A novel cascade allows Metarhizium robertsii to distinguish cuticle and hemocoel microenvironments during infection of insects

Histone deacetylase HDAC3 regulates ergosterol production for oxidative stress tolerance in the entomopathogenic and endophytic fungus Metarhizium robertsii

Oxidative stress is encountered by fungi in almost all niches, resulting in fungal degeneration or even death. Fungal tolerance to oxidative stress has been extensively studied, but the current understanding of the mechanisms regulating oxidative stress tolerance in fungi remains limited. The entomopathogenic and endophytic fungus Metarhizium robertsii encounters oxidative stress when it infects insects and develops a symbiotic relationship with plants, and we found that host reactive oxygen species (ROSs) greatly limited fungal growth in both insects and plants. We identified a histone H3 deacetylase (HDAC3) that catalyzed the deacetylation of lysine 56 of histone H3. Deleting Hdac3 significantly reduced the tolerance of M. robertsii to oxidative stress from insects and plants, thereby decreasing fungal ability to colonize the insect hemocoel and plant roots. HDAC3 achieved this by regulating the expression of three genes in the ergosterol biosynthesis pathway, which includes the lanosterol synthase gene Las1. The deletion of Hdac3 or Las1 reduced the ergosterol content and impaired cell membrane integrity. This resulted in an increase in ROS accumulation in fungal cells that were thus more sensitive to oxidative stress. We further showed that HDAC3 regulated the expression of the three ergosterol biosynthesis genes in an indirect manner. Our work significantly advances insights into the epigenetic regulation of oxidative stress tolerance and the interactions between M. robertsii and its plant and insect hosts.IMPORTANCEOxidative stress is a common challenge encountered by fungi that have evolved sophisticated mechanisms underlying tolerance to this stress. Although fungal tolerance to oxidative stress has been extensively investigated, the current understanding of the mechanisms for fungi to regulate oxidative stress tolerance remains limited. In the model entomopathogenic and plant symbiotic fungus Metarhizium robertsii, we found that the histone H3 deacetylase HDAC3 regulates the production of ergosterol, an essential cell membrane component. This maintains the cell membrane integrity to resist the oxidative stress derived from the insect and plant hosts for successful infection of insects and development of symbiotic associates with plants. Our work provides significant insights into the regulation of oxidative stress tolerance in M. robertsii and its interactions with insects and plants.

Conserved signaling modules regulate filamentous growth in fungi: a model for eukaryotic cell differentiation

Eukaryotic organisms are composed of different cell types with defined shapes and functions. Specific cell types are produced by the process of cell differentiation, which is regulated by signal transduction pathways. Signaling pathways regulate cell differentiation by sensing cues and controlling the expression of target genes whose products generate cell types with specific attributes. In studying how cells differentiate, fungi have proved valuable models because of their ease of genetic manipulation and striking cell morphologies. Many fungal species undergo filamentous growth-a specialized growth pattern where cells produce elongated tube-like projections. Filamentous growth promotes expansion into new environments, including invasion into plant and animal hosts by fungal pathogens. The same signaling pathways that regulate filamentous growth in fungi also control cell differentiation throughout eukaryotes and include highly conserved mitogen-activated protein kinase (MAPK) pathways, which is the focus of this review. In many fungal species, mucin-type sensors regulate MAPK pathways to control filamentous growth in response to diverse stimuli. Once activated, MAPK pathways reorganize cell polarity, induce changes in cell adhesion, and promote the secretion of degradative enzymes that mediate access to new environments. However, MAPK pathway regulation is complicated because related pathways can share components with each other yet induce unique responses (i.e. signal specificity). In addition, MAPK pathways function in highly integrated networks with other regulatory pathways (i.e. signal integration). Here, we discuss signal specificity and integration in several yeast models (mainly Saccharomyces cerevisiae and Candida albicans) by focusing on the filamentation MAPK pathway. Because of the strong evolutionary ties between species, a deeper understanding of the regulation of filamentous growth in established models and increasingly diverse fungal species can reveal fundamentally new mechanisms underlying eukaryotic cell differentiation.

