Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FLOWER SENESCENCE 71
and Woodson, 1989; Arora and Singh, 2004). Experiments with inhibitors of protein and
RNA synthesis have demonstrated that senescence is a genetically programmed process that
requires the selective activation of specific RNAs and proteins, and does not merely result
from the inhibition of cellular metabolism by declining rates of protein and RNA synthesis.
These inhibitor studies have also suggested that transcription and protein synthesis in organelles
is not central to the regulation of senescence. In support of the nuclear regulation
of senescence, nuclear genes have been found to encode almost all of the mRNAs found
to increase during senescence (Nooden, 1988). While the later stages of ripening resemble
senescence, the entire process represents more of an interaction between degradative
and synthetic processes. In contrast to senescing flowers and leaves, protein levels in fruits
remain constant or increase slightly during ripening (Brady, 1988). Specific mRNAs and
proteins that increase during ripening have also been identified (Gray et al., 1992). These
will be referred to as senescence-related (SR) genes.
4.15.1 Plant cysteine proteinases
Proteolysis in plants is a complex process involving many enzymes and multifarious proteolytic
pathways in various cellular compartments, with cysteine proteinases playing an
essential role. Their share in total proteolysis depends on the kind of plant and its organ. It
amounts up to 30% of total proteolytic activity in mature nonsenescing organs. However,
the activities of cysteine proteinases respond dramatically to different internal and external
stimuli, and in some cases, they rise to 90% of the total proteolytic activity (Wisniewski and
Zagdanska, 2001). They are involved in protein maturation, degradation, and protein rebuilt
in response to different external stimuli, and they also play a housekeeping function to
remove abnormal, misfolded proteins. In each case, the proteolysis by cysteine proteinases
is a highly regulated process.
4.15.2 Proteolysis regulates metabolic processes within the cell
Proteolysis is an indispensable process in all living organisms. A continual turnover of proteins
removes functionally impaired proteins (due to biosynthetic errors, improper folding,
thermal denaturation, oxidative damage (Arora et al., 2002), which if left unchecked, may
restrict metabolic activities and jeopardize a cell’s integrity. Proteases also recycle essential
amino acids and are important in the recovery of valuable nutrients. Proteases regulate
metabolic pathways and developmental programs by affecting the rapid turnover of ratelimiting
enzymes and key regulatory proteins (Clarke, 2005). Proteolytic cleavage is thought
to play a significant role in the senescence of flowers because expression of protease genes
is one of the earliest senescence-related gene changes to be identified (Eason et al., 2002).
Upregulated expression of protease genes, raised enzyme activity, and a decline in soluble
protein levels occur consistently during senescence, both of ethylene-sensitive (Wagstaff
et al., 2002; Jones et al., 2005) and ethylene-insensitive flowers (Eason et al., 2002; Arora
and Ezura, 2003; Arora and Singh, 2004). Regulating the senescence-associated activity
of proteases may be achieved with different molecular strategies. First, the interaction between
proteases and their inhibitor proteins have been linked to modulation of cell death
processes in plants (Sin and Chye, 2004). In certain cut flowers (Sandersonia and Iris),
chemical inhibition of protease action delays the onset of senescence (Pak and van Doorn,