MAP kinase and cAMP signaling regulate infection structure formation and pathogenic growth in the rice blast fungus Magnaporthe grisea

Simultaneous transcriptome analysis of Colletotrichum gloeosporioides and tomato fruit pathosystem reveals novel fungal pathogenicity and fruit defense strategies

The fungus Colletotrichum gloeosporioides breaches the fruit cuticle but remains quiescent until fruit ripening signals a switch to necrotrophy, culminating in devastating anthracnose disease. There is a need to understand the distinct fungal arms strategy and the simultaneous fruit response. Transcriptome analysis of fungal-fruit interactions was carried out concurrently in the appressoria, quiescent and necrotrophic stages. Conidia germinating on unripe fruit cuticle showed stage-specific transcription that was accompanied by massive fruit defense responses. The subsequent quiescent stage showed the development of dendritic-like structures and swollen hyphae within the fruit epidermis. The quiescent fungal transcriptome was characterized by activation of chromatin remodeling genes and unsuspected environmental alkalization. Fruit response was portrayed by continued highly integrated massive up-regulation of defense genes. During cuticle infection of green or ripe fruit, fungi recapitulate the same developmental stages but with differing quiescent time spans. The necrotrophic stage showed a dramatic shift in fungal metabolism and up-regulation of pathogenicity factors. Fruit response to necrotrophy showed activation of the salicylic acid pathway, climaxing in cell death. Transcriptome analysis of C. gloeosporioides infection of fruit reveals its distinct stage-specific lifestyle and the concurrent changing fruit response, deepening our perception of the unfolding fungal-fruit arms and defenses race.

Systematic characterization of the bZIP transcription factor gene family in the rice blast fungus, Magnaporthe oryzae

Regulatory roles of the basic leucine zipper (bZIP) transcription factors (TFs) in fungi have been identified in diverse cellular processes such as development, nutrient utilization and various stress responses. In this study, the 22 Magnaporthe oryzae genes encoding bZIP TFs were systematically characterized. Phylogenetic analysis of fungal bZIP TFs revealed that seven MobZIPs are Magnaporthe-specific, while others belongs to 15 clades of orthologous Ascomycota genes. Expression patterns of MobZIPs under various conditions showed that they are highly stress responsive. We generated deletion mutants for 13 MobZIPs: nine with orthologues in other fungal species and four Magnaporthe-specific ones. Seven of them exhibited defects in mycelial growth, development and/or pathogenicity. Consistent with the conserved functions of the orthologues, MobZIP22 and MobZIP13 played a role in sulfur metabolism and iron homeostasis respectively. Along with MobZIP22 and MobZIP13, one Magnaporthe-specific gene, MobZIP11 is essential for pathogenicity in a reactive oxygen species-dependent manner. Taken together, our results will contribute to understanding the regulatory mechanisms of the bZIP TF gene family in fungal development, adaptation to environmental stresses and pathogenicity in the rice blast fungus.

HAM-5 functions as a MAP kinase scaffold during cell fusion in Neurospora crassa

Cell fusion in genetically identical Neurospora crassa germlings and in hyphae is a highly regulated process involving the activation of a conserved MAP kinase cascade that includes NRC-1, MEK-2 and MAK-2. During chemotrophic growth in germlings, the MAP kinase cascade members localize to conidial anastomosis tube (CAT) tips every ∼8 minutes, perfectly out of phase with another protein that is recruited to the tip: SOFT, a recently identified scaffold for the MAK-1 MAP kinase pathway in Sordaria macrospora. How the MAK-2 oscillation process is initiated, maintained and what proteins regulate the MAP kinase cascade is currently unclear. A global phosphoproteomics approach using an allele of mak-2 (mak-2^Q100G) that can be specifically inhibited by the ATP analog 1NM-PP1 was utilized to identify MAK-2 kinase targets in germlings that were potentially involved in this process. One such putative target was HAM-5, a protein of unknown biochemical function. Previously, Δham-5 mutants were shown to be deficient for hyphal fusion. Here we show that HAM-5-GFP co-localized with NRC-1, MEK-2 and MAK-2 and oscillated with identical dynamics from the cytoplasm to CAT tips during chemotropic interactions. In the Δmak-2 strain, HAM-5-GFP localized to punctate complexes that did not oscillate, but still localized to the germling tip, suggesting that MAK-2 activity influences HAM-5 function/localization. However, MAK-2-GFP showed cytoplasmic and nuclear localization in a Δham-5 strain and did not localize to puncta. Via co-immunoprecipitation experiments, HAM-5 was shown to physically interact with NRC-1, MEK-2 and MAK-2, suggesting that it functions as a scaffold/transport hub for the MAP kinase cascade members for oscillation and chemotropic interactions during germling and hyphal fusion in N. crassa. The identification of HAM-5 as a scaffold-like protein will help to link the activation of MAK-2 cascade to upstream factors and proteins involved in this intriguing process of fungal communication.

Cell fusion between genetically identical cells of the fungus Neurospora crassa occurs when germinating asexual cells (conidia) sense each other's proximity and redirect their growth. Chemotropic growth is dependent upon the assembly of a MAPK cascade (NRC-1/MEK-2/MAK-2) at the cell cortex (conidial anastomosis tubes; CATs), followed by disassembly over an ∼8 min cycle. A second protein required for fusion, SO, also assembles and disassembles at CAT tips during chemotropic growth, but with perfectly opposite dynamics to the MAK-2 complex. This process of germling chemotropism, oscillation and cell fusion is regulated by many genes and is poorly understood. Via a phosphoproteomics approach, we identify HAM-5, which functions as a scaffold for the MAK-2 signal transduction complex. HAM-5 is required for assembly/disassembly and oscillation of the MAK-2 complex during chemotropic growth. Our data supports a model whereby regulated modification of HAM-5 controls the disassembly of the MAK-2 MAPK complex and is essential for modulating the tempo of oscillation during chemotropic interactions.

Screening of a synthetic peptide combinatorial library to identify inhibitors of the appressorium formation in Magnaporthe oryzae

The rice blast disease caused by Magnaporthe oryzae is one of the most devastating diseases of cultivated rice. One of the most important stages in the infective cycle of M. oryzae is the formation of the dome-shaped structure called appressorium. The purpose of the present study was to identify novel peptides to control the rice blast disease by blocking the appressorium formation through screening of a synthetic peptide combinatorial library. As result of the screening, a set of 29 putative bioactive peptides were identified, synthesized and assayed in comparison with the previously identified peptide PAF104. The peptides MgAPI24, MgAPI40 and MgAPI47 showed improved inhibitory activity on the M. oryzae appressorium formation. Our data show that these peptides have a differential effect on two developmental structures: appressoria and appressorium-like structures. Antimicrobial assays against M. oryzae and other non-target microorganisms showed a weak or no toxicity of these peptides, demonstrating their specific activity blocking the appressorium formation. Therefore, the outcome of this research would be useful in the development of novel target-oriented peptides to use in plant protection.

