Fusarium Tri8 encodes a trichothecene C-3 esterase

The identification of a key gene highlights macrocyclic ring's role in trichothecene toxicity

Trichothecenes are toxins produced by certain species from several fungal genera, including Aspergillus, Fusarium, Isaria, Paramyrothecium, Stachybotrys, Trichoderma, and Trichothecium. These toxins are of interest because they contribute to the toxigenicity, plant pathogenicity, and/or biological control activities of some fungi. All trichothecenes have the same core (12,13-epoxytrichothec-9-ene or EPT) structure but can differ from one another by the presence or absence of a macrocyclic ring formed from polyketide and isoprenoid substituents esterified to carbon atoms 4 and 15 of EPT, respectively. Genes required for formation and some modifications of EPT have been elucidated, but almost nothing is known about genes specific to the formation of the macrocyclic ring. Therefore, we used genomic, transcriptomic, metabolomic, and gene deletion analyses to identify genes that are required specifically for the formation of the macrocyclic ring. These analyses identified one gene, TRI24, that is predicted to encode an acyltransferase and that is required for macrocyclic ring formation during biosynthesis of macrocyclic trichothecenes by the fungus Paramyrothecium roridum. In addition, a TRI24 deletion mutant of P. roridum caused less severe disease symptoms on common bean and had less antifungal activity than its wild-type progenitor strain. We propose that the reduced aggressiveness and antifungal activity of the mutant resulted from its inability to produce trichothecenes with a macrocyclic ring. To our knowledge, this is the first report of a gene required specifically for the formation of the macrocyclic ring of trichothecenes and that loss of the macrocyclic ring of trichothecenes can alter the biological activities of a fungus. KEY POINTS: • TRI24 gene is found in all known macrocyclic trichothecene-producing fungi. • A tri24-deletion mutant exhibits a reduction in antifungal and plant disease activities. • TRI24 is the first described gene specific to macrocyclic trichothecene biosynthesis.

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.

Type A Trichothecene Metabolic Profile Differentiation, Mechanisms, Biosynthetic Pathways, and Evolution in Fusarium Species-A Mini Review

Trichothecenes are the most common Fusarium toxins detected in grains and related products. Type A trichothecenes are among the mycotoxins of greatest concern to food and feed safety due to their high toxicity. Recently, two different trichothecene genotypes within Fusarium species were reported. The available information showed that Tri1 and Tri16 genes are the key determinants of the trichothecene profiles of T-2 and DAS genotypes. In this review, polymorphisms in the Tri1 and Tri16 genes in the two genotypes were investigated. Meanwhile, the functions of genes involved in DAS and NEO biosynthesis are discussed. The possible biosynthetic pathways of DAS and NEO are proposed in this review, which will facilitate the understanding of the synthesis process of trichothecenes in Fusarium strains and may also inspire researchers to design and conduct further research. Together, the review provides insight into trichothecene profile differentiation and Tri gene evolutionary processes responsible for the structural diversification of trichothecene produced by Fusarium.

Genetic and Transcriptional Regulatory Mechanisms of Lipase Activity in the Plant Pathogenic Fungus Fusarium graminearum

Lipases, which catalyze the hydrolysis of long-chain triglycerides, diglycerides, and monoglycerides into free fatty acids and glycerol, participate in various biological pathways in fungi. In this study, we examined the biological functions and regulatory mechanisms of fungal lipases via two approaches. First, we performed a systemic functional characterization of 86 putative lipase-encoding genes in the plant-pathogenic fungus Fusarium graminearum. The phenotypes were assayed for vegetative growth, asexual and sexual reproduction, stress responses, pathogenicity, mycotoxin production, and lipase activity. Most mutants were normal in the assessed phenotypes, implying overlapping roles for lipases in F. graminearum. In particular, FgLip1 and Fgl1 were revealed as core extracellular lipases in F. graminearum. Second, we examined the lipase activity of previously constructed transcription factor (TF) mutants of F. graminearum and identified three TFs and one histone acetyltransferase that significantly affect lipase activity. The relative transcript levels of FgLIP1 and FGL1 were markedly reduced or enhanced in these TF mutants. Among them, Gzzc258 was identified as a key lipase regulator that is also involved in the induction of lipase activity during sexual reproduction. To our knowledge, this study is the first comprehensive functional analysis of fungal lipases and provides significant insights into the genetic and regulatory mechanisms underlying lipases in fungi. IMPORTANCE Fusarium graminearum is an economically important plant-pathogenic fungus that causes Fusarium head blight (FHB) on wheat and barley. Here, we constructed a gene knockout mutant library of 86 putative lipase-encoding genes and established a comprehensive phenotypic database of the mutants. Among them, we found that FgLip1 and Fgl1 act as core extracellular lipases in this pathogen. Moreover, several putative transcription factors (TFs) that regulate the lipase activities in F. graminearum were identified. The disruption mutants of F. graminearum-lipase regulatory TFs all showed defects in sexual reproduction, which implies a strong relationship between sexual development and lipase activity in this fungus. These findings provide valuable insights into the genetic mechanisms regulating lipase activity as well as its importance to the developmental stages of this plant-pathogenic fungus.

Effect of Amino Acids on Fusarium oxysporum Growth and Pathogenicity Regulated by TORC1-Tap42 Gene and Related Interaction Protein Analysis

Free amino acids (AAs) formed in fermented meat products are important nitrogen sources for the survival and metabolism of contaminating fungi. These AAs are mainly regulated by the TORC1-Tap42 signaling pathway. Fusarium spp., a common contaminant of fermented products, is a potential threat to food safety. Therefore, there is an urgent need to clarify the effect of different AAs on Fusarium spp. growth and metabolism. This study investigated the effect of 18 AAs on Fusarium oxysporum (Fo17) growth, sporulation, T-2 toxin (T-2) synthesis and Tri5 expression through Tap42 gene regulation. Co-immunoprecipitation and Q Exactive LC-MS/MS methods were used to detect the interacting protein of Tap42 during specific AA treatment. Tap42 positively regulated L-His, L-Ile and L-Tyr absorption for Fo17 colony growth. Acidic (L-Asp, L-Glu) and sulfur-containing (L-Cys, L-Met) AAs significantly inhibited the Fo17 growth which was not regulated by Tap42. The L-Ile and L-Pro addition significantly activated the sporulation of ΔFoTap42. L-His and L-Ser inhibited the sporulation of ΔFoTap42. In T-2 synthesis, ΔFoTap42 was increased in GYM medium, but was markedly inhibited in L-Asp and L-Glu addition groups. Dose-response experiments showed that 10-70 mg/mL of neutral AA (L-Thr) and alkaline AA (L-His) significantly increased the T-2 production and Tri5 expression of Fo17, but Tri5 expression was not activated in ΔFoTap42. Inhibition of T-2 synthesis and Tri5 expression were observed in Fo17 following the addition of 30-70 mg/mL L-Asp. KEGG enrichment pathway analysis demonstrated that interacting proteins of Tap42 were from glycerophospholipid metabolism, pentose phosphate pathway, glyoxylate and dicarboxylate metabolism, glycolysis and gluconeogenesis, and were related to the MAPK and Hippo signaling pathways. This study enhanced our understanding of AA regulation in fermented foods and its effect on Fusarium growth and metabolism, and provided insight into potential ways to control fungal contamination in high-protein fermented foods.

