Handbook of Plant and Crop Physiology

(Steven Felgate) #1

catalases, peroxidases, and glutathione reductases, are present in plant cells to prevent the damage by
these toxic species [153–156]. Superoxide is eliminated by the superoxide dismutase, which produces hy-
drogen peroxide. Catalases convert hydrogen peroxide into water and oxygen, whereas peroxidases re-
duce hydrogen peroxide to water and oxidize a variety of substrates. Ascorbate peroxidase is a hydrogen
peroxide–scavenging enzyme that is specific to higher plants and algae [157]. This enzyme protects
chloroplasts and other cell constituents from damage by hydrogen peroxide and derived hydroxyl radi-
cals. Ascorbate peroxidase, glutathione reductase, and dehydroascorbate reductase remove hydrogen per-
oxide through a pathway termed photoscavenging[154]. Nevertheless, in certain cases some of these ac-
tivities decrease with the progression of senescence, thereby favoring a parallel rise of ROIs [158,159].
In other instances, the soluble activity of protective enzymes decreases but there is an increase of wall-
bound activity. This is the case for peroxidase during senescence of stigmas and styles in Citrus[160]. It
has been shown that the reduction of ascorbate contents in transgenic potato plants by antisense inhibi-
tion of the GDP-mannose pyrophosphorylase accelerates senescence [161].
As a consequence of proteolysis, metal cofactors present in metalloproteins are released to the cell
internal environment. Some of these free metal ions, such as iron and copper, may be pernicious to the
plant because they catalyze the production of ROIs [151]. On the other hand, these metals have to be mo-
bilized to growing parts, being needed as nutrients by the developing organs. Metallothioneins, metal-
binding proteins that are known to be induced during senescence [88,162] and are localized in vascular
tissues, may fulfill the task, chelating metal ions (thereby protecting against ROIs) and favoring their di-
rected transport.
The localization of the metabolism of ROIs in specific compartments, such as peroxisomes, could
also serve to protect the cell under normal conditions [149]. It has been stated that the number of peroxi-
somes increases with oxidative stress [163] and superoxide radicals have been localized in glyoxisomes,
a special kind of peroxisomes [164]. Nevertheless, membrane deterioration during senescene and the sub-
sequent loss of compartmentation may contribute to an extension of the effects of ROIs inside the cell.



  1. Membranes


One of the most characteristic changes during senescence is the progressive loss of membrane integrity.
The major classes of lipids in plasma membrane and tonoplast are phospholipids, sterols, and ceramide
monohexosides [165]. Common changes during senescence include a decrease in the total phospholipid
and protein content, an increase in neutral lipids, and generalized oxidation [165]. Sterols also decline
with physiological aging [166]. As a consequence of all of these, the physicochemical properties of the
membranes, such as lipid fluidity, phase transition temperature, and nonbilayer lipid structure, are pro-
gressively altered during senescence [59,167–169]. The bilayer destabilization leads to a generalized fail-
ure of membrane functions, including loss of selective permeability and intracellular compartmentation,
as well as membrane-associated enzymes and receptors [59,170]. Unfortunately, all these alterations
complicate the experimental isolation of senescent membranes due to the changes in density and surface
charge and the loss of marker enzymes [171].
The main enzymes implicated in lipid degradation are phospholipase D, phosphatidate phosphatase,
and lipolytic acyl hydrolase. Most of the enzymes implicated in this process possess both membranous
and cytosolic forms, which are differently regulated. The sequential action of these enzymes produces
polyunsaturated fatty acids (PUFAs), which are substrates for lipoxygenase [172]. Lipoxygenase is a
dioxygenase that catalyzes the oxidation of PUFA to fatty acid hydroperoxide, which is a precursor of
volatile compounds that provide the typical flavor that characterizes wounded tissues [165]. Lipoxyge-
nase activity has been described to increase during senescence in different plant organs [173,174]. The
degree of partitioning of lipoxygenase between cytosol and membranes seems to be an important factor
in the peroxidative damage because this enzyme seems to favor the oxidative injury of membranes by su-
peroxide radicals. Delta 9 desaturase, an enzyme that has been shown to increase in senescing petals
[175], may also play a role in the degradation of saturated fatty acids of membrane lipids.
The fact that some products of lipid peroxidation could serve as Ca^2 ionophores, together with
structural changes in the lipid phase, renders senescent membranes leaky to Ca^2 . Ca^2 is stored in com-
partments such as apoplast and vacuole by the action of adenosinetriphosphatases (ATPases) that main-
tain its cytoplasmic concentration below micromolar levels under steady-state conditions. However, cy-
tosolic Ca^2 increases during senescence as a consequence of the decrease in the efficiency of ATPases,
along with Ca^2 leakage from the storage compartments. In turn, the increase in cytosolic Ca^2 triggers


190 PEÑARRUBIA AND MORENO
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