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

MaNrtB, a Putative Nitrate Transporter, Contributes to Stress Tolerance and Virulence in the Entomopathogenic Fungus Metarhizium acridum

Nitrogen is an essential nutrient that frequently determines the growth rate of fungi. Nitrate transporter proteins (Nrts) play a crucial role in the cellular absorption of nitrate from the environment. Entomopathogenic fungi (EPF) have shown their potential in the biological control of pests. Thus, comprehending the mechanisms that govern the pathogenicity and stress tolerance of EPF is helpful in improving the effectiveness and practical application of these fungal biocontrol agents. In this study, we utilized homologous recombination to create MaNrtB deletion mutants and complementation strains. We systematically investigated the biological functions of the nitrate transporter protein gene MaNrtB in M. acridum. Our findings revealed that the disruption of MaNrtB resulted in delayed conidial germination without affecting conidial production. Stress tolerance assays demonstrated that the MaNrtB disruption strain was more vulnerable to UV-B irradiation, hyperosmotic stress, and cell wall disturbing agents, yet it exhibited increased heat resistance compared to the wild-type strain. Bioassays on the locust Locusta migratoria manilensis showed that the disruption of MaNrtB impaired the fungal virulence owing to the reduced appressorium formation on the insect cuticle and the attenuated growth in the locust hemolymph. These findings provide new perspectives for understanding the pathogenesis of EPF.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

MaPac2, a Transcriptional Regulator, Is Involved in Conidiation, Stress Tolerances and Pathogenicity in Metarhizium acridum

The Gti1/Pac2 protein family, which is highly conserved across fungi, is pivotal in processes such as fungal development, spore formation, protein export, toxin production, and virulence. Despite its importance, the precise functions of Pac2 within entomopathogenic fungi have yet to be fully understood. In our study, the MaPac2 gene from M. acridum was identified, and its functions were explored. Studying the domain of the protein showed that MaPac2 comprises 422 amino acids with a characteristic Gti1/Pac2 family domain (Pfam09729). Additionally, MaPac2 is predicted to have an N-terminal protein kinase A phosphorylation site and a potential cyclin-dependent kinase phosphorylation site, highlighting its potential regulatory roles in the fungus. Our findings indicate that the inactivation of MaPac2 resulted in faster germination of conidia and a marked reduction in conidial production. Furthermore, stress tolerance tests revealed that the absence of MaPac2 significantly bolstered the fungal resilience to UV-B radiation, heat shock, SDS exposure, and stresses induced by hyperosmotic conditions and oxidative challenges. Virulence assessments through bioassays indicated no substantial differences among the WT, MaPac2-disrupted strain, and CP strains in the topical inoculation trials. Interestingly, deletion of MaPac2 increased the fungal virulence by intrahemocoel injection. Furthermore, we found that disruption of MaPac2 impaired fungal cuticle penetration due to the diminished appressorium formation but increased the fungal growth in locust hemolymph. These findings provide further insights into the roles played by Gti1/Pac2 in insect pathogenic fungi.

Melanin in fungi: advances in structure, biosynthesis, regulation, and metabolic engineering

Fungi can synthesize a diverse range of melanins with appropriate physicochemical and biological characteristics for numerous applications in health, environmental protection, energy, and industry. Gaining deeper insights into the chemical structures, biosynthetic pathways, and regulatory mechanisms of fungal melanin would establish a basis for metabolic engineering approaches, aimed at enhancing production efficiency and creating custom-designed melanin with desirable material properties. Due to growing interest in their beneficial effects and applications, research on the structure, biosynthesis, and regulation of fungal melanin has significantly advanced. This review highlighted recent progress in fungal melanin production and applications, concentrating on structure, biosynthesis, and regulatory networks, and suggested how an improved understanding of melanin biosynthesis could enable efficient production for future applications.

