Integration of an insecticidal scorpion toxin (BjαIT) gene into Metarhizium acridum enhances fungal virulence towards Locusta migratoria manilensis

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.

Incidence of putative RNA mycoviruses in entomopathogenic fungi in Korea

Entomopathogenic fungi have potential as biocontrol agents against insect pests, and mycovirus-mediated hypervirulence may enhance their efficacy. Before initiating research on hypervirulence, the presence or absence of double-stranded (ds) RNA elements was determined in 94 Korean entomopathogenic fungi. dsRNA elements varying in size from ca. 0.8 to 7 kbp were found in 14.9% (14/94) of the strains examined, including Beauveria bassiana, Metarhizium pemphigi, M. pinghaense, M. rileyi, and Cordyceps fumosorosea. This study provides information on the incidence and electrophoretic banding patterns of dsRNA elements and is the first report of mycoviruses entomopathogenic fungi in Korea.

Enhancing the Biocontrol Potential of the Entomopathogenic Fungus in Multiple Respects via the Overexpression of a Transcription Factor Gene MaSom1

Entomopathogenic fungi play important roles in the control of populations of agricultural and disease vector pests in nature. The shortcomings of mycoinsecticides for pest management in the field cannot be completely overcome by improving single biocontrol properties of fungi. Therefore, enhancing the biocontrol potential of entomopathogenic fungi in multiple respects by genetic engineering is desirable. Transcription factors are usually involved in various important processes during fungal growth and pathogenesis via regulating a series of genes, and are important candidates for fungal improvement via genetic engineering. Herein, overexpression of MaSom1, a key transcription factor gene in the cAMP/PKA pathway, improves the biocontrol traits of Metarhizium acridum in multiple respects. When compared with WT, the MaSom1-overexpression strains exhibit enhanced tolerances to UV-B and heat shock, with increased mean 50% inhibition times by 66.9% and 155.2%, respectively. Advanced conidiation emerged accompanied by increased conidial yield up to 3.89 times after 3-day incubation for the MaSom1-overexpression strains compared to WT. Furthermore, when compared with WT, the virulence of the MaSom1-overexpression strains was also increased with the mean 50% lethality times reduced by 21.8% to 23.8%. Taken together, the MaSom1-overexpression improved the biocontrol potential of M. acridum in multiple respects. Our results provide insights into the application of key transcription factors for genetic engineering and offer a credible way to further improve the biocontrol potential of entomopathogenic fungi.

Genome-Wide Identification and Analysis of Nilaparvata lugens microRNAs during Challenge with the Entomopathogenic Fungus Metarhizium anisopliae

The resistance of the notorious rice pest Nilaparvata lugens to many insecticides has caused significant concerns. Our previous study demonstrated that the fungus Metarhizium anisopliae CQMa421 shows great potential for the control of this pest, but the interactions between them are still unclear. Thus, we further investigated fungal infection-related microRNAs (miRNAs) in N. lugens during M. anisopliae CQMa421 challenge using Illumina sequencing. In this study, we constructed twenty-four small RNA libraries over different time courses (i.e., 4 h, 8 h, 16 h, and 24 h). A total of 478.62 M clean reads were collected, with each sample producing more than 13.37 M reads, after the removal of low-quality reads. We identified 2324 miRNAs and their 11,076 target genes within the twenty-four libraries by bioinformatics analysis. Differentially expressed miRNAs (DEmiRNAs), including 58 (32 upregulated vs. 26 downregulated), 62 (30 upregulated vs. 32 downregulated), 126 (71 upregulated vs. 55 downregulated), and 109 (40 upregulated vs. 69 downregulated) DEmiRNAs were identified at 4 h, 8 h, 16 h, and 24 h post-infection, respectively. We further conducted Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis to predict the functions of all target genes of DEmiRNAs. These DEmiRNAs targets identified during 24 h of infection were primarily involved in energy metabolism, lysine degradation, the FoxO signaling pathway, ubiquitin-mediated proteolysis, the mRNA surveillance pathway, and the MAPK signaling pathway. Taken together, our results provide essential information for further study of the interactions between the entomopathogenic fungus M. anisopliae and N. lugens at the posttranscriptional level.

