Toxin-dependent utilization of engineered ribosomal protein L3 limits trichothecene resistance in transgenic plants

Type II Fusarium head blight susceptibility conferred by a region on wheat chromosome 4D

Fusarium head blight (FHB) causes significant grain yield and quality reductions in wheat and barley. Most wheat varieties are incapable of preventing FHB spread through the rachis, but disease is typically limited to individually infected spikelets in barley. We point-inoculated wheat lines possessing barley chromosome introgressions to test whether FHB resistance could be observed in a wheat genetic background. The most striking differential was between 4H(4D) substitution and 4H addition lines. The 4H addition line was similarly susceptible to the wheat parent, but the 4H(4D) substitution line was highly resistant, which suggests that there is an FHB susceptibility factor on wheat chromosome 4D. Point inoculation of Chinese Spring 4D ditelosomic lines demonstrated that removing 4DS results in high FHB resistance. We genotyped four Chinese Spring 4DS terminal deletion lines to better characterize the deletions in each line. FHB phenotyping indicated that lines del4DS-2 and del4DS-4, containing smaller deletions, were susceptible and had retained the susceptibility factor. Lines del4DS-3 and del4DS-1 contain larger deletions and were both significantly more resistant, and hence had presumably lost the susceptibility factor. Combining the genotyping and phenotyping results allowed us to refine the susceptibility factor to a 31.7 Mbp interval on 4DS.

Type II Fusarium head blight susceptibility conferred by a region on wheat chromosome 4D

Fusarium head blight (FHB) causes significant grain yield and quality reductions in wheat and barley. Most wheat varieties are incapable of preventing FHB spread through the rachis, but disease is typically limited to individually infected spikelets in barley. We point-inoculated wheat lines possessing barley chromosome introgressions to test whether FHB resistance could be observed in a wheat genetic background. The most striking differential was between 4H(4D) substitution and 4H addition lines. The 4H addition line was similarly susceptible to the wheat parent, but the 4H(4D) substitution line was highly resistant, which suggests that there is an FHB susceptibility factor on wheat chromosome 4D. Point inoculation of Chinese Spring 4D ditelosomic lines demonstrated that removing 4DS results in high FHB resistance. We genotyped four Chinese Spring 4DS terminal deletion lines to better characterize the deletions in each line. FHB phenotyping indicated that lines del4DS-2 and del4DS-4, containing smaller deletions, were susceptible and had retained the susceptibility factor. Lines del4DS-3 and del4DS-1 contain larger deletions and were both significantly more resistant, and hence had presumably lost the susceptibility factor. Combining the genotyping and phenotyping results allowed us to refine the susceptibility factor to a 31.7 Mbp interval on 4DS.

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.

Aphids transform and detoxify the mycotoxin deoxynivalenol via a type II biotransformation mechanism yet unknown in animals

Biotransformation of mycotoxins in animals comprises phase I and phase II metabolisation reactions. For the trichothecene deoxynivalenol (DON), several phase II biotransformation reactions have been described resulting in DON-glutathiones, DON-glucuronides and DON-sulfates made by glutathione-S-transferases, uridine-diphosphoglucuronyl transferases and sulfotransferases, respectively. These metabolites can be easily excreted and are less toxic than their free compounds. Here, we demonstrate for the first time in the animal kingdom the conversion of DON to DON-3-glucoside (DON-3G) via a model system with plant pathogenic aphids. This phase II biotransformation mechanism has only been reported in plants. As the DON-3G metabolite was less toxic for aphids than DON, this conversion is considered a detoxification reaction. Remarkably, English grain aphids (Sitobion avenae) which co-occur with the DON producer Fusarium graminearum on wheat during the development of fusarium symptoms, tolerate DON much better and convert DON to DON-3G more efficiently than pea aphids (Acyrthosiphon pisum), the latter being known to feed on legumes which are no host for F. graminearum. Using a non-targeted high resolution mass spectrometric approach, we detected DON-diglucosides in aphids probably as a result of sequential glucosylation reactions. Data are discussed in the light of an eventual co-evolutionary adaptation of S. avenae to DON.

