Cloning, characterization, and targeted disruption of cpcat1, coding for an in planta secreted catalase of Claviceps purpurea

Penicillium chrysogenum as a fungal factory for feruloyl esterases

Plant biomass is a promising substrate for biorefinery, as well as a source of bioactive compounds, platform chemicals, and precursors with multiple industrial applications. These applications depend on the hydrolysis of its recalcitrant structure. However, the effective biological degradation of plant cell walls requires several enzymatic groups acting synergistically, and novel enzymes are needed in order to achieve profitable industrial hydrolysis processes. In the present work, a feruloyl esterase (FAE) activity screening of Penicillium spp. strains revealed a promising candidate (Penicillium rubens Wisconsin 54-1255; previously Penicillium chrysogenum), where two FAE-ORFs were identified and subsequently overexpressed. Enzyme extracts were analyzed, confirming the presence of FAE activity in the respective gene products (PrFaeA and PrFaeB). PrFaeB-enriched enzyme extracts were used to determine the FAE activity optima (pH 5.0 and 50-55 °C) and perform proteome analysis by means of MALDI-TOF/TOF mass spectrometry. The studies were completed with the determination of other lignocellulolytic activities, an untargeted metabolite analysis, and upscaled FAE production in stirred tank reactors. The findings described in this work present P. rubens as a promising lignocellulolytic enzyme producer. KEY POINTS: • Two Penicillium rubens ORFs were first confirmed to have feruloyl esterase activity. • Overexpression of the ORFs produced a novel P. rubens strain with improved activity. • The first in-depth proteomic study of a P. rubens lignocellulolytic extract is shown.

A secreted catalase contributes to Puccinia striiformis resistance to host-derived oxidative stress

Plants can produce reactive oxygen species (ROS) to counteract pathogen invasion, and pathogens have also evolved corresponding ROS scavenging strategies to promote infection and pathogenicity. Catalases (CATs) have been found to play pivotal roles in detoxifying H2O2 formed by superoxide anion catalyzed by superoxide dismutases (SODs). However, few studies have addressed H2O2 removing during rust fungi infection of wheat. In this study, we cloned a CAT gene PsCAT1 from Puccinia striiformis f. sp. tritici (Pst), which encodes a monofunctional heme-containing catalase. PsCAT1 exhibited a high degree of tolerance to pH and temperature, and forms high homopolymers.Heterologous complementation assays in Saccharomyces cerevisiae reveal that the signal peptide of PsCAT1 is functional. Overexpression of PsCAT1 enhanced S. cerevisiae resistance to H2O2. Transient expression of PsCAT1 in Nicotiana benthamiana suppressed Bax-induced cell death. Knockdown of PsCAT1 using a host-induced gene silencing (HIGS) system led to the reduced virulence of Pst, which was correlated to H2O2 accumulation in HIGS plants. These results indicate that PsCAT1 acts as an important pathogenicity factor that facilitates Pst infection by scavenging host-derived H2O2.

Global Transcriptome Characterization and Assembly of the Thermophilic Ascomycete Chaetomium thermophilum

A correct genome annotation is fundamental for research in the field of molecular and structural biology. The annotation of the reference genome of Chaetomium thermophilum has been reported previously, but it is essentially limited to open reading frames (ORFs) of protein coding genes and contains only a few noncoding transcripts. In this study, we identified and annotated full-length transcripts of C. thermophilum by deep RNA sequencing. We annotated 7044 coding genes and 4567 noncoding genes. Astonishingly, 23% of the coding genes are alternatively spliced. We identified 679 novel coding genes as well as 2878 novel noncoding genes and corrected the structural organization of more than 50% of the previously annotated genes. Furthermore, we substantially extended the Gene Ontology (GO) and Enzyme Commission (EC) lists, which provide comprehensive search tools for potential industrial applications and basic research. The identified novel transcripts and improved annotation will help to understand the gene regulatory landscape in C. thermophilum. The analysis pipeline developed here can be used to build transcriptome assemblies and identify coding and noncoding RNAs of other species.

Characterization of two catalase-peroxidase-encoding genes in Fusarium verticillioides reveals differential responses to in vitro versus in planta oxidative challenges

Catalase-peroxidases (KatGs) are a superfamily of reactive oxygen species (ROS)-degrading enzymes believed to have been horizontally acquired by ancient Ascomycota from bacteria. Subsequent gene duplication resulted in two KatG paralogues in ascomycetes: the widely distributed intracellular KatG1 group and the phytopathogen-dominated extracellular KatG2 group. To functionally characterize FvCP01 (KatG1) and FvCP02 (KatG2) in the maize pathogen Fusarium verticillioides, single and double gene deletion mutants were examined in response to hydrogen peroxide (H₂ O₂ ). Both ΔFvCP01 and ΔFvCP02 were more sensitive to H₂ O₂ than the wild-type in vitro, although their sensitivity differed depending on the type of inoculum. Inoculations using mycelial agar plugs demonstrated an additive effect of the mutants, with the ΔFvCP01/ΔFvCP02 double deletion being the most sensitive to H₂ O₂ . In general, conidia were much more sensitive than agar plugs to H₂ O₂ , and conidial inoculations indicated that FvCP01 conferred more H₂ O₂ tolerance than FvCP02. Transcriptional analysis showed the induction of FvCP01, but decreased expression of FvCP02, in both mycelia and spores in the wild-type after H₂ O₂ exposure, but this trend was reversed when the fungus was grown on germinating maize seeds. This interaction with the plant increased the expression of FvCP02, but not FvCP01, indicating that FvCP02 may be responsive to plant-derived H₂ O₂ . Yet, FvCP01 was induced more than three-fold in the ΔFvCP02 mutant grown on germlings, suggesting that FvCP01 can compensate for the loss of FvCP02. Given the differential responses of these two F. verticillioides genes to in vitro versus in planta challenges, a model is proposed to illustrate the differing roles of FvCP01 and FvCP02 in protective responses against H₂ O₂ -derived oxidative stress.

Cross-talk of the biotrophic pathogen Claviceps purpurea and its host Secale cereale

BACKGROUND

The economically important Ergot fungus Claviceps purpurea is an interesting biotrophic model system because of its strict organ specificity (grass ovaries) and the lack of any detectable plant defense reactions. Though several virulence factors were identified, the exact infection mechanisms are unknown, e.g. how the fungus masks its attack and if the host detects the infection at all.

