Differential susceptibility of blastospores and aerial conidia of entomopathogenic fungi to heat and UV-B stresses

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.

The role of MrUbp4, a deubiquitinase, in conidial yield, thermotolerance, and virulence in Metarhizium robertsii

Ubiquitin-specific proteases (UBPs), the largest subfamily of deubiquitinating enzymes, regulate ubiquitin homeostasis and play diverse roles in eukaryotes. Ubp4 is essential for the growth, development, and pathogenicity of various fungal pathogens. However, its functions in the growth, stress responses, and virulence of entomopathogenic fungi remain unclear. In this study, we elucidated the role of the homolog of Ubp4, MrUbp4, in the entomopathogenic fungus Metarhizium robertsii. Deletion of MrUbp4 led to a notable increase in ubiquitination levels, demonstrating the involvement of MrUbp4 in protein deubiquitination. Furthermore, the ΔMrUbp4 mutant displayed a significant reduction in conidial yield, underscoring the pivotal role of MrUbp4 in conidiation. Additionally, the mutant exhibited heightened resistance to conidial heat treatment, emphasizing the role of MrUbp4 in thermotolerance. Notably, insect bioassays unveiled a substantial impairment in the virulence of the ΔMrUbp4 mutant. This was accompanied by a notable decrease in cuticle penetration ability and appressorium formation upon further analysis. In summary, our findings highlight the essential role of MrUbp4 in regulating the conidial yield, thermotolerance, and contributions to the virulence of M. robertsii.

Metarhizium pingshaense photolyase expression and virulence to Rhipicephalus microplus after UV-B exposure

The current study examined the impact of ultraviolet (UV)-B radiation in Metarhizium pingshaense blastospores' photolyase expression and their virulence against Rhipicephalus microplus. Blastospores were exposed to UV under laboratory and field conditions. Ticks were treated topically with fungal suspension and exposed to UV-B in the laboratory for three consecutive days. The expression of cyclobutane pyrimidine dimmers (CPDs)-photolyase gene maphr1-2 in blastospores after UV exposure followed by white light exposure was accessed after 0, 8, 12, 24, 36, and 48 h. Average relative germination of blastospores 24 h after in vitro UV exposure was 8.4% lower than 48 h. Despite this, the relative germination of blastospores exposed to UV in the field 18 h (95.7 ± 0.3%) and 28 h (97.3 ± 0.8%) after exposure were not different (p > 0.05). Ticks treated with fungus and not exposed to UV exhibited 0% survival 10 days after the treatment, while fungus-treated ticks exposed to UV exhibited 50 ± 11.2% survival. Expression levels of maphr1-2 8, 12, and 24 h after UV-B exposure were not different from time zero. Maphr1-2 expression peak in M. pingshaense blastospores occurred 36 h after UV-B exposure, in the proposed conditions and times analyzed, suggesting repair mechanisms other than CPD-mediated-photoreactivation might be leading blastospores' germination from 0 to 24 h.

Utilization of Metarhizium as an insect biocontrol agent and a plant bioinoculant with special reference to Brazil

Brazil has a long history of using biological control and has the largest program in sugarcane agriculture to which a biocontrol program has been applied. This achievement is at least partly due to the utilization of the entomopathogenic fungus Metarhizium. This well-known fungal genus exhibits pathogenicity against a broad range of arthropod hosts and has been used globally as a biocontrol agent. This fungus is also a root symbiont, and in this capacity, it is a plant growth promoter. However, this feature (i.e., as a plant symbiont) has yet to be fully explored and implemented in Brazil, although the number of reports demonstrating Metarhizium's utility as a plant bioinoculant is increasing. The Brazilian bioproduct industry targets agricultural pests, and is limited to two Metarhizium species represented by four fungal isolates as active ingredients. Entomopathogenic fungi have also been successful in controlling arthropods of public health concern, as shown in their control of mosquitoes, which are vectors of diseases. The isolation of new indigenous Metarhizium isolates from a variety of substrates such as soil, insects, and plants shows the wide genetic diversity within this fungal genus. In this review, we emphasize the significance of Metarhizium spp. for the biological control of insects in Brazil. We also suggest that the experience and success of biological control with fungi in Brazil is an important resource for developing integrated pest management and sustainable strategies for pest control worldwide. Moreover, the future implementation prospects of species of Metarhizium being used as bioinoculants and possible new advances in the utility of this fungus are discussed.

