Fusarium Tri4 encodes a key multifunctional cytochrome P450 monooxygenase for four consecutive oxygenation steps in trichothecene biosynthesis

Citric acid impairs type B trichothecene biosynthesis of Fusarium graminearum but enhances its growth and pigment biosynthesis: transcriptomic and proteomic analyses

Fusarium graminearum is a pathogenic fungus that causes devastating plant diseases such as Fusarium head blight, leading to significant food waste and posing a threat to human health due to the accumulation of mycotoxins. As a soil-borne fungus, F. graminearum interacts closely with plants and soil, completing its life cycle in an intricate environment. In this investigation, citric acid (CA), a plant root exudate and heavy metal chelator, was investigated for its effect on F. graminearum at concentrations of 2.5, 5, 10, and 20 mM. The fungus treated with this CA gradient exhibited different growth conditions and mycelial colors. Integrative analysis of transcriptomics and proteomics found that the growth of the fungus was accelerated by the upregulation of carbon source metabolism-related enzymes such as phosphoenolpyruvate carboxykinase (PEPCK), Glpk, and LAI12, and the mycelial pigments were altered by polyketide biosynthetic enzymes like AurF, AurJ, and AurT, which were responsible for the production of aurofusarin and rubrofusarin. Interestingly, many Tri genes and their corresponding proteins along with type B trichothecene biosynthesis were significantly downregulated, though the fungal growth was promoted. In this study, the CK, 2.5, and 5 mM groups had similar expression patterns of RNA and protein, while the 10 and 20 mM groups were alike. With the validation of CK, 5, and 10 mM groups, the discovery of CA as a type B trichothecene biosynthesis inhibitor and growth promoter was expected to facilitate its reasonable application and contribute to further research for the prevention of F. graminearum.

IMPORTANCE

Fusarium graminearum is a challenging phytopathogen that causes plant disasters worldwide, such as Fusarium head blight and ear rot in small grain crops like wheat and maize. Besides, the invasion of the fungus on plants is often accompanied by the production of health-threatening mycotoxins-trichothecenes. Therefore, the control of Fusarium graminearum has always been a hot area of research. However, currently, the prevalence of these plant diseases around the world and the mycotoxin accumulation beyond safety limits indicate the necessity for more related research. In the last decades, researchers have sought to identify the key substances associated with the growth, propagation, and mycotoxin production of the fungus, such as carbon and nitrogen sources. Organic acids have always been considered antifungal agents due to their abundant H+ content. However, their comprehensive impact on fungi has been rarely investigated. This research focused on the effect of citric acid, a type of crop exudate and a common heavy-metal chelator, on the metabolism of Fusarium graminearum. The result was expected to provide a theoretical basis for further studies on plants-soil-fungi interactions and the reasonable utilization of citric acid in agriculture.

Biosynthesis of the Central Tricyclic Skeleton of Trichothecene Mycotoxins

Trichothecenes are a widespread family of sesquiterpenoid toxins that can pose significant risks to food and feed safety as well as environmental health. A defining feature of all trichothecenes is their central tricyclic 12,13-epoxytrichothec-9-ene (EPT) motif. Although the formation of the EPT central skeleton has long been presumed to be a spontaneous process, the nonenzymatic cyclization reaction forming the tetrahydropyran ring in EPT requires acid catalysis; otherwise, it occurs too slowly to sustain efficient trichothecene biosynthesis under physiological conditions. Here, we resolved this decades-old problem by identifying the missing enzymes for EPT biosynthesis. We demonstrate that the C11 hydroxyl group of universal trichothecene precursors, isotrichodiol and isotrichotriol, must be acetylated by a strictly conserved O-acetyltransferase Tri3 to furnish a better leaving group. These acetylated intermediates preferentially undergo spontaneous allylic rearrangement with water to give shunt products, trichodiol and trichotriol. Therefore, a novel cyclase, Tri14, which was previously annotated as a hypothetical protein, is required to overcome the kinetically unfavored oxide bridge closure and meanwhile suppress the spontaneous formation of any shunt products.

The trichothecene mycotoxin deoxynivalenol facilitates cell-to-cell invasion during wheat-tissue colonization by Fusarium graminearum

Fusarium head blight disease on small-grain cereals is primarily caused by the ascomycete fungal pathogen Fusarium graminearum. Infection of floral spike tissues is characterized by the biosynthesis and secretion of potent trichothecene mycotoxins, of which deoxynivalenol (DON) is widely reported due to its negative impacts on grain quality and consumer safety. The TRI5 gene encodes an essential enzyme in the DON biosynthesis pathway and the single gene deletion mutant, ΔTri5, is widely reported to restrict disease progression to the inoculated spikelet. In this study, we present novel bioimaging evidence revealing that DON facilitates the traversal of the cell wall through plasmodesmata, a process essential for successful colonization of host tissue. Chemical complementation of ΔTri5 did not restore macro- or microscopic phenotypes, indicating that DON secretion is tightly regulated both spatially and temporally. A comparative qualitative and quantitative morphological cellular analysis revealed infections had no impact on plant cell wall thickness. Immunolabelling of callose at plasmodesmata during infection indicates that DON can increase deposits when applied exogenously but is reduced when F. graminearum hyphae are present. This study highlights the complexity of the interconnected roles of mycotoxin production, cell wall architecture and plasmodesmata in this highly specialized interaction.

Divergent Biosynthesis of Bridged Polycyclic Sesquiterpenoids by a Minimal Fungal Biosynthetic Gene Cluster

The bridged polycyclic sesquiterpenoids derived from sativene, isosativene, and longifolene have unique structures, and many chemical synthesis approaches with at least 10 steps have been reported. However, their biosynthetic pathway remains undescribed. A minimal biosynthetic gene cluster (BGC), named bip, encoding a sesquiterpene cyclase (BipA) and a cytochrome P450 (BipB) is characterized to produce such complex sesquiterpenoids with multiple carbon skeletons based on enzymatic assays, heterologous expression, and precursor experiments. BipA is demonstrated as a versatile cyclase with (-)-sativene as the dominant product and (-)-isosativene and (-)-longifolene as minor ones. BipB is capable of hydroxylating different enantiomeric sesquiterpenes, such as (-)-longifolene and (+)-longifolene, at C-15 and C-14 in turn. The C-15- or both C-15- and C-14-hydroxylated products are then further oxidized by unclustered oxidases, resulting in a structurally diverse array of sesquiterpenoids. Bioinformatic analysis reveals the BipB homologues as a discrete clade of fungal sesquiterpene P450s. These findings elucidate the concise and divergent biosynthesis of such intricate bridged polycyclic sesquiterpenoids, offer valuable biocatalysts for biotransformation, and highlight the distinct biosynthetic strategy employed by nature compared to chemical synthesis.

