Pathogenicity of microsclerotia from Metarhizium robertsii against Aedes aegypti larvae and antimicrobial peptides expression by mosquitoes during fungal-host interaction

Aedes aegypti is a vector of various disease-causing arboviruses. Chemical insecticide-based methods for mosquito control have increased resistance in different parts of the world. Thus, alternative control agents such as the entomopathogenic fungi are excellent candidates to control mosquitoes as part of an ecofriendly strategy. There is evidence of the potential of entomopathogenic fungal conidia and blastospores for biological control of eggs, larval and adult stages, as well as the pathogenicity of fungal microsclerotia against adults and eggs. However, there are no studies on the pathogenicity of microsclerotia against either aquatic insects or insects that develop part of their life cycle in the water, such as the A. aegypti larvae. In this study, we assayed the production of microsclerotia and their pathogenicity against A. aegypti larvae of two isolates of Metarhizium robertsii, i.e., CEP 423 isolated in La Plata, Argentina, and the model ARSEF 2575. Both isolates significantly reduced the survival of A. aegypti exposed to their microsclerotia. The fungus-larva interaction resulted in a delayed response in the host. This was evidenced by the expression of some humoral immune system genes such as defensins and cecropin on the 9th day post-infection, when the fungal infection was consolidated as a successful process that culminates in larvae mortality. In conclusion, M. robertsii microsclerotia are promising propagules to be applied as biological control agents against mosquitoes since they produce pathogenic conidia against A. aegypti larvae.

Tolerance to Abiotic Factors of Microsclerotia and Mycelial Pellets From Metarhizium robertsii, and Molecular and Ultrastructural Changes During Microsclerotial Differentiation

Metarhizium species fungi are able to produce resistant structures termed microsclerotia, formed by compact and melanized threads of hyphae. These propagules are tolerant to desiccation and produce infective conidia; thus, they are promising candidates to use in biological control programs. In this study, we investigated the tolerance to both ultraviolet B (UV-B) radiation and heat of microsclerotia of Metarhizium robertsii strain ARSEF 2575. We also adapted the liquid medium and culture conditions to obtain mycelial pellets from the same isolate in order to compare these characteristics between both types of propagules. We followed the peroxisome biogenesis and studied the oxidative stress during differentiation from conidia to microsclerotia by transmission electron microscopy after staining with a peroxidase activity marker and by the expression pattern of genes potentially involved in these processes. We found that despite their twice smaller size, microsclerotia exhibited higher dry biomass, yield, and conidial productivity than mycelial pellets, both with and without UV-B and heat stresses. From the 16 genes measured, we found an induction after 96-h differentiation in the oxidative stress marker genes MrcatA, MrcatP, and Mrgpx; the peroxisome biogenesis factors Mrpex5 and Mrpex14/17; and the photoprotection genes Mrlac1 and Mrlac2; and Mrlac3. We concluded that an oxidative stress scenario is induced during microsclerotia differentiation in M. robertsii and confirmed that because of its tolerance to desiccation, heat, and UV-B, this fungal structure could be an excellent candidate for use in biological control of pests under tropical and subtropical climates where heat and UV radiation are detrimental to entomopathogenic fungi survival and persistence.

The entomopathogenic fungus Beauveria bassiana produces microsclerotia-like pellets mediated by oxidative stress and peroxisome biogenesis

Several filamentous fungi are known to produce macroscopic pigmented hyphal aggregates named sclerotia. In recent years, some entomopathogenic fungi were reported to produce small sclerotia termed 'microsclerotia', becoming new potential propagules for biocontrol strategies. In this study, we described the production of microsclerotia-like pellets by the entomopathogenic fungus Beauveria bassiana. The carbon: nitrogen ratio equal to or higher than 12.5:1 amended with Fe²⁺ induced the germination of conidia, producing hyphal aggregate that formed sclerotial structures in submerged liquid cultures. These aggregates were able to tolerate desiccation as they germinated and subsequently produced viable conidia. Conidia derived from microsclerotial aggregates formulated with diatomaceous earth effectively kill Tribolium castaneum larvae. Optical and transmission microscopical imaging, qPCR and spectrophotometric analysis revealed that an oxidative stress scenario is involved in conidial differentiation into microsclerotia-like pellets, inducing fungal antioxidant response with high peroxidase activity - mainly detected in peroxisomes and mitochondria - and progress with active peroxisome proliferation. The results provide clues about B. bassiana microsclerotial differentiation and indicate that these pigmented aggregates are promising propagules for production, formulation and potentially application in the control of soil-inhabiting arthropod pests.