Iturin A Strongly Inhibits the Growth and T-2 Toxin Synthesis of Fusarium oxysporum: A Morphological, Cellular, and Transcriptomics Study

Fusarium oxysporum (F. oxysporum) is a common contaminant of dried fish, and the T-2 synthesis by this organism in dried fish products poses a serious public health risk. In this study, we investigated the effects of iturin A, a cyclic lipopeptide produced by Bacillus subtilis, on the growth and synthesis of the T-2 toxin of F. oxysporum, and transcriptomics was conducted. Results showed that the inhibitory effect of iturin A on F. oxysporum was significantly enhanced with an increase in iturin A concentrations. More specifically, compared with the control group, all indexes in the iturin A treatment group with 50 μg/mL were decreased to 24.84 mm, 0.33 × 106 cfu/mL, and 5.86 ng/mL for the colony diameter, number of spores, and concentration of T-2 toxin, respectively. Furthermore, iturin A was proven to destroy the integrity of cell membranes and cause a significant increase in ROS at 25 μg/mL or 50 μg/mL. Transcriptomic analysis revealed that with the treatment of iturin A, the genes of the oxidation-reduction process were up-regulated, while the gene expression of mycelial growth, cell integrity, transmembrane transport, energy metabolism, and others were down-regulated. More importantly, the Tri5 gene cluster was significantly inhibited. This study provided new insights into the mechanism for the inhibitory effect of iturin A on the growth and T-2 toxin synthesis of F. oxysporum and theoretical guidance for the application of iturin A in the preservation of dried aquatic products.

Comparative Transcriptomics of Fusarium graminearum and Magnaporthe oryzae Spore Germination Leading up To Infection

For fungal plant pathogens, the germinating spore provides the first interaction with the host. Spore germlings move across the plant surface and use diverse penetration strategies for ingress into plant surfaces. Penetration strategies include pressurized melanized appressoria, which facilitate physically punching through the plant cuticle, and nonmelanized appressoria, which penetrate with the help of enzymes or cuticular damage to breach the plant surface. Two well-studied plant pathogens, Fusarium graminearum and Magnaporthe oryzae, are typical of these two modes of penetration. We applied comparative transcriptomics to Fusarium graminearum and Magnaporthe oryzae to characterize the genetic programming of the early host-pathogen interface. Four sequential stages of development following spore localization on the plant surface, from spore swelling to appressorium formation, were sampled for each species on culture medium and on barley sheaths, and transcriptomic analyses were performed. Gene expression in the prepenetration stages in both species and under both conditions was similar. In contrast, gene expression in the final stage was strongly influenced by the environment. Appressorium formation involved the greatest number of differentially expressed genes. Laser-dissection microscopy was used to perform detailed transcriptomics of initial infection points by F. graminearum. These analyses revealed new and important aspects of early fungal ingress in this species. Expression of the trichothecene genes involved in biosynthesis of deoxynivalenol by F. graminearum implies that toxisomes are not fully functional until after penetration and indicates that deoxynivalenol is not essential for penetration under our conditions. The use of comparative gene expression of divergent fungi promises to advance highly effective targets for antifungal strategies. IMPORTANCE Fusarium graminearum and Magnaporthe oryzae are two of the most important pathogens of cereal grains worldwide. Despite years of research, strong host resistance has not been identified for F. graminearum, so other methods of control are essential. The pathogen takes advantage of multiple entry points to infect the host, including breaches in the florets due to senescence of flower parts and penetration of the weakened trichome bases to breach the epidermis. In contrast, M. oryzae directly punctures leaves that it infects, and resistant cultivars have been characterized. The threat of either pathogen causing a major disease outbreak is ever present. Comparative transcriptomics demonstrated its potential to reveal novel and effective disease prevention strategies that affect the initial stages of disease. Shedding light on the basis of this diversity of infection strategies will result in development of increasingly specific control strategies.

Evaluation of Barley and Wheat Germplasm for Resistance to Head Blight and Mycotoxin Production by Fusarium asiaticum and F. graminearum

Fusarium head blight (FHB) is one of the most serious diseases in barley and wheat, as it is usually accompanied by the production of harmful mycotoxins in the grains. To identify FHB-resistant breeding resources, we evaluated 60 elite germplasm accessions of barley (24) and wheat (36) for FHB and mycotoxin accumulation. Assessments were performed in a greenhouse and five heads per accession were inoculated with both Fusarium asiaticum (Fa73, nivalenol producer) and F. graminearum (Fg39, deoxynivalenol producer) strains. While the accessions varied in disease severity and mycotoxin production, four wheat and one barley showed <20% FHB severity repeatedly by both strains. Mycotoxin levels in these accessions ranged up to 3.9 mg/kg. FHB severity was generally higher in barley than in wheat, and Fa73 was more aggressive in both crops than Fg39. Fg39 itself, however, was more aggressive toward wheat and produced more mycotoxin in wheat than in barley. FHB severity by Fa73 and Fg39 were moderately correlated in both crops (r = 0.57/0.60 in barley and 0.42/0.58 in wheat). FHB severity and toxin production were also correlated in both crops, with a stronger correlation for Fa73 (r = 0.42/0.82 in barley, 0.70 in wheat) than for Fg39.

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.

