Increased insect virulence in Beauveria bassiana strains overexpressing an engineered chitinase

Expression of a viral ecdysteroid UDP-glucosyltransferase enhanced the insecticidal activity of the insect pathogenic fungus Beauveria bassiana

BACKGROUND

Entomopathogenic fungi, such as Beauveria bassiana, hold promise as biological control agents against insect pests. However, the efficacy of these fungi can be hindered by insect immune responses. One strategy to enhance fungal virulence is to manipulate host immune by targeting key regulatory molecules like 20-hydroxyecdysone (20E).

RESULTS

In this study, we engineered B. bassiana strains to constitutively express the enzyme ecdysteroid UDP-glucosyltransferase (EGT), which inactivates 20E, a crucial insect molting hormone. The engineered strain Bb::EGT-1 exhibited robust expression of EGT, leading to a significant reduction in insect 20E levels upon infection. Moreover, infection with Bb::EGT-1 resulted in accelerated larval mortality. Immune responses analysis revealed repression of insect immune response genes and decreased phenoloxidase (PO) activity in larvae infected with Bb::EGT-1. Microbiome analysis indicated alterations in bacterial composition within infected insects, with increased abundance observed during infection with Bb::EGT-1. Additionally, the presence of bacteria hindered hyphal emergence from insect cadavers, suggesting a role for microbial competition in fungal dissemination.

CONCLUSIONS

Constitutive expression of EGT in B. bassiana enhances fungal virulence by reducing insect 20E levels, suppressing immune responses, and altering the insect microbiome. These findings highlighted the potential of engineered fungi as effective biocontrol agents against insect pests and provide insights into the complex interactions between entomopathogenic fungi, their hosts, and associated microbes. © 2024 Society of Chemical Industry.

A fungal pathogen secretes a cell wall-associated β-N-acetylhexosaminidase that is co-expressed with chitinases to contribute to infection of insects

BACKGROUND

β-N-acetylhexosaminidases (HEXs) are widely distributed in fungi and involved in cell wall chitin metabolism and utilization of chitin-containing substrates. However, details of the fungal pathogens-derived HEXs in the interaction with their hosts remain limited.

RESULTS

An insect nutrients-induced β-N-acetylhexosaminidase, BbHex1, was identified from the entomopathogenic fungus Beauveria bassiana, which was involved in cell wall modification and degradation of insect cuticle. BbHex1 was localized to cell wall and secreted, and displayed enzyme activity to degrade the chitinase-hydrolyzed product (GlcNAc)2. Disruption of BbHex1 resulted in a significant decrease in the level of cell wall chitin in the presence of insect nutrients and during infection of insects, with impaired ability to penetrate insect cuticle, accompanying downregulated cell wall metabolism-involved and cuticle-degrading chitinase genes. However, the opposite phenotypes were examined in the gene overexpression strain. Distinctly altered cell wall structures caused by BbHex1 mutation and overexpression led to the easy activation and evasion (respectively) of insect immune response during fungal infection. As a result, BbHex1 contributed to fungal virulence. Bioinformatics analysis revealed that promoters of some co-expressed chitinase genes with the BbHex1 promoter shared conserved transcription factors Skn7, Msn2 and Ste12, and CreA-binding motifs, implying co-regulation of those genes with BbHex1.

CONCLUSION

These data support a mechanism that the fungal pathogen specifically expresses BbHex1, which is co-expressed with chitinases to modify cell wall for evasion of insect immune recognition and to degrade insect cuticle, and contributes to the fungal virulence against insects. © 2024 Society of Chemical Industry.

Expression profiles and characterization of microRNAs responding to chitin in Arthrobotrys oligospora

Extracellular proteases, such as chitinases secreted by Arthrobotrys oligospora (A. oligospora), play a crucial role in the process of nematode infection. However, post-transcriptional regulation of gene expression involving microRNAs (miRNAs) in A. oligospora remains scarcely described. Hereto, transcriptome sequencing was carried out to analyze the expression profiles of chitin-responsive miRNAs in A. oligospora. Based on the RNA-seq data, the differential expression of miRNAs (DEmiRNAs) in response to chitin was screened, identified and characterized in A. oligospora. Meanwhile, the potential target genes were predicted by the online tools miRanda and Targetscan, respectively. Furthermore, the interaction of DEmiRNA with it's target gene was validated by a dual-luciferase reporter assay system. Among 85 novel miRNAs identified, 25 miRNAs displayed significant differences in expression in A. oligospora in response to chitin. Gene Ontology (GO) analysis showed that the potential genes targeted by DEmiRNAs were enriched in the biological processes such as bio-degradation, extracellular components and cell cycle. KEGG analysis revealed that the target genes were mainly involved in Hippo, carbon and riboflavin metabolic pathway. Outstandingly, chitinase AOL_s00004g379, which is involved in the hydrolysis metabolic pathway of chitin, was confirmed to be a target gene of differential miR_70. These findings suggest that chitin-responsive miRNAs are involved in the regulation of cell proliferation, predator hyphae growth and chitinase expression through the mechanisms of post-transcriptional regulation, which provides a new perspective to the molecular mechanisms underlying miRNAs-mediated control of gene expression in A. oligospora.

A life-and-death struggle: interaction of insects with entomopathogenic fungi across various infection stages

Insects constitute approximately 75% of the world's recognized fauna, with the majority of species considered as pests. Entomopathogenic fungi (EPF) are parasitic microorganisms capable of efficiently infecting insects, rendering them potent biopesticides. In response to infections, insects have evolved diverse defense mechanisms, prompting EPF to develop a variety of strategies to overcome or circumvent host defenses. While the interaction mechanisms between EPF and insects is well established, recent findings underscore that their interplay is more intricate than previously thought, especially evident across different stages of EPF infection. This review primarily focuses on the interplay between EPF and the insect defense strategies, centered around three infection stages: (1) Early infection stage: involving the pre-contact detection and avoidance behavior of EPF in insects, along with the induction of behavioral responses upon contact with the host cuticle; (2) Penetration and intra-hemolymph growth stage: involving the initiation of intricate cellular and humoral immune functions in insects, while symbiotic microbes can further contribute to host resistance; (3) Host insect's death stage: involving the ultimate confrontation between pathogens and insects. Infected insects strive to separate themselves from the healthy population, while pathogens rely on the infected insects to spread to new hosts. Also, we discuss a novel pest management strategy underlying the cooperation between EPF infection and disturbing the insect immune system. By enhancing our understanding of the intricate interplay between EPF and the insect, this review provides novel perspectives for EPF-mediated pest management and developing effective fungal insecticides.