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

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

The Forkhead Box Gene, MaSep1, Negatively Regulates UV- and Thermo-Tolerances and Is Required for Microcycle Conidiation in Metarhizium acridum

Insect pathogenic fungi have shown great potential in agricultural pest control. Conidiation is crucial for the survival of filamentous fungi, and dispersal occurs through two methods: normal conidiation, where conidia differentiate from mycelium, and microcycle conidiation, which involves conidial budding. The conidiation process is related to cell separation. The forkhead box gene Sep1 in Schizosaccharomyces pombe plays a crucial role in cell separation. Nevertheless, the function of Sep1 has not been clarified in filamentous fungi. Here, MaSep1, the homolog of Sep1 in Metarhizium acridum, was identified and subjected to functional analysis. The findings revealed that conidial germination of the MaSep1-deletion strain (ΔMaSep1) was accelerated and the time for 50% germination rate of conidial was shortened by 1 h, while the conidial production of ΔMaSep1 was considerably reduced. The resistances to heat shock and UV-B irradiation of ΔMaSep1 were enhanced, and the expression of some genes involved in DNA damage repair and heat shock response was significantly increased in ΔMaSep1. The disruption of MaSep1 had no effect on the virulence of M. acridum. Interestingly, ΔMaSep1 conducted the normal conidiation on the microcycle conidiation medium, SYA. Furthermore, 127 DEGs were identified by RNA-Seq between the wild-type and ΔMaSep1 strains during microcycle conidiation, proving that MaSep1 mediated the conidiation pattern shift by governing some genes associated with conidiation, cell division, and cell wall formation.

MaAzaR, a Zn2Cys6/Fungus-Specific Transcriptional Factor, Is Involved in Stress Tolerance and Conidiation Pattern Shift in Metarhizium acridum

Entomopathogenic fungi are valuable sources of biological pesticides, with conidial yield and quality being pivotal factors determining their broad applications. AzaR, a fungus-specific zinc-cluster transcription factor, is known to regulate the biosynthesis of polyketone secondary metabolites in Aspergillus niger; however, its role in pathogenic fungi remains unclear. This study investigated the role of MaAzaR in the growth, development, and environmental tolerance of Metarhizium acridum. MaAzaR deletion slowed down conidial germination rate, caused reduction in conidial yield, lowered fungal tolerance to UV radiation, did not affect fungal heat-shock tolerance, and increased fungal sensitivity to the cell-wall-destructive agent calcofluor white. Furthermore, MaAzaR deletion transformed microcycle conidiation to normal conidiation on the microcycle conidiation medium. Transcription profile analysis demonstrated that MaAzaR could regulate transformation of the conidiation pattern by controlling the expression of genes related to cell division, mycelium growth and development, and cell wall integrity. Thus, this study identified a new gene related to fungal conidiation and environmental tolerance, enriching our understanding of the molecular mechanism of microcycle conidiation and providing theoretical support and genetic resources for the development of high-yielding strains.

Pathogenicity analysis and comparative genomics reveal the different infection strategies between the generalist Metarhizium anisopliae and the specialist Metarhizium acridum

BACKGROUND

The fungal genera Metarhizium contain many important multiple species that are used as biocontrol agents and as model organisms for exploring insect-fungal interactions. Metarhizium spp. exhibit different traits of pathogenicity, suggesting that the pathogenesis can be quite distinctive. However, the underlying differences in their pathogenesis remain poorly understood.

RESULTS

Pathogenicity analysis showed that Metarhizium anisopliae (strain CQMa421) displayed higher virulence against oriental migratory locusts, Locusta migratoria manilensis (Meyen), than the acridid-specific specie Metarhizium acridum (strain CQMa102). Relative to M. acridum, M. anisopliae possessed a higher conidial hydrophobicity, increased ability to penetrate the host, accelerated growth under hypoxia and enhanced ability for the utilization of different carbon sources. Different distributions of carbohydrate epitopes at cell wall surface of M. anisopliae might also contribute to successful evasion of host immune defenses. Comparative genomics showed that M. anisopliae has 98 more virulence-related secreted proteins (133) than M. acridum (35), which can be functionally classified as hydrolases, virulence effectors, cell wall degradation and stress tolerance-related proteins, and helpful to the cuticle penetration and host internal environment adaption. In addition, differences in genomic clusters specifically related to secondary metabolites, including the clusters of Indole-NRPS hybrid, T1PKS-NRPS like hybrid, Betalactone, Fungal-Ripp and NRPS-Terpene hybrid, may lead to differences in core virulence-related secondary metabolite genes in M. acridum (18) and M. anisopliae (36).

