Shared and distinct functions of two Gti1/Pac2 family proteins in growth, morphogenesis and pathogenicity of Magnaporthe oryzae

The Roles of Gti1/Pac2 Family Proteins in Fungal Growth, Morphogenesis, Stress Response, and Pathogenicity

Gti1/Pac2 is a fungal-specific transcription factor family with a stable and conserved N-terminal domain. Generally, there are two members in this family, named Gti1/Wor1/Rpy1/Mit1/Reg1/Ros1/Sge1 and Pac2, which are involved in fungal growth, development, stress response, spore production, pathogenicity, and so on. The Gti1/Pac2 family proteins share some conserved and distinct functions. For example, in Schizosaccharomyces pombe, Gti1 promotes the initiation of gluconate uptake during glucose starvation, while Pac2 controls the onset of sexual development in a pathway independent of the cAMP cascade. In the last two decades, more attention was focused on the Gti1 and its orthologs because of their significant effect on morphological switching and fungal virulence. By contrast, limited work was published on the functions of Pac2, which is required for stress responses and conidiation, but plays a minor role in fungal virulence. In this review, we present an overview of our current understanding of the Gti1/Pac2 proteins that contribute to fungal development and/or pathogenicity and of the regulation mechanisms during infection related development. Understanding the working networks of the conserved Gti1/Pac2 transcription factors in fungal pathogenicity not only advances our knowledge of the highly elaborate infection process but may also lead to the development of novel strategies for the control of plant disease. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

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/.

Discovery and Transcriptional Profiling of Penicillium digitatum Genes That Could Promote Fungal Virulence during Citrus Fruit Infection

Green mold caused by Penicillium digitatum (Pers.:Fr.) Sacc is the most prevalent postharvest rot concerning citrus fruits. Using the subtractive suppression hybridization (SSH) technique, different P. digitatum genes have been identified that could be involved in virulence during citrus infection in the early stages, a crucial moment that determines whether the infection progresses or not. To this end, a comparison of two P. digitatum strains with high and low virulence has been carried out. We conducted a study on the gene expression profile of the most relevant genes. The results indicate the importance of transcription and regulation processes as well as enzymes involved in the degradation of the plant cell wall. The most represented expressed sequence tag (EST) was identified as PDIP_11000, associated with the FluG domain, which is putatively involved in the activation of conidiation. It is also worth noting that PDIP_02280 encodes a pectin methyl esterase, a cell wall remodeling protein with a high expression level in the most virulent fungal strains, which is notably induced during citrus infection. Furthermore, within the group with the greatest representation and showing significant induction in the early stages of infection, regulatory proteins (PDIP_68700, PDIP_76160) and a chaperone (PDIP_38040) stand out. To a lesser extent, but not less relevant, it is worth distinguishing different regulatory proteins and transcription factors, such as PDIP_00580, PDIP_49640 and PDIP_78930.

Improvement of triterpenoid production in mycelia of Antrodia camphorata through mutagenesis breeding and amelioration of CCl4-induced liver injury in mice

Due to the scarcity of wild fruiting bodies, submerged fermentation of the medicinal fungus Antrodia camphorata is attracting much attention, but the production of bioactive triterpenoids is low. Therefore, there is an urgent need to improve the triterpenoid yield of submerged fermentation. Here, the A. camphorata mutant E3-64 was generated from strain AC16101 through random mutagenesis breeding, producing 172.8 mg triterpenoid per gram of dry mycelia. Further optimization of culture parameters resulted in a yield of 255.5 mg/g dry mycelia (i.e., an additional >1.4-fold increase), which is the highest reported yield thus far. Notably, mutant E3-64 produced 94% and 178% more of the triterpenoid components antcin A and antcamphin A, respectively, while it produced 52% and 15% less antcin B and G, respectively. Mutant E3-64 showed increased expression of key genes involved in triterpenoid biosynthesis, as well as different genome-wide single-nucleotide polymorphisms as compared with AC16101. Triterpenoids of the E3-64 mycelia exhibited remarkably protective activity against acute CCl4-induced liver injury in mice. This study shows the potential of A. camphorata for scientific research and commercial application.

Knockout of amino acid transporter gene OsLHT1 accelerates leaf senescence and enhances resistance to rice blast fungus

Plant amino acid transporters (AATs) regulate not only long-distance transport and reallocation of nitrogen (N) from source to sink organs, but also amount of amino acids in leaves hijacked by invaded pathogens. However, the function of AATs in plant defense responses to pathogen infection remains unknown. In this study, we found that rice amino acid transporter gene OsLHT1 was expressed in leaves and up-regulated by maturing, N starvation and inoculation of blast fungus Magnaporthe oryzae. Knockout of OsLHT1 resulted in development stage- and N supply-dependent premature senescence of leaves at vegetative growth stage. In comparison to wild type, Oslht1 mutant lines showed sustained rusty red spots on fully mature leaf blades irrespective of N supply levels. Notably, no relationship between the severity of leaf rusty red spots and concentration of total N or amino acids was found in Oslht1 mutants at different developmental stages. Disruption of OsLHT1 altered transport and metabolism of amino acids and biosynthesis of flavones and flavonoids, enhanced expression of jasmonic acid- and salicylic acid-related defense genes and production of jasmonic acid and salicylic acid, accumulation of reactive oxygen species. OsLHT1 inactivation dramatically prevented the leaf invasion of M. oryzae, the hemi-biotrophic ascomycete fungus. Overall, these results establish a module connecting the activity of amino acid transporter with leaf metabolism and defense to rice blast fungus.

