Structural analysis of cassiicolin, a host-selective protein toxin from Corynespora cassiicola

Durian Shell-Mediated Simple Green Synthesis of Nanocopper against Plant Pathogenic Fungi

The synthesis of fungicides in eco-friendly and cost-effective ways is significantly essential for agriculture. Plant pathogenic fungi cause many ecological and economic issues worldwide, which must be treated with effective fungicides. Here, this study proposes the biosynthesis of fungicides, which combines copper and Cu₂O nanoparticles (Cu/Cu₂O) synthesized using durian shell (DS) extract as a reducing agent in aqueous media. Sugar and polyphenol compounds contained in DS, as the main phytochemicals acting in the reduction procedure, were extracted under different temperatures and duration conditions to obtain the highest yields. We confirmed the extraction process performed at 70 °C for 60 min to be the most effective in extracting sugar (6.1 g/L) and polyphenols (22.7 mg/L). We determined the suitable conditions for Cu/Cu₂O synthesis using a DS extract as a reducing agent for a synthesis time of 90 min, a volume ratio of DR extract/Cu²⁺ of 15:35, an initial pH solution of 10, a synthesis temperature of 70 °C, and a CuSO₄ concentration of 10 mM. The characterization results of as-prepared Cu/Cu₂O NP showed a highly crystalline structure of Cu₂O and Cu with sizes estimated in the range of 40-25 nm and 25-30 nm, respectively. Through in vitro experiments, the antifungal efficacy of Cu/Cu₂O against Corynespora cassiicola and Neoscytalidium dimidiatum was investigated by the inhibition zone. The green-synthesized Cu/Cu₂O nanocomposites, which are potential antifungals against plant pathogens, exhibited excellent antifungal efficacy against both Corynespora cassiicola (MIC = 0.25 g/L, the diameter of the inhibition zone was 22.00 ± 0.52 mm) and Neoscytalidium dimidiatum (MIC = 0.0625 g/L, the diameter of the inhibition zone was 18.00 ± 0.58 mm). Cu/Cu₂O nanocomosites prepared in this study could be a valuable suggestion for the control of plant pathogenic fungi affecting crop species globally.

Subcutaneous phaeohyphomycosis caused by plant pathogenic Corynespora cassiicola: A case report

Corynespora cassiicola is a common plant pathogen responsible for leaf-spotting diseases in the tropical and subtropical areas. C. cassiicola seldom causes human infections. Here we describe a case of subcutaneous phaeohyphomycosis caused by C. cassiicola in a 76-year-old Chinese man, who presented to our hospital with a purulent discharge and painful sensation on his right leg. Skin biopsy revealed an abscess, and culture confirmed C. cassiicola to be the causative agent. The result was further identified by sequence analysis of the internal transcribed spacer region. The patient was successfully treated with systemic voriconazole and wound debridement: the lesion disappeared after 20 days.

Unraveling the Host-Selective Toxic Interaction of Cassiicolin with Lipid Membranes and Its Cytotoxicity

Cassiicolin (Cas), a toxin produced by Corynespora cassiicola, is responsible for Corynespora leaf fall disease in susceptible rubber trees. Currently, the molecular mechanisms of the cytotoxicity of Cas and its host selectivity have not been fully elucidated. Here, we analyzed the binding of Cas1 and Cas2 to membranes consisting of different plant lipids and their membrane disruption activities. Using high-speed atomic force microscopy and confocal microscopy, we reveal that the binding and disruption activities of Cas1 and Cas2 on lipid membranes are strongly dependent on the specific plant lipids. The negative phospholipids, glycerolipids, and sterols are more sensitive to membrane damage caused by Cas1 and Cas2 than neutral phospholipids and betaine lipids. Mature Cas1 and Cas2 play an essential role in causing membrane disruption. Cytotoxicity tests on rubber leaves of Rubber Research Institute of Vietnam (RRIV) 1, RRIV 4, and Prang Besar (PB) 255 clones suggest that the toxins cause necrosis of rubber leaves, except for the strong resistance of PB 255 against Cas2. Cryogenic scanning electron microscopy analyses of necrotic leaf tissues treated with Cas1 confirm that cytoplasmic membranes are vulnerable to the toxin. Thus, the host selectivity of Cas toxin is attained by the lipid-dependent binding activity of Cas to the membrane, and the cytotoxicity of Cas arises from its ability to form biofilm-like structures and to disrupt specific membranes.

