Acidic pH as a determinant ofTRIgene expression and trichothecene B biosynthesis inFusarium graminearum

Advances in Understanding Fusarium graminearum: Genes Involved in the Regulation of Sexual Development, Pathogenesis, and Deoxynivalenol Biosynthesis

The wheat head blight disease caused by Fusarium graminearum is a major concern for food security and the health of both humans and animals. As a pathogenic microorganism, F. graminearum produces virulence factors during infection to increase pathogenicity, including various macromolecular and small molecular compounds. Among these virulence factors, secreted proteins and deoxynivalenol (DON) are important weapons for the expansion and colonization of F. graminearum. Besides the presence of virulence factors, sexual reproduction is also crucial for the infection process of F. graminearum and is indispensable for the emergence and spread of wheat head blight. Over the last ten years, there have been notable breakthroughs in researching the virulence factors and sexual reproduction of F. graminearum. This review aims to analyze the research progress of sexual reproduction, secreted proteins, and DON of F. graminearum, emphasizing the regulation of sexual reproduction and DON synthesis. We also discuss the application of new gene engineering technologies in the prevention and control of wheat head blight.

Contamination of Fusarium spp. and mycotoxins at different ear physiological stages of maize in Argentina

Maize is one of the most important crops worldwide, being affected by several fungal species under field conditions. The study of plant-pathogen interaction plays a key role because fungal diseases are responsible for reducing grain yield and quality by increasing mycotoxin production. Thus, the present work aimed to evaluate the interaction of F. graminearum and F. verticillioides and mycotoxin production under field conditions along different physiological stages. During the 2019/2020 and 2020/2021 growing seasons, twelve maize genotypes were inoculated at the flowering stage (silking). Four treatments were applied using one isolate of each species: (i) F. graminearum; (ii) F. verticillioides; (iii) a combined inoculum (F. graminearum + F. verticillioides); (iv) and control treatment. Fungal diversity, disease evaluation, and mycotoxin contamination were evaluated at three different physiological stages: T1 (from R2 to R4), T2 (from R4 to R6), and T3 (at harvest time). A total of 15,907 Fusarium isolates were obtained. The results showed a predominance of F. verticillioides over F. graminearum in both years evaluated, reporting an increase in the occurrence of this species at late stages. Regarding mycotoxin contamination, no evidence was found supporting antagonism or synergism regarding isolates of both species used as inoculum under field conditions. The results reported in the present manuscript point out a major influence of climatic conditions on F. verticillioides predominance over F. graminearum, mainly during the late physiological stages. Furthermore, no clear relationship between mycotoxin concentration and physiological stages was established, suggesting that other grain factors, such as water activity and pH, could modulate mycotoxin production and accumulation under field conditions.

Genetic Regulation of Mycotoxin Biosynthesis

Mycotoxin contamination in food poses health hazards to humans. Current methods of controlling mycotoxins still have limitations and more effective approaches are needed. During the past decades of years, variable environmental factors have been tested for their influence on mycotoxin production leading to elucidation of a complex regulatory network involved in mycotoxin biosynthesis. These regulators are putative targets for screening molecules that could inhibit mycotoxin synthesis. Here, we summarize the regulatory mechanisms of hierarchical regulators, including pathway-specific regulators, global regulators and epigenetic regulators, on the production of the most critical mycotoxins (aflatoxins, patulin, citrinin, trichothecenes and fumonisins). Future studies on regulation of mycotoxins will provide valuable knowledge for exploring novel methods to inhibit mycotoxin biosynthesis in a more efficient way.

Fungal diversity and mycotoxins detected in maize stored in silo-bags: a review

Silo-bags are hermetic storage systems that inhibit fungal growth because of their atmosphere with low humidity, as well as low pH and O₂ concentrations, and a high CO₂ concentration. If a silo-bag with stored maize loses its hermetic nature, it favors the development of fungi and the production of mycotoxins. To the best of our knowledge, this is the first review on the diversity of fungal species and mycotoxins that were reported in maize stored under the environmental conditions provided by silo-bags. The genera Penicillium, Aspergillus and Fusarium were found more frequently, whereas Acremonium spp., Alternaria sp., Candida sp., Cladosporium sp., Debaryomyces spp., Epiconum sp., Eupenicillium spp., Eurotium sp., Eurotium amstelodami, Hyphopichia spp., Hyphopichia burtonii, Moniliella sp., Wallemia sp. and genera within the orden Mucorales were reported less recurrently. Despite finding a great fungal diversity, all of the studies focused their investigations on a small group of toxins: fumonisins (FBs), aflatoxins (AFs), deoxynivalenol (DON), zearalenone (ZEA), patulin (PAT), toxin T2 (T2) and ochratoxin (OT). Of the FBs, fumonisin B₁ and fumonisin B₂ presented higher incidence percentages, followed by fumonisin B₃ . Of the AFs, the only one reported was aflatoxin B_1. The mycotoxins DON, ZEA and OT were found with lower incidences, whereas PAT and T2 were not detected. Good management practices of the silo-bags are necessary to achieve a hermetically sealed environment, without exchange of gases and water with the external environment during the storage period. © 2022 Society of Chemical Industry.

