Infection-related development in the rice blast fungus Magnaporthe grisea

A novel MAP kinase-interacting protein MoSmi1 regulates development and pathogenicity in Magnaporthe oryzae

The cell wall is the first barrier against external adversity and plays roles in maintaining normal physiological functions of fungi. Previously, we reported a nucleosome assembly protein, MoNap1, in Magnaporthe oryzae that plays a role in cell wall integrity (CWI), stress response, and pathogenicity. Moreover, MoNap1 negatively regulates the expression of MoSMI1 encoded by MGG_03970. Here, we demonstrated that deletion of MoSMI1 resulted in a significant defect in appressorium function, CWI, cell morphology, and pathogenicity. Further investigation revealed that MoSmi1 interacted with MoOsm1 and MoMps1 and affected the phosphorylation levels of MoOsm1, MoMps1, and MoPmk1, suggesting that MoSmi1 regulates biological functions by mediating mitogen-activated protein kinase (MAPK) signalling pathway in M. oryzae. In addition, transcriptome data revealed that MoSmi1 regulates many infection-related processes in M. oryzae, such as membrane-related pathway and oxidation reduction process. In conclusion, our study demonstrated that MoSmi1 regulates CWI by mediating the MAPK pathway to affect development and pathogenicity of M. oryzae.

Beneficial effects of Bacillus mojavensis strain MTC-8 on plant growth, immunity and disease resistance against Magnaporthe oryzae

Rice blast, a prevalent and highly destructive rice disease that significantly impacts rice yield, is caused by the rice blast fungus. In the present study, a strain named MTC-8, identified as Bacillus mojavensis, was demonstrated has strong antagonistic activity against the rice blast fungus, Rhizoctonia solani, Ustilaginoidea virens, and Bipolaria maydis. The potential biocontrol agents were identified using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) analysis and chromatography. Further investigations elucidated the inhibitory mechanism of the isolated compound and demonstrated its ability to suppress spore germination, alter hyphal morphology, disrupt cell membrane integrity, and induce defense-related gene expression in rice. MTC-8 promoted plant growth and may lead to the development of a biocontrol agent that meets agricultural standards. Overall, the Bacillus mojavensis MTC-8 strain exerted beneficial effects on plant growth, immunity and disease resistance against rice blast fungus. In this study, we isolated and purified a bioactive substance from fermentation broth, and the results provide a foundation for the development and application of biopesticides. Elucidation of the inhibitory mechanism against rice blast fungus provides theoretical support for the identification of molecular targets. The successful development of a biocontrol agent lays the groundwork for its practical application in agriculture.

Sucrose non-fermenting protein kinase gene UvSnf1 is required for virulence in Ustilaginoidea virens

Rice false smut caused by Ustilaginoidea virens is becoming one of the most devastating diseases in rice production areas in the world. Revealing U. virens potential pathogenic mechanisms provides ideas for formulating more effective prevention and control strategies. Sucrose non-fermenting 1 (Snf1) protein kinase plays a critical role in activating transcription and suppressing gene expression, as well as in cellular response to various stresses, such as nutrient limitation. In our study, we identified the Snf1 homolog UvSnf1 and analyzed its biological functions in U. virens. The expression level of UvSnf1 was dramatically up-regulated during invasion, indicating that UvSnf1 may participate in infection. Phenotypic analyses of UvSnf1 deletion mutants revealed that UvSnf1 is necessary for hyphae growth, spore production, and virulence in U. virens. Moreover, UvSnf1 promotes U. virens to use unfavorable carbon sources when the sucrose is insufficient. In addition, deletion of UvSnf1 down-regulates the expression of the cell wall-degrading enzymes (CWDEs) genes under sucrose limitation conditions in U. virens. Further analyses showed that CWDEs (UvCut1 and UvXyp1) are not only involved in growth, spore production, and virulence but are also required for the utilization of carbon sources. In conclusion, this study demonstrates that UvSnf1 plays vital roles in virulence and carbon source utilization in U. virens, and one of the possible mechanisms is playing a role in regulating the expression of CWDE genes.

MoNap1, a Nucleosome Assemble Protein 1, Regulates Growth, Development, and Pathogenicity in Magnaporthe oryzae

Nap1 is an evolutionarily conserved protein from yeast to human and is involved in diverse physiological processes, such as nucleosome assembly, histone shuttling between the nucleus and cytoplasm, transcriptional regulation, and the cell cycle regulation. In this paper, we identified nucleosome assemble protein MoNap1 in Magnaporthe oryzae and investigated its function in pathogenicity. Deletion of MoNAP1 resulted in reduced growth and conidiation, decreased appressorium formation rate, and impaired virulence. MoNap1 affects appressorium turgor and utilization of glycogen and lipid droplets. In addition, MoNap1 is involved in the regulation of cell wall, oxidation, and hyperosmotic stress. The subcellular localization experiments showed that MoNap1 is located in the cytoplasm. MoNap1 interacts with MoNbp2, MoClb3, and MoClb1 in M. oryzae. Moreover, deletion of MoNBP2 and MoCLB3 has no effects on vegetative growth, conidiation, and pathogenicity. Transcriptome analysis reveals that MoNAP1 is involved in regulating pathogenicity, the melanin biosynthetic process. Taken together, our results showed that MoNap1 plays a crucial role in growth, conidiation, and pathogenicity of M. oryzae.

Evolution of pathogenicity in obligate fungal pathogens and allied genera

Obligate fungal pathogens (ascomycetes and basidiomycetes) and oomycetes are known to cause diseases in cereal crop plants. They feed on living cells and most of them have learned to bypass the host immune machinery. This paper discusses some of the factors that are associated with pathogenicity drawing examples from ascomycetes, basidiomycetes and oomycetes, with respect to their manifestation in crop plants. The comparisons have revealed a striking similarity in the three groups suggesting convergent pathways that have arisen from three lineages independently leading to an obligate lifestyle. This review has been written with the intent, that new information on adaptation strategies of biotrophs, modifications in pathogenicity strategies and population dynamics will improve current strategies for breeding with stable resistance.