Post-Application Field Persistence and Efficacy of Cordyceps javanica against Bemisia tabaci

Previously, Cordyceps javanica Wf GA17, a causing agent of whitefly epizootics in southern Georgia, demonstrated superior temperature tolerance and higher virulence against the whitefly Bemisia tabaci than commercial strains in the laboratory. The post-application persistence and efficacy of this fungus against B. tabaci were compared with that of the commercially available C. javanica Apopka97 strain over a two-year field study in cotton and vegetable crops. When blastospores of both strains were applied alone, whitefly populations were not effectively suppressed. Thus, JMS stylet oil was added to fungal treatments for enhancing efficacy and persistence. For 0-day samples, all fungal treatments caused similar but significant levels of immature mortality regardless of fungal strain, propagule form (conidia vs. blastospores), and application method (alone or mixed with JMS). In follow-up samplings, Wf GA17 blastospores + JMS achieved higher control levels than other treatments in some trials, but the efficacy did not last long. The JMS oil alone caused significant mortality and suppressed whiteflies. Over 90% of spores lost viability 24 h after treatment in all fungal treatments. Across evaluation times, there was no difference between the two fungal strains (conidia or blastospores, alone or combined with JMS), but conidia persisted better than blastospores for both strains. Overall, the field persistence and efficacy of C. javanica did not last long; therefore, improved delivery methods and formulations are needed for enhancement.

Interactive effects of changes in UV radiation and climate on terrestrial ecosystems, biogeochemical cycles, and feedbacks to the climate system

Terrestrial organisms and ecosystems are being exposed to new and rapidly changing combinations of solar UV radiation and other environmental factors because of ongoing changes in stratospheric ozone and climate. In this Quadrennial Assessment, we examine the interactive effects of changes in stratospheric ozone, UV radiation and climate on terrestrial ecosystems and biogeochemical cycles in the context of the Montreal Protocol. We specifically assess effects on terrestrial organisms, agriculture and food supply, biodiversity, ecosystem services and feedbacks to the climate system. Emphasis is placed on the role of extreme climate events in altering the exposure to UV radiation of organisms and ecosystems and the potential effects on biodiversity. We also address the responses of plants to increased temporal variability in solar UV radiation, the interactive effects of UV radiation and other climate change factors (e.g. drought, temperature) on crops, and the role of UV radiation in driving the breakdown of organic matter from dead plant material (i.e. litter) and biocides (pesticides and herbicides). Our assessment indicates that UV radiation and climate interact in various ways to affect the structure and function of terrestrial ecosystems, and that by protecting the ozone layer, the Montreal Protocol continues to play a vital role in maintaining healthy, diverse ecosystems on land that sustain life on Earth. Furthermore, the Montreal Protocol and its Kigali Amendment are mitigating some of the negative environmental consequences of climate change by limiting the emissions of greenhouse gases and protecting the carbon sequestration potential of vegetation and the terrestrial carbon pool.

Encapsulation of entomopathogenic fungal conidia: evaluation of stability and control potential of Rhipicephalus microplus

The use of chemical acaricides is the primary strategy to control tick infestations. Nonetheless, chemical resistance in ticks has been reported. Thus, complementary methods such as biological control using entomopathogenic fungi (EPF) have been investigated. EPF, although efficient, have their viability compromised when applied under natural conditions, which indicates that formulation development is essential. Some researchers have demonstrated the efficacy of ionic gelation in protecting EPF against deleterious abiotic factors. In the present study, we conducted the ionic gelation technique to encapsulate Metarhizium anisopliae (Metschn.) Sorokin (Hypocreales: Clavicipitaceae) conidia in 2% (EC 2%) and 3% (EC 3%) sodium alginate. Next, the quantity and viability of encapsulated conidia (EC) were determined. The morphology of particles was characterized by using Scanning Electron Microscopy (SEM). EC and non-encapsulated conidia (NEC) were stored at room temperature (26.8 °C) and in the freezer (-11.9 °C) to shelf-life testing. For UV-B irradiance tolerance and thermotolerance tests, EC and NEC were exposed to UV-B (6.0 or 8.0 kJ m ^- 2) and heat (42 ºC). In addition, biological parameters of Rhipicephalus microplus Canestrini (Acari: Ixodidae) engorged females exposed to EC were evaluated. The particles presented a spherical shape, more homogeneous (EC 2%) or heterogeneous (EC 3%). Encapsulation decreased (4.8×) the conidial concentration and did not affect their viability. On the other hand, encapsulation increased the shelf life of conidia at room temperature as well as their UV-B tolerance and thermotolerance (6 h). The fungal particles decreased the biological parameters of females more significantly than the NEC. As far as we know, we reported for the first time the use of the ionic gelation to encapsulate entomopathogenic fungi toward controlling R. microplus.