Transcriptome profiling in susceptible and tolerant rubber tree clones in response to cassiicolin Cas1, a necrotrophic effector from Corynespora cassiicola

Corynespora cassiicola, a fungal plant pathogen with a large host range, causes important damages in rubber tree (Hevea brasiliensis), in Asia and Africa. A small secreted protein named cassiicolin was previously identified as a necrotrophic effector required for the virulence of C. cassiicola in specific rubber tree clones. The objective of this study was to decipher the cassiicolin-mediated molecular mechanisms involved in this compatible interaction. We comparatively analyzed the RNA-Seq transcriptomic profiles of leaves treated or not with the purified cassiicolin Cas1, in two rubber clones: PB260 (susceptible) and RRIM600 (tolerant). The reads were mapped against a synthetic transcriptome composed of all available transcriptomic references from the two clones. Genes differentially expressed in response to cassiicolin Cas1 were identified, in each clone, at two different time-points. After de novo annotation of the synthetic transcriptome, we analyzed GO enrichment of the differentially expressed genes in order to elucidate the main functional pathways impacted by cassiicolin. Cassiicolin induced qualitatively similar transcriptional modifications in both the susceptible and the tolerant clones, with a strong negative impact on photosynthesis, and the activation of defense responses via redox signaling, production of pathogenesis-related protein, or activation of the secondary metabolism. In the tolerant clone, transcriptional reprogramming occurred earlier but remained moderate. By contrast, the susceptible clone displayed a late but huge transcriptional burst, characterized by massive induction of phosphorylation events and all the features of a hypersensitive response. These results confirm that cassiicolin Cas1 is a necrotrophic effector triggering a hypersensitive response in susceptible rubber clones, in agreement with the necrotrophic-effector-triggered susceptibility model.

Genomic Characteristics and Comparative Genomics Analysis of Two Chinese Corynespora cassiicola Strains Causing Corynespora Leaf Fall (CLF) Disease

Rubber tree Corynespora leaf fall (CLF) disease, caused by the fungus Corynespora cassiicola, is one of the most damaging diseases in rubber tree plantations in Asia and Africa, and this disease also threatens rubber nurseries and young rubber plantations in China. C. cassiicola isolates display high genetic diversity, and virulence profiles vary significantly depending on cultivar. Although one phytotoxin (cassicolin) has been identified, it cannot fully explain the diversity in pathogenicity between C. cassiicola species, and some virulent C. cassiicola strains do not contain the cassiicolin gene. In the present study, we report high-quality gapless genome sequences, obtained using short-read sequencing and single-molecule long-read sequencing, of two Chinese C. cassiicola virulent strains. Comparative genomics of gene families in these two stains and a virulent CPP strain from the Philippines showed that all three strains experienced different selective pressures, and metabolism-related gene families vary between the strains. Secreted protein analysis indicated that the quantities of secreted cell wall-degrading enzymes were correlated with pathogenesis, and the most aggressive CCP strain (cassiicolin toxin type 1) encoded 27.34% and 39.74% more secreted carbohydrate-active enzymes (CAZymes) than Chinese strains YN49 and CC01, respectively, both of which can only infect rubber tree saplings. The results of antiSMASH analysis showed that all three strains encode ~60 secondary metabolite biosynthesis gene clusters (SM BGCs). Phylogenomic and domain structure analyses of core synthesis genes, together with synteny analysis of polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) gene clusters, revealed diversity in the distribution of SM BGCs between strains, as well as SM polymorphisms, which may play an important role in pathogenic progress. The results expand our understanding of the C. cassiicola genome. Further comparative genomic analysis indicates that secreted CAZymes and SMs may influence pathogenicity in rubber tree plantations. The findings facilitate future exploration of the molecular pathogenic mechanism of C. cassiicola.

Genome sequence and spore germination-associated transcriptome analysis of Corynespora cassiicola from cucumber

BACKGROUND

Corynespora cassiicola, as a necrotrophic phytopathogenic ascomycetous fungus, can infect hundreds of species of plants and rarely causes human diseases. This pathogen infects cucumber species and causes cucumber target spot, which has recently caused large cucumber yield losses in China. Genome sequence and spore germination-associated transcriptome analysis will contribute to the understanding of the molecular mechanism of pathogenicity and spore germination of C. cassiicola.

