The PAK family kinase Cla4 is required for budding and morphogenesis in Ustilago maydis

Cla4A, a Novel Regulator of Gene Expression Networks Required for Asexual and Insect-Pathogenic Lifecycles of Beauveria bassiana

Cla4, an orthologous p21-activated kinase crucial for non-entomopathogenic fungal lifestyles, has two paralogs (Cla4A/B) functionally unknown in hypocrealean entomopathogens. Here, we report a regulatory role of Cla4A in gene expression networks of Beauveria bassiana required for asexual and entomopathogenic lifecycles while Cla4B is functionally redundant. The deletion of cla4A resulted in severe growth defects, reduced stress tolerance, delayed conidiation, altered conidiation mode, impaired conidial quality, and abolished pathogenicity through cuticular penetration, contrasting with no phenotype affected by cla4B deletion. In ∆cla4A, 5288 dysregulated genes were associated with phenotypic defects, which were restored by targeted gene complementation. Among those, 3699 genes were downregulated, including more than 1300 abolished at the transcriptomic level. Hundreds of those downregulated genes were involved in the regulation of transcription, translation, and post-translational modifications and the organization and function of the nuclear chromosome, chromatin, and protein-DNA complex. DNA-binding elements in promoter regions of 130 dysregulated genes were predicted to be targeted by Cla4A domains. Samples of purified Cla4A extract were proven to bind promoter DNAs of 12 predicted genes involved in multiple stress-responsive pathways. Therefore, Cla4A acts as a novel regulator of genomic expression and stability and mediates gene expression networks required for insect-pathogenic fungal adaptations to the host and environment.

Intracellular polyamines regulate redox homeostasis with cAMP-PKA signalling during sexual mating/filamentation and pathogenicity of Sporisorium scitamineum

Sugarcane smut caused by Sporisorium scitamineum seriously impairs sugarcane production and quality. Sexual mating/filamentation is a critical step of S. scitamineum pathogenesis, yet the regulatory mechanisms are not fully understood. In this study, we identified the SsAGA, SsODC, and SsSAMDC genes, which are involved in polyamine biosynthesis in S. scitamineum. Deletion of SsODC led to complete loss of filamentous growth after sexual mating, and deletion of SsAGA or SsSAMDC caused reduced filamentation. Double deletion of SsODC and SsSAMDC resulted in auxotrophy for putrescine (PUT) and spermidine (SPD) when grown on minimal medium (MM), indicating that these two genes encode enzymes that are critical for PUT and SPD biosynthesis. We further showed that low PUT concentrations promoted S. scitamineum filamentation, while high PUT concentrations suppressed filamentation. Disrupted fungal polyamine biosynthesis also resulted in a loss of pathogenicity and reduced fungal biomass within infected plants at the early infection stage. SPD formed a gradient from the diseased part to nonsymptom parts of the cane stem, suggesting that SPD is probably favourable for fungal virulence. Mutants of the cAMP-PKA (SsGPA3-SsUAC1-SsADR1) signalling pathway displayed up-regulation of the SsODC gene and elevated intracellular levels of PUT. SsODC directly interacted with SsGPA3, and sporidia of the ss1uac1ΔodcΔ mutant displayed abundant pseudohyphae. Furthermore, we found that elevated PUT levels caused accumulation of intracellular reactive oxygen species (ROS), probably by suppressing transcription of ROS-scavenging enzymes, while SPD played the opposite role. Overall, our work proves that polyamines play important roles in the pathogenic development of sugarcane smut fungus, probably by collaboratively regulating intracellular redox homeostasis with the cAMP-PKA signalling pathway.

Cla4 phosphorylates histone methyltransferase Set1 to prevent its degradation by the APC/CCdh1 complex

H3K4 trimethylation (H3K4me3) is a conserved histone modification catalyzed by histone methyltransferase Set1, and its dysregulation is associated with pathologies. Here, we show that Set1 is intrinsically unstable and elucidate how its protein levels are controlled within cell cycle and during gene transcription. Specifically, Set1 contains a destruction box (D-box) that is recognized by E3 ligase APC/CCdh1 and degraded by the ubiquitin-proteasome pathway. Cla4 phosphorylates serine 228 (S228) within Set1 D-box, which inhibits APC/CCdh1-mediated Set1 proteolysis. During gene transcription, PAF complex facilitates Cla4 to phosphorylate Set1-S228 and protect chromatin-bound Set1 from degradation. By modulating Set1 stability and its binding to chromatin, Cla4 and APC/CCdh1 control H3K4me3 levels, which then regulate gene transcription, cell cycle progression, and chronological aging. In addition, there are 141 proteins containing the D-box that can be potentially phosphorylated by Cla4 to prevent their degradation by APC/CCdh1. We addressed the long-standing question about how Set1 stability is controlled and uncovered a new mechanism to regulate protein stability.

p21-activated kinase is involved in the sporulation, pathogenicity, and stress response of Arthrobotrys oligospora under the indirect regulation of Rho GTPase-activating protein

