Conidium differentiation in Aspergillus nidulans wild-type and wet-white (wetA) mutant strains

Alternaria solani effectors AsCEP19 and AsCEP20 reveal novel functions in pathogenicity and conidiogenesis

Previous work identified a pair of specific effectors AsCEP19 and AsCEP20 in Alternaria solani as contributors to the virulence of A. solani. Here, we constructed AsCEP19 and AsCEP20 deletion mutants in A. solani strain HWC168 to further reveal the effects of these genes on the biology and pathogenicity of A. solani. Deletion of AsCEP19 and AsCEP20 did not affect vegetative growth but did affect conidial maturation, with an increase in the percentage of abnormal conidia produced. Furthermore, we determined the expression patterns of genes involved in the conidiogenesis pathway and found that the regulatory gene abaA was significantly upregulated and chsA, a positive regulator for conidiation, was significantly downregulated in the mutant strains compared to the wild-type strain. These results suggest that AsCEP19 and AsCEP20 indirectly affect the conidial development and maturation of A. solani. Pathogenicity assays revealed significantly impaired virulence of ΔAsCEP19, ΔAsCEP20, and ΔAsCEP19 + AsCEP20 mutants on potato and tomato plants. Moreover, we performed localization assays with green fluorescent protein-tagged proteins in chili pepper leaves. We found that AsCEP19 can specifically localize to the chloroplasts of chili pepper epidermal cells, while AsCEP20 can localize to both chloroplasts and the plasma membrane. Weighted gene co-expression network analysis revealed enrichment of genes of this module in the photosynthesis pathway, with many hub genes associated with chloroplast structure and photosynthesis. These results suggest that chloroplasts are the targets for AsCEP19 and AsCEP20.

IMPORTANCE

Alternaria solani is an important necrotrophic pathogen causing potato early blight. Previous studies have provide preliminary evidence that specific effectors AsCEP19 and AsCEP20 contribute to virulence, but their respective functions, localization, and pathogenic mechanisms during the infection process of A. solani remain unclear. Here, we have systematically studied the specific effectors AsCEP19 and AsCEP20 for the first time, which are essential for conidial maturation. The deletion of AsCEP19 and AsCEP20 can significantly impair fungal pathogenicity. Additionally, we preliminarily revealed that AsCEP19 and AsCEP20 target the chloroplasts of host cells. Our findings further enhance our understanding of the molecular mechanisms underlying the virulence of necrotrophic pathogens.

Effects of MrwetA on Sexual Reproduction and Secondary Metabolism of Monascus ruber M7 Based on Transcriptome Analysis

wetA, one of the conidiation center regulatory genes in many filamentous fungi, plays an important role in promoting asexual spores (conidia) maturation. Our recent research has found that knocking out or overexpressing MrwetA (a homolog of wetA) in Monascus ruber M7 does not affect the development of its asexual spores like other fungi, but both repress the development of its sexual spores (ascospores). However, the mechanism remains unclear. In this study, the function of MrwetA on sexual reproduction and secondary metabolism in M. ruber M7 was confirmed by a complementary experiment. Moreover, the regulatory roles of MrwetA in modulating the expression of genes involved in sexual reproduction, meiosis, and biosynthesis of Monascus pigment and citrinin were analyzed based on the transcriptional data. These results not only contribute to clarifying the regulation of the reproduction and secondary metabolism of Monascus spp., but also to enriching the regulation molecular mechanism of reproduction in filamentous fungi.

Coordination of two regulators SscA and VosA in Aspergillus nidulans conidia

Airborne fungal spores are a major cause of fungal diseases in humans, animals, and plants as well as contamination of foods. Previous studies found a variety of regulators including VosA, VelB, WetA, and SscA for sporogenesis and the long-term viability in Aspergillus nidulans. To gain a mechanistic understanding of the complex regulatory mechanisms in asexual spores, here, we focused on the relationship between VosA and SscA using comparative transcriptomic analysis and phenotypic studies. The ΔsscA ΔvosA double-mutant conidia have lower spore viability and stress tolerance compared to the ΔsscA or ΔvosA single mutant conidia. Deletion of sscA or vosA affects chitin levels and mRNA levels of chitin biosynthetic genes in conidia. In addition, SscA and VosA are required for the dormant state of conidia and conidial germination by modulating the mRNA levels of the cytoskeleton and development-associated genes. Overall, these results suggest that SscA and VosA play interdependent roles in governing spore maturation, dormancy, and germination in A. nidulans.

Regulators of the Asexual Life Cycle of Aspergillus nidulans

The genus Aspergillus, one of the most abundant airborne fungi, is classified into hundreds of species that affect humans, animals, and plants. Among these, Aspergillus nidulans, as a key model organism, has been extensively studied to understand the mechanisms governing growth and development, physiology, and gene regulation in fungi. A. nidulans primarily reproduces by forming millions of asexual spores known as conidia. The asexual life cycle of A. nidulans can be simply divided into growth and asexual development (conidiation). After a certain period of vegetative growth, some vegetative cells (hyphae) develop into specialized asexual structures called conidiophores. Each A. nidulans conidiophore is composed of a foot cell, stalk, vesicle, metulae, phialides, and 12,000 conidia. This vegetative-to-developmental transition requires the activity of various regulators including FLB proteins, BrlA, and AbaA. Asymmetric repetitive mitotic cell division of phialides results in the formation of immature conidia. Subsequent conidial maturation requires multiple regulators such as WetA, VosA, and VelB. Matured conidia maintain cellular integrity and long-term viability against various stresses and desiccation. Under appropriate conditions, the resting conidia germinate and form new colonies, and this process is governed by a myriad of regulators, such as CreA and SocA. To date, a plethora of regulators for each asexual developmental stage have been identified and investigated. This review summarizes our current understanding of the regulators of conidial formation, maturation, dormancy, and germination in A. nidulans.

Chemical stimuli override a temperature-dependent morphological program by reprogramming the transcriptome of a fungal pathogen

The human fungal pathogen Histoplasma changes its morphology in response to temperature. At 37°C it grows as a budding yeast whereas at room temperature it transitions to hyphal growth. Prior work has demonstrated that 15-20% of transcripts are temperature-regulated, and that transcription factors Ryp1-4 are necessary to establish yeast growth. However, little is known about transcriptional regulators of the hyphal program. To identify TFs that regulate filamentation, we utilize chemical inducers of hyphal growth. We show that addition of cAMP analogs or an inhibitor of cAMP breakdown overrides yeast morphology, yielding inappropriate hyphal growth at 37°C. Additionally, butyrate supplementation triggers hyphal growth at 37°C. Transcriptional profiling of cultures filamenting in response to cAMP or butyrate reveals that a limited set of genes respond to cAMP while butyrate dysregulates a larger set. Comparison of these profiles to previous temperature- or morphology-regulated gene sets identifies a small set of morphology-specific transcripts. This set contains 9 TFs of which we characterized three, STU1 , FBC1 , and PAC2 , whose orthologs regulate development in other fungi. We found that each of these TFs is individually dispensable for room-temperature (RT) induced filamentation but each is required for other aspects of RT development. FBC1 and PAC2 , but not STU1 , are necessary for filamentation in response to cAMP at 37°C. Ectopic expression of each of these TFs is sufficient to induce filamentation at 37°C. Finally, PAC2 induction of filamentation at 37°C is dependent on STU1 , suggesting these TFs form a regulatory circuit that, when activated at RT, promotes the hyphal program.

Importance

Fungal illnesses pose a significant disease burden. However, the regulatory circuits that govern the development and virulence of fungi remain largely unknown. This study utilizes chemicals that can override the normal growth morphology of the human pathogen Histoplasma . Using transcriptomic approaches, we identify novel regulators of hyphal morphology and refine our understanding of the transcriptional circuits governing morphology in Histoplasma .

