Host-selective toxins and avirulence determinants: what's in a name?

The role of effectors and host immunity in plant-necrotrophic fungal interactions

Fungal diseases pose constant threats to the global economy and food safety. As the largest group of plant fungal pathogens, necrotrophic fungi cause heavy crop losses worldwide. The molecular mechanisms of the interaction between necrotrophic fungi and plants are complex and involve sophisticated recognition and signaling networks. Here, we review recent findings on the roles of phytotoxin and proteinaceous effectors, pathogen-associated molecular patterns (PAMPs), and small RNAs from necrotrophic fungi. We also consider the functions of damage-associated molecular patterns (DAMPs), the receptor-like protein kinase BIK1, and epigenetic regulation in plant immunity to necrotrophic fungi.

Transcriptomics of the rice blast fungus Magnaporthe oryzae in response to the bacterial antagonist Lysobacter enzymogenes reveals candidate fungal defense response genes

Plants and animals have evolved a first line of defense response to pathogens called innate or basal immunity. While basal defenses in these organisms are well studied, there is almost a complete lack of understanding of such systems in fungal species, and more specifically, how they are able to detect and mount a defense response upon pathogen attack. Hence, the goal of the present study was to understand how fungi respond to biotic stress by assessing the transcriptional profile of the rice blast pathogen, Magnaporthe oryzae, when challenged with the bacterial antagonist Lysobacter enzymogenes. Based on microscopic observations of interactions between M. oryzae and wild-type L. enzymogenes strain C3, we selected early and intermediate stages represented by time-points of 3 and 9 hours post-inoculation, respectively, to evaluate the fungal transcriptome using RNA-seq. For comparative purposes, we also challenged the fungus with L. enzymogenes mutant strain DCA, previously demonstrated to be devoid of antifungal activity. A comparison of transcriptional data from fungal interactions with the wild-type bacterial strain C3 and the mutant strain DCA revealed 463 fungal genes that were down-regulated during attack by C3; of these genes, 100 were also found to be up-regulated during the interaction with DCA. Functional categorization of genes in this suite included those with roles in carbohydrate metabolism, cellular transport and stress response. One gene in this suite belongs to the CFEM-domain class of fungal proteins. Another CFEM class protein called PTH11 has been previously characterized, and we found that a deletion in this gene caused advanced lesion development by C3 compared to its growth on the wild-type fungus. We discuss the characterization of this suite of 100 genes with respect to their role in the fungal defense response.

The necrotrophic effector SnToxA induces the synthesis of a novel phytoalexin in wheat

Stagonospora nodorum and Pyrenophora tritici-repentis produce the effector ToxA that interacts with the dominant susceptibility gene in wheat, Tsn1. However, the way in which ToxA induces cell death and causes disease is unclear. Here, we performed comprehensive metabolite profiling of ToxA-infiltrated wheat (Triticum aestivum) to observe the secondary metabolite response to this effector. A strong induction of secondary metabolism subsequent to SnToxA infiltration was observed, including the monoamine serotonin. We established a novel role for serotonin as a phytoalexin in wheat and demonstrated that serotonin strongly inhibited sporulation of S. nodorum. Microscopy revealed that serotonin interferes with spore formation and maturation within pycnidial structures of the fungus. Subsequent analysis of S. nodorum exposed to serotonin revealed metabolites changes previously associated with sporulation, including trehalose and alternariol. Furthermore, we identified significantly lower concentrations of serotonin during infection compared with infiltration with ToxA, providing evidence that S. nodorum may suppress plant defence. This is the first study demonstrating induction of plant secondary metabolites in response to a necrotrophic effector that have significant antifungal potential against the pathogen. While it is generally accepted that necrotrophs exploit host cell responses, the current research strengthens the notion that necrotrophs require mechanisms to overcome plant defence to survive initial stages of infection.

Genetics of tan spot resistance in wheat

Tan spot is a devastating foliar disease of wheat caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis. Much has been learned during the past two decades about the genetics of wheat-P. tritici-repentis interactions. Research has shown that the fungus produces at least three host-selective toxins (HSTs), known as Ptr ToxA, Ptr ToxB, and Ptr ToxC, that interact directly or indirectly with the products of the dominant host genes Tsn1, Tsc2, and Tsc1, respectively. The recent cloning and characterization of Tsn1 provided strong evidence that the pathogen utilizes HSTs to subvert host resistance mechanisms to cause disease. However, in addition to host-HST interactions, broad-spectrum, race non-specific resistance QTLs and recessively inherited qualitative 'resistance' genes have been identified. Molecular markers suitable for marker-assisted selection against HST sensitivity genes and for race non-specific resistance QTLs have been developed and used to generate adapted germplasm with good levels of tan spot resistance. Future research is needed to identify novel HSTs and corresponding host sensitivity genes, determine if the recessively inherited resistance genes are HST insensitivities, extend the current race classification system to account for new HSTs, and determine the molecular basis of race non-specific resistance QTLs and their relationships with host-HST interactions at the molecular level. Necrotrophic pathogens such as P. tritici-repentis are likely to become increasingly significant under a changing global climate making it imperative to further characterize the wheat-P. tritici-repentis pathosystem and develop tan spot resistant wheat varieties.

Genetic and cellular mechanisms regulating plant responses to necrotrophic pathogens

Necrotrophs are plant pathogens that kill host cells and proliferate on nutrients from dead or dying tissues causing devastating diseases of horticultural and agronomic crops. Their interactions with plants involve a complex network of pathogen disease factors and corresponding plant immune response regulators. Mechanisms of quantitative resistance and the major regulators intersect regardless of pathogen life style. By contrast, some plant immune responses, such as effector-triggered immunity (ETI), a major source of qualitative resistance to biotrophs, are co-opted by necrotrophs to promote disease, which highlights the disparate plant immunity systems. Advances towards understanding mechanisms and processes underlying host responses to necrotrophs are summarized.

Characterization of the LOV1-mediated, victorin-induced, cell-death response with virus-induced gene silencing

Victoria blight, caused by Cochliobolus victoriae, is a disease originally described on oat and recapitulated on Arabidopsis. C. victoriae pathogenesis depends upon production of the toxin victorin. In oat, victorin sensitivity is conferred by the Vb gene, which is genetically inseparable from the Pc2 resistance gene. Concurrently, in Arabidopsis, sensitivity is conferred by the LOCUS ORCHESTRATING VICTORIN EFFECTS1 (LOV1) gene. LOV1 encodes a nucleotide-binding site leucine-rich repeat protein, a type of protein commonly associated with disease resistance, and LOV1 "guards" the defense thioredoxin, TRX-h5. Expression of LOV1 and TRX-h5 in Nicotiana benthamiana is sufficient to confer victorin sensitivity. Virus-induced gene silencing was used to characterize victorin-induced cell death in N. benthamiana. We determined that SGT1 is required for sensitivity and involved in LOV1 protein accumulation. We screened a normalized cDNA library and identified six genes that, when silenced, suppressed LOV1-mediated, victorin-induced cell death and cell death induced by expression of the closely related RPP8 resistance gene: a mitochondrial phosphate transporter, glycolate oxidase, glutamine synthetase, glyceraldehyde 3-phosphate dehydrogenase, and the P- and T-protein of the glycine decarboxylase complex. Silencing the latter four also inhibited cell death and disease resistance mediated by the PTO resistance gene. Together, these results provide evidence that the victorin response mediated by LOV1 is a defense response.

Changes of phenolic secondary metabolite profiles in the reaction of narrow leaf lupin (Lupinus angustifolius) plants to infections with Colletotrichum lupini fungus or treatment with its toxin

Plant interactions with environmental factors cause changes in the metabolism and regulation of biochemical and physiological processes. Plant defense against pathogenic microorganisms depends on an innate immunity system that is activated as a result of infection. There are two mechanisms of triggering this system: basal immunity activated as a result of a perception of microbe-associated molecular patterns through pattern recognition receptors situated on the cell surface and effector-triggered immunity (ETI). An induced biosynthesis of bioactive secondary metabolites, in particular phytoalexins, is one of the mechanisms of plant defense to fungal infection. Results of the study on narrow leaf lupin (Lupinus angustifolius L.) plants infected with the anthracnose fungus Colletotrichum lupini and treated with fungal phytotoxic metabolites are described in the paper. The C. lupini phytotoxins were isolated from liquid cultures, purified and partially characterized with physicochemical methods. Accumulation of secondary metabolites on leaf surface and within the tissues of plants either infected, treated with the fungal phytotoxin or submitted to both treatments was studied using GC-MS and LC-MS, respectively. Substantial differences in isoflavone aglycones and glycoconjugate profiles occurred in response to different ways of plant treatment.

