The tetrameric pheromone module SteC-MkkB-MpkB-SteD regulates asexual sporulation, sclerotia formation and aflatoxin production in Aspergillus flavus

Glucose and HODEs regulate Aspergillus ochraceus quorum sensing through the GprC-AcyA pathway

Aspergillus ochraceus is the traditional ochratoxin A (OTA)-producing fungus with density-dependent behaviors, which is known as quorum sensing (QS) that is mediated by signaling molecules. Individual cells trend to adapt environmental changes in a "whole" flora through communications, allowing fungus to occupy an important ecological niche. Signals perception, transmission, and feedback are all rely on a signal network that constituted by membrane receptors and intracellular effectors. However, the interference of density information in signal transduction, which regulates most life activities of Aspergillus, have yet to be elucidated. Here we show that the G protein-coupled receptor (GPCR) to cAMP pathway is responsible for transmitting density information, and regulates the key point in life cycle of A. ochraceus. Firstly, the quorum sensing phenomenon of A. ochraceus is confirmed, and identified the density threshold is 103 spores/mL, which represents the low density that produces the most OTA in a series quorum density. Moreover, the GprC that classified as sugar sensor, and intracellular adenylate cyclase (AcyA)-cAMP-PKA pathway that in response to ligands glucose and HODEs are verified. Furthermore, GprC and AcyA regulate the primary metabolism as well as secondary metabolism, and further affects the growth of A. ochraceus during the entire life cycle. These studies highlight a crucial G protein signaling pathway for cell communication that is mediated by carbohydrate and oxylipins, and clarified a comprehensive effect of fungal development, which include the direct gene regulation and indirect substrate or energy supply. Our work revealed more signal molecules that mediated density information and connected effects on important adaptive behaviors of Aspergillus ochraceus, hoping to achieve comprehensive prevention and control of mycotoxin pollution from interrupting cell communication.

Fus3 Interacts with Gal83, Revealing the MAPK Crosstalk to Snf1/AMPK to Regulate Secondary Metabolic Substrates in Aspergillus flavus

Aflatoxins (AFs), highly carcinogenic natural products, are produced by the secondary metabolism of fungi such as Aspergillus flavus. Essential for the fungi to respond to environmental changes and aflatoxin synthesis, the pheromone mitogen-activated protein kinase (MAPK) is a potential regulator of aflatoxin biosynthesis. However, the mechanism by which pheromone MAPK regulates aflatoxin biosynthesis is not clear. Here, we showed Gal83, a new target of Fus3, and identified the pheromone Fus3-MAPK signaling pathway as a regulator of the Snf1/AMPK energy-sensing pathway modulating aflatoxins synthesis substrates. The screening for Fus3 target proteins identified the β subunit of Snf1/AMPK complexes using tandem affinity purification and multiomics. This subunit physically interacted with Fus3 both in vivo and in vitro and received phosphorylation from Fus3. Although the transcript levels of aflatoxin synthesis genes were not noticeably downregulated in both gal83 and fus3 deletion mutant strains, the levels of aflatoxin B1 and its synthesis substrates and gene expression levels of primary metabolizing enzymes were significantly reduced. This suggests that both the Fus3-MAPK and Snf1/AMPK pathways respond to energy signals. In conclusion, all the evidence unlocks a novel pathway of Fus3-MAPK to regulate AFs synthesis substrates by cross-talking with the Snf1/AMPK complexes.

Fus3 regulates asexual development and trap morphogenesis in the nematode-trapping fungus Arthrobotrys oligospora

Mitogen-activated protein kinase (MAPK) Fus3 is an essential regulator of cell differentiation and virulence in fungal pathogens of plants and animals. However, the function and regulatory mechanism of MAPK signaling in nematode-trapping (NT) fungi remain largely unknown. NT fungi can specialize in the formation of "traps", an important indicator of transition from a saprophytic to a predatory lifestyle. Here, we characterized an orthologous Fus3 in a typical NT fungus Arthrobotrys oligospora using multi-phenotypic analysis and multi-omics approaches. Our results showed that Fus3 plays an important role in asexual growth and development, conidiation, stress response, DNA damage, autophagy, and secondary metabolism. Importantly, Fus3 plays an indispensable role in hyphal fusion, trap morphogenesis, and nematode predation. Moreover, we constructed the regulatory networks of Fus3 by means of transcriptomic and yeast two-hybrid techniques. This study provides insights into the mechanism of MAPK signaling in asexual development and pathogenicity of NT fungi.

