Gut enzyme in cockchafer larvae activates dandelion defense mechanism

Removing the sugar component from a toxin in dandelion roots increases larval growth while simultaneously deterring the larvae from feeding

A new study shows, for the first time, that the degradation of a plant defense substance by an insect digestive enzyme can influence the insect’s preference for certain food plants. Researchers at the Universities of Münster and Bern (Switzerland) and the Max Planck Institute for Chemical Ecology in Jena investigated this phenomenon in the larvae of cockchafers (Melolontha melolontha) and the plant they feed on, the dandelion (Taraxacum officinale). The new insight contribute to a better understanding of plant-consumer interactions.

In tackling the enemies that feed on it, plants often use a chemical trick: they bind a sugar component to the toxin and thus deactivate it. As a result, the substance is prevented from damaging the plant itself. Only when a herbivore nibbles at the plant is the sugar removed. As a result, the compound becomes toxic or deters the attacker. “Amazingly, during digestion some insects remove the sugar component in the plant defense compound themselves,” says biologist Meret Huber from Münster University, who started this project in the Department of Biochemistry at the Max Planck Institute. “The consequences for the insects, and especially their behavior, are still unclear,” she adds.

One example of a defense compound containing sugar is a substance called taraxinic acid β-D-glucopyranosyl ester (TA-G), which dandelions accumulate in their roots in particularly high concentrations. This dandelion defense deters the larva of the cockchafer from eating.

The team led by Meret Huber and Matthias Erb (University of Bern) demonstrated that the pH conditions in the cockchafer’s gut inhibit the activity of dandelion enzymes which would otherwise remove the sugar from the defense compound. Instead, the cockchafer larva removes the sugar itself with a sugar-splitting enzyme it houses in its gut.

By using biotechnological methods to simultaneously manipulate the production of TA-G and the presence of the insect enzyme, the researchers showed that the insect’s gut enzyme promotes the growth of the larva on dandelion plants containing the defense compound. At the same time, however, separating the sugar from the toxin also leads to dandelion plants containing TA-G being shunned. “This means that, for the first time, we were able to demonstrate that the metabolization of plants’ defense substances by the digestive enzymes of insects changes the choice of food plant,” says Meret Huber. “This finding is important, because the choice of host plant plays a key role in the distribution of herbivores and, as a result, has a strong influence on the damage caused.”

Why the separation of sugar increases growth in the cockchafer larva – while simultaneously deterring the larva – is one of the as yet unanswered questions. Possibly this shunning behavior on the part of the larva helps in finding the side roots of the plant – which have a lower concentration of the defense compound, but a higher nutritional content than the main roots which are more important for the plant’s survival. For this purpose the researchers used different methods of chemical analysis and biotechnology. They also carried out so-called bio-essays to study the effects of TA-G on larval food choice. The results may help improve our understanding of the interactions between plants and their consumers. Future studies in various systems could contribute to the development of medicines or pesticides which are activated only in the presence of certain conditions specific to species or environments.

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