A Role in 15-Deacetylcalonectrin Acetylation in the Non-Enzymatic Cyclization of an Earlier Bicyclic Intermediate in Fusarium Trichothecene Biosynthesis

The trichothecene biosynthesis in Fusarium begins with the cyclization of farnesyl pyrophosphate to trichodiene, followed by subsequent oxygenation to isotrichotriol. This initial bicyclic intermediate is further cyclized to isotrichodermol (ITDmol), a tricyclic precursor with a toxic trichothecene skeleton. Although the first cyclization and subsequent oxygenation are catalyzed by enzymes encoded by Tri5 and Tri4, the second cyclization occurs non-enzymatically. Following ITDmol formation, the enzymes encoded by Tri101, Tri11, Tri3, and Tri1 catalyze 3-O-acetylation, 15-hydroxylation, 15-O-acetylation, and A-ring oxygenation, respectively. In this study, we extensively analyzed the metabolites of the corresponding pathway-blocked mutants of Fusarium graminearum. The disruption of these Tri genes, except Tri3, led to the accumulation of tricyclic trichothecenes as the main products: ITDmol due to Tri101 disruption; a mixture of isotrichodermin (ITD), 7-hydroxyisotrichodermin (7-HIT), and 8-hydroxyisotrichodermin (8-HIT) due to Tri11 disruption; and a mixture of calonectrin and 3-deacetylcalonectrin due to Tri1 disruption. However, the ΔFgtri3 mutant accumulated substantial amounts of bicyclic metabolites, isotrichotriol and trichotriol, in addition to tricyclic 15-deacetylcalonectrin (15-deCAL). The ΔFgtri5ΔFgtri3 double gene disruptant transformed ITD into 7-HIT, 8-HIT, and 15-deCAL. The deletion of FgTri3 and overexpression of Tri6 and Tri10 trichothecene regulatory genes did not result in the accumulation of 15-deCAL in the transgenic strain. Thus, the absence of Tri3p and/or the presence of a small amount of 15-deCAL adversely affected the non-enzymatic second cyclization and C-15 hydroxylation steps.

Functional analysis of a fungal P450 enzyme

Fungal cytochrome P450s participate in various physiological reactions, including the synthesis of internal cellular components, metabolic detoxification of xenobiotic compounds, and oxidative modification of natural products. Although functional analysis reports of fungal P450s continue to grow, there are still some difficulties as compared to prokaryotic P450s, because most of these fungal enzymes are transmembrane proteins. In this chapter, we will describe the methods for heterologous expression, in vivo analysis, enzyme preparation, and in vitro enzyme assays of the fungal P450 enzyme Trt6 and isomerase Trt14, which play important roles in the divergence of the biosynthetic pathway of terretonins, as a model for the functional analysis of fungal P450 enzymes.

Structural diversification of fungal natural products by oxidative enzymes

Ascomycota and basidiomycota fungi are prolific producers of biologically active natural products. Fungal natural products exhibit remarkable structural diversity and complexity, which are generated by the enzymes involved in their biosynthesis. After the formation of core skeletons, oxidative enzymes play a critical role in converting them into mature natural products. Besides simple oxidations, more complex transformations, such as multiple oxidations by single enzymes, oxidative cyclization, and skeletal rearrangement, are often observed. Those oxidative enzymes are of significant interest for the identification of new enzyme chemistry and have the potential to be biocatalysts for the synthesis of complex molecules. This review presents selected examples of unique oxidative transformations that have been found in the biosynthesis of fungal natural products. The development of strategies for refactoring the fungal biosynthetic pathways with an efficient genome-editing method is also introduced.

Reactive Oxygen Species Metabolism and Diacetoxyscirpenol Biosynthesis Modulation in Potato Tuber Inoculated with Ozone-Treated Fusarium sulphureum

Potato dry rot, caused by Fusarium species, is a devastating fungal decay that seriously impacts the yield and quality of potato tubers worldwide. Fusarium sulphureum is a major causal agent causing potato tuber dry rot that leads to trichothecene accumulation in Gansu Province of China. Ozone (O3), a strong oxidant, is widely applied to prevent postharvest disease in fruits and vegetables. In this study, F. sulphureum was first treated with 2 mg L-1 ozone for 0, 30 s, 1 min, and 2 min, then inoculated with the potato tubers. The impact of ozone application on dry rot development and diacetoxyscirpenol (DIA) accumulation and the possible mechanisms involved were analyzed. The results showed that ozone treatment significantly inhibited the development of potato tuber dry rot by activating reactive oxygen species (ROS) metabolism and increased the activities of antioxidant enzymes NADPH oxidase (NOX), superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) by 24.2%, 13.1%, 45.4%, and 15.8%, respectively, compared with their corresponding control. The activities of key enzymes involved in ascorbate-glutathione cycle (AsA-GSH) of ascorbic peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), and glutathione reductase (GR) also increased by 26.6%, 41.5%, 56%, and 24.1%, respectively, compared with the control group, and their corresponding gene expressions. In addition, ozone treatment markedly suppressed DIA accumulation in potato tubers by downregulating the expression of genes associated with DIA biosynthesis pathway. These results suggest that ozone treatment inhibited the occurrence of potato dry rot and the accumulation of DIA in potato tubers inoculated with F. sulphureum by promoting ROS metabolism and modulating DIA biosynthesis pathway.

Regulatory mechanism of trichothecene biosynthesis in Fusarium graminearum

Among the genes involved in the biosynthesis of trichothecene (Tri genes), Tri6 and Tri10 encode a transcription factor with unique Cys₂His₂ zinc finger domains and a regulatory protein with no consensus DNA-binding sequences, respectively. Although various chemical factors, such as nitrogen nutrients, medium pH, and certain oligosaccharides, are known to influence trichothecene biosynthesis in Fusarium graminearum, the transcriptional regulatory mechanism of Tri6 and Tri10 genes is poorly understood. Particularly, culture medium pH is a major regulator in trichothecene biosynthesis in F. graminearum, but it is susceptible to metabolic changes posed by nutritional and genetic factors. Hence, appropriate precautions should be considered to minimize the indirect influence of pH on the secondary metabolism while studying the roles of nutritional and genetic factors on trichothecene biosynthesis regulation. Additionally, it is noteworthy that the structural changes of the trichothecene gene cluster core region exert considerable influence over the normal regulation of Tri gene expression. In this perspective paper, we consider a revision of our current understanding of the regulatory mechanism of trichothecene biosynthesis in F. graminearum and share our idea toward establishing a regulatory model of Tri6 and Tri10 transcription.

A Novel Trichothecene Toxin Phenotype Associated with Horizontal Gene Transfer and a Change in Gene Function in Fusarium

Fusarium trichothecenes are among the mycotoxins of most concern to food and feed safety. Production of these mycotoxins and presence of the trichothecene biosynthetic gene (TRI) cluster have been confirmed in only two multispecies lineages of Fusarium: the Fusarium incarnatum-equiseti (Incarnatum) and F. sambucinum (Sambucinum) species complexes. Here, we identified and characterized a TRI cluster in a species that has not been formally described and is represented by Fusarium sp. NRRL 66739. This fungus is reported to be a member of a third Fusarium lineage: the F. buharicum species complex. Cultures of NRRL 66739 accumulated only two trichothecenes, 7-hydroxyisotrichodermin and 7-hydroxyisotrichodermol. Although these are not novel trichothecenes, the production profile of NRRL 66739 is novel, because in previous reports 7-hydroxyisotrichodermin and 7-hydroxyisotrichodermol were components of mixtures of 6-8 trichothecenes produced by several Fusarium species in Sambucinum. Heterologous expression analysis indicated that the TRI13 gene in NRRL 66739 confers trichothecene 7-hydroxylation. This contrasts the trichothecene 4-hydroxylation function of TRI13 in other Fusarium species. Phylogenetic analyses suggest that NRRL 66739 acquired the TRI cluster via horizontal gene transfer from a close relative of Incarnatum and Sambucinum. These findings provide insights into evolutionary processes that have shaped the distribution of trichothecene production among Fusarium species and the structural diversity of the toxins.

