MPG1, a gene encoding a fungal hydrophobin of Magnaporthe grisea, is involved in surface recognition

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.

Safe-Harbor-Targeted CRISPR/Cas9 System and Cmhyd1 Overexpression Enhances Disease Resistance in Cordyceps militaris

Fungal disease of mushroomCordyceps militaris (CM) caused byCalcarisporium cordycipiticola (CC) is destructive to fruiting body cultivation, resulting in significant economic loss and potential food safety risks. CRISPR/Cas9 genome editing has proven to be a powerful tool for crop improvement but seldom succeeded in mushrooms. Here, the first genomic safe-harbor site, CmSH1 locus, was identified in the CM genome. A safe-harbor-targeted CRISPR/Cas9 system based on an autonomously replicating plasmid was designed to facilitate alien gene integration at the CmSH1 locus. Cmhyd1, one of the hydrophobin genes, was confirmed as a defensive factor against CC infection, and Cmhyd1 overexpression by this system showed enhancement of disease resistance with negligible effect on the agronomic traits of CM. No off-target events and residues of plasmid sequence were tested by PCR and genome resequencing. This study provided the first safe harbor site for genetic manipulations, a safe harbor-targeted CRISPR/Cas9 system, and the first disease-resistant gene-editing breeding system in mushrooms.

MoRts1, a regulatory subunit of PP2A, is required for fungal development and pathogenicity of Magnaporthe oryzae

Protein phosphatase 2 A (PP2A) is a major heterotrimeric serine/threonine protein phosphatase comprised of three subunits, including structural subunits (A), regulatory subunits (B), and catalytic subunits (C). PP2A has been widely shown to involve in a series of cell signal transduction processes such as cell metabolism, cell cycle regulation, DNA replication, gene transcription and protein translation in yeast and mammalian. However, the roles of PP2A in pathogenic fungi Magnaporthe oryzae still remain unclear. We here found that MoRts1, a gene encoding B regulatory subunit of PP2A homologous to Saccharomyces cerevisiae Rts1, showed up-regulated transcription during conidia and initially infectious stage. Subcellular localization revealed that MoRts1-eGFP was localized to the cytoplasm and septum. Targeted disruption of MoRts1 leads to a reduction of mycelial growth and sporulation, as well as the defects of hydrophobicity, melanin pigmentation and cell wall integrity (CWI). The MoRts1 mutants were less pathogenic to the host plants, compared to the Ku80 strain, and the transcriptional levels of several pathogenicity-related Rho GTPase genes, including MoCdc42, MoRho2, MoRho3, MoRho4, MoRhoX and MoRac1, were significantly decreased in the MoRts1 mutants. Besides, two splicing variants of MoRts1 with unique functions of regulating the growth and pathogenicity were identified, and the B56 domain is vital for determining the sporulation and pathogenicity of M. oryzae. Furthermore, MoRts1 was identified to interact with PP2A catalytic subunit MoPPG1 in vivo in M. oryzae. In summary, our results showed that MoRts1 is an important regulator contributing to the fungal development, and pathogenicity of M. oryzae.

The COPII subunit MoSec24B is involved in development, pathogenicity and autophagy in the rice blast fungus

The endoplasmic reticulum (ER) acts as the starting point of the secretory pathway, where approximately one-third of the proteins are correctly folded and modified, loaded into vesicles, and transported to the Golgi for further processing and modification. In this process, COPII vesicles are responsible for transporting cargo proteins from the ER to the Golgi. Here, we identified the inner shell subunit of COPII vesicles (MoSec24B) and explored the importance of MoSec24B in the rice blast fungus. The targeted disruption of MoSec24B led to decreased growth, reduced conidiation, restricted glycogen and lipids utilization, sensitivity to the cell wall and hypertonic stress, the failure of septin-mediated repolarization of appressorium, impaired appressorium turgor pressure, and decreased ability to infect, which resulted in reduced pathogenicity to the host plant. Furthermore, MoSec24B functions in the three mitogen-activated protein kinase (MAPK) signaling pathways by acting with MoMst50. Deletion of MoSec24B caused reduced lipidation of MoAtg8, accelerated degradation of exogenously introduced GFP-MoAtg8, and increased lipidation of MoAtg8 upon treatment with a late inhibitor of autophagy (BafA1), suggesting that MoSec24B regulates the fusion of late autophagosomes with vacuoles. Together, these results suggest that MoSec24B exerts a significant role in fungal development, the pathogenesis of filamentous fungi and autophagy.

The Hydrophobin Gene Family Confers a Fitness Trade-off between Spore Dispersal and Host Colonization in Penicillium expansum

Hydrophobins are small amphipathic surface proteins found exclusively in fungi. In filamentous ascomycetes, one conserved role of a subset of hydrophobins is their requirement for spore dispersal. Other contributions of these proteins to fungal biology are less clear and vary across genera. To determine the functions of hydrophobins in the biology and virulence of this fungus, we created seven single mutants and a septuple-deletion mutant (Δsep) of the entire putative P. expansum hydrophobin gene family. One spore hydrophobin, HfbA, shared 72.56% sequence identity to the Aspergillus fumigatus spore hydrophobin RodA and was required for efficient spore dispersion in P. expansum. The Δsep mutant was likewise reduced in spore dispersal, hypothesized to be due to the aberrant shape and clumping of the Δsep conidia and conidiophores. Additionally, the Δsep mutant presented several differences in physiological traits, including decreased survival in extreme cold temperatures and increased production of several toxic secondary metabolites. Most striking was the unexpected fitness advantage that the Δsep strain displayed in competitive passaging with the wild-type strain on host apple where the mutant significantly increased in percentage of the colonizing population. This work uncovers potential ecological trade-offs of hydrophobin presence in filamentous fungi. IMPORTANCE Hydrophobins are amphipathic secreted proteins uniquely found in filamentous fungi. These proteins self-assemble and constitute the outer most layer of fungal surfaces thus mediating multiple aspects of fungal interactions with their environments. Hydrophobins facilitate spore dispersal, yet a full understanding of the function and need for multiple hydrophobins in fungal species remains elusive. To address the role of this protein family in Penicillium expansum, the causative agent of blue mold disease in pome fruit, all seven putative hydrophobin genes were deleted and the mutant assessed for numerous physiological traits and virulence on fruit. Despite showing a decrease in spore dispersal, the septuple-deletion mutant was more fit than the wild type in competitive pathogenicity tests on apple. Our findings suggest this gene family illustrates a functional trade-off between dispersal and host colonization in P. expansum.

