Biosynthesis of the mycotoxin tenuazonic acid by a fungal NRPS-PKS hybrid enzyme

Tenuazonic acid (TeA) is a well-known mycotoxin produced by various plant pathogenic fungi. However, its biosynthetic gene has been unknown to date. Here we identify the TeA biosynthetic gene from Magnaporthe oryzae by finding two TeA-inducing conditions of a low-producing strain. We demonstrate that TeA is synthesized from isoleucine and acetoacetyl-coenzyme A by TeA synthetase 1 (TAS1). TAS1 is a unique non-ribosomal peptide synthetase and polyketide synthase (NRPS–PKS) hybrid enzyme that begins with an NRPS module. In contrast to other NRPS/PKS hybrid enzymes, the PKS portion of TAS1 has only a ketosynthase (KS) domain and this domain is indispensable for TAS1 activity. Phylogenetic analysis classifies this KS domain as an independent clade close to type I PKS KS domain. We demonstrate that the TAS1 KS domain conducts the final cyclization step for TeA release. These results indicate that TAS1 is a unique type of NRPS–PKS hybrid enzyme.

Tenuazonic acid is a mycotoxin produced by various plant pathogenic fungi but its biosynthetic gene is unknown to date. Here, the authors identify the tenuazonic acid biosynthetic gene encoding a protein with a unique KS domain that conducts cyclization step for tenuazonic acid release in Magnaporthe oryzae.

Phenotypic and phylogenetic segregation of Alternaria infectoria from small-spored Alternaria species isolated from wheat in Germany and Russia

AIMS

To identify the taxonomic differences between phytopathogenic small-spored Alternaria strains isolated from wheat kernels in Germany and Russia by a polyphasic approach.

METHODS AND RESULTS

Ninety-five Alternaria (A.) strains were characterized by their colony colour, their three-dimensional sporulation patterns, mycotoxin production and phylogenetic relationships based on sequence variation in translation elongation factor 1-α (TEF1-α). The examination of toxin profiles and the phylogenetic features via TEF1-α resulted in two distinct clusters, in each case containing Alternaria infectoria isolates (92 and 96% respectively) in the first and the Alternaria alternata, Alternaria arborescens and Alternaria tenuissima isolates (77 and 79% respectively) in the other combined cluster. The production of Alternariol, Altertoxin and Altenuene has not been reported previously in the A. infectoria species group. The isolates from Germany and Russia differ slightly in species composition and mycotoxin production capacity.

CONCLUSIONS

We identified that the A. infectoria species group can be differentiated from the A. alternata, A. arborescens and A. tenuissima species group by colour, low mycotoxin production and by the sequence variation in TEF1-α gene.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results allow a reliable toxic risk assessment when detecting different Alternaria fungi on cereals.

Impact of Alternaria toxins on CYP1A1 expression in different human tumor cells and relevance for genotoxicity

The Alternaria toxins alternariol (AOH) and alternariol monomethyl ether (AME) have been reported previously to act as activators of the aryl hydrocarbon receptor (AhR) in murine hepatoma cells, thus enhancing the expression of cytochrome P450 (CYP) 1A monooxygenases. Concomitantly, both benzopyrones represent substrates of CYP1A, giving rise to catecholic metabolites. The impact of AOH and AME on CYP1A expression in human cells of different tissue origin colon (HT29), esophagus (KYSE510), liver (HepG2) and their effects on cell viability, generation of reactive oxygen species (ROS) and DNA integrity were investigated. ROS production was induced by both mycotoxins in all cell lines with AOH exhibiting the highest potency in esophageal cells concomitant with the most prominent CYP1A induction level. Of note, altertoxin-II (ATX-II), the more potent DNA-damaging mutagen formed by Alternaria alternata, induces CYP1A even at significant lower concentrations. AhR-siRNA knockdown in human esophageal cells supported the hypothesis of AhR-mediated CYP1A1 induction by AOH. However, DNA damage was minor at CYP1A1-inducing AOH concentrations. AhR-depletion did not affect the DNA-damaging properties of AOH indicating no substantial impact of AhR in this regard. However, in combination with xenobiotics prone to metabolic activation by CYP1A the induction of CYP1A by Alternaria toxins deserves further attention.