Identification of polyketide synthase genes required for aspinolide biosynthesis in Trichoderma arundinaceum

The fungus Trichoderma arundinaceum exhibits biological control activity against crop diseases caused by other fungi. Two mechanisms that likely contribute to this activity are upregulation of plant defenses and production of two types of antifungal secondary metabolites: the sesquiterpenoid harzianum A (HA) and the polyketide-derived aspinolides. The goal of the current study was to identify aspinolide biosynthetic genes as part of an effort to understand how these metabolites contribute to the biological control activity of T. arundinaceum. Comparative genomics identified two polyketide synthase genes (asp1 and asp2) that occur in T. arundinaceum and Aspergillus ochraceus, which also produces aspinolides. Gene deletion and biochemical analyses in T. arundinaceum indicated that both genes are required for aspinolide production: asp2 for formation of a 10-member lactone ring and asp1 for formation of a butenoyl subsituent at position 8 of the lactone ring. Gene expression and comparative genomics analyses indicated that asp1 and asp2 are located within a gene cluster that occurs in both T. arundinaceum and A. ochraceus. A survey of genome sequences representing 35 phylogenetically diverse Trichoderma species revealed that intact homologs of the cluster occurred in only two other species, which also produced aspinolides. An asp2 mutant inhibited fungal growth more than the wild type, but an asp1 mutant did not, and the greater inhibition by the asp2 mutant coincided with increased HA production. These findings indicate that asp1 and asp2 are aspinolide biosynthetic genes and that loss of either aspinolide or HA production in T. arundinaceum can be accompanied by increased production of the other metabolite(s). KEY POINTS: • Two polyketide synthase genes are required for aspinolide biosynthesis. • Blocking aspinolide production increases production of the terpenoid harzianum A. • Aspinolides and harzianum A act redundantly in antibiosis of T. arundinaceum.

The Endoplasmic Reticulum Cargo Receptor FgErv14 Regulates DON Production, Growth and Virulence in Fusarium graminearum

Fusarium graminearum is a plant filamentous pathogenic fungi and the predominant causal agent of Fusarium head blight (FHB) in cereals worldwide. The regulators of the secretory pathway contribute significantly to fungal mycotoxin synthesis, development, and virulence. However, their roles in these processes in F. graminearum remain poorly understood. Here, we identified and functionally characterized the endoplasmic reticulum (ER) cargo receptor FgErv14 in F. graminearum. Firstly, it was observed that FgErv14 is mainly localized in the ER. Then, we constructed the FgErv14 deletion mutant (ΔFgerv14) and found that the absence of the FgErv14 caused a serious reduction in vegetative growth, significant defects in asexual and sexual reproduction, and severely impaired virulence. Furthermore, we found that the ΔFgerv14 mutant exhibited a reduced expression of TRI genes and defective toxisome generation, both of which are critical for deoxynivalenol (DON) biosynthesis. Importantly, we found the green fluorescent protein (GFP)-tagged FgRud3 was dispersed in the cytoplasm, whereas GFP-FgSnc1-PEM was partially trapped in the late Golgi in ΔFgerv14 mutant. These results demonstrate that FgErv14 mediates anterograde ER-to-Golgi transport as well as late secretory Golgi-to-Plasma membrane transport and is necessary for DON biosynthesis, asexual and sexual reproduction, vegetative growth, and pathogenicity in F. graminearum.

Branching and converging pathways in fungal natural product biosynthesis

In nature, organic molecules with great structural diversity and complexity are synthesized by utilizing a relatively small number of starting materials. A synthetic strategy adopted by nature is pathway branching, in which a common biosynthetic intermediate is transformed into different end products. A natural product can also be synthesized by the fusion of two or more precursors generated from separate metabolic pathways. This review article summarizes several representative branching and converging pathways in fungal natural product biosynthesis to illuminate how fungi are capable of synthesizing a diverse array of natural products.

Comparative genomics of plant pathogenic Diaporthe species and transcriptomics of Diaporthe caulivora during host infection reveal insights into pathogenic strategies of the genus

Background

Diaporthe caulivora is a fungal pathogen causing stem canker in soybean worldwide. The generation of genomic and transcriptomic information of this ascomycete, together with a comparative genomic approach with other pathogens of this genus, will contribute to get insights into the molecular basis of pathogenicity strategies used by D. caulivora and other Diaporthe species.

Results

In the present work, the nuclear genome of D. caulivora isolate (D57) was resolved, and a comprehensive annotation based on gene expression and genomic analysis is provided. Diaporthe caulivora D57 has an estimated size of 57,86 Mb and contains 18,385 predicted protein-coding genes, from which 1501 encode predicted secreted proteins. A large array of D. caulivora genes encoding secreted pathogenicity-related proteins was identified, including carbohydrate-active enzymes (CAZymes), necrosis-inducing proteins, oxidoreductases, proteases and effector candidates. Comparative genomics with other plant pathogenic Diaporthe species revealed a core secretome present in all Diaporthe species as well as Diaporthe-specific and D. caulivora-specific secreted proteins. Transcriptional profiling during early soybean infection stages showed differential expression of 2659 D. caulivora genes. Expression patterns of upregulated genes and gene ontology enrichment analysis revealed that host infection strategies depends on plant cell wall degradation and modification, detoxification of compounds, transporter activities and toxin production. Increased expression of effectors candidates suggests that D. caulivora pathogenicity also rely on plant defense evasion. A high proportion of the upregulated genes correspond to the core secretome and are represented in the pathogen-host interaction (PHI) database, which is consistent with their potential roles in pathogenic strategies of the genus Diaporthe.

Conclusions

Our findings give novel and relevant insights into the molecular traits involved in pathogenicity of D. caulivora towards soybean plants. Some of these traits are in common with other Diaporthe pathogens with different host specificity, while others are species-specific. Our analyses also highlight the importance to have a deeper understanding of pathogenicity functions among Diaporthe pathogens and their interference with plant defense activation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08413-y.

Distribution, Function, and Evolution of a Gene Essential for Trichothecene Toxin Biosynthesis in Trichoderma

Trichothecenes are terpenoid toxins produced by species in 10 fungal genera, including species of Trichoderma. The trichothecene biosynthetic gene (tri) cluster typically includes the tri5 gene, which encodes a terpene synthase that catalyzes formation of trichodiene, the parent compound of all trichothecenes. The two Trichoderma species, Trichoderma arundinaceum and T. brevicompactum, that have been examined are unique in that tri5 is located outside the tri cluster in a genomic region that does not include other known tri genes. In the current study, analysis of 35 species representing a wide range of the phylogenetic diversity of Trichoderma revealed that 22 species had tri5, but only 13 species had both tri5 and the tri cluster. tri5 was not located in the cluster in any species. Using complementation analysis of a T. arundinaceum tri5 deletion mutant, we demonstrated that some tri5 homologs from species that lack a tri cluster are functional, but others are not. Phylogenetic analyses suggest that Trichoderma tri5 was under positive selection following its divergence from homologs in other fungi but before Trichoderma species began diverging from one another. We propose two models to explain these diverse observations. One model proposes that the location of tri5 outside the tri cluster resulted from loss of tri5 from the cluster in an ancestral species followed by reacquisition via horizontal transfer. The other model proposes that in species that have a functional tri5 but lack the tri cluster, trichodiene production provides a competitive advantage.

