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An international team of researchers has developed a new technique for
increasing pest resistance in transgenic crop plants, they report this week
in PNAS. The strategy, which boosts and broadens the activity of Bacillus
thuringiensis (Bt) toxins, targets previously impervious pest species and
reduces by up to 1000-fold the level of toxin expression needed, said
coauthor Paul Christou at the University of Lleida in Spain, May 2005 by
Charles Q. Choi.


Heavy use of Bt insecticides worldwide has raised concern that insects
might evolve resistance to Bt crops. Strategies to avoid the evolution of
resistance include expressing multiple Bt toxins at high doses or fusing Bt
toxins together, with resistance in both approaches requiring the unlikely
acquisition of multiple simultaneous mutations. Christou and colleagues in
Zimbabwe, Uganda, and Britain instead devised a new strategy that increases
the repertoire of toxin-binding sites a Bt toxin attacks. They fused the
sequence for Bt toxin Cry1Ac with that of the nontoxic B-chain subunit of
ricin (RB) in a recombinant plasmid. RB is a leptin that binds with
galactose- and N-acetylgalactosamine residues with high affinity, the latter
of which are key components of Bt toxin?binding receptors. They then
bombarded embryonic callus from mature maize seeds with this BtRB fusion.

"The Bt toxins currently in crops have a very narrow host specificity, which
is good because it reduces negative effects on beneficial insects and other
nontarget organisms, including people, but it limits the application of any
particular Bt toxin to controlling relatively few pests. So when you combine
Bt toxins together, you're limited to the host specificities of each toxin,"
said Bruce Tabashnik at the University of Arizona in Tucson, who did not
participate in this study. "With this novel technique, you don't have that
limitation?you're broadening the specificity of an existing toxin by
modifying its binding domain, rather than mixing and matching."

The researchers tested their fusion toxin against stem borer Chilo
suppressalis, a pest normally susceptible to Cry1Ac, and found that maize
producing low levels of BtRB killed 75% of larvae, compared with 17% in
Bt-only plants. Similar trials with the cotton leaf worm Spodoptera
littoralis, which is resistant to Bt delta endotoxins, showed that after 4
days, nearly 78% of larvae died on BtRB maize, compared to less than 20% on
Bt-only or nontransformed maize. In the leafhopper Cicadulina mbila, which
like other homopterans is ordinarily unaffected by Bt toxins, 95% of insects
died by day 4 on BtRB maize, compared to 80% survival otherwise. In tests
with the homopteran cereal aphid Rhopalosiphum padi, however, no toxicity
was seen with BtRB or Bt-only maize.

Broadening the spectrum of species that Bt toxins attack could help control
a wider range of pests, but "the potential negative effect is that you could
increase toxicity to nontarget species such as humans or beneficial
insects," Tabashnik told The Scientist.

Christou said his group was engaged in large-scale, collaborative
experiments to determine exactly which insects are susceptible to these
novel fusion proteins. "These results need to be validated repeatedly in
multiple experiments, first in the laboratory, then in greenhouse
experiments, and ultimately in controlled field experiments," he said. "If
any of these experiments indicate toxicity to nontarget or beneficial
insects, this toxicity needs to be studied, understood, and remedied before
experiments progress forward."

Richard ffrench-Constant at the University of Bath in England, who did not
participate in this study, cautioned that the increased toxicity of the
fusion protein might instead be due to RB improving membrane insertion. "I'd
like to see a Scatchard Plot of the binding of this fusion protein to brush
border membrane preparations of different insect pests in order to confirm
that activity against an increased range of insects is associated with
altered binding," he told The Scientist.


Links for this article
L. Mehlo et al., "An alternative strategy for sustainable pest resistance in
genetically enhanced crops." PNAS, May 16, 2005. [www.pnas.org]

Paul Christou [www.icrea.es]

C. Holding, "Insect resistance to insect resistance," The Scientist,
November 10, 2003. [www.the-scientist.com]

C. Holding, "Resistance found in GM refuges," The Scientist, May 11, 2004.
[www.the-scientist.com]

S.B. Maqbool et al., "Expression of multiple insecticidal genes confers
broad resistance against a range of different rice pests," Mol Breed,
7:85-93, January 2001.
[www.springerlink.com]

S. Naimov et al. "A hybrid Bacillus thuringiensis delta-endotoxin gives
resistance against a coleopteran and a lepidopteran pest in transgenic
potato," Plant Biotechnol J, 1:51-7, January 2003.
[www.blackwellpublishing.com]

Bruce Tabashnik [ag.arizona.edu]

Richard ffrench-Constant [www.bath.ac.uk]

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