Common genetic pathways regulate organ-specific infection-related development in the rice blast fungus

Host-induced gene silencing is a promising biological tool to characterize the pathogenicity of Magnaporthe oryzae and control fungal disease in rice

The rice blast fungus Magnaporthe oryzae is a devastating plant pathogen that threatens rice production worldwide. Host-induced gene silencing (HIGS) has been effectively applied to study pathogenic gene function during host-microbe interactions and control fungal diseases in various crops. In this study, the HIGS system of M. oryzae was established using transgenic fungus expressing green fluorescence protein (GFP), KJ201::eGFP and 35S::dsRNAi plants, which produce small interfering RNAs targeting fungal genes. Through this system, we verified the HIGS of rice blast fungus quantitatively and qualitatively in both Arabidopsis and rice. Then, we showed that the HIGS of M. oryzae's pathogenic genes, including RGS1, MgAPT2 and LHS1, significantly alter its virulence. Both 35S::dsRNAi_MgAPT2 and 35S::dsRNAi_LHS1 plants showed a considerably enhanced fungal resistance, characterized by the formation of H2 O2 -containing defensive granules and induction of rice pathogenesis-related (PR) genes. In addition, the enhanced susceptibility of 35S::dsRNAi_RGS1 plants to blast fungus suggested a novel mode of action of this gene during fungal infection. Overall, the results of this study demonstrate that HIGS is a very effective and efficient biological tool not only to accurately characterize the functions of fungal pathogenic genes during rice-M. oryzae interactions, but also to control fungal disease and ensure a successful rice production.

MIF-like domain containing protein orchestrates cellular differentiation and virulence in the fungal pathogen Magnaporthe oryzae

Macrophage migration inhibitory factor (MIF) is a pleiotropic protein with chemotactic, pro-inflammatory, and growth-promoting activities first discovered in mammals. In parasites, MIF homologs are involved in immune evasion and pathogenesis. Here, we present the first comprehensive analysis of an MIF protein from the devastating plant pathogen Magnaporthe oryzae (Mo). The fungal genome encodes a single MIF protein (MoMIF1) that, unlike the human homolog, harbors multiple low-complexity regions (LCRs) and is unique to Ascomycota. Following infection, MoMIF1 is expressed in the biotrophic phase of the fungus, and is strongly down-regulated during subsequent necrotrophic growth in leaves and roots. We show that MoMIF1 is secreted during plant infection, affects the production of the mycotoxin tenuazonic acid and inhibits plant cell death. Our results suggest that MoMIF1 is a novel key regulator of fungal virulence that maintains the balance between biotrophy and necrotrophy during the different phases of fungal infection.

Hyphopodium-Specific Signaling Is Required for Plant Infection by Verticillium dahliae

For successful colonization, fungal pathogens have evolved specialized infection structures to overcome the barriers present in host plants. The morphology of infection structures and pathogenic mechanisms are diverse according to host specificity. Verticillium dahliae, a soil-borne phytopathogenic fungus, generates hyphopodium with a penetration peg on cotton roots while developing appressoria, that are typically associated with leaf infection on lettuce and fiber flax roots. In this study, we isolated the pathogenic fungus, V. dahliae (Vda^Sm), from Verticillium wilt eggplants and generated a GFP-labeled isolate to explore the colonization process of Vda^Sm on eggplants. We found that the formation of hyphopodium with penetration peg is crucial for the initial colonization of Vda^Sm on eggplant roots, indicating that the colonization processes on eggplant and cotton share a similar feature. Furthermore, we demonstrated that the VdNoxB/VdPls1-dependent Ca²⁺ elevation activating VdCrz1 signaling is a common genetic pathway to regulate infection-related development in V. dahliae. Our results indicated that VdNoxB/VdPls1-dependent pathway may be a desirable target to develop effective fungicides, to protect crops from V. dahliae infection by interrupting the formation of specialized infection structures.

Transcription Factor VdCf2 Regulates Growth, Pathogenicity, and the Expression of a Putative Secondary Metabolism Gene Cluster in Verticillium dahliae

Verticillium dahliae is an important soilborne phytopathogen which can ruinously attack numerous host plants and cause significant economic losses. Transcription factors (TFs) were reported to be involved in various biological processes, such as hyphal growth and virulence of pathogenic fungi.

Novel insights into host specificity of Pyricularia oryzae and Pyricularia grisea in the infection of gramineous plant roots

Pyricularia oryzae and Pyricularia grisea are pathogens that cause blast disease in various monocots. It has been reported that P. oryzae infects the leaves and roots of rice via different mechanisms. However, it is unclear to what extent the tissue types affect the host specificities of P. oryzae and P. grisea. Here, we evaluated the tissue-specific infection strategies of P. oryzae and P. grisea in various gramineous plants. Generally, mycelial plug inoculation caused root browning but the degree of browning did not simply follow the disease index on leaves. Interestingly, the Triticum and Digitaria pathotypes caused strong root growth inhibition in rice, wheat, and barley. Moreover, the Digitaria pathotype inhibited root branching only in rice. Culture filtrate reproduced these inhibitory effects on root, suggesting that some secreted molecules are responsible for the inhibitions. Observation of root sections revealed that most of the infection hyphae penetrated intercellular spaces and further extended into root cells, regardless of pathotype and host plant. The infection hyphae of Digitaria and Triticum pathotypes tended to localize in the outer layer of rice roots, but not in those of wheat and barley roots. The infection hyphae of the Oryza pathotype were distributed in both the intercellular and intracellular spaces of rice root cells. Pathogenesis-related genes and reactive oxygen species accumulation were induced after root inoculation with all combinations. These results suggest that resistance reactions were induced in the roots of gramineous plants against the infection with Pyricularia isolates but failed to prevent fungal invasion.

Proteomics Analysis Reveals the Molecular Mechanism of MoPer1 Regulating the Development and Pathogenicity of Magnaporthe oryzae

Glycosylphosphatidylinositol (GPI) anchoring the protein GPI modification post-transcriptionally is commonly seen. In our previous study, MoPer1, a GPI anchoring essential factor, has a critical effect on Magnaporthe oryzae growth, pathogenicity, and conidiogenesis, but its molecular mechanism is not clear. Here, we extracted the glycoproteins from the ΔMoper1 mutant and wild-type Guy11 to analyze their differential levels by quantitative proteomic analysis of TMT markers. After background subtraction, a total of 431 proteins, with significant changes in expression, were successfully identified, and these differential proteins were involved in biological regulation, as well as cellular process and metabolic process, binding, catalytic activity, and other aspects. Moreover, we found that MoPer1 regulates the expression of 14 proteins involved in growth, development, and pathogenicity of M. oryzae. The above findings shed light on MoPer1’s underlying mechanism in regulating growth, development, and pathogenicity of M. oryzae.