4-O-Glucosylation of Trichothecenes by Fusarium Species: A Phase II Xenobiotic Metabolism for t-Type Trichothecene Producers

The t-type trichothecene producers Fusarium sporotrichioides and Fusarium graminearum protect themselves against their own mycotoxins by acetylating the C-3 hydroxy group with Tri101p acetylase. To understand the mechanism by which they deal with exogenously added d-type trichothecenes, the Δtri5 mutants expressing all but the first trichothecene pathway enzymes were fed with trichodermol (TDmol), trichothecolone (TCC), 8-deoxytrichothecin, and trichothecin. LC-MS/MS and NMR analyses showed that these C-3 unoxygenated trichothecenes were conjugated with glucose at C-4 by α-glucosidic linkage. As t-type trichothecenes are readily incorporated into the biosynthetic pathway following the C-3 acetylation, the mycotoxins were fed to the ΔFgtri5ΔFgtri101 mutant to examine their fate. LC-MS/MS and NMR analyses demonstrated that the mutant conjugated glucose at C-4 of HT-2 toxin (HT-2) by α-glucosidic linkage, while the ΔFgtri5 mutant metabolized HT-2 to 3-acetyl HT-2 toxin and T-2 toxin. The 4-O-glucosylation of exogenously added t-type trichothecenes appears to be a general response of the ΔFgtri5ΔFgtri101 mutant, as nivalenol and its acetylated derivatives appeared to be conjugated with hexose to some extent. The toxicities of 4-O-glucosides of TDmol, TCC, and HT-2 were much weaker than their corresponding aglycons, suggesting that 4-O-glucosylation serves as a phase II xenobiotic metabolism for t-type trichothecene producers.

Verification of TRI3 Acetylation of Trichodermol to Trichodermin in the Plant Endophyte Trichoderma taxi

Trichodermin, a trichothecene first isolated in Trichoderma species, is a sesquiterpenoid antibiotic that exhibits significant inhibitory activity to the growth of many pathogenic fungi such as Candida albicans, Rhizoctonia solani, and Botrytis cinerea by inhibiting the peptidyl transferase involved in eukaryotic protein synthesis. Trichodermin has also been shown to selectively induce cell apoptosis in several cancer cell lines and thus can act as a potential lead compound for developing anticancer therapeutics. The biosynthetic pathway of trichodermin in Trichoderma has been identified, and most of the involved genes have been functionally characterized. An exception is TRI3, which encodes a putative acetyltransferase. Here, we report the identification of a gene cluster that contains seven genes expectedly involved in trichodermin biosynthesis (TRI3, TRI4, TRI6, TRI10, TRI11, TRI12, and TRI14) in the trichodermin-producing endophytic fungus Trichoderma taxi. As in Trichoderma brevicompactum, TRI5 is not included in the cluster. Functional analysis provides evidence that TRI3 acetylates trichodermol, the immediate precursor, to trichodermin. Disruption of TRI3 gene eliminated the inhibition to R. solani by T. taxi culture filtrates and significantly reduced the production of trichodermin but not of trichodermol. Both the inhibitory activity and the trichodermin production were restored when native TRI3 gene was reintroduced into the disruption mutant. Furthermore, a His-tag-purified TRI3 protein, expressed in Escherichia coli, was able to convert trichodermol to trichodermin in the presence of acetyl-CoA. The disruption of TRI3 also resulted in lowered expression of both the upstream biosynthesis TRI genes and the regulator genes. Our data demonstrate that T. taxi TRI3 encodes an acetyltransferase that catalyzes the esterification of the C-4 oxygen atom on trichodermol and thus plays an essential role in trichodermin biosynthesis in this fungus.

Genome-wide association study for deoxynivalenol production and aggressiveness in wheat and rye head blight by resequencing 92 isolates of Fusarium culmorum

Background

Fusarium culmorum is an important pathogen causing head blight of cereals in Europe. This disease is of worldwide importance leading to reduced yield, grain quality, and contamination by mycotoxins. These mycotoxins are harmful for livestock and humans; therefore, many countries have strict regulatory limits for raw materials and processed food. Extensive genetic diversity is described among field populations of F. culmorum isolates for aggressiveness and production of the trichothecene mycotoxin deoxynivalenol (DON). However, the causes for this quantitative variation are not clear, yet. We analyzed 92 isolates sampled from different field populations in Germany, Russia, and Syria together with an international collection for aggressiveness and DON production in replicated field experiments at two locations in two years with two hosts, wheat and rye. The 30x coverage whole-genome resequencing of all isolates resulted in the identification of 130,389 high quality single nucleotide polymorphisms (SNPs) that were used for the first genome-wide association study in this phytopathogenic fungus.

Results

In wheat, 20 and 27 SNPs were detected for aggressiveness and DON content, respectively, of which 10 overlapped. Additionally, two different SNPs were significantly associated with aggressiveness in rye that were among those SNPs being associated with DON production in wheat. Most of the SNPs explained only a small proportion of genotypic variance (p_G), however, four SNPs were associated with major quantitative trait loci (QTLs) with p_G ranging from 12 to 48%. The QTL with the highest p_G was involved in DON production and associated with a SNP most probably located within the Tri4 gene.

Conclusions

The diversity of 92 isolates of F. culmorum were captured using a heuristic approach. Key phenotypic traits, SNPs, and candidate genes underlying aggressiveness and DON production were identified. Clearly, many QTLs are responsible for aggressiveness and DON content in wheat, both traits following a quantitative inheritance. Several SNPs involved in DON metabolism, among them the Tri4 gene of the trichothecene pathway, were inferred as important source of variation in fungal aggressiveness. Using this information underlying the phenotypic variation will be of paramount importance in evaluating strategies for successful resistance breeding.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07931-5.

Barley Nepenthesin-Like Aspartic Protease HvNEP-1 Degrades Fusarium Phytase, Impairs Toxin Production, and Suppresses the Fungal Growth

Nepenthesins are categorized under the subfamily of the nepenthesin-like plant aspartic proteases (PAPs) that form a distinct group of atypical PAPs. This study describes the effect of nepenthesin 1 (HvNEP-1) protease from barley (Hordeum vulgare L.) on fungal histidine acid phosphatase (HAP) phytase activity. Signal peptide lacking HvNEP-1 was expressed in Pichia pastoris and biochemically characterized. Recombinant HvNEP-1 (rHvNEP-1) strongly inhibited the activity of Aspergillus and Fusarium phytases, which are enzymes that release inorganic phosphorous from phytic acid. Moreover, rHvNEP-1 suppressed in vitro fungal growth and strongly reduced the production of mycotoxin, 15-acetyldeoxynivalenol (15-ADON), from Fusarium graminearum. The quantitative PCR analysis of trichothecene biosynthesis genes (TRI) confirmed that rHvNEP-1 strongly repressed the expression of TRI4, TRI5, TRI6, and TRI12 in F. graminearum. The co-incubation of rHvNEP-1 with recombinant F. graminearum (rFgPHY1) and Fusarium culmorum (FcPHY1) phytases induced substantial degradation of both Fusarium phytases, indicating that HvNEP-1-mediated proteolysis of the fungal phytases contributes to the HvNEP-1-based suppression of Fusarium.

Cytochrome P450 enzymes in fungal natural product biosynthesis

Covering: 2015 to the end of 2020 Fungal-derived polyketides, non-ribosomal peptides, terpenoids and their hybrids contribute significantly to the chemical space of total natural products. Cytochrome P450 enzymes play essential roles in fungal natural product biosynthesis with their broad substrate scope, great catalytic versatility and high frequency of involvement. Due to the membrane-bound nature, the functional and mechanistic understandings for fungal P450s have been limited for quite a long time. However, recent technical advances, such as the efficient and precise genome editing techniques and the development of several filamentous fungal strains as heterologous P450 expression hosts, have led to remarkable achievements in fungal P450 studies. Here, we provide a comprehensive review to cover the most recent progresses from 2015 to 2020 on catalytic functions and mechanisms, research methodologies and remaining challenges in the fast-growing field of fungal natural product biosynthetic P450s.

