Eight RGS and RGS-like proteins orchestrate growth, differentiation, and pathogenicity of Magnaporthe oryzae

Identification of Nigrospora oryzae Causing Leaf Spot Disease in Tomato and Screening of Its Potential Antagonistic Bacteria

Tomato is a widely cultivated vegetable crop worldwide. It is susceptible to various phytopathogens, including fungi, bacteria, viruses, and nematodes. In 2024, an unknown leaf spot disease outbreak, characterized by distinct brown necrotic lesions on leaves, was observed in tomato plants in Yunnan Province, China. Through rigorous pathogen isolation and the fulfillment of Koch's postulates, it was proved that the fungal isolate could infect tomato leaves and cause typical symptoms. The pathogen isolated from tomato leaves was identified as Nigrospora oryzae based on its morphology and using a multilocus sequence analysis method with the internal transcribed spacer gene (ITS1), beta-tubulin gene (TUB2), and translation elongation factor 1-alpha gene (TEF1-α). This represents the first documented case of N. oryzae infecting tomatoes in the world. Given the damage caused by N. oryzae to tomato plants, we explored biocontrol methods. Through a dual-culture assay on PDA plates, Bacillus velezensis B31 demonstrated significant biocontrol potential, exhibiting strong antagonistic activity toward N. oryzae. In addition, we developed a polyethylene glycol (PEG)-mediated transformation system that successfully introduced pYF11-GFP into the protoplasts of N. oryzae. This achievement provides a foundation for future genetic manipulation studies of N. oryzae.

Virulence of banana wilt-causing fungal pathogen Fusarium oxysporum tropical race 4 is mediated by nitric oxide biosynthesis and accessory genes

Fusarium wilt of banana, caused by Fusarium oxysporum f. sp. cubense (Foc), is one of the most damaging plant diseases known. Foc race 1 (R1) decimated the Gros Michel-based banana (Musa acuminata) trade, and now Foc tropical race 4 (TR4) threatens global production of its replacement, the Cavendish banana. Here population genomics revealed that all Cavendish banana-infecting Foc race 4 strains share an evolutionary origin distinct from that of R1 strains. Although TR4 lacks accessory chromosomes, it contains accessory genes at the ends of some core chromosomes that are enriched for virulence and mitochondria-related functions. Meta-transcriptomics revealed the unique induction of the entire mitochondrion-localized nitric oxide (NO) biosynthesis pathway upon TR4 infection. Empirically, we confirmed the unique induction of a NO burst in TR4, suggesting that nitrosative pressure may contribute to virulence. Targeted mutagenesis demonstrated the functional importance of fungal NO production and the accessory gene SIX4 as virulence factors.

MoRgs3 functions in intracellular reactive oxygen species perception-integrated cAMP signaling to promote appressorium formation in Magnaporthe oryzae

Regulator of G-protein signaling (RGS) proteins exhibit GTPase-accelerating protein activities to govern G-protein function. In the rice blast fungus Magnaporthe oryzae, there is a family of at least eight RGS and RGS-like proteins (MoRgs1 to MoRgs8), each exhibiting distinct or shared functions in the growth, appressorium formation, and pathogenicity. MoRgs3 recently emerged as one of the crucial regulators that senses intracellular oxidation during appressorium formation. To explore this unique regulatory mechanism of MoRgs3, we identified the nucleoside diphosphate kinase MoNdk1 that interacts with MoRgs3. MoNdk1 phosphorylates MoRgs3 under induced intracellular reactive oxygen species levels, and MoRgs3 phosphorylation is required for appressorium formation and pathogenicity. In addition, we showed that MoRgs3 phosphorylation determines its interaction with MoCrn1, a coronin-like actin-binding protein homolog, which regulates MoRgs3 internalization. Finally, we provided evidence demonstrating that MoRgs3 functions in MoMagA-mediated cAMP signaling to regulate normal appressorium induction. By revealing a novel signal perception mechanism, our studies highlighted the complexity of regulation during the appressorium function and pathogenicity of the blast fungus.

IMPORTANCE

We report that MoRgs3 becomes phosphorylated in an oxidative intracellular environment during the appressorium formation stage. We found that this phosphorylation is carried out by MoNdk1, a nucleoside diphosphate kinase. In addition, this phosphorylation leads to a higher binding affinity between MoRgs3 and MoCrn1, a coronin-like actin-binding protein that was implicated in the endocytic transport of several other RGS proteins of Magnaporthe oryzae. We further found that the internalization of MoRgs3 is indispensable for its GTPase-activating protein function toward the Gα subunit MoMagA. Importantly, we characterized how such cellular regulatory events coincide with cAMP signaling-regulated appressorium formation and pathogenicity in the blast fungus. Our studies uncovered a novel intracellular reactive oxygen species signal-transducing mechanism in a model pathogenic fungus with important basic and applied implications.

Phenazine biosynthesis protein MoPhzF regulates appressorium formation and host infection through canonical metabolic and noncanonical signaling function in Magnaporthe oryzae

Microbe-produced secondary metabolite phenazine-1-carboxylic acid (PCA) facilitates pathogen virulence and defense mechanisms against competitors. Magnaporthe oryzae, a causal agent of the devastating rice blast disease, needs to compete with other phyllosphere microbes and overcome host immunity for successful colonization and infection. However, whether M. oryzae produces PCA or it has any other functions remains unknown. Here, we found that the MoPHZF gene encodes the phenazine biosynthesis protein MoPhzF, synthesizes PCA in M. oryzae, and regulates appressorium formation and host virulence. MoPhzF is likely acquired through an ancient horizontal gene transfer event and has a canonical function in PCA synthesis. In addition, we found that PCA has a role in suppressing the accumulation of host-derived reactive oxygen species (ROS) during infection. Further examination indicated that MoPhzF recruits both the endoplasmic reticulum membrane protein MoEmc2 and the regulator of G-protein signaling MoRgs1 to the plasma membrane (PM) for MoRgs1 phosphorylation, which is a critical regulatory mechanism in appressorium formation and pathogenicity. Collectively, our studies unveiled a canonical function of MoPhzF in PCA synthesis and a noncanonical signaling function in promoting appressorium formation and host infection.

Class VI G protein-coupled receptors in Aspergillus oryzae regulate sclerotia formation through GTPase-activating activity

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors in eukaryotes that sense and transduce extracellular signals into cells. In Aspergillus oryzae, 16 canonical GPCR genes are identified and classified into nine classes based on the sequence similarity and proposed functions. Class VI GPCRs (AoGprK-1, AoGprK-2, and AoGprR in A. oryzae), unlike other GPCRs, feature a unique hybrid structure containing both the seven transmembrane (7-TM) and regulator of G-protein signaling (RGS) domains, which is not found in animal GPCRs. We report here that the mutants with double or triple deletion of class VI GPCR genes produced significantly increased number of sclerotia compared to the control strain when grown on agar plates. Interestingly, complementation analysis demonstrated that the expression of the RGS domain without the 7-TM domain is sufficient to restore the phenotype. In line with this, among the three Gα subunits in A. oryzae, AoGpaA, AoGpaB, and AoGanA, forced expression of GTPase-deficient mutants of either AoGpaA or AoGpaB caused an increase in the number of sclerotia formed, suggesting that RGS domains of class VI GPCRs are the negative regulators of these two GTPases. Finally, we measured the expression of velvet complex genes and sclerotia formation-related genes and found that the expression of velB was significantly increased in the multiple gene deletion mutants. Taken together, these results demonstrate that class VI GPCRs negatively regulate sclerotia formation through their GTPase-activating activity in the RGS domains. KEY POINTS: • Class VI GPCRs in A. oryzae regulate sclerotia formation in A. oryzae • RGS function of class VI GPCRs is responsible for regulation of sclerotia formation • Loss of class VI GPCRs resulted in increased expression of sclerotia-related genes.

