Possible source of the high UV-B and heat tolerance of Metarhizium acridum (isolate ARSEF 324)

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.

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

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

Molecular basis and regulatory mechanisms underlying fungal insecticides' resistance to solar ultraviolet irradiation

Resistance to solar ultraviolet (UV) irradiation is crucial for field-persistent control efficacies of fungal formulations against arthropod pests, because their active ingredients are formulated conidia very sensitive to solar UV wavelengths. This review seeks to summarize advances in studies aiming to quantify, understand and improve conidial UV resistance. One focus of studies has been on the many sets of genes that have been revealed in the postgenomic era to contribute to or mediate UV resistance in the insect pathogens serving as main sources of fungal insecticides. Such genetic studies have unveiled the broad basis of UV-resistant molecules including cytosolic solutes, cell wall components, various antioxidant enzymes, and numerous effectors and signaling proteins, that function in developmental, biosynthetic and stress-responsive pathways. Another focus has been on the molecular basis and regulatory mechanisms underlying photorepair of UV-induced DNA lesions and photoreactivation of UV-impaired conidia. Studies have shed light upon a photoprotective mechanism depending on not only one or two photorepair-required photolyases, but also two white collar proteins and other partners that play similar or more important roles in photorepair via interactions with photolyases. Research hotspots are suggested to explore a regulatory network of fungal photoprotection and to improve the development and application strategies of UV-resistant fungal insecticides. © 2021 Society of Chemical Industry.

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.

Virulence of the insect-pathogenic fungi Metarhizium spp. to Mormon crickets, Anabrus simplex (Orthoptera: Tettigoniidae)

The Mormon cricket (MC), Anabrus simplex Haldeman, 1852 (Orthoptera: Tettigoniidae), has a long and negative history with agriculture in Utah and other western states of the USA. Most A. simplex populations migrate in large groups, and their feeding can cause significant damage to forage plants and cultivated crops. Chemical pesticides are often applied, but some settings (e.g. habitats of threatened and endangered species) call for non-chemical control measures. Studies in Africa, South America, and Australia have assessed certain isolates of Metarhizium acridum as very promising pathogens for Orthoptera: Acrididae (locust) biocontrol. In the current study, two isolates of Metarhizium robertsii, one isolate of Metarhizium brunneum, one isolate of Metarhizium guizhouense, and three isolates of M. acridum were tested for infectivity to MC nymphs and adults of either sex. Based on the speed of mortality, M. robertsii (ARSEF 23 and ARSEF 2575) and M. brunneum (ARSEF 7711) were the most virulent to instars 2 to 5 MC nymphs. M. guizhouense (ARSEF 7847) from Arizona was intermediate and the M. acridum isolates (ARSEF 324, 3341, and 3609) were the slowest killers. ARSEF 2575 was also the most virulent to instar 6 and 7 nymphs and adults of MC. All of the isolates at the conidial concentration of 1 × 107 conidia ml-1 induced approximately 100% mortality by 6 days post application of fungal conidia. In conclusion, isolates ARSEF 23, ARSEF 2575, and ARSEF 7711 acted most rapidly to kill MC under laboratory conditions. The M. acridum isolates, however, have much higher tolerance to heat and UV-B radiation, which may be critical to their successful use in field application.

Fungal tolerance to Congo red, a cell wall integrity stress, as a promising indicator of ecological niche

Differential sensitivities to the cell wall stress caused by Congo red (CR) have been observed in many fungal species. In this study, the tolerances and sensitivities to CR was studied with an assorted collection of fungal species from three phylogenetic classes: Sordariomycetes, Dothideomycetes, and Eurotiomycetes, three orders, and eight families. These grouped into different ecological niches, such as insect pathogens, plant pathogens, saprotrophs, and mycoparasitics. The saprotroph Aspergillus niger and the mycoparasite Trichoderma atroviride stood out as the most resistant species to cell wall stress caused by CR, followed by the plant pathogenic fungi, a mycoparasite, and other saprotrophs. The insect pathogens had low tolerance to CR. The insect pathogens Metarhizium acridum and Cordyceps fumosorosea were the most sensitive to CR. In conclusion, Congo red tolerance may reflect ecological niche, accordingly, the tolerances of the fungal species to Congo red were closely aligned with their ecology.

