Transformation of Metarhizium anisopliae mediated by Agrobacterium tumefaciens

MaEng1, an endo-1,3-glucanase, contributes to the conidiation pattern shift through changing the cell wall structure in Metarhizium acridum

Microcycle conidiation has displayed the greater potential than normal conidiation in large-scale production of mycopesticides. Fungi require partial hydrolysis of the cell wall to achieve the necessary plasticity during their morphological changes. Therefore, various cell wall-associated hydrolases are crucial for fungal morphogenesis. Eng1, as an endo-β-1,3-glucanase, is involved in the cell separation of fungi, but its role in morphological changes of entomopathogenic fungi is not yet clear. Here, the endo-β-1,3-glucanase gene MaEng1 was characterized in the model entomopathogenic fungi M. acridum. MaEng1 possesses a typical carbohydrate hydrolase domain and belongs to the GH81 family. The functions of MaEng1 in fungal growth, stress tolerance, pathogenicity, and conidiation capacity were analyzed using targeted gene disruption. The results displayed that the absence of MaEng1 does not affect the fungal growth, stress tolerances, and pathogenicity in M. acridum. However, the knockout of MaEng1 led to the normal conidiation of M. acridum on the SYA medium, which can induce the microcycle conidiation. Moreover, the content of β-1,3-glucan in the cell wall of the MaEng1-disruption strain were significantly reduced and the exposures of β-1,3-glucan on the surface of the mature conidia and mycelia in ΔMaEng1 were declined, indicating that MaEng1 contributes to the conversion of conidiation mode in M. acridum by affecting the cell wall structure.

The role of MrUbp4, a deubiquitinase, in conidial yield, thermotolerance, and virulence in Metarhizium robertsii

Ubiquitin-specific proteases (UBPs), the largest subfamily of deubiquitinating enzymes, regulate ubiquitin homeostasis and play diverse roles in eukaryotes. Ubp4 is essential for the growth, development, and pathogenicity of various fungal pathogens. However, its functions in the growth, stress responses, and virulence of entomopathogenic fungi remain unclear. In this study, we elucidated the role of the homolog of Ubp4, MrUbp4, in the entomopathogenic fungus Metarhizium robertsii. Deletion of MrUbp4 led to a notable increase in ubiquitination levels, demonstrating the involvement of MrUbp4 in protein deubiquitination. Furthermore, the ΔMrUbp4 mutant displayed a significant reduction in conidial yield, underscoring the pivotal role of MrUbp4 in conidiation. Additionally, the mutant exhibited heightened resistance to conidial heat treatment, emphasizing the role of MrUbp4 in thermotolerance. Notably, insect bioassays unveiled a substantial impairment in the virulence of the ΔMrUbp4 mutant. This was accompanied by a notable decrease in cuticle penetration ability and appressorium formation upon further analysis. In summary, our findings highlight the essential role of MrUbp4 in regulating the conidial yield, thermotolerance, and contributions to the virulence of M. robertsii.

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.

Hypothetical protein MAA_07646 is required for stress resistance and pathogenicity in Metarhizium robertsii

Metarhizium robertsii, a vital entomopathogenic fungus for pest management, relies on various virulence-related proteins for infection. Identifying these proteins, especially those with unknown functions, can illuminate the fungus's virulence mechanisms. Through RNA-seq, we discovered that the hypothetical protein MAA_07646 was significantly upregulated during appressorium formation in M. robertsii. In this study, we characterized MAA_07646, finding its presence in both the nucleus and cytoplasm. Surprisingly, it did not affect vegetative growth, conidiation, or chemical tolerance. However, it played a role in heat and UV radiation sensitivity. Notably, ΔMAA_07646 exhibited reduced virulence in Galleria mellonella larvae due to impaired appressorium formation and decreased expression of virulence-related genes. In conclusion, MAA_07646 contributes to thermotolerance, UV resistance, and virulence in M. robertsii. Understanding its function sheds light on the insecticidal potential of M. robertsii's hypothetical proteins.

Hyphal Growth and Conidia Germination Are Induced by Phytohormones in the Root Colonizing and Plant Growth Promoting Fungus Metarhizium guizhouense

Beneficial associations are very important for plants and soil-dwelling microorganisms in different ecological niches, where communication by chemical signals is relevant. Among the chemical signals, the release of phytohormones by plants is important to establish beneficial associations with fungi, and a recently described association is that of the entomopathogenic ascomycete fungus Metarhizium with plants. Here, we evaluated the effect of four different phytohormones, synthetic strigolactone (GR24), sorgolactone (SorL), 3-indolacetic acid (IAA) and gibberellic acid (GA3), on the fungus Metarhizium guizhouense strain HA11-2, where the germination rate and hyphal elongation were determined at three different times. All phytohormones had a positive effect on germination, with GA3 showing the greatest effect, and for hyphal length, on average, the group treated with synthetic strigolactone GR24 showed greater average hyphal length at 10 h of induction. This work expands the knowledge of the effect of phytohormones on the fungus M. guizhouense, as possible chemical signals for the rapid establishment of the fungus-plant association.

Trade-offs between immunity and competitive ability in fighting ant males

BACKGROUND

Fighting disease while fighting rivals exposes males to constraints and trade-offs during male-male competition. We here tested how both the stage and intensity of infection with the fungal pathogen Metarhizium robertsii interfere with fighting success in Cardiocondyla obscurior ant males. Males of this species have evolved long lifespans during which they can gain many matings with the young queens of the colony, if successful in male-male competition. Since male fights occur inside the colony, the outcome of male-male competition can further be biased by interference of the colony's worker force.

RESULTS

We found that severe, but not yet mild, infection strongly impaired male fighting success. In late-stage infection, this could be attributed to worker aggression directed towards the infected rather than the healthy male and an already very high male morbidity even in the absence of fighting. Shortly after pathogen exposure, however, male mortality was particularly increased during combat. Since these males mounted a strong immune response, their reduced fighting success suggests a trade-off between immune investment and competitive ability already early in the infection. Even if the males themselves showed no difference in the number of attacks they raised against their healthy rivals across infection stages and levels, severely infected males were thus losing in male-male competition from an early stage of infection on.

CONCLUSIONS

Males of the ant C. obscurior have a well-developed immune system that raises a strong immune response very fast after fungal exposure. This allows them to cope with mild pathogen exposures without compromising their success in male-male competition, and hence to gain multiple mating opportunities with the emerging virgin queens of the colony. Under severe infection, however, they are weak fighters and rarely survive a combat already at early infection when raising an immune response, as well as at progressed infection, when they are morbid and preferentially targeted by worker aggression. Workers thereby remove males that pose a future disease threat by biasing male-male competition. Our study thus reveals a novel social immunity mechanism how social insect workers protect the colony against disease risk.

Dynamic pathogen detection and social feedback shape collective hygiene in ants

Cooperative disease defense emerges as group-level collective behavior, yet how group members make the underlying individual decisions is poorly understood. Using garden ants and fungal pathogens as an experimental model, we derive the rules governing individual ant grooming choices and show how they produce colony-level hygiene. Time-resolved behavioral analysis, pathogen quantification, and probabilistic modeling reveal that ants increase grooming and preferentially target highly-infectious individuals when perceiving high pathogen load, but transiently suppress grooming after having been groomed by nestmates. Ants thus react to both, the infectivity of others and the social feedback they receive on their own contagiousness. While inferred solely from momentary ant decisions, these behavioral rules quantitatively predict hour-long experimental dynamics, and synergistically combine into efficient colony-wide pathogen removal. Our analyses show that noisy individual decisions based on only local, incomplete, yet dynamically-updated information on pathogen threat and social feedback can lead to potent collective disease defense.

