Contents - Faperta

386 Biotechnological Approaches for Pest Management and Ecological Sustainability
Gene Pyramiding
Several genes can be inserted in the same plant for effective pest management (L. Chen
et al., 1998). To convert transgenics into an effective weapon in pest control, it is important
to deploy genes with different modes of action in the same plant (Zhao et al., 1997; L. Chen
et al., 1998). Many of the candidate genes that have been used in genetic transformation of
crops are highly specifi c or are only mildly effective against the target insect pests. In addition,
crops frequently suffer from a number of primary herbivores. This suggests that
single and multiple transgenes will need to be combined in the same variety with other
sources, mechanisms, and targets of insect pest resistance in order to generate effective
and sustainable seed-based technologies. Several genes such as trypsin inhibitors, secondary
plant metabolites, vegetative insecticidal proteins, plant lectins, and enzymes that are
selectively toxic to insects can be deployed along with the Bt genes to increase the durability
of transgenic resistance to insects.
From an evolutionary point of view, the development of multiple toxin systems in transgenic
plants will be expected to decrease the ability of insect pests to overcome newly
deployed seed-based resistance technologies and thereby prolong the life of such new
varieties (Hadi, McMullen, and Finer, 1996; Karim, Raizuddin, and Dean, 1999). Stacking
or pyramiding of transgenes is an important strategy and involves introducing more than
one insecticidal gene into the same plant. Many advantages can be realized by stacking
two different Bt genes having a different mode of action. For example, Bollgard II contains
two Bt genes, cry1Ac and cry2Ab. The additive effect of both proteins will ensure greater
lethality to bollworms, and the product is also expected to have an expanded host range
[including the tobacco caterpillar, Spodoptera litura (F.)], and delay the development of resistance
in bollworms. Commercial introduction of Bollgard II could form a basic component
of a resistance management strategy in the future. The second gene can also be a non-Bt
gene, such as protease or amylase inhibitors. The following strategies can be used for gene
pyramiding to increase the effectiveness of transgenic crops for pest management and
delay the evolution of insect biotypes capable of surviving on the transgenic crops.
Pyramiding Two or More Bt Genes
Pyramiding two or more Bt genes will help in expanding the spectrum of insecticidal
activity of Bt, thereby providing more protection to crop plants, and also reduce the
possibility of development of resistance. The hybrid gene combining cry1Ac and cry2a
in Bollgard II has not only enhanced insecticidal spectrum, but also decreased the possibility
of development of resistance in bollworms (J.H. Perlak et al., 2001). Greenplate
et al. (2003) studied independent and additive interactive effects of two Bt d-endotoxins
expressed in the transgenic cotton variety 15985 by examining the responses of H. virescens,
H. zea, and S. frugiperda larvae to fi eld- or greenhouse-grown tissue from near-isolines,
which expressed cry1Ac only, cry2Ab only, or both toxins. In all cases, the cry2Ab component
was the larger contributor to total toxicity in the two-toxin isoline. Levels of each
toxin in tissues of the two-toxin isoline were not statistically different from the levels found
in the corresponding tissues of the respective single-toxin isoline. Considering the additive
interaction of toxins, a relatively simple insect resistance-monitoring procedure has
been proposed for the monitoring of commercial cotton varieties expressing both toxins.
A combination of Cry1Aa and Cry1Ac toxins has a synergistic effect, while a combination
of Cry1Aa and Cry1Ab produces an antagonistic effect against the gypsy moth, Lymantria
dispar L. (Lee et al., 1996). In tobacco plants, a combination of cry1Ab and cry1Ac genes has