platform for the accumulation of a structurally and functionally diverse group of proteins, collectively called “MT plus-end tracking proteins” (+TIPs). +TIPs control MT dynamics and link MTs to diverse sub-cellular structures. Neurospora crassaMicroTubule Binding protein-3 (MTB-3) is the homolog of yeast EB1, a highly conserved +TIP. To address the function of MTB-3, we examined strains with mtb-3 deletions, and we tagged MTB-3 with GFP to assess its dynamic behavior. MTB-3-GFP was present as comet-like structures distributed more or less homogeneously within the hyphal cytoplasm, and moving mainly towards the apex at speeds up to 4× faster than the normal hyphal elongation rates. MTB-3-GFP comets were present in all developmental stages, but were most abundant in mature hyphae. MTB-3-GFP comets were observed moving in anterograde and retrograde direction along the hypha. Retrograde movement was also observed as originating from the apical dome. The integrity of the microtubular cytoskeleton affects the presence and dynamics of MTB-3-GFP comets, while actin does not seem to play a role. The size of MTB-3-GFP comets is affected by the absence of dynactin and conventional kinesin. We detected no obvious morphological phenotypes in Δmtb-3 mutants but there were fewer MTs in Δmtb-3, MTs were less bundled and less organized. Compared to WT, both MT polymerization and depolymerization rates were significantly decreased in Δmtb-3. In summary, the lack of MTB-3 affects overall growth and morphological phenotypes of N. crassa only slightly, but deletion of mtb-3 has strong effect on MT dynamics.

5'-Serial Analysis of Gene Expression studies reveal a transcriptomic switch during fruiting body development in Coprinopsis cinerea

Background

The transition from the vegetative mycelium to the primordium during fruiting body development is the most complex and critical developmental event in the life cycle of many basidiomycete fungi. Understanding the molecular mechanisms underlying this process has long been a goal of research on basidiomycetes. Large scale assessment of the expressed transcriptomes of these developmental stages will facilitate the generation of a more comprehensive picture of the mushroom fruiting process. In this study, we coupled 5'-Serial Analysis of Gene Expression (5'-SAGE) to high-throughput pyrosequencing from 454 Life Sciences to analyze the transcriptomes and identify up-regulated genes among vegetative mycelium (Myc) and stage 1 primordium (S1-Pri) of Coprinopsis cinerea during fruiting body development.

Results

We evaluated the expression of >3,000 genes in the two respective growth stages and discovered that almost one-third of these genes were preferentially expressed in either stage. This identified a significant turnover of the transcriptome during the course of fruiting body development. Additionally, we annotated more than 79,000 transcription start sites (TSSs) based on the transcriptomes of the mycelium and stage 1 primoridum stages. Patterns of enrichment based on gene annotations from the GO and KEGG databases indicated that various structural and functional protein families were uniquely employed in either stage and that during primordial growth, cellular metabolism is highly up-regulated. Various signaling pathways such as the cAMP-PKA, MAPK and TOR pathways were also identified as up-regulated, consistent with the model that sensing of nutrient levels and the environment are important in this developmental transition. More than 100 up-regulated genes were also found to be unique to mushroom forming basidiomycetes, highlighting the novelty of fruiting body development in the fungal kingdom.

Conclusions

We implicated a wealth of new candidate genes important to early stages of mushroom fruiting development, though their precise molecular functions and biological roles are not yet fully known. This study serves to advance our understanding of the molecular mechanisms of fruiting body development in the model mushroom C. cinerea.

A mouse neurodegenerative dynein heavy chain mutation alters dynein motility and localization in Neurospora crassa

Cytoplasmic dynein is responsible for the transport and delivery of cargoes in organisms ranging from humans to fungi. Dysfunction of dynein motor machinery due to mutations in dynein or its activating complex dynactin can result in one of several neurological diseases in mammals. The mouse Legs at odd angles (Loa) mutation in the tail domain of the dynein heavy chain has been shown to lead to progressive neurodegeneration in mice. The mechanism by which the Loa mutation affects dynein function is just beginning to be understood. In this work, we generated the dynein tail mutation observed in Loa mice into the Neurospora crassa genome and utilized cell biological and complementing biochemical approaches to characterize how that tail mutation affected dynein function. We determined that the Loa mutation exhibits several subtle defects upon dynein function in N. crassa that were not seen in mice, including alterations in dynein localization, impaired velocity of vesicle transport, and in the biochemical properties of purified motors. Our work provides new information on the role of the tail domain on dynein function and points out areas of future research that will be of interest to pursue in mammalian systems.

