Regulation of cell separation in the dimorphic fungus Ustilago maydis

A Novel Potent Carrier for Unconventional Protein Export in Ustilago maydis

Recombinant proteins are ubiquitously applied in fields like research, pharma, diagnostics or the chemical industry. To provide the full range of useful proteins, novel expression hosts need to be established for proteins that are not sufficiently produced by the standard platform organisms. Unconventional secretion in the fungal model Ustilago maydis is an attractive novel option for export of heterologous proteins without N-glycosylation using chitinase Cts1 as a carrier. Recently, a novel factor essential for unconventional Cts1 secretion termed Jps1 was identified. Here, we show that Jps1 is unconventionally secreted using a fusion to bacterial β-glucuronidase as an established reporter. Interestingly, the experiment also demonstrates that the protein functions as an alternative carrier for heterologous proteins, showing about 2-fold higher reporter activity than the Cts1 fusion in the supernatant. In addition, Jps1-mediated secretion even allowed for efficient export of functional firefly luciferase as a novel secretion target which could not be achieved with Cts1. As an application for a relevant pharmaceutical target, export of functional bi-specific synthetic nanobodies directed against the SARS-CoV2 spike protein was demonstrated. The establishment of an alternative efficient carrier thus constitutes an excellent expansion of the existing secretion platform.

Controlling Unconventional Secretion for Production of Heterologous Proteins in Ustilago maydis through Transcriptional Regulation and Chemical Inhibition of the Kinase Don3

Heterologous protein production is a highly demanded biotechnological process. Secretion of the product to the culture broth is advantageous because it drastically reduces downstream processing costs. We exploit unconventional secretion for heterologous protein expression in the fungal model microorganism Ustilago maydis. Proteins of interest are fused to carrier chitinase Cts1 for export via the fragmentation zone of dividing yeast cells in a lock-type mechanism. The kinase Don3 is essential for functional assembly of the fragmentation zone and hence, for release of Cts1-fusion proteins. Here, we are first to develop regulatory systems for unconventional protein secretion using Don3 as a gatekeeper to control when export occurs. This enables uncoupling the accumulation of biomass and protein synthesis of a product of choice from its export. Regulation was successfully established at two different levels using transcriptional and post-translational induction strategies. As a proof-of-principle, we applied autoinduction based on transcriptional don3 regulation for the production and secretion of functional anti-Gfp nanobodies. The presented developments comprise tailored solutions for differentially prized products and thus constitute another important step towards a competitive protein production platform.

Versatile CRISPR/Cas9 Systems for Genome Editing in Ustilago maydis

The phytopathogenic smut fungus Ustilago maydis is a versatile model organism to study plant pathology, fungal genetics, and molecular cell biology. Here, we report several strategies to manipulate the genome of U. maydis by the CRISPR/Cas9 technology. These include targeted gene deletion via homologous recombination of short double-stranded oligonucleotides, introduction of point mutations, heterologous complementation at the genomic locus, and endogenous N-terminal tagging with the fluorescent protein mCherry. All applications are independent of a permanent selectable marker and only require transient expression of the endonuclease Cas9hf and sgRNA. The techniques presented here are likely to accelerate research in the U. maydis community but can also act as a template for genome editing in other important fungi.

A Novel Factor Essential for Unconventional Secretion of Chitinase Cts1

Subcellular targeting of proteins is essential to orchestrate cytokinesis in eukaryotic cells. During cell division of Ustilago maydis, for example, chitinases must be specifically targeted to the fragmentation zone at the site of cell division to degrade remnant chitin and thus separate mother and daughter cells. Chitinase Cts1 is exported to this location via an unconventional secretion pathway putatively operating in a lock-type manner. The underlying mechanism is largely unexplored. Here, we applied a forward genetic screen based on UV mutagenesis to identify components essential for Cts1 export. The screen revealed a novel factor termed Jps1 lacking known protein domains. Deletion of the corresponding gene confirmed its essential role for Cts1 secretion. Localization studies demonstrated that Jps1 colocalizes with Cts1 in the fragmentation zone of dividing yeast cells. While loss of Jps1 leads to exclusion of Cts1 from the fragmentation zone and strongly reduced unconventional secretion, deletion of the chitinase does not disturb Jps1 localization. Yeast-two hybrid experiments indicate that the two proteins might interact. In essence, we identified a novel component of unconventional secretion that functions in the fragmentation zone to enable export of Cts1. We hypothesize that Jps1 acts as an anchoring factor for Cts1.

A Potential Lock-Type Mechanism for Unconventional Secretion in Fungi

Protein export in eukaryotes can either occur via the classical pathway traversing the endomembrane system or exploit alternative routes summarized as unconventional secretion. Besides multiple examples in higher eukaryotes, unconventional secretion has also been described for fungal proteins with diverse functions in important processes such as development or virulence. Accumulating molecular insights into the different export pathways suggest that unconventional secretion in fungal microorganisms does not follow a common scheme but has evolved multiple times independently. In this study, we review the most prominent examples with a focus on the chitinase Cts1 from the corn smut Ustilago maydis. Cts1 participates in cell separation during budding growth. Recent evidence indicates that the enzyme might be actively translocated into the fragmentation zone connecting dividing mother and daughter cells, where it supports cell division by the degradation of remnant chitin. Importantly, a functional fragmentation zone is prerequisite for Cts1 release. We summarize in detail what is currently known about this potential lock-type mechanism of Cts1 secretion and its connection to the complex regulation of fragmentation zone assembly and cell separation.

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.

Mechanisms of Cytokinesis in Basidiomycetous Yeasts

While mechanisms of cytokinesis exhibit considerable plasticity, it is difficult to precisely define the level of conservation of this essential part of cell division in fungi, as majority of our knowledge is based on ascomycetous yeasts. However, in the last decade more details have been uncovered regarding cytokinesis in the second largest fungal phylum, basidiomycetes, specifically in two yeasts, Cryptococcus neoformans and Ustilago maydis. Based on these findings, and current sequenced genomes, we summarize cytokinesis in basidiomycetous yeasts, indicating features that may be unique to this phylum, species-specific characteristics, as well as mechanisms that may be common to all eukaryotes.

