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

Invasion of the four kingdoms: the parasite journey across plant and non-plant hosts

Parasites have a rich and long natural history among biological entities, and it has been suggested that parasites are one of the most significant factors in the evolution of their hosts. However, it has been emphasized less frequently how co-evolution has undoubtedly also shaped the paths of parasites. It may seem safe to assume that specific differences among the array of potential hosts for particular parasites have restricted and diversified their evolutionary pathways and strategies for survival. Nevertheless, if one looks closely enough at host and parasite, one finds commonalities, both in terms of host defences and parasite strategies to out-manoeuvre them. While such analyses have been the source of numerous reviews, they are generally limited to interactions between, at most, one kingdom of parasite with two kingdoms of host (e.g. similarities in animal and plant host responses against fungi). With the aim of extending this view, we herein critically evaluate the similarities and differences across all four eukaryotic host kingdoms (plants, animals, fungi, and protists) and their parasites. In doing so, we show that hosts tend to share common strategies for defence, including both physical and behavioural barriers, and highly evolved immune responses, in particular innate immunity. Parasites have, similarly, evolved convergent strategies to counter these defences, including mechanisms of active penetration, and evading the host's innate and/or adaptive immune responses. Moreover, just as hosts have evolved behaviours to avoid parasites, many parasites have adaptations to manipulate host phenotype, physiologically, reproductively, and in terms of behaviour. Many of these strategies overlap in the host and parasite, even across wide phylogenetic expanses. That said, specific differences in host physiology and immune responses often necessitate different adaptations for parasites exploiting fundamentally different hosts. Taken together, this review facilitates hypothesis-driven investigations of parasite-host interactions that transcend the traditional kingdom-based research fields.

Exploring Potential Surrogate Systems for Studying the Early Steps of the Sporisorium scitamineum Pathogenesis

Despite its global importance as a primary source of table sugar and bioethanol, sugarcane faces a significant threat to its production due to diseases. One of these diseases, sugarcane smut, involves the emergence of a whip-like structure from the host apical shoot. The slow onset of this pathogenesis is the most substantial challenge for researchers to investigate the molecular events leading to resistance or susceptibility. In this study, we explored the early interaction between the smut fungus Sporisorium scitamineum and foliar tissues of the model plants Arabidopsis thaliana and Nicotiana benthamiana. Upon inoculation with the fungus, A. thaliana showed a compatible reaction, producing lesions during fungus colonization, whereas N. benthamiana showed signs of nonhost resistance. In addition, we propose a sugarcane detached leaf assay using plants cultivated in vitro to reveal sugarcane smut response outcomes. We used two sugarcane genotypes with known contrasting reactions to smut in the field. Although there is no evidence of sugarcane smut fungus infecting host leaves naturally, the sugarcane detached leaf assay enabled a rapid assessment of disease outcomes. Different symptoms in the detached leaves after inoculation distinguished smut-susceptible and smut-resistant sugarcane genotypes. Microscopic observations and gene expression analysis of S. scitamineum candidate effectors confirmed the fungal growth and its restriction on the compatible and incompatible interactions, respectively. These findings offer new prospects into the disease phenotyping of S. scitamineum, which could greatly expedite the comprehension of the initial stages of the pathogenesis and predict smut resistance in sugarcane genotypes.

The mRNA stability factor Khd4 defines a specific mRNA regulon for membrane trafficking in the pathogen Ustilago maydis

Fungal pathogens depend on sophisticated gene expression programs for successful infection. A crucial component is RNA regulation mediated by RNA-binding proteins (RBPs). However, little is known about the spatiotemporal RNA control mechanisms during fungal pathogenicity. Here, we discover that the RBP Khd4 defines a distinct mRNA regulon to orchestrate membrane trafficking during pathogenic development of Ustilago maydis. By establishing hyperTRIBE for fungal RBPs, we generated a comprehensive transcriptome-wide map of Khd4 interactions in vivo. We identify a defined set of target mRNAs enriched for regulatory proteins involved, e.g., in GTPase signaling. Khd4 controls the stability of target mRNAs via its cognate regulatory element AUACCC present in their 3' untranslated regions. Studying individual examples reveals a unique link between Khd4 and vacuole maturation. Thus, we uncover a distinct role for an RNA stability factor defining a specific mRNA regulon for membrane trafficking during pathogenicity.

