Senescence mechanisms

Abstract

Senescence in plants is usually viewed as an internally programmed degeneration leading to death. It is a developmental process that occurs in many different tissues and serves different purposes. Generally, apoptosis refers to programmed death of small numbers of animal cells, and it shows some special features at the cell level. Some senescing plant cells show some symptoms typical of apoptosis, while others do not. This review will focus primarily on leaf senescence with ultimate aim of explaining whole plant senescence (i.e., monocarpic senescence). Traditionally, the ideas on senescence mechanisms fall into two major groupings, nutrient deficiencies (e.g., starvation) and genetic programming (i.e., senescence‐promoting and senescence‐inhibiting genes). Considerable evidence indicates that nutrient deficiencies are not central senescence program components, while increasing evidence supports genetic programming. Because chlorophyll (Chl) and chloroplast (CP) breakdown are so prominent, leaf senescence is generally measured in terms of Chl loss. Although CP breakdown may not be the proximate cause of leaf cell death, it certainly is important as a source of nutrients for use elsewhere, e.g., for developing reproductive structures in monocarpic plants, and this loss limits assimilatory capacity. The CP is dismantled in an orderly sequence. Individual protein complexes seem to be taken out all at once, not one subunit at a time. Removal of any component, e.g., Chl, seems to destabilize the whole complex. It is of special interest that senescing CPs secrete Chl‐containing globules indicating that some CP components are broken down outside the CP. Senescence appears to be imposed on the CP by the nucleus, and all the known senescence‐altering genes except one, cytG in soybean, are nuclear. Only the d1d2 mutation(s) in soybean prevents a broad range of leaf senescence processes. Exactly, what causes cell death is unclear; however, the selective thiol protease inhibitor, E‐64, does delay death, and this suggests that proteases play a key role.

Sistemas de coberturas de solo no inverno e seus efeitos sobre o rendimento de grãos do milho em sucessão

“…A ocorrência de patógenos foliares, associada a deficiências nutricionais, são fatores que estimulam a senescência foliar prematura, restringindo a atividade fotossintética das plantas e seu potencial produtivo (NOODEN et al, 1997).…”

Climate warming alters nitrogen dynamics and total non-structural carbohydrate accumulations of perennial herbs of distinctive functional groups during the plant senescence in autumn in an alpine meadow of the Tibetan Plateau, China

“…In leaves, most of the N is found within the chloroplasts as the necessary component in the formation of thylakoid and enzymes (Evans, 1989), resulting in a strong positive relationship between N content and the capacity of photosynthesis (Hikosaka, 2004;Reich et al, 1994). High N and chlorophyll concentrations enhance photosynthesis and offset nutrient deficiencies, which control senescence (Noodén et al, 1997). Chlorophyll biosynthesis encompasses a series of enzymatic reactions, which are highly affected by temperature (Lambers et al, 2008).…”

Tansley Review No. 118

“…expression (Godiard et al, 1998) and increased protease activity (Minami & Fukuda, 1995 ;Drake et al, 1996 ;Ye & Varner, 1996 ;Beers & Freeman, 1997 ;Gan & Amasino, 1997). In animals, the caspase subfamily of the cysteine proteases plays a key role in apoptotic cascade signalling, together with the appropriate regulatory signals (Vaux, 1993 ;Noode! n et al, 1997).…”

“…Within cells undergoing senescence, losses of chlorophyll and chloroplast integrity are the earliest symptoms and breakdown of the nucleus is the final event (Thomas & Stoddart, 1980 ;Smart, 1994 ;Buchanan-Wollaston, 1997 ;Gan & Amasino, 1997 ;Matile, 1997). Chloroplast breakdown is under nuclear control (Smart, 1994 ;Buchanan-Wollaston, 1997 ;Gan & Amasino, 1997 ;Matile, 1997 ;Noode! n et al, 1997), and the chloroplasts can remain intact until up to 80% of the chlorophyll has been removed (Wittenbach, 1978 ;Zavaleta-Mancera et al, 1999b).…”