Correlation of the aphicidal activity of Beauveria bassiana SFB-205 supernatant with enzymes

Entomopathogenic fungi in crops protection with an emphasis on bioactive metabolites and biological activities

Plant pathogens with their abundance are harmful and cause huge damage to different agricultural crops and economy of a country as well as lead towards the shortage of food for humans. For their management, the utilization of entomopathogenic fungi is an eco-friendly technique, sustainable to the environment, safe for humans and has promising effect over chemical-based pesticides. This process requires a biochemical mechanism, including the production of enzymes, toxins, and other metabolites that facilitate host infection and invasion. Essential enzymes such as chitinase, proteinase, and lipase play a direct role in breaking down the host cuticle, the primary barrier to EPF (Entomopathogenic Fungi) infection. Additionally, secondary metabolites such as destruxins in Metarhizium, beauvericin in Beauveria, hirsutellides in Hirsutella, isarolides in Isaria, cordyols in Cordyceps, and vertihemipterins in Verticillium, among others, act both directly and indirectly to disable the defense mechanisms of insect hosts, thereby accelerating the EPF infection process. The chemical composition of these secondary metabolites varies, ranging from simple non-peptide pigments such as oosporine to highly complex piperazine derivatives such as vertihemiptellides. The biocontrol efficacy of EPF is extensively studied, with numerous fungal strains commercially available on a large scale for managing arthropod pests. This review emphasizes the role of proteins and enzymes against crop pathogens, detailing their mode of action, and describing the metabolites from entomopathogenic fungi and their biological activities. In doing so, these findings contribute to establishing a symbiotic equilibrium between agricultural productivity and environmental conservation.

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.

Selection and characterization of thermotolerant Beauveria bassiana isolates and with insecticidal activity against the cotton-melon aphid Aphis gossypii (Glover) (Hemiptera: Aphididae)

BACKGROUND

Cotton-melon aphid Aphis gossypii (Glover) causes severe damage mainly to cucurbits. Twenty-two Beauveria sp. isolates were simultaneously assessed for their pathogenicity and heat tolerance. The selected isolates were identified molecularly and characterized in terms of conidial germination rate, mycelial growth, conidial yield and endophytic activity.

RESULTS

Screening bioassays showed that the B. bassiana isolates B3, B7, B9 and B12 were the most toxic, inducing mortality equal to or slightly higher than the commercialized strain B. bassiana BNat (70.7%). Median lethal concentration (LC₅₀ ) bioassays revealed that only isolate B12 had a significantly lower LC₅₀ value (5.4 × 10⁵ conidia ml^-1 ) than strain BNat (5 × 10⁶ conidia ml^-1 ). The heat tolerance screening test (1 h of exposure to 45°C) allowed us to select isolates B3, B7, B9 and B12 with germination rates of 57.5% to 80.1% after 24 h incubation at 25°C, all significantly higher than strain BNat (22.1%). The germination rates of all isolates decreased significantly after 2 h of exposure to 45°C, with the exception of isolate B12 which displayed the highest thermotolerance (72% germination). The four selected isolates were able to endophytically colonize cucumber leaves when applied to the foliage. Inoculation of cucumber plants with isolate B12 did not affect cucumber plant growth. However, several plant growth parameters were improved 5 weeks after root inoculation.

CONCLUSION

On the basis of its potent toxicity and thermotolerance, isolate B12 is a good candidate for further development as a biopesticide for use in integrated pest management strategies for aphid control. © 2022 Society of Chemical Industry.

Insect-fungal-interactions: A detailed review on entomopathogenic fungi pathogenicity to combat insect pests

Global food security is threatened by insect pests of economically important crops. Chemical pesticides have been used frequently for the last few decades to manage insect pests throughout the world. However, these chemicals are hazardous for human health as well as the ecosystem. In addition, several pests have evolved resistance to many chemicals. Finding environment friendly alternatives lead the researchers to introduce biocontrol agents such as entomopathogenic fungi (EPF). These fungi include various genera that can infect and kill insects efficiently. Moreover, EPFs have considerable host specificity with a mild effect on non-target organisms and can be produced in bulk quantity quickly. However, insights into the biology of EPF and mechanism of action are of prime significance for their efficient utilization as a biocontrol agent. This review focuses on EPF-mediated insect management by explaining particular EPF strains and their general mode of action. We have comprehensively discussed which criteria should be used for the selection of pertinent EPF, and which aspects can impact the EPF efficiency. Finally, we have outlined various advantages of EPF and their limitations. The article summarizes the prospects related to EPF utilization as biocontrol agents. We hope that future strategies for the management of insects will be safer for our planet.

