Two novel transcriptional regulators are essential for infection-related morphogenesis and pathogenicity of the rice blast fungus Magnaporthe oryzae

Unveiling the functions of the Lim-domain binding protein MaPtaB in Metarhizium acridum

BACKGROUND

The Lim-domain binding protein PtaB, a homolog of Mfg1, governs conidiation and biofilm formation in several fungi. PtaB includes a conserved Lim-binding domain and two predicted nuclear localization sequences at its C terminus, and is co-regulated with the transcription factor Som1 downstream of the cyclic AMP-dependent protein kinase A (cAMP/PKA) pathway. However, the function of PtaB in entomopathogenic fungi remain poorly understood.

RESULTS

Inactivation of PtaB in Metarhizium acridum resulted in delayed conidial germination, reduced conidial yield and increased sensitivities to cell wall disruptors, ultraviolet B irradiation and heat shock. In addition, the fungal virulence was significantly decreased after deletion of MaPtaB because of impairments in appressorium formation, cuticle penetration and evasion of insect immune responses in M. acridum. The MaPtaB-deletion and MaSom1-deletion strains showed similar phenotypes supporting that MaSom1/MaPtaB complex controls M. acridum normal conidiation and pathogenic progress. Upon loss of MaPtaB or MaSom1, the fungal sporulation mode in M. acridium shifted from microcycle conidiation to normal conidiation on SYA, a microcycle conidiation medium. Transcriptional analysis showed that more differentially expression genes were identified in MaSom1 RNA sequencing, and MaSom1 and MaPtaB may regulate the expression of genes for conidiation, nutrient metabolism and the cell cycle to control conidiation pattern shift.

CONCLUSION

These data corroborate a complex control function for MaPtaB as an important central factor interacting with MaSom1 in the cAMP/PKA pathway, which links stress tolerance, conidiation and virulence in the entomopathogenic fungus M. acridum. © 2024 Society of Chemical Industry.

Mitogen-activated protein (MAP) kinase signalling pathway VmMkh1-VmMkk1-VmSpm1 regulates cell wall integrity in Valsamali

Apple Valsa canker disease, caused by Valsa mali Miyabe et Yamada, seriously endangers the healthy growth of apple trees. Mitogen-Activated Protein Kinase (MAPK) signaling pathway is an important pathway to transmit signals stimulated by environmental stress. In this study, we identify and functionally characterize MAPKKK VmMkh1, MAPKK VmMkk1 and MAPK VmSpm1. VmMkh1 and VmMkk1 positively regulate the phosphorylation of VmSpm1. The radial growth rate of the VmSpm1 deletion mutant was reduced by approximately 31 %. There was no significant difference in growth rate between the VmMkh1 and VmMkk1 mutant and the wild-type. VmMkh1 hyphe branches into a curved shape. The VmMkh1, VmMkk1, and VmSpm1 deletion mutant produced fewer conidia than the wild-type strain at 20 days post inoculation. Moreover, the VmMkh1, VmMkk1, and VmSpm1 deletion mutant slows conidial germination. The hyphal growth of VmMkh1, VmMkk1, and VmSpm1 deletion mutants are significantly inhibited on media containing NaCl, KCl, sorbitol (high osmotic stresses). The hyphal growth of VmMkh1, VmMkk1, and VmSpm1 deletion mutants are significantly inhibited on media containing Congo red, CFW, SDS, and Lysing encymes (Cell wall stress agents). A looser distribution of spacers in VmMkh1, VmMkk1, and VmSpm1 deletion mutants compared with the wild-type strain. The size of lesions on apple fruits and branches inoculated with VmSpm1 deletion mutant showed a reduction of approximately 46 % and 43 %, respectively, after 9 dpi. Overall, our findings demonstrate that VmMkh1, VmMkk1, and VmSpm1 are involved in regulating the growth and development, colony surface hydrophobicity, osmotic stress, cell wall integrity maintenance, carbon and nitrogen source utilization, septa formation, and pathogenicity of Valsa mali.

The Mediator complex subunit MoMed15 plays an important role in conferring sensitivity to isoprothiolane by modulating xenobiotic metabolism in M. oryzae

Rice blast caused by Magnaporthe oryzae is one of the most economically important rice diseases. Fungicides such as isoprothiolane (IPT) have been used extensively for rice blast control, but resistance to IPT in M. oryzae is an emerging threat. In this study, molecular mechanisms of resistance in IPT-resistant mutants were identified. Through whole-genome sequencing and genetic transformation, we identified the gene MoMed15, encoding a transcriptional glutamine-rich co-activator Mediator complex subunit, in which mutations or deletion resulted in moderate IPT resistance. Further research found that MoMed15 physically interacted with the IPT resistance regulatory factor MoIRR to simultaneously regulate both MoIRR expression and the expression of multiple xenobiotic-metabolizing enzymes in response to IPT stress. We hypothesize that some xenobiotic-metabolizing enzymes enhance IPT toxicity by modifying the IPT structure. Variation of MoMed15 affected the recruitment of the transcriptional Mediator complex and decreased the expression of these xenobiotic-metabolizing enzymes, resulting in moderate IPT resistance. We also found that MoPGR1, encoding a protein that activates cytochrome P450 enzymes, was essential to confer IPT sensitivity, and its expression was directly regulated by MoIRR.IMPORTANCEIsoprothiolane (IPT) has been used extensively for the management of rice blast disease and IPT-resistant subpopulations have emerged in Chinese rice fields. The emergence of resistant pathogen populations has led to a steep increase in fungicide use, increasing pesticide risk for the applicator and the environment. The molecular mechanisms of IPT resistance in M. oryzae remain elusive. In this study, we demonstrated that transcriptional co-activator MoMed15 interacts with IPT resistance regulator MoIRR to recruit the Mediator complex, which promotes the expression of xenobiotic-metabolizing enzymes, leading to exacerbated IPT toxicity. The MoMed15 could be used for IPT resistance detection in rice fields.

VmSom1 is essential for growth, development, maintenance of cell wall integrity and virulence in Valsa mali

Apple Valsa canker disease, caused by Valsa mali Miyabe et Yamada, severely endangers the healthy growth of apple trees. The Som1, located downstream of the cyclic AMP-dependent protein kinase A (cAMP-PKA) pathway, plays crucial roles in the growth, development, morphological differentiation, and virulence of filamentous fungi. In this study, we identify and functionally characterize VmSom1, a homolog of Som1, in Valsa mali. The VmSom1 gene is located on chromosome 12, encoding an 824 amino acid protein. Phylogenetic analysis reveals VmSom1 as a fungal Som1 homolog. The VmSom1 deletion mutants exhibit slower growth rates and fail to produce pycnidia. Additionally, their hyphal growth is significantly inhibited on media containing Calcofluor White, Congo Red, NaCl, and sorbitol. The growth rate of VmSom1 deletion mutants is reduced on maltose, lactose, sucrose and fructose media but increases on glucose medium. Moreover, the mycelial growth rate of the VmSom1 deletion mutant is significantly lower than that of the wild-type strain in peptone, NH4SO4, NaNO3, and no nitrogen. Notably, the distances between the septa increase, and chitin concentration shifts to the hyphal tip in the VmSom1 deletion mutant. Furthermore, compared with the wild-type strain, the VmSom1 deletion mutant exhibits fewer diseased spots on apple fruit and branches. Overall, our findings demonstrate that VmSom1 is involved in regulating the growth and development, colony surface hydrophobicity, osmotic stress, cell wall integrity maintenance, carbon and nitrogen source utilization, septa formation, and virulence of V. mali.

