A glycan receptor kinase facilitates intracellular accommodation of arbuscular mycorrhiza and symbiotic rhizobia in the legume Lotus japonicus.

Receptors that distinguish the multitude of microbes surrounding plants in the environment enable dynamic responses to the biotic and abiotic conditions encountered. In this study, we identify and characterise a glycan receptor kinase, EPR3a, closely related to the exopolysaccharide receptor EPR3. Epr3a is up-regulated in roots colonised by arbuscular mycorrhizal (AM) fungi and is able to bind glucans with a branching pattern characteristic of surface-exposed fungal glucans. Expression studies with cellular resolution show localised activation of the Epr3a promoter in cortical root cells containing arbuscules. Fungal infection and intracellular arbuscule formation are reduced in epr3a mutants. In vitro, the EPR3a ectodomain binds cell wall glucans in affinity gel electrophoresis assays. In microscale thermophoresis (MST) assays, rhizobial exopolysaccharide binding is detected with affinities comparable to those observed for EPR3, and both EPR3a and EPR3 bind a well-defined ß-1,3/ß-1,6 decasaccharide derived from exopolysaccharides of endophytic and pathogenic fungi. Both EPR3a and EPR3 function in the intracellular accommodation of microbes. However, contrasting expression patterns and divergent ligand affinities result in distinct functions in AM colonisation and rhizobial infection in Lotus japonicus. The presence of Epr3a and Epr3 genes in both eudicot and monocot plant genomes suggest a conserved function of these receptor kinases in glycan perception.

Phosphatidylcholine-deficient suppressor mutant of Sinorhizobium meliloti, altered in fatty acid synthesis, partially recovers nodulation ability in symbiosis with alfalfa (Medicago sativa).

Rhizobial phosphatidylcholine (PC) is thought to be a critical phospholipid for the symbiotic relationship between rhizobia and legume host plants. A PC-deficient mutant of Sinorhizobium meliloti overproduces succinoglycan, is unable to swim, and lacks the ability to form nodules on alfalfa (Medicago sativa) host roots. Suppressor mutants had been obtained which did not overproduce succinoglycan and regained the ability to swim. Previously, we showed that point mutations leading to altered ExoS proteins can reverse the succinoglycan and swimming phenotypes of a PC-deficient mutant. Here, we report that other point mutations leading to altered ExoS, ChvI, FabA, or RpoH1 proteins also revert the succinoglycan and swimming phenotypes of PC-deficient mutants. Notably, the suppressor mutants also restore the ability to form nodule organs on alfalfa roots. However, nodules generated by these suppressor mutants express only low levels of an early nodulin, do not induce leghemoglobin transcript accumulation, thus remain white, and are unable to fix nitrogen. Among these suppressor mutants, we detected a reduced function mutant of the 3-hydoxydecanoyl-acyl carrier protein dehydratase FabA that produces reduced amounts of unsaturated and increased amounts of shorter chain fatty acids. This alteration of fatty acid composition probably affects lipid packing thereby partially compensating for the previous loss of PC and contributing to the restoration of membrane homeostasis.

Timely symbiosis: circadian control of legume-rhizobia symbiosis.

Legumes house nitrogen-fixing endosymbiotic rhizobia in specialised polyploid cells within root nodules. This results in a mutualistic relationship whereby the plant host receives fixed nitrogen from the bacteria in exchange for dicarboxylic acids. This plant-microbe interaction requires the regulation of multiple metabolic and physiological processes in both the host and symbiont in order to achieve highly efficient symbiosis. Recent studies have showed that the success of symbiosis is influenced by the circadian clock of the plant host. Medicago and soybean plants with altered clock mechanisms showed compromised nodulation and reduced plant growth. Furthermore, transcriptomic analyses revealed that multiple genes with key roles in recruitment of rhizobia to plant roots, infection and nodule development were under circadian control, suggesting that appropriate timing of expression of these genes may be important for nodulation. There is also evidence for rhythmic gene expression of key nitrogen fixation genes in the rhizobium symbiont, and temporal coordination between nitrogen fixation in the bacterial symbiont and nitrogen assimilation in the plant host may be important for successful symbiosis. Understanding of how circadian regulation impacts on nodule establishment and function will identify key plant-rhizobial connections and regulators that could be targeted to increase the efficiency of this relationship.