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.

TORC1 Signaling in Fungi: From Yeasts to Filamentous Fungi

Target of rapamycin complex 1 (TORC1) is an important regulator of various signaling pathways. It can control cell growth and development by integrating multiple signals from amino acids, glucose, phosphate, growth factors, pressure, oxidation, and so on. In recent years, it has been reported that TORC1 is of great significance in regulating cytotoxicity, morphology, protein synthesis and degradation, nutrient absorption, and metabolism. In this review, we mainly discuss the upstream and downstream signaling pathways of TORC1 to reveal its role in fungi.

The Nma1 protein promotes long distance transport mediated by early endosomes in Ustilago maydis

Early endosomes (EEs) are part of the endocytic transport pathway and resemble the earliest class of transport vesicles between the internalization of extracellular material, their cellular distribution or vacuolar degradation. In filamentous fungi, EEs fulfill important functions in long distance transport of cargoes as mRNAs, ribosomes, and peroxisomes. Formation and maturation of early endosomes is controlled by the specific membrane-bound Rab-GTPase Rab5 and tethering complexes as CORVET (class C core vacuole/endosome tethering). In the basidiomycete Ustilago maydis, Rab5a is the prominent GTPase to recruit CORVET to EEs; in rab5a deletion strains, this function is maintained by the second EE-associated GTPase Rab5b. The tethering- and core-subunits of CORVET are essential, buttressing a central role for EE transport in U. maydis. The function of EEs in long distance transport is supported by the Nma1 protein that interacts with the Vps3 subunit of CORVET. The interaction stabilizes the binding of Vps3 to the CORVET core complex that is recruited to Rab5a via Vps8. Deletion of nma1 leads to a significantly reduced number of EEs, and an increased conversion rate of EEs to late endosomes. Thus, Nma1 modulates the lifespan of EEs to ensure their availability for the various long distance transport processes.

Benzoic Acid and Its Hydroxylated Derivatives Suppress Early Blight of Tomato (Alternaria solani) via the Induction of Salicylic Acid Biosynthesis and Enzymatic and Nonenzymatic Antioxidant Defense Machinery

Tomato early blight, caused by Alternaria solani, is a destructive foliar fungal disease. Herein, the potential defensive roles of benzoic acid (BA) and two of its hydroxylated derivatives, ρ-hydroxybenzoic acid (HBA), and protocatechuic acid (PCA) against A. solani were investigated. All tested compounds showed strong dose-dependent fungistatic activity against A. solani and significantly reduced the disease development. Benzoic acid, and its hydroxylated derivatives, enhanced vegetative growth and yield traits. Moreover, BA and its derivatives induce the activation of enzymatic (POX, PPO, CAT, SlAPXs, and SlSODs) and non-enzymatic (phenolics, flavonoids, and carotenoids) antioxidant defense machinery to maintain reactive oxygen species (ROS) homeostasis within infected leaves. Additionally, BA and its hydroxylated derivatives induce the accumulation of salicylic acid (SA) and its biosynthetic genes including isochorismate synthase (SlICS), aldehyde oxidases (SlAO1 and SlAO2), and phenylalanine ammonia-lyases (SlPAL1, SlPAL2, SlPAL3, SlPAL5, and SlPAL6). Higher SA levels were associated with upregulation of pathogenesis-related proteins (SlPR-1, SlPR1a2, SlPRB1-2, SlPR4, SlPR5, SlPR6), nonexpressor of pathogenesis-related protein 1 (SlNPR1), and salicylic acid-binding protein (SlSABP2). These findings outline the potential application of BA and its hydroxylated derivatives as a sustainable alternative control strategy for early blight disease and also deciphering the physiological and biochemical mechanisms behind their protective role.

