Growth of Metarhizium anisopliae on non-preferred carbon sources yields conidia with increased UV-B tolerance

The improved resistance of germinated spores to ultraviolet irradiation: Comparison with chlorine

Fungi outbreaks in water will include a series of processes, including spore aggregation, germination, biofilm, and finally present in a mixed state in the aquatic environment. More attention is paid to the control of dispersed fungal spores, however, there was little knowledge of the control of germinated spores. This study investigated the inactivation kinetics and mechanism of ultraviolet (UV) treatment for fungal spores with different germination percentages compared with dormant spores. The results indicated that the inactivation rate constants (k) of spores with 5%-45% germination were 0.0278-0.0299 cm2/mJ for Aspergillus niger and 0.0588-0.0647 cm2/mJ for Penicillium polonicum, which were lower than those of dormant spores. It suggested that germinated spores were more tolerant to UV irradiation than dormant spores, and it may be due to the defensive barrier (upregulated pigments) and some reductive substance (upregulated enoyl reductase) by absorbing UV or reacting with reactive oxygen species according to transcriptome analysis. Compared to dormant spores, the k-UV of germinated spores decreased by 18.17%-26.56% for Aspergillus niger, which was less than k-chlorine (62.33%-69.74%). A slighter decrease in k-UV showed UV irradiation can efficiently control fungi contamination, especially when dormant spores and germinated spores coexisted in actual water systems. This study indicates that more attention should be paid to germinated spores.

Conidial mass production of entomopathogenic fungi and tolerance of their mass-produced conidia to UV-B radiation and heat

We investigated conidial mass production of eight isolates of six entomopathogenic fungi (EPF), Aphanocladium album (ARSEF 1329), Beauveria bassiana (ARSEF 252 and 3462), Lecanicillium aphanocladii (ARSEF 6433), Metarhizium anisopliae sensu lato (ARSEF 2341), Metarhizium pingshaense (ARSEF 1545), and Simplicillium lanosoniveum (ARSEF 6430 and 6651) on white or brown rice at four moisture conditions (75-100%). The tolerance of mass-produced conidia of the eight fungal isolates to UV-B radiation and heat (45 °C) were also evaluated. For each moisture content compared, a 20-g sample of rice in a polypropylene bag was inoculated with each fungal isolate in three replicates and incubated at 28 ± 1 °C for 14 days. Conidia were then harvested by washing the substrate, and conidial concentrations determined by haemocytometer counts. Conidial suspensions were inoculated on PDAY with 0.002% benomyl in Petri plates and exposed to 978 mW m-2 of Quaite-weighted UV-B for 2 h. Additionally, conidial suspensions were exposed to 45 °C for 3 h, and aliquots inoculated on PDAY with benomyl. The plates were incubated at 28 ± 1 °C, and germination was assessed at 400 × magnification after 48 h. Conidial production was generally higher on white rice than on brown rice for all fungal species, except for L. aphanocladii ARSEF 6433, regardless of moisture combinations. The 100% moisture condition provided higher conidial production for B. bassiana (ARSEF 252 and ARSEF 3462) and M. anisopliae (ARSEF 2341) isolates, while the addition of 10% peanut oil enhanced conidial yield for S. lanosoniveum isolate ARSEF 6430. B. bassiana ARSEF 3462 on white rice with 100% water yielded the highest conidial production (approximately 1.3 × 1010 conidia g-1 of substrate). Conidia produced on white rice with the different moisture conditions did not differ in tolerance to UV-B radiation or heat. However, high tolerance to UV-B radiation and heat was observed for B. bassiana, M. anisopliae, and A. album isolates. Heat-treated conidia of S. lanosoniveum and L. aphanocladii did not germinate.

Congo red induces trans-priming to UV-B radiation in Metarhizium robertsii

Metarhizium spp. is used as a biocontrol agent but is limited because of low tolerance to abiotic stress. Metarhizium robertsii is an excellent study model of fungal pathogenesis in insects, and its tolerance to different stress conditions has been extensively investigated. Priming is the time-limited pre-exposure of an organism to specific stress conditions that increases adaptive response to subsequent exposures. Congo red is a water-soluble azo dye extensively used in stress assays in fungi. It induces morphological changes and weakens the cell wall at sublethal concentrations. Therefore, this chemical agent has been proposed as a stressor to induce priming against other stress conditions in entomopathogenic fungi. This study aimed to evaluate the capacity of Congo red to induce priming in M. robertsii. Conidia were grown on potato dextrose agar with or without Congo red.The tolerance of conidia produced from mycelia grown in these three conditions was evaluated against stress conditions, including osmotic, oxidative, heat, and UV-B radiation. Conidia produced on medium supplemented with Congo red were significantly more tolerant to UV-B radiation but not to the other stress conditions assayed. Our results suggest that Congo red confers trans-priming to UV-B radiation but not for heat, oxidative, or osmotic stress.

