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Chapter 20

Senescence and
Programmed Cell Death

CHAPTER OUTLINE
Introduction
20.1 Types of cell death observed in animals and plants
20.2 PCD in the life cycle of plants
20.3 Overview of senescence
20.4 Pigment metabolism during senescence
20.5 Protein metabolism in senescence
20.6 Impact of senescence on photosynthesis
20.7 Impact of senescence on oxidative metabolism
20.8 Degradation of nucleic acids during senescence
20.9 Regulation of metabolic activity in senescing cells
20.10 Endogenous plant growth regulators and senescence
20.11 Environmental influences on senescence
20.12 Examples of developmental PCD in plants
20.13 Examples of PCD as a plant response to stress
20.14 Further questions and future directions for PCD research

Jeffery L. Dangl
Robert A. Dietrich
Howard Thomas

 

 

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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