When nibbled, the leaves of one type of sweet potato release a strong-smelling chemical warning that prompts other leaves—on the same plant and those nearby—to produce defensive proteins that make them hard to digest. New research tracks this odorous alert system.
“It’s sort of a shortcut,” says Axel Mithöfer, a plant ecologist at the Max Planck Institute for Chemical Ecology in Jena, Germany, and co-author of the study, which appeared last November in Scientific Reports. Other plants have chemical warning systems that prompt neighbors to prepare for attack, but individual leaves often wait to manufacture defensive compounds until bitten themselves. But this plant’s leaves produce the compound immediately when neighbors are bitten, he says.
To investigate this response, Mithöfer and his colleagues released caterpillars on the pest-resistant sweet potato strain Tainong (TN) 57 and its more susceptible cousin TN66, both native to Taiwan. Each “exhaled” at least 40 chemicals when attacked, but the TN57 leaves released twice the amount of a compound called DMNT, also found in other plant-defense responses.
Next, the scientists placed a healthy TN57 plant in a closed glass tank with one whose leaves had been pierced with tweezers. Within 24 hours high levels of a protein called sporamin formed in both plants’ uninjured leaves. Sporamin, also found in sweet potato tubers themselves, is what makes it difficult for humans to digest them uncooked—and it causes trouble in insect guts, too. When researchers released synthesized DMNT into a tank with healthy plants, the leaves again readily formed sporamin.
Mithöfer’s team is now probing the mechanism TN57 leaves use to “smell” and “recognize” DMNT. The researchers also hope to test whether other chemicals the leaves release also elicit defenses. 
Cesar Rodriguez-Saona, an entomologist at Rutgers University, who was not involved in the study, says this research showcases an intriguing defense mechanism—although he cautions that DMNT exposure in closed tanks could be higher than what plants experience in open, windy fields. It is also possible, he notes, that unattacked TN57s may not always expend the energy to use this direct defense “shortcut.”