4.3
Effects of Legume-Rhizobium Symbiosis on Heavy Metal
Toxicity
Elevated levels of heavy metals in soils have deleterious effects not only for plants
but also for soil microbiota. Heavy metals may cause changes in microbial composition and decrease beneficial activities of microsymbionts [156]. The decline
in plant growth and symbiosis was found in white clover plants, which were
grown in soils contaminated with cadmium, lead, and zinc [54, 157]. Many metals
(Cu, Ni, Zn, Cd, Cr, As) were found to inhibit the growth, morphology, and activities
of various symbiotic N 2 -fixing bacteria like R. leguminosarum, Mesorhizobium
ciceri, Bradyrhizobium sp. [53, 158–161]. A strong inhibitory effect of copper
on growth and enzyme activities of Bradyrhizobium BMP1 strain was found [59].
Hirsch and coauthors showed that R. leguminosarum bv. trifolii population was
significantly altered by long-term exposure to heavy metals, and this rhizobia lost the
ability to establish functional symbiosis with white and red clover [162]. In addition
to the toxicity of heavy metals on the growth and survival of Rhizobia, nodulation
defects in legumes were also observed [163]. Effective R. leguminosarum bv. trifolii
population did not survive during long-term incubation in soils containing 7.1 mg
Cd kg
À1 [62].
Heavy metals can disturb redox balance in both the host and rhizobia affecting
their growth and decreasing efficacy of legume-rhizobium symbiosis [28, 49, 50,
52–56]. Several studies have reported that nitrogen-fixing bacteria can diminish the
toxicity of heavy metals (Cd, Cr, Cu, Pb, etc.) on host plants, since the root nodules
can be the major accumulators of heavy metals from the soil [57–59, 62]. At the
same time, legume-rhizobium symbiosis seems to be also sensitive to heavy metals,
and its protective effects against metal toxicity are not fully clear. It should be noted
that resistance of the bacteria to heavy metals is both species- and strain-specific
[59]. We can surmise that rhizobia with powerful protective systems can be successfully used for effective soil bioremediation [28, 51, 58, 60–62].
5
Conclusions and Perspectives
Legume-rhizobium symbiosis seems to be a great example of plant and bacterial
coevolution. Root secretion and plant immunity are key factors determining interaction of rhizobia with plant roots. There are many members of the microbiota
in the soil, and rhizobia must compete with them before infecting legumes and
forming nitrogen-fixing bacteroids. The ability to respond to plant signals and
chemoattractants and to colonize root surfaces is crucial for the competitive success
of these bacteria. There tends to be strict species specificity between legumes and
their compatible symbionts. Genetic and molecular mechanisms that regulate symbiotic specificity are diverse, involving a wide range of host and bacterial genes/
signals with various modes of action. A variety of secondary metabolites released by
both the host plant and bacteria are involved in mutual recognition and nodule
development. In particular, the specificity is determined largely by the structure of
312
U. Y. Stambulska and M. M. Bayliak
Effects of Legume-Rhizobium Symbiosis on Heavy Metal
Toxicity
Elevated levels of heavy metals in soils have deleterious effects not only for plants
but also for soil microbiota. Heavy metals may cause changes in microbial composition and decrease beneficial activities of microsymbionts [156]. The decline
in plant growth and symbiosis was found in white clover plants, which were
grown in soils contaminated with cadmium, lead, and zinc [54, 157]. Many metals
(Cu, Ni, Zn, Cd, Cr, As) were found to inhibit the growth, morphology, and activities
of various symbiotic N 2 -fixing bacteria like R. leguminosarum, Mesorhizobium
ciceri, Bradyrhizobium sp. [53, 158–161]. A strong inhibitory effect of copper
on growth and enzyme activities of Bradyrhizobium BMP1 strain was found [59].
Hirsch and coauthors showed that R. leguminosarum bv. trifolii population was
significantly altered by long-term exposure to heavy metals, and this rhizobia lost the
ability to establish functional symbiosis with white and red clover [162]. In addition
to the toxicity of heavy metals on the growth and survival of Rhizobia, nodulation
defects in legumes were also observed [163]. Effective R. leguminosarum bv. trifolii
population did not survive during long-term incubation in soils containing 7.1 mg
Cd kg
À1 [62].
Heavy metals can disturb redox balance in both the host and rhizobia affecting
their growth and decreasing efficacy of legume-rhizobium symbiosis [28, 49, 50,
52–56]. Several studies have reported that nitrogen-fixing bacteria can diminish the
toxicity of heavy metals (Cd, Cr, Cu, Pb, etc.) on host plants, since the root nodules
can be the major accumulators of heavy metals from the soil [57–59, 62]. At the
same time, legume-rhizobium symbiosis seems to be also sensitive to heavy metals,
and its protective effects against metal toxicity are not fully clear. It should be noted
that resistance of the bacteria to heavy metals is both species- and strain-specific
[59]. We can surmise that rhizobia with powerful protective systems can be successfully used for effective soil bioremediation [28, 51, 58, 60–62].
5
Conclusions and Perspectives
Legume-rhizobium symbiosis seems to be a great example of plant and bacterial
coevolution. Root secretion and plant immunity are key factors determining interaction of rhizobia with plant roots. There are many members of the microbiota
in the soil, and rhizobia must compete with them before infecting legumes and
forming nitrogen-fixing bacteroids. The ability to respond to plant signals and
chemoattractants and to colonize root surfaces is crucial for the competitive success
of these bacteria. There tends to be strict species specificity between legumes and
their compatible symbionts. Genetic and molecular mechanisms that regulate symbiotic specificity are diverse, involving a wide range of host and bacterial genes/
signals with various modes of action. A variety of secondary metabolites released by
both the host plant and bacteria are involved in mutual recognition and nodule
development. In particular, the specificity is determined largely by the structure of
312
U. Y. Stambulska and M. M. Bayliak