A MADS box protein interacts with a mating-type protein and is required for fruiting body development in the homothallic ascomycete Sordaria macrospora

Molecular mechanisms underlying phenotypic degeneration in Cordyceps militaris: insights from transcriptome reanalysis and osmotic stress studies

Phenotypic degeneration in Cordyceps militaris poses a significant concern for producers, yet the mechanisms underlying this phenomenon remain elusive. To address this concern, we isolated two strains that differ in their abilities to form fruiting bodies. Our observations revealed that the degenerated strain lost the capacity to develop fruiting bodies, exhibited limited radial expansion, increased spore density, and elevated intracellular glycerol levels. Transcriptome reanalysis uncovered dysregulation of genes involved in the MAPK signaling pathway in the degenerate strain. Our RT-qPCR results demonstrated reduced expression of sexual development genes, along with upregulation of genes involved in asexual sporulation, glycerol synthesis, and MAPK regulation, when compared to the wild-type strain. Additionally, we discovered that osmotic stress reduced radial growth but increased conidia sporulation and glycerol accumulation in all strains. Furthermore, hyperosmotic stress inhibited fruiting body formation in all neutralized strains. These findings indicate dysregulation of the MAPK signaling pathway, the possibility of the activation of the high-osmolarity glycerol and spore formation modules, as well as the downregulation of the pheromone response and filamentous growth cascades in the degenerate strain. Overall, our study sheds light on the mechanisms underlying Cordyceps militaris degeneration and identifies potential targets for improving cultivation practices.

Mutations in Podospora anserina MCM1 and VelC Trigger Spontaneous Development of Barren Fruiting Bodies

The ascomycete Podospora anserina is a heterothallic filamentous fungus found mainly on herbivore dung. It is commonly used in laboratories as a model system, and its complete life cycle lasting eight days is well mastered in vitro. The main objective of our team is to understand better the global process of fruiting body development, named perithecia, induced normally in this species by fertilization. Three allelic mutants, named pfd3, pfd9, and pfd23 (for "promoting fruiting body development") obtained by UV mutagenesis, were selected in view of their abilities to promote barren perithecium development without fertilization. By complete genome sequencing of pfd3 and pfd9, and mutant complementation, we identified point mutations in the mcm1 gene as responsible for spontaneous perithecium development. MCM1 proteins are MADS box transcription factors that control diverse developmental processes in plants, metazoans, and fungi. We also identified using the same methods a mutation in the VelC gene as responsible for spontaneous perithecium development in the vacua mutant. The VelC protein belongs to the velvet family of regulators involved in the control of development and secondary metabolite production. A key role of MCM1 and VelC in coordinating the development of P. anserina perithecia with gamete formation and fertilization is highlighted.

Fus3/Kss1-MAP kinase and Ste12-like control distinct biocontrol-traits besides regulation of insect cuticle penetration via phosphorylation cascade in a filamentous fungal pathogen

BACKGROUND

Homolog of the yeast Fus3/Kss1 mitogen-activated protein kinase (MAPK) pathway and its target transcription factor, Ste12-like, are involved in penetration of host cuticle/pathogenicity in many ascomycete pathogens. However, details of their interaction during fungal infection, as well as their controlled other virulence-associated traits, are unclear.

RESULTS

Ste12-like (BbSte12) and Fus3/Kss1 MAPK homolog (Bbmpk1) interacted in nucleus, and phosphorylation of BbSte12 by Bbmpk1 was essential for penetration of insect cuticle in an insect fungal pathogen, Beauveria bassiana. However, some distinct biocontrol-traits were found to be mediated by Ste12 and Bbmpk1. In contrast to ΔBbmpk1 colony that grew more rapid than wild-type strain, inactivation of BbSte12 resulted in the opposite phenotype, which was consistent with their different proliferation rates in insect hemocoel after direct injection of conidia bypass the cuticle. Reduced conidial yield with decreased hydrophobicity was examined in both mutants, however they displayed distinct conidiogenesis, accompanying with differently altered cell cycle, distinct hyphal branching and septum formation. Moreover, ΔBbmpk1 showed increased tolerance to oxidative agent, whereas the opposite phenotype was seen for ΔBbSte12 strain. RNA sequencing analysis revealed that Bbmpk1 controlled 356 genes depending on BbSte12 during cuticle penetration, but 1077 and 584 genes were independently controlled by Bbmpk1 and BbSte12.

CONCLUSION

BbSte12 and Bbmpk1 separately participate in additional pathways for control of conidiation, growth and hyphal differentiation, as well as oxidative stress response besides regulating cuticle penetration via phosphorylation cascade. © 2023 Society of Chemical Industry.

The Role of Chromatin and Transcriptional Control in the Formation of Sexual Fruiting Bodies in Fungi

Fungal fruiting bodies are complex, three-dimensional structures that arise from a less complex vegetative mycelium. Their formation requires the coordinated action of many genes and their gene products, and fruiting body formation is accompanied by major changes in the transcriptome. In recent years, numerous transcription factor genes as well as chromatin modifier genes that play a role in fruiting body morphogenesis were identified, and through research on several model organisms, the underlying regulatory networks that integrate chromatin structure, gene expression, and cell differentiation are becoming clearer. This review gives a summary of the current state of research on the role of transcriptional control and chromatin structure in fruiting body development. In the first part, insights from transcriptomics analyses are described, with a focus on comparative transcriptomics. In the second part, examples of more detailed functional characterizations of the role of chromatin modifiers and/or transcription factors in several model organisms (Neurospora crassa, Aspergillus nidulans, Sordaria macrospora, Coprinopsis cinerea, and Schizophyllum commune) that have led to a better understanding of regulatory networks at the level of chromatin structure and transcription are discussed.

Transcriptome profiling of transcription factors in Ganoderma lucidum in response to methyl jasmonate

Ganoderma lucidum is a traditional Chinese medicine and its major active ingredients are ganoderma triterpenoids (GTs). To screen for transcription factors (TFs) that involved in the biosynthetic pathway of GTs in G. lucidum, the chemical composition in mycelia, primordium and fruiting body were analyzed, and the transcriptomes of mycelia induced by methyl jasmonate (MeJA) were analyzed. In addition, the expression level data of MeJA-responsive TFs in mycelia, primordia and fruiting body were downloaded from the database, and the correlation analysis was carried out between their expression profiles and the content of total triterpenoids. The results showed that a total of 89 components were identified, and the content of total triterpenoids was the highest in primordium, followed by fruiting body and mycelia. There were 103 differentially expressed TFs that response to MeJA-induction including 95 upregulated and 8 downregulated genes. These TFs were classified into 22 families including C2H2 (15), TFII-related (12), HTH (9), fungal (8), bZIP (6), HMG (5), DADS (2), etc. Correlation analysis showed that the expression level of GL23559 (MADS), GL26472 (HTH), and GL31187 (HMG) showed a positive correlation with the GTs content, respectively. While the expression level of GL25628 (fungal) and GL26980 (PHD) showed a negative correlation with the GTs content, respectively. Furthermore, the over expression of the Glmhr1 gene (GL25628) in Pichia pastoris GS115 indicated that it might be a negative regulator of GT biosynthesis through decreasing the production of lanosterol. This study provided useful information for a better understanding of the regulation of TFs involved in GT biosynthesis and fungal growth in G. lucidum.