Beyond Ribosomal Binding: The Increased Polarity and Aberrant Molecular Interactions of 3-epi-deoxynivalenol

Deoxynivalenol (DON) is a secondary fungal metabolite and contaminant mycotoxin that is widely detected in wheat and corn products cultivated around the world. Bio-remediation methods have been extensively studied in the past two decades and promising ways to reduce DON-associated toxicities have been reported. Bacterial epimerization of DON at the C3 carbon was recently reported to induce a significant loss in the bio-toxicity of the resulting stereoisomer (3-epi-DON) in comparison to the parental compound, DON. In an earlier study, we confirmed the diminished bio-potency of 3-epi-DON using different mammalian cell lines and mouse models and mechanistically attributed it to the reduced binding of 3-epi-DON within the ribosomal peptidyl transferase center (PTC). In the current study and by inspecting the chromatographic behavior of 3-epi-DON and its molecular interactions with a well-characterized enzyme, Fusarium graminearum Tri101 acetyltransferase, we provide the evidence that the C3 carbon epimerization of DON influences its molecular interactions beyond the abrogated PTC binding.

Comparison of Volatiles Profile and Contents of Trichothecenes Group B, Ergosterol, and ATP of Bread Wheat, Durum Wheat, and Triticale Grain Naturally Contaminated by Mycobiota

In natural conditions cereals can be infested by pathogenic fungi. These can reduce the grain yield and quality by contamination with mycotoxins which are harmful for plants, animals, and humans. To date, performed studies of the compounds profile have allowed for the distinction of individual species of fungi. The aim of this study was to determine the profile of volatile compounds and trichothecenes of group B, ergosterol, adenosine triphosphate content carried out on a representative sample of 16 genotypes of related cereals: triticale, bread wheat, and durum wheat. Based on an analysis of volatile compounds by means of gas chromatography mass spectrometry and with the use of an electronic nose, volatile profiles for cereals were determined. Differentiation is presented at four levels through discriminant analysis, heatmaps, principal component analysis (PCA), and electronic nose maps. The statistical model was built by subsequent incorporation of chemical groups such as trichothecenes (GC/MS), fungal biomass indicators ergosterol (HPLC) and ATP (luminometric) and volatiles. The results of the discriminatory analyses showed that the volatile metabolites most markedly differentiated grain samples, among which were mainly: lilial, trichodiene, p-xylene. Electronic nose analysis made it possible to completely separate all the analyzed cereals based only on 100 ions from the 50-150 m/z range. The research carried out using chemometric analysis indicated significant differences in the volatile metabolites present in the grain of bread wheat, durum wheat and triticale. The end result of the performed analyses was a complete discrimination of the examined cereals based on the metabolites present in their grain.

Solvent and Water Mediated Structural Variations in Deoxynivalenol and Their Potential Implications on the Disruption of Ribosomal Function

Fusarium head blight (FHB) is a disease of cereal crops caused by trichothecene producing Fusarium species. Trichothecenes, macrocylicic fungal metabolites composed of three fused rings (A-C) with one epoxide functionality, are a class of mycotoxins known to inhibit protein synthesis in eukaryotic ribosomes. These toxins accumulate in the kernels of infected plants rendering them unsuitable for human and animal consumption. Among the four classes of trichothecenes (A-D) A and B are associated with FHB, where the type B trichothecene deoxynivalenol (DON) is most relevant. While it is known that these toxins inhibit protein synthesis by disrupting peptidyl transferase activity, the exact mechanism of this inhibition is poorly understood. The three-dimensional structures and H-bonding behavior of DON were evaluated using one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy techniques. Comparisons of the NMR structure presented here with the recently reported crystal structure of DON bound in the yeast ribosome reveal insights into the possible toxicity mechanism of this compound. The work described herein identifies a water binding pocket in the core structure of DON, where the 3OH plays an important role in this interaction. These results provide preliminary insights into how substitution at C3 reduces trichothecene toxicity. Further investigations along these lines will provide opportunities to develop trichothecene remediation strategies based on the disruption of water binding interactions with 3OH.