Loss of Septation Initiation Network (SIN) kinases blocks tissue invasion and unlocks echinocandin cidal activity against Aspergillus fumigatus

Although considered effective treatment for many yeast fungi, the therapeutic efficacy of the echinocandin class of antifungals for invasive aspergillosis (IA) is limited. Recent studies suggest intense kinase- and phosphatase-mediated echinocandin adaptation in A. fumigatus. To identify A. fumigatus protein kinases required for survival under echinocandin stress, we employed CRISPR/Cas9-mediated gene targeting to generate a protein kinase disruption mutant library in a wild type genetic background. Cell wall and echinocandin stress screening of the 118 disruption mutants comprising the library identified only five protein kinase disruption mutants displaying greater than 4-fold decreased echinocandin minimum effective concentrations (MEC) compared to the parental strain. Two of these mutated genes, the previously uncharacterized A. fumigatus sepL and sidB genes, were predicted to encode protein kinases functioning as core components of the Septation Initiation Network (SIN), a tripartite kinase cascade that is necessary for septation in fungi. As the A. fumigatus SIN is completely uncharacterized, we sought to explore these network components as effectors of echinocandin stress survival. Our data show that mutation of any single SIN kinase gene caused complete loss of hyphal septation and increased susceptibility to cell wall stress, as well as widespread hyphal damage and loss of viability in response to echinocandin stress. Strikingly, mutation of each SIN kinase gene also resulted in a profound loss of virulence characterized by lack of tissue invasive growth. Through the deletion of multiple novel regulators of hyphal septation, we show that the non-invasive growth phenotype is not SIN-kinase dependent, but likely due to hyphal septation deficiency. Finally, we also find that echinocandin therapy is highly effective at eliminating residual tissue burden in mice infected with an aseptate strain of A. fumigatus. Together, our findings suggest that inhibitors of septation could enhance echinocandin-mediated killing while simultaneously limiting the invasive potential of A. fumigatus hyphae.

Coregulation of gene expression by White collar 1 and phytochrome in Ustilago maydis

Ustilago maydis encodes ten predicted light-sensing proteins. The biological functions of only a few of them are elucidated. Among the characterized ones are two DNA-photolyases and two rhodopsins that act as DNA-repair enzymes or green light-driven proton pumps, respectively. Here we report on the role of two other photoreceptors in U. maydis, namely White collar 1 (Wco1) and Phytochrome 1 (Phy1). We show that they bind flavins or biliverdin as chromophores, respectively. Both photoreceptors undergo a photocycle in vitro. Wco1 is the dominant blue light receptor in the saprophytic phase, controlling all of the 324 differentially expressed genes in blue light. U. maydis also responds to red and far-red light. However, the number of red or far-red light-controlled genes is less compared to blue light-regulated ones. Moreover, most of the red and far-red light-controlled genes not only depend on Phy1 but also on Wco1, indicating partial coregulation of gene expression by both photoreceptors. GFP-fused Wco1 is preferentially located in the nucleus, Phy1 in the cytosol, thus providing no hint that these photoreceptors directly interact or operate within the same complex. This is the first report on a functional characterization and coaction of White collar 1 and phytochrome orthologs in basidiomycetes.

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.

How Do Smut Fungi Use Plant Signals to Spatiotemporally Orientate on and In Planta?

Smut fungi represent a large group of biotrophic plant pathogens that cause extensive yield loss and are also model organisms for studying plant-pathogen interactions. In recent years, they have become biotechnological tools. After initial penetration of the plant epidermis, smut fungi grow intra-and intercellularly without disrupting the plant-plasma membrane. Following the colonialization step, teliospores are formed and later released. While some smuts only invade the tissues around the initial penetration site, others colonize in multiple plant organs resulting in spore formation distal from the original infection site. The intimate contact zone between fungal hyphae and the host is termed the biotrophic interaction zone and enables exchange of signals and nutrient uptake. Obviously, all steps of on and in planta growth require fine sensing of host conditions as well as reprogramming of the host by the smut fungus. In this review, we highlight selected examples of smut fungal colonization styles, directional growth in planta, induction of spore formation, and the signals required, pointing to excellent reviews for details, to draw attention to some of the open questions in this important research field.