The transcription factor Ste12 mediates the regulatory role of the Tmk1 MAP kinase in mycoparasitism and vegetative hyphal fusion in the filamentous fungus Trichoderma atroviride

Mycoparasitic species of the fungal genus Trichoderma are potent antagonists able to combat plant pathogenic fungi by direct parasitism. An essential step in this mycoparasitic fungus-fungus interaction is the detection of the fungal host followed by activation of molecular weapons in the mycoparasite by host-derived signals. The Trichoderma atroviride MAP kinase Tmk1, a homolog of yeast Fus3/Kss1, plays an essential role in regulating the mycoparasitic host attack, aerial hyphae formation and conidiation. However, the transcription factors acting downstream of Tmk1 are hitherto unknown. Here we analyzed the functions of the T. atroviride Ste12 transcription factor whose orthologue in yeast is targeted by the Fus3 and Kss1 MAP kinases. Deletion of the ste12 gene in T. atroviride not only resulted in reduced mycoparasitic overgrowth and lysis of host fungi but also led to loss of hyphal avoidance in the colony periphery and a severe reduction in conidial anastomosis tube formation and vegetative hyphal fusion events. The transcription of several orthologues of Neurospora crassa hyphal fusion genes was reduced upon ste12 deletion; however, the Δste12 mutant showed enhanced expression of mycoparasitism-relevant chitinolytic and proteolytic enzymes and of the cell wall integrity MAP kinase Tmk2. Based on the comparative analyses of Δste12 and Δtmk1 mutants, an essential role of the Ste12 transcriptional regulator in mediating outcomes of the Tmk1 MAPK pathway such as regulation of the mycoparasitic activity, hyphal fusion and carbon source-dependent vegetative growth is suggested. Aerial hyphae formation and conidiation, in contrast, were found to be independent of Ste12.

The Aspergillus nidulans signalling mucin MsbA regulates starvation responses, adhesion and affects cellulase secretion in response to environmental cues

In the heterogeneous semi-solid environment naturally occupied by lignocellulolytic fungi the majority of nutrients are locked away as insoluble plant biomass. Hence, lignocellulolytic fungi must actively search for, and attach to, a desirable source of nutrients. During growth on lignocellulose a period of carbon deprivation provokes carbon catabolite derepression and scavenging hydrolase secretion. Subsequently, starvation and/or contact sensing was hypothesized to play a role in lignocellulose attachment and degradation. In Aspergillus nidulans the extracellular signalling mucin, MsbA, influences growth under nutrient-poor conditions including lignocellulose. Cellulase secretion and activity was affected by MsbA via a mechanism that was independent of cellulase transcription. MsbA modulated both the cell wall integrity and filamentous growth MAPK pathways influencing adhesion, biofilm formation and secretion. The constitutive activation of MsbA subsequently enhanced cellulase activity by increasing the secretion of the cellobiohydrolase, CbhA, while improved substrate attachment and may contribute to an enhanced starvation response. Starvation and/or contact sensing therefore represents a new dimension to the already multifaceted regulation of cellulase activity.

Histidine kinases mediate differentiation, stress response, and pathogenicity in Magnaporthe oryzae

The aim of this study is a functional characterization of 10 putative histidine kinases (HIKs)-encoding genes in the phytopathogenic fungus Magnaporthe oryzae. Two HIKs were found to be required for pathogenicity in the fungus. It was found that the mutant strains ΔMohik5 and ΔMohik8 show abnormal conidial morphology and furthermore ΔMohik5 is unable to form appressoria. Both HIKs MoHik5p and MoHik8p appear to be essential for pathogenicity since the mutants fail to infect rice plants. MoSln1p and MoHik1p were previously reported to be components of the HOG pathway in M. oryzae. The ΔMosln1 mutant is more susceptible to salt stress compared to ΔMohik1, whereas ΔMohik1 appears to be stronger affected by osmotic or sugar stress. In contrast to yeast, the HOG signaling cascade in phytopathogenic fungi apparently comprises more elements. Furthermore, vegetative growth of the mutants ΔMohik5 and ΔMohik9 was found to be sensitive to hypoxia-inducing NaNO2 -treatment. Additionally, it was monitored that NaNO2 -treatment resulted in MoHog1p phosphorylation. As a consequence we assume a first simplified model for hypoxia signaling in M. oryzae including the HOG pathway and the HIKs MoHik5p and MoHik9p.

Bypassing both surface attachment and surface recognition requirements for appressorium formation by overactive ras signaling in Magnaporthe oryzae

Magnaporthe oryzae forms a highly specialized infection structure called an appressorium for plant penetration. In M. oryzae and many other plant-pathogenic fungi, surface attachment and surface recognition are two essential requirements for appressorium formation. Development of appressoria in the air has not been reported. In this study, we found that expression of a dominant active MoRAS2(G18V) allele in M. oryzae resulted in the formation of morphologically abnormal appressoria on nonconducive surfaces, in liquid suspensions, and on aerial hyphae without attachment to hard surfaces. Both the Pmk1 mitogen-activated protein kinase cascade and cAMP signaling pathways that regulate surface recognition and appressorium morphogenesis in M. oryzae were overactivated in the MoRAS2(G18V) transformant. In mutants deleted of PMK1 or CPKA, expression of MoRAS2(G18V) had no significant effects on appressorium morphogenesis. Furthermore, expression of dominant MoRAS2 in Colletotrichum graminicola and C. gloeosporioides also caused the formation of appressorium-like structures in aerial hyphae. Overall, our data indicate that MoRas2 functions upstream from both the cAMP-PKA and Pmk1 pathways and overactive Ras signaling leads to improper activation of these two pathways and appressorium formation without surface attachment in appressorium-forming pathogens.