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.

Inhibitor of apoptosis-1 gene as a potential target for pest control and its involvement in immune regulation during fungal infection

BACKGROUND

Environmentally friendly insect management technologies, including RNA interference (RNAi) and entomopathogenic fungi, have attracted increasing attention as options for pest control. Here, we sought to extend RNAi-directed targeting of the inhibitor of apoptosis protein 1 (IAP1) gene to the locust, and to examine its relationship to immune responses and susceptibility to Metarhizium acridum, a locust-specific fungal pathogen.

RESULTS

Expression of the locust LmIAP gene was induced in the hemolymph and fat body after M. acridum infection. RNAi-directed silencing of locust LmIAP1 resulted in increased caspase 3 activity, degeneration of the gut and dose-dependent mortality. Synergistic mortality was seen in RNAi-LmIAP/fungal co-infection experiments with median survival time (MST) values decreasing from ∼ 5 days for RNAi and M. acridum treatments alone, to 2.6 days for co-treatments. Reduced hemocyte numbers and antimicrobial peptide levels were seen in co-treated locusts, with changes in gut opportunistic pathogenic bacteria seen between treatments. Enhanced fungal sporulation on co-treated insect cadavers was also compared with fungal infection alone.

CONCLUSIONS

Silencing of the locust LmIAP1 gene results in direct mortality and increases insect susceptibility to insect fungal pathogens, in part by decreasing immunity and altering the gut microbiome. © 2019 Society of Chemical Industry.

Transcriptomic Analysis of the Brown Planthopper, Nilaparvata lugens, at Different Stages after Metarhizium anisopliae Challenge

Nilaparvata lugens is one of the major pests of rice and results in substantial yield loss every year. Our previous study found that the entomopathogenic fungus Metarhizium anisopliae showed effective potential for controlling this pest. However, the mechanisms underlying M. anisopliae infection of N. lugens are not well known. In the present study, we further examined the transcriptome of N. lugens at 4 h, 8 h, 16 h, and 24 h after M. anisopliae infection by Illumina deep sequencing. In total, 174.17 Gb of data was collected after sequencing, from which 23,398 unigenes were annotated by various databases, including 3694 newly annotated genes. The results showed that there were 246 vs 75, 275 vs 586, 378 vs 1055, and 638 vs 182 up- and downregulated differentially expressed genes (DEGs) at 4 h, 8 h, 16 h, and 24 h after M. anisopliae infection, respectively. The biological functions and associated metabolic processes of these genes were determined with the Clusters of Orthologous Groups (COG), Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. The DEGs data were verified using RT-qPCR. These results indicated that the DEGs during the initial fungal infection appropriately reflected the time course of the response to the fungal infection. Taken together, the results of this study provide new insights into the molecular mechanisms underlying the insect host response to fungal infection, especially during the initial stage of infection, and may improve the potential control strategies for N. lugens.

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.

Effects of the Entomopathogenic Fungus Metarhizium anisopliae on the Mortality and Immune Response of Locusta migratoria