Bioprospecting Phenols as Inhibitors of Trichothecene-Producing Fusarium: Sustainable Approaches to the Management of Wheat Pathogens

Fusarium spp. are ubiquitous fungi able to cause Fusarium head blight and Fusarium foot and root rot on wheat. Among relevant pathogenic species, Fusarium graminearum and Fusarium culmorum cause significant yield and quality loss and result in contamination of the grain with mycotoxins, mainly type B trichothecenes, which are a major health concern for humans and animals. Phenolic compounds of natural origin are being increasingly explored as fungicides on those pathogens. This review summarizes recent research activities related to the antifungal and anti-mycotoxigenic activity of natural phenolic compounds against Fusarium, including studies into the mechanisms of action of major exogenous phenolic inhibitors, their structure-activity interaction, and the combined effect of these compounds with other natural products or with conventional fungicides in mycotoxin modulation. The role of high-throughput analysis tools to decipher key signaling molecules able to modulate the production of mycotoxins and the development of sustainable formulations enhancing potential inhibitors' efficacy are also discussed.

Plant defense compound triggers mycotoxin synthesis by regulating H2B ub1 and H3K4 me2/3 deposition

Fusarium graminearum produces the mycotoxin deoxynivalenol (DON) which promotes its expansion during infection on its plant host wheat. Conditional expression of DON production during infection is poorly characterized. Wheat produces the defense compound putrescine, which induces hypertranscription of DON biosynthetic genes (FgTRIs) and subsequently leads to DON accumulation during infection. Further, the regulatory mechanisms of FgTRIs hypertranscription upon putrescine treatment were investigated. The transcription factor FgAreA regulates putrescine-mediated transcription of FgTRIs by facilitating the enrichment of histone H2B monoubiquitination (H2B ub1) and histone 3 lysine 4 di- and trimethylations (H3K4 me2/3) on FgTRIs. Importantly, a DNA-binding domain (bZIP) specifically within the Fusarium H2B ub1 E3 ligase Bre1 othologs is identified, and the binding of this bZIP domain to FgTRIs depends on FgAreA-mediated chromatin rearrangement. Interestingly, H2B ub1 regulates H3K4 me2/3 via the methyltransferase complex COMPASS component FgBre2, which is different from Saccharomyces cerevisiae. Taken together, our findings reveal the molecular mechanisms by which host-generated putrescine induces DON production during F. graminearum infection. Our results also provide a novel insight into the role of putrescine during phytopathogen-host interactions and broaden our knowledge of H2B ub1 biogenesis and crosstalk between H2B ub1 and H3K4 me2/3 in eukaryotes.

Fusarium Mycotoxins Enniatins: An Updated Review of Their Occurrence, the Producing Fusarium Species, and the Abiotic Determinants of Their Accumulation in Crop Harvests

Cereal grains and their processed food products are frequently contaminated with mycotoxins produced by the Fusarium genus. Enniatins (ENNs), which belong to the so-called "emerging mycotoxins" family, are among the most frequently found in small grain cereals. Health hazards induced by a chronic exposure to ENNs or an association of ENNs with other major mycotoxins is a risk that cannot be excluded given the current toxicological data. Thus, efforts must be pursued to define efficient control strategies to mitigate their presence in cereal grains. A key condition for achieving this aim is to gain deep and comprehensive knowledge of the factors promoting the appearance of ENNs in crop harvests. After an update of ENN occurrence data, this review surveys the scientific literature on the Fusarium species responsible for ENN contamination and covers the recent advances concerning the abiotic determinants and the genetic regulation of ENN biosynthesis.

Transcriptional analysis of wheat seedlings inoculated with Fusarium culmorum under continual exposure to disease defence inductors

A facultative parasite of cereals, Fusarium culmorum is a soil-, air- and seed-borne fungus causing foot and root rot, fusarium seedling blight, and especially Fusarium head blight, a spike disease leading to decreased yield and mycotoxin contamination of grain. In the present study, we tested changes in expression of wheat genes (B2H2, ICS, PAL, and PR2) involved in defence against diseases. We first compared expression of the analysed genes in seedlings of non-inoculated and artificially inoculated wheat (variety Bohemia). The second part of the experiment compared expression of these genes in seedlings grown under various treatment conditions. These treatments were chosen to determine the effects of prochloraz, sodium bicarbonate, ergosterol, aescin and potassium iodide on expression of the analysed defence genes. In addition to the inoculated and non-inoculated cultivar Bohemia, we additionally examined two other varieties of wheat with contrasting resistance to Fusarium sp. infection. These were the blue aleurone layer variety Scorpion that is susceptible to Fusarium sp. infection and variety V2-49-17 with yellow endosperm and partial resistance to Fusarium sp. infection. In this manner, we were able to compare potential effects of inductors upon defence gene expression among three varieties with different susceptibility to infection but also between inoculated and non-inoculated seedlings of a single variety. The lowest infection levels were detected in the sodium bicarbonate treatment. Sodium bicarbonate had not only negative influence on Fusarium growth but also positively affected expression of plant defence genes. Expression of the four marker genes shown to be important in plant defence was significantly affected by the treatments. The greatest upregulation in comparison to the water control was identified under all treatments for the B2H2 gene. Only expression of PAL under the ergosterol and prochloraz treatments were not statistically significant.

