Effects of UVB irradiance on conidia and germinants of the entomopathogenic Hyphomycete Metarhizium anisopliae: a study of reciprocity and recovery

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.

Recovery of insect-pathogenic fungi from solar UV damage: Molecular mechanisms and prospects

Molecular mechanisms underlying insect-pathogenic fungal tolerance to solar ultraviolet (UV) damage have been increasingly understood. This chapter reviews the methodology established to quantify fungal response to solar UV radiation, which consists of UVB and UVA, and characterize a pattern of the solar UV dose (damage) accumulated from sunrise to sunset on sunny summer days. An emphasis is placed on anti-UV mechanisms of fungal insect pathogens in comparison to those well documented in model yeast. Principles are discussed for properly timing the application of a fungal pesticide to improve pest control during summer months. Fungal UV tolerance depends on either nucleotide excision repair (NER) or photorepair of UV-induced DNA lesions to recover UV-impaired cells in the darkness or the light. NER is a slow process independent of light and depends on a large family of anti-UV radiation (RAD) proteins studied intensively in model yeast but rarely in non-yeast fungi. Photorepair is a rapid process that had long been considered to depend on only one or two photolyases in filamentous fungi. However, recent studies have greatly expanded a genetic/molecular basis for photorepair-dependent photoreactivation that serves as a primary anti-UV mechanism in insect-pathogenic fungi, in which photolyase regulators required for photorepair and multiple RAD homologs have higher or much higher photoreactivation activities than do photolyases. The NER activities of those homologs in dark reactivation cannot recover the severe UV damage recovered by their activities in photoreactivation. Future studies are expected to further expand the genetic/molecular basis of photoreactivation and enrich principles for the recovery of insect-pathogenic fungi from solar UV damage.

Microsclerotia from Metarhizium robertsii: Production, ultrastructural analysis, robustness, and insecticidal activity

Microsclerotia (MS) are considered one of the most promising propagules for use as active ingredients in biopesticides due to their tolerance to abiotic factors and ability to produce infective conidia for the control of pests. Therefore, the objective of this research was to establish the conditions required to induce the formation of microsclerotia in Metarhizium robertsii Mt004 and to study its development process, tolerance to abiotic factors and insecticidal activity of MS-derived conidia. M. robertsii started to form hyphal aggregates after 2 days and looked more compact after 8 days. MS were mature and pigmented after 20 days. The final yield was 2.0 × 103 MS/mL and MS size varied between 356.9 and 1348.4 μm. Ultrastructure analysis revealed that mature MS contained only a few live cells embedded in an extracellular matrix. Mature MS were more tolerance to UV-B radiation, heat and storage trials than conidia from Solid State Fermentation. MS-derived conidia were as virulent as conidia against Diatraea saccharalis larvae. These results showed that MS are promising propagules for the development of more persistent and efficient biopesticides for harsh environmental conditions. Our findings provide a baseline for production and a better understanding of microsclerotia development in M. robertsii strains.

Imperfect match between radiation exposure times required for conidial viability loss and infective capacity reduction attenuate UV-B impact on Beauveria bassiana

BACKGROUND

UV-B radiation represents a significant challenge for the widespread use of entomopathogenic fungi in pest management. This study focused on research of the asynchronous response between virulence and conidial viability against Ceratitis capitata adults using specific statistical models. Moreover, it was also investigated whether the observed differences in susceptibility to UV-B radiation in in vitro assays among three selected isolates of Beauveria bassiana were reflected in the above-mentioned asynchrony.

RESULTS

While the irradiation of the three isolates of B. bassiana was associated with a significant loss of conidial viability, their virulence was not significantly affected compared to nonirradiated treatments when exposed to 1200 mW m-2 for 6 h before or after the inoculation of C. capitata. In fact, the irradiation time needed to reduce the mortality to 50% compared to the controls was 34.69 h for EABb 10/225-Fil, 16.36 h for EABb 09/20-Fil, and 24.59 h for EABb 09/28-Fil. Meanwhile, the irradiation time necessary to reduce conidial viability to 50% was 9.89 h for EABb 10/225-Fil, 8.74 h for EABb 09/20-Fil, and 4.71 h for EABb 09/28-Fil.

CONCLUSION

These results highlight the importance of modeling the response of entomopathogenic fungi virulence and conidial susceptibility when exposed to UV-B radiation for the selection of environmentally competent isolates, regardless of the results obtained in previous in vitro assays on conidial germination. This strategic approach is critical in overcoming the challenges posed by UV-B radiation and holds the key to realizing the full potential of entomopathogenic fungi in pest management. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

Physiological and Molecular Response Modifications by Ultraviolet-C Radiation in Plutella xylostella and Its Compatibility with Cordyceps fumosorosea

