Catalase overexpression reduces the germination time and increases the pathogenicity of the fungus Metarhizium anisopliae

Overexpression of cat2 restores antioxidant properties and production traits in degenerated strains of Volvariella volvacea

Strain degeneration is an important factor hindering the development of the edible fungus industry. Strain degeneration is associated with the excessive accumulation of reactive oxygen species (ROS) in vivo. Catalase (CAT), an important antioxidant enzyme, can promote the clearance of ROS. In this study, the cat2 gene of Volvariella volvacea was first cloned into an overexpression plasmid via homologous recombination. Finally, through Agrobacterium-mediated transformation, this plasmid was inserted into degenerated strains of V. volvacea T19. The physiological properties, antioxidant properties, ROS content, matrix degradation activity, and cultivation properties of the transformants were tested. The results showed that the cloned cat2 gene was 99.94% similar to the reference sequence. Screening revealed that six positive transformants were successfully obtained. After the overexpression of cat2, the growth rate and biomass of the mycelium increased significantly in the transformant strains (versus the V. volvacea T19 degenerated strains). Moreover, the accumulation of superoxide radical (O2•-) and hydrogen peroxide (H2O2) was significantly reduced, and the activity of the enzymes CAT, superoxide dismutase (SOD), glutathione reductase (GR), and glutathione peroxidase (GPX) was significantly increased. Meanwhile, the expression of cat2, Mnsod1, Mnsod2, gpx, and gr was significantly upregulated, and the activity of eight matrix degradation-related enzymes was increased to varying degrees. More importantly, the overexpression of the cat2 gene promoted the regrowth of fruiting bodies in degenerated strains of V. volvacea T19. This study provides a new biotechnological strategy to control the degeneration of V. volvacea and other edible fungi.

Identification and characteristic analysis of an extracellular signal-regulated kinase from Ostrinia furnacalis Guenée

The extracellular signal-regulated kinase (ERK) pathway, a critical genetic determinant, controls diverse physiological functions, including innate immunity, development, and stress response. In the current study, a full-length cDNA (1592bp) encoding the ERK gene (OfERK) was cloned from Ostrinia furnacalis Guenée (GenBank accession number: MF797866). The open reading frame of the OfERK gene encoded 364 amino acids and shared 96.43%-98.08% amino acid identities with other insect mitogen-activated protein kinases. For spatiotemporal analysis of the expression pattern, OfERK exhibited a significant peak expression on the 3rd day of the pupa stage and showed the highest expression in hemocytes specifically. Indirect immunofluorescence assays and immuno-electron microscopy revealed a wide distribution of the OfERK protein in hemocytes and epidermis. Moreover, the results demonstrated that the Bt Cry1Ab-activated toxin significantly induces the expression of OfERK. Other genes related to immune response, development, and stress response exhibited dynamic changes in expression after Cry1Ab oral treatment. The expression of OfERK was downregulated through RNA interference, and the correlation of its expression with other related genes was verified using quantitative real-time polymerase chain reaction. Our study provides valuable insights into the regulatory mechanism of ERK in insects for future studies.

Sensing of H2O2-induced oxidative stress by the UPF factor complex is crucial for activation of catalase-3 expression in Neurospora

UPF-1-UPF-2-UPF-3 complex-orchestrated nonsense-mediated mRNA decay (NMD) is a well-characterized eukaryotic cellular surveillance mechanism that not only degrades aberrant transcripts to protect the integrity of the transcriptome but also eliminates normal transcripts to facilitate appropriate cellular responses to physiological and environmental changes. Here, we describe the multifaceted regulatory roles of the Neurospora crassa UPF complex in catalase-3 (cat-3) gene expression, which is essential for scavenging H2O2-induced oxidative stress. First, losing UPF proteins markedly slowed down the decay rate of cat-3 mRNA. Second, UPF proteins indirectly attenuated the transcriptional activity of cat-3 gene by boosting the decay of cpc-1 and ngf-1 mRNAs, which encode a well-studied transcription factor and a histone acetyltransferase, respectively. Further study showed that under oxidative stress condition, UPF proteins were degraded, followed by increased CPC-1 and NGF-1 activity, finally activating cat-3 expression to resist oxidative stress. Together, our data illustrate a sophisticated regulatory network of the cat-3 gene mediated by the UPF complex under physiological and H2O2-induced oxidative stress conditions.

Comparative transcriptomics of growth metabolism and virulence reveal distinct morphogenic profiles of yeast-like cells and hyphae of the fungus Metarhizium rileyi

Metarhizium rileyiis an entomopathogenic fungus with a narrow host range which distinguishes it from other Metarhiziumspecies with broad host ranges. This species is also unique because the initial yeast-like growth on solid media is only observed in liquid culture in other Metharizium species. A lack of knowledge about the metabolism and genetic signatures of M. rileyiduring this yeast-like phase on solid and in liquid media is a bottleneck for its large-scale production as a commercial biocontrol agent.In this study wefound that M. rileyiyeast-like cells produced on solid medium infected and killed the important insect pest Spodoptera frugiperda with comparable efficiency as yeast-like cells grown in liquid medium. Secondly, we used comparative transcriptomic analysis to investigate theactive genes and genomic signatures of the M. rileyi yeast-like morphotypes produced on solid and in liquid media. Yeast-like cells grown in liquid medium had upregulated genes relating specifically to signal transduction andparticular membrane transporters. Thirdly, we compared the transcriptomic profiles of yeast-like phases of M. rileyi with those of M. anisopliae. The yeast-like phase of M. rileyi grown on solid medium upregulated unique genes not found in otherMetarhiziumspecies including specific membrane proteins and several virulence factors. Orthologous genes associated with heat shock protein, iron permease, membrane proteins and key virulence traits (e.g. collagen-like protein Mcl1) were upregulated in both species. Comparative transcriptome analyses of gene expression showed more differences than similarities between M. anisopliae and M. rileyi yeast-like cells.

