A Polyketide Synthase Encoded by the Gene An15g07920 Is Involved in the Biosynthesis of Ochratoxin A in Aspergillus niger

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.

Regulation of nitrogen utilization and mycotoxin biosynthesis by the GATA transcription factor AaAreA in Alternaria alternata

Alternaria alternata is a prevalent postharvest pathogen that generates diverse mycotoxins, notably alternariol (AOH) and alternariol monomethyl ether (AME), which are recurrent severe contaminants. Nitrogen sources modulate fungal growth, development, and secondary metabolism, including mycotoxin production. The GATA transcription factor AreA regulates nitrogen source utilization. However, little is known about its involvement in the regulation of nitrogen utilization in A. alternata. To examine the regulatory mechanism of AaAreA on AOH and AME biosynthesis in A. alternata, we analyzed the impact of diverse nitrogen sources on the fungal growth, conidiation and mycotoxin production. The use of a secondary nitrogen source (NaNO3) enhanced mycelial elongation and sporulation more than the use of a primary source (NH4Cl). NaNO3 favored greater mycotoxin accumulation than did NH4Cl. The regulatory roles of AaAreA were further clarified through gene knockout. The absence of AaAreA led to an overall reduction in growth in minimal media containing any nitrogen source except NH4Cl. AaAreA positively regulates mycotoxin biosynthesis when both NH4Cl and NaNO3 are used as nitrogen sources. Subcellular localization analysis revealed abundant nuclear transport when NaNO3 was the sole nitrogen source. The regulatory pathway of AaAreA was systematically revealed through comprehensive transcriptomic analyses. The deletion of AaAreA significantly impedes the transcription of mycotoxin biosynthetic genes, including aohR, pksI and omtI. The interaction between AaAreA and aohR, a pathway-specific transcription factor gene, demonstrated that AaAreA binds to the aohR promoter sequence (5'-GGCTATGGAAA-3'), activating its transcription. The expressed AohR regulates the expression of downstream synthase genes in the cluster, ultimately impacting mycotoxin production. This study provides valuable information to further understand how AreA regulates AOH and AME biosynthesis in A. alternata, thereby enabling the effective design of control measures for mycotoxin contamination.

Isolation, identification, degradation mechanism and exploration of active enzymes in the ochratoxin A degrading strain Acinetobacter pittii AP19

Ochratoxin A (OTA) is a polyketide mycotoxin that commonly contaminates agricultural products and causes significant economic losses. In this study, the efficient OTA-degrading strain AP19 was isolated from vineyard soil and was identified as Acinetobacter pittii. Compared with growth in nutrient broth supplemented with OTA (OTA-NB), strain AP19 grew faster in nutrient broth (NB), but the ability of the resulting cell lysates to remove OTA was weaker. After cultivation in NB, the cell lysate of strain AP19 was able to remove 100% of 1 mg/L OTA within 18 h. The cell lysate fraction > 30 kDa degraded 100% of OTA within 12 h, while the fractions < 30 kDa were practically unable to degrade OTA. Further anion exchange chromatography of the > 30 kDa fraction yielded two peaks exhibiting significant OTA degradation activity. The degradation product was identified as OTα. Amino acid metabolism exhibited major transcriptional trends in the response of AP19 to OTA. The dacC gene encoding carboxypeptidase was identified as one of the contributors to OTA degradation. Soil samples inoculated with strain AP19 showed significant OTA degradation. These results provide significant insights into the discovery of novel functions in A. pittii, as well as its potential as an OTA decomposer.

Bamboo polysaccharides elicit hypocrellin A biosynthesis of a bambusicolous fungus Shiraia sp. S9

Hypocrellin A (HA), a fungal perylenequinone from bambusicolous Shiraia species, is a newly developed photosensitizer for photodynamic therapy in cancer and other infectious diseases. The lower yield of HA is an important bottleneck for its biomedical application. This study is the first report of the enhancement of HA production in mycelium culture of Shiraia sp. S9 by the polysaccharides from its host bamboo which serve as a strong elicitor. A purified bamboo polysaccharide (BPSE) with an average molecular weight of 34.2 kDa was found to be the most effective elicitor to enhance fungal HA production and characterized as a polysaccharide fraction mainly composed of arabinose and galactose (53.7: 36.9). When BPSE was added to the culture at 10 mg/L on day 3, the highest HA production of 422.8 mg/L was achieved on day 8, which was about 4.0-fold of the control. BPSE changed the gene expressions mainly responsible for central carbon metabolism and the cellular oxidative stress. The induced generation of H2O2 and nitric oxide was found to be involved in both the permeabilization of cell membrane and HA biosynthesis, leading to enhancements in both intra- and extracellular HA production. Our results indicated the roles of plant polysaccharides in host-fungal interactions and provided a new elicitation technique to improve fungal perylenequinone production in mycelium cultures.