The p21-GTPase-activated protein kinases (PAKs) participate in signal transduction downstream of Rho GTPases, which are regulated by Rho GTPase-activating proteins (Rho-GAP). Herein, we characterized two orthologous Rho-GAPs (AoRga1 and AoRga2) and two PAKs (AoPak1 and AoPak2) through bioinformatics analysis and reverse genetics in Arthrobotrys oligospora, a typical nematode-trapping (NT) fungus. The transcription analyses performed at different development stages suggested that Aopaks and Aorga1 play a crucial role during sporulation and trap formation, respectively. In addition, we successfully deleted Aopak1 and Aorga1 via the homologous recombination method. The disruption of Aopak1 and Aorga1 caused a remarkable reduction in spore yield and the number of nuclei per cell, but did not affect mycelial growth. In ∆Aopak1 mutants, the trap number was decreased at 48 h after the introduction of nematodes, but nematode predatory efficiency was not affected because the extracellular proteolytic activity was increased. On the contrary, the number of traps in ∆Aorga1 mutants was significantly increased at 36 h and 48 h. In addition, Aopak1 and Aorga1 had different effects on the sensitivity to cell-wall-disturbing reagent and oxidant. A yeast two-hybrid assay revealed that AoPak1 and AoRga1 both interacted with AoRac, and AoPak1 also interacted with AoCdc42. Furthermore, the Aopaks were up-regulated in ∆Aorga1 mutants, and Aorga1 was down-regulated in ∆Aopak1 mutants. These results reveal that AoRga1 indirectly regulated AoPAKs by regulating small GTPases.

The emerging role of septins in fungal pathogenesis

Fungal pathogens undergo specific morphogenetic transitions in order to breach the outer surfaces of plants and invade the underlying host tissue. The ability to change cell shape and switch between non-polarised and polarised growth habits is therefore critical to the lifestyle of plant pathogens. Infection-related development involves remodelling of the cytoskeleton, plasma membrane and cell wall at specific points during fungal pathogenesis. Septin GTPases are components of the cytoskeleton that play pivotal roles in actin remodelling, micron-scale plasma membrane curvature sensing and cell polarity. Septin assemblages, such as rings, collars and gauzes, are known to have important roles in cell shape changes and are implicated in formation of specialised infection structures to enter plant cells. Here, we review and compare the reported functions of septins of plant pathogenic fungi, with a special focus on invasive growth. Finally, we discuss septins as potential targets for broad-spectrum antifungal plant protection strategies.

Live-cell imaging of septins and cell polarity proteins in the growing dikaryotic vegetative hypha of the model mushroom Coprinopsis cinerea

The developmental biology underlying the morphogenesis of mushrooms remains poorly understood despite the essential role of fungi in the terrestrial environment and global carbon cycle. The mushroom Coprinopsis cinerea is a leading model system for the molecular and cellular basis of fungal morphogenesis. The dikaryotic vegetative hyphae of this fungus grow by tip growth with clamp cell formation, conjugate nuclear division, septation, subapical peg formation, and fusion of the clamp cell to the peg. Studying these processes provides many opportunities to gain insights into fungal cell morphogenesis. Here, we report the dynamics of five septins, as well as the regulators CcCla4, CcSpa2, and F-actin, visualized by tagging with fluorescent proteins, EGFP, PA-GFP or mCherry, in the growing dikaryotic vegetative hyphae. We also observed the nuclei using tagged Sumo proteins and histone H1. The five septins colocalized at the hyphal tip in the shape of a dome with a hole (DwH). CcSpa2-EGFP signals were observed in the hole, while CcCla4 signals were observed as the fluctuating dome at the hyphal tip. Before septation, CcCla4-EGFP was also occasionally recruited transiently around the future septum site. Fluorescent protein-tagged septins and F-actin together formed a contractile ring at the septum site. These distinct specialized growth machineries at different sites of dikaryotic vegetative hyphae provide a foundation to explore the differentiation program of various types of cells required for fruiting body formation.

Cytochrome P450 Sterol 14 Alpha-Demethylase Gene SsCI72380 Is Required for Mating/Filamentation and Pathogenicity in Sporisorium scitamineum

Sugarcane smut is a significant sugarcane disease caused by Sporisorium scitamineum and is a large threat to the sugar industry in China and the world. Accordingly, it is important to study the pathogenic mechanism by which this disease occurs to identify effective prevention and control strategies. Gene SsCI72380, which encodes cytochrome P450 sterol 14 alpha-demethylase (CYP51), was screened out from the transcriptome of S. scitamineum. In this study, the functions of gene SsCI72380 were identified via the knockout mutants ΔSs72380⁺ and ΔSs72380⁻, which were obtained by polyethylene glycol (PEG)-mediated protoplast transformation technology, as well as the complementary mutants COM72380⁺ and COM72380⁻. The results showed that the CYP51 gene SsCI72380 played an important role in sporidial growth, sexual mating/filamentation, hyphae growth, and pathogenicity in S. scitamineum. Gene SsCI72380 may regulate the biosynthesis process of ergosterol by encoding CYP51 enzymes and then affecting the structure and function of the cell membrane. Gene SsCI72380 also played an important role in the response toward different abiotic stresses, including hyperosmotic stress, oxidative stress, and cell wall stress, by regulating the permeability of the cell membrane. In addition, gene SsCI72380 is a new type of pathogenic gene from S. scitamineum that enhances the pathogenicity of S. scitamineum.

