Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

66 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
(Lycopodium cernuum) (Paull and Chantrachit, 2001). Jasmonic acid and methyl jasmonate,
natural plant growth regulators, are involved in plant defense responses, exhibit direct
antifungal activity as well as increase numerous antifungal compounds in plant tissues
when applied exogenously. They also activate many inducible genes leading to the synthesis
of secondary plant products that function as antimicrobial compounds (Meir et al.,
1998).
4.10 Identification and classification of senescence-related genes
Recent molecular studies have confirmed that senescence and ripening are accompanied by
changes in gene expression. Utilizing differential screening and subtractive hybridization
techniques, a number of cDNAs that are upregulated during senescence have been cloned.
Genes that exhibit enhanced expression during senescence have been cloned from the leaves
of Arabidopsis, asparagus, Brassica napus, barley, maize, radish, and tomato (Smart, 1994;
Buchanan-Wollaston, 1997; Nam, 1997; Weaver et al., 1997; Quirino et al., 2000). Differential
screening of senescence petal cDNA libraries and PCR-based differential display
techniques have been utilized to identify genes that are upregulated during senescence of
carnation and daylily flowers (Woodson et al., 1993; Woodson, 1994; Valpuesta et al., 1995;
Guerrero et al., 1998; Panavas et al., 1999).
Most of the genes that have been identified as senescence-related are expressed at basal
levels in nonsenescing tissues (green leaves and young flowers) and increase in abundance
during senescence. A smaller number of SR genes are only detectable in senescing tissues
and represent senescence-specific genes. An even smaller set of genes have been identified
that have high levels of expression early in development, decreased expression in young
maturing tissue, and increased expression at the onset of senescence. Genes that fit within
this class have only been identified in vegetative tissues and represent genes that have a
similar role in multiple stages of development like germination and senescence (Lohman
et al., 1994; Smart, 1994; Buchanan-Wollaston, 1997).
Weaver et al. (1998) shows the patterns of expression of a selected group of SAGs
(senescence-associated genes) during age-related senescence of Arabidopsis leaves. Most
of these genes exhibit basal levels of expression in green nonsenescing tissues. Within this
broad classification, genes are differentially regulated, with some increasing in abundance
gradually as the leaf matures and others increasing more abruptly at various stages of leaf
development. Only SAG12 and SAG13 show senescence-specific expression. Among the
senescence-specific genes, SAG13 is detected before any visible signs of leaf senescence
and, as such, may be responsible for initiation of the senescence process, while SAG12 is
expressed after the leaf is visibly yellowing.
Many of the genes that have been identified as senescence-related are identified from
a particular plant organ, and it is not known whether they are expressed in other senescing
organs or during other developmental processes. The expression of a number of SAGs was
investigated in roots, stems, flower buds, and mature flowers of Arabidopsis (Quirino et al.,
1999). Expression of SAG12, SAG13, SAG25, SAG26, and SAG29 was not detected in any
nonsenescing tissues but was detected in both senescing flowers and leaves, indicating a
common molecular regulation of senescence in vegetative and floral tissue. Some of the
SAGs show low levels of expression in multiple tissues with upregulation in senescing