PTK1, a mitogen-activated-protein kinase gene, is required for conidiation, appressorium formation, and pathogenicity of Pyrenophora teres on barley

Functional Characterization of the Transcription Factor Gene CgHox7 in Colletotrichum gloeosporioides, Which Is Responsible for Poplar Anthracnose

Colletotrichum gloeosporioides is the main pathogen that causes poplar anthracnose. This hemibiotrophic fungus, which can severely decrease the economic benefits and ecological functions of poplar trees, infects the host by forming an appressorium. Hox7 is an important regulatory factor that functions downstream of the Pmk1 MAPK signaling pathway. In this study, we investigated the effect of deleting CgHox7 on C. gloeosporioides. The conidia of the ΔCgHox7 deletion mutant germinated on a GelBond membrane to form non-melanized hyphal structures, but were unable to form appressoria. The deletion of CgHox7 weakened the ability of hyphae to penetrate a cellophane membrane and resulted in decreased virulence on poplar leaves. Furthermore, deleting CgHox7 affected the oxidative stress response. In the initial stage of appressorium formation, the accumulation of reactive oxygen species differed between the ΔCgHox7 deletion mutant and the wild-type control. Moreover, CgHox7 expression was necessary for maintaining cell wall integrity. Considered together, these results indicate that CgHox7 is a transcription factor with crucial regulatory effects on appressorium formation and the pathogenicity of C. gloeosporioides.

Molecular Mechanisms of the Stripe Rust Interaction with Resistant and Susceptible Wheat Genotypes

Rust fungi cause significant damage to wheat production worldwide. In order to mitigate disease impact and improve food security via durable resistance, it is important to understand the molecular basis of host-pathogen interactions. Despite a long history of research and high agricultural importance, still little is known about the interactions between the stripe rust fungus and wheat host on the gene expression level. Here, we present analysis of the molecular interactions between a major wheat pathogen-Puccinia striiformis f. sp. tritici (Pst)-in resistant and susceptible host backgrounds. Using plants with durable nonrace-specific resistance along with fully susceptible ones allowed us to show how gene expression patterns shift in compatible versus incompatible interactions. The pathogen showed significantly greater number and fold changes of overexpressed genes on the resistant host than the susceptible host. Stress-related pathways including MAPK, oxidation-reduction, osmotic stress, and stress granule formation were, almost exclusively, upregulated in the resistant host background, suggesting the requirement of the resistance-countermeasure mechanism facilitated by Pst. In contrast, the susceptible host background allowed for broad overrepresentation of the nutrient uptake pathways. This is the first study focused on the stripe rust pathogen-wheat interactions, on the whole transcriptome level, from the pathogen side. It lays a foundation for the better understanding of the resistant/susceptible hosts versus pathogenic fungus interaction in a broader sense.

Fus3 regulates asexual development and trap morphogenesis in the nematode-trapping fungus Arthrobotrys oligospora

Mitogen-activated protein kinase (MAPK) Fus3 is an essential regulator of cell differentiation and virulence in fungal pathogens of plants and animals. However, the function and regulatory mechanism of MAPK signaling in nematode-trapping (NT) fungi remain largely unknown. NT fungi can specialize in the formation of "traps", an important indicator of transition from a saprophytic to a predatory lifestyle. Here, we characterized an orthologous Fus3 in a typical NT fungus Arthrobotrys oligospora using multi-phenotypic analysis and multi-omics approaches. Our results showed that Fus3 plays an important role in asexual growth and development, conidiation, stress response, DNA damage, autophagy, and secondary metabolism. Importantly, Fus3 plays an indispensable role in hyphal fusion, trap morphogenesis, and nematode predation. Moreover, we constructed the regulatory networks of Fus3 by means of transcriptomic and yeast two-hybrid techniques. This study provides insights into the mechanism of MAPK signaling in asexual development and pathogenicity of NT fungi.

Management of mung bean leaf spot disease caused by Phoma herbarum through Penicillium janczewskii metabolites mediated by MAPK signaling cascade

Vigna radiata L., an imperative legume crop of Pakistan, faces hordes of damage due to fungi; infecting host tissues by the appressorium. The use of natural compounds is an innovative concern to manage mung-bean fungal diseases. The bioactive secondary metabolites of Penicillium species are well documented for their strong fungi-static ability against many pathogens. Presently, one-month-old aqueous culture filtrates of Penicillium janczewskii, P. digitatum, P. verrucosum, P. crustosum, and P. oxalicum were evaluated to check the antagonistic effect of different dilutions (0, 10, 20, … and 60%). There was a significant reduction of around 7-38%, 46-57%, 46-58%, 27-68%, and 21-51% in Phoma herbarum dry biomass production due to P. janczewskii, P. digitatum, P. verrucosum, P. crustosum, and P. oxalicum, respectively. Inhibition constants determined by a regression equation demonstrated the most significant inhibition by P. janczewskii. Finally, using real-time reverse transcription PCR (qPCR) the effect of P. Janczewskii metabolites was determined on the transcript level of StSTE12 gene involved in the development and penetration of appressorium. The expression pattern of the StSTE12 gene was determined by percent Knockdown (%KD) expression that was found to be decreased i.e. 51.47, 43.22, 40.67, 38.01, 35.97, and 33.41% for P. herbarum with an increase in metabolites concentrations viz., 10, 20, 30, 40, 50 and 60% metabolites, respectively. In silico studies were conducted to analyze the role of Ste12 a transcriptional factor in the MAPK signaling pathway. The present study concludes a strong fungicidal potential of Penicillium species against P. herbarum. Further studies to isolate the effective fungicidal constituents of Penicillium species through GCMS analysis and determination of their role in signaling pathways are requisite.

Knockdown of Bmp1 and Pls1 Virulence Genes by Exogenous Application of RNAi-Inducing dsRNA in Botrytis cinerea

Botrytis cinerea is a pathogen of wide agronomic and scientific importance partly due to its tendency to develop fungicide resistance. Recently, there has been great interest in the use of RNA interference as a control strategy against B. cinerea. In order to reduce the possible effects on non-target species, the sequence-dependent nature of RNAi can be used as an advantage to customize the design of dsRNA molecules. We selected two genes related to virulence: BcBmp1 (a MAP kinase essential for fungal pathogenesis) and BcPls1 (a tetraspanin related to appressorium penetration). After performing a prediction analysis of small interfering RNAs, dsRNAs of 344 (BcBmp1) and 413 (BcPls1) nucleotides were synthesized in vitro. We tested the effect of topical applications of dsRNAs, both in vitro by a fungal growth assay in microtiter plates and in vivo on artificially inoculated detached lettuce leaves. In both cases, topical applications of dsRNA led to gene knockdown with a delay in conidial germination for BcBmp1, an evident growth retardation for BcPls1, and a strong reduction in necrotic lesions on lettuce leaves for both genes. Furthermore, a strongly reduced expression of the BcBmp1 and BcPls1 genes was observed in both in vitro and in vivo experiments, suggesting that these genes could be promising targets for the development of RNAi-based fungicides against B. cinerea.

Molecular characterization and elucidation of the function of Hap38 MAPK in the response of Helicoverpa armigera (Hübner) to UV-A stress

The cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), an important pest of cotton, is detrimental to cotton production. Light from UV-A ultraviolet lamps is regarded as a form of environmental stress for insects. In order to investigate the response of H. armigera exposed to UV-A, we explored Hap38 MAPK expression and functions. We hope that the findings of this study will lay the foundation for future investigations into the insect's phototaxis mechanism. A p38 MAPK was cloned and named Hap38 MAPK. A phylogenetic tree showed that Hap38 MAPK was highly conserved. The gene was highly expressed in the thorax and females. Under UV-A stress, the expression of the gene decreased significantly. After silencing Hap38 MAPK, the activity of the antioxidant enzymes SOD, POD, CAT, and GR decreased. This study suggested that Hap38 MAPK responds to UV-A irradiation and plays critical roles in the defense response to environmental stresses.