A 1-aminocyclopropane-1-carboxylate deaminase MrACCD from Metarhizium robertsii is associated with plant growth promotion for Metarhizium spp

Besides killing insects, Metarhizium spp. have been showing another realistic ecology role as plant associates. Partial genra and groups of these entomopathogenic fungi act as plant growth promoters during root colonization. Here, we report that Metarhizium robertsii produces a 1-aminocyclopropane-1-carboxylate (ACC) deaminase (ACCD encoded by MracdS, MrACCD), which is involved in promoting wheat early vegetative growth, while Metarhizium acridum lacks the genuine ACCD though a MracdS homologue exists in the species. MracdS expression was up-regulated by a max 10.7-fold with 3 mM ACC and high ACCD enzymatic activities were induced by either ACC (7.5-fold) or wheat root (3.2-fold). In contrast, no ACCD activity was detected in M. acridum in the presence of both inducers. In pot assay, wheat seeds were treated with wild-type M. robertsii (Mr23), wild-type M. acridum (Mac324), MracdS disruption mutant (ΔMracdS) and M. acridum transformant harboring heterologous MracdS (Mac324-MracdS). Relative to the control seeds treated with heat-killed conidia, Mr23, ΔMracdS and Mac324-MracdS increased root length (by 66.2, 31.8 and 40.2%), and plant biomass (by 56.6, 42.1 and 40.9%). Nevertheless, ΔMracdS deficient in ACCD activity heavily impaired its capability of wheat growth promotion by decrease of 20.7% in root length relative to Mr23. In addition, Mr23 and Mac324-MracdS also increased shoot growth (by 42.3, and 42.7%) while ΔMracdS failed. Mac324 showed no effect on plant growth during the test. These data suggest a role for ACCD in the plant growth promotion effect by M. robertsii, which is irrelevant to Metarhizium colonization of roots since rhizosphere competency of both Mr23 and Mac324 are unaffected by the change of ACCD activity.

Three Methods Assessing the Association of the Endophytic Entomopathogenic Fungus Metarhizium robertsii with Non-Grafted Grapevine Vitis vinifera

Characterizing the association of endophytic insect pathogenic fungi (EIPF) with plants is an important step in order to understand their ecology before using them in biological control programs. Since several methods are available, it is challenging to identify the most appropriate for such investigations. Here, we used two strains of Metarhizium robertsii: EF3.5(2) native to the French vineyard environment and ARSEF-2575-GFP a laboratory strain expressing a green fluorescent protein, to compare their potential of association with non-grafted grapevine Vitis vinifera. Three methods were used to evaluate the kinetics of rhizosphere and grapevine endospheric colonization: (i) Droplet Digital (ddPCR), a sensitive molecular method of M. robertsii DNA quantification in different plant parts, (ii) culture-based method to detect the live fungal propagules from plant tissues that grew on the medium, (iii) confocal imaging to observe roots segments. Both strains showed evidence of establishment in the rhizosphere of grapevines according to the culture-based and ddPCR methods, with a significantly higher establishment of strain EF3.5(2) (40% positive plants and quantified median of exp(4.61) c/μL) compared to strain ARSEF-2575-GFP (13% positive plants and quantified median of exp(2.25) c/μL) at 96-98 days post-inoculation. A low incidence of association of both strains in the grapevine root endosphere was found with no significant differences between strains and evaluation methods (15% positive plants inoculated with strain EF3.5(2) and 5% with strain ARSEF-2575-GFP according to culture-based method). ddPCR should be used more extensively to investigate the association between plants and EIPF but always accompanied with at least one method such as culture-based method or confocal microscopy.

Development of Genetic Tools in Glucoamylase-Hyperproducing Industrial Aspergillus niger Strains

Simple Summary

Glucoamylase is one of the most needed industrial enzymes in the food and biofuel industries. Aspergillus niger is a commonly used cell factory for the production of commercial glucoamylase. For decades, genetic manipulation has promoted significant progress in industrial fungi for strain engineering and in obtaining deep insights into their genetic features. However, genetic engineering is more laborious in the glucoamylase-producing industrial strains A. niger N1 and O1 because their fungal features of having few conidia (N1) or of being aconidial (O1) make them difficult to perform transformation on. In this study, we targeted A. niger N1 and O1 and successfully developed high-efficiency transformation tools. We also constructed a clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 editing marker-free system using an autonomously replicating plasmid to express Cas9 protein and to guide RNA and the selectable marker. By using the genetic tools developed here, we generated nine albino deletion mutants. After three rounds of sub-culturing under nonselective conditions, the albino deletions lost the autonomously replicating plasmid. Together, the tools and optimization process above provided a good reference to manipulate the tough working industrial strain, not only for the further engineering these two glucoamylase-hyperproducing strains, but also for other industrial strains.

Abstract

The filamentous fungus Aspergillus niger is widely exploited by the fermentation industry for the production of enzymes, particularly glucoamylase. Although a variety of genetic techniques have been successfully used in wild-type A. niger, the transformation of industrially used strains with few conidia (e.g., A. niger N1) or that are even aconidial (e.g., A. niger O1) remains laborious. Herein, we developed genetic tools, including the protoplast-mediated transformation and Agrobacterium tumefaciens-mediated transformation of the A. niger strains N1 and O1 using green fluorescent protein as a reporter marker. Following the optimization of various factors for protoplast release from mycelium, the protoplast-mediated transformation efficiency reached 89.3% (25/28) for N1 and 82.1% (32/39) for O1. The A. tumefaciens-mediated transformation efficiency was 98.2% (55/56) for N1 and 43.8% (28/64) for O1. We also developed a marker-free CRISPR/Cas9 genome editing system using an AMA1-based plasmid to express the Cas9 protein and sgRNA. Out of 22 transformants, 9 albA deletion mutants were constructed in the A. niger N1 background using the protoplast-mediated transformation method and the marker-free CRISPR/Cas9 system developed here. The genome editing methods improved here will accelerate the elucidation of the mechanism of glucoamylase hyperproduction in these industrial fungi and will contribute to the use of efficient targeted mutation in other industrial strains of A. niger.

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.

The Deubiquitinating Enzyme MrUbp14 Is Involved in Conidiation, Stress Response, and Pathogenicity in Metarhizium robertsii

Protein ubiquitination, which is involved in various biological processes in eukaryotic cells, is a reversible modification of proteins. Deubiquitinases can maintain ubiquitin homeostasis by removing ubiquitin or modulating protein degradation via the ubiquitin-proteasome system (UPS). Metarhizium robertsii, an entomopathogenic fungus, has become a model fungus for investigating the interactions between insects and fungal pathogens. To explore the possible effects of the deubiquitination process on the development, stress response, and virulence of M. robertsii, disruption of MrUbp14 (an ortholog of the yeast ubiquitin-specific protease gene, Ubp14) was performed. The results of this study showed that the deletion of MrUbp14 led to accelerated conidial germination, reduced conidial yields, and decreased expression levels of some genes involved in conidiation. Furthermore, the MrUbp14 mutant (ΔMrUbp14) exhibited decreased tolerance to cell wall-damaging stressors (Congo red and SDS) and heat stress. Importantly, the results of the bioassay demonstrated that the fungal virulence of the ΔMrUbp14 strain was largely reduced in cuticle infection, but not in direct injection, which was accompanied by a significant decline in appressorium formation and cuticle penetration. Moreover, our results demonstrated that the disruption of MrUbp14 resulted in significantly increased ubiquitination levels of total protein, suggesting that MrUbp14 acts as a deubiquitinating enzyme in M. robertsii. In summary, our phenotypic changes in the gene disruption mutants suggest that MrUbp14 is important for conidiation, stress response, and fungal virulence in M. robertsii.

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.

