Transgenic Sunflower - Genetics, Genomics and Breeding of Crop Plants - page 298

metabolic inhibitors. The most widespread Bt proteins show strong activity
against Lepidoptera, although some bacterial variants have also proven
effective against Diptera ( B. th. var. israeliensis ) and Coleoptera ( B. th. var.
tenebrionis ). There is a strong specificity in the action and expression of Bt
endotoxins. Not all the genes that codify Bt proteins are expressed in different
plant species. Similarly, not all the Lepidoptera found in a crop are controlled
by the same event.
On the other hand, the CpT1 agent is very active against Coleoptera
and Orthoptera (Boulter et al. 1989) and is already available in GM crops.
Modern biotechnological strategies incorporate the expression of a carrier
to improve the toxin penetration and its influx into the insect's
haemolymph (Fitches et al. 2004). The ideal GM technology should be
environment-friendly, with a wide spectrum of activity with respect to the
target insects, but with few, if any, effects on beneficial insects (Hilder and
Boulter 1999).
The GM sunflowers released in the environment and authorized for
research include two groups of events for insect control. The reported
Lepidoptera-resistant varieties express the Bt insecticide protein, which is
codified by the Cry1F gene. If expressed in the early stages of crop
development, this could be a valuable tool for controlling polyphagous moth
larvae of the genera Agrotis and Euoxa , which are present in the main
sunflower growing regions (Rogers 1992; Charlet et al. 1997). Control of
Suleima helianthana , which bores sunflower roots and stems in North
America is difficult by the use of GM varieties because the damage caused is
seldom significant (Charlet and Brewer 2001).
Bt proteins could also offer excellent possibilities for controlling insect
damage to aerial tissues. Lepidoptera that cause important crop damage
include H. armigea , Spodoptera spp., Diabrotica spp., Spilosoma spp., Colias
lesbia , Rachiplusia nu , and Vanessa cardui . These species could be controlled
through GM technology based on Cry1 variants of the Bt gene. As these
species are highly polyphagous, refuges to prevent the selection pressure
for insect resistance would not be indispensable, except in cases where all
the crops in a given region were GM varieties with the same expression of Bt
proteins.
On the other hand, CpT1 could improve stand establishment in cases
where failure is due to the Coleoptera of the genera including Agriotes spp.,
Melolontha sp., Annoxia sp., and Orthoptera of Gryllotalpa spp., Calolampra
spp. and Teleogryllus spp. Larvae of these species exhibit subterranean habits
and eat plant roots at different stages of crop development, causing the
death of seedlings in early attacks (Rogers 1992; Charlet et al. 1997). To
achieve the required impact at crop establishment, the expression of CpT1
toxin should take place early in crop development and involve
concentrations that are lethal for the plague. Seedlings are very sensitive to