Investigating the Smuts: Common Cues, Signaling Pathways, and the Role of MAT in Dimorphic Switching and Pathogenesis

The corn smut fungus Ustilago maydis serves as a model species for studying fungal dimorphism and its role in phytopathogenic development. The pathogen has two growth phases: a saprobic yeast phase and a pathogenic filamentous phase. Dimorphic transition of U. maydis involves complex processes of signal perception, mating, and cellular reprogramming. Recent advances in improvement of reference genomes, high-throughput sequencing and molecular genetics studies have been expanding research in this field. However, the biology of other non-model species is frequently overlooked. This leads to uncertainty regarding how much of what is known in U. maydis is applicable to other dimorphic fungi. In this review, we will discuss dimorphic fungi in the aspects of physiology, reproductive biology, genomics, and molecular genetics. We also perform comparative analyses between U. maydis and other fungi in Ustilaginomycotina, the subphylum to which U. maydis belongs. We find that lipid/hydrophobicity is a potential common cue for dimorphic transition in plant-associated dimorphic fungi. However, genomic profiles alone are not adequate to explain dimorphism across different fungi.

Identification of a novel member of the pH responsive pathway Pal/Rim in Ustilago maydis

The most important signal transduction mechanism related to environmental pH responses in fungi is the Pal/Rim pathway. Our knowledge of this pathway came initially from studies on Ascomycota species where it is made by seven members divided into two complexes, one located at the plasma membrane, and other at the endosomal membrane. In Basidiomycota sepecies only the homologs of the endosomal membrane complex (genes PalA/Rim20, PalB/ Rim13, and PalC/ Rim23), plus the transcription factor PacC/Rim101 have been identified. In this study, we describe the identification in Ustilago maydis of a gene encoding a Rho-like protein (tentatively named RHO4) as a novel member of this pathway. The RHO4 gene possibly plays, among other functions, a role in the second proteolytic cleavage that leads to the activation of the transcription factor PacC/Rim101. Mutants in this gene showed a pleiotropic phenotype, displaying similar characteristics to the Pal/Rim mutants, such as a lower growth rate at alkaline pH, high sensitivity to ionic and osmotic stresses, and impairment in protease secretion, but no alteration of the yeast-to-mycelium dimorphic transition induced by acid pH whereas it has a function in the dimorphic transition induced by fatty acids.

The Botrytis cinerea PAK kinase BcCla4 mediates morphogenesis, growth and cell cycle regulating processes downstream of BcRac

Rac proteins are involved in a variety of cellular processes. Effector proteins that interact with active Rac convey the GTPase-generated signal to downstream developmental cascades and processes. Here we report on the analysis of the main effector and signal cascade downstream of BcRac, the Rac homolog of the grey mold fungus Botrytis cinerea. Several lines of evidence highlighted the p21-activated kinase Cla4 as an important effector of Rac in fungi. Analysis of Δbccla4 strains revealed that the BcCla4 protein was sufficient to mediate all of the examined BcRac-driven processes, including hyphal growth and morphogenesis, conidia production and pathogenicity. In addition, the Δbccla4 strains had altered nuclei content, a phenomenon that was previously observed in Δbcrac isolates, thus connecting the BcRac/BcCla4 module with cell cycle control. Further analyses revealed that BcRac/BcCla4 control mitotic entry through changes in phosphorylation status of the cyclin dependent kinase BcCdk1. The complete cascade includes the kinase BcWee1, which is downstream of BcCla4 and upstream of BcCdk1. These results provide a mechanistic insight on the connection of cell cycle, morphogenesis and pathogenicity in fungi, and position BcCla4 as the most essential effector and central regulator of all of these processes downstream of BcRac.

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.

Dephosphorylation of the Core Septin, AspB, in a Protein Phosphatase 2A-Dependent Manner Impacts Its Localization and Function in the Fungal Pathogen Aspergillus fumigatus

Septins are a conserved family of GTPases that form hetero–oligomeric complexes and perform diverse functions in higher eukaryotes, excluding plants. Our previous studies in the human fungal pathogen Aspergillus fumigatus revealed that the core septin, AspB, a CDC3 ortholog, is required for septation, conidiation, and conidial cell wall organization. Although AspB is important for these cellular functions, nothing is known about the role of kinases or phosphatases in the posttranslational regulation and localization of septins in A. fumigatus. In this study, we assessed the function of the Gin4 and Cla4 kinases and the PP2A regulatory subunit ParA, in the regulation of AspB using genetic and phosphoproteomic approaches. Gene deletion analyses revealed that Cla4 and ParA are indispensable for hyphal extension, and Gin4, Cla4, and ParA are each required for conidiation and normal septation. While deletion of gin4 resulted in larger interseptal distances and hypervirulence, a phenotype mimicking aspB deletion, deletion of cla4 and parA caused hyperseptation without impacting virulence, indicating divergent roles in regulating septation. Phosphoproteomic analyses revealed that AspB is phosphorylated at five residues in the GTPase domain (S134, S137, S247, T297, and T301) and two residues at its C-terminus (S416 and S461) in the wild-type, Δgin4 and Δcla4 strains. However, concomitant with the differential localization pattern of AspB and hyperseptation in the ΔparA strain, AspB remained phosphorylated at two additional residues, T68 in the N-terminal polybasic region and S447 in the coiled-coil domain. Generation of nonphosphorylatable and phosphomimetic strains surrounding each differentially phosphorylated residue revealed that only AspB^mt-T68E showed increased interseptal distances, suggesting that dephosphorylation of T68 is important for proper septation. This study highlights the importance of septin phosphorylation/dephosphorylation in the regulation of A. fumigatus hyphal septation.