Whole Genome Sequencing and Comparative Genomics of Indian Isolates of Wheat Spot Blotch Pathogen Bipolaris sorokiniana Reveals Expansion of Pathogenicity Gene Clusters

Spot blotch is a highly destructive disease in wheat caused by the fungal pathogen Bipolaris sorokiniana (teleomorph, Cochliobolus sativus). It is prevalent in warm and humid areas, including Africa, Asia, Latin America, and the USA. In the present study, twelve isolates of B. sorokiniana were collected from wheat fields in three different geographical locations in India. The pathogenicity of seven sporulating isolates was assessed on 'DDK 1025', a spot blotch-susceptible wheat variety under greenhouse conditions. The isolate 'D2' illustrated the highest virulence, followed by 'SI' and 'BS52'. These three isolates were sequenced using the Illumina HiSeq1000 platform. The estimated genome sizes of the isolates BS52, D2, and SI were 35.19 MB, 39.32 MB, and 32.76 MB, with GC contents of 48.48%, 50.43%, and 49.42%, respectively. The numbers of pathogenicity genes identified in BS52, D2, and SI isolates were 2015, 2476, and 2018, respectively. Notably, the isolate D2 exhibited a relatively larger genome with expanded arsenals of Biosynthetic Gene Clusters (BGCs), CAZymes, secretome, and pathogenicity genes, which could have contributed to its higher virulence among the tested isolates. This study provides the first comparative genome analysis of the Indian isolates of B. sorokiniana using whole genome sequencing.

Nanosheet-Facilitated Spray Delivery of dsRNAs Represents a Potential Tool to Control Rhizoctonia solani Infection

Rhizoctonia solani is one of the important pathogenic fungi causing several serious crop diseases, such as maize and rice sheath blight. Current methods used to control the disease mainly depend on spraying fungicides because there is no immunity or high resistance available in crops. Spraying double-strand RNA (dsRNA) for induced-gene silencing (SIGS) is a new potentially sustainable and environmentally friendly tool to control plant diseases. Here, we found that fluorescein-labelled EGFP-dsRNA could be absorbed by R. solani in co-incubation. Furthermore, three dsRNAs, each targeting one of pathogenicity-related genes, RsPG1, RsCATA, and RsCRZ1, significantly downregulated the transcript levels of the target genes after co-incubation, leading to a significant reduction in the pathogenicity of the fungus. Only the spray of RsCRZ1 dsRNA, but not RsPG1 or RsCATA dsRNA, affected fungal sclerotium formation. dsRNA stability on leaf surfaces and its efficiency in entering leaf cells were significantly improved when dsRNAs were loaded on layered double hydroxide (LDH) nanosheets. Notably, the RsCRZ1-dsRNA-LDH approach showed stronger and more lasting effects than using RsCRZ1-dsRNA alone in controlling pathogen development. Together, this study provides a new potential method to control crop diseases caused by R. solani.

A secreted catalase contributes to Puccinia striiformis resistance to host-derived oxidative stress

Plants can produce reactive oxygen species (ROS) to counteract pathogen invasion, and pathogens have also evolved corresponding ROS scavenging strategies to promote infection and pathogenicity. Catalases (CATs) have been found to play pivotal roles in detoxifying H2O2 formed by superoxide anion catalyzed by superoxide dismutases (SODs). However, few studies have addressed H2O2 removing during rust fungi infection of wheat. In this study, we cloned a CAT gene PsCAT1 from Puccinia striiformis f. sp. tritici (Pst), which encodes a monofunctional heme-containing catalase. PsCAT1 exhibited a high degree of tolerance to pH and temperature, and forms high homopolymers.Heterologous complementation assays in Saccharomyces cerevisiae reveal that the signal peptide of PsCAT1 is functional. Overexpression of PsCAT1 enhanced S. cerevisiae resistance to H2O2. Transient expression of PsCAT1 in Nicotiana benthamiana suppressed Bax-induced cell death. Knockdown of PsCAT1 using a host-induced gene silencing (HIGS) system led to the reduced virulence of Pst, which was correlated to H2O2 accumulation in HIGS plants. These results indicate that PsCAT1 acts as an important pathogenicity factor that facilitates Pst infection by scavenging host-derived H2O2.

Functional Analysis of Keto-Acid Reductoisomerase ILVC in the Entomopathogenic Fungus Metarhizium robertsii

Ketol-acid reductoisomerase (ILVC) is the second enzyme in the branched-chain amino acid (BCAA) biosynthesis, which regulates many physiological activities in a variety of organisms from bacteria to fungi and plants. In this work, function mechanisms of ILVC in Metarhizium robertsii Metchnikoff (Hypocreales: Clavicipitaceae) were explored with site-directed mutagenesis, reductase activity assays and transcriptomics analysis. The reductase activity assays showed that ILVC from phytopathogenic fungi exhibited significantly higher activities than those from entomopathogenic fungi but lower than those from yeast. Site-directed mutagenesis and enzymatic activities of MrILVC with different active-site mutants (Arg-113, Ser-118, Asp-152, Asp-260, and Glu-264) confirmed that active sites of MrILVC are conserved with plant and bacterial ILVCs. Deleting MrilvC causes the complete failures of vegetative growth and conidial germination, feeding with branched-chain amino acids (BCAAs) recovers the fungal growth but not conidial germination, while both characteristics are restored when supplemented with yeast extract. Compared to ΔMrilvC cultured in czapek agar (CZA), plenty of genes involved in the biosynthesis of antibiotics and amino acids were up- or down-regulated in the wild type or ΔMrilvC feeding with either BCAAs or yeast extract. Further analysis showed some genes, such as catalase A, participate in mycelial growth and conidial germination was down-regulated in ΔMrilvC from CZA, revealing that MrILVC might control the fungal development by gene regulation and BCAAs or yeast extract could play partial roles of MrILVC. This study will advance our understanding of ILVC function mechanisms in fungi.