Mitogen-Activated Protein Kinase CsPMK1 Is Essential for Pepper Fruit Anthracnose by Colletotrichum scovillei

The phytopathogenic fungus Colletotrichum scovillei, belonging to the Colletotrichum acutatum species complex, causes severe anthracnose disease on several fruits, including chili pepper (Capsicum annuum). However, the molecular mechanisms underlying the development and pathogenicity of Colletotrichum scovillei are unclear. The conserved Fus3/Kss1-related MAPK regulates fungal development and pathogenicity. Here, the role of CsPMK1, orthologous to Fus3/Kss1, was characterized by phenotypic comparison of a target deletion mutant (ΔCspmk1). The mycelial growth and conidiation of ΔCspmk1 were normal compared to that of the wild type. ΔCspmk1 produced morphologically abnormal conidia, which were delayed in conidial germination. Germinated conidia of ΔCspmk1 failed to develop appressoria on inductive surfaces of hydrophobic coverslips and host plants. ΔCspmk1 was completely defective in infectious growth, which may result from failure to suppress host immunity. Furthermore, ΔCspmk1 was impaired in nuclear division and lipid mobilization during appressorium formation, in response to a hydrophobic surface. CsPMK1 was found to interact with CsHOX7, a homeobox transcription factor essential for appressorium formation, via a yeast two-hybridization analysis. Taken together, these findings suggest that CsPMK1 is required for fungal development, stress adaptation, and pathogenicity of C. scovillei.

Effect of Osmotic Stress on the Growth, Development and Pathogenicity of Setosphaeria turcica

Osmotic stress is a severe condition frequently encountered by microorganisms; however, there is limited knowledge on the influence of hyperosmotic stress on the growth, development and pathogenicity of phytopathogenic fungi. Here, three osmotic conditions (0.4 M NaCl, 0.4 M KCl, and 0.6 M sorbitol supplemented in potato dextrose agar medium) were used to identify the effect of osmotic stress on the growth, development and pathogenicity of Setosphaeria turcica which is a plant pathogenic fungus and causes northern corn leaf blight disease in maize, sorghum, and related grasses. In osmotic stress, the growth rate of mycelium was decreased, and the number of vesicular structures and flocculent secretion outside the hypha cell wall were significantly increased. The qRT-PCR results showed that the osmotic stress quickly activated the HOG-MAPK pathway, up-regulated the expression of the downstream genes, and these genes were most highly expressed within 30 min of exposure to osmotic stress. Furthermore, the germination rate and the yield of conidia were significantly higher under osmotic stress than in the control. A pathogenicity analysis confirmed that pathogenicity of the conidia which were cultured under osmotic stress was significantly enhanced. By analyzing the knock-out mutants of an osmotic stress responsed gene StFPS1, an aquaglyceroporin downstream of the HOG-MAPK pathway, we found that StFPS1 was involved in the formation of appressorium and penetration peg, which affected the penetration ability of S. turcica. In summary, our work explained the correlation between osmotic stress and growth, development, and pathogenicity in S. turcica.

Pyrenophora teres: Taxonomy, Morphology, Interaction With Barley, and Mode of Control

Net blotch, induced by the ascomycete Pyrenophora teres, has become among the most important disease of barley (Hordeum vulgare L.). Easily recognizable by brown reticulated stripes on the sensitive barley leaves, net blotch reduces the yield by up to 40% and decreases seed quality. The life cycle, the mode of dispersion and the development of the pathogen, allow a quick contamination of the host. Crop residues, seeds, and wild grass species are the inoculum sources to spread the disease. The interaction between the barley plant and the fungus is complex and involves physiological changes with the emergence of symptoms on barley and genetic changes including the modulation of different genes involved in the defense pathways. The genes of net blotch resistance have been identified and their localizations are distributed on seven barley chromosomes. Considering the importance of this disease, several management approaches have been performed to control net blotch. One of them is the use of beneficial bacteria colonizing the rhizosphere, collectively referred to as Plant Growth Promoting Rhizobacteria. Several studies have reported the protective role of these bacteria and their metabolites against potential pathogens. Based on the available data, we expose a comprehensive review of Pyrenophora teres including its morphology, interaction with the host plant and means of control.

Expression Analysis of Cell Wall-Related Genes in the Plant Pathogenic Fungus Drechslera teres

Drechslera teres (D. teres) is an ascomycete, responsible for net blotch, the most serious barley disease causing an important economic impact. The cell wall is a crucial structure for the growth and development of fungi. Thus, understanding cell wall structure, composition and biosynthesis can help in designing new strategies for pest management. Despite the severity and economic impact of net blotch, this is the first study analyzing the cell wall-related genes in D. teres. We have identified key genes involved in the synthesis/remodeling of cell wall polysaccharides, namely chitin, β-(1,3)-glucan and mixed-linkage glucan synthases, as well as endo/exoglucanases and a mitogen-activated protein kinase. We have also analyzed the differential expression of these genes in D. teres spores and in the mycelium after cultivation on different media, as well as in the presence of Paraburkholderia phytofirmans strain PsJN, a plant growth-promoting bacterium (PGPB). The targeted gene expression analysis shows higher gene expression in the spores and in the mycelium with the application of PGPB. Besides analyzing key cell-wall-related genes, this study also identifies the most suitable reference genes to normalize qPCR results in D. teres, thus serving as a basis for future molecular studies on this ascomycete.

The MAP Kinase Kinase Gene AbSte7 Regulates Multiple Aspects of Alternaria brassicicola Pathogenesis

Mitogen-activated protein kinase (MAPK) cascades in fungi are ubiquitously conserved signaling pathways that regulate stress responses, vegetative growth, pathogenicity, and many other developmental processes. Previously, we reported that the AbSte7 gene, which encodes a mitogen-activated protein kinase kinase (MAPKK) in Alternaria brassicicola, plays a central role in pathogenicity against host cabbage plants. In this research, we further characterized the role of AbSte7 in the pathogenicity of this fungus using ΔAbSte7 mutants. Disruption of the AbSte7 gene of A. brassicicola reduced accumulation of metabolites toxic to the host plant in liquid culture media. The ΔAbSte7 mutants could not efficiently detoxify cruciferous phytoalexin brassinin, possibly due to reduced expression of the brassinin hydrolase gene involved in detoxifying brassinin. Disruption of the AbSte7 gene also severely impaired fungal detoxification of reactive oxygen species. AbSte7 gene disruption reduced the enzymatic activity of cell wall-degrading enzymes, including cellulase, β-glucosidase, pectin methylesterase, polymethyl-galacturonase, and polygalacturonic acid transeliminase, during host plant infection. Altogether, the data strongly suggest the MAPKK gene AbSte7 plays a pivotal role in A. brassicicola during host infection by regulating multiple steps, and thus increasing pathogenicity and inhibiting host defenses.