Identification ofSaccharomyces cerevisiaeRibosomal Protein L3 as a Target of Curvularol, a G1-Specific Inhibitor of Mammalian Cells

The cellular target of curvularol, a G1-specific cell-cycle inhibitor of mammalian cells, was identified by a genetic approach in Saccharomyces cerevisiae. Since the wild-type W303 strain was highly resistant to curvularol, a drug hypersensitive parental strain was constructed in which various genes implicated in general drug resistance had been disrupted. Curvularol resistant mutants were isolated, and strains that exhibited a semi-dominant, curvularol-specific resistance phenotype were selected. All five strains examined were classified into a single genetic complementation group designated YCR1. A mutant gene responsible for curvularol resistance was identified as an allele of the RPL3 gene encoding the ribosomal protein L3. Sequence analysis of the mutant genes revealed that Trp255Cys and Trp255Leu substitutions of Rpl3p are responsible for curvularol resistance. Rpl3p mutants in which Trp255 residue was replaced by other amino acids were constructed. All of these replacements led to varying degrees of increased resistance to curvularol and growth defects.

Rapamycin induces apoptosis when autophagy is inhibited in T-47D mammary cells and both processes are regulated by Phlda1

Autophagy is an evolutionarily conserved lysosomal degradation pathway and plays a critical role in the homeostatic process of recycling proteins and organelles. Functional relationships have been described between apoptosis and autophagy. Perturbations in the apoptotic machinery have been reported to induce autophagic cell deaths. Inhibition of autophagy in cancer cells has resulted in cell deaths that manifested hallmarks of apoptosis. However, the molecular relationships and the circumstances of which molecular pathways dictate the choice between apoptosis and autophagy are currently unknown. This study aims to identify specific gene expression of rapamycin-induced autophagy and the effects of rapamycin when the autophagy process is inhibited. In this study, we have demonstrated that rapamycin is capable of inducing autophagy in T-47D breast carcinoma cells. However, when the autophagy process was inhibited by 3-MA, the effects of rapamycin became apoptotic. The Phlda1 gene was found to be up-regulated in both autophagy and apoptosis and silencing this gene was found to reduce both activities, strongly suggests that Phlda1 mediates and positively regulates both autophagy and apoptosis pathways.

First results of GEN-AU: Cloning of Deoxynivalenol- and Zearalenone-inactivating UDP-glucosyltransferase genes fromArabidopsis thaliana and expression in yeast for production of mycotoxin-glucosides

First results of the GEN-AU pilot project "Fusarium virulence and plant resistance mechanisms" are reported. Employing genetically engineered yeast strains we have been able to clone genes from the model plantArabidopsis thaliana encoding UDP-glucosyltransferases which can inactivate deoxynivalenol (DON) and zearalenone (ZON). The structure of the metabolites produced by the transformed yeast strains were determined by LC-MS/MS as DON-3O-glucoside and ZON-4O-glucoside, respectively. ZON and derivatives added to glucosyltransferase expressing yeast cultures are converted into the corresponding glucosides in very high yield, opening an efficient way to produce reference materials for these masked mycotoxins.

Transgenic Arabidopsis thaliana expressing a barley UDP-glucosyltransferase exhibit resistance to the mycotoxin deoxynivalenol

Fusarium head blight (FHB), caused by Fusarium graminearum, is a devastating disease of small grain cereal crops. FHB causes yield reductions and contamination of grain with trichothecene mycotoxins such as deoxynivalenol (DON). DON inhibits protein synthesis in eukaryotic cells and acts as a virulence factor during fungal pathogenesis, therefore resistance to DON is considered an important component of resistance against FHB. One mechanism of resistance to DON is conversion of DON to DON-3-O-glucoside (D3G). Previous studies showed that expression of the UDP-glucosyltransferase genes HvUGT13248 from barley and AtUGt73C5 (DOGT1) from Arabidopsis thaliana conferred DON resistance to yeast. Over-expression of AtUGt73C5 in Arabidopsis led to increased DON resistance of seedlings but also to dwarfing of transgenic plants due to the formation of brassinosteroid-glucosides. The objectives of this study were to develop transgenic Arabidopsis expressing HvUGT13248, to test for phenotypic changes in growth habit, and the response to DON. Transgenic lines that constitutively expressed the epitope-tagged HvUGT13248 protein exhibited increased resistance to DON in a seed germination assay and converted DON to D3G to a higher extent than the untransformed wild-type. By contrast to the over-expression of DOGT1 in Arabidopsis, which conjugated the brassinosteriod castasterone with a glucoside group resulting in a dwarf phenotype, expression of the barley HvUGT13248 gene did not lead to drastic morphological changes. Consistent with this observation, no castasterone-glucoside formation was detectable in yeast expressing the barley HvUGT13248 gene. This barley UGT is therefore a promising candidate for transgenic approaches aiming to increase DON and Fusarium resistance of crop plants without undesired collateral effects.