The G protein subunit α1, CaGα1, mediates ethylene sensing of mango anthracnose pathogen Colletotrichum asianum to regulate fungal development and virulence and mediates surface sensing for spore germination

Mango is an important tropic fruit, but its production is highly restricted by anthracnose diseases. Mango anthracnose development is related to the fruit-ripening hormone ethylene, but how the pathogen senses ethylene and affects the infection remains largely unknown. In this study, mango pathogen Colletotrichum asianum strain TYC-2 was shown to sense ethylene to enhance spore germination, appressorium formation and virulence. Upon further analysis of ethylene sensing signaling, three histidine kinase genes (CaHKs) and a G-protein gene (CaGα1) were functionally characterized. Ethylene upregulated the expression of the three CaHKs but had no influence on CaGα1 expression. No function in ethylene sensing was identified for the three CaHKs. Ethylene enhanced spore germination and multiple appressorium formation of the wild-type TYC-2 but not CaGα1 mutants. TYC-2 has extremely low germination in water, where self-inhibition may play a role in ethylene sensing via CaGα1 signaling. Self-inhibitors extracted from TYC-2 inhibited spore germination of TYC-2 and CaGα1 mutants, but ethylene could not rescue the inhibition, indicating that the self-inhibition was not mediated by CaGα1 and had no interactions with ethylene. Interestingly, spore germination of CaGα1 mutants was significantly enhanced in water on hydrophobic but not hydrophilic surfaces, suggesting that CaGα1 is involved in surface sensing. In the pathogenicity assay, CaGα1 mutants showed less virulence with delayed germination and little appressorium formation at early infection on mango leaves and fruit. Transcriptome and qRT-PCR analyses identified several pathogenicity-related genes regulated by ethylene, indicating that ethylene may regulate TYC-2 virulence partially by regulating the expression of these genes.

MoDHX35, a DEAH-Box Protein, Is Required for Appressoria Formation and Full Virulence of the Rice Blast Fungus, Magnaporthe oryzae

The DExD/H-box protein family encompasses a large number of RNA helicases that are involved in RNA metabolism and a variety of physiological functions in different species. However, there is limited knowledge of whether DExD/H-box proteins play a role in the pathogenicity of plant fungal pathogens. In the present work, the DExD/H-box protein MoDHX35, which belongs to the DEAH subfamily, was shown to be crucial in appressoria formation and full virulence of the rice blast fungus, Magnaporthe oryzae. The predicted protein sequence of MoDHX35 had typical DEAH-box domains, showed 47% identity to DHX35 in Homo species, but had no orthologs in Saccharomyces cerevisiae. Deletion of the MoDHX35 gene resulted in reduced tolerance of the mutants to doxorubicin, a nucleic acid synthesis disturbing agent, suggesting the involvement of MoDHX35 in RNA metabolism. MoDHX35-deleted mutants exhibited normal vegetative growth, conidia generation and conidial germination, but showed a reduced appressorium formation rate and attenuated virulence. Our work demonstrates the involvement of DEAH-box protein functions in the pathogenicity of plant fungal pathogens.

Characterization of two infection-induced transcription factors of Magnaporthe oryzae reveals their roles in regulating early infection and effector expression

The initial stage of rice blast fungus, Magnaporthe oryzae, infection, before 36 h postinoculation, is a critical timespan for deploying pathogen effectors to overcome the host's defences and ultimately cause the disease. However, how this process is regulated at the transcription level remains largely unknown. This study functionally characterized two M. oryzae Early Infection-induced Transcription Factor genes (MOEITF1 and MOEITF2) and analysed their roles in this process. Target gene deletion and mutant phenotype analysis showed that the mutants Δmoeitf1 and Δmoeitf2 were only defective for infection growth but not for vegetative growth, asexual/sexual sporulation, conidial germination, and appressoria formation. Gene expression analysis of 30 putative effectors revealed that most effector genes were down-regulated in mutants, implying a potential regulation by the transcription factors. Artificial overexpression of two severely down-regulated effectors, T1REP and T2REP, in the mutants partially restored the pathogenicity of Δmoeitf1 and Δmoeitf2, respectively, indicating that these are directly regulated. Yeast one-hybrid assay and electrophoretic mobility shift assay indicated that Moeitf1 specifically bound the T1REP promoter and Moeitf2 specifically bound the T2REP promoter. Both T1REP and T2REP were predicted to be secreted during infection, and the mutants of T2REP were severely reduced in pathogenicity. Our results indicate crucial roles for the fungal-specific Moeitf1 and Moeitf2 transcription factors in regulating an essential step in M. oryzae early establishment after penetrating rice epidermal cells, highlighting these as possible targets for disease control.

Adenylsuccinate Synthetase MoADE12 Plays Important Roles in the Development and Pathogenicity of the Rice Blast Fungus

Purines are basic components of nucleotides in living organisms. In this study, we identified the ortholog of adenylosuccinate synthase MoADE12 in Magnaporthe oryzae by screening for growth-defective T-DNA insertional mutants. Gene replacement was performed to investigate the biological role of MoADE12. Δmoade12 mutants were adenine auxotrophs that failed to produce conidia, and showed reduced perithecia formation and pathogenicity. Moreover, the Δmoade12 mutant was hypersensitive to Congo red and oxidants, indicating that MoADE12 was required for cell wall integrity and oxidative stress resistance. Transcriptomic analysis identified the underlying mechanisms and indicated that several pathogenicity-related genes were regulated in the Δmoade12 mutant. Therefore, our data suggest that the adenylosuccinate synthase MoADE12 is involved in the de novo AMP biosynthesis pathway and is important for conidiation and pathogenicity in the rice blast fungus.

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.

Research on the Molecular Interaction Mechanism between Plants and Pathogenic Fungi

Plant diseases caused by fungi are one of the major threats to global food security and understanding the interactions between fungi and plants is of great significance for plant disease control. The interaction between pathogenic fungi and plants is a complex process. From the perspective of pathogenic fungi, pathogenic fungi are involved in the regulation of pathogenicity by surface signal recognition proteins, MAPK signaling pathways, transcription factors, and pathogenic factors in the process of infecting plants. From the perspective of plant immunity, the signal pathway of immune response, the signal transduction pathway that induces plant immunity, and the function of plant cytoskeleton are the keys to studying plant resistance. In this review, we summarize the current research progress of fungi–plant interactions from multiple aspects and discuss the prospects and challenges of phytopathogenic fungi and their host interactions.

