Contents - Faperta

30 Biotechnological Approaches for Pest Management and Ecological Sustainability
phosphatase have larger leaves, altered stem growth, and improved response to stress
(Goddijn et al., 1997; Pilon Smits et al., 1998). Over-expression of various glutamate dehydrogenases
(GDH) also improves plant growth and stress tolerance. Plants have been
specifi cally transformed with genes encoding a- and b-subunits of the chloroplast-located
GDH from the alga, Chlorella sorokiniana (Shihira and Krauss) (Schmidt and Miller, 1997).
Similar improvements in plant performance have been reported for rice plants transformed
with the barley late embryogenesis (LEA) gene (Wu and Ho, 1997). Plants with an ability
to produce more citric acid in roots provide tolerance to aluminum in acid soils (De la
Fuente et al., 1997). Introduction of functional calcineurin activity provides tolerance to
salinity (Pardo, Hasegawa, and Bressan, 1999; McCourt et al., 1999) involving the introduction
of a gene encoding a plant farnesyltransferse (Pei et al., 1998) and inhibitors of this
enzyme when expressed in plants enhance drought tolerance, improve resistance to senescence,
and modify the growth habit. A salt tolerance gene isolated from mangrove,
Avicennia marina (Forssk.) Vierh., has been cloned, and can be transferred into other crop
plants (Swaminathan, 2000). The gutD gene from Escherichia coli Escherich can also be used
to provide salt tolerance (Liu et al., 1999). These genes hold a great potential for increasing
crop production in marginal lands.
Increased Starch and Sugar Production
Sucrose phosphate synthase (SPS) is a key enzyme in the regulation of sucrose metabolism.
Transgenic plants expressing the maize SPS under the control of a promoter from the
small subunit of tobacco (Rubisco) has shown increased foliar sucrose or starch ratios
in leaves, and decreased amounts of foliar carbohydrates when plants were grown with
CO2 enrichment (Signora et al., 1998). Modifi cation of the activity of metabolites of the
TCA (tricarboxylic acid) cycle by reducing the amount of the NAD-malic enzyme can also
be used for increasing starch concentrations (Leaver et al., 1998). Introduction of the E. coli
inorganic pyrophosphatase to alter the amount of sugar (Sonnewald and Willmitzer, 1996),
and modifi cation of hexokinases (Sheen, 1998), which affect the sugar-sensing capacities of
a plant, and sucrose-binding proteins (Grimes and Chao, 1998), and a class of cupin protein
(Dunwell, 1998) have been implicated in sugar unloading in developing legume seeds.
This has opened up exciting possibilities for changing the chemical composition of the
food grains to meet specifi c requirements.
Altering Senescence and Drought Resistance
It has long been argued that a reduction in senescence (Smart et al., 1996; De Nijs, Broer,
and Van Doorn, 1997) would improve the performance of plants and thereby increase crop
yield. Introduction of farnesyl transferase and the isopentenyl transferase genes delays
senescence. Senescence associated promoters SAG1 and SAG2 will be useful for producing
transgenic plants with improved performance (Amasino and Gan, 1997). Delayed leaf
senescence is also associated with resistance to drought.
Increased Photosynthetic Efficiency, Crop Growth, and Yield
An exciting experimental approach to radically change plant metabolism is currently being
investigated with respect to introducing the C 4 type of photosynthesis into C 3 plants such
as A. thaliana (Ishimaru et al., 1997) and potato (Ishimaru et al., 1998). The C 3 photosynthesis
suffers from O 2 inhibition due to the oxygenase reaction of ribulose 1,5-biophosphate