Effects of mercury stress on methylmercury production in rice rhizosphere, methylmercury uptake in rice and physiological changes of leaves

Methylmercury (MeHg) in rice is presumed to be derived from MeHg formed in the soil, although it is still controversial. Moderate soil mercury (Hg) concentration can affect the diversity of soil microorganisms and may also impact the physiological changes and MeHg absorption of rice. In this study, the pot experiment was conducted to explore the effects of Hg concentration gradients (0, 0.3, 3, and 30 mg kg^-1) stress on Hg transformation in the rhizosphere, Hg translocation in rice, and physiological changes in rice leaves during the whole rice growing season. Moderate soil Hg concentration (3 mg kg^-1) greatly increased the MeHg/THg (1.69%) of rhizosphere, while 30 mg kg^-1 soil Hg concentration sharply reduced the MeHg/THg (0.29%) of rhizosphere. Highest MeHg/THg of the four groups all appeared at the blooming or filling stage. There was a significant positive correlation between Fe²⁺ in rhizosphere and MeHg/THg, but no significant correlation between SO₄^2- and MeHg/THg was observed. Although the 3 mg kg^-1 soil Hg concentration significantly enhanced MeHg concentrations in seeds, it considerably reduced the bioaccumulation factors of MeHg in roots, stalks, old leaves and young leaves. Soil Hg concentration of 30 mg kg^-1, to a certain extent, curtailed MeHg concentrations in seeds, while MeHg concentrations in the husk were significantly increased. Consistent with the result that there was no significant difference for THg concentrations in old and young leaves among the four Hg treatment groups, the content of chlorophyll, H₂O₂, malondialdehyde and antioxidant substances, and the activities of antioxidant enzyme in old and young leaves varied indistinctly among groups. MAIN FINDING: Moderate soil mercury concentration (3 mg kg^-1) could extremely enhance MeHg production in the rhizosphere soil and its accumulation in rice; MeHg production in the rhizosphere soil increased greatly at the blooming or filling stage, whereas little effect on antioxidant systems in leaves was observed.

Synergy in Efficacy of Artemisia sieversiana Crude Extract and Metarhizium anisopliae on Resistant Oedaleus asiaticus

In order to explore the synergistic control effect of crude extracts of Artemisia sieversiana and Metarhizium anisopliae on Oedaleus asiaticus, we used different doses of M. anisopliae and crude extracts of A. sieversiana singly and in combination, to determine their toxicities to fourth instar O. asiaticus. The results showed that the combination of 10% crude extract of A. sieversiana with 10⁷ and 10⁸ spores/g M. anisopliae concentrations and the combination of 20% crude extract of A. sieversiana with 10⁷ and 10⁸ spores/g M. anisopliae concentrations had significant effects on the mortality, body weight gain, body length gain, growth rate, and overall performance of O. asiaticus than those of the crude extract of A. sieversiana and M. anisopliae alone. Among them, the 20% A. sieversiana crude extract mixed with 10⁸ spores/g M. anisopliae and 10% A. sieversiana crude extract combined with 10⁷ spores/g M. anisopliae, had the best control efficacy. In order to clarify the biochemical mechanism underlying the immune responses of O. asiaticus to the pesticide treatments, we monitored the activities of four enzymes: superoxidase dismutase (SOD), peroxidase (POD), catalase (CAT), and polyphenol oxidase (PPO). The results showed that the activities of three enzymes (SOD, CAT, and PPO) were significantly increased from the treatment with the combination of M. anisopliae mixed with crude extract of A. sieversiana. Interestingly, compared to the crude extract, the combination treatment did not significantly induce the expression of POD enzyme activity, which may be a biochemical factor for increasing the control effect of the combination treatment. Our results showed that the combination treatment had synergistic and antagonistic effects on host mortality, growth, development, and enzyme activities in O. asiaticus.

Exposure to a sublethal menadione concentration modifies the mycelial secretome and conidial enzyme activities of Metarhizium anisopliae sensu lato and increases its virulence against Rhipicephalus microplus

Menadione (MND) is known to induce oxidative stress in fungal cells. Here, we explore how exposure to this molecule alters conidial enzyme activities, fungal efficacy against Rhipicephalus microplus, and mycelial secretion (secretome) of an isolate of Metarhizium anisopliae sensu lato. First, the fungus was exposed to different MND concentrations in potato-dextrose-agar (PDA) to determine the LC₅₀ by evaluating conidia germination (38μM). To ensure high cell integrity, a sublethal dose of MND (half of LC₅₀) was added to solid (PDA MND) and liquid media (MS MND). Changes in colony growth, a slight reduction in conidia production, decreases in conidial surface Pr1 and Pr2 activities as well as improvements in proteolytic and antioxidant (catalase, superoxide dismutase, and peroxidase) conidial intracellular activities were observed for PDA MND conidia. Additionally, PDA MND conidia had the best results for killing tick larvae, with the highest mortality rates until 15 days after treatment, which reduces both LC₅₀ and LT₅₀, particularly at 10⁸ conidia mL^-1. The diversity of secreted proteins after growth in liquid medium + R. microplus cuticle (supplemented or not with half of MND LC₅₀), was evaluated by mass spectrometry-based proteomics. A total of 654 proteins were identified, 31 of which were differentially regulated (up or down) and mainly related to antioxidant activity (catalase), pathogenicity (Pr1B, Pr1D, and Pr1K), cell repair, and morphogenesis. In the exclusively MS MND profile, 48 proteins, mostly associated with cellular signaling, nutrition, and antioxidant functions, were distinguished. Finally, enzymatic assays were performed to validate some of these proteins. Overall, supplementation with MND in the solid medium made conidia more efficient at controlling R. microplus larvae, especially by increasing, inside the conidia, the activity of some infection-related enzymes. In the liquid medium (a consolidated study model that mimics some infection conditions), proteins were up- and/or exclusively-regulated in the presence of MND, which opens a spectrum of new targets for further study to improve biological control of ticks using Metarhizium species.

Interaction between Metarhizium anisopliae and Its Host, the Subterranean Termite Coptotermes curvignathus during the Infection Process

Metarhizium anisopliae (Metchnikoff) Sorokin, a pathogenic fungus to insects, infects the subterranean termite, Coptotermes curvignathus Holmgren, a devastating pest of plantation trees in the tropics. Electron microscopy and proteomics were used to investigate the infection and developmental process of M. anisopliae in C. curvignathus. Fungal infection was initiated by germ tube penetration through the host's cuticle as observed at 6 h post-inoculation (PI), after which it elongated into the host's integumental tissue. The colonization process continued as seen from dissemination of blastospores in the hemocoel at 96 h PI. At this time point, the emergent mycelia had mummified the host and forty-eight hours later, new conidia were dispersed on the termites' body surface. Meanwhile, hyphal bodies were observed in abundance in the intercellular space in the host's body. The proteomes of the pathogen and host were isolated separately using inoculated termite samples withdrawn at each PI-time point and analyzed in two-dimensional electrophoresis (2-DE) gels. Proteins expressed in termites showed evidence of being related to cell regulation and the immune response, while those expressed in M. anisopliae, to transportation and fungal virulence. This study provides new information on the interaction between termites and its entomopathogen, with potential utilization for developing future biopesticide to control the termite population.

