The toxic substances that plants use to keep herbivores at bay are stored in a non-toxic form until they need to be used
Plants produce toxic substances and chemicals to protect themselves from destructive herbivores.
These plant chemicals attack certain parts of the cell membrane of leaf chewers such as caterpillars. But researchers have long wondered how plants protect their own cell membranes from the toxins they produce.
Scientists at the Max Planck Institute for Chemical Ecology in Jena, Germany, and the University of Münster recently described the production and action of defensive substances that plants use to fend off herbivores. These substances are called diterpene glycosides.
The research team focused on the autotoxicity in a wild tobacco plant species. It was chosen because the diterpene glycosides occur at high concentrations in tobacco leaves. However, the researchers at the outset had no idea why they were such effective defences or why they could be so toxic to produce. The other abundant toxin the plant produces is nicotine, a specific neurotoxin.
In the course of the research, scientists discovered that plants prevent self-harm by storing the toxic defensive substances in a non-toxic form, which is synthesized in a particular way.
Professor Ian Baldwin of the Max Planck Institute for Chemical Ecology said the diterpene glycoside defence targets a cellular process in harmful pests called sphingolipid biosynthesis.
Sphingolipids are a group of lipids found in all animals and plants, including the enemies of wild tobacco, such as the larvae of the tobacco hawkmoth.
The research team discovered that, in the case of the tobacco plant, the diterpene glycosides stored in the leaves that are chewed by the caterpillar become toxic as soon as the compound reaches the insect’s digestive processes in the gut. Once there, the compound produces the toxic metabolites that poisons the sphingolipid.
Baldwin said in the news release that because plants lack nerves and muscles, they offer no target for the toxin, so producing and storing nicotine (the neurotoxin) does not harm plants.
To test the impact of the toxin on the plant itself, the researchers conducted tests to genetically modify tobacco plants. They removed the proteins involved in the biosynthesis of the diterpene glycosides.
Without those defensive substances stored in the leaves, the plants showed symptoms of self-poisoning because the toxin had turned on its host. They were sick, unable to grow normally, and could no longer reproduce.
The research showed that certain components of the cell membrane, the sphingolipids, had been attacked.
As well, caterpillars that had fed on plants without diterpene glycosides grew much better than the larvae that had fed on the control plants, which still contained the compound.
The scientists then analyzed the frass, or feces, of the larvae using chemical methods.
The diterpene glycosides apparently protect against a variety of pest insects. Baldwin referred to past research that showed that plants without the defensive compounds were more heavily attacked by grasshoppers, mirid bugs, and flea beetles.
Researchers now plan to use a genetic procedure called forward genetics to continue the digestive study.
The research paper was published in the journal Science.