Current and future experimental strategies for structural analysis of trichothecene mycotoxins--a prospectus

Fungal toxins, such as those produced by members of the order Hypocreales, have widespread effects on cereal crops, resulting in yield losses and the potential for severe disease and mortality in humans and livestock. Among the most toxic are the trichothecenes. Trichothecenes have various detrimental effects on eukaryotic cells including an interference with protein production and the disruption of nucleic acid synthesis. However, these toxins can have a wide range of toxicity depending on the system. Major differences in the phytotoxicity and cytotoxicity of these mycotoxins are observed for individual members of the class, and variations in toxicity are observed among different species for each individual compound. Furthermore, while diverse toxicological effects are observed throughout the whole cellular system upon trichothecene exposure, the mechanism of toxicity is not well understood. In order to comprehend how these toxins interact with the cell, we must first have an advanced understanding of their structure and dynamics. The structural analysis of trichothecenes was a subject of major interest in the 1980s, and primarily focused on crystallographic and solution-state Nuclear Magnetic Resonance (NMR) spectroscopic studies. Recent advances in structural determination through solution- and solid-state NMR, as well as computation based molecular modeling is leading to a resurgent interest in the structure of these and other mycotoxins, with the focus shifting in the direction of structural dynamics. The purpose of this work is to first provide a brief overview of the structural data available on trichothecenes and a characterization of the methods commonly employed to obtain such information. A summary of the current understanding of the relationship between structure and known function of these compounds is also presented. Finally, a prospectus on the application of new emerging structural methods on these and other related systems is discussed.

Hydrogen-bonding interactions in T-2 toxin studied using solution and solid-state NMR

The structure of T-2 toxin in the solid-state is limited to X-ray crystallographic studies, which lack sufficient resolution to provide direct evidence for hydrogen-bonding interactions. Furthermore, its solution-structure, despite extensive Nuclear Magnetic Resonance (NMR) studies, has provided little insight into its hydrogen-bonding behavior, thus far. Hydrogen-bonding interactions are often an important part of biological activity. In order to study these interactions, the structure of T-2 toxin was compared in both the solution- and solid-state using NMR Spectroscopy. It was determined that the solution- and solid-state structure differ dramatically, as indicated by differences in their carbon chemical shifts, these observations are further supported by solution proton spectral parameters and exchange behavior. The slow chemical exchange process and cross-relaxation dynamics with water observed between the hydroxyl hydrogen on C-3 and water supports the existence of a preferential hydrogen bonding interaction on the opposite side of the molecule from the epoxide ring, which is known to be essential for trichothecene toxicity. This result implies that these hydrogen-bonding interactions could play an important role in the biological function of T-2 toxin and posits towards a possible interaction for the trichothecene class of toxins and the ribosome. These findings clearly illustrate the importance of utilizing solid-state NMR for the study of biological compounds, and suggest that a more detailed study of this whole class of toxins, namely trichothecenes, should be pursued using this methodology.

Validation of a candidate deoxynivalenol-inactivating UDP-glucosyltransferase from barley by heterologous expression in yeast

Resistance to the virulence factor deoxynivalenol (DON) due to formation of DON-3-O-glucoside (D3G) is considered to be an important component of resistance against Fusarium spp. which produce this toxin. Multiple candidate UDP-glycosyltransferase (UGT) genes from different crop plants that are either induced by Fusarium spp. or differentially expressed in cultivars varying in Fusarium disease resistance have been described. However, UGT are encoded by a very large gene family in plants. The study of candidate plant UGT is highly warranted because of the potential relevance for developing Fusarium-spp.-resistant crops. We tested Arabidopsis thaliana genes closely related to a previously identified DON-glucosyltransferase gene by heterologous expression in yeast and showed that gene products with very high sequence similarity can have pronounced differences in detoxification capabilities. We also tested four candidate barley glucosyltransferases, which are highly DON inducible. Upon heterologous expression of full-length cDNAs, only one gene, HvUGT13248, conferred DON resistance. The conjugate D3G accumulated in the supernatant of DON-treated yeast transformants. We also present evidence that the product of the TaUGT3 gene recently proposed to encode a DON-detoxification enzyme of wheat does not protect yeast against DON.

