A scorpion neurotoxin increases the potency of a fungal insecticide

Engineered fungus containing a caterpillar gene kills insects rapidly by disrupting their ecto- and endo-microbiomes

Similar to the physiological importance of gut microbiomes, recent works have shown that insect ectomicrobiotas can mediate defensive colonization resistance against fungal parasites that infect via cuticle penetration. Here we show that engineering the entomopathogenic fungus Metarhizium robertsii with a potent antibacterial moricin gene from silkworms substantially enhances the ability of the fungus to kill mosquitos, locusts, and two Drosophila species. Further use of Drosophila melanogaster as an infection model, quantitative microbiome analysis reveals that engineered strains designed to suppress insect cuticular bacteria additionally disrupt gut microbiomes. An overgrowth of harmful bacteria such as the opportunistic pathogens of Providencia species is detected that can accelerate insect death. In support, quantitative analysis of antimicrobial genes in fly fat bodies and guts indicates that topical fungal infections result in the compromise of intestinal immune responses. In addition to providing an innovative strategy for improving the potency of mycoinsecticides, our data solidify the importance of both the ecto- and endo-microbiomes in maintaining insect wellbeing.

Bioefficacy of engineered Beauveria bassiana with scorpion neurotoxin, LqqIT1 against cotton mealybug, Phenacoccus solenopsis and cowpea aphid, Aphis craccivora

Cotton mealybug, Phenacoccus solenopsis (Tinsley) and cowpea aphid Aphis craccivora (Koch) are notorious polyphagous, hemipteran sap sucking insect pests. A recombinant toxin gene 'LqqIT1' from the scorpion Leiurus quinquestriatus quinquestriatus (Ehrenberg) was cloned in the pAL1 fungal expression vector and then expressed in the entomopathogenic fungus Beauveria bassiana (Balasmo) using genetic modification techniques. The genetically transformed B. bassiana strain (BbLqqIT1-3) and its un-transformed parent strain (Bb-C) were screened to infect the third instar nymphs of P. solenopsis and first instar nymph of A. craccivora through leaf treatment and topical application (spray) method at 1 * 107 spores per ml concentration. The recombinant strain BbLqqIT1-3 was highly pathogenic against A. craccivora but non pathogenic to P. solenopsis. BbLqqIT1-3 induced 72 and 43.33% mortality in A. craccivora nymphs 96 h after leaf treatment and topical application, respectively. The nymphs of A. craccivora infected with BbLqqIT1-3 displayed classical neurotoxic symptoms such as sluggishness, solublize and liquification of the body. Crude soluble toxin protein, BbLqqIT1a-CSE and Bb-WT-CSE was extracted from the BbLqqIT1-3 and Bb-C, respectively using ammonium sulphate precipitation method, and their oral toxicity was analyzed at 5 µg/ml concentration. The survival of the studied insects was negatively affected by the crude soluble toxin extracts. The LT50 values of BbLqqIT1a-CSE against P. solenopsis and A. craccivora were 22.18 and 17.69 h, respectively. Exposure to crude soluble toxin extracts also accounted for the imbalance of ionic concentrations in the hemolymph of treated insects such as hyperpotassemia (3.53-8.18 meq/ml) in the P. solenopsis and hypopotassemia (7.52-0.47 meq/ml) in A. craccivora. The transformed fungus BbLqqIT1-3 strain exhibited promising results in invitro study.

The Registration Situation and Use of Mycopesticides in the World

Mycopesticides are living preparations that use fungal cells, such as spores and hyphae, as active ingredients. They mainly include mycoinsecticides, mycofungicides, mycoherbicides and nematophagous fungi. The utilization of fungi for controlling agricultural pests can be traced back to approximately 1880, when entomopathogenic fungi were initially employed for this purpose. However, it was not until 1965 that the world's first mycopesticide, Beauveria bassiana, was registered as Boverin® in the former Soviet Union. In past decades, numerous novel mycopesticides have been developed for their lower R&D costs, as well as the environmentally friendly and safe nature. In this review, we investigated the mycopesticides situation of registration in USA, EU, China, Canada and Australia. Superisingly, it was found that the registered mycopesticides are extremely raised in recent years. Currently, the insecticides, fungicides (nematocides) and herbicides were respectively registered 27, 53 and 8 fungal strains. This paper also analyzes the main problems currently faced by mycopesticides and offers suggestions for their future development.

Two Novel Mosquitocidal Peptides Isolated from the Venom of the Bahia Scarlet Tarantula (Lasiodora klugi)

Effective control of diseases transmitted by Aedes aegypti is primarily achieved through vector control by chemical insecticides. However, the emergence of insecticide resistance in A. aegypti undermines current control efforts. Arachnid venoms are rich in toxins with activity against dipteran insects and we therefore employed a panel of 41 spider and 9 scorpion venoms to screen for mosquitocidal toxins. Using an assay-guided fractionation approach, we isolated two peptides from the venom of the tarantula Lasiodora klugi with activity against adult A. aegypti. The isolated peptides were named U-TRTX-Lk1a and U-TRTX-Lk2a and comprised 41 and 49 residues with monoisotopic masses of 4687.02 Da and 5718.88 Da, respectively. U-TRTX-Lk1a exhibited an LD50 of 38.3 pmol/g when injected into A. aegypti and its modeled structure conformed to the inhibitor cystine knot motif. U-TRTX-Lk2a has an LD50 of 45.4 pmol/g against adult A. aegypti and its predicted structure conforms to the disulfide-directed β-hairpin motif. These spider-venom peptides represent potential leads for the development of novel control agents for A. aegypti.

Cysteine-rich peptides: From bioactivity to bioinsecticide applications

Greater levels of insect resistance and constraints on the use of current pesticides have recently led to increased crop losses in agricultural production. Further, the health and environmental impacts of pesticides now restrict their application. Biologics based on peptides are gaining popularity as efficient crop protection agents with low environmental toxicity. Cysteine-rich peptides (whether originated from venoms or plant defense substances) are chemically stable and effective as insecticides in agricultural applications. Cysteine-rich peptides fulfill the stability and efficacy requirements for commercial uses and provide an environmentally benign alternative to small-molecule insecticides. In this article, cysteine-rich insecticidal peptide classes identified from plants and venoms will be highlighted, focusing on their structural stability, bioactivity and production.

A 1-aminocyclopropane-1-carboxylate deaminase MrACCD from Metarhizium robertsii is associated with plant growth promotion for Metarhizium spp

Besides killing insects, Metarhizium spp. have been showing another realistic ecology role as plant associates. Partial genra and groups of these entomopathogenic fungi act as plant growth promoters during root colonization. Here, we report that Metarhizium robertsii produces a 1-aminocyclopropane-1-carboxylate (ACC) deaminase (ACCD encoded by MracdS, MrACCD), which is involved in promoting wheat early vegetative growth, while Metarhizium acridum lacks the genuine ACCD though a MracdS homologue exists in the species. MracdS expression was up-regulated by a max 10.7-fold with 3 mM ACC and high ACCD enzymatic activities were induced by either ACC (7.5-fold) or wheat root (3.2-fold). In contrast, no ACCD activity was detected in M. acridum in the presence of both inducers. In pot assay, wheat seeds were treated with wild-type M. robertsii (Mr23), wild-type M. acridum (Mac324), MracdS disruption mutant (ΔMracdS) and M. acridum transformant harboring heterologous MracdS (Mac324-MracdS). Relative to the control seeds treated with heat-killed conidia, Mr23, ΔMracdS and Mac324-MracdS increased root length (by 66.2, 31.8 and 40.2%), and plant biomass (by 56.6, 42.1 and 40.9%). Nevertheless, ΔMracdS deficient in ACCD activity heavily impaired its capability of wheat growth promotion by decrease of 20.7% in root length relative to Mr23. In addition, Mr23 and Mac324-MracdS also increased shoot growth (by 42.3, and 42.7%) while ΔMracdS failed. Mac324 showed no effect on plant growth during the test. These data suggest a role for ACCD in the plant growth promotion effect by M. robertsii, which is irrelevant to Metarhizium colonization of roots since rhizosphere competency of both Mr23 and Mac324 are unaffected by the change of ACCD activity.

