Multiple upstream signals converge on the adaptor protein Mst50 in Magnaporthe grisea

The Magnaporthe grisea species complex and plant pathogenesis

TAXONOMY

Kingdom Fungi; Phylum Ascomycota; Class Sordariomycetes; Order Magnaporthales; Family Pyriculariaceae (anamorph)/Magnaporthaceae (teleomorph); Genus Pyricularia (anamorph)/Magnaporthe (teleomorph); Species P. grisea (anamorph)/M. grisea (teleomorph).

HOST RANGE

Very broad at the species level, including rice, wheat, barley, millet and other species of the Poaceae (Gramineae).

DISEASE SYMPTOMS

Can be found on all parts of the plant, including leaves, leaf collars, necks, panicles, pedicels, seeds and even the roots. Initial symptoms are white to grey-green lesions or spots with darker borders, whereas older lesions are elliptical or spindle-shaped and whitish to grey with necrotic borders. Lesions may enlarge and coalesce to eventually destroy the entire leaf.

DISEASE CONTROL

Includes cultural strategies, genetic resistance and the application of chemical fungicides.

GEOGRAPHICAL DISTRIBUTION

Widespread throughout the rice-growing regions of the globe and has been reported in more than 85 countries.

GENOMIC STRUCTURE

Different isolates possess similar genomic sizes and overall genomic structures. For the laboratory strain 70-15: assembly size, 40.98 Mb; number of chromosomes, seven; number of predicted genes, 13 032; G + C composition, 51.6%; average gene contains 451.6 amino acids; mitochondrion genome size, 34.87 kb.

USEFUL WEBSITE

http://www.broadinstitute.org/annotation/genome/magnaporthe_comparative/MultiHome.html.

Skp1, a component of E3 ubiquitin ligase, is necessary for growth, sporulation, development and pathogenicity in rice blast fungus (Magnaporthe oryzae)

Ubiqitination is an important process in eukaryotic cells involving E3 ubiquitin ligase, which co-ordinates with cell cycle proteins and controls various cell functions. Skp1 (S-phase kinase-associated protein 1) is a core component of the SCF (Skp1-Cullin 1-F-box) E3 ubiquitin ligase complex necessary for protein degradation by the 26S proteasomal pathway. The rice blast fungus Magnaporthe oryzae has a single MoSKP1(MGG_04978) required for viability. Skp1 has multiple functions; however, its roles in growth, sporulation and appressorial development are not understood. MoSKP1 complements Skp1 function in the fission yeast temperature-sensitive mutant skp1 A7, restoring the normal length of yeast cells at restrictive temperature. The MoSkp1 protein in M. oryzae is present in spores and germ tubes, and is abundantly expressed in appressoria. Various RNA interference (RNAi) and antisense transformants of MoSKP1 in B157 show reduced sporulation, defective spore morphology, lesser septation and diffuse nuclei. Further, they show elongated germ tubes and are unable to form appressoria. Transformants arrested in G1/S stage during initial spore germination show a similar phenotype to wild-type spores treated with hydroxyurea (HU). Reduced MoSkp1 transcript and protein levels in knockdown transformants result in atypical germ tube development. MoSkp1 interacts with the putative F-box protein (MGG_06351) revealing the ability to form protein complexes. Our investigation of the role of MoSKP1 suggests that a decrease in MoSkp1 manifests in decreased total protein ubiquitination and, consequently, defective cell cycle and appressorial development. Thus, MoSKP1 plays important roles in growth, sporulation, appressorial development and pathogenicity of M. oryzae.

Thioredoxins are involved in the activation of the PMK1 MAP kinase pathway during appressorium penetration and invasive growth in Magnaporthe oryzae

In Magnaporthe oryzae, the Mst11-Mst7-Pmk1 MAP kinase pathway is essential for appressorium formation and invasive growth. To determine their roles in Pmk1 activation and plant infection, we characterized the two thioredoxin genes, TRX1 and TRX2, in M. oryzae. Whereas the Δtrx1 mutants had no detectable phenotypes, deletion of TRX2 caused pleiotropic defects in growth, conidiation, light sensing, responses to stresses and plant infection progresses. The Δtrx1 Δtrx2 double mutant had more severe defects than the Δtrx2 mutant and was non-pathogenic in infection assays. The Δtrx2 and Δtrx1 Δtrx2 mutant rarely formed appressoria on hyphal tips and were defective in invasive growth after penetration. Pmk1 phosphorylation was barely detectable in the Δtrx2 and Δtrx1 Δtrx2 mutants. Deletion of TRX2 affected proper folding or intra-/inter-molecular interaction of Mst7 and expression of the dominant active MST7 allele partially rescued the defects of the Δtrx1 Δtrx2 mutant. Furthermore, Cys305 is important for Mst7 function and Trx2 directly interacts with Mst7 in co-IP assays. Our data indicated that thioredoxins play important roles in intra-cellular ROS signalling and pathogenesis in M. oryzae. As the predominant thioredoxin gene, TRX2 may regulate the activation of Pmk1 MAPK via its effects on Mst7.

Clg2p interacts with Clf and ClUrase to regulate appressorium formation, pathogenicity and conidial morphology in Curvularia lunata

Ras is a small GTPase that regulates numerous processes in the cellular development and morphogenesis of many organisms. In this study, we identified and functionally characterized the Clg2p gene of Curvularia lunata, which is homologous with the Ras protein. The Clg2p deletion mutant (ΔClg2p) had altered appressorium formation and conidial morphology and produced fewer, smaller lesions compared with the wild-type strain. When a dominant Clg2p allele was introduced into the mutant, all of these defective phenotypes were completely restored. To further understand the regulation of Clg2p in appressorium formation and conidial morphology, and its role in pathogenicity, seven Clg2p-interacting proteins were screened using a yeast two-hybrid assay. Two of these proteins, Clf, a homologue of Mst11, which corresponds to MAP kinase kinase kinase in Magnaporthe oryzae, and urate oxidase (designated ClUrase) were functionally characterized. Clg2p specifically interacted with Clf through its RA domain to regulate appressorium formation and pathogenicity, whereas the Clg2p-ClUrase interaction regulated conidial morphology without affecting fungal pathogenicity. This report is the first to elucidate the regulatory mechanism of the key Ras protein Clg2p in C. lunata.

Penetration Peg Formation and Invasive Hyphae Development Require Stage-Specific Activation of MoGTI1 in Magnaporthe oryzae

The hemibiotrophic pathogen Magnaporthe oryzae causes one of the most destructive diseases in cultivated rice. Complex infection-related morphogenesis and production of various effectors are known to be important for successful colonization and disease development. In this study, we characterized the activation of the MoGTI1 transcription factor and its role in infection-related morphogenesis and effector gene expression. The Mogti1 mutant was nonpathogenic, although it was normal in appressorium formation and turgor generation. Close examination showed that Mogti1 was defective in penetration and growth of normal invasive hyphae. Deletion of MoGTI1 affected the expression of the majority of effector genes. The expression of MoGti1 appeared to be controlled by the Mps1 but not Pmk1 mitogen-activated protein kinase (MAPK), and the mps1 and Mogti1 mutants had similar phenotypes in plant infection and cell wall integrity defects. However, lack of MAPK phosphorylation sites and dispensability of the putative MAPK docking site suggested that MoGti1 is not a direct target of Mps1. Site-specific mutagenesis analyses showed that the putative protein kinase A phosphorylation site was not essential for localization of MoGti1 to the nucleus but important for its normal function. Although the cyclin-dependent kinase (CDK) phosphorylation site of MoGti1 is dispensable during vegetative growth and appressorium formation, the S77A mutation affected penetration and invasive growth. Localization of MoGti1(S77A)-green fluorescent protein to the nucleus in late stages of appressorium formation and during invasive growth was not observed, suggesting a stage-specific CDK phosphorylation of MoGti1. Overall, our data indicate that Mps1 may indirectly regulate the expression of MoGti1 in maintaining cell wall integrity, conidiation, and plant infection. MoGti1 is likely a stage-specific target of CDK and plays a crucial role in effector gene expression and morphogenesis related to the development of penetration pegs and invasive hyphae.