MoOpy2 is essential for fungal development, pathogenicity, and autophagy in Magnaporthe oryzae

The development and pathogenicity of the fungus Magnaporthe oryzae, the causal agent of destructive rice blast disease, require it to perceive external environmental signals. Opy2, an overproduction-induced pheromone-resistant protein 2, is a crucial protein for sensing external signals in Saccharomyces cerevisiae. However, the biological functions of the homolog of Opy2 in M. oryzae are unclear. In this study, we identified that MoOPY2 is involved in fungal development, pathogenicity, and autophagy in M. oryzae. Deletion of MoOPY2 resulted in pleiotropic defects in hyphal growth, conidiation, germ tube extension, appressorium formation, appressorium turgor generation, and invasive growth, therefore leading to attenuated pathogenicity. Furthermore, MoOpy2 participates in the Osm1 MAPK pathway and the Mps1 MAPK pathway by interacting with the adaptor protein Mst50. The interaction sites of Mst50 and MoOpy2 colocalized with the autophagic marker protein MoAtg8 in the preautophagosomal structure sites (PAS). Notably, the ΔMoopy2 mutant caused cumulative MoAtg8 lipidation and rapid GFP-MoAtg8 degradation in response to nitrogen starvation, showing that MoOpy2 is involved in the negative regulation of autophagy activity. Taken together, our study revealed that MoOpy2 of M. oryzae plays an essential role in the orchestration of fungal development, appressorium penetration, autophagy and pathogenesis. This article is protected by copyright. All rights reserved.

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.

Hydrophobin CmHYD1 Is Involved in Conidiation, Infection and Primordium Formation, and Regulated by GATA Transcription Factor CmAreA in Edible Fungus, Cordyceps militaris

Hydrophobins are a family of small proteins exclusively secreted by fungi, and play a variety of roles in the life cycle. Cmhyd1, one of the hydrophobin class II members in Cordyceps militaris, has been shown to have a high transcript level during fruiting body development. Here, deletion of Cmhyd1 results in reduction in aerial mycelia, conidiation, hydrophobicity and infection ability, and complete inhibition of pigmentation and primordium differentiation. Cmhyd1 plays roles in conidiation and cuticle-bypassing infection by regulating the transcripts of frequency clock protein, Cmfrq, and velvet protein, Cmvosa, as well as primordium formation via the mitogen-activated protein kinase signaling pathway. Cmhyd1 also participates in stress response, including tolerance of mycelia to osmotic and oxidative stresses, and conidia to high or low temperatures. CmAreA, a transcription factor of nitrogen regulatory, is recruited to the promoter of Cmhyd1 and activates the transcription of Cmhyd1 with coactivator CmOTam using electrophoretic mobility shift assays and transient luciferase expression in tobacco. Furthermore, CmHYD1 is proved to regulate the transcription of Cmarea at different developmental stages via a positive feedback loop. These results reveal the diverse roles and regulation of Cmhyd1 in C. militaris, and provide insights into the developmental regulatory mechanism of mushrooms.

The Redox Proteome of Thiol Proteins in the Rice Blast Fungus Magnaporthe oryzae

Redox modification, a post-translational modification, has been demonstrated to be significant for many physiological pathways and biological processes in both eukaryotes and prokaryotes. However, little is known about the global profile of protein redox modification in fungi. To explore the roles of redox modification in the plant pathogenic fungi, a global thiol proteome survey was performed in the model fungal pathogen Magnaporthe oryzae. A total of 3713 redox modification sites from 1899 proteins were identified through a mix sample containing mycelia with or without oxidative stress, conidia, appressoria, and invasive hyphae of M. oryzae. The identified thiol-modified proteins were performed with protein domain, subcellular localization, functional classification, metabolic pathways, and protein–protein interaction network analyses, indicating that redox modification is associated with a wide range of biological and cellular functions. These results suggested that redox modification plays important roles in fungal growth, conidium formation, appressorium formation, as well as invasive growth. Interestingly, a large number of pathogenesis-related proteins were redox modification targets, suggesting the significant roles of redox modification in pathogenicity of M. oryzae. This work provides a global insight into the redox proteome of the pathogenic fungi, which built a groundwork and valuable resource for future studies of redox modification in fungi.

Characterisation and comparative analysis of hydrophobin isolated from Pleurotus floridanus (PfH)

Hydrophobins are a class of small cysteine rich surface active proteins produced exclusively by filamentous fungi. It forms a nano layer in the cell-water interface, thereby protecting the emerging fungal hyphae from surrounding water. Even though hydrophobins have similar functions in fungi, they share less sequence similarity. In the current study, we made a comparative study of the hydrophobin produced by the mushroom Pleurotus floridanus (PfH). Mushroom P. floridanus was cultured in PD broth. The hydrophobin was purified by foam fractionation and characterized in terms of molecular weight, solubility and glycosylation. In the RP-HPLC analysis, the hydrophobin eluted at a retention time of 45.56 min. The molecular weight of the PfH was found to be 13.52 kDa by MALDI-TOF MS and the LC-MS/MS showed no similar sequence in MASCOT database. The hydrophobin gene of P. floridanus was amplified using custom-designed primers and the BLAST analysis showed 80% sequence similarity with the Vmh2-1 gene of Pleurotus ostreatus. The sequence was translated into protein using ExPASy, secondary and tertiary structure predictions were carried out using Jpred4 and Phyre2. The tertiary structure showed 91.5% similarity with the HYD1 hydrophobin of Schizophyllum commune. A comparative study of PfH with Vmh2-1 and HYD1 was performed using bioinformatics tools. Hydrophobic cluster analysis revealed that three of these proteins have uniformity in terms of amphiphilic and non-amphiphilic α-helices, whereas PfH has a unique proline clustering. Physicochemical analysis by ProtParam revealed that PfH shares similar properties with HYD1 and Vmh2-1, which can be correlated with its function.

Evolutionary compromises in fungal fitness: hydrophobins can hinder the adverse dispersal of conidiospores and challenge their survival

Fungal evolutionary biology is impeded by the scarcity of fossils, irregular life cycles, immortality, and frequent asexual reproduction. Simple and diminutive bodies of fungi develop inside a substrate and have exceptional metabolic and ecological plasticity, which hinders species delimitation. However, the unique fungal traits can shed light on evolutionary forces that shape the environmental adaptations of these taxa. Higher filamentous fungi that disperse through aerial spores produce amphiphilic and highly surface-active proteins called hydrophobins (HFBs), which coat spores and mediate environmental interactions. We exploited a library of HFB-deficient mutants for two cryptic species of mycoparasitic and saprotrophic fungi from the genus Trichoderma (Hypocreales) and estimated fungal development, reproductive potential, and stress resistance. HFB4 and HFB10 were found to be relevant for Trichoderma fitness because they could impact the spore-mediated dispersal processes and control other fitness traits. An analysis in silico revealed purifying selection for all cases except for HFB4 from T. harzianum, which evolved under strong positive selection pressure. Interestingly, the deletion of the hfb4 gene in T. harzianum considerably increased its fitness-related traits. Conversely, the deletion of hfb4 in T. guizhouense led to the characteristic phenotypes associated with relatively low fitness. The net contribution of the hfb4 gene to fitness was found to result from evolutionary tradeoffs between individual traits. Our analysis of HFB-dependent fitness traits has provided an evolutionary snapshot of the selective pressures and speciation process in closely related fungal species.