Comparative transcriptome analysis to unveil genes affecting the host cuticle destruction in Metarhizium rileyi

Insect pathogenic fungi, also known as entomopathogenic fungi, are one of the largest insect pathogenic microorganism communities, represented by Beauveria spp. and Metarhizium spp. Entomopathogenic fungi have been proved to be a great substitute for chemical pesticide in agriculture. In fact, a lot of functional genes were also already characterized in entomopathogenic fungi, but more depth of exploration is still needed to reveal their complicated pathogenic mechanism to insects. Metarhizium rileyi (Nomuraea rileyi) is a great potential biocontrol fungus that can parasitize more than 40 distinct species (mainly Lepidoptera: Noctuidae) to cause large-scale infectious diseases within insect population. In this study, a comparative analysis of transcriptome profile was performed with topical inoculation and hemolymph injection to character the infectious pattern of M. rileyi. Appressorium and multiple hydrolases are indispensable constituents to break the insect host primary cuticle defense in entomopathogenic fungi. Within our transcriptome data, numerous transcripts related to destruction of insect cuticle rather growth regulations were obtained. Most importantly, some unreported ribosomal protein genes and novel unannotated protein (hypothetical protein) genes were proved to participate in the course of pathogenic regulation. Our current data provide a higher efficiency gene library for virulence factors screen in M. rileyi, and this library may be also useful for furnishing valuable information on entomopathogenic fungal pathogenic mechanisms to host.

Chitinases as key virulence factors in microbial pathogens: Understanding their role and potential as therapeutic targets

Chitinases are crucial for the survival of bacterial and fungal pathogens both during host infection and outside the host in the environment. Chitinases facilitate adhesion onto host cells, act as virulence factors during infection, and provide protection from the host immune system, making them crucial factors in the survival of microbial pathogens. Understanding the mechanisms behind chitinase action is beneficial to design novel therapeutics to control microbial infections. This review explores the role of chitinases in the pathogenesis of bacterial, fungal, and viral infections. The mechanisms underlying the action of chitinases of bacterial, fungal, and viral pathogens in host cells are thoroughly reviewed. The evolutionary relationships between chitinases of various bacterial, fungal, and viral pathogens are discussed to determine their involvement in processes, such as adhesion and host immune system modulation. Gaining a better understanding of the distribution and activity of chitinases in these microbial pathogens can help elucidate their role in the invasion and infection of host cells.

Dual RNA Sequencing of Beauveria bassiana-Infected Spodoptera frugiperda Reveals a Fungal Protease with Entomopathogenic and Antiphytopathogenic Activities

Insect pests and phytopathogens significantly impact crop yield and quality. The fall armyworm (FAW) Spodoptera frugiperda and the phytopathogen Fusarium graminearum cause substantial economic losses in crops like barley and wheat. However, the entomopathogen Beauveria bassiana shows limited efficacy against FAW, and its antiphytopathogenic activities against F. graminearum remain unclear. Here, dual RNA sequencing was performed to identify differentially expressed genes in B. bassiana-infected FAW larvae. We found that the BbAorsin gene was significantly upregulated at 36 and 48 h post-infection. BbAorsin encodes a serine-carboxyl protease and is mainly expressed in blastospores and hyphae. Overexpression of BbAorsin in B. bassiana ARSEF2860 enhanced virulence against Galleria mellonella and FAW larvae and inhibited F. graminearum growth. The recombinant BbAorsin protein induced apoptosis and necrosis in FAW hemocytes and inhibited F. graminearum spore germination. These findings shed light on transcriptomic mechanisms governing insect-pathogen interactions, which could aid in developing dual-functional entomopathogens and anti-phytopathogens.

Shell disease syndromes of decapod crustaceans

The term shell disease subsumes a number of debilitating conditions affecting the outer integument (the carapace) of decapod crustaceans, such as lobsters and crabs. Herein, we seek to find commonality in the aetiology and pathology of such conditions, and those cases that result in the progressive erosion of the cuticle through to the visceral tissues by a cocktail of microbial-derived enzymes including lipases, proteases and chitinases. Aquimarina spp. are involved in shell disease in many different crustaceans across a wide geographical area, but the overall view is that the condition is polymicrobial in nature leading to dysbiosis within the microbial consortium of the damaged cuticle. The role of environment, decapod behaviour and physiology in triggering this disease is also reviewed. Finally, we provide a conceptual model for disease aetiology and suggest several avenues for future research that could improve our understanding of how such factors trigger, or exacerbate, this condition.

Identification, Pathogenicity, and Culture Conditions of a New Isolate of Cordyceps javanica (Hypocreales: Cordycipitaceae) From Soil

This study decribes a highly effective insecticidal isolate of Cordyceps javanica (Frieder. & Bally) (Hypocreales: Cordycipitaceae) named IJ-tg19, which was isolated from soil. Spray bioassays were performed with IJ-tg19 on Myzus persicae (Sulzer) (Hemiptera: Aphididae) adults, third-instar nymphs of Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae), and third-instar larvae of Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae) to determine the pathogenicity of the isolate. The corrected mortality rates for all three pests were 100% when the conidia concentration was 1 × 106 conidia/ml, the lowest concentration in this study, and the median survival times (MST) were 4, 4, and 3 d. The MST shortens with increasing conidia concentration. The effects of laboratory culture conditions on the sporulation and growth of the isolate were also studied. This isolate had the greatest conidia production and fastest growth rate on malt extract agar medium at 25°C. The amount of conidia produced had positive correlation to light duration, with the highest production at 24 hr light. The growth of mycelium can adapt to a moderately alkaline environment, but the optimum conidial production occurred at the pH of 7. Our finding and research will be useful in biocontrol programs that are considering using the new isolate of C. javanica against greenhouse pests.