The LAMMER Kinase MoKns1 Regulates Growth, Conidiation and Pathogenicity in Magnaporthe oryzae

Magnaporthe oryzae is an important pathogen that causes a devastating disease in rice. It has been reported that the dual-specificity LAMMER kinase is conserved from yeast to animal species and has a variety of functions. However, the functions of the LAMMER kinase have not been reported in M. oryzae. In this study, we identified the unique LAMMER kinase MoKns1 and analyzed its function in M. oryzae. We found that in a MoKNS1 deletion mutant, growth and conidiation were primarily decreased, and pathogenicity was almost completely lost. Furthermore, our results found that MoKns1 is involved in autophagy. The ΔMokns1 mutant was sensitive to rapamycin, and MoKns1 interacted with the autophagy-related protein MoAtg18. Compared with the wild-type strain 70−15, autophagy was significantly enhanced in the ΔMokns1 mutant. In addition, we also found that MoKns1 regulated DNA damage stress pathways, and the ΔMokns1 mutant was more sensitive to hydroxyurea (HU) and methyl methanesulfonate (MMS) compared to the wild-type strain 70−15. The expression of genes related to DNA damage stress pathways in the ΔMokns1 mutant was significantly different from that in the wild-type strain. Our results demonstrate that MoKns1 is an important pathogenic factor in M. oryzae involved in regulating autophagy and DNA damage response pathways, thus affecting virulence. This research on M. oryzae pathogenesis lays a foundation for the prevention and control of M. oryzae.

Nucleosome Assembly Protein 1, Nap1, Is Required for the Growth, Development, and Pathogenicity of Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast, leading to significant reductions in rice and wheat productivity. Nap1 is a conserved protein in eukaryotes involved in diverse physiological processes, such as nucleosome assembly, histone shuttling between the nucleus and cytoplasm, transcriptional regulation, and the cell cycle. Here, we identified Nap1 and characterized its roles in fungal development and virulence in M. oryzae. MoNap1 is involved in aerial hyphal and conidiophore differentiation, sporulation, appressorium formation, plant penetration, and virulence. ΔMonap1 generated a small, elongated, and malformed appressorium with an abnormally organized septin ring on hydrophobic surfaces. ΔMonap1 was more sensitive to cell wall integrity stresses but more resistant to microtubule stresses. MoNap1 interacted with histones H₂A and H₂B and the B-type cyclin (Cyc1). Moreover, a nuclear export signal (NES) domain is necessary for Nap1’s roles in the regulation of the growth and pathogenicity of M. oryzae. In summary, NAP1 is essential for the growth, appressorium formation, and pathogenicity of M. oryzae.

Development of a LAMP method for detecting F129L mutant in azoxystrobin-resistant Pyricularia oryzae

Azoxystrobin has been widely used since 1996 to control rice blast caused by Pyricularia oryzae. Azoxystrobin resistance related to mutations at the P. oryzae target protein (F129L of Cytb) has been reported worldwide. To quickly identify and detect resistant strains in the field, this research established a rapid loop-mediated isothermal amplification (LAMP) detection system for the F129L mutation. The system could detect the P. oryzae F129L (TTC-TTA) mutation at 62 °C within 60 min, with a detection limit of 100 fg/μL, which is 10 times higher than for conventional PCR. The method had high specificity and repeatability and could detect the F129L (TTC-TTA) mutation in plant tissues within 3 h. The LAMP method established in this study will be useful to detect azoxystrobin-resistant P. oryzae F129L mutant strains and generate significant data for the management of resistant P. oryzae isolates.

Host-Induced Gene Silencing of a Multifunction Gene Sscnd1 Enhances Plant Resistance Against Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a devastating necrotrophic fungal pathogen and has a substantial economic impact on crop production worldwide. Magnaporthe appressoria-specific (MAS) proteins have been suggested to be involved in the appressorium formation in Magnaporthe oryzae. Sscnd1, an MAS homolog gene, is highly induced at the early infection stage of S. sclerotiorum. Knock-down the expression of Sscnd1 gene severely reduced the virulence of S. sclerotiorum on intact rapeseed leaves, and their virulence was partially restored on wounded leaves. The Sscnd1 gene-silenced strains exhibited a defect in compound appressorium formation and cell integrity. The instantaneous silencing of Sscnd1 by tobacco rattle virus (TRV)-mediated host-induced gene silencing (HIGS) resulted in a significant reduction in disease development in tobacco. Three transgenic HIGS Arabidopsis lines displayed high levels of resistance to S. sclerotiorum and decreased Sscnd1 expression. Production of specific Sscnd1 siRNA in transgenic HIGS Arabidopsis lines was confirmed by stem-loop qRT-PCR. This study revealed that the compound appressorium-related gene Sscnd1 is required for cell integrity and full virulence in S. sclerotiorum and that Sclerotinia stem rot can be controlled by expressing the silencing constructs of Sscnd1 in host plants.

Appressoria

Appressoria are specialised infection structures used by many disease-causing microorganisms to breach the outer surface of a host. Nick Talbot tells us about these small but incredibly powerful structures.

A Report on Fungal (1→3)-α-d-glucans: Properties, Functions and Application

The cell walls of fungi are composed of glycoproteins, chitin, and α- and β-glucans. Although there are many reports on β-glucans, α-glucan polysaccharides are not yet fully understood. This review characterizes the physicochemical properties and functions of (1→3)-α-d-glucans. Particular attention has been paid to practical application and the effect of glucans in various respects, taking into account unfavourable effects and potential use. The role of α-glucans in plant infection has been proven, and collected facts have confirmed the characteristics of Aspergillus fumigatus infection associated with the presence of glucan in fungal cell wall. Like β-glucans, there are now evidence that α-glucans can also stimulate the immune system. Moreover, α-d-glucans have the ability to induce mutanases and can thus decompose plaque.

Engineering a peptide aptamer to target calmodulin for the inhibition of Magnaporthe oryzae

To develop an antimicrobial agent for preventing the devasting damage caused by rice blast, a novel peptide aptamer was identified to interact with calmodulin (CaM) for the inhibition of the spore development in the pathogen Magnaporthe oryzae. A peptide aptamer designated as SNP-D4, consisted of the scaffold protein Staphylococcus aureus nuclease (SN) and an exposed surface loop of 16 random amino acids, was screened from the constructed peptide aptamer libraries by bacterial two-hybrid system using CaM of M. oryzae as the bait. The preliminary inhibition in the sporulation development was observed after treating with the crude extracts expressing SNP-D4. The inhibition efficacies of the purified SNP-D4 were quantified at the stages of conidial germination, germ tube elongation, and appressorium formation in M. oryzae. The binding affinity analysis revealed that SNP-D4 interacted with CaM at a dissociation constant (K_d) of about 20 μM. Moreover, the N-terminus of CaM was identified as the key binding region.

