Magnaporthe as a model for understanding host-pathogen interactions

Two phosphodiesterase genes, PDEL and PDEH, regulate development and pathogenicity by modulating intracellular cyclic AMP levels in Magnaporthe oryzae

Cyclic AMP (cAMP) signaling plays an important role in regulating multiple cellular responses, such as growth, morphogenesis, and/or pathogenicity of eukaryotic organisms such as fungi. As a second messenger, cAMP is important in the activation of downstream effector molecules. The balance of intracellular cAMP levels depends on biosynthesis by adenylyl cyclases (ACs) and hydrolysis by cAMP phosphodiesterases (PDEases). The rice blast fungus Magnaporthe oryzae contains a high-affinity (PdeH/Pde2) and a low-affinity (PdeL/Pde1) PDEases, and a previous study showed that PdeH has a major role in asexual differentiation and pathogenicity. Here, we show that PdeL is required for asexual development and conidial morphology, and it also plays a minor role in regulating cAMP signaling. This is in contrast to PdeH whose mutation resulted in major defects in conidial morphology, cell wall integrity, and surface hydrophobicity, as well as a significant reduction in pathogenicity. Consistent with both PdeH and PdeL functioning in cAMP signaling, disruption of PDEH only partially rescued the mutant phenotype of ΔmagB and Δpka1. Further studies suggest that PdeH might function through a feedback mechanism to regulate the expression of pathogenicity factor Mpg1 during surface hydrophobicity and pathogenic development. Moreover, microarray data revealed new insights into the underlying cAMP regulatory mechanisms that may help to identify potential pathogenicity factors for the development of new disease management strategies.

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

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

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

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.

MoVam7, a conserved SNARE involved in vacuole assembly, is required for growth, endocytosis, ROS accumulation, and pathogenesis of Magnaporthe oryzae

Soluble NSF attachment protein receptor (SNARE) proteins play a central role in membrane fusion and vesicle transport of eukaryotic organisms including fungi. We previously identified MoSce22 as a homolog of Saccharomyces cerevisiae SNARE protein Sec22 to be involved in growth, stress resistance, and pathogenicity of Magnaporthe oryzae. Here, we provide evidences that MoVam7, an ortholog of S. cerevisiae SNARE protein Vam7, exerts conserved functions in vacuolar morphogenesis and functions in pathogenicity of M. oryzae. Staining with neutral red and FM4-64 revealed the presence of abnormal fragmented vacuoles and an absence of the Spitzenkörper body in the ΔMovam7 mutant. The ΔMovam7 mutant also exhibited reduced vegetative growth, poor conidiation, and failure to produce the infection structure appressorium. Additionally, treatments with cell wall perturbing agents indicated weakened cell walls and altered distributions of the cell wall component chitin. Furthermore, the ΔMovam7 mutant showed a reduced accumulation of reactive oxygen species (ROS) in the hyphal apex and failed to cause diseases on the rice plant. In summary, our studies indicate that MoVam7, like MoSec22, is a component of the SNARE complex whose functions in vacuole assembly also underlies the growth, conidiation, appressorium formation, and pathogenicity of M. oryzae. Further studies of MoVam7, MoSec22, and additional members of the SNARE complex are likely to reveal critical mechanisms in vacuole formation and membrane trafficking that is linked to fungal pathogenicity.

Differential roles of the phospholipase C genes in fungal development and pathogenicity of Magnaporthe oryzae

Calcium plays a critical role in a variety of cellular processes in cells. However, relatively little is known about the biological effects of Ca²+ signaling on morphogenesis and pathogenesis in the rice blast fungus Magnaporthe oryzae compared to other signaling pathways. We have previously demonstrated that MoPLC1-mediated calcium regulation is important for infection-related development and pathogenicity in M. oryzae. In the present study, four genes encoding phospholipase C (PLC) isozymes (MoPLC2 to MoPLC5), which differ from MoPLC1 in their domain organization, were additionally identified. The C2 domain involved in Ca²+-dependent membrane binding is found only in MoPLC2 and MoPLC3. Detailed functional analysis using deletion mutants for MoPLC2 and MoPLC3 indicated that MoPLC2 and MoPLC3 play essential roles in development. The two deletion mutants for MoPLC2 and MoPLC3 showed reduced conidiation and a defect in appressorium-mediated penetration. Reintroduction of the genes restored defects of ΔMoplc2 and ΔMoplc3. Notably, ΔMoplc2 and ΔMoplc3 mutants developed multiple appressoria on separate germ tubes of a conidium, indicating that MoPLC2- and MoPLC3-regulated signaling suppresses a feedback loop of a pathway for appressorial development. The similarity in phenotypic defects between the two mutants indicates that both MoPLC2 and MoPLC3 are important for regulation of appropriate levels of signaling molecules in a similar manner. Comparative analysis indicated that the two MoPLCs-mediated signaling pathways have interrelated, but distinct, roles in the development of M. oryzae.