Pyridoxine biosynthesis protein MoPdx1 affects the development and pathogenicity of Magnaporthe oryzae

B vitamins are essential micro-organic compounds for the development of humans and animals. Vitamin B6 comprises a group of components including pyridoxine, pyridoxal, and pyridoxamine. In addition, vitamin B6 acts as the coenzymes in amino acid biosynthesis, decarboxylation, racemic reactions, and other biological processes. In this study, we found that the expressions of a gene encoding pyridoxine biosynthesis protein (PDX1) were significantly upregulated in the early infectious stages in M. oryzae. Furthermore, deletion of MoPDX1 slowed vegetative growth on different media, especially on MM media, and the growth defect was rescued when MoPdx1-protein was expressed in mutants strains and when commercial VB6 (pyridoxine) was added exogenously. However, VB6 content in different strains cultured in CM media has no significant difference, suggested that MoPdx1 was involved in de novo VB6 biosynthesis not in uptake process, and VB6 regulates the vegetative growth of M. oryzae. The ΔMopdx1 mutants presented abnormal appressorium turgor, slowed invasive growth and reduced virulence on rice seedlings and sheath cells. MoPdx1 was located in the cytoplasm and present in spore and germ tubes at 14 hours post inoculation (hpi) and then transferred into the appressorium at 24 hpi. Addition of VB6 in the conidial suspentions could rescue the defects of appressorium turgor pressure at 14 hpi or 24 hpi, invasive growth and pathogenicity of the MoPDX1 deletion mutants. Indicated that MoPdx1 affected the appressorium turgor pressure, invasive growth and virulence mainly depended on de novo VB6, and VB6 was biosynthesized in conidia, then transported into the appressorium, which play important roles in substances transportation from conidia to appressorium thus to regulate the appressorium turgor pressure. However, deletion of MoPDX1 did not affect the ability that scavenge ROS produced by rice cells, and the mutant strains were unable to activate host defense responses. In addition, co-immunoprecipitation (Co-IP) assays investigating potential MoPdx1-interacting proteins suggested that MoPdx1 might take part in multiple pathways, especially in the ribosome and in biosynthesis of some substances. These results indicate that vitamins are involved in the development and pathogenicity of M. oryzae.

Regulatory mechanism of trichothecene biosynthesis in Fusarium graminearum

Among the genes involved in the biosynthesis of trichothecene (Tri genes), Tri6 and Tri10 encode a transcription factor with unique Cys₂His₂ zinc finger domains and a regulatory protein with no consensus DNA-binding sequences, respectively. Although various chemical factors, such as nitrogen nutrients, medium pH, and certain oligosaccharides, are known to influence trichothecene biosynthesis in Fusarium graminearum, the transcriptional regulatory mechanism of Tri6 and Tri10 genes is poorly understood. Particularly, culture medium pH is a major regulator in trichothecene biosynthesis in F. graminearum, but it is susceptible to metabolic changes posed by nutritional and genetic factors. Hence, appropriate precautions should be considered to minimize the indirect influence of pH on the secondary metabolism while studying the roles of nutritional and genetic factors on trichothecene biosynthesis regulation. Additionally, it is noteworthy that the structural changes of the trichothecene gene cluster core region exert considerable influence over the normal regulation of Tri gene expression. In this perspective paper, we consider a revision of our current understanding of the regulatory mechanism of trichothecene biosynthesis in F. graminearum and share our idea toward establishing a regulatory model of Tri6 and Tri10 transcription.

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

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

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.

Vacuolar Protein-Sorting Receptor MoVps13 Regulates Conidiation and Pathogenicity in Rice Blast Fungus Magnaporthe oryzae

Magnaporthe oryzae (synonym Pyricularia oryzae) is a filamentous fungal pathogen that causes major yield losses in cultivated rice worldwide. However, the mechanisms of infection of M. oryzae are not well characterized. The VPS13 proteins play vital roles in various biological processes in many eukaryotic organisms, including in the organization of actin cytoskeleton, vesicle trafficking, mitochondrial fusion, and phagocytosis. Nevertheless, the function of the Vps13 protein in plant pathogenic fungi has not been explored. Here, we analysed the biological functions of the Vps13 protein in the development and pathogenicity of M. oryzae. Deletion mutants of MoVps13 significantly reduced the conidiation and decreased the rate of fungal infection on hosts. Moreover, the loss of MoVps13 resulted in defective cell wall integrity (CWI) and plasma membrane (PM) homeostasis when treated with chemicals for inducing cell wall stress (200 mg/mL Congo Red or 0.005% SDS) and sphingolipid synthesis inhibitors (2 μM myriocin or 2 μM amphotericin B). This indicated that MoVps13 is also involved in cell wall synthesis and sphingolipid synthesis. Through immunoblotting, autophagic flux detection, co-localization, and chemical drug sensitivity assays, we confirmed the involvement of Movps13 in ER-phagy and the response to ER stress. Additionally, we generated the C-terminal structure of MoVps13 with high accuracy using the alphaflod2 database. Our experimental evidence indicates that MoVps13 is an important virulence factor that regulates the pathogenicity of M. oryzae by controlling CWI, lipid metabolism and the ER-phagy pathway. These results have expanded our knowledge about pathogenic fungi and will help exploration for novel therapeutic strategies against the rice blast fungus.