The phosphorylation landscape of infection-related development by the rice blast fungus

Many of the world's most devastating crop diseases are caused by fungal pathogens that elaborate specialized infection structures to invade plant tissue. Here, we present a quantitative mass-spectrometry-based phosphoproteomic analysis of infection-related development by the rice blast fungus Magnaporthe oryzae, which threatens global food security. We mapped 8,005 phosphosites on 2,062 fungal proteins following germination on a hydrophobic surface, revealing major re-wiring of phosphorylation-based signaling cascades during appressorium development. Comparing phosphosite conservation across 41 fungal species reveals phosphorylation signatures specifically associated with biotrophic and hemibiotrophic fungal infection. We then used parallel reaction monitoring (PRM) to identify phosphoproteins regulated by the fungal Pmk1 MAPK that controls plant infection by M. oryzae. We define 32 substrates of Pmk1 and show that Pmk1-dependent phosphorylation of regulator Vts1 is required for rice blast disease. Defining the phosphorylation landscape of infection therefore identifies potential therapeutic interventions for the control of plant diseases.

Protein kinase A regulatory subunit is required for normal growth, zoosporogenesis, and pathogenicity in Phytophthora sojae

Phytophthora sojae, one of the most devastating Oomycete pathogens, causes severe diseases that lead to economic loss in the soybean industry. The production of zoospores play a crucial role during the development of Phytophthora disease. In this work, CRISPR/Cas9 genome editing technology were used to obtain protein kinase A regulatory subunit (PsPkaR) knockout mutants. The role of PsPkaR in the production of zoospores and pathogenicity of P. sojae was analyzed. The overall findings indicate that PsPkaR is involved in regulating the growth process of P. sojae, primarily affecting the hyphal morphology and growth rate. Additionally, PsPkaR participates in the regulation of the release process of zoospores. Specifically, knocking-out PsPkaR resulted in incomplete cytoplasmic differentiation and uneven protoplast division, leading to abnormal release of zoospores. Furthermore, when the PsPkaR knockout mutants were inoculated on soybean leaves, the pathogenicity was significantly reduced compared to that of the wild-type and control strains. These findings of this study provide important clues and evidence regarding the role of the cAMP-PKA signaling pathway in the interaction between P. sojae and its host. This work contributes to a better understanding of the pathogenic mechanism of P. sojae and the development of corresponding prevention and control strategies.

Pyricularia oryzae: Lab star and field scourge

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

TAXONOMY

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

HOST RANGE

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

DISEASE SYMPTOMS

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

The basic leucine zipper transcription factor MeaB is critical for biofilm formation, cell wall integrity, and virulence in Aspergillus fumigatus

The regulation of fungal cell wall biosynthesis is crucial for cell wall integrity maintenance and directly impacts fungal pathogen virulence. Although numerous genes are involved in fungal cell wall polysaccharide biosynthesis through multiple pathways, the underlying regulatory mechanism is still not fully understood. In this study, we identified and functionally characterized a direct downstream target of SomA, the basic-region leucine zipper transcription factor MeaB, playing a certain role in Aspergillus fumigatus cell wall integrity. Loss of meaB reduces hyphal growth, causes severe defects in galactosaminogalactan-mediated biofilm formation, and attenuates virulence in a Galleria mellonella infection model. Furthermore, the meaB null mutant strain exhibited hypersensitivity to cell wall-perturbing agents and significantly alters the cell wall structure. Transcriptional profile analysis revealed that MeaB positively regulates the expression of the galactosaminogalactan biosynthesis and β-1,3-glucanosyltransferase genes uge3, agd3, and sph3 and gel1, gel5, and gel7, respectively, as well as genes involved in amino sugar and nucleotide sugar metabolism. Further study demonstrated that MeaB could respond to cell wall stress and contribute to the proper expression of mitogen-activated protein kinase genes mpkA and mpkC in the presence of different concentrations of congo red. In conclusion, A. fumigatus MeaB plays a critical role in cell wall integrity by governing the expression of genes encoding cell wall-related proteins, thus impacting the virulence of this fungus.IMPORTANCEAspergillus fumigatus is a common opportunistic mold that causes life-threatening infections in immunosuppressed patients. The fungal cell wall is a complex and dynamic organelle essential for the development of pathogenic fungi. Genes involved in cell wall polysaccharide biosynthesis and remodeling are crucial for fungal pathogen virulence. However, the potential regulatory mechanism for cell wall integrity remains to be fully defined in A. fumigatus. In the present study, we identify basic-region leucine zipper transcription factor MeaB as an important regulator of cell wall galactosaminogalactan biosynthesis and β-1,3-glucan remodeling that consequently impacts stress response and virulence of fungal pathogens. Thus, we illuminate a mechanism of transcriptional control fungal cell wall polysaccharide biosynthesis and stress response. As these cell wall components are promising therapeutic targets for fungal infections, understanding the regulatory mechanism of such polysaccharides will provide new therapeutic opportunities.

The Alternaria alternata StuA transcription factor interacting with the pH-responsive regulator PacC for the biosynthesis of host-selective toxin and virulence in citrus

IMPORTANCE

In this study, we used Alternaria alternata as a biological model to report the role of StuA in phytopathogenic fungi. Our findings indicated that StuA is required for Alternaria citri toxin (ACT) biosynthesis and fungal virulence. In addition, StuA physically interacts with PacC. Disruption of stuA or pacC led to decreased expression of seven toxin biosynthetic genes (ACCT) and toxin production. PacC could recognize and bind to the promoter regions of ACTT6 and ACTTR. Our results revealed a previously unrecognized (StuA-PacC)→ACTTR module for the biosynthesis of ACT in A. alternata, which also provides a framework for the study of StuA in other fungi.

Transcription Factor VM1G_06867: A Requirement for Growth, Pathogenicity, Development, and Maintenance of Cell Wall Integrity in Valsa mali

Apple canker disease, caused by Valsa mali, is one of the most serious apple tree diseases in China. VmSom1 is an important transcription factor that acts on the cyclic adenosine signaling pathway (cAMP/PKA), regulating the growth, development, morphological differentiation, and pathogenic forces of the pathogen. We perform transcriptome analysis of the VmSom1 deletion mutant and the wild-type strain 11-175 and identify a significantly differentially expressed gene, VM1G_06867, a zinc finger motif transcription factor in V. mali. In this study, we obtain the VM1G_06867 gene using the single deletion mutant via homologous recombination. To determine the relationship between VmSom1 and VM1G_06867, we also obtain a double deletion mutant ΔVmSom1/06867. Compared to the wild-type strain 11-175, the single deletion mutant VM1G_06867 shows a drastic reduction in growth rate and forms more pycnidia on the PDA medium. Additionally, the growth of the mutant is inhibited by SDS, Congo red, and fluorescent brighteners. In comparison to the single deletion mutant VmSom1, the double deletion mutant ΔVmSom1/06867 shows no significant change in growth or conidiation and is unable to produce conidia. The growth rate is significantly increased in Congo red, NaCl, and Sorbitol mediums. These results demonstrate that VM1G_06867 plays important roles in growth, pathogenicity, asexual development, and maintenance of cell wall integrity. VM1G_06867 can recover osmotic stress and cell wall integrity defects caused by the deletion of VmSom1, as well as restore the loss of pathogenicity caused by the deletion of the VmSom1 gene, but not completely.

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.

Conservation and Expansion of Transcriptional Factor Repertoire in the Fusarium oxysporum Species Complex

The Fusarium oxysporum species complex (FOSC) includes both plant and human pathogens that cause devastating plant vascular wilt diseases and threaten public health. Each F. oxysporum genome comprises core chromosomes (CCs) for housekeeping functions and accessory chromosomes (ACs) that contribute to host-specific adaptation. This study inspects global transcription factor profiles (TFomes) and their potential roles in coordinating CC and AC functions to accomplish host-specific interactions. Remarkably, we found a clear positive correlation between the sizes of TFomes and the proteomes of an organism. With the acquisition of ACs, the FOSC TFomes were larger than the other fungal genomes included in this study. Among a total of 48 classified TF families, 14 families involved in transcription/translation regulations and cell cycle controls were highly conserved. Among the 30 FOSC expanded families, Zn2-C6 and Znf_C2H2 were most significantly expanded to 671 and 167 genes per family including well-characterized homologs of Ftf1 (Zn2-C6) and PacC (Znf_C2H2) that are involved in host-specific interactions. Manual curation of characterized TFs increased the TFome repertoires by 3% including a disordered protein Ren1. RNA-Seq revealed a steady pattern of expression for conserved TF families and specific activation for AC TFs. Functional characterization of these TFs could enhance our understanding of transcriptional regulation involved in FOSC cross-kingdom interactions, disentangle species-specific adaptation, and identify targets to combat diverse diseases caused by this group of fungal pathogens.

