Resistance in wheat to Fusarium infection and trichothecene formation

The state of the art in Fusarium head blight resistance research is reviewed with reference to breeding for genetic resistance to Fusarium in wheat in practice. Fusarium graminearum and F. culmorum produce the trichothecene mycotoxin deoxynivalenol (DON). DON has phytotoxic properties and is an important aggressiveness factor in head blight. Head blight resistance in wheat is not specific for either F. graminearum or F. culmorum. Resistance components include resistance to penetration, resistance to colonization and mechanisms that influence kernel DON content. The resistance to Fusarium in wheat is a quantitative trait with relative high heritability and controlled by a few genes with major effect. A major quantitative trait locus (QTL) for head blight resistance from the Chinese variety Sumai 3 has been identified and verified by several research groups via molecular marker analysis. Research is now directed at identifying additional QTLs to make accumulation of resistance genes in elite wheat lines possible. The policy of official variety list trials may affect the head blight resistant level of future wheat varieties by excluding candidate varieties that are a too susceptible to Fusarium. A higher level of Fusarium head blight will guarantee lower risks for the farmer of crop loss due to reduced grain yield, low quality and mycotoxin contamination.

Workshop on trichothecenes with a focus on DON: summary report

A number of mycotoxins of the class of trichothecenes are produced by a variety of Fusarium fungi commonly found on cereals. Unfavourable weather conditions may lead to a high level of Fusarium infections in crops such as wheat and correspondingly high trichothecene contents. The ILSI Europe Natural Toxin Task Force therefore organised a workshop on trichothecenes with a special focus on deoxynivalenol (DON). A number of experts reviewed the current knowledge on trichothecenes with respect to occurrence, including aspects of mould growth, toxin formation, storage and effects of processing; prevention; analytical methodologies, including sampling; surveillance and exposure assessments; and toxicology and risk assessment. A number of recommendations were given under the headings: prevention, sampling and analytical methods, exposure assessment, and toxicology. Gaps in knowledge were also identified.

Influence of environmental factors on the biosynthesis of type B trichothecenes by isolates of Fusarium spp. from Spanish crops

Various species of Fusarium can produce trichothecene mycotoxins that contaminate food commodities and can represent a risk for human and animal health. In this paper, a full factorial design was applied to study the influence of incubation temperature, water activity (a(w)) and type of isolate on the production of deoxynivalenol (DON), nivalenol (NIV) and 3-acetyldeoxynivalenol (3-AcDON) in corn kernel cultures by three isolates of Fusarium graminearum and three isolates of Fusarium culmorum from crops grown in Spain. The tested temperatures were 15, 20, 28 and 32 degrees C. The a(w)-values were 0.960, 0.970 and 0.980. Moisture of cultures (within the studied range) did not affect significantly production of trichothecenes; however, the temperature affected significantly mycotoxin production and the optimal values were 28, 20 and 15 degrees C for DON, NIV and 3-AcDON, respectively. Four additional isolates of F. graminearum and two additional isolates of F. culmorum were examined for production of these mycotoxins at the optimal temperatures. Of the seven isolates of F. graminearum, four produced DON (0.88-3.97 microg/g), seven produced NIV (1.53-124 microg/g), and three produced 3-AcDON (0.65-10.6 microg/g). Of the five isolates of F. culmorum, four produced DON (1.20-4.93 microg/g), four produced NIV (6.94-701 microg/g), and four produced 3-AcDON (0.83-7.70 microg/g). Practically all isolates seem to belong to the NIV-chemotype. This is the first study done with regard to interaction between strain and ecological variables on type B trichothecene production by isolates of these two species from crops grown in Spain.