Entomopathogenic fungi are the key regulators of insect populations and some of them are important biological agents used in integrated pest management strategies. Compared with their ability to become resistant to insecticides, insect pests do not easily become resistant to the infection by entomopathogenic fungi. In this study, we evaluated the mortality and immune response of the serious crop pest Locusta migratoria manilensis after exposure to a new entomopathogenic fungus strain, Metarhizium anisopliae CQMa421. M. anisopliae CQMa421 could effectively infect and kill the L. migratoria adults and nymphs. The locust LT₅₀ under 1 × 10⁸ conidia/mL concentration of M. anisopliae was much lower than that under conidial concentration 1 × 10⁵ conidia/mL (i.e., 6.0 vs. 11.2 and 5.0 vs. 13.8 for adults and nymphs, respectively). The LC₅₀ (log₁₀) of M. anisopliae against locust adults and nymphs after 10 days was 5.2 and 5.6, respectively. Although the number of hemocytes in L. migratoria after exposure to M. anisopliae did not differ with that in the controls, the enzymatic activity of superoxide dismutase (SOD) and prophenoloxidase (ProPO) did differ between the two treatments. The activities of both SOD and ProPO under the M. anisopliae treatment were lower than that in the controls, except for the ProPO activity at 72 h and the SOD activity at 96 h. Further, the expression of the L. migratoria immune-related genes defensin, spaetzle, and attacin differed after exposure to M. anisopliae for 24 h to 96 h. Taken together, this study indicated that infection with M. anisopliae CQMa421 could cause the death of L. migratoria by interacting with the immune responses of the host, demonstrating that this fungal strain of M. anisopliae can be an efficient biocontrol agent against L. migratoria.

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.

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.

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.

High-yield production of spider short-chain insecticidal neurotoxin Tx4(6-1) in Pichia pastoris and bioactivity assays in vivo

Short-chain insecticidal neurotoxin Tx4(6-1) from the spider Phoneutria nigriventer can be prepared by reversed-phase high-performance liquid-chromatography (HPLC) fractionation of PhTx4, but this is difficult and represents an obstacle preventing analyses of its insecticidal activity against agricultural insect pests. Herein, we performed secretory expression of recombinant Tx4(6-1) using Pichia pastoris strain X33 as the host, and screened transformants using enzyme-linked immunosorbent assay (ELISA). In flasks, ∼5 mg/l rTx4(6-1) was expressed as a secreted protein following induction with methanol, and this was increased to 45 mg/l rTx4(6-1) in a fed-batch reactor. Approximately 4 mg of high-purity rTx4(6-1) was purified from a 400 ml fed-batch culture supernatant by Ni⁺-nitriloacetic acid affinity chromatography, followed by carboxymethyl (CM) sepharose ion-exchange chromatography. Purified rTx4(6-1) was determined by mass spectrometry (MS) analysis, revealing a molecular weight (MW) of 7660.5 Da, larger than the expected size due to O-linked glycosylation. Insect bioactivity tests of rTx4(6-1)-treated fifth-instar silkworm larvae (Bombyx mori Linnaeus) showed neurotoxin symptoms such as contraction paralysis, abdominal contraction, and mouth movement syndrome, with a half lethal dose at 12 h post-injection of ∼4.5-8.5 μg/g body weight. Dietary toxicity was not observed in silkworm larvae.

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.

The interplay between dose and immune system activation determines fungal infection outcome in the African malaria mosquito, Anopheles gambiae

The Toll pathway is a central regulator of antifungal immunity in insects. In mosquitoes, the Toll pathway affects infections with the fungal entomopathogen, Beauveria bassiana, which is considered a potential mosquito biopesticide. We report here the use of B. bassiana strain I93-825 in Anopheles gambiae to analyze the impact of Toll pathway modulation on mosquito survival. Exposure to a narrow dose range of conidia by direct contact decreased mosquito longevity and median survival. In addition, fungal exposure dose correlated positively and linearly with hazard ratio. Increased Toll signaling by knockdown of its inhibitor, cactus, decreased survivorship of uninfected females, increased mosquito survival after low dose B. bassiana exposure, but had little effect following exposure to higher doses. This observed trade-off could have implications for development of B. bassiana as a prospective vector control tool. On the one hand, selection for small increases in mosquito immune signaling across a narrow dose range could impair efficacy of B. bassiana. On the other hand, costs of immunity and the capacity for higher doses of fungus to overwhelm immune responses could limit evolution of resistance.

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.