Improvement of Akanthomyces lecanii resistance to tebuconazole through UV-C radiation and selective pressure on microbial evolution and growth arenas

Tebuconazole (TEB) is a fungicide widely used in agriculture; however, its constant application has increased the emergence of resistant plant pathogenic fungal strains and reduced the effectiveness of fungi as biological control agents; for instance, the entomopathogenic and hyperparasitic fungus Akanthomyces lecanii, suitable for simultaneous biological control of insect pest and plant pathogenic fungi, is highly sensitive to fungicides. We carried out the induction of resistance to TEB in two wild type strains of A. lecanii by UV radiation and selective pressure in increasing fungicide gradients using a modified Microbial Evolution and Growth Arena (MEGA), to produce A. lecanii strains that can be used as biological control agent in the presence of tebuconazole. Nine UV-induced and three naturally adapted A. lecanii strains were resistant to TEB at the agriculturally recommended dose, and three irradiated strains were resistant to TEB concentration ten times higher; moreover, growth, sporulation rates, production of hydrolytic enzymes, and virulence against the hemipteran Coccus viridis, a major pest of coffee crops, were not affected in the TEB-resistant strains. These A. lecanii TEB-resistant strains would have a greater opportunity to develop and to establish themselves in fields where the fungicide is present and can be used in a combined biological-chemical strategy to improve insect and plant pathogenic fungal control in agriculture. Also, the selective pressure through modified MEGA plate methodology can be used for the adaptation of entomopathogenic filamentous fungi to withstand other chemical or abiotic stresses that limits its effectiveness for pest control.

Genomic Determinants of Entomopathogenic Fungi and Their Involvement in Pathogenesis

Entomopathogenic fungi offer an effective and eco-friendly alternative to curb insect populations in biocontrol strategy. The evolutionary history of selected entomopathogenic fungi indicates their ancestral relationship with plant endophytes. During this host shifting, entomopathogenic fungi must have acquired multiple mechanisms, including a combination of various biomolecules that make them distinguishable from other fungi. In this review, we focus on understanding various biochemical and molecular mechanisms involved in entomopathogenesis. In particular, we attempt to explain the indispensable role of enlarged gene families of various virulent factors, viz. chitinases, proteases, lipases, specialized metabolites, and cytochrome P450, in entomopathogenesis. Our analysis suggests that entomopathogenic fungi recruit a different set of gene products during the progression of pathogenesis. Knowledge of these bio-molecular interactions between fungi and insect hosts will allow researchers to execute pointed efforts towards the development of improved entomopathogenic fungal strains.

Strain improvement of Trichoderma harzianum for enhanced biocontrol capacity: Strategies and prospects

In the control of plant diseases, biocontrol has the advantages of being efficient and safe for human health and the environment. The filamentous fungus Trichoderma harzianum and its closely related species can inhibit the growth of many phytopathogenic fungi, and have been developed as commercial biocontrol agents for decades. In this review, we summarize studies on T. harzianum species complex from the perspective of strain improvement. To elevate the biocontrol ability, the production of extracellular proteins and compounds with antimicrobial or plant immunity-eliciting activities need to be enhanced. In addition, resistance to various environmental stressors should be strengthened. Engineering the gene regulatory system has the potential to modulate a variety of biological processes related to biocontrol. With the rapidly developing technologies for fungal genetic engineering, T. harzianum strains with increased biocontrol activities are expected to be constructed to promote the sustainable development of agriculture.

Expression of mosquito miRNAs in entomopathogenic fungus induces pathogen-mediated host RNA interference and increases fungal efficacy

The growing threat of insecticide resistance prompts the urgent need to develop additional tools for mosquito control. Entomopathogenic fungi provide an eco-friendly alternative to chemical insecticides. One limitation to the use of mycoinsecticides is their relatively low virulence. Here, we report an approach for suppressing mosquito immunity and increasing fungal virulence. We engineered Beauveria bassiana to express Aedes immunosuppressive microRNAs (miRNAs) to induce host RNA interference (RNAi) immune responses. We show that engineered strains can produce and deliver the miRNAs into host cells to activate cross-kingdom RNAi during infection and suppress mosquito immunity by targeting multiple host genes, thereby dramatically increasing fungal virulence against Aedes aegypti and Galleria mellonella larvae. Importantly, expressing host miRNAs also significantly increases fungal virulence against insecticide-resistant mosquitoes, creating potential for insecticide-resistance management. This pathogen-mediated RNAi (pmRNAi)-based approach provides an innovative strategy to enhance the efficacy of fungal insecticides and eliminate the likelihood of resistance development.

Stress-Mediated Responses of Aedes aegypti (Diptera: Culicidae) Larvae When Exposed to Metarhizium brunneum (Hypocreales: Clavicipitaceae) and Toxorhynchites brevipalpis (Diptera: Culicidae)

Aedes aegypti mosquitoes are capable of vectoring a wide range of diseases including dengue, yellow fever, and Zika viruses, with approximately half of the worlds' population at risk from such diseases. Development of combined predator-parasite treatments for the control of larvae consistently demonstrates increased efficacy over single-agent treatments, however, the mechanism behind the interaction remains unknown. Treatments using the natural predator Toxorhynchites brevipalpis and the entomopathogenic fungus Metarhizium brunneum were applied in the laboratory against Ae. aegypti larvae as both individual and combined treatments to determine the levels of interaction between control strategies. Parallel experiments involved the removal of larvae from test arenas at set intervals during the course of the trial to record whole body caspase and phenoloxidase activities. This was measured via luminometric assay to measure larval stress factors underlying the interactions. Combined Metarhizium and Toxorhynchites treatments were seen to drastically reduce lethal times as compared to individual treatments. This was accompanied by increased phenoloxidase and caspase activities in combination treatments after 18 h (p < 0.001). The sharp increases in caspase and phenoloxidase activities suggest that combined treatments act to increase stress factor responses in the larvae that result in rapid mortality above that of either control agent individually. This work concludes that the underlying mechanism for increased lethality in combined parasite-predator treatments may be related to additive stress factors induced within the target host larvae.

Fungal insecticidal activity elevated by non-risky markerless overexpression of an endogenous cysteine-free protein gene in Beauveria bassiana

BACKGROUND

Fungal insecticides are notorious for slow kill action, an intrinsic trait that can be improved by the genetic engineering of an exogenous or endogenous virulence factor. However, transgenic insecticides expressing exogenous toxin and herbicide-resistant marker genes may cause unexpected ecological risks and are hardly permitted for field release due to strict regulatory hurdles. It is necessary to improve biotechnology that can speed up fungal insect-killing action and exclude ecological risk source.

RESULTS

A markerless transformation system of Beauveria bassiana, a main source of wide-spectrum fungal insecticides, was reconstructed based on the fungal uridine auxotrophy (Δura3). The system was applied for overexpression of the small cysteine-free protein (120 amino acids) gene cfp previously characterized as a regulator of the fungal virulence and gene expression. Three cfp-overexpressed strains showed much faster kill action to Galleria mellonella larvae than the parental wild-type via normal cuticle infection but no change in vegetative growth and aerial condition. The faster kill action was achieved due to not only significant increases in conidial adherence to insect cuticle and total activity of secreted cuticle-degrading Pr1 proteases and of antioxidant enzymes crucial for collapse of insect immune defense but acceleration of hemocoel localization, proliferation in vivo and host death from mummification.