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

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

Peroxisome dynamics determines host-derived ROS accumulation and infectious growth of the rice blast fungus

IMPORTANCE

The interplay between plant and pathogen is a dynamic process, with the host's innate defense mechanisms serving a crucial role in preventing infection. In response to many plant pathogen infections, host cells generate the key regulatory molecule, reactive oxygen species (ROS), to limit the spread of the invading organism. In this study, we reveal the effects of fungal peroxisome dynamics on host ROS homeostasis, during the rice blast fungus Magnaporthe oryzae infection. The elongation of the peroxisome appears contingent upon ROS and links to the accumulation of ROS within the host and the infectious growth of the pathogen. Importantly, we identify a peroxisomal 3-ketoacyl-CoA thiolase, MoKat2, responsible for the elongation of the peroxisome during the infection. In response to host-derived ROS, the homodimer of MoKat2 undergoes dissociation to bind peroxisome membranes for peroxisome elongation. This process, in turn, inhibits the accumulation of host ROS, which is necessary for successful infection. Overall, our study is the first to highlight the intricate relationship between fungal organelle dynamics and ROS-mediated host immunity, extending the fundamental knowledge of pathogen-host interaction.

Establishment of RNA Interference Genetic Transformation System and Functional Analysis of FlbA Gene in Leptographium qinlingensis

Leptographium qinlingensis is a pathogenic fungus of Pinus armandii that is epidemic in the Qinling Mountains. However, an effective gene interference strategy is needed to characterize the pathogenic genes in this fungus on a functional level. Using the RNA silencing vector pSilent-1 as a template, we established an RNA interference genetic transformation system mediated by Agrobacterium tumefaciens GV3101, which is suitable for the gene study for Leptographium qinlingensis by homologous recombination and strain interference system screening. The LqFlbA gene was silenced using the RNA interference approach described above, and the resulting transformants displayed various levels of silencing with a gene silencing effectiveness ranging from 41.8% to 91.4%. The LqFlbA-RNAi mutant displayed altered colony morphology, sluggish mycelium growth, and diminished pathogenicity toward the host P. armandii in comparison to the wild type. The results indicate that this method provides a useful reverse genetic system for studying the gene function of L. qinlingensis, and that LqFlbA plays a crucial role in the growth, development, and pathogenicity of L. qinlingensis.

The Signal Peptidase FoSpc2 Is Required for Normal Growth, Conidiation, Virulence, Stress Response, and Regulation of Light Sensitivity in Fusarium odoratissimum

Signal peptidase (SPase) is responsible for cleavage of N-terminal signal peptides in most secretory precursor proteins and many membrane proteins during maturation. In this study, we identified four components of the SPase complex (FoSec11, FoSpc1, FoSpc2, and FoSpc3) in the banana wilt fungal pathogen Fusarium odoratissimum. We proved that interactions exist among the four SPase subunits by bimolecular fluorescence complementation (BiFC) and affinity purification and mass spectrometry (AP-MS) assays. Among the four SPase genes, FoSPC2 was successfully deleted. FoSPC2 deletion caused defects in vegetative growth, conidiation, and virulence. Loss of FoSPC2 also affected the secretion of some pathogenicity-related extracellular enzymes, suggesting that SPase without FoSpc2 may have a lower efficiency in managing the maturation of the extracellular enzymes in F. odoratissimum. In addition, we found that the ΔFoSPC2 mutant had increased sensitivity to light, and the colonies of the mutant grew faster under all-dark conditions than under all-light conditions. We further observed that deletion of FoSPC2 affected expression of the blue light photoreceptor gene FoWC2, leading to cytoplasmic accumulation of FoWc2 under all-light conditions. Since FoWc2 has signal peptides, FoSpc2 may regulate the expression and subcellular localization of FoWc2 indirectly. Contrary to its response to light, the ΔFoSPC2 mutant displayed a significant decreased sensitivity to osmotic stress, and culturing the mutant under osmotic stress conditions restored both the localization of FoWc2 and light sensitivity of the ΔFoSPC2, suggesting that a cross talk between osmotic stress and light response pathways in F. odoratissimum and FoSpc2 takes part in these processes. IMPORTANCE In this study, we identified four components of SPase in the banana wilt pathogen Fusarium odoratissimum and characterized the SPase FoSpc2. Loss of FoSPC2 affected the secretion of extracellular enzymes, suggesting that SPase without FoSpc2 may have a lower efficiency in managing the maturation of the extracellular enzymes in F. odoratissimum. In addition, this is the first time that we have found a relationship between the SPase and fungal light response. Deletion of FoSPC2 resulted in decreased sensitivity to the osmotic stresses but with increased sensitivity to light. Continuous light inhibited the growth rate of the ΔFoSPC2 mutant and affected the cellular localization of the blue light photoreceptor FoWc2 in this mutant, but culturing the mutant under osmotic stress both restored the localization of FoWc2 and eliminated the light sensitivity of the ΔFoSPC2 mutant, suggesting that loss of FoSPC2 may affect a cross talk between the osmotic stress and light response pathways in F. odoratissimum.

Accessory genes in tropical race 4 contributed to the recent resurgence of the devastating disease of Fusarium wilt of banana

Fusarium wilt of banana, caused by Fusarium oxysporum f. sp. cubense (Foc), is one of the most damaging plant diseases recorded. Foc race 1 (R1) decimated the Gros Michel-based banana trade. Currently, tropical race 4 (TR4) is threatening the global production of its replacement cultivar, Cavendish banana. Population genomics and phylogenetics revealed that all Cavendish banana-infecting race 4 strains shared an evolutionary origin that is distinct from R1 strains. The TR4 genome lacks accessory or pathogenicity chromosomes, reported in other F. oxysporum genomes. Accessory genes-enriched for virulence and mitochondrial-related functions-are attached to ends of some core chromosomes. Meta-transcriptomics revealed the unique induction of the entire mitochondria-localized nitric oxide (NO) biosynthesis pathway upon TR4 infection. Empirically, we confirmed the unique induction of NO burst in TR4,suggesting the involvement of nitrosative pressure in its virulence. Targeted mutagenesis demonstrated the functional importance of accessory genes SIX1 and SIX4 as virulent factors.

Hydrophobic cue-induced appressorium formation depends on MoSep1-mediated MoRgs7 phosphorylation and internalization in Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae forms specialized infectious structures called appressoria that breach host cells to initiate infection. Previous studies demonstrated that the regulator of G-protein signaling (RGS)-like protein MoRgs7 undergoes endocytosis upon fungal sensing of hydrophobic environmental cues to activate cAMP signaling required for appressorium formation. However, the mechanism by which MoRgs7 internalizes and its fate remains undetermined. We here show that MoSep1, a conserved protein kinase of Mitotic Exit Network (MEN), phosphorylates MoRgs7 to regulate its function. MoRgs7 phosphorylation determines its interaction with MoCrn1, a coronin-like actin-binding protein homolog that also modulates the internalization of MoRgs7. Importantly, the endocytic transport of MoRgs7 is critical for its GTPase-activating protein (GAP) function important in cAMP signaling. Together, our findings revealed a novel mechanism by which M. oryzae activates MoRgs7-mediated hydrophobic cue-sensing signal transduction involving protein phosphorylation and endocytic transport to govern appressorium formation and fungal pathogenicity.

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.

MoErv14 mediates the intracellular transport of cell membrane receptors to govern the appressorial formation and pathogenicity of Magnaporthe oryzae

Magnaporthe oryzae causes rice blasts posing serious threats to food security worldwide. During infection, M. oryzae utilizes several transmembrane receptor proteins that sense cell surface cues to induce highly specialized infectious structures called appressoria. However, little is known about the mechanisms of intracellular receptor tracking and their function. Here, we described that disrupting the coat protein complex II (COPII) cargo protein MoErv14 severely affects appressorium formation and pathogenicity as the ΔMoerv14 mutant is defective not only in cAMP production but also in the phosphorylation of the mitogen-activated protein kinase (MAPK) MoPmk1. Studies also showed that either externally supplementing cAMP or maintaining MoPmk1 phosphorylation suppresses the observed defects in the ΔMoerv14 strain. Importantly, MoErv14 is found to regulate the transport of MoPth11, a membrane receptor functioning upstream of G-protein/cAMP signaling, and MoWish and MoSho1 function upstream of the Pmk1-MAPK pathway. In summary, our studies elucidate the mechanism by which the COPII protein MoErv14 plays an important function in regulating the transport of receptors involved in the appressorium formation and virulence of the blast fungus.

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.

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.