The sensor histidine kinase (SLN1) and acetyl-CoA carboxylase (ACC1) coordinately regulate the response of Neurospora crassa to the springtail Sinella curviseta (Collembola: Entomobryidae) attack

IMPORTANCE

Understanding the regulatory pathways by which fungi respond to environmental signals through interlinked genes provides insights into the interactions between fungi and insects. The coordinated optimization of the regulatory networks is necessary for fungi to adapt to their habitats. We demonstrated that the synergistic regulation of sensor histidine kinase (SLN1) and acetyl-CoA carboxylase (ACC1) plays a critical role in regulating the fungal response to Sinella curviseta stress. Furthermore, we found that the enhanced production of trehalose, carotenoids, and 5-MTHF plays crucial role in the resistance to the fungivore. Our results provide insights into the understanding of the adaptation of N. crassa to environmental stimuli.

The IV International Symposium on Fungal Stress and the XIII International Fungal Biology Conference

For the first time, the International Symposium on Fungal Stress was joined by the XIII International Fungal Biology Conference. The International Symposium on Fungal Stress (ISFUS), always held in Brazil, is now in its fourth edition, as an event of recognized quality in the international community of mycological research. The event held in São José dos Campos, SP, Brazil, in September 2022, featured 33 renowned speakers from 12 countries, including: Austria, Brazil, France, Germany, Ghana, Hungary, México, Pakistan, Spain, Slovenia, USA, and UK. In addition to the scientific contribution of the event in bringing together national and international researchers and their work in a strategic area, it helps maintain and strengthen international cooperation for scientific development in Brazil.

Tolerance to UV-B radiation of the entomopathogenic fungus Metarhizium rileyi

Soybean, corn, and cotton crops are afflicted by several noctuid pests and the development of bioinsecticides could help control these pests. The fungus Metarhizium rileyi has the greatest potential because its epizootics decimate caterpillar populations in the absence of insecticide applications. However, insect-pathogenic fungi when used for insect control in agriculture have low survival mainly due to the deleterious effects of ultraviolet radiation and heat from solar radiation. In this study, fourteen isolates of M. rileyi were studied and compared with isolates ARSEF 324 and ARSEF 2575 of Metarhizium acridum and Metarhizium robertsii, respectively, whose sensitivity to UV-B radiation had previously been studied. Conidia were exposed at room temperature (ca. 26 °C) to 847.90 mWm-2 of Quaite-weighted UV-B using two fluorescent lamps. The plates containing the conidial suspensions were irradiated for 1, 2, and 3 h, providing doses of 3.05, 6.10, and 9.16 kJ m2, respectively. A wide variability in conidial UV-B tolerance was found among the fourteen isolates of M. rileyi. Isolate CNPSo-Mr 150 was the most tolerant isolate (germination above 80% after 2 h exposure), which was comparable to ARSEF 324 (germination above 90% after 2 h exposure), the most tolerant Metarhizium species. The least tolerant isolates were CNPSo-Mr 141, CNPSo-Mr 142, CNPSo-Mr 156, and CNPSo-Mr 597. Nine M. rileyi isolates exhibited similar tolerance to UV-B radiation as ARSEF 2575 (germination above 50% after 2 h exposure). In conclusion, the majority of M. rileyi isolates studied can endure 1 or 2 h of UV-B radiation exposure. However, after 3 h of exposure, the germination of all studied isolates reduced below 40%, except for CNPSo-Mr 150 and ARSEF 324.

Asphyxiation of Metarhizium robertsii during mycelial growth produces conidia with increased stress tolerance via increased expression of stress-related genes

Little is known about the impact of hypoxia and anoxia during mycelial growth on tolerance to different stress conditions of developing fungal conidia. Conidia of the insect-pathogenic fungus Metarhizium robertsii were produced on potato dextrose agar (PDA) medium under normoxia (control = normal oxygen concentrations), continuous hypoxia, and transient anoxia, as well as minimal medium under normoxia. The tolerance of the conidia produced under these different conditions was evaluated in relation to wet heat (heat stress), menadione (oxidative stress), potassium chloride (osmotic stress), UV radiation, and 4-nitroquinoline-1-oxide (=4-NQO genotoxic stress). Growth under hypoxic condition induced higher conidial tolerance of M. robertsii to menadione, KCl, and UV radiation. Transient anoxic condition induced higher conidial tolerance to KCl and UV radiation. Nutritional stress (i.e., minimal medium) induced higher conidial tolerance to heat, menadione, KCl, and UV radiation. However, neither of these treatments induced higher tolerance to 4-NQO. The gene hsp30 and hsp101 encoding a heat shock protein was upregulated under anoxic condition. In conclusion, growth under hypoxia and anoxia produced conidia with higher stress tolerances than conidia produced in normoxic condition. The nutritive stress generated by minimal medium, however, induced much higher stress tolerances. This condition also caused the highest level of gene expression in the hsp30 and hsp101 genes. Thus, the conidia produced under nutritive stress, hypoxia, and anoxia had greater adaptation to stress.