Fungal model systems and the elucidation of pathogenicity determinants

Fungi have the capacity to cause devastating diseases of both plants and animals, causing significant harvest losses that threaten food security and human mycoses with high mortality rates. As a consequence, there is a critical need to promote development of new antifungal drugs, which requires a comprehensive molecular knowledge of fungal pathogenesis. In this review, we critically evaluate current knowledge of seven fungal organisms used as major research models for fungal pathogenesis. These include pathogens of both animals and plants; Ashbya gossypii, Aspergillus fumigatus, Candida albicans, Fusarium oxysporum, Magnaporthe oryzae, Ustilago maydis and Zymoseptoria tritici. We present key insights into the virulence mechanisms deployed by each species and a comparative overview of key insights obtained from genomic analysis. We then consider current trends and future challenges associated with the study of fungal pathogenicity.

The Sch9 kinase regulates conidium size, stress responses, and pathogenesis in Fusarium graminearum

Fusarium head blight caused by Fusarium graminearum is an important disease of wheat and barley worldwide. In a previous study on functional characterization of the F. graminearum kinome, one protein kinase gene important for virulence is orthologous to SCH9 that is functionally related to the cAMP-PKA and TOR pathways in the budding yeast. In this study, we further characterized the functions of FgSCH9 in F. graminearum and its ortholog in Magnaporthe oryzae. The ΔFgsch9 mutant was slightly reduced in growth rate but significantly reduced in conidiation, DON production, and virulence on wheat heads and corn silks. It had increased tolerance to elevated temperatures but became hypersensitive to oxidative, hyperosmotic, cell wall, and membrane stresses. The ΔFgsch9 deletion also had conidium morphology defects and produced smaller conidia. These results suggest that FgSCH9 is important for stress responses, DON production, conidiogenesis, and pathogenesis in F. graminearum. In the rice blast fungus Magnaporthe oryzae, the ΔMosch9 mutant also was defective in conidiogenesis and pathogenesis. Interestingly, it also produced smaller conidia and appressoria. Taken together, our data indicate that the SCH9 kinase gene may have a conserved role in regulating conidium size and plant infection in phytopathogenic ascomycetes.

Epoxy alcohol synthase of the rice blast fungus represents a novel subfamily of dioxygenase-cytochrome P450 fusion enzymes

The genome of the rice blast fungus Magnaporthe oryzae codes for two proteins with N-terminal dioxygenase (DOX) and C-terminal cytochrome P450 (CYP) domains, respectively. One of them, MGG_13239, was confirmed as 7,8-linoleate diol synthase by prokaryotic expression. The other recombinant protein (MGG_10859) possessed prominent 10R-DOX and epoxy alcohol synthase (EAS) activities. This enzyme, 10R-DOX-EAS, transformed 18:2n-6 sequentially to 10(R)-hydroperoxy-8(E),12(Z)-octadecadienoic acid (10R-HPODE) and to 12S(13R)-epoxy-10(R)-hydroxy-8(E)-octadecenoic acid as the end product. Oxygenation at C-10 occurred by retention of the pro-R hydrogen of C-8 of 18:2n-6, suggesting antarafacial hydrogen abstraction and oxygenation. Experiments with (18)O2 and (16)O2 gas confirmed that the epoxy alcohol was formed from 10R-HPODE, likely by heterolytic cleavage of the dioxygen bond with formation of P450 compound I, and subsequent intramolecular epoxidation of the 12(Z) double bond. Site-directed mutagenesis demonstrated that the cysteinyl heme ligand of the P450 domain was required for the EAS activity. Replacement of Asn(965) with Val in the conserved AsnGlnXaaGln sequence revealed that Asn(965) supported formation of the epoxy alcohol. 10R-DOX-EAS is the first member of a novel subfamily of DOX-CYP fusion proteins of devastating plant pathogens.

Genome sequence and virulence variation-related transcriptome profiles of Curvularia lunata, an important maize pathogenic fungus

BACKGROUND

Curvularia lunata is an important maize foliar fungal pathogen that distributes widely in maize growing area in China. Genome sequencing of the pathogen will provide important information for globally understanding its virulence mechanism.

RESULTS

We report the genome sequences of a highly virulent C. lunata strain. Phylogenomic analysis indicates that C. lunata was evolved from Bipolaris maydis (Cochliobolus heterostrophus). The highly virulent strain has a high potential to evolve into other pathogenic stains based on analyses on transposases and repeat-induced point mutations. C. lunata has a smaller proportion of secreted proteins as well as B. maydis than entomopathogenic fungi. C. lunata and B. maydis have a similar proportion of protein-encoding genes highly homologous to experimentally proven pathogenic genes from pathogen-host interaction database. However, relative to B. maydis, C. lunata possesses not only many expanded protein families including MFS transporters, G-protein coupled receptors, protein kinases and proteases for transport, signal transduction or degradation, but also many contracted families including cytochrome P450, lipases, glycoside hydrolases and polyketide synthases for detoxification, hydrolysis or secondary metabolites biosynthesis, which are expected to be crucial for the fungal survival in varied stress environments. Comparative transcriptome analysis between a lowly virulent C. lunata strain and its virulence-increased variant induced by resistant host selection reveals that the virulence increase of the pathogen is related to pathways of toxin and melanin biosynthesis in stress environments, and that the two pathways probably have some overlaps.

CONCLUSIONS

The data will facilitate a full revelation of pathogenic mechanism and a better understanding of virulence differentiation of C. lunata.

Regulation of cellular diacylglycerol through lipid phosphate phosphatases is required for pathogenesis of the rice blast fungus, Magnaporthe oryzae

Considering implication of diacylglycerol in both metabolism and signaling pathways, maintaining proper levels of diacylglycerol (DAG) is critical to cellular homeostasis and development. Except the PIP2-PLC mediated pathway, metabolic pathways leading to generation of DAG converge on dephosphorylation of phosphatidic acid catalyzed by lipid phosphate phosphatases. Here we report the role of such enzymes in a model plant pathogenic fungus, Magnaporthe oryzae. We identified five genes encoding putative lipid phosphate phosphatases (MoLPP1 to MoLPP5). Targeted disruption of four genes (except MoLPP4) showed that MoLPP3 and MoLPP5 are required for normal progression of infection-specific development and proliferation within host plants, whereas MoLPP1 and MoLPP2 are indispensable for fungal pathogenicity. Reintroduction of MoLPP3 and MoLPP5 into individual deletion mutants restored all the defects. Furthermore, exogenous addition of saturated DAG not only restored defect in appressorium formation but also complemented reduced virulence in both mutants. Taken together, our data indicate differential roles of lipid phosphate phosphatase genes and requirement of proper regulation of cellular DAGs for fungal development and pathogenesis.

