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

The bZIP transcription factor MoAP1 mediates the oxidative stress response and is critical for pathogenicity of the rice blast fungus Magnaporthe oryzae

Saccharomyces cerevisiae Yap1 protein is an AP1-like transcription factor involved in the regulation of the oxidative stress response. An ortholog of Yap1, MoAP1, was recently identified from the rice blast fungus Magnaporthe oryzae genome. We found that MoAP1 is highly expressed in conidia and during invasive hyphal growth. The Moap1 mutant was sensitive to H₂O₂, similar to S. cerevisiae yap1 mutants, and MoAP1 complemented Yap1 function in resistance to H₂O₂, albeit partially. The Moap1 mutant also exhibited various defects in aerial hyphal growth, mycelial branching, conidia formation, the production of extracellular peroxidases and laccases, and melanin pigmentation. Consequently, the Moap1 mutant was unable to infect the host plant. The MoAP1-eGFP fusion protein is localized inside the nucleus upon exposure to H₂O₂, suggesting that MoAP1 also functions as a redox sensor. Moreover, through RNA sequence analysis, many MoAP1-regulated genes were identified, including several novel ones that were also involved in pathogenicity. Disruption of respective MGG_01662 (MoAAT) and MGG_02531 (encoding hypothetical protein) genes did not result in any detectable changes in conidial germination and appressorium formation but reduced pathogenicity, whereas the mutant strains of MGG_01230 (MoSSADH) and MGG_15157 (MoACT) showed marketed reductions in aerial hyphal growth, mycelial branching, and loss of conidiation as well as pathogenicity, similar to the Moap1 mutant. Taken together, our studies identify MoAP1 as a positive transcription factor that regulates transcriptions of MGG_01662, MGG_02531, MGG_01230, and MGG_15157 that are important in the growth, development, and pathogenicity of M. oryzae.

Magnaporthe oryzae is a causal agent of rice blast disease and an important model for understanding of fungal development and pathogenicity. To examine the molecular mechanisms involved in conidium formation and appressorium development of M. oryzae, we identified the transcriptional factor MoAP1 as a regulator of the oxidative stress response. Our results indicated that MoAP1 is a stage-specific regulator for conidium formation, morphology, aerial hyphal growth, and also growth in planta. Additionally, we identified four novel genes whose functions were linked to MoAP1 and pathogenicity. Disruption of MGG_01662 (encoding aminobutyrate aminotransferase, MoAat) and MGG_02531 (hypothetical protein) caused minor phenotypic changes but attenuated virulence, and disruption of MGG_01230 (encoding succinic semialdehyde dehydrogenase, MoSsadh) and MGG_15157 (encoding acetyltransferase, MoAct) resulted in drastic reductions in the growth of aerial hyphae and hyphal branching as well as loss of conidiation and pathogenicity. Our studies extend the current understanding of AP1 functions in fungi and reveal that the MoAP1-mediated regulatory network is associated with the pathogenicity of M. oryzae.

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.

The basic leucine zipper transcription factor Moatf1 mediates oxidative stress responses and is necessary for full virulence of the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast disease, leading to enormous losses of rice production. Here, we characterized a basic leucine zipper (bZIP) transcription factor, Moatf1, in M. oryzae, a homolog of Schizosaccharomyces pombe ATF/CREB that regulates the oxidative stress response. Moatf1 deletion caused retarded vegetative growth of mycelia, and the Moatf1 mutant exhibited higher sensitivity to hydrogen peroxide (H(2)O(2)) than did the wild-type strain. The mutant showed severely reduced activity of extracellular enzymes and transcription level of laccases and peroxidases and exhibited significantly reduced virulence on rice cultivar CO-39. On rice leaf sheath, most of the infectious hyphae of the mutant became swollen and displayed restricted growth in primary infected cells. Defense response was strongly activated in plants infected by the mutant. Diamino benzidine staining revealed an accumulation of H(2)O(2) around Moatf1 mutant appressoria and rice cells with Moatf1 hyphae that was absent in the wild type. Inhibition of the plant NADPH oxidase by diphenyleneiodonium prevented host-derived H(2)O(2) accumulation and restored infectious hyphal growth of the mutant in rice cells. Thus, we conclude that Moatf1 is necessary for full virulence of M. oryzae by regulating the transcription of laccases and peroxidases to impair reactive oxygen species-mediated plant defense.