Characterization and Evaluation of Metarhizium spp. (Metsch.) Sorokin Isolates for Their Temperature Tolerance

A field survey was done in teak (Tectona grandis F.) forests in South India to explore the entomopathogenic effect of Metarhizium anisopliae (Ascomycota: Sordariomycetes) against teak defoliator, Hyblaea puera (Lepidoptera: Hyblaeidae). About 300 soils and infected insect samples were collected during the survey and thirty-six fungal isolates were isolated from soil and insect samples and characterized. The fungi were cultured on PDAY with dodine and antibiotics. Generally, the EPF culture was incubated at 27 °C in darkness for 15 days. Virulence of the Entomopathogenic Fungi (EPF) ability to germinate under cold and heat temperatures was assessed in a culture impregnated with conidia. In the experiment, it was found that for the first time Metarhizium quizhouense, Metarhizium robertsii, and Metarhizium majus species caused significantly higher mortality to hosts. These isolates of M. anisopliae, M. robertsii, M. majus, and M. quizhouense were all considered to be effective virulent and environmentally adaptive. The Metarhizium isolates were recommended as effective bio-control agents through the field investigation of teak defoliator Hyblaea puera from South India forest. This study paves the way to utilize the indigenous isolates of EPF for the control of teak defoliator and to combat the pests thatare resistant to insecticide.

Low- or high-white light irradiance induces similar conidial stress tolerance in Metarhizium robertsii

White light during mycelial growth influences high conidial stress tolerance of the insect-pathogenic fungus Metarhizium robertsii, but little is known if low- or high-white light irradiances induce different stress tolerances. The fungus was grown either in the dark using two culture media: on minimal medium (Czapek medium without sucrose = MM) or on potato dextrose agar (PDA) or PDA medium under five different continuous white light irradiances. The stress tolerances of conidia produced on all treatments were evaluated by conidial germination on PDA supplemented with KCl for osmotic stress or on PDA supplemented with menadione for oxidative stress. Conidia produced on MM in the dark were more tolerant to osmotic and oxidative stress than conidia produced on PDA in the dark or under the light. For osmotic stress, growth under the lower to higher irradiances produced conidia with similar tolerances but more tolerant than conidia produced in the dark. For oxidative stress, conidia produced under the white light irradiances were generally more tolerant to menadione than conidia produced in the dark. Moreover, conidia produced in the dark germinated at the same speed when incubated in the dark or under lower irradiance treatment. However, at higher irradiance, conidial germination was delayed compared to germination in the dark, which germinated faster. Therefore, growth under light from low to high irradiances induces similar conidial higher stress tolerances; however, higher white light irradiances cause a delay in germination speed.

Conidiation under illumination enhances conidial tolerance of insect-pathogenic fungi to environmental stresses

Light is an important signal for fungi in the environment and induces many genes with roles in stress and virulence responses. Conidia of the entomopathogenic fungi Aschersonia aleyrodis, Beauveria bassiana, Cordyceps fumosorosea, Lecanicillium aphanocladii, Metarhizium anisopliae, Metarhizium brunneum, Metarhizium robertsii, Simplicillium lanosoniveum, Tolypocladium cylindrosporum, and Tolypocladium inflatum were produced on potato dextrose agar (PDA) medium under continuous white light, on PDA medium in the dark, or under nutritional stress (= Czapek medium without sucrose = MM) in the dark. The conidial tolerance of these species produced under these different conditions were evaluated in relation to heat stress, oxidative stress (menadione), osmotic stress (KCl), UV radiation, and genotoxic stress caused by 4-nitroquinoline 1-oxide (4-NQO). Several fungal species demonstrated greater stress tolerance when conidia were produced under white light than in the dark; for instance white light induced higher tolerance of A. aleyrodis to KCl and 4-NQO; B. bassiana to KCl and 4-NQO; C. fumosorosea to UV radiation; M. anisopliae to heat and menadione; M. brunneum to menadione, KCl, UV radiation, and 4-NQO; M. robertsii to heat, menadione, KCl, and UV radiation; and T. cylindrosporum to menadione and KCl. However, conidia of L. aphanocladii, S. lanosoniveum, and T. inflatum produced under white light exhibited similar tolerance as conidia produced in the dark. When conidia were produced on MM, a much stronger stress tolerance was found for B. bassiana to menadione, KCl, UV radiation, and 4-NQO; C. fumosorosea to KCl and 4-NQO; Metarhizium species to heat, menadione, KCl, and UV radiation; T. cylindrosporum to menadione and UV radiation; and T. inflatum to heat and UV radiation. Again, conidia of L. aphanocladii and S. lanosoniveum produced on MM had similar tolerance to conidia produced on PDA medium in the dark. Therefore, white light is an important factor that induces higher stress tolerance in some insect-pathogenic fungi, but growth in nutritional stress always provides in conidia with stronger stress tolerance than conidia produced under white light.

Isolation, fermentation, and formulation of entomopathogenic fungi virulent against adults of Diaphorina citri

BACKGROUND

Mycopesticides are important for integrated management of the Asian citrus psyllid, Diaphorina citri. However, there are few reports on the fermentation and formulation for mycopesticides with high virulence against D. citri.