An overview of Parafrankia (Nod+/Fix+) and Pseudofrankia (Nod+/Fix-) interactions through genome mining and experimental modeling in co-culture and co-inoculation of Elaeagnus angustifolia.

In many frankia, the ability to nodulate host plants (Nod+) and fix nitrogen (Fix+) is a common strategy. However, some frankia within the Pseudofrankia genus lack one or two of these traits. This phenomenon has been consistently observed across various actinorhizal nodule isolates, displaying Nod- and/or Fix- phenotypes. Yet, the mechanisms supporting the colonization and persistence of these inefficient frankia within nodules, both with and without symbiotic strains (Nod+/Fix+), remain unclear. It is also uncertain whether these associations burden or benefit host plants. This study delves into the ecological interactions between Parafrankia EUN1f and Pseudofrankia inefficax EuI1c, isolated from Elaeagnus umbellata nodules. EUN1f (Nod+/Fix+) and EuI1c (Nod+/Fix-) display contrasting symbiotic traits. While the prediction suggests a competitive scenario, the absence of direct interaction evidence implies that the competitive advantage of EUN1f and EuI1c is likely contingent on contextual factors such as substrate availability and the specific nature of stressors in their respective habitats. In co-culture, EUN1f outperforms EuI1c, especially under specific conditions, driven by its nitrogenase activity. Iron-depleted conditions favor EUN1f, emphasizing iron's role in microbial competition. Both strains benefit from host root exudates in pure culture, but EUN1f dominates in co-culture, enhancing its competitive traits. Nodulation experiments show that host plant preferences align with inoculum strain abundance under nitrogen-depleted conditions, while consistently favoring EUN1f in nitrogen-supplied media. This study unveils competitive dynamics and niche exclusion between EUN1f and EuI1c, suggesting that host plant may penalize less effective strains and even all strains. These findings highlight the complex interplay between strain competition and host selective pressure, warranting further research into the underlying mechanisms shaping plant-microbe-microbe interactions in diverse ecosystems. IMPORTANCE: While Pseudofrankia strains typically lack the common traits of ability to nodulate the host plant (Nod-) and/or fix nitrogen (Fix-), they are still recovered from actinorhizal nodules. The enigmatic question of how and why these unconventional strains establish themselves within nodule tissue, thriving either alongside symbiotic strains (Nod+/Fix+) or independently, while considering potential metabolic costs to the host plant, remains a perplexing puzzle. This study endeavors to unravel the competitive dynamics between Pseudofrankia inefficax strain EuI1c (Nod+/Fix-) and Parafrankia strain EU1Nf (Nod+/Fix+) through a comprehensive exploration of genomic data and empirical modeling, conducted both in controlled laboratory settings and within the host plant environment.

The native distribution of a common legume shrub is limited by the range of its nitrogen-fixing mutualist.

Plant-microbe mutualisms, such as the legume-rhizobium symbiosis, are influenced by the geographical distributions of both partners. However, limitations on the native range of legumes, resulting from the absence of a compatible mutualist, have rarely been explored. We used a combination of a large-scale field survey and controlled experiments to determine the realized niche of Calicotome villosa, an abundant and widespread legume shrub. Soil type was a major factor affecting the distribution and abundance of C. villosa. In addition, we found a large region within its range in which neither C. villosa nor Bradyrhizobium, the bacterial genus that associates with it, were present. Seedlings grown in soil from this region failed to nodulate and were deficient in nitrogen. Inoculation of this soil with Bradyrhizobium isolated from root nodules of C. villosa resulted in the formation of nodules and higher growth rate, leaf N and shoot biomass compared with un-inoculated plants. We present evidence for the exclusion of a legume from parts of its native range by the absence of a compatible mutualist. This result highlights the importance of the co-distribution of both the host plant and its mutualist when attempting to understand present and future geographical distributions of legumes.

Cell-layer specific roles for gibberellins in nodulation and root development.

