Programmed Cell Death during Plant Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 97

mobilization are also found associated with senescence (Perez-Amador et al., 2000; Lers
et al., 2001). Interestingly, the tomato ribonuclease LX contains an HDEL signal for ER
retention (Lehmann et al., 2001). LX is expressed not only during leaf senescence but also
during germination and xylem differentiation. It is assumed that RNase LX accumulates in
an ER-derived compartment and is released by membrane disruption into the cytoplasm of
those cells that are intended to undergo autolysis, in a similar fashion as reported in the case
of castor bean KDEL-cysteine protease described earlier. Also, SAG-encoding enzymes
for glycolysis/gluconeogenesis are identified, suggesting the activation of alternative path-
ways for obtaining energy to sustain the senescence program (Buchanan-Wollaston, 1997;
Buckner et al., 1998).
A large number of genes are known to be downregulated during senescence. Some of
them refer to the photosynthesis-associated genes, but others such as AGL15 (Fernandez
et al., 2000) may have a regulatory role and, therefore, can be important in the modulation
or establishment of the program.
5.7.2 Cell preservation subprogram—protection against oxidative stress
Senescent cells are especially sensitive to different stresses (Arora et al., 2002). Protection
of the senescent cell against oxidative damage is exerted at many levels: induction of
protective antioxidative activities, elimination of photooxidative molecules, production of
sunscreens, etc. All of these are upregulated during senescence. One of the reasons for
this appears to be the high phototoxicity (ROS generated) of unbound chlorophyll and its
products (Mach et al., 2001). In the presence of light and oxygen, the unbound chlorophyll
released from the membrane during senescence would produce singlet oxygen and cause
photooxidative damage thus jeopardizing nutrient retrieval from these cells. To prevent
this, chlorophyll is not metabolized and mobilized as nutrients, but is stored in the vacuole
as any other xenophobic substance (Hortensteiner and Feller, 2002). Consistent with this,
gene expression profiles during senescence overlap with signaling pathways related to stress
(Buchanan-Wollaston, 1997; Rubinstein, 2000). Examples of stress-related SAGs include
glyoxalase II gene from Arabidopsis (Quirino et al., 1999), together with a variety of genes
involved in oxidative stress like Fe(II) ascorbate oxidase in Arabidopsis (Callard et al.,
1996), catalase in Brassica (Buchanan-Wollaston and Ainsworth, 1997), among others.
The working hypothesis is that antioxidant activities are elevated to keep the cell in a viable
stage for as long as the nutrient mobilization mechanism is in place. The chloroplastic
form of glutamine synthase from pea leaves is degraded more rapidly than the rest of
the chloroplast enzymes involved in carbon assimilation (Thoenen and Feller, 1998). In
contrast, the cytoplasmic form remains stable and localized in the vascular elements of
leaves (Sakurai et al., 1996) whose cells show a much delayed senescence to help in the
remobilization of nutrients.
5.7.3 Transcriptional activation cascade in senescence PCD
Early searches for genes induced during senescence failed to identify transcription factors
associated with senescence. This, together with the lack of detection of obvious common
upstream sequence homologies between different SAGs identified at that time (Buchanan-
Wollaston, 1997; Gan and Amasino, 1997) casts some doubts on the existence of a real