Rhizobium bacteria and legumes symbiotic relationship

rhizobium bacteria and legumes symbiotic relationship

The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution . management of the symbiotic relationship between plants and bacteria. In microbiology, there are many examples of mutualistic bacteria in the gut that .. Legumes have a symbiotic relationship with bacteria called rhizobia, which. OtherUPDATE ON RHIZOBIA-LEGUME SYMBIOSIS Moreover, other plant families can establish interactions with nitrogen-fixing bacteria exclusive of rhizobia. have been utilized as well to study the relationships of angiosperm genera.

Ammonium is then converted into amino acids like glutamine and asparagine before it is exported to the plant. This process keeps the nodule oxygen poor in order to prevent the inhibition of nitrogenase activity. Nature of the mutualism[ edit ] The legume—rhizobium symbiosis is a classic example of mutualism —rhizobia supply ammonia or amino acids to the plant and in return receive organic acids principally as the dicarboxylic acids malate and succinate as a carbon and energy source.

rhizobium bacteria and legumes symbiotic relationship

However, because several unrelated strains infect each individual plant, a classic tragedy of the commons scenario presents itself. Cheater strains may hoard plant resources such as polyhydroxybutyrate for the benefit of their own reproduction without fixing an appreciable amount of nitrogen. The sanctions hypothesis[ edit ] There are two main hypotheses for the mechanism that maintains legume-rhizobium symbiosis though both may occur in nature.

The sanctions hypothesis theorizes that legumes cannot recognize the more parasitic or less nitrogen fixing rhizobia, and must counter the parasitism by post-infection legume sanctions. In response to underperforming rhizobia, legume hosts can respond by imposing sanctions of varying severity to their nodules. Within a nodule, some of the bacteria differentiate into nitrogen fixing bacteroids, which have been found to be unable to reproduce. This ability to reinforce a mutual relationship with host sanctions pushes the relationship toward a mutualism rather than a parasitism and is likely a contributing factor to why the symbiosis exists.

However, other studies have found no evidence of plant sanctions. There is evidence for sanctions in soybean plants, which reduce rhizobium reproduction perhaps by limiting oxygen supply in nodules that fix less nitrogen. Some studies support the partner choice hypothesis. The partner choice hypothesis is not exclusive from the host sanctions hypothesis, as it is apparent that both of them are prevalent in the symbiotic relationship. To understand the evolutionary history of this symbiosis, it is helpful to compare the rhizobia-legume symbiosis to a more ancient symbiotic relationship, such as that between endomycorrhizae fungi and land plants, which dates back to almost million years ago.

Instead the rhizobia simply needed to evolve mechanisms to take advantage of the symbiotic signaling processes already in place from endomycorrhizal symbiosis. Other diazotrophs[ edit ] Many other species of bacteria are able to fix nitrogen diazotrophsbut few are able to associate intimately with plants and colonize specific structures like legume nodules.

Bacteria that do associate with plants include the actinobacteria Frankiawhich form symbiotic root nodules in actinorhizal plantsalthough these bacteria have a much broader host range implying the association is less specific than in legumes.

Diazotrophic bacterial endophytes have very broad host ranges, in some cases colonizing both monocots and dicots. The cytoplasm of a nitrogen-fixing symbiotic cell hosts about 50, bacteroids. To accommodate such a high number of endosymbionts, the host cells grow. 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.

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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.

rhizobium bacteria and legumes symbiotic relationship

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.

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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.

rhizobium bacteria and legumes symbiotic relationship

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.

rhizobium bacteria and legumes symbiotic relationship

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?

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What can be the advantage of such a diverse peptide repertoire? Is it necessary for interaction of the host with various bacteria?

rhizobium bacteria and legumes symbiotic relationship

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. AMPs with broad spectrum of microbial cell-killing activity are most frequently cationic provoking cell death by pore formation, membrane disruption and consequent lysis of microbial cells. The fact that the cell division ability is definitively lost during endosymbiont differentiation indicates that at least certain symPEPs have antimicrobial activities.

Treatment of bacteria with synthetic cationic NCRs indeed provoked rapid and efficient dose-dependent elimination of various Gram-negative and Gram-positive bacteria including important human and plant pathogens Van de Velde et al. This ex-planta killing effect correlated with permeabilization of microbial membranes, however, symPEPs in their natural environment — in the nodule cells — do not permeabilize the bacterial membranes and do not kill the endosymbionts.

Most likely the peptide concentrations in the nodules are significantly lower than those applied in the in vitro assays.

Moreover cationic peptides are produced together with anionic and neutral peptides in the same cell, and possible combination of a few tens or hundreds of peptides with various charge and hydrophobicity might neutralize the direct bactericidal effect of the cationic peptides. In the weevil Sitophilus, the symbiotic cells produce the antimicrobial peptide coleoptericin-A ColA which provokes the development of giant filamentous endosymbionts by inhibiting cell division and protects the neighboring insect tissues from bacterial invasion Login et al.

In this system a single peptide is sufficient for differentiation of the obligate vertically transmitted endosymbiont unlike nodules that operate with hundreds of symPEPs and can host innumerable strain variants as their endosymbionts.

In the aphid-Buchnera symbiosis, the host cells also produce bacteriocyte-specific peptides including cysteine rich peptides BCRs which resemble the Medicago NCR peptides, however the functions of these symbiotic peptides have not been reported yet Shigenobu and Stern, NCR is expressed in the older cell layers of zone II and in the interzone where bacterial cell division stops and remarkable elongation of the endosymbionts occurs Farkas et al.

This small cationic peptide effectively killed various microbes in vitro and the in silico analysis indicated its extreme protein binding capacities. FITC-labeled NCR entered the bacterial cytosol where its interactions with numerous bacterial proteins were possible. Binding partners were identified by treatment of S.

One of the interactors was the FtsZ cell division protein playing a crucial primary role in cell division.

A number of antibiotic peptides are known to exert bactericidal or bacteriostatic effect through the interaction with FtsZ, inhibiting its polymerization thereby hindering proper Z-ring and septum formation Handler et al. NCR was co-purified with FtsZ from the bacterial cytoplasm and was shown to disrupt septum formation. NCR exhibiting in vitro also bactericidal effect and produced in the same symbiotic cells as NCR accumulates at the division septum which indicates simultaneous or consecutive action of these peptides and evolution of multiple host strategies to inhibit endosymbiont proliferation.

Another study showed that expression of important cell division genes, including genes required for Z-ring function, were strongly attenuated in cells treated by NCR Penterman et al.

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Ribosomal proteins were the most abundant NCR interacting partners.