Tri1 in Fusarium graminearum encodes a P450 oxygenase

Gibberella zeae (asexual state Fusarium graminearum) is a major causal agent of wheat head blight and maize ear rot in North America and is responsible for contamination of grain with deoxynivalenol and related trichothecene mycotoxins. To identify additional trichothecene biosynthetic genes, cDNA libraries were prepared from fungal cultures under trichothecene-inducing conditions in culture and in planta. A gene designated LH1 that was highly expressed under these conditions exhibited only moderate (59%) similarity to known trichothecene biosynthetic cytochrome P450s. To determine the function of LH1, gene disruptants were produced and assessed for trichothecene production. Gene disruptants no longer produced 15-acetyldeoxynivalenol, which is oxygenated at carbon 7 (C-7) and C-8, but rather accumulated calonectrin and 3-deacetylcalonectrin, which are not oxygenated at either C-7 or C-8. These results indicate that gene LH1 encodes a cytochrome P450 responsible for oxygenation at one or both of these positions. Despite the relatively low level of DNA and amino acid sequence similarity between the two genes, LH1 from G. zeae is the probable homologue of Tri1, which encodes a cytochrome P450 required for C-8 oxygenation in F. sporotrichioides.

Diversity in metabolite production by Fusarium langsethiae, Fusarium poae, and Fusarium sporotrichioides

The production of mycotoxins and other metabolites by 109 strains of Fusarium langsethiae, Fusarium poae, Fusarium sporotrichioides, and F. kyushuense was investigated independently in four laboratories by liquid or gas chromatography analyses of cultural extracts with UV diode array, electron capture, or mass spectrometric detection systems. From the compiled results, it was found that F. langsethiae consistently produced the trichothecenes diacetoxyscirpenol (DAS), T-2 toxin (T-2), HT-2 toxin (HT-2), and neosolaniol (NEO) and, to a lesser extent, some additional trichothecene derivatives. F. langsethiae also produced culmorins, chrysogine (CHRYS), aurofusarin (AUF), and enniatin (EN). F. sporotrichioides showed a metabolite profile similar to that of F. langsethiae, while F. poae had a different profile as 41 of 49 strains produced nivalenol (NIV) and other 8-keto trichothecenes, in addition to DAS and derivatives of this metabolite. Only a trace amount of NIV was detected from one strain of F. kyushuense. In summary, all the three core taxa of this joint study were found to produce trichothecenes. Fusarin C (F-C) was not detected from F. langsethiae, but it was produced by F. poae and F. sporotrichioides. Aurofusarin was only detected from a few strains of F. langsethiae, while nearly all strains of F. poae and F. sporotrichioides produced this compound. In contrast, chrysogine was not detected from F. poae, but was produced by the other two taxa. Production of enniatins was scattered among the three main taxa of this study, whereas beauvericin (BEA) was produced by many strains of F. poae and F. sporotrichioides. Only one odd strain of F. langsethiae (IBT 9959) produced beauvericin. However, the status of this strain is uncertain. By a polyphasic approach using species-specific metabolite profiles, the fruity odour of F. poae, and morphological observations, it was concluded that F. langsethiae, F. poae, and F. sporotrichioides should be regarded as three significant taxa at a species level.

Functional demarcation of the Fusarium core trichothecene gene cluster

Many Fusarium species produce toxic sesquiterpenoids known as trichothecenes, including deoxynivalenol and nivalenol by Fusarium graminearum and T-2 toxin by Fusarium sporotrichioides. These toxins are potent inhibitors of protein synthesis and are a significant agricultural problem due to their adverse affect on human, animal, and plant health. Previously, 10-12 co-regulated orthologous genes within a 26-kb region were identified in F. graminearum and F. sporotrichioides, respectively. A majority of these clustered genes have been shown to be involved in different aspects of trichothecene metabolism including 7 of 15 biosynthetic steps. Three other biosynthetic steps are carried out by genes located elsewhere in the genome. In this study, we sequenced 14-16 kb of DNA on both sides of the core clusters and identified 12 new ORFs in both Fusarium species. Although the predicted functions of some of the new ORFs are consistent with some unassigned biochemical reactions, gene expression and gene deletion studies indicate that none are required for trichothecene biosynthesis. These results provide evidence to demarcate both ends of the core trichothecene gene cluster. Index descriptors: Fungal secondary metabolite, Pathogenic fungi, Gene cluster, Fusarium, Trichothecene, DON

