Divergent cAMP signaling pathways regulate growth and pathogenesis in the rice blast fungus Magnaporthe grisea

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.

Determination of cyclic adenosine phosphate and protein content in dormant chlamydospore and nondormant chlamydospore of Arthrobotrys flagrans

Arthrobotrys flagrans, a nematode-eating fungus, is an effective component of animal parasitic nematode biocontrol agents. In the dried formulation, the majority of spores are in an endogenous dormant state. This study focuses on dormant chlamydospore and nondormant chlamydospore of A. flagrans to investigate the differences in cyclic adenosine monophosphate (cAMP) and protein content between the two types of spores. cAMP and soluble proteins were extracted from the nondormant chlamydospore and dormant chlamydospore of two isolates of A. flagrans. The cAMP Direct Immunoassay Kit and Bradford protein concentration assay kit (Coomassie brilliant blue method) were used to detect the cAMP and protein content in two types of spores. Results showed that the content of cAMP in dormant spores of both isolates was significantly higher than that in nondormant spores (p < 0.05). The protein content of dormant spores in DH055 bacteria was significantly higher than that of nondormant spores (p < 0.05). In addition, the protein content of dormant spores of the SDH035 strain was slightly higher than that of nondormant spores, but the difference was not significant (p > 0.05). The results obtained in this study provide evidence for the biochemical mechanism of chlamydospore dormancy or the germination of the nematophagous fungus A. flagrans.

Deciphering the Genomic Landscape and Virulence Mechanisms of the Wheat Powdery Mildew Pathogen Blumeria graminis f. sp. tritici Wtn1: Insights from Integrated Genome Assembly and Conidial Transcriptomics

A high-quality genome sequence from an Indian isolate of Blumeria graminis f. sp. tritici Wtn1, a persistent threat in wheat farming, was obtained using a hybrid method. The assembly of over 9.24 million DNA-sequence reads resulted in 93 contigs, totaling a 140.61 Mb genome size, potentially encoding 8480 genes. Notably, more than 73.80% of the genome, spanning approximately 102.14 Mb, comprises retro-elements, LTR elements, and P elements, influencing evolution and adaptation significantly. The phylogenomic analysis placed B. graminis f. sp. tritici Wtn1 in a distinct monocot-infecting clade. A total of 583 tRNA anticodon sequences were identified from the whole genome of the native virulent strain B. graminis f. sp. tritici, which comprises distinct genome features with high counts of tRNA anticodons for leucine (70), cysteine (61), alanine (58), and arginine (45), with only two stop codons (Opal and Ochre) present and the absence of the Amber stop codon. Comparative InterProScan analysis unveiled "shared and unique" proteins in B. graminis f. sp. tritici Wtn1. Identified were 7707 protein-encoding genes, annotated to different categories such as 805 effectors, 156 CAZymes, 6102 orthologous proteins, and 3180 distinct protein families (PFAMs). Among the effectors, genes like Avra10, Avrk1, Bcg-7, BEC1005, CSEP0105, CSEP0162, BEC1016, BEC1040, and HopI1 closely linked to pathogenesis and virulence were recognized. Transcriptome analysis highlighted abundant proteins associated with RNA processing and modification, post-translational modification, protein turnover, chaperones, and signal transduction. Examining the Environmental Information Processing Pathways in B. graminis f. sp. tritici Wtn1 revealed 393 genes across 33 signal transduction pathways. The key pathways included yeast MAPK signaling (53 genes), mTOR signaling (38 genes), PI3K-Akt signaling (23 genes), and AMPK signaling (21 genes). Additionally, pathways like FoxO, Phosphatidylinositol, the two-component system, and Ras signaling showed significant gene representation, each with 15-16 genes, key SNPs, and Indels in specific chromosomes highlighting their relevance to environmental responses and pathotype evolution. The SNP and InDel analysis resulted in about 3.56 million variants, including 3.45 million SNPs, 5050 insertions, and 5651 deletions within the whole genome of B. graminis f. sp. tritici Wtn1. These comprehensive genome and transcriptome datasets serve as crucial resources for understanding the pathogenicity, virulence effectors, retro-elements, and evolutionary origins of B. graminis f. sp. tritici Wtn1, aiding in developing robust strategies for the effective management of wheat powdery mildew.

A single laccase acts as a key component of environmental sensing in a broad host range fungal pathogen

Secreted laccases are important enzymes on a broad ecological scale for their role in mediating plant-microbe interactions, but within ascomycete fungi these enzymes have been primarily associated with melanin biosynthesis. In this study, a putatively secreted laccase, Sslac2, was characterized from the broad-host-range plant pathogen Sclerotinia sclerotiorum, which is largely unpigmented and is not dependent on melanogenesis for plant infection. Gene knockouts of Sslac2 demonstrate wide ranging developmental phenotypes and are functionally non-pathogenic. These mutants also displayed indiscriminate growth behaviors and enhanced biomass formation, seemingly as a result of their inability to respond to canonical environmental growth cues, a phenomenon further confirmed through chemical stress, physiological, and transcriptomic analyses. Transmission and scanning electron microscopy demonstrate apparent differences in extracellular matrix structure between WT and mutant strains that likely explain the inability of the mutants to respond to their environment. Targeting Sslac2 using host-induced gene silencing significantly improved resistance to S. sclerotiorum, suggesting that fungal laccases could be a valuable target of disease control. Collectively, we identified a laccase critical to the development and virulence of the broad-host-range pathogen S. sclerotiorum and propose a potentially novel role for fungal laccases in modulating environmental sensing.

The RasGEF MoCdc25 regulates vegetative growth, conidiation and appressorium-mediated infection in the rice blast fungus Magnaporthe oryzae

Ras guanine nucleotide exchange factors (RasGEFs) can trigger Ras GTPase activities and play important roles in controlling various cellular processes in eukaryotes. Recently, it has been exhibited that RasGEF Cdc25 regulates morphological differentiation and pathogenicity in several plant pathogenic fungi. However, the role of RasGEFs in Magnaporthe oryzae is largely unknown. In this study, we identified and functionally characterized a RasGEF gene MoCDC25 in M. oryzae, which is orthologous to Saccharomyces cerevisiae CDC25. Targeted gene deletion mutants (ΔMocdc25) were completely nonpathogenic and were severely impaired in hyphal growth, conidiation and appressorium formation. The mutants exhibited highly sensitive response to osmotic, cell wall integrity or oxidative stresses. MoCdc25 physically interacts with the MAPK scaffold Mst50 and the putative Cdc42GEF MoScd1 in yeast two-hybrid assays. Moreover, we found that MoCdc25 was involved in regulating the phosphorylation of the MAP kinases (Pmk1, Mps1, and Osm1). In addition, the intracellular cAMP content in hyphae of the ΔMocdc25 mutants was significantly reduced compared to the parent strain Ku80 and the defect of appressorium formation of the mutants could be partially restored by the supplement of exogenous cAMP. Taken together, we conclude that the RasGEF MoCdc25 regulates vegetative growth, conidiation, appressorium formation and pathogenicity via MAPK and cAMP response pathways in M. oryzae.

