Functional roles of LaeA, polyketide synthase, and glucose oxidase in the regulation of ochratoxin A biosynthesis and virulence in Aspergillus carbonarius

Gene function characterization in Aspergillus niger using a dual resistance marker transformation system mediated by Agrobacterium tumefaciens

Aspergillus niger is a well-known workhorse for the industrial production of enzymes and organic acids. This fungus can also cause postharvest diseases in fruits. Although Agrobacterium tumefaciens-mediated transformation (ATMT) based on antibiotic resistance markers has been effectively exploited for inspecting functions of target genes in wild-type fungi, it still needs to be further improved in A. niger. In the present study, we re-examined the ATMT in the wild-type A. niger strains using the hygromycin resistance marker and introduced the nourseothricin resistance gene as a new selection marker for this fungus. Unexpectedly, our results revealed that the ATMT method using the resistance markers in A. niger led to numerous small colonies as false-positive transformants on transformation plates. Using the top agar overlay technique to restrict false positive colonies, a transformation efficiency of 87 ± 18 true transformants could be achieved for 106 conidia. With two different selection markers, we could perform both the deletion and complementation of a target gene in a single wild-type A. niger strain. Our results also indicated that two key regulatory genes (laeA and veA) of the velvet complex are required for A. niger to infect apple fruits. Notably, we demonstrated for the first time that a laeA homologous gene from the citrus postharvest pathogen Penicillium digitatum was able to restore the acidification ability and pathogenicity of the A. niger ΔlaeA mutant. The dual resistance marker ATMT system from our work represents an improved genetic tool for gene function characterization in A. niger.

Two Polyketide Synthase Genes, VpPKS10 and VpPKS33, Regulated by VpLaeA Are Essential to the Virulence of Valsa pyri

Valsa pyri, the causal agent of pear canker disease, typically induces cankers on the bark of infected trees and even leads to tree mortality. Secondary metabolites produced by pathogenic fungi play a crucial role in the pathogenic process. In this study, secondary metabolic regulator VpLaeA was identified in V. pyri. VpLaeA was found to strongly affect the pathogenicity, fruiting body formation, and toxicity of secondary metabolites of V. pyri. Additionally, VpLaeA was found to be required for the response of V. pyri to some abiotic stresses. Transcriptome data analysis revealed that many of differentially expressed genes were involved in the secondary metabolite biosynthesis. Among them, about one third of secondary metabolite biosynthesis core genes were regulated by VpLaeA at different periods. Seven differentially expressed secondary metabolite biosynthesis core genes (VpPKS9, VpPKS10, VpPKS33, VpNRPS6, VpNRPS7, VpNRPS16, and VpNRPS17) were selected for knockout. Two modular polyketide synthase genes (VpPKS10 and VpPKS33) that were closely related to the virulence of V. pyri from the above seven genes were identified. Notably, VpPKS10 and VpPKS33 also affected the production of fruiting body of V. pyri but did not participate in the resistance of V. pyri to abiotic stresses. Overall, this study demonstrates the multifaceted biological functions of VpLaeA in V. pyri and identifies two toxicity-associated polyketide synthase genes in Valsa species fungi for the first time.

Involvement of LaeA and Velvet Proteins in Regulating the Production of Mycotoxins and Other Fungal Secondary Metabolites

Fungi are rich sources of secondary metabolites of agrochemical, pharmaceutical, and food importance, such as mycotoxins, antibiotics, and antitumor agents. Secondary metabolites play vital roles in fungal pathogenesis, growth and development, oxidative status modulation, and adaptation/resistance to various environmental stresses. LaeA contains an S-adenosylmethionine binding site and displays methyltransferase activity. The members of velvet proteins include VeA, VelB, VelC, VelD and VosA for each member with a velvet domain. LaeA and velvet proteins can form multimeric complexes such as VosA-VelB and VelB-VeA-LaeA. They belong to global regulators and are mainly impacted by light. One of their most important functions is to regulate gene expressions that are responsible for secondary metabolite biosynthesis. The aim of this mini-review is to represent the newest cognition of the biosynthetic regulation of mycotoxins and other fungal secondary metabolites by LaeA and velvet proteins. In most cases, LaeA and velvet proteins positively regulate production of fungal secondary metabolites. The regulated fungal species mainly belong to the toxigenic fungi from the genera of Alternaria, Aspergillus, Botrytis, Fusarium, Magnaporthe, Monascus, and Penicillium for the production of mycotoxins. We can control secondary metabolite production to inhibit the production of harmful mycotoxins while promoting the production of useful metabolites by global regulation of LaeA and velvet proteins in fungi. Furthermore, the regulation by LaeA and velvet proteins should be a practical strategy in activating silent biosynthetic gene clusters (BGCs) in fungi to obtain previously undiscovered metabolites.