Rapamycin induces morphological and physiological changes without increase in lipid content in Ustilago maydis

The evolutionarily conserved serine/threonine kinase TOR recruits different subunits to assemble the Target of Rapamycin Complex 1 (TORC1), which is inhibited by rapamycin and regulates ribosome biogenesis, autophagy, and lipid metabolism by regulating the expression of lipogenic genes. In addition, TORC1 participates in the cell cycle, increasing the length of the G2 phase. In the present work, we investigated the effect of rapamycin on cell growth, cell morphology and neutral lipid metabolism in the phytopathogenic fungus Ustilago maydis. Inhibition of TORC1 by rapamycin induced the formation of septa that separate the nuclei that were formed after mitosis. Regarding neutral lipid metabolism, a higher accumulation of triacylglycerols was not detected, but the cells did contain large lipid bodies, which suggests that small lipid bodies became fused into big lipid droplets. Vacuoles showed a similar behavior as the lipid bodies, and double labeling with Blue-CMAC and BODIPY indicates that vacuoles and lipid bodies were independent organelles. The results suggest that TORC1 has a role in cell morphology, lipid metabolism, and vacuolar physiology in U. maydis.

Conditional gene expression reveals stage-specific functions of the unfolded protein response in the Ustilago maydis-maize pathosystem

Ustilago maydis is a model organism for the study of biotrophic plant-pathogen interactions. The sexual and pathogenic development of the fungus are tightly connected since fusion of compatible haploid sporidia is prerequisite for infection of the host plant, maize (Zea mays). After plant penetration, the unfolded protein response (UPR) is activated and required for biotrophic growth. The UPR is continuously active throughout all stages of pathogenic development in planta. However, since development of UPR deletion mutants stops directly after plant penetration, the role of an active UPR at later stages of development remained to be determined. Here, we established a gene expression system for U. maydis that uses endogenous, conditionally active promoters to either induce or repress expression of a gene of interest during different stages of plant infection. Integration of the expression constructs into the native genomic locus and removal of resistance cassettes were required to obtain a wild-type-like expression pattern. This indicates that genomic localization and chromatin structure are important for correct promoter activity and gene expression. By conditional expression of the central UPR regulator, Cib1, in U. maydis, we show that a functional UPR is required for continuous plant defence suppression after host infection and that U. maydis relies on a robust control system to prevent deleterious UPR hyperactivation.

The Unfolded Protein Response Regulates Pathogenic Development of Ustilago maydis by Rok1-Dependent Inhibition of Mating-Type Signaling

The unfolded protein response (UPR) is crucial for endoplasmic reticulum (ER) homeostasis and disease development in fungal pathogens. In the plant-pathogenic fungus Ustilago maydis, the UPR supports fungal proliferation in planta and effector secretion for plant defense suppression. In this study, we uncovered that UPR activity, which is normally restricted to the biotrophic stage in planta, inhibits mating and the formation of infectious filaments by Rok1-dependent dephosphorylation of the pheromone responsive mitogen-activated protein kinase (MAPK) Kpp2. This observation is relevant for understanding how the fungal virulence program is regulated by cellular physiology. UPR-mediated control of mating-type signaling pathways predicts that effector gene expression and the virulence potential are controlled by ER stress levels.

Fungal pathogens require the unfolded protein response (UPR) to maintain protein homeostasis of the endoplasmic reticulum (ER) during pathogenic development. In the corn smut fungus Ustilago maydis, pathogenic development is controlled by the a and b mating-type loci. The UPR is specifically activated after plant penetration and required for efficient secretion of effectors and suppression of the plant defense response. The interaction between the UPR regulator Cib1 and the central developmental regulator Clp1 modulates the pathogenic program and triggers fungal colonization of the host plant. By contrast, when activated before plant penetration, the UPR interferes with fungal virulence by reducing expression of bE and bW, the central regulators of pathogenic development encoded by the b mating-type locus. Here, we show that this inhibitory effect results from UPR-mediated suppression of the pheromone response pathway upstream of the b regulatory network. UPR activity prompts dephosphorylation of the pheromone-responsive mitogen-activated protein kinase (MAPK) Kpp2, reducing activity of the pheromone response factor Prf1 that regulates expression of bE and bW. Deletion of the dual specificity phosphatase rok1 fully suppressed UPR-dependent inhibition of Kpp2 phosphorylation, formation of infectious filaments, and fungal virulence. Rok1 determines the activity of mating-type signaling pathways and thus the degree of fungal virulence. We propose that UPR-dependent regulation of Rok1 aligns ER physiology with fungal aggressiveness and effector gene expression during biotrophic growth of U. maydis in the host plant.