Transcription factor control of virulence in phytopathogenic fungi

Plant-pathogenic fungi are a significant threat to economic and food security worldwide. Novel protection strategies are required and therefore it is critical we understand the mechanisms by which these pathogens cause disease. Virulence factors and pathogenicity genes have been identified, but in many cases their roles remain elusive. It is becoming increasingly clear that gene regulation is vital to enable plant infection and transcription factors play an essential role. Efforts to determine their regulatory functions in plant-pathogenic fungi have expanded since the annotation of fungal genomes revealed the ubiquity of transcription factors from a broad range of families. This review establishes the significance of transcription factors as regulatory elements in plant-pathogenic fungi and provides a systematic overview of those that have been functionally characterized. Detailed analysis is provided on regulators from well-characterized families controlling various aspects of fungal metabolism, development, stress tolerance, and the production of virulence factors such as effectors and secondary metabolites. This covers conserved transcription factors with either specialized or nonspecialized roles, as well as recently identified regulators targeting key virulence pathways. Fundamental knowledge of transcription factor regulation in plant-pathogenic fungi provides avenues to identify novel virulence factors and improve our understanding of the regulatory networks linked to pathogen evolution, while transcription factors can themselves be specifically targeted for disease control. Areas requiring further insight regarding the molecular mechanisms and/or specific classes of transcription factors are identified, and direction for future investigation is presented.

Current progress on pathogenicity-related transcription factors in Fusarium oxysporum

Fusarium oxysporum is a well-known soilborne plant pathogen that causes severe vascular wilt in economically important crops worldwide. During the infection process, F. oxysporum not only secretes various virulence factors, such as cell wall-degrading enzymes (CWDEs), effectors, and mycotoxins, that potentially play important roles in fungal pathogenicity but it must also respond to extrinsic abiotic stresses from the environment and the host. Over 700 transcription factors (TFs) have been predicted in the genome of F. oxysporum, but only 26 TFs have been functionally characterized in various formae speciales of F. oxysporum. Among these TFs, a total of 23 belonging to 10 families are required for pathogenesis through various mechanisms and pathways, and the zinc finger TF family is the largest family among these 10 families, which consists of 15 TFs that have been functionally characterized in F. oxysporum. In this review, we report current research progress on the 26 functionally analysed TFs in F. oxysporum and sort them into four groups based on their roles in F. oxysporum pathogenicity. Furthermore, we summarize and compare the biofunctions, involved pathways, putative targets, and homologs of these TFs and analyse the relationships among them. This review provides a systematic analysis of the regulation of virulence-related genes and facilitates further mechanistic analysis of TFs important in F. oxysporum virulence.

A Sge1 homolog in Cytospora chrysosperma governs conidiation, virulence and the expression of putative effectors

SIX Gene Expression 1 (Sge1) is an important and well-recognized fungal-specific transcription regulator from the Gti1/Pac2 family that exhibits a conserved function in the vegetative growth, regulating the expression of effector genes and pathogenicity in plant pathogenic fungi. However, its functions in Cytospora chrysosperma, a notorious phytopathogenic fungus in forestry, remain poorly understood. Here, we characterized a Sge1 orthologue, CcSge1, in C. chrysosperma and deleted its Gti1/Pac2 domain for functional analysis. The CcSge1 deletion mutants showed obvious defects in hyphal growth, conidial production and response to hydrogen peroxide. Correspondingly, significantly lower expression of conidiation related genes were found in deletion mutants compared to that of the wild type. Importantly, the CcSge1 deletion mutants totally lost their pathogenicity to the host. Further analysis demonstrated that CcSge1 was responsible for the expression of putative effector genes and the transcription of CcSge1 was under tight control by pathogenicity-related MAP Kinase 1 (CcPmk1). What's more, one of the putative effector gene CCG_07874 was positively regulated by both CcSge1 and CcPmk1. Taken together, these data indicate that CcSge1is indispensable for hyphal radial growth, conidiation, the expression of effector genes and fungal virulence.

Transcriptome data reveal conserved patterns of fruiting body development and response to heat stress in the mushroom-forming fungus Flammulina filiformis

Mushroom-forming fungi are complex multicellular organisms that form the basis of a large industry, yet, our understanding of the mechanisms of mushroom development and its responses to various stresses remains limited. The winter mushroom (Flammulina filiformis) is cultivated at a large commercial scale in East Asia and is a species with a preference for low temperatures. This study investigated fruiting body development in F. filiformis by comparing transcriptomes of 4 developmental stages, and compared the developmental genes to a 200-genome dataset to identify conserved genes involved in fruiting body development, and examined the response of heat sensitive and -resistant strains to heat stress. Our data revealed widely conserved genes involved in primordium development of F. filiformis, many of which originated before the emergence of the Agaricomycetes, indicating co-option for complex multicellularity during evolution. We also revealed several notable fruiting-specific genes, including the genes with conserved stipe-specific expression patterns and the others which related to sexual development, water absorption, basidium formation and sporulation, among others. Comparative analysis revealed that heat stress induced more genes in the heat resistant strain (M1) than in the heat sensitive one (XR). Of particular importance are the hsp70, hsp90 and fes1 genes, which may facilitate the adjustment to heat stress in the early stages of fruiting body development. These data highlighted novel genes involved in complex multicellular development in fungi and aid further studies on gene function and efforts to improve the productivity and heat tolerance in mushroom-forming fungi.

