Agriculture Reference
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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 rate-
limiting 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-bisphosphatecarboxylase-oxygenase(Rubisco)smallsubunit(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
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