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

The sensor protein AaSho1 regulates infection structures differentiation, osmotic stress tolerance and virulence via MAPK module AaSte11-AaPbs2-AaHog1 in Alternaria alternata

The high-osmolarity-sensitive protein Sho1 functions as a key membrane receptor in phytopathogenic fungi, which can sense and respond to external stimuli or stresses, and synergistically regulate diverse fungal biological processes through cellular signaling pathways. In this study, we investigated the biological functions of AaSho1 in Alternaria alternata, the causal agent of pear black spot. Targeted gene deletion revealed that AaSho1 is essential for infection structure differentiation, response to external stresses and synthesis of secondary metabolites. Compared to the wild-type (WT), the ∆AaSho1 mutant strain showed no significant difference in colony growth, morphology, conidial production and biomass accumulation. However, the mutant strain exhibited significantly reduced levels of melanin production, cellulase (CL) and ploygalacturonase (PG) activities, virulence, resistance to various exogenous stresses. Moreover, the appressorium and infection hyphae formation rates of the ∆AaSho1 mutant strain were significantly inhibited. RNA-Seq results showed that there were four branches including pheromone, cell wall stress, high osmolarity and starvation in the Mitogen-activated Protein Kinase (MAPK) cascade pathway. Furthermore, yeast two-hybrid experiments showed that AaSho1 activates the MAPK pathway via AaSte11-AaPbs2-AaHog1. These results suggest that AaSho1 of A. alternata is essential for fungal development, pathogenesis and osmotic stress response by activating the MAPK cascade pathway via Sho1-Ste11-Pbs2-Hog1.

Conserved signaling modules regulate filamentous growth in fungi: a model for eukaryotic cell differentiation

Eukaryotic organisms are composed of different cell types with defined shapes and functions. Specific cell types are produced by the process of cell differentiation, which is regulated by signal transduction pathways. Signaling pathways regulate cell differentiation by sensing cues and controlling the expression of target genes whose products generate cell types with specific attributes. In studying how cells differentiate, fungi have proved valuable models because of their ease of genetic manipulation and striking cell morphologies. Many fungal species undergo filamentous growth-a specialized growth pattern where cells produce elongated tube-like projections. Filamentous growth promotes expansion into new environments, including invasion into plant and animal hosts by fungal pathogens. The same signaling pathways that regulate filamentous growth in fungi also control cell differentiation throughout eukaryotes and include highly conserved mitogen-activated protein kinase (MAPK) pathways, which is the focus of this review. In many fungal species, mucin-type sensors regulate MAPK pathways to control filamentous growth in response to diverse stimuli. Once activated, MAPK pathways reorganize cell polarity, induce changes in cell adhesion, and promote the secretion of degradative enzymes that mediate access to new environments. However, MAPK pathway regulation is complicated because related pathways can share components with each other yet induce unique responses (i.e. signal specificity). In addition, MAPK pathways function in highly integrated networks with other regulatory pathways (i.e. signal integration). Here, we discuss signal specificity and integration in several yeast models (mainly Saccharomyces cerevisiae and Candida albicans) by focusing on the filamentation MAPK pathway. Because of the strong evolutionary ties between species, a deeper understanding of the regulation of filamentous growth in established models and increasingly diverse fungal species can reveal fundamentally new mechanisms underlying eukaryotic cell differentiation.

The receptor kinase RiSho1 in Rhizophagus irregularis regulates arbuscule development and drought tolerance during arbuscular mycorrhizal symbiosis

In terrestrial ecosystems, most plant species can form beneficial associations with arbuscular mycorrhizal (AM) fungi. Arbuscular mycorrhizal fungi benefit plant nutrient acquisition and enhance plant tolerance to drought. The high osmolarity glycerol 1 mitogen-activated protein kinase (HOG1-MAPK) cascade genes have been characterized in Rhizophagus irregularis. However, the upstream receptor of the HOG1-MAPK cascade remains to be investigated. We identify the receptor kinase RiSho1 from R. irregularis, containing four transmembrane domains and one Src homology 3 (SH3) domain, corresponding to the homologue of Saccharomyces cerevisiae. Higher expression levels of RiSho1 were detected during the in planta phase in response to drought. RiSho1 protein was localized in the plasma membrane of yeast, and interacted with the HOG1-MAPK module RiPbs2 directly by protein-protein interaction. RiSho1 complemented the growth defect of the yeast mutant ∆sho1 under sorbitol conditions. Knock-down of RiSho1 led to the decreased expression of downstream HOG1-MAPK cascade (RiSte11, RiPbs2, RiHog1) and drought-resistant genes (RiAQPs, RiTPSs, RiNTH1 and Ri14-3-3), hampered arbuscule development and decreased plants antioxidation ability under drought stress. Our study reveals the role of RiSho1 in regulating arbuscule development and drought-resistant genes via the HOG1-MAPK cascade. These findings provide new perspectives on the mechanisms by which AM fungi respond to drought.

Fungal Cell Wall-Associated Effectors: Sensing, Integration, Suppression, and Protection

The cell wall (CW) of plant-interacting fungi, as the direct interface with host plants, plays a crucial role in fungal development. A number of secreted proteins are directly associated with the fungal CW, either through covalent or non-covalent interactions, and serve a range of important functions. In the context of plant-fungal interactions many are important for fungal development in the host environment and may therefore be considered fungal CW-associated effectors (CWAEs). Key CWAE functions include integrating chemical/physical signals to direct hyphal growth, interfering with plant immunity, and providing protection against plant defenses. In recent years, a diverse range of mechanisms have been reported that underpin their roles, with some CWAEs harboring conserved motifs or functional domains, while others are reported to have novel features. As such, the current understanding regarding fungal CWAEs is systematically presented here from the perspective of their biological functions in plant-fungal interactions. An overview of the fungal CW architecture and the mechanisms by which proteins are secreted, modified, and incorporated into the CW is first presented to provide context for their biological roles. Some CWAE functions are reported across a broad range of pathosystems or symbiotic/mutualistic associations. Prominent are the chitin interacting-effectors that facilitate fungal CW modification, protection, or suppression of host immune responses. However, several alternative functions are now reported and are presented and discussed. CWAEs can play diverse roles, some possibly unique to fungal lineages and others conserved across a broad range of plant-interacting fungi. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Ecological inducers of the yeast filamentous growth pathway reveal environment-dependent roles for pathway components

Signaling modules, such as mitogen-activated protein kinase (MAPK) pathways, are evolutionarily conserved drivers of cell differentiation and stress responses. In many fungal species including pathogens, MAPK pathways control filamentous growth, where cells differentiate into an elongated cell type. The convenient model budding yeast Saccharomyces cerevisiae undergoes filamentous growth by the filamentous growth (fMAPK) pathway; however, the inducers of the pathway remain unclear, perhaps because pathway activity has been mainly studied in laboratory conditions. To address this knowledge gap, an ecological framework was used, which uncovered new fMAPK pathway inducers, including pectin, a material found in plants, and the metabolic byproduct ethanol. We also show that induction by a known inducer of the pathway, the non-preferred carbon source galactose, required galactose metabolism and induced the pathway differently than glucose limitation or other non-preferred carbon sources. By exploring fMAPK pathway function in fruit, we found that induction of the pathway led to visible digestion of fruit rind through a known target, PGU1, which encodes a pectolytic enzyme. Combinations of inducers (galactose and ethanol) stimulated the pathway to near-maximal levels, which showed dispensability of several fMAPK pathway components (e.g., mucin sensor, p21-activated kinase), but not others (e.g., adaptor, MAPKKK) and required the Ras2-protein kinase A pathway. This included a difference between the transcription factor binding partners for the pathway, as Tec1p, but not Ste12p, was partly dispensable for fMAPK pathway activity. Thus, by exploring ecologically relevant stimuli, new modes of MAPK pathway signaling were uncovered, perhaps revealing how a pathway can respond differently to specific environments. IMPORTANCE Filamentous growth is a cell differentiation response and important aspect of fungal biology. In plant and animal fungal pathogens, filamentous growth contributes to virulence. One signaling pathway that regulates filamentous growth is an evolutionarily conserved MAPK pathway. The yeast Saccharomyces cerevisiae is a convenient model to study MAPK-dependent regulation of filamentous growth, although the inducers of the pathway are not clear. Here, we exposed yeast cells to ecologically relevant compounds (e.g., plant compounds), which identified new inducers of the MAPK pathway. In combination, the inducers activated the pathway to near-maximal levels but did not cause detrimental phenotypes associated with previously identified hyperactive alleles. This context allowed us to identify conditional bypass for multiple pathway components. Thus, near-maximal induction of a MAPK pathway by ecologically relevant inducers provides a powerful tool to assess cellular signaling during a fungal differentiation response.

