Rhizobia are unique in that they are the only nitrogen-fixing bacteria living in a symbiotic relationship with legumes. Nitrogen Fixation Root Hair Nodule Development Infection Thread Legume Root Roth J, Stacey G, Newton W (eds) Nitrogen fixation: achievements and objectives. Mellor RB () Bacteroids in the Rhizobium-legume symbiosis inhabit a. One striking feature of the legume – rhizobial symbiosis is its high . by compatible legume hosts for the purpose of symbiosis development.
RP5 enhancement of Ni and Zn bioavailability to leguminous plants through the secretion of siderophores Wani et al. A variety of metal loid resistance genes as well as genes reported to promote host plant growth were identified in a draft genome sequence, indicating their potential for aiding phytoremediation Hao et al.
For example, inoculation with Bradyrhizobium sp. Mixed systems with multiple plant species or microbial symbionts have also been used for restoration, such as in PAH-spiked chrysene-amended agricultural soil Johnson et al. Moreover, different legume species respond distinctly to the inoculation of metal-resistant rhizobia. Cadmium concentrations in the roots of L.
YL-6, but significantly decreased in the roots and shoots of Glycine max Guo and Chi, Moreover, Camargo et al. Other Strategies to Address Recalcitrant Pollutants Optimization of Pollutant-Degrading Microbial Consortia The augmentation of the diversity and richness of degrading microbial consortia in contaminated sites has been regarded as one of the key reasons that rhizobia enhance the biodegradation of organic pollutants.
By manipulating sterile and non-sterile soils, Johnson et al. Many studies have suggested that the symbiotic association between alfalfa and rhizobia i. Rhizobia have the potential to directly modify rhizosphere microflora by improving environmental conditions and nutrient availability. Nitrogen is a major limiting factor in bioremediation and is often added to contaminated soils to stimulate the existing microbial communities Terrence et al.
Thus, the organic nitrogen resource fixed by rhizobia is one of the key factors that facilitates the growth and activity of other soil biodegraders.
That being said, changes in the amounts and constituents of root exudates i. Synergistic Interactions With Other Microbes The collaboration between multiple beneficial microbes has been exploited for more comprehensive and sustainable rehabilitation. Simultaneous inoculation of multiple beneficial microbes often provides complementary and additive benefits to plants, revealing the compatibility, and synergy between distinct mutualists Larimer et al.
The synergistic promotion of plant biomass and activities of indigenous microbial species caused by dual colonization of rhizobia and other microbes could be a potent tool to further intensify bioremediation efficiency. For example, arbuscular mycorrhizal fungus AMF could form extended mycelial networks that not only provide organic nitrogen to host plants but also possess the catabolic capacity to remove organic pollutants Harms et al.
This could be due to their combined contribution to plant growth and development by improving nutrient conditions Saini et al. The positive synergism of these interactions could be limited by plant species and soil conditions. Additionally, combined approaches using both biostimulation with exogenous carbon sources and bioaugmentation may be necessary to sustain the timely and effective in situ microbial biodegradation of pollutants Andeer et al. However, questions remain due to the considerable complexity of the soil environment.
For example, the determination of the best method to establish appropriate numbers of foreign pollutant-degrading bacteria in the contaminated sites and to ensure that multiple degraders co-work during the entire rehabilitation process requires further studies Segura and Ramos, Bioengineering could potentially be used to manipulate the tolerance, accumulation, and degradation potentials of plants and microbes against pollutants.
Rhizobial transgenics can be harnessed for the accumulation of inorganic contaminants and detoxification of organic pollutants. The results from this study showed that the two recombinant strains accumulated more cadmium compared with the free-living cells.
When Astragalus sinicus was inoculated with the two recombinant strains, the increased cadmium accumulation in nodules was observed.
However, the accumulation of copper was not promoted by the expression of MTL4 in M. Genetic horizontal transfer of plasmid pJP4 that encodes genes for mercury resistance and 2,4-D degradation into Bradyrhizobium in non-sterile soil Kinkle et al. The work of Chen et al.
The dechlorination of PCBs by alfalfa inoculated with the S. Therefore, the effectiveness of phytostabilization could be improved by developing such downstream functional strategies. Studies have shown that the simultaneous expression of multiple genes related to degradation or metal accumulation exerts additive effects on the removal of pollutants Ike et al.
However, it is difficult to control the expression levels of the transferred genes in the recipient cells and the effect of certain individual genes may be limited to a narrow scope of impact as a consequence, thereby restricting its application. Further exploitation of the genomic databases of these rhizobia and the identification of the various functions of different genes are required in order to identify the most effective genes for bioremediation.