Pleiotropic function of the putative zinc-finger protein MoMsn2 in Magnaporthe oryzae

The mitogen-activated protein kinase MoOsm1-mediated osmoregulation pathway plays crucial roles in stress responses, asexual and sexual development, and pathogenicity in Magnaporthe oryzae. Utilizing an affinity purification approach, we identified the putative transcriptional activator MoMsn2 as a protein that interacts with MoOsm1 in vivo. Disruption of the MoMSN2 gene resulted in defects in aerial hyphal growth, conidial production, and infection of host plants. Quantitative reverse transcription-polymerase chain reaction analysis showed that the expression of several genes involved in conidiophore formation was reduced in ΔMomsn2, suggesting that MoMsn2 might function as a transcriptional regulator of these genes. Subsequently, MoCos1 was identified as one of the MoMsn2 targets through yeast one-hybrid analysis in which MoMsn2 binds to the AGGGG and CCCCT motif of the MoCOS1 promoter region. Phenotypic characterization showed that MoMsn2 was required for appressorium formation and penetration and pathogenicity. Although the ΔMomsn2 mutant was tolerant to the cell-wall stressor Calcofluor white, it was sensitive to common osmotic stressors. Further analysis suggests that MoMsn2 is involved in the regulation of the cell-wall biosynthesis pathway. Finally, transcriptome data revealed that MoMsn2 modulates numerous genes participating in conidiation, infection, cell-wall integrity, and stress response. Collectively, our results led to a model in which MoMsn2 mediates a series of downstream genes that control aerial hyphal growth, conidiogenesis, appressorium formation, cell-wall biosynthesis, and infection and that also offer potential targets for the development of new disease management strategies.

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.

N-glycosylation of effector proteins by an α-1,3-mannosyltransferase is required for the rice blast fungus to evade host innate immunity

Plant pathogenic fungi deploy secreted effectors to suppress plant immunity responses. These effectors operate either in the apoplast or within host cells, so they are putatively glycosylated, but the posttranslational regulation of their activities has not been explored. In this study, the ASPARAGINE-LINKED GLYCOSYLATION3 (ALG3)-mediated N-glycosylation of the effector, Secreted LysM Protein1 (Slp1), was found to be essential for its activity in the rice blast fungus Magnaporthe oryzae. ALG3 encodes an α-1,3-mannosyltransferase for protein N-glycosylation. Deletion of ALG3 resulted in the arrest of secondary infection hyphae and a significant reduction in virulence. We observed that Δalg3 mutants induced massive production of reactive oxygen species in host cells, in a similar manner to Δslp1 mutants, which is a key factor responsible for arresting infection hyphae of the mutants. Slp1 sequesters chitin oligosaccharides to avoid their recognition by the rice (Oryza sativa) chitin elicitor binding protein CEBiP and the induction of innate immune responses, including reactive oxygen species production. We demonstrate that Slp1 has three N-glycosylation sites and that simultaneous Alg3-mediated N-glycosylation of each site is required to maintain protein stability and the chitin binding activity of Slp1, which are essential for its effector function. These results indicate that Alg3-mediated N-glycosylation of Slp1 is required to evade host innate immunity.

Ectoparasitic growth of Magnaporthe on barley triggers expression of the putative barley wax biosynthesis gene CYP96B22 which is involved in penetration resistance

BACKGROUND

Head blast caused by the fungal plant pathogen Magnaporthe oryzae is an upcoming threat for wheat and barley cultivation. We investigated the nonhost response of barley to an isolate of the Magnaporthe species complex which is pathogenic on Pennisetum spp. as a potential source for novel resistance traits.

RESULTS

Array experiments identified a barley gene encoding a putative cytochrome P450 monooxygenase whose transcripts accumulate to a higher concentration in the nonhost as compared to the host interaction. The gene clusters within the CYP96 clade of the P450 plant gene family and is designated as CYP96B22. Expression of CYP96B22 was triggered during the ectoparasitic growth of the pathogen on the outside of the leaf. Usage of a fungicidal treatment and a Magnaporthe mutant confirmed that penetration was not necessary for this early activation of CYP96B22. Transcriptional silencing of CYP96B22 using Barley stripe mosaic virus led to a decrease in penetration resistance of barley plants to Magnaporthe host and nonhost isolates. This phenotype seems to be specific for the barley-Magnaporthe interaction, since penetration of the adapted barley powdery mildew fungus was not altered in similarly treated plants.

CONCLUSION

Taken together our results suggest a cross-talk between barley and Magnaporthe isolates across the plant surface. Since members of the plant CYP96 family are known to be involved in synthesis of epicuticular waxes, these substances or their derivatives might act as signal components. We propose a functional overlap of CYP96B22 in the execution of penetration resistance during basal and nonhost resistance of barley against different Magnaporthe species.

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.

Development of a real-time microchip PCR system for portable plant disease diagnosis

Rapid and accurate detection of plant pathogens in the field is crucial to prevent the proliferation of infected crops. Polymerase chain reaction (PCR) process is the most reliable and accepted method for plant pathogen diagnosis, however current conventional PCR machines are not portable and require additional post-processing steps to detect the amplified DNA (amplicon) of pathogens. Real-time PCR can directly quantify the amplicon during the DNA amplification without the need for post processing, thus more suitable for field operations, however still takes time and require large instruments that are costly and not portable. Microchip PCR systems have emerged in the past decade to miniaturize conventional PCR systems and to reduce operation time and cost. Real-time microchip PCR systems have also emerged, but unfortunately all reported portable real-time microchip PCR systems require various auxiliary instruments. Here we present a stand-alone real-time microchip PCR system composed of a PCR reaction chamber microchip with integrated thin-film heater, a compact fluorescence detector to detect amplified DNA, a microcontroller to control the entire thermocycling operation with data acquisition capability, and a battery. The entire system is 25 × 16 × 8 cm(3) in size and 843 g in weight. The disposable microchip requires only 8-µl sample volume and a single PCR run consumes 110 mAh of power. A DNA extraction protocol, notably without the use of liquid nitrogen, chemicals, and other large lab equipment, was developed for field operations. The developed real-time microchip PCR system and the DNA extraction protocol were used to successfully detect six different fungal and bacterial plant pathogens with 100% success rate to a detection limit of 5 ng/8 µl sample.