Dissecting the labdane-related diterpenoid biosynthetic gene clusters in rice reveals directional cross-cluster phytotoxicity

Rice (Oryza sativa) is a staple food crop and serves as a model cereal plant. It contains two biosynthetic gene clusters (BGCs) for the production of labdane-related diterpenoids (LRDs), which serve important roles in combating biotic and abiotic stress. While plant BGCs have been subject to genetic analyses, these analyses have been largely confined to the investigation of single genes. CRISPR/Cas9-mediated genome editing was used to precisely remove each of these BGCs, as well as simultaneously knock out both BGCs. Deletion of the BGC from chromosome 2 (c2BGC), which is associated with phytocassane biosynthesis, but not that from chromosome 4 (c4BGC), which is associated with momilactone biosynthesis, led to a lesion mimic phenotype. This phenotype is dependent on two closely related genes encoding cytochrome P450 (CYP) mono-oxygenases, CYP76M7 and CYP76M8, from the c2BGC. However, rather than being redundant, CYP76M7 has been associated with the production of phytocassanes, whereas CYP76M8 is associated with momilactone biosynthesis. Intriguingly, the lesion mimic phenotype is not present in a line with both BGCs deleted. These results reveal directional cross-cluster phytotoxicity, presumably arising from the accumulation of LRD intermediates dependent on the c4BGC in the absence of CYP76M7 and CYP76M8, further highlighting their interdependent evolution and the selective pressures driving BGC assembly.

Characterization of the MYB Genes Reveals Insights Into Their Evolutionary Conservation, Structural Diversity, and Functional Roles in Magnaporthe oryzae

The myeloblastosis (MYB) transcription factor family is evolutionarily conserved among plants, animals, and fungi, and contributes to their growth and development. We identified and analyzed 10 putative MYB genes in Magnaporthe oryzae (MoMYB) and determined their phylogenetic relationships, revealing high divergence and variability. Although MYB domains are generally defined by three tandem repeats, MoMYBs contain one or two weakly conserved repeats embedded in extensive disordered regions. We characterized the secondary domain organization, disordered segments, and functional contributions of each MoMYB. During infection, MoMYBs are distinctively expressed and can be subdivided into two clades of being either up- or down-regulated. Among these, MoMYB1 and MoMYB8 are up-regulated during infection and vegetative growth, respectively. We found MoMYB1 localized predominantly to the cytosol during the formation of infection structures. ΔMomyb1 exhibited reduced virulence on intact rice leaves corresponding to the diminished ability to form hypha-driven appressorium (HDA). We discovered that MoMYB1 regulates HDA formation on hard, hydrophobic surfaces, whereas host surfaces partially restored HDA formation in ΔMomyb1. Lipid droplet accumulation in hyphal tips and expression of HDA-associated genes were strongly perturbed in ΔMomyb1 indicating genetic interaction of MoMYB1 with downstream components critical to HDA formation. We also found that MoMYB8 is necessary for fungal growth, dark-induced melanization of hyphae, and involved in higher abiotic stress tolerance. Taken together, we revealed a multifaceted picture of the MoMYB family, wherein a low degree of conservation has led to the development of distinct structures and functions, ranging from fungal growth to virulence.

Pyricularia sp. jiangsuensis, a new cryptic rice panicle blast pathogen from rice fields in Jiangsu Province, China

Pyricularia oryzae is a multi-host pathogen causing cereal disease, including the devastating rice blast. Panicle blast is a serious stage, leading to severe yield loss. Thirty-one isolates (average 4.1%) were collected from the rice panicle lesions at nine locations covering Jiangsu province from 2010 to 2017. These isolates were characterized as Pyricularia sp. jiangsuensis distinct from known Pyricularia species. The representative strain 18-2 can infect rice panicle, root and five kinds of grasses. Intriguingly, strain 18-2 can co-infect rice leaf with P. oryzae Guy11. The whole genome of P. sp. jiangsuensis 18-2 was sequenced. Nine effectors were distributed in translocation or inversion region, which may link to the rapid evolution of effectors. Twenty-one homologues of known blast-effectors were identified in strain 18-2, seven effectors including the homologues of SLP1, BAS2, BAS113, CDIP2/3, MoHEG16 and Avr-Pi54, were upregulated in the sample of inoculated panicle with strain 18-2 at 24 hpi compared with inoculation at 8 hpi. Our results provide evidences that P. sp. jiangsuensis represents an addition to the mycobiota of blast disease. This study advances our understanding of the pathogenicity of P. sp. jiangsuensis to hosts, which sheds new light on the adaptability in the co-evolution of pathogen and host.

Comparative Analysis of Transcriptome and sRNAs Expression Patterns in the Brachypodium distachyon- Magnaporthe oryzae Pathosystems

The hemibiotrophic fungus Magnaporthe oryzae (Mo) is the causative agent of rice blast and can infect aerial and root tissues of a variety of Poaceae, including the model Brachypodium distachyon (Bd). To gain insight in gene regulation processes occurring at early disease stages, we comparatively analyzed fungal and plant mRNA and sRNA expression in leaves and roots. A total of 310 Mo genes were detected consistently and differentially expressed in both leaves and roots. Contrary to Mo, only minor overlaps were observed in plant differentially expressed genes (DEGs), with 233 Bd-DEGs in infected leaves at 2 days post inoculation (DPI), compared to 4978 at 4 DPI, and 138 in infected roots. sRNA sequencing revealed a broad spectrum of Mo-sRNAs that accumulated in infected tissues, including candidates predicted to target Bd mRNAs. Conversely, we identified a subset of potential Bd-sRNAs directed against fungal cell wall components, virulence genes and transcription factors. We also show a requirement of operable RNAi genes from the DICER-like (DCL) and ARGONAUTE (AGO) families for fungal virulence. Overall, our work elucidates the extensive reprogramming of transcriptomes and sRNAs in both plant host (Bd) and fungal pathogen (Mo), further corroborating the critical role played by sRNA species in the establishment of the interaction and its outcome.

Structural specificity in plant-filamentous pathogen interactions

Plant diseases bear names such as leaf blights, root rots, sheath blights, tuber scabs, and stem cankers, indicating that symptoms occur preferentially on specific parts of host plants. Accordingly, many plant pathogens are specialized to infect and cause disease in specific tissues and organs. Conversely, others are able to infect a range of tissues, albeit often disease symptoms fluctuate in different organs infected by the same pathogen. The structural specificity of a pathogen defines the degree to which it is reliant on a given tissue, organ, or host developmental stage. It is influenced by both the microbe and the host but the processes shaping it are not well established. Here we review the current status on structural specificity of plant-filamentous pathogen interactions and highlight important research questions. Notably, this review addresses how constitutive defence and induced immunity as well as virulence processes vary across plant organs, tissues, and even cells. A better understanding of the mechanisms underlying structural specificity will aid targeted approaches for plant health, for instance by considering the variation in the nature and the amplitude of defence responses across distinct plant organs and tissues when performing selective breeding.

Karyopherin MoKap119-mediated nuclear import of cyclin-dependent kinase regulator MoCks1 is essential for Magnaporthe oryzae pathogenicity

Nuclear import of proteins relies on nuclear import receptors called importins/karyopherins (Kaps), whose functions were reported in yeasts, fungi, plants, and animal cells, including cell cycle control, morphogenesis, stress sensing/response, and also fungal pathogenecity. However, limited is known about the physiological function and regulatory mechanism of protein import in the rice-blast fungus Magnaporthe oryzae. Here, we identified an ortholog of β-importin in M. oryzae encoded by an ortholog of KAP119 gene. Functional characterisation of this gene via reverse genetics revealed that it is required for vegetative growth, conidiation, melanin pigmentation, and pathogenicity of M. oryzae. The mokap119Δ mutant was also defective in formation of appressorium-like structure from hyphal tips. By affinity assay and liquid chromatography-tandem mass spectrometry, we identified potential MoKap119-interacting proteins and further verified that MoKap119 interacts with the cyclin-dependent kinase subunit MoCks1 and mediates its nuclear import. Transcriptional profiling indicated that MoKap119 may regulate transcription of infection-related genes via MoCks1 regulation of MoSom1. Overall, our findings provide a novel insight into the regulatory mechanism of M. oryzae pathogenesis likely by MoKap119-mediated nuclear import of the cyclin-dependent kinase subunit MoCks1.

