Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

ISOPRENOID BIOSYNTHESIS IN FRUITS AND VEGETABLES 289
plastid-targeting sequences with an abundance of serine and threonine and low numbers of
aspartate and glutamate.
13.8.1 Developmental regulation of DXS, PSY1, PSY2, and PDS
DXS transcripts are abundant in young, developing and fully expanded leaves, inflorescences
and stems, and are undetectable in roots (Lois et al., 2000). DXS transcript levels in young
and mature green fruits are similar to those of other photosynthetically active tissues, but are
enhanced greatly during fruit ripening. In situ hybridization studies show a clear correlation
between the spatial distribution of DXS transcripts (localized to the outer pericarp layers)
and the distribution of carotenoids (Lois et al., 2000). PSY1 is involved primarily in orange
or red tissues and PSY2 in green tissues, though both genes seem to be expressed in all
tissues (at least in low levels) (Bartley and Scolnik, 1993; Fraser et al., 1994; Fraser et al.,
1999). Phytoene desaturase (PDS) is also developmentally regulated, but it is not a ratelimiting
enzyme. Its transcripts appear to be low in roots, stems, leaves, and immature/green
floral parts, and greater accumulation is observed in mature floral stages and ripening fruit
(Bartley and Scolnik, 1993; Giuliano et al., 1993).
The induction of PSY1 expression begins gradually, before the onset of ripening, at the
mature green stage (Lois et al., 2000). The strongest PSY1 induction occurs at the orange
stage, with PSY1 and PDS mRNA levels increasing approximately 25-fold and 16-fold,
respectively, from the immature green stage to the orange stage (Ronen et al., 1999). Others
confirm increases in PSY1 and PDS mRNA levels at the breaker stage, albeit with varying
magnitudes (Pecker et al., 1992; Giuliano et al., 1993; Fraser et al., 1994; Corona et al.,
1996; Lois et al., 2000). Concurrent with patterns of carotenoid accumulation, DXS transcript
levels only begin to increase at the orange stage and then decrease as ripening continues
(Lois et al., 2000). DXS induction matches most closely with the pattern of carotenoid
accumulation, as opposed to PSY1 induction, which begins at the mature green stage.
Therefore, Lois et al. (2000) propose that DXS, rather than PSY1, controls the initiation of
carotenoid accumulation during ripening. There does appear to be some mutual regulation
of the two enzymes. Experiments with r,r-mutants (that possess a truncated, nonfunctional
form of the PSY1 protein) demonstrate that PSY1 is required for the final downregulation,
but not the initial upregulation of ripening-related DXS expression (Lois et al., 2000).
Interestingly, the addition of the DXS product, 1-deoxy-D-xylulose (dephosphorylated to
improve cellular incorporation), appeared to induce a number of ripening-related processes
in fruit such as DXS and PSY1 induction, chlorophyll a degradation, and downregulation
of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) small subunit (rbcS2) gene
expression (Lois et al., 2000).
Patterns of enzyme activity tend to follow the transcripts levels, although DXS activity
has not been measured in tomato. Fraser et al. (1994) report decreases in both PSY and PDS
activity of approximately 80% between the mature green and breaker stages. These activities
stay relatively constant thereafter, with the exception of PSY, which increases approximately
fourfold 7 days postbreaker (d.p.b.). A ripening-related reduction in carotenoid catabolism
would result from the destruction of the photosynthetic apparatus, thereby eliminating the
carotenoid turnover associated with photosynthesis as well as the reduction of abscisic
acid biosynthesis, which uses carotenoid precursors (Scolnik, 1987). Fraser et al. (1994)
hypothesize that this near-elimination of carotenoid consumption could allow for their