Recombinant production of the insecticidal scorpion toxin BjαIT in Escherichia coli

Scorpion long-chain insect neurotoxins have important potential application value in agricultural pest control. The difficulty of obtaining natural toxins is the major obstacle preventing analyses of their insecticidal activity against more agricultural insect pests. Here we cloned the insect neurotoxin BjαIT gene into the pET32 expression vector and expressed the resulting thioredoxin (Trx)-BjαIT fusion protein in Escherichia coli. Soluble Trx-BjαIT was expressed at a high level when induced at 18 °C with 0.1 mM isopropyl β-d-1-thiogalactopyranoside, and it was purified by Ni²⁺-nitriloacetic acid affinity chromatography. After cleaving the Trx tag with recombinant enterokinase, the digestion products were purified by CM Sepharose FF ion-exchange chromatography, and 1.5 mg of purified recombinant BjαIT (rBjαIT) was obtained from 100 ml of induced bacterial cells. Injecting rBjαIT induced obvious neurotoxic symptoms and led to death in locust (Locusta migratoria) larvae. Dietary toxicity was not observed in locusts. The results demonstrate that active rBjαIT could be obtained efficiently from an E. coli expression system, which is helpful for determining its insecticidal activity against agricultural insect pests.

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.

Establishment of polyethylene-glycol-mediated protoplast transformation for Lecanicillium lecanii and development of virulence-enhanced strains against Aphis gossypii

BACKGROUND

Lecanicillium lecanii has been developed as a biopesticide and used in biological control of several agricultural insects. To improve fungal virulence, an optimised polyethylene glycol (PEG)-mediated protoplast transformation system was established for L. lecanii. Pr1A-like cuticle-degrading protease gene (Cdep1) from Beauveria bassiana was transferred into L. lecanii, and its resulting activity against Aphis gossypii was assessed.

RESULTS

The optimised protoplast generation yielded 2.5 × 10(8) protoplasts g(-1) wet mycelium of fungi, and gave nearly 98% viability and 80% regeneration on plates. Protease activities were increased about fivefold in transformants expressing CDEP1. The median lethal concentration (LC50 ) for transformants expressing CDEP1 was twofold lower than that for the wild type (WT). The median survival time (LT50 ) for transformants expressing CDEP1 was also 14.2% shorter than that for WT, though no significant difference. There were no significant differences in conidial germination as colony growth and conidial yield on plates between transformants expressing CDEP1 and WT. The transformants expressing CDEP1 grew significantly quicker than WT in insects. The transformants expressing CDEP1 were lower in conidial yields on insect cadavers, but insignificantly different from WT.

CONCLUSION

The PEG-mediated protoplast transformation system was effective for L. lecanii, and the expression of CDEP1 significantly enhanced fungal virulence against cotton aphids. © 2016 Society of Chemical Industry.

Entomopathogenic Fungi: New Insights into Host-Pathogen Interactions

Although many insects successfully live in dangerous environments exposed to diverse communities of microbes, they are often exploited and killed by specialist pathogens. Studies of host-pathogen interactions (HPI) provide valuable insights into the dynamics of the highly aggressive coevolutionary arms race between entomopathogenic fungi (EPF) and their arthropod hosts. The host defenses are designed to exclude the pathogen or mitigate the damage inflicted while the pathogen responds with immune evasion and utilization of host resources. EPF neutralize their immediate surroundings on the insect integument and benefit from the physiochemical properties of the cuticle and its compounds that exclude competing microbes. EPF also exhibit adaptations aimed at minimizing trauma that can be deleterious to both host and pathogen (eg, melanization of hemolymph), form narrow penetration pegs that alleviate host dehydration and produce blastospores that lack immunogenic sugars/enzymes but facilitate rapid assimilation of hemolymph nutrients. In response, insects deploy an extensive armory of hemocytes and macromolecules, such as lectins and phenoloxidase, that repel, immobilize, and kill EPF. New evidence suggests that immune bioactives work synergistically (eg, lysozyme with antimicrobial peptides) to combat infections. Some proteins, including transferrin and apolipophorin III, also demonstrate multifunctional properties, participating in metabolism, homeostasis, and pathogen recognition. This review discusses the molecular intricacies of these HPI, highlighting the interplay between immunity, stress management, and metabolism. Increased knowledge in this area could enhance the efficacy of EPF, ensuring their future in integrated pest management programs.