Comparative transcriptome analysis of abnormal cap and healthy fruiting bodies of the edible mushroom Lentinula edodes

Lentinula edodes, a commercially important mushroom, is cultivated worldwide. Artificially cultivated L. edodes often present with abnormal symptoms in the fruiting body, which affect their commercial value and reduce production efficiency. In this study, we carried out a comparative transcriptome analysis of normal fruiting body pileus (LeNP), normal margin in abnormal fruiting body pileus (LeAPNM), and abnormal margin in abnormal fruiting body pileus (LeAPAM). Metabolic pathways such as those involved in transmembrane transport, ribosome production, tryptophan metabolism, arginine and proline metabolism, and the metabolism of other amino acids were significantly enriched in LeAPAM. F-box, short-chain dehydrogenases/reductases, the major facilitator superfamily, and the FMN_red superfamily are related to malformation in L. edodes. Genes encoding heat shock proteins, G protein, and β-1,3-glucanase in the GH5 family showed different expression patterns, suggesting that these genes are involved in the development of L. edodes fruiting bodies. In particular, CAZymes, which are involved in the development of cell walls in L. edodes, were highly expressed in LeAPAM. According to TEM observation, the cell wall of LeAPAM samples showed significant thickening compared to the other samples. These results suggested that cell wall anabolism in LeAPAM samples was more active than that in normal fruiting bodies, enhancing the environmental adaptability of the fungus. This study provides preliminary data for future research aimed at solving the phenomenon of abnormal fruiting bodies of L. edodes.

The MADS-box transcription factor GlMADS1 regulates secondary metabolism in Ganoderma lucidum

MADS-box transcription factors play crucial roles in regulating development processes and biosynthesis of secondary metabolites in eukaryotes. However, the role of MADS-box transcription factors vary among fungal species, and their function remains unclear in the medicinally and economically important fungus Ganoderma lucidum. In this study, we characterized a MADS-box gene, GlMADS1, in G. lucidum. Analyses using quantitative real-time polymerase chain reaction (qRT-PCR) showed that GlMADS1 expression levels were up-regulated from the mycelia to the primordia stage. In order to further evaluate the effect of MADS-box transcription factors on secondary metabolism, we utilized RNA interference (RNAi) to silence GlMADS1 in G. lucidum. Ganoderic acid (GA) and flavonoid contents were enhanced in GlMADS1-silenced strains, suggesting that GlMADS1 negatively regulates GA and flavonoid accumulation.

Sordaria macrospora: 25 years as a model organism for studying the molecular mechanisms of fruiting body development

Abstract

Fruiting bodies are among the most complex multicellular structures formed by fungi, and the molecular mechanisms that regulate their development are far from understood. However, studies with a number of fungal model organisms have started to shed light on this developmental process. One of these model organisms is Sordaria macrospora, a filamentous ascomycete from the order Sordariales. This fungus has been a genetic model organism since the 1950s, but its career as a model organism for molecular genetics really took off in the 1990s, when the establishment of a transformation protocol, a mutant collection, and an indexed cosmid library provided the methods and resources to start revealing the molecular mechanisms of fruiting body development. In the 2000s, “omics” methods were added to the S. macrospora tool box, and by 2020, 58 developmental genes have been identified in this fungus. This review gives a brief overview of major method developments for S. macrospora, and then focuses on recent results characterizing different processes involved in regulating development including several regulatory protein complexes, autophagy, transcriptional and chromatin regulation, and RNA editing.

Key points

•Sordaria macrospora is a model system for analyzing fungal fruiting body development.

•More than 100 developmental mutants are available for S. macrospora.

•More than 50 developmental genes have been characterized in S. macrospora.

MADS-box transcription factor Mcm1 controls cell cycle, fungal development, cell integrity and virulence in the filamentous insect pathogenic fungus Beauveria bassiana

MADS-box transcription factor Mcm1 plays crucial roles in regulating mating processes and pathogenesis in some fungi. However, its roles are varied in fungal species, and its function remains unclear in entomopathogenic fungi. Here, Mcm1 orthologue, Bbmcm1, was characterized in a filamentous entomopathogenic fungus, Beauveria bassiana. Disruption of Bbmcm1 resulted in a distinct reduction in growth with abnormal conidiogenesis, and a significant decrease in conidial viability with abnormal germination. ΔBbmcm1 displayed impaired cell integrity, with distorted cell wall structure and altered cell wall component. Abnormal cell cycle was detected in ΔBbmcm1 with longer G₂ /M phase but shorter G₁ /G₀ and S phases in unicellular blastospores, and sparser septa in multicellular hyphae, which might be responsible for defects in development and differentiation due to the regulation of cell cycle-involved genes, as well as the corresponding cellular events-associated genes. Dramatically decreased virulence was examined in ΔBbmcm1, with impaired ability to escape haemocyte encapsulation, which was consistent with markedly reduced cuticle-degrading enzyme production by repressing their coding genes, and downregulated fungal effector protein-coding genes, suggesting a novel role of Mcm1 in interaction with host insect. These data indicate that Mcm1 is a crucial regulator of development, cell integrity, cell cycle and virulence in insect fungal pathogens.

NBR1 is involved in selective pexophagy in filamentous ascomycetes and can be functionally replaced by a tagged version of its human homolog

Macroautophagy/autophagy is a conserved degradation process in eukaryotic cells involving the sequestration of proteins and organelles within double-membrane vesicles termed autophagosomes. In filamentous fungi, its main purposes are the regulation of starvation adaptation and developmental processes. In contrast to nonselective bulk autophagy, selective autophagy is characterized by cargo receptors, which bind specific cargos such as superfluous organelles, damaged or harmful proteins, or microbes, and target them for autophagic degradation. Herein, using the core autophagy protein ATG8 as bait, GFP-Trap analysis followed by liquid chromatography mass spectrometry (LC/MS) identified a putative homolog of the human autophagy cargo receptor NBR1 (NBR1, autophagy cargo receptor) in the filamentous ascomycete Sordaria macrospora (Sm). Fluorescence microscopy revealed that SmNBR1 colocalizes with SmATG8 at autophagosome-like structures and in the lumen of vacuoles. Delivery of SmNBR1 to the vacuoles requires SmATG8. Both proteins interact in an LC3 interacting region (LIR)-dependent manner. Deletion of Smnbr1 leads to impaired vegetative growth under starvation conditions and reduced sexual spore production under non-starvation conditions. The human NBR1 homolog partially rescues the phenotypic defects of the fungal Smnbr1 deletion mutant. The Smnbr1 mutant can neither use fatty acids as a sole carbon source nor form fruiting bodies under oxidative stress conditions. Fluorescence microscopy revealed that degradation of a peroxisomal reporter protein is impaired in the Smnbr1 deletion mutant. Thus, SmNBR1 is a cargo receptor for pexophagy in filamentous ascomycetes.

