bacterial Nod factors, whose synthesis is induced by plant flavonoids. A variety of
rhizobial cell-surface exopolysaccharides and lipopolysaccharides and secreted proteins appear to function as signals from the rhizobia to its host. Recent studies have
just begun to disclose the underlying molecular mechanisms that regulate this
specificity, and there are many challenging questions waiting to be answered.
Increasing evidence has shown that ROS/RNS, especially H 2 O 2 and
•
NO, play an
important signaling role in the establishment of legume-rhizobium symbiosis, the
functioning and senescence steps in mature nodules. Changes in the levels of these
reactive species in both partners impair either the development of the nodules or their
N 2 -fixing activity. At low levels, ROS/RNS activate expression of genes involved
in progression of infection threads, nodule development, and differentiation of
bacteroids. High levels of ROS/RNS lead to the development of intense oxidative
stress and accelerated senescence of nodules. Since nitrogenase of bacteroids is very
sensitive to oxidation, enhanced ROS/RNS levels may decrease N 2 -fixing activity of
bacteroids. Elevated levels of heavy metals in soils can increase significantly ROS
production in both legumes and their microsymbionts. In legume-rhizobium symbiosis, rhizobia seem to be more stressed due to preferential accumulation of heavy
metals in root nodules. Data available suggest that using rhizobia can be an effective
approach to minimize toxic effects heavy metals have on agricultural plants. At the
same time, the protective efficacy of nodule bacteria depends on many factors such
as type and concentrations of heavy metals, compatibility of partners, bacterial
virulence, adaptive capacity of both partners, N 2 -fixing activity of bacteria, etc.
Therefore, the study of effects of heavy metal on legume-rhizobium symbiosis and
search of ways to enhance metal resistance of nodule bacteria are perspective
directions for future research. The genetic construction of rhizobia strains better
adapted to field conditions and with enhanced stress resistance and compatibility
with legumes may be a great opportunity to increase the benefit from their use
in bioremediation of soils polluted with heavy metals.
References
1. Gibson KE, Kobayashi H, Walker GC (2008) Molecular determinants of a symbiotic chronic
infection.
Annu
Rev
Genet
42:413–441.
https://doi.org/10.1146/annurev.
genet.42.110807.091427
2. Dos Santos P, Fang Z, Mason SW, Setubal JC, Dixon R (2012) Distribution of nitrogen
fixation and nitrogenase-like sequences amongst microbial genomes. BMC Genomics
13:1–12. https://doi.org/10.1186/1471-2164-13-162
3. Mus F, Crook MB, Garcia K, Garcia Costas A, Geddes BA, Kouri ED, Paramasivan P,
Ryu M-H, Oldroyd GED, Poole PS, Udvardi MK, Voigt CA, Ané J-M, Peters JW (2016)
Symbiotic nitrogen fixation and the challenges to its extension to nonlegumes. Appl Environ
Microbiol 82:3698–3710. https://doi.org/10.1128/AEM.01055-16
4. Martinez-Romero E (2009) Controversies in science coevolution in Rhizobium-legume
symbiosis? DNA Cell Biol 28:361–370. https://doi.org/10.1126/science.ns-21.524.95-c
5. Coba de la Peña T, Fedorova E, Pueyo JJ, Lucas MM (2018) The symbiosome: legume and
rhizobia co-evolution toward a nitrogen-fixing organelle? Front Plant Sci 8:1–26. https://doi.
org/10.3389/fpls.2017.02229
13 Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes. . .
313
rhizobial cell-surface exopolysaccharides and lipopolysaccharides and secreted proteins appear to function as signals from the rhizobia to its host. Recent studies have
just begun to disclose the underlying molecular mechanisms that regulate this
specificity, and there are many challenging questions waiting to be answered.
Increasing evidence has shown that ROS/RNS, especially H 2 O 2 and
•
NO, play an
important signaling role in the establishment of legume-rhizobium symbiosis, the
functioning and senescence steps in mature nodules. Changes in the levels of these
reactive species in both partners impair either the development of the nodules or their
N 2 -fixing activity. At low levels, ROS/RNS activate expression of genes involved
in progression of infection threads, nodule development, and differentiation of
bacteroids. High levels of ROS/RNS lead to the development of intense oxidative
stress and accelerated senescence of nodules. Since nitrogenase of bacteroids is very
sensitive to oxidation, enhanced ROS/RNS levels may decrease N 2 -fixing activity of
bacteroids. Elevated levels of heavy metals in soils can increase significantly ROS
production in both legumes and their microsymbionts. In legume-rhizobium symbiosis, rhizobia seem to be more stressed due to preferential accumulation of heavy
metals in root nodules. Data available suggest that using rhizobia can be an effective
approach to minimize toxic effects heavy metals have on agricultural plants. At the
same time, the protective efficacy of nodule bacteria depends on many factors such
as type and concentrations of heavy metals, compatibility of partners, bacterial
virulence, adaptive capacity of both partners, N 2 -fixing activity of bacteria, etc.
Therefore, the study of effects of heavy metal on legume-rhizobium symbiosis and
search of ways to enhance metal resistance of nodule bacteria are perspective
directions for future research. The genetic construction of rhizobia strains better
adapted to field conditions and with enhanced stress resistance and compatibility
with legumes may be a great opportunity to increase the benefit from their use
in bioremediation of soils polluted with heavy metals.
References
1. Gibson KE, Kobayashi H, Walker GC (2008) Molecular determinants of a symbiotic chronic
infection.
Annu
Rev
Genet
42:413–441.
https://doi.org/10.1146/annurev.
genet.42.110807.091427
2. Dos Santos P, Fang Z, Mason SW, Setubal JC, Dixon R (2012) Distribution of nitrogen
fixation and nitrogenase-like sequences amongst microbial genomes. BMC Genomics
13:1–12. https://doi.org/10.1186/1471-2164-13-162
3. Mus F, Crook MB, Garcia K, Garcia Costas A, Geddes BA, Kouri ED, Paramasivan P,
Ryu M-H, Oldroyd GED, Poole PS, Udvardi MK, Voigt CA, Ané J-M, Peters JW (2016)
Symbiotic nitrogen fixation and the challenges to its extension to nonlegumes. Appl Environ
Microbiol 82:3698–3710. https://doi.org/10.1128/AEM.01055-16
4. Martinez-Romero E (2009) Controversies in science coevolution in Rhizobium-legume
symbiosis? DNA Cell Biol 28:361–370. https://doi.org/10.1126/science.ns-21.524.95-c
5. Coba de la Peña T, Fedorova E, Pueyo JJ, Lucas MM (2018) The symbiosome: legume and
rhizobia co-evolution toward a nitrogen-fixing organelle? Front Plant Sci 8:1–26. https://doi.
org/10.3389/fpls.2017.02229
13 Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes. . .
313