RESULTS

We present a first dual transcriptome analysis using an RNA-Seq approach. We studied both, fungal and plant gene expression in young ovaries infected by the wild-type and two virulence-attenuated mutants. We can show that the plant recognizes the fungus, since defense related genes are upregulated, especially several phytohormone genes. We present a survey of in planta expressed fungal genes, among them several confirmed virulence genes. Interestingly, the set of most highly expressed genes includes a high proportion of genes encoding putative effectors, small secreted proteins which might be involved in masking the fungal attack or interfering with host defense reactions. As known from several other phytopathogens, the C. purpurea genome contains more than 400 of such genes, many of them clustered and probably highly redundant. Since the lack of effective defense reactions in spite of recognition of the fungus could very well be achieved by effectors, we started a functional analysis of some of the most highly expressed candidates. However, the redundancy of the system made the identification of a drastic effect of a single gene most unlikely. We can show that at least one candidate accumulates in the plant apoplast. Deletion of some candidates led to a reduced virulence of C. purpurea on rye, indicating a role of the respective proteins during the infection process.

CONCLUSIONS

We show for the first time that- despite the absence of effective plant defense reactions- the biotrophic pathogen C. purpurea is detected by its host. This points to a role of effectors in modulation of the effective plant response. Indeed, several putative effector genes are among the highest expressed genes in planta.

Proteome catalog of Zymoseptoria tritici captured during pathogenesis in wheat

Zymoseptoria tritici is an economically important pathogen of wheat. However, the molecular basis of pathogenicity on wheat is still poorly understood. Here, we present a global survey of the proteins secreted by this fungus in the apoplast of resistant (cv. Shafir) and susceptible (cv. Obelisk) wheat cultivars after inoculation with reference Z. tritici strain IPO323. The fungal proteins present in apoplastic fluids were analyzed by gel electrophoresis and by data-independent acquisition liquid chromatography/mass spectrometry (LC/MS(E)) combined with data-dependent acquisition LC-MS/MS. Subsequent mapping mass spectrometry-derived peptide sequence data against the genome sequence of strain IPO323 identified 665 peptides in the MS(E) and 93 in the LC-MS/MS mode that matched to 85 proteins. The identified fungal proteins, including cell-wall degrading enzymes and proteases, might function in pathogenicity, but the functions of many remain unknown. Most fungal proteins accumulated in cv. Obelisk at the onset of necrotrophy. This inventory provides an excellent basis for future detailed studies on the role of these genes and their encoded proteins during pathogenesis in wheat.

Highly Active and Stable Large Catalase Isolated from a Hydrocarbon Degrading Aspergillus terreus MTCC 6324

A hydrocarbon degrading Aspergillus terreus MTCC 6324 produces a high level of extremely active and stable cellular large catalase (CAT) during growth on n-hexadecane to combat the oxidative stress caused by the hydrocarbon degrading metabolic machinery inside the cell. A 160-fold purification with specific activity of around 66 × 10⁵ U mg⁻¹ protein was achieved. The native protein molecular mass was 368 ± 5 kDa with subunit molecular mass of nearly 90 kDa, which indicates that the native CAT protein is a homotetramer. The isoelectric pH (pI) of the purified CAT was 4.2. BLAST aligned peptide mass fragments of CAT protein showed its highest similarity with the catalase B protein from other fungal sources. CAT was active in a broad range of pH 4 to 12 and temperature 25°C to 90°C. The catalytic efficiency (K_cat/K_m) of 4.7 × 10⁸ M⁻¹ s⁻¹ within the studied substrate range and alkaline pH stability (half-life, t_1/2 at pH 12~15 months) of CAT are considerably higher than most of the extensively studied catalases from different sources. The storage stability (t_1/2) of CAT at physiological pH 7.5 and 4°C was nearly 30 months. The haem was identified as haem b by electrospray ionization tandem mass spectroscopy (ESI-MS/MS).

Recognition- and defense-related gene expression at 3 resynthesis stages in lichen symbionts

Recognition and defense responses are early events in plant-pathogen interactions and between lichen symbionts. The effect of elicitors on responses between lichen symbionts is not well understood. The objective of this study was to compare the difference in recognition- and defense-related gene expression as a result of culture extracts (containing secreted water-soluble elicitors) from compatible and incompatible interactions at each of 3 resynthesis stages in the symbionts of Cladonia rangiferina. This study investigated gene expression by quantitative PCR in cultures of C. rangiferina and its algal partner, Asterochloris glomerata/irregularis, after incubation with liquid extracts from cultures of compatible and incompatible interactions at 3 early resynthesis stages. Recognition-related genes were significantly upregulated only after physical contact, demonstrating symbiont recognition in later resynthesis stages than expected. One of 3 defense-related genes, chit, showed significant downregulation in early resynthesis stages and upregulation in the third resynthesis stage, demonstrating a need for the absence of chitinase early in thallus formation and a need for its presence in later stages as an algal defense reaction. This study revealed that recognition- and defense-related genes are triggered by components in culture extracts at 3 stages of resynthesis, and some defense-related genes may be induced throughout thallus growth. The parasitic nature of the interaction shows parallels between lichen symbionts and plant pathogenic systems.

Pre-illumination of rice blast conidia induces tolerance to subsequent oxidative stress

Many environmental factors, alone or combined, affect organisms by changing a pro-/antioxidant balance. Here we tested rice blast fungus (Magnaporthe oryzae) for possible cross-adaptations caused by relatively intense light and protecting from artificially formed reactive oxygen species (ROS) and ROS-dependent fungitoxic response of the host plant. Spore germination was found to be suppressed under 4-h and, to larger extent, 5-h illumination. The effect was diminished by antioxidants and, therefore, suggests involvement of ROS. One-hour of light did not affect spore germination, but stimulated their chemically assayed superoxide production. The illuminated spores were more tolerant (than non-illuminated ones) to artificially generated H(2)O(2), O(2)(-), or OH or to toxic diffusate of rice leaf. They also caused more severe disease symptoms if applied to leaves of the susceptible rice cultivar at low concentration. Spore diffusates decomposed hydrogen peroxide. They detoxified exogenous H(2)O(2) and superoxide radical as well as leaf diffusates. Spore illumination increased some of these protective effects. It is suggested that short-term light led to mild oxidative stress, which induced spore antioxidant capacity, enhancing spore tolerance to subsequent stronger oxidative stress and its aggressiveness in planta. Such tolerance depends partly on the antidotal action of spore extracellular compounds, which may also be light-stimulated. Therefore, a certain ROS-related environmental factor may adapt a fungus to other factors and so modulate its pathogenic properties.