TmpL, a transmembrane protein required for intracellular redox homeostasis and virulence in a plant and an animal fungal pathogen

The regulation of intracellular levels of reactive oxygen species (ROS) is critical for developmental differentiation and virulence of many pathogenic fungi. In this report we demonstrate that a novel transmembrane protein, TmpL, is necessary for regulation of intracellular ROS levels and tolerance to external ROS, and is required for infection of plants by the necrotroph Alternaria brassicicola and for infection of mammals by the human pathogen Aspergillus fumigatus. In both fungi, tmpL encodes a predicted hybrid membrane protein containing an AMP-binding domain, six putative transmembrane domains, and an experimentally-validated FAD/NAD(P)-binding domain. Localization and gene expression analyses in A. brassicicola indicated that TmpL is associated with the Woronin body, a specialized peroxisome, and strongly expressed during conidiation and initial invasive growth in planta. A. brassicicola and A. fumigatus ΔtmpL strains exhibited abnormal conidiogenesis, accelerated aging, enhanced oxidative burst during conidiation, and hypersensitivity to oxidative stress when compared to wild-type or reconstituted strains. Moreover, A. brassicicola ΔtmpL strains, although capable of initial penetration, exhibited dramatically reduced invasive growth on Brassicas and Arabidopsis. Similarly, an A. fumigatus ΔtmpL mutant was dramatically less virulent than the wild-type and reconstituted strains in a murine model of invasive aspergillosis. Constitutive expression of the A. brassicicola yap1 ortholog in an A. brassicicola ΔtmpL strain resulted in high expression levels of genes associated with oxidative stress tolerance. Overexpression of yap1 in the ΔtmpL background complemented the majority of observed developmental phenotypic changes and partially restored virulence on plants. Yap1-GFP fusion strains utilizing the native yap1 promoter exhibited constitutive nuclear localization in the A. brassicicola ΔtmpL background. Collectively, we have discovered a novel protein involved in the virulence of both plant and animal fungal pathogens. Our results strongly suggest that dysregulation of oxidative stress homeostasis in the absence of TmpL is the underpinning cause of the developmental and virulence defects observed in these studies.

The critical roles of reactive oxygen species (ROS) in fungal development and virulence have been well established over the past half a century since the first experimental detection of hydrogen peroxide in fungal cells by Bach (1950). In the cell, ROS act as signaling molecules regulating physiological responses and developmental processes and are also involved in sophisticated virulence processes for many pathogenic fungi. Therefore, uncovering the biological roles of cellular ROS appears to be very important in understanding fungal development and virulence. Currently we have limited knowledge of how intracellular ROS are generated by fungal cells and which cellular ROS regulatory mechanisms are involved in establishing homeostasis. In this study we describe a novel protein, TmpL, involved in development and virulence in both plant and animal pathogenic fungi. In the absence of TmpL, dysregulation of oxidative stress homeostasis in both fungi caused developmental and virulence defects. Therefore, elucidating the role of TmpL presents an opportunity to uncover a common pathogenicity mechanism employed by both plant and animal pathogens and to develop efficient and novel therapeutics for both plant and animal fungal disease. Our findings provide new insights into mechanisms underlying the complex web of interactions between ROS and cell differentiation and the involvement of ROS for both plant and animal fungal pathogenesis.

Effects of catalase on the accumulation of H(2)O(2) in rice cells inoculated with rice blast fungus, Magnaporthe oryzae

Roles of H(2)O(2) in the infection process of Magnaporthe oryzae on rice were investigated. In a leaf sheath assay for up to 48 h post-inoculation, the absence or presence of catalase in the conidia suspension was correlated with the level of accumulated H(2)O(2) in infected leaf cells, as observed by staining with 3',3-diaminobenzidine tetrahydrochloride. In the incompatible interaction, the appearance of autofluorescence or frequency of cell death characterized by granulation (symptoms characteristic of hypersensitive responses) was not significantly affected by the presence of catalase in the conidia suspension. In the leaf blade assay, inoculation of compatible conidia in the presence of catalase produced more severe symptoms than that of conidia in the absence of catalase at 6 days post-inoculation. These results suggest that, in this host-parasite interaction, the primary role of host-produced H(2)O(2) is in limiting hyphal growth after penetration through toxic action. Furthermore, in incompatible interactions, H(2)O(2) is implied not to be a major mediator of hypersensitive cell death.