Conclusion Rhizobia have been recognized as a potential strategy to simultaneously enhance soil nitrogen content, reduce the use of fertilizers, and increase H2 concentration hydrogen fertilizers in the rhizosphere through symbiotic nitrogen fixation. Rhizobia also possess the biochemical and ecological capacity to degrade environmental organic chemicals and to decrease the risk associated with metals and metalloids in contaminated sites.
Rhizobia-assisted phytoremediation provides further environmental and economic benefits for bioremediation. The exploitation of microbe—microbe or plant—microbe interactions between intra-species and inter-species communication in the rhizosphere could represent more integrative approaches to further facilitate bioremediation.
Researchers have proposed that the wide adoption of these biological adaptation strategies would result in the development of environmentally friendly management techniques i. Symbiotic nitrogen fixation and the reductive dechlorination of organic pollutants are both oxygen-sensitive and energetically costly for rhizobia Morris and Schmidt, The presence of leghemoglobin maintains the low O2 concentrations in the root nodules within the nanomolar range and protects nitrogenase from inhibition by O2 Becker et al.
Therefore, researchers have proposed that the micro-oxic environment formed in nodules might also provide the proper conditions for the reductive degradation of organic pollutants. Some reports have provided evidence of NAR-mediated dechlorination of PCB in free-living cells of Phanerochaete chrysosporium and crude enzyme extracts from Medicago sativa leaves, even under aerobic conditions De et al.
However, Becker et al. Therefore, the involvement of metabolic enzymes in the potential degradation of organic compounds in rhizobia requires further study. Our understanding of the genetic and molecular influences of bioremediation effects is not complete, and the goal of transforming this strategy into practice has not yet been fully achieved.
The suitable selection of rhizobial strains or consortia in combination with plant hosts, indicators of successful bioremediation under field conditions and the mechanisms involved constitute future work that should be pursued for the initiation of successful efforts in this area. The successful execution of this versatile bioremediation strategy also requires a thorough understanding of the factors regulating the growth, metabolism, and functions of degradative rhizobia and indigenous microbial communities at contaminated sites.
The selection of suitable rhizobial strains will be necessary for the remediation of certain polluted sites. In conclusion, this review provides a comprehensive framework for applying the versatile rhizobia to revitalize contaminated soils. The selective introduction of degradative rhizobia into hyperaccumulator plants could facilitate the accelerated removal of mixed pollutants from soils.
Using this approach, the exploitation of these degradative, nitrogen-fixing and endophytic pollutant-cleaners could become a highly efficient, eco-friendly and low-input bioremediation technology for the future. Conflict of Interest Statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The cytoplasm of a nitrogen-fixing symbiotic cell hosts about 50, bacteroids. To accommodate such a high number of endosymbionts, the host cells grow.Legumes and Rhizobium bacteria - Sharon Long (Stanford)
The growth of infected cells occurs stepwise in zone II and is the consequence of repeated endoreduplication ER of the genome without mitosis. In zone II the cell cycle machinery is still active but the lack of mitotic cyclins inhibits mitosis and transforms the mitotic cycles to endoreduplication cycles Cebolla et al. This is achieved by the cell cycle switch CCS52A protein that by the destruction of the mitotic cyclins induces repeated rounds of genome duplication leading to the formation of gradually growing polyploid cells Roudier et al.
Interestingly, cortical cells containing AM fungi are also polyploid, as well as the nematode-feeding giant root cells Favery et al. Similarly, insect symbiotic cells, the bacteriocytes harboring intracellular endosymbionts are also large and polyploid Nakabachi et al. In angiosperm plants, polyploidy is frequent and the specific inherited pattern of polyploidy in different organs, tissues and cell types suggest that it could be a major source of the specialized physiology of host cells Nagl, ; Edgar et al.
Beside cell growth, the multiple gene copies, lack of chromosome condensation can contribute to higher transcriptional and metabolic activities. However, association of polyploidy with different cell functions suggests an impact of polyploidy also on the architecture of nucleosomes and on the epigenome controlling activation or repression of specific genomic regions.
Accordingly, the polyploid genome content of symbiotic cells appears to be a prerequisite for nodule differentiation and for the expression of most symbiotic host genes Maunoury et al.
Different Fates of Nitrogen Fixing Bacteroids The bacteria released from the IT are present in the host cytoplasm as organelle-like structures, called symbiosomes. The bacteria have no direct contact with cytoplasm as they are surrounded by a peribacteroid membrane, known also as symbiosome membrane SM.
The bacteroid, the SM and the space between them comprise the symbiosome Catalano et al. The SM during its formation reflects its plasma membrane origin, later modifications of its composition open new, specialized roles at the host-endosymbiont interface Limpens et al.
