Microbial acetyl conjugation of T-2 toxin and its derivatives

Diacetoxyscirpenol, a Fusarium exometabolite, prevents efficiently the incidence of the parasitic weed Striga hermonthica

BACKGROUND

Certain Fusarium exometabolites have been reported to inhibit seed germination of the cereal-parasitizing witchweed, Striga hermonthica, in vitro. However, it is unknown if these exometabolites will consistently prevent S. hermonthica incidence in planta. The study screened a selection of known, highly phytotoxic Fusarium exometabolites, in identifying the most potent/efficient candidate (i.e., having the greatest effect at minimal concentration) to completely hinder S. hermonthica seed germination in vitro and incidence in planta, without affecting the host crop development and yield.

RESULTS

In vitro germination assays of the tested Fusarium exometabolites (i.e., 1,4-naphthoquinone, equisetin, fusaric acid, hymeglusin, neosolaniol (Neo), T-2 toxin (T-2) and diacetoxyscirpenol (DAS)) as pre-Striga seed conditioning treatments at 1, 5, 10, 20, 50 and 100 µM, revealed that only DAS, out of all tested exometabolites, completely inhibited S. hermonthica seed germination at each concentration. It was followed by T-2 and Neo, as from 10 to 20 µM respectively. The remaining exometabolites reduced S. hermonthica seed germination as from 20 µM (P < 0. 0001). In planta assessment (in a S. hermonthica-sorghum parasitic system) of the exometabolites at 20 µM showed that, although, none of the tested exometabolites affected sorghum aboveground dry biomass (P > 0.05), only DAS completely prevented S. hermonthica incidence. Following a 14-d incubation of DAS in the planting soil substrate, bacterial 16S ribosomal RNA (rRNA) and fungal 18S rRNA gene copy numbers of the soil microbial community were enhanced; which coincided with complete degradation of DAS in the substrate. Metabolic footprinting revealed that the S. hermonthica mycoherbicidal agent, Fusarium oxysporum f. sp. strigae (isolates Foxy-2, FK3), did not produce DAS; a discovery that corresponded with underexpression of key genes (Tri5, Tri4) necessary for Fusarium trichothecene biosynthesis (P < 0.0001).

CONCLUSIONS

Among the tested Fusarium exometabolites, DAS exhibited the most promising herbicidal potential against S. hermonthica. Thus, it could serve as a new biocontrol agent for efficient S. hermonthica management. Further examination of DAS specific mode of action against the target weed S. hermonthica at low concentrations (≤ 20 µM), as opposed to non-target soil organisms, is required.

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.

The phytotoxicity ofFusarium metabolites: An update since 1989

The present article summarises the published phytotoxic effects of severalFusarium metabolites (mycotoxins, phytotoxins, antibiotics and pigments) since 1989. The phytotoxicity of many of the commonly isolated metabolites cannot be disputed, but their role in pathogenesis ofFusarium-induced plant diseases is uncertain. Plant species/varieties differ in their susceptibililty resistance to these toxinsin vitro, as well as toFusarium pathogens under field conditions. Such variations in plant response may reflect resistance mechanisms that operate at several levels, including an initial ability to prevent fungal invasion; prevention of fungal spread and toxin tolerance or degradation. Little is known about the mode of action of most of these metabolites on either animal or plant cells. Several novelFusarium metabolites have been isolated in the past few years. Many are toxic to animals and cell lines, but assessment of their phytotoxicity has largely been neglected. Since many plant pathogenic Fusaria produce a plethora of metabolites, the additive or synergistic actions of toxins in combination must be considered in plant pathology.

Natural occurrence of masked deoxynivalenol in Chinese wheat and wheat-based products during 2008-2011

A total of 697 samples of wheat and wheat-based products collected from 24 provinces in China during 2008-2011 were analysed for deoxynivalenol (DON), deoxynivalenol-3-glucoside (DON-3G), 3-O-acetyldeoxynivalenol (3-ADON) and 15-O-acetyldeoxynivalenol (15-ADON) by ultra-performance liquid chromatography tandem mass spectrometry (LC-MS/MS). DON was the predominant toxin detected abundantly and frequently. Nine wheat flour samples were positive for DON at levels exceeding the Chinese regulatory limit of 1000 μg/kg for DON in wheat. Moderate concentrations of DON-3G and lower concentrations of both 3-ADON and 15-ADON were found in the presence of high DON levels. DON-3G was detected at concentrations of 4-238 μg/kg (mean 52 μg/kg) in wheat kernels and 3-39 μg/kg (mean 11 μg/kg) in wheat flour in 2008; and 3-235 μg/kg (mean 22 μg/kg), 3-53 μg/kg (mean 14 μg/kg) and 3-87 μg/kg (mean19 μg/kg) in wheat products in 2009, 2010 and 2011, respectively. The average relative ratio of DON-3G to DON was 33±3% in wheat kernels and 10±1% in wheat flour in 2008; and 22±7%, 9±4% and 14±7% in wheat products in 2009, 2010 and 2011, respectively. The natural occurrence of DON-3G is positively correlated with that of DON in all samples over the 4-year period. DON-3G was present at a level higher than 3-ADON and 15-ADON in all samples examined, possibly due to the fact that a higher level of DON, the precursor of DON-3G, was found compared to either 3-ADON or 15-ADON. As for the toxin proportion, DON-3G accounted for 30% (wheat kernel) and 13% (wheat flour), as well as 14%, 11% and 13% of the total 4 toxins tested in 2008, 2009, 2010 and 2011 samples, respectively. These results indicate the importance of considering DON-3G with regard to setting a regulatory limit for DON in Chinese wheat and wheat-based products.