Efficient approaches for generating GFP fusion and epitope-tagging constructs in filamentous fungi

For functional characterization of predicted genes encoding hypothetical proteins in fungal genomes, it is complementary to genetic studies to determine their expression and subcellular localization patterns in different developmental or infection stages. It is also important to identify and characterize other proteins that are physically associated with or functionally related to these genes in vivo by co-immunoprecipitation or affinity purification analyses. In this chapter, we described a set of yeast shuttle vectors and protocols to generate fusion constructs by the yeast gap repair approach. Because of the simplicity and efficiency of yeast gap repair, these vectors and the general methods described in this chapter are suitable for functional genomics studies in filamentous fungi.

The isolation and characterization of Pik, a rice blast resistance gene which emerged after rice domestication

• The rice-rice blast pathosystem is of great interest, not only because of the damaging potential of rice blast to the rice crop, but also because both the pathogen and its host are experimentally amenable. The rice blast resistance gene Pik, which is one of the five classical alleles located at the Pik locus on the long arm of chromosome 11, confers high and stable resistance to many Chinese rice blast isolates. • The isolation and functional characterization of Pik were performed in the present study through genetic and genomic approaches. • A combination of Pik-1 and Pik-2 is required for the expression of Pik resistance. Both Pik-1 and Pik-2 encode coiled-coil nucleotide binding site leucine-rich repeat (NBS-LRR) proteins, and each shares a very high level of protein identity with corresponding proteins encoded by the Pik-m and Pik-p alleles. Pik could be distinguished from other Pik alleles, including Pik-m and Pik-p, by the allele-specific, single-nucleotide polymorphism T1-2944G. • The coupled genes probably did not evolve as a result of a duplication event, and are far from any NBS-LRR R gene characterized. Pik is a younger allele at the locus that probably emerged after rice domestication.

R-SNARE homolog MoSec22 is required for conidiogenesis, cell wall integrity, and pathogenesis of Magnaporthe oryzae

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins mediate intracellular vesicle fusion, which is an essential cellular process of the eukaryotic cells. To investigate the role of SNARE proteins in the rice blast fungus Magnaporthe oryzae, MoSec22, an ortholog of Saccharomyces cerevisiae SNARE protein Sec22, was identified and the MoSEC22 gene disrupted. MoSec22 restored a S. cerevisiae sec22 mutant in resistance to cell wall perturbing agents, and the ΔMosec22 mutant also exhibited defects in mycelial growth, conidial production, and infection of the host plant. Treatment with oxidative stress inducers indicated a breach in cell wall integrity, and staining and quantification assays suggested abnormal chitin deposition on the lateral walls of hyphae of the ΔMosec22 mutant. Furthermore, hypersensitivity to the oxidative stress correlates with the reduced expression of the extracellular enzymes peroxidases and laccases. Our study thus provides new evidence on the conserved function of Sec22 among fungal organisms and indicates that MoSec22 has a role in maintaining cell wall integrity affecting the growth, morphogenesis, and virulence of M. oryzae.

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.

Fungal secondary metabolites as modulators of interactions with insects and other arthropods

Fungi share a diverse co-evolutionary history with animals, especially arthropods. In this review, we focus on the role of secondary metabolism in driving antagonistic arthropod-fungus interactions, i.e., where fungi serve as a food source to fungal grazers, compete with saprophagous insects, and attack insects as hosts for growth and reproduction. Although a wealth of studies on animal-fungus interactions point to a crucial role of secondary metabolites in deterring animal feeding and resisting immune defense strategies, causal evidence often remains to be provided. Moreover, it still remains an unresolved puzzle as to what extent the tight regulatory control of secondary metabolite formation in some model fungi represents an evolved chemical defense system favored by selective pressure through animal antagonists. Given these gaps in knowledge, we highlight some co-evolutionary aspects of secondary metabolism, such as induced response, volatile signaling, and experimental evolution, which may help in deciphering the ecological importance and evolutionary history of secondary metabolite production in fungi.

