produced oxygen-binding protein. Leghemoglobin accumulates to millimolar
concentrations of O 2 in the cytoplasm of infected cells prior to nitrogen fixation
and buffers the free O 2 concentration at around 7–11 nM, while maintaining high O 2
flux for respiration [7]. Ammonia synthetized in nitrogenase reaction reacts with
intracellular keto acids, such as α-ketoglutaric, pyruvic acid, or oxalic acids in
dehydrogenase- and transaminase-catalyzed reactions forming respective amino
acids, such as glutamine, alanine, or asparagine [115]. Nitrogen-containing substances in the form of free ammonia, amino acids, or amides are transported from
nodules to the roots and, then, to the aboveground parts of plants [37].
In all nodule types, the N 2 -fixation period is optimal between 4 and 5 weeks after
infection. Beyond this period, first reductions of N 2 -fixing bacteroid capacity are
detectable, and a senescence process occurs in the N-fixing nodule zone. Generally,
dynamics of the senescence process in nodules include decrease of N 2 -fixing activity
and leghemoglobin content, modifications in the nodule components of regulating
redox state, and an increase of proteolytic activity, ultimately leading to the death of
infected cells [7].
3
Role of Free Radical Processes in Legume-Rhizobium
Symbiosis
3.1
Reactive Oxygen/Nitrogen Species as Components of Plant
Aerobic Metabolism and Plant Immunity
Production of reactive oxygen species (ROS) is a part of normal aerobic cellular
metabolism of living organisms, including plants. These reactive species include
singlet oxygen (
1 O 2 ), superoxide anion radical O • À
2
À
Á
, hydrogen peroxide (H 2 O 2 ),
and hydroxyl radical (HО
• ). The most short-lived ROS is the hydroxyl radical
(HО
• ), whereas O • À
2 and Н 2 О 2 are more stable compounds. Hydroxyl radical is the
most reactive ROS, and Н 2 О 2 is the least reactive among other reactive species. In
the organism, conversion of less reactive ROS into more reactive compounds is
possible. Thus, O • À
2
can be a starting compound for the synthesis of other ROS:
hydrogen peroxide, peroxynitrite (ONOO
À
), singlet oxygen, and hydroxyl radical
HО
• [10, 116–119]. In leaves, chloroplasts and peroxisomes are the main ROS
producers in the presence of light [120]. Conversely, in non-green plant tissues or
in the darkness, the mitochondria appear to be the main ROS producers [10,
117]. ROS can also be produced in some enzymatic reactions in plants; in particular NAPDH oxidase located in plasma membrane is considered as an important
source of O • À
2 in the cell [121]. Besides ROS, plant cells produce reactive nitrogen
species (RNS) with
• NO being the most important among them [41, 116]. The main
source of
• NO is a reaction catalyzed by the plant NO synthases, which convert
L-arginine to
• NO and L-citrulline [116, 122]. In addition to NO synthases, nitrate
reductases also contribute to NO production in plants [41, 122]. NO is a ubiquitous
signaling molecule in plants, controlling physiological processes as diverse as
304
U. Y. Stambulska and M. M. Bayliak
concentrations of O 2 in the cytoplasm of infected cells prior to nitrogen fixation
and buffers the free O 2 concentration at around 7–11 nM, while maintaining high O 2
flux for respiration [7]. Ammonia synthetized in nitrogenase reaction reacts with
intracellular keto acids, such as α-ketoglutaric, pyruvic acid, or oxalic acids in
dehydrogenase- and transaminase-catalyzed reactions forming respective amino
acids, such as glutamine, alanine, or asparagine [115]. Nitrogen-containing substances in the form of free ammonia, amino acids, or amides are transported from
nodules to the roots and, then, to the aboveground parts of plants [37].
In all nodule types, the N 2 -fixation period is optimal between 4 and 5 weeks after
infection. Beyond this period, first reductions of N 2 -fixing bacteroid capacity are
detectable, and a senescence process occurs in the N-fixing nodule zone. Generally,
dynamics of the senescence process in nodules include decrease of N 2 -fixing activity
and leghemoglobin content, modifications in the nodule components of regulating
redox state, and an increase of proteolytic activity, ultimately leading to the death of
infected cells [7].
3
Role of Free Radical Processes in Legume-Rhizobium
Symbiosis
3.1
Reactive Oxygen/Nitrogen Species as Components of Plant
Aerobic Metabolism and Plant Immunity
Production of reactive oxygen species (ROS) is a part of normal aerobic cellular
metabolism of living organisms, including plants. These reactive species include
singlet oxygen (
1 O 2 ), superoxide anion radical O • À
2
À
Á
, hydrogen peroxide (H 2 O 2 ),
and hydroxyl radical (HО
• ). The most short-lived ROS is the hydroxyl radical
(HО
• ), whereas O • À
2 and Н 2 О 2 are more stable compounds. Hydroxyl radical is the
most reactive ROS, and Н 2 О 2 is the least reactive among other reactive species. In
the organism, conversion of less reactive ROS into more reactive compounds is
possible. Thus, O • À
2
can be a starting compound for the synthesis of other ROS:
hydrogen peroxide, peroxynitrite (ONOO
À
), singlet oxygen, and hydroxyl radical
HО
• [10, 116–119]. In leaves, chloroplasts and peroxisomes are the main ROS
producers in the presence of light [120]. Conversely, in non-green plant tissues or
in the darkness, the mitochondria appear to be the main ROS producers [10,
117]. ROS can also be produced in some enzymatic reactions in plants; in particular NAPDH oxidase located in plasma membrane is considered as an important
source of O • À
2 in the cell [121]. Besides ROS, plant cells produce reactive nitrogen
species (RNS) with
• NO being the most important among them [41, 116]. The main
source of
• NO is a reaction catalyzed by the plant NO synthases, which convert
L-arginine to
• NO and L-citrulline [116, 122]. In addition to NO synthases, nitrate
reductases also contribute to NO production in plants [41, 122]. NO is a ubiquitous
signaling molecule in plants, controlling physiological processes as diverse as
304
U. Y. Stambulska and M. M. Bayliak