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.

Fusarium Toxins in Chinese Wheat since the 1980s

Wheat Fusarium head blight (FHB), caused by Fusarium species, is a widespread and destructive fungal disease. In addition to the substantial yield and revenue losses, diseased grains are often contaminated with Fusarium mycotoxins, making them unsuitable for human consumption or use as animal feed. As a vital food and feed ingredient in China, the quality and safety of wheat and its products have gained growing attention from consumers, producers, scientists, and policymakers. This review supplies detailed data about the occurrence of Fusarium toxins and related intoxications from the 1980s to the present. Despite the serious situation of toxin contamination in wheat, the concentration of toxins in flour is usually lower than that in raw materials, and food-poisoning incidents have been considerably reduced. Much work has been conducted on every phase of toxin production and wheat circulation by scientific researchers. Regulations for maximum contamination limits have been established in recent years and play a substantial role in ensuring the stability of the national economy and people's livelihoods.

Silage review: Mycotoxins in silage: Occurrence, effects, prevention, and mitigation

Ensiled forage, particularly corn silage, is an important component of dairy cow diets worldwide. Forages can be contaminated with several mycotoxins in the field pre-harvest, during storage, or after ensiling during feed-out. Exposure to dietary mycotoxins adversely affects the performance and health of livestock and can compromise human health. Several studies and surveys indicate that ruminants are often exposed to mycotoxins such as aflatoxins, trichothecenes, ochratoxin A, fumonisins, zearalenone, and many other fungal secondary metabolites, via the silage they ingest. Problems associated with mycotoxins in silage can be minimized by preventing fungal growth before and after ensiling. Proper silage management is essential to reduce mycotoxin contamination of dairy cow feeds, and certain mold-inhibiting chemical additives or microbial inoculants can also reduce the contamination levels. Several sequestering agents also can be added to diets to reduce mycotoxin levels, but their efficacy varies with the type and level of mycotoxin contamination. This article gives an overview of the types, prevalence, and levels of mycotoxin contamination in ensiled forages in different countries, and describes their adverse effects on health of ruminants, and effective prevention and mitigation strategies for dairy cow diets. Future research priorities discussed include research efforts to develop silage additives or rumen microbial innocula that degrade mycotoxins.

Intracellular superoxide level controlled by manganese superoxide dismutases affects trichothecene production in Fusarium graminearum

The intracellular superoxide level is a clue to clarification of the regulatory mechanism for mycotoxin production in Fusarium graminearum. In this study, we focused on two manganese superoxide dismutases (SODs) of the fungus, FgSOD2 and FgSOD3, to investigate the relationship of the superoxide level to trichothecene production. Recombinant FgSOD2 and FgSOD3 showed SOD activity, and they were localized mainly in the mitochondria and cytoplasm, respectively. Trichothecene production and mRNA levels of Tri5 and Tri6, which encode a trichothecene biosynthetic enzyme and a key regulator of trichothecene production, respectively, were greatly reduced in gene-deletion mutants of FgSod2 and FgSod3 (ΔFgSod2 and ΔFgSod3). Significant increases in the cytosolic and mitochondrial superoxide levels were observed in ΔFgSod2 and ΔFgSod3, respectively. These results suggested that the cellular superoxide level affects trichothecene production in F. graminearum.

Mycotoxin Biosynthesis and Central Metabolism Are Two Interlinked Pathways in Fusarium graminearum, as Demonstrated by the Extensive Metabolic Changes Induced by Caffeic Acid Exposure

Fusarium graminearum is a major plant pathogen that causes devastating diseases of cereals and produces type B trichothecene (TCTB) mycotoxins in infected grains. A comprehensive understanding of the molecular and biochemical mechanisms underlying the regulation of TCTB biosynthesis is required for improving strategies to control the TCTB contamination of crops and ensuring that these strategies do not favor the production of other toxic metabolites by F. graminearum Elucidation of the association of TCTB biosynthesis with other central and specialized processes was the focus of this study. Combined 1H nuclear magnetic resonance (1H NMR) and liquid chromatography-quadrupole time of flight-mass spectrometry (LC-QTOF-MS) analyses were used to compare the exo- and endometabolomes of F. graminearum grown under toxin-inducing and -repressing caffeic acid conditions. Ninety-five metabolites were putatively or unambiguously identified, including 26 primary and 69 specialized metabolites. Our data demonstrated that the inhibition of TCTB production induced by caffeic acid exposure was associated with significant changes in the secondary and primary metabolism of F. graminearum, although the fungal growth was not affected. The main metabolic changes were an increase in the accumulation of several polyketides, including toxic ones, alterations in the tricarboxylic organic acid cycle, and modifications in the metabolism of several amino acids and sugars. While these findings provide insights into the mechanisms that govern the inhibition of TCTB production by caffeic acid, they also demonstrate the interdependence between the biosynthetic pathway of TCTB and several primary and specialized metabolic pathways. These results provide further evidence of the multifaceted role of TCTB in the life cycle of F. graminearumIMPORTANCEFusarium graminearum is a major plant pathogen that causes devastating diseases of cereal crops and produces type B trichothecene (TCTB) mycotoxins in infected grains. The best way to restrict consumer exposure to TCTB is to limit their production before harvest, which requires increasing the knowledge on the mechanisms that regulate their biosynthesis. Using a metabolomics approach, we investigated the interconnection between the TCTB production pathway and several fungal metabolic pathways. We demonstrated that alteration in the TCTB biosynthetic pathway can have a significant impact on other metabolic pathways, including the biosynthesis of toxic polyketides, and vice versa. These findings open new avenues for identifying fungal targets for the design of molecules with antimycotoxin properties and therefore improving sustainable strategies to fight against diseases caused by F. graminearum Our data further demonstrate that analyses should consider all fungal toxic metabolites rather than the targeted family of mycotoxins when assessing the efficacy of control strategies.