The tig1 histone deacetylase complex regulates infectious growth in the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae is the most damaging fungal pathogen of rice (Oryza sativa). In this study, we characterized the TIG1 transducin beta-like gene required for infectious growth and its interacting genes that are required for plant infection in this model phytopathogenic fungus. Tig1 homologs in yeast and mammalian cells are part of a conserved histone deacetylase (HDAC) transcriptional corepressor complex. The tig1 deletion mutant was nonpathogenic and defective in conidiogenesis. It had an increased sensitivity to oxidative stress and failed to develop invasive hyphae in plant cells. Using affinity purification and coimmunoprecipitation assays, we identified several Tig1-associated proteins, including two HDACs that are homologous to components of the yeast Set3 complex. Functional analyses revealed that TIG1, SET3, SNT1, and HOS2 were core components of the Tig1 complex in M. oryzae. The set3, snt1, and hos2 deletion mutants displayed similar defects as those observed in the tig1 mutant, but deletion of HST1 or HOS4 had no detectable phenotypes. Deletion of any of these core components of the Tig1 complex resulted in a significant reduction in HDAC activities. Our results showed that TIG1, like its putative yeast and mammalian orthologs, is one component of a conserved HDAC complex that is required for infectious growth and conidiogenesis in M. oryzae and highlighted that chromatin modification is an essential regulatory mechanism during plant infection.

Combining ChIP-chip and expression profiling to model the MoCRZ1 mediated circuit for Ca/calcineurin signaling in the rice blast fungus

Significant progress has been made in defining the central signaling networks in many organisms, but collectively we know little about the downstream targets of these networks and the genes they regulate. To reconstruct the regulatory circuit of calcineurin signal transduction via MoCRZ1, a Magnaporthe oryzae C2H2 transcription factor activated by calcineurin dephosphorylation, we used a combined approach of chromatin immunoprecipitation - chip (ChIP-chip), coupled with microarray expression studies. One hundred forty genes were identified as being both a direct target of MoCRZ1 and having expression concurrently differentially regulated in a calcium/calcineurin/MoCRZ1 dependent manner. Highly represented were genes involved in calcium signaling, small molecule transport, ion homeostasis, cell wall synthesis/maintenance, and fungal virulence. Of particular note, genes involved in vesicle mediated secretion necessary for establishing host associations, were also found. MoCRZ1 itself was a target, suggesting a previously unreported autoregulation control point. The data also implicated a previously unreported feedback regulation mechanism of calcineurin activity. We propose that calcium/calcineurin regulated signal transduction circuits controlling development and pathogenicity manifest through multiple layers of regulation. We present results from the ChIP-chip and expression analysis along with a refined model of calcium/calcineurin signaling in this important plant pathogen.

All organisms have the innate ability to perceive their environment and respond to it, largely through controlling gene expression. Tailored specificity of a response is primarily achieved through signal cascades involving unique receptors, downstream transcription factors (proteins that bind to DNA to regulate gene expression), and the genes these transcription factors regulate. For fungal plant pathogens, signal transduction cascades are involved in perception of hosts, transgression of physical barriers, suppression or elicitation of host defenses, in vivo nutrient acquisition, and completion of their life cycle. We know that the Ca²⁺/calcineurin signaling pathway is a central conduit regulating these aspects of the life cycle for fungal pathogens of plants and animals. In this study, we used advanced ChIP-chip and microarray gene expression technologies to identify the genes that the Ca²⁺/calcineurin responsive transcription factor MoCRZ1 directly binds to and regulates the expression of. Our findings show conservations and divergence in this pathway within the fungal kingdom. It also identifies points of control in the pathway that were previously unidentified. Most importantly, this study implicates this pathway in the establishment of host associations and virulence for the causal agent of rice blast disease, Magnaporthe oryzae, the most important disease of rice worldwide.

A homeobox gene is essential for conidiogenesis of the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae starts its infection by the attachment of pyriform conidia on rice tissues, and severity of the disease epidemic is proportional to the quantity of conidia produced in the rice blast lesions. However, the mechanism of conidial production is not well understood. Homeodomain proteins play critical roles in regulating various growth and developmental processes in fungi and other eukaryotes. Through targeted gene replacement, we find that deletion of HTF1, one of seven homeobox genes in the fungal genome, does not affect mycelial growth but causes total defect of conidial production. Further observation revealed that the Deltahtf1 mutant produces significantly more conidiophores, which curve slightly near the tip but could not develop sterigmata-like structures. Although the Deltahtf1 mutant fails to form conidia, it could still develop melanized appressoria from hyphal tips and infect plants. The expression level of HTF1 is significantly reduced in the Deltamgb1 G-beta and DeltacpkA deletion mutant, and the ACR1 but not CON7 gene that encodes transcription factor required for normal conidiogenesis is significantly downregulated in the Deltahtf1 mutant. These data suggest that the HTF1 gene is essential for conidiogenesis, and may be functionally related to the trimeric G-protein signaling and other transcriptional regulators that are known to be important for conidiation in M. oryzae.