Effect of Fusarium virguliforme phytotoxin on soybean gene expression suggests a role in multidimensional defence

Sudden death syndrome (SDS), caused by Fusarium virguliforme, is an important yield-limiting disease of soybean. This soil-borne fungus colonizes soybean roots causing root rot, and also releases a phytotoxin that is translocated to leaves causing interveinal chlorosis and necrosis leading to defoliation and early maturation. The objective of our study was to compare gene expression profiles during the early response of soybean leaves exposed to sterile culture filtrates of F. virguliforme in soybean genotypes with different levels of resistance to SDS. The analysis identified SDS-related defence genes that were induced in the most resistant genotype, but not in the other genotypes. Further functional annotations based on sequence homology suggested that some of the induced genes probably encode proteins involved in cell wall modification, detoxification, defence responses, primary metabolism and membrane transport. Quantitative real-time reverse-transcribed polymerase chain reaction confirmed the differential transcript accumulation of a subset of these genes. In addition, in silico mapping of differentially expressed genes to SDS-resistant quantitative trait loci allowed for the identification of new potential defence genes that could be genetically mapped to the soybean genome, and could be used further in a marker-assisted selection programme. A comparison of the response of soybean to F. virguliforme phytotoxin (Fv toxin) relative to other biotic and abiotic stresses revealed that the resistance response to Fv toxin is quite similar to the response to inoculation with an incompatible Pseudomonas syringae pv. glycinea strain, suggesting that Fv toxin might induce hypersensitive response pathways in soybean leaf tissues in the absence of pathogen in these tissues.

Comparative genome structure, secondary metabolite, and effector coding capacity across Cochliobolus pathogens

The genomes of five Cochliobolus heterostrophus strains, two Cochliobolus sativus strains, three additional Cochliobolus species (Cochliobolus victoriae, Cochliobolus carbonum, Cochliobolus miyabeanus), and closely related Setosphaeria turcica were sequenced at the Joint Genome Institute (JGI). The datasets were used to identify SNPs between strains and species, unique genomic regions, core secondary metabolism genes, and small secreted protein (SSP) candidate effector encoding genes with a view towards pinpointing structural elements and gene content associated with specificity of these closely related fungi to different cereal hosts. Whole-genome alignment shows that three to five percent of each genome differs between strains of the same species, while a quarter of each genome differs between species. On average, SNP counts among field isolates of the same C. heterostrophus species are more than 25× higher than those between inbred lines and 50× lower than SNPs between Cochliobolus species. The suites of nonribosomal peptide synthetase (NRPS), polyketide synthase (PKS), and SSP-encoding genes are astoundingly diverse among species but remarkably conserved among isolates of the same species, whether inbred or field strains, except for defining examples that map to unique genomic regions. Functional analysis of several strain-unique PKSs and NRPSs reveal a strong correlation with a role in virulence.

Centrality of host cell death in plant-microbe interactions

Programmed cell death (PCD) is essential for proper growth, development, and cellular homeostasis in all eukaryotes. The regulation of PCD is of central importance in plant-microbe interactions; notably, PCD and features associated with PCD are observed in many host resistance responses. Conversely, pathogen induction of inappropriate cell death in the host results in a susceptible phenotype and disease. Thus, the party in control of PCD has a distinct advantage in these battles. PCD processes appear to be of ancient origin, as indicated by the fact that many features of cell death strategy are conserved between animals and plants; however, some of the details of death execution differ. Mammalian core PCD genes, such as caspases, are not present in plant genomes. Similarly, pro- and antiapoptotic mammalian regulatory elements are absent in plants, but, remarkably, when expressed in plants, successfully impact plant PCD. Thus, subtle structural similarities independent of sequence homology appear to sustain operational equivalence. The vacuole is emerging as a key organelle in the modulation of plant PCD. Under different signals for cell death, the vacuole either fuses with the plasmalemma membrane or disintegrates. Moreover, the vacuole appears to play a key role in autophagy; evidence suggests a prosurvival function for autophagy, but other studies propose a prodeath phenotype. Here, we describe and discuss what we know and what we do not know about various PCD pathways and how the host integrates signals to activate salicylic acid and reactive oxygen pathways that orchestrate cell death. We suggest that it is not cell death as such but rather the processes leading to cell death that contribute to the outcome of a given plant-pathogen interaction.

Comparative genomics of a plant-pathogenic fungus, Pyrenophora tritici-repentis, reveals transduplication and the impact of repeat elements on pathogenicity and population divergence

Pyrenophora tritici-repentis is a necrotrophic fungus causal to the disease tan spot of wheat, whose contribution to crop loss has increased significantly during the last few decades. Pathogenicity by this fungus is attributed to the production of host-selective toxins (HST), which are recognized by their host in a genotype-specific manner. To better understand the mechanisms that have led to the increase in disease incidence related to this pathogen, we sequenced the genomes of three P. tritici-repentis isolates. A pathogenic isolate that produces two known HSTs was used to assemble a reference nuclear genome of approximately 40 Mb composed of 11 chromosomes that encode 12,141 predicted genes. Comparison of the reference genome with those of a pathogenic isolate that produces a third HST, and a nonpathogenic isolate, showed the nonpathogen genome to be more diverged than those of the two pathogens. Examination of gene-coding regions has provided candidate pathogen-specific proteins and revealed gene families that may play a role in a necrotrophic lifestyle. Analysis of transposable elements suggests that their presence in the genome of pathogenic isolates contributes to the creation of novel genes, effector diversification, possible horizontal gene transfer events, identified copy number variation, and the first example of transduplication by DNA transposable elements in fungi. Overall, comparative analysis of these genomes provides evidence that pathogenicity in this species arose through an influx of transposable elements, which created a genetically flexible landscape that can easily respond to environmental changes.

SnTox5-Snn5: a novel Stagonospora nodorum effector-wheat gene interaction and its relationship with the SnToxA-Tsn1 and SnTox3-Snn3-B1 interactions

The Stagonospora nodorum-wheat interaction involves multiple pathogen-produced necrotrophic effectors that interact directly or indirectly with specific host gene products to induce the disease Stagonospora nodorum blotch (SNB). Here, we used a tetraploid wheat mapping population to identify and characterize a sixth effector-host gene interaction in the wheat-S. nodorum system. Initial characterization of the effector SnTox5 indicated that it is a proteinaceous necrotrophic effector that induces necrosis on host lines harbouring the Snn5 sensitivity gene, which was mapped to the long arm of wheat chromosome 4B. On the basis of ultrafiltration, SnTox5 is probably in the size range 10-30 kDa. Analysis of SNB development in the mapping population indicated that the SnTox5-Snn5 interaction explains 37%-63% of the variation, demonstrating that this interaction plays a significant role in disease development. When the SnTox5-Snn5 and SnToxA-Tsn1 interactions occurred together, the level of SNB was increased significantly. Similar to several other interactions in this system, the SnTox5-Snn5 interaction is light dependent, suggesting that multiple interactions may exploit the same pathways to cause disease.

Comparison of Alternaria spp. collected in Italy from apple with A. mali and other AM-toxin producing strains

Since 1999, a disease of apple caused by an Alternaria sp. has been affecting orchards in northern Italy resulting in necrotic spots on leaves and on fruit. Forty-four single-spored isolates were obtained from diseased plant materials to investigate the diversity of this fungus in Italy and to compare these isolates to isolates of Alternaria associated with apple disease in previous studies, including A. mali, causal agent of apple blotch. All isolates, including the reference strains, were tested for pathogenicity utilizing in vitro bioassays on detached leaf or on fruit ('Golden Delicious'). In addition, morphological characterizations were conducted describing both the three-dimensional sporulation pattern and the colony morphology of each isolate. In order to assess the genetic diversity within the Italian Alternaria population, sequence characterization of specific loci and anonymous regions (endoPG, OPA1-3, OPA2-1, and OPA10-2) and genetic fingerprinting based on amplified fragment length polymorphism and inter simple sequence repeat markers were performed. The single spore isolates exhibited differential pathogenicity, which did not correlate with the morphological groupings or to groupings defined by molecular approaches. Moreover, 10 pathogenic isolates out of the 44 single-spored tested were positive for the host-specific AM-toxin gene based upon polymerase chain reaction amplification using specific primers for the AM-toxin gene. This suggests that the production of the AM-toxin may be involved in pathogenesis by some of the Italian isolates of A. alternata from apple. However, this research also suggests that a number of different Alternaria genotypes and morphotypes may be responsible for the apple disease in Italy and that a single taxon cannot be defined as the sole causal agent.