Infection of the Stable Fly, Stomoxys calcitrans, L. 1758 (Diptera: Muscidae) by the Entomopathogenic Fungi Metarhizium anisopliae (Hypocreales: Clavicipitaceae) Negatively Affects Its Survival, Feeding Propensity, Fecundity, Fertility, and Fitness Parameters

Entomopathogenic fungi can cause substantial mortality in harmful insects. Before killing the insect, these pathogens start by negatively affecting the biological parameters of the host. Prior to our study, the information about how fungal exposure affects the biological parameters of the stable fly, Stomoxys calcitrans was still elusive. Therefore, we aimed to assess the infection of S. calcitrans with some Metarhizium anisopliae strains, and their impact on feeding, fecundity, fertility and other life-history traits of this fly. Among the 11 M. anisopliae strains screened, we identified ICIPE 30 as the most virulent strain against S. calcitrans. We observed that the infectivity of this strain was sex and age-dependent. Infected male S. calcitrans died earlier than their counterpart females. Older infected S. calcitrans died faster than infected young ones. Also, male and female S. calcitrans successfully transmitted ICIPE 30 conidia to their mates. We demonstrated that infection by ICIPE 30 extended the feeding time of S. calcitrans and consequently reduced the feeding probability of the fly and the amount of blood taken. Using a dual test oviposition bioassay, we determined that uninfected gravid female S. calcitrans avoided laying eggs on substrates amended with ICIPE 30 conidia. We showed that these conidia could lower the hatchability of the eggs deposited by gravid females. Using, a no-choice test, we showed that gravid female S. calcitrans infected with ICIPE 30 laid fewer eggs than uninfected females and those eggs hatched less. Using 11 strains of M. anisopliae and four high concentrations of ICIPE 30 conidia, we verified that S. calcitrans larvae were not susceptible to fungal infection. Further, we showed that though these larvae were tolerant to fungal infection, there was a significant effect on their fitness, with contaminated larvae having a small bodyweight coupled with longer developmental time as compared to uncontaminated larvae. Our study provides detailed information on how fungal infection affects the biology of S. calcitrans and the potential of using M. anisopliae ICIPE 30 as a biopesticide to reduce the fly population. Such knowledge can assist in developing fungal-based control strategies against this harmful fly.

Virulence Factors of the Entomopathogenic Genus Metarhizium

The fungal genus Metarhizium has been used as an entomopathogen worldwide for approximately 140 years, and its mechanism of infection and its virulence factors have been studied. The present review is a compilation of virulence factors described in the literature to date and their participation in specific stages of the infection process.

Gene expression profiling of reactive oxygen species (ROS) and antioxidant defense system following Sugarcane mosaic virus (SCMV) infection

BACKGROUND

Viruses are infectious pathogens, and plant virus epidemics can have devastating consequences to crop yield and quality. Sugarcane mosaic virus (SCMV, belonging to family Potyviridae) is one of the leading pathogens that affect the sugarcane crop every year. To combat the pathogens' attack, plants generate reactive oxygen species (ROS) as the first line of defense whose sophisticated balance is achieved through well-organized antioxidant scavenging pathways.

RESULTS

In this study, we investigated the changes occurring at the transcriptomic level of ROS associated and ROS detoxification pathways of SCMV resistant (B-48) and susceptible (Badila) sugarcane genotypes, using Saccharum spontaneum L. genome assembly as a reference genome. Transcriptomic data highlighted the significant upregulation of ROS producing genes such as NADH oxidase, malate dehydrogenase and flavin-binding monooxygenase, in Badila genotype after SCMV pathogenicity. To scavenge the ROS, the Badila genotype illustrated a substantial enhancement of antioxidants i.e. glutathione s-transferase (GST), as compared to its resistant counterpart. GST is supposed to be a key indicator of pathogen attacks on the plant. A remarkably lower GST expression in B-48, as compared to Badila, indicated the development of resistance in this genotype. Additionally, we characterized the critical transcription factors (TFs) involved in endowing resistance to B-48. Among these, WRKY, AP2, NAC, bZIP, and bHLH showed enhanced expression in the B-48 genotype. Our results also confirmed the linkage of transcriptomic data with the enzymatic and qPCR data. The estimation of enzymatic activities for superoxide dismutase, catalase, ascorbate peroxidase, and phenylalanine ammonia-lyase supported the transcriptomic data and evinced higher resistance in B-48 genotype.

CONCLUSION

The current study supported the efficiency of the B-48 genotype under SCMV infection. Moreover, comparative transcriptomic data has been presented to highlight the role of significant transcription factors conferring resistance to this genotype. This study provides an in-depth knowledge of the expression profiling of defense mechanisms in sugarcane.

The PAC-3 transcription factor critically regulates phenotype-associated genes in Neurospora crassa

Transcription factors play an important role in fungal environmental adaptive process by promoting adjustment to challenging stimuli via gene modulation and activation of signaling networks. The transcription factor encoded by the pac-3/rim101/pacC gene is involved in pH regulation and is associated with a wide variety of cellular functions. The deletion of pac-3 affects fungal development. In Neurospora crassa, the Δpac-3 strain presents diminished aerial growth and reduced conidiation. However, the PAC-3-regulated genes associated with this altered phenotype have not been elucidated. In this study, we used RNA-seq to analyze the phenotypic plasticity induced after pac-3 deletion in the filamentous fungus N. crassa cultivated in media supplemented with sufficient or limited inorganic phosphate. Genes related to morphology, hyphal development, and conidiation were of particular interest in this study. Our results suggest a pac-3 dependency in gene regulation in a Pi-dependent manner. Furthermore, our analysis suggested that the fungus attempts to overcome the deletion effects in a Δpac-3 mutant through a complex combined regulatory mechanism. Finally, the modulatory responses observed in the Δpac-3 strain, a double mutant generated based on the Δmus-52 mutant strain, is strain-specific, highlighting that the phenotypic impact may be attributed to pac-3 absence despite the combined mus-52 deletion.