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.

Identification of Exoenzymes Secreted by Entomopathogenic Fungus Beauveria pseudobassiana RGM 2184 and Their Effect on the Degradation of Cocoons and Pupae of Quarantine Pest Lobesia botrana

Beauveria pseudobassiana RGM 2184 has shown 80% maximum efficacy against the pest Lobesia botrana in the autumn and winter seasons. This suggests that the strain possesses an interesting battery of enzymes that are cold-adapted to penetrate the thick and hydrophobic cocoon of L. botrana. In this study, screening of the proteolytic, lipolytic, and chitinolytic activity of enzyme extracts secreted by the RGM 2184 strain was carried out in various culture media. The enzyme extracts with the highest activity were subjected to zymography and mass spectrometry. These analyses allowed the identification of two proteases, two lipases, and three chitinases. Comparative analysis indicated that the degree of similarity between these enzymes was substantially reduced when the highest degree of taxonomic relatedness between RGM 2184 and the entomopathogenic fungus strain was at the family level. These results suggest that there is a wide variety of exoenzymes in entomopathogenic fungi species belonging to the order Hypocreales. On the other hand, exoenzyme extract exposure of cocoons and pupae of L. botrana provoked damage at 10 °C. Additionally, an analysis of the amino acid composition of the RGM 2184 exoenzyme grouped them close to the cold-adapted protein cluster. These results support the use of this strain to control pests in autumn and winter. Additionally, these antecedents can form a scaffold for the future characterization of these exoenzymes along with the optimization of the strain’s biocontrol ability by overexpressing them.

Host–Pathogen Interactions between Metarhizium spp. and Locusts

The progress in research on the interactions between Metarhizium spp. and locusts has improved our understanding of the interactions between fungal infection and host immunity. A general network of immune responses has been constructed, and the pathways regulating fungal pathogenicity have also been explored in depth. However, there have been no systematic surveys of interaction between Metarhizium spp. and locusts. The pathogenesis of Metarhizium comprises conidial attachment, germination, appressorial formation, and colonization in the body cavity of the host locusts. Meanwhile, the locust resists fungal infection through humoral and cellular immunity. Here, we summarize the crucial pathways that regulate the pathogenesis of Metarhizium and host immune defense. Conidial hydrophobicity is mainly affected by the contents of hydrophobins and chitin. Appressorial formation is regulated by the pathways of MAPKs, cAMP/PKA, and Ca²⁺/calmodulin. Lipid droplets degradation and secreted enzymes contributed to fungal penetration. The humoral response of locust is coordinated by the Toll pathway and the ecdysone. The regulatory mechanism of hemocyte differentiation and migration is elusive. In addition, behavioral fever and density-dependent population immunity have an impact on the resistance of hosts against fungal infection. This review depicts a prospect to help us understand host–pathogen interactions and provides a foundation for the engineering of entomopathogenic fungi and the discovery of insecticidal targets to control insect pests.

Unveiling a Novel Role of Cdc42 in Pyruvate Metabolism Pathway to Mediate Insecticidal Activity of Beauveria bassiana

The small GTPase Cdc42 acts as a molecular switch essential for cell cycles and polar growth in model yeast, but has not been explored in Beaurveria bassiana, an insect-pathogenic fungus serving as a main source of fungal formulations against arthropod pests. Here, we show the indispensability of Cdc42 for fungal insecticidal activity. Deletion of cdc42 in B. bassiana resulted in a great loss of virulence to Galleria mellonella, a model insect, via normal cuticle infection as well as defects in conidial germination, radial growth, aerial conidiation, and conidial tolerance to heat and UVB irradiation. The deleted mutant’s hyphae formed fewer or more septa and produced unicellular blastospores with disturbed cell cycles under submerged-culture conditions. Transcriptomic analysis revealed differential expression of 746 genes and dysregulation of pyruvate metabolism and related pathways, which were validated by marked changes in intracellular pyruvate content, ATP content, related enzyme activities, and in extracellular beauvericin content and Pr1 protease activity vital for fungal virulence. These findings uncover a novel role for Cdc42 in the pathways of pyruvate metabolism and the pyruvate-involved tricarboxylic acid cycle (TCA cycle) and a linkage of the novel role with its indispensability for the biological control potential of B. bassiana against arthropod pests.

Entomopathogenic fungi tested in planta on pepper and in field on sorghum, to control commercially important species of aphids

Given the aphids high reproductive capacity, assessing their biocontrol by using entomopathogenic fungi is crucial; to determine their potential, fungi were tested in planta and in field conditions. Significant decrease of Myzus persicae (Sulzer) population was observed in planta after applying Beauveria bassiana (strain 7R), Trichoderma gamsii (strain Z) or Metarhizium brunneum (strain Meta Br1) at 1 × 10⁷ or 1 × 10⁸ conidia/mL on pepper plants. Significant differences of aphids' populations were detected between fungus concentration and control (F = 68.743, df = 6.980, P < 0.001), where M. brunneum at 1 × 10⁸ conidia/mL reduced aphids population close to zero. At 20 °C, dead aphids' mycosis by B. bassiana and T. gamsii was 78% and 84%; at 25 °C was 83% and 88%; and at 30 °C was 75% and 79%, respectively. In field conditions, Mexican PTG4 and commercial GHA B. bassiana strains were tested [(1 × 10⁶ conidia/mL + corn starch) seed treatments] against the Melanaphis sacchari (Zehntner) aphid populations, on naturally infested sorghum plants. Results showed that plant germination and emergence were not affected, whereas yield (grams of sugar/plant) was significantly higher among treated compared with untreated plants. The aphid population decreased in plants from PTG4 treated seeds; indeed, this treatment had a significant positive effect on the flowering index, whereas the stem fresh weight and juice volume was significantly increased among plants from GHA treated seeds. Taken together, tested strains can be used as a tool to control aphids' population on several crops such as pepper and even increase the yield in sorghum.