MaAts, an Alkylsulfatase, Contributes to Fungal Tolerances against UV-B Irradiation and Heat-Shock in Metarhizium acridum

Sulfatases are commonly divided into three classes: type I, type II, and type III sulfatases. The type III sulfatase, alkylsulfatase, could hydrolyze the primary alkyl sulfates, such as sodium dodecyl sulfate (SDS) and sodium octyl sulfate. Thus, it has the potential application of SDS biodegradation. However, the roles of alkylsulfatase in biological control fungus remain unclear. In this study, an alkylsulfatase gene MaAts was identified from Metarhizium acridum. The deletion strain (ΔMaAts) and the complemented strain (CP) were constructed to reveal their functions in M. acridum. The activity of alkylsulfatase in ΔMaAts was dramatically reduced compared to the wild-type (WT) strain. The loss of MaAts delayed conidial germination, conidiation, and significantly declined the fungal tolerances to UV-B irradiation and heat-shock, while the fungal conidial yield and virulence were unaffected in M. acridum. The transcription levels of stress resistance-related genes were significantly changed after MaAts inactivation. Furthermore, digital gene expression profiling showed that 512 differential expression genes (DEGs), including 177 up-regulated genes and 335 down-regulated genes in ΔMaAts, were identified. Of these DEGs, some genes were involved in melanin synthesis, cell wall integrity, and tolerances to various stresses. These results indicate that MaAts and the DEGs involved in fungal stress tolerances may be candidate genes to be adopted to improve the stress tolerances of mycopesticides.

Establishment of High-Efficiency Screening System for Gene Deletion in Fusarium venenatum TB01

Genetic engineering is one of the most effective methods to obtain fungus strains with desirable traits. However, in some filamentous fungi, targeted gene deletion transformant screening on primary transformation plates is time-consuming and laborious due to a relatively low rate of homologous recombination. A strategy that compensates for the low recombination rate by improving screening efficiency was performed in F. venenatum TB01. In this study, the visualized gene deletion system that could easily distinguish the fluorescent randomly inserted and nonfluorescent putative deletion transformants using green fluorescence protein (GFP) as the marker and a hand-held lamp as the tool was developed. Compared to direct polymerase chain reaction (PCR) screening, the screening efficiency of gene deletion transformants in this system was increased approximately fourfold. The visualized gene deletion system developed here provides a viable method with convenience, high efficiency, and low cost for reaping gene deletion transformants from species with low recombination rates.

Role of Two G-Protein α Subunits in Vegetative Growth, Cell Wall Integrity, and Virulence of the Entomopathogenic Fungus Metarhizium robertsii

Heterotrimeric G-proteins are crucial for fungal growth and differentiation. The α subunits of heterotrimeric G-proteins play an essential role in controlling signal transduction. However, the function of G-protein α subunits in entomopathogenic fungi remains poorly understood. Two group II Gα subunits (MrGPA2 and MrGPA4) were characterized in the entomopathogenic fungus, Metarhizium robertsii. Bioinformatics analysis showed that the relationship between MrGPA2 and MrGPA4 was closer than that of other MrGPAs. Both green fluorescent protein (GFP)-tagged MrGPA2 and MrGPA4 were localized at the cytoplasm. Furthermore, ∆MrGpa2∆MrGpa4 double mutants showed remarkably reduced vegetative growth compared to the wild-type and single-mutant strains, which was accompanied by the downregulation of several growth-related genes, such as ssk2, pbs2, stuA, hog1, and ac. Only the ∆MrGpa2∆MrGpa4 double mutant was sensitive to Congo red stress. The insect bioassay demonstrated significantly attenuated virulence for the ∆MrGpa2∆MrGpa4 double mutant compared to the wild-type and single-mutant strains. Further analysis indicated that double deletion of MrGpa2 and MrGpa4 had no effect on appressorium formation but suppressed the expression levels of several virulence-related genes in the insect hemocoel. These findings demonstrate that MrGpa2 and MrGpa4 exhibit functional redundancy and contribute to the vegetative growth, stress tolerance, and pest control potential in M. robertsii.

A Novel Nitrogen and Carbon Metabolism Regulatory Cascade Is Implicated in Entomopathogenicity of the Fungus Metarhizium robertsii

The entomopathogenic fungus Metarhizium robertsii can switch among parasitic, saprophytic, and symbiotic lifestyles in response to changing nutritional conditions, which is attributed to its extremely versatile metabolism. Here, we found that the Fus3–mitogen-activated protein kinase (MAPK) and the transcription factor regulator of nutrient selection 1 (RNS1) constitute a novel fungal cascade that regulates the degradation of insect cuticular lipids, proteins, and chitin to obtain nutrients for hyphal growth and enter the insect hemocoel for subsequent colonization. On the insect cuticle, Fus3-MAPK physically contacts and phosphorylates RNS1, which facilitates the entry of RNS1 into nuclei. The phosphorylated RNS1 binds to the DNA motif BM2 (ACCAGAC) in its own promoter to self-induce expression, which then activates the expression of genes for degrading cuticular proteins, chitin, and lipids. We further found that the Fus3-MAPK/RNS1 cascade also activates genes for utilizing complex and less-favored nitrogen and carbon sources (casein, colloid chitin, and hydrocarbons) that were not derived from insects, which is repressed by favored organic carbon and nitrogen nutrients, including glucose and glutamine. In conclusion, we discovered a novel regulatory cascade that controls fungal nitrogen and carbon metabolism and is implicated in the entomopathogenicity of M. robertsii.

IMPORTANCE Penetration of the cuticle, the first physical barrier to pathogenic fungi, is the prerequisite for fungal infection of insects. In the entomopathogenic fungus Metarhizium robertsii, we found that the Fus3–mitogen-activated protein kinase (MAPK) and the transcription factor regulator of nutrient selection 1 (RNS1) constitute a novel cascade that controls cuticle penetration by regulating degradation of cuticular lipids, proteins, and chitin to obtain nutrients for hyphal growth and entry into the insect hemocoel. In addition, during saprophytic growth, the Fus3-MAPK/RNS1 cascade also activates utilization of complex and less-favored carbon and nitrogen sources that are not derived from insects. The homologs of Fus3-MAPK and RNS1 are widely found in ascomycete filamentous fungi, including saprophytes and pathogens with diverse hosts, suggesting that the regulation of utilization of nitrogen and carbon sources by the Fus3-MAPK/RNS1 cascade could be widespread. Our work provides significant insights into regulation of carbon and nitrogen metabolism in fungi and fungal pathogenesis in insects.

Abscisic acid implicated in differential plant responses of Phaseolus vulgaris during endophytic colonization by Metarhizium and pathogenic colonization by Fusarium

Metarhizium robertsii is an insect pathogen as well as an endophyte, and can antagonize the phytopathogen, Fusarium solani during bean colonization. However, plant immune responses to endophytic colonization by Metarhizium are largely unknown. We applied comprehensive plant hormone analysis, transcriptional expression and stomatal size analysis in order to examine plant immune responses to colonization by Metarhizium and/or Fusarium. The total amount of abscisic acid (ABA) and ABA metabolites decreased significantly in bean leaves by plant roots colonized by M. robertsii and increased significantly with F. solani compared to the un-inoculated control bean plant. Concomitantly, in comparison to the un-inoculated bean, root colonization by Metarhizium resulted in increased stomatal size in leaves and reduced stomatal size with Fusarium. Meanwhile, expression of plant immunity genes was repressed by Metarhizium and, alternately, triggered by Fusarium compared to the un-inoculated plant. Furthermore, exogenous application of ABA resulted in reduction of bean root colonization by Metarhizium but increased colonization by Fusarium compared to the control without ABA application. Our study suggested that ABA plays a central role in differential responses to endophytic colonization by Metarhizium and pathogenic colonization by Fusarium and, we also observed concomitant differences in stomatal size and expression of plant immunity genes.

