Engineered bakers yeast as a sensitive bioassay indicator organism for the trichothecene toxin deoxynivalenol

Development of a fumonisin-sensitive Saccharomyces cerevisiae indicator strain and utilization for activity testing of candidate detoxification genes

IMPORTANCE

Fumonisins can cause diseases in animals and humans consuming Fusarium-contaminated food or feed. The search for microbes capable of fumonisin degradation, or for enzymes that can detoxify fumonisins, currently relies primarily on chemical detection methods. Our constructed fumonisin B1-sensitive yeast strain can be used to phenotypically detect detoxification activity and should be useful in screening for novel fumonisin resistance genes and to elucidate fumonisin metabolism and resistance mechanisms in fungi and plants, and thereby, in the long term, help to mitigate the threat of fumonisins in feed and food.

The Ribosome-Binding Mode of Trichothecene Mycotoxins Rationalizes Their Structure-Activity Relationships

Trichothecenes are the most prevalent mycotoxins contaminating cereal grains. Some of them are also considered as the virulence factors of Fusarium head blight disease. However, the mechanism behind the structure-activity relationship for trichothecenes remains unexplained. Filling this information gap is a crucial step for developing strategies to manage this large family of mycotoxins in food and feed. Here, we perform an in-depth re-examination of the existing structures of Saccharomyces cerevisiae ribosome complexed with three different trichothecenes. Multiple binding interactions between trichothecenes and 25S rRNA, including hydrogen bonds, nonpolar pi stacking interactions and metal ion coordination interactions, are identified as important binding determinants. These interactions are mainly contributed by the key structural elements to the toxicity of trichothecenes, including the oxygen in the 12,13-epoxide ring and a double bond between C9 and C10. In addition, the C3-OH group also participates in binding. The comparison of three trichothecenes binding to the ribosome, along with their binding pocket architecture, suggests that the substitutions at different positions impact trichothecenes binding in two different patterns. Moreover, the binding of trichothecenes induced conformation changes of several nucleotide bases in 25S rRNA. This then provides a structural framework for understanding the structure-activity relationships apparent in trichothecenes. This study will facilitate the development of strategies aimed at detoxifying mycotoxins in food and feed and at improving the resistance of cereal crops to Fusarium fungal diseases.

Mitigation of nivalenol using alcoholic fermentation and magnetic field application

This study aimed at evaluating mitigation of nivalenol (NIV) in alcoholic fermentation with magnetic field application (MF). Mitigation was related to both the glutathione (GSH) redox molecule and the enzyme peroxidase (PO), which were synthesized by Saccharomyces cerevisiae US-05. Conditions under evaluation were NIV (0.2 µg mL^-1), MF application (35 mT) and simultaneous use of mycotoxin and MF. The GSH content and the PO activity were increased when the culture contained NIV and the alcohol profile was altered after 48 h of fermentation. At the end of the alcoholic fermentation, NIV was mitigated by 56.5%. Therefore, this process is a promising method to reduce contamination by NIV, although the mycotoxin affects the chemical characteristics of the final product.

Outstanding questions in flower metabolism

The great diversity of flowers, their color, odor, taste, and shape, is mostly a result of the metabolic processes that occur in this reproductive organ when the flower and its tissues develop, grow, and finally die. Some of these metabolites serve to advertise flowers to animal pollinators, other confer protection towards abiotic stresses, and a large proportion of the molecules of the central metabolic pathways have bioenergetic and signaling functions that support growth and the transition to fruits and seeds. Although recent studies have advanced our general understanding of flower metabolism, several questions still await an answer. Here, we have compiled a list of open questions on flower metabolism encompassing molecular aspects, as well as topics of relevance for agriculture and the ecosystem. These questions include the study of flower metabolism through development, the biochemistry of nectar and its relevance to promoting plant-pollinator interaction, recycling of metabolic resources after flowers whiter and die, as well as the manipulation of flower metabolism by pathogens. We hope with this review to stimulate discussion on the topic of flower metabolism and set a reference point to return to in the future when assessing progress in the field.

