Cell polarity in filamentous fungi: shaping the mold

Morphological Engineering of Filamentous Fungi: Research Progress and Perspectives

Filamentous fungi are important cell factories for the production of high-value enzymes and chemicals for the food, chemical, and pharmaceutical industries. Under submerged fermentation, filamentous fungi exhibit diverse fungal morphologies that are influenced by environmental factors, which in turn affect the rheological properties and mass transfer of the fermentation system, and ultimately the synthesis of products. In this review, we first summarize the mechanisms of mycelial morphogenesis and then provide an overview of current developments in methods and strategies for morphological regulation, including physicochemical and metabolic engineering approaches. We also anticipate that rapid developments in synthetic biology and genetic manipulation tools will accelerate morphological engineering in the future.

Twinfilin regulates actin assembly and Hexagonal peroxisome 1 (Hex1) localization in the pathogenesis of rice blast fungus Magnaporthe oryzae

Actin assembly at the hyphal tip is key for polar growth and pathogenesis of the rice blast fungus Magnaporthe oryzae. The mechanism of its precise assemblies and biological functions is not understood. Here, we characterized the role of M. oryzae Twinfilin (MoTwf) in M. oryzae infection through organizing the actin cables that connect to Spitzenkörper (Spk) at the hyphal tip. MoTwf could bind and bundle the actin filaments. It formed a complex with Myosin2 (MoMyo2) and the Woronin body protein Hexagonal peroxisome 1 (MoHex1). Enrichment of MoMyo2 and MoHex1 in the hyphal apical region was disrupted in a ΔMotwf loss-of-function mutant, which also showed a decrease in the number and width of actin cables. These findings indicate that MoTwf participates in the virulence of M. oryzae by organizing Spk-connected actin filaments and regulating MoHex1 distribution at the hyphal tip.

Molecular Mechanisms Involved in the Multicellular Growth of Ustilaginomycetes

Multicellularity is defined as the developmental process by which unicellular organisms became pluricellular during the evolution of complex organisms on Earth. This process requires the convergence of genetic, ecological, and environmental factors. In fungi, mycelial and pseudomycelium growth, snowflake phenotype (where daughter cells remain attached to their stem cells after mitosis), and fruiting bodies have been described as models of multicellular structures. Ustilaginomycetes are Basidiomycota fungi, many of which are pathogens of economically important plant species. These fungi usually grow unicellularly as yeasts (sporidia), but also as simple multicellular forms, such as pseudomycelium, multicellular clusters, or mycelium during plant infection and under different environmental conditions: Nitrogen starvation, nutrient starvation, acid culture media, or with fatty acids as a carbon source. Even under specific conditions, Ustilago maydis can form basidiocarps or fruiting bodies that are complex multicellular structures. These fungi conserve an important set of genes and molecular mechanisms involved in their multicellular growth. In this review, we will discuss in-depth the signaling pathways, epigenetic regulation, required polyamines, cell wall synthesis/degradation, polarized cell growth, and other cellular-genetic processes involved in the different types of Ustilaginomycetes multicellular growth. Finally, considering their short life cycle, easy handling in the laboratory and great morphological plasticity, Ustilaginomycetes can be considered as model organisms for studying fungal multicellularity.

Current views on endocytosis in filamentous fungi

Filamentous fungi grow by adding cell wall and membrane exclusively at the apex of tubular structures called hyphae. Growth was previously believed to occur only through exocytosis at the Spitzenkörper, an organised body of secretory macro- and microvesicles found only in growing hyphae. More recent work has indicated that an area deemed the sub-apical collar is enriched for endocytosis and is also required for hyphal growth. It is now generally believed that polarity of filamentous fungi is achieved through the balancing of the processes of endocytosis and exocytosis at these two areas. This review is an update on the current progress and understanding surrounding the occurrence of endocytosis and its spatial regulation as they pertain to growth and pathogenicity in filamentous fungi.

RNAi-Mediated Gene Silencing of Trcot1 Induces a Hyperbranching Phenotype in Trichoderma reesei

Trichoderma reesei is the major filamentous fungus used to produce cellulase and there is huge interest in promoting its ability to produce higher titers of cellulase. Among the many factors affecting cellulase production in T. reesei, the mycelial phenotype is important but seldom studied. Herein, a close homolog of the Neurospora crassa COT1 kinase was discovered in T. reesei and designated TrCOT1, which is of 83.3% amino acid sequence identity. Functional disruption of Trcot1 in T. reesei by RNAi-mediated gene silencing resulted in retarded sporulation on potato dextrose agar and dwarfed colonies on minimal medium agar plates containing glucose, xylan, lactose, xylose, or glycerol as the sole carbon source. The representative mutant strain, SUS2/Trcot1i, also displayed reduced mycelia accumulation but hyperbranching in the MM glucose liquid medium, with hyphal growth unit length values decreased to 73.0 µm/tip compared to 239.8 µm/tip for the parent strain SUS2. The hyperbranching phenotype led to slightly but significantly increased cellulase secretion from 24 to 72 h in a batch culture. However, the cellulase production per unit of mycelial biomass was much more profoundly improved from 24 to 96 h.

Molecular Mechanisms of Conidial Germination in Aspergillus spp

Aspergilli produce conidia for reproduction or to survive hostile conditions, and they are highly effective in the distribution of conidia through the environment. In immunocompromised individuals, inhaled conidia can germinate inside the respiratory tract, which may result in invasive pulmonary aspergillosis. The management of invasive aspergillosis has become more complex, with new risk groups being identified and the emergence of antifungal resistance. Patient survival is threatened by these developments, stressing the need for alternative therapeutic strategies. As germination is crucial for infection, prevention of this process might be a feasible approach. A broader understanding of conidial germination is important to identify novel antigermination targets. In this review, we describe conidial resistance against various stresses, transition from dormant conidia to hyphal growth, the underlying molecular mechanisms involved in germination of the most common Aspergillus species, and promising antigermination targets. Germination of Aspergillus is characterized by three morphotypes: dormancy, isotropic growth, and polarized growth. Intra- and extracellular proteins play an important role in the protection against unfavorable environmental conditions. Isotropically expanding conidia remodel the cell wall, and biosynthetic machineries are needed for cellular growth. These biosynthetic machineries are also important during polarized growth, together with tip formation and the cell cycle machinery. Genes involved in isotropic and polarized growth could be effective antigermination targets. Transcriptomic and proteomic studies on specific Aspergillus morphotypes will improve our understanding of the germination process and allow discovery of novel antigermination targets and biomarkers for early diagnosis and therapy.

