Diamondback moth uses plant defense substances as oviposition cues


Date:
September 10, 2020
Source:
Max Planck Institute for Chemical Ecology
Summary:
Researchers showed that isothiocyanates produced by cruciferous
plants to fend off pests serve as oviposition cues. The scientists
identified two olfactory receptors whose sole function is to detect
these defense substances and to guide female moths to the ideal
sites to lay their eggs. They uncovered the molecular mechanism that
explains why some insects that specialize in feeding on certain
host plants are attracted by substances that are supposed to keep
pests away.



FULL STORY ==========================================================================
A research team from the Nanjing Agricultural University in Nanjing,
China, and the Max Planck Institute for Chemical Ecology in Jena, Germany, showed that isothiocyanates produced by cruciferous plants to fend off
pests serve as oviposition cues. The plant defense substances serve as
odor signals for females of the diamondback moth to lay their eggs on
these plants. The scientists identified two olfactory receptors whose
sole function is to detect these defense substances and to guide the
moths to the ideal oviposition sites.

They uncovered the molecular mechanism that explains why some insects
that specialize in feeding on certain host plants are attracted by
substances that are supposed to keep pests away.


==========================================================================
From repellent to attractant Cruciferous plants, such as cabbage, rape (canola), mustard and horseradish, produce glucosinolates. Upon mechanical damage of the plant tissues, e.g.

caused by a chewing insect, glucosinolates are hydrolyzed by the
endogenous plant enzyme myrosinase. This leads to the formation of
a variety of toxic breakdown products, mainly isothiocyanates, to
defend themselves against voracious insects. This defense mechanism is
very effective against most herbivores. The diamondback moth Plutella xylostella, however, has evolved mechanisms of its own to outwit this
defense: It is able to feed successfully on plants of the cabbage family
and make use of the plants for its own reproductive purposes.

"We wanted to know whether the moths use isothiocyanates as odor cues
to locate their host plants. In fact, behavioral experiments showed
that three isothiocyanates are key signals for female moths to locate
and lay eggs on cruciferous plants," says study leader Shuang-Lin Dong
from Nanjing Agricultural University.

Two olfactory receptors specialized on isothiocyanates control egg-laying
The main scientific question was, what are the molecular mechanisms
on which female Plutella xylostella moths base their choice of the
oviposition site? The researchers therefore analyzed, which olfactory
receptors were highly expressed in female moths, and studied the
function of these receptors in the frog oocytes. "With this method,
we were able to investigate which odors an individual receptor was
responding to. We showed that two receptors, OR35 and OR49, responded
to the three isothiocyanates that we had previously identified as
being crucial for oviposition," says Markus Knaden from the Max Planck Institute in Jena. These two receptors did not respond to any other
plant- related odors or to the sex pheromones of the moths. Presumably,
OR35 and OR49 evolved to detect precisely those egg-laying signals. "We
were surprised that even two receptors are specifically tuned to the isothiocyanates. The two receptors, however, detect the isothiocyanates
with different sensitivities. We hypothesize that the more sensitive
receptor could make sure that female moths locate plants from a distance,
while the other may help to provide a more accurate detection of the isothiocyanate concentration. This will give the female moths more
information about the substrate on which they will lay their eggs,"
says Shuang-Lin Dong.

Validation of gene function using CRISPR-Cas9 gene knockout techniques
The researchers used the CRISPR-Cas9 genetic scissors to knock out the
genes encoding the two receptors in moths. This method is used to test
the function of a specific gene. For egg-laying assays, they used plants
of the thale cress Arabidopsis thaliana, a model plant that belongs to
the cruciferous plant family. Some of these plants were unmodified and
produced isothiocyanates that were attractive to the moths, whereas the
others were mutants that were unable to produce isothiocyanates. When one
of the two receptors was inactivated, the moths laid considerably fewer
eggs on the isothiocyanates-emitting plants. When both receptors were
knocked out, the moths were unable to discriminate between unmodified Arabidopsis plants and the mutant plants.

Cheaters in plant-insect interactions In the course of evolution,
plants have developed various strategies to defend themselves
against herbivores. A crucial part of plant-insect interaction is
chemical communication. "In most cases, it is useful for a plant
to communicate to potential herbivores that its defense system is
already activated. However, there will be always someone who misuses
the communication for its own benefit, like in our case the diamondback
moth, which uses a plant defense signal as an attractant and lays eggs
and spreads on this plant," says Markus Knaden.

Finding out how these "cheaters" outwit plant defenses and even use
these defenses for their own purposes could help improve the control of
global crop pests (such as the diamondback moth): "Our results offer
various approaches to control this pest: On the one hand, we could
use the identified isothiocyanates or other attractive substances as attractants to trap these pests. On the other hand, we could try to
develop chemical agents to interrupt or block the perception of the isothiocyanates and thus interfere with the females' location of their
host plants," summarizes Shuang-Lin Dong.

Further investigations are planned to study whether other insects
that attack cruciferous plants also use special receptors to detect isothiocyanates and to locate the plants for oviposition. The results
may provide information on the extent to which the perception of these
odors by specialized receptors is also conserved in other species.


========================================================================== Story Source: Materials provided by
Max_Planck_Institute_for_Chemical_Ecology. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Xiao-Long Liu, Jin Zhang, Qi Yan, Chun-Li Miao, Wei-Kang Han,
Wen Hou, Ke
Yang, Bill S. Hansson, Ying-Chuan Peng, Jin-Meng Guo, Hao Xu,
Chen-Zhu Wang, Shuang-Lin Dong, Markus Knaden. The Molecular Basis
of Host Selection in a Crucifer-Specialized Moth. Current Biology,
2020; DOI: 10.1016/j.cub.2020.08.047 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200910120123.htm

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