Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

PROGRAMMED CELL DEATH DURING PLANT SENESCENCE 109
Table 5.1
Comparison of signaling and possible mechanisms of PCD in floral organs
Floral organ Intercellular signals Mechanism for PCD References
Sex organ abortion GA and brassinosteroids ? Wu and Cheung (2000)
Tapetum in cytoplasmic Mitochondrial
Cytochrome C release Balk and Leaver (2001)
male sterility lines dysfunction
followed by loss of
mitochondrial function
Synergids
Pollination in some
species
Requires mitochondrial
function
Christenson et al. (2002)
Petal senescence Ethylene in some species Calcium/phosphate
signaling, ROS
increases
Jasmonate (via ethylene) Activation of vacuolar
lytic enzymes through
vacuolar-processing
enzyme (caspase 1
activity)
Pollen tube
Cytokinin (via sugar
transport)
During selfincompatibility
Activation of vesicle
bound proteases,
vacuolar leakage
Increased calcium,
cytochrome C release
and caspase-3 activity
Porat and Halevy (1993),
Kinoshita et al. (1999)
Orzaez et al. (1999),
Schmid et al. (1999)
Wagstaff et al. (2003),
Lara et al. (2004)
Thomas and Franklin
(2004)
can be developed. Table 5.1 shows the comparison of signaling and possible mechanisms
of PCD in various floral organs.
5.16 A proteomic analysis of plant PCD
Despite the fundamental importance of PCD in plants, comparatively little is known about
the molecular mechanisms of plant PCD. Plant genomes do not contain obvious homologs
to the key animal cell death proteins such as the Bcl-2 family proteins (Arabidopsis Genome
Initiative, 2000), although a family of genes related to mammalian caspases has been identified
in plants (Uren et al., 2000). Biochemical studies of plants have been able to establish
a causal role for events such as the translocation of cytochrome C from the mitochondria
to the cytosol (Balk et al., 1999; Sun et al., 1999; Zhao et al., 1999; Xu and Hanson, 2000).
In an attempt to identify key plant PCD genes that may function universally during different
types of plant PCD, Swidzinski et al. (2002) have previously undertaken a custom
microarray analysis of gene expression during PCD in an Arabidopsis cell suspension culture.
By identifying mRNA transcripts that changed in abundance following two unrelated
PCD-inducing treatments (a brief, mild heat treatment for 10 min at 55 ◦ C and culture senescence),
they have been able to discriminate between genes that may be common to a core
plant cell death program and those that are specifically related to the inducing stimulus itself.
While this study was successful in identifying several candidate genes whose common
up- or downregulation during PCD may indicate a role for their products in plant PCD, it
was restricted to elements of the PCD process that are transcriptionally regulated and ignored
posttranscriptional and posttranslational regulation. Indeed, posttranslational events
such as proteolytic cleavage and activation and modifications such as phosphorylation are
key regulatory events in animal PCD (Reed, 2000). As a first step toward identifying such