The ADP-ribosylation factor-like small GTPase FgArl1 participates in growth, pathogenicity and DON production in Fusarium graminearum

Fusarium graminearum is the main pathogen of Fusarium head blight (FHB) in wheat and related species, which causes serious production decreases and economic losses and produces toxins such as deoxynivalenol (DON), which endangers the health of humans and livestock. Vesicle transport is a basic physiological process required for cell survival in eukaryotes. Many regulators of vesicle transport are reported to be involved in the pathogenicity of fungi. In yeast and mammalian cells, the ADP-ribosylation factor-like small GTPase Arl1 and its orthologs are involved in regulating vesicular trafficking, cytoskeletal reorganization and other significant biological processes. However, the role of Arl1 in F. graminearum is not well understood. In this study, we characterized the Arl1-homologous protein FgArl1 in F. graminearum and showed that FgArl1 is located in the trans-Golgi apparatus. The deletion of FgARL1 resulted in a significant decrease in vegetative growth and pathogenicity. Further analyses of the ΔFgarl1 mutant revealed defects in the production of DON. Taken together, these results indicate that FgArl1 is important in the development and pathogenicity of F. graminearum.

The type II phosphoinositide 4-kinase FgLsb6 is important for the development and virulence of Fusarium graminearum

Fusarium graminearum is the main pathogenic fungus causing Fusarium head blight (FHB), which is a wheat disease with a worldwide prevalence. In eukaryotes, phosphatidylinositol 4-phosphate (PI4P), which participates in many physiological processes, is located primarily in different organelles, including the trans-Golgi network (TGN), plasma membrane and endosomes. Type II phosphatidylinositol 4-kinases (PI4Ks) are involved in regulating the production of PI4P in yeast, plants and mammalian cells. However, the role of these proteins in phytopathogenic fungi is not well understood. In this study, we characterized the type II PI4K protein FgLsb6 in F. graminearum, a homolog of Lsb6 in Saccharomyces cerevisiae. Unlike Lsb6, FgLsb6 localizes to the vacuoles and endosomes. The ΔFglsb6 mutant displayed defects in vegetative growth, deoxynivalenol (DON) production and pathogenicity. Furthermore, the ΔFglsb6 deletion mutant also exhibited increased resistance to osmotic, oxidative and cell wall stresses. Further analyses of the ΔFglsb6 mutant showed that it was defective in the generation of PI4P on endosomes and endocytosis. Collectively, our data suggest that the decreased vegetative growth and pathogenicity of ΔFglsb6 was due to the conservative roles of FgLsb6 in the generation of PI4P on endosomes and endocytosis.

Recent Advances on Macrocyclic Trichothecenes, Their Bioactivities and Biosynthetic Pathway

Macrocyclic trichothecenes are an important group of trichothecenes bearing a large ring. Despite the fact that many of trichothecenes are of concern in agriculture, food contamination, health care and building protection, the macrocyclic ones are becoming the research hotspot because of their diversity in structure and biologic activity. Several researchers have declared that macrocyclic trichothecenes have great potential to be developed as antitumor agents, due to the plenty of their compounds and bioactivities. In this review we summarize the newly discovered macrocyclic trichothecenes and their bioactivities over the last decade, as well as identifications of genes tri17 and tri18 involved in the trichothecene biosynthesis and putative biosynthetic pathway. According to the search results in database and phylogenetic trees generated in the review, the species of the genera Podostroma and Monosporascus would probably be great sources for producing macrocyclic trichothecenes. Moreover, we propose that the macrocyclic trichothecene roridin E could be formed via acylation or esterification of the long side chain linked with C-4 to the hydroxyl group at C-15, and vice versa. More assays and evidences are needed to support this hypothesis, which would promote the verification of the proposed pathway.

The Dynamin-Like GTPase FgSey1 Plays a Critical Role in Fungal Development and Virulence in Fusarium graminearum

Fusarium graminearum, the main pathogenic fungus causing Fusarium head blight (FHB), produces deoxynivalenol (DON), a key virulence factor, which is synthesized in the endoplasmic reticulum (ER). Sey1/atlastin, a dynamin-like GTPase protein, is known to be required for homotypic fusion of ER membranes, but the functions of this protein are unknown in pathogenic fungi. Here, we characterized Sey1/atlastin homologue FgSey1 in F. graminearum Like Sey1/atlastin, FgSey1 is located in the ER. The FgSEY1 deletion mutant exhibited significantly reduced vegetative growth, asexual development, DON biosynthesis, and virulence. Moreover, the ΔFgsey1 mutant was impaired in the formation of normal lipid droplets (LDs) and toxisomes, both of which participate in DON biosynthesis. The GTPase, helix bundle (HB), transmembrane segment (TM), and cytosolic tail (CT) domains of FgSey1 are essential for its function, but only the TM domain is responsible for its localization. Furthermore, the mutants FgSey1^K63A and FgSey1^T87A lacked GTPase activity and failed to rescue the defects of the ΔFgsey1 mutant. Collectively, our data suggest that the dynamin-like GTPase protein FgSey1 affects the generation of LDs and toxisomes and is required for DON biosynthesis and pathogenesis in F. graminearum IMPORTANCE Fusarium graminearum is a major plant pathogen that causes Fusarium head blight (FHB) of wheats worldwide. In addition to reducing the plant yield, F. graminearum infection of wheats also results in the production of deoxynivalenol (DON) mycotoxins, which are harmful to humans and animals and therefore cause great economic losses through pollution of food products and animal feed. At present, effective strategies for controlling FHB are not available. Therefore, understanding the regulation mechanisms of fungal development, pathogenesis, and DON biosynthesis is important for the development of effective control strategies of this disease. In this study, we demonstrated that a dynamin-like GTPase protein Sey1/atlastin homologue, FgSey1, is required for vegetative growth, DON production, and pathogenicity in F. graminearum Our results provide novel information on critical roles of FgSey1 in fungal pathogenicity; therefore, FgSey1 could be a potential target for effective control of the disease caused by F. graminearum.

Molecular Properties and New Potentials of Plant Nepenthesins

Nepenthesins are aspartic proteases (APs) categorized under the A1B subfamily. Due to nepenthesin-specific sequence features, the A1B subfamily is also named nepenthesin-type aspartic proteases (NEPs). Nepenthesins are mostly known from the pitcher fluid of the carnivorous plant Nepenthes, where they are availed for the hydrolyzation of insect protein required for the assimilation of insect nitrogen resources. However, nepenthesins are widely distributed within the plant kingdom and play significant roles in plant species other than Nepenthes. Although they have received limited attention when compared to other members of the subfamily, current data indicates that they have exceptional molecular and biochemical properties and new potentials as fungal-resistance genes. In the current review, we provide insights into the current knowledge on the molecular and biochemical properties of plant nepenthesins and highlights that future focus on them may have strong potentials for industrial applications and crop trait improvement.

Fungal P450 monooxygenases - the diversity in catalysis and their promising roles in biocontrol activity

The fungal P_450s catalyze vital monooxygenation reactions in primary and secondary metabolism, which may lead to the production of diverse secondary metabolites. Many of these, such as from the family of trichothecenes, involve in biocontrol activities. The diversified nature of fungal P₄₅₀ monooxygenases makes their host organisms adoptable to various ecological niches. The available genome data analysis provided an insight into the activity and mechanisms of the fungal P_450s. However, still more structural and functional studies are needed to elucidate the details of its catalytic mechanism, and the advance studies are also required to decipher further about their dynamic role in various aspects of trichothecene oxygenations. This mini review will provide updated information on different fungal P₄₅₀ monooxygenases, their genetic diversity, and their role in catalyzing various biochemical reactions leading to the production of plant growth promoting secondary metabolites.