Effects of deoxynivalenol on content of chloroplast pigments in barley leaf tissues

To understand further the role of deoxynivalenol (DON) in development of Fusarium head blight (FHB), we investigated effects of the toxin on uninfected barley tissues. Leaf segments, 1 to 1.2 cm long, partially stripped of epidermis were floated with exposed mesophyll in contact with DON solutions. In initial experiments with the leaf segments incubated in light, DON at 30 to 90 ppm turned portions of stripped tissues white after 48 to 96 h. The bleaching effect was greatly enhanced by addition of 1 to 10 mM Ca(2+), so that DON at 10 to 30 ppm turned virtually all stripped tissues white within 48 h. Content of chlorophylls a and b and of total carotenoid pigment was reduced. Loss of electrolytes and uptake of Evans blue indicated that DON had a toxic effect, damaging plasmalemmas in treated tissues before chloroplasts began to lose pigment. When incubated in the dark, leaf segments also lost electrolytes, indicating DON was toxic although the tissues remained green. Thus, loss of chlorophyll in light was due to photobleaching and was a secondary effect of DON, not required for toxicity. In contrast to bleaching effects, some DON treatments that were not toxic kept tissues green without bleaching or other signs of injury, indicating senescence was delayed compared with slow yellowing of untreated leaf segments. Cycloheximide, which like DON, inhibits protein synthesis, also bleached some tissues and delayed senescence of others. Thus, the effects of DON probably relate to its ability to inhibit protein synthesis. With respect to FHB, the results suggest DON may have multiple roles in host cells of infected head tissues, including delayed senescence in early stages of infection and contributing to bleaching and death of cells in later stages.

Intracellular expression of a single domain antibody reduces cytotoxicity of 15-acetyldeoxynivalenol in yeast

15-Acetyldeoxynivalenol (15-AcDON) is a low molecular weight sesquiterpenoid trichothecene mycotoxin associated with Fusarium ear rot of maize and Fusarium head blight of small grain cereals. The accumulation of mycotoxins such as deoxynivalenol (DON) and 15-AcDON within harvested grain is subject to stringent regulation as both toxins pose dietary health risks to humans and animals. These toxins inhibit peptidyltransferase activity, which in turn limits eukaryotic protein synthesis. To assess the ability of intracellular antibodies (intrabodies) to modulate mycotoxin-specific cytotoxocity, a gene encoding a camelid single domain antibody fragment (V(H)H) with specificity and affinity for 15-AcDON was expressed in the methylotropic yeast Pichia pastoris. Cytotoxicity and V(H)H immunomodulation were assessed by continuous measurement of cellular growth. At equivalent doses, 15-AcDON was significantly more toxic to wild-type P. pastoris than was DON. In turn, DON was orders of magnitude more toxic than 3-acetyldeoxynivalenol. Intracellular expression of a mycotoxin-specific V(H)H within P. pastoris conveyed significant (p = 0.01) resistance to 15-AcDON cytotoxicity at doses ranging from 20 to 100 mug.ml(-1). We also documented a biochemical transformation of DON to 15-AcDON to account for the attenuation of DON cytotoxicity at 100 and 200 mug.ml(-1). The proof of concept established within this eukaryotic system suggests that in planta V(H)H expression may lead to enhanced tolerance to mycotoxins and thereby limit Fusarium infection of commercial agricultural crops.

Double mutation in tomato ribosomal protein L3 cDNA confers tolerance to deoxynivalenol (DON) in transgenic tobacco

The contamination of mycotoxins associated with head blight of wheat and other grains caused by Fusarium graminearum is chronic threat to crop, human and animal health throughout the world. Deoxinevalenol (DON), produced by the fungus, belonging to class trichothecene is believed to act as a virulence factor in fungal pathogenesis by inhibiting eukaryotic protein synthesis, thereby blocking or delaying the expression of defense related proteins induced by host plant. The putative site of action of DON is 60s ribosomal protein L3 (RPL3). In order to reduce the effects of DON in the host plants, we modified tomato RPL3 (LeRPL3) to introduce W25R/H259Y mutations so that amino acid residue 258 is changed from tryptophan to arginine and 259 from histidine to tyrosine. Transgenic tobacco plants expressing these modified LeRPL3 cDNAs were tested for growth pattern of T1 seedlings in presence of DON. When seedling of these transgenic tobacco plants were compared for growth in the presence of DON, a significant difference in growth rate and the ability to undergo differentiation was observed among those plants expressing the modified version of the Rp13 gene, compared to those expressing the wild-type Rp13 gene. Expression of the tagged gene product indicates that is was not due to somaclonal variation. These results indicate a possible mechanism of host plant resistance to the fungal pathogen F. graminearum among the susceptible cereal species based on the expression ofmodified Rp13 genes.

