Relationship between thermotolerance and hydrophobin-like proteins in aerial conidia of Beauveria bassiana and Paecilomyces fumosoroseus as fungal biocontrol agents

Complex nitrogen sources from agro-industrial byproducts: impact on production, multi-stress tolerance, virulence, and quality of Beauveria bassiana blastospores

We investigated the impact of various complex organic nitrogen sources on the submerged liquid fermentation of Beauveria bassiana, a versatile entomopathogenic fungus known for producing hydrophilic yeast-like single cells called blastospores. Specifically, we examined yeast extract, autolyzed yeast, inactive yeast, cottonseed flour, corn bran, and corn gluten meal as nitrogen compounds with different carbon-to-nitrogen (C:N) ratios. Our comprehensive analysis encompassed blastospore production, tolerance to abiotic stresses, shelf stability after drying, and virulence against mealworm larvae, crucial attributes for developing effective blastospore-based biopesticides. Notably, cottonseed flour emerged as the optimal nitrogen source, yielding up to 2.5 × 109 blastospores/mL within 3 days in a bioreactor. These blastospores exhibited the highest tolerance to heat stress and UV-B radiation exposure. The endogenous C:N ratio in blastospore composition was also impacted by nitrogen sources. Bioassays with mealworm larvae demonstrated that blastospores from cottonseed flour were the most virulent, achieving faster lethality (lower LT50) and requiring a lower inoculum (LC50). Importantly, blastospores produced with cottonseed flour displayed extended viability during storage, surpassing the retention of viability compared to those from autolyzed yeast over 180 days at 4°C. Despite differences in storage viability, both nitrogen sources conferred similar long-term blastospore bioactivity against mealworms. In summary, this research advances our understanding of the crucial impact of complex organic nitrogen selection on the phenotypic traits of blastospores in association with their intracellular C:N ratio, contributing to the production of ecologically fit, shelf-stable, and virulent propagules for effective pest biocontrol programs.

IMPORTANCE

Biological control through entomopathogenic fungi provides essential ecological services in the integrated management of agricultural pests. In the context of submerged liquid fermentation, the nutritional composition significantly influences the ecological fitness, virulence and quality of these fungi. This study specifically explores the impact of various complex organic nitrogen sources derived from agro-industrial byproducts on the submerged liquid fermentation of Beauveria bassiana, a versatile entomopathogenic fungus known for producing hydrophilic yeast-like blastospores. Notably, manipulating the nitrogen source during submerged cultivation can influence the quality, fitness, and performance of blastospores. This research identifies cottonseed flour as the optimal low-cost nitrogen source, contributing to increased production yields, enhanced multi-stress tolerance, heightened virulence with extended shelf life and long-term bioactivity. These findings deepen our understanding of the critical role of nitrogen compound selection in liquid media formulation, facilitating the production of ecologically fit and virulent blastospores for more effective pest biocontrol programs.

Physiological response of Metarhizium rileyi with linoleic acid supplementation

Metarhizium rileyi has a broad biocontrol spectrum but is highly sensitive to abiotic factors. A Colombian isolate M. rileyi Nm017 has shown notorious potential against Helicoverpa zea. However, it has a loss of up to 22 % of its conidial germination after drying, which limits its potential as a biocontrol agent and further commercialization. Conidial desiccation resistance can be enhanced by nutritional supplements, which promotes field adaptability and facilitates technological development as a biopesticide. In this study, the effect of culture medium supplemented with linoleic acid on desiccation tolerance in Nm017 conidia was evaluated. Results showed that using a 2 % linoleic acid-supplemented medium increased the relative germination after drying by 41 % compared to the control treatment, without affecting insecticidal activity on H. zea. Also, the fungus increased the synthesis of trehalose, glucose, and erythritol during drying, independently of linoleic acid use. Ultrastructural analyses of the cell wall-membrane showed a loss of thickness by 22 % and 25 %, in samples obtained from 2 % linoleic acid supplementation and the control, respectively. Regarding its morphological characteristics, conidia inner area from both treatments did not change after drying. However, conidia from the control had a 24 % decrease in length/width ratio, whereas there was no alteration in conidia from acid linoleic. The average value of dry conidia elasticity coefficient from linoleic acid treatment was 200 % above the control. Medium supplementation with linoleic acid is a promising fermentation strategy for obtaining more tolerant conidia without affecting production and biocontrol parameters, compatible solutes synthesis, or modifying its cell configuration.