An endophytic fungus isolated from finger millet (Eleusine coracana) produces anti-fungal natural products

Finger millet is an ancient African cereal crop, domesticated 7000 years ago in Ethiopia, reaching India at 3000 BC. Finger millet is reported to be resistant to various fungal pathogens including Fusarium sp. We hypothesized that finger millet may host beneficial endophytes (plant-colonizing microbes) that contribute to the antifungal activity. Here we report the first isolation of endophyte(s) from finger millet. Five distinct fungal species were isolated from roots and predicted taxonomically based on 18S rDNA sequencing. Extracts from three putative endophytes inhibited growth of F. graminearum and three other pathogenic Fusarium species. The most potent anti-Fusarium strain (WF4, predicted to be a Phoma sp.) was confirmed to behave as an endophyte using pathogenicity and confocal microscopy experiments. Bioassay-guided fractionation of the WF4 extract identified four anti-fungal compounds, viridicatol, tenuazonic acid, alternariol, and alternariol monomethyl ether. All the purified compounds caused dramatic breakage of F. graminearum hyphae in vitro. These compounds have not previously been reported to have anti-Fusarium activity. None of the compounds, except for tenuazonic acid, have previously been reported to be produced by Phoma. We conclude that the ancient, disease-tolerant crop, finger millet, is a novel source of endophytic anti-fungal natural products. This paper suggests the value of the crops grown by subsistence farmers as sources of endophytes and their natural products. Application of these natural chemicals to solve real world problems will require further validation.

Identification of six new Alternaria sulfoconjugated metabolites by high-resolution neutral loss filtering

RATIONALE

Many species of Alternaria damage important agricultural crops, including small grains and tomatoes. These fungi can produce a variety of secondary metabolites, some of which are toxic to humans and animals. Interest in screening for conjugated or 'modified' mycotoxins has increased because of their tendency to evade traditional analytical screening methods. Two sulfoconjugated Alternaria toxins have been reported and the potential exists for many more.

METHODS

One hundred and forty-eight Canadian strains of Alternaria spp., about half of them isolated from grain, were grown on Potato Dextrose Agar in Petri dishes for 7 days. Plugs of each strain were removed, extracted and screened by a rapid liquid chromatography (LC)/data-dependent tandem mass spectrometry (MS(2)) method in negative electrospray ionization mode. Data generated on an Orbitrap Q-Exactive mass spectrometer was processed by post-acquisition neutral loss filtering (NLF). Seven isolates that produced sulfoconjugates of known Alternaria toxins were selected for growth on three additional types of fermentation media.

RESULTS

Collision-induced dissociation of sulfoconjugated ions displayed a distinctive neutral loss of SO3 (79.957 Da) that was detected in the MS(2) datasets using post-acquisition NLF. A total of 108 of the 148 isolates screened produced sulfoconjugated metabolites on agar plates. Analysis of the seven isolates grown in liquid culture, on rice and Cheerios, led to the discovery of six new, two previously reported and 30 unidentified sulfoconjugated compounds.

CONCLUSIONS

NLF of HRMS(2) data from an Orbitrap Q-Exactive is a powerful tool for the rapid discovery of sulfoconjugated fungal metabolites. This technique could also be applied to the detection of other important conjugated mycotoxins such as glucosides. The majority of the Canadian isolates of Alternaria spp. studied produced sulfoconjugated metabolites, some of which had no known 'free' Alternaria precursor metabolite, indicating that they are possibly new metabolites. The advantage of sulfoconjugation to Alternaria spp. is unknown, and warrants further study into the mechanisms behind the sulfur assimilatory pathways.

Quantitative Determination of Tenuazonic Acid in Pig and Broiler Chicken Plasma by LC-MS/MS and Its Comparative Toxicokinetics

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to quantitate tenuazonic acid (TeA) in pig and broiler chicken plasma was successfully developed and validated. Linear matrix-matched calibration curves ranged between 5 and 200 ng/mL. Correlation coefficients, goodness-of-fit coefficients, and within-day and between-day precision and accuracy fell well within the acceptance criteria. The limit of quantitation was 5.0 ng/mL in both pig and broiler chicken plasma. The LC-MS/MS method was applied in a comparative toxicokinetic study in both pigs and broiler chickens. TeA was completely bioavailable after oral administration in both animal species. However, absorption was deemed to be slower in broiler chickens (mean tmax 0.32 h in pigs vs 2.60 h in chickens). TeA was more slowly eliminated in broiler chickens (mean t1/2el 0.55 h in pigs vs 2.45 h in chickens after oral administration), mainly due to the significantly lower total body clearance (mean Cl 446.1 mL/h/kg in pigs vs 59.2 mL/h/kg in chickens after oral administration). Tissue residue studies and further research to elucidate the biotransformation and excretion processes of TeA in pigs, broiler chickens, and other animal species are imperative.

Survey of Alternaria toxin contamination in food from the German market, using a rapid HPLC-MS/MS approach

A HPLC-MS/MS-based method for the quantification of nine mycotoxins produced by fungi of the genus Alternaria in various food matrices was developed. The method relies on a single-step extraction, followed by dilution of the raw extract and direct analysis. In combination with an analysis time per sample of 12 min, the sample preparation is cost-effective and easy to handle. The method covers alternariol (AOH), alternariol monomethyl ether (AME), tenuazonic acid (TeA), altenuene (ALT), iso-altenuene (isoALT), tentoxin (TEN), altertoxin-I (ATX-I), and the AAL toxins TA1 and TA2. Some Alternaria toxins which are either not commercially available or very expensive, namely AOH, AME, ALT, isoALT, and ATX-I, were isolated as reference compounds from fungal cultures. The method was extensively validated for tomato products, bakery products, sunflower seeds, fruit juices, and vegetable oils. AOH, AME, TeA, and TEN were found in quantifiable amounts and 92.1% of all analyzed samples (n = 96) showed low level contamination with one or more Alternaria toxins. Based on the obtained results, the average daily exposure to Alternaria toxins in Germany was calculated.