CONCLUSION

The comparative study provided new insights into the different infection strategies between M. anisopliae and M. acridum, and further facilitate the identification of virulence-related genes for the improvement of mycoinsecticides. © 2023 Society of Chemical Industry.

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

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

Arginine metabolism governs microcycle conidiation by changing nitric oxide content in Metarhizium acridum

Microcycle conidiation commonly exists in filamentous fungi and has great potential for mass production of mycoinsecticides. L-Arginine metabolism is essential for conidiation and conditional growth and virulence, but its role in microcycle conidiation has not been explored. Here, a unique putative arginase (MaAGA) was characterized in the entomopathogenic fungus Metarhizium acridum. Conidial germination and thermotolerance were facilitated by the disruption of MaAGA. Despite little impact on fungal growth and virulence, the disruption resulted in normal conidiation after a 60-h incubation on microcycle conidiation medium (SYA) under normal culture conditions. In the MaAGA-disruption mutant (ΔMaAGA), intracellular arginine accumulation was sharply increased. Replenishment of the direct metabolites of arginase, namely ornithine and/or urea, was unable to restore the disruption mutant's microcycle conidiation on SYA. Interestingly, nitric oxide synthase (NOS) activity and nitric oxide (NO) levels of the ΔMaAGA strain were markedly decreased in the 60-h-old SYA cultures. Finally, adding N^ω-nitro-L-arginine, an inhibitor of NOS, into the SYA converted the microcycle conidiation of the wild-type strain to normal conidiation. In contrast, adding sodium nitroprusside, an NO donor, into the SYA recovered the mutant's microcycle conidiation. The results indicate that arginine metabolism controls microcycle conidiation by changing the content of NO. KEY POINTS: • The MaAGA-disruption led to normal conidiation on microcycle conidiation medium SYA. • Nitric oxide (NO) level of the ΔMaAGA strain was markedly decreased. • Adding an NO donor into the SYA recovered the microcycle conidiation of ΔMaAGA.

MaCts1, an Endochitinase, Is Involved in Conidial Germination, Conidial Yield, Stress Tolerances and Microcycle Conidiation in Metarhizium acridum

Entomopathogenic fungi are promising biocontrol agents of insect-mediated crop damage. Microcycle conidiation has shown great potential in enhancing the conidial yield and quality of entomopathogenic fungi. Homologs of Cts1, an endochitinase of Saccharomyces cerevisiae, participate in cell separation in several fungal spp. and may contribute to the morphological differences that occur during the shift to microcycle conidiation. However, the precise functions of Cts1 in entomopathogenic fungi remain unclear. Herein, the endochitinase gene, MaCts1, was characterized in the model entomopathogen, Metarhizium acridum. A loss of function line for MaCts1 led to a delay of 1 h in the median germination time, a 28% reduction in conidial yield and significant defects in fungal resistances to UV-irradiation (18%) and heat-shock (15%), while fungal tolerances to cell wall stressors, oxidative and hyperosmotic stresses and virulence remained unchanged. The MaCts1-disruption strain displayed typical conidiation on the microcycle conidiation induction medium, SYA. In contrast, deletion of key genes in the morphogenesis-related NDR kinase network (MOR pathway)/regulation of Ace2 and morphogenesis (RAM pathway) did not affect the SYA-induction of microcycle conidiation. This indicates that MaCts1 makes contributions to the microcycle conidiation, which may not be dependent on the MOR/RAM pathway in M. acridum.