RESULTS

First, we reported the draft genome sequences of the cucumber-sampled C. cassiicola isolate HGCC with high virulence. Although conspecific, HGCC exhibited distinct genome sequence differences from a rubber tree-sampled isolate (CCP) and a human-sampled isolate (UM591). The proportion of secreted proteins was 7.2% in HGCC. A total of 28.9% (4232) of HGCC genes, 29.5% (4298) of CCP genes and 28.6% (4214) of UM591 genes were highly homologous to experimentally proven virulence-associated genes, respectively, which were not significantly different (P = 0.866) from the average (29.7%) of 10 other phytopathogenic fungi. Thousands of putative virulence-associated genes in various pathways or families were identified in C. cassiicola. Second, a global view of the transcriptome of C. cassiicola spores during germination was evaluated using RNA sequencing (RNA-Seq). A total of 3288 differentially expressed genes (DEGs) were identified. The majority of KEGG-annotated DEGs were involved in metabolism, genetic information processing, cellular processes, the organismal system, human diseases and environmental information processing.

CONCLUSIONS

These results facilitate the exploration of the molecular pathogenic mechanism of C. cassiicola in cucumbers and the understanding of molecular and cellular processes during spore germination.

Development of a LAMP method for detecting the N75S mutant in SDHI-resistant Corynespora cassiicola

The replacement of asparagine with serine at codon 75 of the sdhC gene (N75S) confers succinate dehydrogenase inhibitor resistance in Corynespora cassiicola, which caused by consecutive fungicide application. To rapidly detect the mutation of N75S, a method based on loop-mediated isothermal amplification (LAMP) was developed in this study. The optimal primer set among the six primer sets designed could clearly identify N75S from the wild-type genotype. The detection threshold of the optimized LAMP mixture (10 μL) was 8.8 fg of target DNA at 63 °C within 60 min. This method specifically showed a color change and ladder-like band only when DNA extracted from isolates containing the N75S mutation was added. The results of stability tests suggested a satisfactory repeatability of this method. Additionally, the assay could positively distinguish N75S mutants from crude DNA isolated from conidia and mycelia of C. cassiicola. Given the high efficiency, sensitivity, specificity, repeatability and simplicity of operation, the LAMP method established here could be useful to evaluate the shift in the sensitivity of C. cassiicola to SDHIs and will provide significant data for the management of Corynespora leaf spot.

Gene deletion of Corynespora cassiicola cassiicolin Cas1 suppresses virulence in the rubber tree

Corynespora cassiicola is an ascomycete fungus causing important damages in a wide range of plant hosts, including rubber tree. The small secreted protein cassiicolin is suspected to play a role in the onset of the disease in rubber tree, based on toxicity and gene expression profiles. However, its exact contribution to virulence, compared to other putative effectors, remains unclear. We created a deletion mutant targeting the cassiicolin gene Cas1 from the highly aggressive isolate CCP. Wild-type CCP and mutant ccpΔcas1 did not differ in terms of mycelium growth, sporulation, and germination rate in vitro. Cas1 gene deletion induced a complete loss of virulence on the susceptible clones PB260 and IRCA631, as revealed by inoculation experiments on intact (non-detached) leaves. However, residual symptoms persisted when inoculations were conducted on detached leaves, notably with longer incubation times. Complementation with exogenous cassiicolin restored the mutant capacity to colonize the leaf tissues. We also compared the toxicity of CCP and ccpΔcas1 culture filtrates, through electrolyte leakage measurements on abraded detached leaves, over a range of clones as well as an F1 population derived from the cross between the clones PB260 (susceptible) and RRIM600 (tolerant). On average, filtrate toxicity was lower but not fully suppressed in ccpΔcas1 compared to CCP, with clone-dependent variations. The two QTL, previously found associated with sensitivity to CPP filtrate or to the purified cassiicolin, were no longer detected with the mutant filtrate, while new QTL were revealed. Our results demonstrate that: (1) cassiicolin is a necrotrophic effector conferring virulence to the CCP isolate in susceptible rubber clones and (2) other effectors produced by CCP contribute to residual filtrate toxicity and virulence in senescing/wounded tissues. These other effectors may be involved in saprotrophy rather than necrotrophy.