Reduced Toxicity of Trichothecenes, Isotrichodermol, and Deoxynivalenol, by Transgenic Expression of the Tri101 3-O-Acetyltransferase Gene in Cultured Mammalian FM3A Cells

In trichothecene-producing fusaria, isotrichodermol (ITDol) is the first intermediate with a trichothecene skeleton. In the biosynthetic pathway of trichothecene, a 3-O-acetyltransferase, encoded by Tri101, acetylates ITDol to a less-toxic intermediate, isotrichodermin (ITD). Although trichothecene resistance has been conferred to microbes and plants transformed with Tri101, there are no reports of resistance in cultured mammalian cells. In this study, we found that a 3-O-acetyl group of trichothecenes is liable to hydrolysis by esterases in fetal bovine serum and FM3A cells. We transfected the cells with Tri101 under the control of the MMTV-LTR promoter and obtained a cell line G3 with the highest level of C-3 acetylase activity. While the wild-type FM3A cells hardly grew in the medium containing 0.40 μM ITDol, many G3 cells survived at this concentration. The IC₅₀ values of ITDol and ITD in G3 cells were 1.0 and 9.6 μM, respectively, which were higher than the values of 0.23 and 3.0 μM in the wild-type FM3A cells. A similar, but more modest, tendency was observed in deoxynivalenol and 3-acetyldeoxynivalenol. Our findings indicate that the expression of Tri101 conferred trichothecene resistance in cultured mammalian cells.

Trichothecenes in Cereal Grains - An Update

Trichothecenes are sesquiterpenoid mycotoxins produced by fungi from the order Hypocreales, including members of the Fusarium genus that infect cereal grain crops. Different trichothecene-producing Fusarium species and strains have different trichothecene chemotypes belonging to the Type A and B class. These fungi cause a disease of small grain cereals, called Fusarium head blight, and their toxins contaminate host tissues. As potent inhibitors of eukaryotic protein synthesis, trichothecenes pose a health risk to human and animal consumers of infected cereal grains. In 2009, Foroud and Eudes published a review of trichothecenes in cereal grains for human consumption. As an update to this review, the work herein provides a comprehensive and multi-disciplinary review of the Fusarium trichothecenes covering topics in chemistry and biochemistry, pathogen biology, trichothecene toxicity, molecular mechanisms of resistance or detoxification, genetics of resistance and breeding strategies to reduce their contamination of wheat and barley.

Fusarium graminearum Trichothecene Mycotoxins: Biosynthesis, Regulation, and Management

Fusarium head blight (FHB) of small grain cereals caused by Fusarium graminearum and other Fusarium species is an economically important plant disease worldwide. Fusarium infections not only result in severe yield losses but also contaminate grain with various mycotoxins, especially deoxynivalenol (DON). With the complete genome sequencing of F. graminearum, tremendous progress has been made during the past two decades toward understanding the basis for DON biosynthesis and its regulation. Here, we summarize the current understanding of DON biosynthesis and the effect of regulators, signal transduction pathways, and epigenetic modifications on DON production and the expression of biosynthetic TRI genes. In addition, strategies for controlling FHB and DON contamination are reviewed. Further studies on these biosynthetic and regulatory systems will provide useful knowledge for developing novel management strategies to prevent FHB incidence and mycotoxin accumulation in cereals.

Amylose-Free ("waxy") Wheat Colonization by Fusarium spp. and Response to Fusarium Head Blight

Hexaploid waxy wheat (Triticum aestivum L.) has null mutations in Wx genes and grain lacking amylose with increased digestibility and usability for specialty foods. The waxy cultivar Mattern is susceptible to Fusarium head blight (FHB) caused by Fusarium graminearum species complex, which produces the mycotoxin deoxynivalenol (DON). In experiment 1, conducted during low natural FHB, grain from waxy breeding lines, Mattern, and wild-type breeding lines and cultivars were assessed for Fusarium infection and DON concentration. Nine Fusarium species and species complexes were detected from internally infected (disinfested) grain; F. graminearum infections were not different between waxy and wild-type. Surface- and internally infected grain (nondisinfested) had greater numbers of Fusarium isolates across waxy versus wild-type, but F. graminearum-like infections were similar; however, DON levels were higher in waxy. In experiment 2, conducted during a timely epidemic, disease severity, Fusarium-damaged kernels (FDK), and DON were assessed for waxy breeding lines, Mattern, and wild-type cultivars. Disease severity and FDK were not significantly different from wild-type, but DON was higher in waxy than wild-type lines. Across both experiments, waxy breeding lines, Plant Introductions 677876 and 677877, responded similarly to FHB as moderately resistant wild-type cultivar Overland, showing promise for breeding advanced waxy cultivars with reduced FHB susceptibility.

Complete biosynthetic pathways of ascofuranone and ascochlorin in Acremonium egyptiacum

Ascofuranone (AF) and ascochlorin (AC) are meroterpenoids produced by various filamentous fungi, including Acremonium egyptiacum (synonym: Acremonium sclerotigenum), and exhibit diverse physiological activities. In particular, AF is a promising drug candidate against African trypanosomiasis and a potential anticancer lead compound. These compounds are supposedly biosynthesized through farnesylation of orsellinic acid, but the details have not been established. In this study, we present all of the reactions and responsible genes for AF and AC biosyntheses in A. egyptiacum, identified by heterologous expression, in vitro reconstruction, and gene deletion experiments with the aid of a genome-wide differential expression analysis. Both pathways share the common precursor, ilicicolin A epoxide, which is processed by the membrane-bound terpene cyclase (TPC) AscF in AC biosynthesis. AF biosynthesis branches from the precursor by hydroxylation at C-16 by the P450 monooxygenase AscH, followed by cyclization by a membrane-bound TPC AscI. All genes required for AC biosynthesis (ascABCDEFG) and a transcriptional factor (ascR) form a functional gene cluster, whereas those involved in the late steps of AF biosynthesis (ascHIJ) are present in another distantly located cluster. AF is therefore a rare example of fungal secondary metabolites requiring multilocus biosynthetic clusters, which are likely to be controlled by the single regulator, AscR. Finally, we achieved the selective production of AF in A. egyptiacum by genetically blocking the AC biosynthetic pathway; further manipulation of the strain will lead to the cost-effective mass production required for the clinical use of AF.

A Promiscuous Cytochrome P450 Hydroxylates Aliphatic and Aromatic C-H Bonds of Aromatic 2,5-Diketopiperazines

Cytochrome P450 enzymes generally functionalize inert C-H bonds, and thus, they are important biocatalysts for chemical synthesis. However, enzymes that catalyze both aliphatic and aromatic hydroxylation in the same biotransformation process have rarely been reported. A recent biochemical study demonstrated the P450 TxtC for the biosynthesis of herbicidal thaxtomins as the first example of this unique type of enzyme. Herein, the detailed characterization of substrate requirements and biocatalytic applications of TxtC are reported. The results reveal the importance of N-methylation of the thaxtomin diketopiperazine (DKP) core on enzyme reactions and demonstrate the tolerance of the enzyme to modifications on the indole and phenyl moieties of its substrates. Furthermore, hydroxylated, methylated, aromatic DKPs are synthesized through a biocatalytic route comprising TxtC and the promiscuous N-methyltransferase Amir_4628; thus providing a basis for the broad application of this unique P450.

