MAPK pathway-related tyrosine phosphatases regulate development, secondary metabolism and pathogenicity in fungus Aspergillus flavus

Importance of benzoyltransferase GcnE and lysine benzoylation of alcohol dehydrogenase AdhB in pathogenesis and aflatoxin production in Aspergillus flavus

Lysine benzoylation (Kbz) is a newly identified post-translational modification associated with active transcription and metabolism in eukaryotes. However, whether Kbz exists in pathogenic fungi and its function remains unknown. Here, we demonstrated for the first time that Kbz is present in Aspergillus flavus and identified 60 benzoylated sites on 46 benzoylated proteins by global benzoylome analysis. Our data demonstrated that alcohol dehydrogenase B (AdhB) is regulated by benzoylation on lysine 321 (K321), and mutations of Kbz site in AdhB significantly reduced the alcohol dehydrogenase activity in vivo and in vitro. Both adhB deletion mutant and benzoylated site mutants (K321R and K321A) exhibited similar phenotype, including decreased conidiation and seed colonization, increased sclerotia formation and aflatoxin production, and more sensitive to cell wall damage stress. We also found that GcnE has benzoyltransferase activity in vitro and in vivo, and its repression leads to decreased Kbz level and enzymatic activity of AdhB. The catalytic site E139 is important for the benzoyltransferase function of GcnE. Our study uncovers a previously unknown mechanism by which benzoylation regulates AdhB activity to affect the development, secondary metabolism, pathogenicity, and stress response of A. flavus. Meanwhile, it points out the important role of Kbz in the pathogenicity of pathogenic fungi.IMPORTANCEAspergillus flavus is a ubiquitous opportunistic pathogen of plants and animals, which produces carcinogenic and toxic secondary metabolite aflatoxin. A. flavus and aflatoxin contamination have emerged as a global food safety concern. Currently, post-translational modification plays crucial modulatory roles in the fungal development and virulence, but the role of benzoylation in fungal pathogenicity remains undetermined, which limits the development of prevention and control technique. Here, we first identified 46 benzoylated proteins in A. flavus, and found that benzoyltransferase GcnE exerted effects on pathogenicity and aflatoxin production by regulating the benzoylation of AdhB. This finding not only provided valuable information for prevention and control of A. flavus contamination, but also offered basic knowledge for investigation of the regulation mechanism of secondary metabolism in other fungi.

ChIP-Seq Analysis of AtfA Interactions in Aspergillus flavus Reveals Its Involvement in Aflatoxin Metabolism and Virulence Under Oxidative Stress

The risk of Aspergillus flavus contamination is expanding with global warming. Targeting the pathogenicity of A. flavus at its source and diminishing its colonization within the host may be a potential control strategy. Oxidative stress transcription factor AtfA plays a pivotal role in A. flavus pathogenicity by combating reactive oxygen species (ROS) generated by host immune cells. This study employed chromatin immunoprecipitation sequencing to elucidate the binding sites and epigenetic mechanisms of AtfA under oxidative stress. Among the total 1022 identified potential AtfA-binding peaks, a 10-bp region predominated by 5'-DRTGTTGCAA-3', which is highly similar to the AP-1 binding motif was predicted. The significantly regulated genes exhibited a variety of biological functions, including regulation of filamentous growth, response to extracellular stimulus, and regulation of gene expression. Moreover, AtfA indirectly influenced these processes via the MAPK signaling pathway, carbon metabolism, and fatty acid metabolism in response to oxidative stress. The absence of atfA contributed to the decrease in the growth and development, sporulation, AFB1 biosynthesis, and invasion ability of A. flavus under oxidative stress. These findings suggest that AtfA is critical to overcome oxidative stress induced by the host immune cells during the infection, providing a novel target for early prevention of A. flavus contamination.

