Calpains are involved in asexual and sexual development, cell wall integrity and pathogenicity of the rice blast fungus

Dihydroorotase MoPyr4 is required for development, pathogenicity, and autophagy in rice blast fungus

Dihydroorotase (DHOase) is the third enzyme in the six enzymatic reaction steps of the endogenous pyrimidine nucleotide de novo biosynthesis pathway, which is a metabolic pathway conserved in both bacteria and eukaryotes. However, research on the biological function of DHOase in plant pathogenic fungi is very limited. In this study, we identified and named MoPyr4, a homologous protein of Saccharomyces cerevisiae DHOase Ura4, in the rice blast fungus Magnaporthe oryzae and investigated its ability to regulate fungal growth, pathogenicity, and autophagy. Deletion of MoPYR4 led to defects in growth, conidiation, appressorium formation, the transfer and degradation of glycogen and lipid droplets, appressorium turgor accumulation, and invasive hypha expansion in M. oryzae, which eventually resulted in weakened fungal pathogenicity. Long-term replenishment of exogenous uridine-5'-phosphate (UMP) can effectively restore the phenotype and virulence of the ΔMopyr4 mutant. Further study revealed that MoPyr4 also participated in the regulation of the Pmk1-MAPK signaling pathway, co-localized with peroxisomes for the oxidative stress response, and was involved in the regulation of the Osm1-MAPK signaling pathway in response to hyperosmotic stress. In addition, MoPyr4 interacted with MoAtg5, the core protein involved in autophagy, and positively regulated autophagic degradation. Taken together, our results suggested that MoPyr4 for UMP biosynthesis was crucial for the development and pathogenicity of M. oryzae. We also revealed that MoPyr4 played an essential role in the external stress response and pathogenic mechanism through participation in the Pmk1-MAPK signaling pathway, peroxisome-related oxidative stress response mechanism, the Osm1-MAPK signaling pathway and the autophagy pathway.

Deciphering the Role of PIG1 and DHN-Melanin in Scedosporium apiospermum Conidia

Scedosporium apiospermum is a saprophytic filamentous fungus involved in human infections, of which the virulence factors that contribute to pathogenesis are still poorly characterized. In particular, little is known about the specific role of dihydroxynaphtalene (DHN)-melanin, located on the external layer of the conidia cell wall. We previously identified a transcription factor, PIG1, which may be involved in DHN-melanin biosynthesis. To elucidate the role of PIG1 and DHN-melanin in S. apiospermum, a CRISPR-Cas9-mediated PIG1 deletion was carried out from two parental strains to evaluate its impact on melanin biosynthesis, conidia cell-wall assembly, and resistance to stress, including the ability to survive macrophage engulfment. ΔPIG1 mutants did not produce melanin and showed a disorganized and thinner cell wall, resulting in a lower survival rate when exposed to oxidizing conditions, or high temperature. The absence of melanin increased the exposure of antigenic patterns on the conidia surface. PIG1 regulates the melanization of S. apiospermum conidia, and is involved in the survival to environmental injuries and to the host immune response, that might participate in virulence. Moreover, a transcriptomic analysis was performed to explain the observed aberrant septate conidia morphology and found differentially expressed genes, underlining the pleiotropic function of PIG1.

De Novo Purine Nucleotide Biosynthesis Pathway Is Required for Development and Pathogenicity in Magnaporthe oryzae

Purine nucleotides are indispensable compounds for many organisms and participate in basic vital activities such as heredity, development, and growth. Blocking of purine nucleotide biosynthesis may inhibit proliferation and development and is commonly used in cancer therapy. However, the function of the purine nucleotide biosynthesis pathway in the pathogenic fungus Magnaporthe oryzae is not clear. In this study, we focused on the de novo purine biosynthesis (DNPB) pathway and characterized MoAde8, a phosphoribosylglycinamide formyltransferase, catalyzing the third step of the DNPB pathway in M. oryzae. MoAde8 was knocked out, and the mutant (∆Moade8) exhibited purine auxotroph, defects in aerial hyphal growth, conidiation, and pathogenicity, and was more sensitive to hyperosmotic stress and oxidative stress. Moreover, ∆Moade8 caused decreased activity of MoTor kinase due to blocked purine nucleotide synthesis. The autophagy level was also impaired in ∆Moade8. Additionally, MoAde5, 7, 6, and 12, which are involved in de novo purine nucleotide biosynthesis, were also analyzed, and the mutants showed defects similar to the defects of ∆Moade8. In summary, de novo purine nucleotide biosynthesis is essential for conidiation, development, and pathogenicity in M. oryzae.