Optimization for the Production of Deoxynivalenoland Zearalenone by Fusarium graminearum UsingResponse Surface Methodology

Fusarium mycotoxins deoxynivalenol (DON) and zearalenone (ZEN) are the most common contaminants in cereals worldwide, causing a wide range of adverse health effects on animals and humans. Many environmental factors can affect the production of these mycotoxins. Here, we have used response surface methodology (RSM) to optimize the Fusarium graminearum strain 29 culture conditions for maximal toxin production. Three factors, medium pH, incubation temperature and time, were optimized using a Box-Behnken design (BBD). The optimized conditions for DON production were pH 4.91 and an incubation temperature of 23.75 °C for 28 days, while maximal ZEN production required pH 9.00 and an incubation temperature of 15.05 °C for 28 days. The maximum levels of DON and ZEN production were 2811.17 ng/mL and 23789.70 ng/mL, respectively. Considering the total level of DON and ZEN, desirable yields of the mycotoxins were still obtained with medium pH of 6.86, an incubation temperature of 17.76 °C and a time of 28 days. The corresponding experimental values, from the validation experiments, fitted well with these predictions. This suggests that RSM could be used to optimize Fusarium mycotoxin levels, which are further purified for use as potential mycotoxin standards. Furthermore, it shows that acidic pH is a determinant for DON production, while an alkaline environment and lower temperature (approximately 15 °C) are favorable for ZEN accumulation. After extraction, separation and purification processes, the isolated mycotoxins were obtained through a simple purification process, with desirable yields, and acceptable purity. The mycotoxins could be used as potential analytical standards or chemical reagents for routine analysis.

The microtubule end-binding protein FgEB1 regulates polar growth and fungicide sensitivity via different interactors in Fusarium graminearum

In yeasts, the end-binding protein 1 (EB1) homologs regulate microtubule dynamics, cell polarization, and chromosome stability. However, functions of EB1 orthologs in plant pathogenic fungi have not been characterized yet. Here, we observed that the FgEB1 deletion mutant (ΔFgEB1) of Fusarium graminearum exhibits twisted hyphae, increased hyphal branching and curved conidia, indicating that FgEB1 is involved in the regulation of cellular polarity. Microscopic examination further showed that the microtubules of ΔFgEB1 exhibited less organized in comparison with those of the wild type. In addition, the lack of FgEB1 also altered the distribution of polarity-related class I myosin via the interaction with the actin. On the other hand, we identified four core septins as FgEB1-interacting proteins, and found that FgEB1 and septins regulated conidial polar growth in the opposite orientation. Interestingly, FgEB1 and FgKar9 constituted another complex that modulated the response to carbendazim, a microtubule-damaging agent specifically. In addition, the deletion of FgEB1 led to dramatically decreased deoxynivalenol (DON) biosynthesis. Taken together, results of this study indicate that FgEB1 regulates cellular polarity, fungicide sensitivity and DON biosynthesis via different interactors in F. graminarum, which provides a novel insight into understanding of the biological functions of EB1 in filamentous fungi.

The chitin synthase FgChs2 and other FgChss co-regulate vegetative development and virulence in F. graminearum

Fusarium graminearum contains eight chitin synthase (Chs) genes belonging to seven classes. Previous studies have found that deletion of FgChs3b is lethal to F. graminearum, and deletion of FgChs1, FgChs2, FgChs7 and FgChs5 caused diverse defects in chitin content, mycelial growth, conidiation, virulence or stress responses. However, little is known about the functional relationships among these FgChss. In this study, FgChs2 deletion mutant ΔFgChs2 exhibited reduced mycelial growth and virulence as reported previously. In addition, we found that the mutant produced thickened and “wavy” septa. Quantitative real-time PCR (qRT-PCR) assays showed that the expression levels of FgChs1, FgChs3a, FgChs4, FgChs7, FgChs5 and FgChs6 in ΔFgChs2 were significantly higher than those in the wild type. Therefore, we generated six double deletion mutants of FgChs2 and each of the above six FgChss, and found that FgChs2 shares a function with FgChs1 in regulating mycelial growth, and co-regulates conidiation with FgChs1, FgChs4, FgChs7 and FgChs5. Furthermore, FgChs2 and other six FgChss have overlapped functions in virulence, DON production and septum formation. Taken together, these results indicate that although each chitin synthase of F. graminearum plays certain roles, FgChss may co-regualte various cellular processes in F. graminearum.

