plast genomes. In contrast to its mRNA, protein levels of ERD1, one of these Clp-like protease subunits,
decline during senescence, suggesting that it is unlikely to play a major role in chloroplast senescence
[192]. Moreover, other Clp protease subunits are constitutively expressed throughout Phaseolusdevel-
opment [182], thus questioning their specific function in senescence.
Several RNase genes from different species have been shown to be up-regulated during senescence
[40]. In Arabidopsis thaliana, RNS2 is a message coding for an S-like RNase that has similarity to self-
incompatibility RNases (S-RNases). Unlike these, RNS2 is expressed in leaf and petals during senescence
[95]. Because RNS2 and other RNase clones from Arabidopsisand tomato are also expressed under phos-
phate starvation [96,97], it has been proposed that they could play a role in the mobilization of this com-
pound during senescent processes.
The genes encoding the 3-ketoacyl-CoA thiolase and other glyoxysomal proteins, which are proba-
bly implicated in fatty acid mobilization, have been reported to have increased expression during leaf
senescence [193,194]. Besides, messages for a cytosolic glutamine synthetase and a -glucosidase are
also known to be induced [195,196].
B. SAGs Involved in Cell Protection
In order to optimize the recycling of materials from the decaying tissues, senescing cells must stay alive
during the general mobilization phase and have to be protected against multiple senescence-induced
stresses. In this regard, the expression of many genes related to stress, or to stress signaling through the
ethylene pathway, is known to be induced during senescence. Among them are genes implicated in the
antioxidative response, such as ascorbate oxidase [196], anionic peroxidase [184], and glutathione S-
transferase [180,197]; several genes encoding pathogenesis-related proteins [7,181]; and dark-inducible
genes [198,199]. Besides, metallothioneins have been reported to be up-regulated in senescent leaves
[88,162], where they may act in detoxifying metals (released from degraded metalloenzymes) and pro-
tecting against oxidative stress [178,200]. In addition to metallothioneins, other metal-binding proteins,
such as a blue copper-binding protein (BCB) and a copper chaperone (CCH), are distinctly expressed dur-
ing senescence, although their particular roles remain unclear [201,202].
C. SAGs of Unknown Function
Among genes of unidentified function, SAG13is an excellent senescence marker because of its timely ex-
pression pattern. Sequence comparison studies indicate a certain similarity of this clone to short-chain al-
cohol dehydrogenase genes. Curiously, a member of the same highly diverged family is TASSELSEED2,
a gene involved in programmed cell death in maize [203].
VI. SENESCENCE REGULATION AND SIGNALING
Senescence, in a broad sense, encompasses a number of processes with a common final outcome: the
breakdown of cells through an endogenously organized program that optimizes nutrient economy and en-
sures functional takeover. Temporal and spatial coordination of senescence events requires a complex sig-
naling network, whose analysis is further complicated by the intrinsic diversity of senescence processes.
Generally speaking, three types of factors are known to affect the onset and progression of plant senes-
cence: (1) internal factors, related to development and aging; (2) environmental conditions, especially
those causing plant stress; and (3) hormonal factors related to intercommunication with the rest of the
plant.
Developmental processes involving cell death related to differentiation of tissues and directed to
morphogenetic goals (e.g., xylogenesis or postpollination changes in ovaries) are typical cases of apop-
tosis, most likely governed by morphogens. In contrast, natural senescence related to aging may be trig-
gered by nutritional and metabolic imbalances. The senescence of old leaves is a paradigmatic case. It has
been suggested that, regarding the onset of senescence, plant leaves may be considered as autonomous,
their life span being evolutively fixed from a compromise between recurse allocation and payoff expec-
tations [41,204]. Leaves usually produce photosynthetic assimilates in excess of their own needs, export-
ing them to the rest of the plant. However, because their productivity declines with aging, old leaves may
barely contribute to their own nutrition, no longer benefiting the plant as a whole. In that instance, they
192 PEÑARRUBIA AND MORENO