Conservation and Expansion of Transcriptional Factor Repertoire in the Fusarium oxysporum Species Complex

The Fusarium oxysporum species complex (FOSC) includes both plant and human pathogens that cause devastating plant vascular wilt diseases and threaten public health. Each F. oxysporum genome comprises core chromosomes (CCs) for housekeeping functions and accessory chromosomes (ACs) that contribute to host-specific adaptation. This study inspected global transcription factor profiles (TFomes) and their potential roles in coordinating CCs and ACs functions to accomplish host-specific pathogenicity. Remarkably, we found a clear positive correlation between the sizes of TFome and proteome of an organism, and FOSC TFomes are larger due to the acquisition of ACs. Among a total of 48 classified TF families, 14 families involved in transcription/translation regulations and cell cycle controls are highly conserved. Among 30 FOSC expanded families, Zn2-C6 and Znf_C2H2 are most significantly expanded to 671 and 167 genes per family, including well-characterized homologs of Ftf1 (Zn2-C6) and PacC (Znf_C2H2) involved in host-specific interactions. Manual curation of characterized TFs increased the TFome repertoires by 3%, including a disordered protein Ren1. Expression profiles revealed a steady expression of conserved TF families and specific activation of AC TFs. Functional characterization of these TFs could enhance our understanding of transcriptional regulation involved in FOSC cross-kingdom interactions, disentangle species-specific adaptation, and identify targets to combat diverse diseases caused by this group of fungal pathogens.

The Adaptor Protein UvSte50 Governs Fungal Pathogenicity of Ustilaginoidea virens via the MAPK Signaling Pathway

The mitogen-activated protein kinase (MAPK) signaling pathways regulate diverse cellular processes and have been partially characterized in the rice false smut fungus Ustilaginoidea virens. UvSte50 has been identified as a homolog to Saccharomyces cerevisiae Ste50, which is known to be an adaptor protein for MAPK cascades. ΔUvste50 was found to be defective in conidiation, sensitive to hyperosmotic and oxidative stresses, and non-pathogenic. The mycelial expansion of ΔUvste50 inside spikelets of rice terminated at stamen filaments, eventually resulting in a lack of formation of false smut balls on spikelets. We determined that UvSte50 directly interacts with both UvSte7 (MAPK kinase; MEK) and UvSte11 (MAPK kinase kinase; MEKK), where the Ras-association (RA) domain of UvSte50 is indispensable for its interaction with UvSte7. UvSte50 also interacts with UvHog1, a MAP kinase of the Hog1-MAPK pathway, which is known to have important roles in hyphal growth and stress responses in U. virens. In addition, affinity capture-mass spectrometry analysis and yeast two-hybrid assay were conducted, through which we identified the interactions of UvSte50 with UvRas2, UvAc1 (adenylate cyclase), and UvCap1 (cyclase-associated protein), key components of the Ras/cAMP signaling pathway in U. virens. Together, UvSte50 functions as an adaptor protein interacting with multiple components of the MAPK and Ras/cAMP signaling pathways, thus playing critical role in plant infection by U. virens.

Flammulina filiformis Pkac Gene Complementing in Neurospora crassa Mutant Reveals Its Function in Mycelial Growth and Abiotic Stress Response

Flammulina filiformis is a popular edible mushroom that easily suffers from heat and oxidative stresses. The cyclic adenylate-dependent protein kinase A (cAMP/PKA) pathway is the main signaling pathway in response to environmental stress, and the PKAC is the terminal catalytic subunit of this pathway. In this study, the Pkac gene was identified in F. filiformis, which was highly conserved in basidiomycetes and ascomycetes. The transcription analysis showed that the Pkac gene was involved in the mycelial growth and the fruiting body development of fungi. In Neurospora crassa, the Pkac gene deletion (ΔPkac) resulted in the slower growth of the mycelia. We complemented the F. filiformis FfPkac to N. crassa ΔPkac mutant to obtain the CPkac strain. The mycelial growth in the CPkac strain was restored to the same level as the WT strain. In addition, the FfPkac gene showed significantly up-regulated expression under heat and oxidative stresses. By analyzing the differentially expressed genes of ΔPkac and Cpkac with WT, respectively, seven downstream genes regulated by Pkac were identified and may be related to mycelial growth. They were mainly focused on microbial metabolism in diverse environments, mitochondrial biogenesis, protein translation and nucleocytoplasmic transport. RT-qPCR results confirmed that the expression patterns of these seven genes were consistent with FfPkac under heat and oxidative stresses. The results revealed the conserved functions of PKAC in filamentous fungi and its regulatory mechanism in response to heat and oxidative stresses.

Adhesion as a Focus in Trichoderma-Root Interactions

Fungal spores, germlings, and mycelia adhere to substrates, including host tissues. The adhesive forces depend on the substrate and on the adhesins, the fungal cell surface proteins. Attachment is often a prerequisite for the invasion of the host, hence its importance. Adhesion visibly precedes colonization of root surfaces and outer cortex layers, but little is known about the molecular details. We propose that by starting from what is already known from other fungi, including yeast and other filamentous pathogens and symbionts, the mechanism and function of Trichoderma adhesion will become accessible. There is a sequence, and perhaps functional, homology to other rhizosphere-competent Sordariomycetes. Specifically, Verticillium dahliae is a soil-borne pathogen that establishes itself in the xylem and causes destructive wilt disease. Metarhizium species are best-known as insect pathogens with biocontrol potential, but they also colonize roots. Verticillium orthologs of the yeast Flo8 transcription factor, Som1, and several other relevant genes are already under study for their roles in adhesion. Metarhizium encodes relevant adhesins. Trichoderma virens encodes homologs of Som1, as well as adhesin candidates. These genes should provide exciting leads toward the first step in the establishment of beneficial interactions with roots in the rhizosphere.

Alternative splicing diversifies the transcriptome and proteome of the rice blast fungus during host infection

Alternative splicing (AS) contributes to diversifying and regulating cellular responses to environmental conditions and developmental cues by differentially producing multiple mRNA and protein isoforms from a single gene. Previous studies on AS in pathogenic fungi focused on profiling AS isoforms under a limited number of conditions. We analysed AS profiles in the rice blast fungus Magnaporthe oryzae, a global threat to rice production, using high-quality transcriptome data representing its vegetative growth (mycelia) and multiple host infection stages. We identified 4,270 AS isoforms derived from 2,413 genes, including 499 genes presumably regulated by infection-specific AS. AS appears to increase during infection, with 32.7% of the AS isoforms being produced during infection but absent in mycelia. Analysis of the isoforms observed at each infection stage showed that 636 AS isoforms were more abundant than corresponding annotated mRNAs, especially after initial hyphal penetration into host cell. Many such dominant isoforms were predicted to encode regulatory proteins such as transcription factors and phospho-transferases. We also identified the genes encoding distinct proteins via AS and confirmed the translation of some isoforms via a proteomic analysis, suggesting potential AS-mediated neo-functionalization of some genes during infection. Comprehensive profiling of the pattern of genome-wide AS during multiple stages of rice-M. oryzae interaction established a foundational resource that will help investigate the role and regulation of AS during rice infection.