Unlocking the Transcriptional Reprogramming Repertoire between Variety-Dependent Responses of Grapevine Berries to Infection by Aspergillus carbonarius

Aspergillus carbonarius causes severe decays on berries in vineyards and is among the main fungal species responsible for grape contamination by ochratoxin A (OTA), which is the foremost mycotoxin produced by this fungus. The main goal of this study was to investigate at the transcriptome level the comparative profiles between two table grape varieties (Victoria and Fraoula, the white and red variety, respectively) after their inoculation with a virulent OTA-producing A. carbonarius strain. The two varieties revealed quite different transcriptomic signatures and the expression profiles of the differential expressed genes (DEGs) highlighted distinct and variety-specific responses during the infection period. The significant enrichment of pathways related to the modulation of transcriptional dynamics towards the activation of defence responses, the triggering of the metabolic shunt for the biosynthesis of secondary metabolites, mainly phenylpropanoids, and the upregulation of DEGs encoding phytoalexins, transcription factors, and genes involved in plant-pathogen interaction and immune signaling transduction was revealed in an early time point in Fraoula, whereas, in Victoria, any transcriptional reprogramming was observed after a delay. However, both varieties, to some extent, also showed common expression dynamics for specific DEG families, such as those encoding for laccases and stilbene synthases. Jasmonate (JA) may play a critical modulator role in the defence machinery as various JA-biosynthetic DEGs were upregulated. Along with the broader modulation of the transcriptome that was observed in white grape, expression profiles of specific A. carbonarius genes related to pathogenesis, fungal sporulation, and conidiation highlight the higher susceptibility of Victoria. Furthermore, the A. carbonarius transcriptional patterns directly associated with the regulation of the pathogen OTA-biosynthesis gene cluster were more highly induced in Victoria than in Fraoula. The latter was less contaminated by OTA and showed substantially lower sporulation. These findings contribute to uncovering the interplay beyond this plant-microbe interaction.

Ochratoxin A-The Current Knowledge Concerning Hepatotoxicity, Mode of Action and Possible Prevention

Ochratoxin A (OTA) is considered as the most toxic of the other ochratoxins synthesized by various fungal species belonging to the Aspergillus and Penicillium families. OTA commonly contaminates food and beverages, resulting in animal and human health issues. The toxicity of OTA is known to cause liver damage and is still being researched. However, current findings do not provide clear insights into the toxin mechanism of action. The current studies focusing on the use of potentially protective compounds against the effects of the toxin are insufficient as they are mainly conducted on animals. Further research is required to fill the existing gaps in both fields (namely the exact OTA molecular mechanism and the prevention of its toxicity in the human liver). This review article is a summary of the so far obtained results of studies focusing on the OTA hepatotoxicity, its mode of action, and the known approaches of liver cells protection, which may be the base for expanding other research in near future.

Metabolomic Analysis Demonstrates the Impacts of Polyketide Synthases PKS14 and PKS15 on the Production of Beauvericins, Bassianolide, Enniatin A, and Ferricrocin in Entomopathogen Beauveria bassiana