Proteomic dissection of the role of GliZ in gliotoxin biosynthesis in Aspergillus fumigatus

Gliotoxin (GT) biosynthesis in fungi is encoded by the gli biosynthetic gene cluster. While GT addition autoinduces biosynthesis, Zn²⁺ has been shown to attenuate cluster activity, and it was speculated that identification of Zn2Cys6 binuclear transcription factor GliZ binding partners might provide insight into this observation. Using the Tet-ON induction system, doxycycline (DOX) presence induced GliZ fusion protein expression in, and recovery of GT biosynthesis by, A. fumigatus ΔgliZ::HA-gliZ and ΔgliZ::TAP-gliZ strains, respectively. Quantitative RT-PCR confirmed that DOX induces gli cluster gene expression (n = 5) in both A. fumigatus HA-GliZ and TAP-GliZ strains. GT biosynthesis was evident in Czapek-Dox and in Sabouraud media, however tagged GliZ protein expression was more readily detected in Sabouraud media. Unexpectedly, Zn²⁺ was essential for GliZ fusion protein expression in vivo, following 3 h DOX induction. Moreover, HA-GliZ abundance was significantly higher in either DOX/GT or DOX/Zn²⁺, compared to DOX-only. This suggests that while GT induction is still intact, Zn²⁺ inhibition of HA-GliZ production in vivo is lost. Co-immunoprecipitation revealed that GT oxidoreductase GliT associates with GliZ in the presence of GT, suggesting a potential protective role. Additional putative HA-GliZ interacting proteins included cystathionine gamma lyase, ribosomal protein L15 and serine hydroxymethyltransferase (SHMT). Total mycelial quantitative proteomic data revealed that GliT and GtmA, as well as several other gli cluster proteins, are increased in abundance or uniquely expressed with GT addition. Proteins involved in sulphur metabolism are also differentially expressed with GT or Zn²⁺ presence. Overall, we disclose that under DOX induction GliZ functionality is unexpectedly evident in zinc-replete media, subject to GT induction and that GliT appears to associate with GliZ, potentially to prevent DTG-mediated GliZ inactivation by zinc ejection.

SfgA Renders Aspergillus flavus More Stable to the External Environment

sfgA is known as a key negative transcriptional regulator gene of asexual sporulation and sterigmatocystin production in Aspergillus nidulans. However, here, we found that the homolog sfgA gene shows a broad and complex regulatory role in governing growth, conidiation, sclerotia formation, secondary metabolism, and environmental stress responses in Aspergillus flavus. When sfgA was deleted in A. flavus, the fungal growth was slowed, but the conidiation was significantly increased, and the sclerotia formation displayed different behavior at different temperatures, which increased at 30 °C but decreased at 36 °C. In addition, sfgA regulated aflatoxin biosynthesis in a complex way that was associated with the changes in cultured conditions, and the increased production of aflatoxin in the ∆sfgA mutant was associated with a decrease in sclerotia size. Furthermore, the ∆sfgA mutant exhibited sensitivity to osmotic, oxidative, and cell wall stresses but still produced dense conidia. Transcriptome data indicated that numerous development- and secondary-metabolism-related genes were expressed differently when sfgA was deleted. Additionally, we also found that sfgA functions downstream of fluG in A. flavus, which is consistent with the genetic position in FluG-mediated conidiation in A. nidulans. Collectively, sfgA plays a critical role in the development, secondary metabolism, and stress responses of A. flavus, and sfgA renders A. flavus more stable to the external environment.

