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May 9, 2007
Source:
American Society of Plant Biologists
Rooted in place, plants can’t run
from herbivores—but they can fight back. Sensing attack, plants
frequently generate toxins, emit volatile chemicals to attract
the pest’s natural enemies, or launch other defensive tactics.
Now, for the first time, researchers reporting in the June 2007
issue of Plant Physiology
have identified a specific class of small peptide elicitors, or
plant defense signals, that help plants react to insect attack.
In this colorful self-defense strategy, proteins already present
in the plant are ingested by insect attackers. Digesting the
proteins, the insects unwittingly convert this food into a
peptide elicitor, which gets secreted back onto plants during
later feedings. Recognizing the secreted elicitor as a kind of
“SOS,” plants launch defensive chemistry. This defense discovery
opens the door for the development and genetic manipulation of
plants with improved protection against pests.
Although researchers have long known that some plants
distinguish different insect attackers, this defensive behavior
has proven difficult to describe at the molecular level.
Exceedingly few model systems have been utilized to characterize
the potential interactions between what researchers estimate to
be at least four million insects and 230,000 flowering plant
species. Moreover, highly active plant defense signals can occur
at trace levels, too small to easily detect or isolate.
Still, scientists have determined that insect herbivory,
mechanical damage, and pathogens such as bacteria and fungi can
all set off a variety of peptide warning signals in plants,
which respond by increasing phytohormones, particularly
ethylene, jasmonic acid, or salicylic acid, that regulate
defensive responses. But which peptide signals act as alarms—and
how?
To address those questions, Dr. Eric Schmelz at the United
States Department of Agriculture’s Center for Medical,
Agricultural and Veterinary Entomology operated by the U.S.
Department of Agriculture’s Agricultural Research Service in
Gainesville, Florida, led a research team that spent three years
systematically analyzing the biochemical response of cowpea
(Vigna unguiculata), a legume, to herbivory and oral secretions
of fall armyworm (Spodoptera frugiperda), a general crop pest.
During the extensive project, the researchers conducted over
10,000 leaf bioassays, testing for plant phytohormone production
after exposure to successively fractionated insect oral
secretions, among other experiments. Painstakingly collected
just a few microliters at a time, the team tested approximately
one full liter of caterpillar secretions.
As previously reported, the scientists identified and isolated
an 11 amino acid peptide, inceptin, that plays a pivotal warning
role in cowpea plants being attacked by the fall armyworm.
Inceptin is part of a larger, essential enzyme, chloroplastic
ATP synthase, in plants. When the fall armyworm feeds on cowpea,
the insect ingests ATP synthase and breaks it down, releasing
inceptin, which then becomes part of the armyworm’s oral
secretions. When the worm next feeds on cowpea, trace amounts of
inceptin recontact the wounded leaf and alerts plants to
generate a burst of defensive phytohormones.
In the June issue of Plant Physiology, Schmelz and his USDA
collaborators, including Sherry LeClere, Mark Carroll, Hans
Alborn, and Peter Teal, take the analysis further. They confirm
inceptin’s role as the dominant (and most stable) peptide in the
cowpea’s defense to fall armyworm. In addition, the researchers
identify two related but less abundant peptide fragments
(Vu-GE+In and Vu-E+In) that provoke similar defense responses in
cowpea and a third (Vu-In-A) with no apparent effect. They also
show that inceptin-related peptides spark a consistent,
sequential cascade of phytohormone increases in cowpea,
beginning with jasmonic acid, followed by ethylene and, lastly,
salicyclic acid. Finally, the researchers determine critical
features of inceptin’s structure: To work as a plant defense
signal, the peptide must contain a penultimate C-terminal
aspartic acid, though the structure is considerably more
flexible at its N-terminal. Notably, a number of the general
characteristics of inceptin are similar to another known plant
defensive peptide signal, systemin.
The new work challenges researchers to reconsider plant-insect
interactions. “Scientists searching for defense elicitors need
to realize those elicitors may not be synthesized by—or even
exist within—the insect pest species,” Schmelz said. “Instead,
the attacker’s proteases may interact with the host proteins,
generating an elicitor.” Building on this work, Schmelz is now
recruiting a post-doctoral scientist to help the team
biochemically purify and identify the inceptin receptor from
legumes.
The June issue of Plant Physiology will be the Legume Focus
Issue. Published by the American Society of Plant Biologists,
Plant Physiology is the world’s most frequently cited plant
science journal.
The research paper cited in this report is available at the
following link:
http://www.plantphysiol.org/cgi/content/abstract/pp.107.097154v1 |
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