The 14-3-3 Protein Homolog ArtA Regulates Development and Secondary Metabolism in the Opportunistic Plant Pathogen Aspergillus flavus

Redundant and Distinct Roles of Two 14-3-3 Proteins in Fusarium sacchari, Pathogen of Sugarcane Pokkah Boeng Disease

Fusarium sacchari, a key pathogen of sugarcane, is responsible for the Pokkah boeng disease (PBD) in China. The 14-3-3 proteins have been implicated in critical developmental processes, including dimorphic transition, signal transduction, and carbon metabolism in various phytopathogenic fungi. However, their roles are poorly understood in F. sacchari. This study focused on the characterization of two 14-3-3 protein-encoding genes, FsBmh1 and FsBmh2, within F. sacchari. Both genes were found to be expressed during the vegetative growth stage, yet FsBmh1 was repressed at the sporulation stage in vitro. To elucidate the functions of these genes, the deletion mutants ΔFsBmh1 and ΔFsBmh2 were generated. The ΔFsBmh2 exhibited more pronounced phenotypic defects, such as impaired hyphal branching, septation, conidiation, spore germination, and colony growth, compared to the ΔFsBmh1. Notably, both knockout mutants showed a reduction in virulence, with transcriptome analysis revealing changes associated with the observed phenotypes. To further investigate the functional interplay between FsBmh1 and FsBmh2, we constructed and analyzed mutants with combined deletion and silencing (ΔFsBmh/siFsBmh) as well as overexpression (O-FsBmh). The combinations of ΔFsBmh1/siFsBmh2 or ΔFsBmh2/siFsBmh1 displayed more severe phenotypes than those with single allele deletions, suggesting a functional redundancy between the two 14-3-3 proteins. Yeast two-hybrid (Y2H) assays identified 20 proteins with pivotal roles in primary metabolism or diverse biological functions, 12 of which interacted with both FsBmh1 and FsBmh2. Three proteins were specifically associated with FsBmh1, while five interacted exclusively with FsBmh2. In summary, this research provides novel insights into the roles of FsBmh1 and FsBmh2 in F. sacchari and highlights potential targets for PBD management through the modulation of FsBmh functions.

The Histone Deacetylase HstD Regulates Fungal Growth, Development and Secondary Metabolite Biosynthesis in Aspergillus terreus

Histone acetylation modification significantly affects secondary metabolism in filamentous fungi. However, how histone acetylation regulates secondary metabolite synthesis in the lovastatin (a lipid-lowering drug) producing Aspergillus terreus remains unknown because protein is involved and has been identified in this species. Here, the fungal-specific histone deacetylase gene, hstD, was characterized through functional genomics in two marine-derived A. terreus strains, Mj106 and RA2905. The results showed that the ablation of HstD resulted in reduced mycelium growth, less conidiation, and decreased lovastatin biosynthesis but significantly increased terrein biosynthesis. However, unlike its homologs in yeast, HstD was not required for fungal responses to DNA damage agents, indicating that HstD likely plays a novel role in the DNA damage repair process in A. terreus. Furthermore, the loss of HstD resulted in a significant upregulation of H3K56 and H3K27 acetylation when compared to the wild type, suggesting that epigenetic functions of HstD, as a deacetylase, target H3K27 and H3K56. Additionally, a set of no-histone targets with potential roles in fungal growth, conidiation, and secondary metabolism were identified for the first time using acetylated proteomic analysis. In conclusion, we provide a comprehensive analysis of HstD for its targets in histone or non-histone and its roles in fungal growth and development, DNA damage response, and secondary metabolism in A. terreus.