MoLEU1, MoLEU2, and MoLEU4 regulated by MoLEU3 are involved in leucine biosynthesis, fungal development, and pathogenicity in Magnaporthe oryzae

Amino acids are vital components in cell metabolism. Leucine is a regulatory factor that generates significant impact on protein synthesis/turnover, modulates diverse cellular signalling pathways and participates in oxidative processes and immune responses. Here, we identified and characterized the functions of a leucine-associated Zn₂ Cys₆ -type transcription factor, MoLeu3. Disruption of MoLEU3 resulted in significantly reduced pathogenicity in barley and rice. Quantitative RT-PCR showed that the expression levels of the putative leucine biosynthesis-related genes, MoLEU1, MoLEU2 and MoLEU4 were downregulated in the ΔMoleu3 mutant. We used high-throughput gene knockout method to generate the null mutants of MoLEU1, MoLEU2 and MoLEU4 respectively. The ΔMoleu1, ΔMoleu2 and ΔMoleu4 mutants are leucine auxotroph and showed similar phenotypic characterizations, including reduced conidiation, delayed mobilization and degradation of glycogen and lipid droplets, limited appressorium-mediated penetration, and restricted invasive hyphae growth within host cells. Collectively, MoLEU1, MoLEU2, and MoLEU4 regulated by MoLEU3 play crucial roles in fungal development and infectious processes through modulation of leucine biosynthesis in Magnaporthe oryzae.

Magnaporthe oryzae CK2 Accumulates in Nuclei, Nucleoli, at Septal Pores and Forms a Large Ring Structure in Appressoria, and Is Involved in Rice Blast Pathogenesis

Magnaporthe oryzae (Mo) is a model pathogen causing rice blast resulting in yield and economic losses world-wide. CK2 is a constitutively active, serine/threonine kinase in eukaryotes, having a wide array of known substrates, and involved in many cellular processes. We investigated the localization and role of MoCK2 during growth and infection. BLAST search for MoCK2 components and targeted deletion of subunits was combined with protein-GFP fusions to investigate localization. We found one CKa and two CKb subunits of the CK2 holoenzyme. Deletion of the catalytic subunit CKa was not possible and might indicate that such deletions are lethal. The CKb subunits could be deleted but they were both necessary for normal growth and pathogenicity. Localization studies showed that the CK2 holoenzyme needed to be intact for normal localization at septal pores and at appressorium penetration pores. Nuclear localization of CKa was however not dependent on the intact CK2 holoenzyme. In appressoria, CK2 formed a large ring perpendicular to the penetration pore and the ring formation was dependent on the presence of all CK2 subunits. The effects on growth and pathogenicity of deletion of the b subunits combined with the localization indicate that CK2 can have important regulatory functions not only in the nucleus/nucleolus but also at fungal specific structures such as septa and appressorial pores.

Different Hydrophobins of Fusarium graminearum Are Involved in Hyphal Growth, Attachment, Water-Air Interface Penetration and Plant Infection

Hydrophobins (HPs) are small secreted fungal proteins possibly involved in several processes such as formation of fungal aerial structures, attachment to hydrophobic surfaces, interaction with the environment and protection against the host defense system. The genome of the necrotrophic plant pathogen Fusarium graminearum contains five genes encoding for HPs (FgHyd1-5). Single and triple FgHyd mutants were produced and characterized. A reduced growth was observed when the ΔFghyd2 and the three triple mutants including the deletion of FgHyd2 were grown in complete or minimal medium. Surprisingly, the growth of these mutants was similar to wild-type when grown under ionic, osmotic or oxidative stress conditions. All the mutant strains confirmed the ability to develop conidia and perithecia, suggesting that the FgHyds are not involved in normal development of asexual and sexual structures. A reduction in the ability of hyphae to penetrate through the water-air interface was observed for the single mutants ΔFghyd2 and ΔFghyd3 as well as for the triple mutants including the deletion of FgHyd2 and FgHyd3. Besides, ΔFghyd3 and the triple mutant ΔFghyd234 were also affected in the attachment to hydrophobic surface. Indeed, wheat infection experiments showed a reduction of symptomatic spikelets for ΔFghyd2 and ΔFghyd3 and the triple mutants only when spray inoculation was performed. This result could be ascribed to the affected ability of mutants deleted of FgHyd2 and FgHyd3 to penetrate through the water-air interface and to attach to hydrophobic surfaces such as the spike tissue. This hypothesis is strengthened by a histological analysis, performed by fluorescence microscopy, showing no defects in the morphology of infection structures produced by mutant strains. Interestingly, triple hydrophobin mutants were significantly more inhibited than wild-type by the treatment with a systemic triazole fungicide, while no defects at the cell wall level were observed.

Glycerol-3-Phosphate Shuttle Is Involved in Development and Virulence in the Rice Blast Fungus Pyricularia oryzae

The glycerol-3-phosphate (G-3-P) shuttle is an important pathway for delivery of cytosolic reducing equivalents into mitochondrial oxidative phosphorylation, and plays essential physiological roles in yeast, plants, and animals. However, its role has been unclear in filamentous and pathogenic fungi. Here, we characterize the function of the G-3-P shuttle in Pyricularia oryzae by genetic and molecular analyses. In P. oryzae, a glycerol-3-phosphate dehydrogenase 1 (PoGpd1) is involved in NO production, conidiation, and utilization of several carbon sources (pyruvate, sodium acetate, glutamate, and glutamine). A glycerol-3-phosphate dehydrogenase 2 (PoGpd2) is essential for glycerol utilization and fungal development. Deletion of PoGPD2 led to delayed aerial hyphal formation, accelerated aerial hyphal collapse, and reduced conidiation on complete medium (CM) under a light-dark cycle. Aerial mycelial surface hydrophobicity to water and Tween 20 was decreased in ΔPogpd2. Melanin synthesis genes required for cell wall construction and two transcription factor genes (COS1 and CONx2) required for conidiation and/or aerial hyphal differentiation were down-regulated in the aerial mycelia of ΔPogpd2 and ΔPogpd1. Culturing under continuous dark could complement the defects of aerial hyphal differentiation of ΔPogpd2 observed in a light-dark cycle. Two light-sensitive protein genes (PoSIR2 encoding an NAD⁺-dependent deacetylase and TRX2 encoding a thioredoxin 2) were up-regulated in ΔPogpd2 cultured on CM medium in a light-dark cycle. ΔPogpd2 showed an increased intracellular NAD⁺/NADH ratio and total NAD content, and alteration of intracellular ATP production. Culturing on minimal medium also could restore aerial hyphal differentiation of ΔPogpd2, which is deficient on CM medium in a light-dark cycle. Two glutamate synthesis genes, GDH1 and PoGLT1, which synthesize glutamate coupled with oxidation of NADH to NAD⁺, were significantly up-regulated in ΔPogpd2 in a light-dark cycle. Moreover, deletion of PoGpd1 or PoGpd2 led to reduced virulence of conidia or hyphae on rice. The glycerol-3-phosphate shuttle is involved in cellular redox, fungal development, and virulence in P. oryzae.