Multiplex real-time PCR detection of fumonisin-producing and trichothecene-producing groups of Fusarium species

Some species of Fusarium can produce mycotoxins during food processing procedures that facilitate fungal growth, such as the malting of barley. The objectives of this study were to develop a 5' fluorogenic (Taqman) real-time PCR assay for group-specific detection of trichothecene- and fumonisin-producing Fusarium spp. and to identify Fusarium graminearum and Fusarium verticillioides in field-collected barley and corn samples. Primers and probes were designed from genes involved in mycotoxin biosynthesis (TRI6 and FUM1), and for a genus-specific internal positive control, primers and a probe were designed from Fusarium rDNA sequences. Real-time PCR conditions were optimized for amplification of the three products in a single reaction format. The specificity of the assay was confirmed by testing 9 Fusarium spp. and 33 non-Fusarium fungal species. With serial dilutions of purified genomic DNA from F. verticillioides, F. graminearum, or both as the template, the detection limit of the assay was 5 pg of genomic DNA per reaction. The three products were detectable over four orders of magnitude of template concentration (5 pg to 5 ng of genomic DNA per reaction); at 50 ng template per reaction, only the TRI6 and FUM1 PCR products were detected. Barley and corn samples were evaluated for the presence of Fusarium spp. with traditional microbiological methods and with the real-time PCR assay. The 20 barley samples and 1 corn sample that contained F. graminearum by traditional methods of analysis tested positive for the TRI6 and internal transcribed spacer (ITS) PCR products. The five corn samples that tested positive for F. verticillioides by traditional methods also were positive for the FUMI and ITS PCR products. These results indicate that the described multiplex real-time PCR assay provides sensitive and accurate differential detection of fumonisin- and trichothecene-producing groups of Fusarium spp. in complex matrices.

Stachybotrys chartarum: a fungus for our time

Stachybotrys chartarum, a fungus found in damp buildings and sometimes ascribed a role in building-related illnesses, produces a variety of secondary metabolites including trichothecenes, triprenylated phenolics, and a new class of diterpenoids called atranones. A related fungus, Memnoniella echinata also produces trichothecenes and the triprenylated phenolics. Herein the production of these compounds from cultures of the above are reviewed.

Two-dimensional environmental profiles of growth, deoxynivalenol and nivalenol production by Fusarium culmorum on a wheat-based substrate

AIMS

To determine the effect of interacting conditions of water activity (aw, 0.99-0.85), temperature (15, 25 degrees C) and time (40 days) on growth and production of the mycotoxins deoxynivalenol (DON) and nivalenol (NIV) by Fusarium culmorum on a wheat-based agar medium.

METHODS AND RESULTS

Fusarium culmorum grew optimally at 0.995aw and minimally at 0.90 at both 15 and 25 degrees C. No growth was observed at <0.90aw. Overall, temperature, aw and their interaction had a statistically significant effect on the growth rate of F. culmorum. Production of both DON and NIV were over a much narrower range (0.995-0.95aw) than that for growth. The highest concentrations of DON and NIV levels were produced at 0.995aw and 0.981aw at 25 degrees C, respectively, after 40 days of incubation. Statistically, aw, temperature and incubation time, and aw x temperature and temperature x incubation time had a statistically significant effect on DON/NIV production.

CONCLUSIONS

This is the first detailed report on the two-dimensional environmental profiles for DON/NIV production by F. culmorum in the UK.

SIGNIFICANCE AND IMPACT OF THE STUDY

As part of a hazard analysis critical control point (HACCP) approach, this type of information is critical in monitoring critical control points for prevention of DON/NIV entering the wheat production chain.