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.

AnAzf1 acts as a positive regulator of ochratoxin A biosynthesis in Aspergillus niger

Aspergillus niger produces genotoxic and carcinogenic ochratoxin A (OTA) that severely threatens human and animal health. Transcription factor Azf1 is essential in regulating fungal cell development and primary metabolism. However, its effect and mechanism on secondary metabolism are unclear. Here, we characterized and deleted a Azf1 homolog gene, An15g00120 (AnAzf1), in A. niger, which completely blocked OTA production, and repressed the OTA cluster genes, p450, nrps, hal, and bzip at the transcriptional level. The results indicated that AnAzf1 was a positive regulator of OTA biosynthesis. Transcriptome sequencing results showed that the AnAzf1 deletion significantly upregulated antioxidant genes and downregulated oxidative phosphorylation genes. Enzymes involved in reactive oxygen species (ROS) scavenging, including catalase (CAT) and peroxidase (POD) were increased, and the corresponding ROS levels were decreased. Upregulation of genes (cat, catA, hog1, and gfd) in the MAPK pathway and downregulation of genes in iron homeostasis were associated with decreased ROS levels, linking the altered MAPK pathway and iron homeostasis to lower ROS levels caused by AnAzf1 deletion. Additionally, enzymes including complex I (NADH-ubiquinone oxidoreductase), and complex V (ATP synthase), as well as ATP levels, were significantly decreased, indicating impaired oxidative phosphorylation caused by the AnAzf1-deletion. During lower ROS levels and impaired oxidative phosphorylation, OTA was not produced in ∆AnAzf1. Together, these results strongly suggested that AnAzf1 deletion blocked OTA production in A. niger by a synergistic interference of ROS accumulation and oxidative phosphorylation. KEY POINTS: • AnAzf1 positively regulated OTA biosynthesis in A. niger. • Deletion of AnAzf1 decreased ROS levels and impaired oxidative phosphorylation. • An altered MAPK pathway and iron homeostasis were associated with lower ROS levels.

Contrasting regulation of live Bacillus cereus No.1 and its volatiles on Shiraia perylenequinone production

BACKGROUND

Fungal perylenequinones (PQs) are a class of photoactivated polyketide mycotoxins produced by plant-associated fungi. Hypocrellins, the effective anticancer photodynamic therapy (PDT) agents are main bioactive PQs isolated from a bambusicolous Shiraia fruiting bodies. We found previously that bacterial communities inhabiting fungal fruiting bodies are diverse, but with unknown functions. Bacillus is the most dominant genus inside Shiraia fruiting body. To understand the regulation role of the dominant Bacillus isolates on host fungus, we continued our work on co-culture of the dominant bacterium B. cereus No.1 with host fungus Shiraia sp. S9 to elucidate bacterial regulation on fungal hypocrellin production.

RESULTS

Results from "donut" plate tests indicated that the bacterial culture could promote significantly fungal PQ production including hypocrellin A (HA), HC and elsinochrome A-C through bacterial volatiles. After analysis by gas chromatograph/mass spectrometer and confirmation with commercial pure compounds, the volatiles produced by the bacterium were characterized. The eliciting roles of bacterial volatile organic compounds (VOCs) on HA production via transcriptional regulation of host Shiraia fungus were confirmed. In the established submerged bacterial volatile co-culture, bacterial volatiles could not only promote HA production in the mycelium culture, but also facilitate the release of HA into the medium. The total production of HA was reached to 225.9 mg/L, about 1.87 times that of the fungal mono-culture. In contrast, the live bacterium suppressed markedly fungal PQ production in both confrontation plates and mycelium cultures by direct contact. The live bacterium not only down-regulated the transcript levels of HA biosynthetic genes, but also degraded extracellular HA quickly to its reductive product.

CONCLUSION

Our results indicated that bacterial volatile release could be a long-distance signal to elicit fungal PQ production. Biodegradation and inhibition by direct contact on fungal PQs were induced by the dominate Bacillus to protect themselves in the fruiting bodies. This is the first report on the regulation of Bacillus volatiles on fungal PQ production. These findings could be helpful for both understanding the intimate fungal-bacterial interactions in a fruiting body and establishing novel cultures for the enhanced production of bioactive PQs.