A mitogen-activated protein kinase gene (VmPmk1) regulates virulence and cell wall degrading enzyme expression in Valsa mali

Mitogen-activated protein kinases (MAPKs) play critical roles in the regulation of different developmental processes and hydrolytic enzyme production in many fungal plant pathogens. In this study, an FUS3/KSS1-related MAPK gene, VmPmk1, was identified and characterized in Valsa mali, which causes a highly destructive canker disease on apple. VmPmk1 deletion mutant showed a significant reduction in growth rate in vitro, and could not produce pycnidium, indicating that the MAPK gene is important for growth and asexual development. Also, VmPmk1 played a significant role in response to oxidative stress and in the maintenance of cell wall integrity. More importantly, when deletion mutant was inoculated onto detached apple leaves and twigs, an obvious decrease in lesion size was observed. Furthermore, expression of many cell wall degrading enzyme (CWDE) genes declined in the VmPmk1 deletion mutant during infection. VmPmk1 deletion mutant also showed a significant reduction in activities of CWDEs in both induced media and infection process. Finally, the determination of immunogold labeling of pectin demonstrated that the capacity of degradation pectin was attenuated due to the deletion of VmPmk1. These results indicated that VmPmk1 plays important roles in growth, asexual development, response to oxidative stress, and maintenance of cell wall integrity. More importantly, VmPmk1 is involved in pathogenicity of V. mali mainly by regulating CWDE genes expression.

Three Fusarium oxysporum mitogen-activated protein kinases (MAPKs) have distinct and complementary roles in stress adaptation and cross-kingdom pathogenicity

Mitogen-activated protein kinase (MAPK) cascades mediate cellular responses to environmental signals. Previous studies in the fungal pathogen Fusarium oxysporum have revealed a crucial role of Fmk1, the MAPK orthologous to Saccharomyces cerevisiae Fus3/Kss1, in vegetative hyphal fusion and plant infection. Here, we genetically dissected the individual and combined contributions of the three MAPKs Fmk1, Mpk1 and Hog1 in the regulation of development, stress response and virulence of F. oxysporum on plant and animal hosts. Mutants lacking Fmk1 or Mpk1 were affected in reactive oxygen species (ROS) homeostasis and impaired in hyphal fusion and aggregation. Loss of Mpk1 also led to increased sensitivity to cell wall and heat stress, which was exacerbated by simultaneous inactivation of Fmk1, suggesting that both MAPKs contribute to cellular adaptation to high temperature, a prerequisite for mammalian pathogens. Deletion of Hog1 caused increased sensitivity to hyperosmotic stress and resulted in partial rescue of the restricted colony growth phenotype of the mpk1Δ mutant. Infection assays on tomato plants and the invertebrate animal host Galleria mellonella revealed distinct and additive contributions of the different MAPKs to virulence. Our results indicate that positive and negative cross-talk between the three MAPK pathways regulates stress adaptation, development and virulence in the cross-kingdom pathogen F. oxysporum.

Host Delivered RNAi, an efficient approach to increase rice resistance to sheath blight pathogen (Rhizoctonia solani)

Rhizoctonia solani, the causal agent of rice sheath blight disease, causes significant losses worldwide as there are no cultivars providing absolute resistance to this fungal pathogen. We have used Host Delivered RNA Interference (HD-RNAi) technology to target two PATHOGENICITY MAP KINASE 1 (PMK1) homologues, RPMK1-1 and RPMK1-2, from R. solani using a hybrid RNAi construct. PMK1 homologues in other fungal pathogens are essential for the formation of appressorium, the fungal infection structures required for penetration of the plant cuticle, as well as invasive growth once inside the plant tissues and overall viability of the pathogen within the plant. Evaluation of transgenic rice lines revealed a significant decrease in fungal infection levels compared to non-transformed controls and the observed delay in disease symptoms was further confirmed through microscopic studies. Relative expression levels of the targeted genes, RPMK1-1 and RPMK1-2, were determined in R. solani infecting either transgenic or control lines with significantly lower levels observed in R. solani infecting transgenic lines carrying the HD-RNAi constructs. This is the first report demonstrating the effectiveness of HD-RNAi against sheath blight and offers new opportunities for durable control of the disease as it does not rely on resistance conferred by major resistance genes.

RNAi-mediated silencing of MAP kinase signalling genes (Fmk1, Hog1, and Pbs2) in Fusarium oxysporum reduces pathogenesis on tomato plants

Fusarium oxysporum is a soil-borne plant fungal pathogen, and causes colossal losses in several crop plants including tomato. Effective control measures include the use of harmful fungicides and resistant cultivars, but these methods have shown limited success. Conventional methods to validate fungal pathogenic genes are labour intensive. Therefore, an alternative strategy is required to efficiently characterize unknown pathogenic genes. RNA interference (RNAi) has emerged as a potential tool to functionally characterize novel fungal pathogenic genes and also to control fungal diseases. Here, we report an efficient method to produce stable RNAi transformants of F. oxysporum using Agrobacterium-mediated transformation (AMT). We have transformed F. oxysporum spores using RNAi constructs of Fmk1, Hog1, and Pbs2 MAP kinase signalling genes. Fmk1 RNAi fungal transformants showed loss of surface hydrophobicity, reduced invasive growth on tomato fruits and hypo-virulence on tomato seedlings. Hog1 and Pbs2 RNAi transformants showed altered conidial size, and reduced invasive growth and pathogenesis. These results showed that AMT using RNAi constructs is an effective approach for dissecting the role of genes involved in pathogenesis in F. oxysporum and this could be extended for other fungal systems. The obtained knowledge can be easily translated for developing fungal resistant crops by RNAi.

StPBS2, a MAPK kinase gene, is involved in determining hyphal morphology, cell wall development, hypertonic stress reaction as well as the production of secondary metabolites in Northern Corn Leaf Blight pathogen Setosphaeria turcica

Mitogen activated protein kinase kinase (MAPKK) is a crucial component in the MAPK signaling pathway. However, the functions of MAPKKs in foliar pathogens remain poorly understood. In the current study, a MAPKK gene designated as StPBS2 was cloned from Setosphaeria turcica and the functions of this gene were investigated by RNAi technology. Four independent StPBS2 gene silence transformants with different efficiencies were confirmed by real time PCR. Compared to the wild type strain (WT), these transformants showed decreased colony growth, shortened hyphae cell length, broadened cell width and an obvious reduction in conidium yield. Moreover, the cell wall of the transformants was thicker and they were also more sensitive to substances that interfere with cell wall biosynthesis than WT. Additionally, the transformants displayed higher sensitivity to hypertonic stress than WT and the sensitivity was associated with the level of silencing of StPBS2. They were also resistant to the fungicides iprodione, procymidone and fludioxonil, to which WT almost completely sensitive. The transformants produced more red secondary metabolites than WT and the production was enhanced with increasing silencing level and increased glucose content in PDA medium. Our results suggest that StPBS2 is involved in morphogenesis, condiogenesis, cell wall development, hypertonic stress reaction and resistance to fungicides, as well as in the biosynthesis of secondary metabolites in S. turcica.

Role of Trichoderma reesei mitogen-activated protein kinases (MAPKs) in cellulase formation

BACKGROUND

Despite being the most important cellulase producer, the cellulase-regulating carbon source signal transduction processes in Trichoderma reesei are largely unknown. Elucidating these processes is the key for unveiling how external carbon sources regulate cellulase formation, and ultimately for the improvement of cellulase production and biofuel production from lignocellulose.

RESULTS

In this work, the role of the mitogen-activated protein kinase (MAPK) signal transduction pathways on cellulase formation was investigated. The deletion of yeast FUS3-like tmk1 in T. reesei leads to improved growth and significantly improved cellulase formation. However, tmk1 deletion has no effect on the transcription of cellulase-coding genes. The involvement of the cell wall integrity maintenance governing yeast Slt2-like Tmk2 in cellulase formation was investigated by overexpressing tmk3 in T. reesei Δtmk2 to restore cell wall integrity. Transcriptional analysis found little changes in cellulase-coding genes between T. reesei parent, Δtmk2, and Δtmk2::OEtmk3 strains. Cell wall integrity decreased in T. reesei Δtmk2 over the parent strain and restored in Δtmk2::OEtmk3. Meanwhile, cellulase formation is increased in T. reesei Δtmk2 and then decreased in T. reesei Δtmk2::OEtmk3.