Coupling of transcriptional response to oxidative stress and secondary metabolism regulation in filamentous fungi

To survive sudden and potentially lethal changes in their environment, filamentous fungi must sense and respond to a vast array of stresses, including oxidative stresses. The generation of reactive oxygen species, or ROS, is an inevitable aspect of existence under aerobic conditions. In addition, in the case of fungi with pathogenic lifestyles, ROS are produced by the infected hosts and serve as defense weapons via direct toxicity, as well as effectors in fungal cell death mechanisms. Filamentous fungi have thus developed complex and sophisticated responses to evade oxidative killing. Several steps are determinant in these responses, including the activation of transcriptional regulators involved in the control of the antioxidant machinery. Gathering and integrating the most recent advances in knowledge of oxidative stress responses in fungi are the main objectives of this review. Most of the knowledge coming from two models, the yeast Saccharomyces cerevisiae and fungi of the genus Aspergillus, is summarized. Nonetheless, recent information on various other fungi is delivered when available. Finally, special attention is given on the potential link between the functional interaction between oxidative stress and secondary metabolism that has been suggested in recent reports, including the production of mycotoxins.

The role of catalase-peroxidase secreted by Magnaporthe oryzae during early infection of rice cells

The biological role of a secretory catalase of the rice blast fungus Magnaporthe oryzae was studied. The internal amino acid sequences of the partially purified catalase in the culture filtrate enabled us to identify its encoding gene as a catalase-peroxidase gene, CPXB, among four putative genes for catalase or catalase-peroxidase in M. oryzae. Knockout of the gene drastically reduced the level of catalase activity in the culture filtrate and supernatant of conidial suspension (SCS), and increased the sensitivity to exogenously added H₂O₂ compared with control strains, suggesting that CPXB is the major gene encoding the secretory catalase and confers resistance to H₂O₂ in hyphae. In the mutant, the rate of appressoria that induced accumulation of H₂O₂ in epidermal cells of the leaf sheath increased and infection at early stages was delayed; however, the formation of lesions in the leaf blade was not affected compared with the control strain. These phenotypes were complimented by reintroducing the putative coding regions of CPXB driven by a constitutive promoter. These results suggest that CPXB plays a role in fungal defense against H₂O₂ accumulated in epidermal cells of rice at the early stage of infection but not in pathogenicity of M. oryzae.

Reactive oxygen species in phytopathogenic fungi: signaling, development, and disease

Reactive oxygen species (ROS) play a major role in pathogen-plant interactions: recognition of a pathogen by the plant rapidly triggers the oxidative burst, which is necessary for further defense reactions. The specific role of ROS in pathogen defense is still unclear. Studies on the pathogen so far have focused on the importance of the oxidative stress response (OSR) systems to overcome the oxidative burst or of its avoidance by effectors. This review focuses on the role of ROS for fungal virulence and development. In the recent years, it has become obvious that (a) fungal OSR systems might not have the predicted crucial role in pathogenicity, (b) fungal pathogens, especially necrotrophs, can actively contribute to the ROS level in planta and even take advantage of the host's response, (c) fungi possess superoxide-generating NADPH oxidases similar to mammalian Nox complexes that are important for pathogenicity; however, recent data indicate that they are not directly involved in pathogen-host communication but in fungal differentiation processes that are necessary for virulence.

Ergot: from witchcraft to biotechnology

The ergot diseases of grasses, caused by members of the genus Claviceps, have had a severe impact on human history and agriculture, causing devastating epidemics. However, ergot alkaloids, the toxic components of Claviceps sclerotia, have been used intensively (and misused) as pharmaceutical drugs, and efficient biotechnological processes have been developed for their in vitro production. Molecular genetics has provided detailed insight into the genetic basis of ergot alkaloid biosynthesis and opened up perspectives for the design of new alkaloids and the improvement of production strains; it has also revealed the refined infection strategy of this biotrophic pathogen, opening up the way for better control. Nevertheless, Claviceps remains an important pathogen worldwide, and a source for potential new drugs for central nervous system diseases.

The small GTPase Rac and the p21-activated kinase Cla4 in Claviceps purpurea: interaction and impact on polarity, development and pathogenicity

Claviceps purpurea, the ergot fungus, is a highly specialized pathogen of grasses; its colonization of host ovarian tissue requires an extended period of strictly polarized, oriented growth towards the vascular tissue. To understand this process, we study the role of signalling factors affecting polarity and differentiation. We showed that the small GTPase Cdc42 is involved in polarity, sporulation and in planta growth in C. purpurea. Here we present evidence that the GTPase Rac has an even stronger and, in some aspects, inverse impact on growth and development: Deltarac mutants form coralline-like colonies, show hyper-branching, loss of polarity, sporulation and ability to penetrate. Functional analyses and yeast two-hybrid studies prove that the p21-activated kinase Cla4 is a major downstream partner of Rac. Phosphorylation assays of MAP kinases and expression studies of genes encoding reactive oxygen species (ROS)-scavenging and -generating enzymes indicate a function of Rac and Cla4 in fungal ROS homoeostasis which could contribute to their drastic impact on differentiation.