Roles of BrlA and AbaA in Mediating Asexual and Insect Pathogenic Lifecycles of Metarhizium robertsii

BrlA and AbaA are key activators of the central developmental pathway (CDP) that controls asexual development in Aspergillus but their roles remain insufficiently understood in hypocerealean insect pathogens. Here, regulatory roles of BrlA and AbaA orthologs in Metarhizium robertsii (Clavicipitaceae) were characterized for comparison to those elucidated previously in Beauveria bassiana (Cordycipitaceae) at phenotypic and transcriptomic levels. Time-course transcription profiles of brlA, abaA, and the other CDP activator gene wetA revealed that they were not so sequentially activated in M. robertsii as learned in Aspergillus. Aerial conidiation essential for fungal infection and dispersal, submerged blastospore production mimicking yeast-like budding proliferation in insect hemocoel, and insect pathogenicity via cuticular penetration were all abolished as a consequence of brlA or abaA disruption, which had little impact on normal hyphal growth. The disruptants were severely compromised in virulence via cuticle-bypassing infection (intrahemocoel injection) and differentially impaired in cellular tolerance to oxidative and cell wall-perturbing stresses. The ΔbrlA and ΔabaA mutant shad 255 and 233 dysregulated genes (up/down ratios: 52:203 and 101:122) respectively, including 108 genes co-dysregulated. These counts were small compared with 1513 and 2869 dysregulated genes (up/down ratios: 707:806 and 1513:1356) identified in ΔbrlA and ΔabaA mutants of B. bassiana. Results revealed not only conserved roles for BrlA and AbaA in asexual developmental control but also their indispensable roles in fungal adaptation to the insect-pathogenic lifecycle and host habitats. Intriguingly, BrlA- or AbaA-controlled gene expression networks are largely different between the two insect pathogens, in which similar phenotypes were compromised in the absence of either brlA or abaA.

Identification of Three Novel Conidiogenesis-Related Genes in the Nematode-Trapping Fungus Arthrobotrys oligospora

For filamentous fungi, conidiogenesis is the most common reproductive strategy for environmental dispersal, invasion, and proliferation. Understanding the molecular mechanisms controlling conidiation and increasing conidium yield may provide promising applications in commercial development in the future for nematode-trapping fungi. However, the molecular mechanism for regulating conidium production of filamentous fungi is not fully understood. In this study, we characterized three novel conidiogenesis-related genes via gene knockout in A. oligospora. The absence of the genes AoCorA and AoRgsD caused significant increases in conidia production, while the absence of AoXlnR resulted in a decrease in conidiogenesis. Moreover, we characterized the ortholog of AbaA, a well-known conidiogenesis-related gene in Aspergillus nidulans. The deletion of AoAbaA not only completely abolished conidium production but also affected the production of nematode-trapping traps.

MaNsdD regulates conidiation negatively by inhibiting the AbaA expression required for normal conidiation in Metarhizium acridum

Conidiation necessary for filamentous fungal survival and dispersal, proceeds in two fashions, namely normal conidiation through conidiophores differentiated from hyphae, and microcycle conidiation through conidial budding. Normal conidiation has been well studied whereas mechanisms underlying microcycle conidiation are still largely unknown. Here, we report that a gene (MaNsdD) homologous to NsdD in Aspergillus nidulans serves as a suppressor of normal conidiation but a positive regulator of hyphal development in Metarhizium acridum. Disruption of MaNsdD (ΔMaNsdD) resulted in microcycle conidiation and significantly descended in conidial resistance to heat while improved to UV irradiation. Transcriptomic analysis revealed that many genes involved in conidiation, cell division and cell wall formation were differentially expressed in ΔMaNsdD, and likely associated with the conidiation process. We found that a gene (MaAbaA) homologous to the core asexual development regulator AbaA in A. nidulans, was negatively controlled by MaNsdD. Disruption of MaAbaA led to the abolition of the conidiation process of M. acridum. These findings unravel a novel regulatory mechanism of microcycle conidiation, and add a knowledge to the asexual conidiation pathway of filamentous fungi.

Mechanism of a Volatile Organic Compound (6-Methyl-2-Heptanone) Emitted From Bacillus subtilis ZD01 Against Alternaria solani in Potato

The antagonistic mechanisms of soluble non-volatile bioactive compounds, such as proteins and lipopeptides emitted from Bacillus have been widely studied. However, there are limited studies on the antifungal mechanisms of volatile organic compounds (VOCs) produced by Bacillus against plant fungal diseases. In this study, the antagonistic mechanisms of one specific VOC, 6-methyl-2-heptanone, against Alternaria solani were investigated. To optimize the extraction conditions of headspace solid-phase microextraction, a 50/30-μm divinylbenzene/carboxen/polydimethylsiloxane fiber at 50°C for 40 min was used. For gas chromatography-mass spectrometry using a free fatty acid phase capillary column, 6-methyl-2-heptanone accounted for the highest content, at 22.27%, of the total VOCs from Bacillus subtilis ZD01, which inhibited A. solani mycelial growth strongly in vitro. Therefore, 6-methyl-2-heptanone was selected as the main active chemical to elucidate the action mechanisms against A. solani. Scanning and transmission electron microscopy analyses revealed that after exposure to an EC₅₀ dose of 6-methyl-2-heptanone, A. solani hyphal cells had a wide range of abnormalities. 6-Methyl-2-heptanone also caused the capture of cellular fluorescent green label and the release of adenosine triphosphate (ATP) from outer membranes A. solani cells, which may enhance 6-methyl-2-heptanone ability to reach the cytoplasmic membrane. In addition, 6-methyl-2-heptanone showed strong inhibitory effect on A. solani conidial germination. It also damaged conidial internal structures, with the treated group having collapsed shrunken small vesicles as observed by transmission electron microscopy. Because 6-methyl-2-heptanone showed strong effects on mycelial integrity and conidial structure, the expression levels of related pathogenic genes in A. solani treated with 6-methyl-2-heptanone were investigated. The qRT-PCR results showed that transcriptional expression levels of slt2 and wetA genes were strongly down-regulated after exposure to 6-methyl-2-heptanone. Finally, because identifying the functions of pathogenic genes will be important for the biological control of A. solani, the wetA gene was identified as a conidia-associated gene that plays roles in regulating sporulation yield and conidial maturation. These findings provide further insights into the mechanisms of VOCs secreted by Bacillus against A. solani.

Mr-AbaA Regulates Conidiation by Interacting with the Promoter Regions of Both Mr-veA and Mr-wetA in Metarhizium robertsii

Conidiation is a pivotal strategy for fungi to resist adverse environments and disperse to new habitats, which is especially important for entomopathogenic fungi whose conidia are infective as fungal pesticide propagules. However, the molecular mechanism for regulating conidiation in entomopathogenic fungi is not fully understood. Here, we characterized the regulatory mechanism of the key developmental transcription factor Mr-AbaA. Bioinformatic analysis, transcriptional profiles, and subcellular localization of Mr-abaA indicated that AbaA functioned as a transcription factor in the conidiophore development and conidium stages. Microscopic examination showed that the null mutant of Mr-abaA differentiated into defective phialides to produce an abacus structure instead of conidia. Loss of Mr-abaA resulted in the inhibition of submerged blastospore separation in vitro. Moreover, yeast (Saccharomyces cerevisiae) one-hybrid assays of interactions between genes and deletion of Mr-veA showed that Mr-AbaA regulates conidiation by interacting with the promoter regions of Mr-veA and Mr-wetA. These results demonstrate that Mr-AbaA positively regulates conidiation in Metarhizium robertsii by regulating the velvet family ortholog gene Mr-veA and contributes to the separation of blastospores in submerged culture. IMPORTANCE Metarhizium robertsii is an emerging model entomopathogenic fungus for developing biopesticides; therefore, a comprehensive understanding of its conidiation is very important for its application. In this study, we revealed that the transcription factor Mr-AbaA is involved in the control of aerial conidiation and blastospore separation in submerged culture. Further yeast one-hybrid assays demonstrated that Mr-AbaA interacts with the promoter regions of Mr-veA and Mr-wetA, which code for proteins involved in the control of conidiation. This finding provides new insight into the regulation of the conidiation of this important entomopathogenic fungi.