The Role of the Fungal Cell Wall in the Infection of Plants

The polysaccharide-rich wall, which envelopes the fungal cell, is pivotal to the maintenance of cellular integrity and for the protection of the cell from external aggressors - such as environmental fluxes and during host infection. This review considers the commonalities in the composition of the wall across the fungal kingdom, addresses how little is known about the assembly of the polysaccharide matrix, and considers changes in the wall of plant-pathogenic fungi during on and in planta growth, following the elucidation of infection structures requiring cell wall alterations. It highlights what is known about the phytopathogenic fungal wall and what needs to be discovered.

The Colletotrichum gloeosporioides RhoB regulates cAMP and stress response pathways and is required for pathogenesis

Rho GTPases regulate morphology and multiple cellular functions such as asexual development, polarity establishment, and differentiation in fungi. To determine the roles of CgRhoB, a Rho GTPase protein, here we characterized CgRhoB in the poplar anthracnose fungus Colletotrichum gloeosporioides. First of all, we determined that conidial germination was inhibited and intracellular cyclic AMP (cAMP) level was increased in the CgRhoB deletion mutants. Loss of CgRhoB resulted in shorter germ tubes and enhanced appressoria formation after germination on the hydrophobic surface. Exogenous addition of cAMP to the wild type generated the similar phenotypes of ΔCgRhoB inoculated in CM liquid. Furthermore, deletion of CgRhoB had discernible effect upon the sensitivity of C. gloeosporioides to cell wall perturbing agents and altered the distribution of chitin on the cell wall. H₂O₂ sensitivity assay showed the hypersensitive effect on the oxidative stress, and transcriptional analysis revealed that transcription of genes involved in peroxidase activities was altered in the mutants. Finally, virulence assay revealed that CgRhoB was required for pathogenicity. Taken together, our results showed that CgRhoB was associated with appressoria formation and pathogenicity, and affected cAMP level and stress pathways.

Chitosan Mediates Germling Adhesion in Magnaporthe oryzae and Is Required for Surface Sensing and Germling Morphogenesis

The fungal cell wall not only plays a critical role in maintaining cellular integrity, but also forms the interface between fungi and their environment. The composition of the cell wall can therefore influence the interactions of fungi with their physical and biological environments. Chitin, one of the main polysaccharide components of the wall, can be chemically modified by deacetylation. This reaction is catalyzed by a family of enzymes known as chitin deacetylases (CDAs), and results in the formation of chitosan, a polymer of β1,4-glucosamine. Chitosan has previously been shown to accumulate in the cell wall of infection structures in phytopathogenic fungi. Here, it has long been hypothesized to act as a 'stealth' molecule, necessary for full pathogenesis. In this study, we used the crop pathogen and model organism Magnaporthe oryzae to test this hypothesis. We first confirmed that chitosan localizes to the germ tube and appressorium, then deleted CDA genes on the basis of their elevated transcript levels during appressorium differentiation. Germlings of the deletion strains showed loss of chitin deacetylation, and were compromised in their ability to adhere and form appressoria on artificial hydrophobic surfaces. Surprisingly, the addition of exogenous chitosan fully restored germling adhesion and appressorium development. Despite the lack of appressorium development on artificial surfaces, pathogenicity was unaffected in the mutant strains. Further analyses demonstrated that cuticular waxes are sufficient to over-ride the requirement for chitosan during appressorium development on the plant surface. Thus, chitosan does not have a role as a 'stealth' molecule, but instead mediates the adhesion of germlings to surfaces, thereby allowing the perception of the physical stimuli necessary to promote appressorium development. This study thus reveals a novel role for chitosan in phytopathogenic fungi, and gives further insight into the mechanisms governing appressorium development in M.oryzae.

Magnaporthe oryzae is a filamentous fungal pathogen which causes devastating crop losses in rice. Successful invasion of the host is dependent upon the ability of the fungus to remain undetected by the innate immune system of the plant, which recognizes conserved components of the fungal cell wall, such as chitin. Previous studies have demonstrated that infection-related changes in cell wall composition are necessary to allow the fungus to remain undetected during infection. One such change that has long been hypothesized to have a role as a 'stealth mechanism' is the deacetylation of the polysaccharide chitin by enzymes known as chitin deacetylases. The deacetylation of chitin produces a polysaccharide known as chitosan, which has previously been shown to accumulate specifically on infection structures in plant pathogenic fungi. However, in this study, we show that germling-localized chitosan is not required for pathogenicity, arguing against a role as a 'stealth mechanism' at this stage. Instead, chitosan is required for the development of the appressorium, a critical fungal infection structure required for the penetration of plant cells. This requirement can be attributed to chitosan mediating the adhesion of germlings to surfaces, which is required for the perception of physical stimuli.

Differences between appressoria formed by germ tubes and appressorium-like structures developed by hyphal tips in Magnaporthe oryzae

Melanized appressoria are highly specialized infection structures formed by germ tubes of the rice blast fungus Magnaporthe oryzae for plant infection. M. oryzae also forms appressorium-like structures on hyphal tips. Whereas appressorium formation by conidial germ tubes has been well characterized, formation of appressorium-like structures by hyphal tips is under-investigated. In a previous study, we found that the chs7 deletion mutant failed to form appressoria on germ tubes but were normal in the development of appressorium-like structures on artificial hydrophobic surfaces. In this study, we compared the differences between the formation of appressoria by germ tubes and appressorium-like structures by hyphal tips in M. oryzae. Structurally, both appressoria and appressorium-like structures had a melanin layer that was absent in the pore region. In general, the latters were 1.4-fold larger in size but had lower turgor pressure than appressoria, which is consistent with its lower efficiency in plant penetration. Treatments with cAMP, IBMX, or a cutin monomer efficiently induced appressorium formation but not the development of appressorium-like structures. In contrast, coating surfaces with waxes stimulated the formation of both infection structures. Studies with various signaling mutants indicate that Osm1 and Mps1 are dispensable but Pmk1 is essential for both appressorium formation and development of appressorium-like structures on hyphal tips. Interestingly, the cpkA mutant was reduced in the differentiation of appressorium-like structures but not appressorium formation. We also observed that the con7 mutant generated in our lab failed to form appressorium-like structures on hyphal tips but still produced appressoria by germ tubes on hydrophobic surfaces. Con7 is a transcription factor regulating the expression of CHS7. Overall, these results indicate that the development of appressorium-like structures by hyphal tips and formation of appressoria by germ tubes are not identical differentiation processes in M. oryzae and may involve different molecular mechanisms.

