a profusion of secondary effects. Among them, Ca^2 may influence directly phospholipase D and phos-
phatidic acid phosphatase and indirectly affect lipoxygenase [176]. In addition, calcium and spermine
have been shown to cause a decrease in membrane fluidity of tomato microsomes and increase phospho-
lipase D activity by a mechanism attributable to the biophysical effect of the cations on the membranes
[177].
V. CHANGES IN GENE EXPRESSION
Senescence is a controlled process of disorganization that must be regulated by a set of genes acting in
concert. Even if senescence is characterized by a global decrease in total RNA, specific mRNAs have
been described to decrease or increase their levels on both a leaf area basis and an RNA mass basis
[88,178]. Thus, senescence seems to begin with turning off and on of specific genes. The expression of
genes can be regulated at several levels. Among them, transcriptional activation and repression are better
known because they can be easily detected through the differential screening of cDNA libraries (for re-
views, see Refs. 11, 40, 178). However, posttranscriptional regulation at the level of poly(A) tail short-
ening [179] or ribosome inactivating proteins [92] has been also reported during senescence. It is not our
aim in this section to include an exhaustive list of genes that are transcriptionally regulated during senes-
cence; therefore, only general trends and some illustrative examples are discussed.
Among the wealth of down-regulated genes, those coding for ATP sulfurylase [180], a photosystem
II polypeptide [181], and a few stromal enzymes [182] have been characterized. On the other hand, tran-
scriptionally activated genes are expected to be more relevant in defining the typical features of senes-
cence and are, therefore, more intensely studied. Genes up-regulated during senescence are often termed
senescence-associated genes (SAGs). SAGs may be classified according to their pattern of expression
during leaf development or according to their putative functions based on sequence homology with other
already cloned genes [40,178]. Regarding temporal patterns of expression, 10 different classes of senes-
cence-related genes have been described in Brassica napus[178]. Some of these genes are also expressed
in other phases of leaf development, especially in young tissues with elevated rates of metabolism. Al-
though the pattern of expression can provide some hints about their function, sequence similarity to other
characterized genes is usually more revealing. According to this latter (admitedly presumptive) evidence,
SAGs can be divided into two main functional categories: those related to nutrient mobilization and those
involved in cell protection. Exceptionally, some SAGs could play both roles, and the function of some
others remains unknown.
A. SAGs Involved in Nutrient Mobilization
According to the central role played by nutrient mobilization during senescence, SAGs related to this pro-
cess are the most numerous. Moreover, these messages are relatively abundant (as corresponds to their
extensive function) and, thus, they are easier to detect. Indeed, most of the clones that have been described
to be up-regulated during senescence have been shown to be related to macromolecular breakdown and/or
nutrient transport from senescent tissues to growing organs.
Among degradative enzymes, enhanced expression of cysteine proteases has been described in dif-
ferent plant species and senescence systems [117,118,120,122,123,180,183–185]. Certain clones show
senescence-specific expression (typically SAG12fromArabidopsis thaliana) [17,88], and others respond
selectively to specific senescence-inducing factors [10].
Other types of proteases, such as peroxisomal serine proteases [125], and vacuolar processing en-
zymes [126] (probably involved in regulatory activation of hydrolases in lytic vacuoles) are also up-reg-
ulated during senescence processes. Expression of proteins involved in the ubiquitin-mediated proteolytic
pathway (which targets proteins for specific proteasome degradation) is also enhanced [186–188]. How-
ever, the expression of subunits of the 26S proteasome has been described to decline during senescence
of tobacco leaves and flowers [129] but to increase during spinach cotyledon senescence [189], leading
to controversial assumptions about the implication of proteasomal degradation in these processes.
In protein degradation in senescent chloroplasts, expression of some subunits of the protease system
denominated Clp has been reported to be increased during natural senescence [190,191]. These protease
complexes are assembled from a combination of subunits that are encoded in both the nuclear and chloro-
SENESCENCE IN PLANTS AND CROPS 191