Biotechnological Approaches to Enhancing Tropical Fruit Quality - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 375

technologies to these plants (Petri and Burgos, 2005). In addition, very few genotypes of a
particular species have been transformed and, in many instances, these genotypes are not
commercially important (Petri and Burgos, 2005). With the tools available, it is currently
possible to genetically alter practically any gene, as long as a suitable molecular probe is
available, and analyze the function during fruit ripening. The ability to apply this technology
to various fruits is expanding as the number of fruit-bearing plants that can be transformed
and regenerated is also increasing. Currently, the list includes apple, muskmelon, papaya,
strawberry, banana, raspberry, mango, avocado, and other tropical and subtropical crops
species (Gomez Lim and Litz, 2004).
Evaluating the performance of transgenic fruit tree cultivars requires approximately
12 years or until fruiting and flowering have been observed, depending on the species
(Gomez Lim and Litz, 2004). Thus, molecular breeding represents a highly efficient ap-
proach for developing improved, perennial fruit cultivars. At present, insertion of foreign
genes into plant DNA occurs in a random fashion, which may lead to accidental inactivation
of nontarget genes and to variable and unpredictable expression of the transgene itself and
even to gene silencing (Kohli et al., 2003). The use of matrix attachment region (MAR)
sequences has been proposed to minimize transgene silencing and uniformize transgene ex-
pression (Allen et al., 2000). MARs are DNA sequences that bind to the cell's proteinaceous
nuclear matrix to form DNA loop domains. Transgenes flanked with MARs are thought to
be able to form their own chromatic domain and thus be insulated from the influences of
factors in the chromatin adjacent to its site of insertion (Hall et al., 1991). Because a large
majority of plant chromatin is in an inactive conformation at any given time, insulating the
transgenes with MARs may reduce the incidence of gene silencing and enhance transgene
expression (Lorence and Verpoorte, 2004).
The usual approach is to produce a large of number of lines derived from independent
transformation events and to select the best genotype among the transformants. A description
of the current methods employed to transfer foreign genes to fruit crops is beyond the scope
of this chapter, and the interested reader is referred to recently published reviews (Lorence
and Verpoorte, 2004; Cotsaftis and Guiderdoni, 2005).
18.4 Metabolic processes related to fruit quality susceptible
to manipulation by genetic transformation
In the past, traditional plant breeding centered on improving crop yield. However, con-
sumers are increasingly paying more attention to product quality and composition. As an
example, consumers prefer fruits with increased nutritional properties (vitamins, sugars,
proteins, minerals, etc.) and ingredients that may help reduce the risk of certain cancer
or cardiovascular disorders (e.g., lycopene and antioxidants). These requirements have in-
creased the need for fruits with a longer shelf life and increased quality. In what follows, a
discussion of different metabolic processes related to fruit quality that have been or can be
genetically manipulated will be made.
18.4.1 Lipid metabolism
Even though they are not a main component of fruits, except avocado, lipids are actively
metabolized during ripening, and they can even be employed as indicators of the progress