Contents - Faperta

Deployment of Insect-Resistant Transgenic Crops for Pest Management 329
Furner, 1992; Assaad, Tucker, and Signer, 1993). Homology-induced silencing plays a
role in regulation of endogenous genes whereby the endogenous repeats are frequently
hypermethylated and packaged into transcriptionally inactive heterochromatin. These
inactivations are also trans acting (Mayer, Heidmann, and Niedenhof, 1993). Common
promoter sequences between two genes occupying nonallelic locations may result in
trans-inactivation of the primary transgene (Matzke, Neuhuber, and Matzke, 1993). A mere
90 bp of homology within a 35S promoter fragment between a second inserted gene and
the hypermethylated copies of a primary transgene with 35S promoter sequences result in
silencing of the inserted second gene (Vaucheret, 1993). Homology-based gene silencing
(co-suppression) has been observed to occur only in a fraction of transformants that carry
a sense copy of the endogenous gene. At the molecular level, co-suppression involves transcriptional
inhibition (Ingelbrecht et al., 1994) and posttranscriptional RNA degradation,
while transcription is unaffected (Mueller et al., 1995). Even single copy transgenes get
inactivated due to DNA methylation (Antequera, Boyes, and Bird, 1990). Condensation of
chromatin structures is mediated by proteins that specifi cally recognize methylated DNA
(Lewis et al., 1992). The methylation pattern of the integration region also has a signifi cant
infl uence on the methylation state of the transgene (Prols and Meyer, 1992).
Performance Limitations
The Bt toxins cannot produce the same dramatic effects on insect mortality as the
synthetic insecticides. The farmers need to be educated about the effi cacy and mode of
action of transgenic crops. The expectations have to be real, and remedial measures should
be taken as the situation warrants. The effects of the transgenic crops on insects will be
relatively slower, but cumulative over time. Transgenic crops may not be able to withstand
the pest density in some seasons. Therefore, careful monitoring of pest populations is an
essential component of pest management involving the transgenic crops. The value of the
transgenic crops can be best realized when deployed as a component of pest management
for sustainable crop production (Sharma and Ortiz, 2000). However, enough information
has not been generated involving transgenic crops in a genuine IPM system to demonstrate
long-term benefi ts of the transgenic crops, especially if environmental and human
health hazards are taken into account. Currently deployed transgenic crops produce only
one Bt toxin protein, while the Bt strains used for commercial formulations produce
several toxins in addition to other factors that increase insect mortality. The current cryIAb
construct employs PEP-carboxylase promoter, which enables expression in green tissue
and, as a result, the expression is greater in young plants. Insects that migrate into the
plant whorl or stem tissue with incomplete chlorophyll formation may escape the toxin
protein. If the toxin is expressed in insuffi cient amounts in such tissues, the insects can
develop mechanisms to withstand low levels of toxins in the transgenic plants. Behavioral
avoidance of the tissue expressing the toxin gene can be another component in insect
resistance to the transgenic plants. Therefore, care should be taken to express the toxins in
suffi cient amounts at the site of damage or feeding by the insects.
Secondary Pest Problems
Most crops are attacked by a large number of insect pests. In the absence of competition
from the major insect pests, the secondary pests tend to assume a major pest status (Hilder
and Boulter, 1999). The Bt toxins may be ineffective against some insect pests, for example,
leaf hoppers, mirid bugs, root feeders, mites, etc. This will offset some of the advantages