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.

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.

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.

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.

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

Entomopathogenic fungi utilize specific secondary metabolites to defend against insect immunity, thereby enabling colonization of their specific hosts. We are particularly interested in the polyketide synthesis gene pks15, which is involved in metabolite production, and its role in fungal virulence. Targeted disruption of pks15 followed by genetic complementation with a functional copy of the gene would allow for functional characterization of this secondary metabolite biosynthesis gene. Using a Beauveria bassiana ∆pks15 mutant previously disrupted by a bialophos-resistance (bar) cassette, we report here an in-cis complementation at bar cassette using CRISPR/Cas9 gene editing. A bar-specific short guide RNA was used to target and cause a double-strand break in bar, and a donor DNA carrying a wild-type copy of pks15 was co-transformed with the guide RNA. Isolate G6 of ∆pks15 complemented with pks15 was obtained and verified by PCR, Southern analyses and DNA sequencing. Compared to ∆pks15 which showed a marked reduction in sporulation and insect virulence, the complementation in G6 restored with insect virulence, sporulation and conidial germination to wild-type levels. Atomic force and scanning electron microscopy revealed that G6 and wild-type conidial wall surfaces possessed the characteristic rodlet bundles and rough surface while ∆pks15 walls lacked the bundles and were relatively smoother. Conidia of ∆pks15 were larger and more elongated than that of G6 and the wild type, indicating changes in their cell wall organization. Our data indicate that PKS15 and its metabolite are likely not only important for fungal virulence and asexual reproduction, but also cell wall formation.