Physiological and antioxidant response by Beauveria bassiana Bals (Vuill.) to different oxygen concentrations

Production of Escovopsis conidia and the potential use of this parasitic fungus as a biological control agent of leaf-cutting ant fungus gardens

This study was carried out to investigate the use of different substrates for the production of Escovopsis conidia and verify the virulence of four different isolates cultured on four types of substrates using a novel bioassay. Escovopsis isolates were molecularly identified, based on Internal Transcribed Spacer (ITS) nucleotide sequences. To evaluate conidial production, suspensions (1 × 106 conidia mL-1) of each Escovopsis isolate were inoculated onto four substrates (parboiled rice, white rice, rolled oats, and corn grits). After 14 days, conidial yields were assessed. The virulence of each isolate cultured on the four substrates was tested against Leucoagaricus fungus garden fragments, by directly applying 500 µL of each conidial suspension (1 × 107 conidia mL-1), and the development of the parasite was monitored daily until it completely colonized the fungus garden. It was observed that rolled oats were the best substrate for conidial production, with a yield of 1.7 × 107 to 2.0 × 108 conidia mL-1. Furthermore, isolate AT-01 produced the highest number of conidia when compared with the other isolates. Regardless of the substrate used to produce AT-01 conidia, this isolate completely colonized the fungus garden 6 days post inoculation (dpi), followed by AT-02, AC-01, and AC-2. High levels of both conidial production and virulence against the leaf-cutting ant fungus garden were observed here.

Biological Characteristics of Beauveria majiangensis Strain MJ1015 and Optimization of Solid Medium Technology for Sporulation

The entomopathogenic fungus Beauveria majiangensis strain MJ1015, recently isolated from white grubs on a blueberry farm in Guizhou, China, could be used as a biocontrol agent. As a first step toward determining the effect of different solid culture media, temperature, and pH on colony growth rate and sporulation, we evaluated the optimum solid medium for mycelial growth and conidia production on a commercial scale. Subsequently, we also used single-factor analysis and response surface optimization to optimize the composition of the solid culture medium. On potato dextrose agar (PDA) medium, MJ1015 grew fastest and produced the highest spore yield at 29°C and pH 5. The best solid medium for the growth and sporulation of strain MJ1015 comprised 64.70 g/l of rice, 13.00 g/l of wheat, 0.30 g/l of NaNO3, 0.36 g/l of K2HPO4 · 3H2O, and 1.00 g/l of CaCO3. Rice, NaNO3, and K2HPO4 · 3H2O were the main influencing factors. The predicted value of cultured spores using the optimal medium was 4.56 x 1010 conidia/l. The validation test results showed that the average growth rate of strain MJ1015 on the optimal medium was 85% and 96% faster than that on Sabouraud dextrose agar with yeast extracts medium (SDAY) and PDA, respectively. Sporulation was 43.90 times and 9.65 times of that produced on SDAY and PDA, respectively. Our findings provide a theoretical basis for the commercial production of B. majiangensis to control white grubs.

Biocontrol of Xyleborus affinis (Curculionidae: Scolitinae) Females and Progeny by Beauveria bassiana (Hypocreales: Cordycipitaceae) in a Sawdust Artificial Diet Model

The ambrosia beetle Xyleborus affinis, recently reported affecting avocado trees in Mexico, represents one of the most widespread insects worldwide. Previous reports have shown that Xyleborus genera members are susceptible to Beauveria bassiana and other entomopathogenic fungus strains. However, their effect on borer beetles' progeny has not been fully investigated. The aim of the present study was to determine the insecticidal activity of B. bassiana on X. affinis adult females and their progeny in an artificial sawdust diet bioassay model. The B. bassiana strains CHE-CNRCB 44, 171, 431, and 485 were individually tested on females at concentrations ranging from 2 × 10⁶ to 1 × 10⁹ conidia mL^-1. After 10 d of incubation, diet was evaluated to count laid eggs, larvae, and adults. Insect conidia loss after exposure was determined by attached conidia to each insect after 12 h of exposure. The results showed that females' mortality ranged between 3.4% and 50.3% in a concentration-response manner. Furthermore, we did not observe statistical differences among strains at the highest concentration. CHE-CNRCB 44 showed the highest mortality at the lowest concentration and reduced larvae and laid eggs at the highest concentration (p < 0.01). Strains CHE-CNRCB 44, 431, and 485 significantly decreased larvae, as compared with the untreated control. After 12 h, up to 70% of conidia was removed by the effect of the artificial diet. In conclusion, B. bassiana has the potential to control X. affinis adult females and progeny.

