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

An unconventional effector MoRpa12 targeting host nuclei is essential for the development and pathogenicity of Magnaporthe oryzae

RNA polymerase I (Pol I) is a multi-subunit protein complex associated with the transcription of most ribosomal RNA molecules in all eukaryotes. Rpa12 is a small subunit of the Pol I catalytic core and plays a critical role in RNA cleavage, transcription initiation and elongation during proliferation in yeast and mammals. However, the function of Rpa12 in phytopathogenic fungi has not yet been characterized. Here, we present the functional characterization of MoRpa12, a homologue of the yeast Rpa12, in Magnaporthe oryzae. MoRpa12 shows upregulation during the infection phase, and MoRpa12-GFP exhibits nuclear localization at different developmental stages of M. oryzae and translocates into the nuclei of plant cells after fungal penetration. The MoRpa12 mutants also exhibit significant defects on mitosis, autophagy, oxidative stress tolerance, cell wall integrity, septin ring assembly, lipid and glycogen metabolism, and pathogenicity. The four cysteine residues at the amino terminus of this protein are critical for the nuclear localization of MoRpa12, and their site-directed mutagenesis affects the localization, fungal invasion, and full virulence of M. oryzae. In conclusion, our findings indicate that MoRpa12 functions as an unconventional secreted effector targeting host nuclei and is essential for the fungal growth and plant infection of M. oryzae.

Cell cycle arrest via DNA Damage Response (DDR) pathway induced by extracellular self-DNA (esDNA) application in rice root

Conspecific plant growth is inhibited by extracellular fragments in a concentration-dependent manner. Although several reports have addressed this self-DNA inhibition, the underlying mechanism remains unclear. In this investigation, we evaluated the progression of cell cycle of rice roots in responding to extracellular-self DNA (esDNA). We analyzed root growth, hydrogen peroxide (H2O2) production, Catalase (CAT) and Ascorbate Peroxidase (APX) enzyme activities, DNA Damage Response (DDR)-related gene expression, and cell cycle progression. Our results suggest that esDNA-induced root growth inhibition on days 7 and 10 and might associated with cell cycle arrest initiated several hours after esDNA treatment. The esDNA-induced cell cycle arrest is facilitated through the DDR pathway, activated by DNA damage resulting from elevated reactive oxygen species (ROS) induced by esDNA. Specifically, esDNA upregulates DDR-related gene expression including OsATM (Oryza sativa ataxia telangiectasia mutated), OsATR (Oryza sativa ATM and Rad3-related), OsSOG1 (Oryza sativa SUPPRESSOR OF GAMMA RESPONSE 1), OsWEE1 (Oryza sativa WEE1-like kinase 1), and OsSMR4 (Oryza sativa SIAMESE-RELATED 4), leading to cell cycle arrest. Finally, we propose that cell cycle arrest might be a plausible explanation for the phenomenon of root growth inhibition by esDNA. This result highlights the significance of DDR signaling in the plant's response to esDNA. This finding will be helpful as initial information for developing green herbicides to control monocot weeds in agriculture.

Mechanisms of regulated cell death during plant infection by the rice blast fungus Magnaporthe oryzae

Fungi are the most important group of plant pathogens, responsible for many of the world's most devastating crop diseases. One of the reasons they are such successful pathogens is because several fungi have evolved the capacity to breach the tough outer cuticle of plants using specialized infection structures called appressoria. This is exemplified by the filamentous ascomycete fungus Magnaporthe oryzae, causal agent of rice blast, one of the most serious diseases affecting rice cultivation globally. M. oryzae develops a pressurized dome-shaped appressorium that uses mechanical force to rupture the rice leaf cuticle. Appressoria form in response to the hydrophobic leaf surface, which requires the Pmk1 MAP kinase signalling pathway, coupled to a series of cell-cycle checkpoints that are necessary for regulated cell death of the fungal conidium and development of a functionally competent appressorium. Conidial cell death requires autophagy, which occurs within each cell of the spore, and is regulated by components of the cargo-independent autophagy pathway. This results in trafficking of the contents of all three cells to the incipient appressorium, which develops enormous turgor of up to 8.0 MPa, due to glycerol accumulation, and differentiates a thickened, melanin-lined cell wall. The appressorium then re-polarizes, re-orienting the actin and microtubule cytoskeleton to enable development of a penetration peg in a perpendicular orientation, that ruptures the leaf surface using mechanical force. Re-polarization requires septin GTPases which form a ring structure at the base of the appressorium, which delineates the point of plant infection, and acts as a scaffold for actin re-localization, enhances cortical rigidity, and forms a lateral diffusion barrier to focus polarity determinants that regulate penetration peg formation. Here we review the mechanism of regulated cell death in M. oryzae, which requires autophagy but may also involve ferroptosis. We critically evaluate the role of regulated cell death in appressorium morphogenesis and examine how it is initiated and regulated, both temporally and spatially, during plant infection. We then use this synopsis to present a testable model for control of regulated cell death during appressorium-dependent plant infection by the blast fungus.

PARylation of 14-3-3 proteins controls the virulence of Magnaporthe oryzae

Magnaporthe oryzae is a devastating fungal pathogen that causes the rice blast disease worldwide. The post-translational modification of ADP-ribosylation holds significant importance in various fundamental biological processes. However, the specific function of this modification in M. oryzae remains unknown. This study revealed that Poly(ADP-ribosyl)ation (PARylation) executes a critical function in M. oryzae. M. oryzae Poly(ADP-ribose) polymerase 1 (PARP1) exhibits robust PARylation activity. Disruption of PARylation by PARP1 knock-out or chemical inhibition reveals its involvement in M. oryzae virulence, particularly in appressorium formation. Furthermore, we identified two M. oryzae 14-3-3 proteins, GRF1 and GRF2, as substrates of PARP1. Deletion of GRF1 or GRF2 results in delayed and dysfunctional appressorium, diminished plant penetration, and reduced virulence of the fungus. Biochemical and genetic evidence suggest that PARylation of 14-3-3s is essential for its function in M. oryzae virulence. Moreover, PARylation regulates 14-3-3 dimerization and is required for the activation of the mitogen-activated protein kinases (MAPKs), Pmk1 and Mps1. GRF1 interacts with both Mst7 and Pmk1, and bridges their interaction in a PARylation-dependent manner. This study unveils a distinctive mechanism that PARylation of 14-3-3 proteins controls appressorium formation through MAPK activation, and could facilitate the development of new strategies of rice blast disease control.

The dual-specificity kinase MoLKH1-mediated cell cycle, autophagy, and suppression of plant immunity is critical for development and pathogenicity of Magnaporthe oryzae

Cell cycle progression, autophagic cell death during appressorium development, and ROS degradation at the infection site are important for the development of rice blast disease. However, the association of cell cycle, autophagy and ROS detoxification remains largely unknown in M. oryzae. Here, we identify the dual-specificity kinase MoLKH1, which serves as an important cell cycle regulator required for appressorium formation by regulating cytokinesis and cytoskeleton in M. oryzae. MoLKH1 is transcriptionally activated by H2O2 and required for H2O2-induced autophagic cell death and suppression of ROS-activated plant defense during plant invasion of M. oryzae. In addition, the Molkh1 mutant also showed several phenotypic defects, including delayed growth, abnormal conidiation, damaged cell wall integrity, impaired glycogen and lipid transport, reduced secretion of extracellular enzymes and effectors, and attenuated virulence of M. oryzae. Nuclear localization of MoLKH1 requires the nuclear localization sequence, Lammer motif, as well as the kinase active site and ATP-binding site in this protein. Site-directed mutagenesis showed that each of them plays crucial roles in fungal growth and pathogenicity of M. oryzae. In conclusion, our results demonstrate that MoLKH1-mediated cell cycle, autophagy, and suppression of plant immunity play crucial roles in development and pathogenicity of M. oryzae.