In planta stage-specific fungal gene profiling elucidates the molecular strategies of Fusarium graminearum growing inside wheat coleoptiles

The ascomycete Fusarium graminearum is a destructive fungal pathogen of wheat (Triticum aestivum). To better understand how this pathogen proliferates within the host plant, we tracked pathogen growth inside wheat coleoptiles and then examined pathogen gene expression inside wheat coleoptiles at 16, 40, and 64 h after inoculation (HAI) using laser capture microdissection and microarray analysis. We identified 344 genes that were preferentially expressed during invasive growth in planta. Gene expression profiles for 134 putative plant cell wall-degrading enzyme genes suggest that there was limited cell wall degradation at 16 HAI and extensive degradation at 64 HAI. Expression profiles for genes encoding reactive oxygen species (ROS)-related enzymes suggest that F. graminearum primarily scavenges extracellular ROS before a later burst of extracellular ROS is produced by F. graminearum enzymes. Expression patterns of genes involved in primary metabolic pathways suggest that F. graminearum relies on the glyoxylate cycle at an early stage of plant infection. A secondary metabolite biosynthesis gene cluster was specifically induced at 64 HAI and was required for virulence. Our results indicate that F. graminearum initiates infection of coleoptiles using covert penetration strategies and switches to overt cellular destruction of tissues at an advanced stage of infection.

Tricking the guard: exploiting plant defense for disease susceptibility

Typically, pathogens deploy virulence effectors to disable defense. Plants defeat effectors with resistance proteins that guard effector targets. We found that a pathogen exploits a resistance protein by activating it to confer susceptibility in Arabidopsis. The guard mechanism of plant defense is recapitulated by interactions among victorin (an effector produced by the necrotrophic fungus Cochliobolus victoriae), TRX-h5 (a defense-associated thioredoxin), and LOV1 (an Arabidopsis susceptibility protein). In LOV1's absence, victorin inhibits TRX-h5, resulting in compromised defense but not disease by C. victoriae. In LOV1's presence, victorin binding to TRX-h5 activates LOV1 and elicits a resistance-like response that confers disease susceptibility. We propose that victorin is, or mimics, a conventional pathogen virulence effector that was defeated by LOV1 and confers virulence to C. victoriae solely because it incites defense.

A polyketide synthase gene, ACRTS2, is responsible for biosynthesis of host-selective ACR-toxin in the rough lemon pathotype of Alternaria alternata

The rough lemon pathotype of Alternaria alternata produces host-selective ACR-toxin and causes Alternaria leaf spot disease of rough lemon (Citrus jambhiri). The structure of ACR-toxin I (MW = 496) consists of a polyketide with an α-dihydropyrone ring in a 19-carbon polyalcohol. Genes responsible for toxin production were localized to a 1.5-Mb chromosome in the genome of the rough lemon pathotype. Sequence analysis of this chromosome revealed an 8,338-bp open reading frame, ACRTS2, that was present only in the genomes of ACR-toxin-producing isolates. ACRTS2 is predicted to encode a putative polyketide synthase of 2,513 amino acids and belongs to the fungal reducing type I polyketide synthases. Typical polyketide functional domains were identified in the predicted amino acid sequence, including β-ketoacyl synthase, acyl transferase, methyl transferase, dehydratase, β-ketoreductase, and phosphopantetheine attachment site domains. Combined use of homologous recombination-mediated gene disruption and RNA silencing allowed examination of the functional role of multiple paralogs in ACR-toxin production. ACRTS2 was found to be essential for ACR-toxin production and pathogenicity of the rough lemon pathotype of A. alternata.

Pathogenicity Spectra and Screening for Resistance in Barley Against Tunisian Pyrenophora teres f. teres

This work aimed to determine patterns of pathogenicity in Pyrenophora teres f. teres and to identify potentially effective resistance sources that could be used as breeding material to control net blotch in Tunisia. Extensive pathogenic variability was detected in 85 isolates of P. teres causing net blotch of barley in Tunisia. Based on unweighted pair-group method with arithmetic averaging clustering and mean disease rating scores, three distinct virulence groups were identified. The isolates were classified into 23 pathotypes. Pathogenic variability within the groups was higher than that between the groups, a finding that can guide a rational choice of isolates for screening lines as part of a breeding program. Conversely, studying the relationship between geographic and pathotypic structure allowed us to detect a significant isolation by distance pattern, suggesting a regular and gradual dispersal of the pathogen over this spatial scale. Using specific resistance properties of individual barley genotypes as virulence markers, all the differential barley genotypes were shown to be distinct, and no single source of resistance was totally effective against all isolates.

Tools to kill: genome of one of the most destructive plant pathogenic fungi Macrophomina phaseolina

BACKGROUND

Macrophomina phaseolina is one of the most destructive necrotrophic fungal pathogens that infect more than 500 plant species throughout the world. It can grow rapidly in infected plants and subsequently produces a large amount of sclerotia that plugs the vessels, resulting in wilting of the plant.

RESULTS

We sequenced and assembled ~49 Mb into 15 super-scaffolds covering 92.83% of the M. phaseolina genome. We predict 14,249 open reading frames (ORFs) of which 9,934 are validated by the transcriptome. This phytopathogen has an abundance of secreted oxidases, peroxidases, and hydrolytic enzymes for degrading cell wall polysaccharides and lignocelluloses to penetrate into the host tissue. To overcome the host plant defense response, M. phaseolina encodes a significant number of P450s, MFS type membrane transporters, glycosidases, transposases, and secondary metabolites in comparison to all sequenced ascomycete species. A strikingly distinct set of carbohydrate esterases (CE) are present in M. phaseolina, with the CE9 and CE10 families remarkably higher than any other fungi. The phenotypic microarray data indicates that M. phaseolina can adapt to a wide range of osmotic and pH environments. As a broad host range pathogen, M. phaseolina possesses a large number of pathogen-host interaction genes including those for adhesion, signal transduction, cell wall breakdown, purine biosynthesis, and potent mycotoxin patulin.

CONCLUSIONS

The M. phaseolina genome provides a framework of the infection process at the cytological and molecular level which uses a diverse arsenal of enzymatic and toxin tools to destroy the host plants. Further understanding of the M. phaseolina genome-based plant-pathogen interactions will be instrumental in designing rational strategies for disease control, essential to ensuring global agricultural crop production and security.

Geraniol synthase whose mRNA is induced by host-selective ACT-toxin in the ACT-toxin-insensitive rough lemon (Citrus jambhiri)

Host-selective toxins (HSTs) produced by some strains of Alternaria alternata are selectively toxic to certain cultivars of plants. However, the role of HSTs in toxin-insensitive plants is currently unknown. Here, we studied the role of ACT-toxin using an ACT-toxin producing A. alternata strain SH20 and the ACT-toxin-insensitive plant rough lemon. Induction of some defense related genes in response to SH20 were faster or stronger than in response to the ACT-toxin deficient SH20 mutant. By sequencing subtractive PCR clones obtained from mRNA of rough lemon leaves inoculated with SH20 after subtraction with that of the ACT-toxin deficient SH20 mutant, we isolated the SH20-responsive genes in rough lemon. Among the SH20-responsive genes analyzed in this study, we isolated a terpene synthase (TPS) gene, RlemTPS3. We also determined that RlemTPS3 localizes to the chloroplast and produces the monoterpene geraniol.