Suitability of Formulated Entomopathogenic Fungi Against Hibiscus Mealybug, Nipaecoccus viridis (Hemiptera: Pseudococcidae), Deployed Within Mesh Covers Intended to Protect Citrus From Huanglongbing

Mesh exclusion bags are increasingly being adopted by Florida citrus growers to protect young citrus trees from Asian citrus psyllid, Diaphorina citri Kuwayama, and Huanglongbing disease. These mesh bags exclude larger insects such as D. citri but may allow entry of minute insects. Hibiscus mealybugs, Nipaecoccus viridis (Newstead), have been observed thriving in the micro-habitat created by these covers on trees. Entomopathogenic fungi (EPF) and insect growth regulators (IGRs) are effective against several mealybug species under various growing conditions, but their efficacy against N. viridis or within the microclimate within exclusion bags is unknown. Therefore, we evaluated various formulations of entomopathogenic fungi with and without IGR against N. viridis using laboratory bioassays. We then conducted semifield bioassays to determine effectiveness of EPF formulations alone and in combination with an IGR applied to citrus trees enveloped within mesh bags under field conditions. Survival probabilities of N. viridis nymphs exposed to all Beauveria bassiana-based products tested were comparable to malathion under laboratory conditions and reduced survival as compared to controls (water only). Under field conditions, mortality of N. viridis nymphs on leaves sprayed with each fungal formulation tested was significantly greater than on control treatments (Water, Suffoil X, 435 oil) up to four weeks post application. There were no differences in the colony forming units per leaf area amongst all fungal treatments. Formulated B. bassiana-based products applied alone or combined with an IGR should be effective tools for managing N. viridis populations on young citrus trees protected with mesh exclusion bags.

Bombyx mori Apolipophorin-III inhibits Beauveria bassiana directly and through regulating expression of genes relevant to immune signaling pathways

Insect Apolipophorin-III is a multifunctional protein and also plays an important role in insect innate immunity. Early transcriptome and proteome studies indicated that the gene expression level of Bombyx mori Apolipophorin-III (BmApoLp-III) in silkworm larvae infected with Beauveria bassiana was significantly up-regulated. In this study, BmApoLp-III gene was cloned, its expression patterns in different larval tissues investigated, the BmApoLp-III protein was successfully expressed with prokaryotic expression system and its antifungal effect was verified. The results showed that the BmApoLp-III gene was expressed in all the tested tissues of the 5th instar larvae infected by B. bassiana, with the highest expression in fat body. The fungistatic zone test showed that the recombinant BmApoLp-III has a significant antifungal effect on B. bassiana. Injecting purified BmApoLp-III to the larvae delayed the onset and death of the infected larvae. Conversely, silencing BmApoLp-III gene by RNAi resulted in early morbidity and death of the infected larvae. At the same time, injecting BmApoLp-III up-regulated the expression of genes including BmβGRP4 and BmMyd88 in the Toll signaling pathway, BmCTL5 and BmHOP in the Jak/STAT signaling pathway, serine proteinase inhibitor BmSerpin5, and antimicrobial peptide BmCecA, but down-regulated the expression of BmTak1 of Imd signaling pathway; while silencing BmApoLp-III gene down-regulated the expression of BmβGRP1 and BmSpaetzle, BmCTL5 and BmHOP, BmSerpin2 and BmSerpin5, BmBAEE and BmPPO2 of relevant pathways and BmCecA, but up-regulated the expression of BmPGRP-Lc and BmTak1 of Imd pathway. These results indicate that the BmApoLp-III could not only directly inhibit B. bassiana, but also participate in regulation of the expression of immune signaling pathway related genes, promote the expression of immune effectors, and indirectly inhibit the reproduction of B. bassiana in the silkworm.

The Msn2 Transcription Factor Regulates Acaricidal Virulence in the Fungal Pathogen Beauveria bassiana

Beauveria bassiana holds promise as a feasible biological control agent for tick control. The B. bassiana stress–response transcription factor Msn2 is known to contribute to fungal growth, conidiogenesis, stress–response and virulence towards insects; however, little is known concerning whether Msn2 is involved in infection across Arthropoda classes. We evaluated the effects of Msn2 on B. bassiana virulence against Rhipicephalus microplus (Acari, Ixodidae) using wild-type, targeted gene knockout (ΔBbmsn2) and complemented mutant (ΔBbmsn2/Bbmsn2) strains. Reproductive parameters of R. microplus engorged females treated topically or by an intra-hemocoel injection of conidial suspensions were assessed. Treated cuticles of engorged females were analyzed by microscopy, and proteolytic activity of B. bassiana on cuticles was assessed. Topically treated engorged females showed high mean larval hatching (>84%) in control and ΔBbmsn2 treatments, whereas treatment with the wild-type or ΔBbmsn2/Bbmsn2 strains resulted in significantly decreased (lowered egg viability) larval hatching. Percent control of R. microplus topically treated with ΔBbmsn2 was lower than in the groups treated with wild-type (56.1%) or ΔBbmsn2/Bbmsn2 strains. However, no differences on reproductive parameters were detected when R. microplus were treated by intra-hemocoel injection using low (800 conidia/tick) doses for all strains tested; R. microplus injected with high doses of wild-type or mutant strains (10⁶ conidia/tick) died before laying eggs (~48 h after treatment). SEM analyses of B. bassiana infection showed similar conidial germination and formation of pseudo-appressoria on tick cuticle. Histological sections of ticks treated with the wild-type or ΔBbmsn2/Bbmsn2 strains showed fungal penetration through the cuticle, and into the tick interior. Hyphae of ΔBbmsn2, however, did not appear to penetrate or breach the tick exocuticle 120 h after treatment. Protease activity was lower on tick cuticles treated with ΔBbmsn2 than those treated with the wild-type or ΔBbmsn2/Bbmsn2 strains. These data show that loss of the Msn2 transcription factor reduced B. bassiana virulence against R. microplus, but did not interfere with conidial germination, appressoria formation or sporulation on tick cadavers, and plays only a minimal role once the cuticle is breached. Our results indicate that the BbMsn2 transcription factor acts mainly during the fungal penetration process and that decreased protease production may be one mechanism that contributes to the inability of the mutant strain to breach the tick cuticle.