How to enter the state of dormancy? A suggestion by Trichoderma atroviride conidia

It is generally accepted that conidia, propagules of filamentous fungi, exist in the state of dormancy. This state is defined mostly phenomenologically, e.g., by germination requirements. Its molecular characteristics are scarce and are concentrated on the water or osmolyte content, and/or respiration. However, a question of whether conidia are metabolic or ametabolic forms of life cannot be answered on the basis of available experimental data. In other words, are mature conidia open thermodynamic systems as are mycelia, or do they become closed upon the transition to the dormant state? In this article, we present observations which may help to define the transition of freshly formed conidia to the putative dormant forms using measurements of selected enzyme activities, ¹H- and ¹³C-NMR and LC-MS-metabolomes, and ¹⁴C-bicarbonate or ⁴⁵Ca²⁺ inward transport. We have found that Trichoderma atroviride and Aspergillus niger conidia arrest the ⁴⁵Ca²⁺ uptake during the development stopping thereby the cyclic (i.e., bidirectional) Ca²⁺ flow existing in vegetative mycelia and conidia of T. atroviride across the cytoplasmic membrane. Furthermore, we have found that the activity of α-ketoglutarate dehydrogenase was rendered completely inactive after 3 weeks from the conidia formation unlike of other central carbon metabolism enzymes. This may explain the loss of conidial respiration. Finally, we found that conidia take up the H¹⁴CO₃^- and convert it into few stable compounds within 80 d of maturation, with minor quantitative differences in the extent of this process. The uptake of H¹³CO₃^- confirmed these observation and demonstrated the incorporation of H¹³CO₃^- even in the absence of exogenous substrates. These results suggest that T. atroviride conidia remain metabolically active during first ten weeks of maturation. Under these circumstances, their metabolism displays features similar to those of chemoautotrophic microorganisms. However, the Ca²⁺ homeostasis changed from the open to the closed thermodynamic state during the early period of conidial maturation. These results may be helpful for studying the conidial ageing and/or maturation, and for defining the conidial dormant state in biochemical terms.

Transcription Factor MaMsn2 Regulates Conidiation Pattern Shift under the Control of MaH1 through Homeobox Domain in Metarhizium acridum

The growth pattern of filamentous fungi can switch between hyphal radial polar growth and non-polar yeast-like cell growth depending on the environmental conditions. Asexual conidiation after radial polar growth is called normal conidiation (NC), while yeast-like cell growth is called microcycle conidiation (MC). Previous research found that the disruption of MaH1 in Metarhizium acridum led to a conidiation shift from NC to MC. However, the regulation mechanism is not clear. Here, we found MaMsn2, an Msn2 homologous gene in M. acridum, was greatly downregulated when MaH1 was disrupted (ΔMaH1). Loss of MaMsn2 also caused a conidiation shift from NC to MC on a nutrient-rich medium. Yeast one-hybrid (Y1H) and electrophoretic mobility shift assay (EMSA) showed that MaH1 could bind to the promoter region of the MaMsn2 gene. Disrupting the interaction between MaH1 and the promoter region of MaMsn2 significantly downregulated the transcription level of MaMsn2, and the overexpression of MaMsn2 in ΔMaH1 could restore NC from MC of ΔMaH1. Our findings demonstrated that MaMsn2 played a role in maintaining the NC pattern directly under the control of MaH1, which revealed the molecular mechanisms that regulated the conidiation pattern shift in filamentous fungi for the first time.

Characterization of the asexual developmental genes brlA and wetA in Monascus ruber M7

Monascus spp. are widely used in the production of monacolin K and food- grade pigments in East Asia. In Aspergillus species, the three transcription factors BrlA → AbaA → WetA sequentially function as the central activators of asexual development (conidiation), leading to the formation of conidiophores. Unlike their close relative Aspergillus spp., Monascus spp. produce basipetospora-type asexual spores (conidia), and their genomes contain homologs of brlA and wetA but not abaA. In the present study, to investigate their roles in Monascus conidiation, MrbrlA and MrwetA were functionally characterized by gene knockout and overexpression in Monascus ruber M7. The results revealed that the deletion and overexpression of MrbrlA and/or MrwetA caused no apparent changes in the morphology, size, number, structure, or germination of conidia. However, deletion and overexpression of MrwetA severely repressed sexual development and affected the production of secondary metabolites. Taken together, these results suggest that the well-established central regulatory model of conidiation in Aspergillus is not applicable in their Monascus relatives. The results of the present study could enrich our understanding of the asexual development regulatory networks in filamentous fungi.

Transcriptomic, Protein-DNA Interaction, and Metabolomic Studies of VosA, VelB, and WetA in Aspergillus nidulans Asexual Spores

Filamentous fungi produce a vast number of asexual spores that act as efficient propagules. Due to their infectious and/or allergenic nature, fungal spores affect our daily life. Aspergillus species produce asexual spores called conidia; their formation involves morphological development and metabolic changes, and the associated regulatory systems are coordinated by multiple transcription factors (TFs).

In filamentous fungi, asexual development involves cellular differentiation and metabolic remodeling leading to the formation of intact asexual spores. The development of asexual spores (conidia) in Aspergillus is precisely coordinated by multiple transcription factors (TFs), including VosA, VelB, and WetA. Notably, these three TFs are essential for the structural and metabolic integrity, i.e., proper maturation, of conidia in the model fungus Aspergillus nidulans. To gain mechanistic insight into the complex regulatory and interdependent roles of these TFs in asexual sporogenesis, we carried out multi-omics studies on the transcriptome, protein-DNA interactions, and primary and secondary metabolism employing A. nidulans conidia. RNA sequencing and chromatin immunoprecipitation sequencing analyses have revealed that the three TFs directly or indirectly regulate the expression of genes associated with heterotrimeric G-protein signal transduction, mitogen-activated protein (MAP) kinases, spore wall formation and structural integrity, asexual development, and primary/secondary metabolism. In addition, metabolomics analyses of wild-type and individual mutant conidia indicate that these three TFs regulate a diverse array of primary metabolites, including those in the tricarboxylic acid (TCA) cycle, certain amino acids, and trehalose, and secondary metabolites such as sterigmatocystin, emericellamide, austinol, and dehydroaustinol. In summary, WetA, VosA, and VelB play interdependent, overlapping, and distinct roles in governing morphological development and primary/secondary metabolic remodeling in Aspergillus conidia, leading to the production of vital conidia suitable for fungal proliferation and dissemination.

HbxB Is a Key Regulator for Stress Response and β-Glucan Biogenesis in Aspergillus nidulans

Homeobox transcription factors are conserved in eukaryotes and act as multi-functional transcription factors in filamentous fungi. Previously, it was demonstrated that HbxB governs fungal development and spore viability in Aspergillus nidulans. Here, the role of HbxB in A. nidulans was further characterized. RNA-sequencing revealed that HbxB affects the transcriptomic levels of genes associated with trehalose biosynthesis and response to thermal, oxidative, and radiation stresses in asexual spores called conidia. A phenotypic analysis found that hbxB deletion mutant conidia were more sensitive to ultraviolet stress. The loss of hbxB increased the mRNA expression of genes associated with β-glucan degradation and decreased the amount of β-glucan in conidia. In addition, hbxB deletion affected the expression of the sterigmatocystin gene cluster and the amount of sterigmatocystin. Overall, these results indicated that HbxB is a key transcription factor regulating trehalose biosynthesis, stress tolerance, β-glucan degradation, and sterigmatocystin production in A.nidulans conidia.