Simultaneous DHA and organic selenium production by Schizochytrium sp.: a theoretical basis

Docosahexaenoic acid (DHA) and selenium (Se) are nutrients that confer several health benefits to both humans and animals. Widespread use of DHA in milk powder and health products requires large-scale mass production via Schizochytrium sp., while Se intended for human consumption is produced as organic Se via yeast. However, producing these nutrients on an industrial scale is constrained by various factors. We found that supplementing Schizochytrium sp. with Na2SeO3 (0.5 mg/L) improves its biomass and DHA production and also provides organic Se. De novo assembled transcriptome and biochemical indicators showed that Na2SeO3 promotes forming acetyl coenzyme A and L-cysteine via the glycerol kinase and cysteine synthase pathways, promoting DHA synthesis through the polyketide synthase pathway. However, high doses of Na2SeO3 (5 mg/L) limited the biomass of Schizochytrium sp. and DHA content. This study provided a theoretical basis for the simultaneous production of organic Se and DHA via Schizochytrium sp.

Frequent transitions in mating-type locus chromosomal organization in Malassezia and early steps in sexual reproduction

Fungi in the basidiomycete genus Malassezia are the most prevalent eukaryotic microbes resident on the skin of human and other warm-blooded animals and have been implicated in skin diseases and systemic disorders. Analysis of Malassezia genomes revealed that key adaptations to the skin microenvironment have a direct genomic basis, and the identification of mating/meiotic genes suggests a capacity to reproduce sexually, even though no sexual cycle has yet been observed. In contrast to other bipolar or tetrapolar basidiomycetes that have either two linked mating-type-determining (MAT) loci or two MAT loci on separate chromosomes, in Malassezia species studied thus far the two MAT loci are arranged in a pseudobipolar configuration (linked on the same chromosome but capable of recombining). By generating additional chromosome-level genome assemblies, and an improved Malassezia phylogeny, we infer that the pseudobipolar arrangement was the ancestral state of this group and revealed six independent transitions to tetrapolarity, seemingly driven by centromere fission or translocations in centromere-flanking regions. Additionally, in an approach to uncover a sexual cycle, Malassezia furfur strains were engineered to express different MAT alleles in the same cell. The resulting strains produce hyphae reminiscent of early steps in sexual development and display upregulation of genes associated with sexual development as well as others encoding lipases and a protease potentially relevant for pathogenesis of the fungus. Our study reveals a previously unseen genomic relocation of mating-type loci in fungi and provides insight toward the identification of a sexual cycle in Malassezia, with possible implications for pathogenicity.

Frequent transitions in mating-type locus chromosomal organization in Malassezia and early steps in sexual reproduction

Fungi in the basidiomycete genus Malassezia are the most prevalent eukaryotic microbes resident on the skin of human and other warm-blooded animals and have been implicated in skin diseases and systemic disorders. Analysis of Malassezia genomes revealed that key adaptations to the skin microenvironment have a direct genomic basis, and the identification of mating/meiotic genes suggests a capacity to reproduce sexually, even though no sexual cycle has yet been observed. In contrast to other bipolar or tetrapolar basidiomycetes that have either two linked mating-type-determining ( MAT ) loci or two MAT loci on separate chromosomes, in Malassezia species studied thus far the two MAT loci are arranged in a pseudobipolar configuration (linked on the same chromosome but capable of recombining). By incorporating newly generated chromosome-level genome assemblies, and an improved Malassezia phylogeny, we infer that the pseudobipolar arrangement was the ancestral state of this group and revealed six independent transitions to tetrapolarity, seemingly driven by centromere fission or translocations in centromere- flanking regions. Additionally, in an approach to uncover a sexual cycle, Malassezia furfur strains were engineered to express different MAT alleles in the same cell. The resulting strains produce hyphae reminiscent of early steps in sexual development and display upregulation of genes associated with sexual development as well as others encoding lipases and a protease potentially relevant for pathogenesis of the fungus. Our study reveals a previously unseen genomic relocation of mating-type loci in fungi and provides insight towards the discovery of a sexual cycle in Malassezia , with possible implications for pathogenicity.

Significance Statement

Malassezia , the dominant fungal group of the mammalian skin microbiome, is associated with numerous skin disorders. Sexual development and yeast-to-hyphae transitions, governed by genes at two mating-type ( MAT ) loci, are thought to be important for fungal pathogenicity. However, Malassezia sexual reproduction has never been observed. Here, we used chromosome-level assemblies and comparative genomics to uncover unforeseen transitions in MAT loci organization within Malassezia , possibly related with fragility of centromeric-associated regions. Additionally, by expressing different MAT alleles in the same cell, we show that Malassezia can undergo hyphal development and this phenotype is associated with increased expression of key mating genes along with other genes known to be virulence factors, providing a possible connection between hyphal development, sexual reproduction, and pathogenicity.

Progress in pathogenesis research of Ustilago maydis, and the metabolites involved along with their biosynthesis

Ustilago maydis is a pathogenic fungus that causes corn smut. Because of its easy cultivation and genetic transformation, U. maydis has become an important model organism for plant-pathogenic basidiomycetes. U. maydis is able to infect maize by producing effectors and secreted proteins as well as surfactant-like metabolites. In addition, the production of melanin and iron carriers is also associated with its pathogenicity. Here, advances in our understanding of the pathogenicity of U. maydis, the metabolites involved in the pathogenic process, and the biosynthesis of these metabolites, are reviewed and discussed. This summary will provide new insights into the pathogenicity of U. maydis and the functions of associated metabolites, as well as new clues for deciphering the biosynthesis of metabolites.

Transcriptome Comparison between Two Strains of Ustilago esculenta during the Mating

Ustilago esculenta is a smut fungus that obligately infects Zizania latifolia and stimulates tissue swelling to form galls. Unlike T-type, MT-type U. esculenta can only proliferate within plant tissues and infect the offspring of their host. Production of telispores, haploid life, and plant cuticle penetration are not essential for it, which may lead to the degeneration in these processes. Transcriptome changes during the mating of T- and MT-type U. esculenta were studied. The functions of several secreted proteins were further confirmed by knock-out mutants. Our results showed that MT-type U. esculenta can receive environmental signals in mating and circumstance sensing as T-type does. However, MT-type U. esculenta takes a longer time for conjunction tube formation and cytoplasmic fusion. A large number of genes encoding secreted proteins are enriched in the purple co-expression module. They are significantly up-regulated in the late stage of mating in T-type U. esculenta, indicating their relationship with infecting. The knock-out of g6161 (xylanase) resulted in an attenuated symptom. The knock-out of g943 or g4344 (function unidentified) completely blocked the infection at an early stage. This study provides a comprehensive comparison between T- and MT-type during mating and identifies two candidate effectors for further study.