Systematic Characterization of bZIP Transcription Factors Required for Development and Aflatoxin Generation by High-Throughput Gene Knockout in Aspergillus flavus

The basic leucine zipper (bZIP) is an important transcription factor required for fungal development, nutrient utilization, biosynthesis of secondary metabolites, and defense against various stresses. Aspergillus flavus is a major producer of aflatoxin and an opportunistic fungus on a wide range of hosts. However, little is known about the role of most bZIP genes in A. flavus. In this study, we developed a high-throughput gene knockout method based on an Agrobacterium-mediated transformation system. Gene knockout construction by yeast recombinational cloning and screening of the null mutants by double fluorescence provides an efficient way to construct gene-deleted mutants for this multinucleate fungus. We deleted 15 bZIP genes in A. flavus. Twelve of these genes were identified and characterized in this strain for the first time. The phenotypic analysis of these mutants showed that the 15 bZIP genes play a diverse role in mycelial growth (eight genes), conidiation (13 genes), aflatoxin biosynthesis (10 genes), oxidative stress response (11 genes), cell wall stress (five genes), osmotic stress (three genes), acid and alkali stress (four genes), and virulence to kernels (nine genes). Impressively, all 15 genes were involved in the development of sclerotia, and the respective deletion mutants of five of them did not produce sclerotia. Moreover, MetR was involved in this biological process. In addition, HapX and MetR play important roles in the adaptation to excessive iron and sulfur metabolism, respectively. These studies provide comprehensive insights into the role of bZIP transcription factors in this aflatoxigenic fungus of global significance.

Fus3, as a Critical Kinase in MAPK Cascade, Regulates Aflatoxin Biosynthesis by Controlling the Substrate Supply in Aspergillus flavus, Rather than the Cluster Genes Modulation

The Fus3-MAP kinase module is a conserved phosphorylation signal system in eukaryotes that responds to environmental stress and transduction of external signals from the outer membrane to the nucleus. Aspergillus flavus can produce aflatoxins (AF), which seriously threaten human and animal health. In this study, we determined the functions of Fus3, confirmed Ste50-Ste11-Ste7-Fus3 protein interactions and phosphorylation, and explored the possible phosphorylation motifs and potential targets of Fus3. The regulatory mechanism of Fus3 on the biosynthesis of AF was partly revealed in this study. AF production was downregulated in Δfus3, but the transcriptional expression of most AF cluster genes was upregulated. It is notable that the levels of acetyl-CoA and malonyl-CoA, the substrates of AF, were significantly decreased in fus3 defective strains. Genes involved in acetyl-CoA and malonyl-CoA biosynthesis were significantly downregulated at transcriptional or phosphorylation levels. Specifically, AccA might be a direct target of Fus3, which led to acetyl-CoA carboxylase activities were decreased in null-deletion and site mutagenesis strains. The results concluded that Fus3 could regulate the expression of acetyl-CoA and malonyl-CoA biosynthetic genes directly or indirectly, and then affect the AF production that relies on the regulation of AF substrate rather than the modulation of AF cluster genes.

IMPORTANCEAspergillus flavus is an important saprophytic fungus that produces aflatoxins (AF), which threaten food and feed safety. MAP (mitogen-activated protein) kanases are essential for fungal adaptation to diverse environments. Fus3, as the terminal kinase of a MAPK cascade, interacts with other MAPK modules and phosphorylates downstream targets. We provide evidence that Fus3 could affect AF biosynthesis by regulating the production of acetyl-CoA and malonyl-CoA, but this does not depend on the regulation of AF biosynthetic genes. Our results partly reveal the regulatory mechanism of Fus3 on AF biosynthesis and provide a novel AF modulation pattern, which may contribute to the discovery of new strategies in controlling A. flavus and AF contamination.