The MaCreA Gene Regulates Normal Conidiation and Microcycle Conidiation in Metarhizium acridum

As a C₂H₂ type zinc finger transcription factor, CreA is the key in Carbon Catabolism Repression (CCR) pathway, which negatively regulates the genes in carbon sources utilization. As conidiation in filamentous fungi is affected by nutritional conditions, CreA may contribute to fungal conidiation, which has been well studied in filamentous fungi, especially Aspergillus spp., but researches on entomopathogenic fungi are not enough. In this study, we found a homologous gene MaCreA in Metarhizium acridum, and the MaCreA deletion strain showed delayed conidiation, significant decrease in conidial yield, and 96.88% lower conidial production, when compared with the wild-type strain, and the normal conidiation and microcycle conidiation pattern shift was blocked. RT-qPCR showed that the transcription levels of the genes FlbD and LaeA (related to asexual development) were significantly altered, and those of most of the conidiation-related genes were higher in ΔMaCreA strain. The results of RNA-Seq revealed that MaCreA regulated the two conidiation patterns by mediating genes related to cell cycle, cell division, cell wall, and cell polarity. In conclusion, CreA, as a core regulatory gene in conidiation, provides new insight into the mechanism of conidiation in entomopathogenic fungi.

Transcriptional profiling and localization of GUL-1, a COT-1 pathway component, in Neurospora crassa

Impairment of theNeurospora crassaCOT-1 kinase results in defects in hyphal polarity. Some of these effects are partially suppressed by inactivation of gul-1 (encoding an mRNA-binding protein involved in translational regulation). Here, we report on the transcriptional profiling of cot-1 inactivation and demonstrate that gul-1 affects transcript abundance of multiple genes in the COT-1 pathway, including processes such as cell wall remodeling, nitrogen and amino acid metabolism. The GUL-1 protein itself was found to be distributed within the entire hyphal cell, along with a clear presence of aggregates that traffic within the cytoplasm. Live imaging of GUL-1-GFP demonstrated that GUL-1 transport is microtubule-dependent. Cellular stress, as imposed by the presence of the cell wall biosynthesis inhibitor Nikkomycin Z or by nitrogen limitation, resulted in a 2-3-fold increase of GUL-1 aggregate association with nuclei. Taken together, this study demonstrates that GUL-1 affects multiple processes, its function is stress-related and linked with cellular traffic and nuclear association.

UmTea1, a Kelch and BAR domain-containing protein, acts at the cell cortex to regulate cell morphogenesis in the dimorphic fungus Ustilago maydis

The spatial organization of a cell is crucial for distribution of cell components and for cell morphogenesis in all organisms. Ustilago maydis, a basidiomycete fungus, has a yeast-like and a filamentous form. The former buds once per cell cycle at one of the cell poles, and can use the same site repeatedly or choose a new site at the same pole or opposite pole. The filamentous form consists of a long apical cell with short septate basal compartments lacking cytoplasm. It grows at the apex and can reverse growth forming a new growth zone at the basal end. We are interested in understanding how these different morphologies are generated. Here we present identification and characterization of U. maydis Tea1, a homologue of the fission yeast cell end marker Tea1. We demonstrate that UmTea1, a Kelch domain protein, interacts with itself and is an important determinant of the site of polarized growth: tea1 mutants bud simultaneously from both cell poles and form bifurcate buds. UmTea1 also regulates septum positioning, cell wall deposition, cell and neck width, coordination of nuclear division and cell separation, and localization of sterol-rich membrane domains. Some of these functions are shared with UmTea4, another cell end marker. We show that Tea1::GFP localizes to sites of polarized or potential polarized growth and to the septation site in the yeast-like form. Additionally, localization of Tea1::GFP as rings along the filament suggests that the filament undergoes septation. We hypothesize that Tea1 may act as a scaffold for the assembly of proteins that determine the site of polarized growth.

Identification of differentially expressed genes involved in spore germination of Penicillium expansum by comparative transcriptome and proteome approaches

In this study, Penicillium expansum, a common destructive phytopathogen and patulin producer was isolated from naturally infected apple fruits and identified by morphological observation and rDNA-internal transcribed spacer analysis. Subsequently, a global view of the transcriptome and proteome alteration of P. expansum spores during germination was evaluated by RNA-seq (RNA sequencing) and iTRAQ (isobaric tags for relative and absolute quantitation) approaches. A total of 3,026 differentially expressed genes (DEGs), 77 differentially expressed predicted transcription factors and 489 differentially expressed proteins (DEPs) were identified. The next step involved screening out 130 overlapped candidates through correlation analysis between the RNA-seq and iTRAQ datasets. Part of them showed a different expression trend in the mRNA and protein levels, and most of them were involved in metabolism and genetic information processing. These results not only highlighted a set of genes and proteins that were important in deciphering the molecular processes of P. expansum germination but also laid the foundation to develop effective control methods and adequate environmental conditions.

Molecular Insights Into Development and Virulence Determinants of Aspergilli: A Proteomic Perspective

Aspergillus species are the major cause of health concern worldwide in immunocompromised individuals. Opportunistic Aspergilli cause invasive to allergic aspergillosis, whereas non-infectious Aspergilli have contributed to understand the biology of eukaryotic organisms and serve as a model organism. Morphotypes of Aspergilli such as conidia or mycelia/hyphae helped them to survive in favorable or unfavorable environmental conditions. These morphotypes contribute to virulence, pathogenicity and invasion into hosts by excreting proteins, enzymes or toxins. Morphological transition of Aspergillus species has been a critical step to infect host or to colonize on food products. Thus, we reviewed proteins from Aspergilli to understand the biological processes, biochemical, and cellular pathways that are involved in transition and morphogenesis. We majorly analyzed proteomic studies on A. fumigatus, A. flavus, A. terreus, and A. niger to gain insight into mechanisms involved in the transition from conidia to mycelia along with the role of secondary metabolites. Proteome analysis of morphotypes of Aspergilli provided information on key biological pathways required to exit conidial dormancy, consortia of virulent factors and mycotoxins during the transition. The application of proteomic approaches has uncovered the biological processes during development as well as intermediates of secondary metabolite biosynthesis pathway. We listed key proteins/ enzymes or toxins at different morphological types of Aspergillus that could be applicable in discovery of novel therapeutic targets or metabolite based diagnostic markers.

