If it is necessary to use exclusively biochemical assays, a combination of different parameters
may be the best solution for precise monitoring of senescence. For example, Pastori and Trippi [236]
have utilized chlorophyll loss, lipid peroxidation, and cell electrolyte leakage to study senescence in
maize, and Oh et al. [237] used amount of Rubisco large subunit, RNase, and peroxidase activities, to-
gether with photosystem II efficiency and chlorophyll content, to monitor senescence in Arabidopsis
thaliana.
Whenever possible, gene expression analysis of selected senescence markers will provide more
sensitive and accurate monitoring of the senescence process. Because expression patterns display some
variability between species and between senescence-inducing treatments, it might be advisable to check
the particular case by screening an array of selected markers in order to choose the most appropriate
one. In principle, one may use any gene product (at the level of mRNA or protein) that shows a sig-
nificant variation throughout the relevant senescence process. Among markers of declining mRNA lev-
els, the most widely utilized are the messengers corresponding to the chlorophyll a/b binding protein
(CAB) and the small subunit of the Rubisco enzyme (rbcS) [88,201,215] because homologous oligonu-
cleotide probes for these genes are readily available in a wide variety of plants (where at least one of
these genes has been sequenced). Among the messages whose levels are increased during senescence,
those corresponding to the genes LSC54fromBrassica napusandSAG12andSAG13fromArabidop-
sis thalianaare claimed to be the most senescence specific [4]. LSC54(encoding a metallothionein) is
expressed in leaves and flowers of B. napusexclusively during senescence [89]. In contrast,
the homologous gene in A. thaliana(SAG17) is constitutively expressed at a moderate level that rises
with senescence [88]. Another useful gene, SEN1fromA. thaliana, exhibits senescence-dependent
expression but with a different intensity in natural or hormone (abscisic acid or ethylene) induced
senescence [199]. The practical utility of these markers in other plants is somewhat hindered by the
need for identification and characterization of the homologous genes or the use of less specific het-
erologous probes.
VIII. SUMMARY AND CONCLUDING REMARKS
Senescence appears as an ordered dismantling of structures and components from plant parts whose func-
tional contribution has become unnecessary and which are therefore directed to abscission and death.
Aside from functional advantages that may be derived in special cases from senescence of certain struc-
tures, the principal goal of senescence is to recover nutrients from the decaying tissues, withdrawing them
to the surviving parts before abscission. Thus, senescence is essentially a physiological strategy of nutri-
tional economy.
Natural senescence of plant organs is probably triggered by a nutritional imbalance leading to cer-
tain metabolic alterations (sensed locally by an unknown mechanism), which begin a transduction cas-
cade involving multiple intermediate signals (hormones, ROIs, Ca^2 , transcription factors, etc.). Primary
signals activate a set of endogenous adaptive responses (mostly directed to nutrient salvage before pro-
grammed death or to protection of nonsenescing nearby tissues), which are executed through secondary
signals switching off and on specific genes in a functionally coordinated temporal and spatial pattern.
Stress-induced senescence appears to elicit the same adaptative responses through interference by adverse
environmental conditions or pathogenesis somewhere along the signal transduction pathway of natural
senescence. In any case, these endogenous responses lead to the typical alterations that are characteristic
of all senescence processes, including breakdown of photosynthetic pigments and selected macro-
molecules, progressive deterioration and loss of functions of membranes, and, in the final stage, degen-
eration of cell internal structure.
Considering the wealth of information currently being gathered through molecular analysis at the
gene expression level, our understanding of senescence is expected to improve in the coming years. This
will probably uncover the signaling mechanisms, clarify the bounds between senescence and related pro-
cesses such as stress responses and fruit ripening, and extend the possibilities of genetic engineering of
the senescence features of crops for nutritional and commercial benefit.
ACKNOWLEDGMENTS
This work was supported by grants BIO99-1201-C02-02 and PB98-1445 from DGICYT.
SENESCENCE IN PLANTS AND CROPS 195