MoCbp7, a Novel Calcineurin B Subunit-Binding Protein, Is Involved in the Calcium Signaling Pathway and Regulates Fungal Development, Virulence, and ER Homeostasis in Magnaporthe oryzae

Calcineurin, a key regulator of the calcium signaling pathway, is involved in calcium signal transduction and calcium ion homeostasis. Magnaporthe oryzae is a devastating filamentous phytopathogenic fungus in rice, yet little is known about the function of the calcium signaling system. Here, we identified a novel calcineurin regulatory-subunit-binding protein, MoCbp7, which is highly conserved in filamentous fungi and was found to localize in the cytoplasm. Phenotypic analysis of the MoCBP7 gene deletion mutant (ΔMocbp7) showed that MoCbp7 influenced the growth, conidiation, appressorium formation, invasive growth, and virulence of M. oryzae. Some calcium-signaling-related genes, such as YVC1, VCX1, and RCN1, are expressed in a calcineurin/MoCbp7-dependent manner. Furthermore, MoCbp7 synergizes with calcineurin to regulate endoplasmic reticulum homeostasis. Our research indicated that M. oryzae may have evolved a new calcium signaling regulatory network to adapt to its environment compared to the fungal model organism Saccharomyces cerevisiae.

Three Novel Adenylate Cyclase Genes Show Significant Biological Functions in Plant

Adenylate cyclase is the key enzyme solely synthesizing cAMP which participates in cell metabolism regulations and functions as an intracellular second messenger. However, the biological functions of plant ACs have not been elucidated clearly for their poor conservative sequences and low detectable cAMP. We performed a systematic study of plant ACs by using Chinese jujube, whose fruit exhibits the highest cAMP content among plants. Three novel ACs were identified from Chinese jujube, and two types of methods including in vitro and in vivo were used to certificate ZjAC1-3 which can catalyze the conversion of ATP into cAMP. The biological functions of significant accelerations of seed germination, root growth, and flowering were found via overexpression of these AC genes in Arabidopsis, and these functions of ACs were further demonstrated by treating the AC-overexpressing transgenic lines and wild type Arabidopsis with bithionol and dibutyryl-cAMP. At last, transcriptome data revealed that the underlying mechanism of the biological functions of ACs might be regulation of the key genes involved in the circadian rhythm pathway and the hormone signal transduction pathway. This research established a foundation for further investigating plant AC genes and provided strong evidence for cAMP serving as a signaling molecule in plants.

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

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

The cAMP-PKA signalling pathway regulates hyphal growth, conidiation, trap morphogenesis, stress tolerance, and autophagy in Arthrobotrys oligospora

The cyclic adenosine monophosphate-protein kinase A (cAMP-PKA) signalling pathway is evolutionarily conserved in eukaryotes and plays a crucial role in defending against external environmental challenges, which can modulate the cellular response to external stimuli. Arthrobotrys oligospora is a typical nematode-trapping fungus that specializes in adhesive networks to kill nematodes. To elucidate the biological roles of the cAMP-PKA signalling pathway, we characterized the orthologous adenylate cyclase AoAcy, a regulatory subunit (AoPkaR), and two catalytic subunits (AoPkaC1 and AoPkaC2) of PKA in A. oligospora by gene disruption, transcriptome, and metabolome analyses. Deletion of Aoacy significantly reduced the levels of cAMP and arthrobotrisins. Results revealed that Aoacy, AopkaR, and AopkaC1 were involved in hyphal growth, trap morphogenesis, sporulation, stress resistance, and autophagy. In addition, Aoacy and AopkaC1 were involved in the regulation of mitochondrial morphology, thereby affecting energy metabolism, whereas AopkaC2 affected sporulation, nuclei, and autophagy. Multi-omics results showed that the cAMP-PKA signalling pathway regulated multiple metabolic and cellular processes. Collectively, these data highlight the indispensable role of cAMP-PKA signalling pathway in the growth, development, and pathogenicity of A. oligospora, and provide insights into the regulatory mechanisms of signalling pathways in sporulation, trap formation, and lifestyle transition.

Protein kinase A participates in hyphal and appressorial development by targeting Efg1-mediated transcription of a Rab GTPase in Setosphaeria turcica

The cyclic adenosine monophosphate (cAMP) signalling pathway plays an important role in the regulation of the development and pathogenicity of filamentous fungi. cAMP-dependent protein kinase A (PKA) is the conserved element downstream of cAMP, and its diverse mechanisms in multiple filamentous fungi are not well known yet. In the present study, gene knockout mutants of two catalytic subunits of PKA (PKA-C) in Setosphaeria turcica were created to illustrate the regulatory mechanisms of PKA-Cs on the development and pathogenicity of S. turcica. As a result, StPkaC2 was proved to be the main contributor of PKA activity in S. turcica. In addition, it was found that both StPkaC1 and StPkaC2 were necessary for conidiation and invasive growth, while only StPkaC2 played a negative role in the regulation of filamentous growth. We reveal that only StPkaC2 could interact with the transcription factor StEfg1, and it inhibited the transcription of StRAB1, a Rab GTPase homologue coding gene in S. turcica, whereas StPkaC1 could specifically interact with a transcriptional regulator StFlo8, which could rescue the transcriptional inhibition of StEfg1 on StRAB1. We also demonstrated that StRAB1 could positively influence the biosynthesis of chitin in hyphae, thus changing the filamentous growth. Our findings clarify that StPkaC2 participates in chitin biosynthesis to modulate mycelium development by targeting the Efg1-mediated transcription of StRAB1, while StFlo8, interacting with StPkaC1, acts as a negative regulator during this process.

Role of the cAMP signaling pathway in the dissemination and development on pepper fruit anthracnose disease caused by Colletotrichum scovillei

The ascomycete fungus Colletotrichum scovillei causes severe anthracnose disease on the fruit of sweet pepper and chili pepper (Capsicum annuum L.) worldwide. Understanding the biology of C. scovillei would improve the management of fruit anthracnose diseases. The cyclic adenosine monophosphate (cAMP) signaling pathway regulates diverse cellular and physiological processes in several foliar fungal pathogens. We investigated the roles of the cAMP signaling pathway in C. scovillei using pharmaceutical and genetic approaches. Exogenous cAMP was found to increase conidiation, appressorium formation, and anthracnose disease development in C. scovillei. CsAc1, CsCap1, and CsPdeH, which regulate the intracellular cAMP level, were deleted by homology-dependent gene replacement. Expectedly, the intracellular cAMP level was significantly decreased in ΔCsac1 and ΔCscap1 but increased in ΔCspdeh. All three deletion mutants exhibited serious defects in multiple fungal developments and pathogenicity, suggesting regulation of the intracellular cAMP level is important for C. scovillei. Notably, exogenous cAMP recovered the defect of ΔCsac1 in appressorium development, but not penetration, which was further recovered by adding CaCl2. This result suggests that CsAc1 is associated with both the cAMP and Ca2+ signaling pathways in C. scovillei. ΔCscap1 produced morphologically abnormal conidia with reduced tolerance to thermal stress. ΔCspdeh was completely defective in conidiation in C. scovillei, unlike other foliar pathogens. Taken together, these results demonstrate the importance of cAMP signaling in anthracnose disease caused by C. scovillei.