Gibberellins (GA) have a profound influence on the formation of lateral root organs. However, the precise role this hormone plays in the cell-specific events during lateral root formation, rhizobial infection and nodule organogenesis, including interactions with auxin and cytokinin (CK), is not clear. We performed epidermal- and endodermal-specific complementation of the severely GA-deficient na pea (Pisum sativum) mutant with Agrobacterium rhizogenes. Gibberellin mutants were used to examine the spatial expression pattern of CK (TCSn)- and auxin (DR5)-responsive promoters and hormone levels. We found that GA produced in the endodermis promote lateral root and nodule organogenesis and can induce a mobile signal(s) that suppresses rhizobial infection. By contrast, epidermal-derived GA suppress infection but have little influence on root or nodule development. GA suppress the CK-responsive TCSn promoter in the cortex and are required for normal auxin activation during nodule primordia formation. Our findings indicate that GA regulate the checkpoints between infection thread (IT) penetration of the cortex and invasion of nodule primordial cells and promote the subsequent progression of nodule development. It appears that GA limit the progression and branching of IT in the cortex by restricting CK response and activate auxin response to promote nodule primordia development.

Rhizobium symbiotic efficiency meets CEP signaling peptides.

C-terminally encoded peptides (CEP) signaling peptides are drivers of systemic pathways regulating nitrogen (N) acquisition in different plants, from Arabidopsis to legumes, depending on mineral N availability (e.g. nitrate) and on the whole plant N demand. Recent studies in the Medicago truncatula model legume revealed how root-produced CEP peptides control the root competence for endosymbiosis with N fixing rhizobia soil bacteria through the activity of the Compact Root Architecture 2 (CRA2) CEP receptor in shoots. Among CEP genes, MtCEP7 was shown to be tightly linked to nodulation, and the dynamic temporal regulation of its expression reflects the plant ability to maintain a different symbiotic root competence window depending on the symbiotic efficiency of the rhizobium strain, as well as to reinitiate a new window of root competence for nodulation.

Is there a latitudinal diversity gradient for symbiotic microbes? A case study with sensitive partridge peas.

Mutualism is thought to be more prevalent in the tropics than temperate zones and may therefore play an important role in generating and maintaining high species richness found at lower latitudes. However, results on the impact of mutualism on latitudinal diversity gradients are mixed, and few empirical studies sample both temperate and tropical regions. We investigated whether a latitudinal diversity gradient exists in the symbiotic microbial community associated with the legume Chamaecrista nictitans. We sampled bacteria DNA from nodules and the surrounding soil of plant roots across a latitudinal gradient (38.64-8.68 °N). Using 16S rRNA sequence data, we identified many non-rhizobial species within C. nictitans nodules that cannot form nodules or fix nitrogen. Species richness increased towards lower latitudes in the non-rhizobial portion of the nodule community but not in the rhizobial community. The microbe community in the soil did not effectively predict the non-rhizobia community inside nodules, indicating that host selection is important for structuring non-rhizobia communities in nodules. We next factorially manipulated the presence of three non-rhizobia strains in greenhouse experiments and found that co-inoculations of non-rhizobia strains with rhizobia had a marginal effect on nodule number and no effect on plant growth. Our results suggest that these non-rhizobia bacteria are likely commensals-species that benefit from associating with a host but are neutral for host fitness. Overall, our study suggests that temperate C. nictitans plants are more selective in their associations with the non-rhizobia community, potentially due to differences in soil nitrogen across latitude.

Soluble humic acid suppresses plant immunity and ethylene to promote soybean nodulation.

Symbiotic nitrogen fixation (SNF) is a crucial process for nitrogen geochemical cycling and plant-microbe interactions. Water-soluble humic acid (WSHM), an active component of soil humus, has been shown to promote SNF in the legume-rhizobial symbiosis, but its molecular mechanism remains largely unknown. To reveal the SNF-promoting mechanism, we conducted transcriptomic analysis on soybean treated with WSHM. Our findings revealed that up- and downregulated differentially expressed genes (DEGs) were mainly involved in plant cell-wall/membrane formation and plant defence/immunity in the early stage, while the late stage was marked by the flavonoid synthesis and ethylene biosynthetic process. Further study on representative DEGs showed that WSHM could inhibit GmBAK1d-mediated immunity and BR signalling, thereby promoting rhizobial colonisation, infection, and nodulation, while not favoring pathogenic bacteria colonisation on the host plant. Additionally, we also found that the ethylene pathway is necessary for promoting the soybean nodulation by WSHM. This study not only provides a significant advance in our understanding of the molecular mechanism of WSHM in promoting SNF, but also provides evidence of the beneficial interactions among the biostimulator, host plant, and soil microbes, which have not been previously reported.