The histidine kinase CpHK2 has impact on spore germination, oxidative stress and fungicide resistance, and virulence of the ergot fungus Claviceps purpurea

SUMMARY Histidine kinases are important mediators for adaptation of bacteria and plants to environmental signals. Genome analyses of filamentous fungi have revealed the presence of a high number of potential hybrid histidine kinase (HK)-encoding genes; the role of most of these potential sensors is so far unclear, though some members of the class III histidine kinases were shown to be involved in osmostress responses. Here we present a functional analysis of cphk2, a histidine kinase-encoding gene in the biotrophic grass pathogen Claviceps purpurea. The putative product of cphk2 (CpHK2) was shown to group within family X of fungal HKs and it had high homology to the oxidative stress sensors SpMAK2/3 of Schizosaccharomyces pombe. Analysis of a cphk2 deletion mutant indicated that this histidine kinase is involved in spore germination, sensitivity to oxidative stress and fungicide resistance. In addition, virulence of the Dcphk2 mutant on rye was significantly reduced compared with the wild-type strain, even if the conidial titre was adjusted to the lower germination rate. This is the first report of a role for a class X histidine kinase in a filamentous fungus.

A novel role for catalase B in the maintenance of fungal cell-wall integrity during host invasion in the rice blast fungus Magnaporthe grisea

Asexual spores of the rice blast fungus germinate to produce a specialized and melanized infection structure, the appressorium, which is pivotal to successful plant penetration. To investigate whether Magnaporthe grisea counteracts the toxic burst of H2O2 localized beneath the site of attempted invasion, we examined the temporal expression of five candidate antioxidant genes. Of these, the putatively secreted large subunit catalase CATB gene was 600-fold up-regulated in vivo, coincident with penetration, and moderately up-regulated in vitro, in response to exogenous H2O2. Targeted gene replacement of CATB led to compromised pathogen fitness; the catB mutant displayed paler pigmentation and accelerated hyphal growth but lower biomass, poorer sporulation, fragile conidia and appressoria, and impaired melanization. The catB mutant was severely less pathogenic than Guy 11 on barley and rice, and its infectivity was further reduced on exposure to H2O2. The wild-type phenotype was restored by the reintroduction of CATB into the catB mutant We found no evidence to support a role for CATB in detoxification of the host-derived H2O2 at the site of penetration. Instead, we demonstrated that CATB plays a part in strengthening the fungal wall, a role of particular importance during forceful entry into the host.

Analysis of Claviceps africana and C. sorghi from India using AFLPs, EF-1alpha gene intron 4, and beta-tubulin gene intron 3

Isolates of Claviceps causing ergot on sorghum in India were analysed by AFLP analysis, and by analysis of DNA sequences of the EF-1alpha gene intron 4 and beta-tubulin gene intron 3 region. Of 89 isolates assayed from six states in India, four were determined to be C. sorghi, and the rest C. africana. A relatively low level of genetic diversity was observed within the Indian C. africana population. No evidence of genetic exchange between C. africana and C. sorghi was observed in either AFLP or DNA sequence analysis. Phylogenetic analysis was conducted using DNA sequences from 14 different Claviceps species. A multigene phylogeny based on the EF-1alpha gene intron 4, the beta-tubulin gene intron 3 region, and rDNA showed that C. sorghi grouped most closely with C. gigantea and C. africana. Although the Claviceps species we analysed were closely related, they colonize hosts that are taxonomically very distinct suggesting that there is no direct coevolution of Claviceps with its hosts.

Blumeria graminis secretes an extracellular catalase during infection of barley: potential role in suppression of host defence

SUMMARY The obligate biotrophic fungal pathogen of barley, Blumeria graminis f.sp. hordei (Bgh), elicits a burst of H(2)O(2) in its host barley at sites of germ tube invasion. To evaluate whether this specialized pathogen has any antioxidant response to this oxidative burst, the Bgh catB gene was characterized and transcript-profiled together with other genes implicated in the management of oxidative stress (catalase-peroxidase, cpx; glutathione peroxidase, gpx; superoxide dismutase, sod1) and in comparison with the constitutively expressed Bghbeta-tubulin and elongation factor1 (ef1) genes. Gel-based and real-time RT-PCR revealed enhanced numbers of catB transcripts at mature primary germ tube and appressorium germ tube (AGT) stages in a susceptible host. Moreover, an anti-CATB polyclonal antibody, from Aspergillus fumigatus, which recognizes both native and recombinant Bgh CATB, revealed an intense circle of immunofluorescence at the host-pathogen interface at the AGT tip and within the halo area surrounding the host papilla. A new diaminobenzidine-based 'scavenger' assay revealed areas of H(2)O(2) clearing at sites of fungal invasion, provoking speculation that Bgh catalase activity may contribute to pathogenicity in Bgh.

Claviceps purpurea: molecular aspects of a unique pathogenic lifestyle

SUMMARY Claviceps purpurea is a ubiquitous pathogen of cereals and grasses, causing Ergot disease, which results in substitution of grains by sclerotia. These overwintering structures contain ergot-alkaloids, which can cause severe intoxication in mammals. C. purpurea is an interesting model system for the study of host-pathogen interaction. It displays strict organ specificity, attacking exclusively young grass ovaries. It is optimally adapted to this special niche of infection, probably by mimicry of pollen tubes: there are no resistance genes known, and no effective resistance reactions can be detected in the early steps of infection. In this early phase of host tissue colonization the fungus shows directed, almost unbranched growth towards the base of the ovary. Thus, C. purpurea represents one of the few systems in which directed growth in filamentous fungi can be studied. Finally, the fungus behaves as a true biotroph in planta, although it can be easily grown in axenic culture. We describe here the tools available to study this interesting pathogen, report on recent molecular investigations concerning the role of cell-wall-degrading enzymes and of reactive oxygen species in this specialized interaction, and present an update of the signalling cascades involved in early events of pathogenesis.

CPTF1, a CREB-like transcription factor, is involved in the oxidative stress response in the phytopathogen Claviceps purpurea and modulates ROS level in its host Secale cereale

CPTF1, a transcription factor with significant homology to ATF/CREB bZIP factors, was identified during an expressed sequence tag (EST) analysis of in planta-expressed genes of the phytopathogen Claviceps purpurea. Using a gene-replacement approach, deletion mutants of cptf1 were created. Expression studies in axenic culture showed that the H2O2-inducible gene cpcat1 (encoding a secreted catalase) had a reduced basal expression level and no longer responded to oxidative stress in the delta cptf1 mutant. Biochemical analyses indicated that CPTF1 is a general regulator of catalase activity. Delta cptf1 mutants showed significantly reduced virulence on rye. Electron microscopical in situ localization revealed significant amounts of H2O2 in delta cptf1-infected rye epidermal tissues, indicating that the plant tissue displayed an oxidative burst-like reaction, an event not detected in wild-type infections. These data indicate that CPTF1 is involved not only in oxidative stress response in the fungus but also in modulation of the plant's defense reactions.

