References

84 S.G. Thomas et al.
6.3
Programmed Cell Death and Self-Incompatibility
6.3.1
Key Features of Programmed Cell Death
PCD is a conserved process used to remove unwanted cells in plants and
animals during development and in response to external stimuli. In animal
cells, apoptosis (a form of PCD) can be divided into initiation and execution
phases. During the initiation phase, signalling cascades are triggered that
prepare the cell for death. Mitochondria are a key target for the initial signalling
cascade. After receiving appropriate signals, mitochondria become
depolarized and cytochrome c is released from the mitochondrial intermembrane
space into the cytosol (Jiang and Wang 2004; Yao et al. 2004). In
animal cells, cytosolic cytochrome c forms the apoptosome, a protein complex
that activates executioner caspases, the key proteases involved in cell
death (reviewed by Strasser et al. 2000). Cytochrome c release is therefore
a classic marker of PCD in many organisms (Adrain and Martin 2001).
Caspases are proteases that cleave target proteins after an aspartate
residue (reviewed by Riedl and Shi 2004). They are present as inactive
zymogens and are activated rapidly during apoptosis. Caspase-3 is the
main executioner protease activated during apoptosis and is responsible
for cleavage of many cellular proteins (reviewed by Fischer et al. 2003).
Various tetra-peptide inhibitors of caspases are available and these have
aided the study of caspase-dependent apoptosis. Caspase-3 has the recognition
sequence DxxD and the caspase-3 inhibitor, DEVD, is based on this.
Inhibition of protease activity using DEVD implicates the involvement of
a caspase-3 like activity. The peptide YVAD is often used to demonstrate
specificity. The nuclear DNA repair protein, poly(ADP-ribose) polymerase
(PARP), was one of the first caspase-3 substrates to be identified (Lazebnik
et al. 1994) and is cleaved into 89- and 24-kDa fragments during apoptosis.
Caspases also cleave endogenous nuclease inhibitors, which activates
nucleases and leads to the fragmentation of nuclear DNA (Nagata 2000).
Thus, the identification of cleavage fragments of a known caspase substrate,
the inhibition of a caspase-like activity using DEVD and an increase
in the incidence of DNA fragmentation are all considered diagnostic for
apoptosis.
PCD in plants is less well studied compared with PCD in animals, but
many examples of developmental PCD (reviewed by Kuriyama and Fukuda
2002) and PCD in response to external stimuli have been identified. These
include pathogen attack (Lam et al. 2001; Greenberg and Yao 2004), temperature
stress (Swidzinski et al. 2002), reactive oxygen species (Clarke et al.
2000; Overmyer et al. 2005) and UV radiation (Danon et al. 2004). However,