Contents - Faperta

Genetic Transformation of Crops for Resistance to Insect Pests 225
proteinase inhibitor (CpTi) does not impose a cost in yield in transgenic plants (Hilder and
Gatehouse, 1991).
Monoterpenes, the C10 isoprenoids, are a large family of natural products that are best
known as constituents of the essential oils and defensive oleoresins of aromatic plants
(Mahmoud and Croteau, 2002). In addition to ecological roles in pollinator attraction,
allelopathy, and plant defense, monoterpenes are also used extensively in the food,
cosmetic, and pharmaceutical industries. The importance of these plant products has
prompted studies on the monoterpene biosynthetic pathway, cloning of the relevant genes,
and development of genetic transformation techniques for agronomically signifi cant
monoterpene-producing plants. Metabolic engineering of monoterpene biosynthesis in
the model plant peppermint, Mentha piperita L., has resulted in yield increase and compositional
improvement of the essential oil, and also provided strategies for manipulating
fl avor and fragrance production, and plant defense (Mahmoud and Croteau, 2002).
Expression of a bacterial cytokinin biosynthesis gene (PI-II-ipt) in Nicotiana plumbaginifolia
(L.) plants has been correlated with enhanced resistance to M. sexta and M. persicae
(Smigocki, Heu, and Buta, 2000). The PI-II-ipt gene has also been expressed in N. tabacum
and L. esculentum, and similar antifeedant effects were observed with the transgenic
tobacco, but not in tomato. A 30 to 50% reduction in larval weight gain was observed with
some of the tomato plants, but these results could not be repeated consistently. Leaf surface
extracts from transgenic N. plumbaginifolia leaves killed 100% of M. sexta second instars at
concentrations of 0.05%, whereas the N. tabacum extracts were at least 20 times less active.
Extract suspensions were stable for up to two days at ambient temperatures below 42°C,
and for at least three months at 4°C when stored in the dark. High performance liquid
chromatography (HPLC) analysis of the N. plumbaginifolia extracts yielded an active fraction
that reduced hatching of M. sexta eggs by 30% and killed fi rst, second, and third
instars within 24, 48, and 72 hours of exposure, respectively. The activity appears to be
associated with oxygen-containing aliphatic compounds, possibly diterpenes. Based on
partial characterization of activity, the production, secretion, or accumulation of secondary
metabolites in leaves of cytokinin-producing PI-II-ipt N. plumbaginifolia plants appears
to be responsible for the observed insect resistance.
Protease Inhibitors
The enzyme inhibitors act on key insect gut digestive enzymes such as a-amylase and
proteinases. Several kinds of a-amylase and proteinase inhibitors present in seeds
and vegetative organs in plants infl uence food utilization by the phytophagous insects
(Ryan, 1990; Konarev, 1996; Chrispeels, Grossi-de-Sa, and Higgins, 1998; Gatehouse and
Gatehouse, 1998). Protease inhibitors (PIs) of plants are involved in a number of functions,
including the control of endogenous proteolytic enzymes (Richardson, 1977), the reserve
of ammonia and sulfur amino acids within the storage organs (Pusztai, 1972; Tan-Wilson
et al., 1985), and the plant defense against insect and nematode attack (Sijmons, 1993;
Thomas et al., 1994; Urwin et al., 1995; Lawrence and Koundal, 2002). In tomato and tobacco
plants, protease inhibitors have been found to accumulate in response to infection by
pathogenic microorganisms (Peng and Black, 1976; Rickauer, Fournier, and Esquerre-
Tugaye, 1989). Since protease inhibitors are primary gene products, they are excellent candidates
for engineering insect resistance into plants. Disruption of amino acid metabolism
by inhibition of protein digestion has been one of the targets for use in insect control
( Johnson et al., 1989). Genes encoding inhibitors specifi c for serine-proteases are the main
digestive proteases in most lepidopteran insects (Boulter, 1993). Deployment of protease