Plants "smell" the world to defend themselves
Plants "smell" the world to defend themselves
Plants communicate through a chemical language of volatile molecules.
Hello dear readers,
Thank you for your patience and understanding as I’ve taken some time to process my emotions following my father’s passing and to handle the logistics of his estate. Over the past few weeks, I’ve been back and forth to Florida busy settling his affairs, managing a herd of cattle, and making repairs around the ranch. I’m bruised, bug-bitten, and sore, but I’m finally finding a sense of peace.
Classes at Emory are now back in session, and I’m excited to share that my course, Food, Health, and Society, has the largest enrollment I’ve ever experienced—180 undergraduate students! Typically, elective classes here have closer to 20-30 students. I’m not sure if it’s the later start time (I usually teach at 8:30 AM, but this semester the class meets at 11:30 AM) or if more students are simply more curious about the connections between food and health. Either way, I’m loving the energy in the lecture hall!
In fact, it was my most recent lecture and class discussion on chemical ecology that inspired this post. So, let’s dive in.
What is chemical ecology?
Chemical ecology combines the tools of chemistry, biology, and ecology to explore how organisms in an ecosystem produce, detect, and respond to chemicals. These naturally occurring chemicals, called "natural products," include substances like pheromones, toxins, and attractants. You can think of it as the chemical language of life—a phenomenon you’ve likely already observed in nature. Some examples include:
Insects producing pheromones for mating behaviors;
Plants creating defensive compounds to protect against herbivores;
Bacteria using chemical signals for coordinating group behavior (quorum sensing); and
Animals marking their territory with scent glands.
How do plants communicate?
Plants communicate using secondary metabolites, which are distinct from primary metabolites—molecules produced for essential life functions like photosynthesis (energy production). Secondary metabolites serve various purposes for plants, such as:
Defending against herbivores or infections. A good example is the alkaloid nicotine produced by the tobacco plant. While nicotine is an addictive stimulant for humans, it is toxic to insects. Tobacco plants can increase nicotine levels in their leaves in response to insect attacks.
Attracting pollinators and seed dispersers. Plants are sessile, meaning they can’t move away from threats or toward resources. Instead, they produce molecules that create aromas or vibrant colors to lure insects and animals to pollinate their flowers or consume their fruit. This helps move seeds to new locations as they pass through the digestive tracts of animals. A classic example is the color change in maturing fruit—like a ripe red apple. The color and aroma signal to consumers that the fruit is ready to eat, ensuring the seeds are dispersed to other locations.
Protecting their resources. Some plants release allelochemicals (defense molecules) into the soil to prevent the seeds of competing species from germinating nearby. This helps the allelochemical-producing plants maintain exclusive access to resources like water or nutrients by reducing competition with other plants in their territory.
Alerting others of danger. Plants produce volatile molecules—think of the strong aromas associated with essential oils. Examples of volatile organic compounds VOCs, such as terpenoids, phenylpropanoids, and benzenoids) include the scent released when scratching a lemon rind or the smell of freshly cut grass. That "cut grass" smell is actually a mix of distress signals (green leaf volatiles) produced by the plants, essentially shouting, "Danger! We are under attack!"
Plants are responsive
Plants are highly responsive to signals—whether they are physical (i.e., a leaf being chewed) or chemical (i.e., a VOC released by one plant and detected by another plant in the environment). There was an interesting study conducted a the University of Wisconsin, Madison using fluorescent-labeled calcium to illustrate just how quickly defense responses are triggered in plants. Researchers showed how the leaves of a mustard plant respond to a cabbage caterpillar as it chomps away. You can see the movement of the green light (representing calcium) as other leaves in the plant far away from the caterpillar are alerted to the danger.
But what about communication between different plants? This is where volatile organic compounds (VOCs—scented molecules) come into play.
Plants are thought to detect volatile signals through receptors similar to odorant-binding proteins (OBPs) found in animals and insects. These proteins bind to VOCs and transport them to potential olfactory receptors. However, the exact nature of these receptors in plants is still being researched and remains not fully understood.
What we do know, however, is both fascinating and exciting! A 2022 article by Brosset and Blande offers a well-balanced review of current findings. One key takeaway is that the emission and detection of VOCs can influence plant performance, including basic life functions, defense mechanisms, and stress tolerance. Here's a helpful figure from the paper that illustrates this point:
The ratio and concentration of VOCs are also important in signaling. Think of it like the difference between a whisper and a shout—a shout is much more likely to get your attention, especially if it's in a language you understand. For plants, this means that a high concentration of VOC signals from another individual of the same species can trigger specific defense mechanisms—activating genes that control production of molecules to deter a known threat in the plant receiving the the VOC signal. In contrast, when plants pick up signals from unrelated species ("speaking" a different language), they may exhibit a more general defense response as they haven’t been “told” what the specific threat is.
The Takeaway
While plants may not be mobile like animals, they do leverage an incredible system of communication useful for attracting or deterring other creatures. The next time you stop to smell the roses or cut your lawn, take a moment to consider why these plants smell the way they do. Plant communication is a vast and exciting frontier in science, and I can’t wait to see what new studies reveal in the coming years.
Yours in health, Dr. Quave
Cassandra L. Quave, Ph.D. is a Guggenheim Fellow, CNN Champion for Change, Fellow of the National Academy of Inventors, recipient of The National Academies Award for Excellence in Science Communication, and award-winning author of The Plant Hunter. Her day job is as professor and herbarium curator at Emory University School of Medicine, where she leads a group of research scientists studying medicinal plants to find new life-saving drugs from nature. She hosts the Foodie Pharmacology podcast and writes the Nature’s Pharmacy newsletter to share the science behind natural medicines. To support her effort, consider a paid or founding subscription or donation to her lab research.