The MAP kinase FvHog1 regulates FB1 synthesis and Ca2+ homeostasis in Fusarium verticillioides

The high osmolarity glycerol 1 mitogen-activated protein kinase (Hog1-MAPK) cascade genes are important for diverse biological processes. The activated Hog1 upon multiple environmental stress stimuli enters into the nucleus where it directly phosphorylates transcription factors to regulate various physiological processes in phytopathogenic fungi. However, their roles have not been well-characterized in Fusarium verticillioides. In this study, FvHog1 is identified and functionally analyzed. The findings reveal that the phosphorylation level and nuclear localization of FvHog1 are increased in Fumonisin B1 (FB1)-inducing condition to regulate the expression of FB1 biosynthesis FUM genes. More importantly, the deletion mutants of Hog1-MAPK pathway show increased sensitivity to Ca2+ stress and elevated intracellular Ca2+ content. The phosphorylation level and nuclear localization of FvHog1 are increased with Ca2+ treatment. Furthermore, our results show that FvHog1 can directly phosphorylate Ca2+-responsive zinc finger transcription factor 1 (FvCrz1) to regulate Ca2+ homeostasis. In conclusion, our findings indicate that FvHog1 is required for FB1 biosynthesis, pathogenicity and Ca2+ homeostasis in F. verticillioides. It provides a theoretical basis for effective prevention and control maize ear and stalk rot disease.

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.

Role of Pmk1, Mpk1, or Hog1 in the mitogen-activated protein kinase pathway of Aspergillus cristatus

Aspergillus cristatus is a probiotic fungus known for its safety and abundant secondary metabolites, making it a promising candidate for various applications. However, limited progress has been made in researching A. cristatus due to challenges in genetic manipulation. The mitogen-activated protein kinase (MAPK) signaling pathway is involved in numerous physiological processes, but its specific role in A. cristatus remains unclear. In this study, we successfully developed an efficient polyethylene glycol (PEG)-mediated protoplast transformation method for A. cristatus, enabling us to investigate the function of Pmk1, Mpk1, and Hog1 in the MAPK signaling pathway. Our findings revealed that Pmk1, Mpk1, and Hog1 are crucial for sexual reproduction, melanin synthesis, and response to external stress in A. cristatus. Notably, the deletion of Pmk1, Mpk1, or Hog1 resulted in the loss of sexual reproduction capability in A. cristatus. Overall, this research on MAPK will contribute to the continued understanding of the reproductive strategy and melanin synthesis mechanism of A. cristatus.

The Development of Aspergillus flavus and Biosynthesis of Aflatoxin B1 are Regulated by the Golgi-Localized Mn2+ Transporter Pmr1

Microorganisms rely on diverse ion transport and trace elements to sustain growth, development, and secondary metabolism. Manganese (Mn2+) is essential for various biological processes and plays a crucial role in the metabolism of human cells, plants, and yeast. In Aspergillus flavus, we confirmed that Pmr1 localized in cis- and medial-Golgi compartments was critical in facilitating Mn2+ transport, fungal growth, development, secondary metabolism, and glycosylation. In comparison to the wild type, the Δpmr1 mutant displayed heightened sensitivity to environmental stress, accompanied by inhibited synthesis of aflatoxin B1, kojic acid, and a substantial reduction in pathogenicity toward peanuts and maize. Interestingly, the addition of exogenous Mn2+ effectively rectified the developmental and secondary metabolic defects in the Δpmr1 mutant. However, Mn2+ supplement failed to restore the growth and development of the Δpmr1Δgdt1 double mutant, which indicated that the Gdt1 compensated for the functional deficiency of pmr1. In addition, our results showed that pmr1 knockout leads to an upregulation of O-glycosyl-N-acetylglucose (O-GlcNAc) and O-GlcNAc transferase (OGT), while Mn2+ supplementation can restore the glycosylation in A. flavus. Collectively, this study indicates that the pmr1 regulates Mn2+ via Golgi and maintains growth and metabolism functions of A. flavus through regulation of the glycosylation.