Septin-Dependent Assembly of the Exocyst Is Essential for Plant Infection by Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast disease, the most devastating disease of cultivated rice (Oryza sativa) and a continuing threat to global food security. To cause disease, the fungus elaborates a specialized infection cell called an appressorium, which breaches the cuticle of the rice leaf, allowing the fungus entry to plant tissue. Here, we show that the exocyst complex localizes to the tips of growing hyphae during vegetative growth, ahead of the Spitzenkörper, and is required for polarized exocytosis. However, during infection-related development, the exocyst specifically assembles in the appressorium at the point of plant infection. The exocyst components Sec3, Sec5, Sec6, Sec8, and Sec15, and exocyst complex proteins Exo70 and Exo84 localize specifically in a ring formation at the appressorium pore. Targeted gene deletion, or conditional mutation, of genes encoding exocyst components leads to impaired plant infection. We demonstrate that organization of the exocyst complex at the appressorium pore is a septin-dependent process, which also requires regulated synthesis of reactive oxygen species by the NoxR-dependent Nox2 NADPH oxidase complex. We conclude that septin-mediated assembly of the exocyst is necessary for appressorium repolarization and host cell invasion.

FgCDC14 regulates cytokinesis, morphogenesis, and pathogenesis in Fusarium graminearum

Members of Cdc14 phosphatases are common in animals and fungi, but absent in plants. Although its orthologs are conserved in plant pathogenic fungi, their functions during infection are not clear. In this study, we showed that the CDC14 ortholog is important for pathogenesis and morphogenesis in Fusarium graminearum. FgCDC14 is required for normal cell division and septum formation and FgCdc14 possesses phosphatase activity with specificity for a subset of Cdk-type phosphorylation sites. The Fgcdc14 mutant was reduced in growth, conidiation, and ascospore formation. It was defective in ascosporogenesis and pathogenesis. Septation in Fgcdc14 was reduced and hyphal compartments contained multiple nuclei, indicating defects in the coordination between nuclear division and cytokinesis. Interestingly, foot cells of mutant conidia often differentiated into conidiogenous cells, resulting in the production of inter-connected conidia. In the interphase, FgCdc14-GFP localized to the nucleus and spindle-pole-body. Taken together, our results indicate that Cdc14 phosphatase functions in cell division and septum formation in F. graminearum, likely by counteracting Cdk phosphorylation, and is required for plant infection.

Regulation of appressorium development in pathogenic fungi

Many plant pathogenic fungi have the capacity to breach the intact cuticles of their plant hosts using specialised infection cells called appressoria. These cells exert physical force to rupture the plant surface, or deploy enzymes in a focused way to digest the cuticle and plant cell wall. They also provide the means by which focal secretion of effectors occurs at the point of plant infection. Development of appressoria is linked to re-modelling of the actin cytoskeleton, mediated by septin GTPases, and rapid cell wall differentiation. These processes are regulated by perception of plant cell surface components, and starvation stress, but also linked to cell cycle checkpoints that control the overall progression of infection-related development.

Activation of Mst11 and Feedback Inhibition of Germ Tube Growth in Magnaporthe oryzae

Appressorium formation and invasive growth are two important steps in the infection cycle of Magnaporthe oryzae that are regulated by the Mst11-Mst7-Pmk1 mitogen-activated protein kinase (MAPK) pathway. However, the molecular mechanism involved in the activation of Mst11 MAPK kinase kinase is not clear in the rice blast fungus. In this study, we functionally characterized the regulatory region of Mst11 and its self-inhibitory binding. Deletion of the middle region of Mst11, which contains the Ras-association (RA) domain and two conserved phosphorylation sites (S453 and S458), blocked Pmk1 activation and appressorium formation. However, the MST11(ΔRA) transformant MRD-2 still formed appressoria, although it was reduced in virulence. Interestingly, over 50% of its germ tubes branched and formed two appressoria by 48 h, which was suppressed by treatments with exogenous cAMP. The G18V dominant active mutation enhanced the interaction of Ras2 with Mst11, suggesting that Mst11 has stronger interactions with the activated Ras2. Furthermore, deletion and site-directed mutagenesis analyses indicated that phosphorylation at S453 and S458 of Mst11 is important for appressorium formation and required for the activation of Pmk1. We also showed that the N-terminal region of Mst11 directly interacted with its kinase domain, and the S789G mutation reduced their interactions. Expression of the MST11(S789G) allele rescued the defect of the mst11 mutant in plant infection and resulted in the formation of appressoria on hydrophilic surfaces, suggesting the gain-of-function effect of the S789G mutation. Overall, our results indicate that the interaction of Mst11 with activated Ras2 and phosphorylation of S453 and S458 play regulatory roles in Mst11 activation and infection-related morphogenesis, possibly by relieving its self-inhibitory interaction between its N-terminal region and the C-terminal kinase domain. In addition, binding of Mst11 to Ras2 may be involved in the feedback inhibition of cAMP signaling and further differentiation of germ tubes after appressorium formation.

Phosphoproteome Analysis Links Protein Phosphorylation to Cellular Remodeling and Metabolic Adaptation during Magnaporthe oryzae Appressorium Development

The rice pathogen, Magnaporthe oryzae, undergoes a complex developmental process leading to formation of an appressorium prior to plant infection. In an effort to better understand phosphoregulation during appressorium development, a mass spectrometry based phosphoproteomics study was undertaken. A total of 2924 class I phosphosites were identified from 1514 phosphoproteins from mycelia, conidia, germlings, and appressoria of the wild type and a protein kinase A (PKA) mutant. Phosphoregulation during appressorium development was observed for 448 phosphosites on 320 phosphoproteins. In addition, a set of candidate PKA targets was identified encompassing 253 phosphosites on 227 phosphoproteins. Network analysis incorporating regulation from transcriptomic, proteomic, and phosphoproteomic data revealed new insights into the regulation of the metabolism of conidial storage reserves and phospholipids, autophagy, actin dynamics, and cell wall metabolism during appressorium formation. In particular, protein phosphorylation appears to play a central role in the regulation of autophagic recycling and actin dynamics during appressorium formation. Changes in phosphorylation were observed in multiple components of the cell wall integrity pathway providing evidence that this pathway is highly active during appressorium development. Several transcription factors were phosphoregulated during appressorium formation including the bHLH domain transcription factor MGG_05709. Functional analysis of MGG_05709 provided further evidence for the role of protein phosphorylation in regulation of glycerol metabolism and the metabolic reprogramming characteristic of appressorium formation. The data presented here represent a comprehensive investigation of the M. oryzae phosphoproteome and provide key insights on the role of protein phosphorylation during infection-related development.

Orchestration of Morphogenesis in Filamentous Fungi: Conserved Roles for Ras Signaling Networks

Filamentous fungi undergo complex developmental programs including conidial germination, polarized morphogenesis, and differentiation of sexual and asexual structures. For many fungi, the coordinated completion of development is required for pathogenicity, as specialized morphological structures must be produced by the invading fungus. Ras proteins are highly conserved GTPase signal transducers and function as major regulators of growth and development in eukaryotes. Filamentous fungi typically express two Ras homologues, comprising distinct groups of Ras1-like and Ras2-like proteins based on sequence homology. Recent evidence suggests shared roles for both Ras1 and Ras2 homologues, but also supports the existence of unique functions in the areas of stress response and virulence. This review focuses on the roles played by both Ras protein groups during growth, development, and pathogenicity of a diverse array of filamentous fungi.