SsAGM1-Mediated Uridine Diphosphate-N-Acetylglucosamine Synthesis Is Essential for Development, Stress Response, and Pathogenicity of Sclerotinia sclerotiorum

The necrotrophic fungus Sclerotinia sclerotiorum is a devastating pathogen. S. sclerotiorum can cause Sclerotinia stem rot in more than 600 species of plants, which results in serious economic losses every year. Chitin is one of the most important polysaccharides in fungal cell walls. Chitin and β-Glucan form a scaffold that wraps around the cell and determines the vegetative growth and pathogenicity of pathogens. UDP-GlcNAc is a direct precursor of chitin synthesis. During the synthesis of UDP-GlcNAc, the conversion of GlcNAc-6P to GlcNAc-1P that is catalyzed by AGM1 (N-acetylglucosamine-phosphate mutase) is a key step. However, the significance and role of AGM1 in phytopathogenic fungus are unclear. We identified a cytoplasm-localized SsAGM1 in S. sclerotiorum, which is homologous to AGM1 of Saccharomyces cerevisiae. We utilized RNA interference (RNAi) and overexpression to characterize the function of SsAGM1 in S. sclerotiorum. After reducing the expression of SsAGM1, the contents of chitin and UDP-GlcNAc decreased significantly. Concomitantly, the gene-silenced transformants of SsAGM1 slowed vegetative growth and, importantly, lost the ability to produce sclerotia and infection cushion; it also lost virulence, even on wounded leaves. In addition, SsAGM1 was also involved in the response to osmotic stress and inhibitors of cell wall synthesis. Our results revealed the function of SsAGM1 in the growth, development, stress response, and pathogenicity in S. sclerotiorum.

Cell Wall Integrity and Its Industrial Applications in Filamentous Fungi

Signal transduction pathways regulating cell wall integrity (CWI) in filamentous fungi have been studied taking into account findings in budding yeast, and much knowledge has been accumulated in recent years. Given that the cell wall is essential for viability in fungi, its architecture has been analyzed in relation to virulence, especially in filamentous fungal pathogens of plants and humans. Although research on CWI signaling in individual fungal species has progressed, an integrated understanding of CWI signaling in diverse fungi has not yet been achieved. For example, the variety of sensor proteins and their functional differences among different fungal species have been described, but the understanding of their general and species-specific biological functions is limited. Our long-term research interest is CWI signaling in filamentous fungi. Here, we outline CWI signaling in these fungi, from sensor proteins required for the recognition of environmental changes to the regulation of cell wall polysaccharide synthesis genes. We discuss the similarities and differences between the functions of CWI signaling factors in filamentous fungi and in budding yeast. We also describe the latest findings on industrial applications, including those derived from studies on CWI signaling: the development of antifungal agents and the development of highly productive strains of filamentous fungi with modified cell surface characteristics by controlling cell wall biogenesis.

My Personal Journey from the Fascination for Phages to a Tumor-Inducing Fungal Pathogen of Corn

My path in science began with a fascination for microbiology and phages and later involved a switch of subjects to the fungus Ustilago maydis and how it causes disease in maize. I will not provide a review of my work but rather focus on decisive findings, serendipitous, lucky moments when major advances made the U. maydis-maize system what it is now-a well-established model for biotrophic fungi. I also want to share with you the joy of finding the needle in a haystack at the very end of my scientific career, a fungal structure likely used for effector delivery, and how we were able to translate this into a potential application in agriculture. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Transcriptional Network in Colletotrichum gloeosporioides Mutants Lacking Msb2 or Msb2 and Sho1

Colletotrichum gloeosporioides is a hemibiotrophic ascomycetous fungus that causes anthracnose in many plants worldwide. During infections, C. gloeosporioides produces an appressorium in response to various plant surface signals. However, the mechanism mediating host surface signal recognition remains unclear. In this study, C. gloeosporioides ΔCgMsb2 and ΔCgMsb2Sho1 mutants lacking hypothetical sensors of plant surface signals were examined. The mutations in ΔCgMsb2 and ΔCgMsb2Sho1 adversely affected conidial size and sporulation, while also inhibiting growth. Significant transcriptional changes were detected for nearly 19% and 26% of the genes in ΔCgMsb2 and ΔCgMsb2Sho1, respectively. The lack of these plasma membrane receptors altered the expression of specific genes, especially those encoding hydrolases, ABC transporters, and mitogen-activated protein kinases (MAPKs). The encoded MAPKs participate in the signal transduction of ERK and JNK signaling pathways, activate downstream signals, and contribute to metabolic regulation. Our data demonstrate that the C. gloeosporioides membrane proteins Msb2 and Sho1 affect gene regulation, thereby influencing conidial growth, metabolism, and development. These findings provide new insights into the regulation of C. gloeosporioides's development and infection of plant hosts.

Mucin Msb2 cooperates with the transmembrane protein Sho1 in various plant surface signal sensing and pathogenic processes in the poplar anthracnose fungus Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is a hemibiotrophic ascomycete fungus that causes anthracnose on numerous plants worldwide and forms a specialized infection structure known as an appressorium in response to various plant surface signals. However, the associated mechanism of host surface signal recognition remains unclear. In the present study, three putative sensors, namely the mucin Msb2, the membrane sensor protein Sho1, and the G-protein-coupled receptor Pth11, were identified and characterized. The results showed that CgMsb2 plays a major role in the recognition of various host surface signals; deletion of CgMsb2 resulted in significant defects in appressorium formation, appressorium penetration, cellophane membrane penetration, and pathogenicity. CgSho1 plays a minor role and together with CgMsb2 cooperatively regulates host signal recognition, cellophane membrane penetration, and pathogenicity; deletion of CgSho1 resulted in an expansion defect of infection hyphae. Deletion of CgPth11 in wildtype, ΔCgMsb2, and ΔCgSho1 strains only resulted in a slight defect in appressorium formation at the early stage, and CgPth11 was dispensable for penetration and pathogenicity. However, exogenous cAMP failed to restore the defect of appressorium formation in ΔCgPth11 at the early stage. CgMsb2 contributed to the phosphorylation of the mitogen-activated protein kinase CgMk1, which is essential for infection-associated functions, while CgSho1 was unable to activate CgMk1 alone but rather cooperated with CgMsb2 to activate CgMk1. These data suggest that CgMsb2 contributes to the activation of CgMk1 and has overlapping functions with CgSho1 in plant surface sensing, appressorium formation, and pathogenicity.

Regulation of biotic interactions and responses to abiotic stresses by MAP kinase pathways in plant pathogenic fungi

Like other eukaryotes, fungi use MAP kinase (MAPK) pathways to mediate cellular changes responding to external stimuli. In the past two decades, three well-conserved MAP kinase pathways have been characterized in various plant pathogenic fungi for regulating responses and adaptations to a variety of biotic and abiotic stresses encountered during plant infection or survival in nature. The invasive growth (IG) pathway is homologous to the yeast pheromone response and filamentation pathways. In plant pathogens, the IG pathway often is essential for pathogenesis by regulating infection-related morphogenesis, such as appressorium formation, penetration, and invasive growth. The cell wall integrity (CWI) pathway also is important for plant infection although the infection processes it regulates vary among fungal pathogens. Besides its universal function in cell wall integrity, it often plays a minor role in responses to oxidative and cell wall stresses. Both the IG and CWI pathways are involved in regulating known virulence factors as well as effector genes during plant infection and mediating defenses against mycoviruses, bacteria, and other fungi. In contrast, the high osmolarity growth (HOG) pathway is dispensable for virulence in some fungi although it is essential for plant infection in others. It regulates osmoregulation in hyphae and is dispensable for appressorium turgor generation. The HOG pathway also plays a major role for responding to oxidative, heat, and other environmental stresses and is overstimulated by phenylpyrrole fungicides. Moreover, these three MAPK pathways crosstalk and coordinately regulate responses to various biotic and abiotic stresses. The IG and CWI pathways, particularly the latter, also are involved in responding to abiotic stresses to various degrees in different fungal pathogens, and the HOG pathway also plays a role in interactions with other microbes or fungi. Furthermore, some infection processes or stress responses are co-regulated by MAPK pathways with cAMP or Ca2+/CaM signaling. Overall, functions of individual MAP kinase pathways in pathogenesis and stress responses have been well characterized in a number of fungal pathogens, showing the conserved genetic elements with diverged functions, likely by rewiring transcriptional regulatory networks. In the near future, applications of genomics and proteomics approaches will likely lead to better understanding of crosstalk among the MAPKs and with other signaling pathways as well as roles of MAPKs in defense against other microbes (biotic interactions).