T-2 toxin, a trichothecene mycotoxin: review of toxicity, metabolism, and analytical methods

This review focuses on the toxicity and metabolism of T-2 toxin and analytical methods used for the determination of T-2 toxin. Among the naturally occurring trichothecenes in food and feed, T-2 toxin is a cytotoxic fungal secondary metabolite produced by various species of Fusarium. Following ingestion, T-2 toxin causes acute and chronic toxicity and induces apoptosis in the immune system and fetal tissues. T-2 toxin is usually metabolized and eliminated after ingestion, yielding more than 20 metabolites. Consequently, there is a possibility of human consumption of animal products contaminated with T-2 toxin and its metabolites. Several methods for the determination of T-2 toxin based on traditional chromatographic, immunoassay, or mass spectroscopy techniques are described. This review will contribute to a better understanding of T-2 toxin exposure in animals and humans and T-2 toxin metabolism, toxicity, and analytical methods, which may be useful in risk assessment and control of T-2 toxin exposure.

Biological detoxification of fungal toxins and its use in plant breeding, feed and food production

Enzymatic inactivation of fungal toxins is an attractive strategy for the decontamination of agricultural commodities and for the protection of crops from phytotoxic effects of fungal metabolites. This review summarizes research on the biological detoxification of fungal toxins by microorganisms and plants and its practical applications. Some mycotoxins are detoxified during ensiling and other fermentation processes (aflatoxins, alternariol, mycophenolic acid, patulin, PR toxin) while others are transformed into toxic products or survive fermentation unchanged. Plants can detoxify fomannoxin, fusaric acid, HC-toxin, ochratoxin A and oxalate but the degradation of deoxynivalenol has yet to be proven. Microflora of the digestive tract of vertebrates and invertebrates exhibit detoxification activities towards aflatoxins, ochratoxin A, oxalate and trichothecenes. Some toxin-producing fungi are able to degrade or transform their own products under suitable conditions. Pure cultures of bacteria and fungi which detoxify mycotoxins have been isolated from complex microbial populations by screening and enrichment culture techniques. Genes responsible for some of the detoxification activities have been cloned and expressed in heterologous hosts. The detoxification of aflatoxins, cercosporin, fumonisins, fusaric acid, ochratoxin A, oxalic acid, patulin, trichothecenes and zearalenone by pure cultures is reviewed. Finally, current application of these results in food and feed production and plant breeding is summarized and expected future developments are outlined.

The mystery of the trichothecene 3-O-acetyltransferase gene. Analysis of the region around Tri101 and characterization of its homologue from Fusarium sporotrichioides

The trichothecene 3-O-acetyltransferase gene, Tri101, plays a pivotal role for the well-being of the type B trichothecene producer Fusarium graminearum. We have analyzed the cosmids containing Tri101 and found that this resistance gene is not in the biosynthetic gene cluster reported so far. It was located between the UTP-ammonia ligase gene and the phosphate permease gene which are not related to trichothecene biosynthesis. These two 'house-keeping' genes were also linked in Fusarium species that do not produce trichothecenes. The result suggests that the isolated occurrence of Tri101 is attributed to horizontal gene transfer and not to the reciprocal translocation of the chromosome containing the gene cluster. Interestingly, 3-O-acetylation was not always a primary self-defensive strategy for all the t-type trichothecene producers; i.e. the type A trichothecene producer Fusarium sporotrichioides did not acetylate T-2 toxin in vivo although the fungus possessed a functional 3-O-acetyltransferase gene. Thus Tri101 appears to be a defense option which the producers have independently acquired in addition to their original resistance mechanisms.

