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Engineering insect resistance into maize using Bacillus thuringiensis (Bt) cryiac gene fused to the c-terminal
(galactose-binding) of the ricin B-chain
D.
Gahakwa, L. Mehlo, A. Gatehouse, J.P. Du, N.T. Loc, J. Gatehous & P.
Christou
The bacterium Bacillus
thuringiensis (Bt) is the source of d-endotoxins
(or Cry proteins) with potent insecticidal activities. These proteins
have been widely used for crop protection, both through topical
application and the expression of Bt genes in transgenic plants. To
avoid the evolution of insects showing resistance to Bt toxins, much
effort has been directed towards engineering crystal proteins with novel
activities. The binding property of CryIAc was modified by adding the
galactose-binding domain of the non-toxic ricin B-chain. Transgenic
maize plants expressing the CryIAc-ricin fusion protein were generated
by the bombardment of embryogenic calli with fusion constructs. Southern
and western blot analyses showed that the transgene was stably
integrated and expressed into the maize genome and was transmitted to
progeny over at least three generations. The efficacy of the transgenic
maize plants was tested using insect bioassays. The fusion protein was
found to be toxic to Chilo
supprelissalis (rice stem borer), Spodoptera
littoralis (cotton leaf worm, which has been reported to show
tolerance to some Cry1 toxins), Cicadulina
mbila (leafhoppers) and
Rhopalosiphum padi (bird
cherry oat aphid).
The last two represent
homopteran insects that are not normally affected by the CryIAc toxin.
These results show that the activity of the CryIAc toxin was enhanced
and the range of target insects was broadened.
Molecular, expression and bioassay data will be discussed in
the light of our findings.
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