Genomic and Transcriptomic Insight of Giant Sclerotium Formation of Wood-Decay Fungi

Many fungi form persistent and dormant sclerotia with compact hardened mycelia during unfavorable circumstances. While most of these sclerotia are small in size, Wolfiporia cocos, a wood-decay fungus, grows into giant sclerotia, which are mainly composed of polysaccharides of linear (1→3)-β-D-glucans. To explore the underlying mechanism of converting sophisticated wood polysaccharides for biosynthesis of highly homogenized glucans in W. cocos, we sequenced and assembled the genome of a cultivated W. cocos strain (WCLT) in China. The 62-Mb haploid genome contains 44.2% repeat sequences, of which, 48.0% are transposable elements (TEs). Contrary to the genome of W. cocos from North America, WCLT has independently undergone a partial genome duplication (PGD) event. The large-scale TE insertion and PGD occurrence overlapped with an archeological Pleistocene stage of low oxygen and high temperature, and these stresses might have induced the differences in sclerotium due to geographical distribution. The wood decomposition enzymes, as well as sclerotium-regulator kinases, aquaporins, and highly expanded gene families such as NAD-related families, together with actively expressed 1,3-β-glucan synthase for sclerotium polysaccharides, all have contributed to the sclerotium formation and expansion. This study shall inspire further exploration on how fungi convert wood into simple glucans in the sclerotium of W. cocos.

Turning Inside Out: Filamentous Fungal Secretion and Its Applications in Biotechnology, Agriculture, and the Clinic

Filamentous fungi are found in virtually every marine and terrestrial habitat. Vital to this success is their ability to secrete a diverse range of molecules, including hydrolytic enzymes, organic acids, and small molecular weight natural products. Industrial biotechnologists have successfully harnessed and re-engineered the secretory capacity of dozens of filamentous fungal species to make a diverse portfolio of useful molecules. The study of fungal secretion outside fermenters, e.g., during host infection or in mixed microbial communities, has also led to the development of novel and emerging technological breakthroughs, ranging from ultra-sensitive biosensors of fungal disease to the efficient bioremediation of polluted environments. In this review, we consider filamentous fungal secretion across multiple disciplinary boundaries (e.g., white, green, and red biotechnology) and product classes (protein, organic acid, and secondary metabolite). We summarize the mechanistic understanding for how various molecules are secreted and present numerous applications for extracellular products. Additionally, we discuss how the control of secretory pathways and the polar growth of filamentous hyphae can be utilized in diverse settings, including industrial biotechnology, agriculture, and the clinic.

CcPmk1 is a regulator of pathogenicity in Cytospora chrysosperma and can be used as a potential target for disease control

Fus3/Kss1, also known as Pmk1 in several pathogenic fungi, is a component of the mitogen-activated protein kinase (MAPK) signalling pathway that functions as a regulator in fungal development, stress response, mating, and pathogenicity. Cytospora chrysosperma, a notorious woody plant-pathogenic fungus, causes canker disease in many species, and its Pmk1 homolog, CcPmk1, is required for fungal development and pathogenicity. However, the global regulation network of CcPmk1 is still unclear. In this study, we compared transcriptional analysis between a CcPmk1 deletion mutant and the wild type during the simulated infection process. A subset of transcription factor genes and putative effector genes were significantly down-regulated in the CcPmk1 deletion mutant, which might be important for fungal pathogenicity. Additionally, many tandem genes were found to be regulated by CcPmk1. Eleven out of 68 core secondary metabolism biosynthesis genes and several gene clusters were significantly down-regulated in the CcPmk1 deletion mutant. GO annotation of down-regulated genes showed that the ribosome biosynthesis-related processes were over-represented in the CcPmk1 deletion mutant. Comparison of the CcPmk1-regulated genes with the Pmk1-regulated genes from Magnaporthe oryzae revealed only a few overlapping regulated genes in both CcPmk1 and Pmk1, while the enrichment GO terms in the ribosome biosynthesis-related processes were also found. Subsequently, we calculated that in vitro feeding artificial small interference RNAs of CcPmk1 could silence the target gene, resulting in inhibited fungal growth. Furthermore, silencing of BcPmk1 in Botrytis cinerea with conserved CcPmk1 and BcPmk1 fragments could significantly compromise fungal virulence using the virus-induced gene silencing system in Nicotiana benthamiana. These results suggest that CcPmk1 functions as a regulator of pathogenicity and can potentially be designed as a target for broad-spectrum disease control, but unintended effects on nonpathogenic fungi need to be avoided.