Plant pathogenic fungi Colletotrichum and Magnaporthe share a common G1 phase monitoring strategy for proper appressorium development

To breach the plant cuticle, many plant pathogenic fungi differentiate specialized infection structures (appressoria). In Colletotrichum orbiculare (cucumber anthracnose fungus), this differentiation requires unique proper G₁ /S phase progression, regulated by two-component GTPase activating protein CoBub2/CoBfa1 and GTPase CoTem1. Since their homologues regulate mitotic exit, cytokinesis, or septum formation from yeasts to mammals, we asked whether the BUB2 function in G₁ /S progression is specific to plant pathogenic fungi. Colletotrichum higginsianum and Magnaporthe oryzae were genetically analyzed to investigate conservation of BUB2 roles in cell cycle regulation, septum formation, and virulence. Expression profile of cobub2Δ was analyzed using a custom microarray. In bub2 mutants of both fungi, S phase initiation was earlier, and septum formation coordinated with a septation initiation network protein and contractile actin ring was impaired. Earlier G₁ /S transition in cobub2Δ results in especially high expression of DNA replication genes and differing regulation of virulence-associated genes that encode proteins such as carbohydrate-active enzymes and small secreted proteins. The virulence of chbub2Δ and mobub2Δ was significantly reduced. Our evidence shows that BUB2 regulation of G₁ /S transition and septum formation supports its specific requirement for appressorium development in plant pathogenic fungi.

Opsin 1 and Opsin 2 of the Corn Smut Fungus Ustilago maydis Are Green Light-Driven Proton Pumps

In fungi, green light is absorbed by rhodopsins, opsin proteins carrying a retinal molecule as chromophore. The basidiomycete Ustilago maydis, a fungal pathogen that infects corn plants, encodes three putative photoactive opsins, called ops1 (UMAG_02629), ops2 (UMAG_00371), and ops3 (UMAG_04125). UmOps1 and UmOps2 are expressed during the whole life cycle, in axenic cultures as well as in planta, whereas UmOps3 was recently shown to be absent in axenic cultures but highly expressed during plant infection. Here we show that expression of UmOps1 and UmOps2 is induced by blue light under control of white collar 1 (Wco1). UmOps1 is mainly localized in the plasma membrane, both when expressed in HEK cells and U. maydis sporidia. In contrast, UmOps2 was mostly found intracellularly in the membranes of vacuoles. Patch-clamp studies demonstrated that both rhodopsins are green light-driven outward rectifying proton pumps. UmOps1 revealed an extraordinary pH dependency with increased activity in more acidic environment. Also, UmOps1 showed a pronounced, concentration-dependent enhancement of pump current caused by weak organic acids (WOAs), especially by acetic acid and indole-3-acetic acid (IAA). In contrast, UmOps2 showed the typical behavior of light-driven, outwardly directed proton pumps, whereas UmOps3 did not exhibit any electrogenity. With this work, insights were gained into the localization and molecular function of two U. maydis rhodopsins, paving the way for further studies on the biological role of these rhodopsins in the life cycle of U. maydis.

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.

Ustilago maydis effectors and their impact on virulence

Biotrophic fungal plant pathogens establish an intimate relationship with their host to support the infection process. Central to this strategy is the secretion of a range of protein effectors that enable the pathogen to evade plant immune defences and modulate host metabolism to meet its needs. In this Review, using the smut fungus Ustilago maydis as an example, we discuss new insights into the effector repertoire of smut fungi that have been gained from comparative genomics and discuss the molecular mechanisms by which U. maydis effectors change processes in the plant host. Finally, we examine how the expression of effector genes and effector secretion are coordinated with fungal development in the host.