PKR Protects the Major Catalytic Subunit of PKA Cpk1 from FgBlm10-Mediated Proteasome Degradation in Fusarium graminearum

For optimal proteolytic function, the proteasome core (CP or 20S) must associate with activators. The cAMP-PKA pathway is reported to affect the activity of the proteasome in humans. However, the relationship between the proteasome and PKA is not well characterized. Our results showed that the major catalytic subunit Cpk1 was degraded without the protection of Pkr. Eleven (out of 67) pkr suppressors had FgBlm10 C-terminal truncation, one suppressor had an amino acid change mutation in the PRE6 ortholog (FGRRES_07282), and one in the PRE5 ortholog (FGRRES_05222). These mutations rescued the defects in growth and conidial morphology, Cpk1 stability, and PKA activities in the pkr mutant. The interaction of FgBlm10 with FgPre5 and FgPre6 were detected by co-immunoprecipitation, and the essential elements for their interaction were characterized, including the FgBlm10 C-terminus, amino acid D82 of FgPre6 and K62 of FgPre5. Additional FgBlm10-interacting proteins were identified in the wild type and pkr mutant, suggesting that PKA regulates the preference of FgBlm10-mediated proteasome assembly. In addition, PKA indirectly affected the phosphorylation of FgBlm10, and its localization in the nucleus. The truncation of the FgBlm10 C terminus also enhanced nuclear import and bleomycin resistance, suggesting its role in proteasome assembly at DNA damage sites. Collectively, our data demonstrated that regulation between PKA and proteasome degradation is critical for the vegetative growth of F. graminearum.

MoDHX35, a DEAH-Box Protein, Is Required for Appressoria Formation and Full Virulence of the Rice Blast Fungus, Magnaporthe oryzae

The DExD/H-box protein family encompasses a large number of RNA helicases that are involved in RNA metabolism and a variety of physiological functions in different species. However, there is limited knowledge of whether DExD/H-box proteins play a role in the pathogenicity of plant fungal pathogens. In the present work, the DExD/H-box protein MoDHX35, which belongs to the DEAH subfamily, was shown to be crucial in appressoria formation and full virulence of the rice blast fungus, Magnaporthe oryzae. The predicted protein sequence of MoDHX35 had typical DEAH-box domains, showed 47% identity to DHX35 in Homo species, but had no orthologs in Saccharomyces cerevisiae. Deletion of the MoDHX35 gene resulted in reduced tolerance of the mutants to doxorubicin, a nucleic acid synthesis disturbing agent, suggesting the involvement of MoDHX35 in RNA metabolism. MoDHX35-deleted mutants exhibited normal vegetative growth, conidia generation and conidial germination, but showed a reduced appressorium formation rate and attenuated virulence. Our work demonstrates the involvement of DEAH-box protein functions in the pathogenicity of plant fungal pathogens.

The appressorium at a glance

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

Structured Framework and Genome Analysis of Magnaporthe grisea Inciting Pearl Millet Blast Disease Reveals Versatile Metabolic Pathways, Protein Families, and Virulence Factors

Magnaporthe grisea (T.T. Herbert) M.E. Barr is a major fungal phytopathogen that causes blast disease in cereals, resulting in economic losses worldwide. An in-depth understanding of the basis of virulence and ecological adaptation of M. grisea is vital for devising effective disease management strategies. Here, we aimed to determine the genomic basis of the pathogenicity and underlying biochemical pathways in Magnaporthe using the genome sequence of a pearl millet-infecting M. grisea PMg_Dl generated by dual NGS techniques, Illumina NextSeq 500 and PacBio RS II. The short and long nucleotide reads could be draft assembled in 341 contigs and showed a genome size of 47.89 Mb with the N50 value of 765.4 Kb. Magnaporthe grisea PMg_Dl showed an average nucleotide identity (ANI) of 86% and 98% with M. oryzae and Pyricularia pennisetigena, respectively. The gene-calling method revealed a total of 10,218 genes and 10,184 protein-coding sequences in the genome of PMg_Dl. InterProScan of predicted protein showed a distinct 3637 protein families and 695 superfamilies in the PMg_Dl genome. In silico virulence analysis revealed the presence of 51VFs and 539 CAZymes in the genome. The genomic regions for the biosynthesis of cellulolytic endo-glucanase and beta-glucosidase, as well as pectinolytic endo-polygalacturonase, pectin-esterase, and pectate-lyases (pectinolytic) were detected. Signaling pathways modulated by MAPK, PI3K-Akt, AMPK, and mTOR were also deciphered. Multicopy sequences suggestive of transposable elements such as Type LTR, LTR/Copia, LTR/Gypsy, DNA/TcMar-Fot1, and Type LINE were recorded. The genomic resource presented here will be of use in the development of molecular marker and diagnosis, population genetics, disease management, and molecular taxonomy, and also provide a genomic reference for ascomycetous genome investigations in the future.

cAMP Signalling Pathway in Biocontrol Fungi

Biocontrol is a complex process, in which a variety of physiological and biochemical characteristics are altered. The cAMP signalling pathway is an important signal transduction pathway in biocontrol fungi and consists of several key components. The G-protein system contains G-protein coupled receptors (GPCRs), heterotrimeric G-proteins, adenylate cyclase (AC), cAMP-dependent protein kinase (PKA), and downstream transcription factors (TFs). The cAMP signalling pathway can regulate fungal growth, development, differentiation, sporulation, morphology, secondary metabolite production, environmental stress tolerance, and the biocontrol of pathogens. However, few reviews of the cAMP signalling pathway in comprehensive biocontrol processes have been reported. This work reviews and discusses the functions and applications of genes encoding each component in the cAMP signalling pathway from biocontrol fungi, including the G-protein system components, AC, PKA, and TFs, in biocontrol behaviour. Finally, future suggestions are provided for constructing a complete cAMP signalling pathway in biocontrol fungi containing all the components and downstream effectors involved in biocontrol behavior. This review provides useful information for the understanding the biocontrol mechanism of biocontrol fungi by utilising the cAMP signalling pathway.

CsPOM1, a DYRK Family Kinase, Plays Diverse Roles in Fungal Development, Virulence, and Stress Tolerance in the Anthracnose Pathogen Colletotrichum scovillei

Colletotrichum scovillei is the major anthracnose fungus of sweet pepper and chili pepper (Capsicum annuum L.), causing significant losses in the yield and quality of the pepper fruits. Molecular mechanisms governing development and pathogenicity have been widely studied in many foliar fungal pathogens, but the information on fruit diseases is still limited. In this study, we determined the functional roles of the dual-specificity tyrosine phosphorylation-regulated kinase CsPOM1 in C. scovillei. Knockout mutant for CsPOM1 gene was obtained via homology-dependent gene replacement. The ΔCspom1 mutant exhibited a reduction in vegetative growth on osmotic stress, surface hydrophobicity, and conidiation compared with wild-type. Conidia of the ΔCspom1 mutant were already two-celled before inoculation on an induction surface, indicating that CsPOM1 negatively regulates conidial cell division. The ΔCspom1 mutant, similar to wild-type, formed appressoria on the plant surface, but was significantly reduced on hydrophobic coverslips, probably due to a defect in the recognition of surface hydrophobicity. Treatment of conidia with cutin monomers restored appressorium formation on hydrophobic coverslips in the ΔCspom1 mutant. On pepper fruits, the ΔCspom1 mutant exhibited delayed penetration and invasive growth, leading to significantly reduced virulence. Collectively, the results showed that CsPOM1 is important for stress tolerance, conidiation, surface hydrophobicity, appressorium formation, and virulence in C. scovillei.