RESULTS

From four different locations in South China, 12 fungal strains were isolated and classified into Beauveria bassiana (two isolates), Fusarium fujikuroi (five isolates), and Cordyceps javanica (five isolates) based on the phylogenetic analysis of ITS1-5.8S rDNA-ITS4 and morphology of colonies and conidia. Based on the highest initial virulence (86.52 ± 2.35%) and best growth characteristics, F-HY002-ACPHali was further optimized for biphasic fermentation (7.85 ± 2.62 × 10⁹  g^-1  dry substrate) with soybean meal + cottonseed flour (1:1) as the solid substrate and full light as the light/dark cycle. Furthermore, the oil-based conidial formulation was optimized with sesame oil as an effective carrier, which significantly enhanced conidial shelf life up to 16 weeks at 26 °C and 4 °C, thermo-stress (50 °C) and UV-B stress resistance, and virulence against adults of D. citri with increased mortalities and decreased LT₅₀ in comparison to that of unformulated conidia.

CONCLUSION

Our results not only provide a valuable native strain with high virulence against adults of D. citri, but also imply the soybean meal + cottonseed flour as valuable solid substrate, full-light exposure as environmental stimuli for solid-state fermentation, and the sesame oil as an effective carrier for the formulation of the C. javanica. These findings will facilitate the development of a potential mycopesticide for the biological control of Asian citrus psyllid.

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

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

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

Directed evolution of Metarhizium fungus improves its biocontrol efficacy against Varroa mites in honey bee colonies

Entomopathogenic fungi show great promise as pesticides in terms of their relatively high target specificity, low non-target toxicity, and low residual effects in agricultural fields and the environment. However, they also frequently have characteristics that limit their use, especially concerning tolerances to temperature, ultraviolet radiation, or other abiotic factors. The devastating ectoparasite of honey bees, Varroa destructor, is susceptible to entomopathogenic fungi, but the relatively warm temperatures inside honey bee hives have prevented these fungi from becoming effective control measures. Using a combination of traditional selection and directed evolution techniques developed for this system, new strains of Metarhizium brunneum were created that survived, germinated, and grew better at bee hive temperatures (35 °C). Field tests with full-sized honey bee colonies confirmed that the new strain JH1078 is more virulent against Varroa mites and controls the pest comparable to current treatments. These results indicate that entomopathogenic fungi are evolutionarily labile and capable of playing a larger role in modern pest management practices.

UV-B tolerances of conidia, blastospores, and microsclerotia of Metarhizium spp. entomopathogenic fungi

The aim of the present study was to analyze ten native Metarhizium spp. isolates as to their UV-B tolerances. Comparisons included: different fungal propagules (conidia, blastospores, or microsclerotia [MS]); conidia in aqueous suspensions or in 10% mineral oil-in-water emulsions; and conidia mixed with different types of soil. The UV-B effect was expressed as the germination of conidia or culturability of blastospores and MS relative to nongerminated propagules. Metarhizium anisopliae LCM S05 exhibited high tolerance as blastospores and/or MS, but not as conidia; LCM S10 and LCM S08 had positive results with MS or conidia but not blastospores. The formulations with 10% mineral oil did not always protect Metarhizium conidia against UV-B. Conidia of LCM S07, LCM S08, and LCM S10 exhibited the best results when in aqueous suspensions, 24 h after UV-B exposure. In general, conidia mixed with soil and exposed to UV-B yielded similar number of colony forming units as conidia from unexposed soil, regardless the soil type. It was not possible to predict which type of propagule would be the most UV-B tolerant for each fungal isolate; in conclusion, many formulations and propagule types should be investigated early in the development of new fungal biocontrol products.

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

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

The Third International Symposium on Fungal Stress - ISFUS

Stress is a normal part of life for fungi, which can survive in environments considered inhospitable or hostile for other organisms. Due to the ability of fungi to respond to, survive in, and transform the environment, even under severe stresses, many researchers are exploring the mechanisms that enable fungi to adapt to stress. The International Symposium on Fungal Stress (ISFUS) brings together leading scientists from around the world who research fungal stress. This article discusses presentations given at the third ISFUS, held in São José dos Campos, São Paulo, Brazil in 2019, thereby summarizing the state-of-the-art knowledge on fungal stress, a field that includes microbiology, agriculture, ecology, biotechnology, medicine, and astrobiology.

History of the International Symposium on Fungal Stress - ISFUS, a dream come true!

The International Symposium on Fungal Stress (ISFUS) was born in a dream that Drauzio Eduardo Naretto Rangel had in 2013. This article reviews the first three ISFUSs and prospects for the future meetings. Although ISFUS was born as a small family organized meeting, since the first meeting, ISFUS has achieved great success, receiving very important research grants from FAPESP, FAPEG, and CAPES to bring international scientists to Brazil. Moreover, three special issues in leading journals have been published with articles relating to the talks presented at each ISFUS. For the first meeting, most speakers published in a special issue in Current Genetics. From the second and third meeting, articles from the speakers were published in special issues of the top mycology journal, Fungal Biology, published by Elsevier on behalf of the British Mycological Society. Here we show that following the dreams with a full heart and adding lots of love, passion, and hard work can achieve success.