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

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

Isolation and evaluation of endophytic Bacillus tequilensis GYLH001 with potential application for biological control of Magnaporthe oryzae

Biological control is a promising measure in the control of plant disease. In the present study, we isolated 13 endophytic strains from Angelica dahurica. Among them, an endophytic strain which was named GYLH001 exhibited remarkable activity against Magnaporthe oryzae. 16S rRNA sequence analysis, biochemical and physiological proved that it is Bacillus tequilensis. The sterilized culture filtrate of GYLH001 can inhibit the growth of M.oryzae, which suggests the presence of secondary metabolites. Proved by experiment, GYLH001 can produce cellulase, protease, gelatinase, indole-3-acetic acid and 1-amino-cyclopropane-1-carboxylate deaminase. In addition, the temperature experiment showed that secondary metabolites produced by GYLH001 had good thermal stability. They can remain activity even heated at 100°C for 30 min. They also had good acid-resistance in heavily acidic condition. But under alkaline condition, the antifungal effect decreased significantly. By simulative field tests, the spraying of GYLH001 spore solution could prevent and treat rice blast. Through continuous separation and purification of sterilized culture filtrate and identification by mass spectrometry, the molecular weight of an active substance is 364.26. In the control of rice blast, B. tequilensis GYLH001 has potential as a biological control agent in agriculture.

New findings on phosphodiesterases, MoPdeH and MoPdeL, in Magnaporthe oryzae revealed by structural analysis

The cyclic adenosine monophosphate (cAMP) signalling pathway mediates signal communication and sensing during infection-related morphogenesis in eukaryotes. Many studies have implicated cAMP as a critical mediator of appressorium development in the rice blast fungus, Magnaporthe oryzae. The cAMP phosphodiesterases, MoPdeH and MoPdeL, as key regulators of intracellular cAMP levels, play pleiotropic roles in cell wall integrity, cellular morphology, appressorium formation and infectious growth in M. oryzae. Here, we analysed the roles of domains of MoPdeH and MoPdeL separately or in chimeras. The results indicated that the HD and EAL domains of MoPdeH are indispensable for its phosphodiesterase activity and function. Replacement of the MoPdeH HD domain with the L1 and L2 domains of MoPdeL, either singly or together, resulted in decreased cAMP hydrolysis activity of MoPdeH. All of the transformants exhibited phenotypes similar to that of the ΔMopdeH mutant, but also revealed that EAL and L1 play additional roles in conidiation, and that L1 is involved in infectious growth. We further found that the intracellular cAMP level is important for surface signal recognition and hyphal autolysis. The intracellular cAMP level negatively regulates Mps1-MAPK and positively regulates Pmk1-MAPK in the rice blast fungus. Our results provide new information to better understand the cAMP signalling pathway in the development, differentiation and plant infection of the fungus.

The thioredoxin MoTrx2 protein mediates reactive oxygen species (ROS) balance and controls pathogenicity as a target of the transcription factor MoAP1 in Magnaporthe oryzae

We have shown previously that the transcription factor MoAP1 governs the oxidative response and is important for pathogenicity in the rice blast fungus Magnaporthe oryzae. To explore the underlying mechanism, we have identified thioredoxin MoTrx2 as a target of MoAP1 in M. oryzae. Thioredoxins are highly conserved 12-kDa oxidoreductase enzymes containing a dithiol-disulfide active site, and function as antioxidants against free radicals, such as reactive oxygen species (ROS). In yeast and fungi, thioredoxins are important for oxidative stress tolerance and growth. To study the functions of MoTrx2, we generated ΔMotrx2 mutants that exhibit various defects, including sulfite assimilation, asexual and sexual differentiation, infectious hyphal growth and pathogenicity. We found that ΔMotrx2 mutants possess a defect in the scavenging of ROS during host cell invasion and in the active suppression of the rice defence response. We also found that ΔMotrx2 mutants display higher intracellular ROS levels during conidial germination, but lower peroxidase and laccase activities, which contribute to the attenuation in virulence. Given that the function of MoTrx2 overlaps that of MoAP1 in the stress response and pathogenicity, our findings further indicate that MoTrx2 is a key thioredoxin protein whose function is subjected to transcriptional regulation by MoAP1 in M. oryzae.

MoYcp4 is required for growth, conidiogenesis and pathogenicity in Magnaporthe oryzae

The transcription factor MoAP1 has been shown previously to be required for pathogenicity in Magnaporthe oryzae via mediation of the oxidative stress response. In the serial analysis gene expression database, it was found that expression of MoYcp4, a homologue of the Saccharomyces cerevisiae flavodoxin-like protein ScYcp4, was affected by MoAP1. Transcriptional analysis demonstrated that MoYCP4 was significantly up-regulated during conidiation, appressorium formation and infection. The growth rate of a ΔMoycp4 mutant was reduced slightly, but conidial production was increased significantly (more than 10-fold), compared with the wild-type strain. Although the rate of appressorium formation was unaffected, the appressorial turgor was abnormal and the ability to infect rice and barley was reduced, resulting in decreased pathogenicity. In summary, MoYcp4, a target of MoAP1, is involved in the growth, conidiogenesis and pathogenicity of M. oryzae. Our studies provide a comprehensive analysis of flavodoxin-like proteins and will aid in the study of pathogen-related molecular mechanisms.

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.