Beauveria bassiana is a globally distributed entomopathogenic fungus that produces various secondary metabolites to support its pathogenesis in insects. Two polyketide synthase genes, pks14 and pks15, are highly conserved in entomopathogenic fungi and are important for insect virulence. However, understanding of their mechanisms in insect pathogenicity is still limited. Here, we overexpressed these two genes in B. bassiana and compared the metabolite profiles of pks14 and pks15 overexpression strains to those of their respective knockout strains in culture and in vivo using tandem liquid chromatography-mass spectrometry (LC-MS/MS) with Global Natural Products Social Molecular Networking (GNPS). The pks14 and pks15 clusters exhibited crosstalk with biosynthetic clusters encoding insect-virulent metabolites, including beauvericins, bassianolide, enniatin A, and the intracellular siderophore ferricrocin under certain conditions. These secondary metabolites were upregulated in the pks14-overexpressing strain in culture and the pks15-overexpressing strain in vivo. These data suggest that pks14 and pks15, their proteins or their cluster components might be directly or indirectly associated with key pathways in insect pathogenesis of B. bassiana, particularly those related to secondary metabolism. Information about interactions between the polyketide clusters and other biosynthetic clusters improves scientific understanding about crosstalk among biosynthetic pathways and mechanisms of pathogenesis.

Transcriptomic analysis reveals the inhibition mechanism of pulsed light on fungal growth and ochratoxin A biosynthesis in Aspergillus carbonarius

Pulsed light (PL) technology has a good effect on the control of fungi in postharvest fruit. In this present work, PL inhibited the growth of Aspergillus carbonarius in a dose-dependent manner, the mycelial growth decreased by 4.83 %, 13.91 % and 30.01 % at a fluence of 4.5 J·cm^-2 (PL5), 9 J·cm^-2 (PL10) and 13.5 J·cm² (PL15), respectively. When inoculated with PL15 treated A. carbonarius, the scab diameter of the pears, ergosterol and OTA content was reduced by 23.2 %, 27.9 % and 80.7 % after 7 days, respectively. The third-generation sequencing technique was applied to study the transcriptome response of A. carbonarius treated with PL. Compared with the blank control, a total number of 268 and 963 differentially expressed genes (DEGs) were discovered in the group of PL10 and PL15, respectively. To be specific, a large amount of DEGs involved in DNA metabolism were up-regulated, while most of DEGs related to cell integrity, energy and glucose metabolism, ochratoxin A (OTA) biosynthesis and transport were down-regulated. In addition, the stress response of A. carbonarius was imbalanced, including up-regulation of Catalase and PEX12 and down-regulation of taurine and subtaurine metabolism, alcohol dehydrogenase and glutathione metabolism. Meanwhile, the results of transmission electron microscopy, mycelium cellular leakage and DNA electrophoresis indicated that PL15 treatment caused mitochondrial swelling, the destroyed cell membrane permeability and imbalance of DNA metabolism. The expression of P450 and Hal involved in OTA biosynthesis pathway were down-regulated in PL treated samples detected by qRT-PCR. In conclusion, this study reveals the molecular mechanism of pulsed light on inhibiting the growth, development and toxin production of A. carbonarius.

The global regulator FpLaeB is required for the regulation of growth, development, and virulence in Fusarium pseudograminearum

Fusarium pseudograminearum is a soil-borne pathogen that is capable of causing a highly destructive crown disease in wheat. Secondary metabolites (SMs), especially deoxynivalenol (DON), are the primary virulence factors during infection. Here, we characterised the global regulator FpLaeB, an orthologue of LaeB protein function, to regulate the SM in Aspergillus nidulans. Through the utility of the gene targeting approach, we found that the vegetative growth of the FpLaeB deletion mutant was drastically reduced compared to that of the wild type. FpLaeB was also important for conidiation because the FpLaeB deletion mutant formed fewer conidia in induced medium. In addition, the sensitivity of the FpLaeB deletion mutant to the cell wall integrity inhibitor was decreased, while its growth was more severely inhibited by the cell membrane inhibitor sodium dodecyl sulfate (SDS) than that of the wild type. More importantly, the virulence was decreased when the FpLaeB deletion mutant was inoculated onto the wheat stem base or head. Through genome-wide gene expression profiling, FpLaeB was found to regulate several processes related to the above phenotypes such as the carbohydrate metabolic process, which is an integral and intrinsic component of membranes, especially SMs. Furthermore, the generation of DON was impaired in the FpLaeB deletion mutant via ultraperformance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) assay. These results showed that FpLaeB plays an important role in the growth, development, and maintenance of the cell wall, and in membrane integrity. More importantly, FpLaeB is required for SMs and full virulence in F. pseudograminearum.