The putative Gγ subunit gene MGG1 is required for conidiation, appressorium formation, mating and pathogenicity in Magnaporthe oryzae

Heterotrimeric G-proteins play key roles in the transduction of extracellular signals to various downstream effectors in eukaryotes. In our previous study, a T-DNA insertional mutant A1-412, in which the promoter of a putative Gγ subunit gene MGG1 was disrupted, was impaired in asexual/sexual sporulation, appressorium formation, and pathogenicity in Magnaporthe oryzae. Here the roles of MGG1 in regulating fungal development and plant infection were further investigated and verified using a gene deletion strategy. Targeted gene deletion mutants of MGG1 exhibited similar phenotypes to those of A1-412. The Δmgg1 mutants were unable to differentiate appressorium on hydrophobic surfaces and nonpathogenic to susceptible hosts. The defects of the Δmgg1 mutants in appressorium formation were partially restored by adding exogenous cAMP or IBMX (a phosphodiesterase inhibitor), although the induced appressoria were still nonfunctional. Expressing Mgg1-GFP fusion protein in an Δmgg1 mutant could complement all phenotypes of the mutant, and bright GFP fluorescence was observed at the periphery of fungal cells, indicating that Mgg1 mainly localizes to plasma membrane. Quantitative RT-PCR analysis revealed that deletion of MGG1 resulted in a significant reduction in mRNA levels of the genes encoding Gα (MagA, MagB, and MagC), Gβ (Mgb1), and adenylate cyclase (Mac1). Moreover, intracellular cAMP accumulation was significantly reduced in Δmgg1 mutants compared to that in the wild-type strain. Taken together, our results suggested that Gγ subunit Mgg1 might act upstream of cAMP signaling pathway and play critical roles in regulation of conidiation, appressorium formation, mating, and plant infection in M. oryzae.

Activation of the signalling mucin MoMsb2 and its functional relationship with Cbp1 in Magnaporthe oryzae

Various surface signals are recognized by Magnaporthe oryzae to activate the Pmk1 MAP kinase that is essential for appressorium formation and invasive growth. One of upstream sensors of the Pmk1 pathway is the MoMsb2 signalling mucin. However, the activation of MoMsb2 and its relationship with other sensors is not clear. In this study, we showed that the cleavage and transmembrane domains are essential for MoMsb2 functions. Cleavage of MoMsb2 was further confirmed by western blot analysis, and five putative cleavage sites were functionally characterized. Expression of the extracellular region alone partially rescued the defects of Momsb2 in appressorium formation and virulence. The cytoplasmic region of MoMsb2, although dispensable for appressorium formation, was more important for penetration and invasive growth. Interestingly, the Momsb2 cbp1 double mutant deleted of both mucin genes was blocked in Pmk1 activation. It failed to form appressoria on artificial surfaces and was non-pathogenic. In addition, we showed that MoMsb2 interacts with Ras2 but not with MoCdc42 in co-immunoprecipitation assays. Overall, results from this study indicated that the extracellular and cytoplasmic regions of MoMsb2 have distinct functions in appressorium formation, penetration and invasive growth, and MoMsb2 has overlapping functions with Cbp1 in recognizing environmental signals for Pmk1 activation.

The transmembrane protein FgSho1 regulates fungal development and pathogenicity via the MAPK module Ste50-Ste11-Ste7 in Fusarium graminearum

The mitogen-activated protein kinase (MAPK) signaling pathways have been characterized in Fusarium graminearum. Currently, the upstream sensors of these pathways are unknown. Biological functions of a transmembrane protein FgSho1 were investigated using a target gene deletion strategy. The relationship between FgSho1 and the MAPK cassette FgSte50-Ste11-Ste7 was analyzed in depth. The transmembrane protein FgSho1 is required for conidiation, full virulence, and deoxynivalenol (DON) biosynthesis in F. graminearum. Furthermore, FgSho1 and FgSln1 have an additive effect on virulence of F. graminearum. The yeast two-hybrid, coimmunoprecipitation, colocalization and affinity capture-mass spectrometry analyses strongly indicated that FgSho1 physically interacts with the MAPK module FgSte50-Ste11-Ste7. Similar to the FgSho1 mutant, the mutants of FgSte50, FgSte11, and FgSte7 were defective in conidiation, pathogenicity, and DON biosynthesis. In addition, FgSho1 plays a minor role in the response to osmotic stress but it is involved in the cell wall integrity pathway, which is independent of the module FgSte50-Ste11-Ste7 in F. graminearum. Collectively, results of this study strongly indicate that FgSho1 regulates fungal development and pathogenicity via the MAPK module FgSte50-Ste11-Ste7 in F. graminearum, which is different from what is known in the budding yeast Saccharomyces cerevisiae.

The MADS-box transcription factor FgMcm1 regulates cell identity and fungal development in Fusarium graminearum

In eukaryotic cells, MADS-box genes are known to play major regulatory roles in various biological processes by combinatorial interactions with other transcription factors. In this study, we functionally characterized the FgMCM1 MADS-box gene in Fusarium graminearum, the causal agent of wheat and barley head blight. Deletion of FgMCM1 resulted in the loss of perithecium production and phialide formation. The Fgmcm1 mutant was significantly reduced in virulence, deoxynivalenol biosynthesis and conidiation. In yeast two-hybrid assays, FgMcm1 interacted with Mat1-1-1 and Fst12, two transcription factors important for sexual reproduction. Whereas Fgmcm1 mutants were unstable and produced stunted subcultures, Fgmcm1 mat1-1-1 but not Fgmcm1 fst12 double mutants were stable. Furthermore, spontaneous suppressor mutations occurred frequently in stunted subcultures to recover growth rate. Ribonucleic acid sequencing analysis indicated that a number of sexual reproduction-related genes were upregulated in stunted subcultures compared with the Fgmcm1 mutant, which was downregulated in the expression of genes involved in pathogenesis, secondary metabolism and conidiation. We also showed that culture instability was not observed in the Fvmcm1 mutants of the heterothallic Fusarium verticillioides. Overall, our data indicate that FgMcm1 plays a critical role in the regulation of cell identity, sexual and asexual reproduction, secondary metabolism and pathogenesis in F. graminearum.

A transcription factor FgSte12 is required for pathogenicity in Fusarium graminearum

A conserved mitogen-activated protein kinase (MAPK) cascade homologous to the yeast Fus3/Kss1 mating/filamentation pathway is involved in the regulation of vegetative development and pathogenicity in Fusarium graminearum. However, little is known about the downstream transcription factors of this pathway. In Saccharomyces cerevisiae, the homeodomain protein Ste12 is a key transcription factor activated by Fus3/Kss1. In this study, we characterized a Ste12 orthologue FgSte12 in F. graminearum. The FgSTE12 deletion mutant (ΔFgSte12) was impaired in virulence and in the secretion of cellulase and protease, although it did not show recognizable phenotype changes in hyphal growth, conidiation or deoxynivalenol (DON) biosynthesis. In addition, ΔFgSte12 and the FgGPMK1 (a FUS3/KSS1-related MAPK gene) mutant shared several phenotypic traits. Furthermore, we found that FgGpmk1 controls the nuclear localization of FgSte12. Yeast two-hybrid and affinity capture assays indicated that FgSte12 interacts with the FgSte11-Ste7-Gpmk1 complex. Taken together, these results indicate that FgSte12 is a downstream target of FgSte11-Ste7-Gpmk1 and plays an important role in pathogenicity in F. graminearum.