De novo genome assembly and functional annotation for Fusarium langsethiae

BACKGROUND

Fusarium langsethiae is a T-2 and HT-2 mycotoxins producing species firstly characterised in 2004. It is commonly isolated from oats in Northern Europe. T-2 and HT-2 mycotoxins exhibit immunological and haemotological effects in animal health mainly through inhibition of protein, RNA and DNA synthesis. The development of a high-quality and comprehensively annotated assembly for this species is therefore essential in providing the molecular understanding and the mechanism of T-2 and HT-2 biosynthesis in F. langsethiae to help develop effective control strategies.

RESULTS

The F. langsethiae assembly was produced using PacBio long reads, which were then assembled independently using Canu, SMARTdenovo and Flye. A total of 19,336 coding genes were identified using RNA-Seq informed ab-initio gene prediction. Finally, predicting genes were annotated using the basic local alignment search tool (BLAST) against the NCBI non-redundant (NR) genome database and protein hits were annotated using InterProScan. Genes with blast hits were functionally annotated with Gene Ontology.

CONCLUSIONS

We developed a high-quality genome assembly of a total length of 59 Mb and N50 of 3.51 Mb. Raw sequence reads and assembled genome is publicly available and can be downloaded from: GenBank under the accession JAFFKB000000000. All commands used to generate this assembly are accessible via GitHub: https://github.com/FadyMohareb/fusarium_langsethiae .

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.

Comparative Genomics of Eight Fusarium graminearum Strains with Contrasting Aggressiveness Reveals an Expanded Open Pangenome and Extended Effector Content Signatures

Fusarium graminearum, the primary cause of Fusarium head blight (FHB) in small-grain cereals, demonstrates remarkably variable levels of aggressiveness in its host, producing different infection dynamics and contrasted symptom severity. While the secreted proteins, including effectors, are thought to be one of the essential components of aggressiveness, our knowledge of the intra-species genomic diversity of F. graminearum is still limited. In this work, we sequenced eight European F. graminearum strains of contrasting aggressiveness to characterize their respective genome structure, their gene content and to delineate their specificities. By combining the available sequences of 12 other F. graminearum strains, we outlined a reference pangenome that expands the repertoire of the known genes in the reference PH-1 genome by 32%, including nearly 21,000 non-redundant sequences and gathering a common base of 9250 conserved core-genes. More than 1000 genes with high non-synonymous mutation rates may be under diverse selection, especially regarding the trichothecene biosynthesis gene cluster. About 900 secreted protein clusters (SPCs) have been described. Mostly localized in the fast sub-genome of F. graminearum supposed to evolve rapidly to promote adaptation and rapid responses to the host's infection, these SPCs gather a range of putative proteinaceous effectors systematically found in the core secretome, with the chloroplast and the plant nucleus as the main predicted targets in the host cell. This work describes new knowledge on the intra-species diversity in F. graminearum and emphasizes putative determinants of aggressiveness, providing a wealth of new candidate genes potentially involved in the Fusarium head blight disease.

Isolation, Identification, and Complete Genome Assembly of an Endophytic Bacillus velezensis YB-130, Potential Biocontrol Agent Against Fusarium graminearum

Wheat scab caused by F. graminearum is a highly destructive disease that leads to yield reduction and mycotoxin contamination of grains. In this study, an endophytic bacterium of strain YB-130 was isolated from surface sterilized wheat spikes with scab symptoms and identified as Bacillus velezensis by whole genome annotation, 16S rRNA gene and average nucleotide identities analysis. The whole-genome sequence of strain YB-130 was obtained by PacBio sequencing. 88 putative Carbohydrate-Active Enzymes and 12 gene clusters encoding for secondary metabolites were identified in the YB-130 genome, including one gene cluster for the synthesis of lanthipeptide only found in strain YB-130 genome. In dual cultures, strain YB-130 significantly inhibited the growth of F. graminearum PH-1 and other eight fungal plant pathogens, indicating a broad antifungal activity. Furthermore, strain YB-130 was able to significantly inhibit spore morphology and hyphal development of F. graminearum PH-1. Strain YB-130 also reduced deoxynivalenol production by F. graminearum PH-1 in dual cultures, possibly due to its ability to suppress the expression of tri5, tri3, and tri8 that are required for deoxynivalenol production in F. graminearum. Overall, B. velezensis YB-130 is a promising biological control agent of both F. graminearum infection and mycotoxin production.

Signatures of TRI5, TRI8 and TRI11 Protein Sequences of Fusarium incarnatum-equiseti Species Complex (FIESC) Indicate Differential Trichothecene Analogue Production

The variability and phylogeny among TRI5, TRI8 and TRI11 nucleotide and translated protein sequences of isolates from Trinidad belonging to Fusarium incarnatum-equiseti species complex (FIESC) were compared with FIESC reference sequences. Taxa appeared to be more divergent when DNA sequences were analyzed compared to protein sequences. Neutral and non-neutral mutations in TRI protein sequences that may correspond to variability in the function and structure of the selected TRI proteins were identified. TRI5p had the lowest amino acid diversity with zero predicted non-neutral mutations. TRI5p had potentially three protein disorder regions compared to TRI8p with five protein disorder regions. The deduced TRI11p was more conserved than TRI8p of the same strains. Amino acid substitutions that may be non-neutral to protein function were only detected in diacetoxyscirpenol (DAS) and fusarenon-X (FUS-X) producers of the reference sequence subset for TRI8p and TRI11p. The deduced TRI5 and TRI8 amino acid sequences were mapped to known 3D-structure models and indicated that variations in specific protein order/disorder regions exist in these sequences which affect the overall structural conservation of TRI proteins. Assigning single or combination non-neutral mutations to a particular toxicogenic phenotype may be more representative of potential compared to using genotypic data alone, especially in the absence of wet-lab, experimental validation.

