Microbial Response to Fungal Infection in a Fungus-Growing Termite, Odontotermes formosanus (Shiraki)

An odorant binding protein is involved in counteracting detection-avoidance and Toll-pathway innate immunity

INTRODUCTION

Odorant-binding proteins (OBPs) are a class of small molecular weight soluble proteins that exist as expanded gene families in all insects, acting as ligand carriers mediating olfaction and other physiological processes. During fungal infection, a subset of insect OBPs were shown to be differentially expressed.

OBJECTIVES

We tested whether the altered expression of insect OBPs during pathogenic infection plays a role in behavioral or immune interactions between insect hosts and their pathogens.

METHODS

A wide range of techniques including RNAi-directed knockdown, heterologous protein expression, electrophysiological/behavioral analyses, transcriptomics, gut microbiome analyses, coupled with tandem mass spectrometry ion monitoring, were used to characterize the function of a locust OBP in host behavioral and immune responses.

RESULTS

The entomopathogenic fungus Metarhizium anisopliae produces the volatile compound phenylethyl alcohol (PEA) that causes behavioral avoidance in locusts. This is mediated by the locust odorant binding protein 11 (LmOBP11). Expression of LmOBP11 is induced by M. anisopliae infection and PEA treatment. LmOBP11 participates in insect detection of the fungal-produced PEA and avoidance of PEA-contaminated food, but the upregulation of LmOBP11 upon M. anisopliae infection negatively affects the insect immune responses to ultimately benefit successful mycosis by the pathogen. RNAi knockdown of LmOBP11 increases the production of antimicrobial peptides and enhances locust resistance to M. anisopliae infection, while reducing host antennal electrophysiological responses to PEA and locust avoidance of PEA treated food. Also, transcriptomic and gut microbiome analyses reveal microbiome dysbiosis and changes in host genes involved in behavior and immunity. These results are consistent with the elevated expression of LmOBP11 leading to enhanced volatile detection and suppression of immune responses.

CONCLUSION

These findings suggest a crosstalk between olfaction and immunity, indicating manipulation of host OBPs as a novel target exploited by fungal pathogens to alter immune activation and thus promote the successful infection of the host.

Topical Fungal Infection Induces Shifts in the Gut Microbiota Structure of Brown Planthopper, Nilaparvata lugens (Homoptera: Delphacidae)

Simple Summary

Fungal entomopathogens are important natural enemies of insect pests and widely applied for biocontrol. Gut microbiota play important roles in mediating insect physiology and behavior. There is growing evidence that alteration of gut microbial communities due to pathological and environmental exposure can have detrimental impacts on host health and pathogen resistance. Here, we investigated the effects of topical infection with Metarhizium anisopliae fungus on the gut microbial community structure of the brown planthopper (Nilaparvata lugens, BPH), a destructive insect pest of rice. Our results demonstrated dramatic changes of gut bacterial community structure after topical fungal infection in BPH, as indicated by a significant increase in bacterial load, a significant decrease in bacterial community evenness and significant shifts in dominant bacterial abundance at the taxonomic level below the class. The dysbiosis of the gut bacteria might partly be due to the suppression of gut immunity caused by topical fungal infection. Our results highlighted the importance of the gut microbial community in fungal pathogenesis in insects.

Abstract

The brown planthopper (Nilaparvata lugens, BPH) is a destructive insect pest posing a serious threat to rice production. The fungal entomopathogen Metarhizium anisopliae is a promising alternative that can be used for BPH biocontrol. Recent studies have highlighted the significant involvement of gut microbiota in the insect–fungus interactions. In the presented study, we investigated the effects of topical fungal infection on the gut microbial community structure in BPH. Our results revealed that topical infection with M. anisopliae increased the bacterial load and altered the bacterial community structure in the gut of BPH. The relative abundances of the dominant gut bacteria at the order, family and genus level were significantly different between fungus-infected and uninfected groups. At the genus level, the uninfected BPH harbored high proportions of Pantoea and Enterobacter in the gut, whereas the fungus-infected BPH gut was absolutely dominated by Acinetobacter. Moreover, topical fungal infection significantly inhibited the expressions of immune-related genes encoding anti-microbial protein and dual oxidase that were involved in the maintenance of gut microbiota homeostasis, indicating that gut bacteria imbalance might be attributed in part to the suppression of gut immunity caused by fungal pathogen. Our results highlighted the importance of the gut microbial community during interactions between fungal pathogens and insect hosts.