Disruption of putative short-chain acyl-CoA dehydrogenases compromised free radical scavenging, conidiogenesis, and pathogenesis of Magnaporthe oryzae

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/.

MoLrp1-mediated signaling induces nuclear accumulation of MoMsn2 to facilitate fatty acid oxidation for infectious growth of the rice blast fungus

Fatty acid β-oxidation is critical for fatty acid degradation and cellular development. In the rice blast fungus Magnaporthe oryzae, fatty acid β-oxidation is reported to be important mainly for turgor generation in the appressorium. However, the role of fatty acid β-oxidation during invasive hyphal growth is rarely documented. We demonstrated that blocking peroxisomal fatty acid β-oxidation impaired lipid droplet (LD) degradation and infectious growth of M. oryzae. We found that the key regulator of pathogenesis, MoMsn2, which we identified previously, is involved in fatty acid β-oxidation by targeting MoDCI1 (encoding dienoyl-coenzyme A [CoA] isomerase), which is also important for LD degradation and infectious growth. Cytological observations revealed that MoMsn2 accumulated from the cytosol to the nucleus during early infection or upon treatment with oleate. We determined that the low-density lipoprotein receptor-related protein MoLrp1, which is also involved in fatty acid β-oxidation and infectious growth, plays a critical role in the accumulation of MoMsn2 from the cytosol to the nucleus by activating the cyclic AMP signaling pathway. Our results provide new insights into the importance of fatty acid oxidation during invasive hyphal growth, which is modulated by MoMsn2 and its related signaling pathways in M. oryzae.

Appressoria-Small but Incredibly Powerful Structures in Plant-Pathogen Interactions

Plant-pathogenic fungi are responsible for many of the most severe crop diseases in the world and remain very challenging to control. Improving current protection strategies or designating new measures based on an overall understanding of molecular host-pathogen interaction mechanisms could be helpful for disease management. The attachment and penetration of the plant surface are the most important events among diverse plant-fungi interactions. Fungi evolved as small but incredibly powerful infection structure appressoria to facilitate attachment and penetration. Appressoria are indispensable for many diseases, such as rusts, powdery mildews, and blast diseases, as well as devastating oomycete diseases. Investigation into the formation of plant-pathogen appressoria contributes to improving the understanding of the molecular mechanisms of plant-pathogen interactions. Fungal host attachment is a vital step of fungal pathogenesis. Here, we review recent advances in the molecular mechanisms regulating the formation of appressoria. Additionally, some biocontrol agents were revealed to act on appressorium. The regulation of fungal adhesion during the infective process by acting on appressoria formation is expected to prevent the occurrence of crop disease caused by some pathogenic fungi.

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.

Magnaporthe oryzae Chloroplast Targeting Endo-β-1,4-Xylanase I MoXYL1A Regulates Conidiation, Appressorium Maturation and Virulence of the Rice Blast Fungus

Endo-β-1,4-Xylanases are a group of extracellular enzymes that catalyze the hydrolysis of xylan, a principal constituent of the plant primary cell wall. The contribution of Endo-β-1,4-Xylanase I to both physiology and pathogenesis of the rice blast fungus M. oryzae is unknown. Here, we characterized the biological function of two endoxylanase I (MoXYL1A and MoXYL1B) genes in the development of M. oryzae using targeted gene deletion, biochemical analysis, and fluorescence microscopy. Phenotypic analysis of ∆Moxyl1A strains showed that MoXYL1A is required for the full virulence of M. oryzae but is dispensable for the vegetative growth of the rice blast fungus. MoXYL1B, in contrast, did not have a clear role in the infectious cycle but has a critical function in asexual reproduction of the fungus. The double deletion mutant was severely impaired in pathogenicity and virulence as well as asexual development. We found that MoXYL1A deletion compromised appressorium morphogenesis and function, leading to failure to penetrate host cells. Fluorescently tagged MoXYL1A and MoXYL1B displayed cytoplasmic localization in M. oryzae, while analysis of MoXYL1A-GFP and MoXYL1B-GFP in-planta revealed translocation and accumulation of these effector proteins into host cells. Meanwhile, sequence feature analysis showed that MoXYL1A possesses a transient chloroplast targeting signal peptide, and results from an Agrobacterium infiltration assay confirmed co-localization of MoXYL1A-GFP with ChCPN10C-RFP in the chloroplasts of host cells. MoXYL1B, accumulated to the cytoplasm of the host. Taken together, we conclude that MoXYL1A is a secreted effector protein that likely promotes the virulence of M. oryzae by interfering in the proper functioning of the host chloroplast, while the related xylanase MoXYL1B does not have a major role in virulence of M. oryzae.

