In scientific circles, these images have been described as “simply incredible.” Unlike scientists, not everyone is aware that plants are surrounded by a delicate mist of airborne compounds. They use these compounds for communication and protection. Much like scents, these substances repel hungry herbivores and alert neighboring plants to approaching predators. Researchers have known about these protective properties of plants since the 1980s, and since then, they have been identified in over 80 species of flora. Now, a team of researchers from Saitama University in Japan has employed real-time visualization techniques to uncover how plants receive these airborne alarm signals and respond to them. For a long time, scientists understood how plants send messages but did not grasp how they receive them.
How the Research Was Conducted
Molecular biologists Yuri Aratani and Takuya Uemura, along with their colleagues, set up a pump to transfer compounds released by damaged plants under insect attack to their undamaged neighbors. To observe the process, the team utilized a fluorescent microscope.
They placed caterpillars (Spodoptera litura) on tomato leaves and Arabidopsis thaliana—commonly known as thale cress, a widespread weed in the mustard family. They then monitored the response of an undamaged Arabidopsis plant, which had no caterpillars on it, to the danger signals. This plant was not just any ordinary weed; it had been genetically modified. Its cells contained a biosensor that emitted green light upon detecting a surge of calcium ions. Interestingly, calcium ion signals are also used for communication in human cells, as reported by Science Alert.
The team visualized how the undamaged specimens reacted to the volatile compounds released within seconds after their neighbors were harmed. The healthy plants clearly perceived the messages from their injured counterparts, responding with bursts of calcium signals that coursed through their leaves. By analyzing the airborne compounds, the researchers discovered that two of them—Z-3-HAL and E-2-HAL—induced calcium ion signals indicating danger. They also identified which specific cells were the first to respond to these danger signals. To do this, the scientists outfitted the Arabidopsis plants with fluorescent sensors in guard cells, mesophyll cells, and epidermal cells. Guard cells are bean-shaped cells on the surface of plants that form stomata—tiny pores that open to the atmosphere when plants “breathe” in carbon dioxide. Mesophyll cells make up the internal tissue of leaves, while epidermal cells form their outer layer or skin.
When the Arabidopsis plants were exposed to Z-3-HAL, the guard cells generated calcium signals within a minute, after which the mesophyll cells picked up the message. The researchers noted that pre-treating the plants with a phytohormone that closes the pores significantly reduced the transmission of calcium signals. This suggests that stomata act like the “nostrils” of plants. As senior author Masatsugu Toyota pointed out, the scientists ultimately uncovered a complex story of how, when, and where plants respond to warning messages from their endangered neighbors. The findings of this study were published in the journal Nature Communications.