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Sweet potato plants don't have spines or
poisons to defend themselves. But some have evolved a clever way to let hungry
herbivores (食草动物) know they aren't an
all-you-can-eat buffet, a new study finds. When one leaf injured, it produces a
chemical that warms the rest of the plant and its neighbors to make themselves
inedible (不宜食用的)to bugs. Sweet potato
breeders could potentially engineer plants to produce the chemical as an
all-natural pest defense.
Plant ecologists led by Axel Mithofer 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 (造成麻烦) by insect pests.
To find out why, the team offered up Tainong
57 and 66 plants to hungry African cotton leafworm caterpillars (毛虫).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 smell, the team details this
month in Scientific Reports. ("The smell is not nice," Mithofer says.
"You wouldn't want it as a perfume.")
DMNT isn't a new compound; researchers have
isolated (分离出) 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 so it produced DMNT. Then, they exposed
healthy Tainong 57 plants to DMNT they had synthesized (合成).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 caterpillar's snack on sporamin, "they
immediately stop eating because they don't feel well," Mithofer says.
Sporamin is the main protein in sweet potato
tubers (块茎),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," Mithofer 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.
Mithofer himself has no plans now to create
genetically engineered sweet potato plants, because they would not be a viable
(能活下去的) 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.