Mycotoxigenic Fusarium and deoxynivalenol production repress chitinase gene expression in the biocontrol agent Trichoderma atroviride P1

Mycotoxin contamination associated with head blight of wheat and other grains caused by Fusarium culmorum and F. graminearum is a chronic threat to crop, human, and animal health throughout the world. One of the most important toxins in terms of human exposure is deoxynivalenol (DON) (formerly called vomitoxin), an inhibitor of protein synthesis with a broad spectrum of toxigenicity against animals. Certain Fusarium toxins have additional antimicrobial activity, and the phytotoxin fusaric acid has recently been shown to modulate fungus-bacterium interactions that affect plant health (Duffy and Défago, Phytopathology 87:1250-1257, 1997). The potential impact of DON on Fusarium competition with other microorganisms has not been described previously. Any competitive advantage conferred by DON would complicate efforts to control Fusarium during its saprophytic growth on crop residues that are left after harvest and constitute the primary inoculum reservoir for outbreaks in subsequent plantings. We examined the effect of the DON mycotoxin on ecological interactions between pathogenic Fusarium and Trichoderma atroviride strain P1, a competitor fungus with biocontrol activity against a wide range of plant diseases. Expression of the Trichoderma chitinase genes, ech42 and nag1, which contribute to biocontrol activity, was monitored in vitro and on crop residues of two maize cultivars by using goxA reporter gene fusions. We found that DON-producing F. culmorum and F. graminearum strains repressed expression of nag1-gox. DON-negative wild-type Fusarium strains and a DON-negative mutant with an insertional disruption in the tricothecene biosynthetic gene, tri5, had no effect on antagonist gene expression. The role of DON as the principal repressor above other pathogen factors was confirmed. Exposure of Trichoderma to synthetic DON or to a non-DON-producing Fusarium mutant resulted in the same level of nag1-gox repression as the level observed with DON-producing FUSARIUM: DON repression was specific for nag1-gox and had no effect, either positive or negative, on expression of another key chitinase gene, ech42. This is the first demonstration that a target pathogen down-regulates genes in a fungal biocontrol agent, and our results provide evidence that mycotoxins have a novel ecological function as factors in Fusarium competitiveness.

Identification of new genes positively regulated by Tri10 and a regulatory network for trichothecene mycotoxin production

Tri10, a regulatory gene in trichothecene mycotoxin-producing Fusarium species, is required for trichothecene biosynthesis and the coordinated expression of four trichothecene pathway-specific genes (Tri4, Tri5, Tri6, and Tri101) and the isoprenoid biosynthetic gene for farnesyl pyrophosphate synthetase (FPPS). We showed that six more trichothecene genes (Tri3, Tri7, Tri8, Tri9, Tri11, and Tri12) are regulated by Tri10. We also constructed a cDNA library from a strain of Fusarium sporotrichioides that overexpresses Tri10 ( upward arrow Tri10) and used cDNA derived from the upward arrow Tri10 strain and a non-Tri10-expressing strain (DeltaTri10) to differentially screen macroarrays prepared from the cDNA library. This screen identified 15 additional Tri10-regulated transcripts. Four of these transcripts represent Tri1, Tri13, and Tri14 and a gene designated Tri15. Three other sequences are putative orthologs of genes for isoprenoid biosynthesis, the primary metabolic pathway preceding trichothecene biosynthesis. The remaining eight sequences have been designated Ibt (influenced by Tri10) genes. Of the 26 transcripts now known to be positively regulated by Tri10, 22 are positively coregulated by Tri6, a gene that encodes a previously characterized trichothecene pathway-specific transcription factor. These 22 Tri10- and Tri6-coregulated sequences include all of the known Tri genes (except for Tri10), the FPPS gene, and the other three putative isoprenoid biosynthetic genes. Tri6 also regulates a transcript that is not regulated by Tri10. Thus, Tri10 and Tri6 regulate overlapping sets of genes that include a common group of multiple genes for both primary and secondary metabolism.

