cysteine or tyrosine residues, respectively [46]. In nodules of M. truncatula,
• NO has
been shown to activate two genes encoding proteins involved in H 2 O 2 metabolism
(peroxidase and germin-like oxalate oxidase), suggesting a cross talk between ROS
and RNS signaling [123, 129]. Peroxynitrite (ONOO
À ) is a signaling molecule
formed when
• NO reacts with O • À
2 . Its function may be mediated by the selective
nitration of tyrosine residues in a small number of proteins [46].
Nitrogenase complex in bacteroids is very sensitive to ROS attack; therefore,
it is not surprising that legume nodules have efficient mechanisms to maintain proper
redox balance and low ROS levels. Because of susceptibility of N 2 fixation
to oxidative damage, legume nodules have evolved a complex and wide range
of defense mechanisms aimed at destroying ROS or preventing their formation
[42]. Nodules possess a powerful antioxidant system, which includes antioxidant
enzymes (superoxide dismutase, catalase, and various peroxidases), enzymes of the
ascorbate-glutathione cycle, and low-molecular mass antioxidant metabolites such
as ascorbate, glutathione, and tocopherols [40, 123]. The capacity of nodule antioxidant system affects largely nitrogen-fixing efficiency; in particular, nodules may not
function without ascorbate-glutathione cycle [123].
3.3
Redox Balance and Nodule Senescence
The lifespan of the rhizobia-plant symbiotic relationship is relatively short, and the
disruption of this symbiosis affects the yield of the crop. Average nodule lifespan is
10–12 weeks, but their N 2 -fixing capacity starts to decline 3–5 weeks after initiation,
and this decline is caused by nodule senescence [130]. Puppo et al. [130] concluded
that nodule senescence is an active and programmed process in development, in
which ROS, antioxidants, hormones, and proteinases have key roles. A typically
visible sign of nodule senescence is its color, changing from pink to green because of
disruption of Lb activity. In aging soybean nodules, green Lb arises from heme
nitration, underlining the critical role of RNS in the senescence process [45].
Nitrogen-fixing nodules are particularly rich in antioxidant defense mechanisms,
which are sufficient to cope with ROS toxicity. During the natural senescence of
nodules, levels or activity of some antioxidants (glutathione, catalase), but not others
(ascorbate peroxidase, α-tocopherol), significantly decreases [131]. The redox
imbalance followed by oxidative stress promotes oxidation of lipids and proteins
and the degradation of membranes. Aging nodules accumulate oxidized thiols,
lipids, proteins, and DNA. Aging was shown to cause a 50% decrease of homoglutathione in soybean and bean nodules and an 82% decrease of glutathione in pea
nodules [131].
In mature nodules,
•
NO was shown to inhibit N 2 fixation and to trigger nodule
senescence [45]. S. meliloti degrades
•
NO in Medicago nodules, leading to a delay in
nodule senescence [94, 132]. O • À
2 and Н 2 О 2 were also supposed to act as signaling
molecules involved in senescence of legume-rhizobium symbiosis [44]. At the later
stages of symbiotic formation, when the amount of rhizobia in the roots reaches
308
U. Y. Stambulska and M. M. Bayliak
• NO has
been shown to activate two genes encoding proteins involved in H 2 O 2 metabolism
(peroxidase and germin-like oxalate oxidase), suggesting a cross talk between ROS
and RNS signaling [123, 129]. Peroxynitrite (ONOO
À ) is a signaling molecule
formed when
• NO reacts with O • À
2 . Its function may be mediated by the selective
nitration of tyrosine residues in a small number of proteins [46].
Nitrogenase complex in bacteroids is very sensitive to ROS attack; therefore,
it is not surprising that legume nodules have efficient mechanisms to maintain proper
redox balance and low ROS levels. Because of susceptibility of N 2 fixation
to oxidative damage, legume nodules have evolved a complex and wide range
of defense mechanisms aimed at destroying ROS or preventing their formation
[42]. Nodules possess a powerful antioxidant system, which includes antioxidant
enzymes (superoxide dismutase, catalase, and various peroxidases), enzymes of the
ascorbate-glutathione cycle, and low-molecular mass antioxidant metabolites such
as ascorbate, glutathione, and tocopherols [40, 123]. The capacity of nodule antioxidant system affects largely nitrogen-fixing efficiency; in particular, nodules may not
function without ascorbate-glutathione cycle [123].
3.3
Redox Balance and Nodule Senescence
The lifespan of the rhizobia-plant symbiotic relationship is relatively short, and the
disruption of this symbiosis affects the yield of the crop. Average nodule lifespan is
10–12 weeks, but their N 2 -fixing capacity starts to decline 3–5 weeks after initiation,
and this decline is caused by nodule senescence [130]. Puppo et al. [130] concluded
that nodule senescence is an active and programmed process in development, in
which ROS, antioxidants, hormones, and proteinases have key roles. A typically
visible sign of nodule senescence is its color, changing from pink to green because of
disruption of Lb activity. In aging soybean nodules, green Lb arises from heme
nitration, underlining the critical role of RNS in the senescence process [45].
Nitrogen-fixing nodules are particularly rich in antioxidant defense mechanisms,
which are sufficient to cope with ROS toxicity. During the natural senescence of
nodules, levels or activity of some antioxidants (glutathione, catalase), but not others
(ascorbate peroxidase, α-tocopherol), significantly decreases [131]. The redox
imbalance followed by oxidative stress promotes oxidation of lipids and proteins
and the degradation of membranes. Aging nodules accumulate oxidized thiols,
lipids, proteins, and DNA. Aging was shown to cause a 50% decrease of homoglutathione in soybean and bean nodules and an 82% decrease of glutathione in pea
nodules [131].
In mature nodules,
•
NO was shown to inhibit N 2 fixation and to trigger nodule
senescence [45]. S. meliloti degrades
•
NO in Medicago nodules, leading to a delay in
nodule senescence [94, 132]. O • À
2 and Н 2 О 2 were also supposed to act as signaling
molecules involved in senescence of legume-rhizobium symbiosis [44]. At the later
stages of symbiotic formation, when the amount of rhizobia in the roots reaches
308
U. Y. Stambulska and M. M. Bayliak