Regulator of G Protein Signaling Proteins Control Growth, Development and Cellulase Production in Neurospora crassa

Heterotrimeric (αβγ) G protein signaling pathways are critical environmental sensing systems found in eukaryotic cells. Exchange of GDP for GTP on the Gα subunit leads to its activation. In contrast, GTP hydrolysis on the Gα is accelerated by Regulator of G protein Signaling (RGS) proteins, resulting in a return to the GDP-bound, inactive state. Here, we analyzed growth, development and extracellular cellulase production in strains with knockout mutations in the seven identified RGS genes (rgs-1 to rgs-7) in the filamentous fungus, Neurospora crassa. We compared phenotypes to those of strains with either knockout mutations or expressing predicted constitutively activated, GTPase-deficient alleles for each of the three Gα subunit genes (gna-1^Q204L, gna-2^Q205L or gna-3^Q208L). Our data revealed that six RGS mutants have taller aerial hyphae than wild type and all seven mutants exhibit reduced asexual sporulation, phenotypes shared with strains expressing the gna-1^Q204L or gna-3^Q208L allele. In contrast, Δrgs-1 and Δrgs-3 were the only RGS mutants with a slower growth rate phenotype, a defect in common with gna-1^Q204L strains. With respect to female sexual development, Δrgs-1 possessed defects most similar to gna-3^Q208L strains, while those of Δrgs-2 mutants resembled strains expressing the gna-1^Q204L allele. Finally, we observed that four of the seven RGS mutants had significantly different extracellular cellulase levels relative to wild type. Of interest, the Δrgs-2 mutant had no detectable activity, similar to the gna-3^Q208L strain. In contrast, the Δrgs-1 and Δrgs-4 mutants and gna-1^Q204L and gna-2^Q205L strains exhibited significantly higher cellulase activity than wild type. With the exception of sexual development, our results demonstrate the greatest number of genetic interactions between rgs-1 and gna-1 and rgs-2 and gna-3 in N. crassa.

Advancement in the Breeding, Biotechnological and Genomic Tools towards Development of Durable Genetic Resistance against the Rice Blast Disease

Rice production needs to be sustained in the coming decades, as the changeable climatic conditions are becoming more conducive to disease outbreaks. The majority of rice diseases cause enormous economic damage and yield instability. Among them, rice blast caused by Magnaportheoryzae is a serious fungal disease and is considered one of the major threats to world rice production. This pathogen can infect the above-ground tissues of rice plants at any growth stage and causes complete crop failure under favorable conditions. Therefore, management of blast disease is essentially required to sustain global food production. When looking at the drawback of chemical management strategy, the development of durable, resistant varieties is one of the most sustainable, economic, and environment-friendly approaches to counter the outbreaks of rice blasts. Interestingly, several blast-resistant rice cultivars have been developed with the help of breeding and biotechnological methods. In addition, 146 R genes have been identified, and 37 among them have been molecularly characterized to date. Further, more than 500 loci have been identified for blast resistance which enhances the resources for developing blast resistance through marker-assisted selection (MAS), marker-assisted backcross breeding (MABB), and genome editing tools. Apart from these, a better understanding of rice blast pathogens, the infection process of the pathogen, and the genetics of the immune response of the host plant are very important for the effective management of the blast disease. Further, high throughput phenotyping and disease screening protocols have played significant roles in easy comprehension of the mechanism of disease spread. The present review critically emphasizes the pathogenesis, pathogenomics, screening techniques, traditional and molecular breeding approaches, and transgenic and genome editing tools to develop a broad spectrum and durable resistance against blast disease in rice. The updated and comprehensive information presented in this review would be definitely helpful for the researchers, breeders, and students in the planning and execution of a resistance breeding program in rice against this pathogen.

An appressorium membrane protein, Pams1, controls infection structure maturation and virulence via maintaining endosomal stability in the rice blast fungus

The rice blast fungus Magnaporthe oryzae spores differentiate and mature into functional appressoria by sensing the host surface signals. Environmental stimuli are transduced into cells through internalization during appressorium formation, such as in the cAMP-PKA pathway. Here, we describe a novel contribution to how appressoria mature on the surface of a leaf, and its connection to endosomes and the cAMP-PKA pathway. An appressorium membrane-specific protein, Pams1, is required for maintaining endosomal structure, appressorium maturation, and virulence in M. oryzae. During appressorium development, Pams1 was translocated from the cell membrane to the endosomal membrane. Deletion of PAMS1 led to the formation of two types of abnormal appressoria after 8 h post inoculation (hpi): melanized type I had a reduced virulence, while pale type II was dead. Before 8 hpi, Δpams1 formed appressoria that were similar to those of the wild type. After 8 hpi, the appressoria of Δpams1 was differentiated into two types: (1) the cell walls of type I appressoria were melanized, endosomes were larger, and had a different distribution from the wild type and (2) Type II appressoria gradually stopped melanization and began to die. The organelles, including the nucleus, endosomes, mitochondria, and endoplasmic reticula, were degraded, leaving only autophagic body-like vesicles in type II appressoria. The addition of exogenous cAMP to Δpams1 led to the formation of a greater proportion of type I appressoria and a smaller proportion of type II appressoria. Thus, defects in endosomal structure and the cAMP-PKA pathway are among the causes of the defective appressorium maturation and virulence of Δpams1.

Phylogeny of Regulators of G-Protein Signaling Genes in Leptographium qinlingensis and Expression Levels of Three RGSs in Response to Different Terpenoids

Leptographium qinlingensis is a bark beetle-vectored pine pathogen in the Chinese white pine beetle (Dendroctonus armandi) epidemic in Northwest China. L. qinlingensis colonizes pines despite the trees’ massive oleoresin terpenoid defenses. Regulators of G-protein signaling (RGS) proteins modulate heterotrimeric G-protein signaling negatively and play multiple roles in the growth, asexual development, and pathogenicity of fungi. In this study, we have identified three L. qinlingensis RGS genes, and the phylogenetic analysis shows the highest homology with the regulators of G-protein signaling proteins sequence from Ophiostoma piceae and Grosmannia clavigera. The expression profiles of three RGSs in the mycelium of L. qinlingensis treated with six different terpenoids were detected, as well as their growth rates. Under six terpenoid treatments, the growth and reproduction in L. qinlingensis were significantly inhibited, and the growth inflection day was delayed from 8 days to 12–13 days. By analyzing the expression level of three RGS genes of L. qinlingensis with different treatments, results indicate that LqFlbA plays a crucial role in controlling fungal growth, and both LqRax1 and LqRgsA are involved in overcoming the host chemical resistances and successful colonization.

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.

MoIug4 is a novel secreted effector promoting rice blast by counteracting host OsAHL1-regulated ethylene gene transcription

Magnaporthe oryzae secretes several effectors that modulate and hijack rice processes to colonize host cells, but the underlying mechanisms remain unclear. We report on a novel cytoplasmic effector MoIug4 that targets the rice ethylene pathway as a transcription repressor to subvert host immunity. We found that MoIug4 binds to the promoter of the host OsEIN2 gene that encodes a central signal transducer in the ethylene-signaling pathway. We also identified a MoIug4 interacting protein, OsAHL1, which acts as an AT-hook motif-containing protein binding to the A/T-rich promoter regions. Our knockout and overexpression studies showed that OsAHL1 positively regulates plant immunity in response to M. oryzae infection. OsAHL1 exhibits transcriptional regulatory activities by binding the OsEIN2 promoter region, similar to MoIug4. Intriguingly, we found that MoIug4 exhibits a higher binding affinity than OsAHL1 to the OsEIN2 promoter, suggesting differential regulatory specificities. These results revealed a counter-defense strategy by which the pathogen effector suppresses the activation of host defense genes by interfering with host transcription activator functions.

Adenylsuccinate Synthetase MoADE12 Plays Important Roles in the Development and Pathogenicity of the Rice Blast Fungus

Purines are basic components of nucleotides in living organisms. In this study, we identified the ortholog of adenylosuccinate synthase MoADE12 in Magnaporthe oryzae by screening for growth-defective T-DNA insertional mutants. Gene replacement was performed to investigate the biological role of MoADE12. Δmoade12 mutants were adenine auxotrophs that failed to produce conidia, and showed reduced perithecia formation and pathogenicity. Moreover, the Δmoade12 mutant was hypersensitive to Congo red and oxidants, indicating that MoADE12 was required for cell wall integrity and oxidative stress resistance. Transcriptomic analysis identified the underlying mechanisms and indicated that several pathogenicity-related genes were regulated in the Δmoade12 mutant. Therefore, our data suggest that the adenylosuccinate synthase MoADE12 is involved in the de novo AMP biosynthesis pathway and is important for conidiation and pathogenicity in the rice blast fungus.