Proteomics analysis of resistance mechanism of Trichoderma harzianum under U(VI) stress

Trichoderma harzianum has a certain resistance to Hexavalent Uranium (U(VI)), but its resistance mechanism is unknown. Based on proteomics sequencing using DIA mode, differentially expressed proteins (DEPs) of Trichoderma harzianum under U(VI) stress were identified. GO enrichment, KEGG annotation analysis and DEPs annotation were performed. The results showed that 8 DEPs, 8 DEPs and 15 DEPs were obtained in the low-dose, medium-dose and high-dose groups, respectively. The functional classification of GO demonstrated that DEPs were associated with 17 molecular functions, 5 biological processes, and 5 cellular components. Furthermore, DEPs were enriched in transport and catabolism, energy metabolism, translation, and signal transduction. These findings showed that Trichoderma harzianum was significantly changed in protein expression and signaling pathway after U(VI) exposure. Therefore, these results have provided Trichoderma harzianum with a theoretical background that can be applied to environmental cleanup.

PP2C phosphatases Ptc1 and Ptc2 dephosphorylate PGK1 to regulate autophagy and aflatoxin synthesis in the pathogenic fungus Aspergillus flavus

IMPORTANCE

Aspergillus flavus is a model filamentous fungus that can produce aflatoxins when it infects agricultural crops. This study evaluated the protein phosphatase 2C (PP2C) family as a potential drug target with important physiological functions and pathological significance in A. flavus. We found that two redundant PP2C phosphatases, Ptc1 and Ptc2, regulate conidia development, aflatoxin synthesis, autophagic vesicle formation, and seed infection. The target protein phosphoglycerate kinase 1 (PGK1) that interacts with Ptc1 and Ptc2 is essential to regulate metabolism and the autophagy process. Furthermore, Ptc1 and Ptc2 regulate the phosphorylation level of PGK1 S203, which is important for influencing aflatoxin synthesis. Our results provide a potential target for interdicting the toxicity of A. flavus.

Metabolomic and Transcriptome Analysis of the Inhibitory Effects of Bacillus subtilis Strain Z-14 against Fusarium oxysporum Causing Vascular Wilt Diseases in Cucumber

Controlling cucumber Fusarium wilt caused by Fusarium oxysporum f. sp. cucumerinum (FOC) with Bacillus strains is a hot research topic. However, the molecular mechanism of Bacillus underlying the biocontrol of cucumber wilt is rarely reported. In this study, B. subtilis strain Z-14 showed significant antagonistic activity against FOC, and the control effect reached 88.46% via pot experiment. Microscopic observations showed that strain Z-14 induced the expansion and breakage of FOC hyphae. The cell wall thickness was uneven, and the organelle structure was degraded. The combined analysis of metabolome and transcriptome showed that strain Z-14 inhibited the FOC infection by inhibiting the synthesis of cell wall and cell membrane, energy metabolism, and amino acid synthesis of FOC mycelium, inhibiting the clearance of reactive oxygen species (ROS) and the secretion of cell wall-degrading enzymes (CWDEs), thereby affecting mitogen-activated protein kinase (MAPK) signal transduction and inhibiting the transport function.

A putative terpene cyclase gene (CcPtc1) is required for fungal development and virulence in Cytospora chrysosperma

Cytospora chrysosperma is a destructive plant pathogenic fungus, which causes canker disease on numerous woody plants. However, knowledge concerning the interaction between C. chrysosperma and its host remains limited. Secondary metabolites produced by phytopathogens often play important roles in their virulence. Terpene cyclases (TC), polyketide synthases (PKS) and non-ribosomal peptide synthetases (NRPS) are the key components for the synthesis of secondary metabolites. Here, we characterized the functions of a putative terpene type secondary metabolite biosynthetic core gene CcPtc1 in C. chrysosperma, which was significantly up-regulated in the early stages of infection. Importantly, deletion of CcPtc1 greatly reduced fungal virulence to the poplar twigs and they also showed significantly reduced fungal growth and conidiation compared with the wild-type (WT) strain. Furthermore, toxicity test of the crude extraction from each strain showed that the toxicity of crude extraction secreted by ΔCcPtc1 were strongly compromised in comparison with the WT strain. Subsequently, the untargeted metabolomics analyses between ΔCcPtc1 mutant and WT strain were conducted, which revealed 193 significantly different abundant metabolites (DAMs) inΔCcPtc1 mutant compared to the WT strain, including 90 significantly downregulated metabolites and 103 significantly up-regulated metabolites, respectively. Among them, four key metabolic pathways that reported to be important for fungal virulence were enriched, including pantothenate and coenzyme A (CoA) biosynthesis. Moreover, we also detected significant alterations in a series of terpenoids, among which (+)-ar-turmerone, pulegone, ethyl chrysanthemumate, and genipin were significantly down-regulated, while cuminaldehyde and (±)-abscisic acid were significantly up-regulated. In conclusion, our results demonstrated that CcPtc1 acts as a virulence-related secondary metabolism factor and provides new insights into the pathogenesis of C. chrysosperma.