McmA-dependent and -independent regulatory systems governing expression of ClrB-regulated cellulase and hemicellulase genes in Aspergillus nidulans

Fungal cellulolytic and hemicellulolytic enzymes are promising tools for industrial hydrolysis of cellulosic biomass; however, the regulatory network underlying their production is not well understood. The recent discovery of the transcriptional activators ClrB and McmA in Aspergillus nidulans implied a novel regulatory mechanism driven by their interaction, experimental evidence for which was obtained from transcriptional and DNA-binding analyses in this study. It was found that ClrB was essential for induced expression of all the genes examined in this study, while McmA dependency of their expression was gene-dependent. DNA-binding studies revealed McmA assisted in the recruitment of ClrB to the cellulose-responsive element (CeRE) in the promoters of eglA and eglB, expression of which was significantly reduced in the mcmA mutant. The CCG triplet within the CeRE served as the recognition sequence for the ClrB monomer. In contrast, ClrB did not require McmA for binding as a homodimer to the CGGN₈ CCG sequences in the promoter of mndB, expression of which was affected less in the mcmA mutant than in all other examined genes. Thus, there are two types of ClrB-mediated regulation: McmA-assisted and McmA-independent. This novel McmA-ClrB synergistic system provides new insights into the complex regulatory network involved in cellulase and hemicellulase production.

MADS-Box Transcription Factor VdMcm1 Regulates Conidiation, Microsclerotia Formation, Pathogenicity, and Secondary Metabolism of Verticillium dahliae

Verticillium dahliae, a notorious phytopathogenic fungus, causes vascular wilt diseases in many plant species resulting in devastating yield losses worldwide. Due to its ability to colonize plant xylem and form microsclerotia, V. dahliae is highly persistent and difficult to control. In this study, we show that the MADS-box transcription factor VdMcm1 is a key regulator of conidiation, microsclerotia formation, virulence, and secondary metabolism of V. dahliae. In addition, our findings suggest that VdMcm1 is involved in cell wall integrity. Finally, comparative RNA-Seq analysis reveals 823 significantly downregulated genes in the VdMcm1 deletion mutant, with diverse biological functions in transcriptional regulation, plant infection, cell adhesion, secondary metabolism, transmembrane transport activity, and cell secretion. When taken together, these data suggest that VdMcm1 performs pleiotropic functions in V. dahliae.

Involvement of an SRF-MADS protein McmA in regulation of extracellular enzyme production and asexual/sexual development in Aspergillus nidulans

SRF-MADS proteins are transcription factors conserved among eukaryotes that regulate a variety of cellular functions; however, their physiological roles are still not well understood in filamentous fungi. Effects of a mutation in mcmA gene that encodes the sole SRF-MADS protein in the fungus Aspergillus nidulans were examined by RNA sequencing. Sequencing data revealed that expression levels of cellulase genes were significantly decreased by the mutation as reported previously. However, expression levels of various hemicellulolytic enzyme genes, several extracellular protease genes, the nosA and rosA genes involved in sexual development, and AN4394 encoding an ortholog of EcdR involved in Aspergillus oryzae conidiation, were also significantly decreased by the mutation. As expected from the RNA sequencing data, the mcmA mutant had reduced protease production, cleistothecial development, and conidiation. This is the first report describing the involvement of SRF-MADS proteins in protease production in fungi, and asexual and sexual development in Aspergillus.

The MADS-box transcription factor FgMcm1 regulates cell identity and fungal development in Fusarium graminearum

In eukaryotic cells, MADS-box genes are known to play major regulatory roles in various biological processes by combinatorial interactions with other transcription factors. In this study, we functionally characterized the FgMCM1 MADS-box gene in Fusarium graminearum, the causal agent of wheat and barley head blight. Deletion of FgMCM1 resulted in the loss of perithecium production and phialide formation. The Fgmcm1 mutant was significantly reduced in virulence, deoxynivalenol biosynthesis and conidiation. In yeast two-hybrid assays, FgMcm1 interacted with Mat1-1-1 and Fst12, two transcription factors important for sexual reproduction. Whereas Fgmcm1 mutants were unstable and produced stunted subcultures, Fgmcm1 mat1-1-1 but not Fgmcm1 fst12 double mutants were stable. Furthermore, spontaneous suppressor mutations occurred frequently in stunted subcultures to recover growth rate. Ribonucleic acid sequencing analysis indicated that a number of sexual reproduction-related genes were upregulated in stunted subcultures compared with the Fgmcm1 mutant, which was downregulated in the expression of genes involved in pathogenesis, secondary metabolism and conidiation. We also showed that culture instability was not observed in the Fvmcm1 mutants of the heterothallic Fusarium verticillioides. Overall, our data indicate that FgMcm1 plays a critical role in the regulation of cell identity, sexual and asexual reproduction, secondary metabolism and pathogenesis in F. graminearum.

Neurospora crassa female development requires the PACC and other signal transduction pathways, transcription factors, chromatin remodeling, cell-to-cell fusion, and autophagy

Using a screening protocol we have identified 68 genes that are required for female development in the filamentous fungus Neurospora crassa. We find that we can divide these genes into five general groups: 1) Genes encoding components of the PACC signal transduction pathway, 2) Other signal transduction pathway genes, including genes from the three N. crassa MAP kinase pathways, 3) Transcriptional factor genes, 4) Autophagy genes, and 5) Other miscellaneous genes. Complementation and RIP studies verified that these genes are needed for the formation of the female mating structure, the protoperithecium, and for the maturation of a fertilized protoperithecium into a perithecium. Perithecia grafting experiments demonstrate that the autophagy genes and the cell-to-cell fusion genes (the MAK-1 and MAK-2 pathway genes) are needed for the mobilization and movement of nutrients from an established vegetative hyphal network into the developing protoperithecium. Deletion mutants for the PACC pathway genes palA, palB, palC, palF, palH, and pacC were found to be defective in two aspects of female development. First, they were unable to initiate female development on synthetic crossing medium. However, they could form protoperithecia when grown on cellophane, on corn meal agar, or in response to the presence of nearby perithecia. Second, fertilized perithecia from PACC pathway mutants were unable to produce asci and complete female development. Protein localization experiments with a GFP-tagged PALA construct showed that PALA was localized in a peripheral punctate pattern, consistent with a signaling center associated with the ESCRT complex. The N. crassa PACC signal transduction pathway appears to be similar to the PacC/Rim101 pathway previously characterized in Aspergillus nidulans and Saccharomyces cerevisiae. In N. crassa the pathway plays a key role in regulating female development.