Possible role of plant phenolics in the production of trichothecenes by Fusarium graminearum strains on different fractions of maize kernels

Four trichothecene-producing strains of Fusarium graminearum were grown on three maize grain fractions, whole grain, degermed grain, and the germ, to determine the effect of natural substrates on mycotoxin production. Monitoring the ergosterol content after 25 days of incubation indicated that fungal growth on all grain fractions was comparable. Trichothecene (TCT) production was highest on degermed grain, less on whole grain, and very low or nondetectable on the germ; similar results were found with four different strains. It was concluded that inhibitor(s) of TCT biosynthesis were present in maize germ. The presence of phenolic compounds was investigated in the different fractions. The hydroxamate 4-acetylbenzoxazolin-2-one (4-ABOA), a known inhibitor of mycotoxin production, was found in the degermed and whole grain fractions but not in the germ. Therefore, the TCT inhibition observed on the maize germ fraction used in our study is clearly not linked to 4-ABOA. Other soluble phenolic compounds were found at a much higher concentration in the germ than in the two other fractions. The inhibition property of the soluble ester-bound extracts was tested in liquid culture. A possible role for these compounds is discussed.

Deoxynivalenol-nonproducing fusarium graminearum causes initial infection, but does not cause disease spread in wheat spikes

Fusarium graminearum is a major pathogen that causes fusarium head blight (FHB) in wheat and produces deoxynivalenol (DON) in infected grain. In previous studies, the trichodiene synthase gene (Tri5) in the fungal strain GZ3639 was disrupted to produce the DON-nonproducing strain GZT40. In this report, the virulence of strains GZ3639 and GZT40 was tested on wheat cultivars with various resistance levels by using methods of spray inoculation and injection inoculation with fungal conidia. Under field and greenhouse conditions, strain GZ3639 produced significantly more disease symptoms and reduced more yield than strain GZT40 in all wheat cultivars tested. Conidia of strain GZT40 germinated and infected inoculated spikelets, but disease symptoms were limited to inoculated spikelets without spread to uninoculated spikelets. When strain GZT40 was inoculated using the spray method, multiple initial infection sites in a spike resulted in higher levels of disease symptoms than in spikes inoculated by a single injection. Greenhouse tests confirmed that strain GZT40 did not produce DON in the infected kernels following either inoculation method. The results confirm that DON production plays a significant role in the spread of FHB within a spike, and are the first report that DON production is not necessary for initial infection by the fungus.

Tri1 encodes the cytochrome P450 monooxygenase for C-8 hydroxylation during trichothecene biosynthesis in Fusarium sporotrichioides and resides upstream of another new Tri gene

Many Fusarium species produce one or more agriculturally important trichothecene mycotoxins, and the relative level of toxicity of these compounds is determined by the pattern of oxygenations and acetylations or esterifications on the core trichothecene structure. Previous studies with UV-induced Fusarium sporotrichioides NRRL 3299 trichothecene mutants defined the Tri1 gene and demonstrated that it was required for addition of the oxygen at the C-8 position during trichothecene biosynthesis. We have cloned and characterized the Tri1 gene from NRRL 3299 and found that it encodes a cytochrome P450 monooxygenase. The disruption of Tri1 blocks production of C-8-oxygenated trichothecenes and leads to the accumulation of 4,15-diacetoxyscirpenol, the same phenotype observed in the tri1 UV-induced mutants MB1716 and MB1370. The Tri1 disruptants and the tri1 UV-induced mutants do not complement one another when coinoculated, and the Tri1 gene sequence restores T-2 toxin production in both MB1716 and MB1370. The DNA sequence flanking Tri1 contains another new Tri gene. Thus, Tri1 encodes a C-8 hydroxylase and is located either in a new distal portion of the trichothecene gene cluster or in a second separate trichothecene gene cluster.

Metabolite profiles of Stachybotrys isolates from water-damaged buildings and their induction of inflammatory mediators and cytotoxicity in macrophages

The metabolite profiles of 20 Stachybotrys spp. isolates from Finnish water-damaged buildings were compared with their biological activities. Effects of purified compounds on cytotoxicity and production of inflammatory mediators such as nitric oxide, IL-6 and TNFalpha in murine RAW264.7 macrophage cells were studied. The 11 isolates belonging to the satratoxin-producing chemotype were highly cytotoxic to the macrophages. The isolates inducing inflammatory mediators all belonged to the atranone-producing chemotype, but pure atranones B, and D did not elicit a response in the bioassay. Altogether, cytotoxicity of Stachybotrys sp. isolates appear to be related to satratoxin production whereas the specific component inducing inflammatory responses in atranone-producing isolates remains obscure.