Microsclerotial pellets of Metarhizium spp.: thermotolerance and bioefficacy against the cattle tick

The cattle tick, Rhipicephalus microplus (Acari: Ixodidae), is a multi-billion dollar ectoparasite of global importance affecting beef and milk production. Submerged cultures of cosmopolitan entomopathogenic fungal species of the genus Metarhizium typically produce microsclerotia that provide both long-term survival and environmental resistance. Microsclerotia hold great potential as an unconventional active propagule to control this tick under laboratory and semi-field conditions. However, heat stress caused especially by elevated temperatures poses a critical environmental constraint for the successful development and efficacy of microsclerotia under tropical conditions. First, we screened six strains of Metarhizium anisopliae, Metarhizium robertsii and Metarhizium humberi for their ability to produce microsclerotia by submerged liquid cultivation. In addition, we assessed the biological fitness and bioefficacy of dried microsclerotial pellets under amenable (27 °C) and heat-stressed (32 °C) incubation against engorged adult females of R. microplus. Microsclerotia in pelletized formulation prepared with carriers based on diatomaceous earth and microcrystalline cellulose exhibited conidial production at different extents according to the fungal strain and the incubation temperature, but most strains displayed reduced sporogenesis when exposed to 32 °C. Engorged tick females exposed to sporulated microsclerotia from pelletized M. anisopliae CG47 or IP 119 had fewer number of hatching larvae in comparison to the control group, irrespective of the incubation temperature tested. The minimum dosage of microsclerotial pellets that effectively reduced hatchability of tick larvae was estimated to be 2 mg per plate (equivalent to 6.0 kg per hectare). Metarhizium microsclerotial pellets exhibited significant tolerance to 32 °C and pronounced acaricidal activity against this economically important ectoparasite of cattle, even under simulated environmental heat stress. KEY POINTS: • Heat stress affects conidial production by microsclerotia of most pelletized Metarhizium strains • Heat stress does not impair the acaricidal performance of pelletized microsclerotia • Pellet formulation of Metarhizium microsclerotia is a promising mycoacaricide.

Characterization of Brazilian Cordyceps fumosorosea isolates: Conidial production, tolerance to ultraviolet-B radiation, and elevated temperature

Cordyceps fumosorosea is an entomopathogenic fungus with a global distribution and is used for the biological control of agricultural pests. High conidial productivity and tolerance to abiotic stresses such as elevated temperature and ultraviolet radiation (UV-B) are desired characteristics in candidate isolates for commercial products. Our goal in this study was to characterize promising isolates of C. fumosorosea from five Brazilian biomes regarding conidial production, tolerance to UV-B, and elevated temperature (45°). Seventy-two isolates out of 172 were chosen visually, based on growth and sporulation in culture medium, and grown on parboiled rice. Next, fourteen isolates were selected, based on productivity on rice and origin of isolation, for production in polypropylene bags and submitted to UV-B for 2, 4, 6, and 8 h or to 45 °C for 30, 60, and 90 min. High variations in conidial production were observed among isolates, and a positive correlation was observed between UV-B and heat tolerance. The isolates ESALQ4556 and ESALQ4778 showed the highest yields of conidial production in polypropylene bags (3.51 × 10⁹ conidia/g dry rice), while ESALQ1296, an isolate recovered from insects, was the most tolerant to UV-B and 45 °C. Exposure to radiation for more than 4 h and placed directly at 45 °C for more than 30 min significantly reduced conidial germination for all C. fumosorosea isolates. These results contribute to a better understanding of the tolerance to abiotic factors of Brazilian isolates of C. fumosorosea.