The limpet transcription factors of Triatoma infestans regulate the response to fungal infection and modulate the expression pattern of defensin genes

As part of the innate humoral response to microbial attack, insects activate the expression of antimicrobial peptides (AMP). Understanding the regulatory mechanisms of this response in the Chagas disease vector Triatoma infestans is important since biological control strategies against pyrethroid-resistant insect populations were recently addressed by using the entomopathogenic fungus Beauveria bassiana. By bioinformatics, gene expression, and silencing techniques in T. infestans nymphs, we achieved sequence and functional characterization of two variants of the limpet transcription factor (Tilimpet) and studied their role as regulators of the AMP expression, particularly defensins, in fungus-infected insects. We found that Tilimpet variants may act differentially since they have divergent sequences and different relative expression ratios, suggesting that Tilimpet-2 could be the main regulator of the higher expressed defensins and Tilimpet-1 might play a complementary or more general role. Also, the six defensins (Tidef-1 to Tidef-6) exhibited different expression levels in fungus-infected nymphs, consistent with their phylogenetic clustering. This study aims to contribute to a better understanding of T. infestans immune response in which limpet is involved, after challenge by B. bassiana infection.

Riboflavin induces Metarhizium spp. to produce conidia with elevated tolerance to UV-B, and upregulates photolyases, laccases and polyketide synthases genes

AIMS

The effect of nutritional supplementation of two Metarhizium species with riboflavin (Rb) during production of conidia was evaluated on (i) conidial tolerance (based on germination) to UV-B radiation and on (ii) conidial expression following UV-B irradiation, of enzymes known to be active in photoreactivation, viz., photolyase (Phr), laccase (Lac) and polyketide synthase (Pks).

METHODS AND RESULTS

Metarhizium acridum (ARSEF 324) and Metarhizium robertsii (ARSEF 2575) were grown either on (i) potato dextrose agar medium (PDA), (ii) PDA supplemented with 1% yeast extract (PDAY), (iii) PDA supplemented with Rb (PDA+Rb), or (iv) PDAY supplemented with Rb (PDAY+Rb). Resulting conidia were exposed to 866·7 mW m^-2 of UV-B Quaite-weighted irradiance to total doses of 3·9 or 6·24 kJ m^-2 . Some conidia also were exposed to 16 klux of white light (WL) after being irradiated, or not, with UV-B to investigate the role of possible photoreactivation. Relative germination of conidia produced on PDA+Rb (regardless Rb concentration) or on PDAY and exposed to UV-B was higher compared to conidia cultivated on PDA without Rb supplement, or to conidia suspended in Rb solution immediately prior to UV-B exposure. The expression of MaLac3 and MaPks2 for M. acridum, as well as MrPhr2, MrLac1, MrLac2 and MrLac3 for M. robertsii was higher when the isolates were cultivated on PDA+Rb and exposed to UV-B followed by exposure to WL, or exposed to WL only.

CONCLUSIONS

Rb in culture medium increases the UV-B tolerance of M. robertsii and M. acridum conidia, and which may be related to increased expression of Phr, Lac and Pks genes in these conidia.

SIGNIFICANCE AND IMPACT OF THE STUDY

The enhanced UV-B tolerance of Metarhizium spp. conidia produced on Rb-enriched media may improve the effectiveness of these fungi in biological control programs.

Alkane-grown Beauveria bassiana produce mycelial pellets displaying peroxisome proliferation, oxidative stress, and cell surface alterations

The entomopathogenic fungus Beauveria bassiana is able to grow on insect cuticle hydrocarbons, inducing alkane assimilation pathways and concomitantly increasing virulence against insect hosts. In this study, we describe some physiological and molecular processes implicated in growth, nutritional stress response, and cellular alterations found in alkane-grown fungi. The fungal cytology was investigated using light and transmission electron microscopy while the surface topography was examined using atomic force microscopy. Additionally, the expression pattern of several genes associated with oxidative stress, peroxisome biogenesis, and hydrophobicity were analysed by qPCR. We found a novel type of growth in alkane-cultured B. bassiana similar to mycelial pellets described in other alkane-free fungi, which were able to produce viable conidia and to be pathogenic against larvae of the beetles Tenebrio molitor and Tribolium castaneum. Mycelial pellets were formed by hyphae cumulates with high peroxidase activity, exhibiting peroxisome proliferation and an apparent surface thickening. Alkane-grown conidia appeared to be more hydrophobic and cell surfaces displayed different topography than glucose-grown cells. We also found a significant induction in several genes encoding for peroxins, catalases, superoxide dismutases, and hydrophobins. These results show that both morphological and metabolic changes are triggered in mycelial pellets derived from alkane-grown B. bassiana.