Polymorphism of trichothecene biosynthesis genes in deoxynivalenol- and nivalenol-producing Fusarium graminearum isolates

Diversity in trichothecene mycotoxin production by 167 isolates of Fusarium graminearum was examined by chemical and molecular methods. Isolates from barley, corn, and wheat grown in Korea produced either deoxynivalenol (DON) or nivalenol (NIV), whereas isolates from corn grown in the United States produced DON only. Southern blotting of MseI-digested genomic DNA's from these isolates was performed using a 0.6-kb fragment of Tri5, a key enzyme for trichothecene production, as a probe. This technique revealed a single-band polymorphism between these isolates, with 1.8- and 2.2-kb bands arising from DON and NIV producers, respectively. The same set of isolates was subjected to previously developed PCR assays using primers derived from Tri7 or Tri13. These assays also revealed a single-band polymorphism between NIV- and DON-producing chemotypes. The polymorphisms at Tri5, Tri7, or Tri13 in all of the US isolates were consistent with their chemotypes as identified by GC-MS. However, for seven Korean isolates, chemical and molecular analyses yielded seemingly inconsistent results. This issue was resolved by Southern blot analysis with the Tri5 probe using two other restriction enzymes and sequence comparison of a 3.8-kb region spanning Tri5. In addition, one of these exceptional isolates was found to carry both DON and NIV chemotype-specific regions, possibly resulting from recombination between the two chemotypes.

Patterns of volatile metabolites and nonvolatile trichothecenes produced by isolates of Stachybotrys, Fusarium, Trichoderma, Trichothecium and Memnoniella

We reported previously that trichodiene, a volatile trichothecene derivative, was produced by a Stachybotrys isolate, also known to produce highly cytotoxic, non-volatile, macrocyclic trichothecenes (satrotoxins). We investigated the relationship between the production of trichodiene and various non-volatile trichothecenes for several molds. Volatile metabolites were concentrated by adsorption on Tenax TA and analyzed by GC/MS, while non-volatile metabolites were separated by HPLC, derivatized and analyzed by GC/MS. Stachybotrys chartarum isolates producing macrocyclic trichothecenes secreted significantly larger amounts of trichodiene and other sesquiterpenes than isolates which only produced simple trichothecenes. The amounts of secreted trichodiene were relatively small in all cases. With the exception of Memnoniella, which excreted small amounts of sesquiterpenes, the other isolates produced varying amounts of sesquiterpenes, including trichodiene, as well as simple tricothecenes, no detectable trichodiene, but large amounts of griseofulvin derivatives. In Stachybotrys there is apparently a correlation between trichodiene and macrocyclic trichothecene production. In the remaining isolates, there was no simple relationship between trichodiene and non-volatile trichothecene synthesis. Trichodiene is produced in larger amounts by Stachybotrys isolates, which also produce satratoxins, but it will be difficult to utilize this metabolite to detect toxic isolates in buildings due to the relatively small amounts excreted.

Correlation between DNA of trichothecene-producing Fusarium species and deoxynivalenol concentrations in wheat-samples

AIMS

Correlations between DNA content of trichothecene-producing Fusarium spp. and concentration of the key mycotoxin deoxynivalenol (DON) in cereal samples.

METHODS AND RESULTS

A LightCycler PCR-based assay was used to quantify the DNA from trichothecene-producing Fusarium spp. in 300 wheat samples. DNA concentrations ranged from not detectable to 16.3 mg kg-1 whereas DON concentrations (GC/MS data) varied from not detectable to 34.3 mg kg-1. Data analysis revealed a coefficient of correlation r=0.9557 between DON concentrations and DNA-amounts over all samples. An interval of confidence for P=95% was calculated based on samples with DON concentrations < or = 1.5 mg kg-1 (n=234).

