A genetic mechanism that enables corn plants to "cry
for help" and attract beneficial insects has been clarified by
scientists from the University of Neuchâtel, Switzerland and the Max
Planck Institute for Chemical Ecology in Jena. Corn plants emit a
cocktail of scents when they are attacked by certain pests, such as
a caterpillar known as the Egyptian cotton leaf worm. Parasitic
wasps use these plant scents to localize the caterpillar and deposit
their eggs on it, so that their offspring can feed on the
caterpillar. Soon after, the caterpillar dies and the plant is
relieved from its attacker. In the case of corn, only one gene,
TPS10, has to be activated to attract the parasitic wasps. This gene
carries information for a terpene synthase, an enzyme forming the
sesquiterpene scent compounds that are released by the plant and
attract wasps toward the damaged corn plant. Since this mechanism is
based only on a single gene, it might be useful for the development
of crop plants with a better resistance to pests (PNAS, Early
Edition, January 16-20, 2006).
At least 15 species of plants are known to release scents after
insect damage, thus attracting the enemies of their enemies.
Scientists term this mechanism "indirect defence". A previous
cooperation by the scientists in Neuchatel and Jena showed that
indirect defence functions not only above ground, but also below the
earth’s surface [1].
To understand the biochemistry behind this plant defence,
biologists of the Max Planck institute studied corn plants,
caterpillars of the species Spodoptera littoralis (Egyptian cotton
leaf worm) and parasitic wasps of the species Cotesia
marginiventris. Deciphering the complex mix of scents that the
plants release after damage offered clues as to which classes of
enzymes might be important for scent production.
The researchers isolated various genes encoding terpene
synthases, the enzymes that produce these scents. One of these
genes, TPS10, produced the exact bouquet of nine scent compounds
that was released by the damaged corn plant. To demonstrate that
TPS10 is indeed the important gene, the scientists introduced TPS10
into another plant, called Arabidopsis thaliana, which then released
the same scents that have been observed in corn. To test whether
these scents do attract the parasitic wasps, these plants were
tested in an olfactometer, a device to study insect behaviour.
The researchers placed scent-producing as well as unmodified
plants in the six arms of the olfactometer. When the predatory wasps
were set free in the central cylinder of the olfactometer, they flew
towards the scent-producing plants. The experiments led to an
additional, surprising result: in order to react this way, the wasps
needed a first exposure to both the corn scent and the caterpillar
which led them to associate the two. Young, "naive" wasps without
this experience could not distinguish between scent-producing plants
and control plants, or failed to move at all.
Editor's Note: The original news release can be found here.