Diversification of Fungal Chitinases and Their Functional Differentiation in Histoplasma capsulatum

Chitinases enzymatically hydrolyze chitin, a highly abundant and utilized polymer of N-acetyl-glucosamine. Fungi are a rich source of chitinases; however, the phylogenetic and functional diversity of fungal chitinases are not well understood. We surveyed fungal chitinases from 373 publicly available genomes, characterized domain architecture, and conducted phylogenetic analyses of the glycoside hydrolase (GH18) domain. This large-scale analysis does not support the previous division of fungal chitinases into three major clades (A, B, C) as chitinases previously assigned to the “C” clade are not resolved as distinct from the “A” clade. Fungal chitinase diversity was partly shaped by horizontal gene transfer, and at least one clade of bacterial origin occurs among chitinases previously assigned to the “B” clade. Furthermore, chitin-binding domains (including the LysM domain) do not define specific clades, but instead are found more broadly across clades of chitinases. To gain insight into biological function diversity, we characterized all eight chitinases (Cts) from the thermally dimorphic fungus, Histoplasma capsulatum: six A clade, one B clade, and one formerly classified C clade chitinases. Expression analyses showed variable induction of chitinase genes in the presence of chitin but preferential expression of CTS3 in the mycelial stage. Activity assays demonstrated that Cts1 (B-I), Cts2 (A-V), Cts3 (A-V), Cts4 (A-V) have endochitinase activities with varying degrees of chitobiosidase function. Cts6 (C-I) has activity consistent with N-acetyl-glucosaminidase exochitinase function and Cts8 (A-II) has chitobiase activity. These results suggest chitinase activity is variable even within subclades and that predictions of functionality require more sophisticated models.

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 deletion of chiMaD1, a horizontally acquired chitinase of Metarhizium anisopliae, led to higher virulence towards the cattle tick (Rhipicephalus microplus)

The first line of the Arthropods defense against infections is the hard-structured exoskeleton, a physical barrier, usually rich in insoluble chitin. For entomopathogenic fungi that actively penetrate the host body, an arsenal of hydrolytic enzymes (as chitinases and N-acetylglucosaminidases), that break down chitin, is essential. Notably, twenty-one putative chitinase genes have been identified in the genome of Metarhizium anisopliae, a generalist entomopathogenic fungus. As a multigenic family, with enzymes that, presumably, perform redundant functions, the main goal is to understand the singularity of each one of such genes and to discover their precise role in the fungal life cycle. Specially chitinases that can act as virulence determinants are of interest since these enzymes can lead to more efficient biocontrol agents. Here we explored a horizontally acquired chitinase from M. anisopliae, named chiMaD1. The deletion of this gene did not lead to phenotypic alterations or diminished supernatant's chitinolytic activity. Surprisingly, chiMaD1 deletion enhanced M. anisopliae virulence to the cattle tick (Rhipicephalus microplus) larvae and engorged females, while did not alter the virulence to the mealworm larvae (Tenebrio molitor). These results add up to recent reports of deleted genes that enhanced entomopathogenic virulence, showing the complexity of host-pathogen interactions.

Ergosterol-targeting fusion antifungal peptide significantly increases the Verticillium wilt resistance of cotton

Increasing the targeting ability of antifungal proteins towards specific components of fungal cells has the potential to improve their antifungal activity and reduce harmful effects to nontarget cells. To obtain effective disease resistance genes against cotton Verticillium wilt, we constructed several fusion genes, in which binding domains targeting chitin, sphingolipid or ergosterol in the fungal cell wall or cell membrane were individually fused to the antifungal peptide BbAFP1 from entomopathogenic fungus Beauveria bassiana. Transient expression of fusion genes in cotton cotyledons indicated that the BbAFP1::ErBD fusion peptide with an ergosterol binding domain exhibited better disease resistance against V. dahliae than wild-type BbAFP1 and other fusion genes. BbAFP1::ErBD and BbAFP1 transgenic cotton were obtained and verified by Southern and Western blotting. Compared with BbAFP1-expressing cotton, BbAFP1::ErBD-expressing cotton showed higher disease resistance against V. dahliae, with smaller lesion areas (0.07 cm2 vs. 0.16 cm2 ) on the leaves and a lower disease index (23.9 vs. 34.5). Overexpression of BbAFP1::ErBD by transgenic tobacco also showed enhanced disease resistance against V. dahliae compared with that of the wild-type gene. These results indicated that construction of fusion antifungal peptides that target fungal cells is a powerful strategy to obtain new anti-disease genes, and the obtained fusion gene BbAFP1::ErBD has the potential to defend against plant fungal diseases.

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.

Manipulation of host ecdysteroid hormone levels facilitates infection by the fungal insect pathogen, Metarhizium rileyi

Entomopathogenic fungi such as Metarhizium rileyi and Beauveria bassiana are widely used insect biological control agents. Little, however, is known concerning genetic or enzymatic factors that differentiate the mechanisms employed by these two fungal pathogens to infect target hosts. Infection by either of these organisms is known to increase levels of the growth and molting hormone, ecdysone, which also regulates the expression of a number of innate immune pathways. M. rileyi, but not B. bassiana, has apparently evolved an ecdysteroid-22-oxidase (MrE22O) that inactivate ecdysone. We show that deletion of MrE22O impaired virulence compared with the wild-type strain, with an increase in ecdysone titer seen in hosts that was coupled to an increase in the expression of antimicrobial genes. An M. rileyi strain engineered to overexpress MrE22O (MrE22O^OE ), as well as trans-expression in B. bassiana (Bb::MrE220^OE ) resulted, in strains displaying enhanced virulence and dampening of host immune responses compared with their respective wild-type parental strains. These results indicate that ecdysone plays an important role in mediating responses to fungal infection and that some insect pathogenic fungi have evolved mechanisms for targeting this hormone as a means for facilitating infection.