Functional analysis of Saccharomyces cerevisiae ribosomal protein Rpl3p in ribosome synthesis

Ribosome synthesis in eukaryotes requires a multitude of trans-acting factors. These factors act at many steps as the pre-ribosomal particles travel from the nucleolus to the cytoplasm. In contrast to the well-studied trans-acting factors, little is known about the contribution of the ribosomal proteins to ribosome biogenesis. Herein, we have analysed the role of ribosomal protein Rpl3p in 60S ribosomal subunit biogenesis. In vivo depletion of Rpl3p results in a deficit in 60S ribosomal subunits and the appearance of half-mer polysomes. This phenotype is likely due to the instability of early and intermediate pre-ribosomal particles, as evidenced by the low steady-state levels of 27SA₃, 27SB_S and 7S_L/S precursors. Furthermore, depletion of Rpl3p impairs the nucleocytoplasmic export of pre-60S ribosomal particles. Interestingly, flow cytometry analysis indicates that Rpl3p-depleted cells arrest in the G1 phase. Altogether, we suggest that upon depletion of Rpl3p, early assembly of 60S ribosomal subunits is aborted and subsequent steps during their maturation and export prevented.

Transgenic rice plants expressing trichothecene 3-O-acetyltransferase show resistance to the Fusarium phytotoxin deoxynivalenol

Fusarium head blight (FHB) is a devastating disease of small grain cereal crops caused by the necrotrophic pathogen Fusarium graminearum and Fusarium culmorum. These fungi produce the trichothecene mycotoxin deoxynivalenol (DON) and its derivatives, which enhance the disease development during their interactions with host plants. For the self-protection, the trichothecene producer Fusarium species have Tri101 encoding trichothecene 3-O-acetyltransferase. Although transgenic expression of Tri101 significantly reduced inhibitory action of DON on tobacco plants, there are several conflicting observations regarding the phytotoxicity of 3-acetyldeoxynivalenol (3-ADON) to cereal plants; 3-ADON was reported to be highly phytotoxic to wheat at low concentrations. To examine whether cereal plants show sufficient resistance to 3-ADON, we generated transgenic rice plants with stable expression and inheritance of Tri101. While root growth of wild-type rice plants was severely inhibited by DON in the medium, this fungal toxin was not phytotoxic to the transgenic lines that showed trichothecene 3-O-acetylation activity. This is the first report demonstrating the DON acetylase activity and DON-resistant phenotype of cereal plants expressing the fungal gene.

Cloning and characterization of the ribosomal protein L3 (RPL3) gene family from Triticum aestivum

Plant pathogenic fungi of the genus Fusarium can cause severe diseases on small grain cereals and maize. The contamination of harvested grain with Fusarium mycotoxins is a threat to human and animal health. In wheat production of the toxin deoxynivalenol (DON), which inhibits eukaryotic protein biosynthesis, is a virulence factor of Fusarium, and resistance against DON is considered to be part of Fusarium resistance. Previously, single amino acid changes in RPL3 (ribosomal protein L3) conferring DON resistance have been described in yeast. The goal of this work was to characterize the RPL3 gene family from wheat and to investigate the potential role of naturally existing RPL3 alleles in DON resistance by comparing Fusarium-resistant and susceptible cultivars. The gene family consists of three homoeologous alleles of both RPL3A and RPL3B, which are located on chromosomes 4A (RPL3-B2), 4B (RPL3-B1), 4D (RPL3-B3), 5A (RPL3-A3), 5B (RPL3-A2) and 5D (RPL3-A1). Alternative splicing was detected in the TaRPL3-A2 gene. Sequence comparison revealed no amino acid differences between cultivars differing in Fusarium resistance. While using developed SNP markers we nevertheless found that one of the genes, namely, TaRPL3-A3 mapped close to a Fusarium resistance QTL (Qfhs.ifa-5A). The potential role of the RPL3 gene family in DON resistance of wheat is discussed.