A Small GTPase RHO2 Plays an Important Role in Pre-infection Development in the Rice Blast Pathogen Magnaporthe oryzae

The rice blast pathogen Magnaporthe oryzae is a global threat to rice production. Here we characterized RHO2 gene (MGG_02457) that belongs to the Rho GTPase family, using a deletion mutant. This mutant ΔMorho2 exhibited no defects in conidiation and germination but developed only 6% of appressoria in response to a hydrophobic surface when compared to the wild-type progenitor. This result indicates that MoRHO2 plays a role in appressorium development. Furthermore, exogenous cAMP treatment on the mutant led to appressoria that exhibited abnormal morphology on both hydrophobic and hydrophilic surfaces. These outcomes suggested the involvement of MoRHO2 in cAMP-mediated appressorium development. ΔMorho2 mutation also delayed the development of appressorium-like structures (ALS) at hyphal tips on hydrophobic surface, which were also abnormally shaped. These results suggested that MoRHO2 is involved in morphological development of appressoria and ALS from conidia and hyphae, respectively. As expected, ΔMorho2 mutant was defective in plant penetration, but was still able to cause lesions, albeit at a reduced rate on wounded plants. These results implied that MoRHO2 plays a role in M. oryzae virulence as well.

The Plant Infection Test: Spray and Wound-Mediated Inoculation with the Plant Pathogen Magnaporthe Grisea

Plants possess a powerful system to defend themselves against potential threats by pathogenic fungi. For agriculturally important plants, however, current measures to combat such pathogens have proved too conservative and, thus, not sufficiently effective, and they can potentially pose environmental risks. Therefore, it is extremely necessary to identify host-resistance factors to assist in controlling plant diseases naturally through the identification of resistant germplasm, the isolation and characterization of resistance genes, and the molecular breeding of resistant cultivars. In this regard, there is need to establish an accurate, rapid, and large-scale inoculation method to breed and develop plant resistance genes. The rice blast fungal pathogen Magnaporthe grisea causes severe disease symptoms and yield losses. Recently, M. grisea has emerged as a model organism for studying the mechanisms of plant-fungal pathogen interactions. Hence, we report the development of a plant virulence test method that is specific for M. grisea. This method provides for both spray inoculation with a conidial suspension and wounding inoculation with mycelium cubes or droplets of conidial suspension. The key step of the wounding inoculation method for detached rice leaves is to make wounds on plant leaves, which avoids any interference caused by host penetration resistance. This spray/wounding protocol contributes to the rapid, accurate, and large-scale screening of the pathotypes of M. grisea isolates. This integrated and systematic plant infection method will serve as an excellent starting point for gaining a broad perspective of issues in plant pathology.

Application of the red fluorescent protein mCherry in mycelial labeling and organelle tracing in the dermatophyte Trichophyton mentagrophytes

Trichophyton mentagrophytes is a fungus that causes skin disease in humans and other animals worldwide. Studies on molecular biology and fluorescent labeling of the fungus are limited. Here, we applied mCherry for the first time in T. mentagrophytes to label the fungus and its organelles. We constructed four expression vectors of mCherry or mCherry fusions containing a variety of resistance markers and promoters, which were then integrated, together with two previous mCherry expression vectors, in T. mentagrophytes via Agrobacterium tumefaciens-mediated transformation (AtMT). The resulting transformants emitted bright red fluorescence. We used the histone protein H2B and the peroxisome targeting signal 1 (PTS1) peptide to target the nucleus and peroxisomes, respectively, in T. mentagrophytes. In the transformants expressing mCherry-fused H2B, the fluorescence was distinctly localized to the nuclei in hyphae, spores and the fungal cells in infected animal tissue. In the T. mentagrophytes transformants where the peroxisome was targeted, the mCherry was present as small dots (0.2-1 μm diameter) throughout the spores and the hyphae. We also constructed a T. mentagrophytes AtMT library containing more than 1000 hygromycin-resistant transformants that were genetically stable. Our results provide useful tools for further investigations on molecular pathogenesis of T. mentagrophytes.

Methylmalonate-semialdehyde dehydrogenase mediated metabolite homeostasis essentially regulate conidiation, polarized germination and pathogenesis in Magnaporthe oryzae

Plants generate multitude of aldehydes under abiotic and biotic stress conditions. Ample demonstrations have shown that rice-derived aldehydes enhance the resistance of rice against the rice-blast fungus Magnaporthe oryzae. However, how the fungal pathogen nullifies the inhibitory effects of host aldehydes to establish compatible interaction remains unknown. Here we identified and evaluated the in vivo transcriptional activities of M. oryzae aldehyde dehydrogenase (ALDH) genes. Transcriptional analysis of M. oryzae ALDH genes revealed that the acetylating enzyme Methylmalonate-Semialdehyde Dehydrogenase (MoMsdh/MoMmsdh) elevated activities during host invasion and colonization of the fungus. We further examined the pathophysiological importance of MoMSDH by deploying integrated functional genetics, and biochemical approaches. MoMSDH deletion mutant ΔMomsdh exhibited germination defect, hyper-branching of germ tube and failed to form appressoria on hydrophobic and hydrophilic surface. The MoMSDH disruption caused accumulation of small branch-chain amino acids, pyridoxine and AMP/cAMP in the ΔMomsdh mutant and altered Spitzenkörper organization in the conidia. We concluded that MoMSDH contribute significantly to the pathogenesis of M. oryzae by regulating the mobilization of Spitzenkörper during germ tube morphogenesis, appressoria formation by acting as metabolic switch regulating small branch-chain amino acids, inositol, pyridoxine and AMP/cAMP homeostasis.

A Colletotrichum graminicola mutant deficient in the establishment of biotrophy reveals early transcriptional events in the maize anthracnose disease interaction

BACKGROUND

Colletotrichum graminicola is a hemibiotrophic fungal pathogen that causes maize anthracnose disease. It progresses through three recognizable phases of pathogenic development in planta: melanized appressoria on the host surface prior to penetration; biotrophy, characterized by intracellular colonization of living host cells; and necrotrophy, characterized by host cell death and symptom development. A "Mixed Effects" Generalized Linear Model (GLM) was developed and applied to an existing Illumina transcriptome dataset, substantially increasing the statistical power of the analysis of C. graminicola gene expression during infection and colonization. Additionally, the in planta transcriptome of the wild-type was compared with that of a mutant strain impaired in the establishment of biotrophy, allowing detailed dissection of events occurring specifically during penetration, and during early versus late biotrophy.

RESULTS

More than 2000 fungal genes were differentially transcribed during appressorial maturation, penetration, and colonization. Secreted proteins, secondary metabolism genes, and membrane receptors were over-represented among the differentially expressed genes, suggesting that the fungus engages in an intimate and dynamic conversation with the host, beginning prior to penetration. This communication process probably involves reception of plant signals triggering subsequent developmental progress in the fungus, as well as production of signals that induce responses in the host. Later phases of biotrophy were more similar to necrotrophy, with increased production of secreted proteases, inducers of plant cell death, hydrolases, and membrane bound transporters for the uptake and egress of potential toxins, signals, and nutrients.