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

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

MaNCP1, a C2H2 Zinc Finger Protein, Governs the Conidiation Pattern Shift through Regulating the Reductive Pathway for Nitric Oxide Synthesis in the Filamentous Fungus Metarhizium acridum

Asexual sporulation is the most common reproduction mode of fungi. Most filamentous fungi have two conidiation patterns, normal conidiation and microcycle conidiation, which may be regulated by nutritional conditions. Nitrogen source can affect the fungal conidiation pattern, but the regulatory mechanism is not fully understood. In this study, we report a C2H2 zinc finger protein, MaNCP1, which has typical transcription factor characteristics and is screened from the subtractive library regulated by nitrate in the entomopathogenic fungus Metarhizium acridum. MaNCP1 and its N-terminal play critical roles in the conidiation pattern shift. Further study shows that MaNCP1 interacts with MaNmrA, which also contributes to the conidiation pattern shift and is involved in the reductive pathway of nitric oxide (NO) synthesis. Intriguingly, the conidiation pattern of the MaNCP1-disruption strain (ΔMaNCP1) can be restored to microcycle conidiation when grown on the microcycle conidiation medium, SYA, supplemented with NO donor or overexpressing MaNmrA in ΔMaNCP1. Here, we reveal that MaNCP1 governs the conidiation pattern shift through regulating the reductive synthesis of NO by physically targeting MaNmrA in M. acridum. This work provides new mechanistic insights into how changes in nitrogen utilization are linked to the regulation of fungal morphological changes. IMPORTANCE Fungal conidia play important roles in the response to environmental stimuli and evasion of the host immune system. The nitrogen source is one of the main factors affecting shifts in fungal conidiation patterns, but the regulatory mechanism involved is not fully understood. In this work, we report that the C2H2 zinc finger protein, MaNCP1, governs the conidiation pattern shift in M. acridum by targeting the MaNmrA gene, thereby altering the regulation of the reductive pathway for NO synthesis. This work provides further insights into how the nutritional environment can regulate the morphogenesis of filamentous fungi.

cAMP Signalling Pathway in Biocontrol Fungi

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

Host–Pathogen Interactions between Metarhizium spp. and Locusts

The progress in research on the interactions between Metarhizium spp. and locusts has improved our understanding of the interactions between fungal infection and host immunity. A general network of immune responses has been constructed, and the pathways regulating fungal pathogenicity have also been explored in depth. However, there have been no systematic surveys of interaction between Metarhizium spp. and locusts. The pathogenesis of Metarhizium comprises conidial attachment, germination, appressorial formation, and colonization in the body cavity of the host locusts. Meanwhile, the locust resists fungal infection through humoral and cellular immunity. Here, we summarize the crucial pathways that regulate the pathogenesis of Metarhizium and host immune defense. Conidial hydrophobicity is mainly affected by the contents of hydrophobins and chitin. Appressorial formation is regulated by the pathways of MAPKs, cAMP/PKA, and Ca²⁺/calmodulin. Lipid droplets degradation and secreted enzymes contributed to fungal penetration. The humoral response of locust is coordinated by the Toll pathway and the ecdysone. The regulatory mechanism of hemocyte differentiation and migration is elusive. In addition, behavioral fever and density-dependent population immunity have an impact on the resistance of hosts against fungal infection. This review depicts a prospect to help us understand host–pathogen interactions and provides a foundation for the engineering of entomopathogenic fungi and the discovery of insecticidal targets to control insect pests.

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.

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.

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

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

Molecular basis and regulatory mechanisms underlying fungal insecticides' resistance to solar ultraviolet irradiation

Resistance to solar ultraviolet (UV) irradiation is crucial for field-persistent control efficacies of fungal formulations against arthropod pests, because their active ingredients are formulated conidia very sensitive to solar UV wavelengths. This review seeks to summarize advances in studies aiming to quantify, understand and improve conidial UV resistance. One focus of studies has been on the many sets of genes that have been revealed in the postgenomic era to contribute to or mediate UV resistance in the insect pathogens serving as main sources of fungal insecticides. Such genetic studies have unveiled the broad basis of UV-resistant molecules including cytosolic solutes, cell wall components, various antioxidant enzymes, and numerous effectors and signaling proteins, that function in developmental, biosynthetic and stress-responsive pathways. Another focus has been on the molecular basis and regulatory mechanisms underlying photorepair of UV-induced DNA lesions and photoreactivation of UV-impaired conidia. Studies have shed light upon a photoprotective mechanism depending on not only one or two photorepair-required photolyases, but also two white collar proteins and other partners that play similar or more important roles in photorepair via interactions with photolyases. Research hotspots are suggested to explore a regulatory network of fungal photoprotection and to improve the development and application strategies of UV-resistant fungal insecticides. © 2021 Society of Chemical Industry.