Protein kinases in plant-pathogenic fungi: conserved regulators of infection

Phytopathogenic fungi have evolved an amazing diversity of infection modes and nutritional strategies, yet the signaling pathways that govern pathogenicity are remarkably conserved. Protein kinases (PKs) catalyze the reversible phosphorylation of proteins, regulating a variety of cellular processes. Here, we present an overview of our current understanding of the different classes of PKs that contribute to fungal pathogenicity on plants and of the mechanisms that regulate and coordinate PK activity during infection-related development. In addition to the well-studied PK modules, such as MAPK (mitogen-activated protein kinase) and cAMP (cyclic adenosine monophosphate)-PKA (protein kinase A) cascades, we also discuss new PK pathways that have emerged in recent years as key players of pathogenic development and disease. Understanding how conserved PK signaling networks have been recruited during the evolution of fungal pathogenicity not only advances our knowledge of the highly elaborate infection process but may also lead to the development of novel strategies for the control of plant disease.

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.

Cla4, but not Rac1, regulates the filamentous response of Ustilago maydis to low ammonium conditions

Ustilago maydis, the fungal pathogen of maize, undergoes a dimorphic transition from budding yeast-like growth to filamentous growth, both as part of its program for pathogenesis and distinctly, in response to environmental cues, such as acid pH or low nitrogen availability. Smu1 is a p21-activated protein kinase (PAK) with roles in both the mating response required for the former function, as well as for the nutrient response. Hsl7 may be a negative regulator of Smu1 and appears to play a role in cell length and cell cycle.  Additional proteins that participate in cell polarity and filamentation pathways include the small G protein, Rac1, and its effector PAK kinase, Cla4. Here we describe further experiments that explore the roles of Cla4 and Rac1 in the response to nitrogen availability. While deletion of rac1severely delays filamentous growth on solid media low in ammonium (SLAD), we found that deletion of cla4 does not abolish filamentous cell morphology on solid SLAD. Unexpectedly, however, the Dcla4 mutants also filament in liquid SLAD. The filamentous cell morphology of the cla4 mutant in liquid SLAD has only been seen previously for one other mutant, a strain deleted for hsl7 that simultaneously over-expresses smu1. 

Functional analysis of the kinome of the wheat scab fungus Fusarium graminearum

As in other eukaryotes, protein kinases play major regulatory roles in filamentous fungi. Although the genomes of many plant pathogenic fungi have been sequenced, systematic characterization of their kinomes has not been reported. The wheat scab fungus Fusarium graminearum has 116 protein kinases (PK) genes. Although twenty of them appeared to be essential, we generated deletion mutants for the other 96 PK genes, including 12 orthologs of essential genes in yeast. All of the PK mutants were assayed for changes in 17 phenotypes, including growth, conidiation, pathogenesis, stress responses, and sexual reproduction. Overall, deletion of 64 PK genes resulted in at least one of the phenotypes examined, including three mutants blocked in conidiation and five mutants with increased tolerance to hyperosmotic stress. In total, 42 PK mutants were significantly reduced in virulence or non-pathogenic, including mutants deleted of key components of the cAMP signaling and three MAPK pathways. A number of these PK genes, including Fg03146 and Fg04770 that are unique to filamentous fungi, are dispensable for hyphal growth and likely encode novel fungal virulence factors. Ascospores play a critical role in the initiation of wheat scab. Twenty-six PK mutants were blocked in perithecia formation or aborted in ascosporogenesis. Additional 19 mutants were defective in ascospore release or morphology. Interestingly, F. graminearum contains two aurora kinase genes with distinct functions, which has not been reported in fungi. In addition, we used the interlog approach to predict the PK-PK and PK-protein interaction networks of F. graminearum. Several predicted interactions were verified with yeast two-hybrid or co-immunoprecipitation assays. To our knowledge, this is the first functional characterization of the kinome in plant pathogenic fungi. Protein kinase genes important for various aspects of growth, developmental, and infection processes in F. graminearum were identified in this study.

Fusarium head blight caused by Fusarium graminearum is one of the most important diseases on wheat and barley. Although protein kinases are known to play major regulatory roles in fungi, systematic characterization of fungal kinomes has not been reported in plant pathogens. In this study we generated deletion mutants for 96 protein kinase genes. All of the resulting knockout mutants were assayed for changes in 17 phenotypes, including growth, reproduction, stress responses, and plant infection. Overall, deletion of 64 kinase genes resulted in at least one of the phenotypes examined. In total, 42 kinase mutants were significantly reduced in virulence or non-pathogenic. A number of these protein kinase genes, including two that are unique to filamentous fungi, are dispensable for hyphal growth and likely encode novel fungal virulence factors. Ascospores are the primary inoculum for wheat scab. We identified 26 mutants blocked in ascospore. We also used the in silico approach to predict the kinase-kinase interactions and verified some of them by yeast two-hybrid or co-IP assays. Overall, in this study we functionally characterize the kinome of F. graminearum. Protein kinase genes that are important for various aspects of growth, developmental, and plant infection processes were identified.