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

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

Identification and Verification of Diagnostic Biomarkers for Glomerular Injury in Diabetic Nephropathy Based on Machine Learning Algorithms

Diabetic nephropathy (DN) is regarded as the leading cause of end-stage renal disease worldwide and lacks novel therapeutic targets. To screen and verify special biomarkers for glomerular injury in patients with DN, fifteen datasets were retrieved from the Gene Expression Omnibus (GEO) database, correspondingly divided into training and testing cohorts and then merged. Using the limma package, 140 differentially expressed genes (DEGs) were screened out between 81 glomerular DN samples and 41 normal ones from the training cohort. With the help of the ConsensusClusterPlus and WGCNA packages, the 81 glomerular DN samples were distinctly divided into two subclusters, and two highly associated modules were identified. By using machine learning algorithms (LASSO, RF, and SVM-RFE) and the Venn diagram, two overlapping genes (PRKAR2B and TGFBI) were finally determined as potential biomarkers, which were further validated in external testing datasets and the HFD/STZ-induced mouse models. Based on the biomarkers, the diagnostic model was developed with reliable predictive ability for diabetic glomerular injury. Enrichment analyses indicated the apparent abnormal immune status in patients with DN, and the two biomarkers played an important role in the immune microenvironment. The identified biomarkers demonstrated a meaningful correlation between the immune cells' infiltration and renal function. In conclusion, two robust genes were identified as diagnostic biomarkers and may serve as potential targets for therapeutics of DN, which were closely associated with multiple immune cells.

Endocytic protein Pal1 regulates appressorium formation and is required for full virulence of Magnaporthe oryzae

Endocytosis plays key roles during infection of plant-pathogenic fungi, but its regulatory mechanisms are still largely unknown. Here, we identified a putative endocytosis-related gene, PAL1, which was highly expressed in appressorium of Magnaporthe oryzae, and was found to be important for appressorium formation and maturation. Deletion of PAL1 significantly reduced the virulence of M. oryzae due to defects in appressorial penetration and invasive growth in host cells. The Pal1 protein interacted and colocalized with the endocytosis protein Sla1, suggesting it is involved in endocytosis. The Δpal1 mutant was significantly reduced in appressorium formation, which was recovered by adding exogenous cAMP and 3-isobutyl-1-methylxanthine (IBMX). Moreover, the phosphorylation level of Pmk1 in Δpal1 was also reduced, suggesting Pal1 functions upstream of both the cAMP and Pmk1 signalling pathways. As a consequence, the utilization of glycogen and lipid, appressorial autophagy, actin ring formation, localization of septin proteins, as well as turgor accumulation were all affected in the Δpal1 mutant. Taken together, Pal1 regulates cAMP and the Pmk1 signalling pathway for appressorium formation and maturation to facilitate infection of M. oryzae.

Hybrid de novo genome-reassembly reveals new insights on pathways and pathogenicity determinants in rice blast pathogen Magnaporthe oryzae RMg_Dl

Blast disease incited by Magnaporthe oryzae is a major threat to sustain rice production in all rice growing nations. The pathogen is widely distributed in all rice paddies and displays rapid aerial transmissions, and seed-borne latent infection. In order to understand the genetic variability, host specificity, and molecular basis of the pathogenicity-associated traits, the whole genome of rice infecting Magnaporthe oryzae (Strain RMg_Dl) was sequenced using the Illumina and PacBio (RSII compatible) platforms. The high-throughput hybrid assembly of short and long reads resulted in a total of 375 scaffolds with a genome size of 42.43 Mb. Furthermore, comparative genome analysis revealed 99% average nucleotide identity (ANI) with other oryzae genomes and 83% against M. grisea, and 73% against M. poe genomes. The gene calling identified 10,553 genes with 10,539 protein-coding sequences. Among the detected transposable elements, the LTR/Gypsy and Type LINE showed high occurrence. The InterProScan of predicted protein sequences revealed that 97% protein family (PFAM), 98% superfamily, and 95% CDD were shared among RMg_Dl and reference 70-15 genome, respectively. Additionally, 550 CAZymes with high GH family content/distribution and cell wall degrading enzymes (CWDE) such endoglucanase, beta-glucosidase, and pectate lyase were also deciphered in RMg_Dl. The prevalence of virulence factors determination revealed that 51 different VFs were found in the genome. The biochemical pathway such as starch and sucrose metabolism, mTOR signaling, cAMP signaling, MAPK signaling pathways related genes were identified in the genome. The 49,065 SNPs, 3267 insertions and 3611 deletions were detected, and majority of these varinats were located on downstream and upstream region. Taken together, the generated information will be useful to develop a specific marker for diagnosis, pathogen surveillance and tracking, molecular taxonomy, and species delineation which ultimately leads to device improved management strategies for blast disease.

The Role of Melanin in the Biology and Ecology of Nematophagous Fungi

Melanin is a heteropolymer formed by the polymerization of phenolic and indolic compounds. It occurs in organisms across all biological kingdoms and has a range different of functions, thus indicating its important evolutionary role. The presence of melanin offers several protective advantages, including against ultraviolet radiation, traumatic damage, oxidative stress, extreme temperatures, and pressure. For many species of fungi, melanin also participates directly in the process of virulence and pathogenicity. These organisms can synthesize melanin in two main ways: using a substrate of endogenous origin, involving 1,8-dihydroxynaphthalene (DHN); alternatively, in an exogenous manner with the addition of L-3, 4-dihydroxyphenylalanine (L-DOPA or levodopa). As melanin is an amorphous and complex substance, its study requires expensive and inaccessible technologies and analyses are often difficult to perform with conventional biochemical techniques. As such, details about its chemical structure are not yet fully understood, particularly for nematophagous fungi that remain poorly studied. Thus, this review presents an overview of the different types of melanin, with an emphasis on fungi, and discusses the role of melanin in the biology and ecology of nematophagous fungi.

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.