AGC/AKT Protein Kinase SCH9 Is Critical to Pathogenic Development and Overwintering Survival in Magnaporthe oryzae

Primary inoculum that survives overwintering is one of the key factors that determine the outbreak of plant disease. Pathogenic resting structures, such as chlamydospores, are an ideal inoculum for plant disease. Puzzlingly, Magnaporthe oryzae, a devastating fungal pathogen responsible for blast disease in rice, hardly form any morphologically changed resting structures, and we hypothesize that M. oryzae mainly relies on its physiological alteration to survive overwintering or other harsh environments. However, little progress on research into regulatory genes that facilitate the overwintering of rice blast pathogens has been made so far. Serine threonine protein kinase AGC/AKT, MoSch9, plays an important role in the spore-mediated pathogenesis of M. oryzae. Building on this finding, we discovered that in genetic and biological terms, MoSch9 plays a critical role in conidiophore stalk formation, hyphal-mediated pathogenesis, cold stress tolerance, and overwintering survival of M. oryzae. We discovered that the formation of conidiophore stalks and disease propagation using spores was severely compromised in the mutant strains, whereas hyphal-mediated pathogenesis and the root infection capability of M. oryzae were completely eradicated due to MoSch9 deleted mutants’ inability to form an appressorium-like structure. Most importantly, the functional and transcriptomic study of wild-type and MoSch9 mutant strains showed that MoSch9 plays a regulatory role in cold stress tolerance of M. oryzae through the transcription regulation of secondary metabolite synthesis, ATP hydrolyzing, and cell wall integrity proteins during osmotic stress and cold temperatures. From these results, we conclude that MoSch9 is essential for fungal infection-related morphogenesis and overwintering of M. oryzae.

Revealing the high variability on nonconserved core and mobile elements of Austropuccinia psidii and other rust mitochondrial genomes

Mitochondrial genomes are highly conserved in many fungal groups, and they can help characterize the phylogenetic relationships and evolutionary biology of plant pathogenic fungi. Rust fungi are among the most devastating diseases for economically important crops around the world. Here, we report the complete sequence and annotation of the mitochondrial genome of Austropuccinia psidii (syn. Puccinia psidii), the causal agent of myrtle rust. We performed a phylogenomic analysis including the complete mitochondrial sequences from other rust fungi. The genome composed of 93.299 bp has 73 predicted genes, 33 of which encoded nonconserved proteins (ncORFs), representing almost 45% of all predicted genes. A. psidii mtDNA is one of the largest rust mtDNA sequenced to date, most likely due to the abundance of ncORFs. Among them, 33% were within intronic regions of diverse intron groups. Mobile genetic elements invading intron sequences may have played significant roles in size but not shaping of the rust mitochondrial genome structure. The mtDNAs from rust fungi are highly syntenic. Phylogenetic inferences with 14 concatenated mitochondrial proteins encoded by the core genes placed A. psidii according to phylogenetic analysis based on 18S rDNA. Interestingly, cox1, the gene with the greatest number of introns, provided phylogenies not congruent with the core set. For the first time, we identified the proteins encoded by three A. psidii ncORFs using proteomics analyses. Also, the orf208 encoded a transmembrane protein repressed during in vitro morphogenesis. To the best of our knowledge, we presented the first report of a complete mtDNA sequence of a member of the family Sphaerophragmiacea.

eIF3k Domain-Containing Protein Regulates Conidiogenesis, Appressorium Turgor, Virulence, Stress Tolerance, and Physiological and Pathogenic Development of Magnaporthe oryzae Oryzae