CONCLUSION

The markerless system is free of any foreign DNA fragment as a source of ecologic risk and provides a novel biotechnological approach to enhancing fungal insecticidal activity with non-risky endogenous genes and striding over the regulatory hurdles. © 2022 Society of Chemical Industry.

Current developments in the resistance, quality, and production of entomopathogenic fungi

There is a worldwide concern to achieve food security with a sustainable approach, including the generation and implementation of techniques for the production of high-quality chemical-free crops. This food revolution has promoted the development and consolidation of programmes for integrated pest management. Some of those programmes include the use of diverse organisms (biological control agents) to suppress populations of pests potentially harmful to the crops. Among these biological control agents are entomopathogenic fungi that are highly effective in suppressing a diversity of insects and have, therefore, been produced and marketed throughout the world. However, the bottleneck for applying entomopathogenic fungi is the production of propagules (blastospores and conidia) with resistance to environment conditions and abiotic factors, maintaining high quality in terms of virulence. Therefore, this manuscript presents recent studies related to increasing resistance and quality using different bioreactors to produce conidia. The above presents a global panorama related to current developments that contribute to improving the resistance, quality, and production of entomopathogenic fungal propagules.

Strategies for Heterologous Expression, Synthesis, and Purification of Animal Venom Toxins

Animal venoms are complex mixtures containing peptides and proteins known as toxins, which are responsible for the deleterious effect of envenomations. Across the animal Kingdom, toxin diversity is enormous, and the ability to understand the biochemical mechanisms governing toxicity is not only relevant for the development of better envenomation therapies, but also for exploiting toxin bioactivities for therapeutic or biotechnological purposes. Most of toxinology research has relied on obtaining the toxins from crude venoms; however, some toxins are difficult to obtain because the venomous animal is endangered, does not thrive in captivity, produces only a small amount of venom, is difficult to milk, or only produces low amounts of the toxin of interest. Heterologous expression of toxins enables the production of sufficient amounts to unlock the biotechnological potential of these bioactive proteins. Moreover, heterologous expression ensures homogeneity, avoids cross-contamination with other venom components, and circumvents the use of crude venom. Heterologous expression is also not only restricted to natural toxins, but allows for the design of toxins with special properties or can take advantage of the increasing amount of transcriptomics and genomics data, enabling the expression of dormant toxin genes. The main challenge when producing toxins is obtaining properly folded proteins with a correct disulfide pattern that ensures the activity of the toxin of interest. This review presents the strategies that can be used to express toxins in bacteria, yeast, insect cells, or mammalian cells, as well as synthetic approaches that do not involve cells, such as cell-free biosynthesis and peptide synthesis. This is accompanied by an overview of the main advantages and drawbacks of these different systems for producing toxins, as well as a discussion of the biosafety considerations that need to be made when working with highly bioactive proteins.

Insect pest management in the age of synthetic biology

Arthropod crop pests are responsible for 20% of global annual crop losses, a figure predicted to increase in a changing climate where the ranges of numerous species are projected to expand. At the same time, many insect species are beneficial, acting as pollinators and predators of pest species. For thousands of years, humans have used increasingly sophisticated chemical formulations to control insect pests but, as the scale of agriculture expanded to meet the needs of the global population, concerns about the negative impacts of agricultural practices on biodiversity have grown. While biological solutions, such as biological control agents and pheromones, have previously had relatively minor roles in pest management, biotechnology has opened the door to numerous new approaches for controlling insect pests. In this review, we look at how advances in synthetic biology and biotechnology are providing new options for pest control. We discuss emerging technologies for engineering resistant crops and insect populations and examine advances in biomanufacturing that are enabling the production of new products for pest control.

An efficient Agrobacterium tumefaciens-mediated transformation method for Simplicillium subtropicum (Hypocreales: Cordycipitaceae)

Filamentous fungi are the organisms of choice for most industrial biotechnology. Some species can produce a variety of secondary metabolites and enzymes of commercial interest, and the production of valuable molecules has been enhanced through different molecular tools. Methods for genetic manipulation and transformation have been essential for the optimization of these organisms. The genus Simplicillium has attracted increased attention given several potential biotechnological applications. The Simplicillium genus harbors several entomopathogenic species and some isolates have been explored for bioremediation of heavy metal contaminants. Furthermore, the myriad of secondary metabolites isolated from Simplicillium spp. render these organisms as ideal targets for deep exploration and further biotechnological mining possibilities. However, the lack of molecular tools hampered the exploration of this genus. Thus, an Agrobacterium tumefaciens-mediated transformation method was established for Simplicillium subtropicum, employing the far-red fluorescent protein TURBOFP635/Katushka, as a visual marker, and the selection marker SUR gene, that confers resistance to chlorimuron ethyl. Notably, one round of transformation using the established method yielded almost 400 chlorimuron resistant isolates. Furthermore, these transformants displayed mitotic stability for, at least, five generations. We anticipate that this method can be useful for deep molecular exploration and improvement of strains in the Simplicillium genus.

Model Application of Entomopathogenic Fungi as Alternatives to Chemical Pesticides: Prospects, Challenges, and Insights for Next-Generation Sustainable Agriculture

In the past few decades, the control of pests and diseases of cultivated plants using natural and biological measures has drawn increasing attention in the quest to reduce the level of dependence on chemical products for agricultural production. The use of living organisms, predators, parasitoids, and microorganisms, such as viruses, bacteria, and fungi, has proven to be a viable and sustainable pest management technique. Among the aforementioned, fungi, most importantly the insect-pathogenic species, have been in use for more than 150years. These include the most popular strains belonging to the genera Beauveria, Metarhizium, Isaria, Hirsutella, and Lecanicillium. Their application is usually through an inundative approach, which inherently involves exposure of the fungal spores to unfavorable humidity, temperature, and solar radiation conditions. These abiotic factors reduce the persistence and efficacy of these insect-pathogenic fungi. Despite these limitations, over 170 strains have been formulated as mycopesticides and are available for commercial use. In the last few decades, numerous studies have suggested that these species of entomopathogenic fungi (EPF) offer far more benefits and have broader ecological functions than hitherto presumed. For instance, aside from their roles as insect killers, it has been well established that they also colonize various host plants and, hence, provide other benefits including plant pathogen antagonism and plant growth promotion and serve as sources of novel bioactive compounds and secondary metabolites, etc. In this light, the potential of EPF as alternatives or perhaps as supplements to chemical pesticides in plant protection is discussed in this review. The paper highlights the numerous benefits associated with endophytic fungal entomopathogen and host plant associations, the mechanisms involved in mediating plant defense against pests and pathogens, and the general limitations to the use of EPF in plant protection. A deeper understanding of these plant host-fungus-insect relationships could help unveil the hidden potentials of fungal endophytes, which would consequently increase the level of acceptance and adoption by users as an integral part of pest management programs and as a suitable alternative to chemical inputs toward sustainable crop production.