Selection and characterization of thermotolerant Beauveria bassiana isolates and with insecticidal activity against the cotton-melon aphid Aphis gossypii (Glover) (Hemiptera: Aphididae)

BACKGROUND

Cotton-melon aphid Aphis gossypii (Glover) causes severe damage mainly to cucurbits. Twenty-two Beauveria sp. isolates were simultaneously assessed for their pathogenicity and heat tolerance. The selected isolates were identified molecularly and characterized in terms of conidial germination rate, mycelial growth, conidial yield and endophytic activity.

RESULTS

Screening bioassays showed that the B. bassiana isolates B3, B7, B9 and B12 were the most toxic, inducing mortality equal to or slightly higher than the commercialized strain B. bassiana BNat (70.7%). Median lethal concentration (LC₅₀ ) bioassays revealed that only isolate B12 had a significantly lower LC₅₀ value (5.4 × 10⁵ conidia ml^-1 ) than strain BNat (5 × 10⁶ conidia ml^-1 ). The heat tolerance screening test (1 h of exposure to 45°C) allowed us to select isolates B3, B7, B9 and B12 with germination rates of 57.5% to 80.1% after 24 h incubation at 25°C, all significantly higher than strain BNat (22.1%). The germination rates of all isolates decreased significantly after 2 h of exposure to 45°C, with the exception of isolate B12 which displayed the highest thermotolerance (72% germination). The four selected isolates were able to endophytically colonize cucumber leaves when applied to the foliage. Inoculation of cucumber plants with isolate B12 did not affect cucumber plant growth. However, several plant growth parameters were improved 5 weeks after root inoculation.

CONCLUSION

On the basis of its potent toxicity and thermotolerance, isolate B12 is a good candidate for further development as a biopesticide for use in integrated pest management strategies for aphid control. © 2022 Society of Chemical Industry.

Screening and Virulence of the Entomopathogenic Fungi Associated with Chilo suppressalis Walker

The current study aimed to explore the entomopathogenic fungi associated with the larvae of Chilo suppressalis Walker, a serious pest of rice, in northern Iran. The collected specimens were cultured and identified through morphological and molecular methods. The 38 specimens were identified by microscopic examination and genetic sequencing of the ITS region as follows: twenty-one isolates of Beauveria bassiana, five isolates of Akanthomyces lecanii, four isolates of Akanthomyces muscarious, three isolates of Metarhizium anisopliae, two isolates of Hirsutella subulata, two isolates of Trichoderma sp. and one isolate of Aspergillus sp. All the identified isolates were treated on the larvae through bioassay, evaluating the amount of hydrophobin and the activities of proteases, chitinases and lipase to find their virulence. Moreover, the percentage of thermotolerant and cold activity of the isolates were tested to determine their environmental persistence. The overall results revealed the isolates of B. bassiana, including BBRR1, BBAL1 and BBLN1 as the most virulent and environmental adaptive isolates among the fungi associated with C. suppressalis.

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

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

Genomic Analysis of the Insect-Killing Fungus Beauveria bassiana JEF-007 as a Biopesticide

Insect-killing fungi have high potential in pest management. A deeper insight into the fungal genes at the whole genome level is necessary to understand the inter-species or intra-species genetic diversity of fungal genes, and to select excellent isolates. In this work, we conducted a whole genome sequencing of Beauveria bassiana (Bb) JEF-007 and characterized pathogenesis-related features and compared with other isolates including Bb ARSEF2860. A large number of Bb JEF-007 genes showed high identity with Bb ARSEF2860, but some genes showed moderate or low identity. The two Bb isolates showed a significant difference in vegetative growth, antibiotic-susceptibility, and virulence against Tenebrio molitor larvae. When highly identical genes between the two Bb isolates were subjected to real-time PCR, their transcription levels were different, particularly in heat shock protein 30 (hsp30) gene which is related to conidial thermotolerance. In several B. bassiana isolates, chitinases and trypsin-like protease genes involved in pathogenesis were highly conserved, but other genes showed noticeable sequence variation within the same species. Given the transcriptional and genetic diversity in B. bassiana, a selection of virulent isolates with industrial advantages is a pre-requisite, and this genetic approach could support the development of excellent biopesticides with intellectual property protection.

