Elaboration of vomitoxin and zearalenone by Fusarium isolates and the biological activity of Fusarium-produced toxins

The Cultured Microbiome of Pollinated Maize Silks Shifts after Infection with Fusarium graminearum and Varies by Distance from the Site of Pathogen Inoculation

Styles transmit pollen-derived sperm nuclei from pollen to ovules, but also transmit environmental pathogens. The microbiomes of styles are likely important for reproduction/disease, yet few studies exist. Whether style microbiome compositions are spatially responsive to pathogens is unknown. The maize pathogen Fusarium graminearum enters developing grain through the style (silk). We hypothesized that F. graminearum treatment shifts the cultured transmitting silk microbiome (TSM) compared to healthy silks in a distance-dependent manner. Another objective of the study was to culture microbes for future application. Bacteria were cultured from husk-covered silks of 14 F. graminearum-treated diverse maize genotypes, proximal (tip) and distal (base) to the F. graminearum inoculation site. Long-read 16S sequences from 398 isolates spanned 35 genera, 71 species, and 238 OTUs. More bacteria were cultured from F. graminearum-inoculated tips (271 isolates) versus base (127 isolates); healthy silks were balanced. F. graminearum caused a collapse in diversity of ~20-25% across multiple taxonomic levels. Some species were cultured exclusively or, more often, from F. graminearum-treated silks (e.g., Delftia acidovorans, Klebsiella aerogenes, K. grimontii, Pantoea ananatis, Stenotrophomonas pavanii). Overall, the results suggest that F. graminearum alters the TSM in a distance-dependent manner. Many isolates matched taxa that were previously identified using V4-MiSeq (core and F. graminearum-induced), but long-read sequencing clarified the taxonomy and uncovered greater diversity than was initially predicted (e.g., within Pantoea). These isolates represent the first comprehensive cultured collection from pathogen-treated maize silks to facilitate biocontrol efforts and microbial marker-assisted breeding.

Fungal Pathogens of Maize Gaining Free Passage Along the Silk Road

Silks are the long threads at the tips of maize ears onto which pollen land and sperm nuclei travel long distances to fertilize egg cells, giving rise to embryos and seeds; however fungal pathogens also use this route to invade developing grain, causing damaging ear rots with dangerous mycotoxins. This review highlights the importance of silks as the direct highways by which globally important fungal pathogens enter maize kernels. First, the most important silk-entering fungal pathogens in maize are reviewed, including Fusarium graminearum, Fusarium verticillioides, and Aspergillus flavus, and their mycotoxins. Next, we compare the different modes used by each fungal pathogen to invade the silks, including susceptible time intervals and the effects of pollination. Innate silk defences and current strategies to protect silks from ear rot pathogens are reviewed, and future protective strategies and silk-based research are proposed. There is a particular gap in knowledge of how to improve silk health and defences around the time of pollination, and a need for protective silk sprays or other technologies. It is hoped that this review will stimulate innovations in breeding, inputs, and techniques to help growers protect silks, which are expected to become more vulnerable to pathogens due to climate change.

Fast Screening of Antibacterial Compounds from Fusaria

Bio-guided screening is an important method to identify bioactive compounds from fungi. In this study we applied a fast digital time-lapse microscopic method for assessment of the antibacterial properties of secondary metabolites from the fungal genus Fusarium. Here antibacterial effects could be detected for antibiotic Y, aurofusarin, beauvericin, enniatins and fusaric acid after six hours of cultivation. The system was then used in a bio-guided screen of extracts from 14 different Fusarium species, which had been fractionated by HPLC. In this screen, fractions containing the red pigments aurofusarin and bikaverin showed effects against strains of Lactobacillus and Bifidobacterium. The IC50 for aurofusarin against Lactobacillus acidophilus was 8 µM, and against Bifidobacterium breve it was 64 µM. Aurofusarin only showed an effect on probiotic bacteria, leading to the speculation that only health-promoting bacteria with a positive effect in the gut system are affected.

Exposure of Aspergillus fumigatus to T-2 toxin results in a stress response associated with exacerbation of aspergillosis in poultry