Integration of dsRNA against host immune response genes augments the virulence of transgenic Metarhizium robertsii strains in insect pest species

The slow lethality of fungal biopesticides to insects restrains their widespread application as a strategy of pest control. In this study, unary, binary and ternary transgenic Metarhizium robertsii were created by integrating genes that encode the scorpion neurotoxin BjαIT, the cuticle-degrading protease Pr1A, and a double-stranded RNA (dsRNA) that targets host gnbp3, individually or collectively under a constitutive promoter to enhance virulence. Compared with the parental wild type, all unary transgenic strains had increased virulence against four insect species, Tenebrio molitor, Locusta migratoria, Plutella xylostella and Galleria mellonella, whereas the binary transgenic strain expressing both pr1A and BjαIT had increased virulence to T. molitor and L. migratoria, with no change in virulence against P. xylostella and G. mellonella. Importantly, all ternary transgenic strains simultaneously expressing pr1A, BjαIT, and the dsRNA specific to host gnbp3 exhibited the highest increase in insect-specific virulence. This finding highlights a novel strategy for genetic engineering of dsRNAs that target genes associated with the host immune response alongside virulence genes to maximize fungal virulence and lethality against insect pests.

The Chemical Ecology Approach to Reveal Fungal Metabolites for Arthropod Pest Management

Biorational insecticides (for instance, avermectins, spinosins, azadirachtin, and afidopyropen) of natural origin are increasingly being used in agriculture. The review considers the chemical ecology approach for the search for new compounds with insecticidal properties (entomotoxic, antifeedant, and hormonal) produced by fungi of various ecological groups (entomopathogens, soil saprotrophs, endophytes, phytopathogens, and mushrooms). The literature survey revealed that insecticidal metabolites of entomopathogenic fungi have not been sufficiently studied, and most of the well-characterized compounds show moderate insecticidal activity. The greatest number of substances with insecticidal properties was found to be produced by soil fungi, mainly from the genera Aspergillus and Penicillium. Metabolites with insecticidal and antifeedant properties were also found in endophytic and phytopathogenic fungi. It was noted that insect pests of stored products are mostly low sensitive to mycotoxins. Mushrooms were found to be promising producers of antifeedant compounds as well as insecticidal proteins. The expansion of the number of substances with insecticidal properties detected in prospective fungal species is possible by mining fungal genomes for secondary metabolite gene clusters and secreted proteins with their subsequent activation by various methods. The efficacy of these studies can be increased with high-throughput techniques of extraction of fungal metabolites and their analysis by various methods of chromatography and mass spectrometry.

Genome editing for resistance against plant pests and pathogens

The conventional breeding of crops struggles to keep up with increasing food needs and ever-adapting pests and pathogens. Global climate changes have imposed another layer of complexity to biological systems, increasing the challenge to obtain improved crop cultivars. These dictate the development and application of novel technologies, like genome editing (GE), that assist targeted and fast breeding programs in crops, with enhanced resistance to pests and pathogens. GE does not require crossings, hence avoiding the introduction of undesirable traits through linkage in elite varieties, speeding up the whole breeding process. Additionally, GE technologies can improve plant protection by directly targeting plant susceptibility (S) genes or virulence factors of pests and pathogens, either through the direct edition of the pest genome or by adding the GE machinery to the plant genome or to microorganisms functioning as biocontrol agents (BCAs). Over the years, GE technology has been continuously evolving and more so with the development of CRISPR/Cas. Here we review the latest advancements of GE to improve plant protection, focusing on CRISPR/Cas-based genome edition of crops and pests and pathogens. We discuss how other technologies, such as host-induced gene silencing (HIGS) and the use of BCAs could benefit from CRISPR/Cas to accelerate the development of green strategies to promote a sustainable agriculture in the future.

How a Scorpion Toxin Selectively Captures a Prey Sodium Channel: The Molecular and Evolutionary Basis Uncovered

The growing resistance of insects to chemical pesticides is reducing the effectiveness of conventional methods for pest control and thus, the development of novel insecticidal agents is imperative. Scorpion toxins specific for insect voltage-gated sodium channels (Navs) have been considered as one of the most promising insecticide alternatives due to their host specificity, rapidly evoked toxicity, biodegradability, and the lack of resistance. However, they have not been developed for uses in agriculture and public health, mainly because of a limited understanding of their molecular and evolutionary basis controlling their phylogenetic selectivity. Here, we show that the traditionally defined insect-selective scorpion toxin LqhIT2 specifically captures a prey Nav through a conserved trapping apparatus comprising a three-residue-formed cavity and a structurally adjacent leucine. The former serves as a detector to recognize and bind a highly exposed channel residue conserved in insects and spiders, two major prey items for scorpions; and the latter subsequently seizes the "moving" voltage sensor via hydrophobic interactions to reduce activation energy for channel opening, demonstrating its action in an enzyme-like manner. Based on the established toxin-channel interaction model in combination with toxicity assay, we enlarged the toxic spectrum of LqhIT2 to spiders and certain other arthropods. Furthermore, we found that genetic background-dependent cavity shapes determine the species selectivity of LqhIT2-related toxins. We expect that the discovery of the trapping apparatus will improve our understanding of the evolution and design principle of Nav-targeted toxins from a diversity of arthropod predators and accelerate their uses in pest control.

Expression of a phenoloxidase cascade inhibitor enhances the virulence of the fungus Beauveria bassiana against the insect Helicoverpa armigera

Entomopathogenic fungi have high potential for controlling insect pests, although the slow killing speed has blocked their widespread application. To increase the virulence of entomopathogenic fungi, genetic modification can be employed. Egf1.0 is an immunosuppressive protein encoded by polydnavirus, carried by parasitoid wasp Microplitis demolitor, which blocks the prophenoloxidase (PPO) activation response of host insects. In this study, we explored the feasibility of genetically modifying entomopathogenic fungi with increased virulence by expressing Egf1.0. In comparison with the wild-type parents, the median lethal concentration (LC₅₀) of Beauveria bassiana expressing Egf1.0 against Helicoverpa armigera was reduced by 2.7-fold, and the median lethal time (LT₅₀) was reduced by 22.8%. In vitro assay showed that recombinant Egf1.0 was able to inhibit the PPO activation response of H. armigera. In vivo assay revealed that the expression of Egf1.0 in B. bassiana caused a higher degree of suppression to PPO activation response of H. armigera. These assays suggested that the increased virulence of the transgenic fungi is due to the increased ability to suppress the host insect's immune response. Moreover, colony growth, conidia yield, and germination assays revealed that the expression of Egf1.0 in B. bassiana had no effect on its growth and development. In conclusion, the expression of Egf1.0 can significantly enhance the pathogenicity of B. bassiana against host insects.

Virulence Factors of the Entomopathogenic Genus Metarhizium

The fungal genus Metarhizium has been used as an entomopathogen worldwide for approximately 140 years, and its mechanism of infection and its virulence factors have been studied. The present review is a compilation of virulence factors described in the literature to date and their participation in specific stages of the infection process.

Disruption of an adenylate-forming reductase required for conidiation, increases virulence of the insect pathogenic fungus Metarhizium acridum by enhancing cuticle invasion

BACKGROUND

Metarhizium acridum, is a specific acridid pathogen developed for use against the migratory locust (Locusta migratoria manilensis). Adenylate-forming reductases (AFRs) include enzymes that are involved in natural product biosynthesis. Here, we genetically characterize the functions of a class IV AFR in M. acridum (MaAfr^IV ) on fungal development and virulence.

RESULTS

Gene expression analyses indicated MaAfr^IV was induced on locust wings early during the infection process. Surprisingly, loss of MaAfr^IV increased virulence (25.20% decrease in the median lethal time) against the locust in topical bioassays but was no different than the wild type when the cuticle was bypassed by direct infection of conidia into the insect hemocoel. Virulence markers including protease (Pr1) expression and appressorial turgor pressure were higher in the mutant than the parent strain. No difference was seen in the expression of host immune genes (Toll pathway) or in polyphenol oxidase (PPO) activity in locusts infected by the ΔMaAfr^IV or wild type strains. However, the ΔMaAfr^IV strain was unable to successfully sporulate on dead cadavers.

