Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

PROGRAMMED CELL DEATH DURING PLANT SENESCENCE 91
examination on the senescence program in rice clearly showed the asynchronous patterns
of senescence among three different regions in senescing coleoptiles (Inada et al., 1998).
In other systems, guard cells of the stomata and cells adjacent to the vascular tissue often
remain well and alive, while the rest of the cells are becoming fully senescent (Bieleski,
1995; Sakurai et al., 1996; Willmer and Fricker, 1996). In summary, although we know that
a plant cell/organ has to acquire a senescence-competence stage before it can be induced to
senescence, the molecular basis for this change in cell competence remains unknown.
5.6 Cellular changes during senescence
Apoptosis is recognizable by a series of morphological and biochemical hallmarks including
nuclear condensation, membrane blebbing, oligonucleosomal DNA fragmentation, and the
formation of apoptotic bodies (Hacker, 2000) (Fig. 5.2). A number of noncanonical cell
death processes have been reported in animal systems (Kitanaka and Kuchino, 1999), leading
to the introduction of new morphological categories. Together with the standard apoptotic
cell death (renamed as Type 1 physiological cell death), Type 2 (autophagic degenerative
cell death) and Type 3 (nonlysosomal disintegration) categories are proposed. According to
Bursch (2001), these categories should not be considered as mutually exclusive phenomena.
On the contrary, they could reflect a certain flexibility of the cells to respond to different
external conditions. It is probably in this flexible context of physiological cell death where
the study of the morphological changes associated with plant senescence and its comparison
with animal systems becomes more fruitful. The characterization of a certain morphotype
(or morphotypes) associated with plant senescence should help in the identification of the
molecular pathways that conform it.
(b)
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Apoptosis
Necrosis
Fig. 5.2 Morphological differences between apoptosis and necrosis (Studzinski, 1999).