Ultraviolet-C (UV-C) radiation significantly impacts living organisms. UV-C radiation can also be used as a pest management tool. Therefore, this study was designed to investigate the effect of UV-C radiation on the physiology and gene expression level of Plutella xylostella, a destructive vegetable pest. Results showed that, after exposure to UV-C radiation for 3, 6, 12, and 24 h, the activity of SOD (superoxide dismutase) and CAT (catalase) of P. xylostella increased, while the activity of PPO (polyphenol oxidase), POD (peroxidase), AChE (acetylcholinesterase), CarE (carboxylesterase), and ACP (acid phosphatase) decreased with increased exposure time. Correlation coefficient analyses indicated that the activity of CAT correlated positively, while PPO and CarE correlated negatively, with exposure time. Gene regulation analysis via qRT-PCR confirmed a significant increase in regulation in CAT, CarE, and PPO-related genes. We also investigated the effect of UV-C exposure on the virulence of Cordyceps fumosorosea against P. xylostella. Here, results indicated that when the fungal treatment was applied to larvae before UV-C radiation, the virulence of C. fumosorosea was significantly reduced. However, this decline in virulence of C. fumosorosea due to UV-C exposure remained only for one generation, and no effect was observed on secondary infection. On the other hand, when larvae were exposed to UV-C radiation before fungal application, the mortality rate significantly increased as the exposure time to UV-C radiation increased. From the current study, it could be concluded that UV-C exposure suppressed the immunity to P. xylostella, which later enhanced the virulence of entomopathogenic fungi. Moreover, the study also suggested that UV irradiation is an effective pest management tool that could be incorporated into pest management strategies, which could help reduce pesticide application, be economically beneficial for the farmer, and be environmentally safe.

Persistency of Indigenous and Exotic Entomopathogenic Fungi Isolates under Ultraviolet B (UV-B) Irradiation to Enhance Field Application Efficacy and Obtain Sustainable Control of the Red Palm Weevil

The red palm weevil Rhynchophorus ferrugineus Olivier (Coleoptera: Dryophthoridae) has become a key invasive pest and major threat to the palm tree worldwide. Several entomopathogenic fungi are used in insect biological control programs. In the present study, persistency of different local and exotic fungal isolates of Beauveria bassiana and Metarhizium anisopliae was evaluated under UV-B irradiation with different exposure intervals. Several factors, including ultraviolet (UV) light, significantly decrease germination rate of fungi, as UV penetrates and damages their DNA. Several studies have investigated that UV-resistant conidia germinate better under harsh environmental conditions. Seven local and exotic fungi isolates ("BbSA-1", "BbSA-2", "BbSA-3", "MaSA-1", "BbIDN-1", "MaIDN-1", and "MaIDN-2") were tested in the current study under UV-B irradiation having different UV exposure times (i.e., 15, 30, 60, 120, 180, 240, and 300 min). The colony-forming unit (CFU) in each isolate was used to calculate the survival rate. Results indicated that survival rate of all the isolates decreased under UV-B irradiation for all exposure times compared to no exposure to UV-B irradiation. The CFU number decreased as the exposure time increased. Fungi isolates "MaSA-1", "BbSA-1", "BbSA-2", "MaIDN-1", and "MaIDN-2" could persist after 300 min exposure to UV-B, while the remaining isolates, such as "BbIDN-1", and "BbSA-3", could not persist after 300 min exposure to UV-B. The ultimate objective of the present research was to explore an ultraviolet-tolerant fungal isolate that might be useful in the field application for the sustainable management of the red palm weevil, which has become a key invasive pest in many regions rather than its native range. Most of the fungus isolates studied in the present work were collected from Saudi Arabia's Al-Qatif region, where the red palm weevil has infested more than ten thousand trees, worth millions of riyals.

Photobiology of the keystone genus Metarhizium

Metarhizium fungi are soil-inhabiting ascomycetes which are saprotrophs, symbionts of plants, pathogens of insects, and participate in other trophic/ecological interactions, thereby performing multiple essential ecosystem services. Metarhizium species are used to control insect pests of crop plants and insects that act as vectors of human and animal diseases. To fulfil their functions in the environment and as biocontrol agents, these fungi must endure cellular stresses imposed by the environment, one of the most potent of which is solar ultraviolet (UV) radiation. Here, we examine the cellular stress biology of Metarhizium species in context of their photobiology, showing how photobiology facilitates key aspects of their ecology as keystone microbes and as mycoinsectides. The biophysical basis of UV-induced damage to Metarhizium, and mechanistic basis of molecular and cellular responses to effect damage repair, are discussed and interpreted in relation to the solar radiation received on Earth. We analyse the interplay between UV and visible light and how the latter increases cellular tolerance to the former via expression of a photolyase gene. By integrating current knowledge, we propose the mechanism through which Metarhizium species use the visible fraction of (low-UV) early-morning light to mitigate potentially lethal damage from intense UV radiation later in the day. We also show how this mechanism could increase Metarhizium environmental persistence and improve its bioinsecticide performance. We discuss the finding that visible light modulates stress biology in the context of further work needed on Metarhizium ecology in natural and agricultural ecosystems, and as keystone microbes that provide essential services within Earth's biosphere.