Functional Characterization of Core Regulatory Genes Involved in Sporulation of the Nematophagous Fungus Purpureocillium lavendulum

The nematophagous fungus Purpureocillium lavendulum is a natural enemy of plant-parasitic nematodes, which cause severe economic losses in agriculture worldwide. The production of asexual spores (conidia) in P. lavendulum is crucial for its biocontrol activity against nematodes. In this study, we characterized the core regulatory genes involved in conidiation of P. lavendulum at the molecular level. The central regulatory pathway is composed of three genes, P. lavendulumbrlA (PlbrlA), PlabaA, and PlwetA, which regulate the early, middle, and late stages of asexual development, respectively. The deletion of PlbrlA completely inhibited conidiation, with only conidiophore stalks produced. PlAbaA determines the differentiation of conidia from phialides. The deletion of PlwetA affected many phenotypes related to conidial maturation, including abscission of conidia from conidium strings, thickening of the cell wall layers, vacuole generation inside the cytoplasm, production of trehalose, tolerance to heat shock, etc. Comparative analyses showed that the upstream regulators of the core regulatory pathway of conidiation, especially the “fluffy” genes, were different from those in Aspergillus. Besides their roles in conidiation, the central regulators also influence the production of secondary metabolites, such as the leucinostatins, in P. lavendulum. Our study revealed a set of essential genes controlling conidiation in P. lavendulum and provided a framework for further molecular genetic studies on fungus-nematode interactions and for the biocontrol of plant-parasitic nematodes.

IMPORTANCE Plant-parasitic nematodes cause serious damage to crops throughout the world. Purpureocillium lavendulum is a nematophagous fungus which is a natural enemy of nematodes and a potential biocontrol agent against plant-parasitic nematodes. The conidia play an important role during infection of nematodes. In this study, we identified and characterized genes involved in regulating asexual development of P. lavendulum. We found that these genes not only regulate conidiation but also influence secondary-metabolite production. This work provides a basis for future studies of fungus-nematode interactions and nematode biocontrol.

Molecular Mechanisms of Conidial Germination in Aspergillus spp

Aspergilli produce conidia for reproduction or to survive hostile conditions, and they are highly effective in the distribution of conidia through the environment. In immunocompromised individuals, inhaled conidia can germinate inside the respiratory tract, which may result in invasive pulmonary aspergillosis. The management of invasive aspergillosis has become more complex, with new risk groups being identified and the emergence of antifungal resistance. Patient survival is threatened by these developments, stressing the need for alternative therapeutic strategies. As germination is crucial for infection, prevention of this process might be a feasible approach. A broader understanding of conidial germination is important to identify novel antigermination targets. In this review, we describe conidial resistance against various stresses, transition from dormant conidia to hyphal growth, the underlying molecular mechanisms involved in germination of the most common Aspergillus species, and promising antigermination targets. Germination of Aspergillus is characterized by three morphotypes: dormancy, isotropic growth, and polarized growth. Intra- and extracellular proteins play an important role in the protection against unfavorable environmental conditions. Isotropically expanding conidia remodel the cell wall, and biosynthetic machineries are needed for cellular growth. These biosynthetic machineries are also important during polarized growth, together with tip formation and the cell cycle machinery. Genes involved in isotropic and polarized growth could be effective antigermination targets. Transcriptomic and proteomic studies on specific Aspergillus morphotypes will improve our understanding of the germination process and allow discovery of novel antigermination targets and biomarkers for early diagnosis and therapy.

Aspergillus fumigatus exoβ(1-3)glucanases family GH55 are essential for conidial cell wall morphogenesis

The cell wall of Aspergillus fumigatus is predominantly composed of polysaccharides. The central fibrillar core of the cell wall is composed of a branched β(1-3)glucan, to which the chitin and the galactomannan are covalently bound. Softening of the cell wall is an essential event during fungal morphogenesis, wherein rigid cell wall structures are cleaved by glycosyl hydrolases. In this study, we characterised the role of the glycosyl hydrolase GH55 members in A. fumigatus fungal morphogenesis. We showed that deletion of the six genes of the GH55 family stopped conidial cell wall maturation at the beginning of the development process, leading to abrogation of conidial separation: the shape of conidia became ovoid, and germination was delayed. In conclusion, the reorganisation and structuring of the conidial cell wall mediated by members of the GH55 family is essential for their maturation, normal dissemination, and germination.

BrlA and AbaA Govern Virulence-Required Dimorphic Switch, Conidiation, and Pathogenicity in a Fungal Insect Pathogen

Dimorphic plant and human mycopathogens require a switch from the usual yeast growth to filamentous growth for host tissue penetration, and the switch is controlled by multiple signaling systems other than the central developmental pathway. Unlike these fungi, dimorphic insect mycopathogens usually grow by hyphal extension, infect the host by hyphal penetration through the insect cuticle, and switch to unicellular blastospores from the penetrating hyphae only after entry into the host hemocoel, where blastospore propagation by yeast-like budding accelerates host mummification. Here, we report a dependence of the virulence-required dimorphic transition on the central pathway activators BrlA and AbaA in Beauveria bassiana Deletion of brlA or abaA abolished both aerial conidiation and submerged blastospore formation in vitro despite no negative impact on hyphal growth in various media, including a broth mimic of insect hemolymph. The hyphae of either deletion mutant lost insect pathogenicity through normal cuticle penetration, contrasting with a high infectivity of wild-type hyphae. The mutant hyphae injected into the host hemocoel failed to form blastospores, resulting in slower lethal action. Uncovered by transcriptomic analysis, several genes involved in host adhesion and cuticle degradation were sharply repressed in both deletion mutants versus wild type. However, almost all signaling genes homologous to those acting in the dimorphic switch of other fungi were not differentially expressed at a significant level and hence unlikely to be involved in shutting down the dimorphic switch of each deletion mutant. Therefore, like aerial conidiation, the submerged dimorphic switch in vitro and in vivo is a process of asexual development governed by the two central pathway activators in B. bassiana IMPORTANCE Dimorphic insect mycopathogens infect the host by hyphal penetration through the host cuticle and switch from the penetrating hyphae to unicellular blastospores after entry into the host hemocoel, where blastospore propagation by yeast-like budding accelerates host mummification to death. The fungal virulence-required dimorphic switch is confirmed as a process of asexual development directly regulated by BrlA and AbaA, two key activators of the central developmental pathway in an insect mycopathogen. This finding unveils a novel mechanism distinct from the control of the dimorphic switch by multiple signaling systems other than the central developmental pathway in dimorphic plant and human mycopathogens, which switch from the usual yeast growth to filamentous growth required for pathogenicity through host tissue penetration.

Rewiring of transcriptional networks as a major event leading to the diversity of asexual multicellularity in fungi

Complex multicellularity (CM) is characterized by the generation of three-dimensional structures that follow a genetically controlled program. CM emerged at least five times in evolution, one of them in fungi. There are two types of CM programs in fungi, leading, respectively, to the formation of sexual or asexual spores. Asexual spores foment the spread of mycoses, as they are the main vehicle for dispersion. In spite of this key dependence, there is great morphological diversity of asexual multicellular structures in fungi. To advance the understanding of the mechanisms that control initiation and progression of asexual CM and how they can lead to such a remarkable morphological diversification, we studied 503 fungal proteomes, representing all phyla and subphyla, and most known classes. Conservation analyses of 33 regulators of asexual development suggest stepwise emergence of transcription factors. While velvet proteins constitute one of the most ancient systems, the central regulator BrlA emerged late in evolution (with the class Eurotiomycetes). Some factors, such as MoConX4, seem to be species-specific. These observations suggest that the emergence and evolution of transcriptional regulators rewire transcriptional networks. This process could reach the species level, resulting in a vast diversity of morphologies.

Genome sequence of the potato pathogenic fungus Alternaria solani HWC-168 reveals clues for its conidiation and virulence

BACKGROUND

Alternaria solani is a known air-born deuteromycete fungus with a polycyclic life cycle and is the causal agent of early blight that causes significant yield losses of potato worldwide. However, the molecular mechanisms underlying the conidiation and pathogenicity remain largely unknown.