The interaction between cuticle free fatty acids (FFAs) of the cockroaches Blattella germanica and Blatta orientalis and hydrolases produced by the entomopathogenic fungus Conidiobolus coronatus

The interactions between entomopathogenic fungi and insects serve a classic example of a co-evolutionary arms race between pathogens and their target host. The cuticle, site of the first contact between insects and entomopathogenic fungus, is an important defensive barrier against pathogens. It is covered by a layer of lipids that appears to play a key role in these processes and cuticular free fatty acid (FFA) profiles are consider as a determinant of susceptibility, or resistance, to fungal infections. These profiles are species-specific. The cockroaches Blattella germanica (Blattodea: Blattidae) and Blatta orientalis (Blattodea: Ectobiidae) are unsusceptible to the soil fungus Conidiobolus coronatus (Entomophthorales: Ancylistaceae) infection, therefore we studied the profiles of FFAs in order to understand the defensive capabilities of the cockroaches. The fungus was cultivated for three weeks in minimal medium. Cell-free filtrate was obtained, assayed for elastase, N-acetylglucosaminidase, chitobiosidase and lipase activity, and then used for in vitro hydrolysis of the cuticle from wings and thoraces of adults and oothecae. The amounts of amino acids, N-glucosamine and FFAs released from the hydrolysed cuticle samples were measured after eight hours of incubation. The FFA profiles of the cuticle of adults, and the wings, thoraces and oothecae of both species were established using GC-MS and the results were correlated with the effectiveness of fungal proteases, chitinases and lipases in the hydrolyzation of cuticle samples. Positive correlations would suggest the existence of compounds used by the fungus as nutrients, whereas negative correlations may indicate that these compounds could be engaged in insect defence.

Characterization of a fungal competition factor: Production of a conidial cell-wall associated antifungal peptide

Competition is one of the fundamental driving forces of natural selection. Beauveria bassiana is a soil and plant phylloplane/root fungus capable of parasitizing insect hosts. Soil and plant environments are often enriched with other fungi against which B. bassiana competes for survival. Here, we report an antifungal peptide (BbAFP1), specifically expressed and localized to the conidial cell wall and is released into the surrounding microenvironment inhibiting growth of competing fungi. B. bassiana strains expressing BbAFP1, including overexpression strains, inhibited growth of Alternaria brassicae in co-cultured experiments, whereas targeted gene deletion of BbAFP1 significantly decreased (25%) this inhibitory effect. Recombinant BbAFP1 showed chitin and glucan binding abilities, and growth inhibition of a wide range of phytopathogenic fungi by disrupting membrane integrity and eliciting reactive oxygen species (ROS) production. A phenylalanine residue (F⁵⁰) contributes to chitin binding and antifungal activity, but was not required for the latter. Expression of BbAFP1 in tomato resulted in transgenic plants with enhanced resistance to plant fungal pathogens. These results highlight the importance of fungal competition in shaping primitive competition strategies, with antimicrobial compounds that can be embedded in the spore cell wall to be released into the environment during the critical initial phases of germination for successful growth in its environmental niche. Furthermore, these peptides can be exploited to increase plant resistance to fungal pathogens.

Microbial competition exerts powerful selective pressures for the development of defensive and offensive methods of suppressing potential competitors. One of the most vulnerable stages for any fungi is the initial germination of resting spores in potentially hostile environments. Currently, we know little about how fungi defend other microbial competitors during the beginning stage of conidial germination. Here, we report on an antifungal peptide from B. bassiana (BbAFP1) that is specifically expressed in mature aerial conidia, with the protein localized exclusively to the conidial cell wall. The “pre-loaded” BbAFP1 is released into the surrounding microenvironment where it can act to inhibit the growth of competing fungi during the initial stages of fungal germination, i.e. largely before actual germ tubes are apparent, thus conferring an advantage to B. bassiana in out-competing susceptible competitors in the microenvironment surrounding the spore. The effects of BbAFP1 on membrane integrity were characterized and a key amino acid (F⁵⁰) was shown to function in chitin binding and antifungal activity. Transgenic tomato overexpressing BbAFP1 were shown to exhibit enhanced resistance to plant fungal pathogens. Our study provides new insights into the microbial competition and genes involved in this process that can be exploited to increase plant disease resistance.

Putative Role of a Yet Uncharacterized Protein Elicitor PeBb1 Derived from Beauveria bassiana ARSEF 2860 Strain against Myzus persicae (Homoptera: Aphididae) in Brassica rapa ssp. pekinensis

This study reports the characterization of protein elicitor PeBb1 derived from entomopathogenic fungus Beauveria bassiana ARSEF-2860 strain and its putative role in induced systemic resistance in Brassica rapa ssp. pekinensis against green peach aphid Myzus persicae. The sequence of purified elicitor protein was matched with the genomic sequence of a hypothetical protein BBA_10269 from B. bassiana ARSEF-2860 (GenBank Accession No. XP_008603588.1). The protein-encoding gene PeBb1 contained 534 bp cDNA encoding a polypeptide of 177 amino acids with a molecular mass of 19 kDa. The recombinant elicitor protein was expressed in Escherichia coli using pET-28a (+) expression vector and induced necrosis in the leaves of tobacco. The effects of elicitor protein on aphid M. persicae was determined by applying three different concentrations of PeBb1 (i.e., 26, 35, 53 μM) on B. rapa plants at 4-leaf stage and the treated plants were exposed to newly emerged (0–6 h old) apterous adult aphids. Bioassay results showed significant (p < 0.05) sub-lethal effects of the exogenous application of PeBb1 elicitor on M. persicae. Moreover, the RT-qPCR gene expression analyses showed a significant up-regulation of most of the key genes linked to ethylene (ET)- and jasmonic acid (JA)-associated plant defense pathways in elicitor-treated plants. These results not only recommend the putative utilization of PeBb1 elicitor protein in future biological pest control strategies against phloem-feeding insect pests such as M. persicae, but also help in better comprehension of the mechanisms through which beneficial fungi trigger the induced plant resistance.