Comparative genomics of Metarhizium brunneum strains V275 and ARSEF 4556: unraveling intraspecies diversity

Entomopathogenic fungi belonging to the Order Hypocreales are renowned for their ability to infect and kill insect hosts, while their endophytic mode of life and the beneficial rhizosphere effects on plant hosts have only been recently recognized. Understanding the molecular mechanisms underlying their different lifestyles could optimize their potential as both biocontrol and biofertilizer agents, as well as the wider appreciation of niche plasticity in fungal ecology. This study describes the comprehensive whole genome sequencing and analysis of one of the most effective entomopathogenic and endophytic EPF strains, Metarhizium brunneum V275 (commercially known as Lalguard Met52), achieved through Nanopore and Illumina reads. Comparative genomics for exploring intraspecies variability and analyses of key gene sets were conducted with a second effective EPF strain, M. brunneum ARSEF 4556. The search for strain- or species-specific genes was extended to M. brunneum strain ARSEF 3297 and other species of genus Metarhizium, to identify molecular mechanisms and putative key genome adaptations associated with mode of life differences. Genome size differed significantly, with M. brunneum V275 having the largest genome amongst M. brunneum strains sequenced to date. Genome analyses revealed an abundance of plant-degrading enzymes, plant colonization-associated genes, and intriguing intraspecies variations regarding their predicted secondary metabolic compounds and the number and localization of Transposable Elements. The potential significance of the differences found between closely related endophytic and entomopathogenic fungi, regarding plant growth-promoting and entomopathogenic abilities, are discussed, enhancing our understanding of their diverse functionalities and putative applications in agriculture and ecology.

The secretory protein COA1 enables Metarhizium robertsii to evade insect immune recognition during cuticle penetration

The interplay between the insect immune system and entomopathogenic fungi during cuticle penetration is not yet fully understood. Here, we show that a secretory protein COA1 (coat of appressorium 1) from Metarhizium robertsii, an entomopathogenic fungus causing diseases in a wide range of insects, is required to avoid host immune recognition during cuticle penetration. COA1 is highly expressed on the cuticle and translocated to the cell surface, where it directly binds with and masks carbohydrates of the fungal cell wall to avoid provoking the host's intense immune response. Deletion of Coa1 results in a robust immune response, leading to a reduction in bacterial load in both the gut and hemocoel and ultimately attenuating fungal virulence. Our work reveals a novel cell surface protein indispensable for fungal pathogenicity via masking cell wall carbohydrates to avert a hypersensitive response from the host.

A novel transcription factor Sdr1 involving sulfur depletion response in fission yeast

In the fission yeast Schizosaccharomyces pombe, the response to sulfur depletion has been less studied compared to the response to nitrogen depletion. Our study reveals that the fission yeast gene, SPCC417.09c, plays a significant role in the sulfur depletion response. This gene encodes a protein with a Zn2Cys6 fungal-type DNA-binding domain and a transcription factor domain, and we have named it sdr1+ (sulfur depletion response 1). Interestingly, while sulfur depletion typically induces autophagy akin to nitrogen depletion, we found that autophagy was not induced under sulfur depletion in the absence of sdr1+. This suggests that sdr1+ is necessary for the induction of autophagy under conditions of sulfur depletion. Although sdr1+ is not essential for the growth of fission yeast, its overexpression, driven by the nmt1 promoter, inhibits growth. This implies that Sdr1 may possess cell growth-inhibitory capabilities. In addition, our analysis of Δsdr1 cells revealed that sdr1+ also plays a role in regulating the expression of genes associated with the phosphate depletion response. In conclusion, our study introduces Sdr1 as a novel transcription factor that contributes to an appropriate cellular nutrient starvation response. It does so by inhibiting inappropriate cell growth and inducing autophagy in response to sulfur depletion.