PRO40 is a scaffold protein of the cell wall integrity pathway, linking the MAP kinase module to the upstream activator protein kinase C

Mitogen-activated protein kinase (MAPK) pathways are crucial signaling instruments in eukaryotes. Most ascomycetes possess three MAPK modules that are involved in key developmental processes like sexual propagation or pathogenesis. However, the regulation of these modules by adapters or scaffolds is largely unknown. Here, we studied the function of the cell wall integrity (CWI) MAPK module in the model fungus Sordaria macrospora. Using a forward genetic approach, we found that sterile mutant pro30 has a mutated mik1 gene that encodes the MAPK kinase kinase (MAPKKK) of the proposed CWI pathway. We generated single deletion mutants lacking MAPKKK MIK1, MAPK kinase (MAPKK) MEK1, or MAPK MAK1 and found them all to be sterile, cell fusion-deficient and highly impaired in vegetative growth and cell wall stress response. By searching for MEK1 interaction partners via tandem affinity purification and mass spectrometry, we identified previously characterized developmental protein PRO40 as a MEK1 interaction partner. Although fungal PRO40 homologs have been implicated in diverse developmental processes, their molecular function is currently unknown. Extensive affinity purification, mass spectrometry, and yeast two-hybrid experiments showed that PRO40 is able to bind MIK1, MEK1, and the upstream activator protein kinase C (PKC1). We further found that the PRO40 N-terminal disordered region and the central region encompassing a WW interaction domain are sufficient to govern interaction with MEK1. Most importantly, time- and stress-dependent phosphorylation studies showed that PRO40 is required for MAK1 activity. The sum of our results implies that PRO40 is a scaffold protein for the CWI pathway, linking the MAPK module to the upstream activator PKC1. Our data provide important insights into the mechanistic role of a protein that has been implicated in sexual and asexual development, cell fusion, symbiosis, and pathogenicity in different fungal systems.

The specific response to environmental cues is crucial for cell differentiation and is often mediated by highly conserved eukaryotic MAP kinase (MAPK) pathways. How these pathways react specifically to huge numbers of different cues is still unclear, and current literature about adapter and scaffolding proteins remains scarce. However, gaining fundamental insight into molecular signaling determinants is pivotal for combating diseases with impaired signal transduction processes, such as Alzheimer's disease or cancer. Importantly, signal transduction can easily be studied in lower eukaryotes like filamentous fungi that are readily genetically tractable. The fungus Sordaria macrospora has a long history as an ideal model system for cell differentiation, and we show here that the proposed cell wall integrity (CWI) MAPK module of this fungus controls differentiation of sexual fruiting bodies, cell fusion, polar growth and cell wall stress response. We further discovered that developmental protein PRO40 binds the MAPK kinase kinase (MAPKKK), the MAPK kinase (MAPKK) and upstream activator protein kinase C (PKC1) of the CWI pathway and is required for MAK1 activity, thereby providing evidence that PRO40 is a scaffold protein. Collectively, our findings reveal a molecular role for a protein implicated in development, cell fusion, symbiosis, and pathogenicity in different fungi.

MADS-box transcription factor SsMADS is involved in regulating growth and virulence in Sclerotinia sclerotiorum

MADS-box proteins, a well-conserved family of transcription factors in eukaryotic organisms, specifically regulate a wide range of cellular functions, including primary metabolism, cell cycle, and cell identity. However, little is known about roles of the MADS-box protein family in the fungal pathogen Sclerotinia sclerotiorum. In this research, the S. sclerotiorum MADS-box gene SsMADS was cloned; it encodes a protein that is highly similar to Mcm1 orthologs from Saccharomyces cerevisiae and other fungi, and includes a highly conserved DNA-binding domain. MADS is a member of the MADS box protein SRF (serum response factor) lineage. SsMADS function was investigated using RNA interference. Silenced strains were obtained using genetic transformation of the RNA interference vectors pS1-SsMADS and pSD-SsMADS. SsMADS expression levels in silenced strains were analyzed using RT-PCR. The results showed that SsMADS mRNA expression in these silenced strains was reduced to different degrees, and growth rate in these silenced strains was significantly decreased. Infecting tomato leaflets with silenced strains indicated that SsMADS was required for leaf pathogenesis in a susceptible host. Our results suggest that the MADS-box transcription factor SsMADS is involved in S. sclerotiorum growth and virulence.

The filamentous fungus Sordaria macrospora as a genetic model to study fruiting body development

Filamentous fungi are excellent experimental systems due to their short life cycles as well as easy and safe manipulation in the laboratory. They form three-dimensional structures with numerous different cell types and have a long tradition as genetic model organisms used to unravel basic mechanisms underlying eukaryotic cell differentiation. The filamentous ascomycete Sordaria macrospora is a model system for sexual fruiting body (perithecia) formation. S. macrospora is homothallic, i.e., self-fertile, easily genetically tractable, and well suited for large-scale genomics, transcriptomics, and proteomics studies. Specific features of its life cycle and the availability of a developmental mutant library make it an excellent system for studying cellular differentiation at the molecular level. In this review, we focus on recent developments in identifying gene and protein regulatory networks governing perithecia formation. A number of tools have been developed to genetically analyze developmental mutants and dissect transcriptional profiles at different developmental stages. Protein interaction studies allowed us to identify a highly conserved eukaryotic multisubunit protein complex, the striatin-interacting phosphatase and kinase complex and its role in sexual development. We have further identified a number of proteins involved in chromatin remodeling and transcriptional regulation of fruiting body development. Furthermore, we review the involvement of metabolic processes from both primary and secondary metabolism, and the role of nutrient recycling by autophagy in perithecia formation. Our research has uncovered numerous players regulating multicellular development in S. macrospora. Future research will focus on mechanistically understanding how these players are orchestrated in this fungal model system.

bZIP transcription factor SmJLB1 regulates autophagy-related genes Smatg8 and Smatg4 and is required for fruiting-body development and vegetative growth in Sordaria macrospora

Autophagy is a precisely controlled degradation process in eukaryotic cells, during which the bulk of the cytoplasm is engulfed by a double membrane vesicle, the autophagosome. Fusion of the autophagosome with the vacuole leads to breakdown of its contents, such as proteins and organelles, and the recycling of nutrients. Earlier studies of autophagic genes of the core autophagic machinery in the filamentous ascomycete Sordaria macrospora elucidated the impact of autophagy on fungal viability, vegetative growth and fruiting-body development. To gain further knowledge about the regulation of autophagy in S. macrospora, we analyzed the function of the bZIP transcription factor SmJLB1, a homolog of the Podospora anserina basic zipper-type transcription factor induced during incompatibility 4 (IDI-4) and the Aspergillus nidulans transcription factor jun-like bZIP A (JlbA). Generation of the homokaryotic deletion mutant demonstrated S. macrospora Smjlb1 is associated with autophagy-dependent processes. Deletion of Smjlb1 abolished fruiting-body formation and impaired vegetative growth. SmJLB1 is localized to the cytoplasm and to nuclei. Quantitative real-time PCR experiments revealed an upregulated expression of autophagy-related genes Smatg8 and Smatg4 in the Smjlb1 deletion mutant, suggesting a transcriptional repression function of SmJLB1.

