Jena, Germany
January 17, 2006
Max-Planck
researchers in Jena, Germany have identified a gene which
produces a chemical 'cry for help' that attracts beneficial
insects to damaged plants
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. |