CONCLUSIONS

This approach revealed, in unprecedented detail, fungal genes specifically expressed during critical phases of host penetration and biotrophic establishment. Many encoded secreted proteins, secondary metabolism enzymes, and receptors that may play roles in host-pathogen communication necessary to promote susceptibility, and thus may provide targets for chemical or biological controls to manage this important disease. The differentially expressed genes could be used as 'landmarks' to more accurately identify developmental progress in compatible versus incompatible interactions involving genetic variants of both host and pathogen.

Twilight, a Novel Circadian-Regulated Gene, Integrates Phototropism with Nutrient and Redox Homeostasis during Fungal Development

Phototropic regulation of circadian clock is important for environmental adaptation, organismal growth and differentiation. Light plays a critical role in fungal development and virulence. However, it is unclear what governs the intracellular metabolic response to such dark-light rhythms in fungi. Here, we describe a novel circadian-regulated Twilight (TWL) function essential for phototropic induction of asexual development and pathogenesis in the rice-blast fungus Magnaporthe oryzae. The TWL transcript oscillates during circadian cycles and peaks at subjective twilight. GFP-Twl remains acetylated and cytosolic in the dark, whereas light-induced phosphorylation (by the carbon sensor Snf1 kinase) drives it into the nucleus. The mRNA level of the transcription/repair factor TFB5, was significantly down regulated in the twl∆ mutant. Overexpression of TFB5 significantly suppressed the conidiation defects in the twl∆ mutant. Furthermore, Tfb5-GFP translocates to the nucleus during the phototropic response and under redox stress, while it failed to do so in the twl∆ mutant. Thus, we provide mechanistic insight into Twl-based regulation of nutrient and redox homeostasis in response to light during pathogen adaptation to the host milieu in the rice blast pathosystem.

A Role of AREB in the Regulation of PACC-Dependent Acid-Expressed-Genes and Pathogenicity of Colletotrichum gloeosporioides

Gene expression regulation by pH in filamentous fungi and yeasts is controlled by the PACC/RIM101 transcription factor. In Colletotrichum gloeosporioides, PACC is known to act as positive regulator of alkaline-expressed genes, and this regulation was shown to contribute to fungal pathogenicity. PACC is also a negative regulator of acid-expressed genes, however; the mechanism of downregulation of acid-expressed genes by PACC and their contribution to C. gloeosporioides pathogenicity is not well understood. RNA sequencing data analysis was employed to demonstrate that PACC transcription factor binding sites (TFBS) are significantly overrepresented in the promoter of PACC-upregulated, alkaline-expressed genes. In contrast, they are not overrepresented in the PACC-downregulated, acid-expressed genes. Instead, acid-expressed genes showed overrepresentation of AREB GATA TFBS in C. gloeosporioides and in homologs of five other ascomycetes genomes. The areB promoter contains PACC TFBS; its transcript was upregulated at pH 7 and repressed in ΔpacC. Furthermore, acid-expressed genes were found to be constitutively upregulated in ΔareB during alkalizing conditions. The areB mutants showed significantly reduced ammonia secretion and pathogenicity on tomato fruit. Present results indicate that PACC activates areB expression, thereby conditionally repressing acid-expressed genes and contributing critically to C. gloeosporioides pathogenicity.

Simultaneous transcriptome analysis of Colletotrichum gloeosporioides and tomato fruit pathosystem reveals novel fungal pathogenicity and fruit defense strategies

The fungus Colletotrichum gloeosporioides breaches the fruit cuticle but remains quiescent until fruit ripening signals a switch to necrotrophy, culminating in devastating anthracnose disease. There is a need to understand the distinct fungal arms strategy and the simultaneous fruit response. Transcriptome analysis of fungal-fruit interactions was carried out concurrently in the appressoria, quiescent and necrotrophic stages. Conidia germinating on unripe fruit cuticle showed stage-specific transcription that was accompanied by massive fruit defense responses. The subsequent quiescent stage showed the development of dendritic-like structures and swollen hyphae within the fruit epidermis. The quiescent fungal transcriptome was characterized by activation of chromatin remodeling genes and unsuspected environmental alkalization. Fruit response was portrayed by continued highly integrated massive up-regulation of defense genes. During cuticle infection of green or ripe fruit, fungi recapitulate the same developmental stages but with differing quiescent time spans. The necrotrophic stage showed a dramatic shift in fungal metabolism and up-regulation of pathogenicity factors. Fruit response to necrotrophy showed activation of the salicylic acid pathway, climaxing in cell death. Transcriptome analysis of C. gloeosporioides infection of fruit reveals its distinct stage-specific lifestyle and the concurrent changing fruit response, deepening our perception of the unfolding fungal-fruit arms and defenses race.

Fungal model systems and the elucidation of pathogenicity determinants

Fungi have the capacity to cause devastating diseases of both plants and animals, causing significant harvest losses that threaten food security and human mycoses with high mortality rates. As a consequence, there is a critical need to promote development of new antifungal drugs, which requires a comprehensive molecular knowledge of fungal pathogenesis. In this review, we critically evaluate current knowledge of seven fungal organisms used as major research models for fungal pathogenesis. These include pathogens of both animals and plants; Ashbya gossypii, Aspergillus fumigatus, Candida albicans, Fusarium oxysporum, Magnaporthe oryzae, Ustilago maydis and Zymoseptoria tritici. We present key insights into the virulence mechanisms deployed by each species and a comparative overview of key insights obtained from genomic analysis. We then consider current trends and future challenges associated with the study of fungal pathogenicity.

Unifying themes in microbial associations with animal and plant hosts described using the gene ontology

Microbes form intimate relationships with hosts (symbioses) that range from mutualism to parasitism. Common microbial mechanisms involved in a successful host association include adhesion, entry of the microbe or its effector proteins into the host cell, mitigation of host defenses, and nutrient acquisition. Genes associated with these microbial mechanisms are known for a broad range of symbioses, revealing both divergent and convergent strategies. Effective comparisons among these symbioses, however, are hampered by inconsistent descriptive terms in the literature for functionally similar genes. Bioinformatic approaches that use homology-based tools are limited to identifying functionally similar genes based on similarities in their sequences. An effective solution to these limitations is provided by the Gene Ontology (GO), which provides a standardized language to describe gene products from all organisms. The GO comprises three ontologies that enable one to describe the molecular function(s) of gene products, the biological processes to which they contribute, and their cellular locations. Beginning in 2004, the Plant-Associated Microbe Gene Ontology (PAMGO) interest group collaborated with the GO consortium to extend the GO to accommodate terms for describing gene products associated with microbe-host interactions. Currently, over 900 terms that describe biological processes common to diverse plant- and animal-associated microbes are incorporated into the GO database. Here we review some unifying themes common to diverse host-microbe associations and illustrate how the new GO terms facilitate a standardized description of the gene products involved. We also highlight areas where new terms need to be developed, an ongoing process that should involve the whole community.