Genetically Engineering Entomopathogenic Fungi

Entomopathogenic fungi have been developed as environmentally friendly alternatives to chemical insecticides in biocontrol programs for agricultural pests and vectors of disease. However, mycoinsecticides currently have a small market share due to low virulence and inconsistencies in their performance. Genetic engineering has made it possible to significantly improve the virulence of fungi and their tolerance to adverse conditions. Virulence enhancement has been achieved by engineering fungi to express insect proteins and insecticidal proteins/peptides from insect predators and other insect pathogens, or by overexpressing the pathogen's own genes. Importantly, protein engineering can be used to mix and match functional domains from diverse genes sourced from entomopathogenic fungi and other organisms, producing insecticidal proteins with novel characteristics. Fungal tolerance to abiotic stresses, especially UV radiation, has been greatly improved by introducing into entomopathogens a photoreactivation system from an archaean and pigment synthesis pathways from nonentomopathogenic fungi. Conversely, gene knockout strategies have produced strains with reduced ecological fitness as recipients for genetic engineering to improve virulence; the resulting strains are hypervirulent, but will not persist in the environment. Coupled with their natural insect specificity, safety concerns can also be mitigated by using safe effector proteins with selection marker genes removed after transformation. With the increasing public concern over the continued use of synthetic chemical insecticides and growing public acceptance of genetically modified organisms, new types of biological insecticides produced by genetic engineering offer a range of environmentally friendly options for cost-effective control of insect pests.

Enhancing the utilization of host trehalose by fungal trehalase improves the virulence of fungal insecticide

Entomopathogenic fungi proliferate in insect hemolymph by using host nutrients after penetrating the cuticle. To improve the virulence of the locust specific fungus, Metarhizium acridum, we genetically modified the fungus to overexpress ATM1, an endogenous hydrolase of trehalose, which is the main carbon source in insect hemolymph. Compared with the wild-type strain, Metarhizium acridum overexpressing ATM1 gene secreted more acid trehalase into locust hemolymph. The trehalose concentrations in locusts infected with the ATM1-overexpressing strain were 5.5 and 6.1 mmol/l, lower than that in locusts infected with the wild-type strain at 3 and 5 days post-inoculation, representing 44.5 and 60.7 % reduction, respectively. Correspondingly, overexpressing ATM1 accelerated the growth of Metarhizium acridum in host hemolymph, and the dose causing 50 % mortality (LD50) of the ATM1-overexpressing strain was reduced by 8.3-fold compared with the wild-type strain, suggesting that increasing the utilization of host nutrients by pathogens could be a promising way to improve the virulence of biopesticides based on parasites of pests.

The unfulfilled promises of scorpion insectotoxins

Since the description and biochemical characterization of the first insect-specific neurotoxins from scorpion venoms, almost all contributions have highlighted their potential application as leads for the development of potent bioinsecticides. Their practical use, however, has been hindered by different factors, some of which are intrinsically related to the toxins and other external determinants. Recent developments in the understanding of the action mechanisms of the scorpion insectotoxins and their bioactive surfaces, coupled with the exploration of novel bioinsecticide delivery systems have renewed the expectations that the scorpion insectotoxins could find their way into commercial applications in agriculture, as part of integrated pest control strategies. Herein, we review the current arsenal of available scorpion neurotoxins with a degree of specificity for insects, the progress made with alternative delivery methods, and the drawbacks that still preclude their practical use.