Phytotoxic secondary metabolites and peptides produced by plant pathogenic Dothideomycete fungi

Many necrotrophic plant pathogenic fungi belonging to the class of Dothideomycetes produce phytotoxic metabolites and peptides that are usually required for pathogenicity. Phytotoxins that affect a broad range of plant species are known as non-host-specific toxins (non-HSTs), whereas HSTs affect only a particular plant species or more often genotypes of that species. For pathogens producing HSTs, pathogenicity and host specificity are largely defined by the ability to produce the toxin, while plant susceptibility is dependent on the presence of the toxin target. Non-HSTs are not the main determinants of pathogenicity but contribute to virulence of the producing pathogen. Dothideomycetes are remarkable for the production of toxins, particularly HSTs because they are the only fungal species known so far to produce them. The synthesis, regulation, and mechanisms of action of the most important HSTs and non-HSTs will be discussed. Studies on the mode of action of HSTs have highlighted the induction of programed cell death (PCD) as an important mechanism. We discuss HST-induced PCD and the plant hypersensitive response upon recognition of avirulence factors that share common pathways. In this respect, although nucleotide-binding-site-leucine-rich repeat types of resistance proteins mediate resistance against biotrophs, they can also contribute to susceptibility toward necrotrophs.

Role of the pathotype-specific ACRTS1 gene encoding a hydroxylase involved in the biosynthesis of host-selective ACR-toxin in the rough lemon pathotype of Alternaria alternata

The rough lemon pathotype of Alternaria alternata produces host-selective ACR-toxin and causes Alternaria leaf spot disease of the rootstock species rough lemon (Citrus jambhiri) and Rangpur lime (C. limonia). Genes controlling toxin production were localized to a 1.5-Mb chromosome carrying the ACR-toxin biosynthesis gene cluster (ACRT) in the genome of the rough lemon pathotype. A genomic BAC clone containing a portion of the ACRT cluster was sequenced which allowed identification of three open reading frames present only in the genomes of ACR-toxin producing isolates. We studied the functional role of one of these open reading frames, ACRTS1 encoding a putative hydroxylase, in ACR-toxin production by homologous recombination-mediated gene disruption. There are at least three copies of ACRTS1 gene in the genome and disruption of two copies of this gene significantly reduced ACR-toxin production as well as pathogenicity; however, transcription of ACRTS1 and production of ACR-toxin were not completely eliminated due to remaining functional copies of the gene. RNA-silencing was used to knock down the remaining ACRTS1 transcripts to levels undetectable by reverse transcription-polymerase chain reaction. The silenced transformants did not produce detectable ACR-toxin and were not pathogenic. These results indicate that ACRTS1 is an essential gene in ACR-toxin biosynthesis in the rough lemon pathotype of A. alternata and is required for full virulence of this fungus.

Host-selective toxins of Pyrenophora tritici-repentis induce common responses associated with host susceptibility

Pyrenophora tritici-repentis (Ptr), a necrotrophic fungus and the causal agent of tan spot of wheat, produces one or a combination of host-selective toxins (HSTs) necessary for disease development. The two most studied toxins produced by Ptr, Ptr ToxA (ToxA) and Ptr ToxB (ToxB), are proteins that cause necrotic or chlorotic symptoms respectively. Investigation of host responses induced by HSTs provides better insight into the nature of the host susceptibility. Microarray analysis of ToxA has provided evidence that it can elicit responses similar to those associated with defense. In order to evaluate whether there are consistent host responses associated with susceptibility, a similar analysis of ToxB-induced changes in the same sensitive cultivar was conducted. Comparative analysis of ToxA- and ToxB-induced transcriptional changes showed that similar groups of genes encoding WRKY transcription factors, RLKs, PRs, components of the phenylpropanoid and jasmonic acid pathways are activated. ROS accumulation and photosystem dysfunction proved to be common mechanism-of-action for these toxins. Despite similarities in defense responses, transcriptional and biochemical responses as well as symptom development occur more rapidly for ToxA compared to ToxB, which could be explained by differences in perception as well as by differences in activation of a specific process, for example, ethylene biosynthesis in ToxA treatment. Results of this study suggest that perception of HSTs will result in activation of defense responses as part of a susceptible interaction and further supports the hypothesis that necrotrophic fungi exploit defense responses in order to induce cell death.

Plants versus fungi and oomycetes: pathogenesis, defense and counter-defense in the proteomics era

Plant-fungi and plant-oomycete interactions have been studied at the proteomic level for many decades. However, it is only in the last few years, with the development of new approaches, combined with bioinformatics data mining tools, gel staining, and analytical instruments, such as 2D-PAGE/nanoflow-LC-MS/MS, that proteomic approaches thrived. They allow screening and analysis, at the sub-cellular level, of peptides and proteins resulting from plants, pathogens, and their interactions. They also highlight post-translational modifications to proteins, e.g., glycosylation, phosphorylation or cleavage. However, many challenges are encountered during in planta studies aimed at stressing details of host defenses and fungal and oomycete pathogenicity determinants during interactions. Dissecting the mechanisms of such host-pathogen systems, including pathogen counter-defenses, will ensure a step ahead towards understanding current outcomes of interactions from a co-evolutionary point of view, and eventually move a step forward in building more durable strategies for management of diseases caused by fungi and oomycetes. Unraveling intricacies of more complex proteomic interactions that involve additional microbes, i.e., PGPRs and symbiotic fungi, which strengthen plant defenses will generate valuable information on how pathosystems actually function in nature, and thereby provide clues to solving disease problems that engender major losses in crops every year.

Genomic characterization of the conditionally dispensable chromosome in Alternaria arborescens provides evidence for horizontal gene transfer

BACKGROUND

Fungal plant pathogens cause serious agricultural losses worldwide. Alternaria arborescens is a major pathogen of tomato, with its virulence determined by the presence of a conditionally dispensable chromosome (CDC) carrying host-specific toxin genes. Genes encoding these toxins are well-studied, however the genomic content and organization of the CDC is not known.

RESULTS

To gain a richer understanding of the molecular determinants of virulence and the evolution of pathogenicity, we performed whole genome sequencing of A. arborescens. Here we present the de-novo assembly of the CDC and its predicted gene content. Also presented is hybridization data validating the CDC assembly. Predicted genes were functionally annotated through BLAST. Gene ontology terms were assigned, and conserved domains were identified. Differences in nucleotide usage were found between CDC genes and those on the essential chromosome (EC), including GC3-content, codon usage bias, and repeat region load. Genes carrying PKS and NRPS domains were identified in clusters on the CDC and evidence supporting the origin of the CDC through horizontal transfer from an unrelated fungus was found.

CONCLUSIONS

We provide evidence supporting the hypothesis that the CDC in A. arborescens was acquired through horizontal transfer, likely from an unrelated fungus. We also identified several predicted CDC genes under positive selection that may serve as candidate virulence factors.

Fumonisin B1, a toxin produced by Fusarium verticillioides, modulates maize β-1,3-glucanase activities involved in defense response

Fusarium verticillioides is an important pathogen in maize that causes various diseases affecting all stages of plant development worldwide. The fungal pathogen could be seed borne or survive in soil and penetrate the germinating seed. Most F. verticillioides strains produce fumonisins, which are of concern because of their toxicity to animals and possibly humans, and because they enhance virulence against seedlings of some maize genotypes. In this work, we studied the action of fumonisin B1 (FB1) on the activity of maize β-1,3-glucanases involved in plant defense response. In maize embryos, FB1 induced an acidic isoform while suppressing the activity of two basic isoforms. This acidic isoform was induced also with 2,6-dichloroisonicotinic acid, an analog of salicylic acid. Repression of the basic isoforms suggested a direct interaction of the enzymes with the mycotoxin as in vitro experiments showed that pure FB1 inhibited the basic β-1,3-glucanases with an IC(50) of 53 μM. When germinating maize embryos were inoculated with F. verticillioides the same dual effect on β-1,3-glucanase activities that we observed with the pure toxin was reproduced. Similar levels of FB1 were recovered at 24 h germination in maize tissue when they were treated with pure FB1 or inoculated with an FB1-producing strain. These results suggest that β-1,3-glucanases are a relevant physiological target and their modulation by FB1 might contribute to F. verticillioides colonization.