A transcription factor-mediated regulatory network controls fungal pathogen colonization of insect body cavities

Successful host tissue colonization is crucial for fungal pathogens to cause mycosis and complete the infection cycle, in which fungal cells undergo a series of morphological transition-included cellular events to combat with hosts. However, many transcription factors (TFs) and their mediated networks regulating fungal pathogen colonization of host tissue are not well characterized. Here, a TF (BbHCR1)-mediated regulatory network was identified in an insect pathogenic fungus, Beauveria bassiana, that controlled insect hemocoel colonization. BbHCR1 was highly expressed in fungal cells after reaching insect hemocoel and controlled the yeast (in vivo blastospores)-to-hyphal morphological switch, evasion of immune defense response, and fungal virulence. Comparative analysis of RNA sequencing and chromatin immunoprecipitation sequencing identified a core set of BbHCR1 target genes during hemocoel colonization, in which abaA and brlA were targeted to limit the rapid switch from blastospores to hyphae and fungal virulence. Two targets encoding hypothetical proteins, HP1 and HP2, were activated and repressed by BbHCR1, respectively, which acted as a virulence factor and repressor, respectively, suggesting that BbHCR1 activated virulence factors but repressed virulence repressors during the colonization of insect hemocoel. BbHCR1 tuned the expression of two dominant hemocoel colonization-involved metabolite biosynthetic gene clusters, which linked its regulatory role in evasion of immune response. Those functions of BbHCR1 were found to be collaboratively regulated by Fus3- and Hog1-MAP kinases via phosphorylation. These findings have drawn a regulatory network in which Fus3- and Hog1-MAP kinases phosphorylate BbHCR1, which in turn controls the colonization of insect body cavities by regulating fungal morphological transition and virulence-implicated genes.IMPORTANCEFungal pathogens adopt a series of tactics for successful colonization in host tissues, which include morphological transition and the generation of toxic and immunosuppressive molecules. However, many transcription factors (TFs) and their linked pathways that regulate tissue colonization are not well characterized. Here, we identified a TF (BbHCR1)-mediated regulatory network that controls the insect fungal pathogen, Beauveria bassiana, colonization of insect hemocoel. During these processes, BbHCR1 targeted the fungal central development pathway for the control of yeast (blastospores)-to-hyphae morphological transition, activated virulence factors, repressed virulence repressors, and tuned the expression of two dominant hemocoel colonization-involved immunosuppressive and immunostimulatory metabolite biosynthetic gene clusters. The BbHCR1 regulatory function was governed by Fus3- and Hog1-MAP kinases. These findings led to a new regulatory network composed of Fus3- and Hog1-MAP kinases and BbHCR1 that control insect body cavity colonization by regulating fungal morphological transition and virulence-implicated genes.

Utilization of plant-derived sugars and lipids are coupled during colonization of rhizoplane and rhizosphere by the fungus Metarhizium robertsii

Plant-derived sugars and lipids are key nutritional sources for plant associated fungi. However, the relationship between utilization of host-derived sugars and lipids during development of the symbiotic association remains unknown. Here we show that the fungus Metarhizium robertsii also needs plant-derived lipids to develop symbiotic relationship with plants. The fatty acid binding proteins FABP1 and FABP2 are important for utilization of plant-derived lipids as the deletion of Fabp1 and Fabp2 significantly reduced the ability of M. robertsii to colonize rhizoplane and rhizosphere of maize and Arabidopsis thaliana. Deleting Fabp1 and Fabp2 increased sugar utilization by upregulating six sugar transporters, and this explains why deleting the monosaccharide transporter gene Mst1, which plays an important role in utilization of plant-derived sugars, had no impact on the ability of the double-gene deletion mutant ΔFabp1::ΔFabp2 to colonize plant roots. FABP1 and FABP2 were also found in other plant-associated Metarhizium species, and they were highly expressed in the medium using the tomato root exudate as the sole carbon and nitrogen source, suggesting that they could be also important for these species to develop symbiotic relationship with plants. In conclusion, we discovered that utilization of plant-derived sugars and lipids are coupled during colonization of rhizoplane and rhizosphere by M. robertsii.

