Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

68 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
detachment of leaves in the dark accelerates senescence and serves as a means of coordinating
the senescence process between leaves, its use as a model system for natural age-related
senescence should be approached with caution. Following the identification of three
cDNAs that showed increased expression in dark-treated leaves, experiments revealed that
only one of the three showed enhanced expression during natural senescence (Becker and
Apel, 1993). Only four of seven genes identified from Arabidopsis as dark-induced also
showed enhanced expression in naturally senescing leaves (Park et al., 1998).
There are many common molecular events occurring among senescing tissues. The
degradation and remobilization of cellular constituents is predominant during senescence
and, correspondingly, the activities of hydrolytic enzymes and their mRNAs increase. A
number of SR genes have also been identified that encode products with homology to PR
proteins. While it is not known what the role of these proteins is in senescence, it appears
that they may serve a protective role similar to their role during the defense response.
These patterns of expression indicate that throughout plant development, common molecular
mechanisms are regulated by the same genes in multiple tissues. A few of these genes have
been identified as having leaf, flower, or fruit senescence-specific expression. Of these
genes, many encode different isoforms of the same enzyme, which may be differentially
regulated within the plant organs. Similar to fruit ripening, the developmental regulation of
germination and senescence also share common molecular mechanisms. This is especially
evident when investigating the expression of genes involved in protein and lipid degradation
and remobilization.
While differential cDNA screening, differential display and cDNA subtraction have
identified a number of senescence-related genes, the expression of most genes has not been
investigated in flowers, leaves, and fruits. The use of enhancer trap lines in Arabidopsis
has resulted in the identification of over 100 lines that have reporter gene expression in
senescing, but not in nonsenescing tissues (He et al., 2001). This technology starts to
reveal the complexity of the network of senescence-regulated pathways, and will allow for
the identification of many additional SR genes. The identification of senescence-specific
promoter elements (Noh and Amasino, 1999a) and the generation of mutants and transgenic
plants will help us to better understand the regulation of SR genes during senescence.
DNA microarrays will allow temporal and spatial expression patterns to be determined for
hundreds of genes involved in senescence. These technologies will lead to an increased
understanding of the initiation and execution of senescence, which will allow us to increase
vase life and horticultural performance of ornamentals, increase yield in agronomic crops,
and decrease postharvest losses of fruits and vegetables.
4.12 Sugar status and cut flower senescence
Sugar solutions are well known for their ability to improve postharvest quality and extend the
vase life of cut flowers, although the hypothesis of a sole sugar starvation or sugar accumulation
signal in inducing petal senescence has not been validated (van Doorn, 2004). Signals
from the lack of, the abundance of, or the metabolism of sugars (i.e., sugar signals) probably
form part of a complex array of exogenous and endogenous signals that initiate senescence,
and there is evidence that in carnation petals sucrose decreases ethylene responsiveness
(Verlinden and Garcia, 2004), and complex interactions occur between sugar- and ethylenesignaling
mechanisms that are tissue dependent (Iordachescu and Verlinden, 2005). In