Methyl jasmonate (usually abbreviated as MeJa and occasionally as MJ) is akin to Edvard Munch’s famous painting The Scream in the plant world. This amazing plant-signaling compound “yells” out to other plants when there is danger afoot (aleaf?). If that weren’t talent enough, it is also a key component in the physical defense of plants and many of its necessary functions unrelated to plant defense.
Let’s get the biochemistry out of the way before we look at all the ways MeJa plays its many roles. Jasmonic acid (JA) is synthesized from a commonly found plant fatty acid known as linolenic acid. The release of linolenic acid within a plant leads to the production of jasmonic acid and its derivative MeJa by way of the process cyclooxygenases. It is present in all healthy higher plant species.
Methyl Jasmonate as Defense Mechanism
Methyl jasmonate is released when a plant is subjected to stressors. This stress may be induced by abiotic factors, such as wounding from an errant mower or damage from an ice storm, or it may be biotical in nature and caused by a predatory insect or pathogen.
Whenever a plant is injured, the alarm is sounded. Methyl jasmonate is sent as a volatile organic compound (VOC) through the stomata. This airborne message is received by the stomata on neighboring plants, signaling them to increase their own production of MeJa in preparation for whatever onslaught is about to be wrought.
Amazingly, this signaling and reception can occur between plants of different species. A 1990 study at Washington State University showed this inter-plant communication between members of the Solonaceae (tomatoes, peppers, egg plants, etc.), Fabaceae (peas, beans, legumes, etc.), and Artemisia (wormwoods, sagebrushes, mugworts, etc.) species.
In plant predators, the effects of MeJa can range from an unpleasant digestive issue to causing cannibalistic tendencies. When MeJa is released within a plant, it causes the plant to produce other compounds known as protease inhibitors.
These inhibitors cause negative, sometimes painful reactions on the invading plant pests. They may just make the plant taste “bad” to the pest, causing it to seek its meal elsewhere. In extreme cases, an abundance of MeJa will make a plant taste so horrific to a pest that it would rather eat other members of its species.
A 2017 University of Wisconsin study that showed tobacco hornworms, normally herbivores, eating each other after sampling plant leaves with inflated levels of MeJa.
A 2001 German study using the same type of hornworms made another interesting discovery. It found that plants that had released MeJa in response to stress from herbivores were less likely to have those same predators lay their eggs on their leaves. In this particular study, plants that had elevated levels of MeJa had 91.7 per cent fewer eggs from predator species on their leaves than plants of the same species that had not released any MeJa.
A 1997 study conducted at Washington State University used another common pest, the fungal gnat. Several approaches were taken in an attempt to understand the complexity of the plant-signaling pathways.
First, an analysis of mortality in mutant plants that had no natural ability to produce MeJa found that 80 per cent of the plants succumbed to predation by fungal gnat larvae.
After those same species of plants were treated with naturally occurring levels of MeJa, mortality rates dropped down to only 12 per cent. The researchers then wanted to determine if the MeJa itself was what killed plant predators. It had no effect on the fungal gnat larvae when used alone.
So, it is critical the plants absorb MeJa in order to develop and use its own defenses to fend off invading insects. Further, this study found that it was possible to block the wound-signaling abilities of MeJa in plants with an antisense gene, which can occur in mutant plants.
This study declared that “Jasmonate is both necessary and sufficient for plant defense.” The ability to protect a plant from predators is not the only defense offered by MeJa. Other research has shown that foliar sprays of MeJa have anti-bacterial effects on plants. More research is still needed to draw positive conclusions, but it seems that the introduction of MeJa through plant stomata puts a plant on “high alert” status that can increase its ability to resist the effects of certain diseases.
A 1998 joint Canadian and American study looked at the disease response of plants affected with the fungal disease pythium. It found that JA, a component of MeJa, activated the genes that act in defense of plant pathogens as opposed to those that aid in the defense against insects.
A particular defensive gene in the plant species Arabidopsis essentially created a protein with anti-fungal properties is equally “activated” by exposure to either a pathogen directly or by the presence of JA. Methyl jasmonate along with the compound ethylene (more on this in the next section) create a synergy in certain gene families within plants that create proteins for defense against diseases.
Similar to the previously mentioned study, mutant plants that did not have natural levels of MeJa were highly susceptible to succumbing from pythium and those plants that either naturally had the ability to produce it or were given foliar MeJa treatments were able to successfully fight off or be less susceptible to the effects of the pathogen.
Even more similar is that this study found that nothing happens when pythium is directly exposed to MeJa. The MeJa needed to be activated by the plant as part of its total defensive response in order to be effective against plant pathogens.
A subsequent 2009 study looked at the susceptibility of corn to pythium. Similar to the Arabidopsis study, it found that the higher the levels of MeJa corresponded to higher level of immunity to pythium.
Methyl Jasmonate as a Plant Hormone
Unrelated to its defensive properties, MeJa is a hormone that plays a role in several aspects of plant development in several stages of a plant’s life. A technicality perhaps, but when it occurs as JA alone, and not the methyl ester MeJa, it might be more properly referred to as an intercellular signal as opposed to a true hormone. Methyl jasmonate aids in seed germination and root tendrilling of developing and mature plants.
It plays a role in proper tassel development and maturation of ears in corn, as discovered in the 2009 study mentioned above. It is also involved in promoting tuber formation and the formation of storage proteins.
It aids in flower development (specifically anther development) and pollen production as well as seed and fruit maturation. The latter is likely due to its appearance in concert with the release of ethylene. Ethylene is a naturally occurring gas that is produced by ripening fruit, which increases the rate at which surrounding fruit ripens. Methyl jasmonate also plays a role in the dormancy of plants and seeds.
Too much of anything is, of course, not always a good thing. An abundance of MeJa within a plant can be detrimental to its proper development as well as that of its neighbors, acting as a growth inhibitor or restrictor. Plants exhibiting stress in the form of yellowing of leaves may in fact have levels of MeJa four times higher than that of a plant of the same species that has healthy, green foliage.
Methyl Jasmonate and Cancer Cells in Humans
The importance of MeJa as a defensive compound goes beyond the world of plants. It has been researched widely for many years as a viable treatment for cancer in humans.
Success has been shown with the mitigation, suppression, metastasis, or otherwise inhibition of cancer cell growth in the treatments of prostate cancer, breast cancer, melanomas, and leukemia. It does this without the unwanted side effect of affecting normal human lymphocytes.
These studies are fairly recent and many of them have been performed only on mice, but the plant stress hormone MeJa may yet prove to be part of an entirely new class of anti-cancer drugs.