As exhaustive mutational analysis in Arabidopsishas failed to provide any mutants that globally and
completely block the senescence process [40,41], it appears that no single nonlethal gene regulates senes-
cence as a whole. However, several mutations partially altering senescence or some aspects of the pro-
cess have been described in Arabidopsisand other plants (reviewed in Ref. 42). Most of the mutations an-
alyzed to date are varieties from natural cultivars of the “stay-green” type [42,43]. A mutation in a nuclear
gene of Festuca pratensisresults in a drastic reduction of chlorophyll loss without other senescence char-
acteristics (such as decrease of protein and RNA content, rise of proteolytic activities, or degradation of
plastid structure) being altered [44–46]. The mutated gene controls the thylakoid membrane disassembly
in senescent leaves, impairing the degradation of thylakoid pigments, protein, and lipids [47]. It has been
shown that this mutant is unable to carry out oxygenolytic cleavage of the porphyrin macrocycle [48].
Mutations in nuclear and organular genes altering chlorophyll loss or gas exchange during monocarpic
senescence have also been described in soybean [49]. Plants mutated in two loci (d1 and d2) experience
a significant delay in degradation of soluble proteins [50], plasma membrane, and chloroplasts [51]. Be-
sides, a mutation that affects the light regulation of seedling development has been shown to interfere with
the onset of leaf senescence [52].
Mutants that seem unable to control the rate and extent of cell death when exposed to different senes-
cence-inducing agents have been described [53]. These mutants (acd1) exhibit accelerated cell death with
rapid spreading of necrotic lesions in response to virulent and avirulent pathogens but also during aging
of aseptic plants. Because these lesions are characteristic of the so-called hypersensitive defense response
to pathogens, analysis of the mutants may provide an understanding at the molecular level of this response
and its relationship to natural senescence.
The progress achieved in plant transformation techniques, and especially the easy Agrobacterium
tumefaciens–mediated protocols for transforming Arabidopsis[54], has made transgenic plants a com-
mon tool. Once a senescence-related gene has been cloned, the function and physiological relevance of
the gene product may be tested in transgenic plants. The most common strategy consists of the modifica-
tion of a target endogenous protein level through either overexpression (introducing new copies of the
gene under strong promoters) or decreasing its transcription by antisense technology. Similar procedures
are employed for introducing heterologous genes under the control of endogenous promoters (or other
suspected regulatory sequences) in host plants in order to test the effect of subtle manipulations at the pro-
moter without an endogenous background of the reporter gene or with the aim of introducing novel and
agriculturally desirable properties in transgenic crops. As an example (discussed in detail in the follow-
ing), the use of transgenic plants whose hormone metabolism has been altered (thereby modifying hor-
monal levels endogenously) has shed light on the role of certain phytohormones, such as ethylene and cy-
tokinin, thereby suggesting successful strategies for manipulating senescence.
IV. ULTRASTRUCTURAL, PHYSIOLOGICAL, AND BIOCHEMICAL
CHANGES DURING SENESCENCEA. Ultrastructural Changes
Characteristic changes ocurring during senescence in different plants share common features at the ultra-
structural level. In green organs, chloroplasts are the organelles in which the first symptoms of senescence
are observable. Following an ordered sequence of events, the chloroplast dismantling begins with
swelling, unstacking, and degradation of thylakoids (first those of the lamellae, then the grana), appear-
ance of lipid droplets and plastoglobuli, and finally fragmentation of the envelope [55]. In some cases,
chloroplasts have been observed to fuse with vacuoles at the late stages of senescence [56]. The number
and size of chloroplasts are reduced during senescence, and the rate of oxygen evolution decreases ap-
proximately in parallel with the chloroplast content [57]. Loss of starch is also characteristic of senes-
cence and may result in deformation of the cells. This may explain the distortion of endocarp and meso-
carp cells observed in senescent ovaries of pea [56].
Some extraplastidic membranes, such as those of the endoplasmic reticulum, also undergo early
degradation, the smooth and rough fractions being degraded nonsimultaneously depending on the species
[58]. Changes in the properties of lipid phase have been observed in senescing membranes using wide-
angle x-ray diffraction and freeze-fracture electron microscopy [59]. Regions of the lipid bilayer switch
from liquid crystalline to gel phase, rendering leaky membranes. Freeze-fracture electron microscopy
SENESCENCE IN PLANTS AND CROPS 185