Casein Kinase 2 Mediates Degradation of Transcription Factor Pcf1 during Appressorium Formation in the Rice Blast Fungus

The appressorium is a specialized structure that is differentiated from Magnaporthe oryzae spores that can infect host cells. In the process of cellular transformation from spore to appressorium, the contents inside the spores are transferred into appressoria, accompanied by major differences in the gene expression model. In this study, we reported a transcription factor (TF), Pcf1, which was depressed at the transcription level and degraded at the protein level in nuclei of incipient appressoria at four hpi (hours post inoculation). To investigate its degradation mechanism, the interacting proteins of Pcf1 were identified using an immunoprecipitation-mass spectrometry (IP-MS) assay. Yeast two-hybrid (Y2H) and co-IP (co-immunoprecipitation) assays confirmed that Pcf1 interacted with the casein kinase 2 (CK2) holoenzyme through direct combination with the CKb2 subunit. Moreover, Pcf1 was ubiquitinated in the hyphae. These changes in Pcf1 protein levels in nuclei provide a new clue of how TFs are degraded during appressorium formation: temporarily unnecessary TFs in spores are phosphorylated through interacting with CK2 enzyme and are then ubiquitinated and digested by the ubiquitin proteasome system (UPS).

A Putative D-Arabinono-1,4-lactone Oxidase, MoAlo1, Is Required for Fungal Growth, Conidiogenesis, and Pathogenicity in Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast outbreaks. L-ascorbic acid (ASC) is a famous antioxidant found in nature. However, while ASC is rare or absent in fungi, a five-carbon analog, D-erythroascorbic acid (EASC), seems to appear to be a substitute for ASC. Although the antioxidant function of ASC has been widely described, the specific properties and physiological functions of EASC remain poorly understood. In this study, we identified a D-arabinono-1,4-lactone oxidase (ALO) domain-containing protein, MoAlo1, and found that MoAlo1 was localized to mitochondria. Disruption of MoALO1 (ΔMoalo1) exhibited defects in vegetative growth as well as conidiogenesis. The ΔMoalo1 mutant was found to be more sensitive to exogenous H₂O₂. Additionally, the pathogenicity of conidia in the ΔMoalo1 null mutant was reduced deeply in rice, and defective penetration of appressorium-like structures (ALS) formed by the hyphal tips was also observed in the ΔMoalo1 null mutant. When exogenous EASC was added to the conidial suspension, the defective pathogenicity of the ΔMoalo1 mutant was restored. Collectively, MoAlo1 is essential for growth, conidiogenesis, and pathogenicity in M. oryzae.

PoMet3 and PoMet14 associated with sulfate assimilation are essential for conidiogenesis and pathogenicity in Pyricularia oryzae

Pyricularia oryzae is the causal agent of blast disease on staple gramineous crops. Sulphur is an essential element for the biosynthesis of cysteine and methionine in fungi. Here, we targeted the P. oryzae PoMET3 encoding the enzyme ATP sulfurylase, and PoMET14 encoding the APS (adenosine-5'-phosphosulphate) kinase that are involved in sulfate assimilation and sulphur-containing amino acids biosynthesis. In P. oryzae, deletion of PoMET3 or PoMET14 separately results in defects of conidiophore formation, significant impairments in conidiation, methionine and cysteine auxotrophy, limited invasive hypha extension, and remarkably reduced virulence on rice and barley. Furthermore, the defects of the null mutants could be restored by supplementing with exogenous cysteine or methionine. Our study explored the biological functions of sulfur assimilation and sulphur-containing amino acids biosynthesis in P. oryzae.

MoSnt2-dependent deacetylation of histone H3 mediates MoTor-dependent autophagy and plant infection by the rice blast fungus Magnaporthe oryzae

Autophagy is essential for appressorium-mediated plant infection by Magnaporthe oryzae, the causal agent of rice blast disease and a major threat to global food security. The regulatory mechanism of pathogenicity-associated autophagy, however, remains largely unknown. Here, we report the identification and functional characterization of a plausible ortholog of yeast SNT2 in M. oryzae, which we term MoSNT2. Deletion mutants of MoSNT2 are compromised in autophagy homeostasis and display severe defects in autophagy-dependent fungal cell death and pathogenicity. These mutants are also impaired in infection structure development, conidiation, oxidative stress tolerance and cell wall integrity. MoSnt2 recognizes histone H3 acetylation through its PHD1 domain and thereby recruits the histone deacetylase complex, resulting in deacetylation of H3. MoSnt2 binds to promoters of autophagy genes MoATG6, 15, 16, and 22 to regulate their expression. In addition, MoTor controls MoSNT2 expression to regulate MoTor signaling which leads to autophagy and rice infection. Our study provides evidence of a direct link between MoSnt2 and MoTor signaling and defines a novel epigenetic mechanism by which MoSNT2 regulates infection-associated autophagy and plant infection by the rice blast fungus.

