Fatal Fungus Turns Beetles’ Chemical Shields Into a Deadly Weakness
Far from being defenseless, plants have acquired all sorts of weird and wacky ways to protect themselves against pests and predators. Some, according to a study by Journal of Chemical Ecologyrelease latex to trap unsuspecting insects. Others recruit “bodyguards” to eliminate enemies for them, according to a study published in Scientific reports. Spruce trees, however, release antimicrobial compounds (phenolic glycosides) which inhibit the growth of pathogenic fungi.
Researchers writing PNAS showed that spruce bark beetles (Typography Ips) are able to hijack these chemical defenses to produce their own toxins. The toxins undergo yet another transformation thanks to Beauveria bassianaan insect-killing fungus, capable of infecting (and killing) beetles by bypassing their defenses.
“We demonstrated that a bark beetle can co-opt a tree’s defensive compounds to defend itself against its own enemies,” lead author Jonathan Gershenzon, a professor at the Max Planck Institute for Chemical Ecology, said in a statement.
“However, as one of the enemies, the fungus Beauveria bassianahas developed the ability to detoxify these antimicrobial defenses, it can successfully infect bark beetles and thus actually help the tree in its fight against bark beetles.”
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Beetles exploit spruce tree defenses
The Eurasian spruce bark beetle is a small brown insect no larger than a pea. The pest is found throughout Europe and northern Asia and can seriously damage pine trees if infestations become severe.
They are known to consume large quantities of spruce tissue, which contains high concentrations of microbial compounds, such as stilbenes and flavonoids, which protect the host against pathogenic fungi. However, until now it was unclear whether these compounds conferred resistance to fungi in beetles.
To find out, Gershenzon and the research team used various chemical analysis techniques, including mass spectrometry and nuclear magnetic resonance. The results suggest that consumption of these compounds not only provides the beetle with a defense against fungi, but that additional processes occur after ingestion to increase the potency of the toxins, making them even more effective at repelling pathogens.
Indeed, the beetle can break down plant-derived compounds (phenolic glycosides) via a water-mediated process called hydrolysis, thereby separating the non-sweet, more potent component of a glycoside (aglucones) from the sugary component.
A fungus exploits the defenses of beetles
But just as the spruce bark beetle is able to manipulate the chemicals produced by spruce trees, the fungus B. bassiana is capable of exploiting those ingested and converted by beetles.
Noting that B. bassiana had naturally infected and killed spruce beetles in the past, the team decided to investigate how this had happened. Using additional analysis and enzyme assays, researchers discovered a two-step detoxification process.
First step: the sugar is attached to the aglucones (a process called glucosylation). Second step: A methyl group is added to the sugar (a process called methylation), producing a methylglycoside that is nontoxic to the fungus and resistant to the beetle’s enzymes, allowing it to bypass the insect’s defenses.
Researchers have confirmed the anti-bug power of B. bassiana by removing the genes that enable the two-step process, showing that those that had been modified were less successful at infecting insects.
Biological control against spruce bark beetles
These beetles aren’t the only insects known to “self-medicate” with plant-based compounds: moths, ants and fruit flies also do it, according to a study published in Science. But exactly how they achieve this is less well understood, according to a study published in Biological journals of the Cambridge Philosophical Society.
Understand the underlying mechanisms involved in the conversion of phenolic glycosides to aglucones by spruce bark beetles, and B. bassianaFurther conversion of aglucones to methylglycosides could support the production of more effective biological controls against spruce bark beetle infestations, which are increasing thanks to rising global temperatures.
“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 effectively,” lead author Ruo Sun, a postdoctoral fellow in the department of biochemistry at the Max Planck Institute, said in a statement.
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