There is a plant-protecting spirit blowing through the air. If you breathe in carefully now you will smell… wood spirit! Long believed to be a simple by-product of degradation of a plant cell wall, new results show that methanol is a signal that tells surrounding plants to activate their defenses against pathogens and predators.
Methanol is the simplest of alcohols (Figure 1), formerly called wood spirit because obtained by distilling wood. Many chemical syntheses use methanol, and its derivatives are widely used in the production of many dyeing products, resins, pharmaceuticals, and synthetic perfumes. Very recently, it was shown that the leaf tissue of flowering plants produces such significant amounts of methanol that it is now considered as one of the most significant organic compounds in the atmosphere.
Figure 1: Methanol molecule, CH3OH. Image courtesy François Bouteau.
Methanol was long considered as a by-product of a plant’s response to mechanical wounding or other stresses that damage the cell wall. Indeed, the source of metabolic methanol formation in plants is the demethylation of cell wall pectins by pectin methylesterases (PMEs). However, it has been shown recently that gaseous methanol emitted by a wounded plant induces defense reactions, not only in its intact leaves but also in neighboring plants.
In order to identify the molecular mechanisms underlying methanol-induced plant defenses, we studied early plant cell responses to exogenously supplied methanol applied as liquid or a volatile. We observed that methanol induces various well-known signaling events related to plant defenses such as cytosolic Ca2+ variations, generation of reactive oxygen species (ROS), and ion fluxes through the plant cell membrane (Figure 2). Moreover, we showed that in tobacco, Arabidopsis, and sunflower, methanol induces the synthesis of ethylene (C2H4), one of the most important volatile plant hormones. The methanol-induced synthesis of ethylene depends on the well-known signaling events induced by methanol treatment.
Strikingly, ethylene is also a volatile involved in plant defenses. In the 1990s in South Africa, it was shown that the acacia could warn its congeners via ethylene. By recognizing a bite or shredding attack, the acacia leaf releases ethylene, which radiates within a radius of 6 m. This gaseous stimulus allows the neighboring acacias to start producing tannins even before the arrival of the pest, the kudu antelope, saving them valuable time. The increasing amounts of tannins make acacias more and more toxic, up to doses that are lethal for browsing animal.
Therefore, methanol is not only a by-product of PME activities. Our study shows that it participates together with ethylene to a volatile communication network, allowing plants to defend against pathogens and predators. In addition, it has not escaped our notice that methanol is used as a drug solvent in a wide array of experiments in plants. Our results raise the question of whether, depending on the dose used, some of the effects detected in these previous studies are due to the drug applied or the solvent.
Figure 2: Methanol-induced early plant responses. Image courtesy François Bouteau.
These findings are described in the article entitled Methanol induces cytosolic calcium variations, membrane depolarization and ethylene production in arabidopsis and tobacco, recently published in the journal Annals of Botany. This research was conducted by Delphine Arbelet-Bonnin, Tingting Zhao, Patrick Laurenti and François Bouteau.
Reference:
- Daniel Tran, Aurélien Dauphin, Patrice Meimoun, Takashi Kadono, Hieu TH Nguyen, Delphine Arbelet-Bonnin, Tingting Zhao, Rafik Errakhi, Arnaud Lehner, Tomonori Kawano, François Bouteau. Methanol induces cytosolic calcium variations, membrane depolarization and ethylene production in arabidopsis and tobacco. Annals of Botany, Volume 122, Issue 5, 3 November 2018, Pages 849–860, https://academic.oup.com/aob/article/122/5/849/4944430