Phototoxicity of Ultraviolet-A against the Whitefly Bemisia tabaci and Its Compatibility with an Entomopathogenic Fungus and Whitefly Parasitoid

Ultraviolet (UV) radiation significantly affects insect life and, as a result, has been widely used to control different invertebrate pests. The current results demonstrate that when Bemisia tabaci first instar nymphs are exposed to UV-A light for 12, 24, 48, and 72 h, their developmental and biological parameters are negatively affected by UV-A exposure; the effect increased with an increase in exposure time. We hypothesized that UV-A light is compatible with other biological control agents. Results showed that when the entomopathogenic fungus Cordyceps fumosorosea was applied to third instar nymphs of B. tabaci previously exposed to UV-A light, the LC50 was 3.4% lower after 72 h of exposure to UV-A light compared to the control. However, when the fungus was exposed to UV-A light, its virulence decreased with an increase in UV-A exposure time. The parasitism rate of Encarsia formosa against 24 h UV-A-exposed third instar nymphs of B. tabaci increased while the adult emergence from parasitized nymphs was not affected after UV-A light exposure. Parasitism rate was significantly reduced however following E. formosa exposure to UV-A light; but again, adult emergence was not affected from parasitized nymphs. The percentage mortality of E. formosa increased with increasing exposure time to UV-A light. The enzyme activity of SOD, CAT, GST, and AChE and the energy reserve contents were negatively affected due to UV-A exposure. Collectively, this study has demonstrated that UV-A light significantly suppresses the immune system of B. tabaci and that UV-A light is compatible with other biological control agents if it is applied separately from the biological agent.

Effect of UV-B Irradiation on Water-Suspended Metarhizium anisopliae s.l. (Hypocreales: Clavicipitaceae) Conidia and Their Larvicidal Activity in Aedes aegypti (Diptera: Culicidae)

Ultraviolet (UV) radiation is a key limiting factor for biological pest control with entomopathogenic fungi. While little is known about the impact of UV on Metarhizium anisopliae Metchnikoff (Sorokin) (Hypocreales: Clavicipitaceae) conidia in aquatic mosquito-breeding sites, this study determined the effect of UV-B on the viability and virulence of M. anisopliae sensu lato (s.l.) strain IP 46 in the laboratory against Aedes aegypti (L.) (Diptera: Culicidae) larvae. Conidia were treated in cups under defined water depths (0, 1, 2, and 3 cm) to six different UV-B doses (0, 0.657, 1.971, 3.942, 7.884, 11.826, or 15.768 kJ m-2) at 27 ± 2°C. The ability of treated conidia to germinate up to 24 h postexposure on PDAY + benomyl + chloramphenicol medium at 25 ± 1°C was adversely affected by higher doses of UV-B radiation regardless of the water depth. Germination, however, did not fall below 70% regardless of the test conditions. In fact, conidial virulence against second-instar larvae was not affected by either the water depth (F3,84 = 0.3, P = 0.85) or any tested levels of UV-B radiation (F6,21 ≤ 1.2, P ≥ 0.39) including those distinctly higher than might be expected for tropical sites. These findings strengthen previous observations that IP 46 has significant potential for use against A. aegypti larvae, even when exposed to elevated UV-B irradiance levels in the small breeding sites that are common for this important vector.

UV-B radiation-related effects on conidial inactivation and virulence against Ceratitis capitata (Wiedemann) (Diptera; Tephritidae) of phylloplane and soil Metarhizium sp. strains

Recent studies have demonstrated the presence of Metarhizium species on the epigeal areas of weeds and woody plants in various Mediterranean ecosystems, and the question arises whether isolates from the phylloplane, which experiences greater exposure to environmental UV-B radiation than soil isolates do, could have better UV-B radiation tolerance. The in vitro response of 18 Metarhizium strains isolated from phylloplane and soil of several Mediterranean ecosystems to UV-B radiation and the in vitro and in vivo effects of UV-B radiation on the viability and virulence of a selected M. brunneum strain against C. capitata were determined. The conidial germination, culturability and colony growth of these strains exposed to 1200mWm^-2 for 2, 4 or 6h were evaluated. Germination rates below 30% and poor conidia recovery rates were observed for all strains. However, no relationship between the Metarhizium species or isolation habitat and the effect of UV-B radiation was found. Strain EAMa 01/58-Su, which showed a high tolerance to UV-B inactivation in terms of relative germination, was subsequently selected to investigate the UV-B related effects on virulence toward C. capitata adults. In a series of bioassays, the virulence and viability was determined using pure dry conidia, which were irradiated with 1200mWm^-2 for 6h prior or after adult flies were inoculated, which resulted in a significant 84.7-86.4% decrease in conidial viability but only a slightly significant reduction of virulence, with 100.0% and 91.4% adult mortality rates and 4.6 and 5.9days average survival time for the no UV-B and UV-B treatments, respectively. A second series of experiments was performed to determine whether the UV-B effects on strain EAMa 01/58-Su were dose- or exposure time-dependent. Adult flies were inoculated with five doses (1.0×10⁴-1.0×10⁸conidiaml^-1) and then irradiated at 1200mWm^-2 for 6h, and similar LC₅₀ values, 3.8×10⁷ and 4.3×10⁷conidiaml^-1, were determined for the UV-B and no UV-B treatments, respectively. However, the LT₅₀ values for flies inoculated with 1.0×10⁸conidiaml^-1 and with1.0×10⁷conidiaml^-1 were 15.1% and 30.8% longer for UV-B treatments than no UV-B treatments, respectively. Next, adult flies were treated with 1.0×10⁸conidiaml^-1 and then exposed to 1200mWm^-2 for 0, 6, 12, 24, 36 and 48h, and the relationships among exposure time and conidia viability and fly mortality losses were determined. The exposure time for adult flies at 1200mWm^-2 to achieve a 50% reduction in fly mortality was 47.2h, which was longer than that of 5.6h required for a 50% reduction in conidia viability. Our results show that the UV-B radiation significantly affected the virulence of EAMa 01/58-Su strain against C. capitata adults, with this effect being dependent on the exposure time but not related to fungal dosage.