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.

Filamentous actin accumulates during plant cell penetration and cell wall plug formation in Phytophthora infestans

The oomycete Phytophthora infestans is the cause of late blight in potato and tomato. It is a devastating pathogen and there is an urgent need to design alternative strategies to control the disease. To find novel potential drug targets, we used Lifeact-eGFP expressing P. infestans for high resolution live cell imaging of the actin cytoskeleton in various developmental stages. Previously, we identified actin plaques as structures that are unique for oomycetes. Here we describe two additional novel actin configurations; one associated with plug deposition in germ tubes and the other with appressoria, infection structures formed prior to host cell penetration. Plugs are composed of cell wall material that is deposited in hyphae emerging from cysts to seal off the cytoplasm-depleted base after cytoplasm retraction towards the growing tip. Preceding plug formation there was a typical local actin accumulation and during plug deposition actin remained associated with the leading edge. In appressoria, formed either on an artificial surface or upon contact with plant cells, we observed a novel aster-like actin configuration that was localized at the contact point with the surface. Our findings strongly suggest a role for the actin cytoskeleton in plug formation and plant cell penetration.

Unfolded Protein Response (UPR) Regulator Cib1 Controls Expression of Genes Encoding Secreted Virulence Factors in Ustilago maydis

The unfolded protein response (UPR), a conserved eukaryotic signaling pathway to ensure protein homeostasis in the endoplasmic reticulum (ER), coordinates biotrophic development in the corn smut fungus Ustilago maydis. Exact timing of UPR activation is required for virulence and presumably connected to the elevated expression of secreted effector proteins during infection of the host plant Zea mays. In the baker’s yeast Saccharomyces cerevisiae, expression of UPR target genes is induced upon binding of the central regulator Hac1 to unfolded protein response elements (UPREs) in their promoters. While a role of the UPR in effector secretion has been described previously, we investigated a potential UPR-dependent regulation of genes encoding secreted effector proteins. In silico prediction of UPREs in promoter regions identified the previously characterized effector genes pit2 and tin1-1, as bona fide UPR target genes. Furthermore, direct binding of the Hac1-homolog Cib1 to the UPRE containing promoter fragments of both genes was confirmed by quantitative chromatin immunoprecipitation (qChIP) analysis. Targeted deletion of the UPRE abolished Cib1-dependent expression of pit2 and significantly affected virulence. Furthermore, ER stress strongly increased Pit2 expression and secretion. This study expands the role of the UPR as a signal hub in fungal virulence and illustrates, how biotrophic fungi can coordinate cellular physiology, development and regulation of secreted virulence factors.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

White collar 1-induced photolyase expression contributes to UV-tolerance of Ustilago maydis

Ustilago maydis is a phytopathogenic fungus causing corn smut disease. It also is known for its extreme tolerance to UV‐ and ionizing radiation. It has not been elucidated whether light‐sensing proteins, and in particular photolyases play a role in its UV‐tolerance. Based on homology analysis, U. maydis has 10 genes encoding putative light‐responsive proteins. Four amongst these belong to the cryptochrome/photolyase family (CPF) and one represents a white collar 1 ortholog (wco1). Deletion mutants in the predicted cyclobutane pyrimidine dimer CPD‐ and (6–4)‐photolyase were impaired in photoreactivation. In line with this, in vitro studies with recombinant CPF proteins demonstrated binding of the catalytic FAD cofactor, its photoreduction to fully reduced FADH⁻ and repair activity for cyclobutane pyrimidine dimers (CPDs) or (6–4)‐photoproducts, respectively. We also investigated the role of Wco1. Strikingly, transcriptional profiling showed 61 genes differentially expressed upon blue light exposure of wild‐type, but only eight genes in the Δwco1 mutant. These results demonstrate that Wco1 is a functional blue light photoreceptor in U. maydis regulating expression of several genes including both photolyases. Finally, we show that the Δwco1 mutant is less tolerant against UV‐B due to its incapability to induce photolyase expression.