Abnormal Ergosterol Biosynthesis Activates Transcriptional Responses to Antifungal Azoles

Fungi transcriptionally upregulate expression of azole efflux pumps and ergosterol biosynthesis pathway genes when exposed to antifungal agents that target ergosterol biosynthesis. To date, these transcriptional responses have been shown to be dependent on the presence of the azoles and/or depletion of ergosterol. Using an inducible promoter to regulate Neurospora crassa erg11, which encodes the major azole target, sterol 14α-demethylase, we were able to demonstrate that the CDR4 azole efflux pump can be transcriptionally activated by ergosterol biosynthesis inhibition even in the absence of azoles. By analyzing ergosterol deficient mutants, we demonstrate that the transcriptional responses by cdr4 and, unexpectedly, genes encoding ergosterol biosynthesis enzymes (erg genes) that are responsive to azoles, are not dependent on ergosterol depletion. Nonetheless, deletion of erg2, which encodes C-8 sterol isomerase, also induced expression of cdr4. Deletion of erg2 also induced the expression of erg24, the gene encoding C-14 sterol reductase, but not other tested erg genes which were responsive to erg11 inactivation. This indicates that inhibition of specific steps of ergosterol biosynthesis can result in different transcriptional responses, which is further supported by our results obtained using different ergosterol biosynthesis inhibitors. Together with the sterol profiles, these results suggest that the transcriptional responses by cdr4 and erg genes are associated with accumulation of specific sterol intermediate(s). This was further supported by the fact that when the erg2 mutant was treated with ketoconazole, upstream inhibition overrode the effects by downstream inhibition on ergosterol biosynthesis pathway. Even though cdr4 expression is associated with the accumulation of sterol intermediates, intra- and extracellular sterol analysis by HPLC-MS indicated that the transcriptional induction of cdr4 did not result in efflux of the accumulated intermediate(s). This study demonstrates, by detailed genetic and chemical analysis, that transcriptional responses by a major efflux pump and genes of the ergosterol biosynthesis pathway to ergosterol biosynthesis inhibitors can be independent of the presence of the drugs and are linked with the accumulation of ergosterol intermediate(s).

The Neurospora crassa PP2A Regulatory Subunits RGB1 and B56 Are Required for Proper Growth and Development and Interact with the NDR Kinase COT1

COT1 is the founding member of the highly conserved nuclear Dbf2-related (NDR) Ser/Thr kinase family and plays a role in the regulation of polar growth and development in Neurospora crassa and other fungi. Changes in COT1 phosphorylation state have been shown to affect hyphal elongation, branching, and conidiation. The function of NDR protein kinases has been shown to be regulated by type 2A protein phosphatases (PP2As). PP2As are heterotrimers comprised of a catalytic and scaffolding protein along with an interchangeable regulatory subunit involved in determining substrate specificity. Inactivation of the N. crassa PP2A regulatory subunits rgb-1 and b56 conferred severe hyphal growth defects. Partial suppression of defects observed in the rgb-1^RIP strain (but not in the Δb56 mutant) was observed in cot-1 phosphomimetic mutants, demonstrating that altering COT1 phosphorylation state can bypass, at least in part, the requirement of a functional RGB1 subunit. The functional fusion proteins RGB1::GFP and B56::GFP predominantly localized to hyphal tips and septa, respectively, indicating that their primary activity is in different cellular locations. COT1 protein forms exhibited a hyperphosphorylated gel migration pattern in an rgb-1^RIP mutant background, similar to that observed when the fungus was cultured in the presence of the PP2A inhibitor cantharidin. COT1 was hypophosphorylated in a Δb56 mutant background, suggesting that this regulatory subunit may be involved in determining COT1 phosphorylation state, yet in an indirect manner. Reciprocal co-immunoprecipitation analyses, using tagged COT1, PPH1, RGB1, and B56 subunits established that these proteins physically interact. Taken together, our data determine the presence of a functional and physical link between PP2A and COT1 and show that two of the PP2A regulatory subunits interact with the kinase and determine COT1 phosphorylation state.

High-density genetic mapping identifies the genetic basis of a natural colony morphology mutant in the root rot pathogen Armillaria ostoyae

Filamentous fungi exhibit a broad spectrum of heritable growth patterns and morphological variations reflecting the adaptation of the different species to distinct ecological niches. But also within species, isolates show considerable variation in growth rates and other morphological characteristics. The genetic basis of this intraspecific variation in mycelial growth and morphology is currently poorly understood. By chance, a growth mutant in the root rot pathogen Armillaria ostoyae was discovered. The mutant phenotype was characterized by extremely compact and slow growth, as well as shorter aerial hyphae and hyphal compartments in comparison to the wildtype phenotype. Genetic analysis revealed that the abnormal phenotype is caused by a recessive mutation, which segregates asa single locus in sexual crosses. In order to identify the genetic basis of the mutant phenotype, we performed a quantitative trait locus (QTL) analysis. A mapping population of 198 haploid progeny was genotyped at 11,700 genome-wide single nucleotide polymorphisms (SNPs) making use of double digest restriction site associated DNA sequencing (ddRADseq). In accordance with the genetic analysis, a single significant QTL was identified for the abnormal growth phenotype. The QTL confidence interval spans a narrow, gene dense region of 87kb in the A. ostoyae genome which contains 37 genes. Overall, our study reports the first high-density genetic map for an Armillaria species and shows its successful application in forward genetics by resolving the genetic basis of a mutant phenotype with a severe defect in hyphal growth.

Evolutionary dynamics in the fungal polarization network, a mechanistic perspective

Polarity establishment underlies proper cell cycle completion across virtually all organisms. Much progress has been made in generating an understanding of the structural and functional components of this process, especially in model species. Here we focus on the evolutionary dynamics of the fungal polarization protein network in order to determine general components and mechanistic principles, species- or lineage-specific adaptations and the evolvability of the network. The currently available genomic and proteomic screens in a variety of fungal species have shown three main characteristics: (1) certain proteins, processes and functions are conserved throughout the fungal clade; (2) orthologous functions can never be assumed, as various cases have been observed of homologous loci with dissimilar functions; (3) species have, typically, various species- or lineage-specific proteins incorporated in their polarization network. Further large-scale comparative and experimental studies, including those on non-model species representing the great fungal diversity, are needed to gain a better understanding of the evolutionary dynamics and generalities of the polarization network in fungi.

RNAi-Based Functional Genomics Identifies New Virulence Determinants in Mucormycosis

Mucorales are an emerging group of human pathogens that are responsible for the lethal disease mucormycosis. Unfortunately, functional studies on the genetic factors behind the virulence of these organisms are hampered by their limited genetic tractability, since they are reluctant to classical genetic tools like transposable elements or gene mapping. Here, we describe an RNAi-based functional genomic platform that allows the identification of new virulence factors through a forward genetic approach firstly described in Mucorales. This platform contains a whole-genome collection of Mucor circinelloides silenced transformants that presented a broad assortment of phenotypes related to the main physiological processes in fungi, including virulence, hyphae morphology, mycelial and yeast growth, carotenogenesis and asexual sporulation. Selection of transformants with reduced virulence allowed the identification of mcplD, which encodes a Phospholipase D, and mcmyo5, encoding a probably essential cargo transporter of the Myosin V family, as required for a fully virulent phenotype of M. circinelloides. Knock-out mutants for those genes showed reduced virulence in both Galleria mellonella and Mus musculus models, probably due to a delayed germination and polarized growth within macrophages. This study provides a robust approach to study virulence in Mucorales and as a proof of concept identified new virulence determinants in M. circinelloides that could represent promising targets for future antifungal therapies.