Novel necrotrophic effectors from Stagonospora nodorum and corresponding host sensitivities in winter wheat germplasm in the southeastern United States

Stagonospora nodorum blotch (SNB), caused by the necrotrophic fungus Stagonospora nodorum (teleomorph: Phaeosphaeria nodorum), is among the most common diseases of winter wheat in the United States. New opportunities in resistance breeding have arisen from the recent discovery of several necrotrophic effectors (NEs, also known as host-selective toxins) produced by S. nodorum, along with their corresponding host sensitivity (Snn) genes. Thirty-nine isolates of S. nodorum collected from wheat debris or grain from seven states in the southeastern United States were used to investigate the production of NEs in the region. Twenty-nine cultivars with varying levels of resistance to SNB, representing 10 eastern-U.S. breeding programs, were infiltrated with culture filtrates from the S. nodorum isolates in a randomized complete block design. Three single-NE Pichia pastoris controls, two S. nodorum isolate controls, and six Snn-differential wheat controls were also used. Cultivar-isolate interactions were visually evaluated for sensitivity at 7 days after infiltration. Production of NEs was detected in isolates originating in each sampled state except Maryland. Of the 39 isolates, 17 produced NEs different from those previously characterized in the upper Great Plains region. These novel NEs likely correspond to unidentified Snn genes in Southeastern wheat cultivars, because NEs are thought to arise under selection pressure from genes for resistance to biotrophic pathogens of wheat cultivars that differ by geographic region. Only 3, 0, and 23% of the 39 isolates produced SnToxA, SnTox1, and SnTox3, respectively, by the culture-filtrate test. A Southern dot-blot test showed that 15, 74, and 39% of the isolates carried the genes for those NEs, respectively; those percentages were lower than those found previously in larger international samples. Only two cultivars appeared to contain known Snn genes, although half of the cultivars displayed sensitivity to culture filtrates containing unknown NEs. Effector sensitivity was more frequent in SNB-susceptible cultivars than in moderately resistant (MR) cultivars (P = 0.008), although some susceptible cultivars did not exhibit sensitivity to NEs produced by isolates in this study and some MR cultivars were sensitive to NEs of multiple isolates. Our results suggest that NE sensitivities influence but may not be the only determinant of cultivar resistance to S. nodorum. Specific knowledge of NE and Snn gene frequencies in this region can be used by wheat breeding programs to improve SNB resistance.

Sfp-type 4'-phosphopantetheinyl transferase is required for lysine synthesis, tolerance to oxidative stress and virulence in the plant pathogenic fungus Cochliobolus sativus

Polyketide synthases (PKSs) and nonribosomal peptide synthetases (NRPSs) are the major enzymes involved in the biosynthesis of secondary metabolites, which have diverse activities, including roles as pathogenicity/virulence factors in plant pathogenic fungi. These enzymes are activated by 4'-phosphopantetheinylation at the conserved serine residues, which is catalysed by 4'-phosphopantetheinyl transferase (PPTase). PPTase is also required for primary metabolism (α-aminoadipate reductase, AAR). In the genome sequence of the cereal fungal pathogen Cochliobolus sativus, we identified a gene (PPT1) orthologous to the PPTase-encoding genes found in other filamentous ascomycetes. The deletion of PPT1 in C. sativus generated mutants (Δppt1) that were auxotrophic for lysine, unable to synthesize melanin, hypersensitive to oxidative stress and significantly reduced in virulence to barley cv. Bowman. To analyse the pleiotropic effects of PPT1, we also characterized deletion mutants for PKS1 (involved in melanin synthesis), AAR1 (for AAR) and NPS6 (involved in siderophore-mediated iron metabolism). The melanin-deficient strain (Δpks1) showed no differences in pathogenicity and virulence compared with the wild-type strain. Lysine-auxotrophic mutants (Δaar1) induced spot blotch symptoms, as produced by the wild-type strain, when inoculated on wounded barley leaves or when lysine was supplemented. The Δnps6 strain showed a slightly reduced virulence compared with the wild-type strain, but exhibited significantly higher virulence than the Δppt1 strain. Our results suggest that an unknown virulence factor, presumably synthesized by PKSs or NRPSs which are activated by PPTase, is directly responsible for high virulence of C. sativus on barley cv. Bowman.

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.

Quantitative variation in effector activity of ToxA isoforms from Stagonospora nodorum and Pyrenophora tritici-repentis

ToxA is a proteinaceous necrotrophic effector produced by Stagonospora nodorum and Pyrenophora tritici-repentis. In this study, all eight mature isoforms of the ToxA protein were purified and compared. Circular dichroism spectra indicated that all isoforms were structurally intact and had indistinguishable secondary structural features. ToxA isoforms were infiltrated into wheat lines that carry the sensitivity gene Tsn1. It was observed that different wheat lines carrying identical Tsn1 alleles varied in sensitivity to ToxA. All ToxA isoforms induced necrosis when introduced into any Tsn1 wheat line but we observed quantitative variation in effector activity, with the least-active version found in isolates of P. tritici-repentis. Pathogen sporulation increased with higher doses of ToxA. The isoforms that induced the most rapid necrosis also induced the most sporulation, indicating that pathogen fitness is affected by differences in ToxA activity. We show that differences in toxin activity encoded by a single gene can contribute to the quantitative inheritance of necrotrophic virulence. Our findings support the hypothesis that the variation at ToxA results from selection that favors increased toxin activity.

The cysteine rich necrotrophic effector SnTox1 produced by Stagonospora nodorum triggers susceptibility of wheat lines harboring Snn1

The wheat pathogen Stagonospora nodorum produces multiple necrotrophic effectors (also called host-selective toxins) that promote disease by interacting with corresponding host sensitivity gene products. SnTox1 was the first necrotrophic effector identified in S. nodorum, and was shown to induce necrosis on wheat lines carrying Snn1. Here, we report the molecular cloning and validation of SnTox1 as well as the preliminary characterization of the mechanism underlying the SnTox1-Snn1 interaction which leads to susceptibility. SnTox1 was identified using bioinformatics tools and verified by heterologous expression in Pichia pastoris. SnTox1 encodes a 117 amino acid protein with the first 17 amino acids predicted as a signal peptide, and strikingly, the mature protein contains 16 cysteine residues, a common feature for some avirulence effectors. The transformation of SnTox1 into an avirulent S. nodorum isolate was sufficient to make the strain pathogenic. Additionally, the deletion of SnTox1 in virulent isolates rendered the SnTox1 mutated strains avirulent on the Snn1 differential wheat line. SnTox1 was present in 85% of a global collection of S. nodorum isolates. We identified a total of 11 protein isoforms and found evidence for strong diversifying selection operating on SnTox1. The SnTox1-Snn1 interaction results in an oxidative burst, DNA laddering, and pathogenesis related (PR) gene expression, all hallmarks of a defense response. In the absence of light, the development of SnTox1-induced necrosis and disease symptoms were completely blocked. By comparing the infection processes of a GFP-tagged avirulent isolate and the same isolate transformed with SnTox1, we conclude that SnTox1 may play a critical role during fungal penetration. This research further demonstrates that necrotrophic fungal pathogens utilize small effector proteins to exploit plant resistance pathways for their colonization, which provides important insights into the molecular basis of the wheat-S. nodorum interaction, an emerging model for necrotrophic pathosystems.

In this manuscript we describe the cloning of SnTox1 from Stagonospora nodorum, the gene encoding the first host selective toxin (SnTox1) identified in this fungus. SnTox1 induces necrosis and promotes disease on wheat lines harboring the Snn1 gene. We verified the function of the SnTox1 gene by expressing it in a yeast culture where the resulting culture filtrate induced necrosis but only on wheat lines that carried a functional Snn1. The SnTox1 gene was also transformed into an avirulent S. nodorum isolate, resulting in an isolate that was virulent on wheat lines harboring Snn1. SnTox1 was also disrupted in virulent S. nodorum isolates resulting in the elimination of disease on Snn1 differential wheat lines. Additionally, we investigated the host response to SnTox1 and S. nodorum strains producing SnTox1 and discovered that several hallmarks of a resistance response were present during the susceptible reaction, showing that the necrotrophic pathogen S. nodorum is likely using SnTox1 to stimulate a host resistance pathway involving Snn1 to induce disease.