Empirical Support for the Pattern of Competitive Exclusion between Insect Parasitic Fungi

Fungal entomopathogens are largely facultative parasites and play an important role in controlling the density of insect populations in nature. A few species of these fungi have been used for biocontrol of insect pests. The pattern of the entomopathogen competition for insect individuals is still elusive. Here, we report the empirical competition for hosts or niches between the inter- and intra-species of the entomopathogens Metarhizium robertsii and Beauveria bassiana. It was found that the synergistic effect of coinfection on virulence increase was not evident, and the insects were largely killed and mycosed by M. robertsii independent of its initial co-inoculation dosage and infection order. For example, >90% dead insects were mycosed by M. robertsii even after immersion in a spore suspension with a mixture ratio of 9:1 for B. bassiana versus M. robertsii. The results thus support the pattern of competitive exclusion between insect pathogenic fungi that occurred from outside to inside the insect hosts. Even being inferior to compete for insects, B. bassiana could outcompete M. robertsii during co-culturing in liquid medium. It was also found that the one-sided mycosis of insects occurred during coinfection with different genotypic strains of either fungi. However, parasexual recombination was evident to take place between the compatible strains after coinfection. The data of this study can help explain the phenomena of the exclusive mycosis of insect individuals, but co-occurrence of entomopathogens in the fields, and suggest that the synergistic effect is questionable regarding the mixed use of fungal parasites for insect pest control.

An efficient genetic transformation system for Chinese medicine fungus Tolypocladium ophioglossoides

Tolypocladium ophioglossoides is a rare and valuable fungus extensively used in Chinese medicine for relieving postmenopausal syndrome in women yet its bioactive molecules are unknown. To explore its molecular mechanisms, we have developed a reliable Agrobacterium-mediated transformation system using the selective marker: the chlorimuron ethyl-resistance gene sur. For this purpose, we firstly constructed a T-DNA binary vector system and then improved the transformation efficiency by optimizing conditional parameters including the Agrobacterium tumefaciens concentration, the conidia number of T. ophioglossoides, the co-culture time and the concentration of acetosyringone. Furthermore, we have knocked-out the ku70 gene，which is a key gene in non-homologous end joining (NHEJ) DNA repair pathway，and the effect of the length of the homologous arms (HA) on the genetic transformation efficacy was also examined, which increased by 60% when HA was about 3 kb in length. Our results suggest that the genetic transformation system is efficient and feasible for the truffle-parasite fungus T. ophioglossoides, which can further be used in large-scale experiments for characterization of genes of interest in future work.

Identification of a key G-protein coupled receptor in mediating appressorium formation and fungal virulence against insects

Fungal G-protein coupled receptors (GPCRs) play essential roles in sensing environmental cues including host signals. The study of GPCR in mediating fungus-insect interactions is still limited. Here we report the evolution of GPCR genes encoded in the entomopathogenic Metarhizium species and found the expansion of Pth11-like GPCRs in the generalist species with a wide host range. By deletion of ten candidate genes MrGpr1-MrGpr10 selected from the six obtained subfamilies in the generalist M. robertsii, we found that each of them played a varied level of roles in mediating appressorium formation. In particular, deletion of MrGpr8 resulted in the failure of appressorium formation on different substrates and the loss of virulence during topical infection of insects but not during injection assays when compared with the wild-type (WT) strain. Further analysis revealed that disruption of MrGpr8 substantially impaired the nucleus translocation of the mitogen-activated protein kinase (MAPK) Mero-Fus3 but not the MAPK Mero-Slt2 during appressorium formation. We also found that the defect of AMrGpr8 could not be rescued with the addition of cyclic AMP for appressorium formation. Relative to the WT, differential expression of the selected genes have also been detected in AMrGpr8. The results of this study may benefit the understanding of fungus-interactions mediated by GPCRs.

High efficiency transformation and mutant screening of the laurel wilt pathogen, Raffaelea lauricola

The fungal pathogen, Raffaelea lauricola, is the causative agent of laurel wilt, a devastating disease affecting the Lauraceae family. The fungus is vectored by ambrosia beetles that carry the fungus in specialized structures (mycangia), with the fungus acting as a symbiont and food source for beetle larvae growing in tree galleries. In order to probe the molecular basis for plant pathogenicity and insect symbiosis of the laurel wilt fungus, molecular tools including establishment of efficient transformation protocols are required. Resistance marker profiling revealed susceptibility of R. lauricola to phosphinothricin, chlorimuron ethyl, hygromycin, and benomyl. Agrobacterium-mediated transformation using either the bar or sur marker resulted in 1-200 transformants/10⁵ spores. A second protocol using lithium acetate-polyethylene glycol (LiAc-PEG) treatment of fungal blastospores yielded 5-60 transformants/μg DNA/10⁸ cells. Transformants were mitotically stable (at least 5 generations), and > 95% of transformants showed a single integration event. R. lauricola strains expressing green and red fluorescent proteins (EGFP and RFP), as well as glucuronidase (GUS), were constructed. Using the Agrobacterium-mediated method, a random T-DNA insertion library was constructed, and genetic screens led to the isolation of developmental mutants as well as mutants displaying enhanced resistance to sodium dodecyl sulfate (SDS) or fluconazole, and those showing decreased susceptibility to biphenol. These results establish simple and reliable genetic tools for transformation of R. lauricola needed for genetic dissection of the symbiotic and virulent lifestyles exhibited by this fungus and establish a library of insertion mutants that can be used in various genetic screens to dissect molecular pathways. KEY POINTS: • Vectors and transformation protocols were developed for Raffaelea lauricola. • Method was used for construction of a random insertion mutant library. • Mutant library was validated by phenotypic screens for resistance and susceptibility to various agents.

G-Protein Subunit Gαi in Mitochondria, MrGPA1, Affects Conidiation, Stress Resistance, and Virulence of Entomopathogenic Fungus Metarhizium robertsii

G proteins are critical modulators or transducers in various transmembrane signaling systems. They play key roles in numerous biological processes in fungi, including vegetative growth, development of infection-related structures, asexual conidiation, and virulence. However, functions of G proteins in entomopathogenic fungi remain unclear. Here, we characterized the roles of MrGPA1, a G-protein subunit Gαi, in conidiation, stress resistance, and virulence in Metarhizium robertsii. MrGPA1 was localized in the mitochondria. MrGpa1 deletion resulted in a significant reduction (47%) in the conidiation capacity, and reduced expression of several key conidiation-related genes, including fluG, flbD, brlA, wetA, phiA, and stuA. Further, MrGpa1 disruption resulted in decreased fungal sensitivity to UV irradiation and thermal stress, as determined based on conidial germination of ΔMrGpa1 and wild-type (WT) strains. Chemical stress analysis indicated that MrGpa1 contributes to fungal antioxidant capacity and cell wall integrity, but is not involved in tolerance to antifungal drug and osmotic stress. Importantly, insect bioassays involving (topical inoculation and injection) of Galleria mellonella larvae revealed decreased virulence of ΔMrGpa1 strain after cuticle infection. This was accompanied by decreased rates of appressorium formation and reduced expression of several cuticle penetration-related genes. Further assays showed that MrGpa1 regulated intracellular cyclic AMP (cAMP) levels, but feeding with cAMP could not recover the appressorium formation rate of ΔMrGpa1. These observations suggest that MrGpa1 contributes to the regulation of conidiation, UV irradiation, thermal stress response, antioxidant capacity, and cell wall integrity in M. robertsii. This gene is also involved in insect cuticle penetration during infection. These findings raise the possibility of designing powerful strategies for genetic improvement of M. robertsii conidiation capacity and virulence for killing pests.

