(rRNA), which is the most abundant cellular RNA in both the chloroplast and cytosol, and a major frac-
tion of mRNAs that decay in parallel with total RNA maintaining their relative abundance [89]. This de-
crease correlates with the decline in protein synthesis. Variations in relative amounts of two phenylalanyl
transfer RNAs (tRNAs) have been detected during senescence [90], and tRNA synthase activities are
greatly reduced, probably limiting the translational capacity of senescing chloroplasts [91]. Ribosome-in-
activating proteins have also been shown to increase in naturally senescent and stressed leaves [92].
A general increase in ribonuclease (RNase) activities, described in senescing tissues [93,94], could
account for the generalized loss of RNA. Moreover, some of these activities are known to be selectively
induced during senescence processes [95–97]. Free nucleotides produced as a result of RNase activity
may be further metabolized to release phosphate. The fate of nitrogen bases resulting from nucleic acid
catabolism is still unclear, although their degradation could take place in peroxisomes [98,99].
- Intracellular Proteins
In terms of total protein content, leaf senescence is characterized by a progressive loss of proteins [85].
This loss may be attenuated if additional nitrogen is supplied to the plant [100] or sink organs are re-
moved. In general, the demand for mineral nutrients by growing structures has been described as a regu-
latory factor in leaf senescence, except for phosphorus nutrition, which does not show any regulatory con-
trol on the process [101].
The patterns of protein loss are characteristic and independent of the cause of senescence. A wide
range of specific proteins are degraded while others remain intact. In green organs, chloroplast proteins
are principal targets of degradation during early phases of senescence. The loss of chlorophyll correlates
with degradation of chlorophyll-carrying thylakoidal proteins, whose lysis is strongly retarded by contin-
uous illumination. However, stromal proteins rapidly disappear under the same conditions, indicating that
breakdown of membrane and soluble proteins is differently regulated by light [30].
The most abundant soluble protein in chloroplasts, ribulose-1,5-bisphosphate carboxylase/oxyge-
nase (Rubisco), represents more than 50% of the chloroplast nitrogen and about 25% of that of the whole
cell [102]. Rubisco is known to be extensively and selectively degraded at early stages of senescence in
many plants [20,21,103–106]. The specific proteolysis of this enzyme accounts for up to 85% of the sol-
uble protein lost in senescing barley leaves [107] and more than 90% of the nitrogen mobilized from
leaves before abscission in apple trees [21]. Experiments with transgenic plants in which the level of Ru-
bisco was decreased using antisense technology have firmly established the natural excess of Rubisco
over the amount needed for performing its catalytic function and the correlation of the amount of enzyme
with the nitrogen status of the plants [108]. Evidence for this “luxury” excess, together with the spectac-
ular contribution of this enzyme to nutrient mobilization during senescence, supports the concept of Ru-
bisco as a nitrogen storage protein [108,109].
Protein turnover is a common feature in every living organism; therefore, the global decrease ob-
served during senescence has to be considered as an imbalance between the rates of protein synthesis and
degradation [110]. Probably, both reduced synthesis and enhanced proteolysis are responsible for the pro-
tein loss associated with senescence. In this regard, synthesis of all thylakoidal proteins is known to be
severely curtailed in senescing bean leaves, except for the D-1 protein of photosystem II [111]. On the
other hand, increased protein breakdown may result from different mechanisms: de novo synthesis of pro-
teolytic enzymes, activation of preexisting proteases, decompartmentalization of proteases and their sub-
strates, or making the protein substrates susceptible to degradation.
References on increased proteolytic activities during senescence are abundant (reviewed in Refs. 2,
112–116); however, they more frequently report enhanced levels of preexistent proteases than the ap-
pearance of new activities specific to senescence [113]. Characteristically, increased expression of cys-
teine (or thiol) proteinases (which are typical apoptotic agents) has been associated with senescence of
different flower parts such as tepals [117], petals [118], and ovaries [119,120] as well as with other de-
velopmental events that include programmed cell death, such as xylogenesis [121,122]. Moreover, levels
of cysteine protease mRNAs have been shown to increase during leaf senescence of tomato [123], and
cysteine proteases are known to be induced in suspension-cultured soybean cells by oxidative treatments
that produce programmed cell death [124]. Although cysteine proteases involved in animal apoptosis hy-
drolyze peptidic bonds at aspartic acid residues (thus called caspases), it appears that plant enzymes do
not follow that rule [124]. Increased levels of proteases have been reported to occur in pea leaf peroxi-
somes [125] and lytic vacuoles of vegetative tissues in Arabidopsis[126] during senescence. However,
188 PEÑARRUBIA AND MORENO