Protein kinase A and fungal virulence: a sinister side to a conserved nutrient sensing pathway

Diverse fungal species are the cause of devastating agricultural and human diseases. As successful pathogenesis is dependent upon the ability of the fungus to adapt to the nutritional and chemical environment of the host, the understanding of signaling pathways required for such adaptation will provide insights into the virulence of these pathogens and the potential identification of novel targets for antifungal intervention. The cAMP-PKA signaling pathway is well conserved across eukaryotes. In the nonpathogenic yeast, S. cerevisiae, PKA is activated in response to extracellular nutrients and subsequently regulates metabolism and growth. Importantly, this pathway is also a regulator of pathogenesis, as defects in PKA signaling lead to an attenuation of virulence in diverse plant and human pathogenic fungi. This review will compare and contrast PKA signaling in S. cerevisiae vs. various pathogenic species and provide a framework for the role of this pathway in regulating fungal virulence.

A P-loop mutation in Gα subunits prevents transition to the active state: implications for G-protein signaling in fungal pathogenesis

Heterotrimeric G-proteins are molecular switches integral to a panoply of different physiological responses that many organisms make to environmental cues. The switch from inactive to active Gαβγ heterotrimer relies on nucleotide cycling by the Gα subunit: exchange of GTP for GDP activates Gα, whereas its intrinsic enzymatic activity catalyzes GTP hydrolysis to GDP and inorganic phosphate, thereby reverting Gα to its inactive state. In several genetic studies of filamentous fungi, such as the rice blast fungus Magnaporthe oryzae, a G42R mutation in the phosphate-binding loop of Gα subunits is assumed to be GTPase-deficient and thus constitutively active. Here, we demonstrate that Gα(G42R) mutants are not GTPase deficient, but rather incapable of achieving the activated conformation. Two crystal structure models suggest that Arg-42 prevents a typical switch region conformational change upon Gα_i1(G42R) binding to GDP·AlF₄⁻ or GTP, but rotameric flexibility at this locus allows for unperturbed GTP hydrolysis. Gα(G42R) mutants do not engage the active state-selective peptide KB-1753 nor RGS domains with high affinity, but instead favor interaction with Gβγ and GoLoco motifs in any nucleotide state. The corresponding Gα_q(G48R) mutant is not constitutively active in cells and responds poorly to aluminum tetrafluoride activation. Comparative analyses of M. oryzae strains harboring either G42R or GTPase-deficient Q/L mutations in the Gα subunits MagA or MagB illustrate functional differences in environmental cue processing and intracellular signaling outcomes between these two Gα mutants, thus demonstrating the in vivo functional divergence of G42R and activating G-protein mutants.

Heterotrimeric G-proteins function as molecular switches to convey cellular signals. When a G-protein coupled receptor encounters its ligand at the cellular membrane, it catalyzes guanine nucleotide exchange on the Gα subunit, resulting in a shift from an inactive to an active conformation. G-protein signaling pathways are conserved from mammals to plants and fungi, including the rice blast fungus Magnaporthe oryzae. A mutation in the Gα subunit (G42R), previously thought to eliminate its GTPase activity, leading to constitutive activation, has been utilized to investigate roles of heterotrimeric G-protein signaling pathways in multiple species of filamentous fungi. Here, we demonstrate through structural, biochemical, and cellular approaches that G42R mutants are neither GTPase deficient nor constitutively active, but rather are unable to transition to the activated conformation. A direct comparison of M. oryzae fungal strains harboring either G42R or truly constitutively activating mutations in two Gα subunits, MagA and MagB, revealed markedly different phenotypes. Our results suggest that activation of MagB is critical for pathogenic development of M. oryzae in response to hydrophobic surfaces, such as plant leaves. Furthermore, the lack of constitutive activity by Gα(G42R) mutants prompts a re-evaluation of its use in previous genetic experiments in multiple fungal species.

Lack of evidence for a role of hydrophobins in conferring surface hydrophobicity to conidia and hyphae of Botrytis cinerea

BACKGROUND

Hydrophobins are small, cysteine rich, surface active proteins secreted by filamentous fungi, forming hydrophobic layers on the walls of aerial mycelia and spores. Hydrophobin mutants in a variety of fungi have been described to show 'easily wettable' phenotypes, indicating that hydrophobins play a general role in conferring surface hydrophobicity to aerial hyphae and spores.

RESULTS

In the genome of the grey mould fungus Botrytis cinerea, genes encoding three hydrophobins and six hydrophobin-like proteins were identified. Expression analyses revealed low or no expression of these genes in conidia, while some of them showed increased or specific expression in other stages, such as sclerotia or fruiting bodies. Bhp1 belongs to the class I hydrophobins, whereas Bhp2 and Bhp3 are members of hydrophobin class II. Single, double and triple hydrophobin knock-out mutants were constructed by consecutively deleting bhp1, bhp2 and bhp3. In addition, a mutant in the hydrophobin-like gene bhl1 was generated. The mutants were tested for germination and growth under different conditions, formation of sclerotia, ability to penetrate and infect host tissue, and for spore and mycelium surface properties. Surprisingly, none of the B. cinerea hydrophobin mutants showed obvious phenotypic defects in any of these characters. Scanning electron microscopy of the hydrophobic conidial surfaces did not reveal evidence for the presence of typical hydrophobin 'rodlet' layers.

CONCLUSIONS

These data provide evidence that in B. cinerea, hydrophobins are not involved in conferring surface hydrophobicity to conidia and aerial hyphae, and challenge their universal role in filamentous fungi. The function of some of these proteins in sclerotia and fruiting bodies remains to be investigated.

Very-long-chain aldehydes promote in vitro prepenetration processes of Blumeria graminis in a dose- and chain length-dependent manner

Surface properties of aerial plant organs have been shown to affect the interaction of fungal plant pathogens and their hosts. Conidial germination and differentiation - the so-called prepenetration processes - of the barley powdery mildew fungus (Blumeria graminis f. sp. hordei) are known to be triggered by n-hexacosanal (C(26)-aldehyde), a minor constituent of barley leaf wax. In order to analyze the differentiation-inducing capabilities of typical aldehyde wax constituents on conidia of wheat and barley powdery mildew, synthetic even-numbered very-long-chain aldehydes (C(22)-C(30)) were assayed, applying an in vitro system based on Formvar(®)/n-hexacosane-coated glass slides. n-Hexacosanal was the most effective aldehyde tested. Germination and differentiation rates of powdery mildew conidia increased with increasing concentrations of very-long-chain aldehydes. Relative to n-hexacosanal, the other aldehyde compounds showed a gradual decrease in germination- and differentiation-inducing capabilities with both decreasing and increasing chain length. In addition to n-hexacosanal, several other ubiquitous very-long-chain aldehyde wax constituents were capable of effectively stimulating B. graminis prepenetration processes in a dose- and chain length-dependent manner. Other wax constituents, such as n-alkanes, primary alcohols (with the exception of n-hexacosanol), fatty acids and alkyl esters, did not affect fungal prepenetration.