Fungal communication requires the MAK-2 pathway elements STE-20 and RAS-2, the NRC-1 adapter STE-50 and the MAP kinase scaffold HAM-5

Intercellular communication is critical for the survival of unicellular organisms as well as for the development and function of multicellular tissues. Cell-to-cell signaling is also required to develop the interconnected mycelial network characteristic of filamentous fungi and is a prerequisite for symbiotic and pathogenic host colonization achieved by molds. Somatic cell–cell communication and subsequent cell fusion is governed by the MAK-2 mitogen activated protein kinase (MAPK) cascade in the filamentous ascomycete model Neurospora crassa, yet the composition and mode of regulation of the MAK-2 pathway are currently unclear. In order to identify additional components involved in MAK-2 signaling we performed affinity purification experiments coupled to mass spectrometry with strains expressing functional GFP-fusion proteins of the MAPK cascade. This approach identified STE-50 as a regulatory subunit of the Ste11p homolog NRC-1 and HAM-5 as cell-communication-specific scaffold protein of the MAPK cascade. Moreover, we defined a network of proteins consisting of two Ste20-related kinases, the small GTPase RAS-2 and the adenylate cyclase capping protein CAP-1 that function upstream of the MAK-2 pathway and whose signals converge on the NRC-1/STE-50 MAP3K complex and the HAM-5 scaffold. Finally, our data suggest an involvement of the striatin interacting phosphatase and kinase (STRIPAK) complex, the casein kinase 2 heterodimer, the phospholipid flippase modulators YPK-1 and NRC-2 and motor protein-dependent vesicle trafficking in the regulation of MAK-2 pathway activity and function. Taken together, these data will have significant implications for our mechanistic understanding of MAPK signaling and for homotypic cell–cell communication in fungi and higher eukaryotes.

Appropriate cellular responses to external stimuli depend on the highly orchestrated activity of interconnected signaling cascades. One crucial level of control arises from the formation of discrete complexes through scaffold proteins that bind multiple components of a given pathway. Central for our understanding of these signaling platforms is the archetypical MAP kinase scaffold Ste5p, a protein that is restricted to budding yeast and close relatives. We identified HAM-5, a protein highly conserved in filamentous ascomycete fungi, as cell–cell communication-specific scaffold protein of the Neurospora crassa MAK-2 cascade (homologous to the budding yeast pheromone pathway). We also describe a network of upstream acting proteins, consisting of two Ste20-related kinases, the small G-protein RAS-2 and the adenylate cyclase capping protein CAP-1, whose signals converge on HAM-5. Our work has implications for the mechanistic understanding of MAP kinase scaffold proteins and their function during intercellular communication in eukaryotic microbes as well as higher eukaryotes.

Ras GTPase activating protein CoIra1 is involved in infection-related morphogenesis by regulating cAMP and MAPK signaling pathways through CoRas2 in Colletotrichum orbiculare

Colletotrichum orbiculare is the causative agent of anthracnose disease on cucurbitaceous plants. Several signaling pathways, including cAMP–PKA and mitogen-activating protein kinase (MAPK) pathways are involved in the infection-related morphogenesis and pathogenicity of C. orbiculare. However, upstream regulators of these pathways for this species remain unidentified. In this study, CoIRA1, encoding RAS GTPase activating protein, was identified by screening the Agrobacterium tumefaciens-mediated transformation (AtMT) mutant, which was defective in the pathogenesis of C. orbiculare. The coira1 disrupted mutant showed an abnormal infection-related morphogenesis and attenuated pathogenesis. In Saccharomyces cerevisiae, Ira1/2 inactivates Ras1/2, which activates adenylate cyclase, leading to the synthesis of cAMP. Increase in the intracellular cAMP levels in coira1 mutants and dominant active forms of CoRAS2 introduced transformants indicated that CoIra1 regulates intracellular cAMP levels through CoRas2. Moreover, the phenotypic analysis of transformants that express dominant active form CoRAS2 in the comekk1 mutant or a dominant active form CoMEKK1 in the coras2 mutant indicated that CoRas2 regulates the MAPK CoMekk1–Cmk1 signaling pathway. The CoRas2 localization pattern in vegetative hyphae of the coira1 mutant was similar to that of the wild-type, expressing a dominant active form of RFP–CoRAS2. Moreover, we demonstrated that bimolecular fluorescence complementation (BiFC) signals between CoIra1 and CoRas2 were detected in the plasma membrane of vegetative hyphae. Therefore, it is likely that CoIra1 negatively regulates CoRas2 in vegetative hyphae. Furthermore, cytological analysis of the localization of CoIraI and CoRas2 revealed the dynamic cellular localization of the proteins that leads to proper assembly of F-actin at appressorial pore required for successful penetration peg formation through the pore. Thus, our results indicated that CoIra1 is involved in infection-related morphogenesis and pathogenicity by proper regulation of cAMP and MAPK signaling pathways through CoRas2.

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.

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.

Putative RhoGAP proteins orchestrate vegetative growth, conidiogenesis and pathogenicity of the rice blast fungus Magnaporthe oryzae

Rho GTPases, acting as molecular switches, are involved in the regulation of diverse cellular functions. Rho GTPase activating proteins (Rho GAPs) function as negative regulators of Rho GTPases and are required for a variety of signaling processes in cell development. But the mechanisms underlying Rho GAPs in Rho-mediated signaling pathways in fungi are still elusive. There are eight RhoGAP domain-containing genes annotated in the Magnaporthe oryzae genome. To understand the function of these RhoGAP genes, we generated knockout mutants of each of the RhoGAP genes through a homologous recombination-based method. Phenotypic analysis showed that growth rate of aerial hyphae of the Molrg1 deletion mutant decreased dramatically. The ΔMolrg1 mutant showed significantly reduced conidiation and appressorium formation by germ tubes. Moreover, it lost pathogenicity completely. Deletion of another Rho GAP (MoRga1) resulted in high percentage of larger or gherkin-shaped conidia and slight decrease in conidiation. Appressorial formation of the ΔMoRga1 mutant was delayed significantly on hydrophobic surface, while the development of mycelial growth and pathogenicity in plants was not affected. Confocal fluorescence microscopy imaging showed that MoRga1-GFP localizes to septal pore of the conidium, and this localization pattern requires both LIM and RhoGAP domains. Furthermore, either deleting the LIM or RhoGAP domain or introducing an inactivating R1032A mutation in the RhoGAP domain of MoRga1 caused similar defects as the Morga1 deletion mutant in terms of conidial morphology and appressorial formation, suggesting that MoRga1 is a stage-specific regulator of conidial differentiation by regulating some specific Rho GTPases. In this regard, MoRga1 and MoLrg1 physically interacted with both MoRac1-CA and MoCdc42-CA in the yeast two-hybrid and pull-down assays, suggesting that the actions of these two GAPs are involved in MoRac1 and MoCdc42 pathways. On the other hand, six other putative Rho GAPs (MoRga2 to MoRga7) were dispensable for conidiation, vegetative growth, appressorial formation and pathogenicity, suggesting that these Rho GAPs function redundantly during fungal development. Taking together, Rho GAP genes play important roles in M. oryzae development and infectious processes through coordination and modulation of Rho GTPases.