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.

Programmed cell cycle arrest is required for infection of corn plants by the fungus Ustilago maydis

Ustilago maydis is a plant pathogen that requires a specific structure called infective filament to penetrate the plant tissue. Although able to grow, this filament is cell cycle arrested on the plant surface. This cell cycle arrest is released once the filament penetrates the plant tissue. The reasons and mechanisms for this cell cycle arrest are unknown. Here, we have tried to address these questions. We reached three conclusions from our studies. First, the observed cell cycle arrest is the result of the cooperation of at least two distinct mechanisms: one involving the activation of the DNA damage response (DDR) cascade; and the other relying on the transcriptional downregulation of Hsl1, a kinase that modulates the G2/M transition. Second, a sustained cell cycle arrest during the infective filament step is necessary for the virulence in U. maydis, as a strain unable to arrest the cell cycle was severely impaired in its ability to infect corn plants. Third, production of the appressorium, a structure required for plant penetration, is incompatible with an active cell cycle. The inability to infect plants by strains defective in cell cycle arrest seems to be caused by their failure to induce the appressorium formation process. In summary, our findings uncover genetic circuits to arrest the cell cycle during the growth of this fungus on the plant surface, thus allowing the penetration into plant tissue.

Plant surface cues prime Ustilago maydis for biotrophic development

Infection-related development of phytopathogenic fungi is initiated by sensing and responding to plant surface cues. This response can result in the formation of specialized infection structures, so-called appressoria. To unravel the program inducing filaments and appressoria in the biotrophic smut fungus Ustilago maydis, we exposed cells to a hydrophobic surface and the cutin monomer 16-hydroxy hexadecanoic acid. Genome-wide transcriptional profiling at the pre-penetration stage documented dramatic transcriptional changes in almost 20% of the genes. Comparisons with the U. maydis sho1 msb2 double mutant, lacking two putative sensors for plant surface cues, revealed that these plasma membrane receptors regulate a small subset of the surface cue-induced genes comprising mainly secreted proteins including potential plant cell wall degrading enzymes. Targeted gene deletion analysis ascribed a role to up-regulated GH51 and GH62 arabinofuranosidases during plant penetration. Among the sho1/msb2-dependently expressed genes were several secreted effectors that are essential for virulence. Our data also demonstrate specific effects on two transcription factors that redirect the transcriptional regulatory network towards appressorium formation and plant penetration. This shows that plant surface cues prime U. maydis for biotrophic development.

A basic requirement for pathogens to infect their hosts and to cause disease is to detect that they are in contact with the host surface. Plant pathogenic fungi typically respond to leaf surface contact with the development of specialized infection structures enabling the fungus to penetrate the leaf cuticle and to enter the plant tissue. In this study we analyzed the response of the corn smut fungus Ustilago maydis to two plant surface cues, such as hydrophobic surface and cutin monomers. Based on genome-wide gene expression analysis we found that these cues trigger the production of secreted plant cell wall degrading enzymes helping the fungus to penetrate the plant surface. In addition, genes were activated that code for a group of secreted proteins, so-called effectors, that affect virulence after penetration. These results demonstrate that plant surface cues trigger fungal penetration of the plant surface and also prime the fungus for later development inside plant tissue. These specific responses required two cell surface proteins that likely function as plant surface sensors.

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.