Phosphorylation of Mad1 at serine 18 by Mps1 is required for the full virulence of rice blast fungus, Magnaporthe oryzae

The spindle assembly checkpoint (SAC) proteins are conserved among eukaryotes safeguarding chromosome segregation fidelity during mitosis. However, their biological functions in plant-pathogenic fungi remain largely unknown. In this study, we found that the SAC protein MoMad1 in rice blast fungus (Magnaporthe oryzae) localizes on the nuclear envelope and is dispensable for M. oryzae vegetative growth and tolerance to microtubule depolymerizing agent treatment. MoMad1 plays an important role in M. oryzae infection-related development and pathogenicity. The monopolar spindle 1 homologue in M. oryzae (MoMps1) interacts with MoMad1 through its N-terminal domain and phosphorylates MoMad1 at Ser-18, which is conserved within the extended N termini of Mad1s from fungal plant pathogens. This phosphorylation is required for maintaining MoMad1 protein abundance and M. oryzae full virulence. Similar to the deletion of MoMad1, treatment with Mps1-IN-1 (an Mps1 inhibitor) caused compromised appressorium formation and decreased M. oryzae virulence, and these defects were dependent on its attenuating MoMad1 Ser-18 phosphorylation. Therefore, our study indicates the function of Mad1 in rice blast fungal pathogenicity and sheds light on the potential of blocking Mad1 phosphorylation by Mps1 to control crop fungal diseases.

Dynamic responses of Fusarium mangiferae to ultra-violet radiation

The repair capacity of ultra-violet (UV) light DNA damage is important for adaptation of fungi to different ecological niches. We previously showed that in the soil-borne pathogen Fusarium oxysporum photo-reactivation dependent UV repair is induced at the germling stage and reduced at the filament stage. Here, we tested the developmental control of the transcription of photolyase, UV survival, UV repair capacity, and UV induced mutagenesis in the foliar pathogen Fusarium mangiferae. Unlike F. oxysporum, neither did we observe developmental control over photo-reactivation dependent repair nor the changes in gene expression of photolyase throughout the experiment. Similarly, photo-reactivation assisted reduction in UV induced mutagenesis was similar throughout the development of F. mangiferae but fluctuated during the development of F. oxysporum. To generate hypotheses regarding the recovery of F. mangiferae after UV exposure, an RNAseq analysis was performed after irradiation at different timepoints. The most striking effect of UV on F. mangiferae was developmental-dependent induction of translation related genes. We further report a complex response that changes during recovery time and involves translation, cell cycle and lipid biology related genes.

Pyricularia oryzae: Lab star and field scourge

Pyricularia oryzae (syn. Magnaporthe oryzae), is a filamentous ascomycete that causes a major disease called blast on cereal crops, as well as on a wide variety of wild and cultivated grasses. Blast diseases have a tremendous impact worldwide particularly on rice and on wheat, where the disease emerged in South America in the 1980s, before spreading to Asia and Africa. Its economic importance, coupled with its amenability to molecular and genetic manipulation, have inspired extensive research efforts aiming at understanding its biology and evolution. In the past 40 years, this plant-pathogenic fungus has emerged as a major model in molecular plant-microbe interactions. In this review, we focus on the clarification of the taxonomy and genetic structure of the species and its host range determinants. We also discuss recent molecular studies deciphering its lifecycle.

TAXONOMY

Kingdom: Fungi, phylum: Ascomycota, sub-phylum: Pezizomycotina, class: Sordariomycetes, order: Magnaporthales, family: Pyriculariaceae, genus: Pyricularia.

HOST RANGE

P. oryzae has the ability to infect a wide range of Poaceae. It is structured into different host-specialized lineages that are each associated with a few host plant genera. The fungus is best known to cause tremendous damage to rice crops, but it can also attack other economically important crops such as wheat, maize, barley, and finger millet.

DISEASE SYMPTOMS

P. oryzae can cause necrotic lesions or bleaching on all aerial parts of its host plants, including leaf blades, sheaths, and inflorescences (panicles, spikes, and seeds). Characteristic symptoms on leaves are diamond-shaped silver lesions that often have a brown margin and whose appearance is influenced by numerous factors such as the plant genotype and environmental conditions. USEFUL WEBSITES Resources URL Genomic data repositories http://genome.jouy.inra.fr/gemo/ Genomic data repositories http://openriceblast.org/ Genomic data repositories http://openwheatblast.net/ Genome browser for fungi (including P. oryzae) http://fungi.ensembl.org/index.html Comparative genomics database https://mycocosm.jgi.doe.gov/mycocosm/home T-DNA mutant database http://atmt.snu.kr/ T-DNA mutant database http://www.phi-base.org/ SNP and expression data https://fungidb.org/fungidb/app/.

Septin-dependent invasive growth by the rice blast fungus Magnaporthe oryzae

Septin GTPases are morphogenetic proteins that are widely conserved in eukaryotic organisms fulfilling diverse roles in cell division, differentiation and development. In the filamentous fungal pathogen Magnaporthe oryzae, the causal agent of the devastating blast diseases of rice and wheat, septins have been shown to be essential for plant infection. The blast fungus elaborates a specialised infection structure called an appressorium with which it mechanically ruptures the plant cuticle. Septin aggregation and generation of a hetero-oligomeric ring structure at the base of the infection cell is indispensable for plant infection. Furthermore, once the fungus enters host tissue it develops another infection structure, the transpressorium, enabling it to move between living host plant cells, which also requires septins for its function. Specific inhibition of septin aggregation-either genetically or with chemical inhibitors-prevents plant infection. Significantly, by screening for inhibitors of septin aggregation, broad spectrum anti-fungal compounds have been identified that prevent rice blast and a number of other cereal diseases in field trials. We review the recent advances in our understanding of septin biology and their potential as targets for crop disease control.

The Microtubule End Binding Protein Mal3 Is Essential for the Dynamic Assembly of Microtubules during Magnaporthe oryzae Growth and Pathogenesis

Cytoskeletal microtubules (MTs) play crucial roles in many aspects of life processes in eukaryotic organisms. They dynamically assemble physiologically important MT arrays under different cell conditions. Currently, aspects of MT assembly underlying the development and pathogenesis of the model plant pathogenic fungus Magnaporthe oryzae (M. oryzae) are unclear. In this study, we characterized the MT plus end binding protein MoMal3 in M. oryzae. We found that knockout of MoMal3 results in defects in hyphal polar growth, appressorium-mediated host penetration and nucleus division. Using high-resolution live-cell imaging, we further found that the MoMal3 mutant assembled a rigid MT in parallel with the MT during hyphal polar growth, the cage-like network in the appressorium and the stick-like spindle in nuclear division. These aberrant MT organization patterns in the MoMal3 mutant impaired actin-based cell growth and host infection. Taken together, these findings showed that M. oryzae relies on MoMal3 to assemble elaborate MT arrays for growth and infection. The results also revealed the assembly mode of MTs in M. oryzae, indicating that MTs are pivotal for M. oryzae growth and host infection and may be new targets for devastating fungus control.

The cell cycle, autophagy, and cell wall integrity pathway jointly governed by MoSwe1 in Magnaporthe oryzae

The cell cycle is pivotal to cellular differentiation in plant pathogenic fungi. Cell wall integrity (CWI) signaling plays an essential role in coping with cell wall stress. Autophagy is a degradation process in which cells decompose their components to recover macromolecules and provide energy under stress conditions. However, the specific association between cell cycle, autophagy and CWI pathway remains unclear in model pathogenic fungi Magnaporthe oryzae. Here, we have identified MoSwe1 as the conserved component of the cell cycle in the rice blast fungus. We have found that MoSwe1 targets MoMps1, a conserved critical MAP kinase of the CWI pathway, through protein phosphorylation that positively regulates CWI signaling. The CWI pathway is abnormal in the ΔMoswe1 mutant with cell cycle arrest. In addition, we provided evidence that MoSwe1 positively regulates autophagy by interacting with MoAtg17 and MoAtg18, the core autophagy proteins. Moreover, the S phase initiation was earlier, the morphology of conidia and appressoria was abnormal, and septum formation and glycogen degradation were impaired in the ΔMoswe1 mutant. Our research defines that MoSWE1 regulation of G1/S transition, CWI pathway, and autophagy supports its specific requirement for appressorium development and virulence in plant pathogenic fungi. Video Abstract.

