Contents - Faperta

388 Biotechnological Approaches for Pest Management and Ecological Sustainability
been used in commercialization of Bt cotton varieties such as SGK 321 (cry1Ac and CpTI
genes) in China. Zhang, Wang, and Guo (2004) reported 92% mortality of fourth-instar
larvae of H. armigera in 3 days in CpTI-Bt cotton, SGK 321. Larvae reared on CpTI-Bt transgenic
cotton showed signifi cantly lower approximate digestibility and higher effi ciency of
conversion of ingested food than those treated with Bt transgenic cotton and Shiyuan 321.
The chitinase gene from S. marcesens has also been shown to act synergistically with Bt toxins
against S. littoralis (Rigev et al., 1996).
Pyramiding Bt Genes with Conventional Host Plant Resistance
Insect-resistant lines derived through conventional host plant resistance and novel genes
can be used effectively to achieve high levels of resistance against the target insect pests
(Bergvinson, Willcox, and Hoisington, 1997; Sharma et al., 2004). Insect-resistant transgenic
plants with different transgenes or with lines derived through conventional breeding with
resistance to the target insects can also be deployed as multilines or synthetics (Bergvinson
et al., 1997). Sachs et al. (1996) crossed the transformed cotton line MON 81 expressing
the cry1Ab gene to glandless (terpenoid-free), wild-type (normal terpenoid level), and
high-glanded (high-terpenoid) plant isolines derived from the “TAMCOT CAMD-E” and
“Stoneville 213” variety backgrounds. Survival of Cry1Ab-susceptible larvae was reduced
more by pyramiding the cry1Ab with the high-terpenoid trait in a single plant than by
either trait alone under no-choice conditions. Pyramiding Cry1Ab with high-terpenoid
content should increase plant resistance to H. virescens and improve the durability of the
Cry1Ab trait in commercial cotton.
Activity of Bt genes in transgenic cotton plants is also enhanced by tannic acid (Gibson
et al., 1995). Olsen and Daly (2000) observed that plant toxin interactions infl uence the
effectiveness of Cry1Ac protein in transgenic cotton, possibly due to tannins. Differences
in LC 50 varied from 2.4- to 726-fold on different cotton genotypes. Genetic background of
the transformed lines exercises considerable effect on the effectiveness of the transgene for
controlling the target pests (Li et al., 2001; Kranthi et al., 2005b). Bollworm, H. armigera
damage in Bt transgenic cotton hybrid Mech 184 is much lower than in Mech 12 and Mech
162 (Sharma and Pampapathy, 2006). Resistance levels of Gossypium arboreum L. varieties
Aravinda and MDL 2450 to the bollworms have been found to be comparable to the
transgenic G. hirsutum hybrids, suggesting that it would be useful to combine transgenic
resistance to H. armigera with genotypes derived through conventional plant breeding to
improve the effectiveness of transgenic plants for pest management. Protease inhibitors
engineered into cotton with high gossypol and/or tannin content may achieve greater
protection against H. armigera (Wang and Qin, 1996). The resistance of 32B to cotton bollworm
showed obvious changes with the developmental stage of the cotton (S. Chen et al.,
2002). Resistance during the seedling stage was quite strong. A sharp decrease of resistance
occurred from the square stage, which coincided with changes in the toxin content.
Bt toxin level might be associated with the protein metabolism in Bt transgenic cotton.
Tannin content was negatively correlated with insect resistance and Bt toxin level. It is
suggested that the increase in tannin content might contri bute to the decrease of Bt toxin
and the resistance to cotton bollworm.
High levels of expression of cryV in potato lines resistant to P. operculella resulted in 96%
mortality of this pest. These transgenic lines provide a germplasm base to combine insect
resistance mechanism from conventional plant resistance and novel genes as a means
to achieve durable resistance against this diffi cult to control pest (Westedt et al., 1998).