Creating Surface Properties Using a Palette of Hydrophobins

Small secreted proteins called hydrophobins play diverse roles in the life cycle of filamentous fungi. For example, the hydrophobin SC3 of Schizophyllum commune is involved in aerial hyphae formation, cell-wall assembly and attachment to hydrophobic surfaces. Hydrophobins are capable of self-assembly at a hydrophilic-hydrophobic interface, resulting in the formation of an amphipathic film. This amphipathic film can make hydrophobic surfaces of a liquid or a solid material wettable, while a hydrophilic surface can be turned into a hydrophobic one. These properties, among others, make hydrophobins of interest for medical and technical applications. For instance, hydrophobins can be used to purify proteins from complex mixtures; to reduce the friction of materials; to increase the biocompatibility of medical implants; to increase the solubility of water insoluble drugs; and to immobilize enzymes, for example, biosensor surfaces.

MoCRZ1, a gene encoding a calcineurin-responsive transcription factor, regulates fungal growth and pathogenicity of Magnaporthe oryzae

Ca(2+)-dependent signaling plays important roles in cellular development and metabolism in fungi. Pharmacological and molecular evidence clearly indicates that Ca(2+)-dependent signaling is required for infection-related development and pathogenicity in the rice blast fungus Magnaporthe oryzae. However, little information is available on downstream regulators in the Ca(2+)-dependent signaling pathway. To understand the role of a calcineurin-dependent transcription factor in the rice blast fungus, an ortholog of Saccharomyces cerevisiae CRZ1 in M. oryzae, MoCRZ1, was identified and functionally characterized. The Deltamocrz1 mutant exhibited impaired growth in the presence of Ca(2+) ions or cell wall perturbing agents. The Deltamocrz1 mutant also showed reduced conidiation and reduced pathogenicity, which is mainly due to a defect in host penetration. MoCRZ1 fused to EGFP was trans-localized into the nucleus in a Ca(2+)/calcineurin-dependent manner. The MoCRZ1 gene is also required for the calcineurin-dependent transcriptional induction of FKS1, a gene encoding a beta-1,3 glucan synthase, CHS2 and CHS4, genes encoding two chitin synthases, and PMC and PMR gene families encoding P-type ATPases in response to Ca(2+). These results suggest that MoCRZ1 is a downstream regulator in Ca(2+)-dependent signaling for pathogenicity in M. oryzae, and its biochemical mechanisms are well conserved among fungal species.

MoSNF1 regulates sporulation and pathogenicity in the rice blast fungus Magnaporthe oryzae

The protein kinase Snf1 is a major component of the glucose derepression pathway in yeast and a regulator of gene expression for the cell wall degrading enzyme (CWDE) in some plant pathogenic fungi. To address the molecular function of Snf1 in Magnaporthe oryzae, which causes the rice blast disease, MoSNF1 was cloned and functionally characterized using gene knock-out strategies. MoSNF1 functionally complemented the growth defect of the yeast snf1 mutant on a non-fermenting carbon source. However, the growth rate of the Deltamosnf1 mutant on various carbon sources was reduced independent of glucose, and the expression of the CWDE genes in the mutant was induced during derepressing condition like the wild type. The pre-penetration stage including conidial germination and appressorium formation of the Deltamosnf1 was largely impaired, and the pathogenicity of the Deltamosnf1 was significantly reduced. Most strikingly, the Deltamosnf1 mutant produced only a few conidia and had a high frequency of abnormally shaped conidia compared to the wild type. Our results suggest that MoSNF1 is a functional homolog of yeast Snf1, but its contribution to sporulation, vegetative growth and pathogenicity is critical in M. oryzae.

Cell cycle and cell death are not necessary for appressorium formation and plant infection in the fungal plant pathogen Colletotrichum gloeosporioides

Background

In order to initiate plant infection, fungal spores must germinate and penetrate into the host plant. Many fungal species differentiate specialized infection structures called appressoria on the host surface, which are essential for successful pathogenic development. In the model plant pathogen Magnaporthe grisea completion of mitosis and autophagy cell death of the spore are necessary for appressoria-mediated plant infection; blocking of mitosis prevents appressoria formation, and prevention of autophagy cell death results in non-functional appressoria.

Results

We found that in the closely related plant pathogen Colletotrichum gloeosporioides, blocking of the cell cycle did not prevent spore germination and appressoria formation. The cell cycle always lagged behind the morphogenetic changes that follow spore germination, including germ tube and appressorium formation, differentiation of the penetrating hypha, and in planta formation of primary hyphae. Nuclear division was arrested following appressorium formation and was resumed in mature appressoria after plant penetration. Unlike in M. grisea, blocking of mitosis had only a marginal effect on appressoria formation; development in hydroxyurea-treated spores continued only for a limited number of cell divisions, but normal numbers of fully developed mature appressoria were formed under conditions that support appressoria formation. Similar results were also observed in other Colletotrichum species. Spores, germ tubes, and appressoria retained intact nuclei and remained viable for several days post plant infection.

Conclusion

We showed that in C. gloeosporioides the differentiation of infection structures including appressoria precedes mitosis and can occur without nuclear division. This phenomenon was also found to be common in other Colletotrichum species. Spore cell death did not occur during plant infection and the fungus primary infection structures remained viable throughout the infection cycle. Our results show that the control of basic cellular processes such as those coupling cell cycle and morphogenesis during fungal infection can be substantially different between fungal species with similar lifestyles and pathogenic strategies.

A multidrug resistance transporter in Magnaporthe is required for host penetration and for survival during oxidative stress

In prokaryotes and eukaryotes, multidrug resistance (MDR) transporters use energy-dependent efflux action to regulate the intracellular levels of antibiotic or xenobiotic compounds. Using mutational analysis of ABC3, we define an important role for such MDR-based efflux during the host penetration step of Magnaporthe grisea pathogenesis. Mutants lacking ABC3 were completely nonpathogenic but were surprisingly capable of penetrating thin cellophane membranes to some extent. The inability of abc3Delta to penetrate the host surface was most likely a consequence of excessive buildup of peroxide and accumulation of an inhibitory metabolite(s) within the mutant appressoria. Treatment with antioxidants partially suppressed the host penetration defects in the abc3Delta mutant. abc3Delta was highly sensitive to oxidative stress and was unable to survive the host environment and invasive growth conditions. ABC3 transcript levels were redox-regulated, and on host surfaces, the activation of ABC3 occurred during initial stages of blast disease establishment. An Abc3-green fluorescent protein fusion localized to the plasma membrane in early appressoria (and in penetration hyphae) but became predominantly vacuolar during appressorial maturity. We propose that ABC3 function helps Magnaporthe to cope with cytotoxicity and oxidative stress within the appressoria during early stages of infection-related morphogenesis and likely imparts defense against certain antagonistic and xenobiotic conditions encountered during pathogenic development.