An expanded subfamily of G-protein-coupled receptor genes in Fusarium graminearum required for wheat infection

The cAMP-PKA and MAP kinase pathways are essential for plant infection in the wheat head blight fungus Fusarium graminearum. To identify upstream receptors of these well-conserved signalling pathways, we systematically characterized the 105 G-protein-coupled receptor (GPCR) genes. Although none were required for vegetative growth, five GPCR genes (GIV1-GIV5) significantly upregulated during plant infection were important for virulence. The giv1 mutant was defective in the formation of specialized infection structures known as infection cushions, which was suppressed by application of exogenous cAMP and dominant active FST7 MEK kinase. GIV1 was important for the stimulation of PKA and Gpmk1 MAP kinase by compounds in wheat spikelets. GIV2 and GIV3 were important for infectious growth after penetration. Invasive hyphae of the giv2 mutant were defective in cell-to-cell spreading and mainly grew intercellularly in rachis tissues. Interestingly, the GIV2-GIV5 genes form a phylogenetic cluster with GIV6, which had overlapping functions with GIV5 during pathogenesis. Furthermore, the GIV2-GIV6 cluster is part of a 22-member subfamily of GPCRs, with many of them having in planta-specific upregulation and a common promoter element; however, only three subfamily members are conserved in other fungi. Taken together, F. graminearum has an expanded subfamily of infection-related GPCRs for regulating various infection processes.

Virulence- and signaling-associated genes display a preference for long 3'UTRs during rice infection and metabolic stress in the rice blast fungus

Generation of mRNA isoforms by alternative polyadenylation (APA) and their involvement in regulation of fungal cellular processes, including virulence, remains elusive. Here, we investigated genome-wide polyadenylation site (PAS) selection in the rice blast fungus to understand how APA regulates pathogenicity. More than half of Magnaporthe oryzae transcripts undergo APA and show novel motifs in their PAS region. Transcripts with shorter 3'UTRs are more stable and abundant in polysomal fractions, suggesting they are being translated more efficiently. Importantly, rice colonization increases the use of distal PASs of pathogenicity genes, especially those participating in signalling pathways like 14-3-3B, whose long 3'UTR is required for infection. Cleavage factor I (CFI) Rbp35 regulates expression and distal PAS selection of virulence and signalling-associated genes, tRNAs and transposable elements, pointing its potential to drive genomic rearrangements and pathogen evolution. We propose a noncanonical PAS selection mechanism for Rbp35 that recognizes UGUAH, unlike humans, without CFI25. Our results showed that APA controls turnover and translation of transcripts involved in fungal growth and environmental adaptation. Furthermore, these data provide useful information for enhancing genome annotations and for cross-species comparisons of PASs and PAS usage within the fungal kingdom and the tree of life.

Inferring Roles in Defense from Metabolic Allocation of Rice Diterpenoids

Among their responses to microbial infection, plants deploy an arsenal of natural antibiotic products. Historically these have been identified on the basis of their antibiotic activity in vitro, which leaves open the question of their relevance to defense in planta. The vast majority of such natural products from the important crop plant rice (Oryza sativa) are diterpenoids whose biosynthesis proceeds via either ent- or syn-copalyl diphosphate (CPP) intermediates, which were isolated on the basis of their antibiotic activity against the fungal blast pathogen Magnaporthe oryzae However, rice plants in which the gene for the syn-CPP synthase Os-CPS4 is knocked out do not exhibit increased susceptibility to M. oryzae Here, we show that knocking out or knocking down Os-CPS4 actually decreases susceptibility to the bacterial leaf blight pathogen Xanthomonas oryzae By contrast, genetic manipulation of the gene for the ent-CPP synthase Os-CPS2 alters susceptibility to both M. oryzae and X. oryzae Despite the secretion of diterpenoids dependent on Os-CPS2 or Os-CPS4 from roots, neither knockout exhibited significant changes in the composition of their rhizosphere bacterial communities. Nevertheless, rice plants allocate substantial metabolic resources toward syn- as well as ent-CPP derived diterpenoids upon infection/induction. Further investigation revealed that Os-CPS4 plays a role in fungal non-host disease resistance. Thus, examination of metabolic allocation provides important clues into physiological function.

Comparative analysis of the root transcriptomes of cultivated and wild rice varieties in response to Magnaporthe oryzae infection revealed both common and species-specific pathogen responses

BACKGROUND

Magnaporthe oryzae, the causal fungus of rice blast disease, negatively impacts global rice production. Wild rice (Oryza rufipogon), a relative of cultivated rice (O. sativa), possesses unique attributes that enable it to resist pathogen invasion. Although wild rice represents a major resource for disease resistance, relative to current cultivated rice varieties, no prior studies have compared the immune and transcriptional responses in the roots of wild and cultivated rice to M. oryzae.

RESULTS

In this study, we showed that M. oryzae could act as a typical root-infecting pathogen in rice, in addition to its common infection of leaves, and wild rice roots were more resistant to M. oryzae than cultivated rice roots. Next, we compared the differential responses of wild and cultivated rice roots to M. oryzae using RNA-sequencing (RNA-seq) to unravel the molecular mechanisms underlying the enhanced resistance of the wild rice roots. Results indicated that both common and genotype-specific mechanisms exist in both wild and cultivated rice that are associated with resistance to M. oryzae. In wild rice, resistance mechanisms were associated with lipid metabolism, WRKY transcription factors, chitinase activities, jasmonic acid, ethylene, lignin, and phenylpropanoid and diterpenoid metabolism; while the pathogen responses in cultivated rice were mainly associated with phenylpropanoid, flavone and wax metabolism. Although modulations in primary metabolism and phenylpropanoid synthesis were common to both cultivated and wild rice, the modulation of secondary metabolism related to phenylpropanoid synthesis was associated with lignin synthesis in wild rice and flavone synthesis in cultivated rice. Interestingly, while the expression of fatty acid and starch metabolism-related genes was altered in both wild and cultivated rice in response to the pathogen, changes in lipid acid synthesis and lipid acid degradation were dominant in cultivated and wild rice, respectively.

CONCLUSIONS

The response mechanisms to M. oryzae were more complex in wild rice than what was observed in cultivated rice. Therefore, this study may have practical implications for controlling M. oryzae in rice plantings and will provide useful information for incorporating and assessing disease resistance to M. oryzae in rice breeding programs.

Genome wide analysis of the transition to pathogenic lifestyles in Magnaporthales fungi

The rice blast fungus Pyricularia oryzae (syn. Magnaporthe oryzae, Magnaporthe grisea), a member of the order Magnaporthales in the class Sordariomycetes, is an important plant pathogen and a model species for studying pathogen infection and plant-fungal interaction. In this study, we generated genome sequence data from five additional Magnaporthales fungi including non-pathogenic species, and performed comparative genome analysis of a total of 13 fungal species in the class Sordariomycetes to understand the evolutionary history of the Magnaporthales and of fungal pathogenesis. Our results suggest that the Magnaporthales diverged ca. 31 millon years ago from other Sordariomycetes, with the phytopathogenic blast clade diverging ca. 21 million years ago. Little evidence of inter-phylum horizontal gene transfer (HGT) was detected in Magnaporthales. In contrast, many genes underwent positive selection in this order and the majority of these sequences are clade-specific. The blast clade genomes contain more secretome and avirulence effector genes, which likely play key roles in the interaction between Pyricularia species and their plant hosts. Finally, analysis of transposable elements (TE) showed differing proportions of TE classes among Magnaporthales genomes, suggesting that species-specific patterns may hold clues to the history of host/environmental adaptation in these fungi.