Comparison of Fusarium graminearum Transcriptomes on Living or Dead Wheat Differentiates Substrate-Responsive and Defense-Responsive Genes

Fusarium graminearum is an opportunistic pathogen of cereals where it causes severe yield losses and concomitant mycotoxin contamination of the grains. The pathogen has mixed biotrophic and necrotrophic (saprophytic) growth phases during infection and the regulatory networks associated with these phases have so far always been analyzed together. In this study we compared the transcriptomes of fungal cells infecting a living, actively defending plant representing the mixed live style (pathogenic growth on living flowering wheat heads) to the response of the fungus infecting identical, but dead plant tissues (cold-killed flowering wheat heads) representing strictly saprophytic conditions. We found that the living plant actively suppressed fungal growth and promoted much higher toxin production in comparison to the identical plant tissue without metabolism suggesting that molecules signaling secondary metabolite induction are not pre-existing or not stable in the plant in sufficient amounts before infection. Differential gene expression analysis was used to define gene sets responding to the active or the passive plant as main impact factor and driver for gene expression. We correlated our results to the published F. graminearum transcriptomes, proteomes, and secretomes and found that only a limited number of in planta- expressed genes require the living plant for induction but the majority uses simply the plant tissue as signal. Many secondary metabolite (SM) gene clusters show a heterogeneous expression pattern within the cluster indicating that different genetic or epigenetic signals govern the expression of individual genes within a physically linked cluster. Our bioinformatic approach also identified fungal genes which were actively repressed by signals derived from the active plant and may thus represent direct targets of the plant defense against the invading pathogen.

The effect of agmatine on trichothecene type B and zearalenone production in Fusarium graminearum, F. culmorum and F. poae

Agmatine and other putrescines are known for being strong inducers of deoxynivalenol (DON) production in Fusarium graminearum. Other important species produce DON and/or other trichothecene type B toxins (3 acetylated DON, 15 acetylated DON, Fusarenon-X, Nivalenol), such as F. culmorum and F. poae. In order to verify whether the mechanism of the regulation of trichothecene type B induction by agmatine is shared by different species of Fusarium, we tested the hypothesis on 19 strains belonging to 3 Fusarium species (F. graminearum, F. culmorum, F. poae) with diverse genetic chemotypes (3ADON, 15ADON, NIV) by measuring trichothecene B toxins such as DON, NIV, Fusarenon-X, 3ADON and 15ADON. Moreover, we tested whether other toxins like zearalenone were also boosted by agmatine. The trichothecene type B boosting effect was observed in the majority of strains (13 out of 19) in all the three species. Representative strains from all three genetic chemotypes were able to boost toxin production after agmatine treatment. We identified the non-responding strains to the agmatine stimulus, which may contribute to deciphering the regulatory mechanisms that link toxin production to agmatine (and, more generally, polyamines).

Is the Fgap1 mediated response to oxidative stress chemotype dependent in Fusarium graminearum?

This study aims to compare the role of the transcription factor Fgap1 in oxidative stress response for two Fusarium graminearum strains belonging to the two chemotypes DON/ADON and NIV/FX. While the response to H2O2 was shown to be chemotype dependent, an opposite result was observed for diamide: whatever the chemotype, the global level of TCTB (i.e. trichothecene B) production was strongly increased by the treatment with diamide. Fgap1 was shown to be involved in this regulation for both chemotypes. Our data show that the response to diamide is mediated by Fgap1 whatever the chemotype of the F. graminearum strains. However, the NIV/FX chemotype has developed higher antioxidant capacities in response to oxidative stress. But when this capacity is overwhelmed by an increment in the H2O2 level, the NIV/FX strains also responds by an increase in toxin accumulation.

The tubulin cofactor A is involved in hyphal growth, conidiation and cold sensitivity in Fusarium asiaticum

BACKGROUND

Tubulin cofactor A (TBCA), one of the members of tubulin cofactors, is of great importance in microtubule functions through participating in the folding of α/β-tubulin heterodimers in Saccharomyces cerevisiae. However, little is known about the roles of TBCA in filamentous fungi.

RESULTS

In this study, we characterized a TBCA orthologue FaTBCA in Fusarium asiaticum. The deletion of FaTBCA caused dramatically reduced mycelial growth and abnormal conidiation. The FaTBCA deletion mutant (ΔFaTBCA-3) showed increased sensitivity to low temperatures and even lost the ability of growth at 4°C. Microscopic observation found that hyphae of ΔFaTBCA-3 exhibited blebbing phenotypes after shifting from 25 to 4°C for 1- or 3-day incubation and approximately 72% enlarged nodes contained several nuclei after 3-day incubation at 4°C. However, hyphae of the wild type incubated at 4°C were phenotypically indistinguishable from those incubated at 25°C. These results indicate that FaTBCA is involved in cell division under cold stress (4°C) in F. asiaticum. Unexpectedly, ΔFaTBCA-3 did not exhibit increased sensitivity to the anti-microtubule drug carbendazim although quantitative real-time assays showed that the expression of FaTBCA was up-regulated after treatment with carbendazim. In addition, pathogenicity assays showed that ΔFaTBCA-3 exhibited decreased virulence on wheat head and on non-host tomato.

