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

The fungal protease BbAorsin contributes to growth, conidiation, germination, virulence, and antiphytopathogenic activities in Beauveria bassiana (Hypocreales: Cordycipitaceae)

The fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), is one of the most destructive agricultural pests. The entomopathogenic fungus Beauveria bassiana (Hypocreales: Clavicipitaceae) is a biopesticide widely used for biocontrol of various pests. Secreted fungal proteases are critical for insect cuticle destruction and successful infection. We have previously shown that the serine protease BbAorsin in B. bassiana has entomopathogenic and antiphytopathogenic activities. However, the contribution of BbAorsin to fungal growth, conidiation, germination, virulence and antiphytopathogenic activities remains unclear. In this study, the deletion (ΔBbAorsin), complementation (Comp), and overexpression (BbAorsinOE) strains of B. bassiana were generated for comparative studies. The results showed that ΔBbAorsin exhibited slower growth, reduced conidiation, lower germination rate, and longer germination time compared to WT and Comp. In contrast, BbAorsinOE showed higher growth rate, increased conidiation, higher germination rate and shorter germination time. Injection of BbAorsinOE showed the highest virulence against S. frugiperda larvae, while injection of ΔBbAorsin showed the lowest virulence. Feeding BbAorsinOE resulted in lower pupation and adult eclosion rates and malformed adults. 16S rRNA sequencing revealed no changes in the gut microbiota after feeding either WT or BbAorsinOE. However, BbAorsinOE caused a disrupted midgut, leakage of gut microbiota into the hemolymph, and upregulation of apoptosis and immunity-related genes. BbAorsin can disrupt the cell wall of the phytopathogen Fusarium graminearum and alleviate symptoms in wheat seedlings and cherry tomatoes infected with F. graminearum. These results highlight the importance of BbAorsin for B. bassiana and its potential as a multifunctional biopesticide.

SfMBP: A novel microbial binding protein and pattern recognition receptor in the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)

The fall armyworm, Spodoptera frugiperda, poses a significant threat as a highly destructive agricultural pest in many countries. Understanding the complex interplay between the insect immune system and entomopathogens is critical for optimizing biopesticide efficacy. In this study, we identified a novel microbial binding protein, SfMBP, in S. frugiperda. However, the specific role of SfMBP in the immune response of S. frugiperda remains elusive. Encoded by the LOC118269163 gene, SfMBP shows significant induction in S. frugiperda larvae infected with the entomopathogen Beauveria bassiana. Consisting of 115 amino acids with a signal peptide, an N-terminal flexible region and a C-terminal β-sheet, SfMBP lacks any known functional domains. It is expressed predominantly during early larval stages and in the larval epidermis. Notably, SfMBP is significantly induced in larvae infected with bacteria and fungi and in SF9 cells stimulated by peptidoglycan. While recombinant SfMBP (rSfMBP) does not inhibit bacterial growth, it demonstrates binding capabilities to bacteria, fungal spores, peptidoglycan, lipopolysaccharides, and polysaccharides. This binding is inhibited by monosaccharides and EDTA. Molecular docking reveals potential Zn2+-interacting residues and three cavities. Furthermore, rSfMBP induces bacterial agglutination in the presence of Zn2+. It also binds to insect hemocytes and SF9 cells, enhancing phagocytosis and agglutination responses. Injection of rSfMBP increased the survival of S. frugiperda larvae infected with B. bassiana, whereas blocking SfMBP with the antibody decreased survival. These results suggest that SfMBP acts as a pattern recognition receptor that enhances pathogen recognition and cellular immune responses. Consequently, this study provides valuable insights for the development of pest control measures.