The role of MADS-box transcription factors in secondary metabolism and sexual development in the maize pathogen Fusarium verticillioides

MADS-box transcription factors (TFs) regulate functionally diverse gene targets in eukaryotes. In select ascomycetes, MADS-box TFs have been shown to play a role in virulence, and vegetative and sexual development. Here, we characterized Fusarium verticillioides MADS-box TFs, Mads1 and Mads2, in terms of their roles in secondary metabolism and sexual mating. Sequence analyses showed that MADS1 and MADS2 encode TFs with a SRF-type dimerization domain and a MEF2-type dimerization domain, respectively. The MADS1 and MADS2 knockout mutants (Fmt1 and Fmt2 strains, respectively) exhibited decreased vegetative growth and FB1 production when compared to the wild-type. Fmt1 showed reduced expression of 14 polyketide synthase (PKS) genes present in the organism, whereas Fmt2 did not display a change in PKS gene expression. Significantly, the deletion of MADS1 and MADS2 in the MAT1-2 genotype (Fmt4 and Fmt5 strains, respectively) led to strains that failed to produce perithecia and ascospores when crossed with the MAT1-1 wild-type strain. Notably, deletion of either gene did not have an effect on the ability of the fungus to colonize maize stalk or kernels. FB1 production and PKS expression data suggest that Mads1 is a broad regulator of secondary metabolism in F. verticillioides, and may target regulons upstream of Mads2 to influence FB1 production. In addition, MADS-box TFs in F. verticillioides play a critical role in the perithecia development.

The MAT locus genes play different roles in sexual reproduction and pathogenesis in Fusarium graminearum

Sexual reproduction plays a critical role in the infection cycle of Fusarium graminearum because ascospores are the primary inoculum. As a homothallic ascomycete, F. graminearum contains both the MAT1-1 and MAT1-2-1 loci in the genome. To better understand their functions and regulations in sexual reproduction and pathogenesis, in this study we assayed the expression, interactions, and mutant phenotypes of individual MAT locus genes. Whereas the expression of MAT1-1-1 and MAT12-1 rapidly increased after perithecial induction and began to decline after 1 day post-perithecial induction (dpi), the expression of MAT1-1-2 and MAT1-1-3 peaked at 4 dpi. MAT1-1-2 and MAT1-1-3 had a similar expression profile and likely are controlled by a bidirectional promoter. Although none of the MAT locus genes were essential for perithecium formation, all of them were required for ascosporogenesis in self-crosses. In outcrosses, the mat11-1-2 and mat11-1-3 mutants were fertile but the mat1-1-1 and mat1-2-1 mutants displayed male- and female-specific defects, respectively. The mat1-2-1 mutant was reduced in FgSO expression and hyphal fusion. Mat1-1-2 interacted with all other MAT locus transcription factors, suggesting that they may form a protein complex during sexual reproduction. Mat1-1-1 also interacted with FgMcm1, which may play a role in controlling cell identity and sexual development. Interestingly, the mat1-1-1 and mat1-2-1 mutants were reduced in virulence in corn stalk rot assays although none of the MAT locus genes was important for wheat infection. The MAT1-1-1 and MAT1-2-1 genes may play a host-specific role in colonization of corn stalks.

Insight into the transcriptional regulation of Msn2 required for conidiation, multi-stress responses and virulence of two entomopathogenic fungi

Msn2/4 transcription factors in some fungi have null effects on virulence and cellular stress responses. Here we found that the transcriptional regulation of Msn2 orthologs is vital for the conidiation, virulence and multi-stress responses of Beauveria bassiana (Bb) and Metarhizium robertsii (Mr), which lack Msn4 orthologs. Compared to wild-type and complemented strains of each fungus with all similar phenotypes, ΔBbmsn2 and ΔMrmsn2 showed remarkable defects in conidial yield (∼40% decrease) and virulence (∼25% decrease). Both delta mutants lost 20-65% of their tolerances to hyperosmolarity, oxidation, carbendazim, cell wall perturbing and high temperature at 34 °C during colony growth. Their conidia were also significantly (18-41%) less tolerant to oxidation, hyperosmolarity, wet-heat stress at 45 °C and UV-B irradiation. Accompanied with the defective phenotypes, several conidiation- and virulence-associated genes were greatly repressed in ΔBbmsn2 and ΔMrmsn2. Moreover, differentially expressed genes in the transcriptomes of ΔBbmsn2 versus wild type were ∼3% more under oxidative stress, but ∼12% fewer under heat shock, than those in the ΔMrmsn2 counterparts. Many stress-responsive effector genes and cellular signaling factors were remarkably downregulated. Taken together, the two entomopathogens could have evolved somewhat distinct stress-responsive mechanisms finely tuned by Msn2, highlighting the biological significance of Msn2 orthologs for filamentous fungi.

Autophagy genes Smatg8 and Smatg4 are required for fruiting-body development, vegetative growth and ascospore germination in the filamentous ascomycete Sordaria macrospora

Autophagy is a tightly controlled degradation process involved in various developmental aspects of eukaryotes. However, its involvement in developmental processes of multicellular filamentous ascomycetes is largely unknown. Here, we analyzed the impact of the autophagic proteins SmATG8 and SmATG4 on the sexual and vegetative development of the filamentous ascomycete Sordaria macrospora. A Saccharomyces cerevisiae complementation assay demonstrated that the S. macrospora Smatg8 and Smatg4 genes can functionally replace the yeast homologs. By generating homokaryotic deletion mutants, we showed that the S. macrospora SmATG8 and SmATG4 orthologs were associated with autophagy-dependent processes. Smatg8 and Smatg4 deletions abolished fruiting-body formation and impaired vegetative growth and ascospore germination, but not hyphal fusion. We demonstrated that SmATG4 was capable of processing the SmATG8 precursor. SmATG8 was localized to autophagosomes, whereas SmATG4 was distributed throughout the cytoplasm of S. macrospora. Furthermore, we could show that Smatg8 and Smatg4 are not only required for nonselective macroautophagy, but for selective macropexophagy as well. Taken together, our results suggest that in S. macrospora, autophagy seems to be an essential and constitutively active process to sustain high energy levels for filamentous growth and multicellular development even under nonstarvation conditions.