A putative PKA phosphorylation site S227 in MoSom1 is essential for infection-related morphogenesis and pathogenicity in Magnaporthe oryzae

In the rice blast fungus Magnaporthe oryzae, the cAMP signalling pathway plays a critical role in regulating leaf surface recognition and the initiation of appressorium development. Direct downstream targets of the cAMP signalling pathway are, however, not well-characterised. The MoSom1 protein functions downstream of the cAMP dependent protein kinase A (cAMP-PKA) and is essential for infection-related morphogenesis and pathogenicity. In this study, we show that mutation of a putative PKA phosphorylation site in MoSom1 is essential for its role in appressorium differentiation and pathogenicity in M. oryzae. Mutation of serine 227 in MoSom1 by deletion or serine (S) substitution to alanine (A), valine (V) or tyrosine (Y), resulted in defects of conidiation, appressorium-like structure formation and fungal pathogenicity. Western blot analysis confirmed that S227 in MoSom1 is a putative PKA phosphorylation site. Furthermore, a ΔMosom1 mutant showed reduced expression of PMK1 and was defective in Pmk1 phosphorylation, indicating that the Pmk1 mitogen-activated protein kinase (MAPK) acts downstream of MoSom1 in M. oryzae. We conclude that the cAMP-PKA pathway may regulate the Pmk1 MAPK pathway through MoSom1 during rice infection by the blast fungus. TAKE AWAYS: S227 is crucial for MoSom1 function in M. oryzae. S227 in MoSom1 was identified as a putative PKA phosphorylation site in M. oryzae. S227 is essential for infection-related morphogenesis and pathogenicity in M. oryzae.

The Membrane-Bound Protein, MoAfo1, Is Involved in Sensing Diverse Signals from Different Surfaces in the Rice Blast Fungus

To establish an infection, fungal pathogens must recognize diverse signals from host surfaces. The rice blast fungus, Magnaporthe oryzae, is one of the best models studying host-pathogen interactions. This fungus recognizes physical or chemical signals from the host surfaces and initiates the development of an infection structure called appressorium. Here, we found that protein MoAfo1(appressorium formation, MGG_10422) was involved in sensing signal molecules such as cutin monomers and long chain primary alcohols required for appressorium formation. The knockout mutant (ΔMoafo1) formed a few abnormal appressoria on the onion and rice sheath surfaces. However, it produced normal appressoria on the surface of rice leaves. MoAfo1 localized to the membranes of the cytoplasm and vacuole-like organelles in conidia and appressoria. Additionally, the ΔMoafo1 mutant showed defects in appressorium morphology, appressorium penetration, invasive growth, and pathogenicity. These multiple defects might be partially due to failure to respond properly to oxidative stress. These findings broaden our understanding of the fungal mechanisms at play in the recognition of the host surface during rice blast infection.

Involvement of the Mitochondrial Protein Tyrosine Phosphatase PTPM1 in the Promotion of Conidiation, Development, and Pathogenicity in Colletotrichum graminicola

The phosphorylation status of proteins, which is determined by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs), governs many cellular actions. In fungal pathogens, phosphorylation-mediated signal transduction has been considered to be one of the most important mechanisms in pathogenicity. Colletotrichum graminicola is an economically important corn pathogen. However, whether phosphorylation is involved in its pathogenicity is unknown. A mitochondrial protein tyrosine phosphatase gene, designated CgPTPM1, was deduced in C. graminicola through the use of bioinformatics and confirmed by enzyme activity assays and observation of its subcellular localization. We then created a CgPTPM1 deletion mutant (ΔCgPTPM1) to analyze its biological function. The results indicated that the loss of CgPTPM1 dramatically affected the formation of conidia and the development and differentiation into appressoria. However, the colony growth and conidial morphology of the ΔCgPTPM1 strains were unaffected. Importantly, the ΔCgPTPM1 mutant strains exhibited an obvious reduction of virulence, and the delayed infected hyphae failed to expand in the host cells. In comparison with the wild-type, ΔCgPTPM1 accumulated a larger amount of H2O2 and was sensitive to exogenous H2O2. Interestingly, the host cells infected by the mutant also exhibited an increased accumulation of H2O2 around the infection sites. Since the expression of the CgHYR1, CgGST1, CgGLR1, CgGSH1 and CgPAP1 genes was upregulated with the H2O2 treatment, our results suggest that the mitochondrial protein tyrosine phosphatase PTPM1 plays an essential role in promoting the pathogenicity of C. graminicola by regulating the excessive in vivo and in vitro production of H2O2.

Controllable Bypass Suppression in Magnaporthe oryzae

Evolutionary adaptation of living organisms is commonly thought to be the result of processes that have taken place over long periods of time. By contrast, we found that the filamentous rice blast fungus Magnaporthe oryzae rapidly suppresses the osmosensitive "loss of function" (lof) phenotype in knockout mutants of the high-osmolarity glycerol (HOG) pathway. That suppression occurs highly reproducibly after 4 weeks of continuous growth upon salt stress. Stable mutants reestablished in osmoregulation arise independently out of individual osmosensitive lof mutants of the HOG pathway. The major compatible solute produced upon salt stress by these suppressor strains was found to be glycerol, whereas it is arabitol in the wildtype strain. We aim to address the molecular or biochemical mechanisms behind this rapid suppression and characterize the associated factors and signaling pathways which enable or prevent suppression. Therefore, we present a protocol to generate these suppressor mutants in M. oryzae easily to study the molecular basis of evolutionary processes or even epigenetic modulation. This protocol may be applicable to many other fungi and will open a door for researchers worldwide since the HOG pathway is worked on intensively in many different model organisms.

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

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

The Impact of Blast Disease: Past, Present, and Future

Rice blast disease is both the most explosive and potentially damaging disease of the world's rice (Oryza sativa) crop and a model system for research on the molecular mechanisms that fungi use to cause plant disease. The blast fungus, Magnaporthe oryzae, is highly evolved to sense when it is on a leaf surface; to develop a pressurized cell, the appressorium, to punch through the leaf cuticle; and then to hijack living rice cells to assist it in causing disease. Host specificity, determining which plants particular fungal strains can infect, is also an important topic for research. The blast fungus is a moving target, quickly overcoming rice resistance genes we deploy to control it, and recently emerging to cause devastating disease on an entirely new cereal crop, wheat. M. oryzae is highly adaptable, with multiple examples of genetic instability at certain gene loci and in certain genomic regions. Understanding the biology of the fungus in the field, and its potential for genetic and genome variability, is key to keep it from adapting to life in the research laboratory and losing relevance to the significant impact it has on global food security.

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.

Detection of putative pathogenicity and virulence genes of Erysiphe pisi using genome-wide in-silico search and their suppression by er2 mediated resistance in garden pea

The biotrophic fungus, Erysiphe pisi is the chief causal agent of powdery mildew disease of garden pea. A genome-wide search using in-silico approach was carried to detect putative pathogenicity and virulence genes of E. pisi, since information about these genes and their interaction with pea is limited. Nineteen putative pathogenicity gene sequences were detected through genome-wide pathogenicity gene-search and confirmed them to be conserved in E. pisi through genomic PCRs. Fifteen of these genes expressed through reverse transcriptase-polymerase chain reaction (RT-PCR) amplifying expected band size along with fungal and plant specific internal controls. Gene sequencing and annotation revealed them to be Erysiphe-specific. A time course study was carried to monitor expression of nine of these genes through real-time quantitative (qRT)-PCR in Erysiphe-challenged plants of powdery mildew resistant pea genotype, JI-2480 carrying er2 gene and susceptible pea cultivar, Arkel. Expression of these genes was differentially and temporally regulated. They were found mostly related to signaling; cAMP-PKA (cPKA, CRP and AC) and MAPK (MST7) pathways along with MFP, TRE and PEX which are reported pathogenicity factors in other ascomycete members indicating that similar conserved pathways function in E. pisi also. These genes expressed at higher level at initial hours post inoculation (hpi) as early as 6 hpi in Arkel compared to JI-2480 implying them as pathogenicity factors. The elevated level of expression of MFP, TRE, CRP and cPKA gene sequences in E. pisi-challenged JI-2480 genotype at 12 hpi alone suggests these genes to possess a role in avirulence in JI-2480, conferring er2 mediated resistance.