Enhancing the Biocontrol Potential of the Entomopathogenic Fungus in Multiple Respects via the Overexpression of a Transcription Factor Gene MaSom1

Entomopathogenic fungi play important roles in the control of populations of agricultural and disease vector pests in nature. The shortcomings of mycoinsecticides for pest management in the field cannot be completely overcome by improving single biocontrol properties of fungi. Therefore, enhancing the biocontrol potential of entomopathogenic fungi in multiple respects by genetic engineering is desirable. Transcription factors are usually involved in various important processes during fungal growth and pathogenesis via regulating a series of genes, and are important candidates for fungal improvement via genetic engineering. Herein, overexpression of MaSom1, a key transcription factor gene in the cAMP/PKA pathway, improves the biocontrol traits of Metarhizium acridum in multiple respects. When compared with WT, the MaSom1-overexpression strains exhibit enhanced tolerances to UV-B and heat shock, with increased mean 50% inhibition times by 66.9% and 155.2%, respectively. Advanced conidiation emerged accompanied by increased conidial yield up to 3.89 times after 3-day incubation for the MaSom1-overexpression strains compared to WT. Furthermore, when compared with WT, the virulence of the MaSom1-overexpression strains was also increased with the mean 50% lethality times reduced by 21.8% to 23.8%. Taken together, the MaSom1-overexpression improved the biocontrol potential of M. acridum in multiple respects. Our results provide insights into the application of key transcription factors for genetic engineering and offer a credible way to further improve the biocontrol potential of entomopathogenic fungi.

Characterization of the MYB Genes Reveals Insights Into Their Evolutionary Conservation, Structural Diversity, and Functional Roles in Magnaporthe oryzae

The myeloblastosis (MYB) transcription factor family is evolutionarily conserved among plants, animals, and fungi, and contributes to their growth and development. We identified and analyzed 10 putative MYB genes in Magnaporthe oryzae (MoMYB) and determined their phylogenetic relationships, revealing high divergence and variability. Although MYB domains are generally defined by three tandem repeats, MoMYBs contain one or two weakly conserved repeats embedded in extensive disordered regions. We characterized the secondary domain organization, disordered segments, and functional contributions of each MoMYB. During infection, MoMYBs are distinctively expressed and can be subdivided into two clades of being either up- or down-regulated. Among these, MoMYB1 and MoMYB8 are up-regulated during infection and vegetative growth, respectively. We found MoMYB1 localized predominantly to the cytosol during the formation of infection structures. ΔMomyb1 exhibited reduced virulence on intact rice leaves corresponding to the diminished ability to form hypha-driven appressorium (HDA). We discovered that MoMYB1 regulates HDA formation on hard, hydrophobic surfaces, whereas host surfaces partially restored HDA formation in ΔMomyb1. Lipid droplet accumulation in hyphal tips and expression of HDA-associated genes were strongly perturbed in ΔMomyb1 indicating genetic interaction of MoMYB1 with downstream components critical to HDA formation. We also found that MoMYB8 is necessary for fungal growth, dark-induced melanization of hyphae, and involved in higher abiotic stress tolerance. Taken together, we revealed a multifaceted picture of the MoMYB family, wherein a low degree of conservation has led to the development of distinct structures and functions, ranging from fungal growth to virulence.

An APSES Transcription Factor Xbp1 Is Required for Sclerotial Development, Appressoria Formation, and Pathogenicity in Ciboria shiraiana

Sclerotinia diseases are important plant fungal diseases that, causes huge economic worldwide losses every year. Ciboria shiraiana is the main pathogen that results in mulberry sclerotia diseases. Sclerotia and appressoria play important roles in long-term pathogen survival and in host infection during life and disease cycles. However, the molecular mechanisms of sclerotial development and appressoria formation in C. shiraiana have not been well studied. Here, an Asm1p, Phd1p, Sok2p, Efg1p and StuAp (APSES)-type transcription factor in C. shiraiana, CsXbp1, involved in sclerotial development and appressoria formation was functionally characterized. Bioinformatics analyses showed that CsXbp1 contained an APSES-type DNA binding domain. The expression levels of CsXbp1 were higher in sclerotia and during later stages of infection. Compared with wild-type strains, hyphal growth was slower, the number and weight of sclerotia were reduced significantly, and appressoria formation was obviously delayed in CsXbp1 RNA interference (RNAi) strains. Moreover, the CsXbp1 RNAi strains showed weakened pathogenicity owing to compound appressoria defects. Tobacco rattle virus-mediated host-induced gene silencing enabled Nicotiana benthamiana to increase its resistance to C. shiraiana by reducing the CsXbp1 transcripts level. Thus, CsXbp1 plays vital roles in sclerotial formation, appressoria formation, and pathogenicity in C. shiraiana. This study provides new insights into the infection mechanisms of C. shiraiana and plant resistance breeding.

Class I myosin mediated endocytosis and polarization growth is essential for pathogenicity of Magnaporthe oryzae

In eukaryotes, myosin provides the necessary impetus for a series of physiological processes, including organelle movement, cytoplasmic flow, cell division, and mitosis. Previously, three members of myosin were identified in Magnaporthe oryzae, with class II and class V myosins playing important roles in intracellular transport, fungal growth, and pathogenicity. However, limited is known about the biological function of the class I myosin protein in the rice blast fungus. Here, we found that Momyo1 is highly expressed during conidiation and infection. Functional characterization of this gene via RNA interference (RNAi) revealed that Momyo1 is required for vegetative growth, conidiation, melanin pigmentation, and pathogenicity of M. oryzae. The Momyo1 knockdown mutant is defective in formation of appressorium-like structures (ALS) at the hyphal tips. In addition, Momyo1 also displays defects on cell wall integrity, hyphal hydrophobicity, extracellular enzyme activities, endocytosis, and formation of the Spitzenkörper. Furthermore, Momyo1 was identified to physically interact with the MoShe4, a She4p/Dim1p orthologue potentially involved in endocytosis, polarization of the actin cytoskeleton. Overall, our findings provide a novel insight into the regulatory mechanism of Momyo1 that is involved in fungal growth, cell wall integrity, endocytosis, and virulence of M. oryzae. KEY POINTS: • Momyo1 is required for vegetative growth and pigmentation of M. oryzae. • Momyo1 is essential for cell wall integrity and endocytosis of M. oryzae. • Momyo1 is involved in hyphal surface hydrophobicity of M. oryzae.

Transcriptional Landscapes of Long Non-coding RNAs and Alternative Splicing in Pyricularia oryzae Revealed by RNA-Seq

Pyricularia oryzae causes the rice blast, which is one of the most devastating crop diseases worldwide, and is a model fungal pathogen widely used for dissecting the molecular mechanisms underlying fungal virulence/pathogenicity. Although the whole genome sequence of P. oryzae is publicly available, its current transcriptomes remain incomplete, lacking the information on non-protein coding genes and alternative splicing. Here, we performed and analyzed RNA-Seq of conidia and hyphae, resulting in the identification of 3,374 novel genes. Interestingly, the vast majority of these novel genes likely transcribed long non-coding RNAs (lncRNAs), and most of them were localized in the intergenic regions. Notably, their expressions were concomitant with the transcription of neighboring genes thereof in conidia and hyphae. In addition, 2,358 genes were found to undergo alternative splicing events. Furthermore, we exemplified that a lncRNA was important for hyphal growth likely by regulating the neighboring protein-coding gene and that alternative splicing of the transcription factor gene CON7 was required for appressorium formation. In summary, results from this study indicate that lncRNA transcripts and alternative splicing events are two important mechanisms for regulating the expression of genes important for conidiation, hyphal growth, and pathogenesis, and provide new insights into transcriptomes and gene regulation in the rice blast fungus.

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.

