Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

292 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
had accumulated 12 mg carotenoids/100 g fresh weight; nearly a 50% increase over D- and
R/FR-treated disks. This R/FR reversibility of carotenoid accumulation was also observed
in PSY activity 8 days postbreaker, where it showed peak activity. Tomato fruits were also
ripened under R, R/FR, and D conditions. DXS and PSY1 transcription was monitored using
relative reverse transcriptase-polymerase chain reaction (RT-PCR), and PSY activity was
estimated using enzyme preparations. However, the R/FR regulation of PSY activity was
not reflected in PSY1 transcript levels. It is not surprising that phytochrome regulation of
carotenogenesis occurs at the level of PSY, since it is a branch point from the isoprenoid
pathway and is the first committed step of carotenoid biosynthesis. As well, PSY is an
important control point for the developmental regulation of carotenogenesis in tomato fruit,
withPSY1 transcript levels and PSY activity increasing during ripening. PSY being a key
enzyme of the pathway, any modulation in its activity is likely to be reflected in the levels
of other downstream products such as lycopene and carotene.
Recent work with phytochrome signal transduction mutant high pigment-1 (Cookson
et al., 2003) revealed similar results. This mutant showed enhanced phytochrome responses
and had higher PSY activity in the ripe fruit, but PSY1 transcript levels were unaffected. This
was somewhat unexpected since it is generally accepted that the control of carotenogenesis
during ripening is at the transcript level (Cunningham and Gantt, 1998). Certainly from a
developmental perspective, carotenoid accumulation was preceded by increased DXS and
PSY1 expression, concomitant with increased PSY activity. One possible explanation is that
during development, carotenoid biosynthesis is coarsely regulated by factors such as fruit
maturity and the ethylene, and respiratory climacterics, which initiate gene transcription.
The fine regulation of this pathway is achieved through other mechanisms, including those
mediated by phytochrome. It would seem that these phytochrome effects take place in the
form of translational or posttranslational modification of the PSY protein.
A second effect of light has been attributed in part to phytochrome action via cis-acting
elements in the promoter region of A. thaliana PSY (Welsh et al., 2003). Continuous light of
all qualities, including FR, R, B (blue), and W (white), increases PSY mRNAs (von Lintig
et al., 1997) and PSY protein abundance in “membrane pellets” (Welsh et al., 2000). Two
tandem ATCTA motifs (−856 to −825) comprise a cis-acting element responsible for basal
promoter activity in all light conditions (D, W, FR, R, B) as well as enhanced activity under
W, FR, R, and B light. This novel ATCTA motif is found in promoter regions of several
genes in carotenoid biosynthetic pathway, including PDS (Arabidopsis and Zea mays) and
DXS (Arabidopsis)—which may explain the positive regulation of DXS by light (Mandel et
al., 1996). A second cis-acting element (−210 to −179) containing two short G-box-like
motifs is responsible for the differential response toward different light qualities. The G1
motif (CACGAG) is responsible for R light responses (but it also enhances expression in
response to W, FR, B), and the G2 motif (CTCGAG) is responsible for W, FR, and B light
responses. The promoter region of PSY1 has not been characterized to explore the existence
of any such elements, however, because of the lack of a transcriptional regulation, it appears
likely that such light-response elements may not be present for PSY1.
13.10 Mevalonate pathway of isoprenoid biosynthesis
A feature, common to all isoprenoids, is their biosynthesis from IPP, the central metabolite
and building block for all isoprenoid compounds. Earlier studies were focused on a single