Preparation of a novel glucose oxidase-N-succinyl chitosan nanospheres and its antifungal mechanism of action against Colletotrichum gloeosporioides

In this work, a new glucose oxidase-N-succinyl chitosan (GOD-NSCS) nanospheres was prepared through the immobilization of glucose oxidase (GOD) on N-succinyl chitosan (NSCS) nanospheres. Compared to the free GOD, GOD-NSCS nanospheres demonstrated the excellent anti-Colletotrichum gloeosporioides activity with the EC₅₀ values of 211.2 and 10.7 μg/mL against mycelial growth and spores germination. The computational biology analysis demonstrated that the substrate presented the similar binding free energy with GOD-NSCS nanospheres (-27.64 kcal/mol) compared with the free GOD (-24.04 kcal/mol), indicating that GOD-NSCS nanospheres had the same oxidation efficiency and produced more H₂O₂. Moreover, the enzyme activity stability of GOD-NSCS nanospheres could be prolonged to 10 d. The cell membrane was destructed by the treatment of H₂O₂ produced by GOD, leading to the cell death. In vivo test, GOD-NSCS nanospheres treatment significantly prolonged the preservation period of mangoes 2-fold. Collectively, these results suggested that GOD-NSCS nanospheres suppresses anthracnose in postharvest mangoes by inhibiting the growth of C. gloeosporioides and might become a potential natural preservative for fruits and vegetables.

Impact of environmental factors on ochratoxin A: From natural occurrence to control strategy

Ochratoxin A (OTA) contamination and the associated issues of food security, food safety and economic loss are widespread throughout the world. The occurrence of OTA depends on ochratoxigenic fungi, foodstuffs and their environment. In this review, natural occurrence and control strategy of OTA, with a focus on the impact of environmental factors, are summarized. First, this manuscript introduces potentially contaminated foodstuffs, including the emerging ones which are not regulated in international legislation. Secondly, it gives an update of native producers based on foodstuffs and OTA biosynthesis. Thirdly, complicated environmental regulation is disassembled into individual factors in order to clarify their regulatory effect and mechanism. Finally, to emphasize control OTA at all stages of foodstuffs from farm to table, strategies used at crop planting, harvest, storage and processing stages are discussed.

CgVeA, a light signaling responsive regulator, is involved in regulation of chaetoglobosin A biosynthesis and conidia development in Chaetomium globosum

Cytochalasans, with diverse structures and pharmacological activities, are a class of compounds containing isoindolinone moieties fused to the tricyclic or tetracyclic ring system. Chaetoglobosin A (cheA), mainly produced by Chaetomium globosum, is the most abundant cytochalasan. However, limited understanding of transcriptional regulation of morphological development and cheA biosynthesis in C. globosum has hindered cheA application in agriculture and biomedical field. This study examined the regulatory role of CgVeA gene in C. globosum. CgVeA had significant effect on secondary metabolites production in C. globosum, similar to that reported in other filamentous fungi. Inactivation of CgVeA caused an obvious decrease in cheA production from 51.32 to 19.76 mg/L under dark conditions. In contrast, CgVeA overexpression resulted in a dramatic increase in cheA production, reaching 206.59 mg/L under light conditions, which was higher than that noted under dark condition. The RT-qPCR results confirmed that CgVeA, as a light responsive regulator, positively regulated cheA biosynthesis by controlling the expression of core genes of the cheA biosynthetic gene cluster and other relevant regulators. Electrophoretic mobility shift assays proved that CgVeA directly regulated LaeA, cheR, and p450, and indirectly regulated PKS. Moreover, CgVeA had a significant effect on the regulation of asexual spores production. When compared with wild-type C. globosum, CgVeA-silenced and CgVeA overexpression mutants presented remarkable differences in sporulation, irrespective of light or dark condition. Besides, CgVeA expression was speculated to negatively regulate spore formation. These findings illustrated the regulatory mechanism of a hypothetical global regulator, CgVeA, in C. globosum, suggesting its potential application in industrial-scale cheA biosynthesis.