Research Progress on Phytopathogenic Fungi and Their Role as Biocontrol Agents

Phytopathogenic fungi decrease crop yield and quality and cause huge losses in agricultural production. To prevent the occurrence of crop diseases and insect pests, farmers have to use many synthetic chemical pesticides. The extensive use of these pesticides has resulted in a series of environmental and ecological problems, such as the increase in resistant weed populations, soil compaction, and water pollution, which seriously affect the sustainable development of agriculture. This review discusses the main advances in research on plant-pathogenic fungi in terms of their pathogenic factors such as cell wall-degrading enzymes, toxins, growth regulators, effector proteins, and fungal viruses, as well as their application as biocontrol agents for plant pests, diseases, and weeds. Finally, further studies on plant-pathogenic fungal resources with better biocontrol effects can help find new beneficial microbial resources that can control diseases.

Modulation of fungal virulence through CRZ1 regulated F-BAR-dependent actin remodeling and endocytosis in chickpea infecting phytopathogen Ascochyta rabiei

Polarized hyphal growth of filamentous pathogenic fungi is an essential event for host penetration and colonization. The long-range early endosomal trafficking during hyphal growth is crucial for nutrient uptake, sensing of host-specific cues, and regulation of effector production. Bin1/Amphiphysin/Rvs167 (BAR) domain-containing proteins mediate fundamental cellular processes, including membrane remodeling and endocytosis. Here, we identified a F-BAR domain protein (ArF-BAR) in the necrotrophic fungus Ascochyta rabiei and demonstrate its involvement in endosome-dependent fungal virulence on the host plant Cicer arietinum. We show that ArF-BAR regulates endocytosis at the hyphal tip, localizes to the early endosomes, and is involved in actin dynamics. Functional studies involving gene knockout and complementation experiments reveal that ArF-BAR is necessary for virulence. The loss-of-function of ArF-BAR gene results in delayed formation of apical septum in fungal cells near growing hyphal tip that is crucial for host penetration, and impaired secretion of a candidate effector having secretory signal peptide for translocation across the endoplasmic reticulum membrane. The mRNA transcripts of ArF-BAR were induced in response to oxidative stress and infection. We also show that ArF-BAR is able to tubulate synthetic liposomes, suggesting the functional role of F-BAR domain in membrane tubule formation in vivo. Further, our studies identified a stress-induced transcription factor, ArCRZ1 (Calcineurin-responsive zinc finger 1), as key transcriptional regulator of ArF-BAR expression. We propose a model in which ArCRZ1 functions upstream of ArF-BAR to regulate A. rabiei virulence through a mechanism that involves endocytosis, effector secretion, and actin cytoskeleton regulation.

Structural, Evolutionary, and Functional Analysis of the Protein O-Mannosyltransferase Family in Pathogenic Fungi

Protein O-mannosyltransferases (Pmts) comprise a group of proteins that add mannoses to substrate proteins at the endoplasmic reticulum. This post-translational modification is important for the faithful transfer of nascent glycoproteins throughout the secretory pathway. Most fungi genomes encode three O-mannosyltransferases, usually named Pmt1, Pmt2, and Pmt4. In pathogenic fungi, Pmts, especially Pmt4, are key factors for virulence. Although the importance of Pmts for fungal pathogenesis is well established in a wide range of pathogens, questions remain regarding certain features of Pmts. For example, why does the single deletion of each pmt gene have an asymmetrical impact on host colonization? Here, we analyse the origin of Pmts in fungi and review the most important phenotypes associated with Pmt mutants in pathogenic fungi. Hence, we highlight the enormous relevance of these glycotransferases for fungal pathogenic development.

CgEnd3 Regulates Endocytosis, Appressorium Formation, and Virulence in the Poplar Anthracnose Fungus Colletotrichum gloeosporioides

The hemibiotrophic ascomycete fungus Colletotrichum gloeosporioides is the causal agent of anthracnose on numerous plants, and it causes considerable economic losses worldwide. Endocytosis is an essential cellular process in eukaryotic cells, but its roles in C. gloeosporioides remain unknown. In our study, we identified an endocytosis-related protein, CgEnd3, and knocked it out via polyethylene glycol (PEG)-mediated protoplast transformation. The lack of CgEnd3 resulted in severe defects in endocytosis. C. gloeosporioides infects its host through a specialized structure called appressorium, and ΔCgEnd3 showed deficient appressorium formation, melanization, turgor pressure accumulation, penetration ability of appressorium, cellophane membrane penetration, and pathogenicity. CgEnd3 also affected oxidant adaptation and the expression of core effectors during the early stage of infection. CgEnd3 contains one EF hand domain and four calcium ion-binding sites, and it is involved in calcium signaling. A lack of CgEnd3 changed the responses to cell-wall integrity agents and fungicide fludioxonil. However, CgEnd3 regulated appressorium formation and endocytosis in a calcium signaling-independent manner. Taken together, these results demonstrate that CgEnd3 plays pleiotropic roles in endocytosis, calcium signaling, cell-wall integrity, appressorium formation, penetration, and pathogenicity in C. gloeosporioides, and it suggests that CgEnd3 or endocytosis-related genes function as promising antifungal targets.

Mitogen-activated protein kinase cascade CgSte50-Ste11-Ste7-Mk1 regulates infection-related morphogenesis in the poplar anthracnose fungus Colletotrichum gloeosporioides

The hemibiotrophic pathogen Colletotrichum gloeosporioides is the causal agent of poplar anthracnose and causes considerable economic losses. This fungus infects its host through a specialized structure called an appressorium. In a previous study, we demonstrated that the mitogen-activated protein kinase (MAPK) CgMk1 plays a critical role in appressorium formation and pathogenicity. In this study, we identified three upstream components of CgMk1, the putative adaptor protein CgSte50, MAPKKK CgSte11, and MAPKK CgSte7, and showed that CgSte50, CgSte11, and CgSte7 positively regulate the phosphorylation of CgMk1. Deletion of CgSte50, CgSte11, and CgSte7 resulted in the loss of appressorium formation, penetration of the cellophane membrane, invasive growth and pathogenicity, similar to the defects observed in the CgMk1 mutant. CgSte50, CgSte11, CgSte7 and CgMk1 were also required for polarity during conidial germination. At the initial stage of appressorium formation, the accumulation of reactive oxygen species (ROS) was altered in the CgSte50, CgSte11, CgSte7 and CgMk1 deletion mutants compared with that in wild type (WT). Furthermore, the CgSte50, CgSte11, CgSte7 and CgMk1 deletion mutants manifested pleiotropic defects during vegetative growth; all mutants exhibited albino colonies, and the aerial hyphae had reduced hydrophobicity. In the mutants, autolysis was detected at the colony edge, and septum formation in the hyphae was elevated compared with that in the WT hyphae. Moreover, deletion of CgSte50, CgSte11, CgSte7 and CgMk1 affected vegetative growth under nitrogen-limiting and osmotic stress conditions. CgSte50, CgSte11, and CgSte7 but not CgMk1 were required for the oxidative stress response. Taken together, these results indicate that the CgMk1 MAPK cascade plays vital roles in various important functions in C. gloeosporioides.

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.

Screening of Marine Bioactive Antimicrobial Compounds for Plant Pathogens

Plant diseases have been threatening food production. Controlling plant pathogens has become an important strategy to ensure food security. Although chemical control is an effective disease control strategy, its application is limited by many problems, such as environmental impact and pathogen resistance. In order to overcome these problems, it is necessary to develop more chemical reagents with new functional mechanisms. Due to their special living environment, marine organisms have produced a variety of bioactive compounds with novel structures, which have the potential to develop new fungicides. In the past two decades, screening marine bioactive compounds to inhibit plant pathogens has been a hot topic. In this review, we summarize the screening methods of marine active substances from plant pathogens, the identification of marine active substances from different sources, and the structure and antibacterial mechanism of marine active natural products. Finally, the application prospect of marine bioactive substances in plant disease control was prospected.