CONCLUSION

Disruption of MaAfr^IV increased fungal virulence by promoting insect cuticle invasion without altering host immune response or fungal immune evasion. Although loss of MaAfr^IV conferred an apparent benefit to the fungus in terms of enhanced virulence, a significant trade-off was seen in the inability of the fungus to sporulate on the cadaver. As conidiation on the cadaver is essential for subsequent propagation in the environment, loss of MaAfr^IV can reduce the engineering strains survivability in the field and improve the safety. © 2019 Society of Chemical Industry.

A review of progress toward field application of transgenic mosquitocidal entomopathogenic fungi

In Africa, adult mosquito populations are primarily controlled with insecticide-impregnated bed nets and residual insecticide sprays. This coupled with widespread applications of pesticides in agriculture has led to increasing insecticide resistance in mosquito populations. We have developed multiple alternative strategies for exploiting transgenic Metarhizium spp. directed at: (i) shortening the lifespan of adult mosquitoes; (ii) reducing transmission potential of Plasmodium spp.; (iii) reducing vector competence via pre-lethal effects. The present challenge is to convert this promising strategy into a validated public health intervention by resolving outstanding issues related to the release of genetically modified organisms. © 2019 Society of Chemical Industry.

Pathogenicity and in vivo Development of Metarhizium rileyi Against Spodoptera litura (Lepidoptera: Noctuidae) Larvae

Metarhizium rileyi, a well-known entomopathogenic fungus, could open up new vistas in biological control of insect pests; however, due to its intrinsic shortcomings, such as long pathogenic process, its application is largely limited. To explore which process, the invasion or the following in vivo development, is the main factor responsible for the long pathogenic process, the lethal effect of M. rileyi against Spodoptera litura (Fabricius) was determined by conidial topical application and hyphae body injection, and the host immune response was also monitored. Results showed when larvae were inoculated by conidial topical application, the pathogenicity of M. rileyi varied greatly depending on the larval instar and conidia concentration, and LC50 values ranged from 6.24 × 106 to 6.06 × 109 conidia/ml while LT50 values fluctuated from 4.35 to 9.43 d. However, in vivo study showed when hyphal bodies (Hbs) of M. rileyi were injected into host hemocoel, they would not be recognized by the host's immune system as invaders. There were no significant differences in the hemocytes and phenoloxidase activity between the infected and control larvae at the initial 44 h, indicated that the fungus was able to successfully avoid the attack from the cellular and humoral immune systems, therefore, it could multiply freely in the hemocoel. The in vivo development time of M. rileyi tended to remain constant for 2-3 d regardless of the initial inoculated numbers. Considering no detectable defense response was observed during in vivo development, it can be concluded that host nonself-recognition system does not respond to the hemolymph borne-Hbs.

Application of the Scorpion Neurotoxin AaIT against Insect Pests

Androctonus australis Hector insect toxin (AaIT), an insect-selective toxin, was identified in the venom of the scorpion Androctonus australis. The exclusive and specific target of the toxin is the voltage-gated sodium channels of the insect, resulting in fast excitatory paralysis and even death. Because of its strict toxic selectivity and high bioactivity, AaIT has been widely used in experiments exploring pest bio-control. Recombinant expression of AaIT in a baculovirus or a fungus can increase their virulence to insect pests and diseases vectors. Likewise, transgenic plants expressing AaIT have notable anti-insect activity. AaIT is an efficient toxin and has great potential to be used in the development of commercial insecticides.

Expression of Bacillus thuringiensis toxin Cyt2Ba in the entomopathogenic fungus Beauveria bassiana increases its virulence towards Aedes mosquitoes

Background

The entomopathogenic fungus Beauveria bassiana has been widely used to kill mosquito larvae and adults in the laboratory and field. However, its slow action of killing has hampered its widespread application. In our study, the B. bassiana fungus was genetically modified to express the Bacillus thuringiensis (Bt) toxin Cyt2Ba to improve its efficacy in killing mosquitoes.

Methodology/Principal findings

The efficacy of the wild type (WT) of B. bassiana and a transgenic strain expressing Cyt2Ba toxin (Bb-Cyt2Ba) was evaluated against larval and adult Aedes mosquitoes (Aedes aegypti and Aedes albopictus) using insect bioassays. The Bb-Cyt2Ba displayed increased virulence against larval and adult Aedes mosquitoes compared with the WT: for Ae. aegypti adults, the median lethal time (LT₅₀) was decreased by 33% at the concentration of 1× 10⁸ conidia/ml, 19% at 1× 10⁷ conidia/ml and 47% at 1× 10⁶ conidia/ml. The LT₅₀ for Ae. albopictus adults was reduced by 20%, 23% and 29% at the same concentrations, respectively. The LT₅₀ for Ae. aegypti larvae was decreased by 42% at 1× 10⁷ conidia/ml and 25% at 1× 10⁶ conidia/ml, and that for Ae. albopictus larvae was reduced by 33% and 31% at the same concentrations, respectively. In addition, infection with Bb-Cyt2Ba resulted in a dramatic reduction in the fecundity of Aedes mosquitoes.

Conclusions/Significance

In conclusion, our study demonstrated that the virulence of B. bassiana against mosquitoes can be significantly improved by introducing the Bt toxin gene Cyt2Ba into the genome to express the exogenous toxin in the fungus. The transgenic strain Bb-Cyt2Ba significantly reduced the survival and fecundity of Ae. aegypti and Ae. albopictus compared with the WT strain, which suggested that this recombinant B. bassiana has great potential for use in mosquito control.

Mosquito vectors transmit many diseases to humans and animals, causing illness and death and resulting in substantial socio-economic burdens in endemic countries. The control of mosquitoes has almost exclusively relied on the use of chemical insecticides, which has recently led to the broad resistance of important mosquito vectors worldwide. Entomopathogenic fungi, such as Beauveria bassiana, are an important alternative or complement to chemical insecticides. However, the relatively slow action of fungal pathogens in killing mosquitoes, compared with chemical insecticides, has hampered their widespread application. To improve the insecticidal efficacy of the entomopathogen B. bassiana, the fungus was genetically modified to express the Bacillus thuringiensis toxin Cyt2Ba. The mitotically stable transformant (Bb-Cyt2Ba) successfully expressed Cyt2Ba toxin, and the virulence of this strain against adults and larvae of Aedes aegypti and Aedes albopictus mosquitoes was significantly improved. In addition, egg laying was significantly affected by Bb-Cyt2Ba infection. Infection with this fungus resulted in a dramatic reduction in fecundity of the target mosquitoes. Therefore, this recombinant B. bassiana has great potential for use in mosquito control.

RNAi-knockdown of the Locusta migratoria nuclear export factor protein results in insect mortality and alterations in gut microbiome

BACKGROUND

The migratory locust Locusta migratoria is one of the most important agricultural pests worldwide. The nuclear export factor 1 (NXF1) protein plays a crucial role in mediating mRNA transport from the nucleus to the cytoplasm. This study evaluates whether NXF1 could be a potential target for RNAi-mediated pest control of L. migratoria.