Insights into the Underlying Mechanism of Ochratoxin A Production in Aspergillus niger CBS 513.88 Using Different Carbon Sources

Aspergillus niger produces carcinogenic ochratoxin A (OTA), a serious food safety and human health concern. Here, the ability of A. niger CBS 513.88 to produce OTA using different carbon sources was investigated and the underlying regulatory mechanism was elucidated. The results indicated that 6% sucrose, glucose, and arabinose could trigger OTA biosynthesis and that 1586 differentially expressed genes (DEGs) overlapped compared to a non-inducing nutritional source, peptone. The genes that participated in OTA and its precursor phenylalanine biosynthesis, including pks, p450, nrps, hal, and bzip, were up-regulated, while the genes involved in oxidant detoxification, such as cat and pod, were down-regulated. Correspondingly, the activities of catalase and peroxidase were also decreased. Notably, the novel Gal4-like transcription factor An12g00840 (AnGal4), which is vital in regulating OTA biosynthesis, was identified. Deletion of AnGal4 elevated the OTA yields by 47.65%, 54.60%, and 309.23% using sucrose, glucose, and arabinose as carbon sources, respectively. Additionally, deletion of AnGal4 increased the superoxide anion and H₂O₂ contents, as well as the sensitivity to H₂O₂, using the three carbon sources. These results suggest that these three carbon sources repressed AnGal4, leading to the up-regulation of the OTA biosynthetic genes and alteration of cellular redox homeostasis, ultimately triggering OTA biosynthesis in A. niger.

Isoenzyme N-Acyl-l-Amino Acid Amidohydrolase NA Increases Ochratoxin A Degradation Efficacy of Stenotrophomonas sp. CW117 by Enhancing Amidohydrolase ADH3 Stability

Ochratoxin A (OTA) is a potent mycotoxin mainly produced by toxicogenic strains of Aspergillus spp. and seriously contaminates foods and feedstuffs. OTA detoxification strategies are significant to food safety. A superefficient enzyme ADH3 to OTA hydrolysis was isolated from the difunctional strain Stenotrophomonas sp. CW117 in our previous study. Here, we identified a gene N-acyl-l-amino acid amidohydrolase NA, which is an isoenzyme of ADH3. However, it is not as efficient a hydrolase as ADH3. The kinetic constant showed that the catalytic efficiency of ADH3 (K_cat/K_m = 30,3938 s^-1 · mM^-1) against OTA was 29,113 times higher than that of NA (K_cat/K_m = 10.4 s^-1 · mM^-1), indicating that ADH3 was the overwhelming superior detoxifying gene in CW117. Intriguingly, when gene na was knocked out from the CW117 genome, degradation activity of the Δna mutant was significantly reduced at the first 6 h, suggesting that the two enzymes might have an interactive effect on OTA transformation. Gene expressions and Western blotting assay showed that the Δna mutant and wild-type CW117 showed similar adh3 expression levels, but na deficiency decreased ADH3 protein level in CW117. Collectively, isoenzyme NA was identified as a factor that improved the stability of ADH3 in CW117 but not as a dominant hydrolase for OTA transformation. IMPORTANCE Ochratoxin A (OTA) is a potent mycotoxin mainly produced by toxicogenic strains of Aspergillus spp. and seriously contaminates foods and feedstuffs. Previous OTA detoxification studies mainly focused on characterizations of degradation strains and detoxifying enzymes. Here, we identified a gene N-acyl-l-amino acid amidohydrolase NA from strain CW117, which is an isoenzyme of the efficient detoxifying enzyme ADH3. Isoenzyme NA was identified as a factor that improved the stability of ADH3 in CW117 and, thus, enhanced the degradation activity of the strain. This is the first study on an isoenzyme improving the stability of another efficient detoxifying enzyme in vivo.

LaeA regulates morphological development and ochratoxin A biosynthesis in Aspergillus niger

The global regulator LaeA and its orthologs govern the morphogenetic development and secondary metabolism of several filamentous ascomycetes. In Aspergillus niger, it has been shown that an LaeA ortholog (AnLaeA) regulates the production of citric acid and secondary metabolites. In this work, we constructed AnlaeA disruption and overexpression strains to investigate the roles of AnLaeA in morphological development and ochratoxin A (OTA) biosynthesis in A. niger. Phenotypic observation, chemical analysis, and gene expression analysis indicated that AnLaeA acts as a negative regulator of conidial morphogenesis and positively regulates gene expression of the OTA cluster in A. niger grown in CYA medium. However, it was observed that the upregulation of gene expression of the OTA cluster does not necessarily increase OTA production. Our results contribute to a better understanding of the AnlaeA regulatory mechanism and suggest the AnlaeA gene as a potential target for developing control strategies for A. niger infection and OTA biosynthesis.

