The polyketide synthase PKS15 has a crucial role in cell wall formation in Beauveria bassiana

The PKS-NRPS Gene BBA_09856 Deletion Mutant of Beauveria bassiana Enhanced Its Virulence Against Ostrinia furnacalis Larvae and Strengthened the Host Plant's Resistance to Botrytis cinerea as an Endotype

Nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) play crucial roles in the development and pathogenicity of the entomopathogenic fungus Beauveria bassiana. However, they are among the few biosynthetic gene clusters with unknown functions in B. bassiana. To investigate the role of the hybrid PKS-NRPS synthetase gene BBA_09856 in B. bassiana, we constructed a mutant strain, ∆BBA09856-WT, by deleting the BBA_09856 gene through Agrobacterium-mediated transformation. We then analyzed the biological characteristics of the mutant strain and the virulence of the mutant strain toward Ostrinia furnacalis larvae, as well as its antagonistic effects against the phytopathogen Botrytis cinerea. We found that the average growth rate of the three mutant strains, ∆BBA09856-WT, was significantly higher compared to the wild-type (WT) strain on the 15th day of culture on potato dextrose agar (PDA) plates (7.01 cm vs. 6.30 cm, p < 0.01). Additionally, the average spore production(3.16 × 107/cm2 vs. 9.95 × 106/cm2, p < 0.001) and germination rate (82.50% vs. 54.72%, 12 h, p < 0.001) were significantly different between the three mutant strains, ∆BBA09856-WT, and the WT strain. The average survival rates of O. furnacalis infected with the WT strain and the three mutant strains, ∆BBA09856-WT, after 8 days were 61.66%, and 30.00%, respectively, indicating that the pathogenicity of the tested mutant strains was significantly greater than that of the WT strain. The results of the dual culture test indicated that the inhibitory rates of the WT and ∆BBA09856-WT strains against B. cinerea were 40.25% and 47.65%, respectively (p < 0.001). Similarly, in the dual culture test, the WT strain reduced the growth of B. cinerea by 9.90%, while the ∆BBA09856-WT exhibited a significantly greater inhibition rate of 28.29% (p < 0.05). The diameters of disease spots, measured 6 d after inoculation with B. cinerea in the tomato treatment groups, revealed significant differences in endophytic colonization between the WT and ∆BBA09856-WT strains in the WT+Bc and ∆BBA09856-WT+Bc treatment groups (15.26 mm vs. 12.16 mm, p < 0.01). Notably, ∆BBA09856-WT exhibited enhanced virulence toward O. furnacalis larvae and increased antagonistic activity against B. cinerea. Our results indicate that the gene BBA_09856 may have a negative correlation with the development and virulence of B. bassiana toward the insect pest O. furnacalis larvae, as well as its antagonism against B. cinerea. These findings suggest that molecular techniques, such as gene editing, could be employed to develop superior strains of B. bassiana for the biological control of plant diseases and insect pests.

Serine Hydrolase-Catalyzed Polyol Lipids are Necessary for Rodlet Layer Formation on the Cell Wall of Entomopathogenic Fungi

Some key secondary metabolism genes are important for driving the infection process of entomopathogenic fungi; however, their chemical substance basis has not been well investigated. Here, mixtures of polyol lipids are discovered, which are synthesized through iterative chain transfer-esterification-hydrolysis cycles catalyzed by serine hydrolase during the release of online highly reducing polyketide intermediates. Importantly, an in vivo gene knockout experiment revealed that the synthesis of polyol lipids is necessary for rodlet layer formation on the cell wall of Beauveria bassiana. Our work uncovers an unexpected way for the synthesis of polyol lipids and illuminates a new perspective on their part in significant physiological processes in entomopathogenic fungi.

Unravelling the Function of the Sesquiterpene Cyclase STC3 in the Lifecycle of Botrytis cinerea

The genome sequencing of Botrytis cinerea supplies a general overview of the map of genes involved in secondary metabolite synthesis. B. cinerea genomic data reveals that this phytopathogenic fungus has seven sesquiterpene cyclase (Bcstc) genes that encode proteins involved in the farnesyl diphosphate cyclization. Three sesquiterpene cyclases (BcStc1, BcStc5 and BcStc7) are characterized, related to the biosynthesis of botrydial, abscisic acid and (+)-4-epi-eremophilenol, respectively. However, the role of the other four sesquiterpene cyclases (BcStc2, BcStc3, BcStc4 and BcStc6) remains unknown. BcStc3 is a well-conserved protein with homologues in many fungal species, and here, we undertake its functional characterization in the lifecycle of the fungus. A null mutant ΔBcstc3 and an overexpressed-Bcstc3 transformant (OvBcstc3) are generated, and both strains show the deregulation of those other sesquiterpene cyclase-encoding genes (Bcstc1, Bcstc5 and Bcstc7). These results suggest a co-regulation of the expression of the sesquiterpene cyclase gene family in B. cinerea. The phenotypic characterization of both transformants reveals that BcStc3 is involved in oxidative stress tolerance, the production of reactive oxygen species and virulence. The metabolomic analysis allows the isolation of characteristic polyketides and eremophilenols from the secondary metabolism of B. cinerea, although no sesquiterpenes different from those already described are identified.