MaNmrA, a Negative Transcription Regulator in Nitrogen Catabolite Repression Pathway, Contributes to Nutrient Utilization, Stress Resistance, and Virulence in Entomopathogenic Fungus Metarhizium acridum

Simple Summary

Nutrient metabolism is closely related to the growth, development, and pathogenicity of pathogenic fungi. The nitrogen catabolite repression (NCR) pathway is a well-known fungal nitrogen source regulation path, in which NmrA plays an important regulatory role. Here, we reported a negative regulatory protein MaNmrA, the NmrA homologous protein, in the entomopathogenic fungus Metarhizium acridum, and found that it played important roles in carbon and nitrogen metabolism, growth, stress tolerance, and virulence of M. acridum. Our work will provide a theoretical basis for further exploring the functions of NCR pathway related genes in entomopathogenic fungi.

Abstract

The NCR pathway plays an important regulatory role in the nitrogen metabolism of filamentous fungi. NmrA, a central negative regulatory protein in the NCR pathway and a key factor in sensing to the carbon metabolism, plays important roles in pathogenic fungal nutrition metabolism. In this study, we characterized the functions of MaNmrA in the insect pathogenic fungus M. acridum. Multiple sequence alignments found that the conserved domain (NAD/NADP binding domain) of MaNmrA was highly conservative with its homologues proteins. Deletion of MaNmrA improved the utilization of multiple carbon sources (such as glucose, mannose, sucrose, and trehalose) and non-preferred nitrogen sources (such as NaNO₃ and urea), significantly delayed the conidial germination rate and reduced the conidial yield. The MaNmrA-disruption strain (ΔMaNmrA) significantly decreased tolerances to UV-B and heat-shock, and it also increased the sensitivity to the hypertonic substance sorbitol, oxygen stress substance H₂O₂, and cell wall destroyer calcofluor white, indicating that loss of MaNmrA affected cell wall integrity, tolerances to hypertonic and oxidative stress. Bioassays demonstrated that disruption of MaNmrA decreased the virulence in both topical inoculation and intrahemocoel injection tests. Further studies revealed that the appressorium formation, turgor pressure, and colonization in hemolymph were significantly reduced in the absence of MaNmrA. Our work will deepen the functional cognition of MaNmrA and make a contribution to the study of its homologous proteins.

MaAreB, a GATA Transcription Factor, Is Involved in Nitrogen Source Utilization, Stress Tolerances and Virulence in Metarhizium acridum

The nitrogen catabolite repression (NCR) pathway is involved in nitrogen utilization, in which the global GATA transcription factor AreA plays an indispensable role and has been reported in many fungi. However, relatively few studies are focused on AreB, another GATA transcription factor in the NCR pathway and the functions of AreB are largely unknown in entomopathogenic fungi. Here, we characterized MaAreB in the model entomopathogenic fungus Metarhizium acridum. Sequence arrangement found that MaAreB had a conserved GATA zinc finger DNA binding domain and a leucine zipper domain. Disruption of MaAreB affected the nitrogen utilization and led to decelerated conidial germination and hyphal growth, decreased conidial yield, and lower tolerances to UV-B irradiation and heat-shock. Furthermore, the MaAreB mutant (ΔMaAreB) exhibited increased sensitivity to CFW (Calcofluor white), decreased cell wall contents (chitin and β-1,3-glucan) and reduced expression levels of some genes related to cell wall integrity, indicating that disruption of MaAreB affected the cell wall integrity. Bioassays showed that the virulence of the ΔMaAreB strain was decreased in topical inoculation but not in intra-hemocoel injection. Consistently, deletion of MaAreB severely impaired the appressorium formation and reduced the turgor pressure of appressorium. These results revealed that MaAreB regulated fungal nitrogen utilization, cell wall integrity and biological control potential, which would contribute to the functional characterization of AreB homologous proteins in other insect fungal pathogens, and even filamentous fungi.