Cgl-SLT2 is required for appressorium formation, sporulation and pathogenicity in Colletotrichum gloeosporioides

The mitogen-activated protein (MAP) kinase pathways has been implicated in the pathogenicity of various pathogenic fungi and plays important roles in regulating pathogenicity-related morphogenesis. This work describes the isolation and characterization of MAP kinase gene, Cgl-SLT2, from Colletotrichum gloeosporioides. A DNA sequence, including 1,633 bp of Cgl-SLT2 open-reading frame and its promoter and terminator regions, was isolated via DNA walking and cloned. To analyze gene function, a gene disruption cassette containing hygromycin-resistant gene was constructed, and Cgl-SLT2 was inactivated via gene deletion. Analysis on Cgl-slt2 mutant revealed a defect in vegetative growth and sporulation as compared to the wild-type strain. When grown under nutrient-limiting conditions, hyperbranched hyphal morphology was observed in the mutant. Conidia induction for germination on rubber wax-coated hard surfaces revealed no differences in the percentage of conidial germination between the wild-type and Cgl-slt2 mutant. However, the percentage of appressorium formation in the mutant was greatly reduced. Bipolar germination in the mutant was higher than in the wild-type at 8-h post-induction. A pathogenicity assay revealed that the mutant was unable to infect either wounded or unwounded mangoes. These results suggest that the Cgl-SLT2 MAP kinase is required for C. gloeosporioides conidiation, polarized growth, appressorium formation and pathogenicity.

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.

Glycogen metabolic genes are involved in trehalose-6-phosphate synthase-mediated regulation of pathogenicity by the rice blast fungus Magnaporthe oryzae

The filamentous fungus Magnaporthe oryzae is the causal agent of rice blast disease. Here we show that glycogen metabolic genes play an important role in plant infection by M. oryzae. Targeted deletion of AGL1 and GPH1, which encode amyloglucosidase and glycogen phosphorylase, respectively, prevented mobilisation of glycogen stores during appressorium development and caused a significant reduction in the ability of M. oryzae to cause rice blast disease. By contrast, targeted mutation of GSN1, which encodes glycogen synthase, significantly reduced the synthesis of intracellular glycogen, but had no effect on fungal pathogenicity. We found that loss of AGL1 and GPH1 led to a reduction in expression of TPS1 and TPS3, which encode components of the trehalose-6-phosphate synthase complex, that acts as a genetic switch in M. oryzae. Tps1 responds to glucose-6-phosphate levels and the balance of NADP/NADPH to regulate virulence-associated gene expression, in association with Nmr transcriptional inhibitors. We show that deletion of the NMR3 transcriptional inhibitor gene partially restores virulence to a Δagl1Δgph1 mutant, suggesting that glycogen metabolic genes are necessary for operation of the NADPH-dependent genetic switch in M. oryzae.

Crossover fungal pathogens: the biology and pathogenesis of fungi capable of crossing kingdoms to infect plants and humans

The outbreak of fungal meningitis associated with contaminated methylprednisolone acetate has thrust the importance of fungal infections into the public consciousness. The predominant pathogen isolated from clinical specimens, Exserohilum rostratum (teleomorph: Setosphaeria rostrata), is a dematiaceous fungus that infects grasses and rarely humans. This outbreak highlights the potential for fungal pathogens to infect both plants and humans. Most crossover or trans-kingdom pathogens are soil saprophytes and include fungi in Ascomycota and Mucormycotina phyla. To establish infection, crossover fungi must overcome disparate, host-specific barriers, including protective surfaces (e.g. cuticle, skin), elevated temperature, and immune defenses. This review illuminates the underlying mechanisms used by crossover fungi to cause infection in plants and mammals, and highlights critical events that lead to human infection by these pathogens. Several genes including veA, laeA, and hapX are important in regulating biological processes in fungi important for both invasive plant and animal infections.

Fungal-specific transcription factor AbPf2 activates pathogenicity in Alternaria brassicicola

Alternaria brassicicola is a successful saprophyte and necrotrophic plant pathogen. To identify molecular determinants of pathogenicity, we created non-pathogenic mutants of a transcription factor-encoding gene, AbPf2. The frequency and timing of germination and appressorium formation on host plants were similar between the non-pathogenic ∆abpf2 mutants and wild-type A. brassicicola. The mutants were also similar in vitro to wild-type A. brassicicola in terms of vegetative growth, conidium production, and responses to a phytoalexin, reactive oxygen species and osmolites. The hyphae of the mutants grew slowly but did not cause disease symptoms on the surface of host plants. Transcripts of the AbPf2 gene increased exponentially soon after wild-type conidia contacted their host plants . A small amount of AbPf2 protein, as monitored using GFP fusions, was present in young, mature conidia. The protein level decreased during saprophytic growth, but increased and was located primarily in fungal nuclei during pathogenesis. Levels of the proteins and transcripts sharply decreased following colonization of host tissues beyond the initial infection site. When expression of the transcription factor was induced in the wild-type during early pathogenesis, 106 fungal genes were also induced in the wild-type but not in the ∆abpf2 mutants. Notably, 33 of the 106 genes encoded secreted proteins, including eight putative effector proteins. Plants inoculated with ∆abpf2 mutants expressed higher levels of genes associated with photosynthesis, the pentose phosphate pathway and primary metabolism, but lower levels of defense-related genes. Our results suggest that AbPf2 is an important regulator of pathogenesis, but does not affect other cellular processes in A. brassicicola.

Shared and distinct functions of two Gti1/Pac2 family proteins in growth, morphogenesis and pathogenicity of Magnaporthe oryzae

Gti1/Pac2 are conserved family proteins that regulate morphogenic transition in yeasts such as Schizosaccharomyces pombe and Candida albicans, and they also control toxin production and pathogenicity in filamentous fungus Fusarium graminearum. To test the functions of Gti1/Pac2 paralogues MoGti1 and MoPac2 in the rice blast fungus Magnaporthe oryzae, we generated respective ΔMogti1 and ΔMopac2 mutant strains. We found that MoGti1 and MoPac2 exhibit shared and distinct roles in hyphal growth, conidiation, sexual reproduction, stress responses, surface hydrophobility, invasive hyphal growth and pathogenicity. Consistent with the putative conserved function of MoGti1, we showed that MoGti1-GFP is localized to the nucleus, whereas MoPac2-GFP is mainly found in the cytoplasm. In addition, we provided evidence that the nuclear localization of MoGti1 could be subject to regulation by MoPmk1 mitogen-activated protein kinase. Moreover, we found that the reduced pathogenicity in the ΔMopac2 mutant corresponds with an increased expression of plant defence genes, including PR1a, AOS2, LOX1, PAD4, and CHT1. Taken together, our studies provide a comprehensive analysis of two similar but distinct Gti1/Pac2 family proteins in M. oryzae, which underlines the important yet conserved functions of these family proteins in plant pathogenic fungi.

