Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

ISOPRENOID BIOSYNTHESIS IN FRUITS AND VEGETABLES 283
and their processed products because consumers prefer tomatoes and products with intense
red color. As well, the carotenoids are nutraceuticals because they play a beneficial role in
regulating health. Inverse associations have been observed between blood lycopene levels
and the risk of cancer development at several sites in the human body (Giovannucci, 1999).
The most beneficial effects have been observed in the reduction of the incidence of prostate,
lung, and stomach cancers, with slightly lower benefits at other sites that include pancreas,
colon, rectum esophagus, oral cavity, and cervix (Giovannucci, 1999). β-Carotene (provitamin
A) is a vitamin A precursor, and considerable efforts have been made to increase dietary
levels of β-carotene by engineering the genes for the carotenoid biosynthetic pathway enzymes
into rice endosperm (Ye et al., 2000). Vitamin A deficiency causes xerophthalmia
and blindness (Mayne, 1996) and has led to over a million deaths annually among children
1–4 years of age (Humphrey et al., 1992). Finally, levels of dietary lutein (Seddon et al.,
1994; Curran-Celentano et al., 2001) and serum lutein (EDCCSG, 1992, 1993) are related
to a reduced risk of age-related macular degeneration. For these reasons, there is a tremendous
interest among the industry and grower sectors to obtain tomatoes with the highest
carotenoid levels possible.
13.3 Carotenoid biosynthesis
Carotenoids are biosynthesized by the chloroplast-localized isoprenoid pathway. The accumulation
of lycopene (C 40 ) during fruit ripening results from increased lycopene synthesis
as well as decreased conversion of lycopene to more complex carotenoids—with the notable
exception being the formation of small amounts of β-carotene at the expense of lycopene
(Ronen et al., 2000). At least two enzymes are known to regulate carotenoid synthesis:
1-deoxy-D-xylulose-5-phosphate synthase (DXS) and phytoene synthase (PSY, which is
coded by PSY1 in ripe fruit). DXS appears to be the paramount of several enzymes involved
in the synthesis of isopentenyl pyrophosphate, which is the key 5-carbon unit of the isoprenoid
pathway (Fig. 13.1). Within this pathway, PSY forms phytoene (C 40 ), which is an
immediate precursor to lycopene (Fig. 13.2). Increased expression of DXS and PSY1 genes
during ripening correlates with the pattern of lycopene accumulation, thus demonstrating
transcriptional regulation of the pathway during ripening (Lois et al., 2000). As expected,
PSY activity also correlates with these patterns of mRNA transcription and lycopene levels
(Fraser et al., 1994). The accumulation of lycopene in tomato fruit has been found to be
regulated by light, and this is mediated by phytochromes (Alba et al., 2000a). Phytochromes
are light receptors involved in responses regulated by red light (R) and far-red light (FR)
resulting in alteration of growth and development. In particular, these modified proteins
change conformations depending on whether they have received R or FR; however, it is R
that activates the protein within the cell to cause physiological responses (Quail, 2002). Most
phytochrome responses occur at low-photon fluence rates (1–1,000 μmol/m 2 ) once a given
number of photons have been intercepted, irrespective of the duration of the light exposure
(Neff et al., 2000). As well, they are commonly characterized by R/FR reversibility.
13.4 Composition of tomato pigments
The ripening of tomato fruit is characterized by a large shift in pigment composition.
Pigmentation of immature and mature green fruit is similar to that of other photosynthetic