Genetic bases for variation in structure and biological activity of trichothecene toxins produced by diverse fungi

Trichothecenes are sesquiterpene toxins produced by diverse but relatively few fungal species in at least three classes of Ascomycetes: Dothideomycetes, Eurotiomycetes, and Sordariomycetes. Approximately 200 structurally distinct trichothecene analogs have been described, but a given fungal species typically produces only a small subset of analogs. All trichothecenes share a core structure consisting of a four-ring nucleus known as 12,13-epoxytrichothec-9-ene. This structure can be substituted at various positions with hydroxyl, acyl, or keto groups to give rise to the diversity of trichothecene structures that has been described. Over the last 30 years, the genetic and biochemical pathways required for trichothecene biosynthesis in several species of the fungi Fusarium and Trichoderma have been elucidated. In addition, phylogenetic and functional analyses of trichothecene biosynthetic (TRI) genes from fungi in multiple genera have provided insights into how acquisition, loss, and changes in functions of TRI genes have given rise to the diversity of trichothecene structures. These analyses also suggest both divergence and convergence of TRI gene function during the evolutionary history of trichothecene biosynthesis. What has driven trichothecene structural diversification remains an unanswered question. However, insight into the role of trichothecenes in plant pathogenesis of Fusarium species and into plant glucosyltransferases that detoxify the toxins by glycosylating them point to a possible driver. Because the glucosyltransferases can have substrate specificity, changes in trichothecene structures produced by a fungus could allow it to evade detoxification by the plant enzymes. Thus, it is possible that advantages conferred by evading detoxification have contributed to trichothecene structural diversification. KEY POINTS : • TRI genes have evolved by diverse processes: loss, acquisition and changes in function. • Some TRI genes have acquired the same function by convergent evolution. • Some other TRI genes have evolved divergently to have different functions. • Some TRI genes were acquired or resulted from diversification in function of other genes. • Substrate specificity of plant glucosyltransferases could drive trichothecene diversity.

Pathogenicity and Virulence Factors of Fusarium graminearum Including Factors Discovered Using Next Generation Sequencing Technologies and Proteomics

Fusarium graminearum is a devasting mycotoxin-producing pathogen of grain crops. F. graminearum has been extensively studied to understand its pathogenicity and virulence factors. These studies gained momentum with the advent of next-generation sequencing (NGS) technologies and proteomics. NGS and proteomics have enabled the discovery of a multitude of pathogenicity and virulence factors of F. graminearum. This current review aimed to trace progress made in discovering F. graminearum pathogenicity and virulence factors in general, as well as pathogenicity and virulence factors discovered using NGS, and to some extent, using proteomics. We present more than 100 discovered pathogenicity or virulence factors and conclude that although a multitude of pathogenicity and virulence factors have already been discovered, more work needs to be done to take advantage of NGS and its companion applications of proteomics.

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.

Comparative Analysis of Deoxynivalenol Biosynthesis Related Gene Expression among Different Chemotypes of Fusarium graminearum in Spring Wheat

Fusarium mycotoxins, deoxynivalenol (DON) and nivalenol (NIV) act as virulence factors and are essential for symptom development after initial infection in wheat. To date, 16 genes have been identified in the DON biosynthesis pathway. However, a comparative gene expression analysis in different chemotypes of Fusarium graminearum in response to Fusarium head blight infection remains to be explored. Therefore, in this study, nine genes that involved in trichothecene biosynthesis were analyzed among 3-acetyldeoxynivalenol (3-ADON), 15-acetyldeoxynivalenol (15-ADON) and nivalenol producing F. graminearum strains in a time course study. Quantitative reverse transcription polymerase chain reaction revealed that the expression of all examined TRI gene transcripts initiated at 2 days post-inoculation (dpi), peaked at three to four dpi and gradually decreased at seven dpi. The early induction of TRI genes indicates that presence of high levels of TRI gene transcripts at early stages is important to initiate the biosynthetic pathway of DON and NIV. Comparison of gene expression among the three chemotypes showed that relative expression of TRI genes was higher in 3-ADON producing strains compared with 15-ADON and NIV strains. Comparatively higher levels of gene expression may contribute to the higher levels of DON produced by 3-ADON strains in infected grains.

Identification and Characterization of Carboxylesterases from Brachypodium distachyon Deacetylating Trichothecene Mycotoxins

Increasing frequencies of 3-acetyl-deoxynivalenol (3-ADON)-producing strains of Fusarium graminearum (3-ADON chemotype) have been reported in North America and Asia. 3-ADON is nearly nontoxic at the level of the ribosomal target and has to be deacetylated to cause inhibition of protein biosynthesis. Plant cells can efficiently remove the acetyl groups of 3-ADON, but the underlying genes are yet unknown. We therefore performed a study of the family of candidate carboxylesterases (CXE) genes of the monocot model plant Brachypodium distachyon. We report the identification and characterization of the first plant enzymes responsible for deacetylation of trichothecene toxins. The product of the BdCXE29 gene efficiently deacetylates T-2 toxin to HT-2 toxin, NX-2 to NX-3, both 3-ADON and 15-acetyl-deoxynivalenol (15-ADON) into deoxynivalenol and, to a lesser degree, also fusarenon X into nivalenol. The BdCXE52 esterase showed lower activity than BdCXE29 when expressed in yeast and accepts 3-ADON, NX-2, 15-ADON and, to a limited extent, fusarenon X as substrates. Expression of these Brachypodium genes in yeast increases the toxicity of 3-ADON, suggesting that highly similar genes existing in crop plants may act as susceptibility factors in Fusarium head blight disease.

Variation in type A trichothecene production and trichothecene biosynthetic genes in Fusarium goolgardi from natural ecosystems of Australia

Fusarium goolgardi, isolated from the grass tree Xanthorrhoea glauca in natural ecosystems of Australia, is closely related to fusaria that produce a subgroup of trichothecene (type A) mycotoxins that lack a carbonyl group at carbon atom 8 (C-8). Mass spectrometric analysis revealed that F. goolgardi isolates produce type A trichothecenes, but exhibited one of two chemotypes. Some isolates (50%) produced multiple type A trichothecenes, including 4,15-diacetoxyscirpenol (DAS), neosolaniol (NEO), 8-acetylneosolaniol (Ac-NEO) and T-2 toxin (DAS-NEO-T2 chemotype). Other isolates (50%) produced only DAS (DAS chemotype). In the phylogenies inferred from DNA sequences of genes encoding the RNA polymerase II largest (RPB1) and second largest (RPB2) subunits as well as the trichothecene biosynthetic genes (TRI), F. goolgardi isolates were resolved as a monophyletic clade, distinct from other type A trichothecene-producing species. However, the relationships of F. goolgardi to the other species varied depending on whether phylogenies were inferred from RPB1 and RPB2, the 12-gene TRI cluster, the two-gene TRI1-TRI16 locus, or the single-gene TRI101 locus. Phylogenies based on different TRI loci resolved isolates with different chemotypes into distinct clades, even though only the TRI1-TRI16 locus is responsible for structural variation at C-8. Sequence analysis indicated that TRI1 and TRI16 are functional in F. goolgardi isolates with the DAS-NEO-T2 chemotype, but non-functional in isolates with DAS chemotype due to the presence of premature stop codons caused by a point mutation.