Genome editing in Ustilago maydis using the CRISPR-Cas system

This communication describes the establishment of the type II bacterial CRISPR-Cas9 system to efficiently disrupt target genes in the fungal maize pathogen Ustilago maydis. A single step transformation of a self-replicating plasmid constitutively expressing the U. maydis codon-optimized cas9 gene and a suitable sgRNA under control of the U. maydis U6 snRNA promoter was sufficient to induce genome editing. On average 70% of the progeny of a single transformant were disrupted within the respective b gene. Without selection the self-replicating plasmid was lost rapidly allowing transient expression of the CRISPR-Cas9 system to minimize potential long-term negative effects of Cas9. This technology will be an important advance for the simultaneous disruption of functionally redundant genes and gene families to investigate their contribution to virulence of U. maydis.

Chitinases Are Essential for Cell Separation in Ustilago maydis

Chitin is an essential component of the fungal cell wall, providing rigidity and stability. Its degradation is mediated by chitinases and supposedly ensures the dynamic plasticity of the cell wall during growth and morphogenesis. Hence, chitinases should be particularly important for fungi with dramatic morphological changes, such as Ustilago maydis. This smut fungus switches from yeast to filamentous growth for plant infection, proliferates as a mycelium in planta, and forms teliospores for spreading. Here, we investigate the contribution of its four chitinolytic enzymes to the different morphological changes during the complete life cycle in a comprehensive study of deletion strains combined with biochemical and cell biological approaches. Interestingly, two chitinases act redundantly in cell separation during yeast growth. They mediate the degradation of remnant chitin in the fragmentation zone between mother and daughter cell. In contrast, even the complete lack of chitinolytic activity does not affect formation of the infectious filament, infection, biotrophic growth, or teliospore germination. Thus, unexpectedly we can exclude a major role for chitinolytic enzymes in morphogenesis or pathogenicity of U. maydis. Nevertheless, redundant activity of even two chitinases is essential for cell separation during saprophytic growth, possibly to improve nutrient access or spreading of yeast cells by wind or rain.

The cell end marker Tea4 regulates morphogenesis and pathogenicity in the basidiomycete fungus Ustilago maydis

Positional cues localized to distinct cell domains are critical for the generation of cell polarity and cell morphogenesis. These cues lead to assembly of protein complexes that organize the cytoskeleton resulting in delivery of vesicles to sites of polarized growth. Tea4, an SH3 domain protein, was first identified in fission yeast, and is a critical determinant of the axis of polarized growth, a role conserved among ascomycete fungi. Ustilago maydis is a badiomycete fungus that exhibits a yeast-like form that is nonpathogenic and a filamentous form that is pathogenic on maize and teozintle. We are interested in understanding how positional cues contribute to generation and maintenance of these two forms, and their role in pathogenicity. We identified a homologue of fission yeast tea4 in a genetic screen for mutants with altered colony and cell morphology and present here analysis of Tea4 for the first time in a basidiomycete fungus. We demonstrate that Tea4 is an important positional marker for polarized growth and septum location in both forms. We uncover roles for Tea4 in maintenance of cell and neck width, cell separation, and cell wall deposition in the yeast-like form, and in growth rate, formation of retraction septa, growth reversal, and inhibition of budding in the filamentous form. We show that Tea4::GFP localizes to sites of polarized or potential polarized growth in both forms, as observed in ascomycete fungi. We demonstrate an essential role of Tea4 in pathogencity in the absence of cell fusion. Basidiomycete and ascomycete Tea4 homologues share SH3 and Glc7 domains. Tea4 in basidiomycetes has additional domains, which has led us to hypothesize that Tea4 has novel functions in this group of fungi.

The SPF27 homologue Num1 connects splicing and kinesin 1-dependent cytoplasmic trafficking in Ustilago maydis

The conserved NineTeen protein complex (NTC) is an integral subunit of the spliceosome and required for intron removal during pre-mRNA splicing. The complex associates with the spliceosome and participates in the regulation of conformational changes of core spliceosomal components, stabilizing RNA-RNA- as well as RNA-protein interactions. In addition, the NTC is involved in cell cycle checkpoint control, response to DNA damage, as well as formation and export of mRNP-particles. We have identified the Num1 protein as the homologue of SPF27, one of NTC core components, in the basidiomycetous fungus Ustilago maydis. Num1 is required for polarized growth of the fungal hyphae, and, in line with the described NTC functions, the num1 mutation affects the cell cycle and cell division. The num1 deletion influences splicing in U. maydis on a global scale, as RNA-Seq analysis revealed increased intron retention rates. Surprisingly, we identified in a screen for Num1 interacting proteins not only NTC core components as Prp19 and Cef1, but several proteins with putative functions during vesicle-mediated transport processes. Among others, Num1 interacts with the motor protein Kin1 in the cytoplasm. Similar phenotypes with respect to filamentous and polar growth, vacuolar morphology, as well as the motility of early endosomes corroborate the genetic interaction between Num1 and Kin1. Our data implicate a previously unidentified connection between a component of the splicing machinery and cytoplasmic transport processes. As the num1 deletion also affects cytoplasmic mRNA transport, the protein may constitute a novel functional interconnection between the two disparate processes of splicing and trafficking.

In eukaryotic cells, nascent mRNA is processed by splicing to remove introns and to join the exon sequences. The processed mRNA is then transported out of the nucleus and employed by ribosomes to synthesize proteins. Splicing is achieved by the spliceosome and associated protein complexes, among them the so-called NineTeen complex (NTC). We have identified the Num1 protein as one of the core components of the NTC in the fungus Ustilago maydis, and could show that it is required for polarized growth of the filamentous fungal cells. Consistent with the NTC function, cells with a num1-deletion show reduced splicing of mRNA. Moreover, we uncover a novel cytoplasmic function of the Num1 protein: It physically interacts with the microtubule-associated Kinesin 1 motor protein, and phenotypic analyses corroborate that both proteins are functionally connected. Our findings reveal a yet unidentified role of a global splicing factor during intracellular trafficking processes. A possible connection between these disparate mechanisms presumably resides in mRNA-export out of the nucleus and/or the transport of mRNA within the cytoplasm.

