Contents - Faperta

184 Biotechnological Approaches for Pest Management and Ecological Sustainability
Molecular Markers and Metabolic Pathways
There is considerable scope for changing the products of secondary metabolites that are
associated with resistance to insect pests through biotechnological approaches. Harnessing
synteny may have maximum benefi t where entire metabolic pathways are dissected and
studied in detail in model systems, thereby identifying the key genes for manipulating
that trait, which can then be traced in the species of interest. Many secondary plant metabolites
such as fl avonoids, alkaloids, and terpenoids have been implicated in host plant
resistance to insect pests. Many compounds of the fl avonoid biosynthetic pathway (fl avanones,
fl avones, fl avanols, and isofl avonoids) accumulate in response to insect damage
(Ebel, 1986; Sharma and Norris, 1991; Heller and Forkman, 1993). Molecular breeding and
genetic engineering can be used to change the metabolic pathways to increase the amounts
of various fl avonoids conferring resistance to insect pests, for example, medicarpin and
sativan in alfalfa, cajanol and stilbene in pigeonpea, and stilbene in chickpea (Heller and
Forkman, 1993). Stilbenes have been expressed in transgenic tobacco plants, exhibiting
various degrees of inhibition of fungal growth (Heller and Forkman, 1993). Maysin, a glycosyl
fl avone in maize silk, is associated with resistance to corn earworm, H. zea (Waiss
et al., 1979). Most of the phenotypic variation in maysin concentration in maize silk is
accounted for by the p1 locus, the transcription activator of the portion of the fl avonoid
pathway leading to maysin synthesis. Reduced function p1 allele results in decreased transcription
of genes encoding enzymes of the p1-controlled portion of the pathway, and thus
reduced maysin synthesis. The marker umc105a corresponds to the brown pericarp (bp1)
locus. The p1 and chromosome 9S regions are the major QTLs controlling silk antibiosis to
the corm earworm (Byrne et al., 1997). Composite interval mapping has shown a major
QTL in the asg20-whp1 interval of chromosome 2, and another near the wx1 locus on chromosome
9 (Byrne et al., 1998). A gene that encodes chalcone synthase (whp1) on chromosome
2 and a silk specifi c gene (sm1) on chromosome 6 affect silk maysin concentration
and resistance to corn earworm in maize (Byrne et al., 1998). The extra chromosome 5A of
Allium cepa L. plays an important role in fl avonoid biosynthesis (Masuzaki, Shigyo, and
Yamauchi, 2006). The fl avonoid 3′-hydroxylase (F3′H) gene controlling quercetin synthesis
from kaempferol is located on chromosome 7A, and an anonymous gene involved in
glucosidation of quercetin is located on chromosome 3A or 4A.
The Transgenic Approach and Gene Pyramiding through MAS
Genetic engineering offers the advantage of rapid introgression of novel genes and traits
into elite agronomic backgrounds (Mohan et al., 1997). Transgenic resistance to insects
has been demonstrated in plants expressing insecticidal genes such as δ-endotoxins
from Bacillus thuringiensis (Bt) Berliner, protease inhibitors, enzymes, secondary plant
metabolites, and plant lectins (Sharma, Sharma, and Crouch, 2004). While transgenic
plants with introduced Bt genes have been deployed in several crops on a global scale,
the alternative genes have received considerably less attention. The potential of some of
the alternative genes can only be realized by deploying them in combination with conventional
host plant resistance and Bt genes (Sharma et al., 2002). Many of the candidate
genes used in genetic transformation of crops are highly specifi c or are only mildly effective