Metarhizium robertsii COH1 functionally complements Schizosaccharomyces pombe Ecl family proteins

The fission yeast Schizosaccharomyces pombe ecl family genes respond to various starvation signals and induce appropriate intracellular responses, including the extension of chronological lifespan and induction of sexual differentiation. Herein, we propose that the colonization of hemocoel 1 (COH1) protein of Metarhizium robertsii, an insect-pathogenic fungus, is a functional homolog of S. pombe Ecl1 family proteins.

Fungal infection of insects: molecular insights and prospects

Entomopathogenic fungi (EPF) distribute in different fungal phyla with variable host ranges and play essential role in regulating insect populations by infecting hosts via cuticle penetration. The representative ascomycete EPF of Metarhizium and Beauveria species have been widely used in mechanistic investigations of fungus-insect interactions and as ecofriendly mycoinsecticides. Here, we review the function of diverse genes, pathways, and secondary metabolites associated with EPF stepwise infections. In particular, emerging evidence has shown that EPF have to outcompete insect ectomicrobiotas prior to penetrating cuticles, and subvert or evade host antifungal immunity by using effector-like proteins and chemicals like plant pathogens. Future prospects are discussed for a better understanding of fungal pathobiology, which will provide novel insights into microbe-animal interactions.

Fights on the surface prior to fungal invasion of insects

Entomopathogenic fungi (EPF) infect insects by landing on and penetrating cuticles. Emerging evidence has shown that, prior to the invasion of insects, fungal cells have to battle and overcome diverse challenges, including the host behavioral defenses, colonization resistance mediated by ectomicrobiotas, host recognition, and generation of enough penetration pressure. The ascomycete EPF such as Metarhizium and Beauveria can thus produce adhesive proteins and/or the exopolysaccharide mucilage to tightly glue fungal cells on cuticles. Producing antimicrobial peptides and chemical compounds can enable EPF to outcompete cuticular defensive microbes. The use of divergent membrane receptors, accumulation, and quick degradation of lipid droplets in conidial cells can help EPF recognize proper hosts and build up cellular turgor to breach cuticles for systematic invasion. Further investigations are still required to unveil the multifaceted and intricate relationships between EPF and insect hosts.

Suppression of Drosophila antifungal immunity by a parasite effector via blocking GNBP3 and GNBP-like 3, the dual receptors for β-glucans

The tactics used by animal pathogens to combat host immunity are largely unclear. Here, we report the depiction of the virulence-required effector Tge1 deployed by the entomopathogen Metarhizium robertsii to suppress Drosophila antifungal immunity. Tge1 can target both GNBP3 and GNBP-like 3 (GL3), and the latter can bind to β-glucans like GNBP3, whereas the glucan binding by both receptors can be attenuated by Tge1. As opposed to the surveillance GNBP3, GL3 is inducible in Drosophila depending on the Toll pathway via a positive feedback loop mechanism. Losses of GNBP3 and GL3 genes result in the deregulations of protease cascade, Spätzle maturation, and antimicrobial gene expressions in Drosophila upon fungal challenges. Fly survival assays confirm that GL3 plays a more essential role than GNBP3 in combating fungal infections. In addition to evidencing the gene-for-gene interactions between fungi and insects, our data advance insights into Drosophila antifungal immunity.

Fungus-insect interactions beyond bilateral regimes: the importance and strategy to outcompete host ectomicrobiomes by fungal parasites

Fungus-insect interactions have long been investigated at the bilateral level to unveil the factors involved in mediating fungal entomopathogenicity and insect antifungal immunity. Emerging evidence has shown that insect cuticles are inhabited by different bacteria that can delay and deter fungal parasite infections. Entomopathogenic fungi (EPF), however, have evolved strategies to combat the colonization resistance mediated by insect ectomicrobiomes by producing antimicrobial peptides or antibiotic compounds. Deprivation of micronutrients may also be employed by EPF to counteract the ectomicrobiome antagonism. Further investigations of insect ectomicrobiome assemblage and fungal factors involved in outcompeting cuticular microbiomes may benefit the development of cost-effective mycoinsecticides while protecting ecologically and economically important insect species.

