Chapter 1: The Language of Plants

Entomologist Richard Karban has mastered the art of eliciting responses from sagebrush. By imitating the actions of grasshoppers or beetles, he carefully snips leaves rather than completely removing them. This method—making numerous small cuts—tricks the plants into thinking they are under attack.

Months later, when Karban revisits the sagebrush, he notices that many leaves have suffered damage from actual insect activity. However, the leaves close to where he made his cuts show significantly less harm. This is because those leaves, sensing danger through Karban's simulated damage, send out chemical warnings that alert nearby leaves to prepare their defenses.

Although it may appear that plants are silent, they are, in fact, engaged in constant communication, sending out chemical signals to fend off insects and warn neighboring plants. They even issue distress signals to summon natural predators to combat insect intruders.

Plants communicate using volatile organic compounds (VOCs), which are carbon-based molecules that evaporate easily into the air. This diverse group of compounds includes over 30,000 varieties produced by plants. Some VOCs create recognizable herbal or floral scents, while others are specifically released in response to particular stimuli. For instance, when a plant is damaged, it emits green leaf volatiles (GLVs), which can be detected by humans as the fresh aroma of cut grass.

Humans may not grasp the full extent of information conveyed by VOCs, but these chemical signals carry intricate messages. Similar to other forms of communication, plant signals can be intercepted, decoded, and even distorted.

Plants release VOCs in reaction to physical damage or chemicals found in insect saliva, vomit, or oviposition fluids. Insect bites can trigger hormones within the plant—such as jasmonic acid, ethylene, and salicylic acid—that enhance the activity of the plant’s defense genes. These hormones can also be emitted as VOCs, alerting both the plant's other leaves and its neighboring plants. Karban has found that methyl jasmonate, a volatile form of jasmonic acid, is particularly effective. He also observed that communication is more efficient among genetically similar plants. When Karban sealed clipped sagebrush leaves in plastic bags to contain the VOCs, neither the remaining leaves nor neighboring plants were able to heighten their defenses.

Plant messages can be intended for themselves or their kin, but sometimes other species can also pick up these signals. For example, sagebrush alerts can initiate defensive actions in both tomato and tobacco plants, although it remains unclear how many plant species can intercept signals from others.

Interestingly, plants may not always wish to broadcast their distress. Amy Trowbridge, a postdoctoral research fellow at Indiana University, Bloomington, explains that it might not benefit a plant to inform a nearby competitor of an attack, as this could allow that plant to survive while putting the informant at greater risk. Nevertheless, plants may unintentionally leak their chemical defenses into the air, leading other plants to evolve the ability to eavesdrop. Predatory insects have also adapted to listen for these distress calls. For instance, when apple trees are infested by spider mites, they emit signals that attract other mites that prey on the invaders. Similarly, Scots pines can call for parasitic wasps when sawflies lay eggs in their needles.

While plants and insects have adapted to exchange these chemical signals, human understanding of this communication is still in its infancy. Trowbridge notes that researchers have yet to fully grasp how plants perceive these compounds or the thresholds for detection. Furthermore, it remains unknown whether these molecules penetrate the leaf surface or enter through stomata. However, it is clear that for plants to respond defensively, they must not only receive but also decode these messages. Trowbridge emphasizes that merely absorbing a compound does not guarantee a response if the signal cannot be interpreted.

The information contained in these messages may be encoded through combinations of molecules. According to Karban, the release of VOCs when sagebrush is clipped can involve hundreds of detectable chemicals. He collects these VOCs in specially designed plastic bags and analyzes them using gas chromatography, but identifying the active components poses significant challenges. Chris Jeffrey, an organic chemist and chemical ecologist at the University of Nevada, Reno, suggests that unraveling the complexities of plant communication will require an understanding of the chemistry within entire ecosystems. He compares this to the human sense of smell, where responses arise from a blend of various molecules rather than a single compound.

Why is it important to decode plant communication? Understanding these signals can provide insights into how plants might respond to environmental changes, particularly those associated with climate change. Scientists warn that such changes could disrupt communication, destabilizing ecosystems. Some signals may become amplified while others might be muted or lost entirely.

Trowbridge points out that temperature fluctuations significantly influence volatility, meaning that a warming world could allow VOCs to disperse more readily. Elevated temperatures may also enhance the enzyme activity responsible for VOC production. Conversely, plants facing drought may close their stomata to conserve water, limiting their ability to produce VOCs. This reduction in communication could leave plants unaware of danger, making them more susceptible to pests.

In conclusion, the next time you find yourself in a tranquil garden, remember that the apparent silence belies a complex tapestry of communication. If only we could hear the vibrant dialogue unfolding around us.

This TEDx talk by Prof. Ariel Novoplansky sheds light on plant communication and their adaptive strategies for survival.

This video explores the nuances of how to articulate "shrubs" effectively, drawing connections to plant communication.