MrPEX33 is involved in infection-related morphogenesis and pathogenicity of Metarhizium robertsii

Peroxisomes, being indispensable organelles, play an important role in different biological processes in eukaryotes. PEX33, a filamentous fungus-specific peroxin of the docking machinery of peroxisomes, is involved in the virulence and development of other fungal pathogens. However, it is not clear whether PEX33 is necessary for the pathogenicity and development of an insect pathogenic fungus. In the present study, we report the presence of homologs of PEX33, namely MrPEX33 (MAA_05331), in the entomopathogenic fungus, Metarhizium robertsii. An M. robertsii transgenic strain expressing the fusion protein with MrPEX33-GFP and mCherry-PTS1 showed that MrPEX33 localizes to peroxisomes. The results also demonstrated that MrPEX33 is involved in the peroxisomal import pathway by peroxisomal targeting signals. Targeted gene deletion of MrPEX33 led to a significant decline in the asexual sporulation capacity, which was accompanied by downregulation of several conidiation-associated genes, such as wetA, abaA, and brlA. More importantly, our bioassay results showed that the virulence of ∆MrPEX33 mutants, against Galleria mellonella through cuticle infection, was greatly reduced. This was further accompanied by a significant drop in appressorium formation and cuticle penetration. Additionally, ∆MrPEX33 mutants showed a significant decrease in tolerance to cell wall integrity and oxidative stress. Taken together, our results suggest that MrPEX33 is involved in the cuticle infection-related morphogenesis and pathogenicity. KEY POINTS: • MrPEX33 is a specific peroxin of the docking machinery of peroxisomes. • MrPEX33 localizes to peroxisomes and is involved in the import of matrix proteins. • MrPEX33 is involved in the pathogenicity associated with cuticle infections.

DNM1, a Dynamin-Related Protein That Contributes to Endocytosis and Peroxisome Fission, Is Required for the Vegetative Growth, Sporulation, and Virulence of Metarhizium robertsii

Although dynamins and dynamin-related proteins (DRPs), a large GTPase superfamily, are involved in the budding of transport vesicles and division of organelles in eukaryotic cells, the function of these proteins in entomopathogenic fungi has not been reported to date. Here, DNM1, a DRP in Metarhizium robertsii, was characterized using gene disruption and complementation strategies. Mutant phenotype assays showed that the ΔDnm1 strain displayed increased defects in radial growth (∼24%) and conidial production (∼42%) compared to those of the wild type (WT), and reduced conidiation levels were accompanied by the repression of several key conidiation-related genes, including flbA, wetA, and flbD Additionally, mutant bioassays revealed that disruption of Dnm1 impaired the virulence (both topical inoculation and injection) of M. robertsii in the insect Galleria mellonella Further analysis demonstrated that deleting Dnm1 in fungi suppressed the transcriptional levels of several virulence genes in the insect hemocoel. Moreover, we found that DNM1 colocalized with peroxisomes and mitochondria. Importantly, disruption of Dnm1 abolished normal fungal endocytosis, resulting in significantly decreased numbers of, as well as morphological changes in, peroxisomes. These findings indicate that deletion of Dnm1 causes significant changes in the vegetative growth, sporulation, and virulence of M. robertsii due to changes in cell function and peroxisomes.IMPORTANCE Dnm1 was found to be involved in fungal development and virulence, mediated peroxisomal fission, and normal endocytosis. This finding provides new insights into the cellular processes and pathogenicity in entomopathogenic fungi.

Phenotypic and molecular insights into heat tolerance of formulated cells as active ingredients of fungal insecticides