Combining laser microdissection and RNA-seq to chart the transcriptional landscape of fungal development

BACKGROUND

During sexual development, filamentous ascomycetes form complex, three-dimensional fruiting bodies for the protection and dispersal of sexual spores. Fruiting bodies contain a number of cell types not found in vegetative mycelium, and these morphological differences are thought to be mediated by changes in gene expression. However, little is known about the spatial distribution of gene expression in fungal development. Here, we used laser microdissection (LM) and RNA-seq to determine gene expression patterns in young fruiting bodies (protoperithecia) and non-reproductive mycelia of the ascomycete Sordaria macrospora.

RESULTS

Quantitative analysis showed major differences in the gene expression patterns between protoperithecia and total mycelium. Among the genes strongly up-regulated in protoperithecia were the pheromone precursor genes ppg1 and ppg2. The up-regulation was confirmed by fluorescence microscopy of egfp expression under the control of ppg1 regulatory sequences. RNA-seq analysis of protoperithecia from the sterile mutant pro1 showed that many genes that are differentially regulated in these structures are under the genetic control of transcription factor PRO1.

CONCLUSIONS

We have generated transcriptional profiles of young fungal sexual structures using a combination of LM and RNA-seq. This allowed a high spatial resolution and sensitivity, and yielded a detailed picture of gene expression during development. Our data revealed significant differences in gene expression between protoperithecia and non-reproductive mycelia, and showed that the transcription factor PRO1 is involved in the regulation of many genes expressed specifically in sexual structures. The LM/RNA-seq approach will also be relevant to other eukaryotic systems in which multicellular development is investigated.

A split luciferase complementation assay for studying in vivo protein-protein interactions in filamentous ascomycetes

Protein-protein interactions play important roles in controlling many cellular events. To date, several techniques have been developed for detection of protein-protein interactions in living cells, among which split luciferase complementation has been applied in animal and plant cells. Here, we examined whether the split luciferase assay could be used in filamentous ascomycetes, such as Gibberella zeae and Cochliobolus heterostrophus. The coding sequences of two strongly interacting proteins (the F-box protein, FBP1, and its partner SKP1) in G. zeae, under the control of the cryparin promoter from Cryphonectria parasitica, were translationally fused to the C- and N-terminal fragments of firefly luciferase (luc), respectively. Each fusion product inserted into a fungal transforming vector carrying the gene for resistance to either geneticin or hygromycin B, was transformed into both fungi. We detected complementation of split luciferase proteins driven by interaction of the two fungal proteins with a high luminescence intensity-to-background ratio only in the fungal transformants expressing both N-luc and C-luc fusion constructs. Using this system, we also confirmed a novel protein interaction between transcription factors, GzMCM1 and FST12 in G. zeae, which could hardly be proven by the yeast two-hybrid method. This is the first study demonstrating that monitoring of split luciferase complementation is a sensitive and efficient method of studying in vivo protein-protein interactions in filamentous ascomycetes.

A MADS-box transcription factor MoMcm1 is required for male fertility, microconidium production and virulence in Magnaporthe oryzae

Appressorium formation is a key step in the infection cycle of Magnaporthe oryzae. Mst12 is a transcription factor essential for appressorium penetration and invasive growth. In this study we used the affinity purification approach to identify proteins that physically associate with Mst12. One of the Mst12-interacting genes identified was MoMCM1, which encodes a MADS-box protein orthologous to yeast Mcm1. MoMcm1 interacted with both Mst12 and Mata-1 in yeast two-hybrid assays. Deletion of MoMCM1 resulted in the loss of male fertility and microconidium production. The Momcm1 mutant was defective in appressorium penetration and formed narrower invasive hyphae, which may be responsible for its reduced virulence. In transformants expressing MoMCM1-eGFP fusion, GFP signals were observed in the nucleus. We also generated the Momcm1 mst12 double mutant, which was defective in penetration and non-pathogenic. On hydrophilic surfaces, germ tubes produced by the double mutant were severely curved, and 20% of them formed appressoria. In contrast, the Momcm1 or mst12 mutant did not form appressoria on hydrophilic surfaces. These results suggest that MoMCM1 and MST12 have overlapping functions to suppress appressorium formation under non-conducive conditions. MoMcm1 may interact with Mst12 and MatA-1 to regulate germ tube identity and male fertility respectively.

The phocein homologue SmMOB3 is essential for vegetative cell fusion and sexual development in the filamentous ascomycete Sordaria macrospora

Members of the striatin family and their highly conserved interacting protein phocein/Mob3 are key components in the regulation of cell differentiation in multicellular eukaryotes. The striatin homologue PRO11 of the filamentous ascomycete Sordaria macrospora has a crucial role in fruiting body development. Here, we functionally characterized the phocein/Mob3 orthologue SmMOB3 of S. macrospora. We isolated the gene and showed that both, pro11 and Smmob3 are expressed during early and late developmental stages. Deletion of Smmob3 resulted in a sexually sterile strain, similar to the previously characterized pro11 mutant. Fusion assays revealed that ∆Smmob3 was unable to undergo self-fusion and fusion with the pro11 strain. The essential function of the SmMOB3 N-terminus containing the conserved mob domain was demonstrated by complementation analysis of the sterile S. macrospora ∆Smmob3 strain. Downregulation of either pro11 in ∆Smmob3, or Smmob3 in pro11 mutants by means of RNA interference (RNAi) resulted in synthetic sexual defects, demonstrating for the first time the importance of a putative PRO11/SmMOB3 complex in fruiting body development.

Proteomic changes associated with deletion of the Magnaporthe oryzae conidial morphology-regulating gene COM1

BACKGROUND

The rice blast disease caused by Magnaporthe oryzae is a major constraint on world rice production. The conidia produced by this fungal pathogen are the main source of disease dissemination. The morphology of conidia may be a critical factor in the spore dispersal and virulence of M. oryzae in the field. Deletion of a conidial morphology regulating gene encoding putative transcriptional regulator COM1 in M. oryzae resulted in aberrant conidial shape, reduced conidiation and attenuated virulence.

RESULTS

In this study, a two-dimensional gel electrophoresis/matrix assisted laser desorption ionization- time of flight mass spectrometry (2-DE/MALDI-TOF MS) based proteomics approach was employed to identify the cellular and molecular components regulated by the COM1 protein (COM1p) that might contribute to the aberrant phenotypes in M. oryzae. By comparing the conidial proteomes of COM1 deletion mutant and its isogenic wild-type strain P131, we identified a potpourri of 31 proteins that exhibited statistically significant alterations in their abundance levels. Of these differentially regulated proteins, the abundance levels of nine proteins were elevated and twelve were reduced in the Δcom1 mutant. Three proteins were detected only in the Δcom1 conidial proteome, whereas seven proteins were apparently undetectable. The data obtained in the study suggest that the COM1p plays a key role in transcriptional reprogramming of genes implicated in melanin biosynthesis, carbon and energy metabolism, structural organization of cell, lipid metabolism, amino acid metabolism, etc. Semi-quantitative RT-PCR analysis revealed the down-regulation of genes encoding enzymes involved in melanin biosynthesis in the COM1 mutant.

