West Lafayette, Indiana
March 7, 2007
Wheat has ways to battle the tiny,
red wormlike insects that nibble on the plant's leaves and can
destroy crops worldwide, but the Hessian fly larvae that survive
eventually evolve methods to overcome plant defenses.
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Purdue researchers, from left, Richard Shukle and
Jonathan Neal examine plots of wheat infested with
Hessian fly larvae. Their research has identified genes
in the insects that produce enzymes in response to
toxins plants use to protect themselves from the flies.
(Purdue Agricultural Communication photo/Tom Campbell) |
Purdue University and
USDA-Agriculture Research Service scientists trying to
thwart the insect have identified Hessian fly genes that nullify
toxins that wheat produces to protect itself from the munching
larvae. The researchers report their findings in the Feb. 6
issue of the journal Proceedings
of the National Academy of Sciences.
Richard Shukle, a USDA-ARS entomologist and Purdue adjunct
assistant professor, is working with colleagues to learn more
about how the flies feed and why they can't establish a feeding
site on plants that are resistant to the larvae.
"The focus of our work is to try to understand how the insect
overcomes plant resistance and use that basic knowledge to
enhance the durability of wheat against Hessian fly attack by
combining several genes for resistance or through some other
novel strategies," said Shukle, senior and corresponding author
of the paper.
The toxic oxygen compound defense mechanism in resistant plants
stresses the larvae, which then fight back, he said. This is
confirmed by the Hessian fly larvae's increased production of
enzymes triggered by their antioxidant genes in order to
detoxify wheat anti-insect poisons.
"Hessian fly larvae are under stress when they encounter
resistant wheat plants," Shukle said. "This stress includes
starvation when they can't establish a feeding site. The larvae
also are under stress from toxic molecules, including poisonous
oxygen compounds."
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No
larger than a pencil point, this shaft of wheat grown in
a Purdue greenhouse is infested with Hessian fly larvae.
The darker puparium is an advanced stage, from which an
adult fly will emerge. (Purdue Agricultural
Communication photo/Tom Campbell) |
Researchers know little about
the biochemical mechanisms involved when larvae - the early
stages of the gnat-sized Hessian fly - try to feed on resistant
wheat plants that are able to defend themselves, Shukle said.
While the insects can't establish feeding sites on resistant
plants, the larvae can alter the physiology and change the level
of certain proteins in susceptible plants. This forces the plant
to feed the larvae.
On resistant plants, Hessian fly larvae encounter defense
mechanisms that include poisonous oxygen compounds, Shukle said.
These compounds, also called reactive oxygen species, can be
produced either by the plant or within the Hessian fly larval
digestive system. These poisons then disrupt the insects' gut
function.
"The larvae probe the plant to open a sustained feeding site,"
said Jonathan Neal, a Purdue entomologist and another author of
the paper. "Resistant plants block the larvae from establishing
a feeding site. It's as if larvae keep trying to open doors
because they are programmed to look for a feeding site.
"In resistant plants, the doors are all locked. Then the larvae
finally crawl down to the base of the plant and die. This is
death by frustration."
Hessian flies can be controlled by using specifically bred wheat
varieties called resistant wheat cultivars. Infestations by the
flies also can be prevented by planting wheat after the fly is
no longer active and laying eggs. This "fly-free date" is set in
different regions according to climate.
The study represents a step in understanding the complexity of
antioxidant defense responses during interaction between Hessian
flies and wheat, he said. It may also be applicable to other
insect-plant interactions.
Hessian flies have been in the United States for more than 200
years, apparently accidentally introduced by German mercenaries
during the Revolutionary War.
The flies are found worldwide, but the U.S. infestation has been
mainly in the Southeast. In Georgia during the 1980s, wheat crop
losses totaled $28 million in one year when the insect overcame
the plant's resistant gene that was being used at the time.
Over the past two years, the pest also has caused extensive
yield losses in southeastern Missouri and a resurgence occurred
in Oklahoma in 2006. Most recently, a Hessian fly infestation
was identified near Lafayette, Ind., where the insect hadn't
been reported for more than a decade.
Purdue University, USDA-ARS and the USDA Current Research
Information System provided support for this project
The study's lead author was Omprakash Mittapalli, a former
graduate student in Shukle's laboratory. Mittapalli is now a
postdoctoral fellow at the Max Planck Institute for Chemical
Ecology in Jena, Germany.
ABSTRACT
Antioxidant Defense Response in a Galling Insect
Omprakash Mittapalli , Jonathan J. Neal , and Richard H.
Shukle
Department of Entomology and U.S. Department of
Agriculture/Agricultural Research Service, Purdue University,
901 W. State St., West Lafayette, IN 47906
Herbivorous insect species
are constantly challenged with reactive oxygen species (ROS)
generated from endogenous and exogenous sources. ROS
produced within insects because of stress and prooxidant
allelochemicals produced by host plants in response to
herbivory require a complex mode of antioxidant defense
during insect/plant interactions. Some insect herbivores
have a midgut-based defense against the suite of ROS
encountered. Because the Hessian fly (Mayetiola destructor)
is the major insect pest of wheat worldwide, and an emerging
model for all gall midges, we investigated its antioxidant
responses during interaction with its host plant.
Quantitative data for two phospholipid glutathione
peroxidases (MdesPHGPX-1 and MdesPHGPX-2), two catalases
(MdesCAT-1 and MdesCAT-2), and two superoxide dismutases
(MdesSOD-1 and MdesSOD-2) revealed high levels of all of the
mRNAs in the midgut of larvae on susceptible wheat
(compatible interaction). During development of the Hessian
fly on susceptible wheat, a differential expression pattern
was observed for all six genes. Analysis of larvae on
resistant wheat (incompatible interaction) compared with
larvae on susceptible wheat showed increased levels of mRNAs
in larvae on resistant wheat for all of the antioxidant
genes except MdesSOD-1 and MdesSOD-2. We postulate that the
increased mRNA levels of MdesPHGPX-1, MdesPHGPX-2,
MdesCAT-1, and MdesCAT-2 reflect responses to ROS
encountered by larvae while feeding on resistant wheat
seedlings and/or ROS generated endogenously in larvae
because of stress/starvation. These results provide an
opportunity to understand the cooperative antioxidant
defense responses in the Hessian fly/wheat interaction and
may be applicable to other insect/plant interactions.
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