Variation of cassiicolin genes among Chinese isolates of Corynespora cassiicola

Corynespora cassiicola is a species of fungus that is a plant pathogen of many agricultural crop plants, including severe target spot disease on cucumber. Cassiicolin is an important effector of pathogenicity of this fungus. In this study, we collected 141 Corynespora isolates from eighteen hosts, and the casscolin gene was detected in 82 C. cassiicola strains. The deduced protein sequences revealed that 72 isolates contained the Cas2 gene, two strains from Gynura bicolor harboured the Cas2.2 gene, and 59 isolates without a cassiicolin gene were classified as Cas0. Phylogenetic analyses was performed for the 141 isolates using four loci (ITS, ga4, caa5, and act1) and revealed two genetic clusters. Cluster A is composed of four subclades: subcluster A1 includes all Cas2 isolates plus 18 Cas0 strains, subcluster A2 includes the eight Cas5 isolates and one Cas0 isolate, and subclusters A3 and A4 contain Cas0 strains. Cluster B consists of 21 Cas0 isolates. Twenty-two C. cassiicola strains from different toxin classes showed varying degrees of virulence against cucumber. Cas0 or Cas2 strains induced diverse responses on cucumber, from no symptoms to symptoms of moderate or severe infection, but all Cas5 isolates exhibited avirulence on cucumber.

Aldaulactone - An Original Phytotoxic Secondary Metabolite Involved in the Aggressiveness of Alternaria dauci on Carrot

Qualitative plant resistance mechanisms and pathogen virulence have been extensively studied since the formulation of the gene-for-gene hypothesis. The mechanisms involved in the quantitative traits of aggressiveness and plant partial resistance are less well-known. Nevertheless, they are prevalent in most plant-necrotrophic pathogen interactions, including the Daucus carota-Alternaria dauci interaction. Phytotoxic metabolite production by the pathogen plays a key role in aggressiveness in these interactions. The aim of the present study was to explore the link between A. dauci aggressiveness and toxin production. We challenged carrot embryogenic cell cultures from a susceptible genotype (H1) and two partially resistant genotypes (I2 and K3) with exudates from A. dauci strains with various aggressiveness levels. Interestingly, A. dauci-resistant carrot genotypes were only affected by exudates from the most aggressive strain in our study (ITA002). Our results highlight a positive link between A. dauci aggressiveness and the fungal exudate cell toxicity. We hypothesize that the fungal exudate toxicity was linked with the amount of toxic compounds produced by the fungus. Interestingly, organic exudate production by the fungus was correlated with aggressiveness. Hence, we further analyzed the fungal organic extract using HPLC, and correlations between the observed peak intensities and fungal aggressiveness were measured. One observed peak was closely correlated with fungal aggressiveness. We succeeded in purifying this peak and NMR analysis revealed that the purified compound was a novel 10-membered benzenediol lactone, a polyketid that we named 'aldaulactone'. We used a new automated image analysis method and found that aldaulactone was toxic to in vitro cultured plant cells at those concentrations. The effects of both aldaulactone and fungal organic extracts were weaker on I2-resistant carrot cells compared to H1 carrot cells. Taken together, our results suggest that: (i) aldaulactone is a new phytotoxin, (ii) there is a relationship between the amount of aldaulactone produced and fungal aggressiveness, and (iii) carrot resistance to A. dauci involves mechanisms of resistance to aldaulactone.

Genome-Wide Analysis of Corynespora cassiicola Leaf Fall Disease Putative Effectors

Corynespora cassiicola is an Ascomycetes fungus with a broad host range and diverse life styles. Mostly known as a necrotrophic plant pathogen, it has also been associated with rare cases of human infection. In the rubber tree, this fungus causes the Corynespora leaf fall (CLF) disease, which increasingly affects natural rubber production in Asia and Africa. It has also been found as an endophyte in South American rubber plantations where no CLF outbreak has yet occurred. The C. cassiicola species is genetically highly diverse, but no clear relationship has been evidenced between phylogenetic lineage and pathogenicity. Cassiicolin, a small glycosylated secreted protein effector, is thought to be involved in the necrotrophic interaction with the rubber tree but some virulent C. cassiicola isolates do not have a cassiicolin gene. This study set out to identify other putative effectors involved in CLF. The genome of a highly virulent C. cassiicola isolate from the rubber tree (CCP) was sequenced and assembled. In silico prediction revealed 2870 putative effectors, comprising CAZymes, lipases, peptidases, secreted proteins and enzymes associated with secondary metabolism. Comparison with the genomes of 44 other fungal species, focusing on effector content, revealed a striking proximity with phylogenetically unrelated species (Colletotrichum acutatum, Colletotrichum gloesporioides, Fusarium oxysporum, nectria hematococca, and Botrosphaeria dothidea) sharing life style plasticity and broad host range. Candidate effectors involved in the compatible interaction with the rubber tree were identified by transcriptomic analysis. Differentially expressed genes included 92 putative effectors, among which cassiicolin and two other secreted singleton proteins. Finally, the genomes of 35 C. cassiicola isolates representing the genetic diversity of the species were sequenced and assembled, and putative effectors identified. At the intraspecific level, effector-based classification was found to be highly consistent with the phylogenomic trees. Identification of lineage-specific effectors is a key step toward understanding C. cassiicola virulence and host specialization mechanisms.

