Hormonal controls singly or in combination are essential for overall control of growth, development and senescence in plants. A number of plant hormones have been implicated in these processes, namely, auxins, cytokinins, gibberellins, abscisic acid, jasmonates, and ethylene. Intracellular levels and the sensitivity of a particular cell type or tissue to hormones control plant metabolism and function. Among the plant hormones, the one most associated with promotion of senescence and cell death is ethylene. Ethylene is a simple gaseous hydrocarbon with myriad roles in plant life, namely, seed germination, diageotropism, flowering, abscission, senescence, fruit ripening, and pathogenesis (Mattoo and Aharoni, 1988; Mattoo and Suttle, 1991; Abeles et al, 1992; Fluhr and Mattoo, 1996). This chapter deals with the role of ethylene in signaling in senescence and cell death.

Technically, senescence refers to all forms of programmed cell death (PCD) in plants; however, PCD is commonly used now to refer to senescence of cells or a group of cells rather than to that of a whole plant organ. To gain insight into ethylene's role in senescence, it is necessary to keep in mind other roles ethylene plays in cellular function and how other hormones and factors override the biosynthesis and action of ethylene. Several discoveries

Plant Cell Death Processes

Copyright 2004, Elsevier, Inc. All rights reserved in the late-1960s and 1970s made available biochemical tools that had a major impact in understanding ethylene action and unraveling the biosynthetic pathway of ethylene. These included the discovery (Lieberman et al., 1966) of methionine as a precursor of ethylene, the discovery (Owens et al., 1971) of rhizobitoxine and its analogue aminoethoxyvinylglycine (AVG) as relatively specific inhibitors of ethylene biosynthesis, the use of silver salts (Beyer, 1976) and cyclic olefins (Sisler, 1977) as inhibitors of ethylene action, the breakthrough discovery (Adams and Yang, 1979; Lurssen etal.,1979)of1-aminocyclopropane-1-carboxylic acid (ACC) as the intermediate between methionine and ethylene, and finally the identification of ethylene receptors (Chang et al., 1993). More recently, mutations are being used to determine the role of ethylene in senescence. Details on the steps regulated in the biosynthetic pathway of ethylene or the way ethylene receptors act can be found in several interesting reviews (Fluhr and Mattoo, 1996; Chang and Stadler, 2001; Hall et al., 2001; Wang et al, 2002).

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