Progress and prospects of arthropod chitin pathways and structures as targets for pest management

Chitin is a structural component of the arthropod cuticular exoskeleton and the peritrophic matrix of the gut, which play crucial roles in growth and development. In the past few decades, our understanding of the composition, biosynthesis, assembly, degradation, and regulation of chitinous structures has increased. Many chemicals have been developed that target chitin biosynthesis (benzoyphenyl ureas, etoxazole), chitin degradation (allosamidin, psammaplin), and chitin regulation (benzoyl hydrazines), thus resulting in molting deformities and lethality. In addition, proteins that disrupt chitin structures, such as lectins, proteases, and chitinases have been utilized to halt feeding and induce mortality. Chitin-degrading enzymes, such as chitinases are also useful for improving the efficacy of bio-insecticides. Transgenic plants, baculoviruses, fungi, and bacteria have been engineered to express chitinases from a variety of organisms for control of arthropod pests. In addition, RNA interference targeting genes involved in chitin pathways and structures are now being investigated for the development of environmentally friendly pest management strategies. This review describes the chemicals and proteins used to target chitin structures and enzymes for arthropod pest management, as well as pest management strategies based upon these compounds, such as plant-incorporated-protectants and recombinant entomopathogens. Recent advances in RNA interference-based pest management, and how this technology can be used to target chitin pathways and structures are also discussed.

A potent chitin-hydrolyzing enzyme from Myrothecium verrucaria affects growth and development of Helicoverpa armigera and plant fungal pathogens

Chitin, a crucial structural and functional component of insects and fungi, serves as a target for pest management by utilizing novel chitinases. Here, we report the biocontrol potential of recombinant Myrothecium verrucaria endochitinase (rMvEChi) against insect pest and fungal pathogens. A complete ORF of MvEChi (1185 bp) was cloned and heterologously expressed in Escherichia coli. Structure based sequence alignment of MvEChi revealed the presence of conserved domains SXGG and DXXDXDXE specific for GH-18 family, involved in substrate binding and catalysis, respectively. rMvEChi (46.6 kDa) showed optimum pH and temperature as 7.0 and 30 °C, respectively. Furthermore, rMvEChi remained stable within the pH range of 6.0 to 8.0 and up to 40 °C. rMvEChi exhibited k_cat/K_m values of 129.83 × 10³ [(g/L)^-1 s^-1] towards 4MU chitotrioside. Hydrolysis of chitooligosaccharides with various degrees of polymerization (DP) using rMvEChi indicated the release of DP2 as main end product with order of reaction as DP6 > DP5 > DP4 > DP3. Bioassay of rMvEChi against Helicoverpa armigera displayed potent anti-feedant activity and induced mortality. In vitro antifungal activity against plant pathogenic fungi (Ustilago maydis and Bipolaris sorokiniana) exhibited significant inhibition of mycelium growth. These results suggest that MvEChi has significant potential in enzyme-based pest and pathogen management.

Label-free LC-MS/MS proteomic analysis of the hemolymph of silkworm larvae infected with Beauveria bassiana

Beauveria bassiana, a pathogen of the economically important silkworm (Bombyx mori), causes serious losses in the sericulture industry; however, the mechanisms underlying B. bassiana infection and the silkworm response are not fully understood. To obtain new insights into the interaction between B. bassiana and its host, hemolymph samples from fifth instar silkworm larvae infected with B. bassiana were analyzed at 36-h post-inoculation using a label-free LC-MS/MS proteomic technique. In total, 671 proteins were identified in the hemolymph, including 87 differentially expressed proteins, 42 up-regulated and 45 down-regulated in infected larvae. Six were detected only in infected larvae, and five were detected only in uninfected larvae. Based on GO annotations, 48 of the differentially expressed proteins were involved in molecular functions, 42 were involved in biological processes, and 39 were involved in cell components. A KEGG pathway analysis indicated that these differentially expressed proteins participate in 85 signal transduction pathways, including the amoebiasis, MAPK signaling, Hippo signaling, Toll and Imd signaling, and lysosome pathways. The silkworm hemolymph is the main site for B. bassiana replication. We identified differentially expressed proteins involved in the regulation of the host response to B. bassiana infection, providing important experimental data for the identification of key factors contributing to the interaction between the pathogenic fungus and its host.

Chitin Synthesis and Degradation in Fungi: Biology and Enzymes

Chitin is one of the most important carbohydrates of the fungal cell wall, and is synthesized by chitin synthases. Chitin can be degraded by chitinases, which are important virulence factors in pathogenic fungi. Knowledge about the biosynthesis and degradation of chitin, and the enzymes responsible, has accumulated in recent years. In this review, we analyze the amino acid sequences of chitin synthases from several typical fungi. These enzymes can be divided into seven groups. While the different chitin synthases from a single fungus share a low degree of similarity, the same type of chitin synthase from different fungi shows high similarity. The number of chitinase genes in fungi display wide variation, from a single gene in Schizosaccharomyces pombe, to 36 genes in Trichoderma virens. Chitinases from different fungi can be divided into four groups. The functions of chitin synthases and chitinases in several typical fungi are summarized, and the crystal structures of chitinases and chitinase modification are also discussed.

How do oral insecticidal compounds cross the insect midgut epithelium?

The use of oral insecticidal molecules (small molecules, peptides, dsRNA) via spray or plant mediated applications represents an efficient way to manage damaging insect species. With the exception of Bt toxins that target the midgut epithelium itself, most of these compounds have targets that lie within the hemocoel (body) of the insect. Because of this, one of the greatest factors in determining the effectiveness of an oral insecticidal compound is its ability to traverse the gut epithelium and enter the hemolymph. However, for many types of insecticidal compounds, neither the pathway taken across the gut nor the specific genes which influence uptake are fully characterized. Here, we review how different types of insecticidal compounds enter or cross the midgut epithelium through passive (diffusion) or active (transporter based, endocytosis) routes. A deeper understanding of how insecticidal molecules cross the gut will help to best utilize current insecticides and also provide for more rational design of future ones.