Ssu72 Regulates Fungal Development, Aflatoxin Biosynthesis and Pathogenicity in Aspergillus flavus

The RNA polymerase II (Pol II) transcription process is coordinated by the reversible phosphorylation of its largest subunit-carboxy terminal domain (CTD). Ssu72 is identified as a CTD phosphatase with specificity for phosphorylation of Ser5 and Ser7 and plays critical roles in regulation of transcription cycle in eukaryotes. However, the biofunction of Ssu72 is still unknown in Aspergillus flavus, which is a plant pathogenic fungus and produces one of the most toxic mycotoxins-aflatoxin. Here, we identified a putative phosphatase Ssu72 and investigated the function of Ssu72 in A. flavus. Deletion of ssu72 resulted in severe defects in vegetative growth, conidiation and sclerotia formation. Additionally, we found that phosphatase Ssu72 positively regulates aflatoxin production through regulating expression of aflatoxin biosynthesis cluster genes. Notably, seeds infection assays indicated that phosphatase Ssu72 is crucial for pathogenicity of A. flavus. Furthermore, the Δssu72 mutant exhibited more sensitivity to osmotic and oxidative stresses. Taken together, our study suggests that the putative phosphatase Ssu72 is involved in fungal development, aflatoxin production and pathogenicity in A. flavus, and may provide a novel strategy to prevent the contamination of this pathogenic fungus.

Aspergillus flavus Exploits Maize Kernels Using an "Orphan" Secondary Metabolite Cluster

Aspergillus flavus is a saprophytic cosmopolitan fungus, capable of infecting crops both pre- and post-harvest and exploiting different secondary metabolites, including aflatoxins. Aflatoxins are known carcinogens to animals and humans, but display no clear effect in host plants such as maize. In a previous study, we mined the genome of A. flavus to identify secondary metabolite clusters putatively involving the pathogenesis process in maize. We now focus on cluster 32, encoding for fungal effectors such as salicylate hydroxylase (SalOH), and necrosis- and ethylene-inducing proteins (npp1 domain protein) whose expression is triggered upon kernel contact. In order to understand the role of this genetic cluster in maize kernel infection, mutants of A. flavus, impaired or enhanced in specific functions (e.g., cluster 32 overexpression), were studied for their ability to cause disease. Within this frame, we conducted histological and histochemical experiments to verify the expression of specific genes within the cluster (e.g., SalOH, npp1), the production of salicylate, and the presence of its dehydroxylated form. Results suggest that the initial phase of fungal infection (2 days) of the living tissues of maize kernels (e.g., aleuron) coincides with a significant increase of fungal effectors such as SalOH and Npp1 that appear to be instrumental in eluding host defences and colonising the starch-enriched tissues, and therefore suggest a role of cluster 32 to the onset of infection.

Biotinylated Surfome Profiling Identifies Potential Biomarkers for Diagnosis and Therapy of Aspergillus fumigatus Infection

Aspergillus fumigatus is one of the most common airborne molds capable of causing mycoses and allergies in humans. During infection, fungal surface proteins mediate the first contact with the human immune system to evade immune responses or to induce hypersensitivity. Several methods have been established for surface proteomics (surfomics). Biotinylation coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) identification of peptides is a particularly efficient method to identify the surface-exposed regions of proteins that potentially mediate interaction with the host. After biotinylation of surface proteins during spore germination, we detected 231 different biotinylated surface proteins (including several well-known proteins such as RodA, CcpA, and DppV; allergens; and heat shock proteins [HSPs]), as well as some previously undescribed surface proteins. The dynamic change of the surface proteome was illustrated by detection of a relatively high number of proteins exclusively at one developmental stage. Using immunofluorescence microscopy, we confirmed the surface localization of several HSPs of the HSP70 family, which may have moonlighting functions. Collectively, by comparing our data with data representative of previously published A. fumigatus surface proteomes, our study generated a comprehensive data set corresponding to the A. fumigatus surfome and uncovered the surface-exposed regions of many proteins on the surface of conidia or hyphae. These surface-exposed regions are candidates for direct interaction with host cells and may represent antigenic epitopes that either induce protective immune responses or mediate immune evasion. Thus, our data sets provided and compiled here represent reasonable immunotherapy and diagnostic targets for future investigations.IMPORTANCEAspergillus fumigatus is the most important airborne human-pathogenic mold, capable of causing both life-threatening invasive pulmonary aspergillosis in immunocompromised patients and allergy-inducing infections in individuals with atopic allergy. Despite its obvious medical relevance, timely diagnosis and efficient antifungal treatment of A. fumigatus infection remain major challenges. Proteins on the surface of conidia (asexually produced spores) and mycelium directly mediate host-pathogen interaction and also may serve as targets for diagnosis and immunotherapy. However, the similarity of protein sequences between A. fumigatus and other organisms, sometimes even including the human host, makes selection of targets for immunological-based studies difficult. Here, using surface protein biotinylation coupled with LC-MS/MS analysis, we identified hundreds of A. fumigatus surface proteins with exposed regions, further defining putative targets for possible diagnostic and immunotherapeutic design.