Occurrence, biochemistry and biological effects of host-selective plant mycotoxins

Host-selective mycotoxins (HSTs) are various secondary metabolites or proteinaceous compounds secreted by pathogenic necrotrophic fungi that feed off on dead tissues of certain plants. Research on the HSTs has not only fundamental but also practical importance. On one hand they are implicated in the onset of devastating crop diseases. On the other hand, they have been studied as a good model for revealing the intricate mechanisms of plant-pathogen interactions. At the cellular level, HSTs target different compartments and in most instances induce programmed cell death (PCD) by a wide range of mechanisms. Often the responses provoked by HSTs resemble the effector-triggered immunity used by plant cells to combat biotrophic pathogens, which suggests that HST-producing fungi exploit the plants' own defensive systems to derive benefits. Although by definition HSTs are active only in tissues of susceptible plant genotypes, it has been demonstrated that some of them are able to influence animal cells as well. The possible effects, like cytotoxicity or cytostasis, can be harmful or beneficial and thus HSTs may either pose a health risk for humans and livestock, or be of prospective use in the fields of pharmacology, medicine and agriculture.

Genetic Determinism of Sensitivity to Corynespora cassiicola Exudates in Rubber Tree (Hevea brasiliensis)

An indirect phenotyping method was developed in order to estimate the susceptibility of rubber tree clonal varieties to Corynespora Leaf Fall (CLF) disease caused by the ascomycete Corynespora cassiicola. This method consists in quantifying the impact of fungal exudates on detached leaves by measuring the induced electrolyte leakage (EL%). The tested exudates were either crude culture filtrates from diverse C. cassiicola isolates or the purified cassiicolin (Cas1), a small secreted effector protein produced by the aggressive isolate CCP. The test was found to be quantitative, with the EL% response proportional to toxin concentration. For eight clones tested with two aggressive isolates, the EL% response to the filtrates positively correlated to the response induced by conidial inoculation. The toxicity test applied to 18 clones using 13 toxinic treatments evidenced an important variability among clones and treatments, with a significant additional clone x treatment interaction effect. A genetic linkage map was built using 306 microsatellite markers, from the F1 population of the PB260 x RRIM600 family. Phenotyping of the population for sensitivity to the purified Cas1 effector and to culture filtrates from seven C. cassiicola isolates revealed a polygenic determinism, with six QTL detected on five chromosomes and percentages of explained phenotypic variance varying from 11 to 17%. Two common QTL were identified for the CCP filtrate and the purified cassiicolin, suggesting that Cas1 may be the main effector of CCP filtrate toxicity. The CCP filtrate clearly contrasted with all other filtrates. The toxicity test based on Electrolyte Leakage Measurement offers the opportunity to assess the sensitivity of rubber genotypes to C. cassiicola exudates or purified effectors for genetic investigations and early selection, without risk of spreading the fungus in plantations. However, the power of this test for predicting field susceptibility of rubber clones to CLF will have to be further investigated.

