Biotechnological Approaches to Enhancing Tropical Fruit Quality - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 374

is based on a rather limited number of cultivars, which are poorly characterized for genetic
traits.
Molecular and biotechnological approaches involving genetic transformation, which
is the main subject of this chapter, offer an attractive alternative to conventional genetic
improvement. In what follows, a description will be made of the different attempts of
application of biotechnology for improvement of tropical fruit.
18.2 Fruit ripening
Fruit ripening is a highly complex process, with marked variations in metabolism occurring
between different types of fruits. Nevertheless, the process is characterized by a series of
coordinated biochemical and physiological changes that lead to the development of (in many
cases) a soft, edible fruit (Giovannoni, 2004). Some of these changes include synthesis of
secondary metabolites associated with flavor and aroma, synthesis of pigments, degradation
of chlorophyll, alterations in organic acids and cell wall metabolism, and a softening of the
fruit tissue. At the molecular level, there are a large number of tightly regulated genes
involved in specific processes in a highly coordinated manner (Giovannoni, 2004).
In general, fruits are classified as climacteric or nonclimacteric depending on their pat-
terns of respiration and ethylene synthesis during ripening. Climacteric fruits are character-
ized by an increased respiration rate at an early stage in the ripening process accompanied
by autocatalytic ethylene production. Many of the economically important fruit crops are
climacteric, and therefore a large amount of research has been devoted to studying the
biochemical and molecular pathways operating during the climacteric ripening of fruits.
Nonclimacteric fruits, on the other hand, show a different respiratory pattern and display
a lack of autocatalytic ethylene synthesis. Research in nonclimacteric fruit has been tradi-
tionally lagging behind climacteric fruit, and although there is considerable information, a
clear picture of the mechanisms governing the ripening process in this class of fruit is still
missing (Adams-Phillips et al., 2004).
Most of the research aimed at modifying ripening has centered on manipulation of fruit
firmness (membrane and cell wall properties) and ripening rate (ethylene production or
perception) in climacteric fruit. These two aspects are discussed at length in other chapters
of this topic and will not be discussed here. Instead, the discussion will focus on approaches
for improving fruit quality.
18.3 Genetic transformation of tropical crops
Most of tropical fruit crops are perennial trees and, because of the limitations described
above, genetic transformation seems to be the only practical solution for improvement of
specific horticultural traits (Gomez Lim and Litz, 2004). Genetic transformation provides
the means for modifying single horticultural traits without altering the phenotype. This
capability is particularly valuable for perennial plants and tree species in which development
of new cultivars is hampered by their long generation time. Targeting specific gene traits
is predicated on the ability to regenerate elite selections of what are generally trees from
cell and tissue cultures. The integrity of the clone would thereby remain unchanged except
for the altered trait. However, the difficulty in regenerating many tree species from elite
or mature-phase selections is one of the most serious obstacles for applying gene transfer