The eukaryotic translation initiation factor 3 (eIF3) complex consists of essential and non-essential sub-complexes. Non-essential eIF3 complex subunits, such as eIF3e, eIF3j, eIF3k, and eIF3l, modulate stress tolerance and enhance the lifespan of Neurospora crassa and Caenorhabditis elegans. However, there is limited knowledge of the role of the non-essential eIF3 sub-complex in the pathophysiological development of plant fungal pathogens. Here, we deployed genetic and biochemical techniques to explore the influence of a hypothetical protein containing eIF3k domain in Magnaporthe oryzae Oryzae (MoOeIF3k) on reproduction, hyphae morphogenesis, stress tolerance, and pathogenesis. Also, the targeted disruption of MoOeIF3k suppressed vegetative growth and asexual sporulation in ΔMoOeif3k strains significantly. We demonstrated that MoOeIF3k promotes the initiation and development of the rice blast disease by positively regulating the mobilization and degradation of glycogen, appressorium integrity, host penetration, and colonization during host-pathogen interaction. For the first time, we demonstrated that the eIF3k subunit supports the survival of the blast fungus by suppressing vegetative growth and possibly regulating the conversions and utilization of stored cellular energy reserves under starvation conditions. We also observed that the deletion of MoOeIF3k accelerated ribosomal RNA (rRNA) generation in the ΔMoOeif3k strains with a corresponding increase in total protein output. In summary, this study unravels the pathophysiological significance of eIF3k filamentous fungi. The findings also underscored the need to systematically evaluate the individual subunits of the non-essential eIF3 sub-complex during host-pathogen interaction. Further studies are required to unravel the influence of synergetic coordination between translation and transcriptional regulatory machinery on the pathogenesis of filamentous fungi pathogens.

Translation Initiation Factor eIF4E Positively Modulates Conidiogenesis, Appressorium Formation, Host Invasion and Stress Homeostasis in the Filamentous Fungi Magnaporthe oryzae

Translation initiation factor eIF4E generally mediates the recognition of the 5'cap structure of mRNA during the recruitment of the ribosomes to capped mRNA. Although the eIF4E has been shown to regulate stress response in Schizosaccharomyces pombe positively, there is no direct experimental evidence for the contributions of eIF4E to both physiological and pathogenic development of filamentous fungi. We generated Magnaporthe oryzae eIF4E (MoeIF4E3) gene deletion strains using homologous recombination strategies. Phenotypic and biochemical analyses of MoeIF4E3 defective strains showed that the deletion of MoeIF4E3 triggered a significant reduction in growth and conidiogenesis. We also showed that disruption of MoeIF4E3 partially impaired conidia germination, appressorium integrity and attenuated the pathogenicity of ΔMoeif4e3 strains. In summary, this study provides experimental insights into the contributions of the eIF4E3 to the development of filamentous fungi. Additionally, these observations underscored the need for a comprehensive evaluation of the translational regulatory machinery in phytopathogenic fungi during pathogen-host interaction progression.

Fatty Acid Synthase Beta Dehydratase in the Lipid Biosynthesis Pathway Is Required for Conidiogenesis, Pigmentation and Appressorium Formation in Magnaporthe oryzae S6

Lipid biosynthesis produces glycerol, which is important in fueling turgor pressure necessary for germination and penetration of plant host by fungi. As the relationship between pathogenicity and the lipid biosynthetic pathway is not fully understood, we have elucidated the role of the fatty acid synthase beta subunit dehydratase (FAS1) gene in lipid biosynthesis. The FAS1 gene was silenced through homologous double crossover in Magnaporthe oryzae strain S6 to study the effect on lipid biosynthesis. The vegetative growth of Δfas1 mutants show the highest drop on oleic acid (between 10 and 50%), while the mycelial dry weight of mutants dropped significantly on all media. Conidiation of FAS1 mutants show a ~10- and ~5-fold reduction on oatmeal and Potato Dextrose Agar (PDA), respectively. Mutants formed mycelium that were mildly pigmented, indicating that the deletion of FAS1 may have affected melanin biosynthesis. Biochemical and gene expression studies concluded that the fatty acid degradation pathway might have been interrupted by FAS1 deletion. FAS1 mutants showed no enzyme activity on glucose or olive oil, suggesting that the mutants may lack functional peroxisomes and be defective in β-oxidation of fatty acids, hence explaining the reduced lipid deposits in the spores.

Mitochondrial FgEch1 is responsible for conidiation and full virulence in Fusarium graminearum

Enoyl-CoA hydratase (Ech) is an important and well-recognized enzyme that functions in the degradation of fatty acids by β-oxidation. However, its functions in plant pathogenic fungi are not well known. We characterized an Ech1 orthologue, FgEch1, in Fusarium graminearum. The FgEch1 deletion mutant was defective in the utilization of short-chain fatty acids and conidiation, but not in hyphal growth on glucose-rich media or in perithecium formation. The FgEch1 deletion mutant showed reduced deoxynivalenol (DON) production and virulence in plants. Deletion of FgEch1 also led to increased production of lipid droplets and autophagy. FgEch1, which was localized in the mitochondrion, required the MTS domain for mitochondrial localization and function in F. graminearum. Taken together, these data indicate that mitochondrial FgEch1 is important for conidiation, DON production, and plant infection.