CONCLUSIONS

Our results suggest that the COM1p may regulate the transcription of genes involved in various cellular processes indispensable for conidial development and appressorial penetration. These functions are likely to contribute to the effects of COM1p upon the aberrant phenotypes of M. oryzae.

Sordaria macrospora, a model organism to study fungal cellular development

During the development of multicellular eukaryotes, the processes of cellular growth and organogenesis are tightly coordinated. Since the 1940s, filamentous fungi have served as genetic model organisms to decipher basic mechanisms underlying eukaryotic cell differentiation. Here, we focus on Sordaria macrospora, a homothallic ascomycete and important model organism for developmental biology. During its sexual life cycle, S. macrospora forms three-dimensional fruiting bodies, a complex process involving the formation of different cell types. S. macrospora can be used for genetic, biochemical and cellular experimental approaches since diverse tools, including fluorescence microscopy, a marker recycling system and gene libraries, are available. Moreover, the genome of S. macrospora has been sequenced and allows functional genomics analyses. Over the past years, our group has generated and analysed a number of developmental mutants which has greatly enhanced our fundamental understanding about fungal morphogenesis. In addition, our recent research activities have established a link between developmental proteins and conserved signalling cascades, ultimately leading to a regulatory network controlling differentiation processes in a eukaryotic model organism. This review summarizes the results of our recent findings, thus advancing current knowledge of the general principles and paradigms underpinning eukaryotic cell differentiation and development.

Functional characterization of MAT1-1-specific mating-type genes in the homothallic ascomycete Sordaria macrospora provides new insights into essential and nonessential sexual regulators

Mating-type genes in fungi encode regulators of mating and sexual development. Heterothallic ascomycete species require different sets of mating-type genes to control nonself-recognition and mating of compatible partners of different mating types. Homothallic (self-fertile) species also carry mating-type genes in their genome that are essential for sexual development. To analyze the molecular basis of homothallism and the role of mating-type genes during fruiting-body development, we deleted each of the three genes, SmtA-1 (MAT1-1-1), SmtA-2 (MAT1-1-2), and SmtA-3 (MAT1-1-3), contained in the MAT1-1 part of the mating-type locus of the homothallic ascomycete species Sordaria macrospora. Phenotypic analysis of deletion mutants revealed that the PPF domain protein-encoding gene SmtA-2 is essential for sexual reproduction, whereas the alpha domain protein-encoding genes SmtA-1 and SmtA-3 play no role in fruiting-body development. By means of cross-species microarray analysis using Neurospora crassa oligonucleotide microarrays hybridized with S. macrospora targets and quantitative real-time PCR, we identified genes expressed under the control of SmtA-1 and SmtA-2. Both genes are involved in the regulation of gene expression, including that of pheromone genes.

Evolutionary dynamics of mating-type loci of Mycosphaerella spp. occurring on banana

The devastating Sigatoka disease complex of banana is primarily caused by three closely related heterothallic fungi belonging to the genus Mycosphaerella: M. fijiensis, M. musicola, and M. eumusae. Previous phylogenetic work showing common ancestry led us to analyze the mating-type loci of these Mycosphaerella species occurring on banana. We reasoned that this might provide better insight into the evolutionary history of these species. PCR and chromosome-walking approaches were used to clone the mating-type loci of M. musicola and M. eumusae. Sequences were compared to the published mating-type loci of M. fijiensis and other Mycosphaerella spp., and a novel organization of the MAT loci was found. The mating-type loci of the examined Mycosphaerella species are expanded, containing two additional Mycosphaerella-specific genes in a unique genomic organization. The proteins encoded by these novel genes show a higher interspecies than intraspecies homology. Moreover, M. fijiensis, M. musicola, and M. eumusae contain two additional mating-type-like loci, containing parts of both MAT1-1-1 and MAT1-2-1. The data indicate that M. fijiensis, M. musicola, and M. eumusae share an ancestor in which a fusion event occurred between MAT1-1-1 and MAT1-2-1 sequences and in which additional genes became incorporated into the idiomorph. The new genes incorporated have since then evolved independently in the MAT1-1 and MAT1-2 loci. Thus, these data are an example of the evolutionary dynamics of fungal MAT loci in general and show the great flexibility of the MAT loci of Mycosphaerella species in particular.

Beta-carbonic anhydrases play a role in fruiting body development and ascospore germination in the filamentous fungus Sordaria macrospora

Carbon dioxide (CO(2)) is among the most important gases for all organisms. Its reversible interconversion to bicarbonate (HCO(3) (-)) reaches equilibrium spontaneously, but slowly, and can be accelerated by a ubiquitous group of enzymes called carbonic anhydrases (CAs). These enzymes are grouped by their distinct structural features into alpha-, beta-, gamma-, delta- and zeta-classes. While physiological functions of mammalian, prokaryotic, plant and algal CAs have been extensively studied over the past years, the role of beta-CAs in yeasts and the human pathogen Cryptococcus neoformans has been elucidated only recently, and the function of CAs in multicellular filamentous ascomycetes is mostly unknown. To assess the role of CAs in the development of filamentous ascomycetes, the function of three genes, cas1, cas2 and cas3 (carbonic anhydrase of Sordaria) encoding beta-class carbonic anhydrases was characterized in the filamentous ascomycetous fungus Sordaria macrospora. Fluorescence microscopy was used to determine the localization of GFP- and DsRED-tagged CAs. While CAS1 and CAS3 are cytoplasmic enzymes, CAS2 is localized to the mitochondria. To assess the function of the three isoenzymes, we generated knock-out strains for all three cas genes (Deltacas1, Deltacas2, and Deltacas3) as well as all combinations of double mutants. No effect on vegetative growth, fruiting-body and ascospore development was seen in the single mutant strains lacking cas1 or cas3, while single mutant Deltacas2 was affected in vegetative growth, fruiting-body development and ascospore germination, and the double mutant strain Deltacas1/2 was completely sterile. Defects caused by the lack of cas2 could be partially complemented by elevated CO(2) levels or overexpression of cas1, cas3, or a non-mitochondrial cas2 variant. The results suggest that CAs are required for sexual reproduction in filamentous ascomycetes and that the multiplicity of isoforms results in redundancy of specific and non-specific functions.

