Spruce bark is rich in phenolic compounds that protect trees from pathogenic fungi. A research team at the Max Planck Institute for Chemical Ecology in Jena investigated how these plant defenses function within the food web, particularly in spruce bark beetles (Ips typographus), which ingest the compounds through their diet. Could the beetles use substances from the spruce's defenses to protect themselves against pathogenic fungi?
Beetles convert plant defenses into even more toxic forms
Using state-of-the-art analytical methods such as mass spectrometry and nuclear magnetic resonance (NMR), the team investigated which chemical compounds spruce trees produce for defense and how these compounds are metabolized by bark beetles. The team demonstrated that bark beetles infesting spruce trees utilize the trees' defensive substances found in the phloem, particularly phenolic glycosides such as stilbenes and flavonoids, to bolster their defense against pathogens. They convert these compounds into more toxic aglycones, which are sugar-free and have increased antimicrobial activity. These aglycones serve as an effective defense against fungi. "We did not expect the beetles to be able to convert the spruce's defenses into more toxic derivatives in such a targeted way," said the lead author Ruo Sun from the Department of Biochemistry.
The fungus neutralizes the beetles' defenses via specific detoxification pathways
Then, the scientists investigated how the beetle defense substances affected the fungus Beauveria bassiana. "Although this fungus has not been effective in controlling bark beetles in the past, we found strains that had naturally infected and killed them. We therefore wanted to investigate more closely how they were able to successfully infect the beetles," Ruo Sun explains.
Further analyses and enzyme assays revealed that the fungus employs a two-step detoxification process. First, there is glycosylation, which involves the re-addition of a sugar to the aglycones. Second, there is methylation, which involves the binding of a methyl group to the sugar. The resulting methylglycoside derivatives are not toxic to Beauveria bassiana. Interestingly, methylglycosylation increases fungal infestation, particularly in beetles that had previously consumed plant tissue with a high phenol content. Additionally, methylglycosides are resistant to beetle enzymes that would restore the compounds' toxicity through hydrolysis.
The scientists tested the function of the detoxification pathway in Beauveria bassiana by knocking out the genes responsible for methylglycosylation. Further experiments revealed that fungi lacking these genes, and thus the detoxification pathway, were far less effective at infesting bark beetles.
An evolutionary balancing act with potential application
The study clearly shows that a tree's chemical defenses can undergo multiple transformations and retransformations throughout the food chain – with far-reaching consequences for the evolutionary arms race between hosts, pests, and pathogens. "We have demonstrated that a bark beetle can co-opt a tree's defensive compounds to make defenses against its own enemies. However, since one of the enemies, the fungus Beauveria bassiana, has developed the ability to detoxify these antimicrobial defenses, it can successfully infect the bark beetles and thus actually help the tree in its battle against bark beetles," summarizes the study's leader Jonathan Gershenzon.
These findings could lead to the development of more effective biological control agents against bark beetles. "Now that we know which strains of the fungus tolerate the bark beetle's antimicrobial phenolic compounds, we can use these strains to combat bark beetles more efficiently," says Ruo Sun, pointing out the potential applications. The study emphasizes the importance of checking for resistance or detoxification strategies developed by the pest against its host when using biological pesticides.
In further experiments, the research team wants to determine how widespread the methylglycosylation detoxification pathway is in different strains of the fungus Beauveria bassiana and in other bark beetle pathogens. They also want to understand how this pathway interacts with other characteristics of pathogens that influence its effectiveness.