A mycotoxin transporter (4D) from a library of deoxynivalenol-tolerant microorganisms

New strategies are needed to mitigate the mycotoxin deoxynivalenol (DON) in feed and food products. Microbial DNA fragments were generated from a library of DON-tolerant microorganisms. These fragments were screened in DON-sensitive yeast strains for their ability to modify or transport DON. Fragments were cloned into a PCR8/TOPO vector, and recombined into the yeast vector, pYES-DEST52. Resulting yeast transformants were screened in the presence of 100 ppm DON. Transformants that were able to grow in the presence of DON were plated on a selective medium, and the cloned microbial DNA fragments were sequenced. BLAST queries of one microbial DNA fragment (4D) showed a high degree of similarity to an ABC transporter. A series of screening and inhibition assays were conducted with a transport inhibitor (propanol), to test the hypothesis that 4D is a mycotoxin transporter. DON concentrations did not change for yeast transformants expressing 4D. The ability of yeast transformants expressing 4D to transport DON was inhibited by the addition of propanol. Moreover, yeast transformants expressing a known efflux pump (PDR5) showed similar trends in propanol transport inhibition compared to 4D. Future work should consider mycotoxin transporters such as 4D to the development of transgenic plants to limit DON accumulation in seeds.

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Microbial DNA fragments were generated from a library of DON-tolerant microorganisms.

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Fragments were screened in DON-sensitive yeast strains for their ability to modify or transport DON.

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BLAST queries of one microbial fragment (4D) showed a high degree of similarity to an ABC transporter.

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Transformants expressing a known efflux pump (PDR5) showed similar trends in propanol transport inhibition compared to 4D.

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Future work should consider mycotoxin transporters such as 4D to the development of transgenic plants.

Stable Isotope-Assisted Plant Metabolomics: Investigation of Phenylalanine-Related Metabolic Response in Wheat Upon Treatment With the Fusarium Virulence Factor Deoxynivalenol

The major Fusarium mycotoxin deoxynivalenol (DON) is a virulence factor in wheat and has also been shown to induce defense responses in host plant tissue. In this study, global and tracer labeling with 13C were combined to annotate the overall metabolome of wheat spikes and to evaluate the response of phenylalanine-related pathways upon treatment with DON. At anthesis, spikes of resistant and susceptible cultivars as well as two related near isogenic wheat lines (NILs) differing in the presence/absence of the major resistance QTL Fhb1 were treated with 1 mg DON or water (control), and samples were collected at 0, 12, 24, 48, and 96 h after treatment (hat). A total of 172 Phe-derived wheat constituents were detected with our untargeted approach employing 13C-labeled phenylalanine and subsequently annotated as flavonoids, lignans, coumarins, benzoic acid derivatives, hydroxycinnamic acid amides (HCAAs), as well as peptides. Ninety-six hours after the DON treatment, up to 30% of the metabolites biosynthesized from Phe showed significantly increased levels compared to the control samples. Major metabolic changes included the formation of precursors of compounds implicated in cell wall reinforcement and presumed antifungal compounds. In addition, also dipeptides, which presumably are products of proteolytic degradation of truncated proteins generated in the presence of the toxin, were significantly more abundant upon DON treatment. An in-depth comparison of the two NILs with correlation clustering of time course profiles revealed some 70 DON-responsive Phe derivatives. While several flavonoids had constitutively different abundance levels between the two NILs differing in resistance, other Phe-derived metabolites such as HCAAs and hydroxycinnamoyl quinates were affected differently in the two NILs after treatment with DON. Our results suggest a strong activation of the general phenylpropanoid pathway and that coumaroyl-CoA is mainly diverted towards HCAAs in the presence of Fhb1, whereas the metabolic route to monolignol(-conjugates), lignans, and lignin seems to be favored in the absence of the Fhb1 resistance quantitative trait loci.

Microbial Detoxification of Deoxynivalenol (DON), Assessed via a Lemna minor L. Bioassay, through Biotransformation to 3-epi-DON and 3-epi-DOM-1

Mycotoxins are toxic metabolites produced by fungi. To mitigate mycotoxins in food or feed, biotransformation is an emerging technology in which microorganisms degrade toxins into non-toxic metabolites. To monitor deoxynivalenol (DON) biotransformation, analytical tools such as ELISA and liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) are typically used. However, these techniques do not give a decisive answer about the remaining toxicity of possible biotransformation products. Hence, a bioassay using Lemna minor L. was developed. A dose-response analysis revealed significant inhibition in the growth of L. minor exposed to DON concentrations of 0.25 mg/L and higher. Concentrations above 1 mg/L were lethal for the plant. This bioassay is far more sensitive than previously described systems. The bioassay was implemented to screen microbial enrichment cultures, originating from rumen fluid, soil, digestate and activated sludge, on their biotransformation and detoxification capability of DON. The enrichment cultures originating from soil and activated sludge were capable of detoxifying and degrading 5 and 50 mg/L DON. In addition, the metabolites 3-epi-DON and the epimer of de-epoxy-DON (3-epi-DOM-1) were found as biotransformation products of both consortia. Our work provides a new valuable tool to screen microbial cultures for their detoxification capacity.

