Programmed Cell Death during Plant Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 92

5.6.1 Degeneration of chloroplasts
In photosynthetic tissues, one of the first observable changes is the degeneration of chloro-
plasts. Usually, these organelles undergo a number of sequential changes that include, in
this order, dilation and breakage of thylakoids, increase in number and size of plastoglobuli
(osmiophilic globules found in electron microscopy analysis), and a decrease in pigment
content (Barton, 1966; Nii et al., 1988; Simeonova et al., 2000). Inada et al. (1998) observed
an early degradation of chloroplast DNA (cpDNA) prior to the degeneration of the organelle
during the senescence of rice coleoptile. cpDNA appears to be degraded within the chloro-
plast itself, and the role of a Zn 2 + -dependent nuclease has been suggested (Sodmergen et al.,
1991). Senescent leaf plastids (gerontoplasts) are smaller than green chloroplasts, with de-
generated membrane systems and stroma, and larger plastoglobuli. Surprisingly enough,
gerontoplasts maintain certain integrity during early senescence and are even able to red-
ifferentiate into chloroplasts under certain conditions (Zavaleta-Mancera et al., 1999). The
number of plastids remains constant until the later stage of senescence by the time RbcS and
LHC have already diminished substantially (Martinoia et al., 1983). Evidence for vacuolar
autophagy of senescing chloroplast has been obtained (Minamikawa et al., 2001), and a mass
exodus from senescing chloroplast has been described (Guiamet et al., 1999). These obser-
vations seem to indicate that, although the degeneration of the chloroplast functions in an
autonomous manner at the early stages, probably later in the pathway, the autophagosomal
mechanism is operational for chloroplast degradation.
5.6.2 Degradation of mitochondria
In contrast, mitochondria seem to maintain their integrity even in late stages of senescence
with little or no degradation observed in mitochondrial DNA (Inada et al., 1998). A possible
explanation is the need for ATP supply for the correct dismantling of cellular constituents.
At least during the senescence of Vignamungo cotyledons, the degradation of mitochondria
is known to be accomplished by autophagosomes (Toyooka et al., 2001). In animal cells,
the outer mitochondrial membranes act as a sensor of cellular stress, releasing apoptogenic
factors (cytochrome C ) that trigger cell death. Evidences in this direction reported in plant
cells are too far to be conclusive, and refer to the PCD events taking place during stress
and defense response (Balk et al., 1999; Sun et al., 1999). Observations made in petunia
concluded that the release of cytochrome C is not a feature of PCD, at least during petal
senescence (Xu and Hanson, 2000).
5.6.3 Degradation of other organelles
The nuclei of senescing cells are also subjected to modifications. Chromatin condensation is
reported in relatively early stages during senescence of rice coleoptiles (Inada et al., 1998),
petal senescence in carnation (Smith et al., 1992), and carpel degeneration in pea (Orzaez
and Granell, 1997b). Condensation is often accompanied by the degradation of nuclear
DNA, as highlighted by the TUNEL reaction (Fig. 5.3), which detects the presence of free
3 OH ends as a result of endonuclease digestion. Senescence-associated TUNEL-positive
nuclei have been found in green tissues (Orzaez and Granell, 1997a; Yen and Yang, 1998;
Kawai and Uchimiya, 2000), as well as in nonphotosynthetic tissues like flower petals