Epitaxial growth of structure-tunable ZnO/ZnS core/shell nanowire arrays using HfO2 as the buffer layer

Synthesis of high-quality ZnO/ZnS heterostructures with tunable phase and controlled structures is in high demand due to their adjustable band gap and efficient electron-hole pair separation. In this report, for the first time, remote heteroepitaxy of single-crystalline ZnO/ZnS core/shell nanowire arrays has been realized using amorphous HfO₂ as the buffer layer. Zinc blende or wurtzite ZnS epilayer can be efficiently fabricated under the same thermal deposition condition by adjusting the buffer layer thickness, even among the same batch of products, respectively. Structural characterization reveals "(01-10)ZnO_wz//(2-20)ZnS_ZB, [0001]ZnO_WZ//[001]ZnS_ZB" and "(01-10)ZnO_WZ//(01-10)ZnS_WZ, [0002]ZnO_WZ//[0002]ZnS_WZ" epitaxial relationships between the core and the shell, respectively. The cathodoluminescence measurement demonstrates that the tuning of the optical properties can be accomplished by preparing a heterostructure with HfO₂, in which a strong green emission increases at the expense of the quenching of UV emission. In addition, the core/shell heterostructure based Schottky diode exhibits an asymmetrical rectifying behavior and an outstanding photo-electronic switching-effect. We believe that the aforementioned results could provide fundamental insights for epitaxial growth of structure-tunable ZnO/ZnS heterostructures on the nanoscale. Furthermore, this promising route buffered by the high-k material can broaden the options for fabricating heterojunctions and promote their application in photoelectric nanodevices.

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.

Roles of AaVeA on Mycotoxin Production via Light in Alternaria alternata

Alternaria alternata is a principal plant pathogen responsible for the biosynthesis of mycotoxins, including tenuazonic acid (TeA), alternariol (AOH), and alternariol monomethyl ether (AME). The velvet gene VeA is involved in fungal growth, development, and secondary metabolism, including mycotoxin biosynthesis via light regulation. In this study, the detailed regulatory roles of AaVeA in A. alternata with various light sources were investigated from the comparative analyses between the wild type and the gene knockout strains. In fungal growth and conidiation, mycelial extension was independent of light regulation in A. alternata. Red light favored conidiation, but blue light repressed it. The absence of AaVeA caused the marked reduction of hyphae extension and conidiophore formation even though red light could not induce more spores in ΔAaVeA mutant. The differentially expressed genes (DEGs) enriched in hyphal growth and conidiation were drastically transcribed from the comparatively transcriptomic profile between the wild type and ΔAaVeA mutant strains with or without light. In mycotoxin production, TeA biosynthesis seems no obvious effect by light regulation, but AOH and AME formation was significantly stimulated by blue light. Nevertheless, the disruption of AaVeA resulted in a marked decrease in mycotoxin production and the action of the stimulation was lost via blue light for the abundant accumulation of AOH and AME in the ΔAaVeA strain. From DEG expression and further verification by RT-qPCR, the loss of AaVeA caused the discontinuous supply of the substrates for mycotoxin biosynthesis and the drastic decline of biosynthetic gene expression. In addition, pathogenicity depends on AaVeA regulation in tomato infected by A. alternata in vivo. These findings provide a distinct understanding of the roles of AaVeA in fungal growth, development, mycotoxin biosynthesis, and pathogenicity in response to various light sources.

A superefficient ochratoxin A hydrolase with promising potential for industrial applications