Changes in the Antioxidant System in Soybean Leaves Infected by Corynespora cassiicola

Considering the importance of target spot, caused by the fungus Corynespora cassiicola, to reduce soybean yield in Brazil and that more basic information regarding the soybean-C. cassiicola interaction is needed, the present study aimed to investigate whether the cellular damage caused by C. cassiicola infection could activate the antioxidant system and whether a more efficient antioxidant system could be associated with an increase in soybean resistance to target spot. The activities of the antioxidant enzymes superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, glutathione peroxidase, glutathione reductase, glutathione S-transferase as well as the concentrations of ascorbate (AsA), hydrogen peroxide (H2O2), superoxide (O2•-), and malondialdehyde (MDA) were measured in soybean plants from two cultivars differing in resistance to the pathogen. The number of lesions per square centimeter was significantly reduced by 14% in plants from cultivar Fundacep 59 compared with plants from cultivar TMG 132. The area under the disease progress curve was significantly lower, by 15%, in plants from Fundacep 59 than in plants from TMG 132. Generally, antioxidant enzyme activities and AsA concentration significantly increased in response to C. cassiicola infection in plants from both cultivars, however more prominent increases were recorded for plants from Fundacep 59. The concentrations of MDA, H2O2, and O2•- also increased, particularly for plants from TMG 132. The results from this study highlight the importance of a more efficient antioxidative system in the removal of reactive oxygen species generated in soybean plants during C. cassiicola infection, contributing to the resistance to target spot.

Toxicity of extracellular proteins from Diplodia seriata and Neofusicoccum parvum involved in grapevine Botryosphaeria dieback

Botryosphaeria dieback, esca and Eutypa dieback are three economic major grapevine trunk diseases that cause severe yield reduction in vineyards worldwide. The frequency of disease symptoms has increased considerably over the past decade, and no efficient treatment is currently available to control these diseases. The different fungi associated with grapevine trunk diseases mainly induce necrotic wood and characteristic foliar symptoms. In this context, fungi virulence factors and host invasion are not well understood. We hypothesise that extracellular proteins produced by Diplodia seriata and Neofusicoccum parvum, two causal agents associated with Botryosphaeria dieback, are virulence factors responsible for the pathogenicity. In our previous work, we demonstrated that the total extracellular compounds produced by N. parvum induced more necrosis on Chardonnay calli and triggered a different defence gene expression pattern than those produced by D. seriata. Furthermore, this aggressiveness was not clearly correlated with the production of mellein, a characteristic phytotoxin of Botryosphaeriaceae, in our in vitro calli model. To characterise other potential virulence factors and to understand the mechanisms of host invasion by the fungus, we evaluated the profile, quantity and the impact of extracellular proteins produced by these fungi on Vitis vinifera calli necrosis and defence gene expression. Our results reveal that, under the same conditions, N. parvum produces more extracellular proteins and in higher concentrations than D. seriata. With Vitis vinifera cv. Chardonnay cells, we showed that equivalent concentrations of proteins secreted by N. parvum were more aggressive than those of D. seriata in producing necrosis and that they clearly induced more grapevine defence genes.

Rare case of fungal keratitis caused by Corynespora cassiicola

We describe a 76-year-old male farmer with no diabetes mellitus and no history of ocular trauma from soil or plants who developed a corneal infection from a plant pathogen. The organism was identified as Corynespora cassiicola based on both the morphological characteristics and the sequence of the internal transcribed spacer region of the ribosomal RNA gene. The patient was treated successfully with a combination of topical and systemic voriconazole. This is the first reported case of keratomycosis caused by C. cassiicola.

Characterization of a cassiicolin-encoding gene from Corynespora cassiicola, pathogen of rubber tree (Hevea brasiliensis)

Corynespora Leaf Fall (CLF) is a major disease of rubber tree (Hevea brasiliensis) caused by the Ascomycota Corynespora cassiicola. Here we describe the cloning and characterization of a gene encoding cassiicolin (Cas), a glycosylated cystein-rich small secreted protein (SSP) identified as a potential CLF disease effector in rubber tree. Three isolates with contrasted levels of aggressiveness were analyzed comparatively. The cassiicolin gene was detected - and the toxin successfully purified - from the isolates with high and medium aggressiveness (CCP and CCAM3 respectively) but not from the isolate with the lowest aggressiveness (CCAM1), suggesting the existence of a different disease effector in the later. CCP and CCAM3 carried strictly identical cassiicolin genes and produced toxins of identical mass, as evidence by mass spectrometry analysis, thus suggesting conserved post-translational modifications in addition to sequence identity. The differences in aggressiveness between CCP and CCAM3 may be attributed to differences in cassiicolin transcript levels rather than qualitative variations in cassiicolin structure. Cassiicolin may play an important role in the early phase of infection since a peak of cassiicolin transcripts occurred in 1 or 2 days after inoculation (before the occurrence of the first symptoms), in both the tolerant and the susceptible cultivars.