Suppressed recombination and unique candidate genes in the divergent haplotype encoding Fhb1, a major Fusarium head blight resistance locus in wheat

Fine mapping and sequencing revealed 28 genes in the non-recombining haplotype containing Fhb1 . Of these, only a GDSL lipase gene shows a pathogen-dependent expression pattern. Fhb1 is a prominent Fusarium head blight resistance locus of wheat, which has been successfully introgressed in adapted breeding material, where it confers a significant increase in overall resistance to the causal pathogen Fusarium graminearum and the fungal virulence factor and mycotoxin deoxynivalenol. The Fhb1 region has been resolved for the susceptible wheat reference genotype Chinese Spring, yet the causal gene itself has not been identified in resistant cultivars. Here, we report the establishment of a 1 Mb contig embracing Fhb1 in the donor line CM-82036. Sequencing revealed that the region of Fhb1 deviates from the Chinese Spring reference in DNA size and gene content, which explains the repressed recombination at the locus in the performed fine mapping. Differences in genes expression between near-isogenic lines segregating for Fhb1 challenged with F. graminearum or treated with mock were investigated in a time-course experiment by RNA sequencing. Several candidate genes were identified, including a pathogen-responsive GDSL lipase absent in susceptible lines. The sequence of the Fhb1 region, the resulting list of candidate genes, and near-diagnostic KASP markers for Fhb1 constitute a valuable resource for breeding and further studies aiming to identify the gene(s) responsible for F. graminearum and deoxynivalenol resistance.

GC-MS based targeted metabolic profiling identifies changes in the wheat metabolome following deoxynivalenol treatment

Fusariumgraminearum and related species commonly infest grains causing the devastating plant disease Fusarium head blight (FHB) and the formation of trichothecene mycotoxins. The most relevant toxin is deoxynivalenol (DON), which acts as a virulence factor of the pathogen. FHB is difficult to control and resistance to this disease is a polygenic trait, mainly mediated by the quantitative trait loci (QTL) Fhb1 and Qfhs.ifa-5A. In this study we established a targeted GC-MS based metabolomics workflow comprising a standardized experimental setup for growth, treatment and sampling of wheat ears and subsequent GC-MS analysis followed by data processing and evaluation of QC measures using tailored statistical and bioinformatics tools. This workflow was applied to wheat samples of six genotypes with varying levels of Fusarium resistance, treated with either DON or water, and harvested 0, 12, 24, 48 and 96 h after treatment. The results suggest that the primary carbohydrate metabolism and transport, the citric acid cycle and the primary nitrogen metabolism of wheat are clearly affected by DON treatment. Most importantly significantly elevated levels of amino acids and derived amines were observed. In particular, the concentrations of the three aromatic amino acids phenylalanine, tyrosine, and tryptophan increased. No clear QTL specific difference in the response could be observed except a generally faster increase in shikimate pathway intermediates in genotypes containing Fhb1. The overall workflow proved to be feasible and facilitated to obtain a more comprehensive picture on the effect of DON on the central metabolism of wheat.

Phytotoxicity evaluation of type B trichothecenes using a Chlamydomonas reinhardtii model system

Type B trichothecenes, which consist of deoxynivalenol (DON) and nivalenol (NIV) as the major end products, are produced by phytotoxic fungi, such as the Fusarium species, and pollute arable fields across the world. The DON toxicity has been investigated using various types of cell systems or animal bioassays. The evaluation of NIV toxicity, however, has been relatively restricted because of its lower level compared with DON. In this study, the Chlamydomonas reinhardtii testing system, which has been reported to have adequate NIV sensitivity, was reinvestigated under different mycotoxin concentrations and light conditions. The best concentration of DON and NIV, and their derivatives, for test conditions was found to be 25 ppm (2.5 × 10(-2) mg/mL). In all light test conditions, DON, NIV, and fusarenon-X (FusX) indicated significant growth inhibition regardless of whether a light source existed, or under differential wavelength conditions. FusX growth was also influenced by changes in photon flux density. These results suggest that C. reinhardtii is an appropriate evaluation system for type B trichothecenes.