The bacteroids multiply in the growing host nodule cells to a certain cell density, adapt to the endosymbiotic life-style and microaerobic conditions and mature to nitrogen-fixing bacteroids. The form and physiology of bacteroids can be, however, strikingly different in the various legumes. In certain legume hosts, the nitrogen-fixing bacteroids have the same morphology as cultured cells; this type of bacteroids can revert to the free-living form.
In other associations, the bacteroids are irreversibly transformed to polyploid, enlarged, non-cultivable endosymbionts.
These terminally differentiated bacteroids can be elongated and even branched and 5- to fold longer than the free-living cells or can be spherical from 8 to at least fold amplified genome depending on the host Mergaert et al. Terminal differentiation of bacteroids is host controlled, evolved in multiple branches of the Leguminosae family indicating host advantage and likely higher symbiotic performance Oono et al.
Terminal bacteroid differentiation is the best elucidated in the S. Multiplication of bacteroids stops in the middle of zone II where cell elongation and uniform amplification of the multiple replicons by endoreduplication cycles begin.
Along 2—3 cell layers at the border of zone II and III called interzone sudden growth of bacteroids is visible reaching practically their final size, however, nitrogen-fixation takes place only in zone III. Host Peptides Govern Bacteroid Differentiation Comparison of nodule transcriptomes of legumes with reversible and irreversible bacteroid differentiation revealed the existence of several hundreds of small genes that were only present in the genome of those host plants where bacteroid differentiation was terminal.
The symPEP genes are only activated in the S. A large portion, more than genes encode nodule-specific cysteine-rich NCR peptides Mergaert et al. The NCR peptides are targeted to the bacteroids and when their delivery to the endosymbionts was blocked, bacteroid differentiation was abolished demonstrating that the peptides are responsible for terminal differentiation of S.
The high sequence variety and the characteristic expression patterns of NCR genes suggest diversity in their functions, modes of action and bacterial targets at different stages of bacteroid maturation Figure 2.
However, why does the host cell produce an arsenal of NCRs? What can be the advantage of such a diverse peptide repertoire? Is it necessary for interaction of the host with various bacteria? The symbiotic partners of M. While a nodule contains a single bacterium type, the different nodules on the same root system may possess distinct bacterial populations. It is possible that the plant recognizing the various endosymbionts manipulates them with a strain-specific repertoire of peptides.
These differences can add an additional control level for host-symbiont specificity and thereby for nodulation efficiency. Differential expression of symPEP genes in M. Horizontal Gene Transfer HGT has been accounted for most of the genetic drifts that empowered the evolution of genomes, particularly in bacteria. Recently after the publication of the full sequence of the human genome, the role of horizontal transfer of genes from bacteria to vertebrates has excited intensive research and heated discussion Stanhope et al.
Using vigorous phylogenetic analysis two possible explanations have been suggested: The use of several genes sequences in phylogenetic studies helps to establish consensus relationships from which it is possible to follow overall recombination, horizontal transfer and mutational events that shape the evolutionary history.
Comparative phylogeny can also be used to determine the extent of recombination, where congruence of gene trees strongly suggests clonality while significant divergence in trees can be accounted by recombination.
The majority of these genes that are located on the chromosome are not extensively used for rhizobial phylogeny; however, some rhizobial representatives included always fall in the expected a-proteobacteria group.
Concurrent evolution was observed in comparative phylogenies of nifH sequences and the 16S sequences Hennecke et al. Phylogenies of atpD and recA also support the Mesorhizobium and Sinorhizobium clades, however, the clustering of Rhizobium galegae with Agrobacterium is not as well supported as in 16S phylogeny.
These results clearly show that other genes also show the uncertain similarities observed in 16S sequences. Increased phylogenetic sampling of housekeeping genes and other ribosomal sequences may provide additional insight about the evolutionary history of R. Characters such as pathogenicity, antibiotic resistance, toxins, symbiotic elements, or genetic information transfer are known to be encoded within these elements.
Most of the rhizobial species harbour large plasmids that vary in number and size.
Rhizobia - Wikipedia
An overview of the fully sequenced symbiotic plasmids from Sinorhizobium sp. NGR Freiberg et al. In most cases genes for the symbiotic association are located within these plasmids commonly known as Sym plasmids. Acquisition and loss of these plasmids, which is a continuous and dynamic process, seem to play a major role in the evolution of rhizobia. Plasmid exchange is not restricted among the rhizobia only.
For instance, transfer of the Agrobacterium tumefaciens Ti plasmid to rhizobium ex planta, expression of the Sym plasmid of Rhizobium trifolii in different rhizobial species and Agrobacterium tumefaciens Hooykaas et al. Moreover, transfer of rhizobial plasmid to A.