Contents - Faperta

Development of Resistance to Transgenic Plants 387
been shown to be effective for controlling the lepidopteran insects (Van der Salm et al.,
1994). The Cry1Ac-resistant pink bollworm had little or no survival on second-generation
transgenic cotton with cry2Ab alone or in combination with cry1Ac plus cry2Ab (Tabashnik
et al., 2002a). A mixture of Cry1Ac and Cry1F decreased the EC 50 to H. armigera by 26 times
(Chakrabarti et al., 1998). The d-endotoxins CryIIAa and CryIIAb are ideal candidates for
gene pyramiding as they differ in structure and mode of action from Cry1A proteins
(Kumar et al., 2004). The expression of cry1Ab–cry1Ac genes resulted in increased protection
against S. exigua, Manduca sexta (L.), and H. virescens. In addition, a chitinase gene
from Serratia marcesens (Bizio) has been shown to act synergistically with Bt toxins against
S. littoralis (Rigev et al., 1996). Broccoli with two Bt genes, cry1Ac and cry1C, is effective
for controlling diamondback moth, P. xylostella, resistant to Cry1A and Cry1C proteins
(Cao et al., 2002).
However, pyramiding different Bt events compared with single events did not improve
control of fall armyworm whorl damage in maize, but they did prevent more ear damage
by corn earworm, H. zea (Buntin, Flanders, and Lynch, 2004). The MON 84006 event singly
and pyramided with MON 810 had superior control of whorl-stage damage by S. frugiperda
and ear damage by H. zea compared with MON 810. Improved control of whorl and ear
infestations by H. zea and S. frugiperda would increase the fl exibility of planting corn and
permit double cropping of corn in areas endemic to these pests.
Bollgard II expressing cry1Ac–cryIIAb provides better control of bollworms than Bollgard.
Survival of H. zea larvae was lower on different plant parts of Bollgard II than on the corresponding
structures of Bollgard and conventional cotton (Gore, Leonard, and Adamczyk,
2001). Bollgard II signifi cantly reduced larval survival and fruit penetration by bollworm
compared to the Bollgard (Jackson, Bradley, and Duyn, 2004). The Cry1Ac-selected bollworm
strain displayed increased larval survival, superfi cial fruit damage, and fruit penetration
compared to the feral strain when averaged across genotypes. However, the single
cry2Ab-producing genotype was similar to both Bollgard and Bollgard II with respect to
fruit penetration.
Cry1Ac-resistant pink bollworm had little or no survival on second-generation transgenic
cotton with cry2Ab alone or with cry1Ac plus cry2Ab (Tabashnik et al., 2002b). Artifi cial
diet bioassays showed that resistance to Cry1Ac did not confer strong cross resistance to
Cry2Aa. Strains with 90% larval survival on a diet with 10 μg of Cry1Ac per milliliter
showed no survival on a diet with 3.2 or 10 μg of Cry2Aa per milliliter. However, average
survival of the larvae fed on a diet with 1 μg of Cry2Aa per milliliter was greater for
Cry1Ac-resistant strains (2% to 10%) than for susceptible strains (0). If plants with cry1Ac
plus cry2Ab are deployed, and if the inheritance of resistance to both toxins is recessive, the
effi cacy of transgenic cotton might be greatly extended. After 24 generations of selection
on broccoli expressing cry1Ac and cry1C genes alone or in combination, resistance to pyramided
two-gene plants was signifi cantly delayed in a P. xylostella resistant to Bt at frequencies
of 0.10 and 0.20 for Cry1Ac and Cry1C, respectively, as compared with resistance to
single-gene plants deployed in mosaics, and to Cry1Ac toxin when it was the fi rst used in
a sequence (Zhao et al., 2003).
Pyramiding Bt with Protease Inhibitor and Lectin Genes
Activity of Bt genes in transgenic plants is also enhanced by protease inhibitors (MacIntosh
et al., 1990; Zhao et al., 1997, 1998). Helicoverpa armigera developed three-fold resistance on
transgenic tobacco containing cry1Ac and cowpea trypsin inhibitor (CpTI) compared to
13-fold resistance to plants containing only cry1Ac (Zhao et al., 1999). This approach has