Mucormycosis is an infectious disease caused by organisms of the order Mucorales. It is a lethal infection that is raising the alarm in the medical and scientific community due to its high mortality rates, unusual antifungal drug resistance and its emerging character. Among the reasons explaining the nescience about this disease is the lack of knowledge on the biology of the organisms that cause mucormycosis, which is encouraged by the reluctance of these species to genetic studies. In this work, we have developed an RNAi-based functional genomic platform to study virulence in Mucorales. It is a powerful tool available for the scientific community that will contribute to solve the reluctance of Mucorales to genetic studies and will help to understand the genetic basis of virulence in these organisms. Secondly, and as a proof of concept, we have used this genetic tool to identify two new virulence determinants in Mucor circinelloides. Lack of function of these determinants delays germination and growth of spores, conceding time to macrophages for the inactivation of the pathogen. The two genes identified, mcplD and mcmyo5, represent promising targets for future development of new antifungal therapies against mucormycosis.

Nano-LC-Q-TOF Analysis of Proteome Revealed Germination of Aspergillus flavus Conidia is Accompanied by MAPK Signalling and Cell Wall Modulation

Aspergillus flavus is the second most leading cause of aspergillosis. The ability of A. flavus to adapt within the host environment is crtical for its colonization. Onset of germination of conidia is one of the crucial events; thus, in order to gain insight into A. flavus molecular adaptation while germination, protein profile of A. flavus was obtained. Approximately 82 % of conidia showed germination at 7 h; thus, samples were collected followed by protein extraction and subjected to nLC-Q-TOF mass spectrometer. Q-TOF data were analysed using Protein Lynx Global Services (PLGS 2.2.5) software. A total of 416 proteins were identified from UniProt Aspergillus species database. Orthologues of A. flavus was observed in A. fumigatus, A. niger, A. terreus, A. oryzae, etc. Proteins were further analysed in NCBI database, which showed that 27 proteins of A. flavus are not reported in UniProt and NCBI database. Functional characterization of proteins resulted majorly to cell wall synthesis and degradation, metabolisms (carbohydrate and amino acid metabolism), protein synthesis and degradation. Proteins/enzymes associated with aflatoxin biosynthesis were observed. We also observed Dicer-like proteins 1, 2 and autophagy-related proteins 2, 9, 18, 13, 11, 22. Expression of protein/enzymes associated with MAPK signalling pathway suggests their role during the germination process. Overall, the data present a catalogue of proteins/enzymes involved in the germination of A. flavus conidia and could also be applied to other Aspergillus species.

Multiple isoforms for the catalytic subunit of PKA in the basal fungal lineage Mucor circinelloides

Protein kinase A (PKA) activity is involved in dimorphism of the basal fungal lineage Mucor. From the recently sequenced genome of Mucor circinelloides we could predict ten catalytic subunits of PKA. From sequence alignment and structural prediction we conclude that the catalytic core of the isoforms is conserved, and the difference between them resides in their amino termini. This high number of isoforms is maintained in the subdivision Mucoromycotina. Each paralogue, when compared to the ones form other fungi is more homologous to one of its orthologs than to its paralogs. All of these fungal isoforms cannot be included in the class I or II in which fungal protein kinases have been classified. mRNA levels for each isoform were measured during aerobic and anaerobic growth. The expression of each isoform is differential and associated to a particular growth stage. We reanalyzed the sequence of PKAC (GI 20218944), the only cloned sequence available until now for a catalytic subunit of M. circinelloides. PKAC cannot be classified as a PKA because of its difference in the conserved C-tail; it shares with PKB a conserved C2 domain in the N-terminus. No catalytic activity could be measured for this protein nor predicted bioinformatically. It can thus be classified as a pseudokinase. Its importance can not be underestimated since it is expressed at the mRNA level in different stages of growth, and its deletion is lethal.

The F-actin capping protein is required for hyphal growth and full virulence but is dispensable for septum formation in Botrytis cinerea

Filamentous (F-) actin is an integral part of the cytoskeleton allowing for cell growth, intracellular motility, and cytokinesis of eukaryotic cells. Its assembly from G-actin monomers and its disassembly are tightly regulated processes involving a number of actin-binding proteins (ABPs) such as F-actin nucleators and cross-linking proteins. F-actin capping protein (CP) is an alpha/beta heterodimer known from yeast and higher eukaryotes to bind to the fast growing ends of the actin filaments stabilizing them. In this study, we identified the orthologs of the two CP subunits, named BcCPA1 and BcCPB1, in the plant pathogenic fungus Botrytis cinerea and showed that the two proteins physically interact in a yeast two-hybrid approach. GFP-BcCPA1 fusion proteins were functional and localized to the assumed sites of F-actin accumulation, i.e. to the hyphal tips and the sites of actin ring formation. Deletion of bccpa1 had a profound effect on hyphal growth, morphogenesis, and virulence indicating the importance of F-actin capping for an intact actin cytoskeleton. As polarized growth - unlike septum formation - is impaired in the mutants, it can be concluded that the organization and/or localization of actin patches and cables are disturbed rather than the functionality of the actin rings.

Distinct Roles of Myosins in Aspergillus fumigatus Hyphal Growth and Pathogenesis

Myosins are a family of actin-based motor proteins found in many organisms and are categorized into classes based on their structures. Class II and V myosins are known to be important for critical cellular processes, including cytokinesis, endocytosis, exocytosis, and organelle trafficking, in the model fungi Saccharomyces cerevisiae and Aspergillus nidulans However, the roles of myosins in the growth and virulence of the pathogen Aspergillus fumigatus are unknown. We constructed single- and double-deletion strains of the class II and class V myosins in A. fumigatus and found that while the class II myosin (myoB) is dispensable for growth, the class V myosin (myoE) is required for proper hyphal extension; deletion of myoE resulted in hyperbranching and loss of hyphal polarity. Both myoB and myoE are necessary for proper septation, conidiation, and conidial germination, but only myoB is required for conidial viability. Infection with the ΔmyoE strain in the invertebrate Galleria mellonella model and also in a persistently immunosuppressed murine model of invasive aspergillosis resulted in hypovirulence, while analysis of bronchoalveolar lavage fluid revealed that tumor necrosis factor alpha (TNF-α) release and cellular infiltration were similar compared to those of the wild-type strain. The ΔmyoE strain showed fungal growth in the murine lung, while the ΔmyoB strain exhibited little fungal burden, most likely due to the reduced conidial viability. These results show, for the first time, the important role these cytoskeletal components play in the growth of and disease caused by a known pathogen, prompting future studies to understand their regulation and potential targeting for novel antifungal therapies.