Biological and chemical diversity go hand in hand: Basidiomycota as source of new pharmaceuticals and agrochemicals

The Basidiomycota constitutes the second largest higher taxonomic group of the Fungi after the Ascomycota and comprises over 30.000 species. Mycelial cultures of Basidiomycota have already been studied since the 1950s for production of antibiotics and other beneficial secondary metabolites. Despite the fact that unique and selective compounds like pleuromutilin were obtained early on, it took several decades more until they were subjected to a systematic screening for antimicrobial and anticancer activities. These efforts led to the discovery of the strobilurins and several hundreds of further compounds that mainly constitute terpenoids. In parallel the traditional medicinal mushrooms of Asia were also studied intensively for metabolite production, aimed at finding new therapeutic agents for treatment of various diseases including metabolic disorders and the central nervous system. While the evaluation of this organism group has in general been more tedious as compared to the Ascomycota, the chances to discover new metabolites and to develop them further to candidates for drugs, agrochemicals and other products for the Life Science industry have substantially increased over the past decade. This is owing to the revolutionary developments in -OMICS techniques, bioinformatics, analytical chemistry and biotechnological process technology, which are steadily being developed further. On the other hand, the new developments in polythetic fungal taxonomy now also allow a more concise selection of previously untapped organisms. The current review is dedicated to summarize the state of the art and to give an outlook to further developments.

Biosynthesis of an anti-tuberculosis sesterterpenoid asperterpenoid A

Biosynthesis of a potent MptpB inhibitor asperterpenoid A by a sesterterpene cyclase AstC and a multifunctional P450 enzyme AstB.

Resistance-Related l-Pyroglutamic Acid Affects the Biosynthesis of Trichothecenes and Phenylpropanoids by F. graminearum Sensu Stricto

Fungicide application remains amongst the most widely used methods of fungal control in agroecosystems. However, the extensive use of fungicides poses hazards to human health and the natural environment and does not always ensure the effective decrease of mycotoxins in food and feed. Nowadays, the rising threat from mycotoxin contamination of staple foods has stimulated efforts in developing alternative strategies to control plant pathogenic fungi. A substantial effort is focused on the identification of plant-derived compounds inhibiting mycotoxin production by plant pathogenic fungi. l-Pyroglutamic acid has recently been suggested as playing a role in the response of barley to toxigenic Fusaria. Considering the above, we studied the response of various strains of F. graminearum sensu stricto to different levels of l-pyroglutamic acid on solid YES (yeast extract sucrose) media. l-Pyroglutamic acid decreased the accumulation of trichothecenes in all examined strains. Gene expression studies addressing Tri genes (Tri4, Tri5, and Tri10), which induce the biosynthesis of trichothecenes, revealed the production of mycotoxins by l-pyroglutamic acid to be inhibited at the transcriptional level. Besides inhibitory effects on mycotoxin production, l-pyroglutamic acid exhibited variable and concentration-related effects on phenylpropanoid production by fungi. Accumulation of most of the fungal-derived phenolic acids decreased in the presence of 100 and 400 µg/g of l-pyroglutamic acid. However, a higher dose (800 µg/g) of l-pyroglutamic acid increased the accumulation of trans-cinnamic acid in the media. The accumulation of fungal-derived naringenin increased in the presence of l-pyroglutamic acid. Contrasting results were obtained for quercetin, apigenin, luteolin, and kaempferol, the accumulation of which decreased in the samples treated with 100 and 400 µg/g of l-pyroglutamic acid, whereas the highest l-pyroglutamic acid concentration (800 µg/g) seemed to induce their biosynthesis. The results obtained in this study provide new insights for breeders involved in studies on resistance against Fusaria.

Heterologous Biosynthesis of the Fungal Sesquiterpene Trichodermol in Saccharomyces cerevisiae

Trichodermol, a fungal sesquiterpene derived from the farnesyl diphosphate pathway, is the biosynthetic precursor for trichodermin, a member of the trichothecene class of fungal toxins produced mainly by the genera of Trichoderma and Fusarium. Trichodermin is a promising candidate for the development of fungicides and antitumor agents due to its significant antifungal and cytotoxic effects. It can also serve as a scaffold to generate new congeners for structure-activity relationship (SAR) study. We reconstructed the biosynthetic pathway of trichodermol in Saccharomyces cerevisiae BY4741, and investigated the effect of produced trichodermol on the host by de novo RNA sequencing (RNA-Seq) and quantitative Real-time PCR analyses. Co-expression of pESC::FgTRI5 using plasmid pLLeu-tHMGR-UPC2.1 led to trichodiene production of 683 μg L^-1, while integration of only the codon-optimized FgTRI5 into the chromosome of yeast improved the production to 6,535 μg L^-1. Subsequent expression of the codon-optimized cytochrome P450 monooxygenase encoding genes, TaTRI4 and TaTRI11, resulted in trichodermol, with an estimated titer of 252 μg L^-1 at shake flask level. RNA-Seq and qPCR analyses revealed that the produced trichodermol downregulated the expression of the genes involved in ergosterol biosynthesis, but significantly upregulated the expression of PDR5 related to membrane transport pathway in S. cerevisiae. Collectively, we achieved the first heterologous biosynthesis of trichodermol by reconstructing its biosynthetic pathway in yeast, and the reconstructed pathway will serve as a platform to generate trichodermin analogs as potential candidates for agrochemicals and anticancer agents through further optimizations.

Identification and Characterization of Small Molecule Compounds That Modulate Trichothecene Production by Fusarium graminearum

From the RIKEN Natural Products Depository (NPDepo) chemical library, we identified small molecules that alter trichothecene 15-acetyldeoxynivalenol (15-ADON) production by Fusarium graminearum. Among trichothecene production activators, a furanocoumarin NPD12671 showed the strongest stimulatory activity on 15-ADON production by the fungus cultured in a 24-well plate. NPD12671 significantly increased the transcription of Tri6, a transcription factor gene necessary for trichothecene biosynthesis, in both trichothecene-inducing and noninducing culture conditions. Dihydroartemisinin (DHA) was identified as the most effective inhibitor of trichothecene production in 24-well plate culture; DHA inhibited trichothecene production (>50% inhibition at 1 μM) without affecting fungal mass by suppressing Tri6 expression. To determine the effect of DHA on trichothecene pathway Tri gene expression, we generated a constitutively Tri6-overexpressing strain that produced 15-ADON in YG_60 medium in Erlenmeyer flasks, conditions under which no trichothecenes are produced by the wild-type. While 5 μM DHA failed to inhibit trichothecene biosynthesis by the overexpressor in trichothecene-inducing YS_60 culture, trichothecene production was suppressed in the YG_60 culture. Regardless of a high Tri6 transcript level in the constitutive overexpressor, the YG_60 culture showed reduced accumulation of Tri5 and Tri4 mRNA upon treatment with 5 μM DHA. Deletion mutants of FgOs2 were also generated and examined; both NPD12671 and DHA modulated trichothecene production as they did in the wild-type strain. These results are discussed in light of the mode of actions of these chemicals on trichothecene biosynthesis.

Exploring an Artificial Metabolic Route in Fusarium sporotrichioides: Production and Characterization of 7-Hydroxy T-2 Toxin

An artificial metabolic route to an unnatural trichothecene was designed by taking advantage of the broad substrate specificities of the T-2 toxin biosynthetic enzymes of Fusarium sporotrichioides. By feeding 7-hydroxyisotrichodermin, a shunt pathway metabolite of F. graminearum, to a trichodiene synthase-deficient mutant of F. sporotrichioides, 7-hydroxy T-2 toxin (1) was obtained as the final metabolite. Such an approach may have future applications in the metabolic engineering of a variety of fungal secondary metabolites. The toxicity of 7-hydroxy T-2 toxin was 10 times lower than that of T-2 toxin in HL-60 cells.