Disruption and molecular characterization of calpains-related (MoCAPN1, MoCAPN3 and MoCAPN4) genes in Magnaporthe oryzae

Calpains are intracellular, cysteine proteases found in plants, animals and fungi functioning as signal transduction components in different cellular pathways including sporulation and alkaline adaptation in fungi. Calpains-related MoCAPN1 (MGG_14872), MoCAPN3 (MGG_15810) and MoCAPN4 (MGG_04818) genes from Magnaporthe oryzae genome which are 2604, 3513 and 771-bp in length and encoding identical proteins of 867, 1170 and 256 amino acids were functionally characterized for different phenotypes through gene disruption method. All the mutants except those for MoCAPN1 showed normal phenotypes. In pathogenicity test, the mutants did not lead to any visible changes in phenotypes causing similar blast lesions on blast susceptible rice and barley leaves as those of the Guy-11 strain suggesting no major role in pathogenicity. Germ tubes formation, appressorium formation, mycelium radial growth and mating with 2539 strain were indistinguishable among the mutants and Guy-11 strains. Cell wall integrity (congo red) test, stress response under chemical pressure (ZnSO4, CuSO4 and CdCl2), osmotic and oxidative (NaCl and H2O2) stress response, growth response on glucose and nitrogen deficient media resulted in similar results in the mutants and Guy-11 strains. However, mutants for ΔMoCAPN1 gene produced reduced (0.57±0.15B and 0.54±0.05B) conidia compared to that (1.69±0.13A) of the Guy-11 strain showing its involvement in conidiation.

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.

Cells in cells: morphogenetic and metabolic strategies conditioning rice infection by the blast fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae is a global food security threat due to its destruction of cultivated rice. Of the world's rice harvest, 10-30 % is lost each year to this pathogen, and changing climates are likely to favor its spread into new areas. Insights into how the fungus might be contained could come from the wealth of molecular and cellular studies that have been undertaken in order to shed light on the biological underpinnings of blast disease, aspects of which we review herein. Infection begins when a three-celled spore lands on the surface of a leaf, germinates, and develops the specialized infection structure called the appressorium. The mature appressorium develops a high internal turgor that acts on a thin penetration peg, forcing it through the rice cuticle and into the underlying epidermal cells. Primary then invasive hyphae (IH) elaborate from the peg and grow asymptomatically from one living rice cell to another for the first few days of infection before host cells begin to die and characteristic necrotic lesions form on the surface of the leaf, from which spores are produced to continue the life cycle. To gain new insights into the biology of rice blast disease, we argue that, conceptually, the infection process can be viewed as two discrete phases occurring in markedly different environments and requiring distinct biochemical pathways and morphogenetic regulation: outside the host cell, where the appressorium develops in a nutrient-free environment, and inside the host cell, where filamentous growth occurs in a glucose-rich, nitrogen-poor environment, at least from the perspective of the fungus. Here, we review the physiological and metabolic changes that occur in M. oryzae as it transitions from the surface to the interior of the host, thus enabling us to draw lessons about the strategies that allow M. oryzae cells to thrive in rice cells.

Construction of a plasmid for heterologous protein expression with a constitutive promoter in Trichoderma reesei

Trichoderma reesei is widely used as a host for homologus and heterologus protein expression because of its well-known capability of protein secretion, especially cellulases secretion. In this study, a binary vector which could be used for protein expression was constructed with a constitutive promoter of rp2 gene. This vector contained the expression cassette Prp2-target gene-Trp2 and hygromycin B selection marker based on pCAMBIA1300 for T-DNA random insertion. The feasibility of the promoter was determined by eGFP (enhanced green fluorescence protein) expression in T. reesei. For heterologus protein expression, the expression of bgla from Aspergillus niger in the transformant was 3 folds higher than that of the wild type strain. The results demonstrate that this constitutive promoter could be applied for protein expression and thus this protein express system may contribute to the industrial protein production.

The MAT locus genes play different roles in sexual reproduction and pathogenesis in Fusarium graminearum

Sexual reproduction plays a critical role in the infection cycle of Fusarium graminearum because ascospores are the primary inoculum. As a homothallic ascomycete, F. graminearum contains both the MAT1-1 and MAT1-2-1 loci in the genome. To better understand their functions and regulations in sexual reproduction and pathogenesis, in this study we assayed the expression, interactions, and mutant phenotypes of individual MAT locus genes. Whereas the expression of MAT1-1-1 and MAT12-1 rapidly increased after perithecial induction and began to decline after 1 day post-perithecial induction (dpi), the expression of MAT1-1-2 and MAT1-1-3 peaked at 4 dpi. MAT1-1-2 and MAT1-1-3 had a similar expression profile and likely are controlled by a bidirectional promoter. Although none of the MAT locus genes were essential for perithecium formation, all of them were required for ascosporogenesis in self-crosses. In outcrosses, the mat11-1-2 and mat11-1-3 mutants were fertile but the mat1-1-1 and mat1-2-1 mutants displayed male- and female-specific defects, respectively. The mat1-2-1 mutant was reduced in FgSO expression and hyphal fusion. Mat1-1-2 interacted with all other MAT locus transcription factors, suggesting that they may form a protein complex during sexual reproduction. Mat1-1-1 also interacted with FgMcm1, which may play a role in controlling cell identity and sexual development. Interestingly, the mat1-1-1 and mat1-2-1 mutants were reduced in virulence in corn stalk rot assays although none of the MAT locus genes was important for wheat infection. The MAT1-1-1 and MAT1-2-1 genes may play a host-specific role in colonization of corn stalks.

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.

Differentiated functions of Ras1 and Ras2 proteins in regulating the germination, growth, conidiation, multi-stress tolerance and virulence of Beauveria bassiana

Ras1 and Ras2 are two distinct Ras GTPases in Beauveria bassiana, an entomopathogenic fungus whose biocontrol potential against insect pests depends largely on virulence and multi-stress tolerance. The functions of both proteins were characterized for the first time by constructing dominant-active (GTP-bound) Ras1(G19V) and dominant-negative (GDP-bound) Ras1(D126A) and integrating them and normal Ras1 into wild type and ΔRas2 for a series of phenotypic and transcriptional analyses. The resultant mutants showed gradient changes of multiple phenotypes but little difference in conidial thermotolerance. Expression of Ras1(D126A) caused vigorous hyphal growth, severely defective conidiation, and increased tolerances to oxidation, cell wall disturbance, fungicide and UV-A/UV-B irradiations, but affected slightly germination, osmosensitivity and virulence. These phenotypes were antagonistically altered by mRas1(G19V) expressed in either wild type or ΔRas2, which was severely defective in conidial germination and hyphal growth and displayed intermediate changes in other mentioned phenotypes between paired mutants expressing Ras1(G19V) or Ras1(D126A) in wild type and ΔRas2. Their growth, UV tolerance or virulence was significantly correlated with cellular response to oxidation or cell wall disturbance. Transcriptional changes of 35 downstream effector genes involved in conidiation and multi-stress responses also related to most of the phenotypic changes among the mutants. Our findings highlight that Ras1 and Ras2 regulate differentially or antagonistically the germination, growth, conidiation, multi-stress tolerance and virulence of B. bassiana, thereby exerting profound effects on the fungal biocontrol potential.

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.