A novel pathogenicity gene is required in the rice blast fungus to suppress the basal defenses of the host

For successful colonization and further reproduction in host plants, pathogens need to overcome the innate defenses of the plant. We demonstrate that a novel pathogenicity gene, DES1, in Magnaporthe oryzae regulates counter-defenses against host basal resistance. The DES1 gene was identified by screening for pathogenicity-defective mutants in a T-DNA insertional mutant library. Bioinformatic analysis revealed that this gene encodes a serine-rich protein that has unknown biochemical properties, and its homologs are strictly conserved in filamentous Ascomycetes. Targeted gene deletion of DES1 had no apparent effect on developmental morphogenesis, including vegetative growth, conidial germination, appressorium formation, and appressorium-mediated penetration. Conidial size of the mutant became smaller than that of the wild type, but the mutant displayed no defects on cell wall integrity. The Δdes1 mutant was hypersensitive to exogenous oxidative stress and the activity and transcription level of extracellular enzymes including peroxidases and laccases were severely decreased in the mutant. In addition, ferrous ion leakage was observed in the Δdes1 mutant. In the interaction with a susceptible rice cultivar, rice cells inoculated with the Δdes1 mutant exhibited strong defense responses accompanied by brown granules in primary infected cells, the accumulation of reactive oxygen species (ROS), the generation of autofluorescent materials, and PR gene induction in neighboring tissues. The Δdes1 mutant displayed a significant reduction in infectious hyphal extension, which caused a decrease in pathogenicity. Notably, the suppression of ROS generation by treatment with diphenyleneiodonium (DPI), an inhibitor of NADPH oxidases, resulted in a significant reduction in the defense responses in plant tissues challenged with the Δdes1 mutant. Furthermore, the Δdes1 mutant recovered its normal infectious growth in DPI-treated plant tissues. These results suggest that DES1 functions as a novel pathogenicity gene that regulates the activity of fungal proteins, compromising ROS-mediated plant defense.

Coevolution of plants and microbial pathogens leads to interactions that resemble a molecular war. Pathogens generate effector molecules to infect their hosts, and plants produce defense molecules against pathogen attacks. Interactions between these molecules results in plant immunity or disease. Plant disease could be likened to a complex and delicate matter of balance, where a number of molecules are involved in the battlefield. Discovering and understanding the tipping points in the battle are vital for developing disease-free crops. In the interaction of rice and rice blast fungus, a microbe sensor on rice stimulates the generation of reactive oxygen species (ROS) at the site of infection. ROS is known as an antimicrobial material and a stimulator for defense signaling that is important for preparing reinforcement in neighboring tissues. This paper presents the counter-defense mechanism of the fungus against plant-driven ROS. We found that a pathogenicity factor from rice blast fungus, DES1 (Defense Suppressor 1), is involved in overcoming oxidative stress for the counter-defense mechanism, suggesting that this gene is required for fungal pathogenicity.

Siderophore synthesis in Magnaporthe grisea is essential for vegetative growth, conidiation and resistance to oxidative stress

The plant pathogenic fungus Magnaporthe grisea excretes siderophores of the coprogen-type for iron acquisition and uses ferricrocin for intracellular iron storage. In the present report we characterize mutants with defects in extracellular siderophore biosynthesis. Deletion of the M. grisea SSM2 gene, which encodes a non-ribosomal peptide synthetase, resulted in a loss of the production of all coprogens. The mutant strains had a reduced growth rate, produced fewer conidia and were more sensitive to oxidative stress. Ferricrocin production was not affected. Upon deletion of M. grisea OMO1, a gene predicted to encode an L-ornithine-N(5)-monooxygenase, no siderophores of any type were detected, the strain was aconidial, growth rate was reduced and sensitivity to oxidative stress was increased. Abundance of several proteins was affected in the mutants. The Deltassm2 and Deltaomo1 mutant phenotypes were complemented by supplementation of the medium with siderophores or reintroduction of the respective genes.

