Sweet potato plants don’t have spines or poisons to defend themselves. But some have evolved a clever way to let hungry herbivores know they aren’t an all-you-can-eat buffet, a new study finds. When one leaf is injured, it produces a chemical that alerts the rest of the plant—and its neighbors—to make themselves inedible to bugs. Sweet potato breeders could potentially engineer plants to produce the chemical as an all-natural pest defense.
Plant ecologists led by Axel Mithöfer of the Max Planck Institute for Chemical Ecology in Jena, Germany, started to look into sweet potato (Ipomoea batatas) defenses after they noticed something interesting about two varieties of the plant grown in Taiwan: The yellow-skinned, yellow-fleshed Tainong 57 is generally herbivore-resistant, but its darker orange cousin, Tainong 66, is plagued by insect pests.
To find out why, the team offered up Tainong 57 and 66 plants to hungry African cotton leafworm caterpillars. Both plants released at least 40 airborne compounds as the caterpillars snacked on their leaves. But Tainong 57 produced a lot more of a chemical called DMNT, which has a very distinct odor, the team details this month in Scientific Reports. (“The smell is not nice,” Mithöfer says. “You wouldn’t want it as a perfume.”)
DMNT isn’t a new compound; researchers have isolated the smelly chemical from other plants such as corn and cabbage, and it is known to induce defense responses in some species.
To determine whether this was happening in sweet potatoes, scientists set up two experiments. First, they put two plants next to each other and wounded one with tweezers so it produced DMNT. Then, they exposed healthy Tainong 57 plants to DMNT they had synthesized. In both cases, the DMNT caused the exposed plants to produce more of a protein called sporamin in their leaves. (Tainong 66 did not have the same reaction.) When the caterpillars snack on sporamin, “their digestion is inhibited and they immediately stop eating because they don’t feel well,” Mithöfer says.
Sporamin is the main protein in sweet potato tubers, and is indigestible raw, which is why sweet potatoes must be cooked for humans to enjoy them. “If the caterpillars could cook it, they could eat it,” Mithöfer says. Theoretically, he says, sweet potato breeders could use genetic engineering to make different varieties of sweet potato produce as much DMNT as Tainong 57, and display the same defense responses.
Still, the research isn’t ready for prime time, cautions plant ecologist Martin Heil. DMNT might work in the lab, but in the field, airborne chemicals can be “blown away in seconds,” says Heil, who studies plant-insect interactions at the National Polytechnic Institute in Irapuato, Mexico.
Mithöfer himself has no plans now to create genetically engineered sweet potato plants, because they would not be a viable crop in Europe, where genetically modified crops are outlawed. So for now, Tainong 66 will have to put up with being a caterpillar salad bar.