DNA methyltransferase implicated in the recovery of conidiation, through successive plant passages, in phenotypically degenerated Metarhizium

Metarhizium robertsii is a fungus with two lifestyles; it is a plant root symbiont and an insect pathogen. A spontaneously phenotypically degenerated strain of M. robertsii strain ARSEF 2575 (M. robertsii lc-2575; lc = low conidiation) showed a reduction in conidiation and fungal virulence after successive subculturing on agar medium. In order to recover conidiation, we experimentally passaged M. robertsii lc-2575 through plant (soldier bean and switchgrass) root or insect (Galleria mellonella) larvae. After five passages, the resultant strains had significantly increased conidial yields on agar and increased virulence in insect bioassays. Concomitantly, DNA methyltransferase, MrDIM-2 expression was downregulated in BR5 (a strain after 5 bean root passages) and isolates after switchgrass and insect passages. Bisulfite sequencing showed little difference in overall genomic DNA methylation levels (~ 0.37%) between M. robertsii lc-2575 and BR5. However, a finer comparison of the different methylated regions (DMRs) showed that DMRs of BR5 were more abundant in the intergenic regions (69.32%) compared with that of M. robertsii lc-2575 (33.33%). The addition of DNA methyltransferase inhibitor, 5-azacytidine, to agar supported the role of DNA methyltransferases and resulted in an increase in conidiation of M. robertsii lc-2575. Differential gene expression was observed in selected DMRs in BR5 when compared with M. robertsii lc-2575. Here we implicated epigenetic regulation in the recovery of conidiation through the effects of DNA methyltransferase and that plant passage could be used as a method to recover fungal conidiation and virulence in a phenotypically degenerated M. robertsii. KEY POINTS: • Passage of Metarhizium through plant root or insect results in increased conidiation. • DNA methyltransferase is downregulated after host passage. • Bisulfite sequencing identified potentially methylated genes involved in conidiation.

Plant microbiome analysis after Metarhizium amendment reveals increases in abundance of plant growth-promoting organisms and maintenance of disease-suppressive soil

The microbial community in the plant rhizosphere is vital to plant productivity and disease resistance. Alterations in the composition and diversity of species within this community could be detrimental if microbes suppressing the activity of pathogens are removed. Species of the insect-pathogenic fungus, Metarhizium, commonly employed as biological control agents against crop pests, have recently been identified as plant root colonizers and provide a variety of benefits (e.g. growth promotion, drought resistance, nitrogen acquisition). However, the impact of Metarhizium amendment on the rhizosphere microbiome has yet to be elucidated. Using Illumina sequencing, we examined the community profiles (bacteria and fungi) of common bean (Phaseolus vulgaris) rhizosphere (loose soil and plant root) after amendment with M. robertsii conidia, in the presence and absence of an insect host. Although alpha diversity was not significantly affected overall, there were numerous examples of plant growth-promoting organisms that significantly increased with Metarhizium amendment (Bradyrhizobium, Flavobacterium, Chaetomium, Trichoderma). Specifically, the abundance of Bradyrhizobium, a group of nitrogen-fixing bacteria, was confirmed to be increased using a qPCR assay with genus-specific primers. In addition, the ability of the microbiome to suppress the activity of a known bean root pathogen was assessed. The development of disease symptoms after application with Fusarium solani f. sp. phaseoli was visible in the hypocotyl and upper root of plants grown in sterilized soil but was suppressed during growth in microbiome soil and soil treated with M. robertsii. Successful amendment of agricultural soils with biocontrol agents such as Metarhizium necessitates a comprehensive understanding of the effects on the diversity of the rhizosphere microbiome. Such research is fundamentally important towards sustainable agricultural practices to improve overall plant health and productivity.

The transmembrane protein MaSho1 negatively regulates conidial yield by shifting the conidiation pattern in Metarhizium acridum

Sho1 is an important membrane sensor upstream of the HOG-MAPK signaling pathway, which plays critical roles in osmotic pressure response, growth, and virulence in fungi. Here, a Sho1 homolog (MaSho1), containing four transmembrane domains and one Src homology (SH3) domain, was characterized in Metarhizium acridum, a fungal pathogen of locusts. Targeted gene disruption of MaSho1 impaired cell wall integrity, virulence, and tolerances to UV-B and oxidative stresses, while none of them was affected when the SH3 domain was deleted. Intriguingly, disruption of MaSho1 significantly increased conidial yield, which was not affected in the SH3 domain mutant. Furthermore, it was found that deletion of MaSho1 led to microcycle conidiation of M. acridum on the normal conidiation medium. Deletion of MaSho1 significantly shortened the hyphal cells but had no effect on conidial germination. Digital gene expression profiling during conidiation indicated that differential expression of genes was associated with mycelial development, cell division, and differentiation between the wild type and the MaSho1 mutant. These data suggested that disruption of MaSho1 shifted the conidiation pattern by altering the transcription of genes to inhibit mycelial growth, thereby promoting the conidiation of M. acridum.

Coordinated regulation of infection-related morphogenesis by the KMT2-Cre1-Hyd4 regulatory pathway to facilitate fungal infection

Entomopathogenic fungi can overcome insecticide resistance and represent promising tools for the control of mosquitoes. Better understanding of fungus-mosquito interactions is critical for improvement of fungal efficacy. Upon insect cuticle induction, pathogenic fungi undergo marked infection-related morphological differentiation. However, regulatory mechanisms of fungal infection-related morphogenesis are poorly understood. Here, we show that a histone lysine methyltransferase KMT2 in Metarhizium robertsii (MrKMT2) is up-regulated upon cuticle induction. MrKMT2 plays crucial roles in regulating infection-related morphogenesis and pathogenicity by up-regulating the transcription factor gene Mrcre1 via H3K4 trimethylation during mosquito cuticle infection. MrCre1 further regulates the cuticle-induced gene Mrhyd4 to modulate infection structure (appressorium) formation and virulence. Overall, the MrKMT2-MrCre1-MrHyd4 regulatory pathway regulates infection-related morphogenesis and pathogenicity in M. robertsii. These findings reveal that the epigenetic regulatory mechanism plays a pivotal role in regulating fungal pathogenesis in insects, and provide new insights into molecular interactions between pathogenic fungi and insect hosts.

MrSt12 implicated in the regulation of transcription factor AFTF1 by Fus3-MAPK during cuticle penetration by the entomopathogenic fungus Metarhizium robertsii

Metarhizium robertsii is a versatile fungus with multifactorial lifestyles, and it is an emerging fungal model for investigating the mechanisms of multiple lifestyle transitions that involve trans-kingdom host jumping. Penetration of the insect cuticle is the necessary step for the transition from saprophytic or symbiotic to pathogenic lifestyle. Previously, we found the transcription factor AFTF1 plays an important role in cuticle penetration, which is precisely regulated by Fus3-MAPK, Slt2-MAPK, and the membrane protein Mr-OPY2. Here, we identified a transcription factor (MrSt12) that directly regulated the transcription of Aftf1 by physically interacting with the cis-acting element (ATGAAACA) in the promoter of Aftf1. The deletion mutant of MrSt12 failed to form the infection structure appressorium and was thus nonpathogenic. We further found that the regulation of Aftf1 by MrSt12 was directly controlled by the Fus3-MAPK. In conclusion, we found a new signaling cascade containing Fus3-MAPK, MrSt12, and AFTF1, which regulates cuticle penetration by M. robertsii.

The polyubiquitin gene MrUBI4 is required for conidiation, conidial germination, and stress tolerance in the filamentous fungus Metarhizium robertsii

The polyubiquitin gene is a highly conserved open reading frame that encodes different numbers of tandem ubiquitin repeats from different species, which play important roles in different biological processes. Metarhizium robertsii is a fungal entomopathogen that is widely applied in the biological control of pest insects. However, it is unclear whether the polyubiquitin gene is required for fungal development, stress tolerance, and virulence in the entomopathogenic fungus. In the present study, the polyubiquitin gene (MrUBI4, MAA_02160) was functionally characterized via gene deletion in M. robertsii. Compared to the control strains, the MrUBI4 deletion mutant showed delayed conidial germination and significantly decreased conidial yields (39% of the wild-type 14 days post-incubation). Correspondingly, the transcript levels of several genes from the central regulatory pathways associated with conidiation, including brlA, abaA, and wetA, were significantly downregulated, which indicated that MrUBI4 played an important role in asexual sporulation. Deletion of MrUBI4 especially resulted in increased sensitivity to ultraviolet (UV) and heat-shock stress based on conidial germination analysis between mutant and control strains. The significant increase in sensitivity to heat-shock was accompanied with reduced transcript levels of genes related to heat-shock protein (hsp), trehalose, and mannitol accumulation (tps, tpp, nth, and mpd) in the MrUBI4 deletion mutant. Deletion of MrUBI4 has no effect on fungal virulence. Altogether, MrUBI4 is involved in the regulation of conidiation, conidial germination, UV stress, and heat-shock response in M. robertsii.