RESULTS

We produced a high-quality genome assembly of A. solani HWC-168 that was isolated from a major potato-producing region of Northern China, which facilitated a comprehensive gene annotation, the accurate prediction of genes encoding secreted proteins and identification of conidiation-related genes. The assembled genome of A. solani HWC-168 has a genome size 32.8 Mb and encodes 10,358 predicted genes that are highly similar with related Alternaria species including Alternaria arborescens and Alternaria brassicicola. We identified conidiation-related genes in the genome of A. solani HWC-168 by searching for sporulation-related homologues identified from Aspergillus nidulans. A total of 975 secreted protein-encoding genes, which might act as virulence factors, were identified in the genome of A. solani HWC-168. The predicted secretome of A. solani HWC-168 possesses 261 carbohydrate-active enzymes (CAZy), 119 proteins containing RxLx[EDQ] motif and 27 secreted proteins unique to A. solani.

CONCLUSIONS

Our findings will facilitate the identification of conidiation- and virulence-related genes in the genome of A. solani. This will permit new insights into understanding the molecular mechanisms underlying the A. solani-potato pathosystem and will add value to the global fungal genome database.

Set1 and Kdm5 are antagonists for H3K4 methylation and regulators of the major conidiation-specific transcription factor gene ABA1 in Fusarium fujikuroi

Here we present the identification and characterization of the H3K4‐specific histone methyltransferase Set1 and its counterpart, the Jumonji C demethylase Kdm5, in the rice pathogen Fusarium fujikuroi. While Set1 is responsible for all detectable H3K4me2/me3 in this fungus, Kdm5 antagonizes the H3K4me3 mark. Notably, deletion of both SET1 and KDM5 mainly resulted in the upregulation of genome‐wide transcription, also affecting a large set of secondary metabolite (SM) key genes. Although H3K4 methylation is a hallmark of actively transcribed euchromatin, several SM gene clusters located in subtelomeric regions were affected by Set1 and Kdm5. While the regulation of many of them is likely indirect, H3K4me2 levels at gibberellic acid (GA) genes correlated with GA biosynthesis in the wild type, Δkdm5 and OE::KDM5 under inducing conditions. Whereas Δset1 showed an abolished GA₃ production in axenic culture, phytohormone biosynthesis was induced in planta, so that residual amounts of GA₃ were detected during rice infection. Accordingly, Δset1 exhibited a strongly attenuated, though not abolished, virulence on rice. Apart from regulating secondary metabolism, Set1 and Kdm5 function as activator and repressor of conidiation respectively. They antagonistically regulate H3K4me3 levels and expression of the major conidiation‐specific transcription factor gene ABA1 in F. fujikuroi.

Systematic Dissection of the Evolutionarily Conserved WetA Developmental Regulator across a Genus of Filamentous Fungi

Asexual sporulation is fundamental to the ecology and lifestyle of filamentous fungi and can facilitate both plant and human infection. In Aspergillus, the production of asexual spores is primarily governed by the BrlA→AbaA→WetA regulatory cascade. The final step in this cascade is controlled by the WetA protein and governs not only the morphological differentiation of spores but also the production and deposition of diverse metabolites into spores. While WetA is conserved across the genus Aspergillus, the structure and degree of conservation of the wetA gene regulatory network (GRN) remain largely unknown. We carried out comparative transcriptome analyses of comparisons between wetA null mutant and wild-type asexual spores in three representative species spanning the diversity of the genus Aspergillus: A. nidulans, A. flavus, and A. fumigatus. We discovered that WetA regulates asexual sporulation in all three species via a negative-feedback loop that represses BrlA, the cascade’s first step. Furthermore, data from chromatin immunoprecipitation sequencing (ChIP-seq) experiments in A. nidulans asexual spores suggest that WetA is a DNA-binding protein that interacts with a novel regulatory motif. Several global regulators known to bridge spore production and the production of secondary metabolites show species-specific regulatory patterns in our data. These results suggest that the BrlA→AbaA→WetA cascade’s regulatory role in cellular and chemical asexual spore development is functionally conserved but that the wetA-associated GRN has diverged during Aspergillus evolution.

The formation of resilient spores is a key factor contributing to the survival and fitness of many microorganisms, including fungi. In the fungal genus Aspergillus, spore formation is controlled by a complex gene regulatory network that also impacts a variety of other processes, including secondary metabolism. To gain mechanistic insights into how fungal spore formation is controlled across Aspergillus, we dissected the gene regulatory network downstream of a major regulator of spore maturation (WetA) in three species that span the diversity of the genus: the genetic model A. nidulans, the human pathogen A. fumigatus, and the aflatoxin producer A. flavus. Our data show that WetA regulates asexual sporulation in all three species via a negative-feedback loop and likely binds a novel regulatory element that we term the WetA response element (WRE). These results shed light on how gene regulatory networks in microorganisms control important biological processes and evolve across diverse species.

Velvet domain protein VosA represses the zinc cluster transcription factor SclB regulatory network for Aspergillus nidulans asexual development, oxidative stress response and secondary metabolism

The NF-κB-like velvet domain protein VosA (viability of spores) binds to more than 1,500 promoter sequences in the filamentous fungus Aspergillus nidulans. VosA inhibits premature induction of the developmental activator gene brlA, which promotes asexual spore formation in response to environmental cues as light. VosA represses a novel genetic network controlled by the sclB gene. SclB function is antagonistic to VosA, because it induces the expression of early activator genes of asexual differentiation as flbC and flbD as well as brlA. The SclB controlled network promotes asexual development and spore viability, but is independent of the fungal light control. SclB interactions with the RcoA transcriptional repressor subunit suggest additional inhibitory functions on transcription. SclB links asexual spore formation to the synthesis of secondary metabolites including emericellamides, austinol as well as dehydroaustinol and activates the oxidative stress response of the fungus. The fungal VosA-SclB regulatory system of transcription includes a VosA control of the sclB promoter, common and opposite VosA and SclB control functions of fungal development and several additional regulatory genes. The relationship between VosA and SclB illustrates the presence of a convoluted surveillance apparatus of transcriptional control, which is required for accurate fungal development and the linkage to the appropriate secondary metabolism.

Velvet domain proteins of filamentous fungi are structurally similar to Rel-homology domains of mammalian NF-κB proteins. Velvet and NF-κB proteins control regulatory circuits of downstream transcriptional networks for cellular differentiation, survival and stress responses. Velvet proteins interconnect developmental programs with secondary metabolism in fungi. The velvet protein VosA binds to more than ten percent of the Aspergillus nidulans promoters and is important for the spatial and temporal control of asexual spore formation from conidiophores. A novel VosA-dependent genetic network has been identified and is controlled by the zinc cluster protein SclB. Although zinc cluster proteins constitute one of the most abundant classes of transcription factors in fungi, only a small amount is characterized. SclB is a repression target of VosA and both transcription factors are part of a mutual control in the timely adjusted choreography of asexual sporulation in A. nidulans. SclB acts at the interphase of asexual development and secondary metabolism and interconnects both programs with an adequate oxidative stress response. This study underlines the complexity of different hierarchical levels of the fungal velvet protein transcriptional network for developmental programs and interconnected secondary metabolism.