The ArfGAP protein MoGlo3 regulates the development and pathogenicity of Magnaporthe oryzae

The ADP ribosylation factor (Arf) and the coat protein complex I (COPI) are involved in vesicle transport. Together with GTPase-activating proteins (ArfGAPs) and guanine exchange factors (ArfGEFs) that regulate the activity of Arf, they govern vesicle formation, COPI trafficking and the maintenance of the Golgi complex. In an ongoing effort to study the role of membrane trafficking in pathogenesis of the rice blast fungus Magnaporthe oryzae, we identified MoGlo3 as an ArfGAP protein that is homologous to Glo3p of the budding yeast Saccharomyces cerevisiae. As suspected, MoGlo3 partially complements the function of yeast Glo3p. Consistent with findings in S. cerevisiae, MoGlo3 is localized to the Golgi, and that the localization is dependent on the conserved BoCCS domain. We found that MoGlo3 is highly expressed during conidiation and early infection stages and is required for vegetative growth, conidial production and sexual development. We further found that the ΔMoglo3 mutant is defective in endocytosis, scavenging of the reactive oxygen species, and in the response to endoplasmic reticulum (ER) stress. The combined effects result in failed appressorium function and decreased pathogenicity. Moreover, we provided evidence showing that the domains including the GAP, BoCCS and GRM are all important for normal MoGlo3 functions. Our studies further illustrate the importance of normal membrane trafficking in the physiology and pathogenicity of the rice blast fungus.

Isolation and evaluation of endophytic Streptomyces endus OsiSh-2 with potential application for biocontrol of rice blast disease

BACKGROUND

Biocontrol is a promising strategy in the control of rice blast disease. In the present study, we isolated and characterized a novel antagonist to the pathogen Magnaporthe oryzae from rice endophytic actinomycetes.

RESULTS

Out of 482 endophytic actinomycetes isolated from rice blast infected and healthy rice, Streptomyces endus OsiSh-2 exhibited remarkable in vitro antagonistic activity. Scanning electron microscopy observations of M. oryzae treated by OsiSh-2 revealed significant morphological alterations in hyphae. In 2-year field tests, the spraying of OsiSh-2 spore solution (10⁷  spores mL^-1 ) is capable of reducing rice blast disease severity by 59.64%. In addition, a fermentation broth of OsiSh-2 and its cell-free filtrates could inhibit the growth of M. oryzae, suggesting the presence of active enzymes and secondary metabolites. OsiSh-2 tested positive for polyketide synthase-I and nonribosomal peptide synthetase genes and can produce cellulase, protease, gelatinase, siderophore, indole-3-acetic acid and 1-amino-cyclopropane-1-carboxylate deaminase. A preliminary separation indicated that the methanol extract of OsiSh-2 could suppress the growth of pathogens. The major active component was identified as nigericin.

CONCLUSION

Endophytic S. endus OsiSh-2 has potential as a biocontrol agent against rice blast in agriculture. © 2016 Society of Chemical Industry.

MoVrp1, a putative verprolin protein, is required for asexual development and infection in the rice blast fungus Magnaporthe oryzae

Endocytosis is a crucial cellular process in eukaryotic cells which involves clathrin and/or adaptor proteins, lipid kinases, phosphatases and the actin cytoskeleton. Verprolin proteins, such as Vrp1 in Saccharomyces cerevisiae, are conserved family proteins that regulate actin binding and endocytosis. Here, we identified and characterized MoVrp1 as the yeast Vrp1 homolog in Magnaporthe oryzae. Deletion of the MoVRP1 gene resulted in defects in vegetative growth, asexual development, and infection of the host plant. The ∆Movrp1 mutants also exhibited decreased extracellular peroxidase and laccase activities and showed defects in colony pigmentation, hyphal surface hydrophobicity, cell wall integrity, autophagy, endocytosis, and secretion of avirulent effector. Our studies provided new evidences that MoVrp1 involved in actin cytoskeleton is important for growth, morphogenesis, cellular trafficking, and fungal pathogenesis.

Transcription Factors Encoded on Core and Accessory Chromosomes of Fusarium oxysporum Induce Expression of Effector Genes

Proteins secreted by pathogens during host colonization largely determine the outcome of pathogen-host interactions and are commonly called 'effectors'. In fungal plant pathogens, coordinated transcriptional up-regulation of effector genes is a key feature of pathogenesis and effectors are often encoded in genomic regions with distinct repeat content, histone code and rate of evolution. In the tomato pathogen Fusarium oxysporum f. sp. lycopersici (Fol), effector genes reside on one of four accessory chromosomes, known as the 'pathogenicity' chromosome, which can be exchanged between strains through horizontal transfer. The three other accessory chromosomes in the Fol reference strain may also be important for virulence towards tomato. Expression of effector genes in Fol is highly up-regulated upon infection and requires Sge1, a transcription factor encoded on the core genome. Interestingly, the pathogenicity chromosome itself contains 13 predicted transcription factor genes and for all except one, there is a homolog on the core genome. We determined DNA binding specificity for nine transcription factors using oligonucleotide arrays. The binding sites for homologous transcription factors were highly similar, suggesting that extensive neofunctionalization of DNA binding specificity has not occurred. Several DNA binding sites are enriched on accessory chromosomes, and expression of FTF1, its core homolog FTF2 and SGE1 from a constitutive promoter can induce expression of effector genes. The DNA binding sites of only these three transcription factors are enriched among genes up-regulated during infection. We further show that Ftf1, Ftf2 and Sge1 can activate transcription from their binding sites in yeast. RNAseq analysis revealed that in strains with constitutive expression of FTF1, FTF2 or SGE1, expression of a similar set of plant-responsive genes on the pathogenicity chromosome is induced, including most effector genes. We conclude that the Fol pathogenicity chromosome may be partially transcriptionally autonomous, but there are also extensive transcriptional connections between core and accessory chromosomes.