Fungal spores for dispersion in space and time

Spores are an integral part of the life cycle of the gross majority of fungi. Their morphology and the mode of formation are both highly variable among the fungi, as is their resistance to stressors. The main aim for spores is to be dispersed, both in space, by various mechanisms or in time, by an extended period of dormancy. Some fungal ascospores belong to the most stress-resistant eukaryotic cells described to date. Stabilization is a process in which biomolecules and complexes thereof are protected by different types of molecules against heat, drought, or other molecules. This review discusses the most important compounds that are known to protect fungal spores and also addresses the biophysics of cell protection. It further covers the phenomena of dormancy, breaking of dormancy, and early germination. Germination is the transition from a dormant cell toward a vegetative cell and includes a number of specific changes. Finally, the applied aspects of spore biology are discussed.

Live Cell Imaging of Actin Dynamics in the Filamentous Fungus Aspergillus nidulans

Hyphal cells of filamentous fungi grow at their tips in a method analogous to pollen tube and root hair elongation. This process, generally referred to as tip growth, requires precise regulation of the actin cytoskeleton, and characterizing the various actin structures in these cell types is currently an active area of research. Here, the actin marker Lifeact was used to document actin dynamics in the filamentous fungus Aspergillus nidulans. Contractile double rings were observed at septa, and annular clusters of puncta were seen subtending growing hyphal tips, corresponding to the well-characterized subapical endocytic collar. However, Lifeact also revealed two additional structures. One, an apical array, was dynamic on the face opposite the tip, while a subapical web was dynamic on the apical face and was located several microns behind the growth site. Each was observed turning into the other over time, implying that they could represent different localizations of the same structure, although hyphae with a subapical web grew faster than those exhibiting an apical array. The subapical web has not been documented in any filamentous fungus to date, and is separate from the networks of F-actin seen in other tip-growing organisms surrounding septa or stationary along the plasmalemma.

Nitric Oxide and Reactive Oxygen Species Coordinately Regulate the Germination of Puccinia striiformis f. sp. tritici Urediniospores

Nitric oxide (NO) and reactive oxygen species (ROS) function as signaling molecules in a number of critical signal transduction pathways in plants, including plant biotic interactions. In addition to the role of plant-derived NO and ROS in plant resistance, which has been well documented, pathogen-produced NO and ROS have recently emerged as important players in fungal development and pathogenesis. However, the effects of pathogenic fungi-derived NO and ROS on signaling pathways during fungal pre-infection development remain unknown. Here, using a combination of pharmacological approaches and confocal microscopy, we investigated the roles of NO and ROS during the germination of Puccinia striiformis Westend f. sp. tritici (Pst) the wheat stripe rust pathogen. Both NO and ROS have a crucial role in uredinial germination. The scavengers of NO and ROS delayed spore germination and decreased the lengths of germ tubes. A similar phenotype was produced after treatment with the promoter. However, the spores germinated and grew normally when the levels of NO and ROS were simultaneously elevated by the application of a promoter of NO and a donor of ROS. Confocal laser microscopy indicated that both NO and ROS preferentially localized at the germ pores and apexes of growing germ tubes when the ROS/NO ratio in the spores was maintained in a specific range. We concluded that both NO and ROS are critical signaling molecules in the pre-infection development of Pst and that the polar growth of the germ tube is coordinately regulated by NO and ROS.

Real-time monitoring of fungal inhibition and morphological changes

Mold growth constitutes a problem in many food and clinical environments and there is therefore focus on studying antifungal activity. Methods for determining growth inhibition by measuring colony growth or biomass are, however, time-taking and rapid methods for evaluation of antifungal effects are needed. Propionic acid and diacetyl are antifungal compounds produced by a range of dairy-associated bacteria. Their activity against Penicillium spp. was monitored real-time using an optical detection system with tilted focus plane to assess growth and morphological changes of Penicillium spp. by image recording inside a 96 well microplate. Images were used for generation of growth curves by using a segmentation and extraction of surface areas (SESA) algorithm and for quantifying morphology changes. Using image analysis growth could be detected within 15 h compared with more than 30 h when using standard optical density measurements. Induced morphological changes of fungi could furthermore be visualized and quantified using morphological descriptors such as circularity, branch points, perimeter and area of spores and growing hyphae. Propionic acid inhibited two out of two Penicillium spp. while morphological changes were strain dependent at the concentrations tested. Diacetyl inhibited six out of six Penicillium spp. strains and increased spore size and number of germination sites in two out of six of the strains prior to germination.

Membrane-Coupled mRNA Trafficking in Fungi

Intracellular logistics are essential for delivery of newly synthesized material during polar growth of fungal hyphae. Proteins and lipids are actively transported throughout the cell by motor-dependent movement of small vesicles or larger units such as endosomes and the endoplasmic reticulum. A remarkably tight link is emerging between active membrane trafficking and mRNA transport, a process that determines the precise subcellular localization of translation products within the cell. Here, we report on recent insights into the mechanism and biological role of these intricate cotransport processes in fungal models such as Saccharomyces cerevisiae, Candida albicans, and Ustilago maydis. In the latter, we focus on the new finding of endosomal mRNA transport and its implications for protein targeting, complex assembly, and septin biology.

Live cell imaging of endosomal trafficking in fungi

Endosomes are multipurpose membranous carriers important for endocytosis and secretion. During membrane trafficking, endosomes transport lipids, proteins, and even RNAs. In highly polarized cells such as fungal hyphae, they shuttle bidirectionally along microtubules mediated by molecular motors like kinesins and dynein. For in vivo studies of these highly dynamic protein/membrane complexes, advanced fluorescence microscopy is instrumental. In this chapter, we describe live cell imaging of endosomes in two distantly related fungal model systems, the basidiomycete Ustilago maydis and the ascomycete Aspergillus nidulans. We provide insights into live cell imaging of dynamic endosomal proteins and RNA, dual-color detection for colocalization studies, as well as fluorescence recovery after photobleaching (FRAP) for quantification and photo-activated localization microscopy (PALM) for super-resolution. These methods described in two well-studied fungal model systems are applicable to a broad range of other organisms.