Translocation of Magnaporthe oryzae effectors into rice cells and their subsequent cell-to-cell movement

Knowledge remains limited about how fungal pathogens that colonize living plant cells translocate effector proteins inside host cells to regulate cellular processes and neutralize defense responses. To cause the globally important rice blast disease, specialized invasive hyphae (IH) invade successive living rice (Oryza sativa) cells while enclosed in host-derived extrainvasive hyphal membrane. Using live-cell imaging, we identified a highly localized structure, the biotrophic interfacial complex (BIC), which accumulates fluorescently labeled effectors secreted by IH. In each newly entered rice cell, effectors were first secreted into BICs at the tips of the initially filamentous hyphae in the cell. These tip BICs were left behind beside the first-differentiated bulbous IH cells as the fungus continued to colonize the host cell. Fluorescence recovery after photobleaching experiments showed that the effector protein PWL2 (for prevents pathogenicity toward weeping lovegrass [Eragrostis curvula]) continued to accumulate in BICs after IH were growing elsewhere. PWL2 and BAS1 (for biotrophy-associated secreted protein 1), BIC-localized secreted proteins, were translocated into the rice cytoplasm. By contrast, BAS4, which uniformly outlines the IH, was not translocated into the host cytoplasm. Fluorescent PWL2 and BAS1 proteins that reached the rice cytoplasm moved into uninvaded neighbors, presumably preparing host cells before invasion. We report robust assays for elucidating the molecular mechanisms that underpin effector secretion into BICs, translocation to the rice cytoplasm, and cell-to-cell movement in rice.

MoRic8 Is a novel component of G-protein signaling during plant infection by the rice blast fungus Magnaporthe oryzae

An insertional mutagenesis screen was used to investigate the biology of plant infection by the devastating rice blast pathogen, Magnaporthe oryzae. Here, we report the identification of a new mutant, LY-130, which is defective in multiple steps during infection-related morphogenesis and pathogenicity. Analysis of the mutation confirmed an insertion into gene MoRIC8, which encodes a 480-amino-acid protein that is a putative homologue of the Ric8 regulator of GTP-binding protein (G-protein) signaling, previously described in animals. Targeted gene deletion mutants of MoRIC8 were nonpathogenic and impaired in cellular differentiation associated with sporulation, sexual development, and plant infection. MoRic8 physically interacts with the Galpha subunit MagB in yeast two-hybrid assays and appears to act upstream of the cyclic AMP response pathway that is necessary for appressorium morphogenesis. Taken together, our results indicate that MoRic8 may act as a novel regulator of the G-protein signaling during infection-related development of rice blast fungus M. oryzae.

Cell cycle-mediated regulation of plant infection by the rice blast fungus

To gain entry to plants, many pathogenic fungi develop specialized infection structures called appressoria. Here, we demonstrate that appressorium morphogenesis in the rice blast fungus Magnaporthe oryzae is tightly regulated by the cell cycle. Shortly after a fungus spore lands on the rice (Oryza sativa) leaf surface, a single round of mitosis always occurs in the germ tube. We found that initiation of infection structure development is regulated by a DNA replication-dependent checkpoint. Genetic intervention in DNA synthesis, by conditional mutation of the Never-in-Mitosis 1 gene, prevented germ tubes from developing nascent infection structures. Cellular differentiation of appressoria, however, required entry into mitosis because nimA temperature-sensitive mutants, blocked at mitotic entry, were unable to develop functional appressoria. Arresting the cell cycle after mitotic entry, by conditional inactivation of the Blocked-in-Mitosis 1 gene or expression of stabilized cyclinB-encoding alleles, did not impair appressorium differentiation, but instead prevented these cells from invading plant tissue. When considered together, these data suggest that appressorium-mediated plant infection is coordinated by three distinct cell cycle checkpoints that are necessary for establishment of plant disease.

Homeobox transcription factors are required for conidiation and appressorium development in the rice blast fungus Magnaporthe oryzae

The appropriate development of conidia and appressoria is critical in the disease cycle of many fungal pathogens, including Magnaporthe oryzae. A total of eight genes (MoHOX1 to MoHOX8) encoding putative homeobox transcription factors (TFs) were identified from the M. oryzae genome. Knockout mutants for each MoHOX gene were obtained via homology-dependent gene replacement. Two mutants, ΔMohox3 and ΔMohox5, exhibited no difference to wild-type in growth, conidiation, conidium size, conidial germination, appressorium formation, and pathogenicity. However, the ΔMohox1 showed a dramatic reduction in hyphal growth and increase in melanin pigmentation, compared to those in wild-type. ΔMohox4 and ΔMohox6 showed significantly reduced conidium size and hyphal growth, respectively. ΔMohox8 formed normal appressoria, but failed in pathogenicity, probably due to defects in the development of penetration peg and invasive growth. It is most notable that asexual reproduction was completely abolished in ΔMohox2, in which no conidia formed. ΔMohox2 was still pathogenic through hypha-driven appressoria in a manner similar to that of the wild-type. However, ΔMohox7 was unable to form appressoria either on conidial germ tubes, or at hyphal tips, being non-pathogenic. These factors indicate that M. oryzae is able to cause foliar disease via hyphal appressorium-mediated penetration, and MoHOX7 is mutually required to drive appressorium formation from hyphae and germ tubes. Transcriptional analyses suggest that the functioning of M. oryzae homeobox TFs is mediated through the regulation of gene expression and is affected by cAMP and Ca²⁺ signaling and/or MAPK pathways. The divergent roles of this gene set may help reveal how the genome and regulatory pathways evolved within the rice blast pathogen and close relatives.