The cyclase-associated protein ChCAP is important for regulation of hyphal growth, appressorial development, penetration, pathogenicity, conidiation, intracellular cAMP level, and stress tolerance in Colletotrichum higginsianum

Colletotrichum higginsianum causes anthracnose disease in a wide range of cruciferous crops and has been used as a model system to study plant-pathogen interactions and pathogenicity of hemibiotrophic plant pathogens. Conidiation, hyphae growth, appressorial development and appressorial penetration are significant steps during the infection process of C. higginsianum. However, the mechanisms of these important steps during infection remain incompletely understood. To further investigate the mechanisms of the plant-C. higginsianum interactions during infection progress, we characterized Cyclase-Associated Protein (ChCAP) gene. Deletion of the ChCAP gene resulted in reduction in conidiation and hyphal growth rate. The pathogenicity of ΔChCAP mutants was significantly reduced with much smaller lesion on the infected leaves compared to that of wild type strain with typically water-soaked and dark necrotic lesions on Arabidopsis leaves. Further study demonstrated that the appressorial formation rate, turgor pressure, penetration ability and switch from biotrophic to necrotrophic phases decreased obviously in ΔChCAP mutants, indicating that the attenuated pathogenicity of ΔChCAP mutants was due to these defective phenotypes. In addition, the ΔChCAP mutants sectored on PDA with abnormal, dark color, vesicle-like colony morphology and hyphae tip. Moreover, the ΔChCAP mutants had a reduced intracellular cAMP levels and exogenous cAMP can partially rescue the defects of ΔChCAP mutants in appressorial formation and penetration rate, but not in colony morphology, conidial shape and virulence, indicating that ChCAP is a key component in cAMP signaling pathway and likely play other roles in biology of C. higginsianum. In summary, our findings support the role of ChCAP in regulating conidiation, intracellular cAMP level, hyphal growth, appressorial formation, penetration ability and pathogenicity of this hemibiotrophic fungus.

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.

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.

Regulatory network of genes associated with stimuli sensing, signal transduction and physiological transformation of appressorium in Magnaporthe oryzae

Rice blast caused by Magnaporthe oryzae is the most destructive disease affecting the rice production (Oryza sativa), with an average global loss of 10-30% per annum. Recent reports have indicated that the fungus also inflicts blast disease on wheat (Triticum aestivum) posing a serious threat to the wheat production. Due to its easily detected infectious process and manoeuvrable genetic manipulation, M. oryzae is considered a model organism for exploring the molecular mechanism underlying fungal pathogenicity during the pathogen-host interaction. M. oryzae utilises an infectious structure called appressorium to breach the host surface by generating high turgor pressure. The appressorium development is induced by physical and chemical cues which are coordinated by the highly conserved cAMP/PKA, MAPK and calcium signalling cascades. Genes involved in the appressorium development have been identified and well studied in M. oryzae, a summary of the working gene network linking stimuli sensing and physiological transformation of appressorium is needed. This review provides a comprehensive discussion regarding the regulatory networks underlying appressorium development with particular emphasis on sensing of appressorium inducing stimuli, signal transduction, transcriptional regulation and the corresponding developmental and physiological responses. We also discussed the crosstalk and interaction of various pathways during the appressorium development.

Conserved and Divergent Functions of the cAMP/PKA Signaling Pathway in Candida albicans and Candida tropicalis

Fungal species undergo many morphological transitions to adapt to changing environments, an important quality especially in fungal pathogens. For decades, Candida albicans has been one of the most prevalent human fungal pathogens, and recently, the prevalence of Candida tropicalis as a causative agent of candidiasis has increased. In C. albicans, the ability to switch between yeast and hyphal forms is thought to be a key virulence factor and is regulated by multiple signaling cascades—including the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA), calcineurin, high-osmolarity glycerol (HOG), and mitogen-activated protein kinases (MAPK) signaling pathways—upon receiving environmental cues. The cAMP/PKA signaling pathway also triggers white-opaque switching in C. albicans. However, studies on C. tropicalis morphogenesis are limited. In this minireview, we discuss the regulation of the yeast-hypha transition, virulence, and white-opaque switching through the cAMP/PKA pathway in the closely related species C. albicans and C. tropicalis.

PRKAR2B promotes prostate cancer metastasis by activating Wnt/β-catenin and inducing epithelial-mesenchymal transition

Castration-resistant prostate cancers (CRPC) that occur after the failure of androgen-blocking therapies cause most of the deaths in prostate cancer (PCa) patients. In a previous study we identified that PRKAR2B expression is upregulated in CRPC and possesses potentials to develop CRPC. Here we further investigated the underlying mechanism of PRKAR2B in regulating prostate cancer metastasis. We established an androgen-independent LNCaPcell line (LNCaP-AI), and investigated the function of PRKAR2B on regulating cell invasion in vitro and in vivo. We found that PRKAR2B expression was markedly increased in LNCaP-AI cells and metastatic CRPC (mCRPC) tissues compared to LNCaP cells and primary PCa specimens, respectively. PRKAR2B level was significantly correlated with the Gleason score and lymph nodes metastasis in PCa. In vitro, PRKAR2B overexpression promoted cell invasion, whereas knockdown of PRKAR2B in CRPC cells inhibited cell invasion. PRKAR2B overexpression also promoted tumor metastasis in vivo. PRKAR2B resulted in a decreased expression of E-cadherin and an increased expression of Vimentin, N-cadherin, Fibronectin, indicating that PRKAR2B induced epithelial-mesenchymal transition (EMT). PRKAR2B activated Wnt/β-catenin signaling in CRPC cells. More important, inhibition of Wnt/β-catenin attenuated PRKAR2B-induced EMT and cancer cells invasion. Our results provided novel insights to PRKAR2B-driven CRPC cell invasion and indicated that PRKAR2B might be served as a potential target for CRPC therapy.

Rise of a Cereal Killer: The Biology of Magnaporthe oryzae Biotrophic Growth

The rice blast fungus, Magnaporthe oryzae, causes one of the most destructive diseases of cultivated rice in the world. Infections caused by this recalcitrant pathogen lead to the annual destruction of approximately 10-30% of the rice harvested globally. The fungus undergoes extensive developmental changes to be able to break into plant cells, build elaborate infection structures, and proliferate inside host cells without causing visible disease symptoms. From a molecular standpoint, we are still in the infancy of understanding how M. oryzae manipulates the host during this complex multifaceted infection. Here, we describe recent advances in our understanding of the cell biology of M. oryzae biotrophic interaction and key molecular factors required for the disease establishment in rice cells.