Integration of Entomopathogenic Fungi into IPM Programs: Studies Involving Weevils (Coleoptera: Curculionoidea) Affecting Horticultural Crops

Weevils are significant pests of horticultural crops and are largely managed with insecticides. In response to concerns about negative impacts of synthetic insecticides on humans and the environment, entomopathogenic fungi (EPF) have been developed as an alternative method of control, and as such appear to be "ready-made" components of integrated pest management (IPM) programs. As the success of pest control requires a thorough knowledge of the biology of the pests, this review summarises our current knowledge of weevil biology on nut trees, fruit crops, plant storage roots, and palm trees. In addition, three groups of life cycles are defined based on weevil developmental habitats, and together with information from studies of EPF activity on these groups, we discuss the tactics for integrating EPF into IPM programs. Finally, we highlight the gaps in the research required to optimise the performance of EPF and provide recommendations for the improvement of EPF efficacy for the management of key weevils of horticultural crops.

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.

The Third International Symposium on Fungal Stress - ISFUS

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

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

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

The polyubiquitin gene MrUBI4 is required for conidiation, conidial germination, and stress tolerance in the filamentous fungus Metarhizium robertsii

The polyubiquitin gene is a highly conserved open reading frame that encodes different numbers of tandem ubiquitin repeats from different species, which play important roles in different biological processes. Metarhizium robertsii is a fungal entomopathogen that is widely applied in the biological control of pest insects. However, it is unclear whether the polyubiquitin gene is required for fungal development, stress tolerance, and virulence in the entomopathogenic fungus. In the present study, the polyubiquitin gene (MrUBI4, MAA_02160) was functionally characterized via gene deletion in M. robertsii. Compared to the control strains, the MrUBI4 deletion mutant showed delayed conidial germination and significantly decreased conidial yields (39% of the wild-type 14 days post-incubation). Correspondingly, the transcript levels of several genes from the central regulatory pathways associated with conidiation, including brlA, abaA, and wetA, were significantly downregulated, which indicated that MrUBI4 played an important role in asexual sporulation. Deletion of MrUBI4 especially resulted in increased sensitivity to ultraviolet (UV) and heat-shock stress based on conidial germination analysis between mutant and control strains. The significant increase in sensitivity to heat-shock was accompanied with reduced transcript levels of genes related to heat-shock protein (hsp), trehalose, and mannitol accumulation (tps, tpp, nth, and mpd) in the MrUBI4 deletion mutant. Deletion of MrUBI4 has no effect on fungal virulence. Altogether, MrUBI4 is involved in the regulation of conidiation, conidial germination, UV stress, and heat-shock response in M. robertsii.

Phenotype responses to abiotic stresses, asexual reproduction and virulence among isolates of the entomopathogenic fungus Cordyceps javanica (Hypocreales: Cordycipitaceae)

Selecting entomopathogenic fungal isolates with resilience to environmental stresses, optimal mass production characteristics, and with high virulence to target pests favors the development of mycopesticides. A case in point, Cordyceps (= Isaria) javanica has been extensively investigated for non-chemical control of whiteflies worldwide. We phylogenetically characterized 11 native C. javanica isolates from Northeastern and Central Brazil. These isolates were screened for tolerance to heat-shock, UV-B radiation, osmotic and oxidative stresses, as well as conidial production on cereal grain and insecticidal activity against the whitefly Bemisia tabaci (MEAM 1) in the laboratory. All isolates were pathogenic to whiteflies and significant (3-fold) differences in median lethal concentration were observed among isolates. Furthermore, pronounced differences among isolates were found for stress factors and conidial production. Using principal component analysis, our results highlighted three major clusters formed by isolates (i) resistant to osmotic and oxidative stress, (ii) resilient to UV-B, and (iii) with high virulence, conidial production and heat tolerance. Overall, isolate CG1228 performed best based on multi-stress resistance, mass production and virulence attributes in the laboratory. This study highlights the importance of exploring natural variation in entomopathogenic fungi for selection of appropriate isolates for effective biocontrol of insect pests coupled with mass production characteristics and abiotic stress tolerances.