Contents - Faperta

Development of Resistance to Transgenic Plants 389
A quantitative trait loci (QTL) conditioning maize earworm resistance in soybean PI 229358
and cry1Ac transgene from the recurrent parent Jack-Bt has been pyramided into BC 2F 3
plants by marker-assisted selection (Walker et al., 2002). Combining transgene- and QTLmediated
resistance to lepidopteran pests along with Bt genes is more effective for insect
control than the transgene alone.
Pyramiding Genes for Resistance to Multiple Pests
Development of genetically engineered plants with resistance to more than one pest will
be the most ideal strategy to use such plants in pest management. 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 highly effective and sustainable seed-based technologies.
In this context, it is important to examine whether co-expression of multiple toxins in
the same plant will have a synergistic or antagonistic effect. The Xa21 gene (resistance to
bacterial blight), the Bt fusion gene (for insect resistance), and the chitinase gene (for tolerance
to sheath blight) have been combined in a single rice line by reciprocal crossing of two
transgenic homozygous IR 72 lines (Datta et al., 2003). The identifi ed lines showed resistance
to bacterial blight, tolerance to sheath blight, and caused 100% mortality of neonate
yellow stem borer, Scirpophaga incertulas (Walker) larvae. Rice plants with cry1Ac and GNA
genes accumulated high levels of insecticidal gene products (Nguyen et al., 2002; Loc et al.,
2002). Transgenic plants expressing GNA showed enhanced resistance to brown planthopper,
Nilaparvata lugens (Stal), while the plants expressing cry1Ac were resistant to striped
stem borer, Chilo suppressalis (Walker). Expression of both transgenes gave protection
against both pests, but did not increase protection against either pest signifi cantly over the
levels observed in plants containing a single insecticidal transgene.
Regulation of Gene Expression and Gene Deployment
There is a need to develop appropriate strategies for gene deployment for different crops
and cropping systems depending on the pest spectrum, their sensitivity to the insecticidal
genes, and interaction with the environment. For effi cient pest control, it is important that
effective levels of insect control proteins are expressed in the plant where the insects feed.
Regulation of gene expression by the use of appropriate promoters is important for durability
and specifi city of resistance. In most cases, resistance genes have been inserted with
constitutive promoters such as CaMV35S, maize ubiquitin, or rice actin 1, which direct
expression in most plant tissues. Limiting the time and place of gene expression by tissuespecifi
c promoters such as PHA-L for seed-specifi c expression, RsS1 for phloem-specifi c
expression, or inducible promoters such as potato pin2 wound-induced promoter might
contribute to resistance management, and avoid unfavorable interactions with the benefi -
cial insects. Greater risk of resistance build up would arise from prolonged exposure to
sublethal levels of the transgene product. Restricted expression in tissues may also contribute
to minimizing the yield penalty associated with transgene expression (Xu et al.,
1993; Schular et al., 1998). There are specifi c situations where specifi c promoters would
have a clear advantage, such as root-feeding insects.