Mitogen activated protein kinases of Aspergillus fumigatus

In microbial eukaryotes, mitogen activated protein kinase (MAPK) pathways play a pivotal role in regulating cellular physiology. In fungi MAPK pathways have established functions in mating-pheromone responses, maintaining cell wall integrity, responding to changes in osmolarity and nutrient sensing. We have been studying MAPK functions in the human pathogenic fungus Aspergillus fumigatus. The genome of A. fumigatus has four MAPK genes, sakA/hogA, mpkA, mpkB and mpkC. Deletion of the sakA gene produces a strain that does not correctly regulate conidial germination, sense environmental nitrogen or responds to hypertonic stress. The function of the remaining MAPK genes is still under investigation, but by analogy to work in other filamentous fungi, we speculate as to their possible functions in A. fumigatus.

Identification of mature appressorium-enriched transcripts in Magnaporthe grisea, the rice blast fungus, using suppression subtractive hybridization

We have constructed a fungal subtractive suppressive library enriched in genes expressed during appressorium maturation in Magnaporthe grisea. Sequencing of 250 clones from the subtracted appressorium cDNA library revealed 142 unique genes, represented by 155 non-redundant ESTs (expressed sequence tags). Of these ESTs, 72 have not been previously isolated in M. grisea. RT-PCR analysis of 105 of the genes discovered found transcripts corresponding to 71 of the ESTs only in mature appressoria while transcripts corresponding to a further 34 of the isolated ESTs were expressed both in appressoria and conidia/mycelia. Genes specifically expressed in appressorium identified by SSH included a number that have been previously implicated in appressorium formation or function including GAS1, GAS3, and PTH11.

A mitogen-activated protein kinase that senses nitrogen regulates conidial germination and growth in Aspergillus fumigatus

We show that the mitogen-activated protein (MAP) kinase pathway that responds to osmotic stress in Aspergillus fumigatus is also involved in nutritional sensing. This MAP kinase regulates conidial germination in response to the nitrogen source and is activated upon starvation for either carbon or nitrogen during vegetative growth.

Regulation of aflatoxin synthesis by FadA/cAMP/protein kinase A signaling in Aspergillus parasiticus

Analysis of fadA and pkaA mutants in the filamentous fungus Aspergillus nidulans demonstrated that FadA (Galpha) stimulates cyclic AMP (cAMP)-dependent protein kinase A (PKA) activity resulting, at least in part, in inhibition of conidiation and sterigmatocystin (ST) biosynthesis. In contrast, cAMP added to the growth medium stimulates aflatoxin (AF) synthesis in Aspergillus parasiticus. Our goal was to explain these conflicting reports and to provide mechanistic detail on the role of FadA, cAMP, and PKA in regulation of AF synthesis and conidiation in A. parasiticus. cAMP or dibutyryl-cAMP (DcAMP) were added to a solid growth medium and intracellular cyclic nucleotide levels, PKA activity, and nor-1 promoter activity were measured in A. parasiticus D8D3 (nor1::GUS reporter) and TJYP1-22 (fadAGA2R, activated allele). Similar to Tice and Buchanan [34], cAMP or DcAMP stimulated AF synthesis (and conidiation) associated with an AflR-dependent increase in nor-1 promoter activity. However, treatment resulted in a 100-fold increase in intracellular cAMP/DcAMP accompanied by a 40 to 80 fold decrease in total PKA activity. ThefadAG42R allele in TJYP1-22 decreased AF synthesis and conidiation, increased basal PKA activity 10 fold, and decreased total PKA activity 2 fold. In TJYP1-22, intracellular cAMP increased 2 fold without cAMP or DcAMP treatment; treatment did not stimulate conidiation or AF synthesis. Based on these data, we conclude that: (1) FadA/PKA regulate toxin synthesis and conidiation via similar mechanisms in Aspergillus spp.; and (2) intracellular cAMP levels, at least in part, mediate a PKA-dependent regulatory influence on conidiation and AF synthesis.

PKA and MAPK phosphorylation of Prf1 allows promoter discrimination in Ustilago maydis

Mating in Ustilago maydis requires cross-talk between cAMP and mitogen-activated protein kinase (MAPK) signalling. During this process, pheromone response factor 1 (Prf1) activates transcription of a and b mating type genes by binding to pheromone response elements (PREs) located in regulatory regions of these genes. Here, we show that PREs are also necessary and sufficient to mediate cAMP-induced gene expression. Prf1 interacts with cAMP-dependent protein kinase A (PKA) Adr1 as well as MAPK Kpp2 in vivo, and its central phosphorylation sites that are functionally important are modified by the respective kinases in vitro. PKA sites in Prf1 are essential for induced expression of a and b mating type genes. In contrast, MAPK sites are not required for pheromone-induced expression of a genes but are crucial for pheromone-responsive b gene expression. This illustrates how a single transcription factor can integrate signals from two pathways and how its phosphorylation status can determine different transcriptional responses.

The tetraspanin BcPls1 is required for appressorium-mediated penetration of Botrytis cinerea into host plant leaves

Animal tetraspanins are membrane proteins controlling cell adhesion, morphology and motility. In fungi, the tetraspanin MgPls1 controls an appressorial function required for the penetration of Magnaporthe grisea into host plants. An orthologue of MgPLS1, BcPLS1, was identified in the necrotrophic fungal plant pathogen Botrytis cinerea. We constructed a Bcpls1::bar null mutant by targeted gene replacement. Bcpls1::bar is not pathogenic on intact plant tissues of bean, tomato or rose, but it infects wounded plant tissues. Both wild type and Bcpls1::bar differentiate appressoria on plant and artificial surfaces, a process involving an arrest of polarized growth, apex swelling and its cell wall reinforcement. Although wild-type appressoria allowed the penetration of the fungus into the host plant within 6-12 h, no successful penetration events were observed with Bcpls1::bar, suggesting that its appressoria are not functional. An eGFP transcriptional fusion showed that BcPLS1 was specifically expressed in conidia, germ tubes and appressoria during host penetration. Our results indicate that BcPLS1 is required for the penetration of B. cinerea into intact host plants. The defect in pathogenicity of Bcpls1::bar also demonstrates that functional B. cinerea appressoria are required for a successful penetration process. As Bcpls1::bar and Mgpls1 Delta::hph penetration defects are similar, fungal tetraspanins are likely to be required for an essential appressorial function widespread among fungi.