Involvement of MoVMA11, a Putative Vacuolar ATPase c' Subunit, in Vacuolar Acidification and Infection-Related Morphogenesis of Magnaporthe oryzae

Many functions of vacuole depend on the activity of vacuolar ATPase which is essential to maintain an acidic lumen and create the driving forces for massive fluxes of ions and metabolites through vacuolar membrane. In filamentous fungus Magnaportheoryzae, subcellular colocalization and quinacrine staining suggested that the V1V0 domains of V-ATPase were fully assembled and the vacuoles were kept acidic during infection-related developments. Targeted gene disruption of MoVMA11 gene, encoding the putative c' subunit of V-ATPase, impaired vacuolar acidification and mimicked the phenotypes of yeast V-ATPase mutants in the poor colony morphology, abolished asexual and sexual reproductions, selective carbon source utilization, and increased calcium and heavy metals sensitivities, however, not in the typical pH conditional lethality. Strikingly, aerial hyphae of the MoVMA11 null mutant intertwined with each other to form extremely thick filamentous structures. The results also implicated that MoVMA11 was involved in cell wall integrity and appressorium formation. Abundant non-melanized swollen structures and rare, small appressoria without penetration ability were produced at the hyphal tips of the ΔMovma11 mutant on onion epidermal cells. Finally, the MoVMA11 null mutant lost pathogenicity on both intact and wounded host leaves. Overall, our data indicated that MoVMA11, like other fungal VMA genes, is associated with numerous cellular functions and highlighted that V-ATPase is essential for infection-related morphogenesis and pathogenesis in M. oryzae.

Global expression profiling of transcription factor genes provides new insights into pathogenicity and stress responses in the rice blast fungus

Because most efforts to understand the molecular mechanisms underpinning fungal pathogenicity have focused on studying the function and role of individual genes, relatively little is known about how transcriptional machineries globally regulate and coordinate the expression of a large group of genes involved in pathogenesis. Using quantitative real-time PCR, we analyzed the expression patterns of 206 transcription factor (TF) genes in the rice blast fungus Magnaporthe oryzae under 32 conditions, including multiple infection-related developmental stages and various abiotic stresses. The resulting data, which are publicly available via an online platform, provided new insights into how these TFs are regulated and potentially work together to control cellular responses to a diverse array of stimuli. High degrees of differential TF expression were observed under the conditions tested. More than 50% of the 206 TF genes were up-regulated during conidiation and/or in conidia. Mutations in ten conidiation-specific TF genes caused defects in conidiation. Expression patterns in planta were similar to those under oxidative stress conditions. Mutants of in planta inducible genes not only exhibited sensitive to oxidative stress but also failed to infect rice. These experimental validations clearly demonstrated the value of TF expression patterns in predicting the function of individual TF genes. The regulatory network of TF genes revealed by this study provides a solid foundation for elucidating how M. oryzae regulates its pathogenesis, development, and stress responses.

The catalytic subunit of cAMP-dependent protein kinase A StPKA-c contributes to conidiation and early invasion in the phytopathogenic fungus Setosphaeria turcica

Cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) is an important mediator of signal transduction in eukaryotic cells. Thus, identifying its function is necessary to understand the cAMP signaling network. StPKA-c, the PKA catalytic subunit gene in Setosphaeria turcica, was investigated by RNA interference technology. Transformant strains M3, M5, and M9 with diverse StPKA-c silencing efficiency were confirmed by reverse transcription polymerase chain reaction and Northern blot. Compared with the wild-type strain 01-23, the transformant strains exhibited increased growth rate and significantly decreased conidium production. In addition, the ratios of spore germination and appressorium formation and penetration were slightly reduced. Relative to the wild-type strain, the transformants demonstrated different colony color, greatly reduced pathogenicity, and similar HT-toxin activity. Further studies showed that the content of intracellular melanin in the transformants significantly decreased, and the transcription of transcriptional factor StMR was down-regulated correspondingly. The transcription and enzyme activity of xylanase was also impaired. Thus, we proposed that StPKA-c was mainly involved in the mycelium growth, conidiation, and pathogenesis of S. turcica. Furthermore, it was positively correlated with the biosyntheses of melanin and xylanase but dispensable for the activity of HT-toxin.

Differences between appressoria formed by germ tubes and appressorium-like structures developed by hyphal tips in Magnaporthe oryzae

Melanized appressoria are highly specialized infection structures formed by germ tubes of the rice blast fungus Magnaporthe oryzae for plant infection. M. oryzae also forms appressorium-like structures on hyphal tips. Whereas appressorium formation by conidial germ tubes has been well characterized, formation of appressorium-like structures by hyphal tips is under-investigated. In a previous study, we found that the chs7 deletion mutant failed to form appressoria on germ tubes but were normal in the development of appressorium-like structures on artificial hydrophobic surfaces. In this study, we compared the differences between the formation of appressoria by germ tubes and appressorium-like structures by hyphal tips in M. oryzae. Structurally, both appressoria and appressorium-like structures had a melanin layer that was absent in the pore region. In general, the latters were 1.4-fold larger in size but had lower turgor pressure than appressoria, which is consistent with its lower efficiency in plant penetration. Treatments with cAMP, IBMX, or a cutin monomer efficiently induced appressorium formation but not the development of appressorium-like structures. In contrast, coating surfaces with waxes stimulated the formation of both infection structures. Studies with various signaling mutants indicate that Osm1 and Mps1 are dispensable but Pmk1 is essential for both appressorium formation and development of appressorium-like structures on hyphal tips. Interestingly, the cpkA mutant was reduced in the differentiation of appressorium-like structures but not appressorium formation. We also observed that the con7 mutant generated in our lab failed to form appressorium-like structures on hyphal tips but still produced appressoria by germ tubes on hydrophobic surfaces. Con7 is a transcription factor regulating the expression of CHS7. Overall, these results indicate that the development of appressorium-like structures by hyphal tips and formation of appressoria by germ tubes are not identical differentiation processes in M. oryzae and may involve different molecular mechanisms.