The Herbivore-Induced Plant Volatiles Methyl Salicylate and Menthol Positively affect Growth and Pathogenicity of Entomopathogenic Fungi

Some herbivore-induced-plant volatiles (HIPVs) compounds are vital for the functioning of an ecosystem, by triggering multi-trophic interactions for natural enemies, plants and herbivores. However, the effect of these chemicals, which play a crucial role in regulating the multi-trophic interactions between plant-herbivore-entomopathogenic fungi, is still unknown. To fill this scientific gap, we therefore investigated how these chemicals influence the entomopathogenic fungi growth and efficacy. In this study, Lipaphis erysimi induced Arabidopsis thaliana HIPVs were collected using headspace system and detected with GC-MS, and then analyzed the effects of these HIPVs chemicals on Lecanicillium lecanii strain V3450. We found that the HIPVs menthol and methyl salicylate at 1 and 10 nmol·ml-1 improved many performance aspects of the fungus, such as germination, sporulation, appressorial formation as well as its pathogenicity and virulence. These findings are not only important for understanding the multi-trophic interactions in an ecosystem, but also would contribute for developing new and easier procedures for conidial mass production as well as improve the pathogenicity and virulence of entomopathogenic fungi in biological pest management strategies.

Heat-stressed Metarhizium anisopliae: viability (in vitro) and virulence (in vivo) assessments against the tick Rhipicephalus sanguineus

The current study investigated the thermotolerance of Metarhizium anisopliae s.l. conidia from the commercial products Metarril® SP Organic and Metarril® WP. The efficacy of these M. anisopliae formulations against the tick Rhipicephalus sanguineus s.l. was studied in laboratory under optimum or heat-stress conditions. The products were prepared in water [Tween® 80, 0.01 % (v/v)] or pure mineral oil. Conidia from Metarril® SP Organic suspended in water presented markedly delayed germination after heating to constant 40 °C (for 2, 4, or 6 h) compared to conidia suspended in mineral oil. Metarril® SP Organic suspended in oil and exposed to daily cycles of heat-stress (40 °C for 4 h and 25 °C for 19 h for 5 consecutive days) presented relative germination of conidia ranging from 92.8 to 87.2 % from day 1 to day 5, respectively. Conversely, germination of conidia prepared in water ranged from 79.3 to 39.1 % from day 1 to day 5, respectively. Culturability of Metarril® WP decreased from 96 % when conidia were cultured for 30 min prior to heat exposure (40 °C for 4 h) to 9 % when conidia were cultured for 8 h. Tick percent control was distinctly higher when engorged females were treated with oil suspensions rather than water suspensions, even when treated ticks were exposed to heat-stress regimen. Oil-based applications protected fungal conidia against heat-stress. Although Metarril® is not registered for tick control, it may be useful for controlling R. sanguineus, especially if it is prepared in mineral oil.

Responses to abiotic environmental stresses among phylloplane and soil isolates of Beauveria bassiana from two holm oak ecosystems

The response of entomopathogenic mitosporic ascomycete (EMAs) to abiotic stresses might be adapted to the microhabitats in which they inhabit. In phylloplane, these organisms are more exposed to such stresses than they are in soil, which may have led to adaptation to this environment. In the present work, we investigate whether Beauveria bassiana genotype or isolation habitat, i.e., soil or phylloplane, within the same geographic area influences their responses to key environmental stresses, such as temperature, moisture and ultraviolet radiation (UV-B), which can affect their successful use in microbial control. Twenty isolates of B. bassiana obtained from the soil and phylloplane in two ecosystems from southern Spain (holm oak dehesa and a reforested area) were selected to study the population distribution of these isolates and evaluate their thermal, humidity and UV-B requirements. Molecular characterization was conducted by using elongation factor-1α (EF-1α), the intergenic nuclear region Bloc and 15 microsatellite primers. The cluster analysis based on concatenated EF-1α and Bloc sequences grouped the 20 isolates into five clades within B. basiana, with Clades a, b, d and e containing both soil and phylloplane isolates and Clade c including three phylloplane isolates. The dendrogram and the minimal spanning network generated from the genetic distances among multilocus genotypes showed four divergent groups corresponding to the five clades obtained based on the sequence data (Clades b and d were represented in the same group), with a high degree of shared alleles within groups and few alleles shared among groups. Although no relationship was found between MLG and the habitat (soil or phylloplane) of isolation, isolates grouped into Clade c, all of which were collected from phylloplane, formed a separate group of MLGs. To investigate our hypothesis, the responses to temperature (germination and colony growth evaluated in the range 15-35°C), water activity (conidia germination evaluated against values of a_w between 1 and 0.862) and UV-B exposure (conidia exposed to 920 or 1200mWm^-2 for 2, 4 or 6h) of the soil and phylloplane isolates from the five clades were investigated. No associations of isolate-specific genetic or physiological characteristics with isolate habitat, i.e., soil or phylloplane, were found. These results provide no support for the hypothesis that EMAs strains from the phylloplane have evolved to resist unfavourable environmental conditions.