MrArk1, an actin-regulating kinase gene, is required for endocytosis and involved in sustaining conidiation capacity and virulence in Metarhizium robertsii

Actin-regulating kinase (Ark) plays an important role in controlling endocytosis, which has been shown to be involved in the development and virulence of several fungal pathogens. However, it remains unclear whether Ark1 is required for the development and pathogenicity of an entomopathogenic fungus. Here, MrArk1 (MAA_03415), a homologue of yeast Ark1, was characterized in the insect pathogenic fungus, Metarhizium robertsii. Disruption of MrArk1 led to defects in endocytosis and a marked reduction (58%) in conidiation capacity. The reduced conidiation level was accompanied by repression of several key conidiation-related genes, including brlA, abaA, and wetA. Additionally, the deletion mutant showed a significant decrease in its tolerance to heat shock, but not to UV-B irradiation. Bioassays demonstrated attenuated virulence for the deletion mutant against Galleria mellonella via normal cuticle infection, accompanied by suppressed appressorium formation and reduced transcript levels of several genes involved in cuticle penetration. Taken together, our results indicate that MrArk1 is involved in the heat tolerance, sporulation, and virulence of M. robertsii, and thus is an important factor for sustaining the fungal potential against insect pests.

MrSVP, a secreted virulence-associated protein, contributes to thermotolerance and virulence of the entomopathogenic fungus Metarhizium robertsii

Background

Metarhizium robertsii, a widely distributed insect pathogen, is presently used as a natural alternative to chemical insecticides. Unfortunately, its worldwide commercial use has been restricted by a short shelf life and inconsistencies in virulence. In our previous study, a gene (GenBank accession number EFZ01626) was found to be significantly upregulated in heat-treated conidia. In the present study, this gene was characterized via gene disruption and complementation strategies.

Results

The gene (amplified by rapid amplification of cDNA ends PCR) was 1219 bp long and contained an open reading frame (ORF) of 777 bp. It encoded a protein of 234 amino acid residues with a 26-residue signal peptide. Bioinformatics analyses did not identify conserved functional domains; therefore, it was assumed to be a secreted virulence-associated protein according to its signal peptide and bioassay results. We found that the conidial germination rate of the ΔMrSVP mutant fungi dramatically decreased after heat shock treatment in a thermotolerance test. In addition, transcription levels of all tested heat shock–related genes were significantly lower in the mutant than in the wild type. We also demonstrated that the mean lethal time to death (LT₅₀) of ΔMrSVP significantly increased relative to the wild type in insect bioassays (both topical inoculation and injection) involving Galleria mellonella. Moreover, similar rates of appressorium formation between ΔMrSVP and the wild type—and the significantly different expression of virulence-related genes such as acid trehalase and sucrose nonfermenting protein kinase in the haemocoel after injection—revealed that MrSVP is required for virulence in the insect haemocoel.

Conclusions

Overall, our data suggest that the Mrsvp gene contributes to thermotolerance and virulence of M. robertsii. Furthermore, this gene is deeply involved in the mycosis of insect cadavers and in immune escape rather than insect cuticle penetration during infection.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1396-8) contains supplementary material, which is available to authorized users.

Differential Functions of Two Metalloproteases, Mrmep1 and Mrmep2, in Growth, Sporulation, Cell Wall Integrity, and Virulence in the Filamentous Fungus Metarhizium robertsii

The Metarhizium genus of filamentous entomopathogenic fungi plays a pivotal role in regulating insect populations. Metalloproteases (MEPs) are a widely distributed and diverse family of hydrolytic enzymes that are important toxicity factors in the interactions between fungi and their hosts. Herein, we characterized two MEPs, Mrmep1 and Mrmep2, in Metarhizium robertsii using gene deletion. Growth rates of the resulting ΔMrmep1 and ΔMrmep2 mutants decreased by 16.2 and 16.5%, respectively, relative to the wild-type (WT) strain. Both mutants were less sensitive to cell wall-perturbing agents, sodium dodecyl sulfate and Congo red than the WT strain, whereas did not show any obvious changes in fungal sensitivity to ultraviolet B irradiation or heat stress. The conidial yield of ΔMrmep1, ΔMrmep2, and ΔMrmep1ΔMrmep2 mutants decreased by 56.0, 23, and 53%, respectively. Insect bioassay revealed that median lethal time values against Galleria mellonella increased by 25.5% (ΔMrmep1), 19% (ΔMrmep2), and 28.8% (ΔMrmep1ΔMrmep2) compared with the WT strain at a concentration of 1 × 10⁷ conidia mL^-1, suggesting attenuated fungal virulence in the ΔMrmep1, ΔMrmep2, and ΔMrmep1ΔMrmep2 strains. During fungal infection, transcription levels of Mrmep1 was 1.6-fold higher than Mrmep2 at 36 h post inoculation. Additionally, transcription levels of gallerimycin gene were 1.2-fold, 2.18-fold, and 2.5-fold higher in insects infected with the ΔMrmep1, ΔMrmep2, or ΔMrmep1ΔMrmep2 mutant than those infected with the WT strain, respectively. Our findings suggest that Mrmep1 and Mrmep2 are differentially contributed to the growth, sporulation, cell wall integrity, and virulence of M. robertsii.

Genetically engineering better fungal biopesticides

Microbial insect pathogens offer an alternative means of pest control with the potential to wean us off our heavy reliance on chemical pesticides. Insect pathogenic fungi play an important natural role in controlling disease vectors and agricultural pests. Most commercial products employ Ascomycetes in the genera Metarhizium and Beauveria. However, their utilization has been limited by inconsistent field results as a consequence of sensitivity to abiotic stresses and naturally low virulence. Other naturally occurring biocontrol agents also face these hurdles to successful application, but the availability of complete genomes and recombinant DNA technologies have facilitated design of multiple fungal pathogens with enhanced virulence and stress resistance. Many natural and synthetic genes have been inserted into entomopathogen genomes. Some of the biggest gains in virulence have been obtained using genes encoding neurotoxic peptides, peptides that manipulate host physiology and proteases and chitinases that degrade the insect cuticle. Prokaryotes, particularly extremophiles, are useful sources of genes for improving entomopathogen resistance to ultraviolet (UV) radiation. These biological insecticides are environmentally friendly and cost-effective insect pest control options. © 2017 Society of Chemical Industry.

Genetic manipulation of Fonsecaea pedrosoi using particles bombardment and Agrobacterium mediated transformation

Fonsecaea pedrosoi, a melanized fungal pathogen that causes Chromoblastomycosis, a human disease with a worldwide distribution. Biolistic is a widely used technique for direct delivery of genetic material into intact cells by particles bombardment. Another well-established transformation method is Agrobacterium-mediated transformation (ATMT), which involves the transfer of a T-DNA from the bacterium to the target cells. In F. pedrosoi there are no reports of established protocols for genetic transformation, which require optimization of physical and biological parameters. In this work, intact conidia of F. pedrosoi were particle bombarded and subjected to ATMT. In addition, we proposed hygromycin B, nourseothricin and neomycin as dominant selective markers for F. pedrosoi and vectors were constructed. We tested two parameters for biolistic: the distance of the particles to the target cells and time of cells recovery in nonselective medium. The biolistic efficiency was 37 transformants/μg of pFpHYG, and 45 transformants/μg of pAN7.1. Transformants expressing GFP were successfully obtained by biolistic. A co-culture ratio of 10: 1 (bacterium: conidia) and co-incubation time of 72 h yielded the largest number of transformants after ATMT. Southern blot analysis showed the number of foreign DNA insertion into the genome is dependent upon the plasmid used to generate the mutants. This work describes for the first time two efficient methods for genetic modification of Fonsecaea and these results open new avenues to better understand the biology and pathogenicity of the main causal agent of this neglected disease.