Aspergillus fumigatus is a ubiquitous airborne pathogen. Saprophytic growth in the presence of environmental mycotoxins might affect its fitness and virulence. T-2 toxin (T-2) is a trichothecene mycotoxin produced by Fusarium spp. in various substrates. This study aimed to evaluate the effects of T-2 on the fitness of A. fumigatus in vitro and its virulence in experimentally inoculated chickens. We cultured A. fumigatus on agar media containing T-2, and examined the changes in viability, morphology, growth rate, proteome expression, and susceptibility to antimycotics and oxidative stress of this fungus. Results showed that exposure to 1000 ng/ml T-2 in the substrate did not reduce the viability of A. fumigatus, but its growth was inhibited, with wrinkling and depigmentation of the colonies. Proteomic analysis revealed 21 upregulated proteins and 33 downregulated proteins, including those involved in stress response, pathogenesis, metabolism, transcription. The proteome seems to have shifted to enhance the glycolysis, catabolism of lipids, and amino acid conversion. Assays on fungal susceptibility to antimycotics and oxidative stress showed that T-2 exposure did not affect the minimal inhibitory concentrations of amphotericin B, itraconazole, voriconazole and terbinafine against A. fumigatus, but increased the susceptibility of A. fumigatus to H2O2 and menadione. Experimental inoculation of chickens with A. fumigatus showed that exposure of A. fumigatus to T-2 significantly exacerbated aspergillosis in chickens exposed to dietary T-2. In conclusion, A. fumigatus is capable of surviving and growing on substrates containing levels of T-2 up to 1000 ng/ml. Growth in presence of T-2 induces a stress response in A. fumigatus, which is associated with exacerbation of aspergillosis in vivo.

Antifungal activity of trichothecenes from Fusarium sp. against clinical isolates of Paracoccidioides brasiliensis

Paracoccidioidomycosis (PCM), a human mycosis caused by the dimorphic fungus Paracoccidioides brasiliensis, is a serious public health problem in several countries of Latin America. In our search we found that the crude extract of the endophytic fungus UFMGCB 551 was able to inhibit several clinical strains of P. brasiliensis, and was also active in the bioautographic assay against Cladosporium sphaerospermum. The endophytic fungus UFMGCB 551 was isolated from the plant Piptadenia adiantoides J.F. Macbr (Fabaceae). The fungus was identified as Fusarium sp. based on its macro- and micro-morphology, and on the sequence of the internally transcribed spacer regions (ITS) of its rRNA gene. The chromatographic fractionation of the fungal extract was guided by the bioautographic assay to afford three known trichothecene mycotoxins: T2-toxin (1) and a mixture of 8-n-butyrylneosolaniol (2) and 8-isobutyrylsolaniol (3). The minimal inhibitory concentrations (MIC) of the these compounds against eleven clinical strains of P. brasiliensis were evaluated and found to be in the range between 75 and 640 nmol l(-1) for 1 and 160-640 nmol l(-1) for the mixture of 2 and 3.

Colonization of maize silks by Fusarium graminearum, the causative organism of gibberella ear rot

One of the most economically important diseases of maize in Canada is gibberella ear rot caused by Fusarium graminearum Schwabe (teleomorph = Gibberella zeae (Schw.) Petch). Understanding how the fungus becomes established will help in developing effective strategies to reduce the incidence of this disease. This study investigates the infection process of F. graminearum on maize silks using both a wild-type F. graminearum as well as a strain transformed with a gene from jellyfish to constitutively express green fluorescent protein. Immature ears of maize were inoculated in the field with wild-type F. graminearum and harvested at specific times post infection, and the silks were stained with Chlorazol Black E for examination. In addition, uninoculated ears were excised, placed on water agar in large Petri dishes, and the silks inoculated with a suspension of macroconidia of the transformed fungus. The progress of fungal growth was then monitored using microscopy. Germination of conidia was observed 4–6 h after inoculation. A variable period of random growth often followed, after which some of the hyphae would grow in more or less straight lines down the silk towards the cob (rachis), and ultimately infect the developing kernels. Access to the cob occurred in 7–9 d in susceptible genotypes and 12–15 d in resistant genotypes. The fungus could penetrate the ovary directly through the silk attachment point or, when the silk was growing over other kernels, the fungus could traverse from the silk to colonize interkernel spaces. Entry into the cob was either through the rachis surface via exterior growth between kernels, or into the rachis via the pedicel.

Long chain alkanes in silk extracts of maize genotypes with varying resistance to Fusarium graminearum

The alkane content of the silks of nine maize genotypes was analyzed to investigate the role of silk wax in resistance to Fusarium graminearum. Silk samples were collected 2, 4, 6, and 8 days after silk emergence and divided into three sections: exposed silk, silk channel silk, and silk that is under the husk and overlying the kernels. Four major unbranched alkanes (C(25), C(27), C(29), and C(31)) and three isoalkanes (C(27i), C(29i), and C(31i)) were identified. Total alkane contents were highest in the exposed silk followed by the silk channel silk, with the lowest in the youngest silk closest to the kernels. In the silk channel and overlying kernel silks, the moderately resistant inbred CO272 consistently had the highest alkane content. None of the other inbreds with improved resistance had as high a level of alkanes as CO272, indicating that alkane content is not a major mechanism of resistance.