Sensibilidade de isolados de fungos entomopatogênicos às radiações solar, ultravioleta e à temperatura

RESUMO: Este trabalho teve por objetivo avaliar a sensibilidade de isolados dos fungos Metarhizium anisopliae (Metsch.) Sorok. e Beauveria bassiana (Bals). Vuill. ao efeito das radiações solar e ultravioleta e da temperatura. Conídios dos isolados foram expostos, por vários períodos, aos raios de um simulador solar em diversas irradiâncias e a uma lâmpada de raios ultravioleta germicida. Os conídios do isolado de M. anisopliae foram também expostos às temperaturas de 19,5; 24,2 e 31,0ºC, e os do isolado de B. bassiana a 19,4; 20,8 e 28,3ºC, e 18,7; 23,8 e 30,9ºC. Avaliou-se a germinação de conídios pelo teste de viabilidade. Os isolados dos fungos se mostraram bastantes sensíveis aos raios do simulador solar e aos raios ultravioleta. A germinação de ambos sofreu significativa redução a partir de 30 minutos de exposição à radiação do simulador solar. O efeito mais severo foi evidenciado pelo isolado de B. bassiana, com grande redução da germinação dos conídios em todas as irradiâncias testadas. A sensibilidade à radiação ultravioleta também foi grande, pois ocorreu acentuada redução da germinação dos conídios do isolado de M. anisopliae (38,2%) e de B. bassiana (65%) já aos 30 segundos de exposição. A temperatura afetou a viabilidade de ambos os fungos. Temperaturas entre 23,8 e 31ºC favoreceram a germinação dos conídios, enquanto temperaturas próximas de 20ºC dificultaram a germinação.

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.

Growth under visible light increases conidia and mucilage production and tolerance to UV-B radiation in the plant pathogenic fungus Colletotrichum acutatum

Light conditions can influence fungal development. Some spectral wavebands can induce conidial production, whereas others can kill the conidia, reducing the population size and limiting dispersal. The plant pathogenic fungus Colletotrichum acutatum causes anthracnose in several crops. During the asexual stage on the host plant, Colletototrichum produces acervuli with abundant mucilage-embedded conidia. These conidia are responsible for fungal dispersal and host infection. This study examined the effect of visible light during C. acutatum growth on the production of conidia and mucilage and also on the UV tolerance of these conidia. Conidial tolerance to an environmentally realistic UV irradiance was determined both in conidia surrounded by mucilage on sporulating colonies and in conidial suspension. Exposures to visible light during fungal growth increased production of conidia and mucilage as well as conidial tolerance to UV. Colonies exposed to light produced 1.7 times more conidia than colonies grown in continuous darkness. The UV tolerances of conidia produced under light were at least two times higher than conidia produced in the dark. Conidia embedded in the mucilage on sporulating colonies were more tolerant of UV than conidia in suspension that were washed free of mucilage. Conidial tolerance to UV radiation varied among five selected isolates.

Liquid production of entomopathogenic fungi and ultraviolet radiation and temperature effects on produced propagules

The purpose of this paper was to evaluate the liquid culture media for the production of Metarhizium anisopliae (IBCB 425) and Beauveria bassiana (IBCB 66), as well as the tolerance of these seedlings to the ultraviolet action and to the temperature. Twelve treatments composed of combinations between carbon and nitrogen concentrations were assessed. In order to determine the effect of ultraviolet radiation, plates with blastos-pores were exposed to it for 25 and 50 seconds. To determine the temperature efect, blastospores from culture media were exposed to 20, 25, 30 and 35°. For the virulence experiments, caterpillars of Diatraea saccharalis were sprayed with 2 mL of fungal suspension with the aid of a Potter tower. Te best media for M. anisopliae are 16.00 g (carbon) + 7.00 g (nitrogen) and 14.40 g (carbon) + 7.00 g (nitrogen), whereas for B. bassiana: 20.00 g (carbon) + 6.30 g (nitrogen) and 20.00 g (carbon) + 7.00 g (nitrogen). Te longer the exposure to ultraviolet radiation, the smaller the number of colonies. At 35°, there is a significant decrease in the formation of colonies. Te produced seedlings of fungi are pathogenic to D. saccharalis.