Alternative transcription start site selection in Mr-OPY2 controls lifestyle transitions in the fungus Metarhizium robertsii

Metarhizium robertsii is a versatile fungus with saprophytic, plant symbiotic and insect pathogenic lifestyle options. Here we show that M. robertsii mediates the saprophyte-to-insect pathogen transition through modulation of the expression of a membrane protein, Mr-OPY2. Abundant Mr-OPY2 protein initiates appressorium formation, a prerequisite for infection, whereas reduced production of Mr-OPY2 elicits saprophytic growth and conidiation. The precise regulation of Mr-OPY2 protein production is achieved via alternative transcription start sites. During saprophytic growth, a single long transcript is produced with small upstream open reading frames in its 5′ untranslated region. Increased production of Mr-OPY2 protein on host cuticle is achieved by expression of a transcript variant lacking a small upstream open reading frame that would otherwise inhibit translation of Mr-OPY2. RNA-seq and qRT-PCR analyses show that Mr-OPY2 is a negative regulator of a transcription factor that we demonstrate is necessary for appressorial formation. These findings provide insights into the mechanisms regulating fungal lifestyle transitions.

The fungus Metarhizium robertsii can act as a saprophyte, plant symbiont and insect pathogen. Here, the authors show that the use of alternative transcription start sites controls the expression of membrane protein Mr-OPY2, which in turn modulates the saprophyte-to-pathogen transition.

A silver bullet in a golden age of functional genomics: the impact of Agrobacterium-mediated transformation of fungi

The implementation of Agrobacterium tumefaciens as a transformation tool revolutionized approaches to discover and understand gene functions in a large number of fungal species. A. tumefaciens mediated transformation (AtMT) is one of the most transformative technologies for research on fungi developed in the last 20 years, a development arguably only surpassed by the impact of genomics. AtMT has been widely applied in forward genetics, whereby generation of strain libraries using random T-DNA insertional mutagenesis, combined with phenotypic screening, has enabled the genetic basis of many processes to be elucidated. Alternatively, AtMT has been fundamental for reverse genetics, where mutant isolates are generated with targeted gene deletions or disruptions, enabling gene functional roles to be determined. When combined with concomitant advances in genomics, both forward and reverse approaches using AtMT have enabled complex fungal phenotypes to be dissected at the molecular and genetic level. Additionally, in several cases AtMT has paved the way for the development of new species to act as models for specific areas of fungal biology, particularly in plant pathogenic ascomycetes and in a number of basidiomycete species. Despite its impact, the implementation of AtMT has been uneven in the fungi. This review provides insight into the dynamics of expansion of new research tools into a large research community and across multiple organisms. As such, AtMT in the fungi, beyond the demonstrated and continuing power for gene discovery and as a facile transformation tool, provides a model to understand how other technologies that are just being pioneered, e.g. CRISPR/Cas, may play roles in fungi and other eukaryotic species.

Metarhizium robertsii produces indole-3-acetic acid, which promotes root growth in Arabidopsis and enhances virulence to insects

The plant root colonizing insect-pathogenic fungus Metarhizium robertsii has been shown to boost plant growth, but little is known about the responsible mechanisms. Here we show that M. robertsii promotes lateral root growth and root hair development of Arabidopsis seedlings in part through an auxin [indole-3-acetic acid (IAA)]-dependent mechanism. M. robertsii, or its auxin-containing culture filtrate promoted root proliferation, activated IAA-regulated gene expression and rescued the root hair defect of the IAA-deficient rhd6 Arabidopsis mutant. Substrate feeding assays suggest that M. robertsii possesses tryptamine (TAM) and indole-3-acetamide tryptophan (Trp)-dependent auxin biosynthetic pathways. Deletion of Mrtdc impaired M. robertsii IAA production by blocking conversion of Trp to TAM but the reduction was not sufficient to affect plant growth enhancement. We also show that M. robertsii secretes IAA on insect cuticle. ∆Mrtdc produced fewer infection structures and was less virulent to insects than the wild-type, whereas M. robertsii spores harvested from culture media containing IAA were more virulent. Furthermore, exogenous application of IAA increased appressorial formation and virulence. Together, these results suggest that auxins play an important role in the ability of M. robertsii to promote plant growth, and the endogenous pathways for IAA production may also be involved in regulating entomopathogenicity. Auxins were also produced by other Metarhizium species and the endophytic insect pathogen Beauveria bassiana suggesting that interplay between plant- and fungal-derived auxins has important implications for plant-microbe-insect interactions.

Dicer and Argonaute Genes Involved in RNA Interference in the Entomopathogenic Fungus Metarhizium robertsii

RNA interference (RNAi) is a gene-silencing mechanism that plays an important role in gene regulation in a number of eukaryotic organisms. Two core components, Dicer and Argonaute, are central in the RNAi machinery. However, the physiological roles of Dicer and Argonaute in the entomopathogenic fungus Metarhizium robertsii have remained unclear. Here, the roles of genes encoding Dicer (M. robertsiidcl1 [Mrdcl1] and Mrdcl2) and Argonaute (Mrago1 and Mrago2) proteins in M. robertsii were investigated. The results showed that the Dicer-like protein MrDCL2 and Argonaute protein MrAGO1 are the major components of the RNAi process occurring in M. robertsii The Dicer and Argonaute genes were not involved in the regulation of growth and diverse abiotic stress response in M. robertsii under the tested conditions. Moreover, our results showed that the Dicer and Argonaute gene mutants demonstrated reduced abilities to produce conidia, compared to the wild type (WT) and the gene-rescued mutant. In particular, the conidial yields in the Δdcl2 and Δago1 mutants were reduced by 55.8% and 59.3%, respectively, compared with those from the control strains. Subsequently, for the WT and Δdcl2 mutant strains, digital gene expression (DGE) profiling analysis of the stage of mycelium growth and conidiogenesis revealed that modest changes occur in development or metabolism processes, which may explain the reduction in conidiation in the Δdcl2 mutant. In addition, we further applied high-throughput sequencing technology to identify small RNAs (sRNAs) that are differentially expressed in the WT and the Δdcl2 mutant and found that 4 known microRNA-like small RNAs (milRNAs) and 8 novel milRNAs were Mrdcl2 dependent in M. robertsiiIMPORTANCE The identification and characterization of components in RNAi have contributed significantly to our understanding of the mechanism and functions of RNAi in eukaryotes. Here, we found that Dicer and Argonaute genes play an important role in regulating conidiation in M. robertsii Our study also demonstrates that diverse small RNA pathways exist in M. robertsii The study provides a theoretical platform for exploration of the functions of Dicer and Argonaute genes involved in RNAi in fungi.

Evolution of the chitin synthase gene family correlates with fungal morphogenesis and adaption to ecological niches

The fungal kingdom potentially has the most complex chitin synthase (CHS) gene family, but evolution of the fungal CHS gene family and its diversification to fulfill multiple functions remain to be elucidated. Here, we identified the full complement of CHSs from 231 fungal species. Using the largest dataset to date, we characterized the evolution of the fungal CHS gene family using phylogenetic and domain structure analysis. Gene duplication, domain recombination and accretion are major mechanisms underlying the diversification of the fungal CHS gene family, producing at least 7 CHS classes. Contraction of the CHS gene family is morphology-specific, with significant loss in unicellular fungi, whereas family expansion is lineage-specific with obvious expansion in early-diverging fungi. ClassV and ClassVII CHSs with the same domain structure were produced by the recruitment of domains PF00063 and PF08766 and subsequent duplications. Comparative analysis of their functions in multiple fungal species shows that the emergence of ClassV and ClassVII CHSs is important for the morphogenesis of filamentous fungi, development of pathogenicity in pathogenic fungi, and heat stress tolerance in Pezizomycotina fungi. This work reveals the evolution of the fungal CHS gene family, and its correlation with fungal morphogenesis and adaptation to ecological niches.