Fungal spores for dispersion in space and time

Spores are an integral part of the life cycle of the gross majority of fungi. Their morphology and the mode of formation are both highly variable among the fungi, as is their resistance to stressors. The main aim for spores is to be dispersed, both in space, by various mechanisms or in time, by an extended period of dormancy. Some fungal ascospores belong to the most stress-resistant eukaryotic cells described to date. Stabilization is a process in which biomolecules and complexes thereof are protected by different types of molecules against heat, drought, or other molecules. This review discusses the most important compounds that are known to protect fungal spores and also addresses the biophysics of cell protection. It further covers the phenomena of dormancy, breaking of dormancy, and early germination. Germination is the transition from a dormant cell toward a vegetative cell and includes a number of specific changes. Finally, the applied aspects of spore biology are discussed.

Beauveria bassiana: Biocontrol Beyond Lepidopteran Pests

Beauveria bassiana has been extensively employed since the last century for biocontrol of lepidopteran pests. B. bassiana has also been explored for diverse functions including bioremediation of toxic industrial effluents and heavy-metal polluted soils. Investigations on multifarious applications of chemically diverse secondary metabolites of this entomopathogenic fungus offer promising implications in pharmaceutical and agricultural sectors. In addition, the development of eco-friendly bioremediation strategies using abiotic stress-tolerant strains of B. bassiana will contribute to maintain the sustainability of agroecosystem.

Morphological changes, chitinolytic enzymes and hydrophobin-like proteins as responses of Lecanicillium lecanii during growth with hydrocarbon

Lecanicillium lecanii, Verticillium chlamydosporium, V. fungicola var flavidum and Beauveria bassiana were evaluated on their growth with pure n-hexane, toluene and n-hexane:toluene 17:83 (v:v) mixture. Another set of treatments were conducted with colloidal chitin as additional carbon source. All the strains of Lecanicillium were able to grow using hydrocarbons with or without the addition of chitin, although the presence of hydrocarbons showed significant inhibition evidenced by measured biomass, radial growth and microscopic analyses. Degradation of n-hexane ranged within 43 and 62 % and it was higher than that with toluene. The strains L460, L157 and L2149, which presented the highest growth, were further selected for determinations of hydrocarbon consumptions in microcosms. Strain L157 showed the highest consumption of n-hexane (55.6 %) and toluene (52.9 %) as sole carbon source and it also displayed activities of endochitinases, N-acetylhexosaminidase and production of hydrophobins class I and II.

Soil eukaryotic microorganism succession as affected by continuous cropping of peanut--pathogenic and beneficial fungi were selected

Peanut is an important oil crop worldwide and shows considerable adaptability but growth and yield are negatively affected by continuous cropping. Soil micro-organisms are efficient bio-indicators of soil quality and plant health and are critical to the sustainability of soil-based ecosystem function and to successful plant growth. In this study, 18S rRNA gene clone library analyses were employed to study the succession progress of soil eukaryotic micro-organisms under continuous peanut cultivation. Eight libraries were constructed for peanut over three continuous cropping cycles and its representative growth stages. Cluster analyses indicated that soil micro-eukaryotic assemblages obtained from the same peanut cropping cycle were similar, regardless of growth period. Six eukaryotic groups were found and fungi predominated in all libraries. The fungal populations showed significant dynamic change and overall diversity increased over time under continuous peanut cropping. The abundance and/or diversity of clones affiliated with Eurotiales, Hypocreales, Glomerales, Orbiliales, Mucorales and Tremellales showed an increasing trend with continuous cropping but clones affiliated with Agaricales, Cantharellales, Pezizales and Pyxidiophorales decreased in abundance and/or diversity over time. The current data, along with data from previous studies, demonstrated that the soil microbial community was affected by continuous cropping, in particular, the pathogenic and beneficial fungi that were positively selected over time, which is commonplace in agro-ecosystems. The trend towards an increase in fungal pathogens and simplification of the beneficial fungal community could be important factors contributing to the decline in peanut growth and yield over many years of continuous cropping.