Comparative genome sequencing reveals chemotype-specific gene clusters in the toxigenic black mold Stachybotrys

BACKGROUND

The fungal genus Stachybotrys produces several diverse toxins that affect human health. Its strains comprise two mutually-exclusive toxin chemotypes, one producing satratoxins, which are a subclass of trichothecenes, and the other producing the less-toxic atranones. To determine the genetic basis for chemotype-specific differences in toxin production, the genomes of four Stachybotrys strains were sequenced and assembled de novo. Two of these strains produce atranones and two produce satratoxins.

RESULTS

Comparative analysis of these four 35-Mbp genomes revealed several chemotype-specific gene clusters that are predicted to make secondary metabolites. The largest, which was named the core atranone cluster, encodes 14 proteins that may suffice to produce all observed atranone compounds via reactions that include an unusual Baeyer-Villiger oxidation. Satratoxins are suggested to be made by products of multiple gene clusters that encode 21 proteins in all, including polyketide synthases, acetyltransferases, and other enzymes expected to modify the trichothecene skeleton. One such satratoxin chemotype-specific cluster is adjacent to the core trichothecene cluster, which has diverged from those of other trichothecene producers to contain a unique polyketide synthase.

CONCLUSIONS

The results suggest that chemotype-specific gene clusters are likely the genetic basis for the mutually-exclusive toxin chemotypes of Stachybotrys. A unified biochemical model for Stachybotrys toxin production is presented. Overall, the four genomes described here will be useful for ongoing studies of this mold's diverse toxicity mechanisms.

Upstream regulation of mycotoxin biosynthesis

Mycotoxins are natural contaminants of food and feed products, posing a substantial health risk to humans and animals throughout the world. A plethora of filamentous fungi has been identified as mycotoxin producers and most of these fungal species belong to the genera Aspergillus, Fusarium, and Penicillium. A number of studies have been conducted to better understand the molecular mechanisms of biosynthesis of key mycotoxins and the regulatory cascades controlling toxigenesis. In many cases, the mycotoxin biosynthetic genes are clustered and regulated by one or more pathway-specific transcription factor(s). In addition, as biosynthesis of many secondary metabolites is coordinated with fungal growth and development, there are a number of upstream regulators affecting biosynthesis of mycotoxins in fungi. This review presents a concise summary of the regulation of mycotoxin biosynthesis, focusing on the roles of the upstream regulatory elements governing biosynthesis of aflatoxin and sterigmatocystin in Aspergillus.

Molecular and Genetic Studies ofFusariumTrichothecene Biosynthesis: Pathways, Genes, and Evolution

Trichothecenes are a large family of sesquiterpenoid secondary metabolites of Fusarium species (e.g., F. graminearum) and other molds. They are major mycotoxins that can cause serious problems when consumed via contaminated cereal grains. In the past 20 years, an outline of the trichothecene biosynthetic pathway has been established based on the results of precursor feeding experiments and blocked mutant analyses. Following the isolation of the pathway gene Tri5 encoding the first committed enzyme trichodiene synthase, 10 biosynthesis genes (Tri genes; two regulatory genes, seven pathway genes, and one transporter gene) were functionally identified in the Tri5 gene cluster. At least three pathway genes, Tri101 (separated alone), and Tri1 and Tri16 (located in the Tri1-Tri16 two-gene cluster), were found outside of the Tri5 gene cluster. In this review, we summarize the current understanding of the pathways of biosynthesis, the functions of cloned Tri genes, and the evolution of Tri genes, focusing on Fusarium species.

Type II myosin gene in Fusarium graminearum is required for septation, development, mycotoxin biosynthesis and pathogenicity

Type II myosin is required for cytokinesis/septation in yeast and filamentous fungi, including Fusarium graminearum, a prevalent cause of Fusarium head blight in China. A type II myosin gene from the Chinese F. graminearum strain 5035, isolated from infected wheat spikes, was identified by screening a mutant library generated by restriction enzyme-mediated integration. Disruption of the Myo2 gene reduced mycelial growth by 50% and conidiation by 76-fold, and abolished sexual reproduction on wheat kernels. The Δmyo2 mutants also had a 97% decrease in their pathogenicity on wheat, and mycotoxin production fell to just 3.4% of the normal level. The distribution of nuclei and septa was abnormal in the mutants, and the septal ultrastructure appeared disorganized. Time-lapse imaging of septation provided direct evidence that Myo2 is required for septum initiation and formation, and revealed the dynamic behavior of GFP-tagged Myo2 during hyphal and macroconidia development, particularly in the delimiting septum of phialides and macroconidial spores. Microarray analysis identified many genes with altered expression profiles in the Δmyo2 mutant, indicating that Myo2 is required for several F. graminearum developmental processes and biological activities.

Evaluation of rapid test kits for quantification of HT-2 and T-2 toxins in naturally contaminated oats

The aim of this study was to evaluate the performance and usefulness of three rapid test kits for analysis of HT-2 and T-2 toxins (HT-2 and T-2), two of the most potent trichothecenes commonly found in European oats. Concentrations of these two toxins combined (HT-2+T-2) were analysed in naturally contaminated oat samples (n=68) using the following test kits: Ridascreen® FAST T-2 Toxin (‘Fast ELISA’), DRAFT Ridascreen® HT-2/T-2 (‘Standard ELISA’, not commercially available), and the lateral flow device ROSA® HT-2-T-2 (‘Rosa LFD’). Mycotoxin analysis by LC-MS/MS was used as a reference method. Rosa LFD offered the best reliability, achieving detection that was stable across toxin levels, whereas detection by both ELISA kits differed significantly among toxin levels (P<0.01). The kits were also evaluated regarding agreement with the reference method (measured as Cohen's kappa) at a HT-2+T-2 concentration of 1000 μg/kg in naturally contaminated oats. Agreement was greatest for Rosa LFD (89.2%), intermediate for Standard ELISA (66.8%), and lowest for Fast ELISA (62.2%). Rosa LFD showed cross-reaction of 100% with both T-2 and HT-2. For the ELISA kits, cross-reactions were 100% with T-2 but below 100% with HT-2. Therefore, to estimate the sum of HT-2 and T-2 in an oat sample, it was necessary to re-calculate the data from both ELISA kits according to the known cross-reaction of each kit with HT-2 and the concentration ratio of HT-2 to T-2 in Norwegian oats. Rosa LFD had the highest correlation with LC-MS/MS (R2=0.94), and the corresponding R2 values for Fast and Standard ELISA were 0.61 and 0.83, respectively. Rosa LFD was well suited for on-site detection. Standard ELISA allows simultaneous testing of several samples that are useful for centralised laboratories.