Review on Mycotoxin Issues in Ruminants: Occurrence in Forages, Effects of Mycotoxin Ingestion on Health Status and Animal Performance and Practical Strategies to Counteract Their Negative Effects

Ruminant diets include cereals, protein feeds, their by-products as well as hay and grass, grass/legume, whole-crop maize, small grain or sorghum silages. Furthermore, ruminants are annually or seasonally fed with grazed forage in many parts of the World. All these forages could be contaminated by several exometabolites of mycotoxigenic fungi that increase and diversify the risk of mycotoxin exposure in ruminants compared to swine and poultry that have less varied diets. Evidence suggests the greatest exposure for ruminants to some regulated mycotoxins (aflatoxins, trichothecenes, ochratoxin A, fumonisins and zearalenone) and to many other secondary metabolites produced by different species of Alternaria spp. (e.g., AAL toxins, alternariols, tenuazonic acid or 4Z-infectopyrone), Aspergillus flavus (e.g., kojic acid, cyclopiazonic acid or β-nitropropionic acid), Aspergillus fuminatus (e.g., gliotoxin, agroclavine, festuclavines or fumagillin), Penicillium roqueforti and P. paneum (e.g., mycophenolic acid, roquefortines, PR toxin or marcfortines) or Monascus ruber (citrinin and monacolins) could be mainly related to forage contamination. This review includes the knowledge of mycotoxin occurrence reported in the last 15 years, with special emphasis on mycotoxins detected in forages, and animal toxicological issues due to their ingestion. Strategies for preventing the problem of mycotoxin feed contamination under farm conditions are discussed.

Simultaneous analysis of Alternaria toxins and citrinin in tomato: an optimised method using liquid chromatography-tandem mass spectrometry

Alternaria toxins and citrinin are mycotoxins produced by fungi growing on different raw materials and agricultural commodities. Maximum levels of these toxins in foods are currently under consideration by the European Commission as a risk management measure. In this study, a new quantitative method is described for the determination of five Alternaria toxins and citrinin in tomato and tomato juice samples based on LC-MS/MS detection. Samples were extracted with pure methanol, followed by a derivatisation step with 2,4-dinitrophenylhydrazine to improve the determination of tenuazonic acid and to decrease the wide polarity difference between the compounds of interest. Samples were purified on hydrophilic-modified styrene polymer solid-phase extraction cartridges. High-performance liquid chromatographic columns packed with different core-shell materials were tested for the separation of toxins and a C-18 phase was in the final method applied to achieve sufficient separation of all relevant analytes. A key element of this approach was to prove successful transferability of the method to three different triple quadrupole mass spectrometers. A full single laboratory method validation was performed on two LC-MS/MS systems and performance characteristics met the predefined requirements. Moreover, the method was used in an international proficiency test and the satisfactory z-scores obtained (-0.1 to 0.8 in tomato juice samples) demonstrated the reliability of the approach described. The method will be validated in an inter-laboratory collaborative study and if the criteria for method precision are met, the method will be proposed as a new Work Item to the European Committee for Standardisation.

Identification and mycotoxigenic capacity of fungi associated with pre- and postharvest fruit rots of pomegranates in Greece and Cyprus

Pre- and postharvest fruit rots of fungal origin are an important burden for the pomegranate industry worldwide, affecting the produce both quantitatively and qualitatively. During 2013, local orchards were surveyed and 280 fungal isolates from Greece (GR) and Cyprus (CY) were collected from pomegranates exhibiting preharvest rot symptoms, and additional 153 isolates were collected postharvest from cold-stored fruit in GR. Molecular identification revealed that preharvest pomegranate fruit rots were caused predominately by species of the genera Aspergillus (Aspergillus niger and Aspergillus tubingensis) and Alternaria (Alternaria alternata, Alternaria tenuissima, and Alternaria arborescens). By contrast, postharvest fruit rots were caused mainly by Botrytis spp. and to a lesser extent by isolates of Pilidiella granati and Alternaria spp. Considering that a significant quota of the fungal species found in association with pomegranate fruit rots are known for their mycotoxigenic capacity in other crop systems, their mycotoxin potential was examined. Alternariol (AOH), alternariol monomethyl-ether (AME) and tentoxin (TEN) production was estimated among Alternaria isolates, whereas ochratoxin A (OTA) and fumonisin B2 (FB2) production was assessed within the black aspergilli identified. Overall in both countries, 89% of the Alternaria isolates produced AOH and AME in vitro, while TEN was produced only by 43.9%. In vivo production of AOH and AME was restricted to 54.2% and 31.6% of the GR and CY isolates, respectively, while none of the isolates produced TEN in vivo. Among black aspergilli 21.7% of the GR and 17.8% of the CY isolates produced OTA in vitro, while in vivo OTA was detected in 8.8% of the isolates from both countries. FB2 was present in vitro in 42.0% of the GR and 22.2% of the CY isolates, while in vivo the production was limited to 27.5% and 4.5% of the GR and the CY isolates, respectively. Our data imply that mycotoxigenic Alternaria and Aspergillus species not only constitute a significant subset of the fungal population associated with pomegranate fruit rots responsible for fruit deterioration, but also pose a potential health risk factor for consumers of pomegranate-based products.