A Group III histidine kinase (mhk1) upstream of high-osmolarity glycerol pathway regulates sporulation, multi-stress tolerance and virulence of Metarhizium robertsii, a fungal entomopathogen

The role of Metarhizium robertsii Group III histidine kinase (mhk1) in regulating various phenotypes of the fungal entomopathogen and the transcripts of 25 downstream genes likely associated with the phenotypes were probed by constructing Δmhk1 and Δmhk1/mhk1 mutants. All examined Δmhk1 phenotypes except unchanged sensitivity to fungicide (dimethachlon) differed significantly from those of wild type and Δmhk1/mhk1, which were similar to each other. Significant phenotypic changes in Δmhk1 included increased conidial yields on two media, increased tolerance to H(2)O(2) , decreased tolerance to menadione, increased tolerance to hyperosmolarity, increased conidial thermotolerance, decreased conidial UV-B resistance and reduced virulence to Tenebrio molitor larvae. The mhk1 disruption elevated the transcripts of nine genes, including two associated with conidiation (flbC and hymA) and three encoding catalases but decreased seven other gene transcripts, including three for superoxide dismultases, under normal conditions. The high-osmolarity glycerol pathway MAPK phosphorylation level in Δmhk1 culture was increased 1.0- to 1.8-fold by KCl, sucrose and menadione stresses but reduced drastically by H(2)O(2) or heat (40°C) stress, accompanied with different transcript patterns of all examined genes under the stresses. Our results confirmed the crucial role of mhk1 in regulating the expression of the downstream genes and associated phenotypes important for the fungal biocontrol potential.

Characterization of a thioredoxin (BbTrx) from the entomopathogenic fungus Beauveria bassiana and its expression in response to thermal stress

A thioredoxin (BbTrx) was identified from the entomopathogenic fungus Beauveria bassiana. The cloned nucleotide sequence consisted of a 423-bp open reading frame encoding a 141-amino-acid thioredoxin, a 1011-bp 5' region, and a 419-bp 3' region. The deduced protein sequence of BbTrx, including a common 95-amino-acid conserved domain and a unique 46-amino-acid carboxy terminal region, was similar (≤38% identity) to that of other thioredoxins and phylogenetically closest to that from Neurospora crassa. In insulin solution containing dithiothreitol at 25 °C, recombinant BbTrx or a truncated form lacking the carboxy terminal region (BbTrxD) exhibited disulfide reduction activity. BbTrxD was more active after pre-incubation at 40-75 °C, and cells expressing BbTrxD showed significantly higher tolerance to thermal stress (51 °C). The BbTrx expression in B. bassiana was greatly elevated when stressed at 40 °C. The results indicate that the new thioredoxin is a potential target for improving the thermotolerance of B. bassiana formulations.

A conidial protein (CP15) of Beauveria bassiana contributes to the conidial tolerance of the entomopathogenic fungus to thermal and oxidative stresses