Cpk2, a Catalytic Subunit of Cyclic AMP-PKA, Regulates Growth and Pathogenesis in Rice Blast

The cAMP-Protein Kinase A signaling, anchored on CpkA, is necessary for appressorium development and host penetration, but indispensable for infectious growth in Magnaporthe oryzae. In this study, we identified and characterized the gene encoding the second catalytic subunit, CPK2, whose expression was found to be lower compared to CPKA at various stages of pathogenic growth in M. oryzae. Deletion of CPK2 caused no alterations in vegetative growth, conidiation, appressorium formation, or pathogenicity. Surprisingly, the cpkAΔcpk2Δ double deletion strain displayed significant reduction in growth rate and conidiation compared to the single deletion mutants. Interestingly, loss of CPKA and CPK2 resulted in morphogenetic defects in germ tubes (with curled/wavy and serpentine growth pattern) on hydrophobic surfaces, and a complete failure to produce appressoria therein, thus suggesting an important role for CPK2-mediated cAMP-PKA in surface sensing and response pathway. CPKA promoter-driven expression of CPK2 partially suppressed the defects in host penetration and pathogenicity in the cpkAΔ. Such ectopic CPK2 expressing strain successfully penetrated the rice leaves, but was unable to produce proper secondary invasive hyphae, thus underscoring the importance of CpkA in growth and differentiation in planta. The Cpk2-GFP localized to the nuclei and cytoplasmic vesicles in conidia and germ tubes. The Cpk2-GFP colocalized with CpkA-mCherry on vesicles in the cytosol, but such overlap was not evident in the nuclei. Our studies indicate that CpkA and Cpk2 share overlapping functions, but also play distinct roles during pathogenesis-associated signaling and morphogenesis in the rice blast fungus.

PKA activity is essential for relieving the suppression of hyphal growth and appressorium formation by MoSfl1 in Magnaporthe oryzae

In the rice blast fungus Magnaporthe oryzae, the cAMP-PKA pathway regulates surface recognition, appressorium turgor generation, and invasive growth. However, deletion of CPKA failed to block appressorium formation and responses to exogenous cAMP. In this study, we generated and characterized the cpk2 and cpkA cpk2 mutants and spontaneous suppressors of cpkA cpk2 in M. oryzae. Our results demonstrate that CPKA and CPK2 have specific and overlapping functions, and PKA activity is essential for appressorium formation and plant infection. Unlike the single mutants, the cpkA cpk2 mutant was significantly reduced in growth and rarely produced conidia. It failed to form appressoria although the intracellular cAMP level and phosphorylation of Pmk1 MAP kinase were increased. The double mutant also was defective in plant penetration and Mps1 activation. Interestingly, it often produced fast-growing spontaneous suppressors that formed appressoria but were still non-pathogenic. Two suppressor strains of cpkA cpk2 had deletion and insertion mutations in the MoSFL1 transcription factor gene. Deletion of MoSFL1 or its C-terminal 93-aa (MoSFL1^ΔCT) was confirmed to suppress the defects of cpkA cpk2 in hyphal growth but not appressorium formation or pathogenesis. We also isolated 30 spontaneous suppressors of the cpkA cpk2 mutant in Fusarium graminearum and identified mutations in 29 of them in FgSFL1. Affinity purification and co-IP assays showed that this C-terminal region of MoSfl1 was essential for its interaction with the conserved Cyc8-Tup1 transcriptional co-repressor, which was reduced by cAMP treatment. Furthermore, the S211D mutation at the conserved PKA-phosphorylation site in MoSFL1 partially suppressed the defects of cpkA cpk2. Overall, our results indicate that PKA activity is essential for appressorium formation and proper activation of Pmk1 or Mps1 in M. oryzae, and phosphorylation of MoSfl1 by PKA relieves its interaction with the Cyc8-Tup1 co-repressor and suppression of genes important for hyphal growth.

The cAMP-PKA signaling pathway plays a critical role in regulating various cellular processes in eukaryotic cells in response to extracellular cues. In the rice blast fungus, this important pathway is involved in surface recognition, appressorium morphogenesis, and infection. However, the exact role of PKA is not clear due to the functional redundancy of two PKA catalytic subunits CPKA and CPK2. To further characterize their functions in growth and pathogenesis, in this study we generated and characterized the cpkA cpk2 double mutant and its suppressor strains. Unlike the single mutants, cpkA cpk2 mutant had severe defects in growth and conidiation and was defective in appressorium formation and plant infection. Interestingly, the double mutant was unstable and produced fast-growing suppressors. In two suppressor strains, mutations were identified in a transcription factor gene orthologous to SFL1, a downstream target of PKA in yeast. Deletion of the entire or C-terminal 93 residues of MoSFL1 could suppress the growth defect of cpkA cpk2. Furthermore, the terminal region of MoSfl1 was found to be essential for its interaction with the MoCyc8 co-repressor, which may be negatively regulated by PKA. Therefore, loss-of-function mutations in MoSFL1 can bypass PKA activity to suppress the growth defect of cpkA cpk2.

The cAMP-PKA Signaling Pathway Regulates Pathogenicity, Hyphal Growth, Appressorial Formation, Conidiation, and Stress Tolerance in Colletotrichum higginsianum

Colletotrichum higginsianum is an economically important pathogen that causes anthracnose disease in a wide range of cruciferous crops. Understanding the mechanisms of the cruciferous plant–C. higginsianum interactions will be important in facilitating efficient control of anthracnose diseases. The cAMP-PKA signaling pathway plays important roles in diverse physiological processes of multiple pathogens. C. higginsianum contains two genes, ChPKA1 and ChPKA2, that encode the catalytic subunits of cyclic AMP (cAMP)-dependent protein kinase A (PKA). To analyze the role of cAMP signaling pathway in pathogenicity and development in C. higginsianum, we characterized ChPKA1 and ChPKA2 genes, and adenylate cyclase (ChAC) gene. The ChPKA1 and ChAC deletion mutants were unable to cause disease and significantly reduced in hyphal growth, tolerance to cell wall inhibitors, conidiation, and appressorial formation with abnormal germ tubes, but they had an increased tolerance to elevated temperatures and exogenous H₂O₂. In contrast, the ChPKA2 mutant had no detectable alteration of phenotypes, suggesting that ChPKA1 contributes mainly to PKA activities in C. higginsianum. Moreover, we failed to generate ΔChPKA1ChPKA2 double mutant, indicating that deletion of both PKA catalytic subunits is lethal in C. higginsianum and the two catalytic subunits possibly have overlapping functions. These results indicated that ChPKA1 is the major PKA catalytic subunit in cAMP-PKA signaling pathway and plays significant roles in hyphal growth, pathogenicity, appressorial formation, conidiation, and stress tolerance in C. higginsianum.