RESULTS

We cloned and characterized the nuclear export factor lm-nxf1 of L. migratoria. Lm-nxf1 was expressed in all tissues examined, including head, fat body, hemolymph, trunk, leg and midgut, with high expression observed in the hemolymph and fat body. Injection of lm-nxf1 dsRNA into hemolymph resulted in inhibition of mRNA export in hemocytes, which were used as a target for observing mRNA export. Total hemocyte levels were reduced by ca. 97% in lm-nxf1-dsRNA-treated locusts, and high insect mortality occurred with LT₅₀ = 7.75 day as compared with 18.15 day for gfp-dsRNA-treated controls. Further, the locust intestine became atrophy, and the opportunistic pathogens Enterobacter aerogenes, Klebsiella pneumoniae and Enterobacter asburiae were specifically detected in midgut after lm-nxf1 dsRNA treatment.

CONCLUSIONS

The results reveal that knockdown of the lm-nxf1 gene affects the survival of L. migratoria, indicating that lm-nxf1 is a potential target for RNAi-mediated pest control. © 2018 Society of Chemical Industry.

Expression of exogenous dsRNA by Lecanicillium attenuatum enhances its virulence to Dialeurodes citri

BACKGROUND

Dialeurodes citri is an important pest in citrus-producing areas of the world. Lecanicillium attenuatum parasitizes D. citri and kills it, suggesting a potential approach for the biological control of pests. However, the low virulence of the fungus and its slow rate of killing have limited its commercial competitiveness. The objective reason for these disadvantages is immunological rejection by the host. Our strategy was to use fungi to express the double-stranded RNA (dsRNA) of the host immune genes. The fungal hyphae release siRNA at the time of infection, thus interfering with the expression of immune genes in the host and facilitating fungal invasion.

RESULTS

We selected prophenoloxidase (DcPPO), prophenoloxidase-activating factor (DcPPO-AF), and lysozyme (DcLZM) as target genes to construct intron-splicing hairpin RNA expression vectors and to successfully obtain transgenic fungi. Two days after infection, the immune genes of D. citri showed varying degrees of silencing compared with those in the positive control group. The median lethal concentration (LC₅₀ ; spores mL^-1 ) values of La::GFP, La::DcPPO, La::DcPPO-AF, and La::DcLZM were 9.63 × 10⁴ , 2.66 × 10⁴ , 1.21 × 10⁵ , and 3.31 × 10⁴ , respectively. The 50% lethal time (LT₅₀ ) values of these fungi were 5.15, 3.60, 5.34, and 4.04 days, respectively. The virulence of La::DcPPO and La::DcLZM increased 3.62- and 2.91-fold, respectively, and their LT₅₀ decreased by 30.10% and 21.55%, respectively.

CONCLUSIONS

The results indicate that this method, which uses tens of thousands of hyphae to inject dsRNA to improve the virulence of transgenic fungi, can play a greater role in the prevention and control of pests in the future. © 2018 Society of Chemical Industry.

Truncated, strong inducible promoter Pmcl1 from Metarhizium anisopliae

In this study, Metarhizium collagen -like protein (MCL1) promoter from Metarhizium anisopliae was analysed and truncated into different sizes through series of targeted and random deletions based on the presence of various transcription factor-binding sites. Synthetic Green Fluorescent Protein (sGFP) was being utilized as a reporter gene to study the relative expression driving capability of unmodified and truncated promoters. Conserved promoter sequence analysis revealed similarity between the paralogous promoters from M. brunneum and M. acridum. sGFP expression in the haemolymph was directed with the help of mcl1 signal peptide sequence. Deleting the promoter region from - 2764 to - 1583 bp increases the promoter mcl1 (Pmcl1) activity by twofolds, while deletions of the regions upstream of - 1150 bp and - 840 bp caused a decrease of sGFP expression level (80% and 70%, respectively). Transcriptional binding sites predicted for the deleted region revealed the loss of upstream repressing sequences such as Matalpha2 along with ROX1 and Rap1 repressor-binding sites located - 2234 bp, - 1754 bp and - 1724 bp from the TSS. Compared with Pmcl1-wild type (2.7 kbp), Pmcl1-1583 bp had a shorter sequence and showed statistically significant expression in M. anisopliae. This study introduces a highly efficient strong inducible promoter for over-expression of target genes in M. anisopliae.

Increased virulence in the locust-specific fungal pathogen Metarhizium acridum expressing dsRNAs targeting the host F1 F0 -ATPase subunit genes

BACKGROUND

Metarhizium acridum is a host-specific fungal pathogen with great potential for locust control. However, the slow killing action of M. acridum has impeded its widespread application. To enhance fungal virulence, we constructed transgenic M. acridum strains that express double-stranded (ds)RNAs targeting the genes of the F₁ F₀ -ATP synthase α and β subunits in Locusta migratoria.

RESULTS

The two host genes were transcriptionally suppressed in L. migratoria nymphs (instar V) infected by RNA interference (RNAi) strains targeting one or two subunit genes of the host ATP synthase, followed by reduced ATPase activity and ATP synthesis. Consequently, the RNAi strain targeting both subunit genes displayed high virulence that was 3.7-fold that in the wild-type strain.

CONCLUSION

Our results demonstrate that dsRNA expression in M. acridum can cause host RNA silencing during infection and greatly enhances the fungal virulence through interference with critical host genes, highlighting a new strategy for augmentation of fungal virulence against insect pests. © 2018 Society of Chemical Industry.

Transgenic Metarhizium pingshaense synergistically ameliorates pyrethroid-resistance in wild-caught, malaria-vector mosquitoes

Transgenic Metarhizium pingshaense expressing the spider neurotoxin Hybrid (Met-Hybrid) kill mosquitoes faster and at lower spore doses than wild-type strains. In this study, we demonstrate that this approach dovetails with the cornerstone of current malaria control: pyrethroid-insecticides, which are the cornerstone of current malaria control. We used World Health Organization (WHO) tubes, to compare the impact on insecticide resistance of Met-Hybrid with red fluorescent M. pingshaense (Met-RFP), used as a proxy for the wild-type fungus. Insecticides killed less than 20% of Anopheles coluzzii and Anopheles gambiae s.s. mosquitoes collected in a malaria endemic region of Burkina Faso where pyrethroid use is common. Seven days post-infection, mortality for insecticide-sensitive and resistant mosquitoes averaged 94% with Met-Hybrid and 64% with Met-RFP, with LT80 values of 5.32±0.199 days and 7.76±0.183 days, respectively. Eighty nine percent of insecticide-resistant mosquitoes exposed to permethrin five days post-infection with Met-Hybrid died within 24 hours: only 22% died from Met-Hybrid alone over this 24-hour period. Compared to Met-RFP, Met-Hybrid also significantly reduced flight capacity of mosquitoes 3 to 5 days post-infection. Based on WHOPES phase I laboratory susceptibility bioassays, transgenic Met-Hybrid provides effective biological control for adult African malaria vectors that may be used to synergistically manage insecticide resistance with current methods.

High-level expression and purification of active scorpion long-chain neurotoxin BjαIT from Pichia pastoris

As an insect-selective neurotoxin, scorpion long-chain BjαIT is a promising prospect for insecticidal application; however, the difficulty of obtaining natural BjαIT represents the major obstacle preventing analysis of its insecticidal activity against agricultural insect pests. Here, we screened recombinant Pichia pastoris transformants showing high levels of secretory recombinant (r)BjαIT. Secreted rBjαIT was expressed at levels as high as 340 mg/L following methanol induction in a fed-batch reactor, with ∼21 mg of pure rBjαIT obtained from 200-mL fed-batch culture supernatant by Ni²⁺-nitriloacetic acid affinity chromatography and CM Sepharose ion-exchange chromatography. Injection of purified rBjαIT induced neurotoxicity symptoms in locust (Locusta migratoria) larvae, and the half-lethal dose of rBjαIT for locusts at 24-h post-injection ranged from 11 to 14 μg/g body weight. These results demonstrated that large amounts of active rBjαIT were efficiently prepared from P. pastoris, suggesting this system as efficacious for determining rBjαIT insecticidal activity against other agricultural insect pests.