Regulator of G Protein Signaling Contributes to the Development and Aflatoxin Biosynthesis in Aspergillus flavus through the Regulation of Gα Activity

Heterotrimeric G-proteins play crucial roles in growth, asexual development, and pathogenicity of fungi. The regulator of G-protein signaling (RGS) proteins function as negative regulators of the G proteins to control the activities of GTPase in Gα subunits. In this study, we functionally characterized the six RGS proteins (i.e., RgsA, RgsB, RgsC, RgsD, RgsE, and FlbA) in the pathogenic fungus Aspergillus flavus. All the aforementioned RGS proteins were also found to be functionally different in conidiation, aflatoxin (AF) biosynthesis, and pathogenicity in A. flavus. Apart from FlbA, all other RGS proteins play a negative role in regulating both the synthesis of cyclic AMP (cAMP) and the activation of protein kinase A (PKA). Additionally, we also found that although RgsA and RgsE play a negative role in regulating the FadA-cAMP/PKA pathway, they function distinctly in aflatoxin biosynthesis. Similarly, RgsC is important for aflatoxin biosynthesis by negatively regulating the GanA-cAMP/PKA pathway. PkaA, which is the cAMP-dependent protein kinase catalytic subunit, also showed crucial influences on A. flavus phenotypes. Overall, our results demonstrated that RGS proteins play multiple roles in the development, pathogenicity, and AF biosynthesis in A. flavus through the regulation of Gα subunits and cAMP-PKA signals. IMPORTANCE RGS proteins, as crucial regulators of the G protein signaling pathway, are widely distributed in fungi, while little is known about their roles in Aspergillus flavus development and aflatoxin. In this study, we identified six RGS proteins in A. flavus and revealed that these proteins have important functions in the regulation of conidia, sclerotia, and aflatoxin formation. Our findings provide evidence that the RGS proteins function upstream of cAMP-PKA signaling by interacting with the Gα subunits (GanA and FadA). This study provides valuable information for controlling the contamination of A. flavus and mycotoxins produced by this fungus in pre- and postharvest of agricultural crops.

Towards resolution of a paradox in plant G-protein signaling

G-proteins are molecular on-off switches that are involved in transmitting a variety of extracellular signals to their intracellular targets. In animal and yeast systems, the switch property is encoded through nucleotides: a GDP-bound state is the "off-state" and the GTP-bound state is the "on-state". The G-protein cycle consists of the switch turning on through nucleotide exchange facilitated by a G-protein coupled receptor and the switch turning off through hydrolysis of GTP back to GDP, facilitated by a protein designated REGULATOR OF G SIGNALING 1 (RGS). In plants, G-protein signaling dramatically differs from that in animals and yeast. Despite stringent conservation of the nucleotide binding and catalytic structures over the 1.6 billion years that separate the evolution of plants and animals, genetic and biochemical data indicate that nucleotide exchange is less critical for this switch to operate in plants. Also, the loss of the single RGS protein in Arabidopsis (Arabidopsis thaliana) confers unexpectedly weaker phenotypes consistent with a diminished role for the G cycle, at least under static conditions. However, under dynamic conditions, genetic ablation of RGS in Arabidopsis results in a strong phenotype. We explore explanations to this conundrum by formulating a mathematical model that takes into account the accruing evidence for the indispensable role of phosphorylation in G-protein signaling in plants and that the G-protein cycle is needed to process dynamic signal inputs. We speculate that the plant G-protein cycle and its attendant components evolved to process dynamic signals through signaling modulation rather than through on-off, switch-like regulation of signaling. This so-called change detection may impart greater fitness for plants due to their sessility in a dynamic light, temperature, and pest environment.

The Peroxisomal-CoA Synthetase MoPcs60 Is Important for Fatty Acid Metabolism and Infectious Growth of the Rice Blast Fungus

Fatty acid metabolism is important for the maintenance of fatty acid homeostasis. Free fatty acids, which are toxic in excess, are activated by esterification with coenzyme A (CoA) and then subjected to β-oxidization. Fatty acid β-oxidation-related genes play critical roles in the development and virulence of several phytopathogens. In this study, we identified and characterized a peroxisomal-CoA synthetase in the rice blast fungus Magnaporthe oryzae, MoPCS60, which is a homolog of PCS60 in budding yeast. MoPCS60 was highly expressed during the conidial and early infectious stages and was induced under oleate treatment. Targeted deletion of MoPCS60 resulted in a significant reduction in growth rate when oleate and olive oil were used as the sole carbon sources. Compared with the wild-type strain Guy11, the ΔMopcs60 mutant exhibited fewer peroxisomes, more lipid droplets, and decreased pathogenicity. The distribution of MoPcs60 varied among developmental stages and was mainly localized to peroxisomes in the hyphae, conidia, and appressoria when treated with oleate. Our results suggest that MoPcs60 is a key peroxisomal-CoA synthetase involved in fatty acid β-oxidation and pathogenicity in rice blast fungi.

Host-Induced Gene Silencing of a G Protein α Subunit Gene CsGpa1 Involved in Pathogen Appressoria Formation and Virulence Improves Tobacco Resistance to Ciboria shiraiana

Hypertrophy sorosis scleroteniosis caused by Ciboria shiraiana is the most devastating disease of mulberry fruit. However, few mulberry lines show any resistance to C. shiraiana. An increasing amount of research has shown that host-induced gene silencing (HIGS) is an effective strategy for enhancing plant tolerance to pathogens by silencing genes required for their pathogenicity. In this study, two G protein α subunit genes, CsGPA1 and CsGPA2, were identified from C. shiraiana. Silencing CsGPA1 and CsGPA2 had no effect on hyphal growth but reduced the number of sclerotia and increased the single sclerotium weight. Moreover, silencing CsGpa1 resulted in increased fungal resistance to osmotic and oxidative stresses. Compared with wild-type and empty vector strains, the number of appressoria was clearly lower in CsGPA1-silenced strains. Importantly, infection assays revealed that the virulence of CsGPA1-silenced strains was significantly reduced, which was accompanied by formation of fewer appressoria and decreased expression of several cAMP/PKA- or mitogen-activated protein-kinase-related genes. Additionally, transgenic Nicotiana benthamiana expressing double-stranded RNA targeted to CsGpa1 through the HIGS method significantly improved resistance to C. shiraiana. Our results indicate that CsGpa1 is an important regulator in appressoria formation and the pathogenicity of C. shiraiana. CsGpa1 is an efficient target to improve tolerance to C. shiraiana using HIGS technology.

Functional analysis of seven regulators of G protein signaling (RGSs) in the nematode-trapping fungus Arthrobotrys oligospora

Regulators of G protein signaling (RGSs) are proteins that negatively regulate G protein signal transduction. In this study, seven putative RGSs were characterized in the nematode-trapping (NT) fungus, Arthrobotrys oligospora. Deleting Rgs genes significantly increased intracellular cAMP levels, and caused defects in mycelia growth, stress resistance, conidiation, trap formation, and nematocidal activity. In particular, the ΔAoFlbA mutant was unable to produce conidia and traps. Transcriptomic analysis showed that amino acid metabolic and biosynthetic processes were significantly enriched in the ΔAoFlbA mutant compared to WT. Interestingly, Gas1 family genes are significantly expanded in A. oligospora and other NT fungi that produce adhesive traps, and are differentially expressed during trap formation in A. oligospora. Disruption of two Gas1 genes resulted in defective conidiation, trap formation, and pathogenicity. Our results indicate that RGSs play pleiotropic roles in regulating A. oligospora mycelial growth, development, and pathogenicity. Further, AoFlbA is a prominent member and required for conidiation and trap formation, possibly by regulating amino acid metabolism and biosynthesis. Our results provide a basis for elucidating the signaling mechanism of vegetative growth, lifestyle transition, and pathogenicity in NT fungi.

