Why Are Plants Not Poisoned By Their Own Defensive Toxins?

Researchers from the Max Planck Institute for Chemical Ecology in Jena and the University of Münster, Germany described how plants could produce defensive toxins without harming themselves.

Their study showed the biosynthesis and the exact mode of action of a group of defensive substances, known as diterpene glycoside, in tobacco plants. These toxins attack certain parts of the plant's cell membrane but plants store these toxins in a non-toxic form to protect themselves.

Science Daily reported that autotoxicity and defense against it play a significant role in the evolution of plant defenses.

Chemical Defenses

According to Lumen, when the mechanical defense of a plant does not work, they resort to the second layer of defense which is its defense toxins and enzymes. It could be very toxic and lethal to animals that ingest them. But it is not toxic to the plants that produce the defensive toxins against the herbivores.

For example, tobacco plants produce diterpene glycosides that are so toxic to produce. Researchers said that this defensive toxin is found at very high concentrations in tobacco plants but they do not know why they have such defenses and why they could be toxic to produce.

But it is different from nicotine's case, which is also abundant in tobacco because these plants lack nerves and muscles so nicotine has no target.

To the surprise of the researchers, they found that transformed tobacco plants that cannot produce the proteins needed in the biosynthesis of diterpene glycosides were also not able to form the defensive substances that are stored in large amounts in the leaves.

In case of self-poisoning in plants that produce defensive toxins, symptoms could include sickness, unable to grow normally, and could no longer reproduce.

Experiments have also shown that these toxins could also attack certain components of the cell membrane, called sphingolipids.

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Defensive Toxins Targetting Cell Membrane

Although plants could protect themselves from the toxins by storing them in a non-toxic form, a part of the non-toxic molecule is cleaved off and activated when insects feed on the plant.

"Interestingly, in both cases, in plants with incomplete diterpene glycoside biosynthesis and in feeding caterpillars, the target of the toxins is the sphingolipid metabolism," Jiancai Li, the first author of the study, said.

Sphingolipids act as mediators in several physiological processes, and the effect of diterpene glycosides on them intrigued the scientists. Shuqing Xu from the Institute for Evolution and Biodiversity at the University of Münster and one of the senior authors of the study said that diterpene glycosides and their derivatives have broad defensive functions against herbivores and pathogenic fungi.

But it also has the same effect on human diseases, like some types of cancers, diabetes, and some neurodegenerative disorders that are also associated with elevated sphingolipid metabolisms, Science Daily reported.

Doctors have been looking for treatments for these diseases by inhibiting the sphingolipid metabolism. The study about diterpene glycosides could potentially become a candidate for further investigations.