Ochratoxin A Defective Aspergillus carbonarius Mutants as Potential Biocontrol Agents

Aspergillus carbonarius is one of the main species responsible for wine, coffee and cocoa toxin contamination. The main mycotoxin produced by this fungus, ochratoxin A (OTA), is a secondary metabolite categorized as a possible carcinogen because of its significant nephrotoxicity and immunosuppressive effects. A polyketide synthase gene (otaA) encodes the first enzyme in the OTA biosynthetic pathway. It is known that the filamentous fungi, growth, development and production of secondary metabolites are interconnected processes governed by global regulatory factors whose encoding genes are generally located outside the gene clusters involved in the biosynthesis of each secondary metabolite, such as the veA gene, which forms part of the VELVET complex. Different fungal strains compete for nutrients and space when they infect their hosts, and safer non-mycotoxigenic strains may be able to outcompete mycotoxigenic strains during colonization. To determine the possible utility of biopesticides based on the competitive exclusion of mycotoxigenic strains by non-toxigenic ones, we used A. carbonarius ΔotaA and ΔveA knockout mutants. Our results showed that during both in vitro growth and infection of grapes, non-mycotoxigenic strains could outcompete the wild-type strain. Additionally, the introduction of the non-mycotoxigenic strain led to a drastic decrease in OTA during both in vitro growth and infection of grapes.

Unveiling ochratoxin a controlling and biodetoxification molecular mechanisms: Opportunities to secure foodstuffs from OTA contamination

Anarchic growth of ochratoxin A (OTA) producing fungi during crop production, prolonged storage, and processing results in OTA contamination in foodstuffs. OTA in food exacerbates the risk of health and economic problems for consumers and farmers worldwide. Although the toxic effects of OTA on human health have not been well established, comprehensive preventive and remedial measures will be essential to eliminate OTA from foodstuffs. Strict regulations, controlling OTA at pre- or post-harvest stage, and decontamination of OTA have been adopted to prevent human and animal OTA exposure. Biological control of OTA and bio-decontamination are the most promising strategies due to their safety, specificity and nutritional value. This review addresses the current understanding of OTA biodegradation mechanisms and recent developments in OTA control and bio-decontamination strategies. Additionally, this review analyses the strength and weaknesses of different OTA control methods and the contemporary approaches to enhance the efficiency of biocontrol agents. Overall, this review will support the implementation of new strategies to effectively control OTA in food sectors. Further studies on efficacy-related issues, production issues and cost-effectiveness of OTA biocontrol are to be carried out to improve the knowledge, develop improved delivery technologies and safeguard the durability of OTA biocontrol approaches.

Overexpression of llm1 Affects the Synthesis of Secondary Metabolites of Aspergillus cristatus

Putative methyltransferases are thought to be involved in the regulation of secondary metabolites in filamentous fungi. Here, we report the effects of overexpression of a predicted LaeA-like methyltransferase gene llm1 on the synthesis of secondary metabolites in Aspergillus cristatus. Our results revealed that overexpression of the gene llm1 in A. cristatus significantly hindered the production of conidia and enhanced sexual development, and reduced oxidative tolerance to hydrogen peroxide. Compared with the wild-type, the metabolic profile of the overexpression transformant was distinct, and the contents of multiple secondary metabolites were markedly increased, mainly including terpenoids and flavonoids, such as (S)-olEuropeic acid, gibberellin A62, gibberellin A95, ovalitenone, PD 98059, and 1-isomangostin. A total of 600 significantly differentially expressed genes (DEGs) were identified utilizing transcriptome sequencing, and the DEGs were predominantly enriched in transmembrane transport and secondary metabolism-related biological processes. In summary, the strategy of overexpressing global secondary metabolite regulators successfully activated the expression of secondary metabolite gene clusters, and the numerous secondary metabolites were greatly strengthened in A. cristatus. This study provides new insights into the in-depth exploitation and utilization of novel secondary metabolites of A. cristatus.