The Exocyst Regulates Hydrolytic Enzyme Secretion at Hyphal Tips and Septa in the Banana Fusarium Wilt Fungus Fusarium odoratissimum

Hyphal polarized growth in filamentous fungi requires tip-directed secretion, while additional evidence suggests that fungal exocytosis for the hydrolytic enzyme secretion can occur at other sites in hyphae, including the septum. In this study, we analyzed the role of the exocyst complex involved in the secretion in the banana wilt fungal pathogen Fusarium odoratissimum. All eight exocyst components in F. odoratissimum not only localized to the tips ahead of the Spitzenkörper in growing hyphae but also localized to the outer edges of septa in mature hyphae. To further analyze the exocyst in F. odoratissimum, we attempted single gene deletion for all the genes encoding the eight exocyst components and only succeeded in constructing the gene deletion mutants for exo70 and sec5; we suspect that the other 6 exocyst components are encoded by essential genes. Deletion of exo70 or sec5 led to defects in vegetative growth, conidiation, and pathogenicity in F. odoratissimum. Notably, the deletion of exo70 resulted in decreased activities for endoglucosidase, filter paper enzymes, and amylase, while the loss of sec5 only led to a slight reduction in amylase activity. Septum-localized α-amylase (AmyB) was identified as the marker for septum-directed secretion, and we found that Exo70 is essential for the localization of AmyB to septa. Meanwhile the loss of Sec5 did not affect AmyB localization to septa but led to a higher accumulation of AmyB in cytoplasm. This suggested that while Exo70 and Sec5 both take part in the septum-directed secretion, the two play different roles in this process. IMPORTANCE The exocyst complex is a multisubunit tethering complex (MTC) for secretory vesicles at the plasma membrane and contains eight subunits, Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84. While the exocyst complex is well defined in eukaryotes from yeast (Saccharomyces cerevisiae) to humans, the exocyst components in filamentous fungi show different localization patterns in the apical tips of hyphae, which suggests that filamentous fungi have evolved divergent strategies to regulate endomembrane trafficking. In this study, we demonstrated that the exocyst components in Fusarium odoratissimum are localized not only to the tips of growing hyphae but also to the outer edge of the septa in mature hyphae, suggesting that the exocyst complex plays a role in the regulation of septum-directed protein secretion in F. odoratissimum. We further found that Exo70 and Sec5 are required for the septum-directed secretion of α-amylase in F. odoratissimum but with different influences.

MoWhi2 regulates appressorium formation and pathogenicity via the MoTor signalling pathway in Magnaporthe oryzae

Magnaporthe oryzae causes rice blast disease, which seriously threatens the safety of food production. Understanding the mechanism of appressorium formation, which is one of the key steps for successful infection by M. oryzae, is helpful to formulate effective control strategies of rice blast. In this study, we identified MoWhi2, the homolog of Saccharomyces cerevisiae Whi2 (Whisky2), as an important regulator that controls appressorium formation in M. oryzae. When MoWHI2 was disrupted, multiple appressoria were formed by one conidium and pathogenicity was significantly reduced. A putative phosphatase, MoPsr1, was identified to interact with MoWhi2 using a yeast two-hybridization screening assay. The knockout mutant ΔMopsr1 displayed similar phenotypes to the ΔMowhi2 strain. Both the ΔMowhi2 and ΔMopsr1 mutants could form appressoria on a hydrophilic surface with cAMP levels increasing in comparison with the wild type (WT). The conidia of ΔMowhi2 and ΔMopsr1 formed a single appressorium per conidium, similar to WT, when the target of rapamycin (TOR) inhibitor rapamycin was present. In addition, compared with WT, the expression levels of MoTOR and the MoTor signalling activation marker gene MoRS3 were increased, suggesting that inappropriate activation of the MoTor signalling pathway is one of the important reasons for the defects in appressorium formation in the ΔMowhi2 and ΔMopsr1 strains. Our results provide insights into MoWhi2 and MoPsr1-mediated appressorium development and pathogenicity by regulating cAMP levels and the activation of MoTor signalling in M. oryzae.

The rice blast fungus MoRgs1 functioning in cAMP signaling and pathogenicity is regulated by casein kinase MoCk2 phosphorylation and modulated by membrane protein MoEmc2

GTP-binding protein (G-protein) and regulator of G-protein signaling (RGS) mediated signal transduction are critical in the growth and virulence of the rice blast pathogen Magnaporthe oryzae. We have previously reported that there are eight RGS and RGS-like proteins named MoRgs1 to MoRgs8 playing distinct and shared regulatory functions in M. oryzae and that MoRgs1 has a more prominent role compared to others in the fungus. To further explore the unique regulatory mechanism of MoRgs1, we screened a M. oryzae cDNA library for genes encoding MoRgs1-interacting proteins and identified MoCkb2, one of the two regulatory subunits of the casein kinase (CK) 2 MoCk2. We found that MoCkb2 and the sole catalytic subunit MoCka1 are required for the phosphorylation of MoRgs1 at the plasma membrane (PM) and late endosome (LE). We further found that an endoplasmic reticulum (ER) membrane protein complex (EMC) subunit, MoEmc2, modulates the phosphorylation of MoRgs1 by MoCk2. Interestingly, this phosphorylation is also essential for the GTPase-activating protein (GAP) function of MoRgs1. The balance among MoRgs1, MoCk2, and MoEmc2 ensures normal operation of the G-protein MoMagA-cAMP signaling required for appressorium formation and pathogenicity of the fungus. This has been the first report that an EMC subunit is directly linked to G-protein signaling through modulation of an RGS-casein kinase interaction.

G-proteins play a significant role in signal perception and transduction during pathogen and host interactions. In the rice blast fungus M. oryzae, previous studies demonstrated that G-protein/cAMP signaling are important for appressorium formation and pathogenicity. One of the eight regulator of G-protein signaling (RGS) and RGS-like proteins, MoRgs1, targets G-protein MoMagA to regulate cAMP levels and growth and virulence of the fungus; however, how MoRgs1 exhibits this function and its own regulation indifferent from other RGS and RGS-like proteins are not clear. We here demonstrated that MoRgs1 is subject to regulation by the casein kinase 2 MoCk2 through protein phosphorylation, and this regulation is also essential for the GTPase-activating protein (GAP) function of MoRgs1. We also showed that the endoplasmic reticulum (ER) membrane complex (EMC) subunit MoEmc2 modulates MoCk2-mediated MoRgs1 phosphorylation. Balanced interactions among MoRgs1, MoEmc2, and MoCk2 ensure normal appressorium formation and pathogenicity of M. oryzae.

The G-protein coupled receptor GPRK contributes to fungal development and full virulence in Metarhizium robertsii

G-protein-coupled receptor K (GPRK), which is a class VI fungal G-protein-coupled receptor (GPCR), plays a critical role in plant immunity against pathogens by mediating the endocytic pathway, influencing metabolism in response to environmental signals, and regulating asexual reproduction and pathogenic development. However, the function of these proteins in entomopathogenic fungi has rarely been investigated. Accordingly, we characterized MrGPRK, a GPCR in the entomopathogenic fungus Metarhizium robertsii containing a C-terminal seven-transmembrane and a conserved regulator of G protein signaling domain, and found that it localized to endosomes. Mutant phenotype assays showed that a ΔMrGprk strain displayed increased defects in radial growth (~28%) and decreased conidial production (~80%) compared with a wild-type strain. Decreased conidiation rates coincided well with the repression of conidiation-related regulatory genes, including three key conidial transcription factors: brlA, abaA, and wetA. MrGprk deficiency impaired full virulence (both topical and injectable inoculations). Further analysis demonstrated that deleting fungal MrGprk decreased the rates of appressorium formation and suppressed the transcription of several genes contributing to appressorial turgor pressure, cuticle penetration, and pH regulation. Additionally, the ΔMrGprk strain showed higher cyclic (cAMP) levels, suggesting that this GPCR is critical for cAMP signal transduction. In summary, MrGPRK was found to contribute to vegetative growth, conidial production, and full virulence of M. robertsii. These findings are conducive to a better understanding of the roles of GPCRs in the development and pathogenicity of entomopathogenic fungi.

