fixation. Both rhizobia and host legumes exhibit a strong specificity, which can
be a result of their coevolution. Symbiotic specificity is provided by the complex
exchange of signals between both symbiotic partners. To initiate symbiosis,
legumes produce a cocktail of flavonoids that trigger synthesis and secretion
of bacterial lipochgitooligosaccharide molecules called Nod factors. Nod factors
together with surface polysaccharides and secreted proteins are proposed to be
major rhizobial determinants of host specificity. Much evidence suggests that
reactive oxygen species (ROS) play a key role in the formation and functioning of
legume-rhizobium symbiosis. Elevated levels of heavy metals in soils can affect
rhizobial growth and host legumes as well as impair legume-rhizobium symbiosis, in particular due to enhanced ROS production. On the other hand, if plants
form symbiosis with rhizobia, heavy metals are accumulated preferentially
in nodules that can be one of the possible ways to reduce toxic effects of heavy
metals to legumes.
Keywords
Rhizobium · Chemotaxis · Nodule development, Nod factors · Surface
polysaccharides · Phytohormones · ROS · Oxidative stress · Bioremediation
Abbreviations
EPS Extracellular polysaccharide
IAA Indole-3-acetic acid
Lb
Leghemoglobin
LPS Lipopolysaccharide
RNS Reactive nitrogen species
ROS Reactive oxygen species
1
Introduction
Nitrogen is an essential element for all living organisms, including plants. It is
a component of main cellular macromolecules, such as proteins and nucleic acids,
and low-molecular mass compounds like chlorophylls, amines, and vitamins.
On Earth, most of nitrogen was found in the inaccessible form of atmospheric
nitrogen gas (N 2 ). Biological fixation plays an important role in the conversion of
chemically inert N 2 into metabolically active ammonia (NH 3 ), which can be utilized
by plants in different ways. The ability to convert N 2 to NH 3 has evolved only among
prokaryotes called collectively diazotrophs. The latter include both free-living (e.g.,
azobacteria and cyanobacteria) and symbiotic nitrogen fixators (rhizobia) [1–3].
During evolution, some plant species, especially from Fabaceae family, have
developed a complex relationship with rhizobia to receive benefits under nitrogenlimiting soil conditions. Formation of nitrogen-fixing nodules on the plant root
seems to be a result of coevolution of legumes and rhizobia. Plants, obviously,
influenced more evolution of nodule bacteria, than bacteria did. Bacterial
genetic plasticity may be indicative of the large capacity of rhizobia to adapt to
292
U. Y. Stambulska and M. M. Bayliak
be a result of their coevolution. Symbiotic specificity is provided by the complex
exchange of signals between both symbiotic partners. To initiate symbiosis,
legumes produce a cocktail of flavonoids that trigger synthesis and secretion
of bacterial lipochgitooligosaccharide molecules called Nod factors. Nod factors
together with surface polysaccharides and secreted proteins are proposed to be
major rhizobial determinants of host specificity. Much evidence suggests that
reactive oxygen species (ROS) play a key role in the formation and functioning of
legume-rhizobium symbiosis. Elevated levels of heavy metals in soils can affect
rhizobial growth and host legumes as well as impair legume-rhizobium symbiosis, in particular due to enhanced ROS production. On the other hand, if plants
form symbiosis with rhizobia, heavy metals are accumulated preferentially
in nodules that can be one of the possible ways to reduce toxic effects of heavy
metals to legumes.
Keywords
Rhizobium · Chemotaxis · Nodule development, Nod factors · Surface
polysaccharides · Phytohormones · ROS · Oxidative stress · Bioremediation
Abbreviations
EPS Extracellular polysaccharide
IAA Indole-3-acetic acid
Lb
Leghemoglobin
LPS Lipopolysaccharide
RNS Reactive nitrogen species
ROS Reactive oxygen species
1
Introduction
Nitrogen is an essential element for all living organisms, including plants. It is
a component of main cellular macromolecules, such as proteins and nucleic acids,
and low-molecular mass compounds like chlorophylls, amines, and vitamins.
On Earth, most of nitrogen was found in the inaccessible form of atmospheric
nitrogen gas (N 2 ). Biological fixation plays an important role in the conversion of
chemically inert N 2 into metabolically active ammonia (NH 3 ), which can be utilized
by plants in different ways. The ability to convert N 2 to NH 3 has evolved only among
prokaryotes called collectively diazotrophs. The latter include both free-living (e.g.,
azobacteria and cyanobacteria) and symbiotic nitrogen fixators (rhizobia) [1–3].
During evolution, some plant species, especially from Fabaceae family, have
developed a complex relationship with rhizobia to receive benefits under nitrogenlimiting soil conditions. Formation of nitrogen-fixing nodules on the plant root
seems to be a result of coevolution of legumes and rhizobia. Plants, obviously,
influenced more evolution of nodule bacteria, than bacteria did. Bacterial
genetic plasticity may be indicative of the large capacity of rhizobia to adapt to
292
U. Y. Stambulska and M. M. Bayliak