The ash dieback invasion of Europe was founded by two genetically divergent individuals

Accelerating international trade and climate change make pathogen spread an increasing concern. Hymenoscyphus fraxineus, the causal agent of ash dieback, is a fungal pathogen that has been moving across continents and hosts from Asian to European ash. Most European common ash trees (Fraxinus excelsior) are highly susceptible to H. fraxineus, although a minority (~5%) have partial resistance to dieback. Here, we assemble and annotate a H. fraxineus draft genome which approaches chromosome scale. Pathogen genetic diversity across Europe and in Japan, reveals a strong bottleneck in Europe, though a signal of adaptive diversity remains in key host interaction genes. We find that the European population was founded by two divergent haploid individuals. Divergence between these haplotypes represents the ancestral polymorphism within a large source population. Subsequent introduction from this source would greatly increase adaptive potential of the pathogen. Thus, further introgression of H. fraxineus into Europe represents a potential threat and Europe-wide biological security measures are needed to manage this disease.

Evolution of structural diversity of trichothecenes, a family of toxins produced by plant pathogenic and entomopathogenic fungi

Trichothecenes are a family of terpenoid toxins produced by multiple genera of fungi, including plant and insect pathogens. Some trichothecenes produced by the fungus Fusarium are among the mycotoxins of greatest concern to food and feed safety because of their toxicity and frequent occurrence in cereal crops, and trichothecene production contributes to pathogenesis of some Fusarium species on plants. Collectively, fungi produce over 150 trichothecene analogs: i.e., molecules that share the same core structure but differ in patterns of substituents attached to the core structure. Here, we carried out genomic, phylogenetic, gene-function, and analytical chemistry studies of strains from nine fungal genera to identify genetic variation responsible for trichothecene structural diversity and to gain insight into evolutionary processes that have contributed to the variation. The results indicate that structural diversity has resulted from gain, loss, and functional changes of trichothecene biosynthetic (TRI) genes. The results also indicate that the presence of some substituents has arisen independently in different fungi by gain of different genes with the same function. Variation in TRI gene duplication and number of TRI loci was also observed among the fungi examined, but there was no evidence that such genetic differences have contributed to trichothecene structural variation. We also inferred ancestral states of the TRI cluster and trichothecene biosynthetic pathway, and proposed scenarios for changes in trichothecene structures during divergence of TRI cluster homologs. Together, our findings provide insight into evolutionary processes responsible for structural diversification of toxins produced by pathogenic fungi.

Toxins produced by pathogens can contribute to infection and/or colonization of hosts. Some toxins consist of a family of metabolites with similar but distinct chemical structures. This structural variation can affect biological activity, which in turn likely contributes to adaptation to different environments, including to different hosts. Trichothecene toxins consist of over 150 structurally distinct molecules produced by certain fungi, including some plant and insect pathogens. In multiple systems that have been examined, trichothecenes contribute to pathogenesis on plants. To elucidate the evolutionary processes that have given rise to trichothecene structural variation, we conducted comparative analyses of nine fungal genera, most of which produce different trichothecene structures. Using genomic, molecular biology, phylogenetic, and analytical chemistry approaches, we obtained evidence that trichothecene structural variation has arisen primarily from gain, loss, and functional changes of trichothecene biosynthetic genes. Our results also indicate that some structural changes have arisen independently in different fungi. Our findings provide insight into genetic and biochemical changes that can occur in toxin biosynthetic pathways as fungi with the pathways adapt to different environmental conditions.

Increased metabolite production by deletion of an HDA1-type histone deacetylase in the phytopathogenic fungi, Magnaporthe oryzae (Pyricularia oryzae) and Fusarium asiaticum

Histone deacetylases (HDACs) play an important role in the regulation of chromatin structure and gene expression. We found that dark pigmentation of Magnaporthe oryzae (anamorph Pyricularia oryzae) ΔMohda1, a mutant strain in which an orthologue of the yeast HDA1 was disrupted by double cross-over homologous recombination, was significantly stimulated in liquid culture. Analysis of metabolites in a ΔMohda1 mutant culture revealed that the accumulation of shunt products of the 1,8-dihydroxynaphthalene melanin and ergosterol pathways were significantly enhanced compared to the wild-type strain. Northern blot analysis of the ΔMohda1 mutant revealed transcriptional activation of three melanin genes that are dispersed throughout the genome of M. oryzae. The effect of deletion of the yeast HDA1 orthologue was also observed in Fusarium asiaticum from the Fusarium graminearum species complex; the HDF2 deletion mutant produced increased levels of nivalenol-type trichothecenes. These results suggest that histone modification via HDA1-type HDAC regulates the production of natural products in filamentous fungi.

SIGNIFICANCE AND IMPACT OF THE STUDY

Natural products of fungi have significant impacts on human welfare, in both detrimental and beneficial ways. Although HDA1-type histone deacetylase is not essential for vegetative growth, deletion of the gene affects the expression of clustered secondary metabolite genes in some fungi. Here, we report that such phenomena are also observed in physically unlinked genes required for melanin biosynthesis in the rice blast fungus. In addition, production of Fusarium trichothecenes, previously reported to be unaffected by HDA1 deletion, was significantly upregulated in another Fusarium species. Thus, the HDA1-inactivation strategy may be regarded as a general approach for overproduction and/or discovery of fungal metabolites.

Functional characterization of a soluble NADPH-cytochrome P450 reductase from Fusarium graminearum

Fusarium head blight is a devastating disease in wheat caused by some fungal pathogens of the Fusarium genus mainly F. graminearum, due to accumulation of toxic trichothecenes. Most of the trichothecene biosynthetic pathway has been mapped, although some proteins of the pathway remain uncharacterized, including an NADPH-cytochrome P450 reductase. We subcloned a F. graminearum cytochrome P450 reductase that might be involved in the trichothecene biosynthesis. It was expressed heterologously in E. coli as N-terminal truncated form with an octahistidine tag for purification. The construct yielded a soluble apoprotein and its incubation with flavins yielded the corresponding monomeric holoprotein. It was characterized for activity in the pH range 5.5-9.5, using thiazolyl blue tetrazolium bromide (MTT) or cytochrome c as substrates. Binding of the small molecule MTT was weaker than for cytochrome c, however, the rate of MTT reduction was faster. Contrary to other studies of cytochrome reductase proteins, MTT reduction proceeded in a cooperative manner in our studies. Optimum kinetic activity was found at pH 7.5-8.5 for bothMTT and cytochrome c. This is the first paper presenting characterization of a cytochrome P450 reductase from F. graminearum which most likely is involved in mycotoxin biosynthesis or some primary metabolic pathway such as sterol biosynthesis in F. graminearum.

Identification of a trichothecene production inhibitor by chemical array and library screening using trichodiene synthase as a target protein

Trichothecene mycotoxins often accumulate in apparently normal grains of cereal crops. In an effort to develop an agricultural chemical to reduce trichothecene contamination, we screened trichothecene production inhibitors from the compounds on the chemical arrays. By using the trichodiene (TDN) synthase tagged with hexahistidine (rTRI5) as a target protein, 32 hit compounds were obtained from chemical library of the RIKEN Natural Product Depository (NPDepo) by chemical array screening. At 10μgmL^-1, none of the 32 chemicals inhibited trichothecene production by Fusarium graminearum in liquid culture. Against the purified rTRI5 enzyme, however, NPD10133 [progesterone 3-(O-carboxymethyl)oxime amide-bonded to phenylalanine] showed weak inhibitory activity at 10μgmL^-1 (18.7μM). For the screening of chemicals inhibiting trichothecene accumulation in liquid culture, 20 analogs of NPD10133 selected from the NPDepo chemical library were assayed. At 10μM, only NPD352 [testosterone 3-(O-carboxymethyl)oxime amide-bonded to phenylalanine methyl ester] inhibited rTRI5 activity and trichothecene production. Kinetic analysis suggested that the enzyme inhibition was of a mixed-type. The identification of NPD352 as a TDN synthase inhibitor lays the foundation for the development of a more potent inhibitor via systematic introduction of wide structural diversity on the gonane skeleton and amino acid residues.