Serial Analysis of Gene Expression (SAGE) of Magnaporthe grisea: genes involved in appressorium formation

Treatment with cyclic AMP (cAMP) induces appressorium formation in the phytopathogenic fungus Magnaporthe grisea, the causative agent of rice blast disease. In a search for the M. grisea genes responsible for appressorium formation and host invasion, SAGE (Serial Analysis of Gene Expression) was carried out using mRNA isolated from fungal conidia germinating in the presence and absence of cAMP. From cAMP-treated conidia 5087 tags including 2889 unique tags were isolated, whereas untreated conidia yielded 2342 unique tags out of total of 3938. cAMP treatment resulted in up- and down-regulation of genes corresponding to 57 and 53 unique tags, respectively. Upon consultation of EST/cDNA databases, 22 tags with higher representation in cAMP-treated conidia were annotated with putative gene names. Furthermore, 28 tags corresponding to cAMP-induced genes could be annotated with the help of the recently published genome sequence of M. grisea. cAMP-induced genes identified by SAGE included many genes that have not been described so far, as well as a number of genes known to be involved in pathogenicity, e.g. MPG1, MAS1 and MAC1. RT-PCR of 13 randomly selected genes confirmed the SAGE results, verifying the fidelity of the SAGE data.

Large scale parallel analysis of gene expression during infection-related morphogenesis of Magnaporthe grisea

SUMMARY The rice blast fungus Magnaporthe grisea causes one of the most destructive diseases of rice. To initiate the infection of host tissues, conidia elaborate germ tubes that differentiate specialized infection structures called appressoria. Microarrays composed of 3500 cDNAs of M. grisea were prepared for the identification of genes that are specifically up- or down-regulated during appressorium formation. Gene expression in ungerminated conidia, during appressorium formation, and during mycelial growth was investigated with a novel highly sensitive dendrimer based detection system. Transcripts of 85 different genes were found to be more abundant in ungerminated conidia and/or in conidia with developing appressoria than in vegetative mycelia. Nineteen of these showed higher expression in both ungerminated conidia and developing appressoria than in mycelia, suggesting that their expression remains elevated during the early stage of fungal infection. The expression of 18 genes was higher in ungerminated conidia than in developing appressoria, indicating their possible role in the germination process or maintaining dormancy. Transcripts of 47 genes were found to be more abundant in developing appressoria than in ungerminated conidia, suggesting that their expression is induced during appressorium formation. Several of these genes, including a chitin binding protein and infection structure specific protein MIF23, were previously shown to be preferentially expressed during appressorium formation. However, the expression of many of these genes has not been reported prior to this analysis. In contrast, transcripts of 38 different genes were found to be more abundant in mycelia than in developing appressoria. A Northern blot analysis of selected genes was consistent with the microarray results. Results from this study provide a powerful resource for furthering our understanding of gene expression during infection-related morphogenesis and for the functional analysis of M. grisea genes involved in fungal infection.

Glycosphingolipids from Magnaporthe grisea cells: expression of a ceramide dihexoside presenting phytosphingosine as the long-chain base

Magnaporthe grisea is a fungal pathogen that infects rice leaves and causes rice blast, a devastating crop disease. M. grisea produces active elicitors of the hypersensitive response in rice that were previously identified as ceramide monohexosides (CMHs). Using several chromatographic approaches, mass spectrometry, and nuclear magnetic resonance, we identified ceramide mono- and dihexosides (CDH) in purified lipid extracts from M. grisea cells. As described by other authors, CMH consists of a ceramide moiety containing 9-methyl-4,8-sphingadienine in amidic linkage to 2-hydroxyoctadecenoic or 2-hydroxyhexadecenoic acids and a carbohydrate segment consisting of one residue of glucose. CDHs, however, contain beta-galactose (1-->4)-linked to beta-glucose as sugar units and phytosphingosine as the long-chain base, bound to a C24 alpha-hydroxylated fatty acid. To our knowledge, this is the first report on the occurrence of CDH in a fungal species and illustrates the existence of an alternative path of ceramide glycosylation in fungal cells.

Transcript profiling in the barley mildew pathogen Blumeria graminis by serial analysis of gene expression (SAGE)

The fungal pathogen Blumeria graminis f. sp. hordei develops on the barley leaf via distinct, morphologically well-defined stages. After landing on a host plant, the conidia rapidly germinate to form a primary germ tube. Subsequently, an appressorial germ tube emerges from the conidium and differentiates an appressorium from which penetration of the host cell wall is attempted. We have used serial analysis of gene expression to provide a measurement of messenger RNA contents in ungerminated conidia, during conidial germination, and during appressorium formation. The resulting data provide a resource for the characterization of changes in transcript accumulation during early development of B. graminis.

A novel gene, CBP1, encoding a putative extracellular chitin-binding protein, may play an important role in the hydrophobic surface sensing of Magnaporthe grisea during appressorium differentiation

The conidial germ tube of the rice blast fungus, Magnaporthe grisea, differentiates a specialized cell, an appressorium, required for penetration into the host plant. Formation of the appressorium is also observed on artificial solid substrata such as polycarbonate. A novel emerging germ tube-specific gene, CBP1 (chitin-binding protein), was found in a cDNA subtractive differential library. CBP1 coded for a putative extracellular protein (signal peptide) with two similar chitin-binding domains at both ends of a central domain with homology to fungal chitin deacetylases and with a C-terminus domain rich in Ser/Thr related extracellular matrix protein such as agglutinin. The consensus sequence of the chitin-binding domain found in CBP1 has never been reported in fungi and is similar to the chitin-binding motif in plant lectins and plant chitinases classes I and IV. CBPI was disrupted in order to identify its function. Null mutants of CBP1 failed to differentiate appressoria normally on artificial surface but succeeded in normally differentiating appressoria on the plant leaf surface. Since the null mutant Cbp1- showed abnormal appressorium differentiation only on artificial surfaces and was sensitive to the chemical inducers, CBP1 seemed to play an important role in the recognition of physical factors on solid surfaces.

Time for a blast: genomics of Magnaporthe grisea

Summary The rice blast fungus Magnaporthe grisea causes one of the most destructive diseases of rice. Genetic studies of this important pathogen during the past decade have made it an excellent system for investigating fungal-plant interactions. Recently, the well coordinated efforts by the rice blast community have also made significant progresses in genomics studies of M. grisea. BAC contigs were assembled and integrated with the high density genetic map, and 12 674 BAC-ends were sequenced as genome survey sequences. Over ten BAC clones have been sequenced by the shot-gun approach. Preliminary analysis with one completely sequenced BAC clone indicated that M. grisea may have a gene density of 4.2 kb/gene and contain approximately 9000 genes. In addition, > 10 000 ESTs have been sequenced from several cDNA libraries representing different stages of fungal growth, differentiation and plant infection. Currently, there are ongoing projects to sequence additional ESTs, telomeres, and 6X coverage of the M. grisea genome. A large-scale functional genomics project on M. grisea and its interaction with rice is also underway. It is an exciting time for genomics studies in M. grisea, and the resources generated in these studies will certainly enhance our understanding of fungal pathogenicity.