Plant immunity to necrotrophs

Plants inhabit environments crowded with infectious microbes that pose constant threats to their survival. Necrotrophic pathogens are notorious for their aggressive and wide-ranging virulence strategies that promote host cell death and acquire nutrients for growth and reproduction from dead cells. This lifestyle constitutes the axis of their pathogenesis and virulence strategies and marks contrasting immune responses to biotrophic pathogens. The diversity of virulence strategies in necrotrophic species corresponds to multifaceted host immune response mechanisms. When effective, the plant immune system disarms the infectious necrotroph of its pathogenic arsenal or attenuates its effect, restricting further ingress and disease symptom development. Simply inherited resistance traits confer protection against host-specific necrotrophs (HSNs), whereas resistance to broad host-range necrotrophs (BHNs) is complex. Components of host genetic networks, as well as the molecular and cellular processes that mediate host immune responses to necrotrophs, are being identified. In this review, recent advances in our understanding of plant immune responses to necrotrophs and comparison with responses to biotrophic pathogens are summarized, highlighting common and contrasting mechanisms.

Whole-genome QTL analysis of Stagonospora nodorum blotch resistance and validation of the SnTox4-Snn4 interaction in hexaploid wheat

Necrotrophic effectors (also known as host-selective toxins) are important determinants of disease in the wheat-Stagonospora nodorum pathosystem. To date, five necrotrophic effector-host gene interactions have been identified in this system. Most of these interactions have additive effects while some are epistatic. The Snn4-SnTox4 interaction was originally identified in a recombinant-inbred population derived from a cross between the Swiss winter wheat cultivars 'Arina' and 'Forno' using the S. nodorum isolate Sn99CH 1A7a. Here, we used a recombinant-inbred population consisting of 121 lines developed from a cross between the hexaploid land race Salamouni and the hexaploid wheat 'Katepwa' (SK population). The SK population was used for the construction of linkage maps and quantitative trait loci (QTL) detection using the Swiss S. nodorum isolate Sn99CH 1A7a. The linkage maps developed in the SK population spanned 3,228 centimorgans (cM) and consisted of 441 simple-sequence repeats, 9 restriction fragment length polymorphisms, 29 expressed sequence tag sequence-tagged site markers, and 5 phenotypic markers. The average marker density was 6.7 cM/marker. Two QTL, designated QSnb.fcu-1A and QSnb.fcu-7A on chromosome arms 1AS and 7AS, respectively, were associated with disease caused by the S. nodorum isolate Sn99CH 1A7a. The effects of QSnb.fcu-1A were determined by the Snn4-SnTox4 interaction and accounted for 23.5% of the phenotypic variation in this population, whereas QSnb.fcu-7A accounted for 16.4% of the phenotypic variation for disease but was not associated with any known effector sensitivity locus. The effects of both QTL were largely additive and collectively accounted for 35.7% of the total phenotypic variation. The results of this research validate the effects of a compatible Snn4-SnTox4 interaction in a different genetic background, and it provides knowledge regarding genomic regions and molecular markers that can be used to improve Stagonospora nodorum blotch resistance in wheat germplasm.

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.

Use of the yeast two-hybrid system to identify targets of fungal effectors

The yeast two-hybrid (Y2H) system is a binary method widely used to determine direct interactions between paired proteins. Although having certain limitations, this method has become one of the two main systemic tools (along with affinity purification/mass spectrometry) for interactome mapping in model organisms including yeast, Arabidopsis, and humans. It has also become the method of choice for investigating host-pathogen interactions in fungal pathosystems involving crop plants. This chapter describes general procedures to use the GAL4-based Y2H system for identification of host proteins that directly interact with proteinaceous fungal effectors, thus being their potential targets. The procedures described include cDNA library construction through in vivo recombination, library screening by yeast mating and cotransformation, as well as methods to analyze positive clones obtained from library screening. These procedures can also be adapted to confirmation of suspected interactions between characterized host and pathogen proteins or determination of interacting domains in partner proteins.

Variable expression of the Stagonospora nodorum effector SnToxA among isolates is correlated with levels of disease in wheat

Most research on host?pathogen interactions is focused on mechanisms of resistance, but less is known regarding mechanisms of susceptibility. The wheat?Stagonospora nodorum pathosystem involves pathogen-produced effectors, also known as host-selective toxins, that interact with corresponding dominant host genes to cause disease. Recognition of the S. nodorum effectors SnToxA and SnTox2 is mediated by the wheat genes Tsn1 and Snn2, respectively. Here, we inoculated a population of wheat recombinant inbred lines that segregates for Tsn1 and Snn2 with conidia from two S. nodorum isolates, Sn4 and Sn5, which both produce SnToxA and SnTox2 to compare the effects of compatible Tsn1?SnToxA and Snn2?SnTox2 interactions between the two isolates. Genetic analysis revealed that the two interactions contribute equally to disease caused by isolate Sn4 but the Tsn1?SnToxA interaction contributed substantially more to disease conferred by Sn5 than did the Snn2?SnTox2 interaction. Sequence analysis of the SnToxA locus from Sn4 and Sn5 indicated that they were 99.5% identical, with no polymorphisms in the coding region or the predicted promoters. Analysis of transcription levels showed that expression levels of SnToxA peaked at 26 h postinoculation for both isolates but SnToxA expression in Sn5 was more than twice that of Sn4. This work demonstrates that necrotrophic effectors of different isolates can be expressed at different levels in planta, and that higher levels of expression lead to increased levels of disease in the wheat?S. nodorum pathosystem.

The Arabidopsis extracellular UNUSUAL SERINE PROTEASE INHIBITOR functions in resistance to necrotrophic fungi and insect herbivory

Protease inhibitors (PIs) function in the precise regulation of proteases, and are thus involved in diverse biological processes in many organisms. Here, we studied the functions of the Arabidopsis UNUSUAL SERINE PROTEASE INHIBITOR (UPI) gene, which encodes an 8.8 kDa protein of atypical sequence relative to other PIs. Plants harboring a loss-of-function UPI allele displayed enhanced susceptibility to the necrotrophic fungi Botrytis cinerea and Alternaria brassicicola as well as the generalist herbivore Trichoplusia ni. Further, ectopic expression conferred increased resistance to B. cinerea and T. ni. In contrast, the mutant has wild-type responses to virulent, avirulent and non-pathogenic strains of Pseudomonas syringae, thus limiting the defense function of UPI to necrotrophic fungal infection and insect herbivory. Expression of UPI is significantly induced by jasmonate, salicylic acid and abscisic acid, but is repressed by ethylene, indicating complex phytohormone regulation of UPI expression. The upi mutant also shows significantly delayed flowering, associated with decreased SOC1 expression and elevated levels of MAF1, two regulators of floral transition. Recombinant UPI strongly inhibits the serine protease chymotrypsin but also weakly blocks the cysteine protease papain. Interestingly, jasmonate induces intra- and extracellular UPI accumulation, suggesting a possible role in intercellular or extracellular functions. Overall, our results show that UPI is a dual-specificity PI that functions in plant growth and defense, probably through the regulation of endogenous proteases and/or those of biotic invaders.

The Fusarium virguliforme toxin FvTox1 causes foliar sudden death syndrome-like symptoms in soybean

Fusarium virguliforme causes sudden death syndrome (SDS) in soybean. The pathogen has never been isolated from diseased foliar tissues; therefore, one or more toxins have been considered to cause foliar SDS development. Cell-free F. virguliforme culture filtrates containing a toxin causes foliar SDS in soybean. A low-molecular-weight protein of approximately 13.5 kDa (FvTox1), purified from F. virguliforme culture filtrates, produces foliar SDS-like symptoms in cut soybean seedlings. Anti-FvTox1 monoclonal antibodies raised against the purified FvTox1 were used in isolating the FvTox1 gene. In the presence of light, recombinant FvTox1 protein expressed in an insect cell line resulted in chlorosis and necrosis in soybean leaf disks that are typical foliar SDS symptoms. SDS-susceptible but not the SDS-resistant soybean lines were sensitive to the baculovirus-expressed toxin. The requirement of light for foliar SDS-like symptom development indicates that FvTox1 induces foliar SDS in soybean, most likely through production of free radicals by interrupting photosynthesis.