Pathogens have evolved diverse strategies to cause disease. Magnaporthe oryzae is the fungal phytopathogen that causes rice blast and is considered an important model for understanding mechanisms in fungal development and pathogenicity. Asexual reproduction and infection-related development play key roles in M. oryzae disease development. The conidium of M. oryzae differentiates a specialized structure, an appressorium. The appressorium generates turgor pressure that allows penetration through the mechanical rupture of host cuticle layers. After colonizing host cells, the fungus produces massive conidia via conidiogenesis, serving as secondary propagules for the polycyclic disease. To elucidate molecular mechanisms in asexual reproduction and appressorium-mediated disease development, we identified eight homeobox transcription factors through a genome-wide in silico analysis. Characterization using deletion mutants revealed that each homeobox TF functions as a stage-specific regulator for conidial shape, hyphal growth, conidiation, appressorium development, and invasive growth during M. oryzae development. Notably, conidiation and appressorium development were entirely abolished in ΔMohox2 and ΔMohox7, respectively. This study also provides evidence that M. oryzae is able to cause rice blast by means of hypha-driven appressoria upon responses to host signaling factors. This study will aid in the understanding of regulatory networks associated with fungal development and pathogenicity.

AGB1 and PMR5 contribute to PEN2-mediated preinvasion resistance to Magnaporthe oryzae in Arabidopsis thaliana

Rice blast, caused by Magnaporthe oryzae, is a devastating disease of rice (Oryza sativa). The mechanisms involved in resistance of rice to blast have been studied extensively and the rice-M. oryzae pathosystem has become a model for plant-microbe interaction studies. However, the mechanisms involved in nonhost resistance (NHR) of other plants to rice blast are still poorly understood. Here, we investigated interactions between Arabidopsis thaliana and M. oryzae to identify the genetic basis of NHR. In A. thaliana accessions, preinvasion resistance to M. oryzae in Col-0 was stronger than that of Ler. To examine the genetic basis underlying the natural variation in the responses, we used a well-established set of recombinant inbred lines (RIL) derived from a Col x Ler cross and identified three quantitative trait loci that govern the expression of NHR in A. thaliana against M. oryzae. Among the penetration (pen) mutants, only the pen2 mutant allowed increased penetration into epidermal cells by M. oryzae. Double mutant analysis indicated that AGB1 and PMR5 contribute to PEN2-mediated preinvasion resistance to M. oryzae in A. thaliana, suggesting a complex genetic network regulating the resistance. Our results demonstrate that A. thaliana can be used to study mechanisms of NHR to M. oryzae.

Disruption of MoCMK1, encoding a putative calcium/calmodulin-dependent kinase, in Magnaporthe oryzae

Ca(2+) is a second messenger in pathways that transduce external signals and activate cellular processes in plants and animals. Ca(2+)-mediated signal transduction is involved in key pathways that contribute to a variety of fundamental physiological processes in eukaryotic cells. However, little is known about the molecular mechanisms of Ca(2+)-mediated signal transduction in filamentous fungi. In this study, the MoCMK1 gene, encoding a putative Ca(2+)/calmodulin-dependent kinase, was identified in the rice blast fungus Magnaporthe oryzae. Three MoCMK1 deletion mutants were obtained by a targeted gene replacement. Colonies of the MoCMK1 mutants had sparse aerial hyphae and fewer conidia than the wild-type strain on complete medium. Conidial germination and appressorial formation were delayed in the DeltaMocmk1 mutants. In spray inoculation tests, DeltaMocmk1 mutants exhibited a weakened ability to infect the susceptible rice cultivar CO-39, compared to the wild-type strain Guy11. These results showed that MoCMK1 plays key roles in the pathogenicity of the rice blast fungus.