Conjugation of the mycotoxins alternariol and alternariol monomethyl ether in tobacco suspension cells

The mycotoxins alternariol (AOH) and alternariol-9-O-methyl ether (AME) carry three and two phenolic hydroxyl groups, respectively, which makes them candidates for the formation of conjugated metabolites in plants. Such conjugates may escape routine methods of analysis and have therefore been termed masked or, more recently, modified mycotoxins. We report now that AOH and AME are extensively conjugated in suspension cultures of tobacco BY-2 cells. Five conjugates of AOH were identified by MS and NMR spectroscopy as β-D-glucopyranosides (attached in AOH 3- or 9-position) as well as their 6'-malonyl derivatives, and as a gentiobiose conjugate. For AME, conjugation resulted in the d-glucopyranoside (mostly attached in the AME 3-position) and its 6'- and 4'-malonyl derivatives. Pronounced differences were noted for the quantitative pattern of AOH and AME conjugates as well as for their phytotoxicity. Our in vitro study demonstrates for the first time that masked mycotoxins of AOH and AME can be formed in plant cells.

Alternaria toxin-induced resistance in rose plants against rose aphid (Macrosiphum rosivorum): effect of tenuazonic acid

Many different types of toxins are produced by the fungus, Alternaria alternata (Fr.) Keissler. Little is known, however, regarding the influence of these toxins on insects. In this study, we investigated the toxin-induced inhibitory effects of the toxin produced by A. alternata on the rose aphid, Macrosiphum rosivorum, when the toxin was applied to leaves of the rose, Rosa chinensis. The results demonstrated that the purified crude toxin was non-harmful to rose plants and rose aphids, but had an intensive inhibitory effect on the multiplication of aphids. The inhibitory index against rose aphids reached 87.99% when rose plants were sprayed with the toxin solution at a low concentration. Further results from bioassays with aphids and high performance liquid chromatography (HPLC) analyses demonstrated that tenuazonic acid (TeA) was one of the most important resistance-related active components in the crude toxin. The content of TeA was 0.1199% in the crude toxin under the HPLC method. Similar to the crude toxin, the inhibitory index of pure TeA reached 83.60% 15 d after the rose plants were sprayed with pure TeA solution at the lower concentration of 0.060 μg/ml, while the contents of residual TeA on the surface and in the inner portion of the rose plants were only 0.04 and 0.00 ng/g fresh weight of TeA-treated rose twigs, respectively, 7 d after the treatment. Our results show that TeA, an active component in the A. alternata toxin, can induce the indirect plant-mediated responses in rose plants to intensively enhance the plant's resistances against rose aphids, and the results are very helpful to understand the plant-mediated interaction between fungi and insects on their shared host plants.

Mycotoxin production in liquid culture and on plants infected with Alternaria spp. isolated from rocket and cabbage

Fungi belonging to the genus Alternaria are common pathogens of fruit and vegetables with some species able to produce secondary metabolites dangerous to human health. Twenty-eight Alternaria isolates from rocket and cabbage were investigated for their mycotoxin production. Five different Alternaria toxins were extracted from synthetic liquid media and from plant material (cabbage, cultivated rocket, cauliflower). A modified Czapek-Dox medium was used for the in vitro assay. Under these conditions, more than 80% of the isolates showed the ability to produce at least one mycotoxin, generally with higher levels for tenuazonic acid. However, the same isolates analyzed in vivo seemed to lose their ability to produce tenuazonic acid. For the other mycotoxins; alternariol, alternariol monomethyl ether, altenuene and tentoxin a good correlation between in vitro and in vivo production was observed. In vitro assay is a useful tool to predict the possible mycotoxin contamination under field and greenhouse conditions.

Natural occurrence of four Alternaria mycotoxins in tomato- and citrus-based foods in China

A total of 70 tomato-based and 86 citrus-based products collected in China were analyzed for alternariol, alternariol monomethyl ether, tentoxin, and tenuazonic acid by ultraperformance liquid chromatography–electrospray ionization–tandem mass spectrometry. No toxins were found in any fresh tomato or citrus fruit samples. Tenuazonic acid was the predominant toxin detected in all tomato ketchup (10.2–1787 μg/kg) and tomato juice samples (7.4–278 μg/kg). Alternariol was quantitated at higher level than alternariol monomethyl ether with the ratio of alternariol/alternariol monomethyl ether ranging from 0.37 to 104 in 14 alternariol-positive tomato ketchup samples. Tentoxin was detected at much lower levels in all samples analyzed. Some citrus juice samples were positive for tenuazonic acid and alternariol monomethyl ether. It is necessary to conduct a systemic surveillance of Alternaria toxins in raw and processed foods to provide the scientific basis for risk assessment of dietary exposure to these toxins in Chinese populations.

