Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FLOWER SENESCENCE 55
cascade (Arora, 2005; Chen et al., 2005). Genes that control ethylene production, ethylene
sensitivity and genes that are affected by the presence of ethylene have been identified in
cut flowers (Kosugi et al., 2000; Muller et al., 2002; Verlinden et al., 2002; Shibuya et al.,
2004; Iordachescu and Verlinden, 2005). Chemical inhibitors have been used to study
biosynthesis of ethylene and ethylene activity; AVG (aminoethoxyvinylglycine) and AOA
(aminooxyacetic acid) inhibit ethylene biosynthesis; STS (silver thiosulfate), NBD (2,5-
norbornadiene), and 1-MCP (1-methyl cyclopropene) bind and block the ethylene receptor.
Currently, the vase life of carnation flowers can be extended by treating cut stems with
inhibitors of ethylene biosynthesis (e.g., AOA), ethylene response or receptor inhibitors
(e.g., STS, 1-MCP), and high temperature (Verlinden and Woodson, 1998) (a treatment
normally exclusive to disinfestation procedures has also been shown to improve the vase
life of carnations by reducing ethylene activity). Although these chemical treatments are
effective at delaying postharvest senescence, comparative analyses of the senescence of
transgenic and wild-type carnations showed that genetic modification for ethylene insensitivity
was more effective than chemical treatment for vase life extension (Bovy et al., 1999).
In recent years, petunia has proved to be an ideal model system for studying the regulation
of postharvest flower senescence. The plants have a relatively short regeneration cycle and
can be grown year-round and manipulated by pruning to produce multiple floral buds per
plant for postharvest experiments. Petunia flower senescence is sensitive to ethylene and
induced following pollination, although emasculated flowers are often used for postharvest
experiments (Jones et al., 2005). The development and senescence of individual flowers
have been fully characterized for this purpose (Xu and Hanson, 2000). It has been possible
to study the ethylene-insensitive regulatory pathways in petunia flowers using plants that
are genetically modified for insensitivity to ethylene (Wilkinson et al., 1997). Comparative
analysis of flower senescence in ethylene-insensitive and wild-type petunia plants has
shown that ethylene differentially regulates individual cysteine protease genes during flower
senescence, supporting the hypothesis that senescence-induced gene expression in petals
occurs via ethylene-dependent and -independent signaling pathways (Jones et al., 2005).
Horticultural performance of transgenic ethylene-insensitive petunias has provided valuable
information about other developmental programs that ethylene regulates, highlighting those
that may hinder the exploitation of ethylene-insensitive cut flower crops in the future, for
exmaple, poor rooting (Clark et al., 1999) and lower disease resistance (Shaw et al., 2002).
Ethylene is perceived by plants when it binds a membrane-localized protein known as a
receptor and is activated upon binding of ethylene and transmits the ethylene signal through
a series of steps (Stepanova and Alonso, 2005). Subsequent activation of a transcription
factors leads to induction of ethylene-responsive genes. Knowledge of ethylene receptors
has advanced our understanding of how ethylene inhibitors (such as cyclopropenes) function.
Cyclopropenes (gaseous ethylene receptor inhibitors) inactivate ethylene receptors
by binding to, and excluding ethylene from the binding site. When the ethylene receptors
of cut flowers are blocked with 1-MCP, 1-hexylcyclopropene or 1-octylcyclopropene, the
flowers have extended vase lives, as they cannot perceive ethylene (Kebenei et al., 2003).
The effectiveness of these compounds in delaying the onset of senescence in cut flowers
is related to the number and turnover rate of ethylene receptors and the concentration and
stability of the gaseous inhibitors. 1-MCP (SmartFresh, AgroFresh Inc., PA) is now commercially
available as a replacement for silver thiosulfate in the floriculture industry, and
sustained-release systems are being developed (Macnish et al., 2004).