Science1 publisher2 min readPublished
Beetle-borne fungi disarm spruce toxins by capping the fragments with ribose
Chemists at the Max Planck Institute for Chemical Ecology report in The ISME Journal that two fungi carried into spruce bark by beetles attach the sugar ribose to degraded defense compounds, leaving them stable and harmless.
The Scientist · Science desk
What happened
- A team at the Max Planck Institute for Chemical Ecology, publishing in The ISME Journal, describes a previously unknown strategy by which certain fungi neutralize spruce defense compounds.
- Bark beetles introduce these fungi into trees, and many of the fungi can metabolize the tree's chemical defenses and spread rapidly through the bark, easing the beetles' invasion.
- Only two of the fungal species go further, attaching ribose, a sugar with five carbon atoms, to the degradation products left by that metabolism.
- The fungus with the strongest ribosylation ability was also the one that grew best on spruce bark, which the authors call a survival advantage.
- The phloem the fungi colonize is among the chemically best-defended plant tissues, with phenolic compounds accounting for up to 5% of the bark's dry weight.
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Why it matters
- capability If a sugar cap is what lets a symbiont survive in phloem, then the fungal community inside a standing spruce becomes something a forest manager could act on, and the authors propose selectively altering it.
- precedent A lab that assigns detoxification products by mass cannot tell a pentose cap from its isomers, so comparable conjugates in other plant-microbe systems may already be sitting unidentified in collected data.
- constraint The chain from a capped phenolic to a beetle attack that fails has not been measured, so anyone designing a microbial intervention for European spruce would be building on a chemical association.
Several of the fungi in the study stop at degradation, and the fragments they leave behind are still toxic [3]. The ribose conjugates behave differently. The team reports they are harmless and stable, and that cleaving the ribose off does not regenerate the original, more toxic substances [5]. The researchers also report that the modification prevents the toxins from being reactivated by the metabolism of another organism [6].
Pinning the sugar down was the hard part. The group cultivated large quantities of the fungi, isolated the compounds and worked out their structures by nuclear magnetic resonance [10]. "However, based on the data, we could not distinguish between ribose, xylose and arabinose since they all have the same molecular weight," said co-author Baoyu Hu, a doctoral student at the institute [12]. "It was only through extensive nuclear magnetic resonance (NMR) analyses that we were able to confirm the presence of ribose and its exact configuration," he said [11].
The fungi were grown both on spruce bark and on artificial culture media, and the previously unknown compounds appeared only when the fungi were exposed to the chemicals found in spruce [9]. So the tree's chemistry is what switches the pathway on.
The growth evidence is weaker than the chemistry. Comparing one species with another leaves ribosylation entangled with every other difference between them [7]. A causal claim needs the trait switched off in the same fungus and the growth measured again. The phys.org account does not name the two ribosylating species or say how many fungi the team screened [17].
The study's framing of the forest link keeps a hedge in it. "We wanted to understand the biochemical tricks these fungi use to overcome such a hostile chemical environment, especially since some of these fungi may contribute to massive bark beetle infestations destroying European forests," said study lead author Ruo Sun [15]. Why the chemistry went unnoticed is a separate question. "We suspect it has been overlooked until now because most researchers assumed that detoxification involved the attachment of glucose," said co-author Yoko Nakamura, a research associate [13].
What to watch
- A knockout or inhibitor of the ribosylation step in the same fungus, with growth in phloem measured again, would turn the species comparison into a causal test.
- The ISME Journal paper itself, for the identities of the two ribosylating fungi and the size of the screen behind them.
- Whether pentose conjugates start turning up in other plant-microbe systems once labs look for them with NMR.