A cyanase is transcriptionally regulated by arginine and involved in cyanate decomposition in Sordaria macrospora

Cyanase degrades toxic cyanate to NH3 and CO2 in a bicarbonate-dependent reaction. High concentrations of cyanate are fairly toxic to organisms. Here, we characterize a eukaryotic cyanase for the first time. We have isolated the cyn1 gene encoding a cyanase from the filamentous ascomycete Sordaria macrospora and functionally characterized the cyn1 product after heterologous expression in Escherichia coli. Site-directed mutagenesis confirmed a predicted catalytic centre of three conserved amino-acids. A Deltacyn1 knockout in S. macrospora was totally devoid of cyanase activity and showed an increased sensitivity to exogenously supplied cyanate in an arginine-depleted medium, defects in ascospore germination, but no other obvious morphological phenotype. By means of real-time PCR we have demonstrated that the transcriptional level of cyn1 is markedly elevated in the presence of cyanate and down-regulated by addition of arginine. The putative functions of cyanase in fungi are discussed.

Asexual cephalosporin C producer Acremonium chrysogenum carries a functional mating type locus

Acremonium chrysogenum, the fungal producer of the pharmaceutically relevant beta-lactam antibiotic cephalosporin C, is classified as asexual because no direct observation of mating or meiosis has yet been reported. To assess the potential of A. chrysogenum for sexual reproduction, we screened an expressed sequence tag library from A. chrysogenum for the expression of mating type (MAT) genes, which are the key regulators of sexual reproduction. We identified two putative mating type genes that are homologues of the alpha-box domain gene, MAT1-1-1 and MAT1-1-2, encoding an HPG domain protein defined by the presence of the three invariant amino acids histidine, proline, and glycine. In addition, cDNAs encoding a putative pheromone receptor and pheromone-processing enzymes, as well as components of a pheromone response pathway, were found. Moreover, the entire A. chrysogenum MAT1-1 (AcMAT1-1) gene and regions flanking the MAT region were obtained from a genomic cosmid library, and sequence analysis revealed that in addition to AcMAT1-1-1 and AcMAT1-1-2, the AcMAT1-1 locus comprises a third mating type gene, AcMAT1-1-3, encoding a high-mobility-group domain protein. The alpha-box domain sequence of AcMAT1-1-1 was used to determine the phylogenetic relationships of A. chrysogenum to other ascomycetes. To determine the functionality of the AcMAT1-1 locus, the entire MAT locus was transferred into a MAT deletion strain of the heterothallic ascomycete Podospora anserina (the PaDeltaMAT strain). After fertilization with a P. anserina MAT1-2 (MAT(+)) strain, the corresponding transformants developed fruiting bodies with mature ascospores. Thus, the results of our functional analysis of the AcMAT1-1 locus provide strong evidence to hypothesize a sexual cycle in A. chrysogenum.

MADS-box transcription factor mig1 is required for infectious growth in Magnaporthe grisea

Magnaporthe grisea is a model fungus for studying fungus-plant interactions. Two mitogen-activated protein (MAP) kinase genes, PMK1 and MPS1, have been implicated in regulating plant infection processes in M. grisea. However, transcription factors activated by these MAP kinases are not well studied. In this study we functionally characterized the MIG1 gene that encodes a MADS-box transcription factor homologous to Saccharomyces cerevisiae Rlm1. In yeast two-hybrid assays, MIG1 interacts with MPS1, suggesting that MIG1 may function downstream from the MPS1 pathway. The mig1 deletion mutant had a normal growth rate and formed melanized appressoria, but it was nonpathogenic and failed to infect rice leaves through wounds. Appressoria formed by the mig1 mutant developed penetration pegs and primary infectious hyphae, but further differentiation of the secondary infectious hyphae inside live plant cells was blocked. However, the mig1 mutant formed infectious hypha-like structures in heat-killed plant cells or cellophane membranes. In transformants expressing the MIG1-GFP fusion, green fluorescent protein (GFP) signals were not detectable in vegetative hyphae and conidiophores. Mig1-GFP was localized to nuclei in conidia, appressoria, and infectious hyphae. Deletion of the MADS box had no effect on the expression and localization of the MIG1-GFP fusion but eliminated its ability to complement the mig1 mutant. These results suggest that MIG1 may be required for overcoming plant defense responses and the differentiation of secondary infectious hyphae in live plant cells. The MADS-box domain is essential for the function of MIG1 but dispensable for its nuclear localization, which may be associated with the activation of MIG1 by MPS1 during conidiation and plant infection.

Regulation of melanin biosynthesis via the dihydroxynaphthalene pathway is dependent on sexual development in the ascomyceteSordaria macrospora

The filamentous ascomycete Sordaria macrospora accumulates melanin during sexual development. The four melanin biosynthesis genes pks, teh, sdh and tih were isolated and their homology to genes involved in 1,8 dihydroxynaphthalene (DHN) melanin biosynthesis was shown. The presence of DHN melanin in S. macrospora was further confirmed by disrupting the pks gene encoding a putative polyketide synthase and by RNA interference-mediated silencing of the sdh gene encoding a putative scytalone dehydratase. Because melanin occurs in fruiting bodies that develop through several intermediate stages within 7 days of growth, a Northern analysis of a developmental time-course was conducted. These data revealed a time-dependent regulation of teh and sdh transcript levels. Comparing the transcriptional expression by real-time PCR of melanin biosynthesis genes in the wild type under conditions allowing or repressing sexual development, a significant downregulation during vegetative growth was detected. Quantitative real-time PCR and Northern blot analysis of melanin biosynthesis gene expression in different developmental mutants confirmed that melanin biosynthesis is linked to fruiting body development and is under the control of specific regulatory genes that participate in sexual differentiation.

A STE12 homologue of the homothallic ascomyceteSordaria macrosporainteracts with the MADS box protein MCM1 and is required for ascosporogenesis

The MADS box protein MCM1 controls diverse developmental processes and is essential for fruiting body formation in the homothallic ascomycete Sordaria macrospora. MADS box proteins derive their regulatory specificity from a wide range of different protein interactions. We have recently shown that the S. macrospora MCM1 is able to interact with the alpha-domain mating-type protein SMTA-1. To further evaluate the functional roles of MCM1, we used the yeast two-hybrid approach to identify MCM1-interacting proteins. From this screen, we isolated a protein with a putative N-terminal homeodomain and C-terminal C2/H2-Zn2+ finger domains. The protein is a member of the highly conserved fungal STE12 transcription factor family of proteins and was therefore termed STE12. Furthermore, we demonstrate by means of two-hybrid and far western analysis that in addition to MCM1, the S. macrospora STE12 protein is able to interact with the mating-type protein SMTA-1. Unlike the situation in the closely related heterothallic ascomycete Neurospora crassa, deletion (Delta) of the ste12 gene in S. macrospora neither affects vegetative growth nor fruiting body formation. However, ascus and ascospore development are highly impaired by the Deltaste12 mutation. Our data provide another example of the functional divergence within the fungal STE12 transcription factor family.