The Conserved MAP Kinase MpkB Regulates Development and Sporulation without Affecting Aflatoxin Biosynthesis in Aspergillus flavus

In eukaryotes, the MAP kinase signaling pathway plays pivotal roles in regulating the expression of genes required for growth, development, and stress response. Here, we deleted the mpkB gene (AFLA_034170), an ortholog of the Saccharomyces cerevisiae FUS3 gene, to characterize its function in Aspergillus flavus, a cosmopolitan, pathogenic, and aflatoxin-producing fungus. Previous studies revealed that MpkB positively regulates sexual and asexual differentiation in Aspergillus nidulans. In A. flavus, mpkB deletion resulted in an approximately 60% reduction in conidia production compared to the wild type without mycelial growth defects. Moreover, the mutant produced immature and abnormal conidiophores exhibiting vesicular dome-immaturity in the conidiophore head, decreased phialide numbers, and very short stalks. Interestingly, the ΔmpkB mutant could not produce sclerotia but produced aflatoxin B1 normally. Taken together, these results suggest that the A. flavus MpkB MAP kinase positively regulates conidiation and sclerotia formation but is not involved in the production of secondary metabolites such as aflatoxin B1.

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

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

The Pheromone Module SteC-MkkB-MpkB-SteD-HamE Regulates Development, Stress Responses and Secondary Metabolism in Aspergillus fumigatus

In order for eukaryotes to efficiently detect and respond to environmental stimuli, a myriad of protein signaling pathways are utilized. An example of highly conserved signaling pathways in eukaryotes are the mitogen-activated protein kinase (MAPK) pathways. In fungi, MAPK pathways have been shown to regulate a diverse array of biological processes, such as asexual and sexual development, stress responses and the production of secondary metabolites (SMs). In the model fungus Aspergillus nidulans, a MAPK pathway known as the pheromone module is utilized to regulate both development and SM production. This signaling cascade consists of the three kinases SteC, MkkB, and MpkB, as well as the SteD adaptor protein and the HamE scaffold. In this study, homologs of each of these proteins have been identified in the opportunistic human pathogen A. fumigatus. By performing epitope tagging and mass spectrometry experiments, we have shown that these proteins form a pentameric complex, similar to what is observed in A. nidulans. This complex has been shown to assemble in the cytoplasm and MpkB enters the nucleus, where it would presumably interact with various transcription factors. Pheromone module mutant strains exhibit drastic reductions in asexual sporulation, vegetative growth rate and production of SMs, such as gliotoxin. Mutants also display increased sensitivity to cell wall and oxidative stress agents. Overall, these data provide evidence of the existence of a conserved MAP kinase signaling pathway in Aspergillus species and suggest that this pathway is critical for the regulation of fungal development and secondary metabolism.

The tetrameric pheromone module SteC-MkkB-MpkB-SteD regulates asexual sporulation, sclerotia formation and aflatoxin production in Aspergillus flavus

For eukaryotes like fungi to regulate biological responses to environmental stimuli, various signalling cascades are utilized, like the highly conserved mitogen‐activated protein kinase (MAPK) pathways. In the model fungus Aspergillus nidulans, a MAPK pathway known as the pheromone module regulates development and the production of secondary metabolites (SMs). This pathway consists five proteins, the three kinases SteC, MkkB and MpkB, the adaptor SteD and the scaffold HamE. In this study, homologs of these five pheromone module proteins have been identified in the plant and human pathogenic fungus Aspergillus flavus. We have shown that a tetrameric complex consisting of the three kinases and the SteD adaptor is assembled in this species. It was observed that this complex assembles in the cytoplasm and that MpkB translocates into the nucleus. Deletion of steC, mkkB, mpkB or steD results in abolishment of both asexual sporulation and sclerotia production. This complex is required for the positive regulation of aflatoxin production and negative regulation of various SMs, including leporin B and cyclopiazonic acid (CPA), likely via MpkB interactions in the nucleus. These data highlight the conservation of the pheromone module in Aspergillus species, signifying the importance of this pathway in regulating fungal development and secondary metabolism.