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.

Incompatibility between proliferation and plant invasion is mediated by a regulator of appressorium formation in the corn smut fungus Ustilago maydis

Plant pathogenic fungi often developed specialized infection structures to breach the outer surface of a host plant. These structures, called appressoria, lead the invasion of the plant by the fungal hyphae. Studies in different phytopathogenic fungi showed that appressorium formation seems to be subordinated to the cell cycle. This subordination ensures the loading in the invading hypha of the correct genetic information to proceed with plant infection. However, how the cell cycle transmits its condition to the genetic program controlling appressorium formation and promoting the plant's invasion is unknown. Our results have uncovered how this process occurs for the appressorium of Ustilago maydis, the agent responsible for corn smut disease. Here, we described that the complex Clb2-cyclin-dependent kinase (Cdk)1, one of the master regulators of G2/M cell cycle progression in U. maydis, interacts and controls the subcellular localization of Biz1, a transcriptional factor required for the activation of the appressorium formation. Besides, Biz1 can arrest the cell cycle by down-regulation of the gene encoding a second b-cyclin Clb1 also required for the G2/M transition. These results revealed a negative feedback loop between appressorium formation and cell cycle progression in U. maydis, which serves as a "toggle switch" to control the fungal decision between infecting the plant or proliferating out of the plant.

The membrane mucin Msb2 regulates aflatoxin biosynthesis and pathogenicity in fungus Aspergillus flavus

As a pathogenic fungus, Aspergillus flavus can produce carcinogenic aflatoxins (AFs), which poses a great threat to crops and animals. Msb2, the signalling mucin protein, is a part of mitogen‐activated protein kinase (MAPK) pathway which contributes to a range of physiological processes. In this study, the roles of membrane mucin Msb2 were explored in A. flavus by the application of gene disruption. The deletion of msb2 gene (Δmsb2) caused defects in vegetative growth, sporulation and sclerotia formation when compared to WT and complement strain (Δmsb2^C) in A. flavus. Using thin‐layer chromatography (TLC) and high‐performance liquid chromatography (HPLC) analysis, it was found that deletion of msb2 down‐regulated aflatoxin B₁ (AFB₁) synthesis and decreased the infection capacity of A. flavus. Consistently, Msb2 responds to cell wall stress and osmotic stress by positively regulating the phosphorylation of MAP kinase. Notably, Δmsb2 mutant exhibited cell wall defect, and it was more sensitive to inhibitor caspofungin when compared to WT and Δmsb2^C. Taking together, these results revealed that Msb2 plays key roles in morphological development process, stresses adaptation, secondary metabolism and pathogenicity in fungus A. flavus.

The High Osmolarity Glycerol (HOG) Pathway Functions in Osmosensing, Trap Morphogenesis and Conidiation of the Nematode-Trapping Fungus Arthrobotrys oligospora

Hog1, a mitogen-activated protein kinase (MAPK), has been identified in diverse fungal species, and it regulates various cellular processes, such as osmoadaptation, nutrient-sensing, and pathogenesis. However, the roles that Hog1 plays in nematode-trapping fungi were previously unclear. Here, we characterized orthologs of Saccharomyces cerevisiae Hog1 and membrane mucin Msb2 in the nematode-trapping fungus Arthrobotrys oligospora. We generated gene deletion mutants of HOG1 and MSB2 in A. oligospora, and characterized their roles in osmosensing, growth, and trap morphogenesis. We found that both hog1 and msb2 mutants were highly sensitive to high osmolarity. Predation analyses further revealed that hog1 and msb2 deletion caused a reduction in trap formation and predation efficiency. Furthermore, HOG1 is required for conidiation in A. oligospora, demonstrating its critical role in this developmental pathway. In summary, this study demonstrated that the conserved Hog1 and Msb2 govern physiology, growth and development in the nematode-trapping fungus A. oligospora.

Corrigendum: Sho1 and Msb2 Play Complementary but Distinct Roles in Stress Responses, Sexual Differentiation, and Pathogenicity of Cryptococcus neoformans

[This corrects the article DOI: 10.3389/fmicb.2018.02958.].

Putative Membrane Receptors Contribute to Activation and Efficient Signaling of Mitogen-Activated Protein Kinase Cascades during Adaptation of Aspergillus fumigatus to Different Stressors and Carbon Sources

The high-osmolarity glycerol (HOG) response pathway is a multifunctional signal transduction pathway that specifically transmits ambient osmotic signals. Saccharomyces cerevisiae Hog1p has two upstream signaling branches, the sensor histidine kinase Sln1p and the receptor Sho1p. The Sho1p branch includes two other proteins, the Msb2p mucin and Opy2p. Aspergillus fumigatus is the leading cause of pulmonary fungal diseases. Here, we investigated the roles played by A. fumigatus SlnA^Sln1p, ShoA^Sho1p, MsbA^Msb2p, and OpyA^Opy2p putative homologues during the activation of the mitogen-activated protein kinase (MAPK) HOG pathway. The shoA, msbA, and opyA singly and doubly null mutants are important for the cell wall integrity (CWI) pathway, oxidative stress, and virulence as assessed by a Galleria mellonella model. Genetic interactions of ShoA, MsbA, and OpyA are also important for proper activation of the SakA^Hog1p and MpkA^Slt2 cascade and the response to osmotic and cell wall stresses. Comparative label-free quantitative proteomics analysis of the singly null mutants with the wild-type strain upon caspofungin exposure indicates that the absence of ShoA, MsbA, and OpyA affects the osmotic stress response, carbohydrate metabolism, and protein degradation. The putative receptor mutants showed altered trehalose and glycogen accumulation, suggesting a role for ShoA, MsbA, and OpyA in sugar storage. Protein kinase A activity was also decreased in these mutants. We also observed genetic interactions between SlnA, ShoA, MsbA, and OpyA, suggesting that both branches are important for activation of the HOG/CWI pathways. Our results help in the understanding of the activation and modulation of the HOG and CWI pathways in this important fungal pathogen.IMPORTANCE Aspergillus fumigatus is an important human-pathogenic fungal species that is responsible for a high incidence of infections in immunocompromised individuals. A. fumigatus high-osmolarity glycerol (HOG) and cell wall integrity pathways are important for the adaptation to different forms of environmental adversity such as osmotic and oxidative stresses, nutrient limitations, high temperatures, and other chemical and mechanical stresses that may be produced by the host immune system and antifungal drugs. Little is known about how these pathways are activated in this fungal pathogen. Here, we characterize four A. fumigatus putative homologues that are important for the activation of the yeast HOG pathway. A. fumigatus SlnA^Sln1p, ShoA^Sho1p, MsbA^Msb2p, and OpyA^Opy2p are genetically interacting and are essential for the activation of the HOG and cell wall integrity pathways. Our results contribute to the understanding of A. fumigatus adaptation to the host environment.

The transmembrane protein MaSho1 negatively regulates conidial yield by shifting the conidiation pattern in Metarhizium acridum

Sho1 is an important membrane sensor upstream of the HOG-MAPK signaling pathway, which plays critical roles in osmotic pressure response, growth, and virulence in fungi. Here, a Sho1 homolog (MaSho1), containing four transmembrane domains and one Src homology (SH3) domain, was characterized in Metarhizium acridum, a fungal pathogen of locusts. Targeted gene disruption of MaSho1 impaired cell wall integrity, virulence, and tolerances to UV-B and oxidative stresses, while none of them was affected when the SH3 domain was deleted. Intriguingly, disruption of MaSho1 significantly increased conidial yield, which was not affected in the SH3 domain mutant. Furthermore, it was found that deletion of MaSho1 led to microcycle conidiation of M. acridum on the normal conidiation medium. Deletion of MaSho1 significantly shortened the hyphal cells but had no effect on conidial germination. Digital gene expression profiling during conidiation indicated that differential expression of genes was associated with mycelial development, cell division, and differentiation between the wild type and the MaSho1 mutant. These data suggested that disruption of MaSho1 shifted the conidiation pattern by altering the transcription of genes to inhibit mycelial growth, thereby promoting the conidiation of M. acridum.