Compatibility of different Metarhizium spp. propagules with synthetic acaricides for controlling Rhipicephalus microplus

The inappropriate use of synthetic acaricides has selected resistant Rhipicephalus microplus populations. The present study evaluated the compatibility of different Metarhizium spp. propagules (conidia, blastospores, and microsclerotia) by incubating them with synthetic acaricides (amitraz, deltamethrin, and a combination of cypermethrin, chlorpyrifos, and citronellal) for 1 h, 5 h, 10 h, and 24 h. Conidia and microsclerotia of the tested isolates were usually more tolerant to synthetic acaricides than blastospores. Our study also analyzed the in vitro effect of deltamethrin associated with fungal propagules for controlling a population of R. microplus females that were not susceptible to this synthetic acaricide. The use of entomopathogenic fungi in association with deltamethrin in this tick population caused a greater tick control than did the use of the fungus or the synthetic acaricide separately.

How Dopamine Influences Survival and Cellular Immune Response of Rhipicephalus microplus Inoculated with Metarhizium anisopliae

Dopamine (DA) is a biogenic monoamine reported to modulate insect hemocytes. Although the immune functions of DA are known in insects, there is a lack of knowledge of DA’s role in the immune system of ticks. The use of Metarhizium anisopliae has been considered for tick control, driving studies on the immune response of these arthropods challenged with fungi. The present study evaluated the effect of DA on the cellular immune response and survival of Rhipicephalus microplus inoculated with M. anisopliae blastospores. Exogenous DA increased both ticks’ survival 72 h after M. anisopliae inoculation and the number of circulating hemocytes compared to the control group, 24 h after the treatment. The phagocytic index of tick hemocytes challenged with M. anisopliae did not change upon injection of exogenous DA. Phenoloxidase activity in the hemolymph of ticks injected with DA and the fungus or exclusively with DA was higher than in untreated ticks or ticks inoculated with the fungus alone, 72 h after treatment. DA was detected in the hemocytes of fungus-treated and untreated ticks. Unveiling the cellular immune response in ticks challenged with entomopathogenic fungi is important to improve strategies for the biological control of these ectoparasites.

Recent advances in photocatalytic removal of airborne pathogens in air

The abatement of airborne pathogens such as bacteria, viruses, and fungi has become an important goal of air-quality management. Efficient and effective treatment techniques such as photocatalysis are essential for disinfection of airborne microorganisms. This review focuses on recent advances in the formulation and development of photocatalytic disinfection, design of efficient photocatalysts, choice of photocatalytic reactor, removal and/or disinfection mechanisms, and the role of reactive ion species. Data from recent studies are analyzed to accurately assess the efficacy of such disinfection approaches. This review also highlights the application of innovative materials in individual and combined abatement systems against airborne bacterial, viral, and fungal pathogens. We discuss the efficiency and benefits presented by such systems, address the challenges, and provide a perspective for future research.

Development of novel spray-dried and air-dried formulations of Metarhizium robertsii blastospores and their virulence against Dalbulus maidis

The present research addressed spray-drying and air-drying techniques applied to Metarhizium robertsii blastospores to develop wettable powder (WP) formulations. We investigated the effect of co-formulants on blastospore viability during drying and assessed the wettability and stability of formulations in water. The effect of oxygen-moisture absorbers was studied on the shelf life of these formulations stored at 26 °C and 4 °C for up to 90 days. Additionally, we determined the virulence of the best spray-dried and air-dried formulations against the corn leafhopper Dalbulus maidis. While sucrose and skim milk played an essential role as osmoprotectants in preserving air-dried blastospores, maltodextrin, skim milk, and bentonite were crucial to attain high cell survival during spray drying. The lowest wettability time was achieved with spray-dried formulations containing less Ca-lignin, while charcoal powder amount was positively associated with formulation stability. The addition of oxygen-moisture absorbers inside sealed packages increased from threefold to fourfold the half-life times of air-dried and spray-dried formulations at both storage temperatures. However, the half-life times of all blastospore-based formulations were shorter than 3 months regardless of temperature and packaging system. Spray-dried and air-dried WP formulations were as virulent as fresh blastopores against D. maydis adults sprayed with 5 × 10⁷ blastospores mL^-1 that induced 87.8% and 70.6% mortality, respectively. These findings bring innovative advancement for M. robertsii blastospore formulation through spray-drying and underpin the importance of adding protective matrices coupled to oxygen-moisture absorbers to extend cell viability during either cold or non-refrigerated storage. KEY POINTS: • Cost-effective wettable powder formulations of M. robertsii blastospores were developed. • Bioefficacy of formulations against the corn leafhopper was comparable to fresh blastospores. • Cold storage and dual oxygen-moisture absorber are critical for extended shelf life.