Mycotoxins as antagonistic or supporting agents in the interaction between phytopathogenic Fusarium and Alternaria fungi

The role of mycotoxins in the microbial competition in an ecosystem or on the same host plant is still unclear. Therefore, a laboratory study was conducted to evaluate the influence of mycotoxins on growth and mycotoxin production of Fusarium and Alternaria fungi. Fusarium culmorum Fc13, Fusarium graminearum Fg23 and two Alternaria tenuissima isolates (At18 and At220) were incubated on wheat kernels supplemented with alternariol (AOH), tetramic acid derivates (TeA), deoxynivalenol (DON) and zearalenone (ZEA) in an in vitro test system. Fungal biomass was quantified by determining ergosterol content. Three Fusarium toxins (DON, nivalenol and ZEA) and three Alternaria toxins (AOH, alternariol methyl ether (AME) and altenuene) were analysed by HPLC-MS/MS. If Alternaria strains grew in wheat kernels spiked with Fusarium mycotoxins, their growth rates were moderately increased, their AOH and AME production was enhanced and they were simultaneously capable of degrading the Fusarium mycotoxins DON and ZEA. In contrast, both Fusarium strains behaved quite differently. The growth rate of Fc13 was not distinctly influenced, while Fg23 increased its growth in wheat kernels spiked with AOH. TeA depressed the ergosterol content in Fc13 as well as in Fg23. The DON production of Fc13 was slightly depressed, whereas the ZEA production was significantly increased. In contrast, Fg23 restricted its ZEA production. Both Fusarium strains were not capable of degrading the Alternaria mycotoxin AOH. Mycotoxins might play an important role in the interfungal competitive processes. They influence growth rates and mycotoxin production of the antagonistic combatants. The observed effects between phytopathogenic Alternaria and Fusarium strains and their mycotoxins aid the understanding of the complexity of microbial competitive behaviour in natural environments.

Characterization of Alternaria strains from Argentinean blueberry, tomato, walnut and wheat

Alternaria species have the ability to produce a variety of secondary metabolite, which plays important roles in food safety. Argentina is the second largest exporter of fresh and processed food products to Europe, however, few studies on Alternaria mycotoxins and other bioactive secondary metabolites have been carried out on Argentinean cereals, fruit and vegetables. Knowing the full chemical potential and the distribution of Alternaria spp. on crops, it is necessary to establish a toxicological risk assessment for food products for human consumption. In the present study, 87 Alternaria strains from different substrates (tomato, wheat, blueberries and walnuts) were characterized according to morphology and metabolite production. Aggressive dereplication (accurate mass, isotopic patterns and lists of all described compounds from Alternaria) was used for high-throughput evaluation of the chemical potential. Four strains belonged to the Alternaria infectoria sp.-grp., 6 to the Alternaria arborescens sp.-grp., 6 showed a sporulation pattern similar to that of "M" according to Simmons, 1 to that of Alternaria vaccinii, and the remaining 70 constituted a diverse group belonging to morphological groups "G" and "H". The cluster analysis yielded 16 almost identical dendrograms and grouped the Alternaria strains into four clusters and 11 singletons and outlier groups. The chemical analysis showed that AOH and AME were the most common metabolites produced, followed by TEN, ALXs and TeA. The A. infectoria sp.-grp. had no metabolites in common with the rest of the strains. Several secondary metabolites isolated from large-spored Alternaria species or other fungal genera were detected, such as dehydrocurvularin, pyrenochaetic acid and alternarienonic acid. The strains isolated from tomato produced lower amounts of metabolites than strains from blueberries, walnut and wheat, although individual strains from tomato produced the highest amount of some metabolites. The A. infectoria sp.-grp. was unique to cereals, whereas strains classified as belonging to the A. arborescens sp.-grp or having sporulation pattern "M" were only isolated from tomatoes. Otherwise, no clear association between substrate and identity could be found. The analyses in the study show that at least 75% of the Argentinean strains are able to produce potential mycotoxins.