New insights into the persistent effect of transient cinnamaldehyde vapor treatment on the growth and aflatoxin synthesis of Aspergillus flavus

The antimicrobial effects of continuous treatment with essential oils (EOs) in both liquid and gaseous phases have been intensively studied. Due to their rapid volatility, the effects of EOs on microorganisms after transient treatment are also worth exploring. In this work, the persistent effects of cinnamaldehyde (CA) vapor on Aspergillus flavus were detected by a series of biochemical analyses. Transcriptome analysis was also conducted to study the gene expression changes between recovered and normal A. flavus. When CA vapor was removed, biochemical analyses showed that the oxidative stress induced by the antimicrobial atmosphere was alleviated, and almost all the damaged functions were restored apart from mitochondrial function. Remarkably, the suppressed aflatoxin production intensified, which was confirmed by the up-regulation of most genes in the aflatoxin synthetic gene cluster, the velvet-related gene FluG and the aflatoxin precursor acetyl-CoA. Transcriptomic analysis also demonstrated significant changes in secondary metabolism, energy metabolism, oxidative stress, and amino acid metabolism in the recovery group. Taken together, these findings provide new insights into the mechanisms underlying the response of A. flavus to CA vapor treatment and will guide the rational application of EOs.

Genomic and Molecular Identification of Genes Contributing to the Caspofungin Paradoxical Effect in Aspergillus fumigatus

Aspergillus fumigatus is a deadly opportunistic fungal pathogen responsible for ~100,000 annual deaths. Azoles are the first line antifungal agent used against A. fumigatus, but azole resistance has rapidly evolved making treatment challenging. Caspofungin is an important second-line therapy against invasive pulmonary aspergillosis, a severe A. fumigatus infection. Caspofungin functions by inhibiting β-1,3-glucan synthesis, a primary and essential component of the fungal cell wall. A phenomenon termed the caspofungin paradoxical effect (CPE) has been observed in several fungal species where at higher concentrations of caspofungin, chitin replaces β-1,3-glucan, morphology returns to normal, and growth rate increases. CPE appears to occur in vivo, and it is therefore clinically important to better understand the genetic contributors to CPE. We applied genomewide association (GWA) analysis and molecular genetics to identify and validate candidate genes involved in CPE. We quantified CPE across 67 clinical isolates and conducted three independent GWA analyses to identify genetic variants associated with CPE. We identified 48 single nucleotide polymorphisms (SNPs) associated with CPE. We used a CRISPR/Cas9 approach to generate gene deletion mutants for seven genes harboring candidate SNPs. Two null mutants, ΔAfu3g13230 and ΔAfu4g07080 (dscP), resulted in reduced basal growth rate and a loss of CPE. We further characterized the dscP phosphatase-null mutant and observed a significant reduction in conidia production and extremely high sensitivity to caspofungin at both low and high concentrations. Collectively, our work reveals the contribution of Afu3g13230 and dscP in CPE and sheds new light on the complex genetic interactions governing this phenotype. IMPORTANCE This is one of the first studies to apply genomewide association (GWA) analysis to identify genes involved in an Aspergillus fumigatus phenotype. A. fumigatus is an opportunistic fungal pathogen that causes hundreds of thousands of infections and ~100,000 deaths each year, and antifungal resistance has rapidly evolved in this species. A phenomenon called the caspofungin paradoxical effect (CPE) occurs in some isolates, where high concentrations of the drug lead to increased growth rate. There is clinical relevance in understanding the genetic basis of this phenotype, since caspofungin concentrations could lead to unintended adverse clinical outcomes in certain cases. Using GWA analysis, we identified several interesting candidate polymorphisms and genes and then generated gene deletion mutants to determine whether these genes were important for CPE. Two of these mutant strains (ΔAfu3g13230 and ΔAfu4g07080/ΔdscP) displayed a loss of the CPE. This study sheds light on the genes involved in clinically important phenotype CPE.