CONCLUSION

Taken together, results of this study indicate that FaTBCA plays crucial roles in vegetative growth, conidiation, temperature sensitivity and virulence in F. asiaticum.

α1-Tubulin FaTuA1 plays crucial roles in vegetative growth and conidiation in Fusarium asiaticum

The filamentous ascomycete Fusarium asiaticum contains two homologous genes FaTUA1 and FaTUA2 encoding α-tubulins. In this study, we found that FaTUA2 was dispensable for vegetative growth and sporulation in F. asiaticum. The deletion of FaTUA1 however led to dramatically reduced mycelial growth, twisted hyphae and abnormal nuclei in apical cells of hyphae. The FaTUA1 deletion mutant (ΔFaTuA1-5) also showed a significant decrease in conidiation, and produced abnormal conidia. Pathogenicity assays showed that ΔFaTuA1-5 exhibited decreased virulence on wheat head. Unexpectedly, the deletion of FaTUA1 led to resistance to high temperatures. In addition, ΔFaTuA2 showed increased sensitivity to carbendazim. Furthermore, increased FaTUA2 expression in ΔFaTuA1-5 partially restored the defects of the mutant in mycelial growth, conidial production and virulence, vice versa, increased FaTUA1 expression in the FaTUA2 deletion mutant also partially relieved the defect of the mutant in the delay of conidial germination. Taken together, these results indicate that FaTuA1 plays crucial roles in vegetative growth and development, and the functions of FaTuA1 and FaTuA2 are partially interchangeable in F. asiaticum.

Plant lignans inhibit growth and trichothecene biosynthesis in Fusarium graminearum

Lignans are a group of diphenolic compounds with anticancer and antioxidant properties which are present in various grains, although their effect on toxigenic fungi has been poorly examined to date. In this study, the impact of the plant lignans pinoresinol and secoisolariciresinol on growth and trichothecene biosynthesis by five Fusarium graminearum strains of different chemotypes was examined in vitro. Both tested lignans exhibited radial growth inhibition against the fungal strains. RT-qPCR analyses of tri4, tri5 and tri11 genes encoding the first steps of the trichothecene biosynthesis pathway revealed a decrease in tri mRNA levels in lignan-treated fungal cultures. Correspondingly, decreased accumulation of toxins in lignan-treated cultures was confirmed by GC-MS analysis. This is the first study to demonstrate the inhibitory effect of both pinoresinol and secoisolariciresinol on growth and trichothecene biosynthesis in F. graminearum.

SIGNIFICANCE AND IMPACT OF THE STUDY

Knowledge of the regulation of trichothecene production in Fusarium graminearum by environmental cues is key to the design of novel strategies to reduce mycotoxin levels in grains. Here, we show that the lignans pinoresinol and secoisolariciresinol, which occur in wheat grains, inhibit radial growth and decrease trichothecene levels in five F. graminearum strains. RT-qPCR analysis reveals that the reduction in trichothecene level in lignan-treated fungal cultures is associated with decreased mRNA transcript levels for the tri4, tri5 and tri11 genes that are involved in the trichothecene biosynthesis pathway.

FgIlv5 is required for branched-chain amino acid biosynthesis and full virulence in Fusarium graminearum

In this study, we characterized FgIlv5, a homologue of the Saccharomyces cerevisiae keto-acid reductoisomerase (KARI) from the important wheat head scab fungus Fusarium graminearum. KARI is a key enzyme in the branched-chain amino acid (BCAA, including leucine, isoleucine and valine) biosynthetic pathway that exists in a variety of organisms from bacteria to fungi and higher plants, but not in mammals. The FgILV5 deletion mutant ΔFgIlv5-4 failed to grow when the culture medium was nutritionally limited for BCAAs. When grown on potato-dextrose agar plates, ΔFgIlv5-4 exhibited a significant decrease in aerial hyphae formation and red pigmentation. Conidia formation was also blocked in ΔFgIlv5-4. Exogenous addition of 1 mM isoleucine and valine was able to rescue the defects of mycelial growth and conidial morphogenesis. Cellular stress assays showed that ΔFgIlv5-4 was more sensitive to osmotic and oxidative stresses than the wild-type strain. In addition, virulence of ΔFgIlv5-4 was dramatically reduced on wheat heads, and a low level of deoxynivalenol production was detected in ΔFgIlv5-4 in wheat kernels. The results of this study indicate that FgIlv5 is involved in valine and isoleucine biosynthesis and is required for full virulence in F. graminearum.

