A research team at the Max Planck Institute for Chemical Ecology has uncovered a previously unknown strategy used by certain fungi to neutralize chemical defenses in spruce trees. These fungi are introduced into the trees by bark beetles, and their success in breaking down the tree’s defenses helps the beetles invade more effectively. The researchers found that many of these fungi can metabolize the tree's defensive compounds and spread quickly through the bark. Two specific fungi were identified as being particularly effective: they can deactivate the breakdown products of these defenses by binding a simple sugar molecule called ribose to them. This process makes the compounds harmless and stable, preventing them from being reactivated into their original, more toxic forms. The fungus that showed the strongest ability to bind ribose thrived especially well in spruce bark, suggesting that this ability gives it a survival advantage. Spruce trees are known for their strong chemical defenses against herbivores and pathogens. The phloem, the living layer of bark that transports nutrients from the needles to the roots, is one of the most chemically protected parts of the tree. It contains high concentrations of phenolic compounds, which can make up as much as 5% of the bark's dry weight. Despite these defenses, many fungi—both harmful pathogens and those that live in partnership with bark beetles—successfully colonize this tissue. "We wanted to understand the biochemical tricks these fungi use to survive in such a hostile environment, especially since some of them may play a role in large-scale bark beetle infestations that threaten European forests," says Ruo Sun, the lead author of the study. To uncover the mechanisms behind this survival, the research team used molecular biology techniques and chemical analysis to study the metabolism of various fungi. They grew the fungi on spruce bark and artificial media, then examined the metabolic byproducts. They found that some fungi could break down the toxic compounds in the bark, but the resulting substances remained harmful. Only two species of fungi took an additional step: they attached ribose, a five-carbon sugar, to the breakdown products. This process, known as ribosylation, reduces the toxicity of the compounds and prevents them from being reactivated by other organisms. The fungus with the strongest ribosylation ability grew best on the bark, indicating that this ability is a significant advantage for survival. Confirming the presence of ribose required detailed nuclear magnetic resonance (NMR) analyses, as it is chemically similar to other sugars like glucose. Ribosylation is a relatively rare and understudied process compared to other sugar-related modifications, such as glucosylation. The researchers believe this strategy may have been overlooked in the past because many scientists assumed detoxification involved the addition of glucose. "Identifying ribose requires careful chemical analysis, including NMR, to distinguish it from other sugars," explains Yoko Nakamura, a research associate involved in the study. Not all fungi can perform this ribosylation. Out of 12 fungal species examined, only two—Coprinellus radians and Cylindrobasidium ipidophilum—had this ability. This may be due to the high energy cost of the process, as the fungus must use ribose for detoxification, which could slow its growth. The study highlights that this strategy is rare but highly effective in the species that use it. In future research, the team plans to identify the enzyme responsible for ribosylation and explore whether this ability exists in other fungal species. They also aim to investigate how ribosylation might influence the composition of microbial communities in bark beetle tunnels and, in turn, affect the beetles’ survival. These findings could lead to new methods for controlling bark beetle infestations in European forests, such as modifying the microbial communities within spruce trees to limit the spread of these pests.