MoLEU1, MoLEU2, and MoLEU4 regulated by MoLEU3 are involved in leucine biosynthesis, fungal development, and pathogenicity in Magnaporthe oryzae

Amino acids are vital components in cell metabolism. Leucine is a regulatory factor that generates significant impact on protein synthesis/turnover, modulates diverse cellular signalling pathways and participates in oxidative processes and immune responses. Here, we identified and characterized the functions of a leucine-associated Zn₂ Cys₆ -type transcription factor, MoLeu3. Disruption of MoLEU3 resulted in significantly reduced pathogenicity in barley and rice. Quantitative RT-PCR showed that the expression levels of the putative leucine biosynthesis-related genes, MoLEU1, MoLEU2 and MoLEU4 were downregulated in the ΔMoleu3 mutant. We used high-throughput gene knockout method to generate the null mutants of MoLEU1, MoLEU2 and MoLEU4 respectively. The ΔMoleu1, ΔMoleu2 and ΔMoleu4 mutants are leucine auxotroph and showed similar phenotypic characterizations, including reduced conidiation, delayed mobilization and degradation of glycogen and lipid droplets, limited appressorium-mediated penetration, and restricted invasive hyphae growth within host cells. Collectively, MoLEU1, MoLEU2, and MoLEU4 regulated by MoLEU3 play crucial roles in fungal development and infectious processes through modulation of leucine biosynthesis in Magnaporthe oryzae.

Karyopherin MoKap119-mediated nuclear import of cyclin-dependent kinase regulator MoCks1 is essential for Magnaporthe oryzae pathogenicity

Nuclear import of proteins relies on nuclear import receptors called importins/karyopherins (Kaps), whose functions were reported in yeasts, fungi, plants, and animal cells, including cell cycle control, morphogenesis, stress sensing/response, and also fungal pathogenecity. However, limited is known about the physiological function and regulatory mechanism of protein import in the rice-blast fungus Magnaporthe oryzae. Here, we identified an ortholog of β-importin in M. oryzae encoded by an ortholog of KAP119 gene. Functional characterisation of this gene via reverse genetics revealed that it is required for vegetative growth, conidiation, melanin pigmentation, and pathogenicity of M. oryzae. The mokap119Δ mutant was also defective in formation of appressorium-like structure from hyphal tips. By affinity assay and liquid chromatography-tandem mass spectrometry, we identified potential MoKap119-interacting proteins and further verified that MoKap119 interacts with the cyclin-dependent kinase subunit MoCks1 and mediates its nuclear import. Transcriptional profiling indicated that MoKap119 may regulate transcription of infection-related genes via MoCks1 regulation of MoSom1. Overall, our findings provide a novel insight into the regulatory mechanism of M. oryzae pathogenesis likely by MoKap119-mediated nuclear import of the cyclin-dependent kinase subunit MoCks1.

Leucine biosynthesis is required for infection-related morphogenesis and pathogenicity in the rice blast fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae causes one of the most devastating crop diseases world-wide and new control strategies for blast disease are urgently required. We have used insertional mutagenesis in M. oryzae to define biological processes that are critical for blast disease. Here, we report the identification of LEU2A by T-DNA mutagenesis, which putatively encodes 3-isopropylmalate dehydrogenase (3-IPMDH) required for leucine biosynthesis, implicating that synthesis of this amino acid is required for fungal pathogenesis. M. oryzae contains a further predicted 3-IPMDH gene (LEU2B), two 2-isopropylmalate synthase (2-IPMS) genes (LEU4 and LEU9) and an isopropylmalate isomerase (IPMI) gene (LEU1). Targeted gene deletion mutants of LEU1, LEU2A or LEU4 are leucine auxotrophs, and severely defective in pathogenicity. All phenotypes associated with mutants lacking LEU1, LEU2A or LEU4 could be overcome by adding exogenous leucine. The expression levels of LEU1, LEU2A or LEU4 genes were significantly down-regulated by deletion of the transcription factor gene LEU3, an ortholog of Saccharomyces cerevisiae LEU3. We also functionally characterized leucine biosynthesis genes in the wheat pathogen Fusarium graminearum and found that FgLEU1, FgLEU3 and FgLEU4 are essential for wheat head blight disease, suggesting that leucine biosynthesis in filamentous fungal pathogens may be a conserved factor for fungal pathogenicity and, therefore, a potential target for disease control.

Host-Induced Gene Silencing of MoAP1 Confers Broad-Spectrum Resistance to Magnaporthe oryzae

Rice blast caused by Magnaporthe oryzae (M. oryzae) is a major threat to global rice production. In recent years, small interference RNAs (siRNAs) and host-induced gene silencing (HIGS) has been shown to be new strategies for the development of transgenic plants to control fungal diseases and proved a useful tool to study gene function in pathogens. We here tested whether in vitro feeding artificial siRNAs (asiRNAs) could compromise M. oryzae virulence and in vivo HIGS technique could improve rice blast resistance. Our data revealed that silencing of M. oryzae MoAP1 by feeding asiRNAs targeting MoAP1 (i.e., asiR1245, asiR1362, and asiR1115) resulted in inhibited fungal growth, abnormal spores, and decreased pathogenicity. Among the asiRNAs, asiR1115 was the most inhibitory toward the rice blast fungus. Conversely, the asiRNAs targeting three other genes (i.e., MoSSADH, MoACT, and MoSOM1) had no effect on fungal growth. Transgenic rice plants expressing RNA hairpins targeting MoAP1 exhibited improved resistance to 11 tested M. oryzae strains. Confocal microscopy also revealed profoundly restricted appressoria and mycelia in rice blast-infected transgenic rice plants. Our results demonstrate that in vitro asiRNA and in vivo HIGS were useful protection approaches that may be valuable to enhance rice blast resistance.

Verticillium dahliae transcription factors Som1 and Vta3 control microsclerotia formation and sequential steps of plant root penetration and colonisation to induce disease

Verticillium dahliae nuclear transcription factors Som1 and Vta3 can rescue adhesion in a FLO8-deficient Saccharomyces cerevisiae strain. Som1 and Vta3 induce the expression of the yeast FLO1 and FLO11 genes encoding adhesins. Som1 and Vta3 are sequentially required for root penetration and colonisation of the plant host by V. dahliae. The SOM1 and VTA3 genes were deleted and their functions in fungus-induced plant pathogenesis were studied using genetic, cell biology, proteomic and plant pathogenicity experiments. Som1 supports fungal adhesion and root penetration and is required earlier than Vta3 in the colonisation of plant root surfaces and tomato plant infection. Som1 controls septa positioning and the size of vacuoles, and subsequently hyphal development including aerial hyphae formation and normal hyphal branching. Som1 and Vta3 control conidiation, microsclerotia formation, and antagonise in oxidative stress responses. The molecular function of Som1 is conserved between the plant pathogen V. dahliae and the opportunistic human pathogen Aspergillus fumigatus. Som1 controls genes for initial steps of plant root penetration, adhesion, oxidative stress response and VTA3 expression to allow subsequent root colonisation. Both Som1 and Vta3 regulate developmental genetic networks required for conidiation, microsclerotia formation and pathogenicity of V. dahliae.

Colletotrichum orbiculare MTF4 Is a Key Transcription Factor Downstream of MOR Essential for Plant Signal-Dependent Appressorium Development and Pathogenesis

The cucumber anthracnose fungus Colletotrichum orbiculare forms a specialized infection structure, called an appressorium. Appressorium differentiation relies on fungal perception of physical and biochemical signals at the plant surface. Our previous report showed that the morphogenesis-related NDR (nuclear Dbf2-related) kinase pathway (MOR) is crucial for translating plant-derived signals for appressorium development. Here, we focused on identifying transcriptional regulators downstream of MOR that are involved in plant signal sensing and transduction for appressorium development. Based on whole-genome transcript profiling, we identified a Zn(II)₂Cys₆ transcription factor, CoMTF4, as a potential downstream factor of MOR. CoMTF4 was expressed in planta rather than in vitro under the control of the NDR kinase CoCbk1. Phenotypes of comtf4 mutants, strains with constitutively active CoCbk1 and strains with constitutive overexpression of CoMTF4 suggested that CoMtf4 acts downstream of MOR. Furthermore, nuclear localization of CoMtf4 was dependent on the MOR and responsive to plant-derived signals that lead to appressorium morphogenesis. Thus, we conclude that CoMtf4 is a transcription factor downstream of MOR that is essential for appressorium morphogenesis and pathogenesis and is regulated in response to plant-derived signals. This study provides insights into fungal sensing of plant signals and subsequent responses critical for appressorium formation.

