How plants produce defensive toxins without harming themselves
Plants produce toxic substances to defend themselves against herbivores. In a new study, scientists from the Max Planck Institute for Chemical Ecology in Jena and the University of Münster in Germany were able to describe in detail the biosynthesis and exact mode of action of a group of important defensive substances, diterpene glycosides, in wild tobacco plants. Diterpene glycosides enable plants to defend themselves against herbivores. Studies have shown that these phytochemicals attack certain parts of the cell membrane. In order to protect themselves from their own toxins and prevent damage to the cell membrane, tobacco plants store these substances in a non-toxic form, which are synthesized in a very specific way. Self-toxicity and protection against it appear to have played a greater role in the evolution of plant defense systems than previously thought. The study was published in the journal Science.
Many plants produce chemical defense systems to protect themselves from being eaten. But little is still known about what makes them toxic to their consumers. Researchers at the Max Planck Institute for Chemical Ecology and the University of Münster have now studied how plants produce toxins and store them in their tissues without harming themselves. In particular, they want to know if the mechanisms of autotoxicity and its prevention have similar mechanisms to those that provide defense against the toxicity characteristics of herbivores.
Autotoxicity and defense
For their experiments, they chose diterpene glycosides from the wild tobacco species Nicotiana attenuata plant." These substances appear in very high concentrations in the leaves of tobacco plants. But we don't know why they are so effective in defense or why they are so toxic. So this situation is completely different from another very abundant toxin produced by this plant, namely nicotine. Nicotine is a special kind of neurotoxin. Since plants lack nerves and muscles, they do not provide a target for the toxin. Therefore, the production and storage of nicotine does not harm the plant." Ian Baldwin from the Department of Molecular Ecology at the Max Planck Institute in Jena, where the study was conducted, said.
The researchers were surprised to find that the modified tobacco plants could no longer produce the two proteins involved in the biosynthesis of diterpene glycosides and thus could not form defensive substances that would otherwise be stored in large quantities in the leaves, showing obvious symptoms of self-intoxication: they became sick, could not grow normally and could no longer reproduce. Further experiments showed that certain components of the cell membrane, the so-called sphingolipids, were already under attack.
Fimbria somatology. Li Jiancai collected frass left on tobacco plants by mandarin larvae for detailed analysis. Credit: Anna Schroll
Targeting cell membranes
Sphingolipids are substances found in all animals and plants, including the larvae of the tobacco hawk moth Manduca sexta, an enemy of wild tobacco. Therefore, the researchers asked whether sphingolipid metabolism could be a target for diterpene glycosides. In fact, Manduca sexta caterpillars that fed on plants without diterpene glycosides grew significantly better than larvae that fed on controls containing defensive chemicals. Analysis of the placentas of Manduca sexta larvae that had ingested diterpene glycosides provided further insight, as the degradation of phytotoxins during larval digestion is in roughly the opposite order to the synthesis of substances in the plant. Plants prevent self-inflicted damage by storing defensive substances in a non-toxic form. However, when the insect feeds on the plant, part of the nontoxic molecule is cleaved off and the chemical is activated or "armed."" Interestingly, in both cases of incomplete diterpene glycoside biosynthesis in plants and feeding on caterpillars, the toxin targets sphingolipid metabolism." said first author Jiancai Li.
Sphingolipids are mediators in many physiological processes. This makes the effect of diterpene glycosides on sphingolipid metabolism so interesting." Diterpene glycosides and their derivatives can have a wide range of defensive functions against many agricultural pests and pathogenic fungi. Also, many human diseases such as diabetes, cancer and some neurodegenerative diseases are associated with elevated sphingolipid metabolism." Shuqing Xu from the Institute for Evolution and Biodiversity at the University of Münster is one of the senior authors of the study. Doctors have been searching for effective substances to treat these diseases by inhibiting sphingolipid metabolism. The diterpene glycosides studied here could be potential candidates for further research.
"Frassomics" - a new and powerful tool for studying interactions between organisms.
The analysis of larval feces proved to be the key to the success of this research. The scientists call this new method "frassomics": combining frass (larval feces) and metabolomics - the analysis of all metabolites in an organism." From this work, we realized that frassomics could be a very powerful research tool. Analysis of larval frass can provide metabolic clues about how something produced by one organism is metabolized by the consuming organism," says Ian Baldwin.
The scientists plan to learn more about the "digestive dichotomy" that occurs between plants and insects to better understand the ecological interactions between plants, insects and microbes.