Tpc1 is an important Zn(II)2Cys6 transcriptional regulator required for polarized growth and virulence in the rice blast fungus

The establishment of polarity is a critical process in pathogenic fungi, mediating infection-related morphogenesis and host tissue invasion. Here, we report the identification of TPC1 (Transcription factor for Polarity Control 1), which regulates invasive polarized growth in the rice blast fungus Magnaporthe oryzae. TPC1 encodes a putative transcription factor of the fungal Zn(II)₂Cys₆ family, exclusive to filamentous fungi. Tpc1-deficient mutants show severe defects in conidiogenesis, infection-associated autophagy, glycogen and lipid metabolism, and plant tissue colonisation. By tracking actin-binding proteins, septin-5 and autophagosome components, we show that Tpc1 regulates cytoskeletal dynamics and infection-associated autophagy during appressorium-mediated plant penetration. We found that Tpc1 interacts with Mst12 and modulates its DNA-binding activity, while Tpc1 nuclear localisation also depends on the MAP kinase Pmk1, consistent with the involvement of Tpc1 in this signalling pathway, which is critical for appressorium development. Importantly, Tpc1 directly regulates NOXD expression, the p22^phox subunit of the fungal NADPH oxidase complex via an interaction with Mst12. Tpc1 therefore controls spatial and temporal regulation of cortical F-actin through regulation of the NADPH oxidase complex during appressorium re-polarisation. Consequently, Tpc1 is a core developmental regulator in filamentous fungi, linking the regulated synthesis of reactive oxygen species and the Pmk1 pathway, with polarity control during host invasion.

Cellular polarity is an intrinsic feature of filamentous fungal growth and pathogenesis. In this study, we identified a gene required for fungal polar growth and virulence in the rice blast fungus Magnaporthe oryzae. This gene has been named TPC1 (Transcription factor for Polarity Control 1). The Tpc1 protein belongs to the fungal Zn(II)₂Cys₆ binuclear cluster family. This DNA-binding motif is present exclusively in the fungal kingdom. We have characterised defects associated with lack of Tpc1 in M. oryzae. We show that Tpc1 is involved in polarised growth and virulence. The M. oryzae Δtpc1 mutant shows a delay in glycogen and lipid metabolism, and infection-associated autophagy–processes that regulate appressorium-mediated M. oryzae plant infection. The saprophytic fungus Neurospora crassa contains a Tpc1 homolog (NcTpc1) involved in vegetative growth and sustained tip elongation, suggesting that Tpc1-like proteins act as core regulators of polarised growth and development in filamentous fungi. A comparative transcriptome analysis has allowed us to identify genes regulated by Tpc1 in M. oryzae including NoxD, an important component of the fungal NADPH complex. Significantly, Tpc1 interacts with Mst12, a component of the Pmk1 signalling pathway essential for appressorium development, and modulates Mst12 binding affinity to NOXD promoter region. We conclude that Tpc1 is a key regulator of polarity in M. oryzae that regulates growth, autophagy and septin-mediated reorientation of the F-actin cytoskeleton to facilitate plant cell invasion.

Secretory proteins are delivered to the septin-organized penetration interface during root infection by Verticillium dahliae

Successful infection of the host requires secretion of effector proteins to evade or suppress plant immunity. Secretion of effectors in root-infecting fungal pathogens, however, remains unexplored. We previously reported that Verticillium dahliae, a root-infecting phytopathogenic fungus, develops a penetration peg from a hyphopodium to infect cotton roots. In this study, we report that a septin ring, requiring VdSep5, partitions the hyphopodium and the invasive hypha and form the specialized fungus-host interface. The mutant strain, VdΔnoxb, in which NADPH oxidase B (VdNoxB) is deleted, impaired formation of the septin ring at the hyphal neck, indicating that NADPH oxidases regulate septin ring organization. Using GFP tagging and live cell imaging, we observed that several signal peptide containing secreted proteins showed ring signal accumulation/secretion at the penetration interface surrounding the hyphal neck. Targeted mutation for VdSep5 reduced the delivery rate of secretory proteins to the penetration interface. Blocking the secretory pathway by disrupting the vesicular trafficking factors, VdSec22 and VdSyn8, or the exocyst subunit, VdExo70, also arrested delivery of the secreted proteins inside the hyphopodium. Reduced virulence was observed when cotton roots were infected with VdΔsep5, VdΔsec22, VdΔsyn8 and VdΔexo70 mutants compared to infection with the isogenic wild-type V592. Taken together, our data demonstrate that the hyphal neck is an important site for protein secretion during plant root infection, and that the multiple secretory routes are involved in the secretion.

Pathogens secrete effector proteins as molecular weapons to evade or suppress plant immunity. However, the mechanism(s) by which root-infecting fungal pathogens secrete secretory effector proteins remains unexplored. We previously reported that Verticillium dahliae, a root-infecting phytopathogenic fungus, forms a specialized infection structure known as a hyphopodium that develops a penetration peg to pierce plant roots. In this study, we observed that after penetration, the penetration peg-developed hyphal neck, partitioning the hyphopodium and invasive hypha, came into close contact with the host, forming the fungus-host penetration interface. NADPH oxidase B (VdNoxB) regulated the cytoskeletal organization of the septin ring at the hyphal neck. Importantly, the penetration interface was a preferential site for secretion of signal peptide-containing proteins. Septin plays an important role in the efficient delivery of secretory proteins to the penetration interface. Moreover, the conventional fungal ER-to-Golgi secretion pathway, endosome-mediated transport and the exocyst complex are involved in the delivery of secretory proteins to the penetration interface. Together, our data demonstrate that the V. dahliae infection structure functions as a key signaling hub during plant infection and is the apparatus that not only breaches host cells but also provides a unique interface for the secretion of fungal effectors.

HvPap-1 C1A Protease Participates Differentially in the Barley Response to a Pathogen and an Herbivore

Co-evolutionary processes in plant-pathogen/herbivore systems indicate that protease inhibitors have a particular value in biotic interactions. However, little is known about the defensive role of their targets, the plant proteases. C1A cysteine proteases are the most abundant enzymes responsible for the proteolytic activity during different processes like germination, development and senescence in plants. To identify and characterize C1A cysteine proteases of barley with a potential role in defense, mRNA and protein expression patterns were analyzed in response to biotics stresses. A barley cysteine protease, HvPap-1, previously related to abiotic stresses and grain germination, was particularly induced by flagellin or chitosan elicitation, and biotic stresses such as the phytopathogenic fungus Magnaporthe oryzae or the phytophagous mite Tetranychus urticae. To elucidate the in vivo participation of this enzyme in defense, transformed barley plants overexpressing or silencing HvPap-1 encoding gene were subjected to M. oryzae infection or T. urticae infestation. Whereas overexpressing plants were less susceptible to the fungus than silencing plants, the opposite behavior occurred to the mite. This unexpected result highlights the complexity of the regulatory events leading to the response to a particular biotic stress.