The path in fungal plant pathogenicity: many opportunities to outwit the intruders?

The number of genes implicated in the infection and disease processes of phytopathogenic fungi is increasing rapidly. Forward genetic approaches have identified mutated genes that affect pathogenicity, host range, virulence and general fitness. Likewise, candidate gene approaches have been used to identify genes of interest based on homology and recently through 'comparative genomic approaches' through analysis of large EST databases and whole genome sequences. It is becoming clear that many genes of the fungal genome will be involved in the pathogen-host interaction in its broadest sense, affecting pathogenicity and the disease process in planta. By utilizing the information obtained through these studies, plants may be bred or engineered for effective disease resistance. That is, by trying to disable pathogens by hitting them where it counts.

Functional analysis of H(2)O(2)-generating systems in Botrytis cinerea: the major Cu-Zn-superoxide dismutase (BCSOD1) contributes to virulence on French bean, whereas a glucose oxidase (BCGOD1) is dispensable

SUMMARY The oxidative burst, a transient and rapid accumulation of reactive oxygen species (ROS), is a widespread defence mechanism of higher plants against pathogen attack. There is increasing evidence that the necrotrophic fungal pathogen Botrytis cinerea itself generates ROS, and that this capability could contribute to the virulence of the fungus. Two potential H(2)O(2)-generating systems were studied with respect to their impact on the interaction of B. cinerea and its host plant Phaseolus vulgaris. A Cu-Zn-superoxide dismutase gene (bcsod1) and a putative glucose oxidase gene (bcgod1) were cloned and characterized, and deletion mutants were created using a gene-replacement methodology. Whereas the Deltabcgod1-mutants displayed normal virulence on bean leaves, the Deltabcsod1 mutants showed a significantly retarded development of lesions, indicating that the Cu-Zn SOD-activity is an important single virulence factor in this interaction system. Whether dismutation of (fungal or host) superoxide, or generation of H(2)O(2) (or both), are important for pathogenesis in this system remains to be elucidated.

Molecular Cloning and Analysis of the Ergopeptine Assembly System in the Ergot Fungus Claviceps purpurea

Claviceps purpurea produces the pharmacological important ergopeptines, a class of cyclol-structured alkaloid peptides containing D-lysergic acid. These compounds are assembled from D-lysergic acid and three different amino acids by the nonribosomal peptide synthetase enzymes LPS1 and LPS2. Cloning of alkaloid biosynthesis genes from C. purpurea has revealed a gene cluster including two NRPS genes, cpps 1 and cpps 2. Protein sequence data had assigned earlier cpps1 to encode the trimodular LPS1 assembling the tripeptide portion of ergopeptines. Here, we show by transcriptional analysis, targeted inactivation, analysis of disruption mutants, and heterologous expression that cpps 2 encodes the monomodular LPS2 responsible for D-lysergic acid activation and incorporation into the ergopeptine backbone. The presence of two distinct NRPS subunits catalyzing formation of ergot peptides is the first example of a fungal NRPS system consisting of different NRPS subunits.

Deletion of all Cochliobolus heterostrophus monofunctional catalase-encoding genes reveals a role for one in sensitivity to oxidative stress but none with a role in virulence

The genome of the maize pathogen Cochliobolus heterostrophus encodes three unlinked monofunctional catalase-encoding (CAT) genes that singly or in combination could offer protection against the harmful effects of oxidative stress. Phylogenetic analysis placed the CAT2 and CAT3 proteins in a cluster with large subunit catalases (CAT3 has a secretory signal sequence and was grouped with known secreted catalases), whereas CAT1 clustered with small subunit catalases. Single, double, and triple cat mutants were created and screened for sensitivity to hydrogen peroxide and altered virulence on maize. All mutants deficient in CAT3 had enhanced sensitivity to hydrogen peroxide, as compared with wild type or with mutants deficient in CAT1, CAT2, or both. All catalase-deficient mutants had normal virulence to maize. Thus, the secreted CAT3 protein protects the fungus from oxidative stress during vegetative growth, but members of this enzyme family, alone or in combination, are not essential for virulence.

Asexual development is increased in Neurospora crassa cat-3-null mutant strains

We use asexual development of Neurospora crassa as a model system with which to determine the causes of cell differentiation. Air exposure of a mycelial mat induces hyphal adhesion, and adherent hyphae grow aerial hyphae that, in turn, form conidia. Previous work indicated the development of a hyperoxidant state at the start of these morphogenetic transitions and a large increase in catalase activity during conidiation. Catalase 3 (CAT-3) increases at the end of exponential growth and is induced by different stress conditions. Here we analyzed the effects of cat-3-null strains on growth and asexual development. The lack of CAT-3 was not compensated by other catalases, even under oxidative stress conditions, and cat-3(RIP) colonies were sensitive to H(2)O(2), indicating that wild-type (Wt) resistance to external H(2)O(2) was due to CAT-3. cat-3(RIP) colonies grown in the dark produced high levels of carotenes as a consequence of oxidative stress. Light exacerbated oxidative stress and further increased carotene synthesis. In the cat-3(RIP) mutant strain, increased aeration in liquid cultures led to increased hyphal adhesion and protein oxidation. Compared to the Wt, the cat-3(RIP) mutant strain produced six times more aerial hyphae and conidia in air-exposed mycelial mats, as a result of longer and more densely packed aerial hyphae. Protein oxidation in colonies was threefold higher and showed more aerial hyphae and conidia in mutant strains than did the Wt. Results indicate that oxidative stress due to lack of CAT-3 induces carotene synthesis, hyphal adhesion, and more aerial hyphae and conidia.

Detection of 3-hydroxykynurenine in a plant pathogenic fungus

A redox-active compound has been purified from the barley powdery mildew fungus Blumeria ( Erysiphe ) graminis f. sp. hordei. A combination of spectrophotometry, MS and NMR has identified it as 3-hydroxykynurenine (3OHKyn). This compound, never previously detected in any fungus or pathogen, is best known for its role in vertebrate cataracts. It is found abundantly in developing and germinating spores and also in runner hyphae. Two roles for 3OHKyn are discussed: first, the presence of active oxygen species would enable 3OHKyn to cross-link the spore chemically with the plant. Secondly, it may be acting as an UV protectant and an antioxidant.