Natural Product Aloesin Significantly Inhibits Spore Germination and Appressorium Formation in Magnaporthe oryzae

This study aims to determine the effects of the natural product aloesin against Magnaporthe oryzae. The results exposed that aloesin had a high inhibitory effect on appressorium formation (the EC50 value was 175.26 μg/mL). Microscopic examination revealed that 92.30 ± 4.26% of M. oryzae spores could be broken down by 625.00 μg/mL of aloesin, and the formation rate of appressoria was 4.74 ± 1.00% after 12 h. M. oryzae mycelial growth was weaker than that on the control. The enzyme activity analysis results indicated that aloesin inhibited the activities of polyketolase (PKS), laccase (LAC), and chain-shortening catalytic enzyme (Aayg1), which are key enzymes in melanin synthesis. The inhibition rate by aloesin of PKS, LAC, and Aayg1 activity was 32.51%, 33.04%, and 43.38%, respectively. The proteomic analysis showed that actin expression was downregulated at 175.62 μg/mL of aloesin, which could reduce actin bundle formation and prevent the polar growth of hyphae in M. oryzae. This is the first report showing that aloesin effectively inhibits conidia morphology and appressorium formation in M. oryzae.

Fus3/Kss1-MAP kinase and Ste12-like control distinct biocontrol-traits besides regulation of insect cuticle penetration via phosphorylation cascade in a filamentous fungal pathogen

BACKGROUND

Homolog of the yeast Fus3/Kss1 mitogen-activated protein kinase (MAPK) pathway and its target transcription factor, Ste12-like, are involved in penetration of host cuticle/pathogenicity in many ascomycete pathogens. However, details of their interaction during fungal infection, as well as their controlled other virulence-associated traits, are unclear.

RESULTS

Ste12-like (BbSte12) and Fus3/Kss1 MAPK homolog (Bbmpk1) interacted in nucleus, and phosphorylation of BbSte12 by Bbmpk1 was essential for penetration of insect cuticle in an insect fungal pathogen, Beauveria bassiana. However, some distinct biocontrol-traits were found to be mediated by Ste12 and Bbmpk1. In contrast to ΔBbmpk1 colony that grew more rapid than wild-type strain, inactivation of BbSte12 resulted in the opposite phenotype, which was consistent with their different proliferation rates in insect hemocoel after direct injection of conidia bypass the cuticle. Reduced conidial yield with decreased hydrophobicity was examined in both mutants, however they displayed distinct conidiogenesis, accompanying with differently altered cell cycle, distinct hyphal branching and septum formation. Moreover, ΔBbmpk1 showed increased tolerance to oxidative agent, whereas the opposite phenotype was seen for ΔBbSte12 strain. RNA sequencing analysis revealed that Bbmpk1 controlled 356 genes depending on BbSte12 during cuticle penetration, but 1077 and 584 genes were independently controlled by Bbmpk1 and BbSte12.

CONCLUSION

BbSte12 and Bbmpk1 separately participate in additional pathways for control of conidiation, growth and hyphal differentiation, as well as oxidative stress response besides regulating cuticle penetration via phosphorylation cascade. © 2023 Society of Chemical Industry.

The Phantom Menace: latest findings on effector biology in the rice blast fungus

Magnaporthe oryzae is a hemibiotrophic fungus responsible for the economically devastating and recalcitrant rice blast disease. However, the blast fungus is not only restricted to rice plants as it can also infect wheat, millet, and other crops. Despite previous outstanding discoveries aimed to understand and control the disease, the fungus remains one of the most important pathogens that threatens global food security. To cause disease, M. oryzae initiates morphological changes to attach, penetrate, and colonize rice cells, all while suppressing plant immune defenses that would otherwise hinder its proliferation. As such, M. oryzae actively secretes a battery of small proteins called "effectors" to manipulate host machinery. In this review, we summarize the latest findings in effector identification, expression, regulation, and functionality. We review the most studied effectors and their roles in pathogenesis. Additionally, we discern the current methodologies to structurally catalog effectors, and we highlight the importance of climate change and its impact on the future of rice blast disease.

Isolation of Bacillus siamensis B-612, a Strain That Is Resistant to Rice Blast Disease and an Investigation of the Mechanisms Responsible for Suppressing Rice Blast Fungus

Rice yield can be significantly impacted by rice blast disease. In this investigation, an endophytic strain of Bacillus siamensis that exhibited a potent inhibitory effect on the growth of rice blast was isolated from healthy cauliflower leaves. 16S rDNA gene sequence analysis showed that it belongs to the genus Bacillus siamensis. Using the rice OsActin gene as an internal control, we analyzed the expression levels of genes related to the defense response of rice. Analysis showed that the expression levels of genes related to the defense response in rice were significantly upregulated 48 h after treatment. In addition, peroxidase (POD) activity gradually increased after treatment with B-612 fermentation solution and peaked 48 h after inoculation. These findings clearly demonstrated that the 1-butanol crude extract of B-612 retarded and inhibited conidial germination as well as the development of appressorium. The results of field experiments showed that treatment with B-612 fermentation solution and B-612 bacterial solution significantly reduced the severity of the disease before the seedling stage of Lijiangxintuan (LTH) was infected with rice blast. Future studies will focus on exploring whether Bacillus siamensis B-612 produces new lipopeptides and will apply proteomic and transcriptomic approaches to investigate the signaling pathways involved in its antimicrobial effects.

The transcriptional landscape of plant infection by the rice blast fungus Magnaporthe oryzae reveals distinct families of temporally co-regulated and structurally conserved effectors

The rice blast fungus Magnaporthe oryzae causes a devastating disease that threatens global rice (Oryza sativa) production. Despite intense study, the biology of plant tissue invasion during blast disease remains poorly understood. Here we report a high-resolution transcriptional profiling study of the entire plant-associated development of the blast fungus. Our analysis revealed major temporal changes in fungal gene expression during plant infection. Pathogen gene expression could be classified into 10 modules of temporally co-expressed genes, providing evidence for the induction of pronounced shifts in primary and secondary metabolism, cell signaling, and transcriptional regulation. A set of 863 genes encoding secreted proteins are differentially expressed at specific stages of infection, and 546 genes named MEP (Magnaportheeffector protein) genes were predicted to encode effectors. Computational prediction of structurally related MEPs, including the MAX effector family, revealed their temporal co-regulation in the same co-expression modules. We characterized 32 MEP genes and demonstrate that Mep effectors are predominantly targeted to the cytoplasm of rice cells via the biotrophic interfacial complex and use a common unconventional secretory pathway. Taken together, our study reveals major changes in gene expression associated with blast disease and identifies a diverse repertoire of effectors critical for successful infection.

MoMih1 is indispensable for asexual development, cell wall integrity, and pathogenicity of Magnaporthe oryzae

Asexual spore serves as essential inoculum of rice blast during the disease cycle, and differentiation of young conidium from conidiophore is intimately regulated by cell cycle. Mih1 encodes a dual-specificity phosphatase that involved in the G2/M transition of the mitotic cell cycle by regulating the Cdk1 activity in eukaryotes. Till now, the roles of Mih1 homologue, however, remain unclear in Magnaporthe oryzae. We here functionally characterized the Mih1 homologue MoMih1 in M. oryzae. MoMih1 is localized to both the cytoplasm and nucleus and can physically interact with the CDK protein MoCdc28 in vivo. Loss of MoMih1 led to delayed nucleus division and a high level of Tyr15 phosphorylation of MoCdc28. The MoMih1 mutants showed retarded mycelial growth with a defective polar growth, less fungal biomass, and shorter distance between diaphragms, compared with the KU80. Asexual reproduction altered in MoMih1 mutants, with both abnormal conidial morphogenesis and decreased conidiation. The MoMih1 mutants severely attenuated the virulence to host plants due to the impaired ability of penetration and biotrophic growth. The incapability of scavenging of host-derived reactive oxygen species, which was possibly ascribed to the severely decreased extracellular enzymes activities, were partially associated with deficiency of pathogenicity. Besides, the MoMih1 mutants displayed also improper localization of retromer protein MoVps26 and polarisome component MoSpa2, and defects of cell wall integrity (CWI), melanin pigmentation, chitin synthesis, and hydrophobicity. In conclusion, our results demonstrate that MoMih1 plays pleiotropic roles during fungal development and plant infection of M. oryzae.