Investigating the Ustilago maydis/Zea mays pathosystem: transcriptional responses and novel functional aspects of a fungal calcineurin regulatory B subunit

The sustainable control of basidiomycete biotrophic plant pathogenesis requires an understanding of host responses to infection, as well as the identification and functional analysis of fungal genes involved in disease development. The creation and analysis of a suppressive subtractive hybridization (SSH) cDNA library from Ustilago maydis-infected Zea mays seedlings enabled the identification of fungal and plant genes expressed during disease development, and uncovered new insights into the interactions of this model system. Candidate U. maydis pathogenesis genes were identified by using the current SSH cDNA library analysis, and by knowledge generated from previous cDNA microarray and comparative genomic analyses. These identifications were supported by the independent determination of transcript level changes in different cell-types and during pathogenic development. The basidiomycete specific um01632, the highly in planta expressed um03046 (zig1), and the calcineurin regulatory B subunit (um10226, cnb1), were chosen for deletion experiments. um01632 and zig1 mutants showed no difference in morphology and did not have a statistically significant impact on pathogenesis. cnb1 mutants had a distinct cell division phenotype and reduced virulence in seedling assays. Infections with reciprocal wild-type×Δcnb1 haploid strain crosses revealed that the wild-type allele was unable to fully compensate for the lack of a second cnb1 allele. This haploinsufficiency was undetected in other fungal cnb1 mutational analyses. The reported data improves U. maydis genome annotation and expands on the current understanding of pathogenesis genes in this model basidiomycete.

Two Rac paralogs regulate polarized growth in the human fungal pathogen Cryptococcus neoformans

A genome wide analysis of the human fungal pathogen Cryptococcus neoformans var. grubii has revealed a number of duplications of highly conserved genes involved in morphogenesis. Previously, we reported that duplicate Cdc42 paralogs provide C. neoformans with niche-specific responses to environmental stresses: Cdc42 is required for thermotolerance, while Cdc420 supports the formation of titan cells. The related Rho-GTPase Rac1 has been shown in C. neoformans var. neoformans to play a major role in filamentation and to share Cdc42/Cdc420 binding partners. Here we report the characterization of a second Rac paralog in C. neoformans, Rac2, and describe its overlapping function with the previously described CnRac, Rac1. Further, we demonstrate that the Rac paralogs play a primary role in polarized growth via the organization of reactive oxygen species and play only a minor role in the organization of actin. Finally, we provide preliminary evidence that pharmacological inhibitors of Rac activity and actin stability have synergistic activity.

Chemical genetics - a versatile method to combine science and higher level teaching in molecular genetics

Phosphorylation is a key event in many cellular processes like cell cycle, transformation of environmental signals to transcriptional activation or polar growth. The chemical genetics approach can be used to analyse the effect of highly specific inhibition in vivo and is a promising method to screen for kinase targets. We have used this approach to study the role of the germinal centre kinase Don3 during the cell division in the phytopathogenic fungus Ustilago maydis. Due to the easy determination of the don3 phenotype we have chosen this approach for a genetic course for M.Sc. students and for IMPRS (International Max-Planck research school) students. According to the principle of “problem-based learning” the aim of this two-week course is to transfer knowledge about the broad spectrum of kinases to the students and that the students acquire the ability to design their own analog-sensitive kinase of interest. In addition to these training goals, we benefit from these annual courses the synthesis of basic constructs for genetic modification of several kinases in our model system U. maydis.

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.

Isolation of UmRrm75, a gene involved in dimorphism and virulence of Ustilago maydis

Ustilago maydis displays dimorphic growth, alternating between a saprophytic haploid yeast form and a filamentous dikaryon, generated by mating of haploid cells and which is an obligate parasite. Induction of the dimorphic transition of haploid strains in vitro by change in ambient pH has been used to understand the mechanisms governing this differentiation process. In this study we used suppression subtractive hybridization to generate a cDNA library of U. maydis genes up-regulated in the filamentous form induced in vitro at acid pH. Expression analysis using quantitative RT-PCR showed that the induction of two unigenes identified in this library coincided with the establishment of filamentous growth in the acid pH medium. This expression pattern suggested that they were specifically associated to hyphal development rather than merely acid pH-induced genes. One of these genes, UmRrm75, encodes a protein containing three RNA recognition motifs and glycine-rich repeats and was selected for further study. The UmRrm75 gene contains 4 introns, and produces a splicing variant by a 3'-alternative splicing site within the third exon. Mutants deleted for UmRrm75 showed a slower growth rate than wild type strains in liquid and solid media, and their colonies showed a donut-like morphology on solid medium. Interestingly, although ΔUmRrm75 strains were not affected in filamentous growth induced by acid pH and oleic acid, they exhibited reduced mating, post-mating filamentous growth and virulence. Our data suggest that UmRrm75 is probably involved in cell growth, morphogenesis, and pathogenicity in U. maydis.

Ste20-related kinases: effectors of signaling and morphogenesis in fungi

The family of Ste20-related kinases is conserved from yeast to mammals and includes the p21 activated kinases (PAKs) and germinal centre kinases (GCKs). These kinases have been shown to be involved in signaling through mitogen activated protein kinase (MAPK) pathways and in morphogenesis through the regulation of cytokinesis and actin-dependent polarized growth. This review concentrates on the role of Ste20-related kinases in fungi where recent research has revealed roles for both PAKs and GCKs in the regulation of cytokinesis and in previously unidentified roles in promoting hyphal growth and differentiation of asexual development structures. In particular, the importance of PAKs during pathogenesis will be examined.

Vesicle trafficking and membrane remodelling in cytokinesis

All cells complete cell division by the process of cytokinesis. At the end of mitosis, eukaryotic cells accurately mark the site of division between the replicated genetic material and assemble a contractile ring comprised of myosin II, actin filaments and other proteins, which is attached to the plasma membrane. The myosin-actin interaction drives constriction of the contractile ring, forming a cleavage furrow (the so-called 'purse-string' model of cytokinesis). After furrowing is completed, the cells remain attached by a thin cytoplasmic bridge, filled with two anti-parallel arrays of microtubules with their plus-ends interdigitating in the midbody region. The cell then assembles the abscission machinery required for cleavage of the intercellular bridge, and so forms two genetically identical daughter cells. We now know much of the molecular detail of cytokinesis, including a list of potential genes/proteins involved, analysis of the function of some of these proteins, and the temporal order of their arrival at the cleavage site. Such studies reveal that membrane trafficking and/or remodelling appears to play crucial roles in both furrowing and abscission. In the present review, we assess studies of vesicular trafficking during cytokinesis, discuss the role of the lipid components of the plasma membrane and endosomes and their role in cytokinesis, and describe some novel molecules implicated in cytokinesis. The present review covers experiments performed mainly on tissue culture cells. We will end by considering how this mechanistic insight may be related to cytokinesis in other systems, and how other forms of cytokinesis may utilize similar aspects of the same machinery.