Differential transcript accumulation in chickpea during early phases of compatible interaction with a necrotrophic fungus Ascochyta rabiei

The initial phases of the disease establishment are very crucial for the compatible interactions. Pathogens must overcome the responses generated by the host for the onset of disease invasion. The compatible interaction is inadequately represented in plant-pathogen interaction studies. To gain broader insight into the early responses elicited by chickpea blight fungus Ascochyta rabiei during compatible interaction; we isolated early responsive genes of chickpea using PCR based suppression subtractive hybridization (SSH) strategy. We obtained ~250 unique genes after homology search and redundancy elimination. Based on their potential cellular functions, these genes were broadly classified into eleven different categories viz. stress, signaling, gene regulation, cellular metabolism and genes of unknown functions. Present study revealed few unexpected genes which have a possible role in induced immunity and disease progression. We employed macroarray, northern blot, real-time PCR and cluster analysis to develop transcript profiles. Most of the genes analyzed were early induced and were transcriptionally upregulated upon 24 h post inoculation. Our approach has rendered the isolation of early responsive genes involved in signaling and regulation of metabolic changes upon fungal infection. The information obtained will help to dissect the molecular mechanisms during compatible chickpea-Ascochyta interactions.

R, Pestka J, Williams D, Glenn A. The current state of mycotoxin biomarker development in humans and animals and the potential for application to plant systems

Filamentous fungi that contaminate livestock feeds and human food supply often produce toxigenic secondary metabolites known as mycotoxins. Among the hundreds of known mycotoxins, aflatoxins, deoxynivalenol, fumonisins, ochratoxin A and zearalenone are considered the most commercially important. Intense research on these mycotoxins, especially aflatoxin, has resulted in the development of 'biomarkers' used to link exposure to disease risk. In the case of aflatoxin this effort has led to the discovery of both exposure and mechanism-based biomarkers, which have proven essential for understanding aflatoxin's potential for causing disease in humans, including subtle effects on growth and immune response. Fumonisin biomarkers have also been used extensively in farm and laboratory animals to study the fumonisin-induced disruption of cellular and systemic physiology which leads to disease. This review summarises the status of mycotoxin biomarker development in humans and animals for the commercially important mycotoxins. Since the fungi responsible for the production of these mycotoxins are often endophytes that infect and colonise living plant tissues, accumulation of mycotoxins in the plant tissues may at times be associated with development of plant disease symptoms. The presence of mycotoxins, even in the absence of disease symptoms, may still have subtle biological effects on the physiology of plants. This review examines the question of whether or not the knowledge gained from mechanistic studies and development of biomarkers in animal and human systems is transferable to the study of mycotoxin effects on plant systems. Thus far, fumonisin has proven amenable to development of mechanism-based biomarkers to study maize seedling disease caused by the fumonisin producer, Fusarium verticillioides. Expanding our knowledge of mechanisms of toxicity and the overt and subtle effects on animal, human, and plant systems through the identification and validation of biomarkers will further our ability to monitor and limit the damage and economic impact of mycotoxins.

Transcriptional analysis of soybean root response to Fusarium virguliforme, the causal agent of sudden death syndrome

Sudden death syndrome (SDS) of soybean can be caused by any of four distinct Fusarium species, with F. virguliforme and F. tucumaniae being the main casual agents in North and South America, respectively. Although the fungal tissue is largely confined to the roots, the fungus releases a toxin that is translocated to leaf tissues, in which it causes interveinal chlorosis and necrosis leading to scorching symptoms and possible defoliation. In this study, we report on an Affymetrix analysis measuring transcript abundances in resistant (PI 567.374) and susceptible (Essex) roots upon infection by F. virguliforme, 5 and 7 days postinoculation. Many of the genes with increased expression were common between resistant and susceptible plants (including genes related to programmed cell death, the phenylpropanoid pathway, defense, signal transduction, and transcription factors), but some genotype-specific expression was noted. Changes in small (sm)RNA levels between inoculated and mock-treated samples were also studied and implicate a role for these molecules in this interaction. In total, 2,467 genes were significantly changing in the experiment, with 1,694 changing in response to the pathogen; 93 smRNA and 42 microRNA that have putative soybean gene targets were identified from infected tissue. Comparing genotypes, 247 genes were uniquely modulating in the resistant host, whereas 378 genes were uniquely modulating in the susceptible host. Comparing locations of differentially expressed genes to known resistant quantitative trait loci as well as identifying smRNA that increased while their putative targets decreased (or vice versa) allowed for the narrowing of candidate SDS defense-associated genes.

Leger RJ. Insertion of an esterase gene into a specific locust pathogen (Metarhizium acridum) enables it to infect caterpillars

An enduring theme in pathogenic microbiology is poor understanding of the mechanisms of host specificity. Metarhizium is a cosmopolitan genus of invertebrate pathogens that contains generalist species with broad host ranges such as M. robertsii (formerly known as M. anisopliae var. anisopliae) as well as specialists such as the acridid-specific grasshopper pathogen M. acridum. During growth on caterpillar (Manduca sexta) cuticle, M. robertsii up-regulates a gene (Mest1) that is absent in M. acridum and most other fungi. Disrupting M. robertsii Mest1 reduced virulence and overexpression increased virulence to caterpillars (Galleria mellonella and M. sexta), while virulence to grasshoppers (Melanoplus femurrubrum) was unaffected. When Mest1 was transferred to M. acridum under control of its native M. robertsii promoter, the transformants killed and colonized caterpillars in a similar fashion to M. robertsii. MEST1 localized exclusively to lipid droplets in M. robertsii conidia and infection structures was up-regulated during nutrient deprivation and had esterase activity against lipids with short chain fatty acids. The mobilization of stored lipids was delayed in the Mest1 disruptant mutant. Overall, our results suggest that expression of Mest1 allows rapid hydrolysis of stored lipids, and promotes germination and infection structure formation by M. robertsii during nutrient deprivation and invasion, while Mest1 expression in M. acridum broadens its host range by bypassing the regulatory signals found on natural hosts that trigger the mobilization of endogenous nutrient reserves. This study suggests that speciation in an insect pathogen could potentially be driven by host shifts resulting from changes in a single gene.

Host selectivity and host switching have been widely documented in diverse pathogens, but in most cases the underlying mechanisms are poorly understood. Entomopathogenic fungi in the genus Metarhizium are being used as environmentally friendly alternatives to chemical insecticides in agricultural and human disease-vector control programs, and as model systems for studying the interactions between invertebrate hosts and pathogenic fungi. In this paper we describe molecular mechanisms controlling the host selectivity of M. robertsii strain Mr2575, a generalist able to infect hundreds of insect species, and M. acridum strain Ma324, a specialist pathogen of grasshoppers and locusts. The esterase gene (Mest1) from Mr2575 is required for virulence against caterpillars but not grasshoppers. Ma324 lacks Mest1, but insertion of Mest1 into Ma324 broadens its host range to include caterpillars. Our results suggest that expression of Mest1 allows rapid mobilization of endogenous lipid reserves and promotes germination and infection structure formation. This study suggests that speciation in insect pathogens can be driven by host shifts due to the gain or loss of a pathogen gene that confers wide host specificity.

High-resolution mapping of Rsn1, a locus controlling sensitivity of rice to a necrosis-inducing phytotoxin from Rhizoctonia solani AG1-IA

Rhizoctonia solani is a necrotrophic fungal pathogen that causes disease on many crop-plant species. Anastomosis group 1-IA is the causal agent of sheath blight of rice (Oryza sativa L.), one of the most important rice diseases worldwide. R. solani AG1-IA produces a necrosis-inducing phytotoxin and rice cultivar's sensitivity to the toxin correlates with disease susceptibility. Unlike genetic analyses of sheath blight resistance where resistance loci have been reported as quantitative trait loci, phytotoxin sensitivity is inherited as a Mendelian trait that permits high-resolution mapping of the sensitivity genes. An F(2) mapping population derived from parent cultivars 'Cypress' (toxin sensitive) and 'Jasmine 85' (toxin insensitive) was used to map Rsn1, the necrosis-inducing locus. Initial mapping based on 176 F(2) progeny and 69 simple sequence repeat (SSR) markers located Rsn1 on the long arm of chromosome 7, with tight linkage to SSR marker RM418. A high-resolution genetic map of the region was subsequently developed using a total of 1,043 F(2) progeny, and Rsn1 was mapped to a 0.7 cM interval flanked by markers NM590 and RM418. Analysis of the corresponding 29 Kb genomic sequences from reference cultivars 'Nipponbare' and '93-11' revealed the presence of four putative genes within the interval. Two are expressed cytokinin-O-glucosyltransferases, which fit an apoptotic pathway model of toxin activity, and are individually being investigated further as potential candidates for Rsn1.