Carbon translocation from a plant to an insect-pathogenic endophytic fungus

Metarhizium robertsii is a common soil fungus that occupies a specialized ecological niche as an endophyte and an insect pathogen. Previously, we showed that the endophytic capability and insect pathogenicity of Metarhizium are coupled to provide an active method of insect-derived nitrogen transfer to a host plant via fungal mycelia. We speculated that in exchange for this insect-derived nitrogen, the plant would provide photosynthate to the fungus. By using ¹³CO₂, we show the incorporation of ¹³C into photosynthate and the subsequent translocation of ¹³C into fungal-specific carbohydrates (trehalose and chitin) in the root/endophyte complex. We determined the amount of ¹³C present in root-associated fungal biomass over a 21-day period by extracting fungal carbohydrates and analysing their composition using nuclear magnetic resonance (NMR) spectroscopy. These findings are evidence that the host plant is providing photosynthate to the fungus, likely in exchange for insect-derived nitrogen in a tripartite, and symbiotic, interaction.

The endophytic fungus Metarhizium robertsii is also an insect pathogen and can facilitate transfer of insect-derived nitrogen to host plants. Here, the authors show that carbon is transferred from plant to fungus suggesting a tripartite interaction where nitrogen is exchanged for photosynthate.

Efficient production of Aschersonia placenta protoplasts for transformation using optimization algorithms

The insect pathogenic fungus Aschersonia placenta is a highly effective pathogen of whiteflies and scale insects. However, few genetic tools are currently available for studying this organism. Here we report on the conditions for the production of transformable A. placenta protoplasts using an optimized protocol based on the response surface method (RSM). Critical parameters for protoplast production were modelled by using a Box-Behnken design (BBD) involving 3 levels of 3 variables that was subsequently tested to verify its ability to predict protoplast production (R(2) = 0.9465). The optimized conditions resulted in the highest yield of protoplasts ((4.41 ± 0.02) × 10(7) cells/mL of culture, mean ± SE) when fungal cells were treated with 26.1 mg/mL of lywallzyme for 4 h of digestion, and subsequently allowed to recover for 64.6 h in 0.7 mol/L NaCl-Tris buffer. The latter was used as an osmotic stabilizer. The yield of protoplasts was approximately 10-fold higher than that of the nonoptimized conditions. Generated protoplasts were transformed with vector PbarGPE containing the bar gene as the selection marker. Transformation efficiency was 300 colonies/(μg DNA·10(7) protoplasts), and integration of the vector DNA was confirmed by PCR. The results show that rational design strategies (RSM and BBD methods) are useful to increase the production of fungal protoplasts for a variety of downstream applications.

Genetically Engineering Entomopathogenic Fungi

Entomopathogenic fungi have been developed as environmentally friendly alternatives to chemical insecticides in biocontrol programs for agricultural pests and vectors of disease. However, mycoinsecticides currently have a small market share due to low virulence and inconsistencies in their performance. Genetic engineering has made it possible to significantly improve the virulence of fungi and their tolerance to adverse conditions. Virulence enhancement has been achieved by engineering fungi to express insect proteins and insecticidal proteins/peptides from insect predators and other insect pathogens, or by overexpressing the pathogen's own genes. Importantly, protein engineering can be used to mix and match functional domains from diverse genes sourced from entomopathogenic fungi and other organisms, producing insecticidal proteins with novel characteristics. Fungal tolerance to abiotic stresses, especially UV radiation, has been greatly improved by introducing into entomopathogens a photoreactivation system from an archaean and pigment synthesis pathways from nonentomopathogenic fungi. Conversely, gene knockout strategies have produced strains with reduced ecological fitness as recipients for genetic engineering to improve virulence; the resulting strains are hypervirulent, but will not persist in the environment. Coupled with their natural insect specificity, safety concerns can also be mitigated by using safe effector proteins with selection marker genes removed after transformation. With the increasing public concern over the continued use of synthetic chemical insecticides and growing public acceptance of genetically modified organisms, new types of biological insecticides produced by genetic engineering offer a range of environmentally friendly options for cost-effective control of insect pests.

MAPK cascade-mediated regulation of pathogenicity, conidiation and tolerance to abiotic stresses in the entomopathogenic fungus Metarhizium robertsii

Metarhizium robertsii has been used as a model to study fungal pathogenesis in insects, and its pathogenicity has many parallels with plant and mammal pathogenic fungi. MAPK (Mitogen-activated protein kinase) cascades play pivotal roles in cellular regulation in fungi, but their functions have not been characterized in M. robertsii. In this study, we identified the full complement of MAPK cascade components in M. robertsii and dissected their regulatory roles in pathogenesis, conidiation and stress tolerance. The nine components of the Fus3, Hog1 and Slt2-MAPK cascades are all involved in conidiation. The Fus3- and Hog1-MAPK cascades are necessary for tolerance to hyperosmotic stress, and the Slt2- and Fus3-MAPK cascades both mediate cell wall integrity. The Hog1 and Slt2-MAPK cascades contribute to pathogenicity; the Fus3-MAPK cascade is indispensable for fungal pathogenesis. During its life cycle, M. robertsii experiences multiple microenvironments as it transverses the cuticle into the haemocoel. RNA-seq analysis revealed that MAPK cascades collectively play a major role in regulating the adaptation of M. robertsii to the microenvironmental change from the cuticle to the haemolymph. The three MAPKs each regulate their own distinctive subset of genes during penetration of the cuticle and haemocoel colonization, but they function redundantly to regulate adaptation to microenvironmental change.

Development of genetic tools for Myceliophthora thermophila

Background

The thermophilic filamentous fungus Myceliophthora thermophila has many suitable characteristics for industrial biotechnology and could be a promising new chassis system for synthetic biology, particularly the ATCC 42464 strain, whose genome was sequenced in 2011. However, metabolic engineering of this strain using genetic approaches has not been reported owing to a lack of genetic tools for this organism.

Results

In the present study, we developed a high efficiency Agrobacterium tumefaciens mediated transformation system for M. thermophila, including an approach for targeted gene deletion using green fluorescence protein (GFP) as a marker for selection. Up to 145 transformants per 10⁵ conidia were obtained in one transformation plate. Moreover, a ku70 deletion mutant was constructed in the ATCC 42464 background using the tools developed in present study and subsequently characterized. The ku70 deletion construct was designed using resistance to phosphinothricin as the selection marker. Additionally, a GFP-encoding cassette was incorporated that allowed for the selection of site-specific (no fluorescence) or ectopic (fluorescence) integration of the ku70 construct. Transformants with ectopically integrated ku70 deletion constructs were therefore identified using the fluorescent signal of GFP. PCR and Southern blotting analyses of non-fluorescent putative ku70 deletion transformants revealed all 11 tested transformants to be correct deletions. The deletion frequency in a pool of 116 transformants analyzed was 58 %. Moreover, the homologous rate improved about 3 folds under ku70 mutant using the pyrG as a test gene to disrupt in M. thermophila.

Conclusions

We successfully developed an efficient transformation and target gene disruption approach for M. thermophila ATCC 42464 mediated by A. tumefaciens. The tools and the ku70 deletion strain developed here should advance the development of M. thermophila as an industrial host through metabolic engineering and accelerate the elucidation of the mechanism of rapid cellulose degradation in this thermophilic fungus.