Programmed Cell Death during Plant Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 95

rupture of the tonoplast, causes the activation and exposure to the cytosol of endopeptidases
stored in newly formed ER-derived vesicles (ricinosomes; Schmid et al., 1999). Once com-
partmentalization is lost, the contents of the lytic organelles are free to digest the remnants
of cell. This represents the latest stage of senescence where homeostasis is irreversibly lost.

5.7 Molecular changes associated with plant senescence

As indicated earlier, the program of senescence requires specific transcription and trans-
lation. Over the past several years, the introduction of novel techniques for the isolation
of differentially regulated genes (i.e., differential display, subtractive hybridization, etc.)
has led to the isolation of many genes whose transcription levels are upregulated during
senescence (John et al., 1995; Buchanan-Wollaston, 1997; Lee et al., 2001). As a result, the
catalog of senescence-associated genes ( SAGs ) reported to date is hard to compile. More
recently, the introduction of DNA microarrays for the profiling of gene expression is starting
to provide large amounts of expression data that will help to complete the picture of genetic
changes associated with plant senescence (Zhu et al., 2001; Chen et al., 2002). Together, the
set of data available so far is providing clues as to the molecular constituents of senescing
cells. There are two important subprograms which define molecular changes during the
plant senescence:

5.7.1 The nutrient salvage subprogram

Many of the genes and activities associated with senescence can be included in this subpro-
gram. Proteases are probably the most conspicuous group of proteins whose mRNA levels
are reported to increase during plant senescence (Granell et al., 1992; Beers et al., 2000;
Arora and Singh, 2004). Many of them belong to the group of cysteine endproteases (C13
class; Granell et al., 1992), which probably participate in the remobilization of N and later
in the salvage program associated with senescence. Cysteine proteases are reported to be
upregulated during the senescence of leaves (Drake et al., 1996), but also in reproductive
organs like flowers (Wagstaff et al., 2002; Arora and Singh, 2004) and other senescent
organs such as unplanted ovaries (Granell et al., 1992; Cercos et al., 1999). Because most
of them contain vacuolar-targeting signals, they are likely to participate in the digestion of
proteins delivered by the autophagosome for N

/

C retrieval or in general digestion once
the tonoplast has been lost at the later stages of senescence. Most of these proteases are
synthesized as proenzymes and sorted to vacuole where they become activated.
A different pathway is followed by a group of cysteine endo peptidases with a C-terminal
KDEL sequence for retention in the ER, as the CysEP protease from castor bean-senescing
endosperm (Schmid et al., 1999). Orthologs of CysEP are also found in senescing daylily
petals (Valpuesta et al., 1995), cotyledons of mungbean ( V. mungo ) and vetch ( Viciasativa ),
and seed pods of maturing French bean ( Phaseolusvulgaris ) (Schmid et al., 1999). CysEP is
accumulated as a precursor in specialized organelles, the ricinosomes, which are originated
as budding vesicles arising from the ER. With the progress of senescence, ricinosomes
disintegrate, liberating the active form of CysEP into the cytoplasm (Schmid et al., 1999).
Although the substrate specificity of CysEP is unknown, its containment in specialized
vesicles and subsequent liberation during senescence seems indicative of a highly regulated