Aerial conidia are central dispersing structures for most fungi and represent the infectious propagule for entomopathogenic fungus Beauveria bassiana, thus the active ingredients of commercial mycoinsecticides. Although a number of formic-acid-extractable (FAE) cell wall proteins from conidia have been characterized, the functions of many such proteins remain obscure. We report that a conidial FAE protein, termed CP15, isolated from B. bassiana is related to fungal tolerance to thermal and oxidative stresses. The full-length genomic sequence of CP15 was shown to lack introns, encoding for a 131 amino acid protein (15.0 kDa) with no sequence identity to any known proteins in the NCBI database. The function of this new gene with two genomic copies was examined using the antisense-RNA method. Five transgenic strains displayed various degrees of silenced CP15 expression, resulting in significantly reduced conidial FAE protein profiles. The FAE protein contents of the strains were linearly correlated to the survival indices of their conidia when exposed to 30-min wet stress at 48°C (r (2) = 0.93). Under prolonged 75-min heat stress, the median lethal times (LT(50)s) of their conidia were significantly reduced by 13.6-29.5%. The CP15 silenced strains were also 20-50% less resistant to oxidative stress but were not affected with respect to UV-B or hyperosmotic stress. Our data indicate that discrete conidial proteins may mediate resistance to some abiotic stresses, and that manipulation of such proteins may be a viable approach to enhancing the environmental fitness of B. bassiana for more persisting control of insect pests in warmer climates.

Carbon catabolite repressor gene BbCre1 influences carbon source uptake but does not have a big impact on virulence in Beauveria bassiana

A gene (BbCre1, GenBank accession number EF108309) encoding a carbon catabolite repressor (CreA) with two Cys(2)His(2) zinc finger regions and a nuclear localization signal was cloned from the entomopathogenic fungus Beauveria bassiana. Overexpression and antisense strategies were used to investigate the biological functions of this gene. Compared with the wild type, the conidial yield and colony growth of BbCre1-overexpression transformants were significantly decreased on the plates with xylose or ethanol as the sole carbon source. With glucose as the sole carbon source, a significant difference was observed in the activity of Pr1A-like protease among BbCre1-overexpression transformants, antisense-BbCre1 transformants and the wild type. However, bioassays showed that knockdown or overexpression of BbCre1 did not have a significant impact on the virulence of B. bassiana to aphids. These results imply that the fungus remains virulent, even when simpler, less expensive nutrients are available, i.e. glucose.

A new non-hydrophobic cell wall protein (CWP10) of Metarhizium anisopliae enhances conidial hydrophobicity when expressed in Beauveria bassiana

A cell wall protein, CWP10, resolved from the conidial formic acid extract of a Metarhizium anisopliae isolate, was characterized as a new 9.9-kDa protein with a 32-aa signal peptide with a central hydrophobic region (ca. 10 residues) at its N-terminus. This protein was proven neither to be hydrophobic nor glycosylated and encoded by a 363-bp, single-copy gene with three introns. CWP10 was existent in the conidial extracts of seven of 18 tested M. anisopliae isolates and much more abundant (immunogold-labeled) on conidial walls than in cytoplasm. Integrating the gene into a CWP10-absent strain of Beauveria bassiana led to excellent expression of CWP10 in aerial conidia, increasing net conidial hydrophobicity by 10.8% or adhesion to hydrophobic Teflon by 1.3-fold. However, the expressed protein had no effect on conidial tolerance to thermal and ultraviolet stresses. This is the first report on a non-hydrophobic cell-wall protein enhancing conidial hydrophobicity and adhesion of the fungal species.

Sprays of emulsifiable Beauveria bassiana formulation are ovicidal towards Tetranychus urticae (Acari: Tetranychidae) at various regimes of temperature and humidity

Aerial conidia of Beauveria bassiana in an emulsifiable formulation germinated by >95% after 24 h exposure to the regimes of 20, 25 and 30 degrees C with 51%, 74% and 95% RH. Ovicidal activities of the formulation towards two-spotted spider mite, Tetranychus urticae, were assayed at the concentrations of 0, 18, 160 and 693 conidia mm(-2) sprayed separately onto fava bean leaves including 39 (25-76) eggs per capita. All the sprayed eggs on the leaves were directly exposed to the different regimes for hatch after 24 h maintenance in covered Petri dishes. Generally, hatched proportions increased over post-spray days and decreased with the elevated fungal concentrations; no more eggs hatched from day 9 or 10 onwards. Based on the counts of the hatched/non-hatched eggs in the different regimes, the final egg mortalities were 15.0-40.4%, 48.9-66.6% and 62.9-87.5% at the low, medium and high concentrations, respectively, but only 5.6-11.3% in blank controls. The RH effect on the fungal action was significant at 20 and 25 degrees C but not at 30 degrees C whereas the effect of temperature was significant at 51% and 74% RH but not at 95% RH. Probit analysis of the egg mortalities versus the fungal sprays generated median lethal concentrations (LC(50)) of 65-320 conidia mm(-2) at all the regimes, and of only 65-78 conidia mm(-2) at 25-30 degrees C with 74-95% RH. The results highlight ovicidal activities of the emulsifiable formulation against the mite species at the tested regimes and its potential use in spider mite control.