The Colletotrichum gloeosporioides RhoB regulates cAMP and stress response pathways and is required for pathogenesis

Rho GTPases regulate morphology and multiple cellular functions such as asexual development, polarity establishment, and differentiation in fungi. To determine the roles of CgRhoB, a Rho GTPase protein, here we characterized CgRhoB in the poplar anthracnose fungus Colletotrichum gloeosporioides. First of all, we determined that conidial germination was inhibited and intracellular cyclic AMP (cAMP) level was increased in the CgRhoB deletion mutants. Loss of CgRhoB resulted in shorter germ tubes and enhanced appressoria formation after germination on the hydrophobic surface. Exogenous addition of cAMP to the wild type generated the similar phenotypes of ΔCgRhoB inoculated in CM liquid. Furthermore, deletion of CgRhoB had discernible effect upon the sensitivity of C. gloeosporioides to cell wall perturbing agents and altered the distribution of chitin on the cell wall. H₂O₂ sensitivity assay showed the hypersensitive effect on the oxidative stress, and transcriptional analysis revealed that transcription of genes involved in peroxidase activities was altered in the mutants. Finally, virulence assay revealed that CgRhoB was required for pathogenicity. Taken together, our results showed that CgRhoB was associated with appressoria formation and pathogenicity, and affected cAMP level and stress pathways.

Hyphopodium-Specific VdNoxB/VdPls1-Dependent ROS-Ca2+ Signaling Is Required for Plant Infection by Verticillium dahliae

Verticillium dahliae is a phytopathogenic fungus obligate in root infection. A few hyphopodia differentiate from large numbers of hyphae after conidia germination on the root surface for further infection. However, the molecular features and role of hyphopodia in the pathogenicity of V. dahliae remain elusive. In this study, we found that the VdPls1, a tetraspanin, and the VdNoxB, a catalytic subunit of membrane-bound NADPH oxidases for reactive oxygen species (ROS) production, were specifically expressed in hyphopodia. VdPls1 and VdNoxB highly co-localize with the plasma membrane at the base of hyphopodia, where ROS and penetration pegs are generated. Mutant strains, VdΔnoxb and VdΔpls1, in which VdPls1 and VdNoxB were deleted, respectively, developed defective hyphpodia incapable of producing ROS and penetration pegs. Defective plasma membrane localization of VdNoxB in VdΔpls1 demonstrates that VdPls1 functions as an adaptor protein for the recruitment and activation of the VdNoxB. Furthermore, in VdΔnoxb and VdΔpls1, tip-high Ca2+ accumulation was impaired in hyphopodia, but not in vegetative hyphal tips. Moreover, nuclear targeting of VdCrz1 and activation of calcineurin-Crz1 signaling upon hyphopodium induction in wild-type V. dahliae was impaired in both knockout mutants, indicating that VdPls1/VdNoxB-dependent ROS was specifically required for tip-high Ca2+ elevation in hyphopodia to activate the transcription factor VdCrz1 in the regulation of penetration peg formation. Together with the loss of virulence of VdΔnoxb and VdΔpls1, which are unable to initiate colonization in cotton plants, our data demonstrate that VdNoxB/VdPls1-mediated ROS production activates VdCrz1 signaling through Ca2+ elevation in hyphopodia, infectious structures of V. dahliae, to regulate penetration peg formation during the initial colonization of cotton roots.

Chitosan Mediates Germling Adhesion in Magnaporthe oryzae and Is Required for Surface Sensing and Germling Morphogenesis

The fungal cell wall not only plays a critical role in maintaining cellular integrity, but also forms the interface between fungi and their environment. The composition of the cell wall can therefore influence the interactions of fungi with their physical and biological environments. Chitin, one of the main polysaccharide components of the wall, can be chemically modified by deacetylation. This reaction is catalyzed by a family of enzymes known as chitin deacetylases (CDAs), and results in the formation of chitosan, a polymer of β1,4-glucosamine. Chitosan has previously been shown to accumulate in the cell wall of infection structures in phytopathogenic fungi. Here, it has long been hypothesized to act as a 'stealth' molecule, necessary for full pathogenesis. In this study, we used the crop pathogen and model organism Magnaporthe oryzae to test this hypothesis. We first confirmed that chitosan localizes to the germ tube and appressorium, then deleted CDA genes on the basis of their elevated transcript levels during appressorium differentiation. Germlings of the deletion strains showed loss of chitin deacetylation, and were compromised in their ability to adhere and form appressoria on artificial hydrophobic surfaces. Surprisingly, the addition of exogenous chitosan fully restored germling adhesion and appressorium development. Despite the lack of appressorium development on artificial surfaces, pathogenicity was unaffected in the mutant strains. Further analyses demonstrated that cuticular waxes are sufficient to over-ride the requirement for chitosan during appressorium development on the plant surface. Thus, chitosan does not have a role as a 'stealth' molecule, but instead mediates the adhesion of germlings to surfaces, thereby allowing the perception of the physical stimuli necessary to promote appressorium development. This study thus reveals a novel role for chitosan in phytopathogenic fungi, and gives further insight into the mechanisms governing appressorium development in M.oryzae.

Magnaporthe oryzae is a filamentous fungal pathogen which causes devastating crop losses in rice. Successful invasion of the host is dependent upon the ability of the fungus to remain undetected by the innate immune system of the plant, which recognizes conserved components of the fungal cell wall, such as chitin. Previous studies have demonstrated that infection-related changes in cell wall composition are necessary to allow the fungus to remain undetected during infection. One such change that has long been hypothesized to have a role as a 'stealth mechanism' is the deacetylation of the polysaccharide chitin by enzymes known as chitin deacetylases. The deacetylation of chitin produces a polysaccharide known as chitosan, which has previously been shown to accumulate specifically on infection structures in plant pathogenic fungi. However, in this study, we show that germling-localized chitosan is not required for pathogenicity, arguing against a role as a 'stealth mechanism' at this stage. Instead, chitosan is required for the development of the appressorium, a critical fungal infection structure required for the penetration of plant cells. This requirement can be attributed to chitosan mediating the adhesion of germlings to surfaces, which is required for the perception of physical stimuli.

A Fungal Effector With Host Nuclear Localization and DNA-Binding Properties Is Required for Maize Anthracnose Development

Plant pathogens have the capacity to manipulate the host immune system through the secretion of effectors. We identified 27 putative effector proteins encoded in the genome of the maize anthracnose pathogen Colletotrichum graminicola that are likely to target the host's nucleus, as they simultaneously contain sequence signatures for secretion and nuclear localization. We functionally characterized one protein, identified as CgEP1. This protein is synthesized during the early stages of disease development and is necessary for anthracnose development in maize leaves, stems, and roots. Genetic, molecular, and biochemical studies confirmed that this effector targets the host's nucleus and defines a novel class of double-stranded DNA-binding protein. We show that CgEP1 arose from a gene duplication in an ancestor of a lineage of monocot-infecting Colletotrichum spp. and has undergone an intense evolution process, with evidence for episodes of positive selection. We detected CgEP1 homologs in several species of a grass-infecting lineage of Colletotrichum spp., suggesting that its function may be conserved across a large number of anthracnose pathogens. Our results demonstrate that effectors targeted to the host nucleus may be key elements for disease development and aid in the understanding of the genetic basis of anthracnose development in maize plants.