CONCLUSIONS

These investigations elucidate the role of Tmk1 and Tmk2 on cellulase formation: they repress cellulase formation, respectively, by repressing growth and maintaining cell wall integrity, while neither MAPK regulates the transcription of cellulase-coding genes. This work, together with the previous investigations, suggests that all MAPKs are involved in cellulase formation, while Tmk3 is the only MAPK involved in signal transduction for the regulation of cellulase expression on the transcriptional level.

Colletotrichum higginsianum Mitogen-Activated Protein Kinase ChMK1: Role in Growth, Cell Wall Integrity, Colony Melanization, and Pathogenicity

Colletotrichum higginsianum is an economically important pathogen that causes anthracnose disease in a wide range of cruciferous crops. To facilitate the efficient control of anthracnose disease, it will be important to understand the mechanism by which the cruciferous crops and C. higginsianum interact. A key step in understanding this interaction is characterizing the mitogen-activated protein kinases (MAPK) signaling pathway of C. higginsianum. MAPK plays important roles in diverse physiological processes of multiple pathogens. In this study, a Fus3/Kss1-related MAPK gene, ChMK1, from C. higginsianum was analyzed. The results showed that the Fus3/Kss1-related MAPK ChMK1 plays a significant role in cell wall integrity. Targeted deletion of ChMK1 resulted in a hypersensitivity to cell wall inhibitors, reduced conidiation and albinistic colonies. Further, the deletion mutant was also unable to form melanized appressorium, a specialized infection structure that is necessary for successful infection. Therefore, the deletion mutant loses pathogenicity on A. thaliana leaves, demonstrating that ChMK1 plays an essential role in the early infection step. In addition, the ChMK1 deletion mutant showed an attenuated growth rate that is different from that of its homolog in Colletotrichum lagenarium, indicating the diverse roles that Fus3/Kss1-related MAPKs plays in phytopathogenic fungi. Furthermore, the expression level of three melanin synthesis associated genes were clearly decreased in the albinistic ChMK1 mutant compared to that of the wild type strain, suggesting that ChMK1 is also required for colony melanization in C. higginsianum.

MAPK cascade-mediated regulation of pathogenicity, conidiation and tolerance to abiotic stresses in the entomopathogenic fungus Metarhizium robertsii

Metarhizium robertsii has been used as a model to study fungal pathogenesis in insects, and its pathogenicity has many parallels with plant and mammal pathogenic fungi. MAPK (Mitogen-activated protein kinase) cascades play pivotal roles in cellular regulation in fungi, but their functions have not been characterized in M. robertsii. In this study, we identified the full complement of MAPK cascade components in M. robertsii and dissected their regulatory roles in pathogenesis, conidiation and stress tolerance. The nine components of the Fus3, Hog1 and Slt2-MAPK cascades are all involved in conidiation. The Fus3- and Hog1-MAPK cascades are necessary for tolerance to hyperosmotic stress, and the Slt2- and Fus3-MAPK cascades both mediate cell wall integrity. The Hog1 and Slt2-MAPK cascades contribute to pathogenicity; the Fus3-MAPK cascade is indispensable for fungal pathogenesis. During its life cycle, M. robertsii experiences multiple microenvironments as it transverses the cuticle into the haemocoel. RNA-seq analysis revealed that MAPK cascades collectively play a major role in regulating the adaptation of M. robertsii to the microenvironmental change from the cuticle to the haemolymph. The three MAPKs each regulate their own distinctive subset of genes during penetration of the cuticle and haemocoel colonization, but they function redundantly to regulate adaptation to microenvironmental change.

The Role of Mitogen-Activated Protein (MAP) Kinase Signaling Components in the Fungal Development, Stress Response and Virulence of the Fungal Cereal Pathogen Bipolaris sorokiniana

Mitogen-activated protein kinases (MAPKs) have been demonstrated to be involved in fungal development, sexual reproduction, pathogenicity and/or virulence in many filamentous plant pathogenic fungi, but genes for MAPKs in the fungal cereal pathogen Bipolaris sorokiniana have not been characterized. In this study, orthologues of three MAPK genes (CsSLT2, CsHOG1 and CsFUS3) and one MAPK kinase kinase (MAPKKK) gene (CsSTE11) were identified in the whole genome sequence of the B. sorokiniana isolate ND90Pr, and knockout mutants were generated for each of them. The ∆Csfus3 and ∆Csste11 mutants were defective in conidiation and formation of appressoria-like structures, showed hypersensitivity to oxidative stress and lost pathogenicity on non-wounded leaves of barley cv. Bowman. When inoculated on wounded leaves of Bowman, the ∆Csfus3 and ∆Csste11 mutants were reduced in virulence compared to the wild type. No morphological changes were observed in the ∆Cshog1 mutants in comparison with the wild type; however, they were slightly reduced in growth under oxidative stress and were hypersensitive to hyperosmotic stress. The ∆Cshog1 mutants formed normal appressoria-like structures but were reduced in virulence when inoculated on Bowman leaves. The ∆Csslt2 mutants produced more vegetative hyphae, had lighter pigmentation, were more sensitive to cell wall degrading enzymes, and were reduced in virulence on Bowman leaves, although they formed normal appressoria like the wild type. Root infection assays indicated that the ∆Cshog1 and ∆Csslt2 mutants were able to infect barley roots while the ∆Csfus3 and ∆Csste11 failed to cause any symptoms. However, no significant difference in virulence was observed for ∆Cshog1 mutants while ∆Csslt2 mutants showed significantly reduced virulence on barley roots in comparison with the wild type. Our results indicated that all of these MAPK and MAPKKK genes are involved in the regulation of fungal development under normal and stress conditions and required for full virulence on barley plants.

Mitogen-activated protein kinases MpkA and MpkB independently affect micafungin sensitivity in Aspergillus nidulans

The transcriptional regulation of the MAPK mpkA and cell wall-related genes in Aspergillus nidulans differs from that of their counterparts in Saccharomyces cerevisiae. The A. nidulans MAPK MpkB is putatively orthologous to the yeast MAPKs Kss1p and Fus3p. To investigate MpkB and its contribution to cell wall integrity in A. nidulans, we constructed mpkB-disruptant (mpkB∆) strains. We previously showed that mpkA∆ strains exhibited reduced colony growth and increased sensitivity to the β-1,3-glucan synthase inhibitor micafungin. Like mpkA∆ strains, mpkB∆ strains exhibited slight growth retardation and increased sensitivity to micafungin. Although MpkB-dependent signaling modulated the transcription of some cell wall-related genes, the sugar composition of cell wall fractions was similar among wild-type, mpkA∆, and mpkB∆ strains. To elucidate the relationship between MpkA and MpkB pathways, we compared conditional mutants of mpkB with those with mpkA deletion. Sensitivity testing suggested that MpkA and MpkB additively contribute to micafungin activity in A. nidulans.

In silico analysis of the core signaling proteome from the barley powdery mildew pathogen (Blumeria graminis f.sp. hordei)

BACKGROUND

Compared to other ascomycetes, the barley powdery mildew pathogen Blumeria graminis f.sp. hordei (Bgh) has a large genome (ca. 120 Mbp) that harbors a relatively small number of protein-coding genes (ca. 6500). This genomic assemblage is thought to be the result of numerous gene losses, which likely represent an evolutionary adaptation to a parasitic lifestyle in close association with its host plant, barley (Hordeum vulgare). Approximately 8% of the Bgh genes are predicted to encode virulence effectors that are secreted into host tissue and/or cells to promote pathogenesis; the remaining proteome is largely uncharacterized at present.