Microbial cytochromes P450: biodiversity and biotechnology. Where do cytochromes P450 come from, what do they do and what can they do for us?

The first eukaryote genome revealed three yeast cytochromes P450 (CYPs), hence the subsequent realization that some microbial fungal genomes encode these proteins in 1 per cent or more of all genes (greater than 100) has been surprising. They are unique biocatalysts undertaking a wide array of stereo- and regio-specific reactions and so hold promise in many applications. Based on ancestral activities that included 14α-demethylation during sterol biosynthesis, it is now seen that CYPs are part of the genes and metabolism of most eukaryotes. In contrast, Archaea and Eubacteria often do not contain CYPs, while those that do are frequently interesting as producers of natural products undertaking their oxidative tailoring. Apart from roles in primary and secondary metabolism, microbial CYPs are actual/potential targets of drugs/agrochemicals and CYP51 in sterol biosynthesis is exhibiting evolution to resistance in the clinic and the field. Other CYP applications include the first industrial biotransformation for corticosteroid production in the 1950s, the diversion into penicillin synthesis in early mutations in fungal strain improvement and bioremediation using bacteria and fungi. The vast untapped resource of orphan CYPs in numerous genomes is being probed and new methods for discovering function and for discovering desired activities are being investigated.

Antifungal activity of phenyl derivative of pyranocoumarin from Psoralea corylifolia L. seeds by inhibition of acetylation activity of trichothecene 3-o-acetyltransferase (Tri101)

Antifungal activity of petroleum ether extract of Psoralea corylifolia L. seed, tested against Fusarium sp. namely, Fusarium oxysporum, Fusarium moniliforme, and Fusarium graminearum, was evaluated by agar well diffusion assay. The chromatographic fractionation of the extract yielded a new phenyl derivative of pyranocoumarin (PDP). The structure of the PDP was confirmed using spectroscopic characterization (GC-MS, IR, and NMR), and a molecular mass of m/z 414 [M-2H](+) with molecular formula C(27)H(28)O(4) was obtained. The PDP had a potent antifungal activity with a minimum inhibitory concentration of 1 mg/mL against Fusarium sp. Molecular docking using Grid-Based Ligand Docking with Energetics (GLIDE, Schrodinger) was carried out with the Tri101, trichothecene 3-O-acetyltransferase, as target protein to propose a mechanism for the antifungal activity. The ligand PDP showed bifurcated hydrogen bond interaction with active site residues at TYR 413 and a single hydrogen bond interaction at ARG 402 with a docking score -7.19 and glide energy of -45.78 kcal/mol. This indicated a strong binding of the ligand with the trichothecene 3-O-acetyltransferase, preventing as a result the acetylation of the trichothecene mycotoxin and destruction of the "self-defense mechanism" of the Fusarium sp.

The predicted secretome of the plant pathogenic fungus Fusarium graminearum: a refined comparative analysis

The fungus Fusarium graminearum forms an intimate association with the host species wheat whilst infecting the floral tissues at anthesis. During the prolonged latent period of infection, extracellular communication between live pathogen and host cells must occur, implying a role for secreted fungal proteins. The wheat cells in contact with fungal hyphae subsequently die and intracellular hyphal colonisation results in the development of visible disease symptoms. Since the original genome annotation analysis was done in 2007, which predicted the secretome using TargetP, the F. graminearum gene call has changed considerably through the combined efforts of the BROAD and MIPS institutes. As a result of the modifications to the genome and the recent findings that suggested a role for secreted proteins in virulence, the F. graminearum secretome was revisited. In the current study, a refined F. graminearum secretome was predicted by combining several bioinformatic approaches. This strategy increased the probability of identifying truly secreted proteins. A secretome of 574 proteins was predicted of which 99% was supported by transcriptional evidence. The function of the annotated and unannotated secreted proteins was explored. The potential role(s) of the annotated proteins including, putative enzymes, phytotoxins and antifungals are discussed. Characterisation of the unannotated proteins included the analysis of Pfam domains and features associated with known fungal effectors, for example, small size, cysteine-rich and containing internal amino acid repeats. A comprehensive comparative genomic analysis involving 57 fungal and oomycete genomes revealed that only a small number of the predicted F. graminearum secreted proteins can be considered to be either species or sequenced strain specific.

Trichothecenes: from simple to complex mycotoxins

As the world's population grows, access to a safe food supply will continue to be a global priority. In recent years, the world has experienced an increase in mycotoxin contamination of grains due to climatic and agronomic changes that encourage fungal growth during cultivation. A number of the molds that are plant pathogens produce trichothecene mycotoxins, which are known to cause serious human and animal toxicoses. This review covers the types of trichothecenes, their complexity, and proposed biosynthetic pathways of trichothecenes.

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.

Accumulation kinetics of three scirpentriol-based toxins in oats inoculated in Vitro with isolates of Fusarium sporotrichioides and Fusarium poae

Autoclaved oats were inoculated with a strain of Fusarium sporotrichioides or Fusarium poae. Moisture content of oats after inoculation was at 38%, incubation took place in standing culture at 28 °C. The A-type trichothecenes, 4,15-diacetoxyscirpenol (4,15-DAS), 15-monoacetoxyscirpenol (15-MAS), and scirpentriol (SCIRP) were analyzed by GC/MS. For each strain, three culture flasks were harvested at 2-3 day intervals starting immediately after inoculation. Total incubation time was 42 days (F. poae) and 56 days (F. sporotrichioides). Following peak accumulation, 4,15-DAS decreased below the detection limit for both strains, 15-MAS decreased below this limit for the isolate of F. sporotrichioides, for the isolate of F. poae it decreased to a level markedly below the peak value. SCIRP, after having peaked, decreased to some extent for the strain F. sporotrichioides, with a significant (P = 0.0029) negative linear regression of toxin content against culture age during this period. The content of 15-MAS, and in part also of 4,15-DAS, decreased along with an increase of SCIRP. This sequential accumulation pattern suggests the successive induction of esterases deacetylating 4,15-DAS and 15-MAS, as well as of enzymes involved in the metabolization of the parent alcohol, SCIRP. The results may explain, at least in part, the somewhat higher incidence in naturally contaminated compounds reported in the literature for SCIRP compared to 4,15-DAS and 15-MAS.