Use of Trichoderma koningiopsis chitinase to enhance the insecticidal activity of Beauveria bassiana against Diatraea saccharalis

Trichoderma is a well-known soil-borne fungus, highly efficient producer of extracellular enzymes including chitinases. The aim of this study was to recover a chitinase from fermentation waste after harvesting Trichoderma koningiopsis Th003 conidia and assess its potential as an enhancer of Beauveria bassiana insecticidal activity against Diatraea saccharalis. T. koningiopsis was produced by solid fermentation, conidia were harvested, and a crude extract (CE) was recovered by washing the residual substrate (rice:wheat bran). The partially purified chitinase (PPC) (75 kDa product) with N-acetyl-β-glucosaminidase activity was obtained by chromatography to 29.3-fold with optimal activity at pH 5 and 55°C. Both the CE and the PPC were mixed with B. bassiana Bv062 conidia and assessed in a bioassay against D. saccharalis larvae. The CE and PPC from T. koningiopsis Th003 did not affect the germination or viability of B. bassiana conidia and enhanced its insecticidal activity when used at 0.06 U/ml enzymatic activity with a 24.5% reduction in B. bassiana lethal time (LT₉₀ ). This study demonstrated the potential of chitinases produced by T. koningiopsis in solid fermentation to be recovered from the waste substrate and used as an additive to enhance B. bassiana, adding value to the main waste from the Trichoderma biopesticide/biofertilizer industries.

Metabolomics of extracellular compounds and parasitic enzymes of Beauveria bassiana associated with biological control of whiteflies (Bemisia tabaci)

The Beauveria spp. were isolated from soil and insect cadavers of crop rhizosphere and characterized for parasitic enzyme activity and virulence against whiteflies (Bemisia tabaci). The colony morphology and molecular identification using ITS specific marker were carried out and confirmed entomopathogenic fungi as Beauveria bassiana. The bioefficacy of B. bassiana against whiteflies demonstrated highest corrected mortality and lowest LC₅₀ in isolate B. bassiana JAU2 (SEM morphology) followed by JAU1 on 6th days. Parasitic enzymes chitinase and lipase were determined highest in JAU2 and protease activity examined higher in isolate JAU4 followed by JAU2 isolate on 6th days after inoculation. Comparative extracellular metabolomics carried out from potent (JAU1 and JAU2), moderate (JAU4 and JAU14) and weak (JAU6) B. bassiana isolates in normal suborder dextrose agar with yeast extrect (SDAY) and chitin induced media. Results illustrated that total 105 metabolites identified common for all five B. bassiana isolates differing in virulence. However, the color intensity of the metabolites changes in heat map showing differential concentration of that extracellular compound compared to other isolates. The volcano plot analysis illustrated 58 compounds significanlty diverse between potent JAU1 and JAU2 under two different culture conditions of which 34 compounds recognized up regulated in most potent JAU2 under chitin induced media. Out of 34 metabolites, ten compounds viz., fumaricine, resazurin, N-methyldioctylamine, penaresidun B, tetralin, squamocin B, oligomycin C, pubesenolide, epirbuterol and gentamicin C1a were recognized significantly upregulated in most potent JAU2 and reported for antimicrobial, nematicidal, larvicidalor insecticidal activities. The mass spectra and fragment structure were elucidated under LCMS-QTOF for some novel and unique compounds recognized in most potent B. bassiana JAU2, involved in parasitic activity against whiteflies.

Lack of involvement of chitinase in direct toxicity of Beauveria bassiana cultures to the aphid Myzus persicae

The fungal insect pathogen Beauveria bassiana produces a range of insecticidal metabolites and enzymes, including chitinases and proteases, which may assist the disease progression. The enzymes often play a predominant role in the pathogenicity pathway and both chitinases and proteases have previously been shown to be important in host infection. Spray application of supernatants of B. bassiana broth cultures of an isolate from New Zealand caused significant mortality in the green peach aphid, Myzus persicae, within 24 h, demonstrating an apparent contact toxicity. Three-day-old broth cultures were the most effective, with less insect mortality seen using six-day-old broth. However, aphicidal activity increased again when treating aphids with seven-day-old broth. Cultures grew substantially better and produced more potent aphicidal cultures when cultured in media with an initial pH above 5.5. Chitinase was produced a day earlier than the serine protease Pr1, but the peak production periods of these enzymes did not correlate with the aphicidal activities of three- or six-day-old cultures. Cultures treated with EDTA or heated to inactivate the enzymes still showed strong insecticidal activity. Neither beauvericin nor bassianolide, two known insecticidal metabolites, were detected in the supernatants. Therefore the key aphicidal components of B. bassiana cultures were not associated with chitinase nor Pr1 and are yet to be identified.

Characterization of the endothiapepsin-like protein in the entomopathogenic fungus Beauveria bassiana and its virulence effect on the silkworm, Bombyx mori

Endothiapepsin is an aspartic proteinase that was first isolated from the plant pathogenic fungus Endothia parasitica. In previous studies, we reported on three endothiapepsin-like proteins in the entomopathogenic fungus Beauveria bassiana; the genes were up-regulated in B. bassiana hyper-virulent strain GXsk1011 at early stage infection in the silkworm. However, whether these proteins play a role in pathogenicity or not remains unknown. In this study, we cloned one protein, BbepnL-1 gene (BBA-07766), that has 98% homology with B. bassiana strain Bb2860, and expressed it in the yeast Pichia pastoris to investigate its function. The endothiapepsin-like protein is a secreted proteinase of molecular weight approximately 40 kDa. It has an N-glycosylation site and a mutation in the C-terminal conserved domain- a Thr was mutated to Gly in B. bassiana GXsk1011 and is different than the endothiapepsin of Endothia parasitica. The recombinant endothiapepsin-like protein showed enzyme activity and degraded the protein components of the silkworm cuticle. To further investigate the activity of the endothiapepsin-like protein, we knocked out the gene BbepnL-1 and showed that the loss of BbepnL-1 reduced the virulence in the silkworm. These results demonstrated that the endothiapepsin-like protein of B. bassiana is a virulence factor.