Fusarium Tri4 encodes a key multifunctional cytochrome P450 monooxygenase for four consecutive oxygenation steps in trichothecene biosynthesis

Fusarium Tri4 encodes a cytochrome P450 monooxygenase (CYP) for hydroxylation at C-2 of the first committed intermediate trichodiene (TDN) in the biosynthesis of trichothecenes. To examine whether this CYP further participates in subsequent oxygenation steps leading to isotrichotriol (4), we engineered Saccharomyces cerevisiae for de novo production of the early intermediates by introducing cDNAs of Fusarium graminearum Tri5 (FgTri5 encoding TDN synthase) and Tri4 (FgTri4). From a culture of the engineered yeast grown on induction medium (final pH 2.7), we identified two intermediates, 2alpha-hydroxytrichodiene (1) and 12,13-epoxy-9,10-trichoene-2alpha-ol (2), and a small amount of non-Fusarium trichothecene 12,13-epoxytrichothec-9-ene (EPT). Other intermediates isotrichodiol (3) and 4 were identified in the transgenic yeasts grown on phosphate-buffered induction medium (final pH 5.5-6.0). When Trichothecium roseum Tri4 (TrTri4) was used in place of FgTri4, 4 was not detected in the culture. The three intermediates, 1, 2, and 3, were converted to 4,15-diacetylnivalenol (4,15-diANIV) when fed to a toxin-deficient mutant of F. graminearum with the FgTri4+ genetic background (viz., by introducing a FgTri5- mutation), but were not metabolized by an FgTri4- mutant. These results provide unambiguous evidence that FgTri4 encodes a multifunctional CYP for epoxidation at C-12,13, hydroxylation at C-11, and hydroxylation at C-3 in addition to hydroxylation at C-2.

Molecular basis of N-acetylglucosaminyltransferase I deficiency in Arabidopsis thaliana plants lacking complex N-glycans

GnTI (N-acetylglucosaminyltransferase I) is a Golgi-resident enzyme essential for the processing of high-mannose to hybrid and complex N-glycans. The Arabidopsis thaliana cgl mutant lacks GnTI activity and as a consequence accumulates oligomannosidic structures. Molecular cloning of cgl GnTI cDNA revealed a point mutation, which causes a critical amino acid substitution (Asp144-->Asn), thereby creating an additional N-glycosylation site. Heterologous expression of cgl GnTI in insect cells confirmed its lack of activity and the use of the N-glycosylation site. Remarkably, introduction of the Asp144-->Asn mutation into rabbit GnTI, which does not result in the formation of a new N-glycosylation site, led to a protein with strongly reduced, but still detectable enzymic activity. Expression of Asn144 rabbit GnTI in cgl plants could partially restore complex N-glycan formation. These results indicate that the complete deficiency of GnTI activity in cgl plants is mainly due to the additional N-glycan, which appears to interfere with the proper folding of the enzyme.

Expression of a truncated form of ribosomal protein L3 confers resistance to pokeweed antiviral protein and the Fusarium mycotoxin deoxynivalenol

The contamination of important agricultural products such as wheat, barley, or maize with the trichothecene mycotoxin deoxynivalenol (DON) due to infection with Fusarium species is a worldwide problem. Trichothecenes inhibit protein synthesis by targeting ribosomal protein L3. Pokeweed antiviral protein (PAP), a ribosome-inactivating protein binds to L3 to depurinate the alpha-sarcin/loop of the large rRNA. Plants transformed with the wild-type PAP show lesions and express very low levels of PAP because PAP autoregulates its expression by destabilizing its own mRNA. We show here that transgenic tobacco plants expressing both the wild-type PAP and a truncated form of yeast L3 (L3delta) are phenotypically normal. PAP mRNA and protein levels are very high in these plants, indicating that L3delta suppresses the autoregulation of PAP mRNA expression. Ribosomes are not depurinated in the transgenic plants expressing PAP and L3delta, even though PAP is associated with ribosomes. The expression of the endogenous tobacco ribosomal protein L3 is up-regulated in these plants and they are resistant to the Fusarium mycotoxin DON. These results demonstrate that expression of an N-terminal fragment of yeast L3 leads to trans-dominant resistance to PAP and the trichothecene mycotoxin DON, providing evidence that both toxins target L3 by a common mechanism.