MoSwi6, an APSES family transcription factor, interacts with MoMps1 and is required for hyphal and conidial morphogenesis, appressorial function and pathogenicity of Magnaporthe oryzae

The Magnaporthe oryzae mitogen-activated protein kinase (MAPK) MoMps1 plays a critical role in the regulation of various developmental processes, including cell wall integrity, stress responses and pathogenicity. To identify potential effectors of MoMps1, we characterized the function of MoSwi6, a homologue of Saccharomyces cerevisiae Swi6 downstream of MAPK Slt2 signalling. MoSwi6 interacted with MoMps1 both in vivo and in vitro, suggesting a possible functional link analogous to Swi6-Slt2 in S. cerevisiae. Targeted gene disruption of MoSWI6 resulted in multiple developmental defects, including reduced hyphal growth, abnormal formation of conidia and appressoria, and impaired appressorium function. The reduction in appressorial turgor pressure also contributed to an attenuation of pathogenicity. The ΔMoswi6 mutant also displayed a defect in cell wall integrity, was hypersensitive to oxidative stress, and showed a significant reduction in transcription and activity of extracellular enzymes, including peroxidases and laccases. Collectively, these roles are similar to those of MoMps1, confirming that MoSwi6 functions in the MoMps1 pathway to govern growth, development and full pathogenicity.

Autophagy vitalizes the pathogenicity of pathogenic fungi

Plant pathogenic fungi utilize a series of complex infection structures, in particular the appressorium, to gain entry to and colonize plant tissue. As a consequence of the accumulation of huge quantities of glycerol in the cell the appressorium generates immense intracellular turgor pressure allowing the penetration peg of the appressorium to penetrate the leaf cuticle. Autophagic processes are ubiquitous in eukaryotic cells and facilitate the bulk degradation of macromolecules and organelles. The study of autophagic processes has been extended from the model yeast Saccharomyces cerevisiae to pathogenic fungi such as the rice blast fungus Magnaporthe oryzae. Significantly, null mutants for the expression of M. oryzae autophagy gene homologs lose their pathogenicity for infection of host plants. Clarification of the functions and network of interactions between the proteins expressed by M. oryzae autophagy genes will lead to a better understanding of the role of autophagy in fungal pathogenesis and help in the development of new strategies for disease control.

Mitogen-activated protein kinase signaling in plant-interacting fungi: distinct messages from conserved messengers

Mitogen-activated protein kinases (MAPKs) are evolutionarily conserved proteins that function as key signal transduction components in fungi, plants, and mammals. During interaction between phytopathogenic fungi and plants, fungal MAPKs help to promote mechanical and/or enzymatic penetration of host tissues, while plant MAPKs are required for activation of plant immunity. However, new insights suggest that MAPK cascades in both organisms do not operate independently but that they mutually contribute to a highly interconnected molecular dialogue between the plant and the fungus. As a result, some pathogenesis-related processes controlled by fungal MAPKs lead to the activation of plant signaling, including the recruitment of plant MAPK cascades. Conversely, plant MAPKs promote defense mechanisms that threaten the survival of fungal cells, leading to a stress response mediated in part by fungal MAPK cascades. In this review, we make use of the genomic data available following completion of whole-genome sequencing projects to analyze the structure of MAPK protein families in 24 fungal taxa, including both plant pathogens and mycorrhizal symbionts. Based on conserved patterns of sequence diversification, we also propose the adoption of a unified fungal MAPK nomenclature derived from that established for the model species Saccharomyces cerevisiae. Finally, we summarize current knowledge of the functions of MAPK cascades in phytopathogenic fungi and highlight the central role played by MAPK signaling during the molecular dialogue between plants and invading fungal pathogens.

StRas2 regulates morphogenesis, conidiation and appressorium development in Setosphaeria turcica

The proteins of Ras family are a large group of monomeric GTPases and act as molecular switches transducing extracellular signals into the cell in higher eukaryotes. However, little is known about roles of Ras family in the foliar pathogens. In this research, we cloned the gene named StRas2 encoding Ras in Setosphaeria turcica and investigated its function by RNA interference technology. We found that the growth rate of RNAi transformants named as R1, R2, R3, R4, R5 and R6, in which the StRas2 silencing efficiency fell in turn. With the highest silencing efficiency, the transformant R1 showed anomalistic hyphae morphology, indicating its growth was significantly affected. The transformants with a middle-silencing efficiency, such as R3, R4, displayed a delay when forming appressoria and invasive hyphae. R1 could not form conidia and appressoria. However, the conidial formation in R5 and R6 was significantly reduced, and these two transformants could form appressoria and penetrate the artificial cellophane, only that its invasive hyphae were fascicular and rarely branched. The HT-toxin biological activity of all transformants showed no difference. All results suggested that StRas2 is involved in the morphogenesis, conidiation, and appressorium development and is not related to the biosynthesis of HT-toxin.

Different chitin synthase genes are required for various developmental and plant infection processes in the rice blast fungus Magnaporthe oryzae

Chitin is a major component of fungal cell wall and is synthesized by chitin synthases (Chs). Plant pathogenic fungi normally have multiple chitin synthase genes. To determine their roles in development and pathogenesis, we functionally characterized all seven CHS genes in Magnaporthe oryzae. Three of them, CHS1, CHS6, and CHS7, were found to be important for plant infection. While the chs6 mutant was non-pathogenic, the chs1 and chs7 mutants were significantly reduced in virulence. CHS1 plays a specific role in conidiogenesis, an essential step for natural infection cycle. Most of chs1 conidia had no septum and spore tip mucilage. The chs6 mutant was reduced in hyphal growth and conidiation. It failed to penetrate and grow invasively in plant cells. The two MMD-containing chitin synthase genes, CHS5 and CHS6, have a similar expression pattern. Although deletion of CHS5 had no detectable phenotype, the chs5 chs6 double mutant had more severe defects than the chs6 mutant, indicating that they may have overlapping functions in maintaining polarized growth in vegetative and invasive hyphae. Unlike the other CHS genes, CHS7 has a unique function in appressorium formation. Although it was blocked in appressorium formation by germ tubes on artificial hydrophobic surfaces, the chs7 mutant still produced melanized appressoria by hyphal tips or on plant surfaces, indicating that chitin synthase genes have distinct impacts on appressorium formation by hyphal tip and germ tube. The chs7 mutant also was defective in appressorium penetration and invasive growth. Overall, our results indicate that individual CHS genes play diverse roles in hyphal growth, conidiogenesis, appressorium development, and pathogenesis in M. oryzae, and provided potential new leads in the control of this devastating pathogen by targeting specific chitin synthases.

Functional analysis of the kinome of the wheat scab fungus Fusarium graminearum

As in other eukaryotes, protein kinases play major regulatory roles in filamentous fungi. Although the genomes of many plant pathogenic fungi have been sequenced, systematic characterization of their kinomes has not been reported. The wheat scab fungus Fusarium graminearum has 116 protein kinases (PK) genes. Although twenty of them appeared to be essential, we generated deletion mutants for the other 96 PK genes, including 12 orthologs of essential genes in yeast. All of the PK mutants were assayed for changes in 17 phenotypes, including growth, conidiation, pathogenesis, stress responses, and sexual reproduction. Overall, deletion of 64 PK genes resulted in at least one of the phenotypes examined, including three mutants blocked in conidiation and five mutants with increased tolerance to hyperosmotic stress. In total, 42 PK mutants were significantly reduced in virulence or non-pathogenic, including mutants deleted of key components of the cAMP signaling and three MAPK pathways. A number of these PK genes, including Fg03146 and Fg04770 that are unique to filamentous fungi, are dispensable for hyphal growth and likely encode novel fungal virulence factors. Ascospores play a critical role in the initiation of wheat scab. Twenty-six PK mutants were blocked in perithecia formation or aborted in ascosporogenesis. Additional 19 mutants were defective in ascospore release or morphology. Interestingly, F. graminearum contains two aurora kinase genes with distinct functions, which has not been reported in fungi. In addition, we used the interlog approach to predict the PK-PK and PK-protein interaction networks of F. graminearum. Several predicted interactions were verified with yeast two-hybrid or co-immunoprecipitation assays. To our knowledge, this is the first functional characterization of the kinome in plant pathogenic fungi. Protein kinase genes important for various aspects of growth, developmental, and infection processes in F. graminearum were identified in this study.