Antioxidant-related catalase CTA1 regulates development, aflatoxin biosynthesis, and virulence in pathogenic fungus Aspergillus flavus

Reactive oxygen species (ROS) induce the synthesis of a myriad of secondary metabolites, including aflatoxins. It raises significant concern as it is a potent environmental contaminant. In Aspergillus flavus., antioxidant enzymes link ROS stress response with coordinated gene regulation of aflatoxin biosynthesis. In this study, we characterized the function of a core component of the antioxidant enzyme catalase (CTA1) of A. flavus. Firstly, we verified the presence of cta1 corresponding protein (CTA1) by Western blot analysis and mass-spectrometry based analysis. Then, the functional study revealed that the growth, sporulation and sclerotia formation significantly increased, while aflatoxins production and virulence were decreased in the cta1 deletion mutant as compared with the WT and complementary strains. Furthermore, the absence of the cta1 gene resulted in a significant rise in the intracellular ROS level, which in turn added to the oxidative stress level of cells. A further quantitative proteomics investigation hinted that in vivo, CTA1 might maintain the ROS level to facilitate the aflatoxin synthesis. All in all, the pleiotropic phenotype of A. flavus CTA1 deletion mutant revealed that the antioxidant system plays a crucial role in fungal development, aflatoxins biosynthesis and virulence.

Members of the nitronate monooxygenase gene family from Metarhizium brunneum are induced during the process of infection to Plutella xylostella

Metarhizium species are the most abundant fungi that can be isolated from soil, with a well-known biopesticide capacity. Metarhizium recognizes their hosts when the conidium interacts with insects, where the fungi are in contact with the hydrocarbons of the outermost lipid layer cuticle. These cuticular hydrocarbons comprise a mixture of n-alkanes, n-alkenes, and methyl-branched chains. Metarhizium can degrade insect hydrocarbons and use these hydrocarbons for energy production and the biosynthesis of cellular components. The metabolism of nitroalkanes involves nitronate monooxygenase activity. In this work, we isolated a family of six genes with potential nitronate monooxygenase activity from Metarhizium brunneum. The six genes were expressed in Escherichia coli, and the nitronate monooxygenase activity was verified in the recombinant proteins. Additionally, when the conidia of M. brunneum were grown in medium with nitroalkanes, virulence against Plutella xylostella increased. Furthermore, we analyzed the expression of the six Npd genes during the infection to this insect, which showed differential expression of the six Npd genes during infection.

The Fungal-Specific Transcription Factor VpFSTF1 Is Required for Virulence in Valsa pyri

Valsa pyri is the causal agent of pear canker disease, which leads to enormous losses of pear production in eastern Asian, especially China. In this study, we identified a fungal-specific transcription factor 1 (termed as VpFSTF1) from V. pyri, which is highly conserved in fungi. To characterize its functions, we generated mutant and complementation strains in V. pyri and found that ΔVpFSTF1 mutants lost the ability to form fruiting bodies along with the reduced virulence. The radial growth of ΔVpFSTF1 mutant was sensitive to increasing concentrations of hydrogen peroxide (H₂O₂) and salicylic acid (SA). Moreover, RNA-sequencing (RNA-Seq) analysis of wild-type (WT) and ΔVpFSTF1 mutant strains was performed, and the results revealed 1,993 upregulated, and 2006 downregulated differentially expressed genes (DEGs) in the mutant. The DEGs were corresponding to the genes that are involved in amino acid metabolism, starch, and sucrose metabolism, gluconeogenesis, citrate cycle, and carbon metabolism. Interestingly, pathogen host interaction (PHI) analysis showed that 69 downregulated genes were related to virulence, suggesting that they might function downstream of VpFSTF1. Nine DEGs were further validated by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), and the results were consistent with RNA-seq analysis. Furthermore, promoter regions were predicted, and VpFSTF1 binding activity was assessed. We demonstrated that five promoters are directly or indirectly targeted by VpFSTF1, including catalase-related peroxidase (VPIG_01209) and P450 family genes. Taken together, these findings indicate that VpFSTF1 is crucial for the virulence of V. pyri via direct or indirect regulation of downstream genes expression and lay an important foundation for understanding the molecular mechanism of V. pyri infection.

Pathogenicity and in vivo Development of Metarhizium rileyi Against Spodoptera litura (Lepidoptera: Noctuidae) Larvae

Metarhizium rileyi, a well-known entomopathogenic fungus, could open up new vistas in biological control of insect pests; however, due to its intrinsic shortcomings, such as long pathogenic process, its application is largely limited. To explore which process, the invasion or the following in vivo development, is the main factor responsible for the long pathogenic process, the lethal effect of M. rileyi against Spodoptera litura (Fabricius) was determined by conidial topical application and hyphae body injection, and the host immune response was also monitored. Results showed when larvae were inoculated by conidial topical application, the pathogenicity of M. rileyi varied greatly depending on the larval instar and conidia concentration, and LC50 values ranged from 6.24 × 106 to 6.06 × 109 conidia/ml while LT50 values fluctuated from 4.35 to 9.43 d. However, in vivo study showed when hyphal bodies (Hbs) of M. rileyi were injected into host hemocoel, they would not be recognized by the host's immune system as invaders. There were no significant differences in the hemocytes and phenoloxidase activity between the infected and control larvae at the initial 44 h, indicated that the fungus was able to successfully avoid the attack from the cellular and humoral immune systems, therefore, it could multiply freely in the hemocoel. The in vivo development time of M. rileyi tended to remain constant for 2-3 d regardless of the initial inoculated numbers. Considering no detectable defense response was observed during in vivo development, it can be concluded that host nonself-recognition system does not respond to the hemolymph borne-Hbs.

Outcome of blue, green, red, and white light on Metarhizium robertsii during mycelial growth on conidial stress tolerance and gene expression

Fungi sense light and utilize it as a source of environmental information to prepare against many stressful conditions in nature. In this study, Metarhizium robertsii was grown on: 1) potato dextrose agar medium (PDA) in the dark (control); 2) under nutritive stress in the dark; and 3) PDA under continuous (A) white light; (B) blue light lower irradiance = LI; (C) blue light higher irradiance = HI; (D) green light; and (E) red light. Conidia produced under these treatments were tested against osmotic stress and UV radiation. In addition, a suite of genes usually involved in different stress responses were selected to study their expression patterns. Conidia produced under nutritive stress in the dark were the most tolerant to both osmotic stress and UV radiation, and the majority of their stress- and virulence-related genes were up-regulated. For osmotic stress tolerance, conidia produced under white, blue LI, and blue HI lights were the second most tolerant, followed by conidia produced under green light. Conidia produced under red light were the least tolerant to osmotic stress and less tolerant than conidia produced on PDA medium in the dark. For UV tolerance, conidia produced under blue light LI were the second most tolerant to UV radiation, followed by the UV tolerances of conidia produced under white light. Conidia produced under blue HI, green, and red lights were the least UV tolerant and less tolerant than conidia produced in the dark. The superoxide dismutases (sod1 and sod2), photolyases (6-4phr and CPDphr), trehalose-phosphate synthase (tps), and protease (pr1) genes were highly up-regulated under white light condition, suggesting a potential role of these proteins in stress protection as well as virulence after fungal exposure to visible spectrum components.