A seven-WD40 protein related to human RACK1 regulates mating and virulence in Ustilago maydis

In mammalian cells RACK1 serves as a scaffold protein that has a role in integrating inputs from different signalling pathways and affects translation through association with ribosomes. Ustilago maydis contains a seven-WD40 repeat motif protein designated Rak1, which shows 68% identity to RACK1 and 51% identity to Asc1p of Saccharomyces cerevisiae. An asc1 mutant could be complemented by introduction of U. maydis rak1. The deletion of rak1 affected cell growth, cell wall integrity and specifically attenuated cell fusion. This latter defect was caused by reduced expression of prf1 encoding the regulator for pheromone (mfa) and pheromone-receptor genes. Rak1 interacts with a variety of ribosomal proteins and microarray analysis revealed that the deletion of rak1 led to severely reduced expression of rop1, a transcriptional activator of prf1. The constitutive expression of rop1 could rescue the defect of mfa1 expression as well as conjugation tube formation in response to pheromone induction in the rak1 mutant. Moreover, a solopathogenic rak1 mutant failed to respond to plant-derived stimuli, resulting in attenuated filamentation and pathogenicity. This could be partially rescued by constitutive expression of the b heterodimer. These data suggest that rak1 is a regulator of rop1 expression with additional roles after cell fusion.

Fungal development of the plant pathogen Ustilago maydis

The maize pathogen Ustilago maydis has to undergo various morphological transitions for the completion of its sexual life cycle. For example, haploid cells respond to pheromone by forming conjugation tubes that fuse at their tips. The resulting dikaryon grows filamentously, expanding rapidly at the apex and inserting retraction septa at the basal pole. In this review, we present progress on the underlying mechanisms regulating such defined developmental programmes. The key findings of the postgenomic era are as follows: (1) endosomes function not only during receptor recycling, but also as multifunctional transport platforms; (2) a new transcriptional master regulator for pathogenicity is part of an intricate transcriptional network; (3) determinants for uniparental mitochondrial inheritance are encoded at the a2 mating-type locus; (4) microtubule-dependent mRNA transport is important in determining the axis of polarity; and (5) a battery of fungal effectors encoded in gene clusters is crucial for plant infection. Importantly, most processes are tightly controlled at the transcriptional, post-transcriptional and post-translational levels, resulting in a complex regulatory network. This intricate system is crucial for the timing of the correct order of developmental phases. Thus, new insights from all layers of regulation have substantially advanced our understanding of fungal development.

Role of Hsl7 in morphology and pathogenicity and its interaction with other signaling components in the plant pathogen Ustilago maydis

The phytopathogenic fungus Ustilago maydis undergoes a dimorphic transition in response to mating pheromone, host, and environmental cues. On a solid medium deficient in ammonium (SLAD [0.17% yeast nitrogen base without ammonium sulfate or amino acids, 2% dextrose, 50 μM ammonium sulfate]), U. maydis produces a filamentous colony morphology, while in liquid SLAD, the cells do not form filaments. The p21-activated protein kinases (PAKs) play a substantial role in regulating the dimorphic transition in fungi. The PAK-like Ste20 homologue Smu1 is required for a normal response to pheromone, via upregulation of pheromone expression, and virulence, and its disruption affects both processes. Our experiments suggest that Smu1 also regulates cell length and the filamentous response on solid SLAD medium. Yeast two-hybrid analysis suggested an Hsl7 homologue as a potential interacting partner of Smu1, and a unique open reading frame for such an arginine methyltransferase was detected in the U. maydis genome sequence. Hsl7 regulates cell length and the filamentous response to solid SLAD in a fashion opposite to that of Smu1, but neither overexpression nor disruption of hsl7 attenuates virulence. Simultaneous disruption of hsl7 and overexpression of smu1 lead to a hyperfilamentous response on solid SLAD. Moreover, only this double mutant strain forms filaments in liquid SLAD. The double mutant strain was also significantly reduced in virulence. A similar filamentous response in both solid and liquid SLAD was observed in strains lacking another PAK-like protein kinase involved in cytokinesis and polar growth, Cla4. Our data suggest that Hsl7 may regulate cell cycle progression, while both Smu1 and Cla4 appear to be involved in the filamentous response in U. maydis.

Septation of infectious hyphae is critical for appressoria formation and virulence in the smut fungus Ustilago maydis

Differentiation of hyphae into specialized infection structures, known as appressoria, is a common feature of plant pathogenic fungi that penetrate the plant cuticle. Appressorium formation in U. maydis is triggered by environmental signals but the molecular mechanism of this hyphal differentiation is largely unknown. Infectious hyphae grow on the leaf surface by inserting regularly spaced retraction septa at the distal end of the tip cell leaving empty sections of collapsed hyphae behind. Here we show that formation of retraction septa is critical for appressorium formation and virulence in U. maydis. We demonstrate that the diaphanous-related formin Drf1 is necessary for actomyosin ring formation during septation of infectious hyphae. Drf1 acts as an effector of a Cdc42 GTPase signaling module, which also consists of the Cdc42-specific guanine nucleotide exchange factor Don1 and the Ste20-like kinase Don3. Deletion of drf1, don1 or don3 abolished formation of retraction septa resulting in reduced virulence. Appressorium formation in these mutants was not completely blocked but infection structures were found only at the tip of short filaments indicating that retraction septa are necessary for appressorium formation in extended infectious hyphae. In addition, appressoria of drf1 mutants penetrated the plant tissue less frequently.

Pathogens exhibit various developmental stages during the process of infection and proliferation. The basidiomycete Ustilago maydis is a model organism for plant pathogenic fungi. On the plant surface U. maydis grows as a cell-cycle arrested filament. Growth of infectious hyphae involves regular formation of retraction septa leaving empty sections behind. The tip cell forms an appressorium and penetrates the cuticle. In this study we identified for the first time a signaling module regulating formation of retraction septa in fungal hyphae. The module consists of the highly conserved small GTPase Cdc42, its activator Don1 and the actin-organizing formin Drf1. After penetration of the plant, cell cycle arrest is released and hyphal septation is resumed in planta but was found to be independent of Cdc42 and Drf1. Thus, during infection Cdc42 signaling and Drf1 coordinate hyphal septation events specifically in infectious hyphae in U. maydis. The inability to form retraction septa affects filament elongation and appressorium formation resulting in significantly reduced virulence. We observed a threshold size of the cytoplasm filled tip compartment above which appressorium formation is blocked. These findings highlight that formation of retraction septa, a common feature of filamentous fungi, is an important virulence determinant of U. maydis.

