Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FLOWER SENESCENCE 73
suggesting that the clp protease does not play a primary role in the programmed disassembly
of the chloroplast during senescence (Weaver et al., 1999). A study by Guiamet et al.
(1999) has reported that the chloroplast of senescing soybean leaves excrete plastoglobulicontaining
constituents of the chloroplast. The dismantling of these chloroplast components
then occurs outside the chloroplast where SR proteases are localized.
Similar to leaf senescence, protein degradation has been demonstrated to be a major part
of petal senescence and the remobilization of N to the developing ovary (Nichols, 1976).
A few of the SR cysteine proteases have been shown to be upregulated in both leaves and
petals (DCCP1, Jones et al., 1995; Peth1, Tournaire et al., 1996; SAG12, Quirino et al.,
1999; GgCyP, Arora and Singh, 2004). Large increases in proteolytic activity during the
senescence of the ephemeral flower, daylily, have been well documented, and this proteolytic
activity was correlated with increases in the expression of two cysteine protease genes
(Sen11 and Sen102) during the senescence of petals (Valpuesta et al., 1995; Guerrero et al.,
1998; Stephenson and Rubinstein, 1998). In contrast to the cysteine proteases from carnation
and petunia, transcripts are not detectable in young daylily flowers (buds) and the level
of transcript does not increase in senescing leaves (Guerrero et al., 1998). Both daylily
cysteine proteases appear to be flower senescence specific. Arora and Singh (2004) studied
the changes in protein content and protease activity in the petals of ethylene-insensitive
gladiolus flowers, during development and senescence. There was a dramatic upregulation
in the expression of GgCyP at the incipient senescent stage of flower development, indicating
that this gene may encode an important enzyme for the proteolytic process in gladiolus.
The gladiolus cysteine protease gene appears to be flower senescence specific.
4.16 Molecular strategies of extending cut flower life
Conventional breeding is still a practical form of increasing the number of flowering buds,
extending the longevity of an inflorescence and improving its postharvest performance,
as has been demonstrated in Lilium (van der Meulen-Muisers et al., 1999). Many of the
molecular mechanisms underlying senescence, and the respective genes involved in protein
degradation, nucleic acid and chlorophyll breakdown, and lipid and nitrogen remobilization
have been extensively covered in other reviews (Buchanan-Wollaston, 1997; Gan and
Amasino, 1997). An understanding of these mechanisms is vital to the use of molecular
techniques to clone genes of interest to reverse, for example, through antisense technology,
the detrimental effects of senescence, aging, or PCD. Maternal inheritance of herbicide
resistance via chloroplast engineering, or hyperexpression of lethal insecticidal proteins
(other than the Bt (Bacillus thuringiensis) gene product) provides new genetic solutions to
biocontrol of infectious agents in development of phytosanitary control.
PCD in plants is well documented, and it is not only synonymous with senescence
(leaf and flower), but is also a fundamental part of a plant’s adaptation to stresses, such as
reactive oxygen species. The termination of a flower involves two overlapping mechanisms
(Rubinstein, 2000), one being petals that abscise before the majority of their cells initiate
a cell death program, and where abscission may occur before or during the mobilization of
food reserves to other parts of the plant. In the second, the petals are more persistent, and
cell deterioration and food remobilization occur while the petals are still part of the flower.
One way of countering the effects of pathogen-induced PCD is through the use of caspase
inhibitors in the cut flower medium (Richael et al., 2001).