The ecl family gene ecl3+ is induced by phosphate starvation and contributes to sexual differentiation in fission yeast

In Schizosaccharomyces pombe, ecl family genes are induced by several signals, such as starvation of various nutrients, including sulfur, amino acids and Mg2+, and environmental stress, including heat or oxidative stress. These genes mediate appropriate cellular responses and contribute to the maintenance of cell viability and induction of sexual differentiation. Although this yeast has three ecl family genes with overlapping functions, any environmental conditions that induce ecl3+ remain unidentified. We demonstrate that ecl3+ is induced by phosphate starvation, similar to its chromosomally neighboring genes, pho1+ and pho84+, which respectively encode an extracellular acid phosphatase and an inorganic phosphate transporter. ecl3+ expression was induced by the transcription factor Pho7 and affected by the cyclin-dependent kinase (CDK)-activating kinase Csk1. Phosphate starvation induced G1 arrest and sexual differentiation via ecl family genes. Biochemical analyses suggested that this G1 arrest was mediated by the stabilization of the CDK inhibitor Rum1, which was dependent on ecl family genes. This study shows that ecl family genes are required for appropriate responses to phosphate starvation and provides novel insights into the diversity and similarity of starvation responses.

Suppression of the insect cuticular microbiomes by a fungal defensin to facilitate parasite infection

Insects can assemble defensive microbiomes on their body surfaces to defend against fungal parasitic infections. The strategies employed by fungal pathogens to combat host cuticular microbiotas remains unclear. Here, we report the identification and functional characterization of the defensin-like antimicrobial gene BbAMP1 encoded by the entomopathogenic fungus Beauveria bassiana. The mature peptide of BbAMP1 can coat fungal spores and can be secreted by the fungus to target and damage Gram-positive bacterial cells. Significant differences in insect survival were observed between the wild-type and BbAMP1 mutant strains during topical infection but not during injection assays that bypassed insect cuticles. Thus, BbAMP1 deletion considerably reduced fungal virulence while gene overexpression accelerated the fungal colonization of insects compared with the wild-type strain in natural infections. Topical infection of axenic Drosophila adults evidenced no difference in fly survivals between strains. However, the gnotobiotic infections with the addition of Gram-positive but not Gram-negative bacterial cells in fungal spore suspensions substantially increased the survival of the flies treated with ∆BbAMP1 compared to those infected by the wild-type and gene-overexpression strains. Bacterial colony counts and microbiome analysis confirmed that BbAMP1 could assist the fungus to manipulate insect surface bacterial loads. This study reveals that fungal defensin can suppress the host surface defensive microbiomes, which underscores the importance to extend the research scope of fungus-host interactions.

Bioremediation of mercury-polluted soil and water by the plant symbiotic fungus Metarhizium robertsii

Fungi are central to every terrestrial and many aquatic ecosystems, but the mechanisms underlying fungal tolerance to mercury, a global pollutant, remain unknown. Here, we show that the plant symbiotic fungus Metarhizium robertsii degrades methylmercury and reduces divalent mercury, decreasing mercury accumulation in plants and greatly increasing their growth in contaminated soils. M. robertsii does this by demethylating methylmercury via a methylmercury demethylase (MMD) and using a mercury ion reductase (MIR) to reduce divalent mercury to volatile elemental mercury. M. robertsii can also remove methylmercury and divalent mercury from fresh and sea water even in the absence of added nutrients. Overexpression of MMD and MIR significantly improved the ability of M. robertsii to bioremediate soil and water contaminated with methylmercury and divalent mercury. MIR homologs, and thereby divalent mercury tolerance, are widespread in fungi. In contrast, MMD homologs were patchily distributed among the few plant associates and soil fungi that were also able to demethylate methylmercury. Phylogenetic analysis suggests that fungi could have acquired methylmercury demethylase genes from bacteria via two independent horizontal gene transfer events. Heterologous expression of MMD in fungi that lack MMD homologs enabled them to demethylate methylmercury. Our work reveals the mechanisms underlying mercury tolerance in fungi, and may provide a cheap and environmentally friendly means of cleaning up mercury pollution.