Fusarium head blight caused by Fusarium graminearum is one of the most important diseases on wheat and barley. Although protein kinases are known to play major regulatory roles in fungi, systematic characterization of fungal kinomes has not been reported in plant pathogens. In this study we generated deletion mutants for 96 protein kinase genes. All of the resulting knockout mutants were assayed for changes in 17 phenotypes, including growth, reproduction, stress responses, and plant infection. Overall, deletion of 64 kinase genes resulted in at least one of the phenotypes examined. In total, 42 kinase mutants were significantly reduced in virulence or non-pathogenic. A number of these protein kinase genes, including two that are unique to filamentous fungi, are dispensable for hyphal growth and likely encode novel fungal virulence factors. Ascospores are the primary inoculum for wheat scab. We identified 26 mutants blocked in ascospore. We also used the in silico approach to predict the kinase-kinase interactions and verified some of them by yeast two-hybrid or co-IP assays. Overall, in this study we functionally characterize the kinome of F. graminearum. Protein kinase genes that are important for various aspects of growth, developmental, and plant infection processes were identified.

Two novel transcriptional regulators are essential for infection-related morphogenesis and pathogenicity of the rice blast fungus Magnaporthe oryzae

The cyclic AMP-dependent protein kinase A signaling pathway plays a major role in regulating plant infection by the rice blast fungus Magnaporthe oryzae. Here, we report the identification of two novel genes, MoSOM1 and MoCDTF1, which were discovered in an insertional mutagenesis screen for non-pathogenic mutants of M. oryzae. MoSOM1 or MoCDTF1 are both necessary for development of spores and appressoria by M. oryzae and play roles in cell wall differentiation, regulating melanin pigmentation and cell surface hydrophobicity during spore formation. MoSom1 strongly interacts with MoStu1 (Mstu1), an APSES transcription factor protein, and with MoCdtf1, while also interacting more weakly with the catalytic subunit of protein kinase A (CpkA) in yeast two hybrid assays. Furthermore, the expression levels of MoSOM1 and MoCDTF1 were significantly reduced in both Δmac1 and ΔcpkA mutants, consistent with regulation by the cAMP/PKA signaling pathway. MoSom1-GFP and MoCdtf1-GFP fusion proteins localized to the nucleus of fungal cells. Site-directed mutagenesis confirmed that nuclear localization signal sequences in MoSom1 and MoCdtf1 are essential for their sub-cellular localization and biological functions. Transcriptional profiling revealed major changes in gene expression associated with loss of MoSOM1 during infection-related development. We conclude that MoSom1 and MoCdtf1 functions downstream of the cAMP/PKA signaling pathway and are novel transcriptional regulators associated with cellular differentiation during plant infection by the rice blast fungus.

Magnaporthe oryzae, the causal agent of rice blast disease, is an important model fungal pathogen for understanding the molecular basis of plant-fungus interactions. In M. oryzae, the conserved cAMP/PKA signaling pathway has been demonstrated to be crucial for regulating infection-related morphogenesis and pathogenicity, including the control of sporulation and appressorium formation. In this study, we report the identification of two novel pathogenicity-related genes, MoSOM1 and MoCDTF1, by T-DNA insertional mutagenesis. Our results show that MoSOM1 or MoCDTF1 are essential for sporulation, appressorium formatiom and pathogenicity, and also play a key role in hyphal growth, melanin pigmentation and cell surface hydrophobicity. Nuclear localization sequences and conserved domains of the MoSom1 and MoCdtf1 proteins are crucial for their biological function. MoSom1 interacts physically with the transcription factors MoCdtf1 and MoStu1. We also show evidence that MoSom1 has the capacity to interact with CpkA, suggesting that MoSom1 may act downstream of the cAMP/PKA signaling pathway to regulate infection-related morphogenesis and pathogenicity in M. oryzae. Our studies extend the current understanding of downstream components of the conserved cAMP/PKA pathway and its precise role in regulating infection-related development and cellular differentiation by M. oryzae.

Roles of Ras1 membrane localization during Candida albicans hyphal growth and farnesol response

Many Ras GTPases localize to membranes via C-terminal farnesylation and palmitoylation, and localization regulates function. In Candida albicans, a fungal pathogen of humans, Ras1 links environmental cues to morphogenesis. Here, we report the localization and membrane dynamics of Ras1, and we characterize the roles of conserved C-terminal cysteine residues, C287 and C288, which are predicted sites of palmitoylation and farnesylation, respectively. GFP-Ras1 is localized uniformly to plasma membranes in both yeast and hyphae, yet Ras1 plasma membrane mobility was reduced in hyphae compared to that in yeast. Ras1-C288S was mislocalized to the cytoplasm and could not support hyphal development. Ras1-C287S was present primarily on endomembranes, and strains expressing ras1-C287S were delayed or defective in hyphal induction depending on the medium used. Cells bearing constitutively activated Ras1-C287S or Ras1-C288S, due to a G13V substitution, showed increased filamentation, suggesting that lipid modifications are differentially important for Ras1 activation and effector interactions. The C. albicans autoregulatory molecule, farnesol, inhibits Ras1 signaling through adenylate cyclase and bears structural similarities to the farnesyl molecule that modifies Ras1. At lower concentrations of farnesol, hyphal growth was inhibited but Ras1 plasma membrane association was not altered; higher concentrations of farnesol led to mislocalization of Ras1 and another G protein, Rac1. Furthermore, farnesol inhibited hyphal growth mediated by cytosolic Ras1-C288SG13V, suggesting that farnesol does not act through mechanisms that depend on Ras1 farnesylation. Our findings imply that Ras1 is farnesylated and palmitoylated, and that the Ras1 stimulation of adenylate cyclase-dependent phenotypes can occur in the absence of these lipid modifications.

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.

MoSfl1 is important for virulence and heat tolerance in Magnaporthe oryzae

The formation of appressoria, specialized plant penetration structures of Magnaporthe oryzae, is regulated by the MST11-MST7-PMK1 MAP kinase cascade. One of its downstream transcription factor, MST12, is important for penetration and invasive growth but dispensable for appressorium formation. To identify additional downstream targets that are regulated by Pmk1, in this study we performed phosphorylation assays with a protein microarray composed of 573 M. oryzae transcription factor (TF) genes. Three of the TF genes phosphorylated by Pmk1 in vitro were further analyzed by coimmunoprecipitation assays. One of them, MoSFL1, was found to interact with Pmk1 in vivo. Like other Sfl1 orthologs, the MoSfl1 protein has the HSF-like domain. When expressed in yeast, MoSFL1 functionally complemented the flocculation defects of the sfl1 mutant. In M. oryzae, deletion of MoSFl1 resulted in a significant reduction in virulence on rice and barley seedlings. Consistent with this observation, the Mosfl1 mutant was defective in invasive growth in penetration assays with rice leaf sheaths. In comparison with that of vegetative hyphae, the expression level of MoSFL1 was increased in appressoria and infected rice leaves. The Mosfl1 mutant also had increased sensitivity to elevated temperatures. In CM cultures of the Mosfl1 and pmk1 mutants grown at 30°C, the production of aerial hyphae and melanization were reduced but their growth rate was not altered. When assayed by qRT-PCR, the transcription levels of the MoHSP30 and MoHSP98 genes were reduced 10- and 3-fold, respectively, in the Mosfl1 mutant. SFL1 orthologs are conserved in filamentous ascomycetes but none of them have been functionally characterized in non-Saccharomycetales fungi. MoSfl1 has one putative MAPK docking site and three putative MAPK phosphorylation sites. Therefore, it may be functionally related to Pmk1 in the regulation of invasive growth and stress responses in M. oryzae.