RESULTS

We provide a comparative analysis of the conceptual Bgh proteome, with an emphasis on proteins with known roles in fungal development and pathogenicity, for example heterotrimeric G proteins and G protein coupled receptors; components of calcium and cAMP signaling; small monomeric GTPases; mitogen-activated protein cascades and transcription factors. The predicted Bgh proteome lacks a number of proteins that are otherwise conserved in filamentous fungi, including two proteins that are required for the formation of anastomoses (somatic hyphal connections). By contrast, apart from minor modifications, all major canonical signaling pathways are retained in Bgh. A family of kinases that preferentially occur in pathogenic species of the fungal clade Leotiomyceta is unusually expanded in Bgh and its close relative, Blumeria graminis f.sp. tritici.

CONCLUSIONS

Our analysis reveals characteristic features of the proteome of a fungal phytopathogen that occupies an extreme habitat: the living plant cell.

Secreted fungal effector lipase releases free fatty acids to inhibit innate immunity-related callose formation during wheat head infection

The deposition of the (1,3)-β-glucan cell wall polymer callose at sites of attempted penetration is a common plant defense response to intruding pathogens and part of the plant's innate immunity. Infection of the Fusarium graminearum disruption mutant Δfgl1, which lacks the effector lipase FGL1, is restricted to inoculated wheat (Triticum aestivum) spikelets, whereas the wild-type strain colonized the whole wheat spike. Our studies here were aimed at analyzing the role of FGL1 in establishing full F. graminearum virulence. Confocal laser-scanning microscopy revealed that the Δfgl1 mutant strongly induced the deposition of spot-like callose patches in vascular bundles of directly inoculated spikelets, while these callose deposits were not observed in infections by the wild type. Elevated concentrations of the polyunsaturated free fatty acids (FFAs) linoleic and α-linolenic acid, which we detected in F. graminearum wild type-infected wheat spike tissue compared with Δfgl1-infected tissue, provided clear evidence for a suggested function of FGL1 in suppressing callose biosynthesis. These FFAs not only inhibited plant callose biosynthesis in vitro and in planta but also partially restored virulence to the Δfgl1 mutant when applied during infection of wheat spikelets. Additional FFA analysis confirmed that the purified effector lipase FGL1 was sufficient to release linoleic and α-linolenic acids from wheat spike tissue. We concluded that these two FFAs have a major function in the suppression of the innate immunity-related callose biosynthesis and, hence, the progress of F. graminearum wheat infection.

StSTE12 is required for the pathogenicity of Setosphaeria turcica by regulating appressorium development and penetration

In filamentous fungi, the pathogenic mitogen-activated protein kinase (PMK) pathway performs an important function in plant infection. STE12-like genes found in higher eukaryotes encode transcription factors and are regulated by the PMK pathway. However, the functions of STE12-like genes in foliar pathogens remain poorly understood. In this study, we cloned StSTE12 from Setosphaeria turcica and investigated its functions by RNA interference. Transformants ste12-3, ste12-2 and, ste12-1, in which the StSTE12 silencing efficiency increased in order, were confirmed by real time PCR. Compared with the wild-type (WT) strain, the transformants showed reduced growth rate, lighter colony color, and obviously decreased conidium production. More importantly, different to WT strain and ste12-3 with lower StSTE12silencing efficiency, ste12-1 and ste12-2 with higher StSTE12 silencing efficiency were nonpathogenic on intact leaves, but pathogenic on wounded leaves. However, the biological activity of HT-toxin from all transformants showed no difference on corn leaves. Furthermore, ste12-1 and ste12-2 did not penetrate artificial cellophane membrane and showed abnormal and delayed development appressoria. Although it could penetrate the cellophane membranes, ste12-3 formed appressoria after 48 h of inoculation more than WT. Therefore, StSTE12 was involved in vegetative growth, conidiation, appressorial development, penetration as well as the pathogenicity, but it was not related to HT-toxin biosynthesis. More interestingly, all the results suggested that StSTE12 was crucial for pathogenicity due to involvement in regulating appressoria development and penetration.

Protein kinases in plant-pathogenic fungi: conserved regulators of infection

Phytopathogenic fungi have evolved an amazing diversity of infection modes and nutritional strategies, yet the signaling pathways that govern pathogenicity are remarkably conserved. Protein kinases (PKs) catalyze the reversible phosphorylation of proteins, regulating a variety of cellular processes. Here, we present an overview of our current understanding of the different classes of PKs that contribute to fungal pathogenicity on plants and of the mechanisms that regulate and coordinate PK activity during infection-related development. In addition to the well-studied PK modules, such as MAPK (mitogen-activated protein kinase) and cAMP (cyclic adenosine monophosphate)-PKA (protein kinase A) cascades, we also discuss new PK pathways that have emerged in recent years as key players of pathogenic development and disease. Understanding how conserved PK signaling networks have been recruited during the evolution of fungal pathogenicity not only advances our knowledge of the highly elaborate infection process but may also lead to the development of novel strategies for the control of plant disease.

MaMk1, a FUS3/KSS1-type mitogen-activated protein kinase gene, is required for appressorium formation, and insect cuticle penetration of the entomopathogenic fungus Metarhizium acridum

Entomopathogenic fungi have great potential for development as insecticides. However, large-scale use of mycoinsecticides is partially limited by poor efficiency. In many fungal pathogens, the yeast and fungal extracellular signal-regulated kinase (YERK1) subfamily is crucial to the fungal pathogenicity. In this study, a Fus3/Kss1-type mitogen-activated protein kinase (MAPK) gene MaMk1 (GenBank accession No. EFY93607) was identified in Metarhizium acridum, which encodes a member of the YERK1 subfamily. Targeted gene disruption was used to analyze the function of MaMk1 in fungal growth, conidial yield and virulence. Growth assays showed that MaMk1 disruption did not affect fungal growth and conidial yield on potato dextrose agar (PDA) plates. Bioassays by topical inoculation showed that a MaMk1-disruption mutant entirely lost its pathogenicity for the locusts, likely because of failure to penetrate the insect cuticle, which might have been caused by inability to form appressoria during infection. However, bioassays by injection showed no significant difference in virulence among the wild type (WT), ΔMaMk1 mutant and complementary transformant. ΔMaMk1 mutant failed to penetrate the cuticle outwards and sporulate on the locust cadaver. These results suggest that MaMk1 is required for penetration of the insect cuticle both into the hemocele and outside from the hemocele, but is dispensable for fungal growth in insect hemolymph. Gene expression pattern analysis showed that MaMk1 disruption downregulated expression of Mad1 and Mpl1, but did not reduce expression of Pr1 in M. acridum.

A MAP kinase gene, Clk1, is required for conidiation and pathogenicity in the phytopathogenic fungus Curvularia lunata

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction pathways, which play a wide variety of important roles in extracellular signal transduction. The first MAPK gene of the maize pathogen Curvularia lunata, Clk1, was isolated via a PCR-based approach with a primer pair designed on the basis of conserved regions of known MAPKs. Southern blot analysis showed that the gene existed in the genome as a single copy. The predicted amino acid sequence (352 amino acids) was highly homologous with MAP kinases of other phytopathogenic fungi. Flanking regions of Clk1 were obtained through RACE and genomic walking technology. To understand the role of Clk1 in C. lunata, targeted gene disruption was adopted to construct Clk1 mutants. It was found that mutants lacking functional domain of Clk1 were not able to produce conidia but tended to form a few special chlamydospore-shaped structures. Clk1 mutants grew slower in adverse environments (at 24°C), produced less cell degrading enzymes (CWDEs) than the wild type, and they were almost unable to infect maize leaves via artificial wounds.

