Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

ISOPRENOID BIOSYNTHESIS IN FRUITS AND VEGETABLES 291
The tomato hp-2 gene encodes a homolog of an Arabidopsis phytochrome signal transduction
gene DEETIOLATED1 (Mustilli et al., 1999). Since hp-1 and hp-2 mutants exhibit
many exaggerated light responses (Mustilli et al., 1999), their wild-type gene products appear
to inhibit phytochrome signal transduction at a downstream location that is common
to both phyA and phyB1 (Kerckhoffs et al., 1997). The fact that increased lycopene accumulation
is one of the responses of these mutants suggests that carotenogenesis is mediated
by phytochrome.
13.9.3 Phytochromes and tomato fruit
Fruit-localized phytochromes have been found to regulate the extent of lycopene accumulation
in tomato fruit; however, they are not required for the initiation of ripening since it will
occur in total darkness (Alba et al., 2000a). Ethylene, on the other hand, is necessary for
ripening and appears to be the initiating factor (Edwards et al., 1983; Theologis et al., 1993;
Bleecker and Kende, 2000). Phytochrome control of lycopene accumulation is not mediated
through ethylene, because phytochromes alter neither the timing nor the characteristics of
the ethylene burst (Alba et al., 2000a). In addition, phytochrome does not regulate other
ethylene-mediated aspects of fruit ripening such as fruit softening and the concentrations of
citrate, malate, fructose, glucose, and sucrose (Alba et al., 2000a). In the fruit, PHYA transcripts
are in greater abundance than transcripts from all other PHYs (Hauser et al., 1997),
and of all the five PHY loci, only PHYA showed substantial differential expression during
ripening (Alba et al., 2000a). The increase in PHYA mRNA accumulation was concurrent
with lycopene accumulation. PHYA mRNA accumulation was first observed at the breaker
stage and increased 11-fold during ripening. It is not known if the increased PHYA mRNA
led to an equivalent increase in functional phyA photoreceptors. Pigment accumulation in
phyA − mutants does not respond to R or R/FR treatments (Alba et al., 2000a). However,
there is still no conclusive evidence that PHYA is the phytochrome responsible for the response.
Hauser et al. (1997) reported that PHYB2 and PHYF were preferentially expressed
in tomato fruit compared with a variety of organs. This raises the possibility that several
PHY loci may be involved in regulating carotenoid synthesis (Alba et al., 2000a). Alba
et al. (2000a) express interest in using PHYA, B1, and B2 mutants to determine the roles
of specific PHYs in tomato ripening.
13.9.4 Mechanism for phytochrome control of carotenogenesis
Phytochrome may regulate carotenogenesis and lycopene accumulation through a number
of mechanisms. DXS has demonstrated light regulation in Arabidopsis thaliana seedlings
(Mandel et al., 1996), and PSY has demonstrated phytochrome regulation in seedlings of
white mustard (Sinapis alba) and A. thaliana (von Lintig et al., 1997; Welsh et al., 2000). As
well, PSY has shown protein activation by light-induced changes to chloroplast membrane
composition (Schledz et al., 1996; Welsh et al., 2000).
Recent studies demonstrate the reversibility of PSY activity under red light and farred
light, thus providing the strongest evidence for the phytochrome control of carotenoid
accumulation (Schofield and Paliyath, 2005). During in vivo studies, pericarp disks from
breaker stage tomatoes were ripened in darkness (D), or D interrupted by daily pulses of
red light (R), or R followed by far-red light (FR). After 14-day incubation, R-treated disks