Improving the secretory expression of active recombinant AaIT in Pichia pastoris by changing the expression strain and plasmid

Scorpion long-chain insect selective neurotoxin AaIT has the potential to be used against agricultural insect pests. However, there is still a lack of a heterologous gene expression system that can express AaIT efficiently. Here, using X33 as the host strain and pPICZαA as the expression vector, one transformant had the highest expression of recombinant AaIT (rAaIT) was obtained, and secreted as high as 240 mg/l rAaIT in fed-batch fermentation. Secretory rAaIT was purified by Ni²⁺-nitriloacetic affinity and CM chromatography, and 8 mg of high purity rAaIT were purified from 200 ml fed-batch fermentation cultures. Injecting silkworm (Bombyx mori Linnaeus) and Galleria mellonella larvae with rAaIT resulted in obvious neurotoxin symptoms and led to death. These results demonstrate that a large amount of anti-insect active rAaIT could be prepared efficiently.

Purification and Characterization of a Novel Insecticidal Toxin, μ-sparatoxin-Hv2, from the Venom of the Spider Heteropoda venatoria

The venom of the spider Heteropoda venatoria produced lethal effect to cockroaches as reported in our previous study, and could be a resource for naturally-occurring insecticides. The present study characterized a novel cockroach voltage-gated sodium channels (Na_Vs) antagonist, μ-sparatoxin-Hv2 (μ-SPRTX-Hv2 for short), from this venom. μ-SPRTX-Hv2 is composed of 37 amino acids and contains six conserved cysteines. We synthesized the toxin by using the chemical synthesis method. The toxin was lethal to cockroaches when intraperitoneally injected, with a LD₅₀ value of 2.8 nmol/g of body weight. Electrophysiological data showed that the toxin potently blocked Na_Vs in cockroach dorsal unpaired median (DUM) neurons, with an IC₅₀ of 833.7 ± 132.2 nM, but it hardly affected the DUM voltage-gated potassium channels (K_Vs) and the DUM high-voltage-activated calcium channels (HVA Ca_Vs). The toxin also did not affect Na_Vs, HVA Ca_Vs, and Kvs in rat dorsal root ganglion (DRG) neurons, as well as Na_V subtypes Na_V1.3–1.5, Na_V1.7, and Na_V1.8. No envenomation symptoms were observed when μ-SPRTX-Hv2 was intraperitoneally injected into mouse at the dose of 7.0 μg/g. In summary, μ-SPRTX-Hv2 is a novel insecticidal toxin from H. venatoria venom. It might exhibit its effect by blocking the insect Na_Vs and is a candidate for developing bioinsecticide.

Comparative Genomics Reveals the Core Gene Toolbox for the Fungus-Insect Symbiosis

Modern genomics has shed light on many entomopathogenic fungi and expanded our knowledge widely; however, little is known about the genomic features of the insect-commensal fungi. Harpellales are obligate commensals living in the digestive tracts of disease-bearing insects (black flies, midges, and mosquitoes). In this study, we produced and annotated whole-genome sequences of nine Harpellales taxa and conducted the first comparative analyses to infer the genomic diversity within the members of the Harpellales. The genomes of the insect gut fungi feature low (26% to 37%) GC content and large genome size variations (25 to 102 Mb). Further comparisons with insect-pathogenic fungi (from both Ascomycota and Zoopagomycota), as well as with free-living relatives (as negative controls), helped to identify a gene toolbox that is essential to the fungus-insect symbiosis. The results not only narrow the genomic scope of fungus-insect interactions from several thousands to eight core players but also distinguish host invasion strategies employed by insect pathogens and commensals. The genomic content suggests that insect commensal fungi rely mostly on adhesion protein anchors that target digestive system, while entomopathogenic fungi have higher numbers of transmembrane helices, signal peptides, and pathogen-host interaction (PHI) genes across the whole genome and enrich genes as well as functional domains to inactivate the host inflammation system and suppress the host defense. Phylogenomic analyses have revealed that genome sizes of Harpellales fungi vary among lineages with an integer-multiple pattern, which implies that ancient genome duplications may have occurred within the gut of insects.

Insect guts harbor various microbes that are important for host digestion, immune response, and disease dispersal in certain cases. Bacteria, which are among the primary endosymbionts, have been studied extensively. However, fungi, which are also frequently encountered, are poorly known with respect to their biology within the insect guts. To understand the genomic features and related biology, we produced the whole-genome sequences of nine gut commensal fungi from disease-bearing insects (black flies, midges, and mosquitoes). The results show that insect gut fungi tend to have low GC content across their genomes. By comparing these commensals with entomopathogenic and free-living fungi that have available genome sequences, we found a universal core gene toolbox that is unique and thus potentially important for the insect-fungus symbiosis. This comparative work also uncovered different host invasion strategies employed by insect pathogens and commensals, as well as a model system to study ancient fungal genome duplication within the gut of insects.

Genetically engineering better fungal biopesticides

Microbial insect pathogens offer an alternative means of pest control with the potential to wean us off our heavy reliance on chemical pesticides. Insect pathogenic fungi play an important natural role in controlling disease vectors and agricultural pests. Most commercial products employ Ascomycetes in the genera Metarhizium and Beauveria. However, their utilization has been limited by inconsistent field results as a consequence of sensitivity to abiotic stresses and naturally low virulence. Other naturally occurring biocontrol agents also face these hurdles to successful application, but the availability of complete genomes and recombinant DNA technologies have facilitated design of multiple fungal pathogens with enhanced virulence and stress resistance. Many natural and synthetic genes have been inserted into entomopathogen genomes. Some of the biggest gains in virulence have been obtained using genes encoding neurotoxic peptides, peptides that manipulate host physiology and proteases and chitinases that degrade the insect cuticle. Prokaryotes, particularly extremophiles, are useful sources of genes for improving entomopathogen resistance to ultraviolet (UV) radiation. These biological insecticides are environmentally friendly and cost-effective insect pest control options. © 2017 Society of Chemical Industry.

Native entomopathogenic Metarhizium spp. from Burkina Faso and their virulence against the malaria vector Anopheles coluzzii and non-target insects

Background

Genetically enhanced Metarhizium pingshaense are being developed for malaria vector control in Burkina Faso. However, not much is known about the local prevalence and pathogenicity of this fungus, so we prospected mosquitoes and plant roots (a common habitat for Metarhizium spp.) for entomopathogenic fungi.

Results

Our investigations showed that Metarhizium spp. represented between 29–74% of fungi isolated from plant root rhizospheres in diverse collection sites. At low spore dosages (1 × 10⁶ conidia/ml), two mosquito-derived M. pingshaense isolates (Met_S26 and Met_S10) showed greater virulence against Anopheles coluzzii (LT₈₀ of ~7 days) than isolates tested in previous studies (LT₈₀ of ~10 days). In addition, the local isolates did not cause disease in non-target insects (honeybees and cockroaches).

Conclusions

Our work provides promising findings for isolating local Metarhizium strains for application in mosquito biological control and for future transgenic biocontrol strategies in Burkina Faso.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2796-6) contains supplementary material, which is available to authorized users.