Calcium-containing phosphopeptides pave the secretory pathway for efficient protein traffic and secretion in fungi

Casein phosphopeptides (CPPs) containing chelated calcium drastically increase the secretion of extracellular homologous and heterologous proteins in filamentous fungi. Casein phosphopeptides released by digestion of alpha - and beta-casein are rich in phosphoserine residues (SerP). They stimulate enzyme secretion in the gastrointestinal tract and enhance the immune response in mammals, and are used as food supplements. It is well known that casein phosphopeptides transport Ca2+ across the membranes and play an important role in Ca2+ homeostasis in the cells. Addition of CPPs drastically increases the production of heterologous proteins in Aspergillus as host for industrial enzyme production. Recent proteomics studies showed that CPPs alter drastically the vesicle-mediated secretory pathway in filamentous fungi, apparently because they change the calcium concentration in organelles that act as calcium reservoirs. In the organelles calcium homeostasis a major role is played by the pmr1 gene, that encodes a Ca2+/Mn2+ transport ATPase, localized in the Golgi complex; this transporter controls the balance between intra-Golgi and cytoplasmic Ca2+ concentrations. A Golgi-located casein kinase (CkiA) governs the ER to Golgi directionality of the movement of secretory proteins by interacting with the COPII coat of secretory vesicles when they reach the Golgi. Mutants defective in the casein-2 kinase CkiA show abnormal targeting of some secretory proteins, including cytoplasmic membrane amino acid transporters that in ckiA mutants are miss-targeted to vacuolar membranes. Interestingly, addition of CPPs increases a glyceraldehyde-3-phpshate dehydrogenase protein that is known to associate with microtubules and act as a vesicle/membrane fusogenic agent. In summary, CPPs alter the protein secretory pathway in fungi adapting it to a deregulated protein traffic through the organelles and vesicles what results in a drastic increase in secretion of heterologous and also of some homologous proteins.

FgFim, a key protein regulating resistance to the fungicide JS399-19, asexual and sexual development, stress responses and virulence in Fusarium graminearum

Fimbrin is an actin-bundling protein found in intestinal microvilli, hair cell stereocilia and fibroblast filopodia. Its homologue Sac6p has been shown to play a critical role in endocytosis and diverse cellular processes in Saccharomyces cerevisiae. FgFim from the wheat scab pathogenic fungus Fusarium graminearum strain Y2021A, which is highly resistant to the fungicide JS399-19, was identified by screening a mutant library generated by HPH-HSV-tk cassette-mediated integration. The functions of FgFim were evaluated by constructing a deletion mutant of FgFim, designated ΔFgFim-15. The deletion mutant exhibited a reduced rate of mycelial growth, reduced conidiation, delayed conidium germination, irregularly shaped hyphae, a lack of sexual reproduction on autoclaved wheat kernels and a dramatic decrease in resistance to JS399-19. ΔFgFim-15 also exhibited increased sensitivity to diverse metal cations, to agents that induce osmotic stress and oxidative stress, and to agents that damage the cell membrane and cell wall. Pathogenicity assays showed that the virulence of the FgFim deletion mutant on flowering wheat heads was impaired, which was consistent with its reduced production of the toxin deoxynivalenol in host tissue. All of these defects were restored by genetic complementation of the mutant with the parental FgFim gene. Quantitative real-time polymerase chain reaction (PCR) assays showed that the basal expression of three Cyp51 genes, which encode sterol 14α-demethylase, was significantly lower in the mutant than in the parental strain. The results of this study indicate that FgFim plays a critical role in the regulation of resistance to JS399-19 and in various cellular processes in F. graminearum.

Functional analysis of BcBem1 and its interaction partners in Botrytis cinerea: impact on differentiation and virulence

In phytopathogenic fungi the establishment and maintenance of polarity is not only essential for vegetative growth and differentiation, but also for penetration and colonization of host tissues. We investigated orthologs of members of the yeast polarity complex in the grey mould fungus Botrytis cinerea: the scaffold proteins Bem1 and Far1, the GEF (guanine nucleotide exchange factor) Cdc24, and the formin Bni1 (named Sep1 in B. cinerea). BcBem1 does not play an important role in regular hyphal growth, but has significant impact on spore formation and germination, on the establishment of conidial anastomosis tubes (CATs) and on virulence. As in other fungi, BcBem1 interacts with the GEF BcCdc24 and the formin BcSep1, indicating that in B. cinerea the apical complex has a similar structure as in yeast. A functional analysis of BcCdc24 suggests that it is essential for growth, since it was not possible to obtain homokaryotic deletion mutants. Heterokaryons of Δcdc24 (supposed to exhibit reduced bccdc24 transcript levels) already show a strong phenotype: an inability to penetrate the host tissue, a significantly reduced growth rate and malformation of conidia, which tend to burst as observed for Δbcbem1. Also the formin BcSep1 has significant impact on hyphal growth and development, whereas the role of the putative ortholog of the yeast scaffold protein Far1 remains open: Δbcfar1 mutants have no obvious phenotypes.

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 N-terminal region of the Neurospora NDR kinase COT1 regulates morphology via its interactions with MOB2A/B

Nuclear Dbf2p-related (NDR) protein kinases are important for cell differentiation and polar morphogenesis in various organisms, yet some of their functions are still elusive. Dysfunction of the Neurospora crassa NDR kinase COT1 leads to cessation of tip extension and hyperbranching. NDR kinases require the physical interaction between the kinase's N-terminal region (NTR) and the MPS1-binding (MOB) proteins for their activity and functions. To study the interactions between COT1 and MOB2 proteins, we mutated several conserved residues and a novel phosphorylation site within the COT1 NTR. The phenotypes of these mutants suggest that the NTR is required for COT1 functions in regulating hyphal elongation and branching, asexual conidiation and germination. Interestingly, while both MOB2A and MOB2B promote proper hyphal growth, they have distinct COT1-dependent roles in regulation of macroconidiation. Immunoprecipitation experiments indicate physical association of COT1 with both MOB2A and MOB2B, simultaneously. Furthermore, the binding of the two MOB2 proteins to COT1 is mediated by different residues at the COT1 NTR, suggesting a hetero-trimer is formed. Thus, although MOB2A/B may have some overlapping functions in regulating hyphal tip extension, their function is not redundant and they are both required for proper fungal development.

