Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

72 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
2005), and the accumulation of cysteine protease mRNAs in senescing carnation flowers
is associated with a corresponding decline in protease inhibitor mRNA (Sugawara et al.,
2002), indicating that inhibitor proteins may play a role in regulating senescence-associated
protease activity in flowers. Second, proteases have been shown to be localized to the plant
vacuole (Sin and Chye, 2004; Lam, 2005), and both posttranslational modification and
subcellular localization provide the cell with a means to regulate protease activity. Another
avenue for extending the display life of cut flowers through modification of proteolysis is
the downregulation of senescence-associated cysteine proteases. In broccoli, this strategy
has produced transgenic plants with delayed postharvest senescence (Eason et al., 2005) and
may offer an alternative strategy for extending vase life of cut flowers. It has been reported
that a single daffodil flower will cause an increase in the vase life of other flowers (van
Doorn et al., 2004). The active compound in the mucilage of daffodil flowers, which delays
tepal senescence in cut Iris flowers, is narciclasine, an alkaloid known to inhibit protein
synthesis (van Doorn et al., 2004). This also provides a new strategy for postharvest vase
life extension, if specific protease inhibitors can be developed in the future for use in vase
solutions.
4.15.3 Genes involved in protein degradation
Decreases in total proteins during senescence result from increases in proteolytic enzyme
activity and decreases in protein synthesis (Brady, 1988). The degradation of proteins and
remobilization of amino acids to developing tissues is the predominant metabolic process
during senescence. Cysteine proteases are believed to be the main proteases involved in
general protein hydrolysis, and recently, a number of cysteine protease have been identified
from senescing leaves, senescing flowers, and ripening fruit.
Of those cysteine proteases identified from senescing tissues, most share sequence
homology with γ -oryzain from rice, a cysteine protease that has been implicated in the
mobilization of reserve proteins during seed germination. These include SAG2, See1, LSC7,
SENU2, and SENU3. The expression patterns of these five genes are similar, with low levels
of expression in young leaves and increased expression during senescence (Buchanan-
Wollaston, 1997; Weaver et al., 1998). Both tomato cysteine protease, SenU2 and SenU3
and See1, from maize also show patterns of upregulation during seed germination, indicating
that these proteases may play similar roles in protein degradation during germination and
leaf senescence (Smart et al., 1995; Drake et al., 1996). While common to germination
and leaf senescence, the SENU2 and SENU3 transcripts were not upregulated during fruit
ripening (Drake et al., 1996). SAG12, which encodes a papainlike cysteine protease, is
one of the few SR genes to the display senescence-specific regulation. SAG12 mRNAs are
not detectable in roots, stems, green leaves, or young flowers, but increase in abundance
in senescing petals as well as leaves (Lohman et al., 1994; Quirino et al., 1999). This
senescence-specific expression suggests that the SAG12 protease might play a key role in
the large-scale increases in protein degradation during senescence.
The dismantling of the chloroplast, which contains greater than 50% of the leaf’s total
protein, is a prominent process in leaf senescence (Thomas and Stoddart, 1980). While
many SR genes have been identified as proteases, only one of these has been found to be localized
to the chloroplast (Erd1; Lohman et al., 1994). Transcript levels of clp protease have
been reported to increase during leaf senescence, but protein levels were found to decline,