CONCLUSIONS

Quantification of 32 samples of Fusarium-contaminated wheat was performed within 45 min. Data analysis allowed estimation of DON contamination from quantitative PCR data in the wheat samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

The method described is useful for the screening of cereals in industrial quality control.

Production of trichothecenes and other secondary metabolites by Fusarium culmorum and Fusarium equiseti on common laboratory media and a soil organic matter agar: an ecological interpretation

Fusarium culmorum and F. equiseti were characterized with regard to production of trichothecenes and other secondary metabolites. Results following growth on laboratory media are interpreted with the aim of increasing the understanding of fungal metabolism in the field environment. While trichothecene production was detected for 94 of 102 F. culmorum isolates, only 8 of 57 F. equiseti isolates were positive. Profiles of secondary metabolites were compared by following growth on yeast extract sucrose agar (YES), potato sucrose agar (PSA), and an agar medium, prepared from soil organic matter (SOM), which was included to simulate growth conditions in soil. SOM supported the production of chrysogine by F. culmorum. The two species utilized the media differently. F. culmorumproduced zearalenone (ZEA) on YES, whereas some F. equiseti isolates produced ZEA on PSA. Other F. equiseti isolates produced equisetin. These differences may reflect that F. culmorum depends on a pathogenic life style while F. equiseti has a more saprotrophic mode of existence.

Multiplex polymerase chain reaction assay for the differential detection of trichothecene- and fumonisin-producing species of Fusarium in cornmeal

The genus Fusarium comprises a diverse group of fungi including several species that produce mycotoxins in food commodities. In this study, a multiplex polymerase chain reaction (PCR) assay was developed for the group-specific detection of fumonisin-producing and trichothecene-producing species of Fusarium. Primers for genus-level recognition of Fusarium spp. were designed from the internal transcribed spacer regions (ITS1 and ITS2) of rDNA. Primers for group-specific detection were designed from the TRI6 gene involved in trichothecene biosynthesis and the FUM5 gene involved in fumonisin biosynthesis. Primer specificity was determined by testing for cross-reactivity against purified genomic DNA from 43 fungal species representing 14 genera, including 9 Aspergillus spp., 9 Fusarium spp., and 10 Penicillium spp. With purified genomic DNA as a template, genus-specific recognition was observed at 10 pg per reaction; group-specific recognition occurred at 100 pg of template per reaction for the trichothecene producer Fusarium graminearum and at 1 ng of template per reaction for the fumonisin producer Fusarium verticillioides. For the application of the PCR assay, a protocol was developed to isolate fungal DNA from cornmeal. The detection of F. graminearum and its differentiation from F. verticillioides were accomplished prior to visible fungal growth at <10(5) CFU/g of cornmeal. This level of detection is comparable to those of other methods such as enzyme-linked immunosorbent assay, and the assay described here can be used in the food industry's effort to monitor quality and safety.

Arabidopsis is susceptible to the cereal ear blight fungal pathogens Fusarium graminearum and Fusarium culmorum

The fungal pathogens Fusarium graminearum and F. culmorum cause ear blight disease on cereal crops worldwide. The disease lowers both grain quality and grain safety. Disease prevalence is increasing due to changes in cropping practices and the difficulties encountered by plant breeders when trying to introgress the polygene-based resistance. The molecular basis of resistance to Fusarium ear blight in cereal species is poorly understood. This is primarily due to the large size of cereal genomes and the expensive resources required to undertake gene function studies in cereals. We therefore explored the possibility of developing various model floral infection systems that would be more amenable to experimental manipulation and high-throughput gene function studies. The floral tissues of tobacco, tomato, soybean and Arabidopsis were inoculated with Fusarium conidia and this resulted in disease symptoms on anthers, anther filaments and petals in each plant species. However, only in Arabidopsis did this initial infection then spread into the developing siliques and seeds. A survey of 236 Arabidopsis ecotypes failed to identify a single genotype that was extremely resistant or susceptible to Fusarium floral infections. Three Arabidopsis floral mutants that failed to develop anthers and/or functional pollen (i.e. agamous-1, apetala1-3 and dad1) were significantly less susceptible to Fusarium floral infection than wild type. Deoxynivalenol (DON) mycotoxin production was also detected in Fusarium-infected flowers at >1 ppm. This novel floral pathosystem for Arabidopsis appears to be highly representative of a serious cereal crop disease.