MoFLP1, encoding a novel fungal fasciclin-like protein, is involved in conidiation and pathogenicity in Magnaporthe oryzae

Fasciclin family proteins have been identified as cell adhesion molecules in various organisms. In this study, a novel Magnaporthe oryzae fasciclin-like protein encoding gene, named MoFLP1, was isolated from a subtractive suppressive cDNA library and functionally analyzed. Sequence analysis showed that the MoFLP1 gene contains an open reading frame (ORF) of 1050 nucleotides encoding 349 amino acids with a calculated molecular weight of 35.85 kDa and a pI of 7.76. The deduced MoFLP1 protein contains a 17-amino acid secretion signal sequence and an 18-amino acid sequence with the characteristics of a glycosylphosphotidylinositol (GPI) anchor additional signal at its N- and C-terminuses, respectively. Potential N-glycosylation sites and domains involving cell adhesion were also identified in MoFLP1. Sequence analysis and subcellular localization by the expression of MoFLP1-GFP fusion construct in M. oryzae indicated that the MoFLP1 protein is probably localized on the vacuole membrane. Two MoFLP1 null mutants generated by targeted gene disruption exhibited marked reduction of conidiation, conidial adhesion, appressorium turgor, and pathogenicity. Our results indicate that fasciclin proteins play important roles in fungal development and pathogenicity in M. oryzae.

Interaction transcriptome analysis identifies Magnaporthe oryzae BAS1-4 as Biotrophy-associated secreted proteins in rice blast disease

Biotrophic invasive hyphae (IH) of the blast fungus Magnaporthe oryzae secrete effectors to alter host defenses and cellular processes as they successively invade living rice (Oryza sativa) cells. However, few blast effectors have been identified. Indeed, understanding fungal and rice genes contributing to biotrophic invasion has been difficult because so few plant cells have encountered IH at the earliest infection stages. We developed a robust procedure for isolating infected-rice sheath RNAs in which approximately 20% of the RNA originated from IH in first-invaded cells. We analyzed these IH RNAs relative to control mycelial RNAs using M. oryzae oligoarrays. With a 10-fold differential expression threshold, we identified known effector PWL2 and 58 candidate effectors. Four of these candidates were confirmed to be fungal biotrophy-associated secreted (BAS) proteins. Fluorescently labeled BAS proteins were secreted into rice cells in distinct patterns in compatible, but not in incompatible, interactions. BAS1 and BAS2 proteins preferentially accumulated in biotrophic interfacial complexes along with known avirulence effectors, BAS3 showed additional localization near cell wall crossing points, and BAS4 uniformly outlined growing IH. Analysis of the same infected-tissue RNAs with rice oligoarrays identified putative effector-induced rice susceptibility genes, which are highly enriched for sensor-transduction components rather than typically identified defense response genes.

Under pressure: investigating the biology of plant infection by Magnaporthe oryzae

The filamentous fungus Magnaporthe oryzae causes rice blast, the most serious disease of cultivated rice. Cellular differentiation of M. oryzae forms an infection structure called the appressorium, which generates enormous cellular turgor that is sufficient to rupture the plant cuticle. Here, we show how functional genomics approaches are providing new insight into the genetic control of plant infection by M. oryzae. We also look ahead to the key questions that need to be addressed to provide a better understanding of the molecular processes that lead to plant disease and the prospects for sustainable control of rice blast.

Common and contrasting themes in host cell-targeted effectors from bacterial, fungal, oomycete and nematode plant symbionts described using the Gene Ontology

A wide diversity of plant-associated symbionts, including microbes, produce proteins that can enter host cells, or are injected into host cells in order to modify the physiology of the host to promote colonization. These molecules, termed effectors, commonly target the host defense signaling pathways in order to suppress the defense response. Others target the gene expression machinery or trigger specific modifications to host morphology or physiology that promote the nutrition and proliferation of the symbiont. When recognized by the host's surveillance machinery, which includes cognate resistance (R) gene products, defense responses are engaged to restrict pathogen proliferation. Effectors from diverse symbionts may be delivered into plant cells via varied mechanisms, including whole organism cellular entry (viruses, some bacteria and fungi), type III and IV secretion (in bacteria), physical injection (nematodes and insects) and protein translocation signal sequences (oomycetes and fungi). This mini-review will summarize both similarities and differences in effectors and effector delivery systems found in diverse plant-associated symbionts as well as how these are described with Plant-Associated Microbe Gene Ontology (PAMGO) terms.

The ER chaperone LHS1 is involved in asexual development and rice infection by the blast fungus Magnaporthe oryzae

In planta secretion of fungal pathogen proteins, including effectors destined for the plant cell cytoplasm, is critical for disease progression. However, little is known about the endoplasmic reticulum (ER) secretion mechanisms used by these pathogens. To determine if normal ER function is crucial for fungal pathogenicity, Magnaporthe oryzae genes encoding proteins homologous to yeast Lhs1p and Kar2p, members of the heat shock protein 70 family in Saccharomyces cerevisiae, were cloned and characterized. Like their yeast counterparts, both LHS1 and KAR2 proteins localized in the ER and functioned in an unfolded protein response (UPR) similar to the yeast UPR. Mutants produced by disruption of LHS1 were viable but showed a defect in the translocation of proteins across the ER membrane and reduced activities of extracellular enzymes. The Deltalhs1 mutant was severely impaired not only in conidiation, but also in both penetration and biotrophic invasion in susceptible rice (Oryza sativa) plants. This mutant also had defects in the induction of the Pi-ta resistance gene-mediated hypersensitive response and in the accumulation of fluorescently-labeled secreted effector proteins in biotrophic interfacial complexes. Our results suggest that proper processing of secreted proteins, including effectors, by chaperones in the ER is requisite for successful disease development and for determining host-pathogen compatibility via the gene-for-gene interaction.