Auxilin-like protein MoSwa2 promotes effector secretion and virulence as a clathrin uncoating factor in the rice blast fungus Magnaporthe oryzae

Plant pathogens exploit the extracellular matrix (ECM) to inhibit host immunity during their interactions with the host. The formation of ECM involves a series of continuous steps of vesicular transport events. To understand how such vesicle trafficking impacts ECM and virulence in the rice blast fungus Magnaporthe oryzae, we characterised MoSwa2, a previously identified actin-regulating kinase MoArk1 interacting protein, as an orthologue of the auxilin-like clathrin uncoating factor Swa2 of the budding yeast Saccharomyces cerevisiae. We found that MoSwa2 functions as an uncoating factor of the coat protein complex II (COPII) via an interaction with the COPII subunit MoSec24-2. Loss of MoSwa2 led to a deficiency in the secretion of extracellular proteins, resulting in both restricted growth of invasive hyphae and reduced inhibition of host immunity. Additionally, extracellular fluid (ECF) proteome analysis revealed that MoSwa2-regulated extracellular proteins include many redox proteins such as the berberine bridge enzyme-like (BBE-like) protein MoSef1. We further found that MoSef1 functions as an apoplastic virulent factor that inhibits the host immune response. Our studies revealed a novel function of a COPII uncoating factor in vesicular transport that is critical in the suppression of host immunity and pathogenicity of M. oryzae.

Two regulators of G-protein signaling (RGS) proteins FlbA1 and FlbA2 differentially regulate fumonisin B1 biosynthesis in Fusarium verticillioides

Fumonisins are a group of mycotoxins produced by maize pathogen Fusarium verticillioides that pose health concerns to humans and animals. Yet we still lack a clear understanding of the mechanism of fumonisins regulation during pathogenesis. The heterotrimeric G protein complex, which consists of canonical subunits and various regulators of G-protein signaling (RGS) proteins, plays an important role in transducing signals under environmental stress. Earlier studies demonstrated that Gα and Gβ subunits are positive regulators of fumonisin B1 (FB1) biosynthesis and that two RGS genes, FvFlbA1 and FvFlbA2, were highly upregulated in Gβ deletion mutant ∆Fvgbb1. Notably, FvFlbA2 has a negative role in FB1 regulation. While many fungi contain a single copy of FlbA, F. verticillioides harbors two putative FvFlbA paralogs, FvFlbA1 and FvFlbA2. In this study, we further characterized functional roles of FvFlbA1 and FvFlbA2. While ∆FvflbA1 deletion mutant exhibited no significant defects, ∆FvflbA2 and ∆FvflbA2/A1 mutants showed thinner aerial hyphal growth while promoting FB1 production. FvFlbA2 is required for proper expression of key conidia regulation genes, including putative FvBRLA, FvWETA, and FvABAA, while suppressing FUM21, FUM1, and FUM8 expression. Split luciferase assays determined that FvFlbA paralogs interact with key heterotrimeric G protein components, which in turn will lead altered G-protein-mediated signaling pathways that regulate FB1 production and asexual development in F. verticillioides.

Magnaporthe oryzae Auxiliary Activity Protein MoAa91 Functions as Chitin-Binding Protein To Induce Appressorium Formation on Artificial Inductive Surfaces and Suppress Plant Immunity

The appressoria that are generated by the rice blast fungus Magnaporthe oryzae in response to surface cues are important for successful colonization. Previous work showed that regulators of G-protein signaling (RGS) and RGS-like proteins play critical roles in appressorium formation. However, the mechanisms by which these proteins orchestrate surface recognition for appressorium induction remain unclear. Here, we performed comparative transcriptomic studies of ΔMorgs mutant and wild-type strains and found that M. oryzae Aa91 (MoAa91), a homolog of the auxiliary activity family 9 protein (Aa9), was required for surface recognition of M. oryzae We found that MoAA91 was regulated by the MoMsn2 transcription factor and that its disruption resulted in defects in both appressorium formation on the artificial inductive surface and full virulence of the pathogen. We further showed that MoAa91 was secreted into the apoplast space and was capable of competing with the immune receptor chitin elicitor-binding protein precursor (CEBiP) for chitin binding, thereby suppressing chitin-induced plant immune responses. In summary, we have found that MoAa91 is a novel signaling molecule regulated by RGS and RGS-like proteins and that MoAa91 not only governs appressorium development and virulence but also functions as an effector to suppress host immunity.IMPORTANCE The rice blast fungus Magnaporthe oryzae generates infection structure appressoria in response to surface cues largely due to functions of signaling molecules, including G-proteins, regulators of G-protein signaling (RGS), mitogen-activated protein (MAP) kinase pathways, cAMP signaling, and TOR signaling pathways. M. oryzae encodes eight RGS and RGS-like proteins (MoRgs1 to MoRgs8), and MoRgs1, MoRgs3, MoRgs4, and MoRgs7 were found to be particularly important in appressorium development. To explore the mechanisms by which these proteins regulate appressorium development, we have performed a comparative in planta transcriptomic study and identified an auxiliary activity family 9 protein (Aa9) homolog that we named MoAa91. We showed that MoAa91 was secreted from appressoria and that the recombinant MoAa91 could compete with a chitin elicitor-binding protein precursor (CEBiP) for chitin binding, thereby suppressing chitin-induced plant immunity. By identifying MoAa91 as a novel signaling molecule functioning in appressorium development and an effector in suppressing host immunity, our studies revealed a novel mechanism by which RGS and RGS-like proteins regulate pathogen-host interactions.

Responses of Leaf Cuticles to Rice Blast: Detection and Identification Using Depth-Profiling Fourier Transform Mid-Infrared Photoacoustic Spectroscopy

Cuticle is the first barrier for rice to resist blast fungus on the surface of the leaf. Studies on how the rice leaf cuticle responds to rice blast and attempts to perform early detection of rice blast are limited, and these two issues were explored in this study via depth-profiling Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS). Rice leaves with four different scales of injury (healthy leaves as CK, asymptomatic leaves from mildly diseased seedlings as S1, infected leaves with fewer than five lesions as S2, and infected leaves with more than 10 lesions as S3) were scanned by three moving mirror velocities 0.32, 0.47, and 0.63 cm/s for the depth profiling of the rice leaf surface. The response patterns were acquired via chemometrics to analyze the variations of the chemical group absorptions in the different layers of a sample and in the same layer between different samples. Results showed that the leaf cuticle tended to be thicker and the relative content of fatty alcohols and cutin, unsaturated compounds, and aromatics in the cuticle increased when rice seedlings were infected by blast fungus. Together with the principal component analysis, the probabilistic neural network was applied to identify the samples in early stages (CK and S1), which reached an accuracy of 90% for the samples in the greenhouse and 82% for the samples in the field. Thus, depth-profiling FTIR-PAS was good at analyzing the variation in cuticle layers and showed great potential in the early detection of rice blast or other diseases in different species.

MicroRNA-like milR236, regulated by transcription factor MoMsn2, targets histone acetyltransferase MoHat1 to play a role in appressorium formation and virulence of the rice blast fungus Magnaporthe oryzae

MicroRNAs (miRNAs) play important roles in various cellular growth and developmental processes through post-transcriptional gene regulation via mRNA cleavage and degradation and the inhibition of protein translation. To explore if miRNAs play a role in appressoria formation and virulence that are also governed by the regulators of G-protein signaling (RGS) proteins in the rice blast fungus Magnaporthe oryzae, we have compared small RNA (sRNA) production between several ΔMorgs mutant and the wild-type strains. We have identified sRNA236 as a microRNA-like milR236 that targets the encoding sequence of MoHat1, a histone acetyltransferase type B catalytic subunit involved in appressorium function and virulence. We have also found that milR236 overexpression induces delayed appressorium formation and virulence attenuation, similar to those displayed by the ΔMohat1 mutant strain. Moreover, we have shown that the transcription factor MoMsn2 binds to the promoter sequence of milR236 to further suppress MoHAT1 transcription and MoHat1-regulated appressorium formation and virulence. In summary, by identifying a novel regulatory role of sRNA in the blast fungus, our studies reveal a new paradigm in the multifaceted regulatory pathways that govern the appressorium formation and virulence of M. oryzae.

Functional analysis of CfSnf1 in the development and pathogenicity of anthracnose fungus Colletotrichum fructicola on tea-oil tree

Background

Tea-oil tree (Camellia oleifera) is a unique edible-oil tree in China, and anthracnose occurs in wherever it is cultivated, causing great economic losses each year. We have previously identified the Ascomycete fungus Colletotrichum fructicola as the major pathogen of anthracnose in Ca.oleifera. The purpose of this study was to characterize the biological function of Snf1 protein, a key component of the AMPK (AMP-activated protein kinase) pathway, for the molecular pathogenic-mechanisms of C. fructicola.