Modeling the Effect of Modified Atmospheres on Conidial Germination of Fungi from Dairy Foods

Modified atmosphere packaging (MAP) is commonly applied to extend food shelf-life. Despite growth of a wide variety of fungal contaminants has been previously studied in relation to modified-atmospheres, few studies aimed at quantifying the effects of dioxygen (O2) and carbon dioxide (CO2) partial pressures on conidial germination in solid agar medium. In the present study, an original culture method was developed, allowing microscopic monitoring of conidial germination under modified-atmospheres in static conditions. An asymmetric model was utilized to describe germination kinetics of Paecilomyces niveus, Mucor lanceolatus, Penicillium brevicompactum, Penicillium expansum, and Penicillium roquefoti, using two main parameters, i.e., median germination time (τ) and maximum germination percentage (Pmax ). These two parameters were subsequently modeled as a function of O2 partial pressure ranging from 0 to 21% and CO2 partial pressure ranging from 0.03 to 70% (8 tested levels for both O2 and CO2). Modified atmospheres with residual O2 or CO2 partial pressures below 1% and up to 70%, respectively, were not sufficient to totally inhibit conidial germination,. However, O2 levels < 1% or CO2 levels > 20% significantly increased τ and/or reduced Pmax , depending on the fungal species. Overall, the present method and results are of interest for predictive mycology applied to fungal spoilage of MAP food products.

WetA bridges cellular and chemical development in Aspergillus flavus

Bridging cellular reproduction and survival is essential for all life forms. Aspergillus fungi primarily reproduce by forming asexual spores called conidia, whose formation and maturation is governed by the central genetic regulatory circuit BrlA→AbaA→WetA. Here, we report that WetA is a multi-functional regulator that couples spore differentiation and survival, and governs proper chemical development in Aspergillus flavus. The deletion of wetA results in the formation of conidia with defective cell walls and no intra-cellular trehalose, leading to reduced stress tolerance, a rapid loss of viability, and disintegration of spores. WetA is also required for normal vegetative growth, hyphal branching, and production of aflatoxins. Targeted and genome-wide expression analyses reveal that WetA exerts feedback control of brlA and that 5,700 genes show altered mRNA levels in the mutant conidia. Functional category analyses of differentially expressed genes in ΔwetA RNA-seq data indicate that WetA contributes to spore integrity and maturity by properly regulating the metabolic pathways of trehalose, chitin, α-(1,3)-glucan, β-(1,3)-glucan, melanin, hydrophobins, and secondary metabolism more generally. Moreover, 160 genes predicted to encode transcription factors are differentially expressed by the absence of wetA, suggesting that WetA may play a global regulatory role in conidial development. Collectively, we present a comprehensive model for developmental control that bridges spore differentiation and survival in A. flavus.

The role of VosA/VelB-activated developmental gene vadA in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans primarily reproduces by forming asexual spores called conidia, the integrity of which is governed by the NF-κB type velvet regulators VosA and VelB. The VosA-VelB hetero-complex regulates the expression of spore-specific structural and regulatory genes during conidiogenesis. Here, we characterize one of the VosA/VelB-activated developmental genes, called vadA, the expression of which in conidia requires activity of both VosA and VelB. VadA (AN5709) is predicted to be a 532-amino acid length fungal-specific protein with a highly conserved domain of unknown function (DUF) at the N-terminus. This DUF was found to be conserved in many Ascomycota and some Glomeromycota species, suggesting a potential evolutionarily conserved function of this domain in fungi. Deletion studies of vadA indicate that VadA is required for proper downregulation of brlA, fksA, and rodA, and for proper expression of tpsA and orlA during sporogenesis. Moreover, vadA null mutant conidia exhibit decreased trehalose content, but increased β(1,3)-glucan levels, lower viability, and reduced tolerance to oxidative stress. We further demonstrate that the vadA null mutant shows increased production of the mycotoxin sterigmatocystin. In summary, VadA is a dual-function novel regulator that controls development and secondary metabolism, and participates in bridging differentiation and viability of newly formed conidia in A. nidulans.

Aspergillus flavus aswA, a gene homolog of Aspergillus nidulans oefC, regulates sclerotial development and biosynthesis of sclerotium-associated secondary metabolites

Aspergillus flavus aswA (AFLA_085170) is a gene encoding a Zn(II)₂Cys₆ DNA-binding domain and a transcriptional activation domain, DUF3468. Disruption of aswA yielded strains that made a truncated gene transcript and generated a fungus that produced a greatly increased number of sclerotia. These sclerotia were odd-shaped and non-pigmented (white) and different from oval and pigmented (dark brown to black) mature sclerotia. Transcriptomic analysis of the ΔaswA strain grown on potato dextrose agar plates and Wickerham agar plates showed that expression of clustering genes involved in the biosynthesis of three sclerotium-associated secondary metabolites was down-regulated. These included gene clusters of asparasone, aflatrem, and aflavarin. In contrast, those of aflatoxin, cyclopiazonic acid and kojic acid were not affected. Metabolite analyses confirmed that the non-pigmented sclerotia contained aflatoxin and cyclopiazonic acid but not other aforementioned metabolites, three asparasone analogs and dihydroxyaflavinine commonly present in mature sclerotia. Impairment in aswA gene function stalls normal sclerotial development, which in turn prevents biosynthesis and accumulation of sclerotium-specific metabolites.

Differential Control of Asexual Development and Sterigmatocystin Biosynthesis by a Novel Regulator in Aspergillus nidulans

The filamentous fungus Aspergillus nidulans primarily reproduces by forming asexual spores called conidia and produces the mycotoxin sterigmatocystin (ST), the penultimate precursor of aflatoxins. It has been known that asexual development and ST production are tightly co-regulated by various regulatory inputs. Here, we report that the novel regulator AslA with a C₂H₂ domain oppositely regulates development and ST biosynthesis. Nullifying aslA resulted in defective conidiation and reduced expression of brlA encoding a key activator of asexual development, which indicates that AslA functions as an upstream activator of brlA expression. aslA deletion additionally caused enhanced ST production and expression of aflR encoding a transcriptional activator for ST biosynthetic genes, suggesting that AslA functions as an upstream negative regulator of aflR. Cellular and molecular studies showed that AslA has a trans-activation domain and is localized in the nuclei of vegetative and developing cells but not in spores, indicating that AslA is likely a transcription factor. Introduction of the aslA homologs from distantly-related aspergilli complemented the defects caused by aslA null mutation in A. nidulans, implying a functional conservancy of AslA. We propose that AslA is a novel regulator that may act at the split control point of the developmental and metabolic pathways.

Transcriptomic Complexity of Aspergillus terreus Velvet Gene Family under the Influence of Butyrolactone I

Filamentous fungi of the Ascomycota phylum are known to contain a family of conserved conidiation regulating proteins with distinctive velvet domains. In Aspergilli, this velvet family includes four proteins, VeA, VelB, VelC and VosA, and is involved in conidiation and secondary metabolism along with a global regulator LaeA. In A. terreus, the overexpression of LaeA has been observed to increase the biogenesis of the pharmaceutically-important secondary metabolite, lovastatin, while the role of the velvet family has not been studied. The secondary metabolism and conidiation of A. terreus have also been observed to be increased by butyrolactone I in a quorum-sensing manner. An enlightenment of the interplay of these regulators will give potential advancement to the industrial use of this fungus, as well as in resolving the pathogenic features. In this study, the Aspergillus terreus MUCL 38669 transcriptome was strand-specifically sequenced to enable an in-depth gene expression analysis to further investigate the transcriptional role of butyrolactone I in these processes. The sequenced transcriptome revealed intriguing properties of the velvet family transcripts, including the regulator laeA, and uncovered the velC gene in A. terreus. The reliability refining microarray gene expression analysis disclosed a positive regulatory role for butyrolactone I in laeA expression, as well as an influence on the expression of the canonical conidiation-regulating genes under submerged culture. All of this supports the suggested regulative role of butyrolactone I in A. terreus secondary metabolism, as well as conidiation.

