Why it matters: Since the 1980s, it's been known that plants possess defense mechanisms that operate through the air. Recently, a team of Japanese researchers has not only demonstrated but also visually captured how this subtle form of communication functions.
When under stress or threatened, plants can "scream" and release a fine, volatile substance that lets other plants know something is wrong. A new study explains how this defense mechanism works, and how green plants can send warning messages through specific volatile organic compounds (VOCs).
VOCs are released by mechanically damaged plants, the study explains, and they can induce various defense responses in neighboring plants. This VOC-based signaling is designed to shield plants from environmental threats. However, the precise mechanics of VOC sensory transduction remained largely unknown until now.
Led by Yuri Aratani, a team at Saitama University specializing in molecular biology has developed an innovative imaging method. The Japanese researchers set up a pump to transfer VOCs emitted by injured or insect-riddled plants to undamaged ones, with a fluorescence microscope watching the ongoing biochemical messaging process.
The scientists let caterpillars (Spodoptera litura) eat leaves from tomato plants and Arabidopsis thaliana, a common weed in the mustard family. The released VOCs were then transferred to a second, insect-free Arabidopsis plant. The plants were genetically modified to express a fluorescent biosensor in their cells, which emitted a green luminescence when a flux of calcium ions was present.
This calcium signaling, a communication method also used by human cells, was confirmed by the researchers to be effective in plant communication. When the undamaged plants received VOCs from injured plants, the study explains, they responded with calcium signaling bursts that rippled through leaves.
The researchers identified two different airborne compounds, or "green leaf volatiles" (Z-3-HAL, E-2-HAL), that were responsible for the increase in calcium concentration in guard cells, mesophyll cells (within the leaves' inner tissue) and epidermal (outermost) cells in leaves. When treated with a phytohormone that shuts stomata, the small pores through which plants "breathe" CO2 from air, the calcium signaling process was significantly reduced.
According to Saitama biologist Masatsugu Toyota, the study finally unveils the intricate story of "when, where, and how" plants respond to airborne warning messages. This ethereal communication network, hidden from human sight, plays a fundamental role in plants' defense mechanisms, Toyota said.