Crosstalk between the unfolded protein response and pathways that regulate pathogenic development in Ustilago maydis

The unfolded protein response (UPR) is a conserved eukaryotic signaling pathway regulating endoplasmic reticulum (ER) homeostasis during ER stress, which results, for example, from an increased demand for protein secretion. Here, we characterize the homologs of the central UPR regulatory proteins Hac1 (for Homologous to ATF/CREB1) and Inositol Requiring Enzyme1 in the plant pathogenic fungus Ustilago maydis and demonstrate that the UPR is tightly interlinked with the b mating-type-dependent signaling pathway that regulates pathogenic development. Exact timing of UPR is required for virulence, since premature activation interferes with the b-dependent switch from budding to filamentous growth. In addition, we found crosstalk between UPR and the b target Clampless1 (Clp1), which is essential for cell cycle release and proliferation in planta. The unusual C-terminal extension of the U. maydis Hac1 homolog, Cib1 (for Clp1 interacting bZIP1), mediates direct interaction with Clp1. The interaction between Clp1 and Cib1 promotes stabilization of Clp1, resulting in enhanced ER stress tolerance that prevents deleterious UPR hyperactivation. Thus, the interaction between Cib1 and Clp1 constitutes a checkpoint to time developmental progression and increased secretion of effector proteins at the onset of biotrophic development. Crosstalk between UPR and the b mating-type regulated developmental program adapts ER homeostasis to the changing demands during biotrophy.

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.

Gene expression and proteomic analysis of the formation of Phakopsora pachyrhizi appressoria

BACKGROUND

Phakopsora pachyrhizi is an obligate fungal pathogen causing Asian soybean rust (ASR). A dual approach was taken to examine the molecular and biochemical processes occurring during the development of appressoria, specialized infection structures by which P. pachyrhizi invades a host plant. Suppression subtractive hybridization (SSH) was utilized to generate a cDNA library enriched for transcripts expressed during appressoria formation. Two-dimensional gel electrophoresis and mass spectroscopy analysis were used to generate a partial proteome of proteins present during appressoria formation.

RESULTS

Sequence analysis of 1133 expressed sequence tags (ESTs) revealed 238 non-redundant ESTs, of which 53% had putative identities assigned. Twenty-nine of the non-redundant ESTs were found to be specific to the appressoria-enriched cDNA library, and did not occur in a previously constructed germinated urediniospore cDNA library. Analysis of proteins against a custom database of the appressoria-enriched ESTs plus Basidiomycota EST sequences available from NCBI revealed 256 proteins. Fifty-nine of these proteins were not previously identified in a partial proteome of P. pachyrhizi germinated urediniospores. Genes and proteins identified fell into functional categories of metabolism, cell cycle and DNA processing, protein fate, cellular transport, cellular communication and signal transduction, and cell rescue. However, 38% of ESTs and 24% of proteins matched only to hypothetical proteins of unknown function, or showed no similarity to sequences in the current NCBI database. Three novel Phakopsora genes were identified from the cDNA library along with six potentially rust-specific genes. Protein analysis revealed eight proteins of unknown function, which possessed classic secretion signals. Two of the extracellular proteins are reported as potential effector proteins.

CONCLUSIONS

Several genes and proteins were identified that are expressed in P. pachyrhizi during appressoria formation. Understanding the role that these genes and proteins play in the molecular and biochemical processes in the infection process may provide insight for developing targeted control measures and novel methods of disease management.

StRas2 regulates morphogenesis, conidiation and appressorium development in Setosphaeria turcica

The proteins of Ras family are a large group of monomeric GTPases and act as molecular switches transducing extracellular signals into the cell in higher eukaryotes. However, little is known about roles of Ras family in the foliar pathogens. In this research, we cloned the gene named StRas2 encoding Ras in Setosphaeria turcica and investigated its function by RNA interference technology. We found that the growth rate of RNAi transformants named as R1, R2, R3, R4, R5 and R6, in which the StRas2 silencing efficiency fell in turn. With the highest silencing efficiency, the transformant R1 showed anomalistic hyphae morphology, indicating its growth was significantly affected. The transformants with a middle-silencing efficiency, such as R3, R4, displayed a delay when forming appressoria and invasive hyphae. R1 could not form conidia and appressoria. However, the conidial formation in R5 and R6 was significantly reduced, and these two transformants could form appressoria and penetrate the artificial cellophane, only that its invasive hyphae were fascicular and rarely branched. The HT-toxin biological activity of all transformants showed no difference. All results suggested that StRas2 is involved in the morphogenesis, conidiation, and appressorium development and is not related to the biosynthesis of HT-toxin.