A quiet revolution is occurring in fields and forests worldwide, where plants employ sophisticated chemical communication to summon bodyguards against attacking insects.
Published on October 5, 2023
Walk through any garden after an insect attack, and you're surrounded by silent screams. The air is filled with invisible distress signals as plants under assault cry for help. But these aren't cries in vain—they're precise chemical messages that bring in predatory insects as airborne cavalry, coming to the rescue of their botanical allies.
This remarkable defense system reveals plants as active participants in their survival, not passive victims. Through sophisticated chemical communication, they can recruit protective insects, signal family members, and mount coordinated defenses that would make any military strategist envious.
The groundbreaking discovery of plant SOS signals began with careful observation of nature's interactions. Researchers noticed that certain plants being eaten by caterpillars suddenly found themselves surrounded by predatory insects that came to their defense. This wasn't a coincidence—it was communication.
U.S. Department of Agriculture scientists identified volicitin, a specific chemical in caterpillar saliva that triggers corn seedlings to send out a distress call 1 . This marked the first time researchers had isolated and identified the precise compound that activates a plant's emergency response system.
This discovery opened up possibilities for developing new, environmentally friendly ways to control crop pests by harnessing natural plant defense systems rather than relying solely on pesticides 1 .
Identification of volicitin in caterpillar saliva as the trigger for plant defense responses.
Potential for developing crops with enhanced natural defense systems against pests.
When a caterpillar begins munching on a leaf, the plant doesn't take this assault lying down. Almost immediately, it mounts a sophisticated defense:
As insects feed, their saliva containing volicitin enters the plant tissue through the feeding wounds 1 .
This caterpillar saliva activates or modifies the "green leaf volatiles"—pungent chemicals that plants produce 4 . The familiar smell of freshly cut grass is actually composed of these volatiles.
The plant releases these specific volatile chemicals into the air, creating an aromatic plume that carries the distress signal .
Predatory insects detect these chemical signals and follow them to the source—the herbivorous insect feeding on the plant 4 .
Research by Silke Allmann and Ian Baldwin at the Max Planck Institute for Chemical Ecology revealed that the caterpillar saliva actually causes a chemical change in the plant's volatiles, creating a signal that's even more effective at attracting the right kind of predatory insects 4 .
To confirm that plants were genuinely sending these distress signals and that insects were responding to them, researchers designed clever experiments that removed any doubt.
Scientists glued caterpillar eggs onto two groups of tobacco plants:
| Treatment Type | Egg Predation Rate | Attracted Predator |
|---|---|---|
| Plant volatiles alone | 8% | Geocoris bugs |
| Plant volatiles + caterpillar saliva | ~24% | Geocoris bugs |
The conclusion was clear: the modified chemical signal effectively betrayed the feeding caterpillar's location to its predators 4 .
Further research has revealed that plant defense communication is far more complex than initially imagined. When plants are attacked, they don't just release generic distress calls—they can send different signals to different parts of the plant and even to neighboring plants.
Research led by Masatsugu Toyota and Simon Gilroy uncovered that when a plant gets wounded, it experiences a wave of calcium ions that spread across the entire plant 8 . This is triggered by the release of glutamate (a neurotransmitter in animals) from the injured leaf, which is then taken up by glutamate-receptor-like ion channels in plant cells 8 .
This systemic signal, propagating through the plant's vascular tissue, alerts even distant, undamaged leaves that the plant is under attack, enabling them to ramp up their defenses preemptively 8 .
Perhaps even more astonishing is the discovery that some plants extend their SOS signals to protect their relatives. Research by Yutaka Kobayashi and Norio Yamamura of Kyoto University revealed that neighboring plants not being eaten also send out distress signals to call in insect "bodyguards" 7 .
Why would plants that aren't under attack cry for help? The researchers proposed this apparent altruism is actually explained by kin selection—the evolutionary strategy that favors the reproductive success of an organism's relatives. The "secondary signalers" are essentially crying for help to save their family 7 .
| Chemical Compound | Role in Defense | Source |
|---|---|---|
| Volicitin | Triggers SOS response in plants | Caterpillar saliva 1 |
| Green Leaf Volatiles (GLVs) | Base compounds for distress signals | Plant tissues 4 |
| Glutamate | Triggers calcium wave defense signaling | Injured plant cells 8 |
| Calcium ions | Systemic signaling across the plant | Plant vascular system 8 |
Understanding plant SOS signals has exciting implications for developing sustainable agricultural practices. Instead of relying solely on pesticides that can harm beneficial insects and the environment, we might enhance crops' natural defense systems.
"Let's think a little bit about what we do in agriculture now. We've bred all these great plants that have incredibly high yields that are basically defenseless" — Ian Baldwin of the Max Planck Institute .
Plant breeders could develop crop varieties with enhanced chemical defense systems that would be better able to attract beneficial insects 1 .
Farmers could then reduce pesticide applications while maintaining effective pest control through natural predator attraction.
"You have to have a crop management procedure that allows for those predators to be there" — Ian Baldwin .
The discovery of plant SOS signals fundamentally changes how we view the botanical world. Plants aren't passive entities merely responding to stimuli—they're active communicators participating in complex ecological networks.
As research continues, scientists are uncovering even more sophistication in how plants perceive threats, distinguish between different types of damage, and tailor their responses accordingly. The initial discovery of volicitin has opened up an entire field of study investigating how plants sense and respond to the world around them.
Next time you see a caterpillar on a leaf, remember—you might be witnessing just one side of a complex interaction that includes silent cries for help and unseen rescuers heeding the call. The air around us is filled with conversations we're only beginning to understand.