Insights Into Triticum aestivum Seedling Root Rot Caused by Fusarium graminearum

Fusarium graminearum is one of the most common and potent fungal pathogens of wheat (Triticum aestivum), known for causing devastating spike infections and grain yield damage. F. graminearum is a typical soil-borne pathogen that builds up during consecutive cereal cropping. Speculation on systemic colonization of cereals by F. graminearum root infection have long existed but have not been proven. We have assessed the Fusarium root rot disease macroscopically in a diverse set of 12 wheat genotypes and microscopically in a comparative study of two genotypes with diverging responses. Here, we show a 'new' aspect of the F. graminearum life cycle, i.e., the head blight fungus uses a unique root-infection strategy with an initial stage typical for root pathogens and a later stage typical for spike infection. Root colonization negatively affects seedling development and leads to systemic plant invasion by tissue-adapted fungal strategies. Another major outcome is the identification of partial resistance to root rot. Disease severity assessments and histological examinations both demonstrated three distinct disease phases that, however, proceeded differently in resistant and susceptible genotypes. Soil-borne inoculum and root infection are considered significant components of the F. graminearum life cycle with important implications for the development of new strategies of resistance breeding and disease control.

Friend or foe: differential responses of rice to invasion by mutualistic or pathogenic fungi revealed by RNAseq and metabolite profiling

The rice endophyte Harpophora oryzae shares a common pathogenic ancestor with the rice blast fungus Magnaporthe oryzae. Direct comparison of the interactions between a single plant species and two closely-related (1) pathogenic and (2) mutualistic fungi species can improve our understanding of the evolution of the interactions between plants and fungi that lead to either mutualistic or pathogenic interactions. Differences in the metabolome and transcriptome of rice in response to challenge by H. or M. oryzae were investigated with GC-MS, RNA-seq, and qRT-PCR. Levels of metabolites of the shikimate and lignin biosynthesis pathways increased continuously in the M. oryzae-challenged rice roots (Mo-roots); these pathways were initially induced, but then suppressed, in the H. oryzae-challenged rice roots (Ho-roots). Compared to control samples, concentrations of sucrose and maltose were reduced in the Ho-roots and Mo-roots. The expression of most genes encoding enzymes involved in glycolysis and the TCA cycle were suppressed in the Ho-roots, but enhanced in the Mo-roots. The suppressed glycolysis in Ho-roots would result in the accumulation of glucose and fructose which was not detected in the Mo-roots. A novel co-evolution pattern of fungi-host interaction is proposed which highlights the importance of plant host in the evolution of fungal symbioses.

MoMyb1 is required for asexual development and tissue-specific infection in the rice blast fungus Magnaporthe oryzae

Background

The Myb super-family of proteins contain a group of functionally diverse transcriptional activators found in plant, animal and fungus. Myb proteins are involved in cell proliferation, differentiation and apoptosis, and have crucial roles in telomeres. The purpose of this study was to characterize the biological function of Myb1 protein in the rice blast fungus Magnaporthe oryzae.

Results

We identified the Saccharomyces cerevisiae BAS1 homolog MYB1 in M. oryzae, named MoMyb1. MoMyb1 encodes a protein of 322 amino acids and has two SANT domains and is well conserved in various organisms. Targeted gene deletion of MoMYB1 resulted in a significant reduction in vegetative growth and showed defects in conidiation and conidiophore development. Quantitative RT-PCR analysis revealed that the transcription levels of several conidiophore-related genes were apparently decreased in the ΔMomyb1 mutant. Inoculation with mycelia mats displayed that the virulence of the ΔMomyb1 mutant was not changed on rice leaves but was non-pathogenic on rice roots in comparison to the wild type Guy11. In addition, ∆Momyb1 mutants showed increased resistance to osmotic stresses but more sensitive to cell wall stressor calcofluor white (CFW). Further analysis revealed that MoMyb1 has an important role in the cell wall biosynthesis pathway.

Conclusion

This study provides the evidence that MoMyb1 is a key regulator involved in conidiogenesis, stress response, cell wall integrity and pathogenesis on rice roots in the filamentous phytopathogen M. oryzae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0375-y) contains supplementary material, which is available to authorized users.

Multilayer regulatory mechanisms control cleavage factor I proteins in filamentous fungi

Cleavage factor I (CFI) proteins are core components of the polyadenylation machinery that can regulate several steps of mRNA life cycle, including alternative polyadenylation, splicing, export and decay. Here, we describe the regulatory mechanisms that control two fungal CFI protein classes in Magnaporthe oryzae: Rbp35/CfI25 complex and Hrp1. Using mutational, genetic and biochemical studies we demonstrate that cellular concentration of CFI mRNAs is a limited indicator of their protein abundance. Our results suggest that several post-transcriptional mechanisms regulate Rbp35/CfI25 complex and Hrp1 in the rice blast fungus, some of which are also conserved in other ascomycetes. With respect to Rbp35, these include C-terminal processing, RGG-dependent localization and cleavage, C-terminal autoregulatory domain and regulation by an upstream open reading frame of Rbp35-dependent TOR signalling pathway. Our proteomic analyses suggest that Rbp35 regulates the levels of proteins involved in melanin and phenylpropanoids synthesis, among others. The drastic reduction of fungal CFI proteins in carbon-starved cells suggests that the pre-mRNA processing pathway is altered. Our findings uncover broad and multilayer regulatory mechanisms controlling fungal polyadenylation factors, which have profound implications in pre-mRNA maturation. This area of research offers new avenues for fungicide design by targeting fungal-specific proteins that globally affect thousands of mRNAs.

Multiple phytohormones and phytoalexins are involved in disease resistance to Magnaporthe oryzae invaded from roots in rice

Blast, caused by the fungus Magnaporthe oryzae, is one of the most devastating diseases of rice worldwide. Phenylalanine ammonia lyase (PAL) is a key enzyme in the phenylpropanoid pathway, which leads to the biosynthesis of defense-related phytohormone salicylic acid (SA) and flavonoid-type phytoalexins sakuranetin and naringenin. However, the roles and biochemical features of individual rice PALs in defense responses to pathogens remain unclear. Here, we report that rice OsPAL06, which can catalyze the formation of trans-cinnamate using l-phenylalanine, is involved in rice root-M. oryzae interaction. OsPAL06-knockout mutant showed increased susceptibility to M. oryzae invaded from roots and developed typical leaf blast symptoms, accompanied by nearly complete disappearance of sakuranetin and naringenin and a two-third reduction of the SA level in roots. This mutant also showed compensatively induced expression of chalcone synthase, which is involved in flavonoid biosynthesis, isochorismate synthase 1, which is putatively involved in SA synthesis via another pathway, reduced jasmonate content and increased ethylene content. These results suggest that OsPAL06 is a positive regulator in preventing M. oryzae infection from roots. It may regulate defense by promoting both phytoalexin accumulation and SA signaling that synergistically and antagonistically interacts with jasmonate- and ethylene-dependent signaling, respectively.

The rice endophyte Harpophora oryzae genome reveals evolution from a pathogen to a mutualistic endophyte

The fungus Harpophora oryzae is a close relative of the pathogen Magnaporthe oryzae and a beneficial endosymbiont of wild rice. Here, we show that H. oryzae evolved from a pathogenic ancestor. The overall genomic structures of H. and M. oryzae were found to be similar. However, during interactions with rice, the expression of 11.7% of all genes showed opposing trends in the two fungi, suggesting differences in gene regulation. Moreover, infection patterns, triggering of host defense responses, signal transduction and nutritional preferences exhibited remarkable differentiation between the two fungi. In addition, the H. oryzae genome was found to contain thousands of loci of transposon-like elements, which led to the disruption of 929 genes. Our results indicate that the gain or loss of orphan genes, DNA duplications, gene family expansions and the frequent translocation of transposon-like elements have been important factors in the evolution of this endosymbiont from a pathogenic ancestor.