Regulation and oxidation of two large monofunctional catalases

The two Neurospora crassa catalase genes cat-1 and cat-3 were shown to encode Cat-1 and Cat-3 large monofunctional catalases. cat-1 and cat-3 genes are regulated differentially during the asexual life cycle and under stress conditions. A stepwise increase in catalase activity occurs during conidiation. Conidia have 60 times more catalase activity than exponentially growing hyphae. Cat-1 activity was predominant in conidia, during germination and early exponential growth. It was induced during prestationary growth and by ethanol or heat shock. Cat-3 activity was predominant during late exponential growth and at the start of the conidiation process. It was induced under stress conditions, such as H(2)O(2), paraquat, cadmium, heat shock, uric acid, and nitrate treatment. In general, Cat-1 activity was associated with nongrowing cells and Cat-3 activity with growing cells. The Cat-3 N-terminus sequence indicates that this catalase is processed and presumably secreted. Paraquat caused modification and degradation of Cat-1. Under heat shock both Cat-1 and Cat-3 were modified and degraded and Cat-1 was resynthesized. Paraquat and heat shock effects were observed only in the presence of air and are probably related to in vivo generation of singlet oxygen. Purified Cat-3 was modified with a photosensitizing reaction in which singlet oxygen is produced.

Polygalacturonase is a pathogenicity factor in the Claviceps purpurea/rye interaction

Claviceps purpurea is a biotrophic, organ-specific pathogen of grasses and cereals, attacking exclusively young ovaries. We have previously shown that its mainly intercellular growth is accompanied by degradation of pectin, and that two endopolygalacturonase genes (cppg1/cppg2) are expressed throughout all stages of infection. We report here on a functional analysis of these genes using a gene-replacement approach. Mutants lacking both polygalacturonase genes are not affected in their vegetative properties, but they are nearly nonpathogenic on rye. Complementation of the mutants with wild-type copies of cppg1 and cppg2 fully restored pathogenicity, proving that the endopolygalacturonases encoded by cppg1 and cppg2 represent pathogenicity factors in the interaction system C. purpurea/Secale cereale, the first unequivocally identified so far in this system.

Functional analysis of an extracellular catalase of Botrytis cinerea

Summary There is evidence that the necrotrophic fungal pathogen Botrytis cinerea is exposed to oxidative processes within plant tissues. The pathogen itself also generates active oxygen species and H(2)O(2) as pathogenicity factors. Our aim was to study how the pathogen may defend itself against cellular damage caused by the accumulation of H(2)O(2) and the role of an extracellular catalase in its detoxification during the infection of tomato and bean plants by B. cinerea. Chloronaphthol staining followed by light microscopy showed that H(2)O(2) accumulates in the infection zone in tomato and bean leaves. An extracellular catalase gene (denominated Bccat2) was cloned from B. cinerea. Exposure of mycelium to H(2)O(2) in liquid culture resulted in increased Bccat2 mRNA levels in a concentration-dependent manner. Bccat2 mRNA was detected at early stages of tomato leaf infection, suggesting that B. cinerea experiences oxidative stress. Bccat2-deficient mutants were generated by transformation-mediated gene disruption. Mutants were more sensitive then the wild-type strain to H(2)O(2)in vitro, but they partly compensated for the absence of BcCAT2 by activating other protective mechanisms in the presence of H(2)O(2). Bccat2-deficient mutants did not display a consistent reduction of virulence on bean and tomato leaves. Cerium chloride staining of infected leaf tissue for ultrastructural studies showed that Bccat2-deficient mutants were exposed to H(2)O(2) comparably to the wild-type. The results suggest that B. cinerea is a robust pathogen adapted to growing in hostile oxidizing environments in host tissues.

The biotrophic, non-appressorium-forming grass pathogen Claviceps purpurea needs a Fus3/Pmk1 homologous mitogen-activated protein kinase for colonization of rye ovarian tissue

Claviceps purpurea is a common pathogen of a wide range of grasses and cereals that is able to establish a stable, balanced interaction with its host plant and is considered a biotroph. It does not form special penetration structures such as appressoria. To study the signaling processes involved in this special host-pathogen interaction, we have cloned a gene, cpmk1, encoding a mitogen-activated protein (MAP) kinase that shows significant homology to Fus3 of Saccharomyces cerevisiae and to pmk1 of Magnaporthe grisea. Using a gene-replacement approach, we isolated a Acpmk1 mutant and characterized it in detail. Loss of CPMK1 has no obvious effect on vegetative properties (such as growth rate, morphology, and conidia formation); however, infection tests on rye show that the mutant is unable to colonize rye tissue, i.e., it appears to be completely nonpathogenic. Complementation of the mutant with a wild-type copy of cpmk1 fully restores its pathogenicity, confirming that this MAP kinase is essential for infection of rye by C. purpurea. Transformation of the delta pmk1 mutant of M. grisea with a complete copy of cpmk1 (including the C. purpurea promoter) fully restored its ability to form appressoria and its pathogenicity on barley. Although both fungi drastically differ in their pathogenic strategies, this result indicates that the signal pathway involving CPMK1 is highly conserved.

Neurospora crassa catalases, singlet oxygen and cell differentiation

The morphogenetic transitions of the N. crassa asexual life cycle are responses to a hyperoxidant state in which probably singlet oxygen is generated. Induction of catalase activity and catalase oxidation by singlet oxygen are consequences of this recurrent hyperoxidant state. Here the biochemical properties and regulation of two large monofunctional catalases are reviewed, and a new catalase-peroxidase gene and activity is described. Catalase-3 is associated to growing and Catalase-1 to non-growing cells. Under stressful conditions one of these catalases is synthesized, depending on whether growth can be continued or a resistant cell has to be made. The catalase-peroxidase Catalase-2 was possibly derived from a bacterial enzyme. In contrast to the other catalases, Catalase-2 had catalase and peroxidase activity. Catalase-2 was expressed under conditions in which vacuolization of hyphae is observed. All three enzymes have a chlorin in its active site instead of ferroprotoheme IX and are resistant to molar concentrations of hydrogen peroxide. These and all other catalases tested so far are oxidized by singlet oxygen, probably at the heme moiety. The catalase activity is virtually unaffected by oxidation, but the enzymes are probably degraded more rapidly than the unmodified ones.