High susceptibility of Tetranychus merganser (Acari: Tetranychidae), an emergent pest of the tropical crop Carica papaya, towards Metarhizium anisopliae s.l. and Beauveria bassiana strains

Background

The mite Tetranychus merganser is considered to be an emerging pest of various crops in tropical countries. It is one of the most detrimental pests in the papaya orchards of some regions of México. The current field control of Tetranychus spp. involves the extensive use of chemicals that have some degree of toxicity to humans or the environment and may cause selective resistance. The use of biological alternatives such as parasitoids and mite predators have limited effectiveness. In order to find effective but non-toxic alternatives for mite pest management, bio-products that are able to be mass produced and applied to large production areas have been sought, including the entomopathogen fungi. B. bassiana and M. anisopliae s.l. are the fungi most extensively used for the biological control of insect pests. Although they do not cause natural epizootic diseases in mites, there are reports that show that they infect T. urticae, and should be evaluated for use in the biological control of papaya's mite pests.

Methods

A T. merganser colony was established and the susceptibility of adult females to 30 entomopathogenic fungi strains was evaluated under laboratory conditions with an in vitro mass screening bioassay. Ten strains of Metarhizium anisopliae sensu lato (s.l.), eleven of Beauveria bassiana, nine of Lecanicillium sp. and one of Hirsutella thompsonii var. sinematosa were tested. The infectivity of adult females was evaluated calculating the percentage of mortality. To calculate the LC₅₀ and LT₅₀ of the most virulent strains, a bioassay was performed using serial concentrations (1×10⁴-1×10⁸ conidia/mL) for each strain. Strains showing ability to infect eggs laid were evaluated with a novel egg-infectivity bioassay. The internal transcribed spacer (ITS) region of the more lethal strains were sequenced.

Results

T. merganser and T. urticae were found in orchards of Carica papaya (Maradol variety and Tainung hybrid) in Campeche, México. All tested strains of M. anisopliae s.l. and B. bassiana were infectious to the adult female of T. merganser at a concentration of 1×10⁸ conidia/mL. Six strains of M. anisopliae (Ma002, Ma003, Ma004, Ma005, Ma014 and Ma034) caused 100% mortality, and one of B. bassiana (Bb016) caused 95% mortality. The most virulent was Ma034, with an LC₅₀ of 1.73×10⁶ conidia/mL followed by Ma005 and Ma003. Ma005 and Ma034 were the fastest strains to reach LT_50,achieving this in less than 3.7 days. Additionally, Ma034 and Ma014 strains were infectious to more than 70% of the eggs.

Conclusions

T. merganser and T. urticae are present in the papaya orchards of Campeche, México. The high susceptibility of T. merganser adult females and eggs toward several M. anisopliae s.l. or B. bassiana strains suggests that these fungi are a viable alternative to control this emergent pest. The most virulent strain, Ma034, was also infective to eggs, and is the most promising to be tested in the field.

Metarhizium: an opportunistic middleman for multitrophic lifestyles

Metarhizium spp. mediate multiple interactions that are usually positive with respect to their long-term plant environment, and negative with respect to short-lived hosts. In particular, their ability to kill a wide range of insects maximizes protection to the plants and provides a resource of nitrogen that the fungus trades with the plant for carbon. Here, we highlight emerging concepts underlying Metarhizium-plant-insect interactions. Experiments on model systems have provided detailed mechanistic knowledge of how these fungi interact with plants and insects, and a greater understanding of the evolutionary forces driving these interactions. However, further integration of studies at the ecological and mechanistic level is needed to evaluate the importance of Metarhizium's multitrophic interactions to the structuring of natural communities.