Septum-associated microtubule organizing centers within conidia support infectious development by the blast fungus Magnaporthe oryzae

Cytoplasmic microtubule arrays play important and diverse roles within fungal cells, including serving as molecular highways for motor-driven organelle motility. While the dynamic plus ends of cytoplasmic microtubules are free to explore the cytoplasm through their stochastic growth and shrinkage, their minus ends are nucleated at discrete organizing centers, composed of large multi-subunit protein complexes. The location and composition of these microtubule organizing centers varies depending on genus, cell type, and in some instances cell-cycle stage. Despite their obvious importance, our understanding of the nature, diversity, and regulation of microtubule organizing centers in fungi remains incomplete. Here, using three-color fluorescence microscopy based live-cell imaging, we investigate the organization and dynamic behavior of the microtubule cytoskeleton within infection-related cell types of the filamentous fungus,Magnaporthe oryzae, a highly destructive pathogen of rice and wheat. We provide data to support the idea that cytoplasmic microtubules are nucleated at septa, rather than at nuclear spindle pole bodies, within the three-celled blast conidium, and provide new insight into remodeling of the microtubule cytoskeleton during nuclear division and inheritance. Lastly, we provide a more complete picture of the architecture and subcellular organization of the prototypical blast appressorium, a specialized pressure-generating cell type used to invade host tissue. Taken together, our study provides new insight into microtubule nucleation, organization, and dynamics in specialized and differentiated fungal cell types.

Acyl-coenzyme A binding protein MoAcb1 regulates conidiation and pathogenicity in Magnaporthe oryzae

Magnaporthe oryzae is a filamentous fungus that causes rice blast. Rice blast seriously threatens the safety of food production. The normal synthesis and metabolism of fatty acids are extremely important for eukaryotes, and acyl-CoA is involved in fatty acid metabolism. Acyl-CoA binding (ACB) proteins specifically bind both medium-chain and long-chain acyl-CoA esters. However, the role of the Acb protein in plant-pathogenic fungi has not yet been investigated. Here, we identified MoAcb1, a homolog of the Acb protein in Saccharomyces cerevisiae. Disruption of MoACB1 causes delayed hyphal growth, significant reduction in conidial production and delayed appressorium development, glycogen availability, and reduced pathogenicity. Using immunoblotting and chemical drug sensitivity analysis, MoAcb1 was found to be involved in endoplasmic reticulum autophagy (ER-phagy). In conclusion, our results suggested that MoAcb1 is involved in conidia germination, appressorium development, pathogenicity and autophagy processes in M. oryzae.

Transcriptome Analysis of Protein Kinase MoCK2, which Affects Acetyl-CoA Metabolism and Import of CK2-Interacting Mitochondrial Proteins into Mitochondria in the Rice Blast Fungus Magnaporthe oryzae

The rice pathogen Magnaporthe oryzae causes severe losses to rice production. Previous studies have shown that the protein kinase MoCK2 is essential for pathogenesis, and this ubiquitous eukaryotic protein kinase might affect several processes in the fungus that are needed for infection. To better understand which cellular processes are affected by MoCK2 activity, we performed a detailed transcriptome sequencing analysis of deletions of the MoCK2 b1 and b2 components in relation to the background strain Ku80 and connected this analysis with the abundance of substrates for proteins in a previous pulldown of the essential CKa subunit of CK2 to estimate the effects on proteins directly interacting with CK2. The results showed that MoCK2 seriously affected carbohydrate metabolism, fatty acid metabolism, amino acid metabolism, and the related transporters and reduced acetyl-CoA production. CK2 phosphorylation can affect the folding of proteins and especially the effective formation of protein complexes by intrinsically disordered or mitochondrial import by destabilizing soluble alpha helices. The upregulated genes found in the pulldown of the b1 and b2 mutants indicate that proteins directly interacting with CK2 are compensatorily upregulated depending on their pulldown. A similar correlation was found for mitochondrial proteins. Taken together, the classes of proteins and the changes in regulation in the b1 and b2 mutants suggest that CK2 has a central role in mitochondrial metabolism, secondary metabolism, and reactive oxygen species (ROS) resistance, in addition to its previously suggested role in the formation of new ribosomes, all of which are processes central to efficient nonself responses as innate immunity. IMPORTANCE The protein kinase CK2 is highly expressed and essential for plants, animals, and fungi, affecting fatty acid-related metabolism. In addition, it directly affects the import of essential mitochondrial proteins into mitochondria. These effects mean that CK2 is essential for lipid metabolism and mitochondrial function and, as shown previously, is crucial for making new translation machinery proteins. Taken together, our new results combined with previously reported results indicate that CK2 is an essential protein necessary for the capacities to launch efficient innate immunity responses and withstand the negative effects of such responses necessary for general resistance against invading bacteria and viruses as well as to interact with plants, withstand plant immunity responses, and kill plant cells.

Transcriptomic Dynamics of Active and Inactive States of Rho GTPase MoRho3 in Magnaporthe oryzae

The small Rho GTPase acts as a molecular switch in eukaryotic signal transduction, which plays a critical role in polar cell growth and vesicle trafficking. Previous studies demonstrated that constitutively active (CA) mutant strains, of MoRho3-CA were defective in appressorium formation. While dominant-negative (DN) mutant strains MoRho3-DN shows defects in polar growth. However, the molecular dynamics of MoRho3-mediated regulatory networks in the pathogenesis of Magnaporthe oryzae still needs to be uncovered. Here, we perform comparative transcriptomic profiling of MoRho3-CA and MoRho3-DN mutant strains using a high-throughput RNA sequencing approach. We find that genetic manipulation of MoRho3 significantly disrupts the expression of 28 homologs of Saccharomyces cerevisiae Rho3-interacting proteins, including EXO70, BNI1, and BNI2 in the MoRho3 CA, DN mutant strains. Functional enrichment analyses of up-regulated DEGs reveal a significant enrichment of genes associated with ribosome biogenesis in the MoRho3-CA mutant strain. Down-regulated DEGs in the MoRho3-CA mutant strains shows significant enrichment in starch/sucrose metabolism and the ABC transporter pathway. Moreover, analyses of down-regulated DEGs in the in MoRho3-DN reveals an over-representation of genes enriched in metabolic pathways. In addition, we observe a significant suppression in the expression levels of secreted proteins suppressed in both MoRho3-CA and DN mutant strains. Together, our results uncover expression dynamics mediated by two states of the small GTPase MoRho3, demonstrating its crucial roles in regulating the expression of ribosome biogenesis and secreted proteins.

Bacillus subtilis KLBMPGC81 suppresses appressorium-mediated plant infection by altering the cell wall integrity signaling pathway and multiple cell biological processes in Magnaporthe oryzae

Magnaporthe oryzae is one of the most destructive crop pathogens in the world, causing huge losses in rice harvest every year. Bacillus subtilis is a potential biocontrol agent that has been explored in many crop systems because it is a potent producer of bioactive compounds. However, the mechanisms by which these agents control rice blasts are not fully understood. We show that B. subtilis KLBMPGC81 (KC81) and its supernatant (SUP) have high antimicrobial activity against M. oryzae strain Guy11. To better exploit KC81 as a biocontrol agent, the mechanism by which KC81 suppresses rice blast pathogens was investigated. This study shows that KC81 SUP is effective in controlling rice blast disease. The SUP has a significant effect on suppressing the growth of M. oryzae and appressorium-mediated plant infection. KC81 SUP compromises cell wall integrity, microtubules and actin cytoskeleton, mitosis, and autophagy, all of which are required for M. oryzae growth, appressorium development, and host infection. We further show that the SUP reduces the activity of the cyclin-dependent kinase Cdc2 by enhancing the phosphorylation of Cdc2 Tyr 15, thereby impairing mitosis in M. oryzae cells. SUP induces the cell wall sensor MoWsc1 to activate the cell wall integrity pathway and Mps1 and Pmk1 mitogen-activated protein kinases. Taken together, our findings reveal that KC81 is an effective fungicide that suppresses M. oryzae growth, appressorium formation, and host infection by abnormally activating the cell wall integrity pathway, disrupting the cytoskeleton, mitosis, and autophagy.

