If you’ve enjoyed the smell of fresh cut grass, you may have unwittingly eavesdropped on a conversation between plants.
The smell is caused by a group of compounds called green leaf volatiles (GLVs) that a plant releases into the air when it is injured. Although it can be a very pleasant smell to humans, to other plants it may be a warning letting them know that danger is near.
Scientists have known for some time that plants can eavesdrop on damage to other plants nearby. Doing so can benefit a plant, which can take steps to defend itself. Scientists are considering harnessing this process to fight agricultural pests without having to use pesticides – although exactly how plants detect danger and protect themselves remains unknown.
A danger-signalling relay
Now, Masatsugu Toyota, a professor at Saitama University with a penchant for making microscopes, has found a way to ‘watch’ plants responding to these warning signals. The new study, published in Nature Communications on October 17, could help unlock long-standing questions in the field of plant defence and pave the way to protect crops without pesticides.
Abdul Rashid War, a scientist working on crop health at NatCo Pharma, explained that plants have two major defence mechanisms, involving a chain of molecular reactions. The reactions are triggered when a plant is damaged, he said, and GLVs are released as by-products. (By mounting a defence response, plants can make themselves less palatable or even indigestible to the insect attackers.) The molecular cascade is mediated by calcium, a common mediator of chemical and electrical signals found throughout biology.
When an insect takes a bite of a plant leaf, calcium ions flood the leaves in the cells. Dr. Toyota inserted a gene into the mustard plant (Arabidopsis thaliana), causing the plants’ cells to glow whenever they were flooded with calcium. When he placed the mutant plant under a special microscope rigged to detect fluorescent signals, he saw it light up in response to being touched, cut with a scissor or eaten by a caterpillar.
So if Dr. Toyota’s mutant mustard could eavesdrop on damage being done to another plant, it should also light up in response to GLVs.
A ‘classic’ marker for defence
To test this, his student Yuri Aratani set up a sensitive experiment. She pumped air laden with GLVs on the mutant mustard plant and watched it light up under the microscope. “This is the first time for human beings to visualise [plants sensing] the volatile components released from the damage to other plants,” Dr. Toyota said.
Dr. Aratani’s painstaking experiments also revealed more about how plants detect volatile compounds. Of all the green leaf volatiles she tested, she found that mustard leaves lit up when exposed to just two of them: E-2-HAL and Z-3-HAL. She found that particular cells in the leaf, called guard cells and mesophylls, had the most calcium in response to GLVs.
Edward Farmer, a professor at the University of Lausanne, said he’s excited about the study because, compared to how much we know about animal senses, “we really don’t have the same level of knowledge at the moment about how plants perceive volatiles. This tells us something about the cells that are responding to the volatiles.”
One check did remain: The plant responded to the GLVs, but did it translate that to danger?
After observing the calcium responses, Dr. Aratani also found that plants exposed to grassy smells expressed their Jaz-7 and OPR-3 genes more. Simon Gilroy, a plant biology professor at the University of Wisconsin-Madison and a former collaborator of Toyota’s, called them “classic gene-level markers for defence”. He wasn’t part of this study.
What the findings mean
The presence of the genes in the mutant mustard after being exposed to GLVs could mean they perceived the compounds as a danger signal and began taking measures to protect themselves. “Everything is pointing in that direction,” Dr. Gilroy said, but he doesn’t think these results definitively show that plants perceive GLVs to be specific danger signals.
Instead, the findings hint at mechanisms by which plants detect and perceive GLVs.
Dr. Toyota said his research could spur the use of GLVs in pest-control. GLVs diffused over crops could, he said, activate plants’ defences.
Other methods of using a plant’s internal defence responses have already been tested. Drs. Farmer and Gilroy said jasmonic acid, a compound that gives jasmine tea its characteristic smell, also activates plants’ defences and effectively repels insect pests – but may also cause plants to put all their energy into defending themselves and too little in making the fruits and vegetables we care about.
‘The edges of a big network’
In his past work with International Crop Research Institute for Semi Arid Tropics, Dr. War studied how groundnut plants defended themselves against bollworm pests. He said GLVs activate internal plant-defence responses and “some of these volatiles repel the insects and some attract” predators of the pests.
He added that GLVs could help mitigate pest damage to crops but that more work was required to understand how specific compounds induced defences against specific pests. Dr. Farmer said he would like to consider more evidence before being convinced that volatile signalling between plants is important in a more natural setting. Such studies are very challenging, he added, because the compounds get diluted in the air.
Dr. Toyota himself has set his sights on how plants “smell” GLVs.
In the final analysis, Dr. Gilroy was optimistic about the sort of questions we could pursue now that we can watch calcium signals in plants. Plants lack brains but obtain information about the world and use it in meaningful ways just like we do. “We’re chipping away”, he said, “at the edges of a big network” of plant sensing.
Siddhant Pusdekar is a science writer and PhD candidate at the University of Minnesota. His research explores insect behaviour and nature communication. He is working with the Biological Purpose Project to produce The Purpose Podcast, which explores the intersection of biology and philosophy.