Physiological implication of intracellular trehalose and mannitol changes in response of entomopathogenic fungus Beauveria bassiana to thermal stress

To explore possible role of intracellular trehalose accumulation in fungal tolerance to summer-like thermal stress, 3-day colonies of Beauveria bassiana grown on a glucose-free medium at 25 degrees C were separately exposed to 35, 37.5 and 40 degrees C for 1-18 h, respectively. Trehalose accumulation in stressed mycelia increased from initial 4.2 to 88.3, 74.7 and 65.5 mg g(-1) biomass after 6-h stress at 35, 37.5 and 40 degrees C, respectively, while intracellular mannitol level generally declined with higher temperatures and longer stress time. The stress-enhanced trehalose level was significantly correlated to decreased trehalase activity (r(2) = 0.73) and mannitol content (r(2) = 0.38), which was inversely correlated to the activity of mannitol dehydrogenase (r(2) = 0.41) or mannitol 1-phosphate dehydrogenase (r(2) = 0.30) under the stresses. All stressed cultures were successfully recovered at 25 degrees C but their vigor depended on stressful temperature, time length and the interaction of both (r (2) = 0.98). The highest level of 6-h trehalose accumulation at 35 degrees C was found enhancing the tolerance of the stressed cultures to the greater stress of 48 degrees C. The results suggest that the trehalose accumulation result partially from metabolized mannitol and contribute to the fungal thermotolerance. Trehalase also contributed to the thermotolerance by hydrolyzing accumulated trehalose under the conditions of thermal stress and recovery.

Intraspecific tolerance of Metarhizium anisopliae conidia to the upper thermal limits of summer with a description of a quantitative assay system

Conidial tolerance to the upper thermal limits of summer is important for fungal biocontrol agents, whose conidia are formulated into mycoinsecticides for field application. To develop an efficient assay system, aerial conidia of eight Metarhizium anisopliae, four M. anisopliae var. anisopliae, and six M. anisopliae var. acridum isolates with different host and geographic origins were wet-stressed for <or=180 min at 48 degrees C or incubated for 14 d colony growths at 10-35 degrees C. The survival ratios (relative to unstressed conidia) of each isolate, examined at 15-min intervals, fit a logistic equation (r2>or=0.975), yielding median lethal times (LT50s) of 14.3-150.3 min for the 18 isolates stressed at 48 degrees C. Seven grasshopper isolates from Africa had a mean LT50 of 110 (73-150) min, but could not grow at 10 or 15 degrees C. The mean LT50 of five non-grasshopper isolates capable of growing at 10-35 degrees C was 16 (10-26) min only. Three isolates with typically low (type I), medium (type II), and high (type III) levels of tolerance to 48 degrees C were further assayed for <or=4-d tolerance of their conidia to the wet stress at 38, 40, 42, or 45 degrees C. The resultant LT50s decreased to 20, 53 and 167 min at 48 degrees C from 507, 1612, and 8256 min at 38 degrees C for types I, II and III, respectively. For the distinguished types, the logarithms of the LT50s were significantly correlated to the temperatures of 38-48 degrees C with an inverse linearity (r2>or=0.88). The method developed to assay quantitatively fungal thermotolerance would be useful for screening of fungal candidates for improved pest control in summer.

