When horseradish flea beetles feed on their host plants, they take up not only nutrients, but also mustard oil glucosides, the characteristic defense compounds of horseradish and other Brassicaceae plants. By using these mustard oil glucosides, beetles turn into “mustard oil bombs” and thus deter predators. A team of researchers at the Max Planck Institute for Chemical Ecology in Jena, Germany, have now been able to demonstrate how the beetle regulates the buildup of mustard oil glucosides in its body. Beetles have special transporters in the excretory system that prevent the excretion of mustard oil glucosides. This mechanism allows the horseradish flea beetle to accumulate large amounts of plant toxins in its body, which it uses for its own defense.
Sequestration: well armed with the weapons of others
Many animals use chemical defense compounds to deter predators. These defense compounds are either produced by the animal itself or by the animal’s symbionts, or they are acquired from food. The ability to acquire defense compounds from food is particularly prevalent in insects that feed on poisonous plants. One example is the horseradish flea beetle (Phyllotreta armoraciae), which can sequester mustard oil glucosides, also known as glucosinolates, in its body.
“The horseradish flea beetle belongs to an economically important group of insects, as several species of Phyllotreta are crop pests. This beetle, which can accumulate large amounts of glucosinolates from the host plant, regulates the levels and composition of glucosinolates in the body at least partially by excretion This suggests that Phyllotreta armoraciae has very efficient transport and storage mechanisms, which we wanted to find out, ”says lead author Zhi-Ling Yang, explaining the purpose of the new study.
The team led by Franziska Beran, head of the insect sequestration and detoxification research group at the Max Planck Institute, has already been able to demonstrate how the horseradish flea beetle effectively uses glucosinolates from its host plant to defend itself against a predatory ladybird.
Special carriers for plant toxins in the excretory system of beetles
Although it has long been known that horseradish flea beetles and related species can accumulate glucosinolates, how the beetle absorbs and stores large amounts of these substances in the body is unknown. The objective of the research team was therefore to identify the transporters of glucosinolates in this insect. “Finding these carriers was literally like looking for a needle in a haystack,” Beran recalls. “We found 1401 putative membrane transporters in the gut and excretory system of this beetle. We have limited our search to specific carriers of horseradish. the flea beetle helped us identify a group of glucosinolate-specific transporters. “
These glucosinolate transporters are located in the excretory system, the so-called Malpighi tubules. The function of Malpighian tubules in insects is similar to the function of the kidneys in vertebrates. Scientists determined the function of the identified transporters using RNA interference, an approach in which the expression of a gene of interest is reduced in order to determine its function in the body: “We have silenced the expression of several transporter genes localized in Malpighi tubules and found that beetles excreted more glucosinolates than a control group of beetles with normal gene expression. Due to the higher rate of excretion, the levels of defense compounds in the body of the beetle decreased. Our study is the first to identify transporters in Malpighian tubules that allow an insect to accumulate plant defense compounds, ”Yang summarizes.
With their study, the researchers show that sequestration is a complex process and much more than the mere absorption of plant metabolites into the animal’s body. The sequestering insect must adapt all of its physiology to use plant defense compounds for its own defense. These adaptations are motivated by the challenges of its environment: predators, parasites and pathogens. “Sequestration is probably one of the most complex adaptations that herbivorous insects have evolved. It certainly also contributes to the evolutionary success of insects that specialize in certain host plants, such as the horseradish flea beetle,” says Beran.
Beran’s team now wants to identify other transporters involved in the sequestration. Scientists also want to know which natural enemies of the horseradish flea beetle glucosinolates protect against. A better understanding of how the horseradish flea beetle sequesters toxins and the effects on its ecological interactions with other organisms in the environment, will improve our understanding of this pest and could potentially lead to better control strategies.
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