AwAreA Regulates Morphological Development, Ochratoxin A Production, and Fungal Pathogenicity of Food Spoilage Fungus Aspergillus westerdijkiae Revealed by an Efficient Gene Targeting System

Aspergillus westerdijkiae, the producer of ochratoxin A (OTA), which is of worldwide concern, is an import fungal species in agriculture, food, and industry. Here, we got the uridine auxotrophic mutant of A. westerdijkiae by deleting AwpyrG. The ΔAwpyrG could be used for bio-transformation with exogenous AfpyrG expression cassette as a selection marker. In order to enhance the efficiency of gene targeting, Awku70 and Awlig4 were homologously deleted from ΔAwpyrG. The efficiencies of homologous replacement for ΔAwku70 and ΔAwlig4 were 95.7 and 87.0% in the deletion of AwAreA, respectively, demonstrating a drastic increase from 4.3% of the wild type (WT) strain. Furthermore, the function of AwAreA was identified with AwAreA deletion mutant and the control strain ΔAwku70. AwAreA regulated the growth and conidiation of A. westerdijkiae in response to nitrogen sources. The concentration of OTA for ΔAwku70 was in the range of 19.4 to 186.9 ng/cm2 on all kinds of nitrogen sources. The OTA production influenced by the deletion of AwAreA was different based on nitrogen sources. Pathogenicity assays on pears, grapes, salted meat, and cheese showed that AwAreA acted as a negative regulator in the infection of food substrates. Therefore, the genetic methods and engineered strains enable us to substantially expand the use of A. westerdijkiae, one of more than twenty OTA-producing fungi, in the study of mycotoxin biosynthesis and regulation, and consequently to aim at providing new ways for controlling this pathogen.

Deletion and Overexpression of the AnOTAbzip Gene, a Positive Regulator of Ochratoxin A Biosynthesis in Aspergillus niger

The ochratoxin A (OTA) biosynthetic gene cluster includes a bZIP transcription factor (TF) gene (OTAbzip) that has been identified in different fungal species. However, most previous studies identified the OTAbzip gene in ochratoxigenic fungi using bioinformatics methods, while few studies focused on deleting the gene, let alone overexpressing it, to characterize the function of the OTAbZIP TF. Here, we characterized the AnOTAbZIP TF in an ochratoxigenic isolate of Aspergillus niger by deleting and overexpressing the AnOTAbzip gene and examining the role of AnOTAbZIP in morphological development, OTA biosynthesis, and pathogenicity. Chemical and gene expression analyses revealed that AnOTAbZIP positively regulates OTA biosynthesis, since the loss of OTA production and the downregulation of the OTA biosynthetic genes were observed in the ΔAnOTAbzip strain, compared with the wild-type (WT) and OE::AnOTAbzip strains. In terms of pathogenicity, the ΔAnOTAbzip strain produced a greater lesion on grape berries, especially with respect to the OE::AnOTAbzip strain, rather than WT. Finally, the ΔAnOTAbzip strain was also more tolerant to oxidative stress with respect to the OE::AnOTAbzip and WT strains in that order. These new findings improve our understanding of the AnOTAbZIP regulatory mechanism and help develop strategies to attenuate plant pathogenicity and reduce OTA biosynthesis of A. niger.

Host Factors Modulating Ochratoxin A Biosynthesis during Fruit Colonization by Aspergillus carbonarius

Aspergillus carbonarius is a strong and consistent ochratoxin A (OTA) producer and considered to be the main source of this toxic metabolite in grapes and grape products such as wine, grape juice and dried vine fruit. OTA is produced under certain growth conditions and its accumulation is affected by several environmental factors, such as growth phase, substrate, temperature, water activity and pH. In this study, we examined the impact of fruit host factors on regulation and accumulation of OTA in colonized grape berries, and assessed in vitro the impact of those factors on the transcriptional levels of the key genes and global regulators contributing to fungal colonization and mycotoxin synthesis. We found that limited sugar content, low pH levels and high malic acid concentrations activated OTA biosynthesis by A. carbonarius, both in synthetic media and during fruit colonization, through modulation of global regulator of secondary metabolism, laeA and OTA gene cluster expression. These findings indicate that fruit host factors may have a significant impact on the capability of A. carbonarius to produce and accumulate OTA in grapes.