Biotechnological Approaches to Enhancing Tropical Fruit Quality - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 376

of the process. There seems to be a correlation between the pattern and timing of ripening
in relation to the appearance of lipid peroxidation products, which involves free radical for-
mation, with ethylene treatment significantly inducing their appearance (Meir et al., 1991).
Genetic manipulation of the lipid composition could potentially improve the nutritional
value of fruits, the profile of aroma compounds, and the ability of plants to withstand low-
temperature stress. At present, two strategies have been used to modify lipid composition
in higher plants: (a) alteration of the major fatty acid level by suppressing or overexpressing
a specific key enzyme in lipid biosynthesis and (b) synthesis of a fatty acid not found
in the host plant. The identification of desaturases, enzymes that introduce a cis -double
bond in saturated fatty acids, has led to the production of plants with an increased level of
polyunsaturated fatty acids (Arondel et al., 1992) or increased chilling tolerance (Ishizaki-
Nishizawa et al., 1996; Khodakovskaya et al., 2006). By suppression of oleate desaturase,
the levels of oleic acid (C18:1) in transgenic soybean and of stearate (CI8:0) in transgenic
canola were increased up to 80 and 30%, respectively (Baldoni and Rugini, 2001). An
example of strategy (b) could be seen in canola that does not naturally produce laurate
(C12:0), whereas transgenic canola contained laurate by introduction of the proper enzyme
(Baldoni and Rugini, 2001).
In avocado, whose outstanding compositional feature is its high fat content, changes
in lipids during ripening, including increases in the monoglyceride and free fatty acid
fractions, probably result from degradation of triglycerides (Kikuta and Erickson, 1968).
Lipid metabolism has been linked with color and flavor development of fruit crops dur-
ing ripening. Storage lipids may be involved in some manner in the metabolic processes
taking place during ripening (Seymour and Tucker, 1993). The majority of lipids found in
many fleshy fruits are esters of long-chain fatty acids. An increased fatty acid oxidizing
activity has been recorded in some fruits as ripening proceeds (Baqui et al., 1977). The
level of total lipids does not normally change during ripening, but the concentration of
individual fatty acids (particularly linoleic and oleic acids) may be altered in a particular
manner depending on the fruit (Wade and Bishop, 1978; Meir et al., 1991). Products of
β

-oxidation are used in the synthesis of both carotenoids and terpenoid volatiles (Baker et
al., 2006), which are important aroma components of many fruits. Interestingly, mRNA of
one enzyme of the -oxidation pathway, peroxisomal thiolase, has been found to be induced

during mango fruit ripening (Bojorquez and Gomez Lim, 1995). This probably reflects an
increased

β

-oxidation pathway activity during ripening whose products are important for
aroma production. Recently, acyl CoA oxidase, the key enzyme of

β

β

-oxidation, has been
identified and also found to be induced during fruit ripening (A. Nila and M.A, Gomez-Lim,
unpublished results). Therefore, the role of these enzymes might be to metabolize fatty acids
to produce volatiles compounds. This is an area barely explored.
The activity of alcohol: acyl CoA acetyl transferase has been correlated with fruit
ripening and the production of aroma volatile compounds in fruits (Shalit et al., 2001). It is
possible that the production of those compounds may be increased or modified by genetic
engineering.

18.4.2 Cold tolerance

Many fruits are sensitive to low temperatures, particularly tropical products. However,
many plants have the ability to increase freezing tolerance in response to low temperature, a