Positional cloning of ds1, the target leaf spot resistance gene against Bipolaris sorghicola in sorghum

Target leaf spot is one of the major sorghum diseases in southern Japan and caused by a necrotrophic fungus, Bipolaris sorghicola. Sorghum resistance to target leaf spot is controlled by a single recessive gene (ds1). A high-density genetic map of the ds1 locus was constructed with simple sequence repeat markers using progeny from crosses between a sensitive variety, bmr-6, and a resistant one, SIL-05, which allowed the ds1 gene to be genetically located within a 26-kb region on the short arm of sorghum chromosome 5. The sorghum genome annotation database for BTx623, for which the whole genome sequence was recently published, indicated a candidate gene from the Leucine-Rich Repeat Receptor Kinase family in this region. The candidate protein kinase gene was expressed in susceptible plants but was not expressed or was severely reduced in resistant plants. The expression patterns of ds1 gene and the phenotype of target leaf spot resistance were clearly correlated. Genomic sequences of this region in parental varieties showed a deletion in the promoter region of SIL-05 that could cause reduction of gene expression. We also found two ds1 alleles for resistant phenotypes with a stop codon in the coding region. The results shown here strongly suggest that the loss of function or suppression of the ds1 protein kinase gene leads to resistance to target leaf spot in sorghum.

Analysis of two in planta expressed LysM effector homologs from the fungus Mycosphaerella graminicola reveals novel functional properties and varying contributions to virulence on wheat

Secreted effector proteins enable plant pathogenic fungi to manipulate host defenses for successful infection. Mycosphaerella graminicola causes Septoria tritici blotch disease of wheat (Triticum aestivum) leaves. Leaf infection involves a long (approximately 7 d) period of symptomless intercellular colonization prior to the appearance of necrotic disease lesions. Therefore, M. graminicola is considered as a hemibiotrophic (or necrotrophic) pathogen. Here, we describe the molecular and functional characterization of M. graminicola homologs of Ecp6 (for extracellular protein 6), the Lysin (LysM) domain-containing effector from the biotrophic tomato (Solanum lycopersicum) leaf mold fungus Cladosporium fulvum, which interferes with chitin-triggered immunity in plants. Three LysM effector homologs are present in the M. graminicola genome, referred to as Mg3LysM, Mg1LysM, and MgxLysM. Mg3LysM and Mg1LysM genes were strongly transcriptionally up-regulated specifically during symptomless leaf infection. Both proteins bind chitin; however, only Mg3LysM blocked the elicitation of chitin-induced plant defenses. In contrast to C. fulvum Ecp6, both Mg1LysM and Mg3LysM also protected fungal hyphae against plant-derived hydrolytic enzymes, and both genes show significantly more nucleotide polymorphism giving rise to nonsynonymous amino acid changes. While Mg1LysM deletion mutant strains of M. graminicola were fully pathogenic toward wheat leaves, Mg3LysM mutant strains were severely impaired in leaf colonization, did not trigger lesion formation, and were unable to undergo asexual sporulation. This virulence defect correlated with more rapid and pronounced expression of wheat defense genes during the symptomless phase of leaf colonization. These data highlight different functions for MgLysM effector homologs during plant infection, including novel activities that distinguish these proteins from C. fulvum Ecp6.

A plant alternative to animal caspases: subtilisin-like proteases

Activities displaying caspase cleavage specificity have been well documented in various plant programmed cell death (PCD) models. However, plant genome analyses have not revealed clear orthologues of caspase genes, indicating that enzyme(s) structurally unrelated yet possessing caspase specificity have functions in plant PCD. Here, we review recent data showing that some caspase-like activities are attributable to the plant subtilisin-like proteases, saspases and phytaspases. These proteases hydrolyze a range of tetrapeptide caspase substrates following the aspartate residue. Data obtained with saspases implicate them in the proteolytic degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) during biotic and abiotic PCD, whereas phytaspase overproducing and silenced transgenics provide evidence that phytaspase regulates PCD during both abiotic (oxidative and osmotic stresses) and biotic (virus infection) insults. Like caspases, phytaspases and saspases are synthesized as proenzymes, which are autocatalytically processed to generate a mature enzyme. However, unlike caspases, phytaspases and saspases appear to be constitutively processed and secreted from healthy plant cells into the intercellular space. Apoplastic localization presumably prevents enzyme-mediated protein fragmentation in the absence of PCD. In response to death-inducing stimuli, phytaspase has been shown to re-localize to the cell interior. Thus, plant PCD-related proteases display both common (D-specific protein fragmentation during PCD) and distinct (enzyme structure and activity regulation) features with animal PCD-related proteases.

The role of the plasma membrane H+-ATPase in plant-microbe interactions

Plasma membrane (PM) H+-ATPases are the primary pumps responsible for the establishment of cellular membrane potential in plants. In addition to regulating basic aspects of plant cell function, these enzymes contribute to signaling events in response to diverse environmental stimuli. Here, we focus on the roles of the PM H+-ATPase during plant-pathogen interactions. PM H+-ATPases are dynamically regulated during plant immune responses and recent quantitative proteomics studies suggest complex spatial and temporal modulation of PM H+-ATPase activity during early pathogen recognition events. Additional data indicate that PM H+-ATPases cooperate with the plant immune signaling protein RIN4 to regulate stomatal apertures during bacterial invasion of leaf tissue. Furthermore, pathogens have evolved mechanisms to manipulate PM H+-ATPase activity during infection. Thus, these ubiquitous plant enzymes contribute to plant immune responses and are targeted by pathogens to increase plant susceptibility.

Mycosphaerella fijiensis, the black leaf streak pathogen of banana: progress towards understanding pathogen biology and detection, disease development, and the challenges of control

BACKGROUND

Banana (Musa spp.) is grown throughout the tropical and subtropical regions of the world. The fruits are a key staple food in many developing countries and a source of income for subsistence farmers. Bananas are also a major, multibillion-dollar export commodity for consumption primarily in developed countries, where few banana cultivars are grown. The fungal pathogen Mycosphaerella fijiensis causes black leaf streak disease (BLSD; aka black Sigatoka leaf spot) on the majority of edible banana cultivars grown worldwide. The fact that most of these cultivars are sterile and unsuitable for the breeding of resistant lines necessitates the extensive use of fungicides as the primary means of disease control. BLSD is a significant threat to the food security of resource-poor populations who cannot afford fungicides, and increases the environmental and health hazards where large-acreage monocultures of banana (Cavendish subgroup, AAA genome) are grown for export.

TAXONOMY

Mycosphaerella fijiensis M. Morelet is a sexual, heterothallic fungus having Pseudocercospora fijiensis (M. Morelet) Deighton as the anamorph stage. It is a haploid, hemibiotrophic ascomycete within the class Dothideomycetes, order Capnodiales and family Mycosphaerellaceae. Its taxonomic placement is based on DNA phylogeny, morphological analyses and cultural characteristics.

DISEASE SYMPTOMS AND HOST RANGE

Mycosphaerella fijiensis is a leaf pathogen that causes reddish-brown streaks running parallel to the leaf veins, which aggregate to form larger, dark-brown to black compound streaks. These streaks eventually form fusiform or elliptical lesions that coalesce, form a water-soaked border with a yellow halo and, eventually, merge to cause extensive leaf necrosis. The disease does not kill the plants immediately, but weakens them by decreasing the photosynthetic capacity of leaves, causing a reduction in the quantity and quality of fruit, and inducing the premature ripening of fruit harvested from infected plants. Although Musa spp. are the primary hosts of M. fijiensis, the ornamental plant Heliconia psittacorum has been reported as an alternative host.

NEW OPPORTUNITIES

Several valuable tools and resources have been developed to overcome some of the challenges of studying this host-pathogen system. These include a DNA-mediated fungal transformation system and the ability to conduct targeted gene disruptions, reliable quantitative plant bioassays, diagnostic probes to detect and differentiate M. fijiensis from related pathogens and to distinguish strains of different mating types, and a genome sequence that has revealed a wealth of gene sequences and molecular markers to be utilized in functional and population biology studies.

USEFUL WEBSITES

http://bananas.bioversityinternational.org/, http://genome.jgi-psf.org/Mycfi2/Mycfi2.home.html, http://www.isppweb.org/names_banana_pathogen.asp#fun, http://www.promusa.org/.