New Downstream Signaling Branches of the Mitogen-Activated Protein Kinase Cascades Identified in the Insect Pathogenic and Plant Symbiotic Fungus Metarhizium robertsii

Fungi rely on major signaling pathways such as the MAPK (Mitogen-Activated Protein Kinase) signaling pathways to regulate their responses to fluctuating environmental conditions, which is vital for fungi to persist in the environment. The cosmopolitan Metarhizium fungi have multiple lifestyles and remarkable stress tolerance. Some species, especially M. robertsii, are emerging models for investigating the mechanisms underlying ecological adaptation in fungi. Here we review recently identified new downstream branches of the MAPK cascades in M. robertsii, which controls asexual production (conidiation), insect infection and selection of carbon and nitrogen nutrients. The Myb transcription factor RNS1 appears to be a central regulator that channels information from the Fus3- and Slt2-MAPK cascade to activate insect infection and conidiation, respectively. Another hub regulator is the transcription factor AFTF1 that transduces signals from the Fus3-MAPK and the membrane protein Mr-OPY2 for optimal formation of the infection structures on the host cuticle. Homologs of these newly identified regulators are found in other Metarhizium species and many non-Metarhizium fungi, indicating that these new downstream signaling branches of the MAPK cascades could be widespread.

Regulatory Roles of Histone Modifications in Filamentous Fungal Pathogens

Filamentous fungal pathogens have evolved diverse strategies to infect a variety of hosts including plants and insects. The dynamic infection process requires rapid and fine-tuning regulation of fungal gene expression programs in response to the changing host environment and defenses. Therefore, transcriptional reprogramming of fungal pathogens is critical for fungal development and pathogenicity. Histone post-translational modification, one of the main mechanisms of epigenetic regulation, has been shown to play an important role in the regulation of gene expressions, and is involved in, e.g., fungal development, infection-related morphogenesis, environmental stress responses, biosynthesis of secondary metabolites, and pathogenicity. This review highlights recent findings and insights into regulatory mechanisms of histone methylation and acetylation in fungal development and pathogenicity, as well as their roles in modulating pathogenic fungi-host interactions.

An inactivating mutation in the vacuolar arginine exporter gene Vae results in culture degeneration in the fungus Metarhizium robertsii

Culture degeneration usually results in great commercial losses in the economically important filamentous fungi, but the genetic causes of the degeneration remain elusive. In the fungus Metarhizium robertsii, we found that deletion of the vacuolar arginine exporter gene Vae caused culture degeneration. Compared to the WT strain, the mutant showed increased apoptosis, reactive oxygen species (ROS) level and mitochondrial membrane potential collapse, reduced conidial yield and abnormal lipid droplet formation. The extent of the degeneration in the mutant gradually increased over the successive subculturing, which eventually became irreversible; compared to the third subculture of the mutant, the seventh subculture showed a lower conidial yield and pathogenicity to insects, stronger apoptosis, higher ROS level and a smaller number of conidial lipid droplets. Incorporation of the genomic clone of Vae could not restore the WT phenotypes in the seventh subculture, but could in the third one. Loss-of-function in Vae resulted in vacuolar arginine accumulation and reduction in the cytosolic arginine. This downregulated the expression of the regulator CAG9 of G protein signalling pathway, which accounted for most of the phenotypic changes associated with the degeneration of the mutant. We identified a deleterious mutation that causes culture degeneration in a filamentous fugus.

Slt2-MAPK/RNS1 Controls Conidiation via Direct Regulation of the Central Regulatory Pathway in the Fungus Metarhizium robertsii