Pre-harvest accumulation of deoxynivalenol in sweet corn ears inoculated with Fusarium graminearum

Three types of commercial sweet corn hybrids [surgary (su1), shrunken or 'supersweet' (sh2) and surgary enhancer (se1)] were silk channel inoculated in 1996 and 1997 with a macroconidial suspension of Fusarium graminearum to determine how early the mycotoxin deoxynivalenol accumulates in kernels. Disease symptoms rapidly developed on all hybrids and were apparent 4 days after inoculation. Symptoms stabilized by 28 days after inoculation. Toxin levels were greater than 1 microgram/g in kernels as early as 2 weeks after silk emergence and rapidly increased to extremely high levels. Susceptibility in all hybrids decreased as the silk dried out. Deoxynivalenol concentrations were correlated to disease severity. There was some indication that the sh2 genotype was more susceptible than the su1 or se1 genotypes. These results suggest that improvement needs to be made in sweet corn with respect to resistance to gibberella ear rot.

Suppression of the biocontrol agent trichoderma harzianum by mycelium of the arbuscular mycorrhizal fungus glomus intraradices in root-free soil

Trichoderma harzianum is an effective biocontrol agent against several fungal soilborne plant pathogens. However, possible adverse effects of this fungus on arbuscular mycorrhizal fungi might be a drawback in its use in plant protection. The objective of the present work was to examine the interaction between Glomus intraradices and T. harzianum in soil. The use of a compartmented growth system with root-free soil compartments enabled us to study fungal interactions without the interfering effects of roots. Growth of the fungi was monitored by measuring hyphal length and population densities, while specific fatty acid signatures were used as indicators of living fungal biomass. Hyphal 33P transport and beta-glucuronidase (GUS) activity were used to monitor activity of G. intraradices and a GUS-transformed strain of T. harzianum, respectively. As growth and metabolism of T. harzianum are requirements for antagonism, the impact of wheat bran, added as an organic nutrient source for T. harzianum, was investigated. The presence of T. harzianum in root-free soil reduced root colonization by G. intraradices. The external hyphal length density of G. intraradices was reduced by the presence of T. harzianum in combination with wheat bran, but the living hyphal biomass, measured as the content of a membrane fatty acid, was not reduced. Hyphal 33P transport by G. intraradices also was not affected by T. harzianum. This suggests that T. harzianum exploited the dead mycelium but not the living biomass of G. intraradices. The presence of external mycelium of G. intraradices suppressed T. harzianum population development and GUS activity. Stimulation of the hyphal biomass of G. intraradices by organic amendment suggests that nutrient competition is a likely means of interaction. In conclusion, it seemed that growth of and phosphorus uptake by the external mycelium of G. intraradices were not affected by the antagonistic fungus T. harzianum; in contrast, T. harzianum was adversely affected by G. intraradices.

Effect of gamma-irradiation on the natural occurrence of Fusarium mycotoxins in wheat, flour and bread

A survey was carried out to obtain data on the occurrence of Fusarium mycotoxin in wheat and flour samples collected from local markets in Egypt and to study the influence of gamma-irradiation on controlling the occurrence of these mycotoxins in wheat, flour and bread. Deoxynivalenol (DON) was detected in five samples of wheat at levels ranging from 103 to 287 micrograms/kg and one sample each of flour and bread at concentrations 188 and 179 micrograms/kg. Zearalenone (ZEN) was detected in ten samples of wheat at levels from 28 to 42 micrograms/kg and four samples each of flour and bread at concentrations of 95 and 34 micrograms/kg, respectively. T-2 toxin was detected only in one sample each of wheat, flour and bread at concentrations of 2.9, 2.2 and 2.3 micrograms/kg, respectively. Gamma-irradiation at dose level of 6 kGy completely eliminated fungal flora in flour and wheat. DON, ZEN and T-2 toxin concentrations are reduced to 85, 20 and 2.0 micrograms/kg for wheat and to 125, 45 and 1.0 micrograms/kg for flour after 4 kGy exposure and a sharp drop in Fusarium toxin levels occurred at 6 kGy and as eliminated at 8 kGy. Bread prepared from 6 kGy was contaminated with Fusarium toxin at levels below 5 microgram/kg. It was noticed that gamma-irradiation reduce greatly the natural occurrence of Fusarium mycotoxins in bread.

Production of deoxynivalenol by Fusarium isolates from samples of wheat associated with a human mycotoxicosis outbreak and from sorghum cultivars

Fusarium isolates from specific diseased sorghum plants and rain-soaked wheat and wheat flour associated with human mycotoxicosis in India have been screened for their toxigenic potential. Of the 322 isolates screened, 11 isolates were found to produce deoxynivalenol in concentrations ranging from 0.01 to 186 micrograms g-1. The occurrence of deoxynivalenol-producing fusaria in a nontemperate region and deoxynivalenol production in low concentrations by Fusarium moniliforme are reported for the first time.