Smut fungal strategies for the successful infection

The smut fungi include a large number of plant pathogens that establish obligate biotrophic relationships with their host. Throughout the whole life inside plant tissue, smut fungi keep plant cells alive and acquire nutrients via biotrophic interfaces. This mini-review mainly summarizes the interactions between smut fungi and their host plants during the infection process. Despite various strategies recruited by plants to defense invading pathogens, smut fungi successfully evolved an arsenal for colonization. Mating of two compatible haploids gives rise to parasitic mycelium, which can sense plant surface cues such as fatty acids and hydrophobic surface, and induce the formation of appressoria for surface penetration. Plants can recognize fungal invading and activate defense response, including callose and lignin deposition, programmed cell death, and SA signaling pathway. To suppress plant immunity and alter the metabolic pathway of host plants, a cocktail of effectors is secreted by smut fungi depending on the plant organ and cell type that is infected.

The Verticillium dahliae Sho1-MAPK pathway regulates melanin biosynthesis and is required for cotton infection

Verticillium dahliae is a soil‐borne fungus that causes vascular wilt on numerous plants worldwide. The fungus survives in the soil for up to 14 years by producing melanized microsclerotia. The protective function of melanin in abiotic stresses is well documented. Here, we found that the V. dahliae tetraspan transmembrane protein VdSho1, a homolog of the Saccharomyces cerevisiae Sho1, acts as an osmosensor, and is required for plant penetration and melanin biosynthesis. The deletion mutant ΔSho1 was incubated on a cellophane membrane substrate that mimics the plant epidermis, revealing that the penetration of ΔSho1 strain was reduced compared to the wild‐type strain. Furthermore, VdSho1 regulates melanin biosynthesis by a signalling mechanism requiring a kinase‐kinase signalling module of Vst50‐Vst11‐Vst7. Strains, ΔVst50, ΔVst7 and ΔVst11 also displayed defective penetration and melanin production like the ΔSho1 strain. Defects in penetration and melanin production in ΔSho1 were restored by overexpression of Vst50, suggesting that Vst50 lies downstream of VdSho1 in the regulatory pathway governing penetration and melanin biosynthesis. Data analyses revealed that the transmembrane portion of VdSho1 was essential for both membrane penetration and melanin production. This study demonstrates that Vst50‐Vst11‐Vst7 module regulates VdSho1‐mediated plant penetration and melanin production in V. dahliae, contributing to virulence.

N-glycosylation of the protein disulfide isomerase Pdi1 ensures full Ustilago maydis virulence

Fungal pathogenesis depends on accurate secretion and location of virulence factors which drive host colonization. Protein glycosylation is a common posttranslational modification of cell wall components and other secreted factors, typically required for correct protein localization, secretion and function. Thus, the absence of glycosylation is associated with animal and plant pathogen avirulence. While the relevance of protein glycosylation for pathogenesis has been well established, the main glycoproteins responsible for the loss of virulence observed in glycosylation-defective fungi have not been identified. Here, we devise a proteomics approach to identify such proteins and use it to demonstrate a role for the highly conserved protein disulfide isomerase Pdi1 in virulence. We show that efficient Pdi1 N-glycosylation, which promotes folding into the correct protein conformation, is required for full pathogenic development of the corn smut fungus Ustilago maydis. Remarkably, the observed virulence defects are reminiscent of those seen in glycosylation-defective cells suggesting that the N-glycosylation of Pdi1 is necessary for the full secretion of virulence factors. All these observations, together with the fact that Pdi1 protein and RNA expression levels rise upon virulence program induction, suggest that Pdi1 glycosylation is important for normal pathogenic development in U. maydis. Our results provide new insights into the role of glycosylation in fungal pathogenesis.

Opposing signaling pathways regulate morphology in response to temperature in the fungal pathogen Histoplasma capsulatum

Phenotypic switching between 2 opposing cellular states is a fundamental aspect of biology, and fungi provide facile systems to analyze the interactions between regulons that control this type of switch. A long-standing mystery in fungal pathogens of humans is how thermally dimorphic fungi switch their developmental form in response to temperature. These fungi, including the subject of this study, Histoplasma capsulatum, are temperature-responsive organisms that utilize unknown regulatory pathways to couple their cell shape and associated attributes to the temperature of their environment. H. capsulatum grows as a multicellular hypha in the soil that switches to a pathogenic yeast form in response to the temperature of a mammalian host. These states can be triggered in the laboratory simply by growing the fungus either at room temperature (RT; which promotes hyphal growth) or at 37 °C (which promotes yeast-phase growth). Prior worked revealed that 15% to 20% of transcripts are differentially expressed in response to temperature, but it is unclear which transcripts are linked to specific phenotypic changes, such as cell morphology or virulence. To elucidate temperature-responsive regulons, we previously identified 4 transcription factors (required for yeast-phase growth [Ryp]1-4) that are required for yeast-phase growth at 37 °C; in each ryp mutant, the fungus grows constitutively as hyphae regardless of temperature, and the cells fail to express genes that are normally induced in response to growth at 37 °C. Here, we perform the first genetic screen to identify genes required for hyphal growth of H. capsulatum at RT and find that disruption of the signaling mucin MSB2 results in a yeast-locked phenotype. RNA sequencing (RNAseq) experiments reveal that MSB2 is not required for the majority of gene expression changes that occur when cells are shifted to RT. However, a small subset of temperature-responsive genes is dependent on MSB2 for its expression, thereby implicating these genes in the process of filamentation. Disruption or knockdown of an Msb2-dependent mitogen-activated protein (MAP) kinase (HOG2) and an APSES transcription factor (STU1) prevents hyphal growth at RT, validating that the Msb2 regulon contains genes that control filamentation. Notably, the Msb2 regulon shows conserved hyphal-specific expression in other dimorphic fungi, suggesting that this work defines a small set of genes that are likely to be conserved regulators and effectors of filamentation in multiple fungi. In contrast, a few yeast-specific transcripts, including virulence factors that are normally expressed only at 37 °C, are inappropriately expressed at RT in the msb2 mutant, suggesting that expression of these genes is coupled to growth in the yeast form rather than to temperature. Finally, we find that the yeast-promoting transcription factor Ryp3 associates with the MSB2 promoter and inhibits MSB2 transcript expression at 37 °C, whereas Msb2 inhibits accumulation of Ryp transcripts and proteins at RT. These findings indicate that the Ryp and Msb2 circuits antagonize each other in a temperature-dependent manner, thereby allowing temperature to govern cell shape and gene expression in this ubiquitous fungal pathogen of humans.