The MAP kinase Bbslt2 controls growth, conidiation, cell wall integrity, and virulence in the insect pathogenic fungus Beauveria bassiana

Entomopathogenic fungi, such as Beauveria bassiana, are key environmental pathogens of insects that have been exploited for biological control of insect pests. Mitogen-activated protein (MAP) kinases play crucial roles in regulating fungal development, growth, and pathogenicity, mediating responses to the environment. Bbslt2, encoding for an Slt2 family MAPK, was isolated and characterized from B. bassiana. Gene disruption of Bbslt2 affected growth, caused a significant reduction in conidial production and viability, and increased sensitivity to Congo Red and fungal cell wall degrading enzymes. ΔBbslt2 mutants were altered in cell wall structure and composition, which included temperature dependent chitin accumulation, reductions in conidial and hyphal hydrophobicity, and alterations in cell surface carbohydrate epitopes. The ΔBbslt2 strain also showed hypersensitivity to heat shock and altered trehalose accumulation, which could only be partially attributed to changes in the expression of trehalase (ntl1). Insect bioassays revealed decreased virulence in the ΔBbslt2 strain using both topical and intrahemoceol injection assays. These results indicate that Bbslt2 plays an important role in conidiation, viability, cell wall integrity and virulence in B. bassiana. Our findings are discussed within the context of the two previous MAP kinases characterized from B. bassiana.

The FUS3/KSS1-type MAP kinase gene FPK1 is involved in hyphal growth, conidiation and plant infection of Fusarium proliferatum

Fusarium proliferatum is an important pathogen of maize that is responsible for ear rots, stalk rots and seeding blight worldwide. During the past decade, F. proliferatum has caused several severe epidemics of maize seedling blight in many areas of China, which led to significant losses in maize. To understand the molecular mechanisms in the fungal developmental regulation and pathogenicity, we isolated and characterized the FPK1 gene (GenBank accession No. HQ844224) encoding a MAP kinase homolog of FUS3/KSS1 in yeast. The gene includes a 1,242-bp DNA sequence from ATG to TAA, with a coding region of 1,068 bp, 3 introns (58 bp, 56 bp and 60 bp) and a predicted protein of 355 aa.The mutant ΔFPK1, which has a disruption of the FPK1 gene, showed reduced vegetative growth, fewer and shorter aerial mycelia, strongly impaired conidiation and spore germination, as well as deviant germ tube outgrowth. When the strain was inoculated in susceptible maize varieties, the infection of the mutant ΔFPK1 was delayed, and the infection efficiency was reduced compared to the wild-type strain. Complementation of the disruptions within the FPK1 open reading frame restored wild-type levels of conidiation, growth rate and virulence to maize seedlings. Our results indicated that the FPK1 gene functioned in hyphal growth, conidiation, spore germination and virulence in F. proliferatum.

A Pmk1-interacting gene is involved in appressorium differentiation and plant infection in Magnaporthe oryzae

In the rice blast fungus Magnaporthe oryzae, the PMK1 mitogen-activated protein (MAP) kinase gene regulates appressorium formation and infectious growth. Its homologs in many other fungi also play critical roles in fungal development and pathogenicity. However, the targets of this important MAP kinase and its interacting genes are not well characterized. In this study, we constructed two yeast two-hybrid libraries of M. oryzae and screened for Pmk1-interacting proteins. Among the nine Pmk1-interacting clones (PICs) identified, two of them, PIC1 and PIC5, were selected for further characterization. Pic1 has one putative nuclear localization signal and one putative MAP kinase phosphorylation site. Pic5 contains one transmembrane domain and two functionally unknown CTNS (cystinosin/ERS1p repeat) motifs. The interaction of Pmk1 with Pic1 or Pic5 was confirmed by coimmunoprecipitation assays. Targeted gene deletion of PIC1 had no apparent effects on vegetative growth and pathogenicity but resulted in a significant reduction in conidiation and abnormal germ tube differentiation on onion epidermal cells. Deletion of PIC5 led to a reduction in conidiation and hyphal growth. Autolysis of aerial hyphae became visible in cultures older than 4 days. The pic5 mutant was defective in germ tube growth and appressorium differentiation. It was reduced in appressorial penetration and virulence on the plant. Both PIC1 and PIC5 are conserved in filamentous ascomycetes, but none of their orthologs have been functionally characterized. Our data indicate that PIC5 is a novel virulence factor involved in appressorium differentiation and pathogenesis in M. oryzae.

Loss of cAMP-dependent protein kinase A affects multiple traits important for root pathogenesis by Fusarium oxysporum

The soilborne fungal pathogen Fusarium oxysporum causes vascular wilt and root rot diseases in many plant species. We investigated the role of cyclic AMP-dependent protein kinase A of F. oxysporum (FoCPKA) in growth, morphology, and root attachment, penetration, and pathogenesis in Arabidopsis thaliana. Affinity of spore attachment to root surfaces of A. thaliana, observed microscopically and measured by atomic force microscopy, was reduced by a loss-of-function mutation in the gene encoding the catalytic subunit of FoCPKA. The resulting mutants also failed to penetrate into the vascular system of A. thaliana roots and lost virulence. Even when the mutants managed to enter the vascular system via physically wounded roots, the degree of vascular colonization was significantly lower than that of the corresponding wild-type strain O-685 and no noticeable disease symptoms were observed. The mutants also had reduced vegetative growth and spore production, and their hyphal growth patterns were distinct from those of O-685. Coinoculation of O-685 with an focpkA mutant or a strain nonpathogenic to A. thaliana significantly reduced disease severity and the degree of root colonization by O-685. Several experimental tools useful for studying mechanisms of fungal root pathogenesis are also introduced.

Pyrenophora teres: profile of an increasingly damaging barley pathogen

Pyrenophora teres, causal agent of net blotch of barley, exists in two forms, designated P. teres f. teres and P. teres f. maculata, which induce net form net blotch (NFNB) and spot form net blotch (SFNB), respectively. Significantly more work has been performed on the net form than on the spot form although recent activity in spot form research has increased because of epidemics of SFNB in barley-producing regions. Genetic studies have demonstrated that NFNB resistance in barley is present in both dominant and recessive forms, and that resistance/susceptibility to both forms can be conferred by major genes, although minor quantitative trait loci have also been identified. Early work on the virulence of the pathogen showed toxin effector production to be important in disease induction by both forms of pathogen. Since then, several laboratories have investigated effectors of virulence and avirulence, and both forms are complex in their interaction with the host. Here, we assemble recent information from the literature that describes both forms of this important pathogen and includes reports describing the host-pathogen interaction with barley. We also include preliminary findings from a genome sequence survey.

TAXONOMY

Pyrenophora teres Drechs. Kingdom Fungi; Phylum Ascomycota; Subphylum Pezizomycotina; Class Dothideomycete; Order Pleosporales; Family Pleosporaceae; Genus Pyrenophora, form teres and form maculata.

IDENTIFICATION

To date, no clear morphological or life cycle differences between the two forms of P. teres have been identified, and therefore they are described collectively. Towards the end of the growing season, the fungus produces dark, globosely shaped pseudothecia, about 1-2mm in diameter, on barley. Ascospores measuring 18-28µm × 43-61µm are light brown and ellipsoidal and often have three to four transverse septa and one or two longitudinal septa in the median cells. Conidiophores usually arise singly or in groups of two or three and are lightly swollen at the base. Conidia measuring 30-174µm × 15-23µm are smoothly cylindrical and straight, round at both ends, subhyaline to yellowish brown, often with four to six pseudosepta. Morphologically, P. teres f. teres and P. teres f. maculata are indistinguishable.