As the most seriously controlled mycotoxin produced by Aspergillus spp. and Penicillium spp., ochratoxin A (OTA) results in various toxicological effects and widely contaminates agro-products. Biological detoxification of OTA is the most priority in food and feed industry, but currently available detoxification enzymes are relatively low effectiveness in time and cost. Here we show a superefficient enzyme ADH3 identified from Stenotrophomonas acidaminiphila with a strong ability to transform OTA into non-toxic ochratoxin-α by acting as an amidohydrolase. Recombinant ADH3 (1.2 μg/mL) completely degrades 50 μg/L OTA within 90 seconds, while the availably most efficient OTA hydrolases takes several hours. The kinetic constant showed that rADH3 (Kcat/Km) catalytic efficiency was 56.7-35000 times higher than those of previous hydrolases rAfOTase, rOTase and commercial carboxypeptidase A (CPA). Protein structure-based assay suggested that ADH3 has a preference for hydrophobic residues to form a larger hydrophobic area than other detoxifying enzymes at the cavity of the catalytic sites, and this structure makes the OTA easier to access to catalytic sites. In addition, ADH3 shows considerable temperature adaptability to exert hydrolytic function at the temperature down to 0°C or up to 70°C. Collectively, we report a superefficient OTA detoxifying enzyme with promising potential for industrial applications. IMPORTANCE Ochratoxin A (OTA) can result in various toxicological effects and widely contaminates agro-products and feedstuffs. OTA detoxifications by microbial strains and bio-enzymes are significant to food safety. Although previous studies showed OTA could be transformed through several pathways, the ochratoxin-α pathway is recognized as the most effective one. However, the most currently available enzymes are not efficient enough. Here, a superefficient hydrolase ADH3 which can completely transform 50 μg/L OTA into ochratoxin-α within 90 seconds was screened and characterized. The hydrolase ADH3 shows considerable temperature adaptability (0-70°C) to exert the hydrolytic function. Findings of this study supplied an efficient OTA detoxifying enzyme and predicted the superefficient degradation mechanism which lay a foundation for future industrial applications.

Aspergillus carbonarius-derived ochratoxins are inhibited by Amazonian Bacillus spp. used as a biocontrol agent in grapes

Bacillus spp. have been used as a biocontrol strategy to eliminate/reduce toxic fungicides in viticulture. Furthermore, the presence of fungi that are resistant to commonly used products is frequent, highlighting the need for new biocontrol strains. Aspergillus carbonarius can produce ochratoxins, including ochratoxin A (OTA), which has a regulatory maximum allowable limit for grape products. The purpose of this study was to assess the ability of four Amazonian strains of Bacillus (P1, P7, P11, and P45) to biocontrol A. carbonarius and various forms of ochratoxins in grapes. Berries treated with strain P1 presented no fungal colonies (100% reduction), while P7, P11 and P45 strains caused a reduction of 95, 95 and 61% on fungal counts, respectively. Six forms of ochratoxin were found in the grapes inoculated with A. carbonarius, including ochratoxin α, ochratoxin β, ochratoxin α methyl-ester, ochratoxin α amide, N-formyl-ochratoxin α amide, and OTA. Four of these ochratoxin forms (ochratoxin β, ochratoxin α methyl-ester, ochratoxin α amide, N-formyl-ochratoxin α amide) are reported for the first time in grapes. These ochratoxins were identified using liquid chromatography coupled to quadrupole time-of-flight mass spectrometry (LC-QToF-MS). All Bacillus strains inhibited the synthesis of OTA, which is the most toxic form of ochratoxin. No ochratoxin form was found when P1 and P7 were used. Although some forms of ochratoxin were detected in grapes treated with Bacillus spp. P11 and P45, the levels decreased by 97%. To our knowledge, this is the first report on the inhibition of Aspergillus carbonarius-derived ochratoxin by Bacillus species. P1 strain, identified as Bacillus velezensis, was found to be the most promising for completely inhibiting fungal growth and production of all ochratoxins.

The signaling role of extracellular ATP in co-culture of Shiraia sp. S9 and Pseudomonas fulva SB1 for enhancing hypocrellin A production

Background

Adenosine 5′-triphosphate (ATP) plays both a central role as an intracellular energy source, and a crucial extracellular signaling role in diverse physiological processes of animals and plants. However, there are less reports concerning the signaling role of microbial extracellular ATP (eATP). Hypocrellins are effective anticancer photodynamic therapy (PDT) agents from bambusicolous Shiraia fungi. The co-culture of Shiraia sp. S9 and a bacterium Pseudomonas fulva SB1 isolated from Shiraia fruiting bodies was established for enhanced hypocrellin A (HA) production. The signaling roles of eATP to mediate hypocrellin biosynthesis were investigated in the co-culture.