Quantification of cyclobutane pyrimidine dimers induced by UVB radiation in conidia of the fungi Aspergillus fumigatus, Aspergillus nidulans, Metarhizium acridum and Metarhizium robertsii

Conidia are responsible for reproduction, dispersal, environmental persistence and host infection of many fungal species. One of the main environmental factors that can kill and/or damage conidia is solar UV radiation. Cyclobutane pyrimidine dimers (CPD) are the major DNA photoproducts induced by UVB. We examined the conidial germination kinetics and the occurrence of CPD in DNA of conidia exposed to different doses of UVB radiation. Conidia of Aspergillus fumigatus, Aspergillus nidulans and Metarhizium acridum were exposed to UVB doses of 0.9, 1.8, 3.6 and 5.4 kJ m(-2). CPD were quantified using T4 endonuclease V and alkaline agarose gel electrophoresis. Most of the doses were sublethal for all three species. Exposures to UVB delayed conidial germination and the delays were directly related both to UVB doses and CPD frequencies. The frequencies of dimers also were linear and directly proportional to the UVB doses, but the CPD yields differed among species. We also evaluated the impact of conidial pigmentation on germination and CPD induction on Metarhizium robertsii. The frequency of dimers in an albino mutant was approximately 10 times higher than of its green wild-type parent strain after exposure to a sublethal dose (1.8 kJ m(-2)) of UVB radiation.

Increased sensitivity of Aspergillus flavus and Aspergillus parasiticus aflatoxin biosynthesis polyketide synthase mutants to UVB light

One strategy to reduce aflatoxin contamination of maize and cottonseed is to introduce spores of non-aflatoxigenic strains as competitors. Using isogenic mutants we show that, upon 5 or 20 min exposure to 302 nm (UVB) light, the viability of conidia of Aspergillus flavus and Aspergillus parasiticus mutants lacking the ability to accumulate any aflatoxin precursor metabolite is reduced five-fold compared to that of aflatoxin-producing strains or pigmented mutants that accumulate aflatoxin precursors. This result suggests that the long-term viability of introduced non-aflatoxigenic competitor strains may be lower than that of natural aflatoxin-producing isolates when exposed to sunlight.

Variability in UVB tolerances of melanized and nonmelanized cells of Cryptococcus neoformans and C. laurentii

Solar radiation is one of the major factors responsible for the control of fungus populations in the environment. Inactivation by UVA and UVB radiation is especially important for the control of fungi that disperse infective units through the air, including fungi such as Cryptococcus spp. that infect their vertebrate hosts by inhalation. Cryptococcus neoformans produces melanin in the presence of certain exogenous substrates such as l-3,4 dihydroxyphenylalanine and melanization may protect the fungus against biotic and abiotic environmental factors. In the present study, we investigated the effect of exposure to an UVB irradiance of 1000 mW m(-2) (biologically effective weighted irradiance) on the survival of melanized and nonmelanized cells of four strains of C. neoformans and four strains of C. laurentii. The relative survival (survival of cells exposed to radiation in relation to cells not exposed) of cells grown 2, 4, 6 or 8 days on medium with or without L-dopa was determined after exposure to UVB doses of 1.8 and 3.6 kJ m(-2). Both the irradiance spectrum and the intensities of those doses are environmentally realistic, and, in fact, occur routinely during summer months in temperate regions. Differences in tolerance to UVB radiation were observed between the C. neoformans and C. laurentii strains. The C. neoformans strains were more susceptible to UVB radiation than the C. laurentii strains. In C. neoformans, differences in tolerance to radiation were observed during development of both melanized and nonmelanized cells. For most treatments (strain, time of growth and UVB dose), there were virtually no differences in tolerances between melanized and nonmelanized cells, but when differences occurred they were smaller than those previously observed with UVC. In tests with two strains of C. laurentii, there was no difference in tolerance to UVB radiation between melanized and nonmelanized cells during 8 days of culture; and in tests with four strains for less culture time (4 days) there were no significant differences in tolerance between melanized and nonmelanized cells of any strain of this species.

Identification and characterization of an alternative oxidase in the entomopathogenic fungusMetarhizium anisopliae

Mitochondria of Metarhizium anisopliae contain an alternative oxidase (AOX), which reduces oxygen to water by accepting electrons directly from ubiquinol. AOX activity is demonstrated in situ as a constitutive enzyme. Greatest activity of AOX appears at the beginning and at the end of the fungal developmental cycle, germination of aerial conidia and the formation of submerged conidia, respectively. Changes in nutritional conditions, e.g., the presence of host insect cuticle or nutrient starvation had no effect on the induction of AOX activity. Antimycin A, an electron transport chain inhibitor, induced AOX activity. Cloning of the AOX DNA and the alignment of the deduced amino acid sequence of a segment of the AOX gene from M. anisopliae shows structural similarities with other AOX sequences with differing levels of variation when compared with homologous sequences from plants, yeasts, and filamentous fungi. Alternative oxidase in entomopathogenic fungi may have a positive contribution to ecological fitness.

Social prophylaxis: group interaction promotes collective immunity in ant colonies

Life in a social group increases the risk of disease transmission. To counteract this threat, social insects have evolved manifold antiparasite defenses, ranging from social exclusion of infected group members to intensive care. It is generally assumed that individuals performing hygienic behaviors risk infecting themselves, suggesting a high direct cost of helping. Our work instead indicates the opposite for garden ants. Social contact with individual workers, which were experimentally exposed to a fungal parasite, provided a clear survival benefit to nontreated, naive group members upon later challenge with the same parasite. This first demonstration of contact immunity in Social Hymenoptera and complementary results from other animal groups and plants suggest its general importance in both antiparasite and antiherbivore defense. In addition to this physiological prophylaxis of adult ants, infection of the brood was prevented in our experiment by behavioral changes of treated and naive workers. Parasite-treated ants stayed away from the brood chamber, whereas their naive nestmates increased brood-care activities. Our findings reveal a direct benefit for individuals to perform hygienic behaviors toward others, and this might explain the widely observed maintenance of social cohesion under parasite attack in insect societies.