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.

Involvement of MaSom1, a downstream transcriptional factor of cAMP/PKA pathway, in conidial yield, stress tolerances, and virulence in Metarhizium acridum

Flo8/Som1, which functions downstream from the cyclic AMP (cAMP)-dependent protein kinase A (PKA) pathway, plays important roles in hyphal development, spore formation, and virulence in yeast and several filamentous fungi. However, the functions of Som1 in entomopathogenic fungi are still a mystery. In this study, MaSom1, a Flo8/Som1 homolog, was identified and functionally characterized in a model entomopathogenic fungus Metarhizium acridum. Similar to Flo8/Som1 in other fungi, MaSom1 mainly localized to the nucleus in M. acridum. Disruption of MaSom1 reduced conidial yield, delayed conidial germination, and impaired the fungal tolerances to heat and UV-B. The expression levels of some genes involved in defenses of heat shock and UV-B radiation were significantly reduced in ΔMaSom1. MaSom1 is also important for cell wall integrity and conidial surface structures in M. acridum. Some genes related to fungal cell wall synthesis were downregulated in ΔMaSom1. Bioassays showed that ΔMaSom1 had a dramatically decreased virulence after both topical inoculation and intrahemocoel injection of the fungus in locusts. Moreover, inactivation of MaSom1 reduced appressorium formation, diminished fungal growth in locust hemolymph in vitro, and enhanced insect immune responses. Taken together, these results indicate that disruption of MaSom1 leads to a decline of fungal virulence because of impairments in conidial germination and appressorium formation, reduction of fungal growth in host hemolymph, and enhancement of insect immune responses owing to the changes in conidial surface structures.

Verticillium dahliae transcription factor VdFTF1 regulates the expression of multiple secreted virulence factors and is required for full virulence in cotton

Fungal transcription factors (TFs) implicated in the regulation of virulence gene expression have been identified in a number of plant pathogens. In Verticillium dahliae, despite its agricultural importance, few regulators of transcription have been characterized. In this study, a T-DNA insertion mutant with significantly reduced virulence towards cotton was identified. The T-DNA was traced to VdFTF1, a gene encoding a TF containing a Fungal_trans domain. Transient expression in onion epidermal cells indicated that VdFTF1 is localized to the nucleus. The VdFTF1-deletion strains displayed normal vegetative growth, mycelial pigmentation and conidial morphology, but exhibited significantly reduced virulence on cotton, suggesting that VdFTF1 is required exclusively for pathogenesis. Comparisons of global transcription patterns of wild-type and VdFTF1-deletion strains indicated that VdFTF1 affected the expression of 802 genes, 233 of which were associated with catalytic processes. These genes encoded 69 potentially secreted proteins, 43 of which contained a carbohydrate enzyme domain known to participate in pathogenesis during infection of cotton. Targeted gene deletion of one VdFTF1-regulated gene resulted in significantly impaired vascular colonization, as measured by quantitative polymerase chain reaction, as well as aggressiveness and symptom severity in cotton. In conclusion, VdFTF1, which encodes a TF containing a Fungal_trans domain, regulates the gene expression of plant cell wall degradation enzymes in V. dahliae, which are required for full virulence on cotton.

The ZtVf1 transcription factor regulates development and virulence in the foliar wheat pathogen Zymoseptoria tritici

The dimorphic fungal pathogen, Zymoseptoria tritici undergoes discrete developmental changes to complete its life cycle on wheat. Molecular mechanisms underlying morphogenesis during infection process of Z. tritici are poorly understood. In this study, we have investigated the role of ZtVf1 gene encoding a transcription factor belonging to C₂-H₂ subfamily. In planta assays revealed that ZtVf1 is required for virulence. Reduced necrotic lesions and low pycnidia density within the lesions resulted in significantly reduced virulence of ZtVf1 mutants. Cytological analysis showed that the impaired virulence of ZtVf1 mutants attributed to reduced penetration and colonization along with hampered pycnidia differentiation. In vitro phenotyping showed that ZtVf1 deletion affects hyphal branching and biomass production suggesting that the reduced tissue colonization by the ZtVf1 mutant might be due to lower hyphal branching and less fungal biomass production. In addition, the majority of infected substomatal cavities by the ZtVf1 mutant filled with compacted mycelia mat that did not differentiate to mature pycnidia indicating that the impaired melanization negatively affected pycnidia formation and maturation. The ZtVf1 might target multiple genes belonging to different cellular processes whose identification is of eminent interest to increase our understanding of this pathosystem. Overall, the data provided in this study indicates that attenuated pathogenicity of ZtVf1 mutant is due to involvement of this gene in the regulation of both early and late stages of infection.

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.

Adaptation to the Host Environment by Plant-Pathogenic Fungi

Many fungi can live both saprophytically and as endophyte or pathogen inside a living plant. In both environments, complex organic polymers are used as sources of nutrients. Propagation inside a living host also requires the ability to respond to immune responses of the host. We review current knowledge of how plant-pathogenic fungi do this. First, we look at how fungi change their global gene expression upon recognition of the host environment, leading to secretion of effectors, enzymes, and secondary metabolites; changes in metabolism; and defense against toxic compounds. Second, we look at what is known about the various cues that enable fungi to sense the presence of living plant cells. Finally, we review literature on transcription factors that participate in gene expression in planta or are suspected to be involved in that process because they are required for the ability to cause disease.

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.

A HOPS Protein, MoVps41, Is Crucially Important for Vacuolar Morphogenesis, Vegetative Growth, Reproduction and Virulence in Magnaporthe oryzae

The homotypic fusion and protein sorting protein complex (HOPS) is the first known tether complex identified in the endocytic system that plays a key role in promoting homotypic vacuolar fusion, vacuolar biogenesis and trafficking in a wide range of organisms, including plant and fungi. However, the exact influence of the HOPS complex on growth, reproduction and pathogenicity of the economically destructive rice blast fungus has not been investigated. In this study, we identified M. oryzae vacuolar protein sorting 41 (MoVps41) an accessory subunit of HOPS complex and used targeted gene deletion approach to evaluate its contribution to growth, reproduction and infectious life cycle of the rice blast fungus. Corresponding results obtained from this study showed that MoVps41 is required for optimum vegetative development of M. oryzae and observed that MoVps41 deletion mutant displayed defective vegetative growth. Our investigation further showed that MoVps41 deletion triggered vacuolar fragmentation, compromised membrane integrity and pathogenesis of the ΔMovps41 mutant. Our studies also showed for the first time that MoVps41 plays an essential role in the regulation of sexual and asexual reproduction of M. oryzae. In summary, our study provides insight into how MoVps41 mediated vacuolar fusion and biogenesis influences reproduction, pathogenesis, and vacuolar integrity in M. oryzae and also underscores the need to holistically investigate the HOPS complex in rice blast pathogen.

The Magnaporthe grisea species complex and plant pathogenesis

TAXONOMY

Kingdom Fungi; Phylum Ascomycota; Class Sordariomycetes; Order Magnaporthales; Family Pyriculariaceae (anamorph)/Magnaporthaceae (teleomorph); Genus Pyricularia (anamorph)/Magnaporthe (teleomorph); Species P. grisea (anamorph)/M. grisea (teleomorph).

HOST RANGE

Very broad at the species level, including rice, wheat, barley, millet and other species of the Poaceae (Gramineae).