Oxidative stress-induced expression of catalases in Comamonas terrigena

When grown under oxidative stress, catalatic as well as peroxidatic activity is increased in the Gram-negative bacterium Comamonas terrigena N3H. Two distinct hydroperoxidases were demonstrated by a specific staining. Based on their molar masses and their sensitivity toward 3-amino-1,2,4-triazole and high temperatures, they were identified as dimeric catalase-1 (Cat-1; 150 kDa), and as a tetrameric catalase-2 (Cat-2; 240 kDa) with enhanced peroxidatic activity, respectively. These two catalases differ in their expression during the bacterial growth; whereas the expression of the smaller enzyme (Cat-1) is induced by 0.5 mmol/L peroxides in the medium, and to a lesser degree by 25 mg/L Cd2+, Cat-2 (typical catalase) is almost specifically induced with cadmium ions.

The major Cu,Zn SOD of the phytopathogen Claviceps purpurea is not essential for pathogenicity

summary Superoxide dismutase (SOD) activities of the biotrophic pathogen Claviceps purpurea, which causes the ergot disease on a wide range of host grasses, were examined in axenic and pathogenic cultures. Almost all SOD activity in axenic culture originated from a single Cu,Zn SOD; a substantial part of this activity could be separated from lyophilized intact mycelia by gentle washing, indicating that this protein is at least partially secreted. The corresponding gene (cpsod1) was cloned and characterized; like other fungal Cu,Zn SOD genes, it groups with the extracellular mammalian Cu,Zn SODs in a phylogenetic tree. Northern analyses showed that cpsod1 is strongly induced by copper and weakly induced by iron; superoxide generated by paraquat, or xanthine and xanthine oxidase, as well as hydrogen peroxide, had no effect on gene expression under axenic conditions. Analysis of the deletion mutant Deltacpsod1 showed that, although growth in axenic culture was generally slower, sensitivity to paraquat was not increased in comparison to the wild-type. Pathogenicity assays showed that this gene is not essential for parasitic growth in rye; no further soluble SOD activity is induced in the mutant.

Haemonchus contortus utilises catalase in defence against exogenous hydrogen peroxide in vitro

The toxicity of activated oxygen species towards adult Haemonchus contortus nematodes was examined in in vitro assays using ingestion of [3H]inulin to assess nematode viability. Both glucose/glucose oxidase (generation of hydrogen peroxide) and xanthine/xanthine oxidase (generation of superoxide anion) systems showed concentration-dependent toxicity to the nematodes. Both adult and larval Haemonchus contortus enzyme preparations showed significant catalase activities. Adult nematodes exposed to aminotriazole for 24 h showed catalase activities reduced to less than 20% of controls. Aminotriazole-treated nematodes exposed to a glucose/glucose oxidase system were significantly more susceptible to the toxic effects of the oxidant-generating system than controls (no aminotriazole pre-treatment). The concentration of glucose oxidase required to inhibit feeding by 50% was decreased 33-fold in aminotriazole-treated nematodes compared with controls. The effect of aminotriazole pre-treatment implicates hydrogen peroxide as a significant toxic agent in the glucose/glucose oxidase system. It is apparent that inhibition of Haemonchus contortus catalase increases the susceptibility of the parasite to the toxic effects of hydrogen peroxide, demonstrating a protective role for this enzyme. This suggests that catalase has the potential to play a significant role in the defence of this parasite against hydrogen peroxide produced as part of the respiratory burst of activated phagocytes within the host during its response to nematode infection.

Molecular and kinetic study of catalase-1, a durable large catalase of Neurospora crassa

Catalase-1 (Cat-1), one of the two monofunctional catalases of Neurospora crassa, increases during asexual spore formation to constitute 0.6% of total protein in conidia. Cat-1 was purified 170-fold with a yield of 48% from conidiating cultures. Like most monofunctional catalases, Cat-1 is a homotetramer, resistant to inactivation by solvents, fully active over a pH range of 4-12, and inactivated by 3-amino-1,2,4-triazole. Unlike most monofunctional catalases, Cat-1 consists of 88 kDa monomers that are glycosylated with alpha-glucose and/or alpha-mannose, is unusually stable, and is not inactivated or inhibited by hydrogen peroxide. Cat-1 was more resistant than other catalases to heat inactivation and to high concentrations of salt and denaturants. Cat-1 exhibited unusual kinetics: at molar concentrations of hydrogen peroxide the apparent V was 10 times higher than at millimolar concentrations. Inactivation of Cat-1 activity with azide and hydroxylamine was according to first order kinetics, while cyanide at micromolar concentrations was a reversible competitive inhibitor.

A catalase gene of Colletotrichum gloeosporioides f. sp. malvae is highly expressed during the necrotrophic phase of infection of round-leaved mallow, Malva pusilla

A small-subunit peroxisomal catalase gene, cgcat1, was cloned from Colletotrichum gloeosporioides f. sp. malvae, a hemibiotrophic pathogen of round-leaved mallow (Malva pusilla). When compared to the expression of an actin gene of the fungus, a much lower level of expression of cgcat1 was detected in the biotrophic phase than in the subsequent necrotrophic phase of infection. In culture, cgcat1was expressed at higher levels when exposed to hydrogen peroxide. Changes in cgcat1 expression during infection may have related to an attempt to prevent damage from hydrogen peroxide from degenerating host cells, and/or resulted from changes in fungal nutrition and development during invasion of the host.