The Aspergillus fumigatus Mucin MsbA Regulates the Cell Wall Integrity Pathway and Controls Recognition of the Fungus by the Immune System

Aspergillus fumigatus is a filamentous fungus which causes invasive pulmonary aspergillosis in immunocompromised individuals. In fungi, cell signaling and cell wall plasticity are crucial for maintaining physiologic processes. In this context, Msb2 is an important signaling mucin responsible for activation of a variety of mitogen-activated protein kinase (MAPK)-dependent signaling pathways that regulate cell growth in several organisms, such as the cell wall integrity (CWI) pathway. Here, we aimed to characterize the MSB2 homologue in A. fumigatus Our results showed that MsbA plays a role in the vegetative and reproductive development of the fungus, in stress adaptation, and in resistance to antifungal drugs by modulating the CWI pathway gene expression. Importantly, cell wall composition is also responsible for activation of diverse receptors of the host immune system, thus leading to a proper immune response. In a model of acute Aspergillus pulmonary infection, results demonstrate that the ΔmsbA mutant strain induced less inflammation with diminished cell influx into the lungs and lower cytokine production, culminating in increased lethality rate. These results characterize for the first time the role of the signaling mucin MsbA in the pathogen A. fumigatus, as a core sensor for cell wall morphogenesis and an important regulator of virulence.IMPORTANCE Aspergillus fumigatus is an opportunistic fungus with great medical importance. During infection, Aspergillus grows, forming hyphae that colonize the lung tissue and invade and spread over the mammal host, resulting in high mortality rates. The knowledge of the mechanisms responsible for regulation of fungal growth and virulence comprises an important point to better understand fungal physiology and host-pathogen interactions. Msb2 is a mucin that acts as a sensor and an upstream regulator of the MAPK pathway responsible for fungal development in Candida albicans and Aspergillus nidulans Here, we show the role of the signaling mucin MsbA in the pathogen A. fumigatus, as a core sensor for cell wall morphogenesis, fungal growth, and virulence. Moreover, we show that cell wall composition, controlled by MsbA, is detrimental for fungal recognition and clearance by immune cells. Our findings are important for the understanding of how fungal sensors modulate cell physiology.

Signal peptide peptidase activity connects the unfolded protein response to plant defense suppression by Ustilago maydis

The corn smut fungus Ustilago maydis requires the unfolded protein response (UPR) to maintain homeostasis of the endoplasmic reticulum (ER) during the biotrophic interaction with its host plant Zea mays (maize). Crosstalk between the UPR and pathways controlling pathogenic development is mediated by protein-protein interactions between the UPR regulator Cib1 and the developmental regulator Clp1. Cib1/Clp1 complex formation results in mutual modification of the connected regulatory networks thereby aligning fungal proliferation in planta, efficient effector secretion with increased ER stress tolerance and long-term UPR activation in planta. Here we address UPR-dependent gene expression and its modulation by Clp1 using combinatorial RNAseq/ChIPseq analyses. We show that increased ER stress resistance is connected to Clp1-dependent alterations of Cib1 phosphorylation, protein stability and UPR gene expression. Importantly, we identify by deletion screening of UPR core genes the signal peptide peptidase Spp1 as a novel key factor that is required for establishing a compatible biotrophic interaction between U. maydis and its host plant maize. Spp1 is dispensable for ER stress resistance and vegetative growth but requires catalytic activity to interfere with the plant defense, revealing a novel virulence specific function for signal peptide peptidases in a biotrophic fungal/plant interaction.

Biotrophic pathogens establish compatible interactions with their host to cause disease.

A critical step in this process is the suppression of plant defense responses by secreted effector proteins. In the maize infecting fungus Ustilago maydis expression of effector encoding genes is coordinately upregulated at defined stages of pathogenic development in so-called effector waves. Efficient secretion of the multitude of effectors relies on the unfolded protein response (UPR) to maintain homeostasis of the endoplasmic reticulum. Activation of the UPR is connected to the control of fungal proliferation through direct protein-protein interactions between the UPR regulator Cib1 and the developmental regulator Clp1. Here, we show that this interaction leads to functional modification of Cib1 and modulation of UPR gene expression to adapt the UPR for long-term activity in the plant. Within a core set of UPR regulated genes we identify the signal peptide peptidase Spp1 as a key factor for fungal virulence. We show that Spp1 requires its conserved catalytic activity to suppress the plant defense and cause disease. The virulence specific function of Spp1 does not involve pathways previously known to be associated with Spp1-like proteins or plant defense suppression, suggesting a novel role for Spp1 substrates in biotrophic interactions.

Sho1 and Msb2 Play Complementary but Distinct Roles in Stress Responses, Sexual Differentiation, and Pathogenicity of Cryptococcus neoformans

The high-osmolarity glycerol response (HOG) pathway is pivotal in environmental stress response, differentiation, and virulence of Cryptococcus neoformans, which causes fatal meningoencephalitis. A putative membrane sensor protein, Sho1, has been postulated to regulate HOG pathway, but its regulatory mechanism remains elusive. In this study, we characterized the function of Sho1 with relation to the HOG pathway in C. neoformans. Sho1 played minor roles in osmoresistance, thermotolerance, and maintenance of membrane integrity mainly in a HOG-independent manner. However, it was dispensable for cryostress resistance, primarily mediated through the HOG pathway. A mucin-like transmembrane (TM) protein, Msb2, which interacts with Sho1 in Saccharomyces cerevisiae, was identified in C. neoformans, but found not to interact with Sho1. MSB2 codeletion with SHO1 further decreased osmoresistance and membrane integrity, but not thermotolerance, of sho1Δ mutant, indicating that both factors play to some level redundant but also discrete roles in C. neoformans. Sho1 and Msb2 played redundant roles in promoting the filamentous growth in sexual differentiation in a Cpk1-independent manner, in contrast to the inhibitory effect of the HOG pathway in the process. However, both factors contributed independently to Cpk1 phosphorylation during vegetative growth and endoplasmic reticulum (ER) stress response. Finally, Sho1 and Msb2 play distinct but complementary roles in the pulmonary virulence of C. neoformans. Overall, Sho1 and Msb2 play complementary but distinct roles in stress response, differentiation, and pathogenicity of C. neoformans.

Transcripts and tumors: regulatory and metabolic programming during biotrophic phytopathogenesis

Biotrophic fungal pathogens of plants must sense and adapt to the host environment to complete their life cycles. Recent transcriptome studies of the infection of maize by the biotrophic pathogen Ustilago maydis are providing molecular insights into an ordered program of changes in gene expression and the deployment of effectors as well as key features of nutrient acquisition. In particular, the transcriptome data provide a deeper appreciation of the complexity of the transcription factor network that controls the biotrophic program of invasion, proliferation, and sporulation. Additionally, transcriptome analysis during tumor formation, a key late stage in the life cycle, revealed features of the remodeling of host and pathogen metabolism that may support the formation of tremendous numbers of spores. Transcriptome studies are also appearing for other smut species during interactions with their hosts, thereby providing opportunities for comparative approaches to understand biotrophic adaptation.

Comparative whole-genome analysis reveals artificial selection effects on Ustilago esculenta genome

Ustilago esculenta, infects Zizania latifolia, and induced host stem swollen to be a popular vegetable called Jiaobai in China. It is the long-standing artificial selection that maximizes the occurrence of favourable Jiaobai, and thus maintaining the plant-fungi interaction and modulating the fungus evolving from plant pathogen to entophyte. In this study, whole genome of U. esculenta was sequenced and transcriptomes of the fungi and its host were analysed. The 20.2 Mb U. esculenta draft genome of 6,654 predicted genes including mating, primary metabolism, secreted proteins, shared a high similarity to related Smut fungi. But U. esculenta prefers RNA silencing not repeat-induced point in defence and has more introns per gene, indicating relatively slow evolution rate. The fungus also lacks some genes in amino acid biosynthesis pathway which were filled by up-regulated host genes and developed distinct amino acid response mechanism to balance the infection-resistance interaction. Besides, U. esculenta lost some surface sensors, important virulence factors and host range-related effectors to maintain the economic endophytic life. The elucidation of the U. esculenta genomic information as well as expression profiles can not only contribute to more comprehensive insights into the molecular mechanism underlying artificial selection but also into smut fungi-host interactions.

