Scientists Reveal How Plants Receive Airborne Warning Messages

Plants are enveloped by an imperceptible haze of airborne compounds, which they use for communication and self-defence. Functioning rather like scents, these chemicals deter hungry herbivores and alert nearby plants to approaching attackers.

Scientists have understood the existence of these plant defences since the 1980s, and have since detected them in more than 80 plant species. More recently, Japanese researchers used real-time imaging methods to show how plants detect and react to these airborne warnings.

This addressed a major missing piece in our understanding of plant communication: researchers knew how plants transmitted messages, but not how they received them.

How plants detect airborne warning compounds

In the study, published last year, molecular biologists Yuri Aratani and Takuya Uemura of Saitama University in Japan, together with colleagues, set up a pump to deliver compounds released by damaged, insect-infested plants to undamaged neighbouring plants. They then used a fluorescence microscope to observe the response.

The researchers allowed caterpillars (Spodoptera litura) to feed on leaves taken from tomato plants and Arabidopsis thaliana, a widespread weed from the mustard family. They imaged how a second, uninjured and insect-free Arabidopsis plant responded to these danger signals.

These were no ordinary weeds. The plants had been genetically modified so that their cells contained a biosensor which glowed green when it detected an influx of calcium ions. Human cells also use calcium signalling to communicate.

Last year, the team used a comparable approach to measure calcium signals in fluorescent Mimosa pudica plants, which rapidly fold their leaves when touched in order to avoid predators.

Calcium signals in Arabidopsis plants

For this research, the team visualised plants' reactions when they were exposed to volatile compounds, which plants emit within seconds of being wounded. The arrangement was not natural: the compounds were collected in a plastic bottle, concentrated, and pumped over the receiving plant at a steady rate. However, this enabled the scientists to examine the compounds within the pungent mixture.

As shown in the video above, undamaged plants clearly received the messages sent by their injured neighbours. Bursts of calcium signalling spread across their extended leaves in response.

After analysing the airborne chemicals, the researchers found that two compounds, Z-3-HAL and E-2-HAL, triggered calcium signals in Arabidopsis.

They also established which cells first react to the danger cues. To do so, they engineered Arabidopsis plants with fluorescent sensors confined to guard cells, mesophyll cells, or epidermal cells.

Guard cells are bean-shaped cells on plant surfaces that create stomata: tiny pores that open to the atmosphere when plants ‘breathe’ in CO₂. Mesophyll cells make up the internal leaf tissue, while epidermal cells form the outermost layer, or skin, of leaves.

When Arabidopsis plants encountered Z-3-HAL, their guard cells produced calcium signals within about a minute. Mesophyll cells then received the message.

Moreover, treating plants in advance with a phytohormone that closes stomata greatly reduced calcium signalling. This indicates that stomata function as the plant's ‘nostrils’.

"We have finally unveiled the intricate story of when, where, and how plants respond to airborne 'warning messages' from their threatened neighbors," Masatsugu Toyota, a molecular biologist at Saitama University in Japan and the study's senior author, explained when the research was published.

"This ethereal communication network, hidden from our view, plays a pivotal role in safeguarding neighboring plants from imminent threats in a timely manner."

The study was published in Nature Communications.

An earlier version of this article was published in January 2024.