Multiple catalase genes are differentially regulated in Aspergillus nidulans

Detoxification of hydrogen peroxide is a fundamental aspect of the cellular antioxidant responses in which catalases play a major role. Two differentially regulated catalase genes, catA and catB, have been studied in Aspergillus nidulans. Here we have characterized a third catalase gene, designated catC, which predicts a 475-amino-acid polypeptide containing a peroxisome-targeting signal. With a molecular mass of 54 kDa, CatC shows high similarity to other small-subunit monofunctional catalases and is most closely related to catalases from other fungi, Archaea, and animals. In contrast, the CatA (approximately 84 kDa) and CatB (approximately 79 kDa) enzymes belong to a family of large-subunit catalases, constituting a unique fungal and bacterial group. The catC gene displayed a relatively constant pattern of expression, not being induced by oxidative or other types of stress. Targeted disruption of catC eliminated a constitutive catalase activity not detected previously in zymogram gels. However, a catalase activity detected in catA catB mutant strains during late stationary phase was still present in catC and catABC null mutants, thus demonstrating the presence of a fourth catalase, here named catalase D (CatD). Neither catC nor catABC triple mutants showed any developmental defect, and both mutants grew as well as wild-type strains in H(2)O(2)-generating substrates, such as fatty acids, and/or purines as the sole carbon and nitrogen sources, respectively. CatD activity was induced during late stationary phase by glucose starvation, high temperature, and, to a lesser extent, H(2)O(2) treatment. The existence of at least four differentially regulated catalases indicates a large and regulated capability for H(2)O(2) detoxification in filamentous fungi.

Identification of two highly divergent catalase genes in the fungal tomato pathogen, Cladosporium fulvum

Catalases of pathogenic micro-organisms have attracted attention as potential virulence factors. Homology-based screens were performed to identify catalase genes in the fungal tomato pathogen Cladosporium fulvum. Two highly divergent genes, Cat1 and Cat2, were isolated and characterized. Cat1 codes for a putative 566-amino-acid catalase subunit and belongs to the gene family that also encodes the mainly peroxisome-localized catalases of animal and yeast species. Cat2 codes for a putative catalase subunit of 745 amino acids and belongs to a different gene family coding for the large-subunit catalases similar to ones found in bacteria and filamentous fungi. Neither catalase had an obvious secretory signal sequence. A search for an extracellular catalase was unproductive. The Cat1 and Cat2 genes showed differential expression, with the Cat1 mRNA preferentially accumulating in spores and the Cat2 mRNA preferentially accumulating in response to external H(2)O(2). With Cat2-deleted strains, activity of the Cat2 gene product (CAT2) was identified among four proteins with catalase activity separated on non-denaturing gels. The CAT2 activity represented a minor fraction of the catalase activity in spores and H(2)O(2)-stressed mycelium, and no phenotype was observed for Cat2-deleted strains, which showed a normal response to H(2)O(2) treatment. These results indicate the existence of a complex catalase system in C. fulvum, with regard to both the structure and regulation of the genes involved. In addition, efficient C. fulvum gene-replacement technology has been established.

The Xylanolytic System of Claviceps purpurea: Cytological Evidence for Secretion of Xylanases in Infected Rye Tissue and Molecular Characterization of Two Xylanase Genes

ABSTRACT Claviceps purpurea is a common phytopathogenic fungus that colonizes ovarian tissue of grasses. A concerted approach involving cytological and molecular techniques was initiated to investigate the role of the fungus' xylanolytic system in the interaction. Using enzyme-gold and immuno-gold electron-microscopic techniques, the presence of arabinoxylans in cell walls of rye ovarian tissues (i.e., along the usual path of infection of C. purpurea) was confirmed; tissue-print and immunostaining analyses indicated the presence of xylanase(s) exclusively in ovaries infected with C. purpurea. This strongly suggests that C. purpurea secretes xylanase while colonizing its host. Two xylanase genes (cpxyl1 and cpxyl2) were isolated from a genomic library of C. purpurea using genes from Cochliobolus carbonum (xyl1) and Magnaporthe grisea (xyn33) as heterologous probes. cpxyl1 of C. purpurea had an open reading frame (ORF) of 832 bp interrupted by a 181-bp intron. The derived gene product (CPXYL1) had a molecular mass of 21.5 kDa and an pI of 8.88; it showed significant homology to family G endo-beta-1,4-xylanases. The cpxyl2 ORF (1,144 bp) contained two introns (76 and 90 bp) and coded for a polypeptide of 33.8 kDa with an pI of 7.01; CPXYL2 belonged to family F xylanases. Southern analyses with genomic DNA demonstrated that both genes were single-copy genes. Using reverse transcription polymerase chain reaction, it could be shown that both genes were expressed in vitro and in planta (during all infection stages). Inactivation of cpxyl1 was achieved by a gene-replacement approach. The mutant strain (Deltacpxyl1) had significantly reduced xylanase activity; Western analyses confirmed that it lacked a polypeptide of approximately 23 kDa.

Secretion of a Fungal Extracellular Catalase by Claviceps purpurea During Infection of Rye: Putative Role in Pathogenicity and Suppression of Host Defense

ABSTRACT Hydrogen peroxide of the host origin accumulates in plant apoplasts in response to pathogen attack and probably functions directly in defense reactions or in signaling, according to a previous study. Since Claviceps purpurea produces compatible interactions with hundreds of host species, we hypothesized that the fungus might interfere with H(2)O(2)-mediated defense by means of secreted catalases. In axenic culture of C. purpurea, catalase activity accumulated in the medium and was inhibited by the catalase inhibitor aminotriazole. Polyacrylamide gel electrophoresis followed by diaminobenzidine (DAB)-mediated activity staining showed that one specific catalase found in culture filtrate was also present in rye ovaries infected with C. purpurea and in honeydew. This catalase form is probably induced during infection. In situ activity staining, using DAB-mediated enzyme-cytochemistry in electron microscopy, located catalase activity in hyphal walls during both axenic culture and infection of rye. Activity staining accumulated in periplasmic spaces and was especially strong at hyphal surfaces; control staining after aminotriazole inhibition was negative. Intracellular activity staining in organelles of the fungal secretory pathway substantiated that catalase was secreted by C. purpurea. With molecular cytology, anticatalase epitopes were localized with different heterologous catalase antibodies at sites corresponding to the activity staining pattern. In all infection phases, immunogold labeling indicated that the putative catalase was secreted via multivesicular bodies into the fungal wall and diffused into the host apoplast exclusively at the hostpathogen interface. The secretion of fungal catalase is a novel finding in phytopathology, and we discuss its role in the ubiquitous ergot disease.