A systems-wide comparison of red rice (Oryza longistaminata) tissues identifies rhizome specific genes and proteins that are targets for cultivated rice improvement

BACKGROUND

The rhizome, the original stem of land plants, enables species to invade new territory and is a critical component of perenniality, especially in grasses. Red rice (Oryza longistaminata) is a perennial wild rice species with many valuable traits that could be used to improve cultivated rice cultivars, including rhizomatousness, disease resistance and drought tolerance. Despite these features, little is known about the molecular mechanisms that contribute to rhizome growth, development and function in this plant.

RESULTS

We used an integrated approach to compare the transcriptome, proteome and metabolome of the rhizome to other tissues of red rice. 116 Gb of transcriptome sequence was obtained from various tissues and used to identify rhizome-specific and preferentially expressed genes, including transcription factors and hormone metabolism and stress response-related genes. Proteomics and metabolomics approaches identified 41 proteins and more than 100 primary metabolites and plant hormones with rhizome preferential accumulation. Of particular interest was the identification of a large number of gene transcripts from Magnaportha oryzae, the fungus that causes rice blast disease in cultivated rice, even though the red rice plants showed no sign of disease.

CONCLUSIONS

A significant set of genes, proteins and metabolites appear to be specifically or preferentially expressed in the rhizome of O. longistaminata. The presence of M. oryzae gene transcripts at a high level in apparently healthy plants suggests that red rice is resistant to this pathogen, and may be able to provide genes to cultivated rice that will enable resistance to rice blast disease.

PAF104, a synthetic peptide to control rice blast disease by blocking appressorium formation in Magnaporthe oryzae

Magnaporthe oryzae is the most devastating pathogen of rice and the main cause of crop losses worldwide. The successful management of blast disease caused by this fungus is a clear necessity. The synthetic peptide PAF104 has been characterized by its inhibition of M. oryzae appressorium formation on hydrophobic surfaces. Growth and the ability of conidia to germinate was not affected by PAF104, indicating the lack of toxicity on fungal conidia. The addition of the cutin monomer 1,16-hexadecanediol does not interfere with the inhibitory effect of PAF104 on in vitro hydrophobic surfaces. On the other hand, inhibition of appressorium formation by PAF104 was nullified by the exogenous addition of cAMP. Our results suggest that PAF104 affects the Pmk1 pathway by repression of the gene expression of MoMSB2, which encodes a sensing surface protein, and the mitogen-activated protein/extracellular signal-regulated kinase kinase kinase MST11. The pathogenicity of M. oryzae was reduced after PAF104 treatment specifically blocking appressorium formation. Our results support PAF104 as a promising compound to control rice blast disease by blocking a specific target related to appressorium formation, a process essential for infection of rice leaves. Moreover, PAF104 is proposed as a lead compound to develop novel specific fungicides with improved properties.

Acetolactate synthases MoIlv2 and MoIlv6 are required for infection-related morphogenesis in Magnaporthe oryzae

Amino acids are important components in the metabolism of a variety of pathogens, plants and animals. Acetolactate synthase (ALS) catalyses the first common step in leucine, isoleucine and valine biosynthesis, and is the target of several classes of inhibitors. Here, MoIlv2, an orthologue of the Saccharomyces cerevisiae ALS catalytic subunit Ilv2, and MoIlv6, an orthologue of the S. cerevisiae ALS regulatory subunit Ilv6, were identified. To characterize MoILV2 and MoILV6 functions, we generated the deletion mutants ΔMoilv2 and ΔMoilv6. Phenotypic analysis showed that both mutants were auxotrophic for leucine, isoleucine and valine, and were defective in conidial morphogenesis, appressorial penetration and pathogenicity. Further studies suggested that MoIlv2 and MoIlv6 play a critical role in maintaining the balance of intracellular amino acid levels. MoIlv2 and MoIlv6 are both localized to the mitochondria and the signal peptide of MoIlv6 is critical for its localization. In summary, our evidence indicates that MoIlv2 plays a crucial role in isoleucine and valine biosynthesis, whereas MoIlv6 contributes to isoleucine and leucine biosynthesis; both genes are required for fungal pathogenicity. This study indicates the potential of targeting branched-chain amino acid biosynthesis for anti-rice blast management.

Physiological stressors and invasive plant infections alter the small RNA transcriptome of the rice blast fungus, Magnaporthe oryzae

Background

The rice blast fungus, Magnaporthe oryzae is a destructive pathogen of rice and other related crops, causing significant yield losses worldwide. Endogenous small RNAs (sRNAs), including small interfering RNAs (siRNAs) and microRNAs (miRNAs) are critical components of gene regulation in many eukaryotic organisms. Recently several new species of sRNAs have been identified in fungi. This fact along with the availability of genome sequence makes M. oryzae a compelling target for sRNA profiling. We have examined sRNA species and their biosynthetic genes in M. oryzae, and the degree to which these elements regulate fungal stress responses. To this end, we have characterized sRNAs under different physiological stress conditions, which had not yet been examined in this fungus.

Results

The resulting libraries are composed of more than 37 million total genome matched reads mapping to intergenic regions, coding sequences, retrotransposons, inverted, tandem, and other repeated regions of the genome with more than half of the small RNAs arising from intergenic regions. The 24 nucleotide (nt) size class of sRNAs was predominant. A comparison to transcriptional data of M. oryzae undergoing the same physiological stresses indicates that sRNAs play a role in transcriptional regulation for a small subset of genes. Support for this idea comes from generation and characterization of mutants putatively involved in sRNAs biogenesis; our results indicate that the deletion of Dicer-like genes and an RNA-Dependent RNA Polymerase gene increases the transcriptional regulation of this subset of genes, including one involved in virulence.

Conclusions

Various physiological stressors and in planta conditions alter the small RNA profile of the rice blast fungus. Characterization of sRNA biosynthetic mutants helps to clarify the role of sRNAs in transcriptional control.

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.

Susceptibility of intact germinating Arabidopsis thaliana to human fungal pathogens Cryptococcus neoformans and C. gattii

The fungus Cryptococcus contributes a large global burden of infectious death in both HIV-infected and healthy individuals. As Cryptococcus is an opportunistic pathogen, much of the evolutionary pressure shaping virulence occurs in environments in contact with plants and soil. The present studies investigated inoculation of intact seeds of the common weed Arabidopsis thaliana with fungal cells over a 21-day period. C. gattii was the more virulent plant pathogen, resulting in disrupted germination as well as increased stem lodging, fungal burden, and plant tissue colocalization. C. neoformans was a less virulent plant pathogen but exhibited prolonged tissue residence within the cuticle and vascular spaces. Arabidopsis mutants of the PRN1 gene, which is involved in abiotic and biotic signaling affecting phenylalanine-derived flavonoids, showed altered susceptibility to cryptoccocal infections, suggesting roles for this pathway in cryptococcal defense. The fungal virulence factor laccase was also implicated in plant pathogenesis, as a cryptococcal lac1Δ strain was less virulent than wild-type fungi and was unable to colonize seedlings. In conclusion, these studies expand knowledge concerning the ecological niche of Cryptococcus by demonstrating the pathogenic capacity of the anamorphic form of cryptococcal cells against healthy seedlings under physiologically relevant conditions. In addition, an important role of laccase in plant as well as human virulence may suggest mechanisms for laccase retention and optimization during evolution of this fungal pathogen.