Phosphorylation-mediated Regulatory Networks in Mycelia of Pyricularia oryzae Revealed by Phosphoproteomic Analyses

Protein phosphorylation is known to regulate pathogenesis, mycelial growth, conidiation and stress response in Pyricularia oryzae However, phosphorylation mediated regulatory networks in the fungal pathogen remain largely to be uncovered. In this study, we identified 1621 phosphorylation sites of 799 proteins in mycelia of P. oryzae, including 899 new p-sites of 536 proteins and 47 new p-sites of 31 pathogenicity-related proteins. From the sequences flanking the phosphorylation sites, 19 conserved phosphorylation motifs were identified. Notably, phosphorylation was detected in 7 proteins that function upstream of Pmk1, but not in Pmk1 and its downstream Mst12 and Sfl1 that have been known to regulate appressorium formation and infection hyphal growth of P. oryzae Interestingly, phosphorylation was detected at the site Ser²⁴⁰ of Pmp1, which is a putative protein phosphatase highly conserved in filamentous fungi but not characterized. We thus generated Δpmp1 deletion mutants and dominant allele PMP1^S240D mutants. Phenotyping analyses indicated that Pmp1 is required for virulence, conidiation and mycelial growth. Further, we observed that phosphorylation level of Pmk1 in mycelia was significantly increased in the Δpmp1 mutant, but decreased in the PMP1^S240D mutant in comparison with the wild type, demonstrating that Pmp1 phosphorylated at Ser²⁴⁰ is important for regulating phosphorylation of Pmk1. To our surprise, phosphorylation of Mps1, another MAP kinase required for cell wall integrity and appressorium formation of P. oryzae, was also significantly enhanced in the Δpmp1 mutant, but decreased in the PMP1^S240D mutant. In addition, we found that Pmp1 directly interacts with Mps1 and the region AA180-230 of Pmp1 is required for the interaction. In summary, this study sheds new lights on the protein phosphorylation mediated regulatory networks in P. oryzae.

MoEnd3 regulates appressorium formation and virulence through mediating endocytosis in rice blast fungus Magnaporthe oryzae

Eukaryotic cells respond to environmental stimuli when cell surface receptors are bound by environmental ligands. The binding initiates a signal transduction cascade that results in the appropriate intracellular responses. Studies have shown that endocytosis is critical for receptor internalization and signaling activation. In the rice blast fungus Magnaporthe oryzae, a non-canonical G-protein coupled receptor, Pth11, and membrane sensors MoMsb2 and MoSho1 are thought to function upstream of G-protein/cAMP signaling and the Pmk1 MAPK pathway to regulate appressorium formation and pathogenesis. However, little is known about how these receptors or sensors are internalized and transported into intracellular compartments. We found that the MoEnd3 protein is important for endocytic transport and that the ΔMoend3 mutant exhibited defects in efficient internalization of Pth11 and MoSho1. The ΔMoend3 mutant was also defective in Pmk1 phosphorylation, autophagy, appressorium formation and function. Intriguingly, restoring Pmk1 phosphorylation levels in ΔMoend3 suppressed most of these defects. Moreover, we demonstrated that MoEnd3 is subject to regulation by MoArk1 through protein phosphorylation. We also found that MoEnd3 has additional functions in facilitating the secretion of effectors, including Avr-Pia and AvrPiz-t that suppress rice immunity. Taken together, our findings suggest that MoEnd3 plays a critical role in mediating receptor endocytosis that is critical for the signal transduction-regulated development and virulence of M. oryzae.

Subcellular compartmentation, interdependency and dynamics of the cyclic AMP-dependent PKA subunits during pathogenic differentiation in rice blast

The cAMP-dependent PKA signalling plays a central role in growth, asexual development and pathogenesis in fungal pathogens. Here, we functionally characterised RPKA, the regulatory subunit of cAMP/PKA and studied the dynamics and organisation of the PKA subunits in the rice blast pathogen Magnaporthe oryzae. The RPKA subunit was essential for proper vegetative growth, asexual sporulation and surface hydrophobicity in M. oryzae. A spontaneous suppressor mutation, SMR19, that restored growth and conidiation in the RPKA deletion mutant was isolated and characterised. SMR19 enhanced conidiation and appressorium formation but failed to suppress the pathogenesis defects in rpkAΔ. The PKA activity was undetectable in the mycelial extracts of SMR19, which showed a single mutation (val242leu) in the highly conserved active site of the catalytic subunit (CPKA) of cAMP/PKA. The two subunits of cAMP/PKA showed different subcellular localisation patterns with RpkA being predominantly nucleocytoplasmic in conidia, while CpkA was largely cytosolic and/or vesicular. The CpkA anchored RpkA in cytoplasmic vesicles, and localisation of PKA in the cytoplasm was governed by CpkA in a cAMP-dependant or independent manner. We show that there exists a tight regulation of PKA subunits at the level of transcription, and the cAMP signalling is differentially compartmentalised in a stage-specific manner in rice blast.

WISH, a novel CFEM GPCR is indispensable for surface sensing, asexual and pathogenic differentiation in rice blast fungus

We have selected and characterized a unique Conserved Fungal-specific Extra-cellular Membrane-spanning (CFEM) domain containing PTH11 like G-protein coupled receptor (GPCR), which is responsible for Water wettability, Infection, Surface sensing and Hyper-conidiation (WISH). The pathogenicity gene WISH is predicted to encode a novel seven transmembrane protein in the rice blast fungus, Magnaporthe oryzae, one of the deadliest pathogens of rice. We generated knockout mutants through a homologous recombination-based method to understand the function of the gene. These mutants are nonpathogenic due to a defect in sensing hydrophobic surface and appressorium differentiation. The mutant failed to undergo early events of pathogenesis, and appressorium development is diminished on inductive hydrophobic surface and was unable to penetrate susceptible rice leaves. The Δwish mutant did not develop any appressorium, suggesting that WISH protein is required for appressorium morphogenesis and is also involved in host surface recognition. We examined various aspects of pathogenesis and the results indicated involvement of WISH in preventing autolysis of vegetative hyphae, determining surface hydrophobicity and maintenance of cell-wall integrity. WISH gene from M. oryzae strain B157 complemented the Δwish mutant, indicating functional authenticity. Exogenous activation of cellular signaling failed to suppress the defects in Δwish mutants. These findings suggest that WISH GPCR senses diverse extracellular signals to play multiple roles and might have effects on PTH11 and MPG1 genes especially as an upstream effector of appressorium differentiation. It is for the first time that a typical GPCR containing seven transmembrane helices involved in the early events of plant pathogenesis of M. oryzae has been functionally characterized.

Using Network Extracted Ontologies to Identify Novel Genes with Roles in Appressorium Development in the Rice Blast Fungus Magnaporthe oryzae

Magnaporthe oryzae is the causal agent of rice blast disease, the most important infection of rice worldwide. Half the world's population depends on rice for its primary caloric intake and, as such, rice blast poses a serious threat to food security. The stages of M. oryzae infection are well defined, with the formation of an appressorium, a cell type that allows penetration of the plant cuticle, particularly well studied. However, many of the key pathways and genes involved in this disease stage are yet to be identified. In this study, I have used network-extracted ontologies (NeXOs), hierarchical structures inferred from RNA-Seq data, to identify pathways involved in appressorium development, which in turn highlights novel genes with potential roles in this process. This study illustrates the use of NeXOs for pathway identification from large-scale genomics data and also identifies novel genes with potential roles in disease. The methods presented here will be useful to study disease processes in other pathogenic species and these data represent predictions of novel targets for intervention in M. oryzae.