The prediction of protein-protein interaction networks in rice blast fungus

Background

Protein-protein interaction (PPI) maps are useful tools for investigating the cellular functions of genes. Thus far, large-scale PPI mapping projects have not been implemented for the rice blast fungus Magnaporthe grisea, which is responsible for the most severe rice disease. Inspired by recent advances in PPI prediction, we constructed a PPI map of this important fungus.

Results

Using a well-recognized interolog approach, we have predicted 11,674 interactions among 3,017 M. grisea proteins. Although the scale of the constructed map covers approximately only one-fourth of the M. grisea's proteome, it is the first PPI map for this crucial organism and will therefore provide new insights into the functional genomics of the rice blast fungus. Focusing on the network topology of proteins encoded by known pathogenicity genes, we have found that pathogenicity proteins tend to interact with higher numbers of proteins. The pathogenicity proteins and their interacting partners in the entire network were then used to construct a subnet called a pathogenicity network. These data may provide further clues for the study of these pathogenicity proteins. Finally, it has been established that secreted proteins in M. grisea interact with fewer proteins. These secreted proteins and their interacting partners were also compiled into a network of secreted proteins, which may be helpful in constructing an interactome between the rice blast fungus and rice.

Conclusion

We predicted the PPIs of M. grisea and compiled them into a database server called MPID. It is hoped that MPID will provide new hints as to the functional genomics of this fungus. MPID is available at .

A genome-wide meta-analysis of rice blast resistance genes and quantitative trait loci provides new insights into partial and complete resistance

The completion of the genome sequences of both rice and Magnaporthe oryzae has strengthened the position of rice blast disease as a model to study plant-pathogen interactions in monocotyledons. Genetic studies of blast resistance in rice were established in Japan as early as 1917. Despite such long-term study, examples of cultivars with durable resistance are rare, partly due to our limited knowledge of resistance mechanisms. A rising number of blast resistance genes and quantitative trait loci (QTL) have been genetically described, and some have been characterized during the last 20 years. Using the rice genome sequence, can we now go a step further toward a better understanding of the genetics of blast resistance by combining all these results? Is such knowledge appropriate and sufficient to improve breeding for durable resistance? A review of bibliographic references identified 85 blast resistance genes and approximately 350 QTL, which we mapped on the rice genome. These data provide a useful update on blast resistance genes as well as new insights to help formulate hypotheses about the molecular function of blast QTL, with special emphasis on QTL for partial resistance. All these data are available from the OrygenesDB database.

MoSNF1 regulates sporulation and pathogenicity in the rice blast fungus Magnaporthe oryzae

The protein kinase Snf1 is a major component of the glucose derepression pathway in yeast and a regulator of gene expression for the cell wall degrading enzyme (CWDE) in some plant pathogenic fungi. To address the molecular function of Snf1 in Magnaporthe oryzae, which causes the rice blast disease, MoSNF1 was cloned and functionally characterized using gene knock-out strategies. MoSNF1 functionally complemented the growth defect of the yeast snf1 mutant on a non-fermenting carbon source. However, the growth rate of the Deltamosnf1 mutant on various carbon sources was reduced independent of glucose, and the expression of the CWDE genes in the mutant was induced during derepressing condition like the wild type. The pre-penetration stage including conidial germination and appressorium formation of the Deltamosnf1 was largely impaired, and the pathogenicity of the Deltamosnf1 was significantly reduced. Most strikingly, the Deltamosnf1 mutant produced only a few conidia and had a high frequency of abnormally shaped conidia compared to the wild type. Our results suggest that MoSNF1 is a functional homolog of yeast Snf1, but its contribution to sporulation, vegetative growth and pathogenicity is critical in M. oryzae.