The Putative Protein Phosphatase MoYvh1 Functions Upstream of MoPdeH to Regulate the Development and Pathogenicity in Magnaporthe oryzae

Protein phosphatases are critical regulators in eukaryotic cells. For example, the budding yeast Saccharomyces cerevisiae dual specificity protein phosphatase (DSP) ScYvh1 regulates growth, sporulation, and glycogen accumulation. Despite such importance, functions of Yvh1 proteins in filamentous fungi are not well understood. In this study, we characterized putative protein phosphatase MoYvh1, an Yvh1 homolog in the rice blast fungus Magnaporthe oryzae. Deletion of the MoYVH1 gene resulted in significant reductions in vegetative growth, conidial production, and virulence. The ΔMoyvh1 mutant also displayed defects in cell-wall integrity and was hyposensitive to the exogenous osmotic stress. Further examination revealed that the ΔMoyvh1 mutant had defects in appressorium function and invasive hyphae growth, resulting attenuated pathogenicity. Interestingly, we found that MoYvh1 affects the scavenging of host-derived reactive oxygen species that promotes M. oryzae infection. Finally, overexpression of the phosphodiesterase MoPDEH suppressed the defects in conidia formation and pathogenicity of the ΔMoyvh1 mutant, suggesting MoYvh1 could regulate MoPDEH for its function. Our study reveals not only the importance of MoYvh1 proteins in growth, differentiation, and virulence of the rice blast fungus but, also, a genetic link between MoYvh1 and MoPDEH-cAMP signaling in this fungus.

The transcription factor VpCRZ1 is required for fruiting body formation and pathogenicity in Valsa pyri

Valsa pyri is a fatal pathogenic fungus that causes pear and apple canker disease. To date, its cellular development and pathogenicity have been poorly understood. In this study, a V. pyri Ca(2+)/calcineurin-dependent transcription factor CRZ1 (VpCRZ1) is identified and functionally characterized. The △VpCRZ1 mutant exhibits impaired pathogenicity and is no longer able to form fruiting body. Interestingly, this mutant also exhibits enhanced pigment deposition and increased resistance to cell wall perturbing agents including SDS, Congo red and calcofluor white (CFW). The expression levels of Congo red resistance genes (VpRCR1 and VpRCR2) and chitin synthetase genes (VpCHS2 and VpCHS6) are upregulated in the △VpCRZ1 mutant compared to the wild type. Furthermore, We show that a VpCRZ1: eGFP fusion protein localizes to the nucleus in a Ca(2+)-dependent manner similar to its homologs in other fungi, and that the VpFKS1, VpPMC1, VpPMC2, VpPMR1, and VpPMA1 genes are regulated by VpCRZ1 in response to Ca(2+) levels. Together, these results suggest that VpCRZ1 is a Ca(2+)-dependent transcription factor and required for regulating mycelial morphology, fruiting body formation, and virulence of this important pear and apple pathogen.

Penetration Peg Formation and Invasive Hyphae Development Require Stage-Specific Activation of MoGTI1 in Magnaporthe oryzae

The hemibiotrophic pathogen Magnaporthe oryzae causes one of the most destructive diseases in cultivated rice. Complex infection-related morphogenesis and production of various effectors are known to be important for successful colonization and disease development. In this study, we characterized the activation of the MoGTI1 transcription factor and its role in infection-related morphogenesis and effector gene expression. The Mogti1 mutant was nonpathogenic, although it was normal in appressorium formation and turgor generation. Close examination showed that Mogti1 was defective in penetration and growth of normal invasive hyphae. Deletion of MoGTI1 affected the expression of the majority of effector genes. The expression of MoGti1 appeared to be controlled by the Mps1 but not Pmk1 mitogen-activated protein kinase (MAPK), and the mps1 and Mogti1 mutants had similar phenotypes in plant infection and cell wall integrity defects. However, lack of MAPK phosphorylation sites and dispensability of the putative MAPK docking site suggested that MoGti1 is not a direct target of Mps1. Site-specific mutagenesis analyses showed that the putative protein kinase A phosphorylation site was not essential for localization of MoGti1 to the nucleus but important for its normal function. Although the cyclin-dependent kinase (CDK) phosphorylation site of MoGti1 is dispensable during vegetative growth and appressorium formation, the S77A mutation affected penetration and invasive growth. Localization of MoGti1(S77A)-green fluorescent protein to the nucleus in late stages of appressorium formation and during invasive growth was not observed, suggesting a stage-specific CDK phosphorylation of MoGti1. Overall, our data indicate that Mps1 may indirectly regulate the expression of MoGti1 in maintaining cell wall integrity, conidiation, and plant infection. MoGti1 is likely a stage-specific target of CDK and plays a crucial role in effector gene expression and morphogenesis related to the development of penetration pegs and invasive hyphae.