Osmotolerance as a determinant of microbial ecology: A study of phylogenetically diverse fungi

Osmotic stress induced by high solute concentration can prevent fungal metabolism and growth due to alterations in properties of the cytosol, changes in turgor, and the energy required to synthesize and retain compatible solutes. We used germination to quantify tolerance/sensitivity to the osmolyte KCl (0.1-4.5 M, in 0.1 M increments) for 71 strains (40 species) of ecologically diverse fungi. These include 11 saprotrophic species (17 strains, including two xerophilic species), five mycoparasitic species (five strains), six plant-pathogenic species (13 strains), and 19 entomopathogenic species (36 strains). A dendrogram obtained from cluster analyses, based on KCl inhibitory concentrations 50 % and 90 % calculated by Probit Analysis, revealed three groups of fungal isolates accordingly to their osmotolerance. The most-osmotolerant group (Group 3) contained the majority of saprotrophic fungi, and Aspergillus niger (F19) was the most tolerant. The highly xerophilic Aspergillus montevidense and Aspergillus pseudoglaucus were the second- and third-most tolerant species, respectively. All Aspergillus and Cladosporium species belonged to Group 3, followed by the entomopathogens Colletotrichum fioriniae, Simplicillium lanosoniveum, and Trichothecium roseum. Group 2 exhibited a moderate osmotolerance, and included plant-pathogens such as Colletotrichum and Fusarium, mycoparasites such as Clonostachys spp, some saprotrophs such as Mucor and Penicillium spp., and some entomopathogens such as Isaria, Lecanicillium, Mariannaea, Simplicillium, and Torrubiella. Group 1 contained the osmo-sensitive strains: the rest of the entomopathogens and the mycoparasitic Gliocladium and Trichoderma. Although stress tolerance did not correlate with their primary ecological niche, classification of these 71 fungal strains was more closely aligned with their ecology than with their phylogenetic relatedness. We discuss the implications for both microbial ecology and fungal taxonomy.

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.

Riboflavin induces Metarhizium spp. to produce conidia with elevated tolerance to UV-B, and upregulates photolyases, laccases and polyketide synthases genes

AIMS

The effect of nutritional supplementation of two Metarhizium species with riboflavin (Rb) during production of conidia was evaluated on (i) conidial tolerance (based on germination) to UV-B radiation and on (ii) conidial expression following UV-B irradiation, of enzymes known to be active in photoreactivation, viz., photolyase (Phr), laccase (Lac) and polyketide synthase (Pks).

METHODS AND RESULTS

Metarhizium acridum (ARSEF 324) and Metarhizium robertsii (ARSEF 2575) were grown either on (i) potato dextrose agar medium (PDA), (ii) PDA supplemented with 1% yeast extract (PDAY), (iii) PDA supplemented with Rb (PDA+Rb), or (iv) PDAY supplemented with Rb (PDAY+Rb). Resulting conidia were exposed to 866·7 mW m^-2 of UV-B Quaite-weighted irradiance to total doses of 3·9 or 6·24 kJ m^-2 . Some conidia also were exposed to 16 klux of white light (WL) after being irradiated, or not, with UV-B to investigate the role of possible photoreactivation. Relative germination of conidia produced on PDA+Rb (regardless Rb concentration) or on PDAY and exposed to UV-B was higher compared to conidia cultivated on PDA without Rb supplement, or to conidia suspended in Rb solution immediately prior to UV-B exposure. The expression of MaLac3 and MaPks2 for M. acridum, as well as MrPhr2, MrLac1, MrLac2 and MrLac3 for M. robertsii was higher when the isolates were cultivated on PDA+Rb and exposed to UV-B followed by exposure to WL, or exposed to WL only.

CONCLUSIONS

Rb in culture medium increases the UV-B tolerance of M. robertsii and M. acridum conidia, and which may be related to increased expression of Phr, Lac and Pks genes in these conidia.

SIGNIFICANCE AND IMPACT OF THE STUDY

The enhanced UV-B tolerance of Metarhizium spp. conidia produced on Rb-enriched media may improve the effectiveness of these fungi in biological control programs.

Metarhizium robertsii illuminated during mycelial growth produces conidia with increased germination speed and virulence

Light conditions during fungal growth are well known to cause several physiological adaptations in the conidia produced. In this study, conidia of the entomopathogenic fungi Metarhizium robertsii were produced on: 1) potato dextrose agar (PDA) medium in the dark; 2) PDA medium under white light (4.98 W m^-2); 3) PDA medium under blue light (4.8 W m^-2); 4) PDA medium under red light (2.8 W m^-2); and 5) minimum medium (Czapek medium without sucrose) supplemented with 3 % lactose (MML) in the dark. The conidial production, the speed of conidial germination, and the virulence to the insect Tenebrio molitor (Coleoptera: Tenebrionidae) were evaluated. Conidia produced on MML or PDA medium under white or blue light germinated faster than conidia produced on PDA medium in the dark. Conidia produced under red light germinated slower than conidia produced on PDA medium in the dark. Conidia produced on MML were the most virulent, followed by conidia produced on PDA medium under white light. The fungus grown under blue light produced more conidia than the fungus grown in the dark. The quantity of conidia produced for the fungus grown in the dark, under white, and red light was similar. The MML afforded the least conidial production. In conclusion, white light produced conidia that germinated faster and killed the insects faster; in addition, blue light afforded the highest conidial production.