Partial purification and characterization of an esterase from Fusarium sporotrichioides

Kinetics analysis of the growth of Fusarium sporotrichioides T-424 at 15 degrees C and 25 degrees C in liquid culture for 35 days revealed that production of deacetylated trichothecenes was associated with an increased activity in fungal esterases. High temperature (25 degrees C) favored enzyme production and enhanced esterase activity. Electrophoresis of crude extracts from the mycelia of F. sporotrichioides T-424 with carboxylesterase staining revealed that several esterases were produced by the fungus. Four carboxylesterase isoenzymes (I-IV) were separated on a DEAE-Sephadex anion exchange column. Type (III) esterase, having activities with the substrate 4-nitrophenylacetate and acetanilide, as well as hydrolytic activity for T-2 toxin and acetyl-T-2 toxin, was partially purified with ammonium sulfate precipitation, immunoaffinity column chromatography, and DEAE-Sephadex A-50 chromatography. The esterase (III) had a molecular weight around 68 kDa in SDS-PAGE. For the deacylation of T-2 toxin and acetyl-T-2 toxin, type (III) esterase had a high specificity for the acetyl group at the C-3 and C-4 positions. The Km values for acetyl-T-2 and T-2 toxin were found to be 41.35 microM and 0.38 microM, respectively. The Km value for the acetyl group at C-3 is 110 times greater than for that at C-4.

Biotransformation and detoxification of T-2 toxin by soil and freshwater bacteria

Bacterial communities isolated from 17 of 20 samples of soils and waters with widely diverse geographical origins utilized T-2 toxin as a sole source of carbon and energy for growth. These isolates readily detoxified T-2 toxin as assessed by a Rhodotorula rubra bioassay. The major degradation pathway of T-2 toxin in the majority of isolates involved side chain cleavage of acetyl moieties to produce HT-2 toxin and T-2 triol. A minor degradation pathway of T-2 toxin that involved conversion to neosolaniol and thence to 4-deacetyl neosolaniol was also detected. Some bacterial communities had the capacity to further degrade the T-2 triol or 4-deacetyl neosolaniol to T-2 tetraol. Two communities, TS4 and KS10, degraded the trichothecene nucleus within 24 to 48 h. These bacterial communities comprised 9 distinct species each. Community KS10 contained 3 primary transformers which were able to cleave acetate from T-2 toxin but which could not assimilate the side chain products, whereas community TS4 contained 3 primary transformers which were able to grow on the cleavage products, acetate and isovalerate. A third community, AS1, was much simpler in structure and contained only two bacterial species, one of which transformed T-2 toxin to T-2 triol in monoculture. In all cases, the complete communities were more active against T-2 toxin in terms of rates of degradation than any single bacterial component. Cometabolic interactions between species is suggested as a significant factor in T-2 toxin degradation.

Toxic fungal metabolites in food

About 100 fungal metabolites may cause cancer, embryological defects, or other histopathological effects in mammals. They are produced by a wide variety of fungi. Few of these metabolites have significant acute toxicity. With the exception of aflatoxin B1 and sterigmatocystin, there is no conclusive evidence that any of them is carcinogenic. However, several of the compounds are mutagenic. Cytochalasin D and T-2 toxin are probably teratogenic. A wide variety of other histopathological effects have been shown. Liver damage has been most frequently reported. In almost all cases the molecular bases of these effects have not been extensively investigated. Although much is known about the routes by which some of the compounds are synthesized in vivo, nothing is known about control at the molecular level of these biosynthetic routes. Little is known about the biological degradation of these compounds or about the levels and incidences of them in food and animal feed. Future work in all these areas will depend on the further development of sensitive assay methods that are applicable to their measurement in food, in animal feed, and in animal tissues and body fluids and on the application of these methods to define exposure to these compounds in the diet.

Metabolism of T-2 toxin in Curtobacterium sp. strain 114-2

The metabolic pathway of T-2 toxin in Curtobacterium sp. strain 114, one of the T-2 toxin-assimilating soil bacteria, was investigated by thin-layer and gas-liquid chromatographic analyses. T-2 toxin added to the basal medium as a single carbon and energy source was biotransformed into HT-2 toxin and an unknown metabolite. Infrared, mass spectrum, proton magnetic resonance, and other physico-chemical analyses identified this new metabolite as T-2 triol. T-2 toxin was first deacetylated by the bacterium into HT-2 toxin, and this metabolite was then biotransformed into T-2 triol without formation of neosolaniol and T-2 tetraol. No trichothecenes remained in the culture medium after prolonged culture. Some properties of T-2 toxin-hydrolyzing enzymes were observed with whole cells, the cell-free soluble fraction, and the culture filtrate. Besides T-2 toxin, trichothecenes such as diacetoxyscirpenol, neosolaniol, nivalenol, and fusarenon-X were also assimilated by this bacterium.