Results

We characterized CfSnf1 as the homolog of Saccharomyces cerevisiae Snf1. Targeted CfSNF1 gene deletion revealed that CfSnf1 is involved in the utilization of specific carbon sources, conidiation, and stress responses. We further found that the ΔCfSnf1 mutant was not pathogenic to Ca.oleifera, resulting from its defect in appressorium formation. In addition, we provided evidence showing crosstalk between the AMPK and the cAMP/PKA pathways for the first time in filamentous fungi.

Conclusion

This study indicate that CfSnf1 is a critical factor in the development and pathogenicity of C. fructicola and, therefore, a potential fungicide target for anthracnose control.

System-Wide Characterization of MoArf GTPase Family Proteins and Adaptor Protein MoGga1 Involved in the Development and Pathogenicity of Magnaporthe oryzae

ADP ribosylation factor (Arf) small GTPase family members are involved in vesicle trafficking and organelle maintenance in organisms ranging from Saccharomyces cerevisiae to humans. A previous study identified Magnaporthe oryzae Arf6 (MoArf6) as one of the Arf proteins that regulates growth and conidiation in the rice blast fungus M. oryzae, but the remaining family proteins remain unknown. Here, we identified six additional Arf proteins, including MoArf1, MoArl1, MoArl3, MoArl8, MoCin4, and MoSar1, as well as their sole adaptor protein, MoGga1, and determined their shared and specific functions. We showed that the majority of these proteins exhibit positive regulatory functions, most notably, in growth. Importantly, MoArl1, MoCin4, and MoGga1 are involved in pathogenicity through the regulation of host penetration and invasive hyphal growth. MoArl1 and MoCin4 also regulate normal vesicle trafficking, and MoCin4 further controls the formation of the biotrophic interfacial complex (BIC). Moreover, we showed that Golgi-cytoplasm cycling of MoArl1 is required for its function. Finally, we demonstrated that interactions between MoArf1 and MoArl1 with MoGga1 are important for Golgi localization and pathogenicity. Collectively, our findings revealed the shared and specific functions of Arf family members in M. oryzae and shed light on how these proteins function through conserved mechanisms to govern growth, transport, and virulence of the blast fungus.IMPORTANCEMagnaporthe oryzae is the causal agent of rice blast, representing the most devastating diseases of rice worldwide, which results in losses of amounts of rice that could feed more than 60 million people each year. Arf (ADP ribosylation factor) small GTPase family proteins are involved in vesicle trafficking and organelle maintenance in eukaryotic cells. To investigate the function of Arf family proteins in M. oryzae, we systematically characterized all seven Arf proteins and found that they have shared and specific functions in governing the growth, development, and pathogenicity of the blast fungus. We have also identified the pathogenicity-related protein MoGga1 as the common adaptor of MoArf1 and MoArl1. Our findings are important because they provide the first comprehensive characterization of the Arf GTPase family proteins and their adaptor protein MoGga1 functioning in a plant-pathogenic fungus, which could help to reveal new fungicide targets to control this devastating disease.

The MaCreA Gene Regulates Normal Conidiation and Microcycle Conidiation in Metarhizium acridum

As a C₂H₂ type zinc finger transcription factor, CreA is the key in Carbon Catabolism Repression (CCR) pathway, which negatively regulates the genes in carbon sources utilization. As conidiation in filamentous fungi is affected by nutritional conditions, CreA may contribute to fungal conidiation, which has been well studied in filamentous fungi, especially Aspergillus spp., but researches on entomopathogenic fungi are not enough. In this study, we found a homologous gene MaCreA in Metarhizium acridum, and the MaCreA deletion strain showed delayed conidiation, significant decrease in conidial yield, and 96.88% lower conidial production, when compared with the wild-type strain, and the normal conidiation and microcycle conidiation pattern shift was blocked. RT-qPCR showed that the transcription levels of the genes FlbD and LaeA (related to asexual development) were significantly altered, and those of most of the conidiation-related genes were higher in ΔMaCreA strain. The results of RNA-Seq revealed that MaCreA regulated the two conidiation patterns by mediating genes related to cell cycle, cell division, cell wall, and cell polarity. In conclusion, CreA, as a core regulatory gene in conidiation, provides new insight into the mechanism of conidiation in entomopathogenic fungi.

The inhibitor of apoptosis protein MoBir1 is involved in the suppression of hydrogen peroxide-induced fungal cell death, reactive oxygen species generation, and pathogenicity of rice blast fungus

The inhibitor of apoptosis protein (IAP) family has been identified in a variety of organisms. All IAPs contain one to three baculoviral IAP repeat (BIR) domains, which are required for anti-apoptotic activity. Here, we identified a type II BIR domain-containing protein, MoBir1, in the rice blast fungus Magnaporthe oryzae. Expression of the MoBIR1 gene in Saccharomyces cerevisiae suppressed hydrogen peroxide-induced cell death and delayed yeast cell chronological aging. Delayed aging was found to require the carboxyl terminus of MoBir1. M. oryzae transformants overexpressing the MoBIR1 gene demonstrated increased growth rate and biomass, delayed mycelial aging, and enhanced resistance to hydrogen peroxide but reduced reactive oxygen species generation and virulence. Moreover, MoBIR1-overexpressing transformants exhibited anti-apoptotic activity. However, MoBIR1 silencing resulted in no obvious phenotypic changes, compared with the wild-type M. oryzae strain Guy11. Our findings broaden the knowledge on fungal type II BIR domain-containing proteins.

The adenylate cyclase UvAc1 and phosphodiesterase UvPdeH control the intracellular cAMP level, development, and pathogenicity of the rice false smut fungus Ustilaginoidea virens

The cyclic adenosine monophosphate (cAMP) signaling pathway plays pleiotropic roles in regulating development and pathogenicity in eukaryotes. cAMP is a second messenger that is important for the activation of downstream pathways. The intracellular cAMP level is modulated mainly by its biosynthesis, which is catalyzed by adenylate cyclases (ACs), and hydrolysis by phosphodiesterases (PDEs). Here, we identified the AC UvAc1 and the cAMP high-affinity PDE UvPdeH in the rice false smut fungus Ustilaginoidea virens; these enzymes are homologs of MoMac1 and MoPdeH in Magnaporthe oryzae (rice blast fungus). A heterogenous complementation assay revealed that UvAc1 and UvPdeH partially or completely rescued the defects in ΔMomac1 and ΔMopdeH mutant M. oryzae. UvAc1 and UvPdeH play important roles in the development and virulence of U. virens. ΔUvac1 and ΔUvpdeH mutant fungi showed defects in conidial production, morphology, and germination; reduced toxicity against germinating rice seeds; and reduced virulence on rice panicles. ΔUvac1 exhibited increased sensitivity to Calcofluor White (CFW) and sodium chloride (NaCl), and decreased sensitivity to Congo Red (CR), while ΔUvpdeH showed increased sensitivity to sodium dodecyl sulfate, CR, sorbitol, and hydrogen peroxide, and decreased sensitivity to CFW and NaCl. High-performance liquid chromatography revealed that the intracellular cAMP level was significantly increased in ΔUvpdeH and decreased in ΔUvac1. Taken together, our results demonstrate that UvAc1 and UvPdeH are conservative components of the cAMP pathway that are important for conidiogenesis, stress responses, virulence, and regulation of the intracellular cAMP level in U. virens.