Genes involved in Beauveria bassiana infection to Galleria mellonella

The ascomycete fungus Beauveria bassiana is a natural pathogen of hundreds of insect species and is commercially produced as an environmentally friendly mycoinsecticide. Many genes involved in fungal insecticide infection have been identified but few have been further explored. In this study, we constructed three transcriptomes of B. bassiana at 24, 48 and 72 h post infection of insect pests (BbI) or control (BbC). There were 3148, 3613 and 4922 genes differentially expressed at 24, 48 and 72 h post BbI/BbC infection, respectively. A large number of genes and pathways involved in infection were identified. To further analyze those genes, expression patterns across different infection stages (0, 12, 24, 36, 48, 60, 72 and 84 h) were studied using quantitative RT-PCR. This analysis showed that the infection-related genes could be divided into four patterns: highly expressed throughout the whole infection process (thioredoxin 1); highly expressed during early stages of infection but lowly expressed after the insect death (adhesin protein Mad1); lowly expressed during early infection but highly expressed after insect death (cation transporter, OpS13); or lowly expressed across the entire infection process (catalase protein). The data provide novel insights into the insect-pathogen interaction and help to uncover the molecular mechanisms involved in fungal infection of insect pests.

Scorpion neurotoxin AaIT-expressing Beauveria bassiana enhances the virulence against Aedes albopictus mosquitoes

To improve the insecticidal efficacy of this entomopathogen Beauveria bassiana, the fungus was genetically modified to express an insect-specific scorpion neurotoxin AaIT. The virulence of the recombinant B. bassiana strain (Bb-AaIT) against Aedes albopictus adults (which occurs via penetration through the cuticle during spore germination or by conidia ingestion), and the larvae (by conidia ingestion) was measured with bioassays. The median lethal concentration (LC50) of Bb-AaIT against A. albopictus larvae was 313.3-fold lower on day 4 and 11.3-fold lower on day 10 than that of the wild type (WT). Through conidia feeding or body contact, Bb-AaIT killed 50% of adult female mosquitoes at 3.9- or 1.9-fold reduced concentrations on day 4 and at 2.1- or 2.4-fold reduced concentrations on day 10. Compared with the results for the WT, the median lethal time (LT50) of Bb-AaIT was reduced by 28.6% at 1 × 107 conidia ml-1 and 34.3% at 1 × 106 conidia ml-1 in the larvae bioassay by conidia ingestion, while it decreased 32.3% at 1 × 107 conidia ml-1 by conidia ingestion and 24.2% at 1 × 108 conidia ml-1 by penetrating through the cuticle in the adult bioassay. All the differences were significant. Our findings indicated that Bb-AaIT had higher virulence and faster action than the WT in killing the larval and adult mosquitoes, and therefore, it is valuable for development as a commercial mosquito pesticide.

The Insect Pathogens

Fungi are the most common disease-causing agents of insects; aside from playing a crucial role in natural ecosystems, insect-killing fungi are being used as alternatives to chemical insecticides and as resources for biotechnology and pharmaceuticals. Some common experimentally tractable genera, such as Metarhizium spp., exemplify genetic diversity and dispersal because they contain numerous intraspecific variants with distinct environmental and insect host ranges. The availability of tools for molecular genetics and multiple sequenced genomes has made these fungi ideal experimental models for answering basic questions on the genetic and genomic processes behind adaptive phenotypes. For example, comparative genomics of entomopathogenic fungi has shown they exhibit diverse reproductive modes that often determine rates and patterns of genome evolution and are linked as cause or effect with pathogenic strategies. Fungal-insect pathogens represent lifestyle adaptations that evolved numerous times, and there are significant differences in host range and pathogenic strategies between the major groups. However, typically, spores landing on the cuticle produce appressoria and infection pegs that breach the cuticle using mechanical pressure and cuticle-degrading enzymes. Once inside the insect body cavity, fungal pathogens face a potent and comprehensively studied immune defense by which the host attempts to eliminate or reduce an infection. The Fungal Kingdom stands alone in the range, extent, and complexity of their manipulation of arthropod behavior. In part, this is because most only sporulate on cadavers, so they must ensure the dying host positions itself to allow efficient transmission.

Improved efficacy of an arthropod toxin expressing fungus against insecticide-resistant malaria-vector mosquitoes

The continued success of malaria control efforts requires the development, study and implementation of new technologies that circumvent insecticide resistance. We previously demonstrated that fungal pathogens can provide an effective delivery system for mosquitocidal or malariacidal biomolecules. Here we compared genes from arthropod predators encoding insect specific sodium, potassium and calcium channel blockers for their ability to improve the efficacy of Metarhizium against wild-caught, insecticide-resistant anophelines. Toxins expressed under control of a hemolymph-specific promoter increased fungal lethality to mosquitoes at spore dosages as low as one conidium per mosquito. One of the most potent, the EPA approved Hybrid (Ca⁺⁺/K⁺ channel blocker), was studied for pre-lethal effects. These included reduced blood feeding behavior, with almost 100% of insects infected with ~6 spores unable to transmit malaria within 5 days post-infection, surpassing the World Health Organization threshold for successful vector control agents. Furthermore, recombinant strains co-expressing Hybrid toxin and AaIT (Na⁺ channel blocker) produced synergistic effects, requiring 45% fewer spores to kill half of the mosquitoes in 5 days as single toxin strains. Our results identify a repertoire of toxins with different modes of action that improve the utility of entomopathogens as a technology that is compatible with existing insecticide-based control methods.

Mode of Infection of Metarhizium spp. Fungus and Their Potential as Biological Control Agents

Chemical insecticides have been commonly used to control agricultural pests, termites, and biological vectors such as mosquitoes and ticks. However, the harmful impacts of toxic chemical insecticides on the environment, the development of resistance in pests and vectors towards chemical insecticides, and public concern have driven extensive research for alternatives, especially biological control agents such as fungus and bacteria. In this review, the mode of infection of Metarhizium fungus on both terrestrial and aquatic insect larvae and how these interactions have been widely employed will be outlined. The potential uses of Metarhizium anisopliae and Metarhizium acridum biological control agents and molecular approaches to increase their virulence will be discussed.

DNA methyltransferases contribute to the fungal development, stress tolerance and virulence of the entomopathogenic fungus Metarhizium robertsii

DNA methylation is an important epigenetic mark in mammals, plants, and fungi and depends on multiple genetic pathways involving de novo and maintenance DNA methyltransferases (DNMTases). Metarhizium robertsii, a model system for investigating insect-fungus interactions, has been used as an environmentally friendly alternative to chemical insecticides. However, little is known concerning the molecular basis for DNA methylation. Here, we report on the roles of two DNMTases (MrRID and MrDIM-2) by characterizing ΔMrRID, ΔMrDIM-2, and ΔRID/ΔDIM-2 mutants. The results showed that approximately 71, 10, and 8% of ^mC sites remained in the ΔMrRID, ΔMrDIM-2, and ΔRID/ΔDIM-2 strains, respectively, compared with the wild-type (WT) strain. Further analysis showed that MrRID regulates the specificity of DNA methylation and MrDIM-2 is responsible for most DNA methylation, implying an interaction or cooperation between MrRID and MrDIM-2 for DNA methylation. Moreover, the ΔMrDIM-2 and ΔRID/ΔDIM-2 strains showed more defects in radial growth and conidial production compared to the WT. Under ultraviolet (UV) irradiation or heat stress, an obvious reduction in spore viability was observed for all the mutant strains compared to the WT. The spore median lethal times (LT₅₀s) for the ΔMrDIM-2 and ΔRID/ΔDIM-2 strains in the greater wax moth, Galleria mellonella, were decreased by 47.7 and 65.9%, respectively, which showed that MrDIM-2 is required for full fungal virulence. Our data advances the understanding of the function of DNMTase in entomopathogenic fungi, which should contribute to future epigenetic investigations in fungi.