Agriculture Reference
In-Depth Information
developmental processes in plants, including xylogenesis, aerenchyma formation
and senescence, as well as in response to pathogens and various abiotic stresses
(Greenberg
et al.
, 1994). The phenotype of PCD in plant cells has a number of
similarities with that of apoptosis in animal cells, including DNA laddering, and
cytochrome
c
release from mitochondria (reviewed in Dannon
et al.
, 2000). The
pattern of depolymerisation of F-actin observed in self-incompatible
Papaver
pollen
tubes is unusual, but is often associated with PCD in animal cells (Korichneva &
Hammerling, 1999; Rao
et al.
, 1999). Indeed, F-actin depolymerisation has been
found to actually stimulate apoptosis in some cell lines (Janmey, 1998). Hence, stud-
ies to investigate the potential role of PCD in
Papaver
-type SI have been initiated.
Caspase-induced cleavage of nuclear DNA at regular intervals to produce oligo-
somal fragments is considered to be a diagnostic characteristic of PCD. Evidence
that nuclear DNA degradation occurs in plant cells undergoing the hypersensitive re-
sponse has been gathered using the technique of Fragment End Labelling (FragEL)
(Ryerson & Heath, 1996; Wang
et al.
, 1996). Using this technique, it has been
established that nuclear DNA fragmentation occurs in self-incompatible but not
self-compatible pollen (Jordan
et al.
, 2000). This nuclear DNA fragmentation is
only detectable several hours after induction of SI, consistent with it being one of
the last steps of PCD and after irreversible termination of pollen tube growth. DNA
fragmentation is also observed after prolonged culture of pollen in which [Ca
2
+
]
i
is artificially elevated by treatment with mastoparan (Jordan
et al.
, 2000). This sug-
gests that the elevation of [Ca
2
+
]
i
observed in the SI response may be involved in
initiating the PCD pathway. In agreement with this, calcium signalling has also been
shown to be involved in the induction of PCD in the hypersensitive response in plant
cells (Levine
et al.
, 1997).
Although nuclear DNA fragmentation is associated with PCD, it is also possible
that it results from necrosis, i.e. is a result of cell death rather than being a cause
of it. To more conclusively affirm that PCD is occurring, it is necessary to identify
upstream components of the PCD pathway, which are more easily separable from
'general death' phenomena. In plant systems, this process has been complicated
by the apparent absence of homologues of a number of the key, highly conserved
components of animal PCD pathways in plant genomes (Rudd & Franklin-Tong,
2003). Particularly notable is the absence of genes with homology to caspases,
which are directly involved in DNA fragmentation, though a family of putative
cysteine proteases that are distantly related to caspases has been recently identified in
plants (termed metacaspases) and demonstrated to exhibit cysteine protease activity
(Uren
et al.
, 2000; Szallies
et al.
, 2002). Inhibitors of caspase activity have been
demonstrated to block pathogenesis in the plant hypersensitive response (Richael
et al.
, 2001), providing further evidence for PCD and providing an alternative assay.
In the
Papaver
SI response, application of the tetrapeptide caspase-3 inhibitor
Ac-DEVD-CHO has been found to block the DNA fragmentation normally observed
in pollen challenged with self-S-protein (Rudd & Franklin-Tong, 2003). In addition,
incompatible pollen tubes treated with Ac-DEVD-CHO grow to approximately the
same length as compatible tubes, suggesting that caspase-like activity is likely to