Response to osmotic stress and temperature of the fungus Ustilago maydis

Ustilago maydis is a fungal pathogen which is exposed during its life cycle to both abiotic and biotic stresses before and after the infection of maize. To cope with extreme environmental changes, microorganisms usually accumulate the disaccharide trehalose. We have investigated both the accumulation of trehalose and the activity of trehalase during the adaptation of U. maydis haploid cells to thermal, sorbitol, and NaCl stresses. Sorbitol and sodium chloride induced sustained accumulation of trehalose, while a transient increase was observed under heat stress. Sorbitol stressed cells showed higher trehalase activity compared with control cells and to those stressed by NaCl and high temperature. Addition of cycloheximide, a protein synthesis inhibitor, did not affect the trehalose accumulation during the first 15 min, but basal levels of trehalose were reached after the second period of 15 min. The proteomic analysis of the response of U. maydis to temperature, sorbitol, and salt stresses indicated a complex pattern which highlights the change of 18 proteins involved in carbohydrate and amino acid metabolism, protein folding, redox regulation, ion homeostasis, and stress response. We hypothesize that trehalose accumulation during sorbitol stress in U. maydis might be related to the adaptation of this organism during plant infection.

Polarity proteins Bem1 and Cdc24 are components of the filamentous fungal NADPH oxidase complex

Regulated synthesis of reactive oxygen species (ROS) by membrane-bound fungal NADPH oxidases (Nox) plays a key role in fungal morphogenesis, growth, and development. Generation of reactive oxygen species (ROS) by the plant symbiotic fungus, Epichloë festucae, requires functional assembly of a multisubunit complex composed of NoxA, a regulatory component, NoxR, and the small GTPase RacA. However, the mechanism for assembly and activation of this complex at the plasma membrane is unknown. We found by yeast two-hybrid and coimmunoprecipitation assays that E. festucae NoxR interacts with homologs of the yeast polarity proteins, Bem1 and Cdc24, and that the Phox and Bem1 (PB1) protein domains found in these proteins are essential for these interactions. GFP fusions of BemA, Cdc24, and NoxR preferentially localized to actively growing hyphal tips and to septa. These proteins interact with each other in vivo at these same cellular sites as shown by bimolecular fluorescent complementation assays. The PB1 domain of NoxR is essential for localization to the hyphal tip. An E. festucae ΔbemA mutant was defective in hyphal morphogenesis and growth in culture and in planta. The changes in fungal growth in planta resulted in a defective symbiotic interaction phenotype. Our inability to isolate a Δcdc24 mutant suggests this gene is essential. These results demonstrate that BemA and Cdc24 play a critical role in localizing NoxR protein to sites of fungal hyphal morphogenesis and growth. Our findings identify a potential shared ancestral link between the protein machinery required for fungal polarity establishment and the Nox complex controlling cellular differentiation.

Divide and ProsPer: the emerging role of PtdIns3P in cytokinesis

Cytokinesis is the final step of cell division whereby the dividing cells separate physically. Failure of this process has been proposed to cause tumourigenesis. Several specific lipids are essential for cytokinesis, and recent evidence has revealed that phosphatidylinositol 3-phosphate (PtdIns3P) - a well-known regulator of endosomal trafficking, receptor signaling, nutrient sensing and autophagy - plays an evolutionarily conserved role during cytokinesis. The emerging picture is that PtdIns3P and its regulators and effectors constitute a novel regulatory mechanism for cytokinesis. Elucidating the role of PtdIns3P in cytokinesis might contribute to insight into mechanisms of tumour development and suppression.

The use of FLP-mediated recombination for the functional analysis of an effector gene family in the biotrophic smut fungus Ustilago maydis

*In the Ustilago maydis genome, several novel secreted effector proteins are encoded by gene families. Because of the limited number of selectable markers, the ability to carry out sequential gene deletions has limited the analysis of effector gene families that may have redundant functions. *Here, we established an inducible FLP-mediated recombination system in U. maydis that allows repeated rounds of gene deletion using a single selectable marker (Hyg(R)). To avoid genome rearrangements via FRT sites remaining in the genome after excision, different mutated FRT sites were introduced. *The FLP-mediated selectable marker-removal technique was successfully applied to delete a family of 11 effector genes (eff1) using five sequential rounds of recombination. We showed that expression of all 11 genes is up-regulated during the biotrophic phase. Strains carrying deletions of 9 or all 11 genes showed a significant reduction in virulence, and this phenotype could be partially complemented by the introduction of different members from the gene family, demonstrating redundancy. *The establishment of the FLP/FRT system in a plant pathogenic fungus paves the way for analyzing multigene families with redundant functions.

Role of the nuclear migration protein Lis1 in cell morphogenesis in Ustilago maydis

Ustilago maydis is a basidiomycete fungus that exhibits a yeast-like and a filamentous form. Growth of the fungus in the host leads to additional morphological transitions. The different morphologies are characterized by distinct nuclear movements. Dynein and alpha-tubulin are required for nuclear movements and for cell morphogenesis of the yeast-like form. Lis1 is a microtubule plus-end tracking protein (+TIPs) conserved in eukaryotes and required for nuclear migration and spindle positioning. Defects in nuclear migration result in altered cell fate and aberrant development in metazoans, slow growth in fungi and disease in humans (e.g. lissencephaly). Here we investigate the role of the human LIS1 homolog in U. maydis and demonstrate that it is essential for cell viability, not previously seen in other fungi. With a conditional null mutation we show that lis1 is necessary for nuclear migration in the yeast-like cell and during the dimorphic transition. Studies of asynchronous exponentially growing cells and time-lapse microscopy uncovered novel functions of lis1: It is necessary for cell morphogenesis, positioning of the septum and cell wall integrity. lis1-depleted cells exhibit altered axes of growth and loss of cell polarity leading to grossly aberrant cells with clusters of nuclei and morphologically altered buds devoid of nuclei. Altered septum positioning and cell wall deposition contribute to the aberrant morphology. lis1-depleted cells lyse, indicative of altered cell wall properties or composition. We also demonstrate, with indirect immunofluorescence to visualize tubulin, that lis1 is necessary for the normal organization of the microtubule cytoskeleton: lis1-depleted cells contain more and longer microtubules that can form coils perpendicular to the long axis of the cell. We propose that lis1 controls microtubule dynamics and thus the regulated delivery of vesicles to growth sites and other cell domains that govern nuclear movements.