Fusarium sibiricum sp. nov, a novel type A trichothecene-producing Fusarium from northern Asia closely related to F. sporotrichioides and F. langsethiae

Production of type A trichothecenes has been reported in the closely related species Fusarium langsethiae and F. sporotrichioides. Here, we characterized a collection of Fusarium isolates from Siberia and the Russian Far East (hereafter Asian isolates) that produce high levels of the type A trichothecene T-2 toxin and are similar in morphology to the type A trichothecene-producing F. langsethiae, and to F. poae which often produces the type B trichothecene nivalenol. The Asian isolates possess unique macroscopic and microscopic characters and have a unique TG repeat in the nuclear ribosomal intergenic spacer (IGS rDNA) region. In Asian isolates, the TRI1-TRI16 locus, which determines type A versus type B trichothecene production in other species, is more similar in organization and sequence to the TRI1-TRI16 locus in F. sporotrichioides and F. langsethiae than to that in F. poae. Phylogenetic analysis of the TRI1 and TRI16 gene coding regions indicates that the genes in the Asian isolates are more closely related to those of F. sporotrichioides than F. langsethiae. Phylogenetic analysis of the beta-tubulin, translation elongation factor, RNA polymerase II and phosphate permease gene sequences resolved the Asian isolates into a well-supported sister lineage to F. sporotrichioides, with F. langsethiae forming a sister lineage to F. sporotrichioides and the Asian isolates. The Asian isolates are conspecific with Norwegian isolate IBT 9959 based on morphological and molecular analyses. In addition, the European F. langsethiae isolates from Finland and Russia were resolved into two distinct subgroups based on analyses of translation elongation factor and IGS rDNA sequences. Nucleotide polymorphisms within the IGS rDNA were used to design PCR primers that successfully differentiated the Asian isolates from F. sporotrichioides and F. langsethiae. Based on these data, we formally propose that the Asian isolates together with Norwegian isolate IBT 9959 comprise a novel phylogenetic species, F. sibiricum, while the two subgroups of F. langsethiae only represent intraspecific groups.

The genetic basis for 3-ADON and 15-ADON trichothecene chemotypes in Fusarium

Certain Fusarium species cause head blight of wheat and other small grains worldwide and produce trichothecene mycotoxins. These mycotoxins can induce toxicoses in animals and humans and can contribute to the ability of some fusaria to cause plant disease. Production of the trichothecene 3-acetyldeoxynivalenol (3-ADON) versus 15-acetyldeoxynivalenol (15-ADON) is an important phenotypic difference within and among some Fusarium species. However, until now, the genetic basis for this difference in chemotype has not been identified. Here, we identified consistent DNA sequence differences in the coding region of the trichothecene biosynthetic gene TRI8 in 3-ADON and 15-ADON strains. Functional analyses of the TRI8 enzyme (Tri8) in F. graminearum, the predominant cause of wheat head blight in North America and Europe, revealed that Tri8 from 3-ADON strains catalyzes deacetylation of the trichothecene biosynthetic intermediate 3,15-diacetyldeoxynivalenol at carbon 15 to yield 3-ADON, whereas Tri8 from 15-ADON strains catalyzes deacetylation of 3,15-diacetyldeoxynivalenol at carbon 3 to yield 15-ADON. Fusarium strains that produce the trichothecene nivalenol have a Tri8 that functions like that in 15-ADON strains. TRI3, which encodes a trichothecene carbon 15 acetyltransferase, was found to be functional in all three chemotypes. Together, our data indicate that differential activity of Tri8 determines the 3-ADON and 15-ADON chemotypes in Fusarium.

CLM1 of Fusarium graminearum encodes a longiborneol synthase required for culmorin production

Fusarium graminearum is a fungal pathogen of cereal crops (e.g., wheat, barley, maize) and produces a number of mycotoxins, including 15-acetyldeoxynivalenol, butenolide, zearalenone, and culmorin. To identify a biosynthetic gene for the culmorin pathway, an expressed-sequence-tag database was examined for terpene cyclase genes. A gene designated CLM1 was expressed under trichothecene-inducing conditions. Expression of CLM1 in yeast (Saccharomyces cerevisiae) resulted in the production of a sesquiterpene alcohol, longiborneol, which has the same ring structure as culmorin. Gene disruption and add-back experiments in F. graminearum showed that CLM1 was required for culmorin biosynthesis. CLM1 gene disruptants were able to convert exogenously added longiborneol to culmorin. Longiborneol accumulated transiently in culmorin-producing strains. The results indicate that CLM1 encodes a longiborneol synthase and is required for culmorin biosynthesis in F. graminearum.

Structural and functional characterization of TRI3 trichothecene 15-O-acetyltransferase from Fusarium sporotrichioides

Fusarium head blight is a devastating disease of cereal crops whose worldwide incidence is increasing and at present there is no satisfactory way of combating this pathogen or its associated toxins. There is a wide variety of trichothecene mycotoxins and they all contain a 12,13-epoxytrichothecene skeleton but differ in their substitutions. Indeed, there is considerable variation in the toxin profile across the numerous Fusarium species that has been ascribed to differences in the presence or absence of biosynthetic enzymes and their relative activity. This article addresses the source of differences in acetylation at the C15 position of the trichothecene molecule. Here, we present the in vitro structural and biochemical characterization of TRI3, a 15-O-trichothecene acetyltransferase isolated from F. sporotrichioides and the "in vivo" characterization of Deltatri3 mutants of deoxynivalenol (DON) producing F. graminearum strains. A kinetic analysis shows that TRI3 is an efficient enzyme with the native substrate, 15-decalonectrin, but is inactive with either DON or nivalenol. The structure of TRI3 complexed with 15-decalonectrin provides an explanation for this specificity and shows that Tri3 and Tri101 (3-O-trichothecene acetyltransferase) are evolutionarily related. The active site residues are conserved across all sequences for TRI3 orthologs, suggesting that differences in acetylation at C15 are not due to differences in Tri3. The tri3 deletion mutant shows that acetylation at C15 is required for DON biosynthesis even though DON lacks a C15 acetyl group. The enzyme(s) responsible for deacetylation at the 15 position of the trichothecene mycotoxins have not been identified.