MoTup1 is required for growth, conidiogenesis and pathogenicity of Magnaporthe oryzae

The general transcriptional repressor Tup1 proteins play important regulatory roles in the growth and development of fungi. In this report, we characterized MoTup1, a protein homologous to Tup1 of Saccharomyces cerevisiae, from M. oryzae. Disruption of MoTUP1 resulted in severe mycelial growth reduction and a defect in conidiogenesis. We found that MoTup1 is required for the maintenance of cell wall integrity by regulating the expression of the genes involved in cell wall biosynthesis. Pathogenicity assays indicated that the ΔMotup1 mutants lost the ability to invade both rice and barley hosts. Moreover, observation of rice epidermis penetration showed that the hyphal tips of the mutants could still form appressorium-like structures, but were unable to invade host cells. Taken together, our results demonstrate that M. oryzae MoTup1 is an important regulatory factor in fungal growth, development and pathogenesis on hosts.

Global genome and transcriptome analyses of Magnaporthe oryzae epidemic isolate 98-06 uncover novel effectors and pathogenicity-related genes, revealing gene gain and lose dynamics in genome evolution

Genome dynamics of pathogenic organisms are driven by pathogen and host co-evolution, in which pathogen genomes are shaped to overcome stresses imposed by hosts with various genetic backgrounds through generation of a variety of isolates. This same principle applies to the rice blast pathogen Magnaporthe oryzae and the rice host; however, genetic variations among different isolates of M. oryzae remain largely unknown, particularly at genome and transcriptome levels. Here, we applied genomic and transcriptomic analytical tools to investigate M. oryzae isolate 98-06 that is the most aggressive in infection of susceptible rice cultivars. A unique 1.4 Mb of genomic sequences was found in isolate 98-06 in comparison to reference strain 70-15. Genome-wide expression profiling revealed the presence of two critical expression patterns of M. oryzae based on 64 known pathogenicity-related (PaR) genes. In addition, 134 candidate effectors with various segregation patterns were identified. Five tested proteins could suppress BAX-mediated programmed cell death in Nicotiana benthamiana leaves. Characterization of isolate-specific effector candidates Iug6 and Iug9 and PaR candidate Iug18 revealed that they have a role in fungal propagation and pathogenicity. Moreover, Iug6 and Iug9 are located exclusively in the biotrophic interfacial complex (BIC) and their overexpression leads to suppression of defense-related gene expression in rice, suggesting that they might participate in biotrophy by inhibiting the SA and ET pathways within the host. Thus, our studies identify novel effector and PaR proteins involved in pathogenicity of the highly aggressive M. oryzae field isolate 98-06, and reveal molecular and genomic dynamics in the evolution of M. oryzae and rice host interactions.

Genetic variations in pathogens, such as the causal agent of rice blast Magnaporthe oryzae, often lead to circumvention of disease-resistance cultivars. Previous genome-wide analyses of model organisms suggest that pathogen effectors are also rapidly evolving, especially in regions with high genome plasticity. However, genetic variations among different isolates remain largely unknown in M. oryzae, particularly at the genome and transcriptome levels. In this study, we provided a systematic genomic and interaction transcriptome profile for a dominant rice blast field isolate, resulting in identification of 134 candidate effectors. Two effectors, Iug6 and Iug9, and one pathogenicity-related (PaR) gene product, Iug18, were subjected to functional characterization. We found that Iug6 and Iug9 are located in the biotrophic interfacial complex (BIC) and their overexpression leads to suppression of defense-related gene expression in rice, while Iug18 appears to be a novel PaR protein. Our studies support the hypothesis that isolate-unique genes may serve as a source of genetic variability in the M. oryzae population encountering different environments. Our studies also facilitate further understanding of effectors and genomic variations in pathogenicity of M. oryzae.

MoMyb1 is required for asexual development and tissue-specific infection in the rice blast fungus Magnaporthe oryzae

Background

The Myb super-family of proteins contain a group of functionally diverse transcriptional activators found in plant, animal and fungus. Myb proteins are involved in cell proliferation, differentiation and apoptosis, and have crucial roles in telomeres. The purpose of this study was to characterize the biological function of Myb1 protein in the rice blast fungus Magnaporthe oryzae.

Results

We identified the Saccharomyces cerevisiae BAS1 homolog MYB1 in M. oryzae, named MoMyb1. MoMyb1 encodes a protein of 322 amino acids and has two SANT domains and is well conserved in various organisms. Targeted gene deletion of MoMYB1 resulted in a significant reduction in vegetative growth and showed defects in conidiation and conidiophore development. Quantitative RT-PCR analysis revealed that the transcription levels of several conidiophore-related genes were apparently decreased in the ΔMomyb1 mutant. Inoculation with mycelia mats displayed that the virulence of the ΔMomyb1 mutant was not changed on rice leaves but was non-pathogenic on rice roots in comparison to the wild type Guy11. In addition, ∆Momyb1 mutants showed increased resistance to osmotic stresses but more sensitive to cell wall stressor calcofluor white (CFW). Further analysis revealed that MoMyb1 has an important role in the cell wall biosynthesis pathway.

Conclusion

This study provides the evidence that MoMyb1 is a key regulator involved in conidiogenesis, stress response, cell wall integrity and pathogenesis on rice roots in the filamentous phytopathogen M. oryzae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0375-y) contains supplementary material, which is available to authorized users.