Tolerance to Abiotic Factors of Microsclerotia and Mycelial Pellets From Metarhizium robertsii, and Molecular and Ultrastructural Changes During Microsclerotial Differentiation

Metarhizium species fungi are able to produce resistant structures termed microsclerotia, formed by compact and melanized threads of hyphae. These propagules are tolerant to desiccation and produce infective conidia; thus, they are promising candidates to use in biological control programs. In this study, we investigated the tolerance to both ultraviolet B (UV-B) radiation and heat of microsclerotia of Metarhizium robertsii strain ARSEF 2575. We also adapted the liquid medium and culture conditions to obtain mycelial pellets from the same isolate in order to compare these characteristics between both types of propagules. We followed the peroxisome biogenesis and studied the oxidative stress during differentiation from conidia to microsclerotia by transmission electron microscopy after staining with a peroxidase activity marker and by the expression pattern of genes potentially involved in these processes. We found that despite their twice smaller size, microsclerotia exhibited higher dry biomass, yield, and conidial productivity than mycelial pellets, both with and without UV-B and heat stresses. From the 16 genes measured, we found an induction after 96-h differentiation in the oxidative stress marker genes MrcatA, MrcatP, and Mrgpx; the peroxisome biogenesis factors Mrpex5 and Mrpex14/17; and the photoprotection genes Mrlac1 and Mrlac2; and Mrlac3. We concluded that an oxidative stress scenario is induced during microsclerotia differentiation in M. robertsii and confirmed that because of its tolerance to desiccation, heat, and UV-B, this fungal structure could be an excellent candidate for use in biological control of pests under tropical and subtropical climates where heat and UV radiation are detrimental to entomopathogenic fungi survival and persistence.

Transcriptional Responses of Beauveria bassiana Blastospores Cultured Under Varying Glucose Concentrations

Culturing the entomopathogenic fungus, Beauveria bassiana, under high glucose concentrations coupled with high aeration results in a fungal developmental shift from hyphal growth to mostly blastospores (yeast-like cells). The underlying molecular mechanisms involved in this shift remain elusive. A systematic transcriptome analysis of the differential gene expression was preformed to uncover the fungal transcriptomic response to osmotic and oxidative stresses associated with the resulting high blastospore yield. Differential gene expression was compared under moderate (10% w/v) and high (20% w/v) glucose concentrations daily for three days. The RNAseq-based transcriptomic results depicted a higher proportion of downregulated genes when the fungus was grown under 20% glucose than 10%. Additional experiments explored a broader glucose range (4, 8, 12, 16, 20% w/v) with phenotype assessment and qRT-PCR transcript abundance measurements of selected genes. Antioxidant, calcium transport, conidiation, and osmosensor-related genes were highly upregulated in higher glucose titers (16-20%) compared to growth in lower glucose (4-6%) concentrations. The class 1 hydrophobin gene (Hyd1) was highly expressed throughout the culturing. Hyd1 is known to be involved in spore coat rodlet layer assembly, and indicates that blastospores or another cell type containing hydrophobin 1 is expressed in the haemocoel during the infection process. Furthermore, we found implications of the HOG signaling pathway with upregulation of homologous genes Ssk2 and Hog1 for all fermentation time points under hyperosmotic medium (20% glucose). These findings expand our knowledge of the molecular mechanisms behind blastospore development and may help facilitate large-scale industrial production of B. bassiana blastospores for pest control applications.

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.