Characterization of a cellobiohydrolase (MoCel6A) produced by Magnaporthe oryzae

Three GH-6 family cellobiohydrolases are expected in the genome of Magnaporthe grisea based on the complete genome sequence. Here, we demonstrate the properties, kinetics, and substrate specificities of a Magnaporthe oryzae GH-6 family cellobiohydrolase (MoCel6A). In addition, the effect of cellobiose on MoCel6A activity was also investigated. MoCel6A contiguously fused to a histidine tag was overexpressed in M. oryzae and purified by affinity chromatography. MoCel6A showed higher hydrolytic activities on phosphoric acid-swollen cellulose (PSC), β-glucan, and cellooligosaccharide derivatives than on cellulose, of which the best substrates were cellooligosaccharides. A tandemly aligned cellulose binding domain (CBD) at the N terminus caused increased activity on cellulose and PSC, whereas deletion of the CBD (catalytic domain only) showed decreased activity on cellulose. MoCel6A hydrolysis of cellooligosaccharides and sulforhodamine-conjugated cellooligosaccharides was not inhibited by exogenously adding cellobiose up to 438 mM, which, rather, enhanced activity, whereas a GH-7 family cellobiohydrolase from M. oryzae (MoCel7A) was severely inhibited by more than 29 mM cellobiose. Furthermore, we assessed the effects of cellobiose on hydrolytic activities using MoCel6A and Trichoderma reesei cellobiohydrolase (TrCel6A), which were prepared in Aspergillus oryzae. MoCel6A showed increased hydrolysis of cellopentaose used as a substrate in the presence of 292 mM cellobiose at pH 4.5 and pH 6.0, and enhanced activity disappeared at pH 9.0. In contrast, TrCel6A exhibited slightly increased hydrolysis at pH 4.5, and hydrolysis was severely inhibited at pH 9.0. These results suggest that enhancement or inhibition of hydrolytic activities by cellobiose is dependent on the reaction mixture pH.

PdeH, a high-affinity cAMP phosphodiesterase, is a key regulator of asexual and pathogenic differentiation in Magnaporthe oryzae

Cyclic AMP-dependent pathways mediate the communication between external stimuli and the intracellular signaling machinery, thereby influencing important aspects of cellular growth, morphogenesis and differentiation. Crucial to proper function and robustness of these signaling cascades is the strict regulation and maintenance of intracellular levels of cAMP through a fine balance between biosynthesis (by adenylate cyclases) and hydrolysis (by cAMP phosphodiesterases). We functionally characterized gene-deletion mutants of a high-affinity (PdeH) and a low-affinity (PdeL) cAMP phosphodiesterase in order to gain insights into the spatial and temporal regulation of cAMP signaling in the rice-blast fungus Magnaporthe oryzae. In contrast to the expendable PdeL function, the PdeH activity was found to be a key regulator of asexual and pathogenic development in M. oryzae. Loss of PdeH led to increased accumulation of intracellular cAMP during vegetative and infectious growth. Furthermore, the pdeHDelta showed enhanced conidiation (2-3 fold), precocious appressorial development, loss of surface dependency during pathogenesis, and highly reduced in planta growth and host colonization. A pdeHDelta pdeLDelta mutant showed reduced conidiation, exhibited dramatically increased (approximately 10 fold) cAMP levels relative to the wild type, and was completely defective in virulence. Exogenous addition of 8-Br-cAMP to the wild type simulated the pdeHDelta defects in conidiation as well as in planta growth and development. While a fully functional GFP-PdeH was cytosolic but associated dynamically with the plasma membrane and vesicular compartments, the GFP-PdeL localized predominantly to the nucleus. Based on data from cAMP measurements and Real-Time RTPCR, we uncover a PdeH-dependent biphasic regulation of cAMP levels during early and late stages of appressorial development in M. oryzae. We propose that PdeH-mediated sustenance and dynamic regulation of cAMP signaling during M. oryzae development is crucial for successful establishment and spread of the blast disease in rice.

Analyses of expressed sequence tags from the maize foliar pathogen Cercospora zeae-maydis identify novel genes expressed during vegetative, infectious, and reproductive growth

Background

The ascomycete fungus Cercospora zeae-maydis is an aggressive foliar pathogen of maize that causes substantial losses annually throughout the Western Hemisphere. Despite its impact on maize production, little is known about the regulation of pathogenesis in C. zeae-maydis at the molecular level. The objectives of this study were to generate a collection of expressed sequence tags (ESTs) from C. zeae-maydis and evaluate their expression during vegetative, infectious, and reproductive growth.

Results

A total of 27,551 ESTs was obtained from five cDNA libraries constructed from vegetative and sporulating cultures of C. zeae-maydis. The ESTs, grouped into 4088 clusters and 531 singlets, represented 4619 putative unique genes. Of these, 36% encoded proteins similar (E value ≤ 10^-05) to characterized or annotated proteins from the NCBI non-redundant database representing diverse molecular functions and biological processes based on Gene Ontology (GO) classification. We identified numerous, previously undescribed genes with potential roles in photoreception, pathogenesis, and the regulation of development as well as Zephyr, a novel, actively transcribed transposable element. Differential expression of selected genes was demonstrated by real-time PCR, supporting their proposed roles in vegetative, infectious, and reproductive growth.

Conclusion

Novel genes that are potentially involved in regulating growth, development, and pathogenesis were identified in C. zeae-maydis, providing specific targets for characterization by molecular genetics and functional genomics. The EST data establish a foundation for future studies in evolutionary and comparative genomics among species of Cercospora and other groups of plant pathogenic fungi.

Cutinase and hydrophobin interplay: A herald for pathogenesis?

Surface-penetrating phytopathogenic fungi frequently form appressoria. These are specialised infection structures pivotal to fungal ingress into the host. Recently, we demonstrated that one member of a family of cutinases in Magnaporthe grisea is involved in surface sensing, mediating appressorium differentiation and penetration peg formation and hence facilitates host penetration. Cutinase2 serves as an upstream activator of cAMP/PKA and DAG/PKC signalling cascades and is essential for full virulence. Here, we speculate on the role of rice blast hydrophobins as surface interactors facilitating fungal cutinase activity.