Results

The co-culture induced release of eATP at 378 nM to the medium around 4 h. The eATP release was interdependent on cytosolic Ca²⁺ concentration and reactive oxygen species (ROS) production, respectively. The eATP production could be suppressed by the Ca²⁺ chelator EGTA or abolished by the channel blocker La³⁺, ROS scavenger vitamin C and NADPH oxidase inhibitor diphenyleneiodonium chloride (DPI). The bacterium-induced H₂O₂ production was strongly inhibited by reactive blue (RB), a specific inhibitor of membrane purinoceptors, but dependent on the induced Ca²⁺ influx in the co-culture. On the other hand, the application of exogenous ATP (exATP) at 10–300 µM to Shiraia cultures also promoted fungal conidiation and HA production, both of which were blocked effectively by the purinoceptor inhibitors pyridoxalphosphate-6-azophenyl-2′, 4′-disulfonic acid (PPADS) and RB, and ATP hydrolase apyrase. Both the induced expression of HA biosynthetic genes and HA accumulation were inhibited significantly under the blocking of the eATP or Ca²⁺ signaling, and the scavenge of ROS in the co-culture.

Conclusions

Our results indicate that eATP release is an early event during the intimate bacterial–fungal interaction and eATP plays a signaling role in the bacterial elicitation on fungal metabolites. Ca²⁺ and ROS are closely linked for activation of the induced ATP release and its signal transduction. This is the first report on eATP production in the fungal–bacterial co-culture and its involvement in the induced biosynthesis of fungal metabolites.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01637-9.

Nitric oxide donor sodium nitroprusside-induced transcriptional changes and hypocrellin biosynthesis of Shiraia sp. S9

Background

Nitric oxide (NO) is a ubiquitous signaling mediator in various physiological processes. However, there are less reports concerning the effects of NO on fungal secondary metabolites. Hypocrellins are effective anticancer photodynamic therapy (PDT) agents from fungal perylenequinone pigments of Shiraia. NO donor sodium nitroprusside (SNP) was used as a chemical elicitor to promote hypocrellin biosynthesis in Shiraia mycelium cultures.

Results

SNP application at 0.01–0.20 mM was found to stimulate significantly fungal production of perylenequinones including hypocrellin A (HA) and elsinochrome A (EA). SNP application could not only enhance HA content by 178.96% in mycelia, but also stimulate its efflux to the medium. After 4 days of SNP application at 0.02 mM, the highest total production (110.34 mg/L) of HA was achieved without any growth suppression. SNP released NO in mycelia and acted as a pro-oxidant, thereby up-regulating the gene expression and activity of reactive oxygen species (ROS) generating NADPH oxidase (NOX) and antioxidant enzymes, leading to the increased levels of superoxide anion (O₂⁻) and hydrogen peroxide (H₂O₂). Gene ontology (GO) analysis revealed that SNP treatment could up-regulate biosynthetic genes for hypocrellins and activate the transporter protein major facilitator superfamily (MFS) for the exudation. Moreover, SNP treatment increased the proportion of total unsaturated fatty acids in the hypha membranes and enhanced membrane permeability. Our results indicated both cellular biosynthesis of HA and its secretion could contribute to HA production induced by SNP.

Conclusions

The results of this study provide a valuable strategy for large-scale hypocrellin production and can facilitate further understanding and exploration of NO signaling in the biosynthesis of the important fungal metabolites.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01581-8.

Dynamic Ochratoxin A Production by Strains of Aspergillus niger Intended Used in Food Industry of China

Thirty strains of Aspergillus niger, including 27 intended used in the food industry of China, were studied for their ochratoxin A (OTA) production on the three natural substrates—corn, rice, and wheat bran—at different time intervals by high-performance liquid chromatography. It was found that the frequencies of OTA for the studied 27 industrial strains ranged from 14.8% (4/27) at day 28 to 25.9% (7/27) at day 7 on corn, 14.8% (4/27) at day 7 to 33.3% (9/27) at day 21 on rice, and 22.2% (6/27) at day 7, 14, and 28 to 44.4% (12/27) at day 21 on wheat bran, respectively. The average concentrations of OTA produced by the studied 27 industrial strains ranged from 5.1 μg/kg at day 28 to 8.7 μg/kg at day 21 on corn, 4.2 μg/kg at day 7 to 17.9 μg/kg at day 14 on rice, and 4.5 μg/kg at day 7 to 7.2 μg/kg at day 21 on wheat bran, respectively. Furthermore, the OTA production in the studied 27 industrial strains of A.niger was strongly associated with their function (or application), culture substrate, and time. The saccharifying enzyme producers produced higher levels of OTA, compared with the organic acid producers, the tannase producers, and the β-galactosidase producer, while concentration differences were also observed in OTA production among strains of A.niger with the same application. In a word, some strains of A.niger intended used in the Chinese food industry indeed have the capability of producing OTA, elevating the risks to food safety associated with their use.

