9
Senescence in Plants and Crops
Lola Peñarrubia and Joaquín Moreno
University of Valencia, Burjassot, Valencia, Spain
181I. INTRODUCTION
Senescencehas been defined as the endogenously controlled deteriorative changes that are natural causes
of death in cells, tissues, organs, or organisms [1]. The differences from the term agingare well estab-
lished, aging being all the degenerative changes that occur in time without reference to death as a conse-
quence. Aging takes place during the entire life span of an organism, whereas senescence is considered
the final developmental phase that culminates in death [1–3]. On the other hand, apoptosisorpro-
grammed cell deathrefers to an intracellular process by which the cell promotes a set of genetically de-
termined self-destructive activities (including specific proteolysis and nuclear chromatin fragmentation)
leading to its own death (i.e., death results from internal activity of the cell and not from ambient injuries).
It is now accepted that many features of stress response and senescence at the cellular and molecular level
are achieved through the operation of programmed cell death [4].
Senescence is a natural developmental process that may be considered as terminal differentiation be-
cause it usually takes place at the end of the life cycle of an organ or organism. However, different kinds
of environmental stress, as well as pathogenesis (i.e., biotic stress), can induce senescence at any stage of
the plant life cycle [5]. In general, the main biochemical changes associated with stress-induced senes-
cence are almost identical to those of natural senescence. Accordingly, gene expression patterns are fre-
quently coincident [6–8], or differ at the relative levels of isoenzymatic activities [9,10], but some par-
ticular genes may display specific expression in senescence processes induced by different factors
(reviewed in Ref. 11). In any case, this differential expression does not usually result in significant vari-
ation at the physiological level. This may be due to the fact that all senescence responses result from trig-
gering the same adaptive mechanisms (most of them aimed at the mobilization and transport of nutrients
out of the decaying tissues) that are constitutively present in plants. These induced adaptive responses
(collectively known as the senescence syndrome) are the hallmark of senescence, whatever the circum-
stances (aging, stress, or pathogen attack) that originated them.
Even if senescence is essentially a degenerative process, it is far from being a chaotic breakdown. On
the contrary, senescence occurs as an orderly loss of functions and structures, comprising an array of bio-
chemical and physiological processes whose ultimate goal is the efficient removal of nutrients from the
decaying tissues. The sequence of events constitutes the senescence syndrome and includes the turnover
of macromolecules and lipids and the transport of mobilized nutrients out of the senescing structures to-