Geneva, New York
November 26, 2003
by
Peter Seem
Entomologists at Cornell
University have provided the first experimental evidence
that breeding plants to produce two different proteins by a
process called "gene pyramiding" delays the development of
resistance in targeted insect pests. The research has
important implications for the long-term protection of
agricultural crops produced through biotechnology,
particularly Bt corn and Bt cotton. The team performed the
research using diamondback moths, one of the world's major
insect pests, and Bt broccoli.
The paper, "Transgenic plants expressing two Bacillus
thuringiensis (Bt) toxins delay insect resistance
evolution," will be published in the journal Nature
Biotechnology on December 1.
Bt, or Bacillus thuringiensis, is a type of bacterium
that produces proteins toxic to many major agricultural
insect pests. Bt was promoted as an environmentally benign
insecticide by Rachel Carson in her 1962 book, Silent
Spring. Even though it is benign, Bt accounts for less
than two percent of the world's insecticides because of its
cost and relatively low effectiveness. When plant breeders
developed the technology to genetically engineer the gene
for Bt into a specific crop, the crop itself became a very
effective method of control. |
 |
|
The principal scientists involved with the Bt pyramid gene
project are (inset photo - left to right) Jun Cao and Lisa
Earle (Plant Breeding,
Ithaca),
Tony Shelton and Jian-Zhou Zhao (Entomology,
Geneva).
The background photo, taken in the greenhouse in Geneva, NY,
shows some of the large cages used in the tests with Bt
broccoli.
Credit: J. Ogrodnick/NYSAES/Cornell |
Bt plants were first commercialized in 1996, and Bt corn and Bt
cotton became widely used alternatives to conventionally bred
corn and cotton. In 2002, Bt crops were grown on 36 million
acres worldwide.
"Breeding plants to express Bt proteins provides positive
economic benefits to growers, and health benefits for the
environment and farm workers," said Tony Shelton, Cornell
University professor of entomology at the New York State
Agricultural Experiment Station, in Geneva, NY, and one of the
paper's authors. "We're moving into the second generation of the
technology now. As techniques have become more sophisticated,
technology allows us to pyramid two Bt genes in a plant."
The paper is the result of 10 years of research by Shelton and
his collaborators to develop transgenic plants as an alternative
to conventional insecticide sprays. Using dual-toxin broccoli
plants developed by Elizabeth Earle and Jun Cao, in the plant
breeding department at Cornell, Shelton's lab examined how
resistance to the two toxins developed in a population of
diamondback moth after 24 generations. Resistance was compared
under several different management strategies.
"Plants containing two Bt toxin genes substantially delayed the
development of resistance compared to two single-toxin plants
used sequentially or in a mosaic," said
Shelton.
"Regulatory agencies and companies now should work together to
promote the development of these pyramid plants and, in the long
term, phase out single gene plants."
Mathematical models of insect resistance suggest that plants
with genes for two different Bt toxins would delay resistance
longer than planting a mixture of two single-toxin plants in the
field (called a mosaic), or using two single-toxin plants
sequentially in crop rotation. Such models have already prompted
one company to develop a variety of cotton that expresses two Bt
proteins.
Shelton's
lab provides the first experimental confirmation of the value of
dual-toxin plants.
Since the commercialization of Bt plants for insect control,
there have been no instances of insect populations developing
resistance in the field, but there is a constant danger that the
pest species will develop resistance to the toxin, as has
happened with many conventional insecticides. To help prevent
insects from developing resistance to transgenic Bt crops, the
EPA has mandated that a portion of acreage next to a Bt crop be
devoted to what is called a "refuge." A refuge is an area in
which the non-transgenic version of the crop is grown. This area
allows some susceptible insects to survive so that the gene that
encodes for resistance does not become abundant in the insect
population.
In
addition to providing better resistance management, plants with
pyramided genes for Bt proteins require less space set aside as
refuge, which helps growers (for whom a refuge can represent a
significant portion of the crop damaged) get a greater return
for their acreage. Preventing insect resistance also extends the
useful life of Bt crops, which helps manufacturers and growers.
"We have based resistance management programs in the United
States on some pretty solid theory, but
Shelton
and his team have given us very useful data," said Fred Gould,
professor of entomology at North Carolina State University, and
a leading expert on Bt crops. "This research should make the EPA
and companies more open to developing pyramided or dual-toxin
plants."
"This work has important implications in the U.S., but also in
Australia, India and China, where millions of acres of the
cotton crop contains Bt," adds Jian-Zhou Zhao, the paper's first
author.
"Using more Bt crops and less insecticide are environmentally
and people friendly strategies in pest control," said Shelton.
"The next step in our research program is to extend the crop's
useful life by having plants express the Bt proteins only when
the crop is most susceptible to insect damage."
This research was funded by the USDA's National Research
Initiative Competitive Grants Program. |