Analyses of dynein heavy chain mutations reveal complex interactions between dynein motor domains and cellular dynein functions

Cytoplasmic dynein transports cargoes for a variety of crucial cellular functions. However, since dynein is essential in most eukaryotic organisms, the in-depth study of the cellular function of dynein via genetic analysis of dynein mutations has not been practical. Here, we identify and characterize 34 different dynein heavy chain mutations using a genetic screen of the ascomycete fungus Neurospora crassa, in which dynein is nonessential. Interestingly, our studies show that these mutations segregate into five different classes based on the in vivo localization of the mutated dynein motors. Furthermore, we have determined that the different classes of dynein mutations alter vesicle trafficking, microtubule organization, and nuclear distribution in distinct ways and require dynactin to different extents. In addition, biochemical analyses of dynein from one mutant strain show a strong correlation between its in vitro biochemical properties and the aberrant intracellular function of that altered dynein. When the mutations were mapped to the published dynein crystal structure, we found that the three-dimensional structural locations of the heavy chain mutations were linked to particular classes of altered dynein functions observed in cells. Together, our data indicate that the five classes of dynein mutations represent the entrapment of dynein at five separate points in the dynein mechanochemical and transport cycles. We have developed N. crassa as a model system where we can dissect the complexities of dynein structure, function, and interaction with other proteins with genetic, biochemical, and cell biological studies.

Subcellular and single-molecule imaging of plant fluorescent proteins using total internal reflection fluorescence microscopy (TIRFM)

Total internal reflection fluorescence microscopy (TIRFM) has been proven to be an extremely powerful technique in animal cell research for generating high contrast images and dynamic protein conformation information. However, there has long been a perception that TIRFM is not feasible in plant cells because the cell wall would restrict the penetration of the evanescent field and lead to scattering of illumination. By comparative analysis of epifluorescence and TIRF in root cells, it is demonstrated that TIRFM can generate high contrast images, superior to other approaches, from intact plant cells. It is also shown that TIRF imaging is possible not only at the plasma membrane level, but also in organelles, for example the nucleus, due to the presence of the central vacuole. Importantly, it is demonstrated for the first time that this is TIRF excitation, and not TIRF-like excitation described as variable-angle epifluorescence microscopy (VAEM), and it is shown how to distinguish the two techniques in practical microscopy. These TIRF images show the highest signal-to-background ratio, and it is demonstrated that they can be used for single-molecule microscopy. Rare protein events, which would otherwise be masked by the average molecular behaviour, can therefore be detected, including the conformations and oligomerization states of interacting proteins and signalling networks in vivo. The demonstration of the application of TIRFM and single-molecule analysis to plant cells therefore opens up a new range of possibilities for plant cell imaging.

Regulation of microtubule dynamics by kinesins

The simple mechanistic and functional division of the kinesin family into either active translocators or non-motile microtubule depolymerases was initially appropriate but is now proving increasingly unhelpful, given evidence that several translocase kinesins can affect microtubule dynamics, whilst non-translocase kinesins can promote microtubule assembly and depolymerisation. Such multi-role kinesins act either directly on microtubule dynamics, by interaction with microtubules and tubulin, or indirectly, through the transport of other factors along the lattice to the microtubule tip. Here I review recent progress on the mechanisms and roles of these translocase kinesins.

Hyphal morphology modification in thermal adaptation by the white-rot fungus Fomes sp. EUM1

A thermotolerant white-rot fungus was identified as Fomes sp. EUM1. The strain exhibited maximum growth at 30 °C, with activation and inactivation energy values of 68 and 32 kJ/mol, respectively. The temperature affected the hyphal morphology, which was related to the thermotolerance of the microorganism: A shift from 30 to 40 °C in the growth temperature caused a decrease (15%) in mycelium branching; also longer (32%) and thinner (13%) hyphae were produced. In addition, as the temperature rose from 25 to 45 °C, an increase was observed in both the hyphal surface area (43%) and the surface growth rate (193%). The modification of the hyphal morphology suggests a strategy to colonize nutrient-rich areas while spending minimal energy for biomass formation under thermal stress.

Bulk segregant analysis followed by high-throughput sequencing reveals the Neurospora cell cycle gene, ndc-1, to be allelic with the gene for ornithine decarboxylase, spe-1

With the advent of high-throughput DNA sequencing, it is now straightforward and inexpensive to generate high-density small nucleotide polymorphism (SNP) maps. Here we combined high-throughput sequencing with bulk segregant analysis to expedite mutation mapping. The general map location of a mutation can be identified by a single backcross to a strain enriched in SNPs compared to a standard wild-type strain. Bulk segregant analysis simultaneously increases the likelihood of determining the precise nature of the mutation. We present here a high-density SNP map between Neurospora crassa Mauriceville-1-c (FGSC2225) and OR74A (FGSC2489), the strains most typically used by Neurospora researchers to carry out mapping crosses. We further have demonstrated the utility of the Mauriceville sequence and our approach by mapping the mutation responsible for the only existing temperature-sensitive (ts) cell cycle mutation in Neurospora, nuclear division cycle-1 (ndc-1). The single T-to-C point mutation maps to the gene encoding ornithine decarboxylase (ODC), spe-1 (NCU01271), and changes a Phe to a Ser residue within a highly conserved motif next to the catalytic site of the enzyme. By growth on spermidine and complementation with a wild-type spe-1 gene, we showed that the defect in spe-1 is responsible for the ts ndc-1 mutation. Based on our results, we propose changing ndc-1 to spe-1(ndc), which reflects that this mutation results in an ODC with a specific nuclear division defect.