Characterization of the Papain-Like Protease p29 of the Hypovirus CHV1-CN280 in Its Natural Host Fungus Cryphonectria parasitica and Nonhost Fungus Magnaporthe oryzae

Cryphonectria hypovirus 1 strain CN280 (CHV1-CN280) was isolated from North China and exhibited typical hypovirulence-associated traits. We previously reported that CHV1-CN280 was more aggressive and had a higher horizontal transmission ability between Cryphonectria parasitica isolates belonging to different vegetative compatibility groups than two other CHV1 hypoviruses (namely, CHV1-EP713 and CHV1-Euro7), thus displaying greater potential for biological control of chestnut blight. The genome sequence of CHV1-CN280 shared approximately 70% identity with three other hypoviruses (CHV1-EP713, CHV1-Euro7, and CHV1-EP721). The coding region for p29, a papain-like protease encoded by CHV1-CN280 hypovirus, displayed an average of only approximately 60% amino acid identity among them, while the identity between the other three CHV1 isolates was higher than 89%. Protease p29 acted as a virus-encoded determinant responsible for altering fungal host phenotypes in other CHV1 isolates. In this study, the impacts of CHV1-CN280 p29 expression in virus-free C. parasitica were investigated. CHV1-CN280 p29 expression in C. parasitica resulted in significantly reduced sporulation, pigmentation, extracellular laccase activities, and pathogenicity, which is consistent with previous investigations. Subsequently, the potential of CHV1-CN280 p29 as a viral determinant responsible for suppression of host phenotypes in other phytopathogenic fungi such as Magnaporthe oryzae, the causal agent of rice blast disease, was discussed. However, heterologous expression of p29 in M. oryzae induced the opposite effect on sporulation, extracellular laccase activities, and pathogenicity; had no significant effect on pigmentation and mycelial growth; and contributed to extracellular peroxidase activities, suggesting that CHV1-CN280 p29 may disturb a unique regulatory pathway in C. parasitica, rather than a basic regulatory pathway conserved in diverse range of fungi. Alternatively, CHV1-CN280 p29-mediated modulation of fungal phenotypes may be facilitated by the specific interaction between p29 and a special fungal-host component, which exists only with C. parasitica but not M. oryzae.

MoImd4 mediates crosstalk between MoPdeH-cAMP signalling and purine metabolism to govern growth and pathogenicity in Magnaporthe oryzae

The high-affinity cyclic adenosine monophosphate (cAMP) phosphodiesterase MoPdeH is important not only for cAMP signalling and pathogenicity, but also for cell wall integrity (CWI) maintenance in the rice blast fungus Magnaporthe oryzae. To explore the underlying mechanism, we identified MoImd4 as an inosine-5'-monophosphate dehydrogenase (IMPDH) homologue that interacts with MoPdeH. Targeted deletion of MoIMD4 resulted in reduced de novo purine biosynthesis and growth, as well as attenuated pathogenicity, which were suppressed by exogenous xanthosine monophosphate (XMP). Treatment with mycophenolic acid (MPA), which specifically inhibits MoImd4 activity, resulted in reduced growth and virulence attenuation. Intriguingly, further analysis showed that MoImd4 promotes the phosphodiesterase activity of MoPdeH, thereby decreasing intracellular cAMP levels, and MoPdeH also promotes the IMPDH activity of MoImd4. Our studies revealed the presence of a novel crosstalk between cAMP regulation and purine biosynthesis in M. oryzae, and indicated that such a link is also important in the pathogenesis of M. oryzae.

The seven transmembrane domain protein MoRgs7 functions in surface perception and undergoes coronin MoCrn1-dependent endocytosis in complex with Gα subunit MoMagA to promote cAMP signaling and appressorium formation in Magnaporthe oryzae

Regulator of G-protein signaling (RGS) proteins primarily function as GTPase-accelerating proteins (GAPs) to promote GTP hydrolysis of Gα subunits, thereby regulating G-protein mediated signal transduction. RGS proteins could also contain additional domains such as GoLoco to inhibit GDP dissociation. The rice blast fungus Magnaporthe oryzae encodes eight RGS and RGS-like proteins (MoRgs1 to MoRgs8) that have shared and distinct functions in growth, appressorium formation and pathogenicity. Interestingly, MoRgs7 and MoRgs8 contain a C-terminal seven-transmembrane domain (7-TM) motif typical of G-protein coupled receptor (GPCR) proteins, in addition to the conserved RGS domain. We found that MoRgs7, but not MoRgs8, couples with Gα MoMagA to undergo endocytic transport from the plasma membrane to the endosome upon sensing of surface hydrophobicity. We also found that MoRgs7 can interact with hydrophobic surfaces via a hydrophobic interaction, leading to the perception of environmental hydrophobiccues. Moreover, we found that MoRgs7-MoMagA endocytosis is regulated by actin patch-associated protein MoCrn1, linking it to cAMP signaling. Our studies provided evidence suggesting that MoRgs7 could also function in a GPCR-like manner to sense environmental signals and it, together with additional proteins of diverse functions, promotes cAMP signaling required for developmental processes underlying appressorium function and pathogenicity.

MoPer1 is required for growth, conidiogenesis, and pathogenicity in Magnaporthe oryzae

BACKGROUND

GPI-anchoring is a prevalent Glycosylphosphatidylinositol modification process of posttranslational protein and is necessary for cell wall integrity in eukaryotes. To date, the function of GPI anchored-related protein remains unknown in phytopathogenic fungi.

RESULTS

We here characterized the functions of MoPer1, a homolog of Saccharomyces cerevisiae ScPer1, from the rice blast fungus Magnaporthe oryzae. Transcriptional analysis demonstrated that MoPER1 was significantly upregulated during conidiation and infection. We found that the ∆Moper1 mutant was defective in conidiation and appressoria formation, and MoPer1 was involved in osmotic stress response and maintaining the cell wall integrity. Pathogenicity assays indicated that deletion of MoPEP1 significant reduction in virulence. Microscopic examination of the lesions revealed that the invasive hyphae of ∆Moper1 mutants were mostly restricted to the primary infected leaf sheath cells.

CONCLUSIONS

Our results indicated that MoPer1 is necessary for growth, conidiogenesis, and pathogenicity of the fungus. Our study facilitated to deep elucidate the pathogenic molecular mechanism of M. oryzae, and also provided a very helpful reference value for developing effective fungicide pointed at as the gene for target.

MoPpe1 partners with MoSap1 to mediate TOR and cell wall integrity signalling in growth and pathogenicity of the rice blast fungus Magnaporthe oryzae

In the rice blast fungus Magnaporthe oryzae, the cell wall integrity (CWI) signalling pathway governs cell wall changes in response to external cues and normal CWI signalling is critical for appressorium function and pathogenicity. We previously characterized the mitogen-activated protein kinase (MAPK) kinase MoMkk1 as an integral component of the CWI pathway. Using the affinity purification approach, we have identified MoMkk1-interacting MoPpe1 as a homologue of Saccharomyces cerevisiae serine/threonine protein phosphatase Sit4/Ppe1. We found that MoPpe1 is required for vegetative growth, conidiation and full virulence. In addition, we found that MoPpe1 interacts with MoSap1, a protein with functions similar to MoPpe1. Intriguingly, we found that MoPpe1-MoSap1 interaction is related to CWI and target of rapamycin (TOR) pathways. We presented evidence suggesting that MoPpe1 and MoSap1 function as an adaptor complex linking CWI and TOR signalling and that the activation of the TOR pathway leads to suppression of CWI signalling, resulting in defects in appressorium function and pathogenicity. Taken together, our studies not only reveal important functions of MoMkk1-MoPpe1-MoSap1 interactions in growth and pathogenicity of the blast fungus, but also highlight the complexity of regulatory networks involving conserved yet novel regulatory mechanisms of CWI and TOR signalling.

Isopropylmalate isomerase MoLeu1 orchestrates leucine biosynthesis, fungal development, and pathogenicity in Magnaporthe oryzae

The biosynthesis of branched-chain amino acids (BCAAs) is conserved in fungi and plants, but not in animals. The Leu1 gene encodes isopropylmalate isomerase that catalyzes the conversion of α-isopropylmalate into β-isopropylmalate in the second step of leucine biosynthesis in yeast. Here, we identified and characterized the functions of MoLeu1, an ortholog of yeast Leu1 in the rice blast fungus Magnaporthe oryzae. The transcriptional level of MoLEU1 was increased during conidiation and in infectious stages. Cellular localization analysis indicated that MoLeu1 localizes to the cytoplasm at all stages of fungal development. Targeted gene deletion of MoLEU1 led to leucine auxotrophy, and phenotypic analysis of the generated ∆Moleu1 strain revealed that MoLeu1-mediated leucine biosynthesis was required for vegetative growth, asexual development, and pathogenesis of M. oryzae. We further observed that invasive hyphae produced by the ∆Moleu1 strain were mainly limited to the primary infected host cells. The application of exogenous leucine fully restored vegetative growth and partially restored conidiation as well as pathogenicity defects in the ∆Moleu1 strain. In summary, our results suggested that MoLeu1-mediated leucine biosynthesis crucially promotes vegetative growth, conidiogenesis, and pathogenicity of M. oryzae. This study helps unveil the regulatory mechanisms that are essential for infection-related morphogenesis and pathogenicity of the rice blast fungus.