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.

Possible additional roles in mating for Ustilago maydis Rho1 and 14-3-3 homologues

Both the Rho GTPases and 14-3-3 proteins each belong to ubiquitous families of proteins involved in a variety of cellular processes, including cytokinesis, cell polarity, cellular differentiation and apoptosis. In fungi, these components of signaling pathways are involved in cell cycle regulation, cytokinesis and virulence. We study cellular differentiation and pathogenesis for Ustilago maydis, the dimorphic fungal pathogen of maize. We have reported on the interactions of Pdc1, a U. maydis homologue of human 14-3-3varepsilon, with Rho1, a small GTP binding protein; these proteins participate in cell polarity and filamentation pathways that include another small G protein, Rac1, and its effector PAK kinase, Cla4. Here we describe additional experiments that explore possible relationships of Pdc1 and Rho1 with another PAK-like kinase pathway and with the a matingtype locus.

Cla4 kinase triggers destruction of the Rac1-GEF Cdc24 during polarized growth in Ustilago maydis

In the dimorphic fungus Ustilago maydis, Rac1 and its activator Cdc24 are essential for hyphal tip growth. Rac1 is shown to stimulate Cla4 kinase, which in turn triggers destruction of Cdc24. Expression of stabilized Cdc24 interferes with cell polarization, indicating that negative feedback regulation of Cdc24 is critical for tip growth.

Dimorphic switching from budding to filamentous growth is a characteristic feature of many pathogenic fungi. In the fungal model organism Ustilago maydis polarized growth is induced by the multiallelic b mating type locus and requires the Rho family GTPase Rac1. Here we show that mating type–induced polarized growth involves negative feedback regulation of the Rac1-specific guanine nucleotide exchange factor (GEF) Cdc24. Although Cdc24 is essential for polarized growth, its concentration is drastically diminished during filament formation. Cdc24 is part of a protein complex that also contains the scaffold protein Bem1 and the PAK kinase Cla4. Activation of Rac1 results in Cla4-dependent degradation of the Rac1-GEF Cdc24, thus creating a regulatory negative feedback loop. We generated mutants of Cdc24 that are resistant to Cla4-dependent destruction. Expression of stable Cdc24 variants interfered with filament formation, indicating that negative feedback regulation of Cdc24 is critical for the establishment of polarized growth.

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.

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.

Activation of the ustilagic acid biosynthesis gene cluster in Ustilago maydis by the C2H2 zinc finger transcription factor Rua1

The phytopathogenic basidiomycetous fungus Ustilago maydis secretes, under conditions of nitrogen starvation, large amounts of the biosurfactant ustilagic acid (UA). This secreted cellobiose glycolipid is toxic for many microorganisms and confers biocontrol activity to U. maydis. Recently, a large gene cluster that is responsible for UA biosynthesis was identified. Here, we show that expression of all cluster genes depends on Rua1, a nuclear protein of the C(2)H(2) zinc finger family, whose gene is located within the gene cluster. While deletion of rua1 results in complete loss of UA production, overexpression of rua1 promotes increased UA synthesis even in the presence of a good nitrogen source. Bioinformatic analysis allowed us to identify a conserved sequence element that is present in the promoters of all structural genes involved in UA biosynthesis. Deletion analysis of several promoters within the cluster revealed that this DNA element serves as an upstream activating sequence (UAS) and mediates Rua1-dependent expression. We used the yeast one-hybrid system to demonstrate specific recognition of this DNA element by Rua1. Introduction of nucleotide exchanges into the consensus sequence interfered with Rua1-dependent activation, suggesting that this sequence element acts as a direct binding site for Rua1.

Two CDC42 paralogues modulate Cryptococcus neoformans thermotolerance and morphogenesis under host physiological conditions

The precise regulation of morphogenesis is a key mechanism by which cells respond to a variety of stresses, including those encountered by microbial pathogens in the host. The polarity protein Cdc42 regulates cellular morphogenesis throughout eukaryotes, and we explore the role of Cdc42 proteins in the host survival of the human fungal pathogen Cryptococcus neoformans. Uniquely, C. neoformans has two functional Cdc42 paralogues, Cdc42 and Cdc420. Here we investigate the contribution of each paralogue to resistance to host stress. In contrast to non-pathogenic model organisms, C. neoformans Cdc42 proteins are not required for viability under non-stress conditions but are required for resistance to high temperature. The paralogues play differential roles in actin and septin organization and act downstream of C. neoformans Ras1 to regulate its morphogenesis sub-pathway, but not its effects on mating. Cdc42, and not Cdc420, is upregulated in response to temperature stress and is required for virulence in a murine model of cryptococcosis. The C. neoformans Cdc42 proteins likely perform complementary functions with other Rho-like GTPases to control cell polarity, septin organization and hyphal transitions that allow survival in the environment and in the host.