Complexity of roles and regulation of the PMK1-MAPK pathway in mycelium development, conidiation and appressorium formation in Magnaporthe oryzae

MST50, MST11, MST7, PMK1 and GAS1/GAS2 genes are the important components in the PMK1-MAPK signal transduction pathway in fungi. Mutants with deletion of these five genes of Magnaporthe oryzae, a pathogen of the rice blast, were constructed. A cDNA array containing 4108 unique genes of M. oryzae was developed and used to analyze the gene expression profiles of these mutants against the wild type to dissect the gene expression regulation networks responsible for conidiation and appressorium formation. With this approach, differentially regulated genes by these five components were identified. The vast majority of the regulated genes were mutant-specific, while only a small proportion were in common for all of the mutants, suggesting that each of these genes has its own regulon. Functional groups and expression patterns of the regulated genes showed that (1) gene members in the PMK1-MAPK pathway are associated with multiple signaling pathways; (2) the regulation of PMK1-mediated signaling pathways is very complex and likely involved in other signaling networks; (3) glucose metabolism and signals are required in mycelium development; and (4) appressorium formation likely shares the mechanisms responsible for sexual conjugation and meiosis, which is affected by carbohydrate metabolism.

Impact of tricyclazole and azoxystrobin on growth, sporulation and secondary infection of the rice blast fungus, Magnaporthe oryzae

BACKGROUND

Rice blast, caused by Magnaporthe oryzae B. Couch sp. nov., is one of the most destructive rice diseases worldwide, causing substantial yield losses every year. In Italy, its management is based mainly on the use of two fungicides, azoxystrobin and tricyclazole, that restrain the disease progress. The aim of this study was to investigate and compare the inhibitory effects of the two fungicides on the growth, sporulation and secondary infection of M. oryzae.

RESULTS

Magnaporthe oryzae mycelium growth was inhibited at low concentrations of azoxystrobin and relatively high concentrations of tricyclazole, while sporulation was more sensitive to both fungicides and was affected at similarly low doses. Furthermore, infection efficiency of conidia obtained from mycelia exposed to tricyclazole was affected to a higher extent than for conidia produced on azoxystrobin-amended media, even though germination of such conidia was reduced after azoxystrobin treatment.

CONCLUSIONS

This study presents for the first time detailed azoxystrobin and tricyclazole growth-response curves for M. oryzae mycelium growth and sporulation. Furthermore, high efficacy of tricyclazole towards inhibition of sporulation and secondary infection indicates an additional possible mode of action of this fungicide that is different from inhibition of melanin biosynthesis.

Characterization and transcriptional regulation of Stachybotrys elegans mitogen-activated-protein kinase gene smkA following mycoparasitism and starvation conditions

Mitogen-activated protein kinase (MAPK) signaling pathways play an important role in the development and conidiation of fungal pathogens on their hosts and the sensing of host-derived cues. Mycoparasitism is a fungus-fungus interaction comprising host-pathogen cross talk. Until now, only little information is available on the role of the MAPK signaling pathway during this interaction. Here, we report on the differential expression of a MAPK/ERK gene in the mycoparasite Stachybotrys elegans in response to direct parasitism of different vegetative structures of the plant pathogen Rhizoctonia solani (i.e., carbon-rich condition) and to nutrient starvation (i.e., carbon-poor condition). Western blot analysis against ERK1/2 highlighted an increase in their phosphorylated forms when S. elegans was grown under starvation condition compared to that detected in response to mycoparasitism. A higher abundance of phosphorylated ERK1/2 at the third day of interaction compared to that estimated under starvation condition was detected applying LC-MS/MS. At the transcriptional level, smkA, a YERK1 class member, was significantly induced in response to hyphal parasitism compared to parasitized sclerotia at 3, 4, and 5 days of interaction. However, under starvation condition, smkA levels were significantly induced after 7 days of growth. Southern blot analysis revealed that smkA is member of a small gene family. Collectively, these results suggest that smkA could be implicated in the mycoparasitic process in S. elegans as well as in stress-activated pathways. These results may be of wider significance in other fungus-fungus interactions.

A serine/threonine-protein phosphatase PP2A catalytic subunit is essential for asexual development and plant infection in Magnaporthe oryzae

Protein phosphatase 2A is a subgroup of widely conserved serine/threonine phosphatases and plays diverse roles in transcription, translation, differentiation, cell cycle, and signal transduction in many organisms. However, its roles in biotrophic and hemi-biotrophic phytopathogenic fungi remain to be investigated. In this study, we isolated an insertional mutant of the rice blast fungus Magnaporthe oryzae that was defective in vegetative hyphal growth. In the mutant, the T-DNA fragment was found to be inserted in the promoter region of a putative serine/threonine protein phosphatase 2A catalytic subunit (PP2Ac) gene MoPPG1. Deletion of MoPPG1 leads to severe defects in vegetative hyphal growth and conidiation. Conidia of the ∆Moppg1 null mutants were misshaped, and most of them were two-celled. The deletion mutants of MoPPG1 did not penetrate into host plant cells and failed to cause any disease lesions on rice leaves. Interestingly, significant reduction was found in the ∆Moppg1 null mutants in expression levels of several Rho GTPase family genes including MgCDC42, MgRHO3, and MgRAC1, which were important for pathogenesis of M. oryzae. Taken together, our results indicated that PP2Ac plays vital roles in asexual development and plant infection by regulating Rho GTPases in the rice blast fungus and perhaps other plant pathogenic fungi.

Why no feeding frenzy? Mechanisms of nutrient acquisition and utilization during infection by the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae is a devastating pathogen of rice and wheat. It is a hemibiotroph that exhibits symptomless biotrophic growth for the first 4 to 5 days of infection of susceptible cultivars before becoming necrotrophic. Here, we review recent advances in our understanding of how M. oryzae is able to grow, acquire nutrients, and interact with the plant cell during infection. In particular, we describe direct mechanisms (such as the integration of carbon and nitrogen metabolism by trehalose-6-phosphate synthase 1) and indirect mechanisms (such as the suppression of host responses) that allow M. oryzae to utilize available host nutrient. We contrast the ability of M. oryzae to voraciously metabolize a wide range of carbon and nitrogen sources in vitro with the carefully orchestrated development it displays during the biotrophic phase of in planta growth and ask how the two observations can be reconciled. We also look at how nutrient acquisition and effector biology might be linked in order to facilitate rapid colonization of the plant host.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

