a certain level, the host plant may also induce mechanisms of ROS generation to
regulate further nodule formation [39]. Furthermore, during nodule senescence high
ROS levels have been detected in senescing symbiosomes suggesting ROS involvement in this process [36].
Developmental nodule senescence is a complex and programmed process,
which induces a decrease of nitrogen-fixing activity and leghemoglobin content,
modifications in the nodule antioxidant components, and an increase of proteolytic
activity, ultimately leading to the death of infected cells. Leghemoglobin, which
plays a fundamental role in nodule functioning, is an important physiological
marker for following the progression of nodule senescence [7]. Content of
leghemoglobin progressively decreases with the onset of senescence. In turn, this
decreases O 2 availability to bacteroids and nitrogenase, releasing free iron to
produce ROS via Fenton reaction. The auto-oxidation of the active form of
leghemoglobin to ferro-Lb-O 2 is associated with O • À
2
and H 2 O 2 generation and
the degradation of the heme group of leghemoglobin by H 2 O 2 that likely allows the
release of the catalytic Fe. The latter may enhance the production of HO
• through
the Fenton and the Haber-Weiss reactions [7]. In senescent soybean nodules, the
high level of H 2 O 2 detected in the cytoplasmic and apoplastic compartments of the
infected tissue was associated with an enhanced expression of cysteine protease
gene suggesting a link between oxidative stress and proteolytic activities under
nodule senescence [7, 133].
In conclusion, ROS and RNS are involved in the regulation of legume-rhizobium
symbiosis. However, they may have different roles, as they are involved in the
establishment and the functioning of the nodule on the one hand and in the regulation
of the nodule number and the onset of senescence on the other hand [93].
4
Toxic Effects of Heavy Metals on Legumes and LegumeRhizobium Symbiosis
4.1
Toxicity of Heavy Metals in Plants: Overview
Plants receive mineral elements from the soil primarily in the form of inorganic
ions [55]. Mineral elements can be divided into two groups: essential nutrients and
toxic non-nutrient elements. The first group includes the macronutrients such as
nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorous (P),
sulfur (S), and silicon (Si) and the micronutrients – chlorine (Cl), iron (Fe), boron
(B), manganese (Mn), sodium (Na), zinc (Zn), copper (Cu), nickel (Ni), and molybdenum (Mo). These elements are essential components of plants, and their absence
or deficiency may cause adverse biochemical perturbations leading to morphological
changes and inhibition of plant growth and reproduction. Micronutrients (Cu, N, Zn,
Mo, etc.) are required in very small quantities. Many anthropogenic activities lead to
excessive accumulation of microelements in the soil that makes them hazardous to
the majority of plant species. Other minerals called heavy metals like cadmium (Cd),
13 Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes. . .
309
regulate further nodule formation [39]. Furthermore, during nodule senescence high
ROS levels have been detected in senescing symbiosomes suggesting ROS involvement in this process [36].
Developmental nodule senescence is a complex and programmed process,
which induces a decrease of nitrogen-fixing activity and leghemoglobin content,
modifications in the nodule antioxidant components, and an increase of proteolytic
activity, ultimately leading to the death of infected cells. Leghemoglobin, which
plays a fundamental role in nodule functioning, is an important physiological
marker for following the progression of nodule senescence [7]. Content of
leghemoglobin progressively decreases with the onset of senescence. In turn, this
decreases O 2 availability to bacteroids and nitrogenase, releasing free iron to
produce ROS via Fenton reaction. The auto-oxidation of the active form of
leghemoglobin to ferro-Lb-O 2 is associated with O • À
2
and H 2 O 2 generation and
the degradation of the heme group of leghemoglobin by H 2 O 2 that likely allows the
release of the catalytic Fe. The latter may enhance the production of HO
• through
the Fenton and the Haber-Weiss reactions [7]. In senescent soybean nodules, the
high level of H 2 O 2 detected in the cytoplasmic and apoplastic compartments of the
infected tissue was associated with an enhanced expression of cysteine protease
gene suggesting a link between oxidative stress and proteolytic activities under
nodule senescence [7, 133].
In conclusion, ROS and RNS are involved in the regulation of legume-rhizobium
symbiosis. However, they may have different roles, as they are involved in the
establishment and the functioning of the nodule on the one hand and in the regulation
of the nodule number and the onset of senescence on the other hand [93].
4
Toxic Effects of Heavy Metals on Legumes and LegumeRhizobium Symbiosis
4.1
Toxicity of Heavy Metals in Plants: Overview
Plants receive mineral elements from the soil primarily in the form of inorganic
ions [55]. Mineral elements can be divided into two groups: essential nutrients and
toxic non-nutrient elements. The first group includes the macronutrients such as
nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorous (P),
sulfur (S), and silicon (Si) and the micronutrients – chlorine (Cl), iron (Fe), boron
(B), manganese (Mn), sodium (Na), zinc (Zn), copper (Cu), nickel (Ni), and molybdenum (Mo). These elements are essential components of plants, and their absence
or deficiency may cause adverse biochemical perturbations leading to morphological
changes and inhibition of plant growth and reproduction. Micronutrients (Cu, N, Zn,
Mo, etc.) are required in very small quantities. Many anthropogenic activities lead to
excessive accumulation of microelements in the soil that makes them hazardous to
the majority of plant species. Other minerals called heavy metals like cadmium (Cd),
13 Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes. . .
309