Pathogen-produced catalase affects immune priming: A potential pathogen strategy

Immune priming in invertebrates occurs when the first contact with a pathogen/parasite enhances resistance after a second encounter with the same strain or species. Although the mechanisms are not well understood, there is evidence that priming the immune response of some hosts leads to greater pro-oxidant production. Parasites, in turn, might counteract the host attack with antioxidants. Virulent pathogen strains may therefore mask invertebrate immune priming. For example, different parasite species overexpress catalase as a virulence factor to resist host pro-oxidants, possibly impairing the immune priming response. The aim of this study was firstly to evaluate the specificity of immune priming in Tenebrio molitor when facing homologous and heterologous challenges. Secondly, homologous challenges were carried out with two Metarhizium anisopliae strains (Ma10 and CAT). The more virulent strain (CAT) overexpresses catalase, an antioxidant that perhaps impairs a host immune response mediated by reactive oxygen species (ROS). Indeed, T. molitor larvae exhibited better immune priming (survival) in response to the Ma10 than CAT homologous challenge. Moreover, the administration of paraquat, an ROS-promoting agent, favoured survival of the host upon exposure to each fungal strain. We propose that some pathogens likely overcome pro-oxidant-mediated immune priming defences by producing antioxidants such as catalase.

Development of an Expression Vector to Overexpress or Downregulate Genes in Curvularia protuberata

Curvularia protuberata, an endophytic fungus in the Ascomycota, provides plants with thermotolerance only when it carries a mycovirus known as Curvularia thermotolerance virus (CThTV), and forms a three-way symbiotic relationship among these organisms. Under heat stress, several genes are expressed differently between virus-free C. protuberata (VF) and C. protuberata carrying CThTV (AN). We developed an expression vector, pM2Z-fun, carrying a zeocin resistance gene driven by the ToxA promoter, to study gene functions in C. protuberata to better understand this three-way symbiosis. Using this new 3.7-kb vector, five genes that are differentially expressed in C. protuberata—including genes involved in the trehalose, melanin, and catalase biosynthesis pathways—were successfully overexpressed or downregulated in VF or AN C. protuberata strains, respectively. The VF overexpression lines showed higher metabolite and enzyme activity than in the control VF strain. Furthermore, downregulation of expression of the same genes in the AN strain resulted in lower metabolite and enzyme activity than in the control AN strain. The newly generated expression vector, pM2Z-fun, has been successfully used to express target genes in C. protuberata and will be useful in further functional expression studies in other Ascomycota fungi.

iTRAQ-based quantitative proteomic analysis of conidia and mycelium in the filamentous fungus Metarhizium robertsii

Metarhizium robertsii is widely applied in biological control via conidia application. To clarify the proteomic differences between conidia and mycelia and explore the underlying mechanisms of conidia as a unit responsible for dispersal and environmental stress, we carried out an iTRAQ (isobaric tags for relative and absolute quantitation)-based quantitative proteomic analysis for two developmental stages from M. robertsii. A total of 2052 proteins were detected, and 90 showed differential protein abundance between the conidia and mycelia. These 90 proteins were primarily associated with stress resistance, amino acid and protein metabolism, and energy metabolism. Further bioinformatics analysis showed that these proteins could be mapped to 52 pathways, five of which were significantly enriched after mapping to KEGG pathways. Interestingly, many proteins involved in the significantly enriched pathway of peroxisome, biosynthesis of secondary metabolites and glyoxylate and dicarboxylate metabolism, including catalase, peroxisomal membrane anchor protein, formate dehydrogenase and isocitrate lyase, were identified with higher abundance in conidia. The results deepened our understanding of the conidia proteome in M. robertsii and provide a basis for further exploration for improving the efficiency of the fungi as biocontrol agents.

De novo transcriptome sequencing and comprehensive analysis of the heat stress response genes in the basidiomycetes fungus Ganoderma lucidum

Ganoderma lucidum is a valuable basidiomycete with numerous pharmacological compounds, which is widely consumed throughout China. We previously found that the polysaccharide content of Ganoderma lucidum fruiting bodies could be significantly improved by 45.63% with treatment of 42 °C heat stress (HS) for 2 h. To further investigate genes involved in HS response and explore the mechanisms of HS regulating the carbohydrate metabolism in Ganoderma lucidum, high-throughput RNA-Seq was conducted to analyse the difference between control and heat-treated mycelia at transcriptome level. We sequenced six cDNA libraries with three from control group (mycelia cultivated at 28 °C) and three from heat-treated group (mycelia subjected to 42 °C for 2 h). A total of 99,899 transcripts were generated using Trinity method and 59,136 unigenes were annotated by seven public databases. Among them, 2790 genes were identified to be differential expressed genes (DEGs) under HS condition, which included 1991 up-regulated and 799 down-regulated. 176 DEGs were then manually classified into five main responsive-related categories according to their putative functions and possible metabolic pathways. These groups include stress resistance-related factors; protein assembly, transportation and degradation; signal transduction; carbohydrate metabolism and energy provision-related process; other related functions, suggesting that a series of metabolic pathways in Ganoderma lucidum are activated by HS and the response mechanism involves a complex molecular network which needs further study. Remarkably, 48 DEGs were found to regulate carbohydrate metabolism, both in carbohydrate hydrolysis for energy provision and polysaccharide synthesis. In summary, this comprehensive transcriptome analysis will provide enlarged resource for further investigation into the molecular mechanisms of basidiomycete under HS condition.