The Aspergillus nidulans peripheral ER: disorganization by ER stress and persistence during mitosis

The genetically amenable fungus Aspergillus nidulans is well suited for cell biology studies involving the secretory pathway and its relationship with hyphal tip growth by apical extension. We exploited live-cell epifluorescence microscopy of the ER labeled with the translocon component Sec63, endogenously tagged with GFP, to study the organization of ‘secretory’ ER domains. The Sec63 A. nidulans ER network includes brightly fluorescent peripheral strands and more faintly labeled nuclear envelopes. In hyphae, the most abundant peripheral ER structures correspond to plasma membrane-associated strands that are polarized, but do not invade the hyphal tip dome, at least in part because the subapical collar of endocytic actin patches constrict the cortical strands in this region. Thus the subapical endocytic ring might provide an attachment for ER strands, thereby ensuring that the growing tip remains ‘loaded’ with secretory ER. Acute disruption of secretory ER function by reductive stress-mediated induction of the unfolded protein response results in the reversible aggregation of ER strands, cessation of exocytosis and swelling of the hyphal tips. The secretory ER is insensitive to brefeldin A treatment and does not undergo changes during mitosis, in agreement with the reports that apical extension continues at normal rates during this period.

The actin-regulating kinase homologue MoArk1 plays a pleiotropic function in Magnaporthe oryzae

Endocytosis is an essential cellular process in eukaryotic cells that involves concordant functions of clathrin and adaptor proteins, various protein and lipid kinases, phosphatases and the actin cytoskeleton. In Saccharomyces cerevisiae, Ark1p is a member of the serine/threonine protein kinase (SPK) family that affects profoundly the organization of the cortical actin cytoskeleton. To study the function of MoArk1, an Ark1p homologue identified in Magnaporthe oryzae, we disrupted the MoARK1 gene and characterized the ΔMoark1 mutant strain. The ΔMoark1 mutant exhibited various defects ranging from mycelial growth and conidial formation to appressorium-mediated host infection. The ΔMoark1 mutant also exhibited decreased appressorium turgor pressure and attenuated virulence on rice and barley. In addition, the ΔMoark1 mutant displayed defects in endocytosis and formation of the Spitzenkörper, and was hyposensitive to exogenous oxidative stress. Moreover, a MoArk1-green fluorescent protein (MoArk1-GFP) fusion protein showed an actin-like localization pattern by localizing to the apical regions of hyphae. This pattern of localization appeared to be regulated by the N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins MoSec22 and MoVam7. Finally, detailed analysis revealed that the proline-rich region within the MoArk1 serine/threonine kinase (S_TKc) domain was critical for endocytosis, subcellular localization and pathogenicity. These results collectively suggest that MoArk1 exhibits conserved functions in endocytosis and actin cytoskeleton organization, which may underlie growth, cell wall integrity and virulence of the fungus.

Aspergillus nidulans translationally controlled tumor protein has a role in the balance between asexual and sexual differentiation and normal hyphal branching

Translationally controlled tumor protein (TCTP) is a highly conserved and ubiquitously expressed protein present in all eukaryotes. Cellular functions of TCTP include growth promoting, allergic response and responses to various cellular stresses, but the functions in filamentous fungi have not been reported. In this report, we characterized an Aspergillus nidulans TCTP (TcpA) with high similarity to TCTP. The level of tcpa mRNA was relatively high, both during vegetative growth stage and at early phases of development. TcpA was found predominantly in the nucleus during germination and mycelial growth, and was localized in cytoplasm and nuclei of vesicles on stipes during conidia development. Deletion of tcpA resulted in abnormal hyphal branch formation during vegetative growth. The tcpA deletion inhibited sexual development, but enhanced asexual development via induction of brlA expression. These results imply that TcpA is involved in normal hyphal branch establishment during vegetative growth and also has a role in the balance between asexual and sexual differentiation.

Germination of conidia of Aspergillus niger is accompanied by major changes in RNA profiles

The transcriptome of conidia of Aspergillus niger was analysed during the first 8 h of germination. Dormant conidia started to grow isotropically two h after inoculation in liquid medium. Isotropic growth changed to polarised growth after 6 h, which coincided with one round of mitosis. Dormant conidia contained transcripts from 4 626 genes. The number of genes with transcripts decreased to 3 557 after 2 h of germination, after which an increase was observed with 4 780 expressed genes 8 h after inoculation. The RNA composition of dormant conidia was substantially different than all the subsequent stages of germination. The correlation coefficient between the RNA profiles of 0 h and 8 h was 0.46. They were between 0.76-0.93 when profiles of 2, 4 and 6 h were compared with that of 8 h. Dormant conidia were characterised by high levels of transcripts of genes involved in the formation of protecting components such as trehalose, mannitol, protective proteins (e.g. heat shock proteins and catalase). Transcripts belonging to the Functional Gene Categories (FunCat) protein synthesis, cell cycle and DNA processing and respiration were over-represented in the up-regulated genes at 2 h, whereas metabolism and cell cycle and DNA processing were over-represented in the up-regulated genes at 4 h. At 6 h and 8 h no functional gene classes were over- or under-represented in the differentially expressed genes. Taken together, it is concluded that the transcriptome of conidia changes dramatically during the first two h and that initiation of protein synthesis and respiration are important during early stages of germination.

Development in Aspergillus

The genus Aspergillus represents a diverse group of fungi that are among the most abundant fungi in the world. Germination of a spore can lead to a vegetative mycelium that colonizes a substrate. The hyphae within the mycelium are highly heterogeneous with respect to gene expression, growth, and secretion. Aspergilli can reproduce both asexually and sexually. To this end, conidiophores and ascocarps are produced that form conidia and ascospores, respectively. This review describes the molecular mechanisms underlying growth and development of Aspergillus.

Evolution of modular conidiophore development in the aspergilli

Conidiophores are reproductive structures that enable filamentous fungi to produce and disseminate large numbers of asexual spores. The diversity in conidiophore morphology is sufficiently large to serve as a basis for fungal systematics. Aspergillus and Penicillium species are members of the family Trichocomaceae that form conidiophores with characteristic architecture. Whereas the Penicillium conidiophore appears to be a modified branched hyphal structure, the Aspergillus conidiophore is seemingly more complex and includes additional cell types. Here, it is proposed that the "aspergillioid" conidiophore may have evolved from a "penicillioid" ancestor via changes in expression of key regulators of the cell cycle and the GTPase Cdc42. Because the transcriptional regulatory network that controls conidiophore development in Aspergillus is well characterized, further study of how this network links to regulators of the cell cycle and Cdc42 should provide fundamental insight into the evolution of developmental morphogenesis in fungi (i.e., fungal evo-devo).