Evaluation of the carbonylation of filamentous fungi proteins by dry immune dot blotting

The development of mycological gerontology requires effective methods for assessing the biological age of fungal cells. This assessment is based on the analysis of a complex of aging and oxidative stress markers. One of the most powerful such markers is the protein carbonylation. In this study, the already known method of dry immune dot blotting is adapted for mycological studies of the content of protein carbonyl groups. After testing the method on a number of filamentous fungi species, some features of the accumulation of carbonylated proteins in mycelium were established. Among these features: (i) a weak effect of exogenous oxidative stress on the accumulation of carbonyls in a number of fungi, (ii) reversibility of the carbonyl accumulation, (iii) possibility of arbitrary regulation of carbonyl content by fungus itself and (iv) the influence of hormesis. In addition, two polar strategies for the accumulation of carbonyl modification were revealed, named Id-strategy (Indifferent) and Cn-strategy (Concern). Thus, even the analysis of one marker allows making some preliminary general assumptions and conclusions. For example, the idea that fungi can freely regulate their biological age is confirmed. This feature makes fungi very flexible in terms of responding to environmental influences and promising objects for gerontology.

Improved conidiation from entomopathogenic fungi through 26% oxygen pulses in solid-state culture depends on a balance between headspace volume and substrate amounts

Oxygen-enriched atmospheres applied as periodic pulses increased conidia production from entomopathogenic fungi in agar surface cultures. However, this advantage has not been obtained in solid-state cultures (SSC), probably as a result of different biomass production between both culture systems. In this work, the biomass formation from two Isaria strains was limited in SSC using 5, 2·5 and 1 initial grams of substrate (gds). In the system with 5 gds, conidia production decreased in 26% oxygen-enriched pulses compared to the normal atmosphere. Conversely, 26% oxygen pulses increased conidiation up to one order magnitude in systems with 2·5 and 1 gds, respective to the normal atmosphere. These results were explained by oxygen depletion and high CO₂ accumulation in the 5 gds system. Whereas in systems with 2·5 or 1 gds, oxygen levels remained high enough to stimulate conidiation. These results were attributed to the headspace volume:gds ratio, which is suggested to be ≥48 ml per gds. This ratio is proposed as a scaling-up criterion for bioreactor design when oxygen-enriched pulses are used in SSC for improvement of conidia production. SIGNIFICANCE AND IMPACT OF THE STUDY: Oxygen-enriched atmospheres applied as periodic pulses increase conidiation in entomopathogenic fungi (EF). However, this remained restricted to agar surface cultures, since conidiation decreased when carried out in solid-state culture (SSC) which is used as large-scale production system. We identified that in SSC the ratio between the headspace volume containing 26% oxygen-enriched pulses and the grams of substrate determines the conidiation response to oxygen-enriched pulses. For the first time, oxygen-enriched pulses increased conidiation in SSC respective to the normal atmosphere in four EF. This ratio is proposed as a bioreactor criterion design for large-scale conidia production of EF using oxygen-enriched pulses.

The endophytic capacity of the entomopathogenic fungus Beauveria bassiana caused inherent physiological response in two barley (Hordeum vulgare) varieties

In this study, the endophytic capacity of B. bassiana was determined for two barley (Hordeum vulgare) varieties, Josefa and Esmeralda, inoculated with a seedling immersion at three different concentrations (1 × 10⁶, 1 × 10⁷ and 1 × 10⁸ conidia/mL). Seedling length and chlorophyll content were found to be not affected when inoculated with the entomopathogenic fungus, in both barley varieties. However, the colonisation percentage was found to be significantly lower with the inoculum concentration 1 × 10⁶ conidia/mL for both barley varieties (P < 0.05) when compared to the other concentrations. Furthermore, a principal component analysis indicated that 96.23% of the variability in the data could be explained with two components. This analysis showed that the seedling length and chlorophyll content were positively correlated in both barley varieties for the 1 × 10⁷ conidia/mL concentration. Likewise, a positive correlation was observed for colonisation percentage and treatment with 1 × 10⁸ conidia/mL in the Josefa variety only. This is the first study in which the endophytic capacity of B. bassiana was evaluated in two different barley varieties, with the Josefa variety found to be the most susceptible.