Applications of Microbial Enzymes in Food Industry

The use of enzymes or microorganisms in food preparations is an age-old process. With the advancement of technology, novel enzymes with wide range of applications and specificity have been developed and new application areas are still being explored. Microorganisms such as bacteria, yeast and fungi and their enzymes are widely used in several food preparations for improving the taste and texture and they offer huge economic benefits to industries. Microbial enzymes are the preferred source to plants or animals due to several advantages such as easy, cost-effective and consistent production. The present review discusses the recent advancement in enzyme technology for food industries. A comprehensive list of enzymes used in food processing, the microbial source of these enzymes and the wide range of their application are discussed.

Mode of Infection of Metarhizium spp. Fungus and Their Potential as Biological Control Agents

Chemical insecticides have been commonly used to control agricultural pests, termites, and biological vectors such as mosquitoes and ticks. However, the harmful impacts of toxic chemical insecticides on the environment, the development of resistance in pests and vectors towards chemical insecticides, and public concern have driven extensive research for alternatives, especially biological control agents such as fungus and bacteria. In this review, the mode of infection of Metarhizium fungus on both terrestrial and aquatic insect larvae and how these interactions have been widely employed will be outlined. The potential uses of Metarhizium anisopliae and Metarhizium acridum biological control agents and molecular approaches to increase their virulence will be discussed.

Managing the plant microbiome for biocontrol fungi: examples from Hypocreales

Feeding an increasing global population requires continued improvements in agricultural efficiency and productivity. Meeting estimated future production levels requires the adoption of practices that increase output without environmental degradation associated with external inputs to supplement nutrition or control pests. Enriching the community of microbes associated with plants in agricultural systems for those providing ecosystem services such as pest control is one possible component towards achieving sustainable productivity increases. In this review we explore the current state of knowledge for Hypocreales fungi used in biological control. Advances in understanding the field ecology, diversity and genetic determinants of host range and virulence of hypocrealean fungi provide the means to improve their efficacy.

Comparison between superficial and solid-state cultures of Isaria fumosorosea: conidial yields, quality and sensitivity to oxidant conditions

Conidia production and quality from mycoinsecticides in solid-state cultures (SSC) are frequently inferred from superficial culture (SC) results. Both parameters were evaluated for two Isaria fumosorosea strains (ARSEF 3302 and CNRCB1), in SC and SSC, using culture media with the same chemical composition. For both strains, conidia production was higher in SC than SSC in terms of conidia per gram of dry substrate. Germination in both strains did not show significant differences between SC and SSC (>90 %). Similarly, conidia viability in ARSEF 3302 strain did not show differences at early stages between SC and SSC, but was higher in SC compared to SSC in the late stage of culture; in contrast, conidia from CNRCB1 strain did not differ between both culture systems. Some infectivity parameters improved in conidia from SSC, compared to SC at the early stages, but these differences disappeared at the final stage, independently of the strain. Both strains showed decreased conidia production when 26 % O2 pulses were applied; nevertheless, conidiation in SSC was two orders of magnitude more sensitive to oxidant pulses. In SC with 26 % O2 pulses, conidia viability for both strains at early stages, was higher than in normal atmospheric conditions. Infectivity towards Galleria mellonella larvae was similar between conidia from normal atmosphere and oxidant conditions; notably, for the strain ARSEF 3302 infectivity decreased at the final stage. This study shows the intrinsic differences between SC and SSC, which should be considered when using SC as a model to design production processes in SSC.

Oxidative stress response of Beauveria bassiana to Bordeaux mixture and its influence on fungus growth and development

BACKGROUND

Chemical fungicides used to manage plant diseases may negatively affect beneficial fungi such as entomopathogens. In this study, the participation of the antioxidative system in the entomopathogenic fungus Beauveria bassiana exposed to the copper-based Bordeaux mixture fungicide and its relation with fungus growth and development were examined. The fungus was grown in submerged culture containing Bordeaux mixture at the recommended dose. Within the first 24 h of elicitation, the vegetative growth, germination, sporulation and activity of amylase and laccase and the antioxidative enzymes catalase (CAT) and superoxide dismutase (SOD), as well as the production of hydrogen peroxide (H2 O2 ), were evaluated.

RESULTS

Bordeaux mixture inhibited B. bassiana germination (between 65 and 88%) and sporulation (between 15 and 57%) and significantly increased laccase production (≥130%), especially within the first 4 h of fungus exposure. By contrast, the mycelial growth was found to be less affected by the fungicide. These effects were accompanied with a significant increase in H2 O2 levels in fungal cells, as well as in SOD activity, but not in CAT, showing clear signs of increased oxidative stress.

CONCLUSION

The effect of Bordeaux mixture on B. bassiana development was probably due to the toxicity of the copper ion itself, and it also induced an oxidative state in fungal cells.

Secretome of the biocontrol agent metarhizium anisopliae induced by the cuticle of the cotton pest Dysdercus peruvianus reveals new insights into infection

Metarhizium anisopliae is an entomopathogenic fungus that has evolved specialized strategies to infect insect hosts. Here we analyzed secreted proteins related to Dysdercus peruvianus infection. Using shotgun proteomics, abundance changes in 71 proteins were identified after exposure to host cuticle. Among these proteins were classical fungal effectors secreted by pathogens to degrade physical barriers and alter host physiology. These include lipolytic enzymes, Pr1A, B, C, I, and J proteases, ROS-related proteins, oxidorreductases, and signaling proteins. Protein interaction networks were generated postulating interesting candidates for further studies, including Pr1C, based on possible functional interactions. On the basis of these results, we propose that M. anisopliae is degrading host components and actively secreting proteins to manage the physiology of the host. Interestingly, the secretion of these factors occurs in the absence of a host response. The findings presented here are an important step in understanding the host–pathogen interaction and developing more efficient biocontrol of D. peruvianus by M. anisopliae.

Oxygen-rich culture conditions enhance the conidial infectivity and the quality of two strains of Isaria fumosorosea for potentially improved biocontrol processes

BACKGROUND

In addition to high production levels of conidia, the success of entomopathogenic fungi as biological control agents depends both on their prevalence under the environmental conditions found in open fields (resistance to stress) and on the capacity of these conidia to infect pests. This study compares conidium production, infectivity and resistance to thermal and osmotic stress in two strains of Isaria fumosorosea (ARSEF 3302 and CNRCB1) grown either under a normal atmosphere (21% O2) or using enriched oxygen pulses (26% O2).