Comparison of Trichothecene Biosynthetic Gene Expression between Fusarium graminearum and Fusarium asiaticum

Nivalenol (NIV) and deoxynivalenol (DON) are predominant Fusarium-producing mycotoxins found in grains, which are mainly produced by Fusarium asiaticum and F. graminearum. NIV is found in most of cereals grown in Korea, but the genetic basis for NIV production by F. asiaticum has not been extensively explored. In this study, 12 genes belonging to the trichothecene biosynthetic gene cluster were compared at the transcriptional level between two NIV-producing F. asiaticum and four DON-producing F. graminearum strains. Chemical analysis revealed that time-course toxin production patterns over 14 days did not differ between NIV and DON strains, excluding F. asiaticum R308, which was a low NIV producer. Both quantitative real-time polymerase chain reaction and Northern analysis revealed that the majority of TRI gene transcripts peaked at day 2 in both NIV and DON producers, which is 2 days earlier than trichothecene accumulation in liquid medium. Comparison of the gene expression profiles identified an NIV-specific pattern in two transcription factor-encoding TRI genes (TRI6 and TRI10) and TRI101, which showed two gene expression peaks during both the early and late incubation periods. In addition, the amount of trichothecenes produced by both DON and NIV producers were correlated with the expression levels of TRI genes, regardless of the trichothecene chemotypes. Therefore, the reduced production of NIV by R308 compared to NIV or DON by the other strains may be attributable to the significantly lower expression levels of the TRI genes, which showed early expression patterns.

Effects of carbon sources and amines on induction of trichothecene production by Fusarium asiaticum in liquid culture

Fusarium asiaticum infects cereal crops and produces trichothecenes such as deoxynivalenol and nivalenol. To determine the trichothecene induction mechanism, effects of carbon sources on the production of deoxynivalenol, nivalenol, 3-acetyl deoxynivalenol (3ADON), and 4-acetyl nivalenol (4ANIV) were examined in liquid cultures incubated with various strains. Sucrose supported significantly higher levels of acetylated trichothecene production in all strains than did the other carbon sources. Structural isomers of sucrose did not induce trichothecene production. The inducing effect of sucrose on trichothecene production was lost after the carbon source in the culture medium changed from sucrose to maltose in the process of incubation. Tri4 and Tri5 expressions were specifically up-regulated in the sucrose-containing medium and down-regulated with sucrose exhaustion. These findings suggest that F. asiaticum recognizes sucrose molecules and regulates Tri gene expression and trichothecene production. Moreover, an accelerating effect on trichothecene production by acidification of the culture medium containing specific amines during fungal incubation was exhibited only in the presence of sucrose in the medium. F. asiaticum induces trichothecene production in the presence of sucrose and accelerates the production when the medium containing specific amines is acidified during incubation.

The velvet gene, FgVe1, affects fungal development and positively regulates trichothecene biosynthesis and pathogenicity in Fusarium graminearum

Trichothecenes are a group of toxic secondary metabolites produced mainly by Fusarium graminearum (teleomorph: Gibberella zeae) during the infection of crop plants, including wheat, maize, barley, oats, rye and rice. Some fungal genes involved in trichothecene biosynthesis have been shown to encode regulatory proteins. However, the global regulation of toxin biosynthesis is still enigmatic. In addition to the production of secondary metabolites belonging to the trichothecene family, F. graminearum produces the red pigment aurofusarin. The gene regulation underlying the production of aurofusarin is not well understood. The velvet gene (veA) is conserved in various genera of filamentous fungi. Recently, the veA gene from Aspergillus nidulans has been shown to be the key component of the velvet complex regulating development and secondary metabolism. Using blast analyses, we identified the velvet gene from F. graminearum, FgVe1. Disruption of FgVe1 causes several phenotypic effects. However, the complementation of this mutant with the FgVe1 gene restores the wild-type phenotypes. The in vitro phenotypes include hyperbranching of the mycelium, suppression of aerial hyphae formation, reduced hydrophobicity of the mycelium and highly reduced sporulation. Our data also show that FgVe1 modulates the production of the aurofusarin pigment and is essential for the expression of Tri genes and the production of trichothecenes. Pathogenicity studies performed on flowering wheat plants indicate that FgVe1 is a positive regulator of virulence in F. graminearum.

On the trail of a cereal killer: recent advances in Fusarium graminearum pathogenomics and host resistance

The ascomycete fungal pathogen Fusarium graminearum (sexual stage: Gibberella zeae) causes the devastating head blight or scab disease on wheat and barley, and cob or ear rot disease on maize. Fusarium graminearum infection causes significant crop and quality losses. In addition to roles as virulence factors during pathogenesis, trichothecene mycotoxins (e.g. deoxynivalenol) produced by this pathogen constitute a significant threat to human and animal health if consumed in respective food or feed products. In the last few years, significant progress has been made towards a better understanding of the processes involved in F. graminearum pathogenesis, toxin biosynthesis and host resistance mechanisms through the use of high-throughput genomic and phenomic technologies. In this article, we briefly review these new advances and also discuss how future research can contribute to the development of sustainable plant protection strategies against this important plant pathogen.

