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Paradoxically, the death of specific sets of
cells is an essential part of the growth and development
of many eukaryotic organisms, including plants
and animals. In addition to its role in development,
cell death can be one component of the response
to biotic and abiotic stresses. Because the organism
controls the initiation and execution of the cell
death process, these types of cell death are referred
to as programmed cell death (PCD). This
broad definition of PCD, however, implies nothing
about the mechanisms involved in the execution
of cell death.
Two examples of
PCD in plants, senescence and the cell
death associated with the hypersensitive response
(HR), demonstrate the range of forms that PCD
can take in plants. Senescence is the relatively
slow cell death of tissues at the end of their
life span. Senescence involves the ordered disassembly
of cellular components in the senescing tissue
and allows for maximum recovery of nutrients from
the senescing tissues for recycling to the parts
of the plant that survive. The PCD seen in the
HR, the cell death that is triggered in plant
cells in and around the point of attempted infection
by some pathogens, is quite different. Because
one function of the localized cell death in HR
might be to block the further spread of infection,
the emphasis is on rapid execution, to kill the
host tissue before the pathogen gets established,
rather than maximum recovery of nutrients. These
are just two examples of PCD that occur in plants,
but they demonstrate how diverse PCD processes
in plants can be.
In this chapter,
the current state of knowledge regarding PCD in
plants will be discussed to provide an understanding
of how death, at the appropriate time and place,
can in fact be an essential part of life.
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