Probing the growth dynamics of Neurospora crassa with microfluidic structures

Despite occupying physically and chemically heterogeneous natural environments, the growth dynamics of filamentous fungi is typically studied on the surface of homogeneous laboratory media. Fungal exploration and exploitation of complex natural environments requires optimal survival and growth strategies at the colony, hyphal, and intra hyphal level, with hyphal space-searching strategies playing a central role. We describe a new methodology for the characterisation and analysis of hyphal space-searching strategies, which uses purposefully designed three-dimensional microfluidics structures mimicking some of the characteristics of natural environments of the fungi. We also demonstrate this new methodology by running a comparative examination of two Neurospora crassa strains, i.e., the wild type of N. crassa -- a commonly used model organism for the study of filamentous fungi -- and the N. crassa ro-1 mutant strain -- which is deficient in hyphal and mycelial growth. Continuous live imaging showed that both strains responded actively to the geometrically confined microstructured environments without any detectable temporal delay or spatial adjustment. While both strains navigated the test structures exhibiting similar geometry-induced space-searching mechanisms, they presented fundamentally different growth patterns that could not be observed on geometrically unconfined, flat agar surfaces.

The hyphal tip structure of Basidiobolus sp.: a zygomycete fungus of uncertain phylogeny

To date, among the zygomycete fungi that have been examined, a Spitzenkörper has not been reported. In this paper, the cytoplasmic order of hyphal tip cells of Basidiobolus sp., a zygomycete genus of uncertain phylogeny, has been examined using light microscopy and transmission electron microscopy methods. With phase-contrast light optics, a phase-dark body was observed at the tips of growing hyphae of Basidiobolus sp. The hyphal apex also showed high affinity for FM4-64 labelling resulting in an intense fluorescence signal. The phase-dark inclusion exhibited independent motility within the hyphal apex and its presence and position were correlated to the rate and direction of hyphal growth. The hyphal apex of Basidiobolus sp. did not contain γ-tubulin. Ultrastructural observations revealed a dense cluster of vesicles at the hyphal apex. These results suggest that the growing hypha of Basidiobolus sp. contains a Spitzenkörper, a character generally attributed to members of the ascomycete and basidiomycete fungi and not to zygomycete fungi.

Live-cell imaging of microtubule dynamics in hyphae of Neurospora crassa

Due to the large number of microtubules in the wild-type strain, total internal reflection fluorescence (TIRF) microscopy was used to study cortical microtubule dynamics in leading hyphae of Neurospora crassa expressing beta-tubulin-GFP. Detection of plus-end dynamics of individual microtubule was much improved with this approach compared to the other commonly used methods such as confocal and widefield fluorescence microscopy. In order to address the roles of motor proteins in microtubule dynamics, microtubule-motor mutant strains, deltankin and ro-1 were examined. Unlike the wild-type strain, there were fewer microtubules in these hyphal cells; therefore, imaging was done using widefield fluorescence microscopy. We have shown that polymerization and depolymerization rates as well as hyphal extension rates were reduced by one half relative to those of wild type. Therefore, we believe that the hyphal extension rates are dependent upon the dynamic characteristics of microtubules, which are then regulated by microtubule motors in N. crassa.

Lack of the GTPase RHO-4 in Neurospora crassa causes a reduction in numbers and aberrant stabilization of microtubules at hyphal tips

The multinucleate hyphae of the filamentous ascomycete fungus Neurospora crassa grow by polarized hyphal tip extension. Both the actin and microtubule cytoskeleton are required for maximum hyphal extension, in addition to other vital processes. Previously, we have shown that the monomeric GTPase encoded by the N. crassa rho-4 locus is required for actin ring formation during the process of septation; rho-4 mutants lack septa. However, other phenotypic aspects of the rho-4 mutant, such as slow growth and cytoplasmic bleeding, led us to examine the hypothesis that the microtubule (MT) cytoskeleton of the rho-4 mutant was affected in morphology and dynamics. Unlike a wild-type strain, the rho-4 mutant had few MTs and these few MTs originated from nuclear spindle pole bodies. rho-4 mutants and rho-4 strains containing a GTP-locked (activated) rho-4 allele showed a reduction in numbers of cytoplasmic MTs and microtubule stabilization at hyphal tips. Strains containing a GDP-biased (negative) allele of rho-4 showed normal numbers of MTs and minor effects on microtubule stabilization. An examination of nuclear dynamics revealed that rho-4 mutants have large, and often, stretched or broken nuclei. These observations indicate that RHO-4 plays important roles in regulating both the actin and MT cytoskeleton, which are essential for optimal hyphal tip growth and in nuclear distribution and morphology.