Since the 1980s, scientists have known that plants can communicate with each other by sensing the presence of volatile organic compounds (VOCs), for example as a warning from nearby plants about damage by herbivores and environmental stressors. However, the mechanisms underlying this “plant eavesdropping” are not yet well understood, particularly in terms of where the signals are emitted or received, and the actual sequence of events happening inside the plant. Masatsugu Toyota, Saitama University, Japan, Suntory Foundation for Life Sciences, Kyoto, Japan, and University of Wisconsin, Madison, USA, and colleagues have shed more light on the details of this inter-plant communication and the factors involved in signal transduction.
The Role of Green Leaf Volatiles
VOCs are emitted by plants in response to being wounded or attacked by herbivores. Emitted VOCs can deter herbivores, attract competitors or predators of the herbivores to aid in defending the plant, and may cause nearby plants to mount a defense themselves. This system of defense and cooperation has been studied in a number of plants, including valuable crops such as beans, tomatoes, and tobacco. Toyota and his colleagues decided to take a closer look at the mechanisms of the sensory transduction of these VOCs in Arabidopsis thaliana (thale cress), a common, quick-growing model organism.
The team focused on a subset of VOCs known as green leaf volatiles (GLVs), which are usually produced virtually instantly after a plant is wounded and that are responsible, for example, for the strong smell when grass is cut. GLVs trigger the induction of genes that help plants tolerate heat stress, and studies have shown that they cause tomato leaves to increase their calcium levels, a typical plant stress response. With this in mind, the team took a closer look at calcium ion concentration in Arabidopsis plants following exposure to GLVs emitted by wounded neighboring plants.
C6 Volatile Aldehydes
The team set up some Arabidopsis plants in a container with the common cutworm (Spodoptera litura), and some in a neighboring container. The two containers were connected so VOCs could pass between them when the first plants were eaten by the worms. The team investigated increases in cytosolic calcium in the worm-free set, and found that the calcium signal was transmitted to all parts of the receiving plant within 20 min of exposure. They visualized the Ca2+ levels using the Ca2+ biosensor GCaMP3, which contains green fluorescent protein (GFP) and emits an easily observable green glow when bound to calcium ions.
The researchers found similar results in Arabidopsis after exposure to the VOCs produced by tomato plants when they were being eaten by the worms. The two GLVs which caused the quickest changes in leaf surface potential, and then calcium concentration, in Arabidopsis were (Z)-3-hexenal (Z-3-HAL) and (E)-2-hexenal (E-2-HAL). These compounds both are C6 volatile aldehydes. The Z-isomer is produced using the enzyme hydroperoxide lyase (HPL) and can be isomerized to E-2-HAL. A lack of active HPL in the “emitting” plants leads to a lack of Ca2+ signals in the “receiving” leaves, suggesting the activation of defense signaling is dependent on the HPL-mediated formation of GLVs. As Toyota posits, “Arabidopsis might possess species-specific VOC recognition systems on the surface of plant cells,” which could be a potential target for new commercial crop treatment methods in the future.
Increases in Gene Expression
About 30–60 mins after exposure to Z-3-HAL and E-2-HAL, the team found that the plants showed increased expression of genes related to heat stress and oxidative stress. Not only that, but their experiments also showed that an increase in cytosolic calcium is necessary to induce these transcriptional changes, hinting more strongly at the sequence of events in the plant.
Stomata as Gatekeepers
The stomata, pores in the surface of plants’ leaves, are key players in VOC perception. By carrying out experiments in wild-type Arabidopsis alongside mutated versions with abnormal stomata function, the team was able to demonstrate their significance in Z-3-HAL perception. When the stomata were closed, the Z-3-HAL-induced calcium concentration was lower than in leaves with open stomata, confirming that VOCs enter plants via the stomata.
Further experimentation involving various plant cell types allowed the researchers to draw the conclusion that calcium concentrations first rose in guard cells within a minute of GLV exposure, then mesophyll cells, then epidermal cells, which are possibly protected from VOCs by the cuticle, acting as a barrier. Overall, this gives a much clearer picture of the order of events in signal transduction. As Toyota puts it, “We finally visualized in real time that plants sense green leaf volatiles (GLVs).”
The researchers suggest that their wide-ranging approach, applied to the model plant Arabidopsis, could be extended to look at VOC signaling networks in other plants and to further investigate signaling between plants of different species. As Toyota states, “This finding is a significant step toward better understanding invisible plant-to-plant communication. The next step is identifying unknown odor receptors in plants … It might be possible to create new volatile agricultural chemicals to protect crops and vegetables from pests by activating plants’ intrinsic defense responses without killing pests and without contributing to pesticide resistance.”
Yuri Aratani, Takuya Uemura, Takuma Hagihara, Kenji Matsui, Masatsugu Toyota, Green leaf volatile sensory calcium transduction in Arabidopsis, Nat. Commun. 2023, 14, 6236. https://doi.org/10.1038/s41467-023-41589-9