The second International Symposium on Fungal Stress: ISFUS

The topic of 'fungal stress' is central to many important disciplines, including medical mycology, chronobiology, plant and insect pathology, industrial microbiology, material sciences, and astrobiology. The International Symposium on Fungal Stress (ISFUS) brought together researchers, who study fungal stress in a variety of fields. The second ISFUS was held in May 8-11 2017 in Goiania, Goiás, Brazil and hosted by the Instituto de Patologia Tropical e Saúde Pública at the Universidade Federal de Goiás. It was supported by grants from CAPES and FAPEG. Twenty-seven speakers from 15 countries presented their research related to fungal stress biology. The Symposium was divided into seven topics: 1. Fungal biology in extreme environments; 2. Stress mechanisms and responses in fungi: molecular biology, biochemistry, biophysics, and cellular biology; 3. Fungal photobiology in the context of stress; 4. Role of stress in fungal pathogenesis; 5. Fungal stress and bioremediation; 6. Fungal stress in agriculture and forestry; and 7. Fungal stress in industrial applications. This article provides an overview of the science presented and discussed at ISFUS-2017.

Maple Bark Biochar Affects Rhizoctonia solani Metabolism and Increases Damping-Off Severity

Many studies have investigated the effect of biochar on plant yield, nutrient uptake, and soil microbial populations; however, little work has been done on its effect on soilborne plant diseases. To determine the effect of maple bark biochar on Rhizoctonia damping-off, 11 plant species were grown in a soilless potting substrate amended with different concentrations of biochar and inoculated or not with Rhizoctonia solani anastomosis group 4. Additionally, the effect of biochar amendment on R. solani growth and metabolism in vitro was evaluated. Increasing concentrations of maple bark biochar increased Rhizoctonia damping-off of all 11 plant species. Using multivariate analyses, we observed positive correlations between biochar amendments, disease severity and incidence, abundance of culturable bacterial communities, and physicochemical parameters. Additionally, biochar amendment significantly increased R. solani growth and hyphal extension in vitro, and altered its primary metabolism, notably the mannitol and tricarboxylic acid cycles and the glycolysis pathway. One or several organic compounds present in the biochar, as identified by gas chromatography-mass spectrometry analysis, may be metabolized by R. solani. Taken together, these results indicate that future studies on biochar should focus on the effect of its use as an amendment on soilborne plant pathogens before applying it to soils.

Modulation of conidia production and expression of the gene bbrgs1 from Beauveria bassiana by oxygen pulses and light

Light and oxidant states affect the conidiation in diverse fungi, although the response has not been described when both stimuli are applied simultaneously. Conidial production and quality in Beauveria bassiana were analysed under four conditions for a wild-type (wt) strain and a previously isolated mutant (mt): normal atmosphere (21% O2; NA) or oxygen-enriched pulses (26% O2; OEP), with either light (L) or darkness (D). The response was complemented by following the expression of the bbrgs1 gene, encoding a regulator of the G-protein signal associated to conidia production. Conidiation was not significantly affected in the mutant strain by any condition (highest value with NA-L: 2.7×10(8)concm(-2)). Relative to maximal levels under NA (NA-D: 4×10(7)concm(2)), the wt strain diminished conidiation by 34-fold under OEP. The expression of bbrgs1 was higher (up to 188 times) in the mutant strain in every condition relative to the wt strain, in fact expression levels were consistent with the conidiation yields between strains. Viability and hydrophobicity were less affected by culture conditions, although pathogenicity parameters improved in conidia from OEP. The response to OEP, either with light or darkness, was strain-dependent for conidial production, viability, hydrophobicity and infectivity of conidia, then these parameters could be modulated in mass production processes.

Fungal stress biology: a preface to the Fungal Stress Responses special edition

There is currently an urgent need to increase global food security, reverse the trends of increasing cancer rates, protect environmental health, and mitigate climate change. Toward these ends, it is imperative to improve soil health and crop productivity, reduce food spoilage, reduce pesticide usage by increasing the use of biological control, optimize bioremediation of polluted sites, and generate energy from sustainable sources such as biofuels. This review focuses on fungi that can help provide solutions to such problems. We discuss key aspects of fungal stress biology in the context of the papers published in this Special Issue of Current Genetics. This area of biology has relevance to pure and applied research on fungal (and indeed other) systems, including biological control of insect pests, roles of saprotrophic fungi in agriculture and forestry, mycotoxin contamination of the food-supply chain, optimization of microbial fermentations including those used for bioethanol production, plant pathology, the limits of life on Earth, and astrobiology.

Tolerance of entomopathogenic fungi to ultraviolet radiation: a review on screening of strains and their formulation

Ultraviolet radiation from sunlight is probably the most detrimental environmental factor affecting the viability of entomopathogenic fungi applied to solar-exposed sites (e.g., leaves) for pest control. Most entomopathogenic fungi are sensitive to UV radiation, but there is great inter- and intraspecies variability in susceptibility to UV. This variability may reflect natural adaptations of isolates to their different environmental conditions. Selecting strains with outstanding natural tolerance to UV is considered as an important step to identify promising biological control agents. However, reports on tolerance among the isolates used to date must be analyzed carefully due to considerable variations in the methods used to garner the data. The current review presents tables listing many studies in which different methods were applied to check natural and enhanced tolerance to UV stress of numerous entomopathogenic fungi, including several well-known isolates of these fungi. The assessment of UV tolerance is usually conducted with conidia using dose-response methods, wherein the UV dose is calculated simply by multiplying the total irradiance by the period (time) of exposure. Although irradiation from lamps seldom presents an environmentally realistic spectral distribution, laboratory tests circumvent the uncontrollable circumstances associated with field assays. Most attempts to increase field persistence of microbial agents have included formulating conidia with UV protectants; however, in many cases, field efficacy of formulated fungi is still not fully adequate for dependable pest control.