MADS-box transcription factor mig1 is required for infectious growth in Magnaporthe grisea

Magnaporthe grisea is a model fungus for studying fungus-plant interactions. Two mitogen-activated protein (MAP) kinase genes, PMK1 and MPS1, have been implicated in regulating plant infection processes in M. grisea. However, transcription factors activated by these MAP kinases are not well studied. In this study we functionally characterized the MIG1 gene that encodes a MADS-box transcription factor homologous to Saccharomyces cerevisiae Rlm1. In yeast two-hybrid assays, MIG1 interacts with MPS1, suggesting that MIG1 may function downstream from the MPS1 pathway. The mig1 deletion mutant had a normal growth rate and formed melanized appressoria, but it was nonpathogenic and failed to infect rice leaves through wounds. Appressoria formed by the mig1 mutant developed penetration pegs and primary infectious hyphae, but further differentiation of the secondary infectious hyphae inside live plant cells was blocked. However, the mig1 mutant formed infectious hypha-like structures in heat-killed plant cells or cellophane membranes. In transformants expressing the MIG1-GFP fusion, green fluorescent protein (GFP) signals were not detectable in vegetative hyphae and conidiophores. Mig1-GFP was localized to nuclei in conidia, appressoria, and infectious hyphae. Deletion of the MADS box had no effect on the expression and localization of the MIG1-GFP fusion but eliminated its ability to complement the mig1 mutant. These results suggest that MIG1 may be required for overcoming plant defense responses and the differentiation of secondary infectious hyphae in live plant cells. The MADS-box domain is essential for the function of MIG1 but dispensable for its nuclear localization, which may be associated with the activation of MIG1 by MPS1 during conidiation and plant infection.

Biochemical and molecular characterization of a putative endoglucanase in Magnaporthe grisea

Microbial pathogens secrete an array of cell wall-degrading enzymes to break down the structure of the host cell wall to facilitate colonization of the host tissue. To better understand their role in the pathogenesis, a putative endoglucanase from Magnaporthe grisea was characterized in this paper. SignalP-3.0 analysis indicates that the protein encoded by gene MGG_02532.5 in M. grisea (named MgEGL1 for M. grisea endoglucanase 1) contains a secretory signal peptide. Multiple alignment shows that MgEGL1 has high level of homology to endoglucanases (EC 3.1.1.4) from Aspergillus nidulans and Trichoderma reesei. The three proteins share a conserved catalytic domain, but only the one from T. reesei contains a cellulose binding module. MgEGL1 was constitutively expressed with the highest level in mycelia and the lowest in conidia. Interestingly, the MgEGL1 RNA could be alternatively processed when cultured in vitro and after infection of rice. Expression analysis confirmed that the MgEGL1 is a secreted protein. Its endoglucanase activity was assayed by Congo red plates, and further confirmed by the dinitrosalicylic acid method. The finding in this paper will provide the basis for further determination of the biochemical properties of the endoglucanase protein and its relevant function in fungal pathogenesis.

Characterization of the model system rice--Magnaporthe for the study of nonhost resistance in cereals

The best characterized form of resistance is gene-for-gene resistance. Less well characterized is nonhost resistance in which an entire plant species is resistant to an entire pathogen species. Here, different rice genotypes were inoculated with host and nonhost strains of Magnaporthe isolated from rice, wheat and crabgrass. The different types of interactions were characterized at a cytological level using a 3,3'-diaminobenzidine (DAB) stain to investigate the occurrence of reactive oxygen intermediates or by observing the occurrence of cellular autofluorescence. Gene expression of a set of selected PR-genes was analysed using quantitative real-time polymerase chain reaction. Inoculation with the isolate from crabgrass resulted in a lack of penetration. The wheat isolate induced a hypersensitive response with varying degrees of pathogen growth inside the invaded cell according to the rice genotype. Expression analysis of our PR-gene set revealed clear differences between the different types of interactions in both kinetic and magnitude of gene induction. Our integrated study opens the way to the dissection of molecular components leading to nonhost reactions to Magnaporthe grisea in rice and points to novel sources of durable resistance to fungal plant pathogens in other cereal crops.

Monitoring autophagy in Magnaporthe oryzae

Autophagy is a ubiquitous degradative pathway for the bulk degradation of eukaryotic macromolecules and organelles in eukaryotic cells (Klionsky, 2005; Levine and Klionsky, 2004). Previously, the role of autophagy in turgor generation in plant pathogenic fungi was unknown. Currently, autophagy is confirmed as an important pathway for turgor accumulation in the appressorium (the tips of the invasive hyphae; Liu et al., 2007b) using a technique of targeted gene replacement, deleting the genes that code for Magnaporthe oryzae homologs of yeast autophagy-related (ATG) genes ATG2, ATG4, ATG5, ATG8, ATG9, and ATG18 (Liu et al., 2007a). All of these null mutants fail to breach the cuticle of the host. This chapter will first look at some methodologies to analyze the functions of autophagy-related gene products at the biological, cellular, and molecular level in this model plant pathogenic fungi, and then provide some research evidence of the role of autophagy in the promotion of the formation of the infection structure and pathogenicity to point out some significant areas for further research in this field.