Temperature, water activity and pH during conidia production affect the physiological state and germination time of Penicillium species

Conidial germination and mycelial growth are generally studied with conidia produced under optimal conditions to increase conidial yield. Nonetheless, the physiological state of such conidia most likely differs from those involved in spoilage of naturally contaminated food. The present study aimed at investigating the impact of temperature, pH and water activity (a_w) during production of conidia on the germination parameters and compatible solutes of conidia of Penicillium roqueforti and Penicillium expansum. Low temperature (5°C) and reduced a_w (0.900 a_w) during sporulation significantly reduced conidial germination times whereas the pH of the sporulation medium only had a slight effect at the tested values (2.5, 8.0). Conidia of P. roqueforti produced at 5°C germinated up to 45h earlier than those produced at 20°C. Conidia of P. roqueforti and P. expansum produced at 0.900 a_w germinated respectively up to 8h and 3h earlier than conidia produced at 0.980 a_w. Furthermore, trehalose and mannitol assessments suggested that earlier germination might be related to delayed conidial maturation even though no ultra-structural modifications were observed by transmission electron microscopy. Taken together, these results highlight the importance of considering environmental conditions during sporulation in mycological studies. The physiological state of fungal conidia should be taken into account to design challenge tests or predictive mycology studies. This knowledge may also be of interest to improve the germination capacity of fungal cultures commonly used in fermented foods.

Negative regulation and developmental competence in Aspergillus

Asexual development (conidiation) in the filamentous fungus Aspergillus nidulans is governed by orchestrated gene expression. The three key negative regulators of conidiation SfgA, VosA, and NsdD act at different control point in the developmental genetic cascade. Here, we have revealed that NsdD is a key repressor affecting the quantity of asexual spores in Aspergillus. Moreover, nullifying both nsdD and vosA results in abundant formation of the development specific structure conidiophores even at 12 h of liquid culture, and near constitutive activation of conidiation, indicating that acquisition of developmental competence involves the removal of negative regulation exerted by both NsdD and VosA. NsdD's role in repressing conidiation is conserved in other aspergilli, as deleting nsdD causes enhanced and precocious activation of conidiation in Aspergillus fumigatus or Aspergillus flavus. In vivo NsdD-DNA interaction analyses identify three NsdD binding regions in the promoter of the essential activator of conidiation brlA, indicating a direct repressive role of NsdD in conidiation. Importantly, loss of flbC or flbD encoding upstream activators of brlA in the absence of nsdD results in delayed activation of brlA, suggesting distinct positive roles of FlbC and FlbD in conidiation. A genetic model depicting regulation of conidiation in A. nidulans is presented.

Genome-Wide Reprogramming of Transcript Architecture by Temperature Specifies the Developmental States of the Human Pathogen Histoplasma

Eukaryotic cells integrate layers of gene regulation to coordinate complex cellular processes; however, mechanisms of post-transcriptional gene regulation remain poorly studied. The human fungal pathogen Histoplasma capsulatum (Hc) responds to environmental or host temperature by initiating unique transcriptional programs to specify multicellular (hyphae) or unicellular (yeast) developmental states that function in infectivity or pathogenesis, respectively. Here we used recent advances in next-generation sequencing to uncover a novel re-programming of transcript length between Hc developmental cell types. We found that ~2% percent of Hc transcripts exhibit 5’ leader sequences that differ markedly in length between morphogenetic states. Ribosome density and mRNA abundance measurements of differential leader transcripts revealed nuanced transcriptional and translational regulation. One such class of regulated longer leader transcripts exhibited tight transcriptional and translational repression. Further examination of these dually repressed genes revealed that some control Hc morphology and that their strict regulation is necessary for the pathogen to make appropriate developmental decisions in response to temperature.

Eukaryotic cells alter their developmental programs in response to environmental signals. Histoplasma capsulatum (Hc), a ubiquitous fungal pathogen of humans, establishes unique transcriptional programs to specify growth in either a multicellular hyphal form or unicellular yeast form in response to temperature. Since hyphae and yeast are specialized to function in infectivity or pathogenesis, respectively, Hc provides a clinically relevant system in which to query eukaryotic regulatory processes. Here we used next-generation sequencing approaches to annotate the transcriptomes of four distinct Hc strains in response to temperature. We found that a fraction of Hc transcripts have differential transcript architecture in hyphae and yeast, exhibiting 5’ leader sequences that differ markedly in length between morphogenetic states. To begin to understand the effect of these differential leader sequences on expression, we performed the first ribosome density and mRNA abundance measurements in Hc, thereby uncovering transcriptional and translational control that contribute to cell-type regulation.

Morphological changes and growth of filamentous fungi in the presence of high concentrations of PAHs

In this study, we evaluated the effect of low and high molecular weight polycyclic aromatic hydrocarbons (PAHs), i.e., Phenanthrene, Pyrene and Benzo[a]pyrene, on the radial growth and morphology of the PAH-degrading fungal strains Aspergillus nomius H7 and Trichoderma asperellum H15. The presence of PAHs in solid medium produced significant detrimental effects on the radial growth of A. nomius H7 at 4,000 and 6,000 mg L⁻¹ and changes in mycelium pigmentation, abundance and sporulation ability at 1,000–6,000 mg L⁻¹. In contrast, the radial growth of T. asperellum H15 was not affected at any of the doses tested, although sporulation was observed only up to 4,000 mg L⁻¹ and as with the H7 strain, some visible changes in sporulation patterns and mycelium pigmentation were observed. Our results suggest that fungal strains exposed to high doses of PAHs significantly vary in their growth rates and sporulation characteristics in response to the physiological and defense mechanisms that affect both pigment production and conidiation processes. This finding is relevant for obtaining a better understanding of fungal adaptation in PAH-polluted environments and for developing and implementing adequate strategies for the remediation of contaminated soils.

Proteomic analyses reveal the key roles of BrlA and AbaA in biogenesis of gliotoxin in Aspergillus fumigatus

The opportunistic human pathogenic fungus Aspergillus fumigatus primarily reproduces by forming a large number of asexual spores (conidia). Sequential activation of the central regulators BrlA, AbaA and WetA is necessary for the fungus to undergo asexual development. In this study, to address the presumed roles of these key developmental regulators during proliferation of the fungus, we analyzed and compared the proteomes of vegetative cells of wild type (WT) and individual mutant strains. Approximately 1300 protein spots were detectable from 2-D electrophoresis gels. Among these, 13 proteins exhibiting significantly altered accumulation levels were further identified by ESI-MS/MS. Markedly, we found that the GliM and GliT proteins associated with gliotoxin (GT) biosynthesis and self-protection of the fungus from GT were significantly down-regulated in the ΔabaA and ΔbrlA mutants. Moreover, mRNA levels of other GT biosynthetic genes including gliM, gliP, gliT, and gliZ were significantly reduced in both mutant strains, and no and low levels of GT were detectable in the ΔbrlA and ΔabaA mutant strains, respectively. As GliT is required for the protection of the fungus from GT, growth of the ΔbrlA mutant with reduced levels of GliT was severely impaired by exogenous GT. Our studies demonstrate that AbaA and BrlA positively regulate expression of the GT biosynthetic gene cluster in actively growing vegetative cells, and likely bridge morphological and chemical development during the life-cycle of A. fumigatus.

Velvet-mediated repression of β-glucan synthesis in Aspergillus nidulans spores

Beta-glucans are a heterologous group of fibrous glucose polymers that are a major constituent of cell walls in Ascomycetes and Basidiomycetes fungi. Synthesis of β (1,3)- and (1,6)-glucans is coordinated with fungal cell growth and development, thus, is under tight genetic regulation. Here, we report that β-glucan synthesis in both asexual and sexual spores is turned off by the NF-kB like fungal regulators VosA and VelB in Aspergillus nidulans. Our genetic and genomic analyses have revealed that both VosA and VelB are necessary for proper down-regulation of cell wall biosynthetic genes including those associated with β-glucan synthesis in both types of spores. The deletion of vosA or velB results in elevated accumulation of β-glucan in asexual spores. Double mutant analyses indicate that VosA and VelB play an inter-dependent role in repressing β-glucan synthesis in asexual spores. In vivo chromatin immuno-precipitation analysis shows that both VelB and VosA bind to the promoter region of the β-glucan synthase gene fksA in asexual spores. Similarly, VosA is required for proper repression of β-glucan synthesis in sexual spores. In summary, the VosA-VelB hetero-complex is a key regulatory unit tightly controlling proper levels of β-glucan synthesis in asexual and sexual spores.