De novo purine nucleotide biosynthesis mediated by MoAde4 is required for conidiation, host colonization and pathogenicity in Magnaporthe oryzae

Amidophosphoribosyltransferase catalyzes the conversion of 5-phosphoribosyl-1-pyrophosphate into 5-phosphoribosyl-1-amine in the de novo purine biosynthetic pathway. Herein, we identified and characterized the functions of MoAde4, an orthologue of yeast Ade4 in Magnaporthe oryzae. MoAde4 is a 537-amino acid protein containing GATase_6 and pribosyltran domains. MoADE4 transcripts were highly expressed during the conidiation, early-infection, and late-infection stages of the fungus. Disruption of the MoADE4 gene resulted in ΔMoade4 exhibiting adenine, adenosine, and hypoxanthine auxotrophy on minimal medium. Conidia quantification assays showed that sporulation was significantly reduced in the ΔMoade4 mutant. The conidia of ΔMoade4 could still form appressoria but mostly failed to penetrate the rice cuticle. Pathogenicity tests showed that ΔMoade4 was completely nonpathogenic on rice and barley leaves, which was attributed to restricted infectious hyphal growth within the primary cells. The ΔMoade4 mutant was defective in the induction of strong host immunity. Exogenous adenine partially rescued conidiation, infectious hyphal growth, and the pathogenicity defects of the ΔMoade4 mutant on barley and rice leaves. Taken together, our results demonstrated that purine nucleotide biosynthesis orchestrated by MoAde4 is required for fungal development and pathogenicity in M. oryzae. These findings therefore act as a suitable target for antifungal development against recalcitrant plant fungal pathogens. KEY POINTS: • MoAde4 is crucial for de novo purine nucleotide biosynthesis. • MoAde4 is pivotal for conidiogenesis and appressorium development of M. oryzae. • MoAde4 is involoved in the pathogenicity of M. oryzae.

The LAMMER Kinase MoKns1 Regulates Growth, Conidiation and Pathogenicity in Magnaporthe oryzae

Magnaporthe oryzae is an important pathogen that causes a devastating disease in rice. It has been reported that the dual-specificity LAMMER kinase is conserved from yeast to animal species and has a variety of functions. However, the functions of the LAMMER kinase have not been reported in M. oryzae. In this study, we identified the unique LAMMER kinase MoKns1 and analyzed its function in M. oryzae. We found that in a MoKNS1 deletion mutant, growth and conidiation were primarily decreased, and pathogenicity was almost completely lost. Furthermore, our results found that MoKns1 is involved in autophagy. The ΔMokns1 mutant was sensitive to rapamycin, and MoKns1 interacted with the autophagy-related protein MoAtg18. Compared with the wild-type strain 70−15, autophagy was significantly enhanced in the ΔMokns1 mutant. In addition, we also found that MoKns1 regulated DNA damage stress pathways, and the ΔMokns1 mutant was more sensitive to hydroxyurea (HU) and methyl methanesulfonate (MMS) compared to the wild-type strain 70−15. The expression of genes related to DNA damage stress pathways in the ΔMokns1 mutant was significantly different from that in the wild-type strain. Our results demonstrate that MoKns1 is an important pathogenic factor in M. oryzae involved in regulating autophagy and DNA damage response pathways, thus affecting virulence. This research on M. oryzae pathogenesis lays a foundation for the prevention and control of M. oryzae.

The appressorium at a glance

Many plant pathogenic fungi have the capacity to infect their plant hosts using specialised cells called appressoria. These structures act as a gateway between the fungus and host, allowing entry to internal tissues. Appressoria apply enormous physical force to rupture the plant surface, or use a battery of enzymes to digest the cuticle and plant cell wall. Appressoria also facilitate focal secretion of effectors at the point of plant infection to suppress plant immunity. These infection cells develop in response to the physical characteristics of the leaf surface, starvation stress and signals from the plant. Appressorium morphogenesis has been linked to septin-mediated reorganisation of F-actin and microtubule networks of the cytoskeleton, and remodelling of the fungal cell wall. In this Cell Science at a Glance and accompanying poster, we highlight recent advances in our understanding of the mechanisms of appressorium-mediated infection, and compare development on the leaf surface to the biology of invasive growth by pathogenic fungi. Finally, we outline key gaps in our current knowledge of appressorium cell biology.

Nucleosome Assembly Protein 1, Nap1, Is Required for the Growth, Development, and Pathogenicity of Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast, leading to significant reductions in rice and wheat productivity. Nap1 is a conserved protein in eukaryotes involved in diverse physiological processes, such as nucleosome assembly, histone shuttling between the nucleus and cytoplasm, transcriptional regulation, and the cell cycle. Here, we identified Nap1 and characterized its roles in fungal development and virulence in M. oryzae. MoNap1 is involved in aerial hyphal and conidiophore differentiation, sporulation, appressorium formation, plant penetration, and virulence. ΔMonap1 generated a small, elongated, and malformed appressorium with an abnormally organized septin ring on hydrophobic surfaces. ΔMonap1 was more sensitive to cell wall integrity stresses but more resistant to microtubule stresses. MoNap1 interacted with histones H₂A and H₂B and the B-type cyclin (Cyc1). Moreover, a nuclear export signal (NES) domain is necessary for Nap1’s roles in the regulation of the growth and pathogenicity of M. oryzae. In summary, NAP1 is essential for the growth, appressorium formation, and pathogenicity of M. oryzae.

Ethylene Promotes Expression of the Appressorium- and Pathogenicity-Related Genes via GPCR- and MAPK-Dependent Manners in Colletotrichum gloeosporioides

Ethylene (ET) represents a signal that can be sensed by plant pathogenic fungi to accelerate their spore germination and subsequent infection. However, the molecular mechanisms of responses to ET in fungi remain largely unclear. In this study, Colletotrichum gloeosporioides was investigated via transcriptomic analysis to reveal the genes that account for the ET-regulated fungal development and virulence. The results showed that ET promoted genes encoding for fungal melanin biosynthesis enzymes, extracellular hydrolases, and appressorium-associated structure proteins at 4 h after treatment. When the germination lasted until 24 h, ET induced multiple appressoria from every single spore, but downregulated most of the genes. Loss of selected ET responsive genes encoding for scytalone dehydratase (CgSCD1) and cerato-platanin virulence protein (CgCP1) were unable to alter ET sensitivity of C. gloeosporioides in vitro but attenuated the influence of ET on pathogenicity. Knockout of the G-protein-coupled receptors CgGPCR3-1/2 and the MAPK signaling pathway components CgMK1 and CgSte11 resulted in reduced ET sensitivity. Taken together, this study in C. gloeosporioides reports that ET can cause transcription changes in a large set of genes, which are mainly responsible for appressorium development and virulence expression, and these processes are dependent on the GPCR and MAPK pathways.

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

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

Heat Shock Transcription Factor CgHSF1 Is Required for Melanin Biosynthesis, Appressorium Formation, and Pathogenicity in Colletotrichum gloeosporioides

Heat shock transcription factors (HSFs) are a family of transcription regulators. Although HSFs’ functions in controlling the transcription of the molecular chaperone heat shock proteins and resistance to stresses are well established, their effects on the pathogenicity of plant pathogenic fungi remain unknown. In this study, we analyze the role of CgHSF1 in the pathogenicity of Colletotrichum gloeosporioides and investigate the underlying mechanism. Failure to generate the Cghsf1 knock-out mutant suggested that the gene is essential for the viability of the fungus. Then, genetic depletion of the Cghsf1 was achieved by inserting the repressive promoter of nitrite reductase gene (PniiA) before its coding sequence. The mutant showed significantly decrease in the pathogenicity repression of appressorium formation, and severe defects in melanin biosynthesis. Moreover, four melanin synthetic genes were identified as direct targets of CgHSF1. Taken together, this work highlights the role of CgHSF1 in fungal pathogenicity via the transcriptional activation of melanin biosynthesis. Our study extends the understanding of fungal HSF1 proteins, especially their involvement in pathogenicity.

Appressorium-mediated plant infection by Magnaporthe oryzae is regulated by a Pmk1-dependent hierarchical transcriptional network

Rice blast is a devastating disease caused by the fungal pathogen Magnaporthe oryzae that threatens rice production around the world. The fungus produces a specialized infection cell, called the appressorium, that enables penetration through the plant cell wall in response to surface signals from the rice leaf. The underlying biology of plant infection, including the regulation of appressorium formation, is not completely understood. Here we report the identification of a network of temporally coregulated transcription factors that act downstream of the Pmk1 mitogen-activated protein kinase pathway to regulate gene expression during appressorium-mediated plant infection. We show that this tiered regulatory mechanism involves Pmk1-dependent phosphorylation of the Hox7 homeobox transcription factor, which regulates genes associated with induction of major physiological changes required for appressorium development-including cell-cycle control, autophagic cell death, turgor generation and melanin biosynthesis-as well as controlling a additional set of virulence-associated transcription factor-encoding genes. Pmk1-dependent phosphorylation of Mst12 then regulates gene functions involved in septin-dependent cytoskeletal re-organization, polarized exocytosis and effector gene expression, which are necessary for plant tissue invasion. Identification of this regulatory cascade provides new potential targets for disease intervention.