In vivo yeast cell morphogenesis is regulated by a p21-activated kinase in the human pathogen Penicillium marneffei

Pathogens have developed diverse strategies to infect their hosts and evade the host defense systems. Many pathogens reside within host phagocytic cells, thus evading much of the host immune system. For dimorphic fungal pathogens which grow in a multicellular hyphal form, a central attribute which facilitates growth inside host cells without rapid killing is the capacity to switch from the hyphal growth form to a unicellular yeast form. Blocking this transition abolishes or severely reduces pathogenicity. Host body temperature (37°C) is the most common inducer of the hyphal to yeast transition in vitro for many dimorphic fungi, and it is often assumed that this is the inducer in vivo. This work describes the identification and analysis of a new pathway involved in sensing the environment inside a host cell by a dimorphic fungal pathogen, Penicillium marneffei. The pakB gene, encoding a p21-activated kinase, defines this pathway and operates independently of known effectors in P. marneffei. Expression of pakB is upregulated in P. marneffei yeast cells isolated from macrophages but absent from in vitro cultured yeast cells produced at 37°C. Deletion of pakB leads to a failure to produce yeast cells inside macrophages but no effect in vitro at 37°C. Loss of pakB also leads to the inappropriate production of yeast cells at 25°C in vitro, and the mechanism underlying this requires the activity of the central regulator of asexual development. The data shows that this new pathway is central to eliciting the appropriate morphogenetic response by the pathogen to the host environment independently of the common temperature signal, thus clearly separating the temperature- and intracellular-dependent signaling systems.

Dimorphic fungal pathogens pose significant health and agricultural problems worldwide. These fungi have the capacity to switch between a multicellular hyphal growth form and a unicellular yeast growth form. Often one form is pathogenic, found in infected hosts, and the other is not. Many dimorphic fungal pathogens of humans produce the yeast form during infection and this form resides within host phagocytic immune cells, where it can tolerate killing by these cells and is not exposed to the acquired immune system. Inhibiting the pathogen's ability to switch growth forms has been shown to block pathogenicity. This study identifies a pathway used by the fungal pathogen to sense the host and switch to the appropriate growth form. This study provides new insights into the molecular mechanisms which are important for pathogenicity and may identify factors which can be targeted to block the ability of the pathogen to successfully reside within host cells.

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.

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.

The RHO1-specific GTPase-activating protein LRG1 regulates polar tip growth in parallel to Ndr kinase signaling in Neurospora

Regulation of Rho GTPase signaling is critical for cell shape determination and polarity. Here, we investigated the role of LRG1, a novel member of the GTPase-activating proteins (GAPs) of Neurospora crassa. LRG1 is essential for apical tip extension and to restrict excessive branch formation in subapical regions of the hypha and is involved in determining the size of the hyphal compartments. LRG1 localizes to hyphal tips and sites of septation via its three LIM domains. The accumulation of LRG1 as an apical cap is dependent on a functional actin cytoskeleton and active growth, and is influenced by the opposing microtubule-dependent motor proteins dynein and kinesin-1. Genetic evidence and in vitro GTPase assays identify LRG1 as a RHO1-specific GAP affecting several output pathways of RHO1, based on hyposensitivity to the glucan inhibitor caspofungin, synthetic lethality with a hyperactive beta1,3-glucan synthase mutant, altered PKC/MAK1 pathway activities, and hypersensitivity to latrunculin A. The morphological defects of lrg-1 are highly reminiscent to the Ndr kinase/RAM pathway mutants cot-1 and pod-6, and genetic evidence suggests that RHO1/LRG1 function in parallel with COT1 in coordinating apical tip growth.

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.

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.

Identification of mating type genes in the bipolar basidiomycetous yeast Rhodosporidium toruloides: first insight into the MAT locus structure of the Sporidiobolales

Rhodosporidium toruloides is a heterothallic, bipolar, red yeast that belongs to the Sporidiobolales, an order within a major lineage of basidiomycetes, the Pucciniomycotina. In contrast to other basidiomycetes, considerably less is known about the nature of the mating type (MAT) loci that control sexual reproduction in this lineage. Three genes (RHA1, RHA2, and RHA3) encoding precursors of the MAT A1 pheromone (rhodotorucine A) were previously identified and formed the basis for a genome walking approach that led to the identification of additional MAT genes in complementary mating strains of R. toruloides. Two mating type-specific alleles encoding a p21-activated kinase (PAK; Ste20 homolog) were found between the RHA2 and RHA3 genes, and identification in MAT A2 strains of a gene encoding a presumptive pheromone precursor enabled prediction of the structure of rhodotorucine a. In addition, a putative pheromone receptor gene (STE3 homolog) was identified upstream of RHA1. Analyses of genomic data from two closely related species, Sporobolomyces roseus and Sporidiobolus salmonicolor, identified syntenic regions that contain homologs of all the above-mentioned genes. Notably, six novel pheromone precursor genes were uncovered, which encoded, similarly to the RHA genes, multiple tandem copies of the peptide moiety. This suggests that this structure, which is unique among fungal lipopeptide pheromones, seems to be prevalent in red yeasts. Species comparisons provided evidence for a large, multigenic MAT locus structure in the Sporidiobolales, but no putative homeodomain transcription factor genes (which are present in all basidiomycetous MAT loci characterized thus far) could be found in any of the three species in the vicinity of the MAT genes identified.