Phage display cDNA cloning and expression analysis of hydrophobins from the entomopathogenic fungus Beauveria (Cordyceps) bassiana

Hydrophobins are small amphipathic proteins that function in a broad range of growth and developmental processes in fungi. They are involved in the formation of aerial structures, the attachment of fungal cells to surfaces, and act in signalling in response to surface cues and pathogenesis. Beauveria bassiana is an important entomopathogenic fungus used as an arthropod biological control agent. To examine the feasibility of using phage display technology to clone cDNAs encoding hydrophobins, biopanning experiments were performed using a variety of affinity resins, including N,N'-diacetylchitobiose-, fucose-, lactose-, maltose- and melibiose-coupled agarose beads. After five rounds of iterative biopanning, cDNAs corresponding to two B. bassiana (class I) hydrophobins were selectively enriched using melibiose- or lactose-coupled agarose beads. Expression analysis revealed that the hyd1 gene was expressed in all samples tested, including aerial conidia, in vitro blastospores, submerged conidia, and cells sporulating on chitin and insect cuticle, with hyd1 expression peaking in growing mycelia. In contrast, the hyd2 gene was not appreciably expressed in any of the single-cell types (aerial conidia, blastospores and submerged conidia), but was constitutively expressed in growing mycelia and when cells were sporulating on chitin and insect cuticle. MS fingerprinting of an approximately 10 kDa protein found in boiling SDS-insoluble, trifluoroacetic acid-soluble extracts from aerial conidia identified the major component of the B. bassiana rodlet layer to be the hyd2 gene product. These results reveal the differential regulation of the isolated hydrophobins and indicate that phage display represents a novel approach to cDNA cloning of hydrophobins.

Rgs1 regulates multiple Galpha subunits in Magnaporthe pathogenesis, asexual growth and thigmotropism

Regulators of G-protein signaling (RGS proteins) negatively regulate heterotrimeric G-protein cascades that enable eukaryotic cells to perceive and respond to external stimuli. The rice-blast fungus Magnaporthe grisea forms specialized infection structures called appressoria in response to inductive surface cues. We isolated Magnaporthe RGS1 in a screen for mutants that form precocious appressoria on non-inductive surfaces. We report that a thigmotropic cue is necessary for initiating appressoria and for accumulating cAMP. Similar to an RGS1-deletion strain, magA(G187S) (RGS-insensitive Galpha(s)) and magA(Q208L) (GTPase-dead) mutants accumulated excessive cAMP and elaborated appressoria on non-inductive surfaces, suggesting that Rgs1 regulates MagA during pathogenesis. Rgs1 was also found to negatively regulate the Galpha(i) subunit MagB during asexual development. Deficiency of MAGB suppressed the hyper-conidiation defect in RGS1-deletion strain, whereas magB(G183S) and magB(Q204L) mutants produced more conidia, similar to the RGS1-deletion strain. Rgs1 physically interacted with GDP.AlF(4)(-)-activated forms of MagA, MagB and MagC (a Galpha(II) subunit). Thus, Rgs1 serves as a negative regulator of all Galpha subunits in Magnaporthe and controls important developmental events during asexual and pathogenic development.

Magnaporthe as a model for understanding host-pathogen interactions

The rice blast pathosystem has been the subject of intense interest in part because of the importance of the disease to world agriculture, but also because both Magnaporthe oryzae and its host are amenable to advanced experimental approaches. The goal of this review is to provide an overview of the system and to point out recent significant studies that update our understanding of the biology of M. oryzae. The genome sequence of M. oryzae has provided insight into how genome structure and pathogen population genetic variability has been shaped by transposable elements. The sequence allows systematic approaches to long-standing areas of investigation, including pathogen development and the molecular basis of compatible and incompatible interactions with its host. Rice blast provides an integrated system to illustrate most of the important concepts governing fungal/plant interactions and serves as an excellent starting point for gaining a broad perspective of issues in plant pathology.

The fungal hydrophobin RolA recruits polyesterase and laterally moves on hydrophobic surfaces

When fungi grow on plant or insect surfaces coated with wax polyesters that protect against pathogens, the fungi generally form aerial hyphae to contact the surfaces. Aerial structures such as hyphae and conidiophores are coated with hydrophobins, which are surface-active proteins involved in adhesion to hydrophobic surfaces. When the industrial fungus Aspergillus oryzae was cultivated in a liquid medium containing the biodegradable polyester polybutylene succinate-coadipate (PBSA), the rolA gene encoding hydrophobin RolA was highly transcribed. High levels of RolA and its localization on the cell surface in the presence of PBSA were confirmed by immunostaining. Under these conditions, A. oryzae simultaneously produced the cutinase CutL1, which hydrolyses PBSA. Pre-incubation of PBSA with RolA stimulated PBSA degradation by CutL1, suggesting that RolA bound to the PBSA surface was required for the stimulation. Immunostaining revealed that PBSA films coated with RolA specifically adsorbed CutL1. Quartz crystal microbalance analyses further demonstrated that RolA attached to a hydrophobic sensor chip specifically adsorbed CutL1. Circular dichroism spectra of soluble-state RolA and bound RolA suggested that RolA underwent a conformational change after its adsorption to hydrophobic surfaces. These results suggest that RolA adsorbed to the hydrophobic surface of PBSA recruits CutL1, resulting in condensation of CutL1 on the PBSA surface and consequent stimulation of PBSA hydrolysis. A fluorescence recovery after photobleaching experiment on PBSA films coated with FITC-labelled RolA suggested that RolA moves laterally on the film. We discuss the novel molecular functions of RolA with regard to plastic degradation.

Gene discovery and gene expression in the rice blast fungus, Magnaporthe grisea: analysis of expressed sequence tags

Over 28,000 expressed sequence tags (ESTs) were produced from cDNA libraries representing a variety of growth conditions and cell types. Several Magnaporthe grisea strains were used to produce the libraries, including a nonpathogenic strain bearing a mutation in the PMK1 mitogen-activated protein kinase. Approximately 23,000 of the ESTs could be clustered into 3,050 contigs, leaving 5,127 singleton sequences. The estimate of 8,177 unique sequences indicates that over half of the genes of the fungus are represented in the ESTs. Analysis of EST frequency reveals growth and cell type-specific patterns of gene expression. This analysis establishes criteria for identification of fungal genes involved in pathogenesis. A large fraction of the genes represented by ESTs have no known function or described homologs. Manual annotation of the most abundant cDNAs with no known homologs allowed us to identify a family of metallothionein proteins present in M. grisea, Neurospora crassa, and Fusarium graminearum. In addition, multiply represented ESTs permitted the identification of alternatively spliced mRNA species. Alternative splicing was rare, and in most cases, the alternate mRNA forms were unspliced, although alternative 5' splice sites were also observed.