Nucleotide excision repair and photoreactivation in the entomopathogenic fungi Beauveria bassiana, Beauveria brongniartii, Beauveria nivea, Metarhizium anisopliae, Paecilomyces farinosus and Verticillium lecanii

AIMS

To compare the DNA repair capabilities of the entomopathogenic fungus (EPF) bassiana to the EPF Beauveria brongniartii, Beauveria nivea, Metarhizium anisopliae, Paecilomyces farinosus, Verticillium lecanii, and the fungi Aspergillus niger and Neurospora crassa.

METHODS AND RESULTS

Germination of B. bassiana conidiospores following ultraviolet (UV) irradiation was used to show that nucleotide excision repair and photoreactivation decrease the post-UV germination delay. These two modes of repair were characterized and compared between the aforementioned EPF, A. niger and N. crassa using a physiological assay where per cent survival post-UV irradiation was scored as colony forming units.

CONCLUSIONS

The results showed B. bassiana and M. anisopliae are the most UV-tolerant EPF. The DNA repair capabilities indicated that EPF do not have all DNA repair options available to fungi, such as A. niger and N. crassa.

SIGNIFICANCE AND IMPACT OF THE STUDY

A key factor detrimental to the survival of EPF in agro-ecosystems is UV light from solar radiation. The EPF literature pertaining to UV irradiation is varied with respect to methodology, UV source, and dose, which prevented comparisons. Here we have characterized the fungi by a standard method and established the repair capabilities of EPF under optimal conditions.

Conidial pigmentation is important to tolerance against solar-simulated radiation in the entomopathogenic fungus Metarhizium anisopliae

The importance of conidial pigmentation to solar UV radiation tolerance in the entomopathogenic fungus Metarhizium anisopliae var. anisopliae, was estimated by comparing the effects of exposure to simulated solar UV radiation on the wild-type parent strain U.S. Department of Agriculture (USDA)-Agricultural Research Service (ARS) Collection of Entomopathogenic Fungal Cultures (ARSEF) 23, which has dark green conidia, and three groups of color mutants with yellow, purple and white conidia. The comparisons included inactivation levels and the kinetics of germination of conidia exposed or not exposed to simulated solar UV radiation. In addition to significantly inactivating the conidia of different mutants, exposure to radiation delayed for several hours the germination of surviving conidia of the wild type and all mutants. In general, mutants with white conidia were more sensitive to simulated solar UV radiation than mutants with purple conidia, which were more sensitive than mutants with yellow conidia, which in turn were more sensitive than the green wild strain. A significant variation in tolerance to simulated solar radiation was observed among mutants within each color group, particularly among mutants with yellow conidia. Revertants with green conidia, DWR 179 and DWR 176, were obtained from the very sensitive UV mutants DWR 148 (yellow conidia) and DWR 149 (purple conidia), respectively. These revertants had levels of tolerance to simulated solar UV radiation similar to those of the wild-type ARSEF 23. This observation is strong evidence of the importance of green conidial pigmentation for tolerance to simulated solar UV radiation, a factor that could be manipulated to produce M. anisopliae strains with more tolerance to solar UV radiation.

Variations in UV-B tolerance and germination speed of Metarhizium anisopliae conidia produced on insects and artificial substrates

Solar ultraviolet radiation (UV-A and UV-B) is a major factor in failure of programs using the insect pathogenic fungus Metarhizium anisopliae as a biological control agent. Studies were conducted to determine if growth conditions, viz. artificial (agar media or rice grain) or natural (infected insects) substrates for conidial production affect two traits that directly influence performance of conidia after field application: tolerance to UV-B radiation and conidial germination speed. Conidia of two isolates (ARSEF 23 and ARSEF 2575) of M. anisopliae var. anisopliae produced on potato dextrose agar plus yeast extract (PDAY) or on fungus-killed larvae of two insect species, Galleria mellonella and Zophobas morio, were inactivated by exposure to UV-B radiation. Conidia of both isolates when produced on insect cadavers were significantly more sensitive to UV-B radiation than conidia produced on PDAY. Also, conidia from insect cadavers germinated slower than those from PDAY cultures. A comparison of conidia from artificial substrates showed that conidia produced on Czapek's and Emerson's YpSs agar media or rice grains had higher tolerance to UV-B radiation and germinated faster than conidia raised on PDA and PDAY. Accordingly, the growth substrate and nutritional environment in which conidia are produced influences M. anisopliae conidial UV-B tolerance and speed of germination; and manipulation of these variables could be used to obtain conidia with increased tolerance to UV-B radiation and shorter germination times.