Transient anoxia during Metarhizium robertsii growth increases conidial virulence to Tenebrio molitor

Little is known about the phenotypic effects of hypoxia and transient anoxia on the virulence of an entomopathogenic fungus. Conidia of Metarhizium robertsii were produced on: (1) potato dextrose agar medium (PDA) under normoxia; (2) PDA medium under continuous hypoxia; (3) PDA medium under transient anoxia; and (4) minimal medium with lactose (MML) under normoxia. Conidia produced under transient anoxia and produced on MML were the most virulent to Tenebrio molitor. Conidia produced under normoxia and hypoxia were the least virulent. Conidial production and germination speed of conidia produced under normoxia, hypoxia, and transient anoxia were similar; however, MML produced less conidia, but germinated faster than any other treatments.

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.

Reactive oxygen species production, induced by atmospheric modification, alter conidial quality of Beauveria bassiana

AIM

The aim of this study was to determine the relationship between reactive oxygen species (ROS) production and conidial infectivity in Beauveria bassiana.

METHODS AND RESULTS

Beauveria bassiana Bb 882.5 was cultured in solid-state culture (SSC) using rice under three oxygen conditions (21%, or pulses at 16 and 26%). Hydrophobicity was determined using exclusion phase assay. Bioassays with larvae or adults of Tenebrio molitor allowed the measurements of infectivity parameters. A fluorometric method was used for ROS quantification (superoxide and total peroxides). NADPH oxidase (NOX) activity was determined by specific inhibition. Conidial hydrophobicity decreased by O2 pulses. Mortality of larvae was only achieved with conidia harvested from cultures under 21% O2 ; whereas for adult insects, the infectivity parameters deteriorated in conidia obtained after pulses at 16 and 26% O2 . At day 7, ROS production increased after 16 and 26% O2 treatments. NOX activity induced ROS production at early stages of the culture.

CONCLUSION

Modification of atmospheric oxygen increases ROS production, reducing conidial quality and infectivity.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study in which conidial infectivity and ROS production in B. bassiana has been related, enhancing the knowledge of the effect of O2 pulses in B. bassiana.

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.

Variations in oxygen concentration cause differential antioxidant response and expression of related genes in Beauveria bassiana

The entomopathogenic fungus Beauveria bassiana is widely used in pest biocontrol strategies. We evaluated both the antioxidant response mediated by compatible solutes, trehalose or mannitol, and the expression of related genes using oxygen pulses at three oxygen concentrations in solid state culture (SSC): normal atmosphere (21% O2), low oxygen (16% O2) and enriched oxygen (26% O2). Trehalose concentration decreased 75% after atmospheric modifications in the cultures, whereas mannitol synthesis was three-fold higher under the 16% O2 pulses relative to normal atmosphere (100 and 30 μg mannitol mg(-1) biomass, respectively). Confirming this result, expression of the mpd gene, coding for mannitol-1-P dehydrogenase (MPD), increased up to 1.4 times after O2 pulses. The expression of the bbrgs1 gene, encoding a regulatory G protein related to conidiation, was analysed to explain previously reported differences in conidial production. Surprisingly, expression of bbrgs1 decreased after atmospheric modification. Finally, principal component analysis (PCA) indicated that 83.39% of the variability in the data could be explained by two components. This analysis corroborated the positive correlation between mannitol concentration and mpd gene expression, as well as the negative correlation between conidial production and bbrgs1 gene expression. This study contributes to understanding of antioxidant and molecular response of B. bassiana induced under oxidant conditions.

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.

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.

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.

Metabolic profiles and free radical scavenging activity of Cordyceps bassiana fruiting bodies according to developmental stage

The metabolic profiles of Cordyceps bassiana according to fruiting body developmental stage were investigated using gas chromatography-mass spectrometry. We were able to detect 62 metabolites, including 48 metabolites from 70% methanol extracts and 14 metabolites from 100% n-hexane extracts. These metabolites were classified as alcohols, amino acids, organic acids, phosphoric acids, purine nucleosides and bases, sugars, saturated fatty acids, unsaturated fatty acids, or fatty amides. Significant changes in metabolite levels were found according to developmental stage. Relative levels of amino acids, purine nucleosides, and sugars were higher in development stage 3 than in the other stages. Among the amino acids, valine, isoleucine, lysine, histidine, glutamine, and aspartic acid, which are associated with ABC transporters and aminoacyl-tRNA biosynthesis, also showed higher levels in stage 3 samples. The free radical scavenging activities, which were significantly higher in stage 3 than in the other stages, showed a positive correlation with purine nucleoside metabolites such as adenosine, guanosine, and inosine. These results not only show metabolic profiles, but also suggest the metabolic pathways associated with fruiting body development stages in cultivated C. bassiana.