Molecular and physiological effects of environmental UV radiation on fungal conidia

Conidia are specialized structures produced at the end of the asexual life cycle of most filamentous fungi. They are responsible for fungal dispersal and environmental persistence. In pathogenic species, they are also involved in host recognition and infection. Conidial production, survival, dispersal, germination, pathogenicity and virulence can be strongly influenced by exposure to solar radiation, although its effects are diverse and often species dependent. UV radiation is the most harmful and mutagenic waveband of the solar spectrum. Direct exposure to solar radiation for a few hours can kill conidia of most fungal species. Conidia are killed both by solar UV-A and UV-B radiation. In addition to killing conidia, which limits the size of the fungal population and its dispersion, exposures to sublethal doses of UV radiation can reduce conidial germination speed and virulence. The focus of this review is to provide an overview of the effects of solar radiation on conidia and on the major systems involved in protection from and repair of damage induced by solar UV radiation. The efforts that have been made to obtain strains of fungi of interest such as entomopathogens more tolerant to solar radiation will also be reviewed.

Stress tolerance and virulence of insect-pathogenic fungi are determined by environmental conditions during conidial formation

The virulence to insects and tolerance to heat and UV-B radiation of conidia of entomopathogenic fungi are greatly influenced by physical, chemical, and nutritional conditions during mycelial growth. This is evidenced, for example, by the stress phenotypes of Metarhizium robertsii produced on various substrates. Conidia from minimal medium (Czapek's medium without sucrose), complex medium, and insect (Lepidoptera and Coleoptera) cadavers had high, moderate, and poor tolerance to UV-B radiation, respectively. Furthermore, conidia from minimal medium germinated faster and had increased heat tolerance and were more virulent to insects than those from complex medium. Low water-activity or alkaline culture conditions also resulted in production of conidia with high tolerance to heat or UV-B radiation. Conidia produced on complex media exhibited lower stress tolerance, whereas those from complex media supplemented with NaCl or KCl (to reduce water activity) were more tolerant to heat and UV-B than those from the unmodified complex medium. Osmotic and nutritive stresses resulted in production of conidia with a robust stress phenotype, but also were associated with low conidial yield. Physical conditions such as growth under illumination, hypoxic conditions, and heat shock before conidial production also induced both higher UV-B and heat tolerance; but conidial production was not decreased. In conclusion, physical and chemical parameters, as well as nutrition source, can induce great variability in conidial tolerance to stress for entomopathogenic fungi. Implications are discussed in relation to the ecology of entomopathogenic fungi in the field, and to their use for biological control. This review will cover recent technologies on improving stress tolerance of entomopathogenic fungi for biological control of insects.

Plant phenolic compounds and oxidative stress: integrated signals in fungal-plant interactions

Upon invasion of a host, fungal pathogens are exposed to a variety of stresses. Plants release reactive oxygen species, and mount a variety of preformed and induced chemical defenses. Phenolic compounds are one example: they are ubiquitous in plants, and an invading pathogen encounters them already at the leaf surface, or for soil-borne pathogens, in the rhizosphere. Phenolic and related aromatic compounds show varying degrees of toxicity to cells. Some compounds are quite readily metabolized, and others less so. It was known already from classical studies that phenolic substrates induce the expression of the enzymes for their degradation. Recently, the ability to degrade phenolics was shown to be a virulence factor. Conversely, phenolic compounds can increase the effectiveness of antifungals. Phenolics are known antioxidants, yet they have been shown to elicit cellular responses that would usually be triggered to counter oxidant stress. Here, we review the evidence for a connection between the fungal response to phenolics as small-molecule signals, and the response to oxidants. The connections proposed here should enable genetic screens to identify specific fungal receptors for plant phenolics. Furthermore, understanding how the pathogen detects plant phenolic compounds as a stress signal may facilitate new antifungal strategies.

Nutritional optimization of a native Beauveria bassiana isolate (HQ917687) pathogenic to housefly, Musca domestica L

A new Beauveria bassiana isolate, showing high activity against Musca domestica L. (Diptera: Muscidae) adults (mortality-100.0 %), larvae (mortality-72.3 %) and pupae (Infection in emerged flies-96.7 %) was used. The isolate was subjected to a combinational approach towards selection of process parameters for its growth optimization. Initial screening of several carbon and nitrogen sources revealed glucose and NaNO3 as the most suitable source for optimal biomass and spore production. Further, optimization through Placket-Burman and a 2(5) full factorial central composite design revealed highly significant effect of glucose and pH. The optimum composition for maximum biomass yield was (g/l): glucose 28; NaNO3 2.43; KH2PO4 1.32; MgSO4 0.60; and pH 7.00. Glucose concentration showed almost linear relationship with biomass yield, indicating its significant contribution in medium composition for fungal growth. Highly significant interactions were observed between glucose and pH, followed by glucose and NaNO3 concentration.