Cloning and Characterization of Two MAPK Genes UeKpp2 and UeKpp6 in Ustilago esculenta

Ustilago esculenta, resembling a fungal endophyte in Zizania latifolia, inhibits the host plant flowering and induces the host stems to swell and form edible galls. It is well believed that when and how the fungus infects and proliferates in the host plants during the host development is of importance in the edible gall formation. Mitogen-activated protein kinases (MAPKs) have been found to play an important role in sensing environment cues and regulating infection. Two MAPK genes UeKpp2 and UeKpp6 from U. esculenta were cloned and suggested to be involved in the Fus3/Kss1 pathway by a phylogenetic analysis with the neighbor-joining method. Quantitative RT-PCR (qRT-PCR) analyses indicated that expression of UeKpp2 and UeKpp6 were induced during mating and infection processes, and their expression patterns displayed differentially under different carbon and nitrogen sources. In addition, subcellular localization of UeKpp2 or UeKpp6 fused with the reporter green fluoresce protein was observed by confocal laser scanning microscope, and yeast two-hybrid assays were carried out. Results showed that both UeKpp2 and UeKpp6 were located in cytoplasm and interacted with UePrf1, indicating their involvement in hyphal growth and host-pathogen regulation. Only UeKpp2 but not UeKpp6 interacted with the upstream MAPK kinase UeFuz7, implying an additional MAPK pathway, in which UeKpp6 involved, existed.

Annotation resource of tandem repeat-containing secretory proteins in sixty fungi

Fungal secretory proteins that interact with host plants are regarded as effectors. Because fungal effectors rarely contain conserved sequence features, identification and annotation of fungal effectors from predicted secretory proteins are difficult using outward comparison methods such as BLAST or hidden Markov model. In desire of more sequence features to prioritize research interests of fungal secretory proteins, this study developed a pipeline to identify tandem repeat (TR) domain within putative secretory proteins and tested a hypothesis that at least one type of TR domain in non-orthologous secretory proteins has emerged from convergent evolution for plant pathogenicity. There were 2804 types of TR domains and a total of 2925 TR-containing secretory proteins found from 60 fungi. There was no conserved type of TR domain shared only by plant pathogens, indicating functional divergence for different types of TR domain and TR-containing secretory proteins. The annotation resource of putative fungal TR-containing secretory proteins provides new sequence features that will be useful for the community interested in fungal effector biology.

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkörper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkörper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.

The Ustilago maydis repetitive effector Rsp3 blocks the antifungal activity of mannose-binding maize proteins

To cause disease in maize, the biotrophic fungus Ustilago maydis secretes a large arsenal of effector proteins. Here, we functionally characterize the repetitive effector Rsp3 (repetitive secreted protein 3), which shows length polymorphisms in field isolates and is highly expressed during biotrophic stages. Rsp3 is required for virulence and anthocyanin accumulation. During biotrophic growth, Rsp3 decorates the hyphal surface and interacts with at least two secreted maize DUF26-domain family proteins (designated AFP1 and AFP2). AFP1 binds mannose and displays antifungal activity against the rsp3 mutant but not against a strain constitutively expressing rsp3. Maize plants silenced for AFP1 and AFP2 partially rescue the virulence defect of rsp3 mutants, suggesting that blocking the antifungal activity of AFP1 and AFP2 by the Rsp3 effector is an important virulence function. Rsp3 orthologs are present in all sequenced smut fungi, and the ortholog from Sporisorium reilianum can complement the rsp3 mutant of U. maydis, suggesting a novel widespread fungal protection mechanism.

The fungus Ustilago maydis secretes many effector proteins to cause disease in maize. Here, Ma et al. show that the repetitive effector Rsp3 is required for virulence by inhibiting the antifungal activity of two mannose-binding proteins that are secreted by the plant cells.

Thigmo Responses: The Fungal Sense of Touch

The growth and development of most fungi take place on a two-dimensional surface or within a three-dimensional matrix. The fungal sense of touch is therefore critical for fungi in the interpretation of their environment and often signals the switch to a new developmental state. Contact sensing, or thigmo-based responses, include thigmo differentiation, such as the induction of invasion structures by plant pathogens in response to topography; thigmonasty, where contact with a motile prey rapidly triggers its capture; and thigmotropism, where the direction of hyphal growth is guided by physical features in the environment. Like plants and some bacteria, fungi grow as walled cells. Despite the well-demonstrated importance of thigmo responses in numerous stages of fungal growth and development, it is not known how fungal cells sense contact through the relatively rigid structure of the cell wall. However, while sensing mechanisms at the molecular level are not entirely understood, the downstream signaling pathways that are activated by contact sensing are being elucidated. In the majority of cases, the response to contact is complemented by chemical cues and both are required, either sequentially or simultaneously, to elicit normal developmental responses. The importance of a sense of touch in the lifestyles and development of diverse fungi is highlighted in this review, and the candidate molecular mechanisms that may be involved in fungal contact sensing are discussed.

Candidate Genes for Aggressiveness in a Natural Fusarium culmorum Population Greatly Differ between Wheat and Rye Head Blight

Fusarium culmorum is one of the species causing Fusarium head blight (FHB) in cereals in Europe. We aimed to investigate the association between the nucleotide diversity of ten F. culmorum candidate genes and field ratings of aggressiveness in winter rye. A total of 100 F. culmorum isolates collected from natural infections were phenotyped for FHB at two locations and two years. Variance components for aggressiveness showed significant isolate and isolate-by-environment variance, as expected for quantitative host-pathogen interactions. Further analysis of the isolate-by-environment interaction revealed the dominant role of the isolate-by-year over isolate-by-location interaction. One single-nucleotide polymorphism (SNP) in the cutinase (CUT) gene was found to be significantly (p < 0.001) associated with aggressiveness and explained 16.05% of the genotypic variance of this trait in rye. The SNP was located 60 base pairs before the start codon, which suggests a role in transcriptional regulation. Compared to a previous study in winter wheat with the same nucleotide sequences, a larger variation of pathogen aggressiveness on rye was found and a different candidate gene was associated with pathogen aggressiveness. This is the first report on the association of field aggressiveness and a host-specific candidate gene codifying for a protein that belongs to the secretome in F. culmorum.

Functional characterization of the upstream components of the Hog1-like kinase cascade in hyperosmotic and carbon sensing in Trichoderma reesei

BACKGROUND

Trichoderma reesei holds a high capacity for protein secretion and represents the most important cellulase producer in industry. However, the external signal sensing and intracellular signal transduction during cellulose induction remain unclear. As one of the most pervasive signal transduction pathways in all eukaryotic species, the mitogen-activated protein kinase (MAPK) pathway and its upstream sensing and signaling components are involved in various physiological processes including stress and nutrient sensing. Particularly, the Hog1-type MAPK Tmk3 has been reported to be involved in the cellulase production in T. reesei.

RESULTS

Here we established the physiological role of two upstream regulatory branches, the Sho1 branch and the Sln1 branch, of the Hog1-type Tmk3 pathway in T. reesei. Deletion of Trste20 of the Sho1 branch or repression of Trypd1 of the Sln1 branch reduced the resistance to high salt stress, whereas TrSho1 showed an opposing effect to that of TrSte20 and the identified TrSln1 seemed to be dispensable in the osmotic regulation. The Sho1 and Sln1 branches also participated in the cell wall integrity maintenance and other stress responses (i.e. oxidative and thermo stresses). Notably, TrSho1 and TrSte20 of the Sho1 branch and TrYpd1 of the Sln1 branch were shown to be differentially involved in the cellulase production of T. reesei. Repression of Trypd1 hardly affected cellulase induction, whereas overexpression of Trypd1 resulted in the reduced production of cellulases. Contrary to the case of Trypd1, repression of Trsho1 or deletion of Trste20 significantly reduced the transcription of cellulase genes.

CONCLUSIONS

TrSho1 and TrSte20 of the Sho1 branch and TrYpd1 of the Sln1 branch are all involved in general stress responses including hyperosmotic regulation and cell wall integrity maintenance. Moreover, our study revealed that the Sho1 and Sln1 osmosensing pathways are differentially involved in the regulation of cellulase production in T. reesei. The Sho1 branch positively regulated the production of cellulases and the transcription of cellulase genes while TrYpd1 of the Sln1 branch negatively controlled the cellulase production, supporting the crosstalks of osmosensing and nutrient sensing.