Regulation of trichothecene biosynthesis in Fusarium: recent advances and new insights

Trichothecenes are toxic secondary metabolites produced by filamentous fungi mainly belonging to the Fusarium genus. Production of these mycotoxins occurs during infection of crops and is a threat to human and animal health. Although the pathway for biosynthesis of trichothecenes is well established, the regulation of the Tri genes implicated in the pathway remains poorly understood. Most of the Tri genes are gathered in a cluster which contains two transcriptional regulators controlling the expression of the other Tri genes. The regulation of secondary metabolites biosynthesis in most fungal genera has been recently shown to be controlled by various regulatory systems in response to external environment. The control of the "Tri cluster" by non-cluster regulators in Fusarium was not clearly demonstrated until recently. This review covers the recent advances concerning the regulation of trichothecene biosynthesis in Fusarium and highlights the potential implication of various general regulatory circuits. Further studies on the role of these regulatory systems in the control of trichothecene biosynthesis might be useful in designing new strategies to reduce mycotoxin accumulation.

Enhancement of trichothecene production in Fusarium graminearum by cobalt chloride

The effects of cobalt chloride on the production of trichothecene and ergosterol in Fusarium graminearum were examined. Incorporation experiments with (13)C-labeled acetate and leucine confirmed that both 3-acetyldeoxynivalenol and ergosterol were biosynthesized via a mevalonate pathway by the fungus, although hydroxymethyl-glutaryl CoA (HMG-CoA) from intact leucine was able to be partially used for ergosterol production. Addition of cobalt chloride at concentrations of 3-30 μM into liquid culture strongly enhanced 3-acetyldeoxynivalenol production by the fungus, whereas the amount of ergosterol and the mycelial weight of the fungus did not change. The mRNA levels of genes encoding trichothecene biosynthetic proteins (TRI4 and TRI6), ergosterol biosynthetic enzymes (ERG3 and ERG25), and enzymes involved in the mevalonate pathway (HMG-CoA synthase (HMGS) and HMG-CoA reductase (HMGR)) were all strongly up-regulated in the presence of cobalt chloride. Precocene II, a specific trichothecene production inhibitor, suppressed the effects of cobalt chloride on Tri4, Tri6, HMGS, and HMGR, but did not affect erg3 and erg25. These results indicate that cobalt chloride is useful for investigating regulatory mechanisms of trichothecene and ergosterol production in F. graminearum.

Early activation of wheat polyamine biosynthesis during Fusarium head blight implicates putrescine as an inducer of trichothecene mycotoxin production

BACKGROUND

The fungal pathogen Fusarium graminearum causes Fusarium Head Blight (FHB) disease on wheat which can lead to trichothecene mycotoxin (e.g. deoxynivalenol, DON) contamination of grain, harmful to mammalian health. DON is produced at low levels under standard culture conditions when compared to plant infection but specific polyamines (e.g. putrescine and agmatine) and amino acids (e.g. arginine and ornithine) are potent inducers of DON by F. graminearum in axenic culture. Currently, host factors that promote mycotoxin synthesis during FHB are unknown, but plant derived polyamines could contribute to DON induction in infected heads. However, the temporal and spatial accumulation of polyamines and amino acids in relation to that of DON has not been studied.

RESULTS

Following inoculation of susceptible wheat heads by F. graminearum, DON accumulation was detected at two days after inoculation. The accumulation of putrescine was detected as early as one day following inoculation while arginine and cadaverine were also produced at three and four days post-inoculation. Transcripts of ornithine decarboxylase (ODC) and arginine decarboxylase (ADC), two key biosynthetic enzymes for putrescine biosynthesis, were also strongly induced in heads at two days after inoculation. These results indicated that elicitation of the polyamine biosynthetic pathway is an early response to FHB. Transcripts for genes encoding enzymes acting upstream in the polyamine biosynthetic pathway as well as those of ODC and ADC, and putrescine levels were also induced in the rachis, a flower organ supporting DON production and an important route for pathogen colonisation during FHB. A survey of 24 wheat genotypes with varying responses to FHB showed putrescine induction is a general response to inoculation and no correlation was observed between the accumulation of putrescine and infection or DON accumulation.

CONCLUSIONS

The activation of the polyamine biosynthetic pathway and putrescine in infected heads prior to detectable DON accumulation is consistent with a model where the pathogen exploits the generic host stress response of polyamine synthesis as a cue for production of trichothecene mycotoxins during FHB disease. However, it is likely that this mechanism is complicated by other factors contributing to resistance and susceptibility in diverse wheat genetic backgrounds.

The pH regulatory factor Pac1 regulates Tri gene expression and trichothecene production in Fusarium graminearum

Fungi manage the adaptation to extra-cellular pH through the PacC transcription factor, a key component of the pH regulatory system. PacC regulates the production of various secondary metabolites in filamentous fungi. In the important cereal pathogen Fusarium graminearum, the production of trichothecene is induced only under acidic pH conditions. Here, we examined the role of the PacC homologue from F. graminearum, FgPac1, on the regulation of trichothecene production. An FgΔPac1 deletion mutant was constructed in F. graminearum which showed a reduced development under neutral and alkaline pH, increased sensitivity to H(2)O(2) and an earlier Tri gene induction and toxin accumulation at acidic pH. A strain expressing the FgPac1(c) constitutively active form of Pac1 exhibited a strongly repressed Tri gene expression and reduced toxin accumulation at acidic pH. These results demonstrate that Pac1 negatively regulates Tri gene expression and toxin production in F. graminearum.