Activation of Ustilaginoidin Biosynthesis Gene uvpks1 in Villosiclava virens Albino Strain LN02 Influences Development, Stress Responses, and Inhibition of Rice Seed Germination

Villosiclava virens (anamorph: Ustilaginoidea virens) is the pathogen of rice false smut (RFS), which is a destructive rice fungal disease. The albino strain LN02 is a natural white-phenotype mutant of V. virens due to its incapability to produce toxic ustilaginoidins. In this study, three strains including the normal strain P1, albino strain LN02, and complemented strain uvpks1C-1 of the LN02 strain were employed to investigate the activation of the ustilaginoidin biosynthesis gene uvpks1 in the albino strain LN02 to influence sporulation, conidia germination, pigment production, stress responses, and the inhibition of rice seed germination. The activation of the ustilaginoidin biosynthesis gene uvpks1 increased fungal tolerances to NaCl-induced osmotic stress, Congo-red-induced cell wall stress, SDS-induced cell membrane stress, and H2O2-induced oxidative stress. The activation of uvpks1 also increased sporulation, conidia germination, pigment production, and the inhibition of rice seed germination. In addition, the activation of uvpks1 was able to increase the mycelial growth of the V. virens albino strain LN02 at 23 °C and a pH from 5.5 to 7.5. The findings help in understanding the effects of the activation of uvpks1 in albino strain LN02 on development, pigment production, stress responses, and the inhibition of rice seed germination by controlling ustilaginoidin biosynthesis.

Effects of Epigenetic Modification and High Hydrostatic Pressure on Polyketide Synthase Genes and Secondary Metabolites of Alternaria alternata Derived from the Mariana Trench Sediments

The hadal biosphere is the most mysterious ecosystem on the planet, located in a unique and extreme environment on Earth. To adapt to extreme environmental conditions, hadal microorganisms evolve special strategies and metabolisms to survive and reproduce. However, the secondary metabolites of the hadal microorganisms are poorly understood. In this study, we focused on the isolation and characterization of hadal fungi, screening the potential strains with bioactive natural products. The isolates obtained were detected further for the polyketide synthase (PKS) genes. Two isolates of Alternaria alternata were picked up as the representatives, which had the potential to synthesize active natural products. The epigenetic modifiers were used for the two A. alternata isolates to stimulate functional gene expression in hadal fungi under laboratory conditions. The results showed that the chemical epigenetic modifier, 5-Azacytidine (5-Aza), affected the phenotype, PKS gene expression, production of secondary metabolites, and antimicrobial activity of the hadal fungus A. alternata. The influence of epigenetic modification on natural products was strongest when the concentration of 5-Aza was 50 μM. Furthermore, the modification of epigenetic agents on hadal fungi under high hydrostatic pressure (HHP) of 40 MPa displayed significant effects on PKS gene expression, and also activated the production of new compounds. Our study demonstrates the high biosynthetic potential of cultivable hadal fungi, but also provides evidence for the utility of chemical epigenetic modifiers on active natural products from hadal fungi, providing new ideas for the development and exploitation of microbial resources in extreme environments.

Metabolomic Analysis Demonstrates the Impacts of Polyketide Synthases PKS14 and PKS15 on the Production of Beauvericins, Bassianolide, Enniatin A, and Ferricrocin in Entomopathogen Beauveria bassiana

Beauveria bassiana is a globally distributed entomopathogenic fungus that produces various secondary metabolites to support its pathogenesis in insects. Two polyketide synthase genes, pks14 and pks15, are highly conserved in entomopathogenic fungi and are important for insect virulence. However, understanding of their mechanisms in insect pathogenicity is still limited. Here, we overexpressed these two genes in B. bassiana and compared the metabolite profiles of pks14 and pks15 overexpression strains to those of their respective knockout strains in culture and in vivo using tandem liquid chromatography-mass spectrometry (LC-MS/MS) with Global Natural Products Social Molecular Networking (GNPS). The pks14 and pks15 clusters exhibited crosstalk with biosynthetic clusters encoding insect-virulent metabolites, including beauvericins, bassianolide, enniatin A, and the intracellular siderophore ferricrocin under certain conditions. These secondary metabolites were upregulated in the pks14-overexpressing strain in culture and the pks15-overexpressing strain in vivo. These data suggest that pks14 and pks15, their proteins or their cluster components might be directly or indirectly associated with key pathways in insect pathogenesis of B. bassiana, particularly those related to secondary metabolism. Information about interactions between the polyketide clusters and other biosynthetic clusters improves scientific understanding about crosstalk among biosynthetic pathways and mechanisms of pathogenesis.