The Cyclase-associated protein Cap1 is important for proper regulation of infection-related morphogenesis in Magnaporthe oryzae

Surface recognition and penetration are critical steps in the infection cycle of many plant pathogenic fungi. In Magnaporthe oryzae, cAMP signaling is involved in surface recognition and pathogenesis. Deletion of the MAC1 adenylate cyclase gene affected appressorium formation and plant infection. In this study, we used the affinity purification approach to identify proteins that are associated with Mac1 in vivo. One of the Mac1-interacting proteins is the adenylate cyclase-associated protein named Cap1. CAP genes are well-conserved in phytopathogenic fungi but none of them have been functionally characterized. Deletion of CAP1 blocked the effects of a dominant RAS2 allele and resulted in defects in invasive growth and a reduced intracellular cAMP level. The Δcap1 mutant was defective in germ tube growth, appressorium formation, and formation of typical blast lesions. Cap1-GFP had an actin-like localization pattern, localizing to the apical regions in vegetative hyphae, at the periphery of developing appressoria, and in circular structures at the base of mature appressoria. Interestingly, Cap1, similar to LifeAct, did not localize to the apical regions in invasive hyphae, suggesting that the apical actin cytoskeleton differs between vegetative and invasive hyphae. Domain deletion analysis indicated that the proline-rich region P2 but not the actin-binding domain (AB) of Cap1 was responsible for its subcellular localization. Nevertheless, the AB domain of Cap1 must be important for its function because CAP1^ΔAB only partially rescued the Δcap1 mutant. Furthermore, exogenous cAMP induced the formation of appressorium-like structures in non-germinated conidia in CAP1^ΔAB transformants. This novel observation suggested that AB domain deletion may result in overstimulation of appressorium formation by cAMP treatment. Overall, our results indicated that CAP1 is important for the activation of adenylate cyclase, appressorium morphogenesis, and plant infection in M. oryzae. CAP1 may also play a role in feedback inhibition of Ras2 signaling when Pmk1 is activated.

In Magnaporthe oryzae, cAMP signaling is known to play an important role in surface recognition and plant penetration. The Mac1 adenylate cyclase is essential for plant infection. To better understand Mac1 activation mechanisms, in this study we used the affinity purification approach to identify proteins that are associated with Mac1 in vivo. One of the Mac1-interacting protein is the adenylate cyclase associated protein (CAP) encoded by the CAP1 gene. Results from our study indicated that Cap1 is important for Mac1 activation and plant infection in M. oryzae. The Δcap1 mutant was defective in germ tube growth and appressorium formation and failed to cause typical blast lesions. Like LifeAct, Cap1 localized to apical patches in vegetative hyphae but not in invasive hyphae. The P2 proline-rich region was important for Cap1 localization but the actin-binding domain played a role in feedback inhibition of Ras signaling. To our knowledge, functional characterization of CAP genes has not been reported in filamentous fungi. Our results indicate that CAP1 is important for regulating adenylate cyclase activities, appressorium morphogenesis, and plant infection. Further characterization of CAP1 will be important to better understand the interaction between cAMP signaling and the PMK1 pathway in M. oryzae.

Polyubiquitin is required for growth, development and pathogenicity in the rice blast fungus Magnaporthe oryzae

Protein ubiquitination, which is highly selective, regulates many important biological processes including cellular differentiation and pathogenesis in eukaryotic cells. Here, we integrated pharmacological, molecular and proteomic approaches to explore the role of ubiquitination in Magnaporthe oryzae, the leading fungal disease of rice world-wide. Inhibition of ubiquitin-mediated proteolysis using the 26S proteasome inhibitor, Bortezomib, significantly attenuated conidia germination, appressorium formation and pathogenicity in M. oryzae. Gene expression analysis revealed that many genes associated with protein ubiquitination were developmentally regulated during conidia germination. Only a few, including a polyubiquitin encoding gene, MGG_01282, were more abundantly expressed during appressorium formation and under nitrogen starvation. Targeted gene deletion of MGG_01282, in addition to a significant reduction in protein ubiquitination as determined by immuno blot assays, resulted in pleiotropic effects on M. oryzae including reduced growth and sporulation, abnormal conidia morphology, reduced germination and appressorium formation, and the inability to cause disease. Mutants were also defective in sexual development and were female sterile. Using mass spectrometry, we identified 63 candidate polyubiquitinated proteins under nitrogen starvation, which included overrepresentation of proteins involved in translation, transport and protein modification. Our study suggests that ubiquitination of target proteins plays an important role in nutrient assimilation, development and pathogenicity of M. oryzae.

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 Top 10 fungal pathogens in molecular plant pathology

The aim of this review was to survey all fungal pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate which fungal pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 495 votes from the international community, and resulted in the generation of a Top 10 fungal plant pathogen list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Magnaporthe oryzae; (2) Botrytis cinerea; (3) Puccinia spp.; (4) Fusarium graminearum; (5) Fusarium oxysporum; (6) Blumeria graminis; (7) Mycosphaerella graminicola; (8) Colletotrichum spp.; (9) Ustilago maydis; (10) Melampsora lini, with honourable mentions for fungi just missing out on the Top 10, including Phakopsora pachyrhizi and Rhizoctonia solani. This article presents a short resumé of each fungus in the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant mycology community, as well as laying down a bench-mark. It will be interesting to see in future years how perceptions change and what fungi will comprise any future Top 10.

The Top 10 fungal pathogens in molecular plant pathology

The aim of this review was to survey all fungal pathologists with an association with the journal Molecular Plant Pathology and ask them to nominate which fungal pathogens they would place in a 'Top 10' based on scientific/economic importance. The survey generated 495 votes from the international community, and resulted in the generation of a Top 10 fungal plant pathogen list for Molecular Plant Pathology. The Top 10 list includes, in rank order, (1) Magnaporthe oryzae; (2) Botrytis cinerea; (3) Puccinia spp.; (4) Fusarium graminearum; (5) Fusarium oxysporum; (6) Blumeria graminis; (7) Mycosphaerella graminicola; (8) Colletotrichum spp.; (9) Ustilago maydis; (10) Melampsora lini, with honourable mentions for fungi just missing out on the Top 10, including Phakopsora pachyrhizi and Rhizoctonia solani. This article presents a short resumé of each fungus in the Top 10 list and its importance, with the intent of initiating discussion and debate amongst the plant mycology community, as well as laying down a bench-mark. It will be interesting to see in future years how perceptions change and what fungi will comprise any future Top 10.