A MADS-box transcription factor MoMcm1 is required for male fertility, microconidium production and virulence in Magnaporthe oryzae

Appressorium formation is a key step in the infection cycle of Magnaporthe oryzae. Mst12 is a transcription factor essential for appressorium penetration and invasive growth. In this study we used the affinity purification approach to identify proteins that physically associate with Mst12. One of the Mst12-interacting genes identified was MoMCM1, which encodes a MADS-box protein orthologous to yeast Mcm1. MoMcm1 interacted with both Mst12 and Mata-1 in yeast two-hybrid assays. Deletion of MoMCM1 resulted in the loss of male fertility and microconidium production. The Momcm1 mutant was defective in appressorium penetration and formed narrower invasive hyphae, which may be responsible for its reduced virulence. In transformants expressing MoMCM1-eGFP fusion, GFP signals were observed in the nucleus. We also generated the Momcm1 mst12 double mutant, which was defective in penetration and non-pathogenic. On hydrophilic surfaces, germ tubes produced by the double mutant were severely curved, and 20% of them formed appressoria. In contrast, the Momcm1 or mst12 mutant did not form appressoria on hydrophilic surfaces. These results suggest that MoMCM1 and MST12 have overlapping functions to suppress appressorium formation under non-conducive conditions. MoMcm1 may interact with Mst12 and MatA-1 to regulate germ tube identity and male fertility respectively.

Multiple plant surface signals are sensed by different mechanisms in the rice blast fungus for appressorium formation

Surface recognition and penetration are among the most critical plant infection processes in foliar pathogens. In Magnaporthe oryzae, the Pmk1 MAP kinase regulates appressorium formation and penetration. Its orthologs also are known to be required for various plant infection processes in other phytopathogenic fungi. Although a number of upstream components of this important pathway have been characterized, the upstream sensors for surface signals have not been well characterized. Pmk1 is orthologous to Kss1 in yeast that functions downstream from Msb2 and Sho1 for filamentous growth. Because of the conserved nature of the Pmk1 and Kss1 pathways and reduced expression of MoMSB2 in the pmk1 mutant, in this study we functionally characterized the MoMSB2 and MoSHO1 genes. Whereas the Momsb2 mutant was significantly reduced in appressorium formation and virulence, the Mosho1 mutant was only slightly reduced. The Mosho1 Momsb2 double mutant rarely formed appressoria on artificial hydrophobic surfaces, had a reduced Pmk1 phosphorylation level, and was nonresponsive to cutin monomers. However, it still formed appressoria and caused rare, restricted lesions on rice leaves. On artificial hydrophilic surfaces, leaf surface waxes and primary alcohols-but not paraffin waxes and alkanes- stimulated appressorium formation in the Mosho1 Momsb2 mutant, but more efficiently in the Momsb2 mutant. Furthermore, expression of a dominant active MST7 allele partially suppressed the defects of the Momsb2 mutant. These results indicate that, besides surface hydrophobicity and cutin monomers, primary alcohols, a major component of epicuticular leaf waxes in grasses, are recognized by M. oryzae as signals for appressorium formation. Our data also suggest that MoMsb2 and MoSho1 may have overlapping functions in recognizing various surface signals for Pmk1 activation and appressorium formation. While MoMsb2 is critical for sensing surface hydrophobicity and cutin monomers, MoSho1 may play a more important role in recognizing rice leaf waxes.

The rice blast fungus is a major pathogen of rice and a model for studying fungal-plant interactions. Like many other fungal pathogens, it can recognize physical and chemical signals present on the rice leaf surface and form a highly specialized infection structure known as appressorium. A well conserved signal transduction pathway involving the protein kinase gene PMK1 is known to regulate appressorium formation and plant penetration in this pathogen. However, it is not clear about the sensor genes that are involved in recognizing various plant surface signals. In this study we functionally characterize two putative sensor genes called MoMSB2 and MoSHO1. Genetic and biochemical analyses indicated that these two genes have overlapping functions in recognizing different physical and chemical signals present on the rice leaf surface for the activation of the Pmk1 pathway and appressorium formation. We found that primary alcohols, a major component of leaf waxes in grasses, can be recognized by the rice blast fungus as chemical cues. While MoMSB2 is critical for sensing hydrophobicity and precursors of cutin molecules of rice leaves, MoSHO1 appears to be more important than MoMSB2 for recognizing wax components.

A novel transcriptional factor important for pathogenesis and ascosporogenesis in Fusarium graminearum

Fusarium head blight or scab caused by Fusarium graminearum is an important disease of wheat and barley. The pathogen not only causes severe yield losses but also contaminates infested grains with mycotoxins. In a previous study, we identified several pathogenicity mutants by random insertional mutagenesis. One of these mutants was disrupted in the ZIF1 gene, which encodes a b-ZIP transcription factor unique to filamentous ascomycetes. The Δzif1 mutant generated by gene replacement was significantly reduced in deoxynivalenol (DON) production and virulence on flowering wheat heads. It was defective in spreading from inoculated florets to the rachis and other spikelets. Deletion of the ZIF1 ortholog MoZIF1 in the rice blast fungus also caused reductions in virulence and in invasive growth. In addition, the Δzif1 mutant is defective in sexual reproduction. Although it had normal male fertility, when selfed or mated as the female in outcrosess, the Δzif1 mutant produced small, pigmented perithecia that were sterile (lack of asci and ascospores), suggesting a female-specific role for ZIF1 during fertilization or ascus development. Similar female-specific defects in sexual reproduction were observed in the ΔMozif1 mutant. When mated as the female, the ΔMozif1 perithecia failed to develop long necks and asci or ascospores. The ZIF1 gene is well conserved in filamentous ascomycetes, particularly in the b-ZIP domain, which is essential for its function. Expression of ZIF1 in Magnaporthe oryzae complemented the defects of the ΔMozif1 mutant. These results indicate that this b-ZIP transcription factor is functionally conserved in these two fungal pathogens for plant infection and sexual reproduction.

Characterization of MoLDB1 required for vegetative growth, infection-related morphogenesis, and pathogenicity in the rice blast fungus Magnaporthe oryzae

An insertional mutagenesis screen in the rice blast fungus, Magnaporthe oryzae, identified a novel mutant, A2-12-3, which is defective in infection-related morphogenesis and pathogenicity. Analysis of the mutation confirmed an insertion into MoLDB1, which putatively encodes an 806-amino-acid protein with a predicted LIM binding domain. Targeted gene deletion mutants of MoLDB1 were unable to produce asexual or sexual spores and were significantly impaired in vegetative growth and fungal virulence. The Δmoldb1 mutants also showed reduced expression of genes coding hydrophobic proteins (e.g. MPG1 and MHP1), resulting in an easily wettable phenotype in vegetative culture. Moreover, the expression of four genes encoding LIM proteins predicted from the M. oryzae genome was significantly downregulated by deletion of MoLDB1. Analysis of an M. oryzae strain expressing a MoLbd1-green fluorescent protein gene fusion was consistent with the protein being nuclear localized. When considered together, MoLdb1 appears to be involved in regulation of cell wall proteins, including hydrophobins and LIM proteins, and is essential for conidiation, sexual development, appressorium formation, and pathogenicity in M. oryzae.