particular, due to stimulation of ethylene synthesis [20, 60, 112, 113]. Details of the
role of phytohormones in legume-rhizobium symbiosis and mechanisms of their
regulation are available in several recent excellent reviews [24, 25, 27, 111].
2.6
Nodule Functioning and Senescence
Nodules can be classified into two main groups according to their mode of development. Determinate nodules have a short-lived root meristem; they initiate from the
outermost one or two layers of cortical cells and grow by plant cell expansion
and division, progressing through well-defined developmental stages. Determinate
nodules usually adopt a globular shape and are formed on Lotus sp., Phaseolus sp.,
G. max, and a number of tropical legumes. The mature nodules contain a homogenous central tissue composed of infected cells fully packed with N 2 -fixing bacteroids
and some uninfected cells. Senescence in these nodules occurs radially, beginning
at the center and extending to the periphery [7]. Indeterminate nodules have
a persistent meristem and elongate, to become cylindrical. New nodule cells are
gradually infected by rhizobia residing in the nodule; this produces more cylindrical
mature nodules separated into distinct developmental zones (Fig. 1): zone I is made
of meristematic cells; zone II is where cells are infected by bacteria which differentiate into bacteroids; zone III is where bacteroids reduce N 2 into ammonia which
is exported to the plant; and zone IV is characterized by the disruption of the
partnership and the onset of senescence [45, 114]. Medicago sp., Vicia sp., Trifolium
sp., P. sativum, and Astragalus are typical legumes with indeterminate nodules [31,
76]. In contrast to bacteroids in determinate nodules, those from indeterminate
nodules have lost their capacity to reproduce [7].
In mature nodules, compatible rhizobia differentiate into bacteroids that express
the enzymes of the nitrogenase complex and begin to fix nitrogen. N 2 reduction
by the bacteroid nitrogenase is the core reaction of the symbiotic process [63].
Incompatible host-strain interactions can also lead to formation of nodules, but the
latter are defective in nitrogen fixation [111]. N 2 -fixation defective phenotype was
not due to a lack of infection but caused by bacteroid degradation after differentiation [9, 12].
Bacteroids receive carbon as dicarboxylates from legumes, and in exchange,
they fix N 2 in a low O 2 environment and secrete ammonia to the plant. To effective
N 2 fixation, bacteroids must balance electron flow to nitrogenase, lipids, polyhydroxybutyrate, and O 2 and coordinate this process with reductant production by
the tricarboxylic acid cycle [74]. In nodules, bacteroids are provided with microaerobic environment required for expression of enzymes of the nitrogenase complex,
which is located on the internal membrane of bacteroids. The nitrogenase reaction
is complex and energetically expensive, since the reduction by nitrogenase of
1 molecule of N 2 to 2 molecules of NH 4
+ requires 16 molecules of ATP and
8 electrons [6, 10]. Paradoxically, despite the N 2 -fixation process requirement of
high O 2 levels, the nitrogenase is an O 2 -sensitive enzyme. Maintaining a very low
concentration of free O 2 is achieved by the presence of leghemoglobin (Lb), a plant13 Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes. . .
303
role of phytohormones in legume-rhizobium symbiosis and mechanisms of their
regulation are available in several recent excellent reviews [24, 25, 27, 111].
2.6
Nodule Functioning and Senescence
Nodules can be classified into two main groups according to their mode of development. Determinate nodules have a short-lived root meristem; they initiate from the
outermost one or two layers of cortical cells and grow by plant cell expansion
and division, progressing through well-defined developmental stages. Determinate
nodules usually adopt a globular shape and are formed on Lotus sp., Phaseolus sp.,
G. max, and a number of tropical legumes. The mature nodules contain a homogenous central tissue composed of infected cells fully packed with N 2 -fixing bacteroids
and some uninfected cells. Senescence in these nodules occurs radially, beginning
at the center and extending to the periphery [7]. Indeterminate nodules have
a persistent meristem and elongate, to become cylindrical. New nodule cells are
gradually infected by rhizobia residing in the nodule; this produces more cylindrical
mature nodules separated into distinct developmental zones (Fig. 1): zone I is made
of meristematic cells; zone II is where cells are infected by bacteria which differentiate into bacteroids; zone III is where bacteroids reduce N 2 into ammonia which
is exported to the plant; and zone IV is characterized by the disruption of the
partnership and the onset of senescence [45, 114]. Medicago sp., Vicia sp., Trifolium
sp., P. sativum, and Astragalus are typical legumes with indeterminate nodules [31,
76]. In contrast to bacteroids in determinate nodules, those from indeterminate
nodules have lost their capacity to reproduce [7].
In mature nodules, compatible rhizobia differentiate into bacteroids that express
the enzymes of the nitrogenase complex and begin to fix nitrogen. N 2 reduction
by the bacteroid nitrogenase is the core reaction of the symbiotic process [63].
Incompatible host-strain interactions can also lead to formation of nodules, but the
latter are defective in nitrogen fixation [111]. N 2 -fixation defective phenotype was
not due to a lack of infection but caused by bacteroid degradation after differentiation [9, 12].
Bacteroids receive carbon as dicarboxylates from legumes, and in exchange,
they fix N 2 in a low O 2 environment and secrete ammonia to the plant. To effective
N 2 fixation, bacteroids must balance electron flow to nitrogenase, lipids, polyhydroxybutyrate, and O 2 and coordinate this process with reductant production by
the tricarboxylic acid cycle [74]. In nodules, bacteroids are provided with microaerobic environment required for expression of enzymes of the nitrogenase complex,
which is located on the internal membrane of bacteroids. The nitrogenase reaction
is complex and energetically expensive, since the reduction by nitrogenase of
1 molecule of N 2 to 2 molecules of NH 4
+ requires 16 molecules of ATP and
8 electrons [6, 10]. Paradoxically, despite the N 2 -fixation process requirement of
high O 2 levels, the nitrogenase is an O 2 -sensitive enzyme. Maintaining a very low
concentration of free O 2 is achieved by the presence of leghemoglobin (Lb), a plant13 Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes. . .
303