What this Book Covers

This book will deal with the developmental processes now widely recognized as senescence and/or programmed cell death (PCD) among plant biologists. Some authors use PCD to refer to death of a small number of cells and senescence for organs and whole plants; however, senescence as conceived in the plant biology literature is in fact programmed cell death. Traditionally, senescence has been viewed broadly including processes involving only a few cells (Leopold, 1961), but not everyone shares that tradition. In many respects, programmed cell death is a better description of what is actually happening. The meanings of these terms will be discussed further in the next section; however, this chapter and most others will refer to senescence and will portray it broadly.

Senescence and programmed cell death play a very wide variety of roles in the life cycle of plants and other organisms (Leopold, 1961). Extensive listings of examples can be found in Barlow (1982), Nooden (1988a), Havel andDurzan (1996), Gray and Johal (1998), Dangl et al. (2000) and in this book. Of particular interest is the more in-depth coverage of cell death as a developmental process in maize (Buckner et al., 2000), differentiation of certain cells (Gray and Johal, 1998; Chapter 2) and the hypersensitive response (Chapters 3 and 13). Endogenously controlled death is expressed at the cell, tissue, organ or even whole plant levels. In all cases, these deaths seem to play integral roles in the life cycle of an organism, even though those roles may not always be completely understood yet. Individual cells may be selectively targeted during plant embryo development, but PCD may occur in a whole

Plant Cell Death Processes

Copyright 2004, Elsevier, Inc. All rights reserved tissue such as the tapetum in anthers or in whole organs such as flower petals or even in whole plants as in monocarpic senescence (Chapter 15). In some cases such as xylem vessel formation (Kuriyama and Fukuda, 2001) and lysigenous aerenchyma formation (Chapter 2), the protoplast is completely cleared out, but more often, a semblance of a cell is left at the end of the senescence process and the mostly emptied organ may be discarded by abscission (Butler and Simon, 1971; Biswal and Biswal, 1988; Noodén, 1988a).

This book examines not only the primary or central processes of programmed cell death/senescence but also some related processes such as chlorophyll degradation, disassembly of the photosynthetic apparatus and metabolism of the released nitrogen that may not be the proximal cause of death.

In addition to the biochemical and cellular perspectives of senescence, the broader implications of whole plant senescence, top senescence, autumnal senescence of trees and postharvest physiology are considered. Although senescence is an endogenous process, it can be induced by environmental factors, particularly those that cause stress, and these factors need to be examined in relation to senescence.

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