Formulated conidia of insect-pathogenic fungi, such as Beauveria and Metarhizium, serve as the active ingredients of fungal insecticides but are highly sensitive to persistent high temperatures (32-35 °C) that can be beyond their upper thermal limits especially in tropical areas and during summer months. Fungal heat tolerance and inter- or intra-specific variability are critical factors and limitations to field applications of fungal pesticides during seasons favoring outbreaks of pest populations. The past decades have witnessed tremendous advances in improving fungal pesticides through selection of heat-tolerant strains from natural isolates, improvements and innovations in terms of solid-state fermentation technologies for the production of more heat-tolerant conidia, and the use of genetic engineering of candidate strains for enhancing heat tolerance. More recently, with the entry into a post-genomic era, a large number of signaling and effector genes have been characterized as important sustainers of heat tolerance in both Beauveria and Metarhizium, which represent the main species used as fungal pesticides worldwide. This review focuses on recent advances and provides an overview into the broad molecular basis of fungal heat tolerance and its multiple regulatory pathways. Emphases are placed on approaches for screening of heat-tolerant strains, methods for optimizing conidial quality linked to virulence and heat tolerance particularly involving cell wall architecture and optimized trehalose/mannitol contents, and how molecular determinants can be exploited for genetic improvement of heat tolerance and pest-control potential. Examples of fungal pesticides with different host spectra and their appropriateness for use in apiculture are given. KEY POINTS: • Heat tolerance is critical for field stability and efficacy of fungal insecticides. • Inter- and intra-specific variability exists in insect-pathogenic fungi. • Optimized production technology and biotechnology can improve heat tolerance. • Fungal heat tolerance is orchestrated by multiple molecular pathways.

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.

Members of chitin synthase family in Metarhizium acridum differentially affect fungal growth, stress tolerances, cell wall integrity and virulence

Chitin is an important component of the fungal cell wall with a family of chitin synthases mediating its synthesis. Here, we report on the genetic characterization of the full suite of seven chitin synthases (MaChsI-VII) identified in the insect pathogenic fungus, Metarhizium acridum. Aberrant distribution of chitin was most evident in targeted gene knockouts of MaChsV and MaChsVII. Mutants of MaChsI, MaChsIII, MaChsIV showed delayed conidial germination, whereas ΔMaChsII and ΔMaChsV mutants germinated more rapidly when compared to the wild-type parent. All MaChs genes impacted conidial yield, but differentially affected stress tolerances. Inactivation of MaChsIII, MaChsV, MaChsVII resulted in cell wall fragility, and ΔMaChsV and ΔMaChsVII mutants showed high sensitivity to Congo red and calcofluor white, suggesting that the three genes are required for cell wall integrity. In addition, ΔMaChsIII and ΔMaChsVII mutants showed the highest sensitivities to heat and UV-B stress. Three of seven chitin synthase genes, MaChsIII, MaChsV, MaChsVII, were found to contribute to fungal virulence. Compared with the wild-type strain, ΔMaChsIII and ΔMaChsV mutants were reduced in virulence by topical inoculation, while the ΔMaChsVII mutant showed more severe virulence defects. Inactivation of MaChsIII, MaChsV, or MaChsVII impaired appressorium formation, affected growth of in insecta produced hyphal bodies, and altered the surface properties of conidia and hyphal bodies, resulting in defects in the ability of the mutant strains to evade insect immune responses. These data provide important links between the physiology of the cell wall and the ability of the fungus to parasitize insects and reveal differential functional consequences of the chitin synthase family in M. acridum growth, stress tolerances, cell wall integrity and virulence.

The fungal cell wall is a dynamic and flexible organelle that modulates the interaction of the pathogen with its host and acts as a critical recognition and evasion interface with host defenses. Chitin is a hallmark constituent of the fungal cell wall and all fungi contain multiple chitin synthase (Chs) genes. However, systematic characterization of chitin synthase genes has not yet been reported in entomopathogenic fungi. By using the insect pathogen Metarhizium acridum as a model, we performed a systematic genetic analysis of the seven member Chs family (ChsI-VII) in the insect pathogenic fungus. Construction of strains bearing targeted single gene mutations revealed differential contributions of specific Chs genes to growth, cell wall integrity, and stress responses. In addition, we revealed that three chitin synthase genes MaChsIII, MaChsV and MaChsVII were shown to be important for fungal appressorium formation and evasion of insect cellular and/or humoral defenses, promoting the fungal dimorphic transition to the production of hyphal bodies that occurs within hosts, and ultimately to virulence. These data provide new insights into the roles of Chs genes and chitin as critical components affecting fungal membrane structure and function.

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.