Biological and biorational management of blast diseases in cereals caused by Magnaporthe oryzae

Blast diseases, caused by the fungal pathogen Magnaporthe oryzae, are among the most destructive diseases that occur on at least 50 species of grasses, including cultivated cereals wheat, and rice. Although fungicidal control of blast diseases has widely been researched, development of resistance of the pathogen against commercially available products makes this approach unreliable. Novel approaches such as the application of biopesticides against the blast fungus are needed for sustainable management of this economically important disease. Antagonistic microorganisms, such as fungi and probiotic bacteria from diverse taxonomic genera were found to suppress blast fungi both in vitro and in vivo. Various classes of secondary metabolites, such as alkaloids, phenolics, and terpenoids of plant and microbial origin significantly inhibit fungal growth and may also be effective in managing blast diseases. Common modes of action of microbial biocontrol agents include: antibiosis, production of lytic enzymes, induction of systemic resistance in host plant, and competition for nutrients or space. However, the precise mechanism of biocontrol of the blast fungus by antagonistic microorganisms and/or their bioactive secondary metabolites is not well understood. Commercial formulations of biocontrol agents and bioactive natural products could be cost-effective and sustainable but their availability at this time is extremely limited. This review updates our knowledge on the infection pathway of the wheat blast fungus, catalogs naturally occurring biocontrol agents that may be effective against blast diseases, and discusses their role in sustainable management of the disease.

Investigating the cell and developmental biology of plant infection by the rice blast fungus Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast disease, the most widespread and serious disease of cultivated rice. Live cell imaging and quantitative 4D image analysis have provided new insight into the mechanisms by which the fungus infects host cells and spreads rapidly in plant tissue. In this video review article, we apply live cell imaging approaches to understanding the cell and developmental biology of rice blast disease. To gain entry to host plants, M. oryzae develops a specialised infection structure called an appressorium, a unicellular dome-shaped cell which generates enormous turgor, translated into mechanical force to rupture the leaf cuticle. Appressorium development is induced by perception of the hydrophobic leaf surface and nutrient deprivation. Cargo-independent autophagy in the three-celled conidium, controlled by cell cycle regulation, is essential for appressorium morphogenesis. Appressorium maturation involves turgor generation and melanin pigment deposition in the appressorial cell wall. Once a threshold of turgor has been reached, this triggers re-polarisation which requires regulated generation of reactive oxygen species, to facilitate septin GTPase-dependent cytoskeletal re-organisation and re-polarisation of the appressorium to form a narrow, rigid penetration peg. Infection of host tissue requires a further morphogenetic transition to a pseudohyphal-type of growth within colonised rice cells. At the same time the fungus secretes an arsenal of effector proteins to suppress plant immunity. Many effectors are secreted into host cells directly, which involves a specific secretory pathway and a specialised structure called the biotrophic interfacial complex. Cell-to-cell spread of the fungus then requires development of a specialised structure, the transpressorium, that is used to traverse pit field sites, allowing the fungus to maintain host cell membrane integrity as new living plant cells are invaded. Thereafter, the fungus rapidly moves through plant tissue and host cells begin to die, as the fungus switches to necrotrophic growth and disease symptoms develop. These morphogenetic transitions are reviewed in the context of live cell imaging studies.

CgEnd3 Regulates Endocytosis, Appressorium Formation, and Virulence in the Poplar Anthracnose Fungus Colletotrichum gloeosporioides

The hemibiotrophic ascomycete fungus Colletotrichum gloeosporioides is the causal agent of anthracnose on numerous plants, and it causes considerable economic losses worldwide. Endocytosis is an essential cellular process in eukaryotic cells, but its roles in C. gloeosporioides remain unknown. In our study, we identified an endocytosis-related protein, CgEnd3, and knocked it out via polyethylene glycol (PEG)-mediated protoplast transformation. The lack of CgEnd3 resulted in severe defects in endocytosis. C. gloeosporioides infects its host through a specialized structure called appressorium, and ΔCgEnd3 showed deficient appressorium formation, melanization, turgor pressure accumulation, penetration ability of appressorium, cellophane membrane penetration, and pathogenicity. CgEnd3 also affected oxidant adaptation and the expression of core effectors during the early stage of infection. CgEnd3 contains one EF hand domain and four calcium ion-binding sites, and it is involved in calcium signaling. A lack of CgEnd3 changed the responses to cell-wall integrity agents and fungicide fludioxonil. However, CgEnd3 regulated appressorium formation and endocytosis in a calcium signaling-independent manner. Taken together, these results demonstrate that CgEnd3 plays pleiotropic roles in endocytosis, calcium signaling, cell-wall integrity, appressorium formation, penetration, and pathogenicity in C. gloeosporioides, and it suggests that CgEnd3 or endocytosis-related genes function as promising antifungal targets.

Turgor-dependent and coronin-mediated F-actin dynamics drive septin disc-to-ring remodeling in the blast fungus Magnaporthe oryzae

The fungus Magnaporthe oryzae uses a specialized pressure-generating infection cell called an appressorium to break into rice leaves and initiate disease. Appressorium functionality is dependent on the formation of a cortical septin ring during its morphogenesis, but precisely how this structure assembles is unclear. Here, we show that F-actin rings are recruited to the circumference of incipient septin disc-like structures in a pressure-dependent manner, and that this is necessary for their contraction and remodeling into rings. We demonstrate that the structural integrity of these incipient septin discs requires both an intact F-actin and microtubule cytoskeleton and provide fundamental new insight into their functional organization within the appressorium. Lastly, using proximity-dependent labeling, we identify the actin modulator coronin as a septin-proximal protein and show that F-actin-mediated septin disc-to-ring remodeling is perturbed in the genetic absence of coronin. Taken together, our findings provide new insight into the dynamic remodeling of infection-specific higher-order septin structures in a globally significant fungal plant pathogen.

Role of Two Metacaspases in Development and Pathogenicity of the Rice Blast Fungus Magnaporthe oryzae

Magnaporthe oryzae causes rice blast disease that threatens global food security by resulting in the severe loss of rice production every year. A tightly regulated life cycle allows M. oryzae to disarm the host plant immune system during its biotrophic stage before triggering plant cell death in its necrotrophic stage.

Rice blast disease caused by Magnaporthe oryzae is a devastating disease of cultivated rice worldwide. Infections by this fungus lead to a significant reduction in rice yields and threats to food security. To gain better insight into growth and cell death in M. oryzae during infection, we characterized two predicted M. oryzae metacaspase proteins, MoMca1 and MoMca2. These proteins appear to be functionally redundant and can complement the yeast Yca1 homologue. Biochemical analysis revealed that M. oryzae metacaspases exhibited Ca²⁺-dependent caspase activity in vitro. Deletion of both MoMca1 and MoMca2 in M. oryzae resulted in reduced sporulation, delay in conidial germination, and attenuation of disease severity. In addition, the double ΔMomca1mca2 mutant strain showed increased radial growth in the presence of oxidative stress. Interestingly, the ΔMomca1mca2 strain showed an increased accumulation of insoluble aggregates compared to the wild-type strain during vegetative growth. Our findings suggest that MoMca1 and MoMca2 promote the clearance of insoluble aggregates in M. oryzae, demonstrating the important role these metacaspases have in fungal protein homeostasis. Furthermore, these metacaspase proteins may play additional roles, like in regulating stress responses, that would help maintain the fitness of fungal cells required for host infection.

The Biology of Invasive Growth by the Rice Blast Fungus Magnaporthe oryzae

This introductory chapter describes the life cycle of Magnaporthe oryzae, the causal agent of rice blast disease. During plant infection, M. oryzae forms a specialized infection structure called an appressorium, which generates enormous turgor, applied as a mechanical force to breach the rice cuticle. Appressoria form in response to physical cues from the hydrophobic rice leaf cuticle and nutrient availability. The signaling pathways involved in perception of surface signals are described and the mechanism by which appressoria function is also introduced. Re-polarization of the appressorium requires a septin complex to organize a toroidal F-actin network at the base of the cell. Septin aggregation requires a turgor-dependent sensor kinase, Sln1, necessary for re-polarization of the appressorium and development of a rigid penetration hypha to rupture the leaf cuticle. Once inside the plant, the fungus undergoes secretion of a large set of effector proteins, many of which are directed into plant cells using a specific secretory pathway. Here they suppress plant immunity, but can also be perceived by rice immune receptors, triggering resistances. M. oryzae then manipulates pit field sites, containing plasmodesmata, to facilitate rapid spread from cell to cell in plant tissue, leading to disease symptom development.