Trichothecenes in cereal grains

Trichothecenes are sesquiterpenoid mycotoxins associated with fusarium head blight (FHB) of cereals, with worldwide economic and health impacts. While various management strategies have been proposed to reduce the mycotoxin risk, breeding towards FHB-resistance appears to be the most effective means to manage the disease, and reduce trichothecene contamination of cereal-based food products. This review provides a brief summary of the trichothecene synthesis in Fusarium species, their toxicity in plants and humans, followed by the current methods of screening and breeding for resistance to FHB and trichothecene accumulation.

The TRI101 story: engineering wheat and barley to resist Fusarium head blight

Fusarium head blight (FHB), caused primarily by Fusarium graminearum, is a major disease of wheat and barley in the United States and Canada. FHB epidemics have been on the increase since 1993 and have caused severe monetary damage for the growers and seed industry. Along with reduced yields, the presence of mycotoxins in moldy grain constitutes a major problem for the grain industry. These mycotoxins pose health hazards to humans and animals upon ingestion. The acute phytotoxicity of these mycotoxins and their occurrence in plant tissues correlates with their role in pathogenesis and the production of plant disease. Transgenic plants incorporating the Fusarium sporotrichioides Tri101 gene, a gene that reduces toxicity of trichothecenes, have reduced levels of disease, thus demonstrating that FHB severity and deoxynivalenol (DON) accumulation can be reduced in small grains by the introduction of a toxin-modification gene.

Structural and functional characterization of the TRI101 trichothecene 3-O-acetyltransferase from Fusarium sporotrichioides and Fusarium graminearum: kinetic insights to combating Fusarium head blight

Fusarium head blight (FHB) is a plant disease with serious economic and health impacts. It is caused by fungal species belonging to the genus Fusarium and the mycotoxins they produce. Although it has proved difficult to combat this disease, one strategy that has been examined is the introduction of an indigenous fungal protective gene into cereals such as wheat barley and rice. Thus far the gene of choice has been tri101 whose gene product catalyzes the transfer of an acetyl group from acetyl coenzyme A to the C3 hydroxyl moiety of several trichothecene mycotoxins. In vitro this has been shown to reduce the toxicity of the toxins by approximately 100-fold but has demonstrated limited resistance to FHB in transgenic cereal. To understand the molecular basis for the differences between in vitro and in vivo resistance the three-dimensional structures and kinetic properties of two TRI101 orthologs isolated from Fusarium sporotrichioides and Fusarium graminearum have been determined. The kinetic results reveal important differences in activity of these enzymes toward B-type trichothecenes such as deoxynivalenol. These differences in activity can be explained in part by the three-dimensional structures for the ternary complexes for both of these enzymes with coenzyme A and trichothecene mycotoxins. The structural and kinetic results together emphasize that the choice of an enzymatic resistance gene in transgenic crop protection strategies must take into account the kinetic profile of the selected protein.

Structure-activity relationships of trichothecene toxins in an Arabidopsis thaliana leaf assay

Many Fusarium species produce trichothecenes, sesquiterpene epoxides that differ in patterns of oxygenation and esterification at carbon positions C-3, C-4, C-7, C-8, and C-15. For the first comprehensive and quantitative comparison of the effects of oxygenation and esterification on trichothecene phytotoxicity, we tested 24 precursors, intermediates, and end products of the trichothecene biosynthetic pathway in an Arabidopsis thaliana detached leaf assay. At 100 microM, the highest concentration tested, only the trichothecene precursor trichodiene was nontoxic. Among trichothecenes, toxicity varied more than 200-fold. Oxygenation at C-4, C-8, C-7/8, or C-15 was, on average, as likely to decrease as to increase toxicity. Esterification at C-4, C-8, or C-15 generally increased toxicity. Esterification at C-3 increased toxicity in one case and decreased toxicity in three of eight cases tested. Thus, the increase in structural complexity along the trichothecene biosynthetic pathway in Fusarium is not necessarily associated with an increase in phytotoxicity.

Simultaneous determination of aflatoxins, ochratoxin A and Fusarium toxins in maize by liquid chromatography/tandem mass spectrometry after multitoxin immunoaffinity cleanup

A liquid chromatography/tandem mass spectrometry method was developed for the simultaneous determination of aflatoxins (B(1), B(2), G(1), G(2)), ochratoxin A, fumonisins (B(1), B(2)), deoxynivalenol, zearalenone, T-2 and HT-2 toxins in maize. A double extraction approach, using a phosphate-buffered solution followed by methanol, was applied to achieve effective co-extraction of the 11 mycotoxins under investigation having quite different polarities and chemical structures. A new multitoxin immunoaffinity column containing antibodies for all these mycotoxins was used to clean up the extract. Detection and quantification of the 11 mycotoxins were performed by reversed-phase liquid chromatography coupled with electrospray ionization triple quadrupole mass spectrometry (LC/ESI-MS/MS) using, as chromatographic mobile phase, a linear gradient of methanol/water containing 0.5% acetic acid and 1 mM ammonium acetate. Method performances were quite satisfactory for all tested mycotoxins at contamination levels close to or below the relevant EU maximum permitted or recommended levels. Limits of detection in maize ranged from 0.3 to 4.2 microg/kg. Recoveries higher than 79% were obtained for all tested mycotoxins with relative standard deviations less than 13%.

Fusarium Tri4 encodes a multifunctional oxygenase required for trichothecene biosynthesis

Fusarium graminearum and Fusarium sporotrichioides produce the trichothecene mycotoxins 15-acetyldeoxynivalenol and T-2 toxin, respectively. In both species, disruption of the P450 monooxygenase-encoding gene, Tri4, blocks production of the mycotoxins and leads to the accumulation of the trichothecene precursor trichodiene. To further characterize its function, the F. graminearum Tri4 (FgTri4) was heterologously expressed in the trichothecene-nonproducing species Fusarium verticillioides. Transgenic F. verticillioides carrying the FgTri4 converted exogenous trichodiene to the trichothecene biosynthetic intermediates isotrichodermin and trichothecene. Conversion of trichodiene to isotrichodermin requires seven biochemical steps. The fifth and sixth steps can occur nonenzymatically. Precursor feeding studies done in the current study indicate that wild-type F. verticillioides has the enzymatic activity necessary to carry out the seventh step, the C-3 acetylation of isotrichodermol to form isotrichodermin. Together, the results of this study indicate that the Tri4 protein catalyzes the remaining four steps and is therefore a multifunctional monooxygenase required for trichothecene biosynthesis.