Guedes HL, Nosanchuk JD, Zancopé-Oliveira RM. Biological function and molecular mapping of M antigen in yeast phase of Histoplasma capsulatum

Histoplasmosis, due to the intracellular fungus Histoplasma capsulatum, can be diagnosed by demonstrating the presence of antibodies specific to the immunodominant M antigen. However, the role of this protein in the pathogenesis of histoplasmosis has not been elucidated. We sought to structurally and immunologically characterize the protein, determine yeast cell surface expression, and confirm catalase activity. A 3D-rendering of the M antigen by homology modeling revealed that the structures and domains closely resemble characterized fungal catalases. We generated monoclonal antibodies (mAbs) to the protein and determined that the M antigen is present on the yeast cell surface and in cell wall/cell membrane preparations. Similarly, we found that the majority of catalase activity was in extracts containing fungal surface antigens and that the M antigen is not significantly secreted by live yeast cells. The mAbs also identified unique epitopes on the M antigen. The localization of the M antigen to the cell surface of H. capsulatum yeast and the characterization of the protein's major epitopes have important implications since it demonstrates that although the protein may participate in protecting the fungus against oxidative stress it is also accessible to host immune cells and antibody.

Evolutionary history of the ancient cutinase family in five filamentous Ascomycetes reveals differential gene duplications and losses and in Magnaporthe grisea shows evidence of sub- and neo-functionalization

* The cuticle is the first barrier for fungi that parasitize plants systematically or opportunistically. Here, the evolutionary history is reported of the multimembered cutinase families of the plant pathogenic Ascomycetes Magnaporthe grisea, Fusarium graminearum and Botrytis cinerea and the saprotrophic Ascomycetes Aspergillus nidulans and Neurospora crassa. * Molecular taxonomy of all fungal cutinases demonstrates a clear division into two ancient subfamilies. No evidence was found for lateral gene transfer from prokaryotes. The cutinases in the five Ascomycetes show significant copy number variation, they form six clades and their extreme sequence diversity is highlighted by the lack of consensus intron. The average ratio of gene duplication to loss is 2 : 3, with the exception of M. grisea and N. crassa, which exhibit extreme family expansion and contraction, respectively. * Detailed transcript profiling in vivo, categorizes the M. grisea cutinases into four regulatory patterns. Symmetric or asymmetric expression profiles of phylogenetically related cutinase genes suggest subfunctionalization and neofunctionalization, respectively. * The cutinase family-size per fungal species is discussed in relation to genome characteristics and lifestyle. The ancestry of the cutinase gene family, together with the expression divergence of its individual members provides a first insight into the drivers for niche differentiation in fungi.

Regulation of catalase activity and gene expression during Phytophthora nicotianae development and infection of tobacco

Plant defence against pathogen attack typically incorporates an oxidative burst involving elevated levels of reactive oxygen species such as hydrogen peroxide. In the present study, we have used an in-gel assay to monitor the activity of the hydrogen peroxide scavenging enzyme, catalase, during asexual development of Phytophthora nicotianae and during infection of host tobacco plants. In vitro, catalase activity is highest in sporulating hyphae; in planta, catalase activity increases dramatically about 8 h after host inoculation. We have cloned and characterized three catalase genes, designated PnCat1, PnCat2 and PnCat3, from P. nicotianae and identified their homologues in P. infestans, P. sojae and P. ramorum. In all three species, Cat2 is predicted to be targeted to the peroxisome and the other catalases are likely to be cytosolic. Quantitative real-time PCR assessment of catalase transcripts during development and infection indicates that peroxisomal PnCat2 is the gene predominantly expressed, with transcript levels peaking in vitro in sporulating hyphae and in planta increasing dramatically during the first 24 h after inoculation of susceptible tobacco seedlings. Levels of tobacco catalase gene expression are significantly down-regulated in susceptible tobacco 4, 8 and 24 h post-inoculation and in resistant plants at 24 h post-inoculation. Together, our results give evidence that during infection P. nicotianae increases its own peroxisomal catalase levels while concurrently down-regulating host catalase expression. This behaviour is consistent with a role of pathogen catalase in counterdefence and protection against oxidative stress and of pathogen-orchestrated enhanced plant cell death to support necrotrophic pathogen growth and plant colonization.

