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Biological Chemistry

To Outwit Plant Defenses, Hungry Caterpillars Turn Chemists


Insects modify a single attachment point to deactivate a toxin in maize

by Carmen Drahl
October 3, 2014 | APPEARED IN VOLUME 92, ISSUE 40

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Credit: Anna Schroll
The fall armyworm (shown) defuses a benzoxazinoid plant toxin by reattaching a sugar to it in the opposite stereochemical configuration.
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Credit: Anna Schroll
The fall armyworm (shown) defuses a benzoxazinoid plant toxin by reattaching a sugar to it in the opposite stereochemical configuration.

Score one for the fall armyworm in the game of survival: Researchers report that the plant-eating caterpillar and its close relatives use a strategy worthy of an organic chemist to defuse a poison found in maize leaves. The finding could be of use in pest control.

Maize and related crops such as wheat and rye contain defense compounds called benzoxazinoids. Plant cells store the highly reactive molecules masked with the sugar β-d-glucose. The plants also produce an enzyme designed to cleave the masking group if leaves are eaten by an insect. The unmasked benzoxazinoid should be toxic to any multilegged freeloaders, but it barely affects fall armyworms or related species.

Daniel G. Vassão set out to learn why. The biochemist at Germany’s Max Planck Institute for Chemical Ecology convinced an international team to help him analyze caterpillar guts and feces with liquid chromatography/mass spectrometry and nuclear magnetic resonance. The team found that the insects’ guts contain an enzyme that reattaches the sugar to a particular benzoxazinoid. However, the enzyme does so with the opposite stereochemical configuration compared with the original plant molecule (Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201406643). The plant can no longer unmask the modified toxin.

“These insects can one-up the plant defensive machinery,” Vassão says. Only a few similar defense strategies are known so far in the insect world. “I think many other examples of the manipulation of stereochemical centers during insect metabolism are waiting to be described,” Vassão adds.

Jocelyn G. Millar, an insect chemical ecologist at the University of California, Riverside, enjoyed reading about how the researchers used LC/MS to watch the active form of the plant toxin be continuously depleted as it travels through the armyworm’s gut. “You have to love the ingenuity of nature!” he says.

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