The small GTPase Rac and the p21-activated kinase Cla4 in Claviceps purpurea: interaction and impact on polarity, development and pathogenicity

Claviceps purpurea, the ergot fungus, is a highly specialized pathogen of grasses; its colonization of host ovarian tissue requires an extended period of strictly polarized, oriented growth towards the vascular tissue. To understand this process, we study the role of signalling factors affecting polarity and differentiation. We showed that the small GTPase Cdc42 is involved in polarity, sporulation and in planta growth in C. purpurea. Here we present evidence that the GTPase Rac has an even stronger and, in some aspects, inverse impact on growth and development: Deltarac mutants form coralline-like colonies, show hyper-branching, loss of polarity, sporulation and ability to penetrate. Functional analyses and yeast two-hybrid studies prove that the p21-activated kinase Cla4 is a major downstream partner of Rac. Phosphorylation assays of MAP kinases and expression studies of genes encoding reactive oxygen species (ROS)-scavenging and -generating enzymes indicate a function of Rac and Cla4 in fungal ROS homoeostasis which could contribute to their drastic impact on differentiation.

A p21-activated kinase is required for conidial germination in Penicillium marneffei

Asexual spores (conidia) are the infectious propagules of many pathogenic fungi, and the capacity to sense the host environment and trigger conidial germination is a key pathogenicity determinant. Germination of conidia requires the de novo establishment of a polarised growth axis and consequent germ tube extension. The molecular mechanisms that control polarisation during germination are poorly understood. In the dimorphic human pathogenic fungus Penicillium marneffei, conidia germinate to produce one of two cell types that have very different fates in response to an environmental cue. At 25 degrees C, conidia germinate to produce the saprophytic cell type, septate, multinucleate hyphae that have the capacity to undergo asexual development. At 37 degrees C, conidia germinate to produce the pathogenic cell type, arthroconidiating hyphae that liberate uninucleate yeast cells. This study shows that the p21-activated kinase pakA is an essential component of the polarity establishment machinery during conidial germination and polarised growth of yeast cells at 37 degrees C but is not required for germination or polarised growth at 25 degrees C. Analysis shows that the heterotrimeric G protein alpha subunit GasC and the CDC42 orthologue CflA lie upstream of PakA for germination at both temperatures, while the Ras orthologue RasA only functions at 25 degrees C. These findings suggest that although some proteins that regulate the establishment of polarised growth in budding yeast are conserved in filamentous fungi, the circuitry and downstream effectors are differentially regulated to give rise to distinct cell types.

Yeast and fungal morphogenesis from an evolutionary perspective

Cellular morphogenesis is a complex process and molecular studies in the last few decades have amassed a large amount of information that is difficult to grasp in any completeness. Fungal systems, in particular the budding and fission yeasts, have been important players in unravelling the basic structural and regulatory elements involved in a wide array of cellular processes. In this article, we address the design principles underlying the various processes of yeast and fungal morphogenesis. We attempt to explain the apparent molecular complexity from the perspective of the evolutionary theory of "facilitated variation". Following a summary of some of the most studied morphogenetic phenomena, we discuss, using recent examples, the underlying core processes and their associated "weak" regulatory linkages that bring about variation in morphogenetic phenotypes.

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 biosynthetic gene cluster for a secreted cellobiose lipid with antifungal activity from Ustilago maydis

The phytopathogenic basidiomycetous fungus Ustilago maydis secretes large amounts of the glycolipid biosurfactant ustilagic acid (UA). UA consists of 15,16-dihydroxypalmitic or 2,15,16-trihydroxypalmitic acid, which is O-glycosidically linked to cellobiose at its terminal hydroxyl group. In addition, the cellobiose moiety is acetylated and acylated with a short-chain hydroxy fatty acid. We have identified a 58 kb spanning gene cluster that contains 12 open reading frames coding for most, if not all, enzymes needed for UA biosynthesis. Using a combination of genetic and mass spectrometric analysis we were able to assign functional roles to three of the proteins encoded by the gene cluster. This allowed us to propose a biosynthesis route for UA. The Ahd1 protein belongs to the family of non-haem diiron reductases and is required for alpha-hydroxylation of palmitic acid. Two P450 monooxygenases, Cyp1 and Cyp2, catalyse terminal and subterminal hydroxylation of palmitic acid. We could demonstrate that infection of tomato leaves by the plant pathogenic fungus Botrytis cinerea is prevented by co-inoculation with wild-type U. maydis sporidia. U. maydis mutants defective in UA biosynthesis were unable to inhibit B. cinerea infection indicating that UA secretion is critical for antagonistic activity.

Sustained cell polarity and virulence in the phytopathogenic fungus Ustilago maydis depends on an essential cyclin-dependent kinase from the Cdk5/Pho85 family

Cyclin-dependent kinases from the Cdk5/Pho85 family are thought to play important roles in morphogenesis in organisms as diverse as yeast and humans. Here we used the corn smut fungus Ustilago maydis to address the role of Cdk5/Pho85 kinases in the morphogenesis and virulence of dimorphic phytopathogens. We found that Cdk5 is essential for growth in U. maydis. A temperature-sensitive cdk5 mutant caused cell wall and morphology defects at the restrictive temperature. Actin patches labeled with a fimbrin-GFP fusion protein were delocalized and a GFP-Myo5 fusion was directed towards the growing cell pole and rapidly dissociated from the tip. These defects were found to be due to an impairment in the maintenance of cell polarity. Our results indicated that Cdk5 is required for the activity of Rac1, probably at the level of the localization of its GEF, Cdc24. Cdk5 was required for full virulence, probably because mutant cells are unable to sustain the dramatic polar growth required for the formation of the infective structures. These results support a major role for morphogenesis in the virulence program of dimorphic fungi.