Incompatibility between proliferation and plant invasion is mediated by a regulator of appressorium formation in the corn smut fungus Ustilago maydis

Plant pathogenic fungi often developed specialized infection structures to breach the outer surface of a host plant. These structures, called appressoria, lead the invasion of the plant by the fungal hyphae. Studies in different phytopathogenic fungi showed that appressorium formation seems to be subordinated to the cell cycle. This subordination ensures the loading in the invading hypha of the correct genetic information to proceed with plant infection. However, how the cell cycle transmits its condition to the genetic program controlling appressorium formation and promoting the plant's invasion is unknown. Our results have uncovered how this process occurs for the appressorium of Ustilago maydis, the agent responsible for corn smut disease. Here, we described that the complex Clb2-cyclin-dependent kinase (Cdk)1, one of the master regulators of G2/M cell cycle progression in U. maydis, interacts and controls the subcellular localization of Biz1, a transcriptional factor required for the activation of the appressorium formation. Besides, Biz1 can arrest the cell cycle by down-regulation of the gene encoding a second b-cyclin Clb1 also required for the G2/M transition. These results revealed a negative feedback loop between appressorium formation and cell cycle progression in U. maydis, which serves as a "toggle switch" to control the fungal decision between infecting the plant or proliferating out of the plant.

A cyclin protein governs the infectious and sexual life cycles of Cryptococcus neoformans

Cell cycle is a fundamental process underlying growth and development in evolutionarily diverse organisms, including fungi. In human fungal pathogens, cell cycle control generally determines their life cycles, either in the environment or during infections. Thus, cell cycle components can potentially serve as important targets for the development of antifungal strategy against fungal infections. Here, in Cryptococcus neoformans, the most common cause of fatal fungal meningitis, we show that a previously uncharacterized B-type cyclin named Cbc1 is essential for both its infectious and sexual cycles. We reveal that Cbc1 coordinates various sexual differentiation and molecular processes, including meiosis. Especially, the absence of Cbc1 abolishes formation of sexual spores in C. neoformans, which are presumed infectious particles. Cbc1 is also required for the major Cryptococcus pathogenic attributes. Virulence assessment using the murine model of cryptococcosis revealed that the cbc1 mutant is avirulent. Together, our results provide an important insight into how C. neoformans employs shared cell cycle regulation to coordinate its infectious and sexual cycles, which are considered crucial for virulence evolution and the production of infectious spores.

Magnaporthe oryzae nucleoside diphosphate kinase is required for metabolic homeostasis and redox-mediated host innate immunity suppression

The fungus Magnaporthe oryzae causes blast, the most devastating disease of cultivated rice. After penetrating the leaf cuticle, M. oryzae grows as a biotroph in intimate contact with living rice epidermal cells before necrotic lesions develop. Biotrophic growth requires maintaining metabolic homeostasis while suppressing plant defenses, but the metabolic connections and requirements involved are largely unknown. Here, we characterized the M. oryzae nucleoside diphosphate kinase-encoding gene NDK1 and discovered it was essential for facilitating biotrophic growth by suppressing the host oxidative burst-the first line of plant defense. NDK enzymes reversibly transfer phosphate groups from tri- to diphosphate nucleosides. Correspondingly, intracellular nucleotide pools were perturbed in M. oryzae strains lacking NDK1 through targeted gene deletion, compared to WT. This affected metabolic homeostasis: TCA, purine and pyrimidine intermediates, and oxidized NADP⁺ , accumulated in Δndk1. cAMP and glutathione were depleted. ROS accumulated in Δndk1 hyphae. Functional appressoria developed on rice leaf sheath surfaces, but Δndk1 invasive hyphal growth was restricted and redox homeostasis was perturbed, resulting in unsuppressed host oxidative bursts that triggered immunity. We conclude Ndk1 modulates intracellular nucleotide pools to maintain redox balance via metabolic homeostasis, thus quenching the host oxidative burst and suppressing rice innate immunity during biotrophy.

Spermine-mediated tight sealing of the Magnaporthe oryzae appressorial pore-rice leaf surface interface

Cellular adhesion mediates many important plant-microbe interactions. In the devastating blast fungus Magnaporthe oryzae¹, powerful glycoprotein-rich mucilage adhesives² cement melanized and pressurized dome-shaped infection cells-appressoria-to host rice leaf surfaces. Enormous internal turgor pressure is directed onto a penetration peg emerging from the unmelanized, thin-walled pore at the appressorial base^1-4, forcing it through the leaf cuticle where it elongates invasive hyphae in underlying epidermal cells⁵. Mucilage sealing around the appressorial pore facilitates turgor build-up², but the molecular underpinnings of mucilage secretion and appressorial adhesion are unknown. Here, we discovered an unanticipated and sole role for spermine in facilitating mucilage production by mitigating endoplasmic reticulum (ER) stress in the developing appressorium. Mutant strains lacking the spermine synthase-encoding gene SPS1 progressed through all stages of appressorial development, including penetration peg formation, but cuticle penetration was unsuccessful due to reduced appressorial adhesion, which led to solute leakage. Mechanistically, spermine neutralized off-target oxygen free radicals produced by NADPH oxidase-1 (Nox1)^3,6 that otherwise elicited ER stress and the unfolded protein response, thereby critically reducing mucilage secretion. Our study reveals that spermine metabolism via redox buffering of the ER underpins appressorial adhesion and rice cell invasion and provides insights into a process that is fundamental to host plant infection.

Autophagy machinery promotes the chaperone-mediated formation and compartmentalization of protein aggregates during appressorium development by the rice blast fungus

The chaperone-mediated sequestration of misfolded proteins into specialized quality control compartments represents an important strategy for maintaining protein homeostasis in response to stress. However, precisely how this process is controlled in time and subcellular space and integrated with the cell's protein refolding and degradation pathways remains unclear. We set out to understand how aggregated proteins are managed during infection-related development by a globally devastating plant pathogenic fungus and to determine how impaired protein quality control impacts cellular differentiation and pathogenesis in this system. Here we show that in the absence of Hsp104 disaggregase activity, aggregated proteins are spatially sequestered into quality control compartments within conidia, but not within terminally differentiated infection cells, and thus spatial protein quality control is cell type–dependent. We demonstrate that impaired aggregate resolution results in a short-term developmental penalty but has no significant impact upon appressorium function. Finally, we show that, somewhat unexpectedly, the autophagy machinery is necessary for the normal formation and compartmentalization of protein aggregates. Taken together, our findings provide important new insight into spatial protein quality control during the process of terminal cellular differentiation by a globally important model eukaryote and reveal a new level of interplay between major proteostasis pathways.

Terminating rice innate immunity induction requires a network of antagonistic and redox-responsive E3 ubiquitin ligases targeting a fungal sirtuin

Fungal phytopathogens can suppress plant immune mechanisms in order to colonize living host cells. Identifying all the molecular components involved is critical for elaborating a detailed systems-level model of plant infection probing pathogen weaknesses; yet, the hierarchy of molecular events controlling fungal responses to the plant cell is not clear. Here we show how, in the blast fungus Magnaporthe oryzae, terminating rice innate immunity requires a dynamic network of redox-responsive E3 ubiquitin ligases targeting fungal sirtuin 2 (Sir2), an antioxidation regulator required for suppressing the host oxidative burst. Immunoblotting, immunopurification, mass spectrometry and gene functional analyses showed that Sir2 levels responded to oxidative stress via a mechanism involving ubiquitination and three antagonistic E3 ubiquitin ligases: Grr1 and Ptr1 maintained basal Sir2 levels in the absence of oxidative stress; Upl3 facilitated Sir2 accumulation in response to oxidative stress. Grr1 and Upl3 interacted directly with Sir2 in a manner that decreased and scaled with oxidative stress, respectively. Deleting UPL3 depleted Sir2 during growth in rice cells, triggering host immunity and preventing infection. Overexpressing SIR2 in the Δupl3 mutant remediated pathogenicity. Our work reveals how redox-responsive E3 ubiquitin ligases in M. oryzae mediate Sir2 accumulation-dependent antioxidation to modulate plant innate immunity and host susceptibility.