The expanding CRISPR toolbox for natural product discovery and engineering in filamentous fungi

Covering: up to May 2022Fungal genetics has transformed natural product research by enabling the elucidation of cryptic metabolites and biosynthetic steps. The enhanced capability to add, subtract, modulate, and rewrite genes via CRISPR/Cas technologies has opened up avenues for the manipulation of biosynthetic gene clusters across diverse filamentous fungi. This review discusses the innovative and diverse strategies for fungal natural product discovery and engineering made possible by CRISPR/Cas-based tools. We also provide a guide into multiple angles of CRISPR/Cas experiment design, and discuss current gaps in genetic tool development for filamentous fungi and the promising opportunities for natural product research.

Microbial polyketides and their roles in insect virulence: from genomics to biological functions

Covering: May 1966 up to January 2022Entomopathogenic microorganisms have potential for biological control of insect pests. Their main secondary metabolites include polyketides, nonribosomal peptides, and polyketide-nonribosomal peptide (PK-NRP) hybrids. Among these secondary metabolites, polyketides have mainly been studied for structural identification, pathway engineering, and for their contributions to medicine. However, little is known about the function of polyketides in insect virulence. This review focuses on the role of bacterial and fungal polyketides, as well as PK-NRP hybrids in insect infection and killing. We also discuss gene distribution and evolutional relationships among different microbial species. Further, the role of microbial polyketides and the hybrids in modulating insect-microbial symbiosis is also explored. Understanding the mechanisms of polyketides in insect pathogenesis, how compounds moderate the host-fungus interaction, and the distribution of PKS genes across different fungi and bacteria will facilitate the discovery and development of novel polyketide-derived bio-insecticides.

The Entomopathogenic Fungus Beauveria bassiana Shows Its Toxic Side within Insects: Expression of Genes Encoding Secondary Metabolites during Pathogenesis

Entomopathogenic fungi are extensively used for the control of insect pests worldwide. Among them, Beauveria bassiana (Ascomycota: Hypocreales) produce a plethora of toxic secondary metabolites that either facilitate fungal invasion or act as immunosuppressive compounds. These toxins have different chemical natures, such as nonribosomal peptides and polyketides. Even though their precise role is poorly understood, they are usually linked to virulence. These fungal secondary metabolites are produced by the expression of gene clusters encoding the various proteins needed for their biosynthesis. Each cluster includes synthetases for nonribosomal peptides (NRPS), polyketides (PKS), or hybrid NRPS–PKS genes. The aim of this review is to summarize the information available from transcriptomics and quantitative PCR studies related to the expression of B. bassiana NRPS and PKS genes inside different insects as the infection progresses; as for the host immune response, to help understand the mechanisms that these toxins trigger as virulence factors, antimicrobials, or immunosuppressives within the context of a fungus–insect interaction.

The Fungus Metarhizium sp. BCC 4849 Is an Effective and Safe Mycoinsecticide for the Management of Spider Mites and Other Insect Pests

Simple Summary

The spider mite is a destructive pest of various crops during warm and dry conditions in tropical countries, including Thailand. The pest is difficult to control despite synthetic acaricide use. In the field, insect populations gradually develop resistance to synthetic pesticides over long-term use. The use of entomopathogenic fungi is more human- and environmentally friendly. We searched and identified a potential fungal isolate from a culture collection, focusing on the genus Metarhizium. Metarhizium sp. BCC 4849 not only controls the spider mite but also plays a significant role as a natural regulator of other insect pests. Here, we investigated its infection process on the mite, optimized a conidial formulation for extended shelf life, and conducted toxicological assays in animals to assess its biosafety in humans. The fungal genome has been sequenced. The genomic data indicated that oxidoreduction proteins; zinc-, heme-, and iron-binding proteins; and transmembrane transporters are abundant in the genome.