ABBREVIATIONS

M. oryzae: Magnaporthe oryzae; S. cerevisiae: Saccharomyces cerevisiae; F. oxysporum: Fusarium oxysporum; U. maydis: Ustilago maydis; Compl.: complemented strains of ΔMosnt2 expressing MoSNT2-GFP; ATG: autophagy-related; HDAC: histone deacetylase complex; Tor: target of rapamycin kinase; MTOR: mechanistic target of rapamycin kinase in mammals; MoSnt2: DNA binding SaNT domain protein in M. oryzae; MoTor: target of rapamycin kinase in M. oryzae; MoAtg8: autophagy-related protein 8 in M. oryzae; MoHos2: hda one similar protein in M. oryzae; MoeIf4G: eukaryotic translation initiation factor 4 G in M. oryzae; MoRs2: ribosomal protein S2 in M. oryzae; MoRs3: ribosomal protein S3 in M. oryzae; MoIcl1: isocitrate lyase in M. oryzae; MoSet1: histone H3K4 methyltransferase in M. oryzae; Asd4: ascus development 4; Abl1: AMP-activated protein kinase β subunit-like protein; Tig1: TBL1-like gene required for invasive growth; Rpd3: reduced potassium dependency; KAT8: lysine (K) acetyltransferase 8; PHD: plant homeodomain; ELM2: Egl-27 and MTA1 homology 2; GFP: green fluorescent protein; YFP: yellow fluorescent protein; YFP^CTF: C-terminal fragment of YFP; YFP^NTF: N-terminal fragment of YFP; GST: glutathione S-transferase; bp: base pairs; DEGs: differentially expressed genes; CM: complete medium; MM-N: minimum medium minus nitrogen; CFW: calcofluor white; CR: congo red; DAPI: 4', 6-diamidino-2-phenylindole; BiFC: bimolecular fluorescence complementation; RT: reverse transcription; PCR: polymerase chain reaction; qPCR: quantitative polymerase chain reaction; RNAi: RNA interference; ChIP: chromatin immunoprecipitation.

Glycerol-3-Phosphate Shuttle Is Involved in Development and Virulence in the Rice Blast Fungus Pyricularia oryzae

The glycerol-3-phosphate (G-3-P) shuttle is an important pathway for delivery of cytosolic reducing equivalents into mitochondrial oxidative phosphorylation, and plays essential physiological roles in yeast, plants, and animals. However, its role has been unclear in filamentous and pathogenic fungi. Here, we characterize the function of the G-3-P shuttle in Pyricularia oryzae by genetic and molecular analyses. In P. oryzae, a glycerol-3-phosphate dehydrogenase 1 (PoGpd1) is involved in NO production, conidiation, and utilization of several carbon sources (pyruvate, sodium acetate, glutamate, and glutamine). A glycerol-3-phosphate dehydrogenase 2 (PoGpd2) is essential for glycerol utilization and fungal development. Deletion of PoGPD2 led to delayed aerial hyphal formation, accelerated aerial hyphal collapse, and reduced conidiation on complete medium (CM) under a light-dark cycle. Aerial mycelial surface hydrophobicity to water and Tween 20 was decreased in ΔPogpd2. Melanin synthesis genes required for cell wall construction and two transcription factor genes (COS1 and CONx2) required for conidiation and/or aerial hyphal differentiation were down-regulated in the aerial mycelia of ΔPogpd2 and ΔPogpd1. Culturing under continuous dark could complement the defects of aerial hyphal differentiation of ΔPogpd2 observed in a light-dark cycle. Two light-sensitive protein genes (PoSIR2 encoding an NAD⁺-dependent deacetylase and TRX2 encoding a thioredoxin 2) were up-regulated in ΔPogpd2 cultured on CM medium in a light-dark cycle. ΔPogpd2 showed an increased intracellular NAD⁺/NADH ratio and total NAD content, and alteration of intracellular ATP production. Culturing on minimal medium also could restore aerial hyphal differentiation of ΔPogpd2, which is deficient on CM medium in a light-dark cycle. Two glutamate synthesis genes, GDH1 and PoGLT1, which synthesize glutamate coupled with oxidation of NADH to NAD⁺, were significantly up-regulated in ΔPogpd2 in a light-dark cycle. Moreover, deletion of PoGpd1 or PoGpd2 led to reduced virulence of conidia or hyphae on rice. The glycerol-3-phosphate shuttle is involved in cellular redox, fungal development, and virulence in P. oryzae.

Autophagy-related protein MoAtg14 is involved in differentiation, development and pathogenicity in the rice blast fungus Magnaporthe oryzae

Autophagy is the major intracellular degradation system by which cytoplasmic materials are delivered to and degraded in the vacuole/lysosome in eukaryotic cells. MoAtg14 in M. oryzae, a hitherto uncharacterized protein, is the highly divergent homolog of the yeast Atg14 and the mammal BARKOR. The MoATG14 deletion mutant exhibited collapse in the center of the colonies, poor conidiation and a complete loss of virulence. Significantly, the ΔMoatg14 mutant showed delayed breakdown of glycogen, less lipid bodies, reduced turgor pressure in the appressorium and impaired conidial autophagic cell death. The autophagic process was blocked in the ΔMoatg14 mutant, and the autophagic degradation of the marker protein GFP-MoAtg8 was interrupted. GFP-MoAtg14 co-localized with mCherry-MoAtg8 in the aerial hypha. In addition, a conserved coiled-coil domain was predicted in the N-terminal region of the MoAtg14 protein, a domain which could mediate the interaction between MoAtg14 and MoAtg6. The coiled-coil domain of the MoAtg14 protein is essential for its function in autophagy and pathogenicity.