veA Gene Acts as a Positive Regulator of Conidia Production, Ochratoxin A Biosynthesis, and Oxidative Stress Tolerance in Aspergillus niger

The veA gene is a key regulator governing morphogenetic development and secondary metabolism in many fungi. Here, we characterized and disrupted a veA orthologue in an ochratoxigenic Aspergillus niger strain. Morphological development, ochratoxin A (OTA) biosynthesis, and oxidative stress tolerance in the wild-type and veA disruption strains were further analyzed. Accordingly, the link between the veA gene and development of specific gene brlA, OTA biosynthesis key gene pks, and oxidative-stress-tolerance-related gene cat was explored. Results demonstrated that the veA gene acts as a positive regulator of conidia production, OTA biosynthesis, and oxidative stress tolerance in A. niger, regardless of light conditions. Darkness promoted conidial production and OTA biosynthesis in the A. niger wild-type strain. Our results contribute to a better understanding of the veA regulatory mechanism and suggest the veA gene as a potential target for developing control strategies for A. niger infection and OTA biosynthesis.

Effects of 5-Azacytidine on Growth and Hypocrellin Production of Shiraia bambusicola

Hypocrellins, fungal perylenequinones of Shiraia bambusicola are developed as important photodynamic therapy agents against cancers and viruses. Due to the limitation of the wild resources, the mycelium culture is a promising alternative for hypocrellin production. As DNA methylation has profound effects on fungal growth, development and secondary metabolism, we used both McrBC cleavage and HPLC analysis to reveal the status of DNA methylation of S. bambusicola mycelium. We found that DNA methylation is absent in mycelia, but DNA methylation inhibitor 5-azacytidine (5-AC) still induced the fluffy phenotype and decreased hypocrellin contents significantly. Simultaneously, a total of 4,046 differentially expressed genes were induced by 5-AC, including up-regulated 2,392 unigenes (59.12%) and down-regulated 1,654 unigenes (40.88%). Gene ontology analysis showed 5-AC treatment changed expression of genes involved in membrane composition and oxidation–reduction process. The fluffy phenotype in 5-AC-treated S. bambusicola was closely related to strong promotion of developmental regulator WetA and the repression of the sexual developmental actor VeA and LaeA. It was a surprise finding that 5-AC reduced reactive oxygen species (ROS) production significantly in the mycelia via the inhibition of NADPH oxidase gene (NOX) expression and NOX activity. With the treatment of vitamin C and H₂O₂, we found that the reduced ROS generation was involved in the down-regulated expression of key genes for hypocrellin biosynthesis and the decreased hypocrellin production. To our knowledge, this is the first attempt to examine DNA methylation level in S. bambusicola. Our results suggested that the mediation of ROS generation could not be ignored in the study using 5-AC as a specific DNA methylation inhibitor.

A Consensus Ochratoxin A Biosynthetic Pathway: Insights from the Genome Sequence of Aspergillus ochraceus and a Comparative Genomic Analysis

Ochratoxin A (OTA) is a toxic secondary metabolite produced by Aspergillus and Penicillium species that widely contaminates food and feed. We sequenced and assembled the complete ∼37-Mb genome of Aspergillusochraceus fc-1, a well-known producer of OTA. Key genes of the OTA biosynthetic pathway were identified by comparative genomic analyses with five other sequenced OTA-producing fungi: A. carbonarius, A. niger, A. steynii, A. westerdijkiae, and Penicillium nordicum OTA production was completely inhibited in the deletion mutants (ΔotaA, ΔotaB, ΔotaC, ΔotaD, and ΔotaR1), and OTA biosynthesis was restored by feeding a postblock substrate to the corresponding mutant. The OTA biosynthetic pathway was unblocked in the ΔotaD mutant by the addition of heterologously expressed halogenase. OTA biosynthesis begins with a polyketide synthase (PKS), OtaA, utilizing acetyl coenzyme A (acetyl-CoA) and malonyl-CoA to synthesize 7-methylmellein, which is oxidized to OTβ by cytochrome P450 monooxygenase (OtaC). OTβ and l-β-phenylalanine are combined by a nonribosomal peptide synthetase (NRPS), OtaB, to form an amide bond to synthesize OTB. Finally, OTB is chlorinated by a halogenase (OtaD) to OTA. The otaABCD genes were expressed at low levels in the ΔotaR1 mutant. A second regulator, otaR2, which is adjacent to the biosynthetic gene, could modulate only the expression of otaA, otaB, and otaD Thus, we have identified a consensus OTA biosynthetic pathway that can be used to prevent and control OTA synthesis and will help us understand the variation and production of the intermediate components in the biosynthetic pathway.IMPORTANCE Ochratoxin A (OTA) is a significant mycotoxin that contaminates cereal products, coffee, grapes, wine, cheese, and meat. OTA is nephrotoxic, carcinogenic, teratogenic, and immunotoxic. OTA contamination is a serious threat to food safety, endangers human health, and can cause huge economic losses. At present, >20 species of the genera Aspergillus and Penicillium are known to produce OTA. Here we demonstrate that a consensus OTA biosynthetic pathway exists in all OTA-producing fungi and is encoded by a gene cluster containing four highly conserved biosynthetic genes and a bZIP transcription factor.

