Programmed Cell Death during Plant Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 94

(Orzaez and Granell, 1997b; Xu and Hanson, 2000). Often, chromatin DNA degradation
takes place preferably at the level of the internucleosomal space, rendering fragments of
discrete size that can be visualized by electrophoresis resulting in a typical DNA ladder.
Internucleosomal DNA cleavage has been considered as one of the hallmarks of apoptosis.
However, its occurrence during plant senescence does not seem to be universal. Even among
different organs within the same plant, the pattern of DNA degradation seems to differ. In
rice, for instance, Kawai and Uchimiya (2000) reported DNA laddering during senescence in
the coleoptile. In contrast, Lee and Chen (2002) found that the nuclear DNA fragmentation
during rice leaf senescence is not accompanied by generation of oligonucleosomal DNA
fragments. During coleoptile senescence in rice, TUNEL-positive cells are restricted to the
epidermis (Kawai and Uchimiya, 2000). During the senescence of unplanted pea carpels,
TUNEL labeling is restricted to well-defined areas such as the funicular-ovule joining
region or the ovule external tegument. The rest of the cells in the degenerating ovule seem
to dismantle its nuclear DNA without showing detectable TUNEL labeling (Orzaez and
Granell, 1997a). In some senescence processes, DNA fragmentation occurs only at the very
last stages (Gietl and Schmid, 2001).
Whereas the fate of the nucleus seems to differ from one form of senescence to another,
virtually all the ultrastructural studies reported significant changes in the vacuolar system.
At very early stages, carnation petals showed an increase in vacuolar-derived vesicles prior
to other visible signs of degeneration (Smith et al., 1992). In a later stage, the central
vacuole enlarges displacing nucleus and cytoplasm. Invaginations of the tonoplast into the
vacuole, occasionally containing cytoplasmic material and organelles, are widely reported
in different stages of senescence (Matile and Winkenbach, 1971; Inada et al., 1998).
Plant vacuoles are multifunctional organelles, and a specific type acts as lytic com-
partments in the degradation of cellular components (Marty, 1999). These lytic vacuoles
accumulate hydrolytic enzymes analogous to the lysosomal enzymes of yeast/animal cells
and are known to play a central role in macroautophagy, a major route in protein and
organelle turnover, which contributes to the maintenance of cellular homeostasis. Dur-
ing macroautophagy, portions of the cytoplasm, including organelles, are surrounded by
double-membrane structures known as autophagosomes and transported to the lytic vac-
uole. In plants, examples of macroautophagy have been described during nutrient starvation
of suspension-cultured cells (Moriyasu and Ohsumi, 1996) and during dark-induced degra-
dation of rubisco in chloroplasts of detached French beans (Minamikawa et al., 2001).
Also, during natural endosperm senescence of germinating castor beans, glyoxysomes, mi-
tochondria, segments of the endoplasmic reticulum (ER), and ribosomes are finally removed
from the cytosol by autophagic vacuoles (Gietl and Schmid, 2001). Moreover, during the
senescence of V. mungo cotyledons, ultrastructural analysis showed that macroautophagic
processes are active in the degradation of both nutrient reserves and cellular components.
Starch granules are sequestered by de novo formed membranous structures and transported
to the lytic vacuole, where its degradation seems to take place. In parallel, autophagosomes-
containing mitochondria and cytoplasm are also fused to the vacuole. Interestingly, while
the cellular dismantling takes place both in natural conditions and in organs detached from
the plant, remobilization of nutrients represented by the breakdown of polysaccharides oc-
curs only when the organs remain attached to the plant (Bieleski, 1995; Toyooka et al.,
2001). In many cases, cell compartmentalization (integrity of membranes) is maintained
until very late in the program. Acidification, probably due to a change in permeability or