Legume-Rhizobium - microbewiki
Legume sanctions maintain Rhizobium mutualism · Current list of rhizobia Nitrogen Fixation and Inoculation of Forage Legumes. The legumes and their association with Rhizobiumspp. in the broad sense have always been extremely important agronomically. The use of crop rotations to. symbiosis with legume plants symbiotic relationships to.
Structure of nitrogen-fixing root nodules formed in S. The different nodules zones are indicated on the longitudinal nodule section: Symbiotic cells in zone II contain the differentiating endosymbionts while in zone III the host cytoplasm is fully packed with long nitrogen-fixing bacteroids. Endosymbionts stained with Syto9 have green fluorescence. Growth of Symbiotic Cells Involves Amplification of the Host Genome by Endoreduplication Cycles Extreme plant cell enlargement can be observed in both the determinate and indeterminate nodules.
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.
Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate
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. 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. Rhizobia colonize the soil in the vicinity of the root hair in response to the flavonoids.
This process is autoregulated where favonoids stimulate Nod factor production, which stimulates flavonoid secretion Russelle, Response to Nod factors is extremely rapid and the disruption of cell wall happens very quickly.
Disruption of crystallization of cell walls take place, thereby allowing entrance by the rhizobia. At the same time Rhizobia multiply in the rhizosphere. The root hair is then stimulated and curls to the side where the bacteria are attached which stimulates cell division in the root cortex. A "shepherd's crook" is formed and entraps the rhizobia which then erode the host cell wall and enter near the root hair tip.
An infection thread is formed as rhizobia digest the root hair cell wall. Free-living Rhizobium bacteria are converted to bacteroids as the infection elongates by tip growth down root hair and toward epidermal cells. Infection thread branches and heads toward the cortex and a visibly evident nodule develops on the root as the plant produces cytokinin and cells divide.
Nodules can contain one or more rhizobial strains and can be either determinant lack a persistent meristem and are spherical or indeterminate located at the distal end of cylindrically shaped lobes Russelle, Many infections are aborted due to a breakdown in communication between rhizobia and the host plant leaving nodule number strictly regulated by the plant. Once inside the nodule, rhizobia are released from the infection thread in a droplet of polysaccharide.
A plant-derived peribacteroid membrane, which regulates the flow of compounds between the plant and bacteroidquickly develops around this droplet via endocytosis.
This process keeps the microbes "outside" the plant where the rhizobia are intracellular but extracytoplasmic Russelle, The loss of the ammonium assimilatory capacity by bacteroids is important for maintaining the symbiotic relationship with legumes. Niche The amount of N2 fixed depends on the soil population of bacterial symbionts, soil acidity, and often overlooked soil nitrogen availability. Nodulation will only be initiated when the plant is in low nitrogen status.
Rhizobium populations are sensitive to changes in environmental conditions. Favorable Environment A balanced pH with high levels of nutrients and good physical properties is favored by rhizobia. A variety of C and N compounds can be utilized by rhizobia. A single rhizobial cell in a favorable environment can infect a root hair and generate progeny Russelle, Unfavorable Environment Rhizobia can be reduced in numbers by strong soil acidity which has high hydrogen ion concentration.
Plant growth can also be limited by toxic levels of aluminum and manganese. A reduction in rhizobial pools can be due to nutrient limitations including deficiencies in calcium, phosphorus, and molybdenum, low or high soil temperatures rhizobia are mesophilesand poor soil physical properties that restrict aeration and moisture supply. Soil acidity reduces nodulation and overall N2 fixation.
Soil nitrate concentration and phosphorus concentration can also affect rhizobia populations. Furthermore, indegenous rhizobial populations are maintained over time and depend on how often host plants are grown and on competitive ability of different rhiboia strains Furseth et al.
Hundreds of microbial species can fix nitrogen as most nitrogen fixing prokaryotes are free-living organisms or associate with plants.
Unfortunately, the environmental factors have to optimal for many of the aerobic, microaerobic, anaerobic, and even photosynthetic bacteria, and actinomycetes to be reliable sources of nitrogen fixation. Evans and Barber, The rates of N2 fixation, unfortunatley, cannot be measured accurately LaRue and Patterson, Legumes prefer to take up available soil nitrogen from soil solution as fixation by bacteria is expensive to the plant.
N2 fixation is constrained in many agricultural soils where nitrogen levels are high from routine addition of fertilizer. Nitrate in the soil reduces fixation where nitrate reduction uses photosynthate. Role of Nitrogenase Nitrogenase is the actual enzyme responsible for conversion of N2 to ammonium. Nitrogenase exists in three forms that include molybdenum nitrogenase, vanadium nitrogenase, and iron nitrogenase.