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 Na+ /H+ antiporter Nhx1 controls vacuolar fusion indispensible for life cycles in vitro and in vivo in a fungal insect pathogen

The sole Na⁺ /H⁺ antiporter Nhx1 has been generally unexplored in filamentous fungi. We characterized Nhx1 in the entomopathogenic fungus Beauveria bassiana. An eGFP-tagged Nhx1 fusion accumulated in small punctuate structures, presumably endosomal and trans-Golgi network compartments, between septum and tubular vacuole of each wild-type cell stained with a vacuole-specific dye. Deletion of nhx1 resulted in significant acidification and severe fusion defect in vacuoles, which were fragmented and distinct from large or tubular wild-type vacuoles. The deletion also caused a drastic reduction in aerial conidiation or submerged blastospore production and more severe defect in vegetative growth than in conidial germination. The Δnhx1 mutant became more sensitive to high osmolarity, heat shock and several metal ions during growth but its conidia showed increased UV-B tolerance. Intriguingly, Δnhx1 was unable to infect a model insect through cuticle penetration or intrahaemocoel injection because it produced much less biomass and cuticle-degrading enzymes in a minimal broth and failed to form blastospores in the insect haemolymph. All changes were completely or largely restored by targeted nhx1 complementation. Our results provide novel insight into an indispensability of Nhx1 for not only vacuolar fusion but also life cycles in vitro and in vivo in B. bassiana.

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.

Measuring Chitinase and Protease Activity in Cultures of Fungal Entomopathogens

Entomopathogenic fungi produce a variety of destructive enzymes and metabolites to overcome the unique defense mechanisms of insects. In a first step, fungal chitinases and proteinases need to break down the insect's cuticle. Both enzyme classes support the infection process by weakening the chitin barrier and by producing nutritional cleavage products for the fungus. In a second step, the pathogen can now mechanically penetrate the weakened cuticle and reach the insect's hemolymph where it starts proliferating. The critical enzymes chitinase and proteinase are also excreted into the supernatants of fungal cultures and can be used as indicators of virulence. Chromogenic assays adapted for 96-well microtiter plates that measure these enzymes provide a sensitive, fast, and easy screening method for evaluating the potential biocontrol activity of fungal isolates and may be considered as an alternative to laborious and time-consuming bioassays. Furthermore, monitoring fungal enzyme production in dependence of time, nutrient sources, or other factors can facilitate in establishing optimal growth and harvesting conditions for selected isolates with the aim of achieving maximum biocontrol activity.

Expression of chitinase genes of Metarhizium anisopliae isolates in lepidopteran pests and on synthetic media

Pathogenecity of the well characterized entomopathogenic fungus Metarhizium anisopliae used for biocontrol of a wide range of insect pests secretes hydrolytic enzymes that degrade the host cuticle. The chitinolytic activity of high and low virulent isolates of M. anisopliae was assayed on minimal medium (MM) + colloidal chitin and MM supplemented with insect cuticles. Ex- pression pattern of four chitinase genes (chitinase (chi), chi 1, chi 2, chi 3) was profiled during pathogenic stages of the entomopathogen under in vitro and in vivo conditions. Reverse-transcription polymerase chain reaction (RT-PCR) analysis confirmed that chitinase cDNAs were expressed during the germination of fungus under nutrient-deprived conditions. RT-PCR analysis performed for the four chitinase genes on the two insect hosts Spodoptera litura and Helicoverpa armigera at six developmental stages of the pathogen displayed up-regulation in S. litura at mycosed and conidiated condition while with H. armigera there was expression only after 48 h of incubation. Differential expression of chi, chi 1 and chi 2 genes in vitro (nitrogen rich and nitrogen limiting media) and in vivo (live insect hosts S. litura and H. armigera) implicate the role of substrate differences in pathogenesis.

Proteases as insecticidal agents

Proteases from a variety of sources (viruses, bacteria, fungi, plants, and insects) have toxicity towards insects. Some of these insecticidal proteases evolved as venom components, herbivore resistance factors, or microbial pathogenicity factors, while other proteases play roles in insect development or digestion, but exert an insecticidal effect when over-expressed from genetically engineered plants or microbial pathogens. Many of these proteases are cysteine proteases, although insect-toxic metalloproteases and serine proteases have also been examined. The sites of protease toxic activity range from the insect midgut to the hemocoel (body cavity) to the cuticle. This review discusses these insecticidal proteases along with their evaluation and use as potential pesticides.