Genomic diversity in ochratoxigenic and non ochratoxigenic strains of Aspergillus carbonarius

Ochratoxin A (OTA) is a mycotoxin with nephrotoxic effects on animals and humans. Aspergillus carbonarius is the main responsible for OTA contamination of grapes and derived products. We present the genome resequencing of four A. carbonarius strains, one OTA producer and three atypical and unique non-OTA producing strains. These strains were sequenced using Illumina technology and compared with a reference genome of this species. We performed some specific bioinformatics analyses in genes involved in OTA biosynthesis. Data obtained in this study revealed the high genomic diversity within A. carbonarius strains. Although some gaps of more than 1,000 bp were identified in non-ochratoxigenic strains, no large deletions in functional genes related with OTA production were found. Moreover, the expression of five genes of the putative OTA biosynthetic cluster was down regulated under OTA-inducing conditions in the non-ochratoxigenic strains. Knowledge of the regulatory mechanisms involved in OTA biosynthesis will provide a deeper understanding of these non-ochratoxigenic strains.

Description of an orthologous cluster of ochratoxin A biosynthetic genes in Aspergillus and Penicillium species. A comparative analysis

Ochratoxin A (OTA) is one of the most important mycotoxins due to its toxic properties and worldwide distribution which is produced by several Aspergillus and Penicillium species. The knowledge of OTA biosynthetic genes and understanding of the mechanisms involved in their regulation are essential. In this work, we obtained a clear picture of biosynthetic genes organization in the main OTA-producing Aspergillus and Penicillium species (A. steynii, A. westerdijkiae, A. niger, A. carbonarius and P. nordicum) using complete genome sequences obtained in this work or previously available on databases. The results revealed a region containing five ORFs which predicted five proteins: halogenase, bZIP transcription factor, cytochrome P450 monooxygenase, non-ribosomal peptide synthetase and polyketide synthase in all the five species. Genetic synteny was conserved in both Penicillium and Aspergillus species although genomic location seemed to be different since the clusters presented different flanking regions (except for A. steynii and A. westerdijkiae); these observations support the hypothesis of the orthology of this genomic region and that it might have been acquired by horizontal transfer. New real-time RT-PCR assays for quantification of the expression of these OTA biosynthetic genes were developed. In all species, the five genes were consistently expressed in OTA-producing strains in permissive conditions. These protocols might favour futures studies on the regulation of biosynthetic genes in order to develop new efficient control methods to avoid OTA entering the food chain.

Biocorrosion of materials and sick building syndrome

The problem of biological damage of mineral building materials and structures based on them is multifaceted and covers all types of industry. The most destructive biocorrosion impacts are on building materials in cities with a large water area. Various types of microorganisms, including pathogens, and especially the filamentous fungi of the genera Aspergillus, Penicillium, Trichoderma, etc., occupy the surfaces of mineral building materials, cause their destruction, disturb the ecological balance of cities and endanger the human health. The term ‘sick building syndrome' (SBS) is used to describe a situation when the residents of a building experience acute health- or comfort-related effects that seem to be linked directly to the time spent in the building wherein no specific illness or cause can be identified. Biological contaminants, in particular micromycetes, can present one of the possible causes of SBS. Here, we assessed the biodeterioration level of structural materials on the basis of fine-grained concrete widely used in construction practice and architecture. First, we determined the strength characteristics of the material that has been biologically damaged; second, we identified the damaging micromycetes and analysed their metabolic activity related both to the induction of biocorrosion and to the impacts of fungi on human health. Applying a new integrated approach, which combines methods of molecular microbiology and genetic toxicology with standard methods for determining the strength of building structures, we confirmed the relation between biodestructive and pathogenic properties of micromycete isolates.