Cold activity of Beauveria and Metarhizium, and thermotolerance of Beauveria

Heat and cold are environmental abiotic factors that restrict the use of entomopathogenic fungi as agents for biological control of insects. The thermotolerance and cold activity of 60 entomopathogenic fungal isolates, including five species of Beauveria and one isolate of Engyodontium albus (=Beauveria alba) were examined as to tolerance of temperatures that might be encountered during field use. In addition, cold activity of eight Metarhizium spp. isolates was evaluated. The isolates were from various geographic regions, arthropod hosts or substrates. High variability in conidial thermotolerance was found among the Beauveria spp. isolates after exposure to 45 degrees C for 2 h, as evidenced by low (0-20%), medium (20-60%), or high germination (60-80%). The thermal death point (0% germination) for three rather thermotolerant B. bassiana isolates (CG 138, GHA and ARSEF 252) was 46 degrees C for 6 h. At low temperatures (5 degrees C), with few exceptions (e.g. CG 66, UFPE 479, CG 227, CG 02), most of the B. bassiana isolates germinated well (ca. 100%). On the other hand, only one isolate of Metarhizium sp. was cold-active (i.e. ARSEF 4343 from Macquarie Island, 54.4 degrees S, Australia). This probably is a M. frigidum isolate. The E. albus isolate (UFPE 3138) was the most susceptible isolate to both heat and cold stress. Isolates ARSEF 252 and GHA of B. bassiana, on the other hand, presented exceptionally high thermotolerance and cold activity. Some isolates with high cold activity, however, were thermosensitive (e.g. ARSEF 1682) and others with high thermotolerance had low cold activity (e.g. CG 227). An attempt to correlate the latitude of origin with thermotolerance or cold activity indicated that B. bassiana isolates from higher latitudes were more cold-active than isolates from nearer the equator, but there was not a similar correlation for heat.

Enhanced frequency of Beauveria bassiana blastospore transformation by restriction enzyme-mediated integration and electroporation

The techniques of restriction enzyme-mediated integration (REMI) and electroporation (EP) were applied for the first time to improving the blastospore transformation of fungal biocontrol agent Beauveria bassiana for higher frequency. The blastospores from < or =24 h incubation in glucose-mineral medium after shaking conidia for 48 h in Subouraud dextrose broth were found most competent for integrating 1 microg plasmid DNA vectoring the phosphinothricin (PPT) resistance gene bar in 360 microL reaction system containing 100 U HindIII or XbaI. Such blastospores were also most suitable for EP transformation at the optimized field strength of 10 kV cm(-1). The optimized REMI and EP generated averagely 39 and 53 transformants microg(-1) plasmid DNA whereas polyethylene glycol (PEG) integration yielded only 22. All transformants grew well on Czapek's agar containing 400 microg PPT mL(-1) after three rounds of cultivation on the same agar excluding PPT but their parental strain showed no resistance. The target gene inserted into the genomes of 10 transformants randomly taken from REMI or EP transformation was consistently detected by both PCR and Southern blotting. Compared to the PEG integration, REMI and EP enhanced the frequency of the blastospore transformation by 73 and 137%, respectively.

Multi-sited mutations of beta-tubulin are involved in benzimidazole resistance and thermotolerance of fungal biocontrol agent Beauveria bassiana

Fungicide resistance and thermotolerance of biocontrol agents in mitosporic fungi are of merits for enhancing fungal formulations against insect pests in the field. Among 20 wild strains of Beauveria bassiana (a well-known fungal biocontrol agent) tested in this study, 19 were sensitive or highly sensitive to carbendazim (methyl 2-benzimidazole carbamate), a typical benzimidazole fungicide, despite low resistance found in one strain. Sequential mutagenesis of a carbendazim-sensitive wild strain [minimal inhibitory concentration (MIC) = 1.32 microg ml(-1)] under artificial selection pressure generated 11 mutants sharing a common MIC of > 1000 microg ml(-1) without visible variation in colony growth and conidiation capacity. This represents at least 758-fold enhancement of the resistance among the mutants. However, accompanied with the enhanced resistance, all the mutants became less thermotolerable. Stressed at 48 degrees C, conidial LT(50)s of the mutants varied from 1.8 to 9.6 min and were lower than the parental LT(50) (36 min). Moreover, the contents of hydrophobin-like proteins in conidial walls declined significantly among the mutants compared with that of the wild parent. Mutations commonly relating to benzimidazole resistance in fungi were located at Q134, F167 and/or E198 around the taxol-binding site of beta-tubulin by sequencing the beta-tubulin of the mutants. Also, mutations of other 37 amino acid residues in the sequences (each having one to five residues mutated) were found for the first time and they were diverse in spatial structure. All mutations restricted to the half of beta-tubulin close to alpha-tubulin were likely involved in variation in each of the traits concerned but their interactions were complicated.