Stagonospora nodorum: from pathology to genomics and host resistance

Stagonospora nodorum is a major necrotrophic pathogen of wheat that causes the diseases S. nodorum leaf and glume blotch. A series of tools and resources, including functional genomics, a genome sequence, proteomics and metabolomics, host-mapping populations, and a worldwide collection of isolates, have enabled the dissection of pathogenicity mechanisms. Metabolic and signaling genes required for pathogenicity have been defined. Interaction with the host is dominated by interplay of fungal effectors that induce necrosis on wheat lines carrying specific sensitivity loci. As such, the pathogen has emerged as a model for the Pleosporales group of pathogens.

Maximizing chemical diversity of fungal metabolites: biogenetically related Heptaketides of the endolichenic fungus Corynespora sp. (1)

In an attempt to explore the biosynthetic potential of the endolichenic fungus Corynespora sp. BA-10763, its metabolite profiles under several culture conditions were investigated. When cultured in potato dextrose agar, it produced three new heptaketides, 9-O-methylscytalol A (1), 7-desmethylherbarin (2), and 8-hydroxyherbarin (3), together with biogenetically related metabolites scytalol A (4), 8-O-methylfusarubin (5), scorpinone (6), and 8-O-methylbostrycoidin (7), which are new to this organism, and herbarin (8), a metabolite previously encountered in this fungal strain. The use of malt extract agar as the culture medium led to the isolation of 6, 8, 1-hydroxydehydroherbarin (9), and 1-methoxydehydroherbarin (10), which was found to be an artifact formed during the extraction of the culture medium with methanol. The structures of all new compounds were determined by interpretation of their spectroscopic data and chemical interconversions.

Identification and characterization of a novel host-toxin interaction in the wheat-Stagonospora nodorum pathosystem

Stagonospora nodorum, casual agent of Stagonospora nodorum blotch (SNB) of wheat, produces a number of host-selective toxins (HSTs) known to be important in disease. To date, four HSTs and corresponding host sensitivity genes have been reported, and all four host-toxin interactions are significant factors in the development of disease. Here, we describe the identification and partial characterization of a fifth S. nodorum produced HST designated SnTox4. The toxin, estimated to be 10-30 kDa in size, was found to be proteinaceous in nature. Sensitivity to SnTox4 is governed by a single dominant gene, designated Snn4, which mapped to the short arm of wheat chromosome 1A in a recombinant inbred (RI) population. The compatible Snn4-SnTox4 interaction is light dependent and results in a mottled necrotic reaction, which is different from the severe necrosis that results from other host-toxin interactions in the wheat-S. nodorum pathosystem. QTL analysis in a population of 200 RI lines derived from the Swiss winter wheat varieties Arina and Forno revealed a major QTL for SNB susceptibility that coincided with the Snn4 locus. This QTL, designated QSnb.fcu-1A, explained 41.0% of the variation in disease on leaves of seedlings indicating that a compatible Snn4-SnTox4 interaction plays a major role in the development of SNB in this population. Additional minor QTL detected on the short arms of chromosomes 2A and 3A accounted for 5.4 and 6.0% of the variation, respectively. The effects of the three QTL were largely additive, and together they explained 50% of the total phenotypic variation. These results provide further evidence that host-toxin interactions in the wheat-S. nodorum pathosystem follow an inverse gene-for-gene model.

Subcutaneous infection caused by Corynespora cassiicola, a plant pathogen

We describe a 69-year-old female farmer with diabetes mellitus who developed subcutaneous infection due to a plant pathogen, Corynespora cassiicola. The organism was identified based on characteristic morphotypes and confirmed by sequence analysis of the internal transcribed spacer (ITS) regions. The patient was treated successfully with amphotericin B therapy.

SnTox3 acts in effector triggered susceptibility to induce disease on wheat carrying the Snn3 gene

