Spruce bark accommodates high levels of phenolic compounds that help protect trees from harmful fungi. Researchers on the Max Planck Institute for Chemical Ecology in Jena wanted to know how these chemical defenses move through the forest food web. Their work focuses on the spruce bark beetle (IPS typographus), which ingests these compounds while feeding on tree tissue. The scientists asked if the beetle could reuse the tree’s defense chemicals to guard itself from fungal infection.
To investigate this process, the team used advanced tools including mass spectrometry and nuclear magnetic resonance (NMR) to discover and track the defense chemicals produced by spruce trees and determine how bark beetles act on them. The researchers found that beetles feeding on spruce trees absorbed defensive compounds from the phloem, particularly phenolic glycosides comparable to stilbenes and flavonoids.
Inside the beetles, these compounds are chemically altered. Insects convert them into aglycones, which not contain sugar molecules and have stronger antimicrobial effects. These modified chemicals provide the beetles with effective protection against fungal pathogens. “We did not expect beetles to be able to convert such targeted spur defenses into more toxic derivatives,” said lead writer Ryu Sun of the Department of Biochemistry.
A fungus that may disable beetle chemical defenses
The scientists then examined how these beetle defenses affected the fungus. “Although this fungus has not been effective in controlling bark beetles in the past, we found strains that naturally infected and killed them. So we wanted to investigate more closely how they were able to successfully infect the beetles,” Ryu Sun explained.
Laboratory evaluation and enzyme tests show that the fungus uses a two-step cleansing strategy. The first step is glycosylation, which attaches a sugar back to toxic aglycones. The second step is methylation, which attaches the methyl group to the sugar. The final compounds, called methylglycosides, aren’t harmful to him.
This chemical modification unexpectedly helps infect fungi, especially people who previously consumed spruce tissue wealthy in phenolic compounds. Methylglycosides are also immune to enzymatic enzymes that might normally break them down and restore their toxicity by hydrolysis.
Gene studies confirm the cleansing mechanism
To confirm the importance of this pathway, the researchers inactivated the gene liable for methylglycosylation. Cocci without these genes were much less successful at infecting bark beetles, indicating that cleansing is vital for overcoming the beetles’ chemical defenses.
The study shows that the defense chemicals of trees will be modified repeatedly as they move from plants to insects after which to pathogens. These changes have major implications for the continued evolutionary struggle between trees, insects and fungi. “We have shown that the bark beetle can use a tree’s defense compounds to defend against its own hosts. However, since an enemy, a fungus, has developed the ability to detoxify these antimicrobial defenses, it can successfully infect the bark beetle and thus help the tree in its fight against the bark beetle,” study leader Summaries said. “
Improving biological control of bark beetles
These findings may help scientists develop more practical biological control methods. “Now that we know which strains of the fungus are tolerant to bark beetle antimicrobial phenolic compounds, we can use these strains to fight bark beetles more effectively.” This study also highlights the necessity to examine whether pests have developed resistance or cleansing strategies when biological pesticides are used.
In future research, the team plans to research how common the methylglycosylation cleansing pathway is in other fungi that infect bark beetles. They also aim to know how this pathway interacts with other pathogen properties that affect infection success.