Reaction of (conjugated) deoxynivalenol with sulphur reagents - novel metabolites, toxicity and application

Recently, three novel deoxynivalenol (DON) sulphonates (DONS-1, -2 and -3) formed by transformation of DON with sulphur reagents were discovered. Their structures were elucidated, factors influencing their formation were investigated and their stability was evaluated. The first aim of the present work was to assess the toxicity of the three DONS. The hypothesis was a loss of toxicity of DONS-1 and reduced toxicity of DONS-2 and DONS-3 compared with DON. Three different toxicity tests confirmed this hypothesis, revealing reduction of toxicity by factors of at least 330, 29 and 33 for DONS-1, -2 and -3, respectively. The second objective was to investigate the reaction of DON conjugates with sulphur reagents both in solution and in cereal samples, and to determine the pattern of the formed DONS in DON-contaminated cereals. Reversed phase ultra high performance liquid chromatography-high resolution tandem mass spectrometry demonstrated that also conjugated mycotoxins (DON-3-glucoside, 3-acetyl-DON, 15-acetyl-DON) are converted into DON conjugate sulphonates upon incubation with sodium metabisulphite (SBS) or sodium sulphite in solution. DON conjugate sulphonates of the series 1, 2 and 3 differed in their fragmentation pattern in a similar way as DONS-1, -2 and -3. Treatment of a barley sample contaminated with DON, 3-acetyl-DON and the masked mycotoxin DON-3-glucoside with SBS revealed that the predominantly formed compounds are DONS-3 and DON conjugate sulphonates of the series 3, followed by those of the series 2. Incubation of DON containing maize samples of different moisture contents with 0.5% of SBS or sodium sulphite for 7 days confirmed DONS-3 as main DON transformation product. Both in barley and in maize, SBS and sodium sulphite showed different efficiency for transformation of DON at low moisture contents. These findings provide novel insights into inactivation of DON-contaminated cereals.

Comprehensive gene expression analysis of type B trichothecenes

Type B trichothecenes, deoxynivalenol (DON) and nivalenol (NIV), are secondary metabolites of Fusarium species and are major pollutants in food and feed products. Recently, the production trend of their derivatives, 3-acetyldeoxynivalenol (3-AcDON), 15-acetyldeoxynivalenol (15-AcDON), and 4-acetylnivalenol (4-AcNIV or fusarenon-X), has been changing in various regions worldwide. Although in vivo behavior has been reported, it is necessary to acquire more detailed information about these derivatives. Here, the yeast PDR5 mutant was used for toxicity evaluation, and the growth test revealed that DON, 15-AcDON, and 4-AcNIV had higher toxicity compared to 3-AcDON and NIV. 15-AcDON exerted the most significant gene expression changes, and cellular localization clustering exhibited repression of mitochondrial ribosomal genes. This study suggests that the toxicity trends of both DON products (DON and its derivatives) and NIV products (NIV and its derivatives) are similar to those observed in mammalian cells, with a notable toxic response to 15-AcDON.

Isolation and characterization of a new less-toxic derivative of the Fusarium mycotoxin diacetoxyscirpenol after thermal treatment

Trichothecenes are an important class of mycotoxins that act as potent protein synthesis inhibitors in eukaryotic organisms. The compound 4,15-diacetoxyscirpenol is highly toxic for plants and animals. Potatoes are especially prone to be contaminated with 4,15-diacetoxyscirpenol after infection with Fusarium sambucinum. In the current study, the reduction of 4,15-diacetoxyscirpenol during thermal treatment in aqueous solution was monitored. A new derivative was detected and named DAS-M1. After isolation, DAS-M1 was characterized with LC-HR-MS and LC-MS/MS and structurally elucidated with (1)H, (13)C, and 2D NMR. Potatoes were inoculated with F. sambucinum, and the infected potatoes were cooked at 100 or 121 °C, respectively. A reduction of 4,15-diacetoxyscirpenol from about 26% (1 h at 100 °C) to 100% (4 h at 121 °C) was detected by means of LC-MS/MS analysis. The effects of different pH values on the reduction of 4,15-diacetoxyscirpenol were investigated, showing higher conversion rates at more acidic pH values. In addition, the toxicity of 4,15-diacetoxyscirpenol and DAS-M1 was compared in vitro using a wheat germ transcription/translation assay and in vivo on Saccharomyces cerevisiae. The results show that the inhibitory effect of DAS-M1 on yeast growth is about 50 times lower and inhibition of protein synthesis is about 100 times lower than that of 4,15-diacetoxyscirpenol.