Comparison of Canadian Fusarium graminearum isolates for aggressiveness, vegetative compatibility, and production of ergosterol and mycotoxins

Fusarium graminearum is the predominant pathogen causing fusarium head blight of cereals in North America. Fifteen Canadian isolates of Fusarium graminearum were highly diverse in terms of vegetative compatibility grouping (VCG) and varied for production of ergosterol and mycotoxin production in rice culture. Aggressiveness was assessed by scoring the disease severity incited in wheat spikes by each isolate. Two inoculation methods, single-floret injection and spray of entire spikes, were used to screen 4 wheat varieties for reaction to the F. graminearum isolates. All isolates were of broadly similar aggressiveness, with disease severity ranging from 17.2 to 39.1 for single floret injection, and 39.1 to 69.0 for spray inoculation. Disease severity, ergosterol production, and mycotoxin development were not correlated. Using nitrate non-utilizing mutants the 15 isolates were grouped into 14 VCGs. Deoxynivalenol (DON) was produced by all isolates in rice culture, at levels between 0.2 and 249 ppm. 15-acetyldeoxynivalenol was produced by 14 of the 15 isolates at levels between 0.4 and 44.6 ppm. These results reveal a high level of diversity for several characteristics among F. graminearum isolates from Canada.

Toxigenic diversity of two different RAPD groups of Stachybotrys chartarum isolates analyzed by potential for trichothecene production and for boar sperm cell motility inhibition

Thirty-one isolates of Stachybotrys chartarum from indoor and outdoor environments were analyzed for the presence of the trichodiene synthase (Tri5) gene, trichothecenes, boar sperm cell motility inhibition, and randomly amplified polymorphic DNA banding patterns (RAPDs). Twenty-two S. chartarum isolates tested positive for the Tri5 gene and nine were negative when tested using novel Tri5 gene-specific PCR primer pair. The Tri5 gene positive isolates contained satratoxins (five isolates) or the simple trichothecene, trichodermol (11 isolates). The Tri5 gene negative isolates did not produce satratoxins or trichodermol. Nineteen S. chartarum isolates, distributed among the Tri5 gene negative and positive groups, inhibited boar spermatozoan motility at concentrations of < or = 60 microg of crude cell extract/mL. The inhibition of motility was independent of satratoxins or atranones. Unweighted pair group method of arithmetic averages (UPGMA) cluster analysis of RAPD fragments clustered the 31 S. chartarum isolates in two distinct groups designated as RAPD groups 1 and 2. The grouping of S. chartarum isolates obtained by UPGMA cluster analysis of RAPD fragments was identical to the grouping obtained by Tri5 gene-specific PCR. This indicates that the S. chartarum isolates belonging to different groups were genetically distinct in a much wider area than just the Tri5 gene.

Identification by PCR of Fusarium culmorum strains producing large and small amounts of deoxynivalenol

Thirty deoxynivalenol-producing F. culmorum strains, isolated from wheat grains, were incubated in vitro and analyzed for trichothecene production. Seventeen strains produced more than 1 ppm of deoxynivalenol and acetyldeoxynivalenol and were considered high-deoxynivalenol-producing strains, whereas 13 F. culmorum strains produced less than 0.07 ppm of trichothecenes and were considered low-deoxynivalenol-producing strains. For all strains, a 550-base portion of the trichodiene synthase gene (tri5) was amplified and sequenced. According to the tri5 data, the F. culmorum strains tested clustered into two groups that correlated with in vitro deoxynivalenol production. For three high-producing and three low-producing F. culmorum strains, the tri5-tri6 intergenic region was then sequenced, which confirmed the two separate clusters within the F. culmorum strains. According to the tri5-tri6 sequence data, specific PCR primers were designed to allow differentiation of high-producing from low-producing F. culmorum strains.