Development and validation of an ultra-high-performance liquid chromatography tandem mass spectrometric method for the simultaneous determination of free and conjugated Alternaria toxins in cereal-based foodstuffs

A UPLC-ESI^+/--MS/MS method for the simultaneous determination of free (alternariol, alternariol monomethyl ether, altenuene, tenuazonic acid, tentoxin, altertoxin-I) and conjugated (sulfates and glucosides of alternariol and alternariol monomethyl ether) Alternaria toxins in cereals and cereal products (rice, oat flakes and barley) was developed. Optimization of the sample preparation and extraction methodology was achieved through experimental design, using full factorial design for extraction solvent composition optimization and fractional factorial design to identify the critical factors in the sample preparation protocol, which were in turn subjected to optimization. Final extracts were analysed using an Waters Acquity UPLC system coupled to a Quattro Premier XE mass spectrometer equipped with an electrospray interface operated in both positive and negative ionization mode. Chromatographic separation was achieved using an Acquity UPLC HSS T3 column, and the applied gradient elution programme allowed for the simultaneous determination of 10 Alternaria toxins in a one-step chromatographic run with a total run time of only 7min. Subsequently, the method, applying isotopically labelled internal standards ([²H₄]-alternariol monomethyl ether and [¹³C₆,¹⁵N]-tenuazonic acid), was validated for several parameters such as linearity, apparent recovery, limit of detection, limit of quantification, precision, measurement uncertainty and specificity (in agreement with the criteria mentioned in Commission Regulation No. 401/2006/EC and Commission Decision No. 2002/657/EC). During validation, quality of the bioanalytical data was improved by counteracting the observed heteroscedasticity through the application of weighted least squares linear regression (WLSLR). Finally, 24 commercially available cereal-based foodstuffs were subjected to analysis, revealing the presence of tenuazonic acid in both rice and oat flake samples (<LOQ - 68±7μgkg^-1) and tentoxin in rice samples (<LOQ - 10.9±2.0μgkg^-1).

Autophagy and senescence, stress responses induced by the DNA-damaging mycotoxin alternariol

The mycotoxin alternariol (AOH), a frequent contaminant in fruit and grain, is known to induce cellular stress responses such as reactive oxygen production, DNA damage and cell cycle arrest. Cellular stress is often connected to autophagy, and we employed the RAW264.7 macrophage model to test the hypothesis that AOH induces autophagy. Indeed, AOH treatment led to a massive increase in acidic vacuoles often observed upon autophagy induction. Moreover, expression of the autophagy marker LC3 was markedly increased and there was a strong accumulation of LC3-positive puncta. Increased autophagic activity was verified biochemically by measuring the degradation rate of long-lived proteins. Furthermore, AOH induced expression of Sestrin2 and phosphorylation of AMPK as well as reduced phosphorylation of mTOR and S6 kinase, common mediators of signaling pathways involved in autophagy. Transmission electron microscopy analyzes of AOH treated cells not only clearly displayed structures associated with autophagy such as autophagosomes and autolysosomes, but also the appearance of lamellar bodies. Prolonged AOH treatment resulted in changed cell morphology from round into more star-shaped as well as increased β-galactosidase activity. This suggests that the cells eventually entered senescence. In conclusion, our data identify here AOH as an inducer of both autophagy and senescence. These effects are suggested to be to be linked to AOH-induced DSB (via a reported effect on topoisomerase activity), resulting in an activation of p53 and the Sestrin2-AMPK-mTOR-S6K signaling pathway.

Oxidative stress of alternariol in Caco-2 cells

Alternariol (AOH) is a mycotoxin produced by fungus Alternaria. It is found in a wide variety of fruits and cereals products. AOH is able to damage human health. The aim of this study was to evaluate the cytotoxicity of AOH in human colon adenocarcinoma (Caco-2) cells. Moreover, some events related to oxidative stress were evaluated: reactive oxygen species (ROS) generated by oxidation of 2',7'-dichlorodihydrofluorescein diacetate; peroxidation of lipid (LPO) by malondialdehyde (MDA) production; and antioxidant enzymatic capability of catalase (CAT) and superoxide dismutase (SOD). Cytotoxicity of AOH (from 3.125 to 100 μM) was determined during 24, 48 and 72 h of exposure by different endpoints. AOH decreased cell viability by MTT, NR and PC assays. However, no IC50 values were obtained by any of the assays tested. AOH induced a strong oxidative stress in Caco-2 cells by generation of ROS production and LPO associated with a rise in the SOD activity at all concentration tested. ROS increased 1.2-fold with respect to the control and MDA production ranged from 130% to 250% compared to control. Our results demonstrated that in spite of AOH showing cytotoxic effect on Caco-2 cells at the highest concentration tested, oxidative stress by LPO and ROS was observed at all concentrations assayed. This could cause an injury and be hazardous to health.

Determination of the absolute configuration of perylene quinone-derived mycotoxins by measurement and calculation of electronic circular dichroism spectra and specific rotations

Altertoxins I-III, alterlosins I and II, alteichin (alterperylenol), stemphyltoxins I-IV, stemphyperylenol, stemphytriol, 7-epi-8-hydroxyaltertoxin I, and 6-epi-stemphytriol are mycotoxins derived from perylene quinone, for which the absolute configuration was not known. Electronic circular dichroism (ECD) spectra were calculated for these compounds and compared with measured spectra of altertoxins I-III, alteichin, and stemphyltoxin III and with reported Cotton effects. Specific rotations were calculated and compared with reported specific rotations. The absolute configuration of all the toxins, except for stemphyltoxin IV, could thus be determined. The validity of the assignment was high whenever reported ECD data were available for comparison, and the validity was lower when the assignment was based only on the comparison of calculated and reported specific rotations. ECD spectra are intrinsically different for toxins with a biphenyl substructure and for toxins derived from dihydroanthracene.