Ste2 receptor-mediated chemotropism of Fusarium graminearum contributes to its pathogenicity against wheat

Fusarium Head Blight of wheat, caused by the filamentous fungus Fusarium graminearum, leads to devastating global food shortages and economic losses. While many studies have addressed the responses of both wheat and F. graminearum during their interaction, the possibility of fungal chemotropic sensing enabling pathogenicity remains unexplored. Based on recent findings linking the pheromone-sensing G-protein-coupled receptor Ste2 to host-directed chemotropism in Fusarium oxysporum, we investigated the role of the Ste2 receptor and its downstream signaling pathways in mediating chemotropism of F. graminearum. Interestingly, a chemotropic response of growing hyphae towards catalytically active Triticum aestivum ‘Roblin’ cultivar secreted peroxidases was detected, with deletion of STE2 in F. graminearum leading to loss of the observed response. At the same time, deletion of STE2 significantly decreased infection on germinating wheat coleoptiles, highlighting an association between Ste2, chemotropism and infection by F. graminearum. Further characterization revealed that the peroxidase-directed chemotropism is associated with stimulation of the fungal cell wall integrity mitogen-activated protein kinase signaling cascade. Altogether, this study demonstrates conservation of Ste2-mediated chemotropism by Fusarium species, and its important role in mediating pathogenicity.

Two 14-3-3 proteins contribute to nitrogen sensing through the TOR and glutamine synthetase-dependent pathways in Fusarium graminearum

Fusarium graminearum responds to environmental cues to modulate its growth and metabolism during wheat pathogenesis. Nitrogen limitation activates virulence-associated behaviours in F. graminearum including mycotoxin production and penetrative growth. In other filamentous fungi, nitrogen sensing is mediated by both the Target of Rapamycin (TOR) and the glutamine synthetase (GS)-dependent signaling pathways. While TOR-dependent nitrogen responses have been demonstrated in F. graminearum, the involvement of GS remains unclear. Our study indicates that both the TOR and GS signalling pathways are involved in nitrogen sensing in F. graminearum and contribute to glutamine-induced mycelial growth. However, neither pathway is required for glutamine-induced repression of the mycotoxin deoxynivalenol (DON) indicating that an additional nitrogen sensing pathway must exist. Further, two genes FgBMH1 and FgBMH2 encoding 14-3-3 proteins regulate nitrogen responses with effects on gene expression, DON production and mycelial growth. Unlike yeast, where 14-3-3s function redundantly in regulating nitrogen sensing, the 14-3-3 proteins have differing functions in F. graminearum. While both FgBMH1 and FgBMH2 regulate early glutamine-induced DON repression, only FgBMH2 is involved in regulating reproduction, virulence and glutamine-induced AreA repression. Together, our findings help to clarify the nitrogen sensing pathways in F. graminearum and highlight the involvement of 14-3-3s in the nitrogen response of filamentous fungi.