A dually located multi-HMG-box protein of Aspergillus nidulans has a crucial role in conidial and ascospore germination

Seven HMG-box proteins of Aspergillus nidulans have been identified in the genomic databases. Three of these have the characteristics of non-specific DNA-binding proteins. One of these, AN1267 (HmbB), comprises one canonical HMG-box in its C-terminus and upstream of the canonical box two structurally related boxes, to be called Shadow-HMG-boxes. This protein defines, together with the Podospora anserina mtHMG1, a clade of proteins present in the Pezizomycotina, with orthologues in some of the Taphrinomycotina. HmbB localizes primarily to the mitochondria but occasionally in nuclei. The deletion of the cognate gene results in a number of pleiotropic effects, including those on hyphal morphology, sensitivity to oxidative stress, absence of sterigmatocystin production and changes in the profile of conidial metabolites. The most striking phenotype of deletion strains is a dramatic decrease in conidial and ascospore viability. We show that this is most likely due to the protein being essential to maintain mitochondrial DNA in spores.

WetA is required for conidiogenesis and conidium maturation in the ascomycete fungus Fusarium graminearum

Fusarium graminearum, a prominent fungal pathogen that infects major cereal crops, primarily utilizes asexual spores to spread disease. To understand the molecular mechanisms underlying conidiogenesis in F. graminearum, we functionally characterized the F. graminearum ortholog of Aspergillus nidulans wetA, which has been shown to be involved in conidiogenesis and conidium maturation. Deletion of F. graminearum wetA did not alter mycelial growth, sexual development, or virulence, but the wetA deletion mutants produced longer conidia with fewer septa, and the conidia were sensitive to acute stresses, such as oxidative stress and heat stress. Furthermore, the survival rate of aged conidia from the F. graminearum wetA deletion mutants was reduced. The wetA deletion resulted in vigorous generation of single-celled conidia through autophagy-dependent microcycle conidiation, indicating that WetA functions to maintain conidial dormancy by suppressing microcycle conidiation in F. graminearum. Transcriptome analyses demonstrated that most of the putative conidiation-related genes are expressed constitutively and that only a few genes are specifically involved in F. graminearum conidiogenesis. The conserved and distinct roles identified for WetA in F. graminearum provide new insights into the genetics of conidiation in filamentous fungi.

The N-terminus region of the putative C2H2 transcription factor Ada1 harbors a species-specific activation motif that regulates asexual reproduction in Fusarium verticillioides

Fusarium verticillioides is an important plant pathogenic fungus causing maize ear and stalk rots. In addition, the fungus is directly associated with fumonisin contamination of food and feeds. Here, we report the functional characterization of Ada1, a putative Cys2-His2 zinc finger transcription factor with a high level of similarity to Aspergillus nidulans FlbC, which is required for the activation of the key regulator of conidiation brlA. ADA1 is predicted to encode a protein with two DNA binding motifs at the C terminus and a putative activator domain at the N terminus region. Deletion of the flbC gene in A. nidulans results in "fluffy" cotton-like colonies, with a defect in transition from vegetative growth to asexual development. In this study we show that Ada1 plays a key role in asexual development in F. verticillioides. Conidia production was significantly reduced in the knockout mutant (Δada1), in which aberrant conidia and conidiophores were also observed. We identified genes that are predicted to be downstream of ADA1, based on A. nidulans conidiation signaling pathway. Among them, the deletion of stuA homologue, FvSTUA, resulted in near absence of conidia production. To further investigate the functional conservation of this transcription factor, we complemented the Δada1 strain with A. nidulans flbC, F. verticillioides ADA1, and chimeric constructs. A. nidulans flbC failed to restore conidia production similar to the wild-type level. However, the Ada1N-terminal domain, which contains a putative activator, fused to A. nidulans FlbC C-terminal motif successfully complemented the Δada1 mutant. Taken together, Ada1 is an important transcriptional regulator of asexual development in F. verticillioides and that the N-terminus domain is critical for proper function of this transcription factor.

Oxygen and an extracellular phase transition independently control central regulatory genes and conidiogenesis in Aspergillus fumigatus

Conidiogenesis is the primary process for asexual reproduction in filamentous fungi. As the conidia resulting from the conidiogenesis process are primarily disseminated via air currents and/or water, an outstanding question has been how fungi recognize aerial environments suitable for conidial development. In this study, we documented the somewhat complex development of the conidia-bearing structures, termed conidiophores, from several Aspergillus species in a subsurface (gel-phase) layer of solid media. A subset of the isolates studied was able to develop conidiophores in a gel-phase environment, but exposure to the aeriform environment was required for the terminal developmental transition from phialide cells to conidia. The remaining Aspergilli could not initiate the conidiogenesis process until they were exposed to the aeriform environment. Our observations of conidiophore development in high or low oxygen conditions in both aeriform and gel-phase environments revealed that oxygen and the aeriform state are positive environmental factors for inducing conidiogenesis in most of the aspergilli tested in this study. Transcriptional analysis using A. fumigatus strain AF293 confined to either the aeriform or gel-phase environments revealed that expression of a key regulatory gene for conidiophore development (AfubrlA) is facilitated by oxygen while expression of another regulatory gene controlling conidia formation from phialides (AfuabaA) was repressed regardless of oxygen levels in the gel-embedded environment. Furthermore, by comparing the developmental behavior of conidiation-defective mutants lacking genes controlling various regulatory checkpoints throughout the conidiogenesis pathway, we propose that this aerial response by the fungus requires both oxygen and the phase transition (solid to aeriform), with these environmental signals integrating into the upstream regulatory pathway and central regulatory pathway of conidiogenesis, respectively. Our findings provide not only novel insight into how fungi respond to an aerial environment to trigger development for airborne conidia production but also the relationship between environmental factors and conidiogenesis regulation in aspergilli.

Gβ-like CpcB plays a crucial role for growth and development of Aspergillus nidulans and Aspergillus fumigatus

Growth, development, virulence and secondary metabolism in fungi are governed by heterotrimeric G proteins (G proteins). A Gβ-like protein called Gib2 has been shown to function as an atypical Gβ in Gpa1-cAMP signaling in Cryptococcus neoformans. We found that the previously reported CpcB (cross pathway control B) protein is the ortholog of Gib2 in Aspergillus nidulans and Aspergillus fumigatus. In this report, we further characterize the roles of CpcB in governing growth, development and toxigenesis in the two aspergilli. The deletion of cpcB results in severely impaired cellular growth, delayed spore germination, and defective asexual sporulation (conidiation) in both aspergilli. Moreover, CpcB is necessary for proper expression of the key developmental activator brlA during initiation and progression of conidiation in A. nidulans and A. fumigatus. Somewhat in accordance with the previous study, the absence of cpcB results in the formation of fewer, but not micro-, cleistothecia in A. nidulans in the presence of wild type veA, an essential activator of sexual development. However, the cpcB deletion mutant cleistothecia contain no ascospores, validating that CpcB is required for progression and completion of sexual fruiting including ascosporogenesis. Furthermore, unlike the canonical GβSfaD, CpcB is not needed for the biosynthesis of the mycotoxin sterigmatocystin (ST) as the cpcB null mutant produced reduced amount of ST with unaltered STC gene expression. However, in A. fumigatus, the deletion of cpcB results in the blockage of gliotoxin (GT) production. Further genetic analyses in A. nidulans indicate that CpcB may play a central role in vegetative growth, which might be independent of FadA- and GanB-mediated signaling. A speculative model summarizing the roles of CpcB in conjunction with SfaD in A. nidulans is presented.