Afper1 contributes to cell development and aflatoxin biosynthesis in Aspergillus flavus

Aspergillus flavus contaminates crops and produces carcinogenic aflatoxins that pose severe threat to food safety and human health. To identify potential targets to control aflatoxin contamination, we characterized a novel Afper1 protein, which regulates cell development and secondary metabolite biosynthesis in A. flavus. Afper1 is localized in the nucleus and is required for hyphal growth, conidial and sclerotial production, and responses to osmotic stress and essential oils such as cinnamaldehyde and thymol. More importantly, aflatoxin production was impaired in the Afper1 deletion mutant. Proteomics analysis revealed that extracellular hydrolases and proteins involved in conidial development, endoplasmic reticulum (ER) homeostasis, and aflatoxin biosynthesis were differentially regulated in ΔAfper1. Unexpectedly, enzymes participated in reactive oxygen species (ROS) scavenging, including catalase (catA, catB) and superoxide dismutase (sodM) were significantly downregulated, and the ROS accumulation and sensitivity to hydrogen peroxide were confirmed experimentally. Additionally, Afper1 deletion significantly upregulated heterochromatin protein HepA and downregulated acetyltransferases involved in heterochromatin formation. Accompanying ROS accumulation and chromatin remodeling, proteins related to aflatoxins, ustiloxin B and gliotoxin were downregulated. These results implied that Afper1 deletion affected chromatin remodeling and disturbed ER homeostasis, leading to ROS accumulation, and ultimately resulting in defective growth and impaired secondary metabolite biosynthesis.

Effect of Water Activity on Conidia Germination in Aspergillus flavus

In this study, we explored the mechanism underlying Aspergillus flavus conidia germination inhibited by decreased water activity. The impact of low water activity was analyzed at 4 h, 8 h and 12 h. Additionally, we demonstrated that low water activity affected cell shape and decreased cell sizes. Transcriptomics found numerous differentially expressed genes (DEGs) during the first 12 h of germination, with 654 DEGs observed among 4 h, 8 h and 12 h. In particular, more DEGs were detected at 8 h of germinating. Therefore, proteomics was performed at 8 h, and 209 differentially expressed proteins (DEPs) were speculated, with 94 up-regulated and 115 down-regulated. Combined analysis of KEGG of transcriptomics and proteomics demonstrated that the dominant pathways were nutrient metabolism and translation. We also found several DEGs and DEPs in the Mitogen Activated Protein Kinase (MAPK) pathway. Therefore, we concluded that low water activity inhibited conidia germination, causing unregular morphology. In addition, low water activity influenced expression of creA, TreB in carbohydrate metabolism, Clr4, RmtA in amino acid metabolism and RPL37, RPL3 in translation in Aspergillus flavus.

Post-translational modifications drive secondary metabolite biosynthesis in Aspergillus: a review

Post‐translational modifications (PTMs) are important for protein function and regulate multiple cellular processes and secondary metabolites (SMs) in fungi. Aspergillus species belong to a genus renown for an abundance of bioactive secondary metabolites, many important as toxins, pharmaceuticals and in industrial production. The genes required for secondary metabolites are typically co‐localized in biosynthetic gene clusters (BGCs), which often localize in heterochromatic regions of genome and are ‘turned off’ under laboratory condition. Efforts have been made to ‘turn on’ these BGCs by genetic manipulation of histone modifications, which could convert the heterochromatic structure to euchromatin. Additionally, non‐histone PTMs also play critical roles in the regulation of secondary metabolism. In this review, we collate the known roles of epigenetic and PTMs on Aspergillus SM production. We also summarize the proteomics approaches and bioinformatics tools for PTM identification and prediction and provide future perspectives on the emerging roles of PTM on regulation of SM biosynthesis in Aspergillus and other fungi.

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.