Medium components and culture conditions affect the thermotolerance of aerial conidia of fungal biocontrol agent Beauveria bassiana

AIMS

To produce more thermotolerable conidia of Beauveria bassiana, a well-known fungal biocontrol agent, by optimizing the medium components and culture conditions.

METHODS AND RESULTS

The conidia produced on media including 0.5-6% glucose, sucrose or starch as carbon source and 50-300-microg ml(-1) Cu2+, Zn2+, Mn2+ or Fe3+ as additive to Sabouraud dextrose medium at 15-30 degrees C, pH 4-8 or KCl-adjusted water availabilities were exposed to 30-min wet heat stress at 48 degrees C. The medium components for conidial production with greatly enhanced thermotolerance included 4% glucose as optimum or 1% starch as alternative for the carbon source and < or =50-microg ml(-1) Mn2+ for the metal additive. The culture conditions were optimized as 25 degrees C and pH 5-6. Conidial thermotolerance decreased remarkably when sucrose and Fe3+ or Cu2+ were used in the cultures, but altered slightly when 50-200-microg ml(-1) Zn2+ were included.

CONCLUSIONS

The tolerance of B. bassiana conidia to the thermal stress was significantly affected by the medium composition and culture conditions under which the conidia were produced.

SIGNIFICANCE AND IMPACT OF THE STUDY

Proper treatment of small grains as mass production substrates for more glucose release and supplement of glucose or 50-microg ml(-1) Mn2+ are possible means to enhancing conidial thermotolerance and field persistence for improved insect control.

A secondary metabolite biosynthesis cluster in Trichoderma virens: evidence from analysis of genes underexpressed in a mutant defective in morphogenesis and antibiotic production

A transcriptional comparison of wild type and a secondary metabolite deficient Trichoderma virens mutant resulted in the identification of six genes similar to those involved in secondary metabolism in other fungi, including four cytochrome P450 genes, one O-methyl transferase and one terpene cylase. Four of the genes (three cytochrome P450s and the cyclase) are located as a cluster. Transcript levels of three of the P450 genes, the O-methyl transferase and the terpene cyclase were measured. These genes are underexpressed in the mutant, which lacks the major secondary metabolites produced by this strain, viridin and viridiol. Expression levels of clones from the differential library with similarity to fungal trehalose synthase and a hydrophobin were also underexpressed in the mutant, while a heat shock protein hsp98 homolog was not. Based on the gene expression pattern and associated secondary metabolite profile, along with similarity to other secondary metabolism pathways in related fungi, we predict that the cluster is associated with the production of a terpene. The terpene could be viridin. This is the first report on cloning of secondary metabolism related genes from T. virens, and of their organization in a cluster, in this biocontrol fungus.

Novel blastospore-based transformation system for integration of phosphinothricin resistance and green fluorescence protein genes into Beauveria bassiana

A novel system was developed for efficient transformation of the fungal biocontrol agent Beauveria bassiana. Competent blastospores were prepared and stored in LiAc- and glycerol-inclusive suspension at -76 degrees C for sequential use in transformation. The system was successfully applied to integrating phosphinothricin resistance gene bar and enhanced green fluorescence protein gene egfp into B. bassiana via blastospore absorption of a plasmid vectoring bar and egfp. A frequency of 24 transformants per microgram of DNA was achieved. The blastospore-based transformation system has proven to be very convenient and would be highly potential for use in genetic manipulation of B. bassiana and other filamentous species.