Characterization of the major Woronin body protein HexA of the human pathogenic mold Aspergillus fumigatus

In filamentous fungi, the septal pore controls the exchange between neighbouring hyphal compartments. Woronin bodies are fungal-specific organelles that plug the pore in case of physical damage. The Hex protein is their major and essential component. Hex proteins of different size are predicted in the data base for pathogenic and non-pathogenic Aspergillus species. However, using specific monoclonal antibodies, we identified 2 dominant HexA protein species of 20 and 25kDa in A. fumigatus, A. terreus, A. nidulans, and A. oryzae. HexA and Woronin bodies were found in A. fumigatus hyphae, but also in resting conidia. Using monoclonal antibodies, a GFP-HexA fusion protein, and an RFP protein fused to the putative peroxisomal targeting sequence of HexA, we analyzed the spatial localization and dynamics of Woronin bodies in A. fumigatus as well as their formation from peroxisomes. In intact hyphae, some Woronin bodies were found in close proximity to the septal pore, while the majority was distributed in the cytoplasm. Septum-associated Woronin bodies show a minimal lateral movement, while the cytosolic Woronin bodies are highly dynamic. The distribution of Woronin bodies and their co-localization pattern with peroxisomes revealed no evidence that Woronin bodies arise predominantly at the apical tip of A. fumigatus hyphae. We found that Woronin bodies are able to plug septal pores of A. fumigatus in case of damage. Woronin bodies therefore contribute to the stress resistance and potentially also to the virulence of A. fumigatus, which renders them a potential target for future anti-fungal strategies.

Microtubule-dependent membrane dynamics in Ustilago maydis: Trafficking and function of Rab5a-positive endosomes

Long-distance trafficking of membranous structures along the cytoskeleton is crucial for secretion and endocytosis in eukaryotes. Molecular motors are transporting both secretory and endocytic vesicles along polarized microtubules. Here, we review the transport mechanism and biological function of a distinct subset of large vesicles marked by the G-protein Rab5a in the model microorganism Ustilago maydis. These Rab5a-positive endosomes shuttle bi-directionally along microtubules mediated by the Unc104/KIF1A-related motor Kin3 and dynein Dyn1/2. Rab5a-positive endosomes exhibit diverse functions during the life cycle of U. maydis. In haploid budding cells they are involved in cytokinesis and pheromone signaling. During filamentous growth endosomes are used for long-distance transport of mRNA, a prerequisite to maintain polarity most likely via local translation of specific proteins at both the apical and distal ends of filaments. Endosomal co-transport of mRNA constitutes a novel function of these membrane compartments supporting the view that endosomes function as multipurpose platforms.

The Saccharomyces cerevisiae BEM1 homologue in Neurospora crassa promotes co-ordinated cell behaviour resulting in cell fusion

Directed growth or movement is a common feature of microbial development and propagation. In polar growing filamentous fungi, directed growth requires the interaction of signal sensing machineries with factors controlling polarity and cell tip extension. In Neurospora crassa an unusual mode of cell-cell signalling mediates mutual attraction of germinating spores, which subsequently fuse. During directed growth of the two fusion partners, the cells co-ordinately alternate between two physiological stages, probably associated with signal sending and receiving. Here, we show that the Saccharomyces cerevisiae BEM1 homologue in N. crassa is essential for the robust and efficient functioning of this MAP kinase-based signalling system. BEM1 localizes to growing hyphal tips suggesting a conserved function as a polarity component. In the absence of BEM1, activation of MAK-2, a MAP kinase essential for germling fusion, is strongly reduced and delayed. Germling interactions become highly instable and successful fusion is greatly reduced. In addition, BEM1 is actively recruited around the forming fusion pore, suggesting potential functions after cell-cell contact has been established. By genetically dissecting the contribution of BEM1 to additional various polarization events, we also obtained first hints that BEM1 might function in different protein complexes controlling polarity and growth direction.

Protein kinase A regulatory subunit isoforms regulate growth and differentiation in Mucor circinelloides: essential role of PKAR4

The protein kinase A (PKA) signaling pathway plays a role in regulating growth and differentiation in the dimorphic fungus Mucor circinelloides. PKA holoenzyme is comprised of two catalytic (C) and two regulatory (R) subunits. In M. circinelloides, four genes encode the PKAR1, PKAR2, PKAR3, and PKAR4 isoforms of R subunits. We have constructed null mutants and demonstrate that each isoform has a different role in growth and differentiation. The most striking finding is that pkaR4 is an essential gene, because only heterokaryons were obtained in knockout experiments. Heterokaryons with low levels of wild-type nuclei showed an impediment in the emission of the germ tube, suggesting a pivotal role of this gene in germ tube emergence. The remaining null strains showed different alterations in germ tube emergence, sporulation, and volume of the mother cell. The pkaR2 null mutant showed an accelerated germ tube emission and was the only mutant that germinated under anaerobic conditions when glycine was used as a nitrogen source, suggesting that pkaR2 participates in germ tube emergence by repressing it. From the measurement of the mRNA and protein levels of each isoform in the wild-type and knockout strains, it can be concluded that the expression of each subunit has its own mechanism of differential regulation. The PKAR1 and PKAR2 isoforms are posttranslationally modified by ubiquitylation, suggesting another regulation point in the specificity of the signal transduction. The results indicate that each R isoform has a different role in M. circinelloides physiology, controlling the dimorphism and contributing to the specificity of cyclic AMP (cAMP)-PKA pathway.

Comparative live-cell imaging analyses of SPA-2, BUD-6 and BNI-1 in Neurospora crassa reveal novel features of the filamentous fungal polarisome

A key multiprotein complex involved in regulating the actin cytoskeleton and secretory machinery required for polarized growth in fungi, is the polarisome. Recognized core constituents in budding yeast are the proteins Spa2, Pea2, Aip3/Bud6, and the key effector Bni1. Multicellular fungi display a more complex polarized morphogenesis than yeasts, suggesting that the filamentous fungal polarisome might fulfill additional functions. In this study, we compared the subcellular organization and dynamics of the putative polarisome components BUD-6 and BNI-1 with those of the bona fide polarisome marker SPA-2 at various developmental stages of Neurospora crassa. All three proteins exhibited a yeast-like polarisome configuration during polarized germ tube growth, cell fusion, septal pore plugging and tip repolarization. However, the localization patterns of all three proteins showed spatiotemporally distinct characteristics during the establishment of new polar axes, septum formation and cytokinesis, and maintained hyphal tip growth. Most notably, in vegetative hyphal tips BUD-6 accumulated as a subapical cloud excluded from the Spitzenkörper (Spk), whereas BNI-1 and SPA-2 partially colocalized with the Spk and the tip apex. Novel roles during septal plugging and cytokinesis, connected to the reinitiation of tip growth upon physical injury and conidial maturation, were identified for BUD-6 and BNI-1, respectively. Phenotypic analyses of gene deletion mutants revealed additional functions for BUD-6 and BNI-1 in cell fusion regulation, and the maintenance of Spk integrity. Considered together, our findings reveal novel polarisome-independent functions of BUD-6 and BNI-1 in Neurospora, but also suggest that all three proteins cooperate at plugged septal pores, and their complex arrangement within the apical dome of mature hypha might represent a novel aspect of filamentous fungal polarisome architecture.