Improving UV resistance and virulence of Beauveria bassiana by genetic engineering with an exogenous tyrosinase gene

Insect pathogenic fungi like Beauveria bassiana have been developed as environmentally friendly biocontrol agents against arthropod pests. However, restrictive environmental factors, including solar ultraviolet (UV) radiation frequently lead to inconsistent field performance. To improve resistance to UV damage, we used Agrobacterium-mediated transformation to engineer B. bassiana with an exogenous tyrosinase gene. The results showed that the mitotically stable transformants produced larger amounts of yellowish pigments than the wild-type strain, and these imparted significantly increased UV-resistance. The virulence of the transgenic isolate was also significantly increased against the silkworm Bombyx mori and the mealworm Tenebrio molitor. This study demonstrated that genetic engineering of B. bassiana with a tyrosinase gene is an effective way to improve fungal tolerance against UV damage.

Cold activity and tolerance of the entomopathogenic fungus Tolypocladium spp. to UV-B irradiation and heat

Studies on the stress resistance of insect-pathogenic fungi are very important to better understand the survival of these organisms in the environment. In this study, we examined the cold activity (8 ± 1°C for 7 days), UV-B tolerance (Quaite-weighted UV-B irradiance at 847.90 mW m(-2) for 1, 2, 3, and 4 h), and wet-heat tolerance (45°C for 1, 2, 3, and 4 h) of two isolates of Tolypocladiumcylindrosporum (ARSEF 3392 and 5558), one isolate of Tolypocladium geodes (ARSEF 3275), and two isolates of Tolypocladium inflatum (ARSEF 4772 and 4877) based on their germination, compared with Metarhizium robertsii (ARSEF 2575). After 3 h of UV-B exposure, T. cylindrosporum germinated at a greater rate than the other Tolypocladium species and had similar viability to that of the M. robertsii. Most Tolypocladium isolates, however, were less UV-B tolerant than M. robertsii. The T.cylindrosporum isolates were also the most thermotolerant, with similar tolerance to the M. robertsii. The isolates of T. inflatum and T. geodes, which had similar heat tolerance, were the least heat tolerant compared with the isolates of T. cylindrosporum and M. robertsii. After 4h of heat exposure, the germination of T. inflatum and T. geodes isolates was not significantly different. For cold activity, both T.cylindrosporum isolates germinated to ca. 100% in only 3 days. Approximately 50% of the two T. inflatum isolates germinated, and less than 5% of T. geodes germinated after 3 days. All fungal isolates, however, completely germinated by the seventh day, except M.robertsii. The isolates of T. cylindrosporum, therefore, were the most heat and UV-B tolerant, and had the highest cold activity compared to the other species. The tolerance of M. robertsii to UV-B radiation and heat was similar to that of T.cylindrosporum.

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.

Visible light during mycelial growth and conidiation of Metarhizium robertsii produces conidia with increased stress tolerance

Light conditions during mycelial growth are known to influence fungi in many ways. The effect of visible-light exposure during mycelial growth was investigated on conidial tolerance to UVB irradiation and wet heat of Metarhizium robertsii, an insect-pathogenic fungus. Two nutrient media and two light regimens were compared. Conidia were produced on (A) potato dextrose agar plus yeast extract medium (PDAY) (A1) under dark conditions or (A2) under continuous visible light (provided by two fluorescent lamps with intensity 5.4 W m(-2)). For comparison, the fungus was also produced on (B) minimal medium (MM) under continuous-dark incubation, which is known to produce conidia with increased tolerance to heat and UVB radiation. The UVB tolerances of conidia produced on PDAY under continuous visible light were twofold higher than conidia produced on PDAY medium under dark conditions, and this elevated UVB tolerance was similar to that of conidia produced on MM in the dark. The heat tolerance of conidia produced under continuous light was, however, similar to that of conidia produced on MM or PDAY in the dark. Conidial yield on PDAY medium was equivalent when the fungus was grown either under continuous-dark or under continuous-light conditions.

Stress induced cross-protection against environmental challenges on prokaryotic and eukaryotic microbes

Prokaryotic and eukaryotic microbes thrive successfully in stressful environments such as high osmolarity, acidic or alkali, solar heat and u.v. radiation, nutrient starvation, oxidative stress, and several others. To live under these continuous stress conditions, these microbes must have mechanisms to protect their proteins, membranes, and nucleic acids, as well as other mechanisms that repair nucleic acids. The stress responses in bacteria are controlled by master regulators, which include alternative sigma factors, such as RpoS and RpoH. The sigma factor RpoS integrates multiple signals, such as the general stress response regulators and the sigma factor RpoH regulates the heat shock proteins. These response pathways extensively overlap and are induced to various extents by the same environmental stresses. In eukaryotes, two major pathways regulate the stress responses: stress proteins, termed heat shock proteins (HSP), which appear to be required only for growth during moderate stress, and stress response elements (STRE), which are induced by different stress conditions and these elements result in the acquisition of a tolerant state towards any stress condition. In this review, the mechanisms of stress resistance between prokaryotic and eukaryotic microbes will be described and compared.