Intracellular expression of a single domain antibody reduces cytotoxicity of 15-acetyldeoxynivalenol in yeast

15-Acetyldeoxynivalenol (15-AcDON) is a low molecular weight sesquiterpenoid trichothecene mycotoxin associated with Fusarium ear rot of maize and Fusarium head blight of small grain cereals. The accumulation of mycotoxins such as deoxynivalenol (DON) and 15-AcDON within harvested grain is subject to stringent regulation as both toxins pose dietary health risks to humans and animals. These toxins inhibit peptidyltransferase activity, which in turn limits eukaryotic protein synthesis. To assess the ability of intracellular antibodies (intrabodies) to modulate mycotoxin-specific cytotoxocity, a gene encoding a camelid single domain antibody fragment (V(H)H) with specificity and affinity for 15-AcDON was expressed in the methylotropic yeast Pichia pastoris. Cytotoxicity and V(H)H immunomodulation were assessed by continuous measurement of cellular growth. At equivalent doses, 15-AcDON was significantly more toxic to wild-type P. pastoris than was DON. In turn, DON was orders of magnitude more toxic than 3-acetyldeoxynivalenol. Intracellular expression of a mycotoxin-specific V(H)H within P. pastoris conveyed significant (p = 0.01) resistance to 15-AcDON cytotoxicity at doses ranging from 20 to 100 mug.ml(-1). We also documented a biochemical transformation of DON to 15-AcDON to account for the attenuation of DON cytotoxicity at 100 and 200 mug.ml(-1). The proof of concept established within this eukaryotic system suggests that in planta V(H)H expression may lead to enhanced tolerance to mycotoxins and thereby limit Fusarium infection of commercial agricultural crops.

ABC transporter Pdr10 regulates the membrane microenvironment of Pdr12 in Saccharomyces cerevisiae

The eukaryotic plasma membrane exhibits both asymmetric distribution of lipids between the inner and the outer leaflet and lateral segregation of membrane components within the plane of the bilayer. In budding yeast (Saccharomyces cerevisiae), maintenance of leaflet asymmetry requires P-type ATPases, which are proposed to act as inward-directed lipid translocases (Dnf1, Dnf2, and the associated protein Lem3), and ATP-binding cassette (ABC) transporters, which are proposed to act as outward-directed lipid translocases (Pdr5 and Yor1). The S. cerevisiae genome encodes two other Pdr5-related ABC transporters: Pdr10 (67% identity) and Pdr15 (75% identity). We report the first analysis of Pdr10 localization and function. A Pdr10-GFP chimera was located in discrete puncta in the plasma membrane and was found in the detergent-resistant membrane fraction. Compared to control cells, a pdr10 mutant was resistant to sorbate but hypersensitive to the chitin-binding agent Calcofluor White. Calcofluor sensitivity was attributable to a partial defect in endocytosis of the chitin synthase Chs3, while sorbate resistance was attributable to accumulation of a higher than normal level of the sorbate exporter Pdr12. Epistasis analysis indicated that Pdr10 function requires Pdr5, Pdr12, Lem3, and mature sphingolipids. Strikingly, Pdr12 was shifted to the detergent-resistant membrane fraction in pdr10 cells. Pdr10 therefore acts as a negative regulator for incorporation of Pdr12 into detergent-resistant membranes, a novel role for members of the ABC transporter superfamily.

Isolation and identification of a strain of Aspergillus tubingensis with deoxynivalenol biotransformation capability

Deoxynivalenol (DON) is one of the most common contaminants of various foodstuffs. A biotransformation system was used in order to lessen the toxicity of DON. A strain of Aspergillus (NJA-1) was isolated from soil and cultured in an inorganic salt medium containing DON. Bt2a/Bt2b primers were used to amplify the β-tubulin gene of NJA-1. Sequence analysis the PCR product and morphology observation indicated that NJA-1 belonged to Aspergillus tubingensis (aerobic fungi). The DNA sequence information of the PCR product was deposited in GenBank (accession number DQ9025790). The DNA sequence had 99% similarity to the Aspergillus tubingensis accession number AY820009. An unknown compound in NJA-1 showed the ability to convert DON into another product. The molecular weight of the bioconversion product was 18.1 D (H2O) larger than that of DON. The analysis showed that DON could be hydrolyzed by NJA-1. The mean DON biotransformation rate was 94.4% after two weeks of cultivation. The finding presents a new method for DON biotransformation.