HOST RANGE

Comprehensive work on the host range of P. teres f. teres has been performed; however, little information on the host range of P. teres f. maculata is available. Hordeum vulgare and H. vulgare ssp. spontaneum are considered to be the primary hosts for P. teres. However, natural infection by P. teres has been observed in other wild Hordeum species and related species from the genera Bromus, Avena and Triticum, including H. marinum, H. murinum, H. brachyantherum, H. distichon, H. hystrix, B. diandrus, A. fatua, A. sativa and T. aestivum (Shipton et al., 1973, Rev. Plant Pathol. 52:269-290). In artificial inoculation experiments under field conditions, P. teres f. teres has been shown to infect a wide range of gramineous species in the genera Agropyron, Brachypodium, Elymus, Cynodon, Deschampsia, Hordelymus and Stipa (Brown et al., 1993, Plant Dis. 77:942-947). Additionally, 43 gramineous species were used in a growth chamber study and at least one of the P. teres f. teres isolates used was able to infect 28 of the 43 species tested. However, of these 28 species, 14 exhibited weak type 1 or 2 reactions on the NFNB 1-10 scale (Tekauz, 1985). These reaction types are small pin-point lesions and could possibly be interpreted as nonhost reactions. In addition, the P. teres f. teres host range was investigated under field conditions by artificially inoculating 95 gramineous species with naturally infected barley straw. Pyrenophora teres f. teres was re-isolated from 65 of the species when infected leaves of adult plants were incubated on nutrient agar plates; however, other than Hordeum species, only two of the 65 host species exhibited moderately susceptible or susceptible field reaction types, with most species showing small dark necrotic lesions indicative of a highly resistant response to P. teres f. teres. Although these wild species have the potential to be alternative hosts, the high level of resistance identified for most of the species makes their role as a source of primary inoculum questionable.

DISEASE SYMPTOMS

Two types of symptom are caused by P. teres. These are net-type lesions caused by P. teres f. teres and spot-type lesions caused by P. teres f. maculata. The net-like symptom, for which the disease was originally named, has characteristic narrow, dark-brown, longitudinal and transverse striations on infected leaves. The spot form symptom consists of dark-brown, circular to elliptical lesions surrounded by a chlorotic or necrotic halo of varying width.

Sho1 and Msb2-related proteins regulate appressorium development in the smut fungus Ustilago maydis

The dimorphic fungus Ustilago maydis switches from budding to hyphal growth on the plant surface. In response to hydrophobicity and hydroxy fatty acids, U. maydis develops infection structures called appressoria. Here, we report that, unlike in Saccharomyces cerevisiae and other fungi where Sho1 (synthetic high osmolarity sensitive) and Msb2 (multicopy suppressor of a budding defect) regulate stress responses and pseudohyphal growth, Sho1 and Msb2-like proteins play a key role during appressorium differentiation in U. maydis. Sho1 was identified through a two-hybrid screen as an interaction partner of the mitogen-activated protein (MAP) kinase Kpp6. Epistasis analysis revealed that sho1 and msb2 act upstream of the MAP kinases kpp2 and kpp6. Furthermore, Sho1 was shown to destabilize Kpp6 through direct interaction with the unique N-terminal domain in Kpp6, indicating a role of Sho1 in fine-tuning Kpp6 activity. Morphological differentiation in response to a hydrophobic surface was strongly attenuated in sho1 msb2 mutants, while hydroxy fatty acid-induced differentiation was unaffected. These data suggest that Sho1 and the transmembrane mucin Msb2 are involved in plant surface sensing in U. maydis.

Requirement of a mitogen-activated protein kinase for appressorium formation and penetration of insect cuticle by the entomopathogenic fungus Beauveria bassiana

Beauveria bassiana is an important insect-pathogenic fungus that invades insects by direct penetration of the host cuticle. To delineate the molecular mechanisms involved in fungal infection, a mitogen-activated protein kinase (MAPK) gene, Bbmpk1, which encodes a YERK1 family MAPK was isolated and characterized. Targeted gene disruption of Bbmpk1 resulted in a complete loss of virulence when applied topically to host insects but did not affect growth of the fungus when conidia were injected directly into the hemocoel. Hyphae of the mutant strain growing in the insect hemocoel were unable to penetrate the cuticle growing outwards and consequently failed to sporulate on the cadaver surface. These data suggest that BbMPK1 is essential for penetration of the insect cuticle both from the outside and from the inside-out in order to escape and disperse from the host. Inactivation of BbMPK1 also caused a significant decrease in fungal adhesion to insect cuticles and eliminated their ability to form appressoria. In order to identify downstream genes regulated by BbMPK1, a suppressive subtractive hybridization (SSH) library was generated comparing mutant and wild-type transcripts isolated during appressorium formation. Thirty-one genes screened from the SSH library were determined to be expressed in the wild-type strain but either significantly reduced or not expressed in the mutant. Ten genes showed high or medium similarity to known protein encoding genes, including proteins involved in cell surface hydrophobicity, lipid metabolism, microtubule dynamics, mitochondrial electron transport, chromatin remodeling, transcription, rRNA processing, small nucleolar RNA accumulation, oxidation of aldehydes, translation, and likely other cellular processes.

The pmk1-like mitogen-activated protein kinase from Lecanicillium (Verticillium) fungicola is not required for virulence on Agaricus bisporus

In plant-pathogenic fungi, the pmk1 mitogen-activated protein kinase (MAPK) signalling pathway plays an essential role in regulating the development of penetration structures and the sensing of host-derived cues, but its role in other pathosystems such as fungal-fungal interactions is less clear. We report the use of a gene disruption strategy to investigate the pmk1-like MAPK, Lf pmk1 in the development of Lecanicillium fungicola (formerly Verticillium fungicola) infection on the cultivated mushroom Agaricus bisporus. Lf pmk1 was isolated using a degenerate PCR-based approach and was shown to be present in a single copy by Southern blot analysis. Quantitative RT-PCR showed the transcript to be fivefold upregulated in cap lesions compared with pure culture. Agrobacterium-mediated targeted disruption was used to delete a central portion of the Lf pmk1 gene. The resulting mutants showed normal symptom development as assessed by A. bisporus mushroom cap assays, sporulation patterns were normal and there were no apparent changes in overall growth rates. Our results indicate that, unlike the situation in fungal-plant pathogens, the pmk1-like MAPK pathway is not required for virulence in the fungal-fungal interaction between the L. fungicola pathogen and A. bisporus host. This observation may be of wider significance in other fungal-fungal and/or fungal-invertebrate interactions.

The role of mitogen-activated protein (MAP) kinase signalling components and the Ste12 transcription factor in germination and pathogenicity of Botrytis cinerea

In all fungi studied so far, mitogen-activated protein (MAP) kinase cascades serve as central signalling complexes that are involved in various aspects of growth, stress response and infection. In this work, putative components of the yeast Fus3/Kss1-type MAP kinase cascade and the putative downstream transcription factor Ste12 were analysed in the grey mould fungus Botrytis cinerea. Deletion mutants of the MAP triple kinase Ste11, the MAP kinase kinase Ste7 and the MAP kinase adaptor protein Ste50 all resulted in phenotypes similar to that of the previously described BMP1 MAP kinase mutant, namely defects in germination, delayed vegetative growth, reduced size of conidia, lack of sclerotia formation and loss of pathogenicity. Mutants lacking Ste12 showed normal germination, but delayed infection as a result of low penetration efficiency. Two differently spliced ste12 transcripts were detected, and both were able to complement the ste12 mutant, except for a defect in sclerotium formation, which was only corrected by the full-sized transcript. Overexpression of the smaller ste12 transcript resulted in delayed germination and strongly reduced infection. Bc-Gas2, a homologue of Magnaporthe grisea Gas2 that is required for appressorial function, was found to be non-essential for growth and infection, but its expression was under the control of both Bmp1 and Ste12. In summary, the role and regulatory connections of the Fus3/Kss1-type MAP kinase cascade in B. cinerea revealed both common and unique properties compared with those of other plant pathogenic fungi, and provide evidence for a regulatory link between the BMP1 MAP kinase cascade and Ste12.

