symbiosis seems to be also sensitive to heavy metals, and its protective effects
against metal toxicity are not fully clear.
This chapter summarizes recent data on the role of signal secondary metabolites
in the initiation and functioning of symbiosis between rhizobia and their hosts,
Fabaceae family plants. Furthermore, we analyze the involvement of ROS/RNS
in both the establishment of effective legume-rhizobium symbiosis and the toxicity
of heavy metals to legume-rhizobium symbiosis. Perspectives of using rhizobia
for remediation of metal contaminated soils are also discussed.
2
The Establishment of Symbiotic Interaction Between
Rhizobia and Leguminous Plants
2.1
Nodule Development
Development of legume-rhizobium symbiosis occurs through a few coordinated
stages, including (Fig. 1) i) preinfection stage, in which both partners produce
chemical signals for mutual recognition and as a result bacteria are attached to the
cell wall of a root hair; (ii) infection process leading to root hair curling and
development of infection threads within the root hair for transport of bacteria to
nodule cells; (iii) formation of nodules, specialized root organs, within which the
bacteria further differentiate into nitrogen-fixing bacteroids; and (iv) functioning the
mature nodules, their senescence and necrosis [3, 7, 12, 19–21, 44]. Each stage
in legume-rhizobium symbiosis is regulated by signals from both the nodule bacteria
and their host plant [1, 5, 14, 17, 19, 20, 44, 63].
The host plant and rhizobia first establish contact with each other at the surface
of the growing tip of a root hair. Rhizobia can persist at low levels as free-living, soil
saprophyte bacteria in the absence of a suitable host plant. If the appropriate host
appears, the legume-rhizobium symbiosis starts with a complex signal exchange
between the host plant and its symbiotic bacteria [12, 23, 64]. Chemical compounds
secreted by both partners play the main role in this early stage of communication.
In particular, plant roots secrete flavonoids, which induce synthesis of rhizobial Nod
factors – a specific group of lipochitooligosaccharides. The latter serve as signaling
molecules that should be appropriately recognized by the host plant [1, 8, 63,
65]. If the initial contact is successful, the root hair curls to trap a small number of
bacteria. This is accompanied by local hydrolysis of the cell wall of the root hair to
allow bacteria to infect plant root cells [66, 67]. From this trap site, the root hair
begins an inverse tip growth, forming a long and narrow passage, called the infection
thread, in which the bacteria “travel” by continuously dividing at the leading edge
[65, 68, 69]. Infection threads are progressive ingrowths of plant cell membranes
containing a matrix composed of plant cell wall material [31].
While the bacteria enter the root hair, host cells in the root cortex restore
properties of stem cells, which undergo active division [7, 70]. It increases
a population of newly generated cells (nodule cells), which form a new root organ,
the nodule (Fig. 2).
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U. Y. Stambulska and M. M. Bayliak
against metal toxicity are not fully clear.
This chapter summarizes recent data on the role of signal secondary metabolites
in the initiation and functioning of symbiosis between rhizobia and their hosts,
Fabaceae family plants. Furthermore, we analyze the involvement of ROS/RNS
in both the establishment of effective legume-rhizobium symbiosis and the toxicity
of heavy metals to legume-rhizobium symbiosis. Perspectives of using rhizobia
for remediation of metal contaminated soils are also discussed.
2
The Establishment of Symbiotic Interaction Between
Rhizobia and Leguminous Plants
2.1
Nodule Development
Development of legume-rhizobium symbiosis occurs through a few coordinated
stages, including (Fig. 1) i) preinfection stage, in which both partners produce
chemical signals for mutual recognition and as a result bacteria are attached to the
cell wall of a root hair; (ii) infection process leading to root hair curling and
development of infection threads within the root hair for transport of bacteria to
nodule cells; (iii) formation of nodules, specialized root organs, within which the
bacteria further differentiate into nitrogen-fixing bacteroids; and (iv) functioning the
mature nodules, their senescence and necrosis [3, 7, 12, 19–21, 44]. Each stage
in legume-rhizobium symbiosis is regulated by signals from both the nodule bacteria
and their host plant [1, 5, 14, 17, 19, 20, 44, 63].
The host plant and rhizobia first establish contact with each other at the surface
of the growing tip of a root hair. Rhizobia can persist at low levels as free-living, soil
saprophyte bacteria in the absence of a suitable host plant. If the appropriate host
appears, the legume-rhizobium symbiosis starts with a complex signal exchange
between the host plant and its symbiotic bacteria [12, 23, 64]. Chemical compounds
secreted by both partners play the main role in this early stage of communication.
In particular, plant roots secrete flavonoids, which induce synthesis of rhizobial Nod
factors – a specific group of lipochitooligosaccharides. The latter serve as signaling
molecules that should be appropriately recognized by the host plant [1, 8, 63,
65]. If the initial contact is successful, the root hair curls to trap a small number of
bacteria. This is accompanied by local hydrolysis of the cell wall of the root hair to
allow bacteria to infect plant root cells [66, 67]. From this trap site, the root hair
begins an inverse tip growth, forming a long and narrow passage, called the infection
thread, in which the bacteria “travel” by continuously dividing at the leading edge
[65, 68, 69]. Infection threads are progressive ingrowths of plant cell membranes
containing a matrix composed of plant cell wall material [31].
While the bacteria enter the root hair, host cells in the root cortex restore
properties of stem cells, which undergo active division [7, 70]. It increases
a population of newly generated cells (nodule cells), which form a new root organ,
the nodule (Fig. 2).
294
U. Y. Stambulska and M. M. Bayliak