Genomic Analysis of the Insect-Killing Fungus Beauveria bassiana JEF-007 as a Biopesticide

Insect-killing fungi have high potential in pest management. A deeper insight into the fungal genes at the whole genome level is necessary to understand the inter-species or intra-species genetic diversity of fungal genes, and to select excellent isolates. In this work, we conducted a whole genome sequencing of Beauveria bassiana (Bb) JEF-007 and characterized pathogenesis-related features and compared with other isolates including Bb ARSEF2860. A large number of Bb JEF-007 genes showed high identity with Bb ARSEF2860, but some genes showed moderate or low identity. The two Bb isolates showed a significant difference in vegetative growth, antibiotic-susceptibility, and virulence against Tenebrio molitor larvae. When highly identical genes between the two Bb isolates were subjected to real-time PCR, their transcription levels were different, particularly in heat shock protein 30 (hsp30) gene which is related to conidial thermotolerance. In several B. bassiana isolates, chitinases and trypsin-like protease genes involved in pathogenesis were highly conserved, but other genes showed noticeable sequence variation within the same species. Given the transcriptional and genetic diversity in B. bassiana, a selection of virulent isolates with industrial advantages is a pre-requisite, and this genetic approach could support the development of excellent biopesticides with intellectual property protection.

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.

Classification and infection mechanism of entomopathogenic fungi

ABSTRACT: Entomopathogenic fungi are important biological control agents throughout the world, have been the subject of intensive research for more than 100 years, and can occur at epizootic or enzootic levels in their host populations. Their mode of action against insects involves attaching a spore to the insect cuticle, followed by germination, penetration of the cuticle, and dissemination inside the insect. Strains of entomopathogenic fungi are concentrated in the following orders: Hypocreales (various genera), Onygenales (Ascosphaera genus), Entomophthorales, and Neozygitales (Entomophthoromycota).

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.

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.

Insect Pathogenic Fungi: Genomics, Molecular Interactions, and Genetic Improvements

Entomopathogenic fungi play a pivotal role in the regulation of insect populations in nature, and representative species have been developed as promising environmentally friendly mycoinsecticides. Recent advances in the genome biology of insect pathogenic fungi have revealed genomic features associated with fungal adaptation to insect hosts and different host ranges, as well as the evolutionary relationships between insect and noninsect pathogens. By using species in the Beauveria and Metarhizium genera as models, molecular biology studies have revealed the genes that function in fungus-insect interactions and thereby contribute to fungal virulence. Taken together with efforts toward genetic improvement of fungal virulence and stress resistance, knowledge of entomopathogenic fungi will potentiate cost-effective applications of mycoinsecticides for pest control in the field. Relative to our advanced insights into the mechanisms of fungal pathogenesis in plants and humans, future studies will be necessary to unravel the gene-for-gene relationships in fungus-insect interactive models.

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.

Up-regulation of carbon metabolism-related glyoxylate cycle and toxin production in Beauveria bassiana JEF-007 during infection of bean bug, Riptortus pedestris (Hemiptera: Alydidae)

Beauveria bassiana (Bb) is used as an environment-friendly biopesticide. However, the molecular mechanisms of Bb-host interactions are not well understood. Herein, RNA isolated from B. bassiana (Bb JEF-007) and Riptortus pedestris (Hemiptera: Alydidae) infected with this strain were firstly subjected to high-throughput next generation sequencing (NGS) to analyze and compare transcriptomes. Due to lack of fungal and host genome information, fungal transcriptome was processed to partially exclude non-infection specific genes and host-flora. Differentially Expressed Gene (DEG) analysis showed that 2381 genes were up-regulated and 2303 genes were down-regulated upon infection. Most DEGs were classified into the categories of single-organism, cellular and metabolism processes by Gene Ontology analysis. Most DEGs were involved in metabolic pathways based on Kyoto Encyclopedia of Genes and Genomes pathway mapping. Carbon metabolism-related enzymes in the glyoxylate cycle were significantly up-regulated, suggesting a possible role for them in Bb growth in the host. Additionally, transcript levels of several fungal genes were dramatically increased after infection, such as cytotoxic lectin-like protein, bacterial-like toxin, proteins related to cell wall formation, hyphal growth, nutrient uptake, and halogenated compound synthesis. This work provides insight into how entomopathogenic B. bassiana grows in agriculturally harmful bean bug at 6 d post infection.

Genome-Wide Host-Pathogen Interaction Unveiled by Transcriptomic Response of Diamondback Moth to Fungal Infection

Genome-wide insight into insect pest response to the infection of Beauveria bassiana (fungal insect pathogen) is critical for genetic improvement of fungal insecticides but has been poorly explored. We constructed three pairs of transcriptomes of Plutella xylostella larvae at 24, 36 and 48 hours post treatment of infection (hpt_I) and of control (hpt_C) for insight into the host-pathogen interaction at genomic level. There were 2143, 3200 and 2967 host genes differentially expressed at 24, 36 and 48 hpt_I/hpt_C respectively. These infection-responsive genes (~15% of the host genome) were enriched in various immune processes, such as complement and coagulation cascades, protein digestion and absorption, and drug metabolism-cytochrome P450. Fungal penetration into cuticle and host defense reaction began at 24 hpt_I, followed by most intensive host immune response at 36 hpt_I and attenuated immunity at 48 hpt_I. Contrastingly, 44% of fungal genes were differentially expressed in the infection course and enriched in several biological processes, such as antioxidant activity, peroxidase activity and proteolysis. There were 1636 fungal genes co-expressed during 24–48 hpt_I, including 116 encoding putative secretion proteins. Our results provide novel insights into the insect-pathogen interaction and help to probe molecular mechanisms involved in the fungal infection to the global pest.

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.