EGCG Alleviates Oxidative Stress and Inhibits Aflatoxin B1 Biosynthesis via MAPK Signaling Pathway

Aflatoxin biosynthesis has established a connection with oxidative stress, suggesting a prevention strategy for aflatoxin contamination via reactive oxygen species (ROS) removal. Epigallocatechin gallate (EGCG) is one of the most active and the richest molecules in green tea with well-known antioxidant effects. Here, we found EGCG could inhibit aflatoxin B1 (AFB1) biosynthesis without affecting mycelial growth in Aspergillus flavus, and the arrest occurred before the synthesis of toxin intermediate metabolites. Further RNA-seq analysis indicated that multiple genes involved in AFB1 biosynthesis were down-regulated. In addition, EGCG exposure facilitated the significantly decreased expression of AtfA which is a bZIP (basic leucine zipper) transcription factor mediating oxidative stress. Notably, KEGG (Kyoto Encyclopedia of Genes and Genomes) analysis indicated that the MAPK signaling pathway target transcription factor was down-regulated by 1 mg/mL EGCG. Further Western blot analysis showed 1 mg/mL EGCG could decrease the levels of phosphorylated SakA in both the cytoplasm and nucleus. Taken together, these data evidently supported that EGCG inhibited AFB1 biosynthesis and alleviated oxidative stress via MAPK signaling pathway. Finally, we evaluated AFB1 contamination in soy sauce fermentation and found that EGCG could completely control AFB1 contamination at 8 mg/mL. Conclusively, our results supported the potential use of EGCG as a natural agent to prevent AFB1 contamination in fermentation industry.

Updates on the Functions and Molecular Mechanisms of the Genes Involved in Aspergillus flavus Development and Biosynthesis of Aflatoxins

Aspergillus flavus (A. flavus) is a ubiquitous and opportunistic fungal pathogen that causes invasive and non-invasive aspergillosis in humans and animals. This fungus is also capable of infecting a large number of agriculture crops (e.g., peanuts, maze, cotton seeds, rice, etc.), causing economic losses and posing serious food-safety concerns when these crops are contaminated with aflatoxins, the most potent naturally occurring carcinogens. In particular, A. flavus and aflatoxins are intensely studied, and they continue to receive considerable attention due to their detrimental effects on humans, animals, and crops. Although several studies have been published focusing on the biosynthesis of the aforementioned secondary metabolites, some of the molecular mechanisms (e.g., posttranslational modifications, transcription factors, transcriptome, proteomics, metabolomics and transcriptome, etc.) involved in the fungal development and aflatoxin biosynthesis in A. flavus are still not fully understood. In this study, a review of the recently published studies on the function of the genes and the molecular mechanisms involved in development of A. flavus and the production of its secondary metabolites is presented. It is hoped that the information provided in this review will help readers to develop effective strategies to reduce A. flavus infection and aflatoxin production.

Mitogen-Activated Protein Kinase MAPKKK7 from Plasmodiophora brassicae Regulates Low-Light-Dependent Nicotiana benthamiana Immunity

MAPKKK is the largest family of mitogen-activated protein kinase (MAPK) cascades and is known to play important roles in plant pathogen interaction by regulating fungal cell proliferation, growth, and pathogenicity. Thus far, only a few have been characterized because of the functional redundancy of MAPKKKs. In this study, it is interesting that Plasmodiophora brassicae (Pb)MAPKKK7 was clustered into the A3 subgroup of plant MAPKKKs by a phylogenetic analysis and also with the BCK1 and STE groups of fungal MAPKKKs. PbMAPKKK7 function in reactive oxygen species accumulation and cell death in Nicotiana benthamiana was characterized. Agroinfiltration with the PbMAPKKK7 mutated protein kinase domain relieved these changes. Interestingly, the induction of cell death was dependent on light intensity. Transcriptional profiling analysis demonstrated that PbMAPKKK7 was highly expressed during cortex infection stages, indicating its important role in P. brassicae infection. These functional analyses of PbMAPKKK7 build knowledge of new roles of the MAPK cascade pathway in N. benthamiana and P. brassicae interactions.

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.