sue mitochondria and apparently uses the same mechanism of proton translocation via fatty acid recycling
[73], thereby regulating respiratory uncoupling and thermogenesis during senescence and fruit ripening.
C. Biochemical Changes
- Photosynthetic Pigments
Color changes are important criteria for the visual evaluation of the advance of senescence, especially in
fruits [27]. Breakdown of chlorophylls may be one of the earliest symptoms of senescence. However,
chlorophyll decline is strongly retarded by continuous illumination in a process regulated by phytochrome
[30]. The chlorophyll a/bratio has been shown to decline with the advance of senescence [74,75], prob-
ably as a result of the nonsynchronous dismantling of lamellae and grana thylakoids and the asymmetri-
cal distribution of photosystems between them. Carotenoids are lost at a much lower rate than chloro-
phylls [76]. This difference in degradation rate accounts for most of the color changes associated with leaf
senescence and may reflect the persistence of the photoprotective role of carotenoids until later phases of
the process. In ripening fruits, senescence is sometimes associated with de novo synthesis of both
carotenoids and anthocyanins [27].
The pathway of chlorophyll degradation may be distinct for different species or even organs. In
senescing barley leaves, chlorophyll breduction seems to be the first and obligatory step of chlorophyll
bbreakdown [77]. This step is carried out by chlorophyll(ide) breductase, a thylakoidal enzymatic ac-
tivity that peaks earlier (day 2) than chlorophyllase (day 4) during dark-induced leaf senescence [77]. The
activity of chlorophyllase, a thylakoidal enzyme that hydrolyzes the phytyl ester group, has been shown
to correlate with chlorophyll degradation in maturing citrus fruits [78]. In Arabidopsis thaliana, two genes
for chlorophyllase (AtCLH1andAtCLH2) have been characterized [79]. AtCLH2encodes a protein with
a typical chloroplast targeting sequence, while ATCLH1has no defined location signal. The expression
ofAtCLH1(but curiously not of AtCLH2) is strongly induced by methyljasmonate, which is known to in-
duce plant senescence and chlorophyll degradation [79]. Chlorophyll oxidase, a complex enzymatic sys-
tem that renders chlorophyll a1 as a first step, may also be involved in chlorophyll turnover. The fact that
chloroplast chlorophyllase is hindered by its membrane localization and that chlorophyll oxidase activity
is dependent on free fatty acids, liberated by lipid hydrolysis, may provide a link between thylakoidal
membrane dismantling and chlorophyll degradation inside the chloroplast [78,80]. Alternatively, the per-
oxidase–hydrogen peroxide pathway, which opens the chlorophyll porphyrin ring, can also be involved
and has been shown to be the main catabolic pathway in detached spinach leaves [81]. In addition, direct
photodamage represents a contribution, even if minor, to chlorophyll breakdown [55].
The existence of a Festuca pratensismutant that does not exhibit chlorophyll loss during senescence
supports the conclusion that the associated decline in photosynthetic capacity is not a result of chlorophyll
breakdown. Studies of the proteins that are abnormally retained in the mutant indicate that all of them pos-
sess an associated tetrapyrrole prosthetic group (heme or chlorophyll) [46]. As it has been proposed that
the degradation of porphyrins and their associated apoproteins is correlated [82], a lesion in the
heme/chlorophyll catabolic pathway may be responsible for the phenotype of the mutant [46].
- Nucleic Acids
Nucleic acids are a rich source of nutrients (especially phosphate), which ought to be mobilized and ex-
ported from senescing organs before abscission. On the other hand, the genetic information stored in
DNA should be preserved throughout the decay process because the completion of the senescence pro-
gram requires the uninterrupted synthesis of specific messenger RNAs (mRNAs) until very late stages.
Thus, the amount and integrity of DNA are usually maintained in senescent cells until the late phase of
chromatin fragmentation. A decrease in nuclear DNA (about 20%) has been described at the final stages
of senescence in soybean cotyledons [83] as well as tobacco and peanut leaves [84]. It has been shown
that repeated sequences are selectively degraded while coding regions of nuclear DNA remain largely in-
tact [85,86].
In general, senescence is a process of overall decline in RNA and protein synthesis, especially in the
chloroplast [87]. Accordingly, total RNA has been found to be around 10-fold lower in a yellow leaf com-
pared with a green one [88,89]. However, a nonspecific decline in RNA synthesis does not cause senes-
cence and, furthermore, selective synthesis of specific mRNAs seems necessary for the progress of senes-
cence (see later). The quantitative decline in RNA is explained mainly by the decrease in ribosomal RNA
SENESCENCE IN PLANTS AND CROPS 187