Signaling by the pathogenicity-related MAP kinase of Cochliobolus heterostrophus correlates with its local accumulation rather than phosphorylation

Phosphorylated mitogen-activated protein kinases (MAPK) transmit signals by activation of their targets. The extent of signal transduction could depend on MAPK phosphorylation level, concentration, and subcellular localization. The pathogenicity MAPK Chk1 of the fungal corn pathogen Cochliobolus heterostrophus is required for central developmental functions, including appressoria formation, conidiation, melanization, virulence, and female fertility. We followed CHK1 transcript level, protein localization, quantity, phosphorylation, and expression of downstream genes during conidial germination on a surface inductive for appressoria formation and in suspension. The Chk1-GFP protein representing a translational fusion of Chk1 and GFP (green fluorescent protein) was very abundant in ungerminated conidia, accumulated in maturating appressoria and appressorial nuclei, but was uniformly distributed in suspension-grown hyphae. Expression of Chk1-dependent genes was upregulated in appressoria-forming hyphae but not in suspension. Despite Chk1 activation, there was no change in its phosphorylation and total protein quantity. Of all conditions tested, a temperature shift caused a decrease whereas hyperosmotic stress caused an increase in Chk1 phosphorylation. Activation of Chk1 during appressoria formation is apparently manifested by its local accumulation but not by significant changes in phosphorylation.

The MAPKK kinase ChSte11 regulates sexual/asexual development, melanization, pathogenicity, and adaptation to oxidative stress in Cochliobolus heterostrophus

All fungi use multiple mitogen-activated protein kinase (MAPK) cascades to respond to external signals to regulate specialized responses. In this study, we cloned and characterized a putative MAPKKK gene ChSte11, orthologous to yeast STE11, of Cochliobolus heterostrophus. DeltaChste11 strains showed defects in conidiation, sexual development, melanization and the formation of appressoria. These mutants were significantly less virulent on corn plants than the wild type. Similar phenotypes were observed in mutants of Chk1-MAPK, a putative downstream protein kinase of ChSte11. These results suggested that ChSte11 regulates various morphological changes and pathogenicity via Chk1 MAPK. Both DeltaChste11 and Deltachk1 strains showed severe sensitivity to oxidative stress, hydrogen peroxide, and heavy metals, cupric or ferric cations. DeltaBmhog1 strains, mutants of the HOG1-type MAPK, did not show sensitivity to these forms of stress. Our results strongly suggested that the Ste11-type MAPKKK regulates not only various morphological changes and pathogenicity, but also adaptations to stress via Chk1-type MAPK in filamentous fungi.

Aspergillus nidulans natural product biosynthesis is regulated by mpkB, a putative pheromone response mitogen-activated protein kinase

The Aspergillus nidulans putative mitogen-activated protein kinase encoded by mpkB has a role in natural product biosynthesis. An mpkB mutant exhibited a decrease in sterigmatocystin gene expression and low mycotoxin levels. The mutation also affected the expression of genes involved in penicillin and terrequinone A synthesis. mpkB was necessary for normal expression of laeA, which has been found to regulate secondary metabolism gene clusters.

RAS2 regulates growth and pathogenesis in Fusarium graminearum

Fusarium graminearum is a ubiquitous pathogen of cereal crops, including wheat, barley, and maize. Diseases caused by F. graminearum are of particular concern because harvested grains frequently are contaminated with harmful mycotoxins such as deoxynivalenol (DON). In this study, we explored the role of Ras GTPases in pathogenesis. The genome of F. graminearum contains two putative Ras GTPase-encoding genes. The two genes (RAS1 and RAS2) showed different patterns of expression under different conditions of nutrient availability and in various mutant backgrounds. RAS2 was dispensable for survival but, when disrupted, caused a variety of morphological defects, including slower growth on solid media, delayed spore germination, and significant reductions in virulence on wheat heads and maize silks. Intracellular cAMP levels were not affected by deletion of RAS2 and exogenous treatment of the ras2 mutant with cAMP did not affect phenotypic abnormalities, thus indicating that RAS2 plays a minor or no role in cAMP signaling. However, phosphorylation of the mitogen-activated protein (MAP) kinase Gpmk1 and expression of a secreted lipase (FGL1) required for infection were reduced significantly in the ras2 mutant. Based on these observations, we hypothesize that RAS2 regulates growth and virulence in F. graminearum by regulating the Gpmk1 MAP kinase pathway.

A MAP kinase gene, BMK1, is required for conidiation and pathogenicity in the rice leaf spot pathogen Bipolaris oryzae

We isolated and characterized BMK1, a gene encoding a mitogen-activated protein kinase (MAPK), from the rice leaf spot pathogen Bipolaris oryzae. The deduced amino acid sequence showed significant homology with Fus3/Kss1 MAPK homologues from other phytopathogenic fungi. The BMK1 disruptants showed impaired hyphal growth, no conidial production, and loss of virulence against rice leaves, indicating that the BMK1 is essential for conidiation and pathogenicity in B. oryzae.

Two mitogen-activated protein kinase signalling cascades mediate basal resistance to antifungal plant defensins in Fusarium graminearum

Antifungal defensins, MsDef1 and MtDef4, from Medicago spp., inhibit the growth of Fusarium graminearum, which causes head blight disease in cereals. In order to determine the signalling cascades that are modulated by these defensins, we have isolated several insertional mutants of F. graminearum that exhibit hypersensitivity to MsDef1, but not to MtDef4. The molecular characterization of two of these mutants, designated enhanced sensitivity to defensin (esd), has revealed that the Mgv1 and Gpmk1 MAP kinase signalling cascades play a major role in regulating sensitivity of F. graminearum to MsDef1, but not to MtDef4. The Hog1 MAP kinase signalling cascade, which is responsible for adaptation of this fungus to hyperosmotic stress, does not participate in the fungal response to these defensins. Significantly, the esd mutants also exhibit hypersensitivity to other tested defensins and are highly compromised in their pathogenesis on wheat heads and tomato fruits. The studies reported here for the first time implicate two MAP kinase signalling cascades in a plant defensin-mediated alteration of fungal growth. Based on our findings, we propose that specific MAP kinase signalling cascades are essential for protection of a fungal pathogen from the antimicrobial proteins of its host plant.

The MAP kinase-encoding gene MgFus3 of the non-appressorium phytopathogen Mycosphaerella graminicola is required for penetration and in vitro pycnidia formation

SUMMARY In eukaryotes, a family of serine/threonine protein kinases known as mitogen-activated protein kinases (MAPKs) is involved in the transduction of a variety of extracellular signals and in the regulation of growth and development. We identified a MAPK-encoding gene in Mycosphaerella graminicola strain IPO323 with high homology to the orthologous Fus3 gene of Saccharomyces cerevisiae and designated it MgFus3. Early colony development of the MgFus3 mutants during in vitro growth was similar to those of the wild-type and ectopic controls, but at the later stages of growth MgFus3 mutants did not become melanized, showed altered polarized growth and were unable to produce aerial mycelia. The MgFus3 mutants were non-pathogenic, and detailed microscopic analyses revealed that they failed to colonize the mesophyll tissue owing to the inability to penetrate stomata. Unlike the wild-type strain, MgFus3 mutants were unable to differentiate pycnidia on plant-derived media. Thus, in addition to the crucial role of MgFus3 in the regulation of penetration, it may also be involved in regulating asexual fructification. Hence, MgFus3 can be regarded as a multifunctional pathogenicity factor of M. graminicola.