Programmed Cell Death during Plant Senescence - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 86

Chapter 5
Programmed Cell Death during
Plant Senescence
Ajay Arora
5.1 Introduction
Programmed cell death (PCD) is now recognized as an important event in shaping plant
organs (Jones and Dangl, 1996; Pennell and Lamb, 1997). In animal cells, programmed cell
death has been defined as cell death that is a part of the normal life cycle of the organism, is
triggered by specific physiological signals, and involves de novo gene transcription (Ellis
et al., 1991). This distinction is important in differentiating developmental PCD from
necrotic cell death that may occur as a result of injury or exposure to toxic substances.
In plants, an important response to pathogenic attack is the hypersensitive response (HR)
in which selected cells around the site of infection undergo PCD to prevent spread of the
pathogen. Although the cellular features of HR-PCD show many similarities to develop-
mental PCD, this area has been reviewed (Greenberg and Yao, 2004) and will not be dealt
with in detail here. In plants, it is also important to distinguish between PCD and senes-
cence, terms that have led to much controversy in this field and have been the focus (van
Doorn and Woltering, 2004). In this chapter, PCD will be used to indicate cellular death
as opposed to the death of whole organs or individuals. In some plant organs, the effect
of PCD is particularly evident. Thus, whereas in animals sculpting of the fingers is often
quoted as the classic example of the action of programmed cell death in organ develop-
ment, in plants an obvious example is the prominent holes in the leaves of the house plant
Monstera (Melville and Wrigley, 1969). In animal cells at least two forms of PCD have
been described: apoptosis and autophagy. A number of cytological features describe apop-
tosis. These include nuclear condensation and marginalization, chromatin condensation,
followed by fragmentation of DNA into nucleosomal units known as DNA laddering, and
the formation of membrane inclusions known as apoptotic bodies. In animals, the apop-
tosed cell remains are finally engulfed by neighboring cells through phagocytosis and the
cell corpse disappears (Cohen, 1993). In a few cases, PCD may be defined as truly au-
tonomous as in the very tightly regulated cell death in Caenorhabditis elegans (Yuan and
Horvitz, 1990). However, in many cases, signals external to the cell, such as changes in
hormone levels, trigger PCD (Cohen, 1993). Once PCD has been triggered, a complex net-
work of regulators is switched on involving increases in cytosolic calcium concentrations,
an oxidative burst, and release of pro-PCD factors such as cytochrome C from the mito-
chondrion. Release of cytochrome C is regulated by a growing family of Bcl-2 proteins
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