DISEASE SYMPTOMS

Can be found on all parts of the plant, including leaves, leaf collars, necks, panicles, pedicels, seeds and even the roots. Initial symptoms are white to grey-green lesions or spots with darker borders, whereas older lesions are elliptical or spindle-shaped and whitish to grey with necrotic borders. Lesions may enlarge and coalesce to eventually destroy the entire leaf.

DISEASE CONTROL

Includes cultural strategies, genetic resistance and the application of chemical fungicides.

GEOGRAPHICAL DISTRIBUTION

Widespread throughout the rice-growing regions of the globe and has been reported in more than 85 countries.

GENOMIC STRUCTURE

Different isolates possess similar genomic sizes and overall genomic structures. For the laboratory strain 70-15: assembly size, 40.98 Mb; number of chromosomes, seven; number of predicted genes, 13 032; G + C composition, 51.6%; average gene contains 451.6 amino acids; mitochondrion genome size, 34.87 kb.

USEFUL WEBSITE

http://www.broadinstitute.org/annotation/genome/magnaporthe_comparative/MultiHome.html.

Characterization of 47 Cys2 -His2 zinc finger proteins required for the development and pathogenicity of the rice blast fungus Magnaporthe oryzae

The Cys2 -His2 (C2H2) zinc finger protein family is the second-largest family of transcription factors (TFs) in Magnaporthe oryzae, the causal fungus responsible for the destructive rice blast disease. However, little is known about the roles of most C2H2 TFs in the development and pathogenicity of M. oryzae. The roles of 47 C2H2 genes in development and pathogenicity were investigated by gene deletion in M. oryzae. The TF-dependent genes in mycelia or appressoria were analyzed with RNA sequencing and quantitative PCR (qPCR). Forty-four C2H2 genes are involved in growth (20 genes), conidiation (28 genes), appressorium formation (four genes) and pathogenicity (22 genes) in M. oryzae. Of these, MGG_14931, named as VRF1, is required for pathogenicity, specifically controlling appressorium maturation by affecting the expression of genes related to appressorial structure and function, including melanin biosynthesis, chitin catabolism, lipid metabolism, proteolysis, transmembrane transport, and response to oxidative stress; MGG_01776, named as VRF2, is required for plant penetration and invasive growth; conidiation-related gene CON7 is required for conidial differentiation; and MoCREA, encoding a carbon catabolite repression protein, is a novel repressor of lipid catabolism when glucose obtainable in M. oryzae. This study provides many insights into the regulation of growth, asexual development, appressorium formation, and pathogenicity by C2H2 TFs in M. oryzae.

Orotate phosphoribosyl transferase MoPyr5 is involved in uridine 5'-phosphate synthesis and pathogenesis of Magnaporthe oryzae

Orotate phosphoribosyl transferase (OPRTase) plays an important role in de novo and salvage pathways of nucleotide synthesis and is widely used as a screening marker in genetic transformation. However, the function of OPRTase in plant pathogens remains unclear. In this study, we characterized an ortholog of Saccharomyces cerevisiae Ura5, the OPRTase MoPyr5, from the rice blast fungus Magnaporthe oryzae. Targeted gene disruption revealed that MoPyr5 is required for mycelial growth, appressorial turgor pressure and penetration into plant tissues, invasive hyphal growth, and pathogenicity. Interestingly, the ∆Mopyr5 mutant is also involved in mycelial surface hydrophobicity. Exogenous uridine 5'-phosphate (UMP) restored vegetative growth and rescued the defect in pathogenicity on detached barley and rice leaf sheath. Collectively, our results show that MoPyr5 is an OPRTase for UMP biosynthesis in M. oryzae and indicate that UTP biosynthesis is closely linked with vegetative growth, cell wall integrity, and pathogenicity of fungus. Our results also suggest that UMP biosynthesis would be a good target for the development of novel fungicides against M. oryzae.

ZNF1 Encodes a Putative C2H2 Zinc-Finger Protein Essential for Appressorium Differentiation by the Rice Blast Fungus Magnaporthe oryzae

The rice blast fungus Magnaporthe oryzae forms specialized infection structures called appressoria which are essential for gaining entry to plant tissue. Here, we report the identification of a novel nonpathogenic T-DNA-tagged mutant XF696 of M. oryzae with a single insertion in the promoter of ZNF1, which encodes a putative transcription factor (TF). Targeted gene deletion mutants of ZNF1 are nonpathogenic and unable to develop appressoria. However, Δznf1 mutants still respond to exogenous cyclic AMP on hydrophilic surfaces and can sense hydrophobic surfaces, initiating the differentiation of germ tubes. Interestingly, Δznf1 mutants also produce significantly more conidia compared with the isogenic wild-type strain. Quantitative reverse-transcription polymerase chain reaction analysis and green fluorescent protein fusion experiments revealed that expression of ZNF1 was highly induced during germination and appressorium development in M. oryzae and potentially regulated by the Pmk1 mitogen-activated protein kinase pathway. We observed that Δznf1 mutants are affected in mitosis and impaired in mobilization and degradation of lipid droplets and glycogen reserves during appressorium differentiation. Site-directed mutagenesis confirmed that three of the four C2H2 zinc-finger domains are essential for the function of Znf1. Taken together, we conclude that a C2H2 zinc-finger TF encoded by ZNF1 is essential for appressorium development by the rice blast fungus.

Transcription Factor SomA Is Required for Adhesion, Development and Virulence of the Human Pathogen Aspergillus fumigatus

The transcription factor Flo8/Som1 controls filamentous growth in Saccharomyces cerevisiae and virulence in the plant pathogen Magnaporthe oryzae. Flo8/Som1 includes a characteristic N-terminal LUG/LUH-Flo8-single-stranded DNA binding (LUFS) domain and is activated by the cAMP dependent protein kinase A signaling pathway. Heterologous SomA from Aspergillus fumigatus rescued in yeast flo8 mutant strains several phenotypes including adhesion or flocculation in haploids and pseudohyphal growth in diploids, respectively. A. fumigatus SomA acts similarly to yeast Flo8 on the promoter of FLO11 fused with reporter gene (LacZ) in S. cerevisiae. FLO11 expression in yeast requires an activator complex including Flo8 and Mfg1. Furthermore, SomA physically interacts with PtaB, which is related to yeast Mfg1. Loss of the somA gene in A. fumigatus resulted in a slow growth phenotype and a block in asexual development. Only aerial hyphae without further differentiation could be formed. The deletion phenotype was verified by a conditional expression of somA using the inducible Tet-on system. A adherence assay with the conditional somA expression strain indicated that SomA is required for biofilm formation. A ptaB deletion strain showed a similar phenotype supporting that the SomA/PtaB complex controls A. fumigatus biofilm formation. Transcriptional analysis showed that SomA regulates expression of genes for several transcription factors which control conidiation or adhesion of A. fumigatus. Infection assays with fertilized chicken eggs as well as with mice revealed that SomA is required for pathogenicity. These data corroborate a complex control function of SomA acting as a central factor of the transcriptional network, which connects adhesion, spore formation and virulence in the opportunistic human pathogen A. fumigatus.

Invasive fungal infections affecting immunocompromised patients are emerging worldwide. Among various human fungal pathogens, Aspergillus fumigatus is one of the most common molds causing severe invasive aspergillosis in immunocompromised patients. The conidia, which can evade from innate immunity and adhere to epithelial cells of alveoli in human lungs will start to germinate and cause the disease. Currently, the understanding of the molecular mechanisms of adherence of fungal cells to hosts is scarce. The transcription factor Flo8 controls adhesion to biotic or abiotic surfaces and morphological development in baker’s yeast. Flo8 homologues in the dimorphic human pathogenic yeast Candida albicans or the filamentous plant pathogen Magnaporthe oryzae are required for development and virulence. We found in this study that the Flo8 homologue SomA of A. fumigatus is required for adhesion and conidiation. Two independent invasive aspergillosis assays using chicken eggs or mouse demonstrated that deletion of the corresponding gene resulted in attenuated virulence. SomA represents an important fungal transcription factor at the interface between adherence, asexual spore formation and pathogenicity in an important opportunistic human pathogen.