Enzyme activities associated with oxidative stress in Metarhizium anisopliae during germination, mycelial growth, and conidiation and in response to near-UV irradiation

Metarhizium anisopliae isolates have a wide insect host range, but an impediment to their commercial use as a biocontrol agent of above-ground insects is the high susceptibility of spores to the near-UV present in solar irradiation. To understand stress responses in M. anisopliae, we initiated studies of enzymes that protect against oxidative stress in two strains selected because their spores differed in sensitivity to UV-B. Spores of the more near-UV resistant strain in M. anisopliae 324 displayed different isozyme profiles for catalase-peroxidase, glutathione reductase, and superoxide dismutase when compared with the less resistant strain 2575. A transient loss in activity of catalase-peroxidase and glutathione reductase was observed during germination of the spores, whereas the intensity of isozymes displaying superoxide dismutase did not change as the mycelium developed. Isozyme composition for catalase-peroxidases and glutathione reductase in germlings changed with growth phase. UV-B exposure from lamps reduced the activity of isozymes displaying catalase-peroxidase and glutathione reductase activities in 2575 more than in 324. The major effect of solar UV-A plus UV-B also was a reduction in catalase-peroxidases isozyme level, a finding confirmed by measurement of catalase specific activity. Impaired growth of M. anisopliae after near-UV exposure may be related to reduced abilities to handle oxidative stress.

Cloning, functional analysis and expression of a scytalone dehydratase gene ( SCD1) involved in melanin biosynthesis of the phytopathogenic fungus Bipolaris oryzae

Scytalone dehydratase is involved in the production of fungal dihydroxynaphthalene melanin. We isolated and characterized SCD1, a gene encoding scytalone dehydratase, from the phytopathogenic fungus Bipolaris oryzae. Sequence analysis showed that SCD1 encodes a putative protein that has 185 amino acids, a molecular weight of 21 kDa and 51-75% sequence identity to other fungal scytalone dehydratases. Targeted disruption of SCD1 showed that this gene is necessary for melanin biosynthesis in B. oryzae. Northern blot analysis showed that SCD1 transcripts are specifically enhanced by near-ultraviolet (300-400 nm) radiation.

Both solar UVA and UVB radiation impair conidial culturability and delay germination in the entomopathogenic fungus Metarhizium anisopliae

The entomopathogenic hyphomycete Metarhizium anisopliae has been used in programs of agricultural pest and disease vector control in several countries. Exposure to simulated solar radiation for a few hours can completely inactivate the conidia of the fungus. In the present study we determined the effect of exposures to full-spectrum sunlight and to solar ultraviolet A radiation at 320-400 nm (UVA) on the conidial culturability and germination of three M. anisopliae strains. The exposures were performed in July and August 2000 in Logan, UT. The strains showed wide variation in tolerance when exposed to full-spectrum sunlight as well as to UVA sunlight. Four-hour exposures to full-spectrum sunlight reduced the relative culturability by approximately 30% for strain ARSEF 324 and by 100% for strains ARSEF 23 and 2575. The relative UV sensitivity of the two more sensitive strains was different under solar UV from that under ultraviolet B radiation at 280-320 nm (UVB) in the laboratory. Four-hour exposures to solar UVA reduced the relative culturability by 10% for strain ARSEF 324, 40% for strain ARSEF 23 and 60% for strain ARSEF 2575. Exposures to both full-spectrum sunlight and UVA sunlight delayed the germination of the surviving conidia of all three strains. These results, in addition to confirming the deleterious effects of UVB, clearly demonstrate the negative effects of UVA sunlight on the survival and germination of M. anisopliae conidia under natural conditions. The negative effects of UVA in sunlight also emphasize that the biological spectral weighting functions for this fungus must not neglect the UVA wavelengths.

Variability in response to UV-B among species and strains of Metarhizium isolated from sites at latitudes from 61 degrees N to 54 degrees S

The effects of irradiances of 920 and 1200 mW m(-2) (biologically effective weighted irradiance) were examined in 2 Metarhizium album strains, 26 M. anisopliae strains, 1 M. flavoviride strain, and 1 M. taii strain isolated from sites located at latitudes from 61 degrees N to 54 degrees S. Conidia were exposed to UV-B from 1 to 6 h and subsequently examined for relative percentage culturability. Total dosage received at the end of the exposure periods ranged from 3.3 to 19.9 kJ m(-2) for the lower irradiance and from 4.3 to 25.9 kJ m(-2) for the higher irradiance. Both the irradiance values and the doses are environmentally realistic and can be observed even in temperate regions. The relationships between latitude of origin and UV-B tolerance were compared for the two levels of irradiance for the data from 1 and 2 h exposure. Exposure to both irradiances drastically reduced the relative percentage culturability of all strains. Tolerance to UV-B varied widely among strains and high variation was observed for both irradiances after all periods of exposure. After 1 h of exposure, a difference between the two irradiance levels was detectable, and this difference was magnified at longer irradiations. A significant quadratic relationship of decreasing UV-B tolerance with increasing latitude was observed after exposure of 1 and 2 h. The shape of the relationship did not differ for the two levels of irradiance. Also, we studied the effect of 1200 mW m(-2) irradiance on conidial germination time in 1 M. album strain, 7 M. anisopliae strains, and 1 M. taii strain. Exposure to UV-B delayed the germination of surviving conidia of all strains. In general, the delay in germination was directly proportional to the dose.