Sho1 and Msb2-related proteins regulate appressorium development in the smut fungus Ustilago maydis

The dimorphic fungus Ustilago maydis switches from budding to hyphal growth on the plant surface. In response to hydrophobicity and hydroxy fatty acids, U. maydis develops infection structures called appressoria. Here, we report that, unlike in Saccharomyces cerevisiae and other fungi where Sho1 (synthetic high osmolarity sensitive) and Msb2 (multicopy suppressor of a budding defect) regulate stress responses and pseudohyphal growth, Sho1 and Msb2-like proteins play a key role during appressorium differentiation in U. maydis. Sho1 was identified through a two-hybrid screen as an interaction partner of the mitogen-activated protein (MAP) kinase Kpp6. Epistasis analysis revealed that sho1 and msb2 act upstream of the MAP kinases kpp2 and kpp6. Furthermore, Sho1 was shown to destabilize Kpp6 through direct interaction with the unique N-terminal domain in Kpp6, indicating a role of Sho1 in fine-tuning Kpp6 activity. Morphological differentiation in response to a hydrophobic surface was strongly attenuated in sho1 msb2 mutants, while hydroxy fatty acid-induced differentiation was unaffected. These data suggest that Sho1 and the transmembrane mucin Msb2 are involved in plant surface sensing in U. maydis.

The germinal centre kinase Don3 triggers the dynamic rearrangement of higher-order septin structures during cytokinesis in Ustilago maydis

The dimorphic phytopathogenic fungus Ustilago maydis grows in its haploid phase by budding. Cytokinesis and separation of daughter cells are accomplished by the consecutive formation of two distinct septa. Here, we show that both septation events involve the dynamic rearrangement of septin assemblies from hourglass-shaped collars into ring-like structures. Using a chemical genetic approach we demonstrate that the germinal centre kinase Don3 triggers this septin reorganization during secondary septum formation. Although chemical inhibition of an analogue-sensitive version of Don3 prevented septation, a stable septin collar was assembled at the presumptive septation site. Interestingly, the essential light chain of type II myosin, Cdc4, was already associated with this septin collar. Release of Don3 kinase inhibition triggered immediate dispersal of septin filaments and concomitant incorporation of Cdc4 into a contractile actomyosin ring, which also contained the F-BAR domain protein Cdc15. Inhibition of actin polymerization or deletion of the cdc15 gene, did not affect assembly of the initial collar consisting of septin and myosin light chain. However, reassembly of septin filaments into a ring-like structure was prevented in the absence of either F-actin or Cdc15, indicating that septin ring formation in U. maydis depends on a functional contractile actomyosin ring.

The fungal RNA-binding protein Rrm4 mediates long-distance transport of ubi1 and rho3 mRNAs

Cytoskeletal transport promotes polar growth in filamentous fungi. In Ustilago maydis, the RNA-binding protein Rrm4 shuttles along microtubules and is crucial for polarity in infectious filaments. Mutations in the RNA-binding domain cause loss of function. However, it was unclear which RNAs are bound and transported. Here, we applied in vivo RNA binding studies and live imaging to determine the molecular function of Rrm4. This new combination revealed that Rrm4 mediates microtubule-dependent transport of distinct mRNAs encoding, for example, the ubiquitin fusion protein Ubi1 and the small G protein Rho3. These transcripts accumulate in ribonucleoprotein particles (mRNPs) that move bidirectionally along microtubules and co-localise with Rrm4. Importantly, the 3' untranslated region of ubi1 containing a CA-rich binding site functions as zipcode during mRNA transport. Furthermore, motile mRNPs are not formed when the RNA-binding domain of Rrm4 is deleted, although the protein is still shuttling. Thus, Rrm4 constitutes an integral component of the transport machinery. We propose that microtubule-dependent mRNP trafficking is crucial for hyphal growth introducing U. maydis as attractive model for studying mRNA transport in higher eukaryotes.

Ustilago maydis Rho1 and 14-3-3 homologues participate in pathways controlling cell separation and cell polarity

Proteins of the 14-3-3 and Rho-GTPase families are functionally conserved eukaryotic proteins that participate in many important cellular processes such as signal transduction, cell cycle regulation, malignant transformation, stress response, and apoptosis. However, the exact role(s) of these proteins in these processes is not entirely understood. Using the fungal maize pathogen, Ustilago maydis, we were able to demonstrate a functional connection between Pdc1 and Rho1, the U. maydis homologues of 14-3-3epsilon and Rho1, respectively. Our experiments suggest that Pdc1 regulates viability, cytokinesis, chromosome condensation, and vacuole formation. Similarly, U. maydis Rho1 is also involved in these three essential processes and exerts an additional function during mating and filamentation. Intriguingly, yeast two-hybrid and epistasis experiments suggest that both Pdc1 and Rho1 could be constituents of the same regulatory cascade(s) controlling cell growth and filamentation in U. maydis. Overexpression of rho1 ameliorated the defects of cells depleted for Pdc1. Furthermore, we found that another small G protein, Rac1, was a suppressor of lethality for both Pdc1 and Rho1. In addition, deletion of cla4, encoding a Rac1 effector kinase, could also rescue cells with Pdc1 depleted. Inferring from these data, we propose a model for Rho1 and Pdc1 functions in U. maydis.

Cbf11 and Cbf12, the fission yeast CSL proteins, play opposing roles in cell adhesion and coordination of cell and nuclear division

The CSL (CBF1/RBP-Jkappa/Suppressor of Hairless/LAG-1) family is comprised of transcription factors essential for metazoan development, mostly due to their involvement in the Notch receptor signaling pathway. Recently, we identified two novel classes of CSL genes in the genomes of several fungal species, organisms lacking the Notch pathway. In this study, we characterized experimentally cbf11+ and cbf12+, the two CSL genes of Schizosaccharomyces pombe, in order to elucidate the CSL function in fungi. We provide evidence supporting their identity as genuine CSL genes. Both cbf11+ and cbf12+ are non-essential; they have distinct expression profiles and code for nuclear proteins with transcription activation potential. Significantly, we demonstrated that Cbf11 recognizes specifically the canonical CSL response element GTGA/GGAA in vitro. The deletion of cbf11+ is associated with growth phenotypes and altered colony morphology. Furthermore, we found that Cbf11 and Cbf12 play opposite roles in cell adhesion, nuclear and cell division and their coordination. Disturbed balance of the two CSL proteins leads to cell separation defects (sep phenotype), cut phenotype, and high-frequency diploidization in heterothallic strains. Our data show that CSL proteins operate in an organism predating the Notch pathway, which should be of relevance to the understanding of (Notch-independent) CSL functions in metazoans.