Role of the MoYAK1 protein kinase gene in Magnaporthe oryzae development and pathogenicity

Conidiation and appressorium differentiation are key processes for polycyclic dissemination and infection in many pathogens. Our previous study using DNA microarray led to the discovery of the MoYAK1 gene in Magnaporthe oryzae that is orthologous to YAK1 in Saccharomyces cerevisiae. Although the mechanistic roles of YAK1 in S. cerevisiae have been described, roles of MoYAK1 in M. oryzae, a phytopathogenic fungus responsible for rice blast, remain uncharacterized. Targeted disruption of MoYAK1 results in pleiotropic defects in M. oryzae development and pathogenicity. The ΔMoyak1 mutant exhibits a severe reduction in aerial hyphal formation and conidiation. Conidia in the ΔMoyak1 are delayed in germination and demonstrate decreased glycogen content in a conidial age-dependent manner. The expression of hydrophobin-coding genes is dramatically changed in the ΔMoyak1 mutant, leading to a loss of surface hydrophobicity. Unlike the complete inability of the ΔMoyak1 mutant to develop appressoria on an inductive surface, the mutant forms appressoria of abnormal morphology in response to exogenous cyclic adenosine-5'-monophosphate and host-driven signals, which are all defective in penetrating host tissues due to abnormalities in glycogen and lipid metabolism, turgor generation and cell wall integrity. These data indicate that MoYAK1 is a protein kinase important for the development and pathogenicity of M. oryzae.

MoARG1, MoARG5,6 and MoARG7 involved in arginine biosynthesis are essential for growth, conidiogenesis, sexual reproduction, and pathogenicity in Magnaporthe oryzae

Arginine is one of the most versatile amino acids in eukaryote cells, which plays important roles in a multitude of processes such as protein synthesis, nitrogen metabolism, nitric oxide (NO) and urea biosynthesis. The de novo arginine biosynthesis pathway is conserved among fungal kingdom, but poorly understood in plant pathogenic fungi. Here, we characterized the functions of three synthetic enzyme-encoding genes MoARG1, MoARG5,6, and MoARG7, which involved the seventh step, second-third step and fifth step of arginine biosynthesis in Magnaporthe oryzae, respectively. Deletion of MoARG1 or MoARG5,6, resulted in arginine auxotrophic mutants, which had a strict requirement for arginine on minimal medium (MM). Both ΔMoarg1 and ΔMoarg5,6 severely reduced in aerial hyphal growth, pigmentation, conidiogenesis, sexual reproduction and pathogenicity. Interestingly, like Saccharomyces cerevisiae, deletion of MoARG7 caused a leaky arginine auxotrophy, and attenuated pathogenicity. Limited appressorium-mediated penetration and restricted invasive hyphae growth in host cells are responsible for the severely attenuated pathogenicity of the Arg(-) mutants. Additionally, we monitored the NO generation during conidial germination and appressorial formation in both Arg(-) mutants and wild type, and demonstrated that NO generation may not occur via arginine-dependent pathway in M. oryzae. In summary, MoARG1, MoARG5,6, and MoARG7 are required for growth, conidiogenesis, sexual reproduction, and pathogenicity in M. oryzae.

Genome analysis of rice-blast fungus Magnaporthe oryzae field isolates from southern India

The Indian subcontinent is the center of origin and diversity for rice (Oryza sativa L.). The O. sativa ssp. indica is a major food crop grown in India, which occupies the first and second position in area and production, respectively. Blast disease caused by Magnaporthe oryzae is a major constraint to rice production. Here, we report the analysis of genome architecture and sequence variation of two field isolates, B157 and MG01, of the blast fungus from southern India. The 40 Mb genome of B157 and 43 Mb genome of MG01 contained 11,344 and 11,733 predicted genes, respectively. Genomic comparisons unveiled a large set of SNPs and several isolate specific genes in the Indian blast isolates. Avr genes were analyzed in several sequenced Magnaporthe strains; this analysis revealed the presence of Avr-Pizt and Avr-Ace1 genes in all the sequenced isolates. Availability of whole genomes of field isolates from India will contribute to global efforts to understand genetic diversity of M. oryzae population and to track the emergence of virulent pathotypes.

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The first genomic study of Magnaporthe from Indian subcontinent

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Provided information about genomic variations in terms of SNPs, InDels and ICVs due to transposable elements

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Identified novel genes specific to Indian isolates

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Genome wide antisense transcripts identified from this study

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Identified Magnaporthe specific pathogenicity genes that are absent in non-pathogenic Ascomycetes fungi

Phosphoproteome Analysis Links Protein Phosphorylation to Cellular Remodeling and Metabolic Adaptation during Magnaporthe oryzae Appressorium Development

The rice pathogen, Magnaporthe oryzae, undergoes a complex developmental process leading to formation of an appressorium prior to plant infection. In an effort to better understand phosphoregulation during appressorium development, a mass spectrometry based phosphoproteomics study was undertaken. A total of 2924 class I phosphosites were identified from 1514 phosphoproteins from mycelia, conidia, germlings, and appressoria of the wild type and a protein kinase A (PKA) mutant. Phosphoregulation during appressorium development was observed for 448 phosphosites on 320 phosphoproteins. In addition, a set of candidate PKA targets was identified encompassing 253 phosphosites on 227 phosphoproteins. Network analysis incorporating regulation from transcriptomic, proteomic, and phosphoproteomic data revealed new insights into the regulation of the metabolism of conidial storage reserves and phospholipids, autophagy, actin dynamics, and cell wall metabolism during appressorium formation. In particular, protein phosphorylation appears to play a central role in the regulation of autophagic recycling and actin dynamics during appressorium formation. Changes in phosphorylation were observed in multiple components of the cell wall integrity pathway providing evidence that this pathway is highly active during appressorium development. Several transcription factors were phosphoregulated during appressorium formation including the bHLH domain transcription factor MGG_05709. Functional analysis of MGG_05709 provided further evidence for the role of protein phosphorylation in regulation of glycerol metabolism and the metabolic reprogramming characteristic of appressorium formation. The data presented here represent a comprehensive investigation of the M. oryzae phosphoproteome and provide key insights on the role of protein phosphorylation during infection-related development.

Transcriptome changes in Fusarium verticillioides caused by mutation in the transporter-like gene FST1

BACKGROUND

Fusarium verticillioides causes an important seed disease on maize and produces the fumonisin group of mycotoxins, which are toxic to humans and livestock. A previous study discovered that a gene (FST1) in the pathogen affects fumonisin production and virulence. Although the predicted amino acid sequence of FST1 is similar to hexose transporters, previous experimental evidence failed to prove function.

RESULTS

Three new phenotypes were identified that are associated with the FST1 mutant of F. verticillioides (Δfst1), namely reduction in macroconidia production, increased sensitivity to hydrogen peroxide, and reduced mycelial hydrophobicity. A transcriptome comparison of the wild type and strain Δfst1 grown on autoclaved maize kernels for six days identified 2677 genes that were differentially expressed. Through gene ontology analysis, 961 genes were assigned to one of 12 molecular function categories. Sets of down-regulated genes in strain Δfst1 were identified that could account for each of the mutant phenotypes.

CONCLUSION

The study provides evidence that disruption of FST1 causes several metabolic and developmental defects in F. verticillioides. FST1 appears to connect the expression of several gene networks, including those involved in secondary metabolism, cell wall structure, conidiogenesis, virulence, and resistance to reactive oxygen species. The results support our hypothesis that FST1 functions within the framework of environmental sensing.