Threonine deaminase MoIlv1 is important for conidiogenesis and pathogenesis in the rice blast fungus Magnaporthe oryzae

Threonine deaminase is the first critical enzyme in the biosynthesis of branched-chain amino acids (BCAAs), which catalyzes threonine into NH2 and ketobutyrate acid. Previously, we identified and characterized two acetolactate synthases MoIlv2 and MoIlv6 that are involved in the second step of BCAA biosynthesis. Deletion of MoILV2 and MoILV6 resulted in auxotrophy for leucine, isoleucine, and valine and defects in conidiation, appressorial penetration, and pathogenicity. Here, we identified a threonine dehydratase, named MoIlv1, from Magnaporthe oryzae. MoIlv1 is a homolog of Saccharomyces cerevisiae Ilv1p, which has an important role in the biosynthesis of isoleucine. To characterize the function of MoIlv1, a ΔMoilv1 knock-out mutant was generated and analyzed. Disruption of MoILV1 resulted in abnormal conidial morphology, reduced conidiation, limited appressorium-mediated penetration, and attenuated virulence on both barley and rice seedlings. Further analysis by domain-specific deletion revealed that the PALP domain is indispensable for MoIlv1 function. Our study indicates that MoIlv1 is a protein involved in isoleucine biosynthesis that underlies the complex process governing morphogenesis, appressorium formation, invasive hyphae growth, and pathogenicity.

Systematic analysis of Zn2Cys6 transcription factors required for development and pathogenicity by high-throughput gene knockout in the rice blast fungus

Because of great challenges and workload in deleting genes on a large scale, the functions of most genes in pathogenic fungi are still unclear. In this study, we developed a high-throughput gene knockout system using a novel yeast-Escherichia-Agrobacterium shuttle vector, pKO1B, in the rice blast fungus Magnaporthe oryzae. Using this method, we deleted 104 fungal-specific Zn(2)Cys(6) transcription factor (TF) genes in M. oryzae. We then analyzed the phenotypes of these mutants with regard to growth, asexual and infection-related development, pathogenesis, and 9 abiotic stresses. The resulting data provide new insights into how this rice pathogen of global significance regulates important traits in the infection cycle through Zn(2)Cys(6)TF genes. A large variation in biological functions of Zn(2)Cys(6)TF genes was observed under the conditions tested. Sixty-one of 104 Zn(2)Cys(6) TF genes were found to be required for fungal development. In-depth analysis of TF genes revealed that TF genes involved in pathogenicity frequently tend to function in multiple development stages, and disclosed many highly conserved but unidentified functional TF genes of importance in the fungal kingdom. We further found that the virulence-required TF genes GPF1 and CNF2 have similar regulation mechanisms in the gene expression involved in pathogenicity. These experimental validations clearly demonstrated the value of a high-throughput gene knockout system in understanding the biological functions of genes on a genome scale in fungi, and provided a solid foundation for elucidating the gene expression network that regulates the development and pathogenicity of M. oryzae.

System-wide characterization of bZIP transcription factor proteins involved in infection-related morphogenesis of Magnaporthe oryzae

The basic leucine zipper (bZIP) domain-containing transcription factors (TFs) function as key regulators of cellular growth and differentiation in eukaryotic organisms including fungi. We have previously identified MoAp1 and MoAtf1 as bZIP TFs in Magnaporthe oryzae and demonstrated that they regulate the oxidative stress response and are critical in conidiogenesis and pathogenicity. Studies of bZIP proteins could provide a novel strategy for controlling rice blast, but a systematic examination of the bZIP proteins has not been carried out. Here, we identified 19 additional bZIP TFs and characterized their functions. We found that the majority of these TFs exhibit active functions, most notably, in conidiogenesis. We showed that MoHac1 regulates the endoplasmic reticulum stress response through a conserved unfolded protein response pathway, MoMetR controls amino acid metabolism to govern growth and differentiation, and MoBzip10 governs appressorium function and invasive hyphal growth. Moreover, MoBzip5 participates in appressorium formation through a pathway distinct from that MoBzip10, and MoMeaB appears to exert a regulatory role through nutrient uptake and nitrogen utilization. Collectively, our results provide insights into shared and specific functions associated with each of these TFs and link the regulatory roles to the fungal growth, conidiation, appressorium formation, host penetration and pathogenicity.

The global regulator FfSge1 is required for expression of secondary metabolite gene clusters but not for pathogenicity in Fusarium fujikuroi

The plant pathogenic fungus Fusarium fujikuroi is the causal agent of bakanae disease on rice due to its ability to produce gibberellins. Besides these phytohormones, F. fujikuroi is able to produce several other secondary metabolites (SMs). Although much progress has been made in the field of secondary metabolism, the transcriptional regulation of SM biosynthesis is complex and still incompletely understood. Environmental conditions, global as well as pathway-specific regulators and chromatin remodelling have been shown to play major roles. Here, the role of FfSge1, a homologue of the morphological switch regulators Wor1 and Ryp1 in Candida albicans and Histoplasma capsulatum, respectively, is explored with emphasis on secondary metabolism. FfSge1 is not required for formation of conidia and pathogenicity but is involved in vegetative growth. Transcriptome analysis of the mutant Δffsge1 compared with the wild type, as well as comparative chemical analysis between the wild type, Δffsge1 and OE:FfSGE1, revealed that FfSge1 functions as a global activator of secondary metabolism in F. fujikuroi. Double mutants of FfSGE1 and other SM regulatory genes brought insights into the hierarchical regulation of secondary metabolism. In addition, FfSge1 is also required for expression of a yet uncharacterized SM gene cluster containing a non-canonical non-ribosomal peptide synthetase.