Coordination of cytokinesis and cell separation by endosomal targeting of a Cdc42-specific guanine nucleotide exchange factor in Ustilago maydis

The small GTPase Cdc42 is a key regulator of cell polarity and cytoskeletal organization in most eukaryotic cells. In Ustilago maydis, Cdc42 and the guanine nucleotide exchange factor (GEF) Don1 regulate cytokinesis and cell separation. Don1 belongs to the FGD1 family of Cdc42-specific GEFs that are characterized by a C-terminal lipid-binding FYVE domain. Although the FGD1/frabin family of Rho-GEFs is evolutionary conserved from fungi to mammals the role of the FYVE domain for its biological function is unknown. Here, we show that the FYVE domain is specific for phosphatidylinositol-3-phosphate (PtdIns(3)P) and targets Don1 to endosomal vesicles. During cytokinesis asymmetric accumulation of Don1-containing vesicles occurs at the site of septation. We could show that FYVE-dependent localization is critical for the function of Don1 at normal expression levels but can be compensated for by overexpression of Don1 lacking a functional FYVE domain. Our results demonstrate that endosomal compartmentalization of a Cdc42-specific exchange factor is involved in the coordination of cytokinesis and cell separation.

Cdk5 kinase regulates the association between adaptor protein Bem1 and GEF Cdc24 in the fungus Ustilago maydis

Cyclin-dependent kinases from the Cdk5/Pho85 family are thought to play important roles in morphogenesis in species as diverse as yeast and humans. In the phytopathogenic fungus Ustilago maydis Cdk5 has a major role in the maintenance of cell polarity and virulence. This role seems to be related to the ability of the guanine-nucleotide exchange factor (GEF) Cdc24 to localize at the cell tips. However, the elements behind the Cdk5-dependent stabilization of Cdc24 at the cell poles are not well understood. Here we investigate the role of the adaptor protein Bem1 in polarity maintenance in U. maydis. We found that Bem1 and Cdc24 physically interact and colocalize at cell tips and that Cdk5 regulates this interaction. Our data suggest a method by which Cdk5 could regulate polar growth in this phytopathogenic fungus.

Spa2 is required for morphogenesis but it is dispensable for pathogenicity in the phytopathogenic fungus Ustilago maydis

The increasing evidence linking regulation of polar growth and pathogenicity in fungi has elicited a significant effort devoted to produce a better understanding of mechanisms determining polarization in pathogenic fungi. Here we characterize in the phytopathogenic basidiomycete Ustilago maydis, the Spa2 protein, a well-known component of polarisome, firstly described in Saccharomyces cerevisiae. U. maydis display a dimorphic switch between budding growth of hapoid cells and filamentous growth of the dikaryon. During yeast growth, a GFP-tagged Spa2 protein localized to distinct growth sites in a cell cycle-specific manner, while during hyphal growth is persistently located to hyphal tips. Deletion of spa2 gene produces rounder budding cells and thicker filaments than wild-type cells, suggesting a role of Spa2 for the determination of the growth area in U. maydis. We also address the connections between Spa2 and the actin- and microtubule-cytoskeleton. We found that the absence of Spa2 does not affect cytoskeleton organization and strikingly, interference with actin filament or microtubule formation does not affect the polar localization of Spa2. In contrast, defects in the small GTPase Rac1 seems to affect the ability of Spa2 to locate to precise sites at the tip cell. Finally, to our surprise, we found that cells defectives in Spa2 function were as pathogenic as wild-type cells.

Pheromone-regulated target genes respond differentially to MAPK phosphorylation of transcription factor Prf1

Pheromone signalling during mating is essential for pathogenicity of Ustilago maydis. The activity of the key transcription factor Prf1 is controlled at the transcriptional level and post-translationally by mitogen-activated protein kinase (MAPK) and protein kinase A (PKA) phosphorylation. However, the precise contribution of these regulatory mechanisms to the transcriptional output is unknown. Here, we genetically dissected the three levels of Prf1 regulation. We performed transcriptional profiling of respective mutants to identify and classify targets. This approach revealed that transcriptional regulation of prf1 had only minor influence on target gene expression stressing the importance of post-translational control. PKA regulation of Prf1 was sufficient to control expression of nine pheromone-responsive genes including the major transcription factor regulating pathogenicity. MAPK regulation was necessary for the pheromone response of a set of 57 genes. In 35 cases, pheromone responsiveness was completely lost, while in the remaining 22 cases regulation was alleviated. This indicated a novel level of complexity in MAPK signalling suggesting that target genes respond differentially to MAPK phosphorylation of the respective transcription factors.

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

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

Selective activation by the guanine nucleotide exchange factor Don1 is a main determinant of Cdc42 signalling specificity in Ustilago maydis

The highly conserved GTP-binding proteins Cdc42 and Rac1 regulate cytokinesis, establishment of cell polarity and vesicular trafficking. In the dimorphic fungus Ustilago maydis, Rac1 is required for cell polarity and budding, while Cdc42 is essential for cell separation during cytokinesis. The same cell separation defect is also observed in mutants that lack Don1, a guanine nucleotide exchange factor (GEF) of the Dbl family. We have generated a series of chimeric GTP-binding proteins consisting of different portions of Cdc42 and Rac1. In vivo complementation analysis revealed that a short region encompassing amino acids 41-56 determines signalling specificity. Remarkably, substitution of a single amino acid at position 56 within this specificity domain is sufficient to confer Cdc42 function to Rac1 in vivo. Expression of Rac1(W56F) in Delta cdc42 mutant cells resulted in complementation of the cell separation defect. In vitro GDP/GTP exchange assays demonstrated that the Dbl family GEF Don1 is highly specific for Cdc42 and cannot activate Rac1. However, if Rac1(W56F) is used as a substrate, Don1 is able to stimulate GDP/GTP exchange. Together these data indicate that activation by the GEF Don1 is an important determinant of Cdc42-specific signalling in vivo.

