Contents - Faperta

Genetic Transformation of Crops for Resistance to Insect Pests 237
The activation of jasmonic acid biosynthesis by cell wall elicitors, the peptide systemin and
other compounds is shown to relate to the function of jasmonates in plants (Creelman and
Mullet, 1997). Jasmonate can modulate gene expression at the level of translation, RNA
processing, and transcription (Creelman and Mullet, 1997). Prosystemin, a compound biologically
active as systemin (Ryan and Pearce, 1998), when assayed for proteinase inhibitor
induction in young tomato plants, has been found to be active in the alkalinization response
in cultured cells (Dombrowski, Pearce, and Ryan, 1999). Prosystemin or large fragments
of prosystemin can be an active inducer of defense responses in tomato leaves. However,
M. sexta larvae feeding on tomato plants constitutively expressing a prosystemin antisense
gene had approximately three times higher growth rates than larvae feeding on nontransformed
control plants (Orozco-Cardenas, McGurl, and Ryan, 1993). Prosystemin mRNA
levels in antisense and control plants were correlated with levels of proteinase inhibitor I
and II protein levels after 6 and 12 days of larval feeding, indicating that plant resistance
to insects can be modulated by genetically engineering a gene encoding a component of
the inducible systemic signaling system regulating a plant defensive response. Despite
several reports on successful protection of plants and trees against phytophagous insects,
defense strategies based on protease inhibitor expression in plants have not resulted in any
commercial applications so far. This could be due to the insects’ capacity to react to protease
inhibitors, and the protease inhibitor expression levels in transgenic plants. Use of
the bacterial isopentenyl transferase (ipt) gene, involved in cytokinin biosynthesis (fused
proteinase inhibitor II [PI-IIK gene]) in N. plumbaginifolia reduces M. sexta feeding by 70%
(Smigocki et al., 1993), and retards the development of peach potato aphid, M. persicae.
Zeatin and zeatin-riboside levels in leaves remaining on PI-II-ipt plants after hornworm
feeding are elevated by about 70-fold. Exogenous application of zeatin to the PI-II-ipt leaves
enhanced the level of resistance to the tobacco hornworm and completely inhibited the
normal development of the green peach aphid. Jin et al. (2000) developed transgenic cabbage
with cry1Ab3 gene under the control of the soybean wound-inducible vspB promoter.
Cabbage plants with cry1Ab3 gene were resistant to larvae of the diamondback moth,
P. xylostella, whereas plants with wild-type Bt gene were susceptible.
Gene Pyramiding
Combining conventional host plant resistance with Bt genes can provide a germplasm
base to achieve durable resistance to insect pests (Table 7.6). Westedt et al. (1998) examined
Lemhi Russet and two lines with resistance to P. operculella. USDA8380-1 (leaf leptines)
TABLE 7.6
Gene Pyramiding to Increase the Effectiveness of Transgenic Plants for Pest Management
Transgene(s) Remarks References
Serine PIs Bt Enhances activity of Bt genes. MacIntosh et al. (1990)
Tannic acid Bt Increases activity of Bt genes. Gibson et al. (1995)
cry1Ac CpTi More effective than plants expressing Bt gene alone. Zhao et al. (1998)
cry1Ac cry1C Plants producing both toxins caused rapid mortality. Cao et al. (2002)
cry1Ac cry1Ab Showed greater insecticidal activity. Stewart et al. (2001)