Abstract

Five isolates of Metarhizium sp. were evaluated for their pathogenicity against the spider mite (Tetranychus truncatus Ehara) (Acari: Tetranychidae) and Metarhizium sp. BCC 4849 resulted in the highest mortality (82%) on the 5th day post-inoculation (DPI). Subsequent insect bioassay data indicated similar high virulence against five other insects: African red mites (Eutetranychus africanus Tucker) (Acari: Tetranychidae), bean aphid (Aphis craccivora Koch) (Hemiptera: Aphididae), cassava mealybug (Phenacoccus manihoti Matile-Ferrero) (Hemiptera: Pseudococcidae), sweet potato weevil (Cylas formicarius Fabricius) (Coleoptera: Brentidae), and oriental fruit fly (Bactrocera dorsalis Hendel) (Diptera: Tephritidae), at mortalities of 92–99%, on 3rd–6th DPI, and in laboratory conditions. The pathogenicity assay against E. africanus in hemp plants under greenhouse conditions indicated 85–100% insect mortality on 10th DPI using the fungus alone or in combination with synthetic acaricide. Genome sequencing of Metarhizium sp. BCC 4849 revealed the high abundance of proteins associated with zinc-, heme-, and iron-binding; oxidation-reduction; and transmembrane transport, implicating its versatile mode of interaction with the environment and adaptation to various ion homeostasis. The light and scanning electron microscopy indicated that at 24 h post inoculation (PI), adhesion and appressorial formation occurred, notably near the setae. Most infected mites had stopped moving and started dying by 48–72 h PI. Elongated hyphal bodies and oval blastospores were detected in the legs. At 96–120 h PI or longer, dense mycelia and conidial mass had colonized the interior and exterior of dead mites, primarily at the bottom than the upper part. The shelf-life study also indicated that conidial formulation combined with an oxygen-moisture absorber markedly enhanced the viability and germination after storage at 35 °C for four months. The fungus was tested as safe for humans and animals, according to our toxicological assays.

The integrative taxonomy of Beauveria asiatica and B. bassiana species complexes with whole-genome sequencing, morphometric and chemical analyses

Fungi are rich in complexes of cryptic species that need a combination of different approaches to be delimited, including genomic information. Beauveria (Cordycipitaceae, Hypocreales) is a well-known genus of entomopathogenic fungi, used as a biocontrol agent. In this study we present a polyphasic taxonomy regarding two widely distributed complexes of Beauveria: B. asiatica and B. bassiana s.lat. Some of the genetic groups as previously detected within both taxa were either confirmed or fused using population genomics. High levels of divergence were found between two clades in B. asiatica and among three clades in B. bassiana, supporting their subdivision as distinct species. Morphological examination focusing on the width and the length of phialides and conidia showed no difference among the clades within B. bassiana while conidial length was significantly different among clades within B. asiatica. The secondary metabolite profiles obtained by liquid chromatography-mass spectrometry (LC-MS) allowed a distinction between B. asiatica and B. bassiana, but not between the clades therein. Based on these genomic, morphological, chemical data, we proposed a clade of B. asiatica as a new species, named B. thailandica, and two clades of B. bassiana to respectively represent B. namnaoensis and B. neobassiana spp. nov. Such closely related but divergent species with different host ranges have potential to elucidate the evolution of host specificity, with potential biocontrol application. Citation: Kobmoo N, Arnamnart N, Pootakham W, et al. 2021. The integrative taxonomy of Beauveria asiatica and B. bassiana species complexes with whole-genome sequencing, morphometric and chemical analyses. Persoonia 47: 136-150. https://doi.org/10.3767/persoonia.2021.47.04.

Genome editing for resistance against plant pests and pathogens

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

Vital roles of Pks11, a highly reducing polyketide synthase, in fungal conidiation, antioxidant activity, conidial cell wall integrity, and UV tolerance of Beauveria bassiana

Fungal polyketide synthases play important and differential roles in synthesizing secondary metabolites and regulating several cell events, including asexual development, environmental adaptation, and pathogenicity. This study shows the important functions of a highly reducing polyketide synthase, Pks11, in Beauveria bassiana, a filamentous fungal insect pathogen used worldwide for pest biocontrol. The deletion of pks11 led to severe defects in conidial yields on different media and a decrease of 36.27% in the mean thickness of conidial cell wall under normal conditions. Compared with the wild-type, Δpks11 showed higher tolerance to oxidation and increased sensitivity to high temperature during colony growth. Moreover, the lack of pks11 caused a decrease in conidial germination after exposure to UV radiation but did not affect the virulence of B. bassiana against Galleria mellonella larvae via typical cuticle infection. These findings concurred with the alteration in the transcript levels of some phenotype-related genes. These data suggested that pks11 played vital roles in the asexual development, cell wall integrity, and fungal responses to oxidation, high temperature, and UV irradiation of B. bassiana.