Secretome analysis of Magnaporthe oryzae using in vitro systems

Magnaporthe oryzae is a devastating blast fungal pathogen of rice (Oryza sativa L.) that causes dramatic decreases in seed yield and quality. During the early stages of infection by this pathogen, the fungal spore senses the rice leaf surface, germinates, and penetrates the cell via an infectious structure known as an appressorium. During this process, M. oryzae secretes several proteins; however, these proteins are largely unknown mainly due to the lack of a suitable method for isolating secreted proteins during germination and appressoria formation. We examined the secretome of M. oryzae by mimicking the early stages of infection in vitro using a glass plate (GP), PVDF membrane, and liquid culture medium (LCM). Microscopic observation of M. oryzae growth revealed appressorium formation on the GP and PVDF membrane resembling natural M. oryzae-rice interactions; however, appresorium formation was not observed in the LCM. Secreted proteins were collected from the GP (3, 8, and 24 h), PVDF membrane (24 h), and LCM (48 h) and identified by two-dimensional gel electrophoresis (2DE) followed by tandem mass spectrometry. The GP, PVDF membrane, and LCM-derived 2D gels showed distinct protein patterns, indicating that they are complementary approaches. Collectively, 53 nonredundant proteins including previously known and novel secreted proteins were identified. Six biological functions were assigned to the proteins, with the predominant functional classes being cell wall modification, reactive oxygen species detoxification, lipid modification, metabolism, and protein modification. The in vitro system using GPs and PVDF membranes applied in this study to survey the M. oryzae secretome, can be used to further our understanding of the early interactions between M. oryzae and rice leaves.

Fusion of a novel genetically engineered chitosan affinity protein and green fluorescent protein for specific detection of chitosan in vitro and in situ

Chitin is the second most abundant polysaccharide, present, e.g., in insect and arthropod exoskeletons and fungal cell walls. In some species or under specific conditions, chitin appears to be enzymatically de-N-acetylated to chitosan-e.g., when pathogenic fungi invade their host tissues. Here, the deacetylation of chitin is assumed to represent a pathogenicity mechanism protecting the fungus from the host's chitin-driven immune response. While highly specific chitin binding lectins are well known and easily available, this is not the case for chitosan-specific probes. This is partly due to the poor antigenicity of chitosan so that producing high-affinity, specific antibodies is difficult. Also, lectins with specificity to chitosan have been described but are not commercially available, and our attempts to reproduce the findings were not successful. We have, therefore, generated a fusion protein between a chitosanase inactivated by site-directed mutagenesis, the green fluorescent protein (GFP), and StrepII, as well as His(6) tags for purification and detection. The recombinant chitosan affinity protein (CAP) expressed in Escherichia coli was shown to specifically bind to chitosan, but not to chitin, and the affinity increased with decreasing degree of acetylation. In vitro, CAP detection was possible either based on GFP fluorescence or using Strep-Tactin conjugates or anti-His(5) antibodies. CAP fluorescence microscopy revealed binding to the chitosan exposing endophytic infection structures of the wheat stem rust fungus, but not the chitin exposing ectophytic infection structures, verifying its suitability for in situ chitosan staining.

Fungal virulence and development is regulated by alternative pre-mRNA 3'end processing in Magnaporthe oryzae

RNA-binding proteins play a central role in post-transcriptional mechanisms that control gene expression. Identification of novel RNA-binding proteins in fungi is essential to unravel post-transcriptional networks and cellular processes that confer identity to the fungal kingdom. Here, we carried out the functional characterisation of the filamentous fungus-specific RNA-binding protein RBP35 required for full virulence and development in the rice blast fungus. RBP35 contains an N-terminal RNA recognition motif (RRM) and six Arg-Gly-Gly tripeptide repeats. Immunoblots identified two RBP35 protein isoforms that show a steady-state nuclear localisation and bind RNA in vitro. RBP35 coimmunoprecipitates in vivo with Cleavage Factor I (CFI) 25 kDa, a highly conserved protein involved in polyA site recognition and cleavage of pre-mRNAs. Several targets of RBP35 have been identified using transcriptomics including 14-3-3 pre-mRNA, an important integrator of environmental signals. In Magnaporthe oryzae, RBP35 is not essential for viability but regulates the length of 3′UTRs of transcripts with developmental and virulence-associated functions. The Δrbp35 mutant is affected in the TOR (target of rapamycin) signaling pathway showing significant changes in nitrogen metabolism and protein secretion. The lack of clear RBP35 orthologues in yeast, plants and animals indicates that RBP35 is a novel auxiliary protein of the polyadenylation machinery of filamentous fungi. Our data demonstrate that RBP35 is the fungal equivalent of metazoan CFI 68 kDa and suggest the existence of 3′end processing mechanisms exclusive to the fungal kingdom.

The rice blast fungus Magnaporthe oryzae is one of the most damaging diseases of cultivated rice worldwide and an emerging disease on wheat, impacting on global food security. We identify a M. oryzae virulence-deficient mutant defective in the production of a RNA-binding protein (called RBP35). Clear orthologues of RBP35 are absent in yeast, plants and metazoans. We find two RBP35 protein isoforms that localise in the nucleus and bind RNA. Notably, we demonstrate that RBP35 interacts in vivo with a highly conserved protein component of the eukaryotic polyadenylation machinery. We show that RBP35 present different diffusional properties in nuclei of distinct fungal structures, and consequently different protein/nucleic acid interactions. Further, we find that RBP35 regulates the length of 3′UTRs of transcripts with developmental and virulence-associated functions. We prove that the Δrbp35 mutant is affected in the TOR (target of rapamycin) signaling pathway showing significant changes in nitrogen metabolism and protein secretion. Nothing it is known about pre-mRNA 3′ end processing in filamentous fungi and our study suggest that their polyadenylation machinery differs from yeast and higher organisms. This study can provide new insights into the evolution of the pre-mRNA maturation and the regulation of gene expression in eukaryotes.

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.

Organ identity and environmental conditions determine the effectiveness of nonhost resistance in the interaction between Arabidopsis thaliana and Magnaporthe oryzae

Mechanisms leading to nonhost resistance of plants against nonadapted pathogens are thought to have great potential for the future management of agriculturally important diseases. In this article, we report an investigation of nonhost resistance motivated by the advantages of studying an interaction between two model organisms, namely Arabidopsis thaliana and Magnaporthe oryzae. During the course of our studies, however, we discovered an unexpected plasticity in the responses of Arabidopsis against this ostensibly nonhost pathogen. Thus, we elucidated that certain experimental conditions, such as the growth of plants under long days at constantly high humidity and the use of high inoculum concentrations of M. oryzae conidia, forced the interaction in leaves of some Arabidopsis ecotypes towards increased compatibility. However, sporulation was never observed. Furthermore, we observed that roots were generally susceptible to M. oryzae, whereas leaves, stems and hypocotyls were not infected. It must be concluded, therefore, that Arabidopsis roots lack an effective defence repertoire against M. oryzae, whereas its leaves possess such nonhost defence mechanisms. In summary, our findings point to organ-specific determinants and environmental conditions influencing the effectiveness of nonhost resistance in plants.

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.