Cdc42 and the Ste20-like kinase Don3 act independently in triggering cytokinesis in Ustilago maydis

In the dimorphic fungus Ustilago maydis the Rho-family GTP-binding protein Cdc42 and the Ste20-like kinase Don3 are both essential for triggering cell separation during cytokinesis. Since Don3 does not contain a Cdc42/Rac interaction and binding domain (CRIB), it is unclear how Cdc42 and Don3 cooperate in the regulation of cytokinesis. To analyse the regulatory network we generated an analogue-sensitive Don3 variant (Don3-as) that allows specific inhibition in vivo. The engineered kinase Don3(M157A) is fully active in vivo and can be specifically inhibited by low concentrations of the ATP-analogue NA-PP1. Inhibition of the Don3-as kinase activity immediately blocked cell separation resulting in the formation of clusters of nonseparated cells. Covalent labelling of cell wall proteins showed that, upon release of inhibition, cytokinesis was resumed instantaneously in all cells. By sequentially activating Don3 and Cdc42 we were able to demonstrate that both proteins act independently of each other and that Don3 activity precedes that of Cdc42. We provide evidence that Don3 and Cdc42 are crucial for the assembly of a contractile actomyosin ring, which is a prerequisite for secondary septum formation. We propose, that Don3 is involved in establishing a landmark, at which the Cdc42-dependent actomyosin ring formation will occur.

A Ras1-Cdc24 signal transduction pathway mediates thermotolerance in the fungal pathogen Cryptococcus neoformans

Pathogenic microorganisms must precisely regulate morphogenesis to survive and proliferate within an infected host. This regulation is often controlled by conserved signal transduction pathways that direct morphological changes in varied species. One such pathway, whose components include Ras proteins and the PAK kinase Ste20, allows the human fungal pathogen Cryptococcus neoformans to grow at high temperature. Previously, we found that Ras1 signalling is required for differentiation, thermotolerance and pathogenesis in C. neoformans. We show here that the guanine nucleotide exchange factor Cdc24 is a Ras1 effector in C. neoformans to mediate the ability of this fungus to grow at high temperature and to cause disease. In addition, we provide evidence that the Ras1-Cdc24 signalling cascade functions specifically through one of the three Cdc42/Rac1 homologues in C. neoformans. In conclusion, our studies illustrate how components of conserved signalling cascades can be specialized for different downstream functions, such as pathogenesis.

Candida albicans Rho-type GTPase-encoding genes required for polarized cell growth and cell separation

Rho proteins are essential regulators of morphogenesis in eukaryotic cells. In this report, we investigate the role of two previously uncharacterized Rho proteins, encoded by the Candida albicans RHO3 (CaRHO3) and CaCRL1/CaRHO4 genes. The CaRHO3 gene was found to contain one intron. Promoter shutdown experiments using a MET3 promoter-controlled RHO3 revealed a strong cell polarity defect and a partially depolarized actin cytoskeleton. Hyphal growth after promoter shutdown was abolished in rho3 mutants even in the presence of a constitutively active ras1(G13V) allele, and existing germ tubes became swollen. Deletion of C. albicans RHO4 indicated that it is a nonessential gene and that rho4 mutants were phenotypically different from rho3. Two distinct phenotypes of rho4 cells were elongated cell morphology and an unexpected cell separation defect generating chains of cells. Colony morphology of crl1/rho4 resulted in a growth-dependent smooth (long cell cycle length) or wrinkled (short cell cycle length) phenotype. This phenotype was additionally dependent on the rho4 cell separation defect and was also found in a Cacht3 chitinase mutant that shows a strong cytokinesis defect. The overexpression of the endoglucanase encoding the ENG1 gene, but not CHT3, suppressed the cell separation defect of crl1/rho4 but could not suppress the cell elongation phenotype. C. albicans Crl1/Rho4 and Bnr1 both localize to septal sites in yeast and hyphal cells but not to the hyphal tip. Deletion of RHO4 and BNR1 produced similar morphological phenotypes. Based on the localization of Rho4 and on the rho4 mutant phenotype, we propose a model in which Rho4p may function as a regulator of cell polarity, breaking the initial axis of polarity found during early bud growth to promote the construction of a septum.

On the move: endosomes in fungal growth and pathogenicity

Fungi invade substrates, such as host tissues, through hyphal tip growth. This article focuses on the corn smut fungus Ustilago maydis, in which tip growth and pathogenicity involve apical endocytic recycling by early endosomes. These organelles rapidly move bi-directionally along microtubules and this movement is mediated by opposing molecular motors. This motility seems to be essential for extended hyphal growth, possibly because it focuses the endocytic machinery at the hyphal tip and mediates communication between the tip and the sub-apical nucleus.

Genetics of morphogenesis and pathogenic development of Ustilago maydis

Ustilago maydis has emerged as an important model system for the study of fungi. Like many fungi, U. maydis undergoes remarkable morphological transitions throughout its life cycle. Fusion of compatible, budding, haploid cells leads to the production of a filamentous dikaryon that penetrates and colonizes the plant, culminating in the production of diploid teliospores within fungal-induced plant galls or tumors. These dramatic morphological transitions are controlled by components of various signaling pathways, including the pheromone-responsive MAP kinase and cAMP/PKA (cyclic AMP/protein kinase A) pathways, which coregulate the dimorphic switch and sexual development of U. maydis. These signaling pathways must somehow cooperate with the regulation of the cytoskeletal and cell cycle machinery. In this chapter, we provide an overview of these processes from pheromone perception and mating to gall production and sporulation in planta. Emphasis is placed on the genetic determinants of morphogenesis and pathogenic development of U. maydis and on the fungus-host interaction. Additionally, we review advances in the development of tools to study U. maydis, including the recently available genome sequence. We conclude with a brief assessment of current challenges and future directions for the genetic study of U. maydis.

A p67Phox-like regulator is recruited to control hyphal branching in a fungal-grass mutualistic symbiosis

Key requirements for microbes to initiate and establish mutualistic symbiotic interactions with plants are evasion of potential host defense responses and strict control of microbial growth. Reactive oxygen species (ROS) produced by a specific NADPH oxidase isoform, NoxA, regulate hyphal growth in the mutualistic interaction between the fungal endophyte Epichloë festucae and its grass host Lolium perenne. Unlike mammalian systems, little is known about the fungal NADPH oxidase complex and its response to differentiation signals. We identify an E. festucae p67(phox)-like regulator, NoxR, dispensable in culture but essential in planta for the symbiotic interaction. Plants infected with a noxR deletion mutant show severe stunting and premature senescence, whereas hyphae in the meristematic tissues show increased branching leading to increased fungal colonization of pseudostem and leaf blade tissue. Inhibition of ROS production or overexpression of noxR recapitulates the hyperbranching phenotype in culture. NoxR interacts in vitro with the small GTP binding protein RacA and requires a functional RacA binding site to complement the noxR mutant and restore the wild-type plant interaction phenotype. These results show that NoxR is a key regulator of NoxA in symbiosis, where it acts together with RacA to spatially regulate ROS production and control hyphal branching and patterning.