The necrotrophic fungus Stagonospora nodorum produces multiple proteinaceous host-selective toxins (HSTs) which act in effector triggered susceptibility. Here, we report the molecular cloning and functional characterization of the SnTox3-encoding gene, designated SnTox3, as well as the initial characterization of the SnTox3 protein. SnTox3 is a 693 bp intron-free gene with little obvious homology to other known genes. The predicted immature SnTox3 protein is 25.8 kDa in size. A 20 amino acid signal sequence as well as a possible pro sequence are predicted. Six cysteine residues are predicted to form disulfide bonds and are shown to be important for SnTox3 activity. Using heterologous expression in Pichia pastoris and transformation into an avirulent S. nodorum isolate, we show that SnTox3 encodes the SnTox3 protein and that SnTox3 interacts with the wheat susceptibility gene Snn3. In addition, the avirulent S. nodorum isolate transformed with SnTox3 was virulent on host lines expressing the Snn3 gene. SnTox3-disrupted mutants were deficient in the production of SnTox3 and avirulent on the Snn3 differential wheat line BG220. An analysis of genetic diversity revealed that SnTox3 is present in 60.1% of a worldwide collection of 923 isolates and occurs as eleven nucleotide haplotypes resulting in four amino acid haplotypes. The cloning of SnTox3 provides a fundamental tool for the investigation of the S. nodorum–wheat interaction, as well as vital information for the general characterization of necrotroph–plant interactions.

The necrotrophic fungus Stagonospora nodorum produces multiple toxins that are effective in causing disease on wheat. Here, we report the characterization of the SnTox3-producing gene, designated SnTox3, as well as the initial characterization of the SnTox3 protein. In order to verify the action of this toxin, we expressed SnTox3 in yeast to show that SnTox3 encodes the SnTox3 protein which interacts directly or indirectly with the product of the corresponding wheat susceptibility gene Snn3. Transformation of a non pathogenic S. nodorum isolate with SnTox3 indicated that expression of the SnTox3 gene is sufficient to render an avirulent isolate virulent in the presence of Snn3. SnTox3 disruption mutants are deficient in the production of SnTox3 and consequently are avirulent on the Snn3 differential wheat line BG220. SnTox3 is present in approximately 60% of a worldwide collection of 923 isolates. The cloning of SnTox3 provides a critical tool for the investigation of the S. nodorum–wheat interaction, but also significantly adds to a necrotrophic effector system that is an exciting contrast to the biotrophic effector models that have been intensively studied.

Host-specific toxins: effectors of necrotrophic pathogenicity

Host-specific toxins (HSTs) are defined as pathogen effectors that induce toxicity and promote disease only in the host species and only in genotypes of that host expressing a specific and often dominant susceptibility gene. They are a feature of a small but well-studied group of fungal plant pathogens. Classical HST pathogens include species of Cochliobolus, Alternaria and Pyrenophora. Recent studies have shown that Stagonospora nodorum produces at least four separate HSTs that interact with four of the many quantitative resistance loci found in the host, wheat. Rationalization of fungal phylogenetics has placed these pathogens in the Pleosporales order of the class Dothideomycetes. It is possible that all HST pathogens lie in this order. Strong evidence of the recent lateral gene transfer of the ToxA gene from S. nodorum to Pyrenophora tritici-repentis has been obtained. Hallmarks of lateral gene transfer are present for all the studied HST genes although definitive proof is lacking. We therefore suggest that the Pleosporales pathogens may have a conserved propensity to acquire HST genes by lateral transfer.

Recent fungal diseases of crop plants: is lateral gene transfer a common theme?

A cursory glance at old textbooks of plant pathology reveals that the diseases which are the current scourge of agriculture in many parts of the world are a different set from those that were prominent 50 or 100 years ago. Why have these new diseases arisen? The traditional explanations subscribe to the "nature abhors a vacuum" principle-that control of one disease creates the condition for the emergence of a replacement-but does little to explain why the new pathogen succeeds. The emergence of a new disease requires a series of conditions and steps, including the enhanced fecundity of the new pathogen, enhanced survival from season to season, and spread around the world. Recently, evidence was obtained that wheat tan spot emerged through a lateral gene transfer event some time prior to 1941. Although there have been sporadic and persistent reports of lateral gene transfer between and into fungal plant pathogens, most examples have been dismissed through incomplete evidence. The completion of whole genome sequences of an increasing number of fungal pathogens no longer allows such proposed cases of lateral gene transfer to be dismissed so easily. How frequent are lateral gene transfers involving fungal plant pathogens, and can this process explain the emergence of many of the new diseases of the recent past? Many of the apparently new diseases are dependant on the expression of host-specific toxins. These are enigmatic molecules whose action requires the presence of plant genes with products that specifically encode sensitivity to the toxin and susceptibility to the disease. It is also notable that many new diseases belong to the fungal taxon dothideomycetes. This review explores the coincidence of new diseases, interspecific gene transfer, host-specific toxins, and the dothideomycete class.