Developmentally Regulated Oscillations in the Expression of UV Repair Genes in a Soilborne Plant Pathogen Dictate UV Repair Efficiency and Survival

Fusarium oxysporum infects plants through the roots and therefore is not exposed to the sun regularly. However, the ability to survive sun exposure expands the distribution of the population. UV from the sun is toxic and mutagenic, and to survive sun exposure, fungi encode several DNA repair mechanisms. We found that Fusarium oxysporum has a gene expression program that activates photolyase at the first hours of germination when the pathogen is not established in the plant tissue. Later on, the expression of photolyase decreases, and the expression of a light-independent UV repair mechanism increases. We suggest a novel point of view to a very fundamental question of how soilborne microorganisms defend themselves against sudden UV exposure.

The ability to withstand UV damage shapes the ecology of microbes. While mechanisms of UV tolerance were extensively investigated in microorganisms regularly exposed to the sun, far less is known about UV repair of soilborne microorganisms. Fusarium oxysporum is a soilborne fungal plant pathogen that is resistant to UV light. We hypothesized that its UV repair capacity is induced to deal with irregular sun exposure. Unlike the SOS paradigm, our analysis revealed only sporadic increases and even decreases in UV repair gene expression following UVC irradiation or exposure to visible light. Strikingly, a major factor determining the expression of UV repair genes was the developmental status of the fungus. At the early stages of germination, the expression of photolyase increased while the expression of UV endonuclease decreased, and then the trend was reversed. These gene expression oscillations were dependent on cell cycle progression. Consequently, the contribution of photoreactivation to UV repair and survival was stronger at the beginning of germination than later when a filament was established. F. oxysporum germinates following cues from the host. Early on in germination, it is most vulnerable to UV; when the filament is established, the pathogen is protected from the sun because it is already within the host tissue.

Two protein kinases UvPmk1 and UvCDC2 with significant functions in conidiation, stress response and pathogenicity of rice false smut fungus Ustilaginoidea virens

Ustilaginoidea virens is an important fungus causing rice false smut, a devastating disease on spikelets of rice. In this study, we identified and characterized two CMGC (CDK/MAPK/GSK3/CLK) kinase genes, UvPmk1 and UvCDC2, in U. virens. Although UvPmk1 and UvCDC2 are, respectively, homologous to Fus3/Kss1 mitogen-activated protein kinases (MAPKs) and cyclin-dependent kinases (CDKs), they all have a conserved serine/threonine protein kinase domain. The qRT-PCR analysis of the relative expression of UvPmk1 and UvCDC2 during the infection of U. virens showed that these two genes were highly expressed during infection. UvPmk1 and UvCDC2 knockout mutants exhibited no significant changes in mycelial vegetative growth but decreases in conidiation. In addition, both UvPmk1 and UvCDC2 knockout mutants showed increases in tolerance to hyperosmotic and cell wall stresses, but they, respectively, exhibited decreases and increases in tolerance to oxidative stress compared with the wild-type strain HWD-2. Pathogenicity and infection assays demonstrated the defective growth of infection hyphae and significant loss of virulence in UvPmk1 and UvCDC2 knockout mutants. Taken together, our results demonstrate that UvPmk1 and UvCDC2 play important roles in the conidiation, stress response, and pathogenicity of U. virens.

Metabolomics Analysis Identifies Sphingolipids as Key Signaling Moieties in Appressorium Morphogenesis and Function in Magnaporthe oryzae

The blast fungus initiates infection using a heavily melanized, dome-shaped infection structure known as the appressorium, which forcibly ruptures the cuticle to enter the rice leaf tissue. How this process takes place remains not fully understood. Here, we used untargeted metabolomics analyses to profile the metabolome of developing appressoria and identified significant changes in six key metabolic pathways, including early sphingolipid biosynthesis. Analyses employing small molecule inhibitors, gene disruption, or genetic and chemical complementation demonstrated that ceramide compounds of the sphingolipid biosynthesis pathway are essential for normal appressorial development controlled by mitosis. In addition, ceramide was found to act upstream from the protein kinase C-mediated cell wall integrity pathway during appressorium repolarization and pathogenicity in rice blast. Further discovery of the sphingolipid biosynthesis pathway revealed that glucosylceramide (GlcCer) synthesized by ceramide is the key substance affecting the pathogenicity of Magnaporthe oryzae Our results provide new insights into the chemical moieties involved in the infection-related signaling networks, thereby revealing a potential target for the development of novel control agents against the major disease of rice and other cereals.IMPORTANCE Our untargeted analysis of metabolomics throughout the course of pathogenic development gave us an unprecedented high-resolution view of major shifts in metabolism that occur in the topmost fungal pathogen that infects rice, wheat, barley, and millet. Guided by these metabolic insights, we demonstrated their practical application by using two different small-molecule inhibitors of sphingolipid biosynthesis enzymes to successfully block the pathogenicity of M. oryzae Our study thus defines the sphingolipid biosynthesis pathway as a key step and potential target that can be exploited for the development of antifungal agents. Furthermore, future investigations that exploit such important metabolic intermediates will further deepen our basic understanding of the molecular mechanisms underlying the establishment of fungal blast disease in important cereal crops.

Ribonucleotide reductase from Fusarium oxysporum does not Respond to DNA replication stress

Ribonucleotide reductase (RNR) catalyzes the rate limiting step in dNTP biosynthesis and is tightly regulated at the transcription and activity levels. One of the best characterized responses of yeast to DNA damage is up-regulation of RNR transcription and activity and consequently, elevation of the dNTP pools. Hydroxyurea is a universal inhibitor of RNR that causes S phase arrest. It is used in the clinic to treat certain types of cancers. Here we studied the response of the fungal plant pathogen Fusarium oxysporum to hydroxyurea in order to generate hypotheses that can be used in the future in development of a new class of pesticides. F. oxysporum causes severe damage to more than 100 agricultural crops and specifically threatens banana cultivation world-wide. Although the recovery of F. oxysporum from transient hydroxyurea exposure was similar to the one of Saccharomyces cerevisiae, colony formation was strongly inhibited in F. oxysporum in comparison with S. cerevisiae. As expected, genomic and phosphoproteomic analyses of F. oxysporum conidia (spores) exposed to hydroxyurea showed hallmarks of DNA replication perturbation and activation of recombination. Unexpectedly and strikingly, RNR was not induced by either hydroxyurea or the DNA-damaging agent methyl methanesulfonate as determined at the RNA and protein levels. Consequently, dNTP concentrations were significantly reduced, even in response to a low dose of hydroxyurea. Methyl methanesulfonate treatment did not induce dNTP pools in F. oxysporum, in contrast to the response of RNR and dNTP pools to DNA damage and hydroxyurea in several tested organisms. Our results are important because the lack of a feedback mechanism to increase RNR expression in F. oxysporum is expected to sensitize the pathogen to a fungal-specific ribonucleotide inhibitor. The potential impact of our observations on F. oxysporum genome stability and genome evolution is discussed.

Plant pathogenic fungi Colletotrichum and Magnaporthe share a common G1 phase monitoring strategy for proper appressorium development

To breach the plant cuticle, many plant pathogenic fungi differentiate specialized infection structures (appressoria). In Colletotrichum orbiculare (cucumber anthracnose fungus), this differentiation requires unique proper G₁ /S phase progression, regulated by two-component GTPase activating protein CoBub2/CoBfa1 and GTPase CoTem1. Since their homologues regulate mitotic exit, cytokinesis, or septum formation from yeasts to mammals, we asked whether the BUB2 function in G₁ /S progression is specific to plant pathogenic fungi. Colletotrichum higginsianum and Magnaporthe oryzae were genetically analyzed to investigate conservation of BUB2 roles in cell cycle regulation, septum formation, and virulence. Expression profile of cobub2Δ was analyzed using a custom microarray. In bub2 mutants of both fungi, S phase initiation was earlier, and septum formation coordinated with a septation initiation network protein and contractile actin ring was impaired. Earlier G₁ /S transition in cobub2Δ results in especially high expression of DNA replication genes and differing regulation of virulence-associated genes that encode proteins such as carbohydrate-active enzymes and small secreted proteins. The virulence of chbub2Δ and mobub2Δ was significantly reduced. Our evidence shows that BUB2 regulation of G₁ /S transition and septum formation supports its specific requirement for appressorium development in plant pathogenic fungi.