Elicitor and calatalse activity of conidia suspensions of various strains of Magnaporthe grisea in suspension-cultured cells of rice

A conidia suspension of Magnaporthe grisea carried elicitor activity that induced the expression of defense-related genes and the production of H(2)O(2) in suspension-cultured rice cells. The levels of H(2)O(2) produced were dependent on fungal isolates and were correlated with the catalase activity in the supernatant fraction of each conidia suspension, not with gene-for-gene interactions.

Differential protein expression in Colletotrichum acutatum: changes associated with reactive oxygen species and nitrogen starvation implicated in pathogenicity on strawberry

The cellular outcome of changes in nitrogen availability in the context of development and early stages of pathogenicity was studied by quantitative analysis of two-dimensional gel electrophoresis of Colletotrichum acutatum infecting strawberry. Significant alterations occurred in the abundance of proteins synthesized during appressorium formation under nitrogen-limiting conditions compared with a complete nutrient supply. Proteins that were up- or down-regulated were involved in energy metabolism, nitrogen and amino acid metabolism, protein synthesis and degradation, response to stress and reactive oxygen scavenging. Members belonging to the reactive oxygen species (ROS) scavenger machinery, superoxide dismutase and glutathione peroxidase, were up-regulated at the appressorium formation stage, as well as under nitrogen-limiting conditions relative to growth with a complete nutrient supply, whereas abundance of bifunctional catalase was up-regulated predominantly at the appressorium formation stage. Fungal ROS were detected within germinating conidia during host pre-penetration, penetration and colonization stages, accompanied by plant ROS, which were abundant in the apoplastic space. Application of exogenous antioxidants quenched ROS production and reduced the frequency of appressorium formation. Up-regulation in metabolic activity was detected during appressorium formation and nutrient deficiency compared with growth under complete nutrient supply. Enhanced levels of proteins related to the glyoxylate cycle and lipid metabolism (malate dehydrogenase, formate dehydrogenase and acetyl-CoA acetyltransferase) were observed at the appressorium formation stage, in contrast to down-regulation of isocitrate dehydrogenase. The present study demonstrates that appressoria formation processes, occurring under nutritional deprivation, are accompanied by metabolic shifts, and that ROS production is an early fungal response that may modulate initial stages of pathogen development.

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.

Magnaporthe grisea cutinase2 mediates appressorium differentiation and host penetration and is required for full virulence

The rice blast fungus Magnaporthe grisea infects its host by forming a specialized infection structure, the appressorium, on the plant leaf. The enormous turgor pressure generated within the appressorium drives the emerging penetration peg forcefully through the plant cuticle. Hitherto, the involvement of cutinase(s) in this process has remained unproven. We identified a specific M. grisea cutinase, CUT2, whose expression is dramatically upregulated during appressorium maturation and penetration. The cut2 mutant has reduced extracellular cutin-degrading and Ser esterase activity, when grown on cutin as the sole carbon source, compared with the wild-type strain. The cut2 mutant strain is severely less pathogenic than the wild type or complemented cut2/CUT2 strain on rice (Oryza sativa) and barley (Hordeum vulgare). It displays reduced conidiation and anomalous germling morphology, forming multiple elongated germ tubes and aberrant appressoria on inductive surfaces. We show that Cut2 mediates the formation of the penetration peg but does not play a role in spore or appressorium adhesion, or in appressorial turgor generation. Morphological and pathogenicity defects in the cut2 mutant are fully restored with exogenous application of synthetic cutin monomers, cAMP, 3-isobutyl-1-methylxanthine, and diacylglycerol (DAG). We propose that Cut2 is an upstream activator of cAMP/protein kinase A and DAG/protein kinase C signaling pathways that direct appressorium formation and infectious growth in M. grisea. Cut2 is therefore required for surface sensing leading to correct germling differentiation, penetration, and full virulence in this model fungus.