NRG1 represses yeast-hypha morphogenesis and hypha-specific gene expression in Candida albicans

We have characterized CaNrg1 from Candida albicans, the major fungal pathogen in humans. CaNrg1 contains a zinc finger domain that is conserved in transcriptional regulators from fungi to humans. It is most closely related to ScNrg1, which represses transcription in a Tup1-dependent fashion in Saccharomyces cerevisiae. Inactivation of CaNrg1 in C.albicans causes filamentous and invasive growth, derepresses hypha-specific genes, increases sensitivity to some stresses and attenuates virulence. A tup1 mutant displays similar phenotypes. However, unlike tup1 cells, nrg1 cells can form normal hyphae, generate chlamydospores at normal rates and grow at 42 degrees C. Transcript profiling of 2002 C.albicans genes reveals that CaNrg1 represses a subset of CaTup1-regulated genes, which includes known hypha-specific genes and other virulence factors. Most of these genes contain an Nrg1 response element (NRE) in their promoter. CaNrg1 interacts specifically with an NRE in vitro. Also, deletion of two NREs from the ALS8 promoter releases it from Nrg1-mediated repression. Hence, CaNrg1 is a transcriptional repressor that appears to target CaTup1 to a distinct set of virulence-related functions, including yeast-hypha morphogenesis.

PLS1, a gene encoding a tetraspanin-like protein, is required for penetration of rice leaf by the fungal pathogen Magnaporthe grisea

We describe in this study punchless, a nonpathogenic mutant from the rice blast fungus M. grisea, obtained by plasmid-mediated insertional mutagenesis. As do most fungal plant pathogens, M. grisea differentiates an infection structure specialized for host penetration called the appressorium. We show that punchless differentiates appressoria that fail to breach either the leaf epidermis or artificial membranes such as cellophane. Cytological analysis of punchless appressoria shows that they have a cellular structure, turgor, and glycogen content similar to those of wild type before penetration, but that they are unable to differentiate penetration pegs. The inactivated gene, PLS1, encodes a putative integral membrane protein of 225 aa (Pls1p). A functional Pls1p-green fluorescent protein fusion protein was detected only in appressoria and was localized in plasma membranes and vacuoles. Pls1p is structurally related to the tetraspanin family. In animals, these proteins are components of membrane signaling complexes controlling cell differentiation, motility, and adhesion. We conclude that PLS1 controls an appressorial function essential for the penetration of the fungus into host leaves.

Expression and sequence analysis of the Blumeria graminis mitogen-activated protein kinase genes, mpk1 and mpk2

Mitogen-activated protein (MAP) kinases represent a group of serine/threonine kinases which play a pivotal role in signal transduction processes in eukaryotic cells. Using degenerate PCR primer design based on published and aligned MAP kinase sequences we have cloned and characterised two MAP kinase genes from the barley powdery mildew fungus, Blumeria graminis. We have utilised 'step down' PCR to attain the full length mildew genomic clones. The single-copy genes, named mpk1 and mpk2, encode putative proteins of 356 and 410 amino acids and carry three and four introns, respectively. Expression studies, using RT-PCR, reveal a differing pattern of tissue gene expression with mpk1 and mpk2 during germling morphogenesis and this is compared with the constitutive expression of the 'control' beta-tubulin gene.

Surface attachment and pre-penetration stage development by plant pathogenic fungi

Fungal pathogens cause many of the most serious crop diseases. One of the principal reasons for the success of this group is their ability to locate and perceive appropriate host surfaces and then to elaborate specialized infection structures. Here we review the processes implicated in surface attachment, germ tube elongation, and development of appressoria. The involvement of surface-acting proteins such as fungal hydrophobins and integrins in these processes is evaluated, along with a description of studies that have revealed the existence of conserved signaling pathways that regulate appressorium formation. Finally, we anticipate the prospect of genome-level analysis of fungal pathogens and the key research questions that will need to be addressed.

Signal transduction cascades regulating fungal development and virulence

Cellular differentiation, mating, and filamentous growth are regulated in many fungi by environmental and nutritional signals. For example, in response to nitrogen limitation, diploid cells of the yeast Saccharomyces cerevisiae undergo a dimorphic transition to filamentous growth referred to as pseudohyphal differentiation. Yeast filamentous growth is regulated, in part, by two conserved signal transduction cascades: a mitogen-activated protein kinase cascade and a G-protein regulated cyclic AMP signaling pathway. Related signaling cascades play an analogous role in regulating mating and virulence in the plant fungal pathogen Ustilago maydis and the human fungal pathogens Cryptococcus neoformans and Candida albicans. We review here studies on the signaling cascades that regulate development of these and other fungi. This analysis illustrates both how the model yeast S. cerevisiae can serve as a paradigm for signaling in other organisms and also how studies in other fungi provide insights into conserved signaling pathways that operate in many divergent organisms.

Threshold level of protein kinase A activity and polarized growth in Mucor rouxii

A model system to study the involvement of cAMP-mediated regulation of a cellular process such as hyphal morphogenesis was investigated. Impairment of polarized growth was observed when Mucor rouxii sporangiospores were grown in the presence of N(6)-cAMP analogues and of SQ 65,442, a cAMP phosphodiesterase inhibitor. Together with an effect on isodiametric growth, there was increased pigmentation, increased cell fragility and loss of cell adhesiveness. The total effect on morphology was attained even after adding the compounds shortly before germ-tube emergence; when added after this time growth continued in a non-polarized form and rounding of the germ tip was observed. The morphological effect was observed under all the nutritional and environmental conditions studied (aerobic conditions and defined medium with maltose or glucose, Casamino acids medium with glucose, or rich medium; anaerobic conditions with rich medium; and following a shift from anaerobiosis to aerobiosis). The time of germ-tube emergence, and the size of the cell at this time, was dependent on the growth medium. Protein kinase A (PKA) specific activity was followed during the germination process under three growth conditions. It was found that the total activity of PKA correlated with differentiation and not with growth, and that the total specific activity at the time of germination was the same, independent of the culture medium. The time of germ-tube emergence correlated with the time of attainment of a threshold level of PKA total specific activity. The concentration of dibutyryl-cAMP needed to promote isodiametric growth correlated with the total units of activity of PKA to be activated per cell. It was concluded that PKA is involved in the morphogenetic process of the fungus grown under all the nutritional and ambient conditions tested.