Programmed Cell Death during Plant Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 88

Fukuda, 1996). However, it is not entirely clear whether there is a master switch for PCD
through which all pathways must go, or whether there are many different switches that are
pathway specific.
In animals, the mitochondrion was found to play a central role in the activation of
different PCD pathways. Studies in plants suggest that mitochondria may also play an
important role in PCD in plants (Lam et al., 2001). However, it is too early to determine
whether the mitochondrion is as central for PCD in plants, as it is for PCD in animals. The
morphological and molecular differences that exist between the different PCD pathways in
plants (i.e., developmental, abiotic stress-induced, disease symptoms, and the hypersensitive
response-PCD; Mittler, 1998), as well as the selective inhibition of specific PCD pathways
by overexpression of Bcl-2-like proteins or p35 in plants (Mitsuhara et al., 1999; Lincoln
et al., 2002), may suggest that there are many switches for PCD activation in plants. It is
possible that plants use PCD pathways that are similar to those found in animals (e.g., the
mitochondrion); however, plants may use different proteins that share only a limited degree
of homology with animal genes such as bcl-2 and caspases (Kawai-Yamada et al., 2001;
Lam et al., 2001).
5.3 Senescence and organ senescence as forms of PCD
S enescence or better the senescencesyndrome refers to those degenerative processes leading
to death that occur under the control of the plant (Nooden, 1988). It is a developmental
program that guides the cell through an ordered schedule of events leading to the death of
the cell/organ and serving, at the same time, a variety of functions selected by evolution
to optimize plant survival (Granell, 1999). Implicit in the definition is the idea that despite
being a degenerative process, it is still organized and remains under control of the cell.
Senescence is an active process that requires energy consumption, and part of the pro-
gram is designed to supply it (Solomos, 1988; Buchanan-Wollaston, 1997). But program
in this case also means that information on how to dismantle the cell is genetically driven
and therefore emerges from the genes. While the programmed nature of senescence is
strongly established, it is also becoming clear that more than one program or variations of
the program may exist. For example, the general loss of membrane function accompanying
senescence in Alstroemeria is not related to lipoxygenase activity or to the accumulation
of lipid hydroperoxides that occurs in other plants (Leverentz et al., 2002). Microarray and
proteomic analyses show commonalities and differences among the different senescence
programs (Swidzinski et al., 2002, 2004).
5.4 Leaf senescence versus petal senescence
The process of senescence and cell death are clearly distinct at a physiological level because
senescence, at least in leaves, can be a reversible process, whereas cell death is considered a
terminal event (Thomas et al., 2003). Leaf senescence has been the focus of several reviews
(Lim et al., 2003) and genomic-wide approaches to identify regulatory networks (Buchanan-
Wollaston et al., 2003; Gepstein et al., 2003). The signals initiating the overall process of
senescence are common to other PCD events. Reduction of ethylene signaling (Grbic and
Bleecker, 1995) and upregulation of cytokinin production (Gan and Amasino, 1995) delay
senescence, indicating that the levels of these two PGRs are involved. Elevated cytoplasmic