Ascomycete fungi usually produce small hydrophobic asexual conidia that are easily dispersed and essential for long-term survival under a variety of environmental conditions. Several upstream signaling regulators have been documented to control conidiation via regulation of the central regulatory pathway that contains the transcription factors BrlA, AbaA and WetA. Here, we showed that the Slt2-MAPK signaling pathway and the transcription factor RNS1 constitute a novel upstream signaling cascade that activates the central regulatory pathway for conidiation in the Ascomycetes fungus M. robertsii. The BrlA gene has two overlapping transcripts BrlAα and BrlAβ; they have the same major ORF, but the 5' UTR of BrlAβ is 835 bp longer than the one of BrlAα. During conidiation, Slt2 phosphorylates the serine residue at the position 306 in RNS1, which self-induces. RNS1 binds to the BM2 motif in the promoter of the BrlA gene and induces the expression of the transcript BlrAα, which in turn activates the expression of the genes AbaA and WetA. In conclusion, the Slt2/RNS1 cascade represents a novel upstream signaling pathway that initiates conidiation via direct activation of the central regulatory pathway. This work provides significant mechanistic insights into the production of asexual conidia in an Ascomycete fungus.

The Sugar Transporter MST1 Is Involved in Colonization of Rhizosphere and Rhizoplane by Metarhizium robertsii

It is widely recognized that plant-symbiotic fungi are supported by photosynthates; however, little is known about the molecular mechanisms underlying the utilization of plant-derived sugars by rhizospheric fungi. In the insect-pathogenic and plant-symbiotic fungus Metarhizium robertsii, we previously showed that the utilization of oligosaccharides by the transporter MRT (Metarhizium raffinose transporter) is important for rhizosphere competency. In this study, we identified a novel monosaccharide transporter (MST1) that is involved in the colonization of the rhizoplane and acts additively with MRT to colonize the rhizosphere. MST1 is not involved in infection of insects by M. robertsii. MST1 is an H⁺ symporter and is able to transport a broad spectrum of monosaccharides, including glucose, sorbose, mannose, rhamnose, and fructose. Deletion of the Mst1 gene impaired germination and mycelial growth in medium containing the sugars that it can transport. Homologs of MST1 were widely found in many fungi, including plant symbionts such as Trichoderma spp. and mycorrhizal fungi and plant pathogens such as Fusarium spp. This work significantly advances insights into the development of symbiotic relationships between plants and fungi. IMPORTANCE Over 90% of all vascular plant species develop an intimate symbiosis with fungi, which has an enormous impact on terrestrial ecosystems. It is widely recognized that plant-symbiotic fungi are supported by photosynthates, but little is known about the mechanisms for fungi to utilize plant-derived carbon sources. In the fungus Metarhizium robertsii, we identified a novel monosaccharide transporter (MST1) that is an H⁺ symporter and can transport a broad spectrum of monosaccharides, including glucose, sorbose, mannose, rhamnose, and fructose. MST1 is involved in the colonization of the rhizoplane and acts additively with the previously characterized oligosaccharide transporter MRT to colonize the rhizosphere. Homologs of MST1 were found in many fungi, including plant symbionts and plant pathogens, suggesting that the utilization of plant-derived sugars by MST1 homologs could also be important for other fungi to develop a symbiotic or parasitic relationship with their respective plant hosts.

Transcriptional Differences Guided Discovery and Genetic Identification of Coprogen and Dimerumic Acid Siderophores in Metarhizium robertsii

Siderophores are small molecular iron chelators and participate in the multiple cellular processes in fungi. In this study, biosynthesis gene clusters of coprogens and dimerumic acids were identified by transcriptional level differences of genes related to iron deficiency conditions in Metarhizium robertsii. This leads to the characterization of new coprogen metachelin C (1) and five known siderophores metachelin A (2), metachelin A-CE (3), metachelin B (4), dimerumic acid 11-mannoside (5), and dimerumic acid (6). The structure of metachelin C (1) was elucidated by NMR spectroscopy and HR-ESI-MS analysis. Genetic deletions of mrsidA, and mrsidD abolished the production of compounds 1–6 that implied their involvement in the biosynthesis of coprogen and dimerumic acid. Interestingly, NRPS gene mrsidD is responsible for biosynthesis of both coprogen and dimerumic acid, thus we proposed plausible biosynthetic pathways for the synthesis of coprogen and dimerumic acid siderophores. Therefore, our study provides the genetic basis for understanding the biosynthetic pathway of coprogen and dimerumic acid in Metarhizium robertsii.