RESULTS

After 180 h, the ARSEF 3302 strain with 26% O2 pulses increased conidium production nearly fivefold compared with the normal atmosphere, while conidium production by the CNRCB1 strain decreased by 50% under O2 pulses, relative to the values measured with the normal atmosphere. The conidia obtained with 26% O2 pulses had a greater germination rate and resistance to thermal and osmotic stress, in addition to improved infectivity against Galleria mellonella (Lepidoptera) larvae. These findings were associated with an increase in catalase activities for both strains.

CONCLUSION

An enriched oxygen atmosphere increases the quality of conidia of both strains of I. fumosorosea, with a variable effect on conidium production.

Inhibition of ecto-phosphatase activity in conidia reduces adhesion and virulence of Metarhizium anisopliae on the host insect Dysdercus peruvianus

Metarhizium anisopliae is an entomopathogenic fungus with the ability to infect a broad range of arthropods, and have evolved distinct strategies for their attachment to hosts. Here, we describe the characterisation of ecto-phosphatase activity on the conidia surface of M. anisopliae and its relevance in the host interaction process. Ecto-phosphatase activity was linear for 60 min and during this time, was linear with the increase of cell density. The optimum pH was in the acidic range and some divalent metals, such as Cu(2+), Cd(2+) and Zn(2+), inhibited ecto-phosphatase activity. The activity was also reduced by phosphatase inhibitors. Importantly, the inhibition of phosphatase activity in conidia reduced the adhesion to Dysdercus peruvianus (Hemiptera: Pyrrhocoridae) integument and, consequently and indirectly, M. anisopliae infection. The results herein presented show, for the first time, the importance of ecto-phosphatase activity in M. anisopliae conidia and provide the first evidence of its direct involvement in adhesion and host infection.

Catalases play differentiated roles in the adaptation of a fungal entomopathogen to environmental stresses

The catalase family of Beauveria bassiana (fungal entomopathogen) consists of catA (spore-specific), catB (secreted), catP (peroxisomal), catC (cytoplasmic) and catD (secreted peroxidase/catalase), which were distinguished in phylogeny and structure and functionally characterized by constructing single-gene disrupted and rescued mutants for enzymatic and multi-phenotypic analyses. Total catalase activity decreased 89% and 56% in ΔcatB and ΔcatP, corresponding to the losses of upper and lower active bands gel-profiled for all catalases respectively, but only 9-12% in other knockout mutants. Compared with wild type and complement mutants sharing similar enzymatic and phenotypic parameters, all knockout mutants showed significant (9-56%) decreases in the antioxidant capability of their conidia (active ingredients of mycoinsecticides), followed by remarkable phenotypic defects associated with the fungal biocontrol potential. These defects included mainly the losses of 40% thermotolerance (45°C) in ΔcatA, 46-48% UV-B resistance in ΔcatA and ΔcatD, and 33-47% virulence to Spodoptera litura larvae in ΔcatA, ΔcatP and ΔcatD respectively. Moreover, the drastic transcript upregulation of some other catalase genes observed in the normal culture of each knockout mutant revealed functionally complimentary effects among some of the catalase genes, particularly between catB and catC whose knockout mutants displayed little or minor phenotypic changes. However, the five catalase genes functioned redundantly in mediating the fungal tolerance to either hyperosmotic or fungicidal stress. The differentiated roles of five catalases in regulating the B. bassiana virulence and tolerances to oxidative stress, high temperature and UV-B irradiation provide new insights into fungal adaptation to stressful environment and host invasion.

Differential expression of insect and plant specific adhesin genes, Mad1 and Mad2, in Metarhizium robertsii

Metarhizium robertsii is an entomopathogenic fungus that is also plant rhizosphere competent. Two adhesin-encoding genes, Metarhizium adhesin-like protein 1 (Mad1) and Mad2, are involved in insect pathogenesis or plant root colonization, respectively. Here we examined the differential expression of the Mad genes when grown on a variety of soluble (carbohydrates and plant root exudate) and insoluble substrates (locust, tobacco hornworm, and cockroach cuticle, chitin, tomato stems, cellulose, and starch) and during insect, Plutella xylostella, infection. On insect cuticles Mad1 was up regulated, whereas bean root exudate and tomato stems resulted in the up regulation of Mad2. During the early stages of insect infection Mad1 was expressed while Mad2 was not expressed until fungal hyphae emerged and conidiated on the insect cadaver. The regulation of Mad2 was compared to that of other stress-related genes (heat shock protein (Hsp)30, Hsp70, and starvation stress gene A (ssgA)). Mad2 was generally up regulated by nutrient starvation (similar to ssgA) but not by pH, temperature, oxidative or osmotic stresses. Whereas Hsp30 and Hsp70 were generally up regulated at 37 °C or by oxidative stress even under nutrient enriched conditions. We fused the promoter of the Mad2 gene to a marker gene (green fluorescent protein (GFP)) and confirmed that Mad2 was up regulated when M. robertsii was grown in the presence of nutrient starvation. Examination of the promoter region of Mad2 revealed that it possessed two copies of a stress-response element (STRE) known to be regulated under the general stress-response pathway.

Isolation, characterization and expression analysis of the ornithine decarboxylase gene (ODC1) of the entomopathogenic fungus, Metarhizium anisopliae

The gene ODC1, which codes for the ornithine decarboxylase enzyme, was isolated from the entomopathogenic fungus, Metarhizium anisopliae. The deduced amino acid sequence predicted a protein of 447 amino acids with a molecular weight of 49.3 kDa that contained the canonical motifs of ornithine decarboxylases. The ODC1 cDNA sequence was expressed in Escherichia coli cells; radiometric enzyme assays showed that the purified recombinant protein had ornithine decarboxylase activity. The optimum pH of the purified Odc1 protein was 8.0-8.5, and the optimum reaction temperature was 37°C. The apparent K(m) for ornithine at a pyridoxal phosphate concentration of 20mM was 22 μM. The competitive inhibitor of ODC activity, 1,4-diamino-2-butanone (DAB), at 0.25 mM inhibited 95% of ODC activity. The ODC1 mRNA showed an increase at the beginning of appressorium formation in vitro. During the M. anisopliae invasion process into Plutella xylostella larvae, the ODC1 mRNA showed a discrete increase within the germinating spore and during appressorium formation. The second expression peak was higher and prolonged during the invasion and death of the insect. The ODC1 gene complements the polyamine auxotrophy of Yarrowia lipolytica odc null mutant.