Isolating fungal pathogens from a dynamic disease outbreak in a native plant population to establish plant-pathogen bioassays for the ecological model plant Nicotiana attenuata

The wild tobacco species Nicotiana attenuata has been intensively used as a model plant to study its interaction with insect herbivores and pollinators in nature, however very little is known about its native pathogen community. We describe a fungal disease outbreak in a native N. attenuata population comprising 873 plants growing in an area of about 1500 m2. The population was divided into 14 subpopulations and disease symptom development in the subpopulations was monitored for 16 days, revealing a waxing and waning of visible disease symptoms with some diseased plants recovering fully. Native fungal N. attenuata pathogens were isolated from diseased plants, characterized genetically, chemotaxonomically and morphologically, revealing several isolates of the ascomycete genera Fusarium and Alternaria, that differed in the type and strength of the disease symptoms they caused in bioassays on either detached leaves or intact soil-grown plants. These isolates and the bioassays will empower the study of N. attenuata-pathogen interactions in a realistic ecological context.

Bioactive secondary metabolites from Phomopsis sp., an endophytic fungus from Senna spectabilis

Chemical investigation of an acetonitrile fraction from the endophytic fungus Phomopsis sp. led to the isolation of the new natural product 2-hydroxy-alternariol (7) together with the known compounds cytochalasins J (1) and H (2), 5'-epialtenuene (3) and the mycotoxins alternariol monomethyl ether (AME, 4), alternariol (AOH, 5) and cytosporone C (6). The structure of the new compound was elucidated by using 1-D and 2-D NMR (nuclear magnetic resonance) and high resolution mass spectrometry. The cytochalasins J (1) and H (2) and AOH (5) exhibited potent inhibition of the total ROS (reactive oxygen species) produced by stimulated human neutrophils and acted as potent potential anti-inflammatory agents. Moreover, cytochalasin H (2) demonstrated antifungal and acetylcholinesterase enzyme (AChE) inhibition in vitro.

Three new resveratrol derivatives from the mangrove endophytic fungus Alternaria sp

Three new resveratrol derivatives, namely, resveratrodehydes A–C (1–3), were isolated from the mangrove endophytic fungus Alternaria sp. R6. The structures of these compounds were elucidated by analysis of their MS, 1D and 2D NMR spectroscopic data. All compounds showed broad-spectrum inhibitory activities against three human cancer cell lines including human breast MDA-MB-435, human liver HepG2, and human colon HCT-116 by MTT assay (IC₅₀ < 50 μM). Among them, compounds 1 and 2 both exhibited marked cytotoxic activities against MDA-MB-435 and HCT-116 cell lines (IC₅₀ < 10 μM). Additionally, compounds 1 and 3 showed moderate antioxidant activity by DPPH radical scavenging assay.

Natural dibenzo-α-pyrones and their bioactivities

Natural dibenzo-α-pyrones are an important group of metabolites derived from fungi, mycobionts, plants and animal feces. They exhibit a variety of biological activities such as toxicity on human and animals, phytotoxicity as well as cytotoxic, antioxidant, antiallergic, antimicrobial, antinematodal, and acetylcholinesterase inhibitory properties. Dibenzo-α-pyrones are biosynthesized via the polyketide pathway in microorganisms or metabolized from plant-derived ellagitannins and ellagic acid by intestinal bacteria. At least 53 dibenzo-α-pyrones have been reported in the past few decades. This mini-review aims to briefly summarize the occurrence, biosynthesis, biotransformation, as well as their biological activities and functions. Some considerations related to synthesis, production and applications of dibenzo-α-pyrones are also discussed.

Role of the Alternaria alternata blue-light receptor LreA (white-collar 1) in spore formation and secondary metabolism

Alternaria alternata is a filamentous fungus that causes considerable loss of crops of economically important feed and food worldwide. It produces more than 60 different secondary metabolites, among which alternariol (AOH) and altertoxin (ATX) are the most important mycotoxins. We found that mycotoxin production and spore formation are regulated by light in opposite ways. Whereas spore formation was largely decreased under light conditions, the production of AOH was stimulated 2- to 3-fold. ATX production was even strictly dependent on light. All light effects observed could be triggered by blue light, whereas red light had only a minor effect. Inhibition of spore formation by light was reversible after 1 day of incubation in the dark. We identified orthologues of genes encoding the Neurospora crassa blue-light-perceiving white-collar proteins, a cryptochrome, a phytochrome, and an opsin-related protein in the genome of A. alternata. Deletion of the white-collar 1 (WC-1) gene (lreA) resulted in derepression of spore formation in dark and in light. ATX formation was strongly induced in the dark in the lreA mutant, suggesting a repressing function of LreA, which appears to be released in the wild type after blue-light exposure. In addition, light induction of AOH formation was partially dependent on LreA, suggesting also an activating function. A. alternata ΔlreA was still able to partially respond to blue light, indicating the action of another blue-light receptor system.