Protein synthesis inhibition by 8-oxo-12,13-epoxytrichothecenes

The Fusarium mycotoxin, 4-deoxynivalenol, is an abundant, natural contaminant of corn and wheat. 8-Oxo-12,13-epoxytrichothecenes related to 4-deoxynivalenol were synthesized; they either lacked the 7-hydroxyl but contained a hydroxyl at C-4 (7-deoxynivalenol) or lacked substituents at C-3 and C-7 (3,7-dideoxynivalenol). The ability of these synthetic analogs and their acetylated derivatives to inhibit protein synthesis by cultured mammalian cells was compared to that of 4-deoxynivalenol. Whereas the 50% inhibitory dose (ID50) for murine erythroleukemia cells was about 1 microgram/ml for 4-deoxynivalenol and 3,7-dideoxynivalenol, all of the other analogs were at least 10-fold less potent. When tested at their ID50 dose, all of the 8-oxotrichothecenes, except 4-deoxynivalenol and 3,7-dideoxynivalenol, caused polysome 'run-off', indicating that, at this dose, they are inhibitors of polypeptide chain initiation. With 4-deoxynivalenol and 3,7-dideoxynivalenol, polysomes remained at control levels indicating that these toxins prevent polypeptide chain elongation. From these results and comparisons to previous studies of 8-oxo-12,13-epoxytrichothecenes (trichothecolone, trichothecin, nivalenol and fusarenone X), trichothecenes with substituents at both C-3 and C-4 predominantly inhibit polypeptide chain initiation, whereas those lacking one substituent at either site are inhibitors of chain elongation.

Production of deoxynivalenol and zearalenone by isolates of Fusarium graminearum Schw

The production of deoxynivalenol (DONI) on rice, corn, wheat, and barley grains by Fusarium graminearum Schw. NRRL 5883 was investigated. Highest yields (91.9-202 ppm) were obtained on rice; yields on the other substrates were: corn (34.1-84.5 ppm), wheat (3.6-24.4 ppm), and barley (0-6.6 ppm). Fifty isolates of Fusarium from corn inoculated in the field in 1979 with a mixture of strains of F. graminearum, originally collected from corn plants infected with stalk rot, were tested for DONI production on corn. Twenty of these were also tested for zearalenone production. One isolate produced more than 200 ppm DONI, 13 produced 20-50 ppm, 17 produced 10-20 ppm, and the rest produced less than 10 ppm. The only isolate that did not produce DONI was not identified as F. graminearum. All 20 isolates tested produced zearalenone; 18 produced higher levels of zearalenone (15.4-369 ppm) than of DONI. The other 2 isolates formed essentially the same levels of zearalenone and DONI-37 and 30 ppm, and 15 and 16 ppm, respectively.

Production of zearalenone, T-2 toxin, and deoxynivalenol by Fusarium spp. isolated from plant materials grown in North Carolina

Fusarium spp. isolated from plant materials grown in the hot, humid climate of North Carolina were tested for production of mycotoxins. Isolates of F. acuminatum, F. graminearum, F. moniliforme, F. oxysporum, and F. solani produced zearalenone while isolates of F. equiseti and F. graminearum produced T-2 toxin and deoxynivalenol, respectively. This is the first report of zearalenone production by F. solani. The toxins were identified by capillary gas chromatography-mass spectrometry. These findings suggest that there are toxigenic strains of Fusarium indigenous to the warmer regions of the USA and that fasariotoxicoses of animals in this region are not necessarily the result of importing toxic grains from the cooler, upper midwestern USA.

Ingestion of vomitoxin (deoxynivalenol)-contaminated wheat by nonlactating dairy cows

Our objective was to determine if there were serious deleterious effects of wheat naturally contaminated with vomitoxin (deoxynivalenol) on nonlactating dairy cows. Comparisons were between two Quebec spring wheat sources contaminated with Fusarium graminearum in a feeding trial involving 10 nonlactating Holstein dairy cattle offered good quality hay for ad libitum intake supplemented with wheat-oats concentrate (1 kg concentrate/100 kg body weight). Initially, for 3 wk all cows were fed hay plus wheat-oats containing 1.5 mg vomitoxin/kg. Each week thereafter two more cows, chosen at random, were switched to a second wheat-oats concentrate containing higher mycotoxin concentration (6.4 mg vomitoxin/kg). After the last two cows had been on the latter diet for 6 wk, all cows were returned to the previous wheat-oats concentrate containing the lower mycotoxin concentration (1.5 mg vomitoxin/kg). No unusual symptoms of illness that might be attributed to the mycotoxin-contaminated wheat were observed. In body weight gains, the data reflected a linear growth pattern with no apparent effect of changing from low to high vomitoxin content in the concentrate. Generally, consumption of the wheat-oats ration containing 6.4 mg vomitoxin/kg was lower than the 1.5 mg vomitoxin/kg concentrate. The progressive, linear decrease of hay consumption was not affected by source of contaminated wheat-oats diet.