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.

Rac1 and Cdc42 regulate hyphal growth and cytokinesis in the dimorphic fungus Ustilago maydis

Small GTP-binding proteins of the highly conserved Rho family act as molecular switches regulating cell signalling, cytoskeletal organization and vesicle trafficking in eukaryotic cells. Here we show that in the dimorphic plant pathogenic fungus Ustilago maydis deletion of either cdc42 or rac1 results in loss of virulence but does not interfere with viability. Cells deleted for cdc42 display a cell separation defect during budding. We have previously shown that the Rho-specific guanine nucleotide exchange factor (GEF) Don1 is required for cell separation in U. maydis. Expression of constitutive active Cdc42 rescues the phenotype of don1 mutant cells indicating that Don1 triggers cell separation by activating Cdc42. Deletion of rac1 affects cellular morphology and interferes with hyphal growth, whereas overexpression of wild-type Rac1 induces filament formation in haploid cells. This indicates that Rac1 is both necessary and sufficient for the dimorphic switch from budding to hyphal growth. Cdc42 and Rac1 share at least one common essential function because depletion of both Rac1 and Cdc42 is lethal. Expression of constitutively active Rac1(Q61L) is lethal and results in swollen cells with a large vacuole. The morphological phenotype, but not lethality is suppressed in cla4 mutant cells suggesting that the PAK family kinase Cla4 acts as a downstream effector of Rac1.

Mycorrhiza helper bacterium Streptomyces AcH 505 induces differential gene expression in the ectomycorrhizal fungus Amanita muscaria

The interaction between the mycorrhiza helper bacteria Streptomyces nov. sp. 505 (AcH 505) and Streptomyces annulatus 1003 (AcH 1003) with fly agaric (Amanita muscaria) and spruce (Picea abies) was investigated. The effects of both bacteria on the mycelial growth of different ectomycorrhizal fungi, on ectomycorrhiza formation, and on fungal gene expression in dual culture with AcH 505 were determined. The fungus specificities of the streptomycetes were similar. Both bacterial species showed the strongest effect on the growth of mycelia at 9 wk of dual culture. The effect of AcH 505 on gene expression of A. muscaria was examined using the suppressive subtractive hybridization approach. The responsive fungal genes included those involved in signalling pathways, metabolism, cell structure, and the cell growth response. These results suggest that AcH 505 and AcH 1003 enhance mycorrhiza formation mainly as a result of promotion of fungal growth, leading to changes in fungal gene expression. Differential A. muscaria transcript accumulation in dual culture may result from a direct response to bacterial substances.

A Rac homolog functions downstream of Ras1 to control hyphal differentiation and high-temperature growth in the pathogenic fungus Cryptococcus neoformans

The Cryptococcus neoformans Ras1 protein serves as a central regulator for several signaling pathways. Ras1 controls the induction of the mating pheromone response cascade as well as a distinct signaling pathway that allows this pathogenic fungus to grow at human physiological temperature. To characterize elements of the Ras1-dependent high-temperature growth pathway, we performed a multicopy suppressor screen, identifying genes whose overexpression allows the ras1 mutant to grow at 37 degrees C. Using this genetic technique, we identified a C. neoformans gene encoding a Rac homolog that suppresses multiple ras1 mutant phenotypes. Deletion of the RAC1 gene does not affect high-temperature growth. However, a rac1 mutant strain demonstrates a profound defect in haploid filamentation as well as attenuated mating. In a yeast two-hybrid assay, Rac1 physically interacts with the PAK kinase Ste20, which similarly regulates hyphal formation in this fungus. Similar to Rac1, overexpression of the STE20alpha gene also restores high-temperature growth to the ras1 mutant. These results support a model in which the small G protein Rac1 acts downstream of Ras proteins and coordinately with Ste20 to control high-temperature growth and cellular differentiation in this human fungal pathogen.

Ectopic expression of a constitutively active Cdc42 small GTPase alters the morphology of haploid and dikaryotic hyphae in the filamentous homobasidiomycete Schizophyllum commune

Cloning of the Cdc42 gene from Schizophyllum commune enabled investigation of the role of ScCdc42 in the regulation of vegetative growth and sexual reproduction in this fungus, which has a well-characterized hyphal cell structure, cytoskeleton, and mating system. Ectopic expression of the constitutively active Sccdc42(G12V) or Sccdc42(Q61L) alleles from native or inducible ScCel1 promoters in haploid hyphae had dramatic effects on hyphal morphology, cytoskeletal structure, and Cdc42 localization. For transformants with constitutively active Sccdc42, polar tip growth of apical cells in the leading hyphae was normal but polar tip growth in side branches was altered, implying different regulation of polarity establishment in the two groups of apical cells. Branch emergence at exceptional sites and isotropic growth of cells near the septum indicated that ScCdc42 regulates branch site selection and subsequent hyphal development. Poor dikaryotization along with irregular clamp connections in mates expressing Sccdc42(G12V) or Sccdc42(Q61L) suggested that Cdc42 also contributes to efficient mating in S. commune.