Epoxide reduction to an alcohol: a novel metabolic pathway for perylene quinone-type alternaria mycotoxins in mammalian cells

The group of perylene quinone-type Alternaria toxins contains several congeners with epoxide groups, for example, altertoxin II (ATX II) and stemphyltoxin III (STTX III). Recent studies in our laboratory have disclosed that the epoxide moieties of ATX II and STTX III are reduced to alcohols in human colon Caco-2 cells, thereby resulting in the formation of altertoxin I (ATX I) and alteichin, respectively. In the present study, this pathway was demonstrated for ATX II in three other mammalian cell lines. Furthermore, the chemical reaction of this toxin with monothiols like glutathione could be shown, and the structures of the reaction products were tentatively elucidated by UV and mass spectrometry. Chemical reaction of ATX II with dithiols capable of forming five- and six-membered rings gave rise to ATX I, thus providing a clue for the molecular mechanism of the epoxide reduction pathway of ATX II. Both epoxide reduction and glutathione conjugation appear to attenuate, but not completely abolish, the genotoxicity of ATX II.

Temperature and water stress impacts on growth and production of altertoxin-II by strains of Alternaria tenuissima from Argentinean wheat

Alternaria tenuissima is commonly isolated from wheat in Argentina. The objective of this study was to examine the effects of temperature (15-34 °C) and water activity (0.98, 0.95 aw) on growth and temporal altertoxin II (ALTX-II) production by two strains over 14-21 days on a milled wheat agar. It was shown that growth occurred over the whole temperature range tested and was optimum at 25-30 °C and 0.98 aw, and 30 °C at 0.95 aw. The incubation time did not show any significant effect on ALTX-II accumulation. The optimum conditions for ALTX-II production were 0.98 aw and 30 °C for both strains. The strains also accumulated significant amounts of this toxin at 34 °C. This is the first study to evaluate the ecology of growth and production of ALTX-II by strains of A. tenuissima.

Conjugation of deoxynivalenol by Alternaria alternata (54028 NRRL), Rhizopus microsporus var. rhizopodiformis (54029 NRRL) and Aspergillus oryzae (5509 NRRL)

Deoxynivalenol (DON, vomitoxin) is a trichothecene mycotoxin which can be considered to be an indicator of Fusarium mycotoxin contamination in grain, feed and food. Recent studies have described the presence of glucose conjugated DON, which is a product of plant metabolism, but there is a lack of information available on DON conjugation by fungi. The aim of the current study was, therefore, to investigate the ability of fungi to metabolize DON into hydrolysable conjugated DON. Alternaria alternata (54028 NRRL) and Rhizopus microsporus var. rhizopodiformis (54029 NRRL) were found to be capable of metabolizing DON into hydrolysable conjugated DON. This ranged from 13-23 % conjugation of DON in potato dextrose agar media and from 11-36 % in corn-based media. There was, however, considerable variation between fungal strains in the ability to conjugate DON as only a slight increase in hydrolysable conjugated DON (1-6 %) was observed when incubating with A. oryzae (5509 NRRL). A. oryzae (5509 NRRL) was also shown to degrade DON (up to 92 %) over 21 days of incubation on corn-based media. The current study shows that conjugation of DON can be achieved through fungal metabolism in addition to being a product of plant metabolism.

[Determination of the profiles of secondary metabolites characteristic of Alternaria strains isolated from tomato]

BACKGROUND

Many Alternaria species have been studied for their ability to produce bioactive secondary metabolites, such as tentoxin (TEN), some of which have toxic properties. The main food contaminant toxins are tenuazonic acid, alternariol (AOH), alternariol monomethyl ether (AME), altenuene, and altertoxins i, ii and iii.

AIMS

To determine the profiles of secondary metabolites characteristic of Alternaria strains isolated from tomato for their chemotaxonomic classification.

METHODS

The profiles of secondary metabolites were determined by HPLC MS.

RESULTS

The Alternaria isolates obtained from spoiled tomatoes belong, according to their morphological characteristics, to the species groups Alternaria alternata, Alternaria tenuissima and Alternaria arborescens, with A. tenuissima being the most frequent. The most frequent profiles of secondary metabolites belonging to the species groups A. alternata (AOH, AME, TEN), A. tenuissima (AOH, AME, TEN, tenuazonic acid) and A. arborescens (AOH, AME, TEN, tenuazonic acid) were determined, with some isolates of the latter being able to synthesize AAL toxins.

CONCLUSIONS

Secondary metabolite profiles are a useful tool for the differentiation of small spored Alternaria isolates not easily identifiable by their morphological characteristics.