Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

62 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
(Jones et al., 1995). These experiments indicate that while many SR genes are regulated by
ethylene, they are also regulated by developmental or temporal cues.
The ability of plant organs to respond to exogenous ethylene appears to be developmentally
regulated as the enhanced expression of SAGs in ethylene-treated leaves is greatest in
old leaves and not detectable or only moderately induced in young green leaves (Weaver et
al., 1998). Immature tomato fruits and flowers also do not respond to exogenous ethylene
with ripening or petal senescence. This ethylene treatment does not induce the expression
of ripening-related genes in immature green fruit or SR genes in petals from flowers in the
bud stage (Lawton et al., 1990). While some flowers like daylily and nonclimacteric fruits
like strawberry are not regulated by ethylene, it is clear that ethylene plays a regulatory role
in both senescence and fruit ripening through the transcriptional regulation of SR genes.
The observed differences in the timing of the response of various SR genes to external
stresses and plant hormones indicate that some of the SR genes may respond directly to
stress, while others may be regulated by senescence that results from the stress or hormone
application (Weaver et al., 1998). Further characterization of the response of SR genes to
various stresses will help to identify those genes that are primarily responsive to senescence
and are thus key regulators of senescence. There are many genes that are upregulated during
senescence and involved in the activation and coordination of senescence; the downregulation
of genes that act as repressors of senescence may play an equally important role
in regulating senescence. Currently, most of the genes identified as downregulated during
senescence are genes involved in photosynthesis (John et al., 1997). Transcript levels for the
pea homolog of the defender against apoptotic death (dad) gene, a gene known to function
as a repressor of programmed cell death (PCD) in Caenorhabditis elegans and mammals,
have been found to decrease during flower development (Orzaez and Granell, 1997), while
the dad-1 cDNA from rice can rescue temperature-sensitive dad-1 mutants of hamster from
PCD. Yamada et al. (2004) isolated a homolog of the potential antiapoptotic gene, defender
against apoptotic death (DAD1) from gladiolus petals as a full-length cDNA (GlDAD1),
and investigated the relationship between its expression and the execution processes of PCD
in senescing petals. RNA gel blotting showed that GlDAD1 expression in petals was drastically
reduced, considerably before the first visible senescence symptom (petal wilting). A
few days after downregulation GlDAD1 expression, DNA and nuclear fragmentation were
observed, both specific for the execution phase of PCD, but the function of the dad gene in
plant senescence is still not very clear.
4.6 Genes involved in ethylene biosynthesis and perception
The biosynthesis and perception of the plant hormone ethylene are known to modulate
specific components of leaf senescence, fruit ripening, and flower senescence (Grbic and
Bleecker, 1995). All three processes are also known to be accompanied by increases
in the synthesis of ethylene (Abeles et al., 1992), and therefore it is reasonable to assume
that SR genes would include those involved in ethylene biosynthesis. Two enzymes,
1-aminocyclopropane-l-carboxylate (ACC) synthase and ACC oxidase, have been identified
as catalyzing rate-limiting steps in ethylene biosynthesis (Kende, 1993). While no
ACC-synthase genes have specifically been isolated by differential screening of senescing
or ripening tissues, three SR clones have been identified that encode ACC oxidase;
these include pTOM13 from tomato (Hamilton et al., 1991); SR120 from carnation petals