Taxonomy of Aspergillus section Flavi and their production of aflatoxins, ochratoxins and other mycotoxins

Aflatoxins and ochratoxins are among the most important mycotoxins of all and producers of both types of mycotoxins are present in Aspergillus section Flavi, albeit never in the same species. Some of the most efficient producers of aflatoxins and ochratoxins have not been described yet. Using a polyphasic approach combining phenotype, physiology, sequence and extrolite data, we describe here eight new species in section Flavi. Phylogenetically, section Flavi is split in eight clades and the section currently contains 33 species. Two species only produce aflatoxin B1 and B2 (A. pseudotamarii and A. togoensis), and 14 species are able to produce aflatoxin B1, B2, G1 and G2: three newly described species A. aflatoxiformans, A. austwickii and A. cerealis in addition to A. arachidicola, A. minisclerotigenes, A. mottae, A. luteovirescens (formerly A. bombycis), A. nomius, A. novoparasiticus, A. parasiticus, A. pseudocaelatus, A. pseudonomius, A. sergii and A. transmontanensis. It is generally accepted that A. flavus is unable to produce type G aflatoxins, but here we report on Korean strains that also produce aflatoxin G1 and G2. One strain of A. bertholletius can produce the immediate aflatoxin precursor 3-O-methylsterigmatocystin, and one strain of Aspergillus sojae and two strains of Aspergillus alliaceus produced versicolorins. Strains of the domesticated forms of A. flavus and A. parasiticus, A. oryzae and A. sojae, respectively, lost their ability to produce aflatoxins, and from the remaining phylogenetically closely related species (belonging to the A. flavus-, A. tamarii-, A. bertholletius- and A. nomius-clades), only A. caelatus, A. subflavus and A. tamarii are unable to produce aflatoxins. With exception of A. togoensis in the A. coremiiformis-clade, all species in the phylogenetically more distant clades (A. alliaceus-, A. coremiiformis-, A. leporis- and A. avenaceus-clade) are unable to produce aflatoxins. Three out of the four species in the A. alliaceus-clade can produce the mycotoxin ochratoxin A: A. alliaceus s. str. and two new species described here as A. neoalliaceus and A. vandermerwei. Eight species produced the mycotoxin tenuazonic acid: A. bertholletius, A. caelatus, A. luteovirescens, A. nomius, A. pseudocaelatus, A. pseudonomius, A. pseudotamarii and A. tamarii while the related mycotoxin cyclopiazonic acid was produced by 13 species: A. aflatoxiformans, A. austwickii, A. bertholletius, A. cerealis, A. flavus, A. minisclerotigenes, A. mottae, A. oryzae, A. pipericola, A. pseudocaelatus, A. pseudotamarii, A. sergii and A. tamarii. Furthermore, A. hancockii produced speradine A, a compound related to cyclopiazonic acid. Selected A. aflatoxiformans, A. austwickii, A. cerealis, A. flavus, A. minisclerotigenes, A. pipericola and A. sergii strains produced small sclerotia containing the mycotoxin aflatrem. Kojic acid has been found in all species in section Flavi, except A. avenaceus and A. coremiiformis. Only six species in the section did not produce any known mycotoxins: A. aspearensis, A. coremiiformis, A. lanosus, A. leporis, A. sojae and A. subflavus. An overview of other small molecule extrolites produced in Aspergillus section Flavi is given.

14-3-3 Proteins: a window for a deeper understanding of fungal metabolism and development

Isoforms of 14-3-3 proteins, similar to their highly conserved homologs in mammals and plants, are both transcriptionally and functionally affected by their extracellular and intracellular environments. These proteins bind to phosphorylated client proteins to modulate their functions in fungi. Since phosphorylation regulates a plethora of different physiological responses in organisms, 14-3-3 proteins play roles in multiple physiological functions, including those controlling metabolisms, cell division, and responses to environmental stimulation. These proteins could also modulate signaling pathways that transduce inputs from the environment and downstream proteins that elicit physiological responses. Increasing evidence supports a prominent role for 14-3-3 proteins in regulating development and metabolism at various levels. In this review, we first provide a brief summary of the molecular structure of 14-3-3 proteins. Second, we discuss the potential roles of 14-3-3 proteins in the regulation of development and metabolism. Third, we review the roles of 14-3-3 proteins in the regulation of their binding partners, including receptors, protein kinases, and some protein kinase substrates. Finally, this review examines recent advances that further elucidate the role of 14-3-3 proteins in signaling transduction in response to environmental stress.