Washington, DC
July 7, 2008
By Stacy Kish,
CSREES
Toxins from the bacterium Bacillus thuringiensis (Bt) kill some
key agricultural pests, but cause little or no harm to people,
wildlife, and even most other insects, including the natural
enemies of pests. For decades, Bt toxins were used successfully
in organic and mainstream agriculture. Widespread exposure to Bt
toxins, however, increases the chances that pests will adapt and
evolve resistance - just as pests have evolved resistance to
conventional insecticides. With funding from Cooperative State
Research, Education, and Extension Service (CSREES), researchers
in Arizona and Mexico have collaborated to design, create and
test genetically-modified Bt toxins that kill insects resistant
to standard Bt toxins.
Since 1996, cotton and corn crops have been genetically
engineered to produce their own Bt toxins. These crops have
grown on more than 490 million acres worldwide, with most of
that acreage in the United States. With Bt toxins increasingly
important for pest control and the threat of resistance also
mounting, the modified toxins could help to protect the nation's
food supply and promote sustainable, environmentally friendly
agriculture.
The Bt toxins used most widely in sprays and, Bt-modified crops
are insecticidal crystal proteins in the Cry1A family, which are
effective against some of the most damaging crop-munching
caterpillars. After Cry1A toxins are ingested by caterpillars,
they are activated by enzymes in the alkaline caterpillar gut.
These activated Cry1A toxins bind to specific receptors on the
insects' midgut membrane. This creates holes in the membrane,
eventually causing the caterpillar's death.
The binding to a caterpillar's midgut receptor by a Cry1A toxin
is like the fit between a lock and key. Each Bt toxin is like a
key that fits only certain receptors, which are like locks. The
best-known mechanism of insect resistance to Bt toxins involves
changes in the receptors. In effect, the lock is altered so the
key won't fit. This allows resistant insects to survive exposure
to the toxin.
Bruce Tabashnik and his colleagues at the University of Arizona
determined that lab-selected resistance to Cry1A toxins in pink
bollworm (Pectinophora gossypiella), a major cotton pest in the
southwestern United States, is tightly linked with specific
genetic mutations. These mutations occur in a gene that carries
instructions for making a receptor protein called cadherin that
binds Cry1A toxins. The researchers determined that resistance
to Bt toxin occurs when mutations inhibit the cadherin gene from
properly binding with Cry1A.
Meanwhile, Mario Soberón, Alejandra Bravo and their colleagues
at the Universidad Nacional Autónoma de Mexico, hypothesized
that this type of resistance to Cry1A toxins could be overcome
by modified toxins that do not require binding to cadherin to
kill caterpillars. Using knowledge from the study, the
scientists designed and created modified toxins called Cry1AMod
toxins. Subsequent tests at the University of Arizona showed
that Cry1A-resistant pink bollworm caterpillars were killed by
the Cry1AMod toxins, suggesting the modified toxins may be
useful for countering or delaying resistance to standard Bt
toxins in caterpillar pests.
CSREES funded this project through the National Research
Initiative Arthropod and Nematode Biology and Management
program. Through federal funding and leadership for research,
education and extension programs, CSREES focuses on investing in
science and solving critical issues impacting people's daily
lives and the nation's future. For more information, visit
www.csrees.usda.gov.
Printable PDF version:
http://www.csrees.usda.gov/newsroom/impact/2008/nri/pdf/bt_toxin.pdf
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