Nodulating another way: what can we learn from lateral root base nodulation in legumes?

Certain legumes provide a special pathway for rhizobia to invade the root and develop nitrogen-fixing nodules, a process known as lateral root base (LRB) nodulation. This pathway involves intercellular infection at the junction of the lateral roots with the taproot, leading to nodule formation in the lateral root cortex. Remarkably, this LRB pathway serves as a backbone for various adaptative symbiotic processes. Here, we describe different aspects of LRB nodulation and highlight directions for future research to elucidate the mechanisms of this as yet little known but original pathway that will help in broadening our knowledge on the rhizobium-legume symbiosis.

A pathogenesis-related protein, PRP1, negatively regulates root nodule symbiosis in Lotus japonicus.

The legume-rhizobium symbiosis represents a unique model within the realm of plant-microbe interactions. Unlike typical cases of pathogenic invasion, the infection of rhizobia and their residence within symbiotic cells do not elicit a noticeable immune response in plants. Nevertheless, there is still much to uncover regarding the mechanisms through which plant immunity influences rhizobial symbiosis. In this study, we identify an important player in this intricate interplay: Lotus japonicus PRP1, which serves as a positive regulator of plant immunity but also exhibits the capacity to decrease rhizobial colonization and nitrogen fixation within nodules. The PRP1 gene encodes an uncharacterized protein and is named Pathogenesis-Related Protein1, owing to its orthologue in Arabidopsis thaliana, a pathogenesis-related family protein (At1g78780). The PRP1 gene displays high expression levels in nodules compared to other tissues. We observed an increase in rhizobium infection in the L. japonicus prp1 mutants, whereas PRP1-overexpressing plants exhibited a reduction in rhizobium infection compared to control plants. Intriguingly, L. japonicus prp1 mutants produced nodules with a pinker colour compared to wild-type controls, accompanied by elevated levels of leghaemoglobin and an increased proportion of infected cells within the prp1 nodules. The transcription factor Nodule Inception (NIN) can directly bind to the PRP1 promoter, activating PRP1 gene expression. Furthermore, we found that PRP1 is a positive mediator of innate immunity in plants. In summary, our study provides clear evidence of the intricate relationship between plant immunity and symbiosis. PRP1, acting as a positive regulator of plant immunity, simultaneously exerts suppressive effects on rhizobial infection and colonization within nodules.

Exploring the role of symbiotic modifier peptidases in the legume - rhizobium symbiosis.

Legumes can establish a mutual association with soil-derived nitrogen-fixing bacteria called 'rhizobia' forming lateral root organs called root nodules. Rhizobia inside the root nodules get transformed into 'bacteroids' that can fix atmospheric nitrogen to ammonia for host plants in return for nutrients and shelter. A substantial 200 million tons of nitrogen is fixed annually through biological nitrogen fixation. Consequently, the symbiotic mechanism of nitrogen fixation is utilized worldwide for sustainable agriculture and plays a crucial role in the Earth's ecosystem. The development of effective nitrogen-fixing symbiosis between legumes and rhizobia is very specialized and requires coordinated signaling. A plethora of plant-derived nodule-specific cysteine-rich (NCR or NCR-like) peptides get actively involved in this complex and tightly regulated signaling process of symbiosis between some legumes of the IRLC (Inverted Repeat-Lacking Clade) and Dalbergioid clades and nitrogen-fixing rhizobia. Recent progress has been made in identifying two such peptidases that actively prevent bacterial differentiation, leading to symbiotic incompatibility. In this review, we outlined the functions of NCRs and two nitrogen-fixing blocking peptidases: HrrP (host range restriction peptidase) and SapA (symbiosis-associated peptidase A). SapA was identified through an overexpression screen from the Sinorhizobium meliloti 1021 core genome, whereas HrrP is inherited extra-chromosomally. Interestingly, both peptidases affect the symbiotic outcome by degrading the NCR peptides generated from the host plants. These NCR-degrading peptidases can shed light on symbiotic incompatibility, helping to elucidate the reasons behind the inefficiency of nitrogen fixation observed in certain groups of rhizobia with specific legumes.

Rhizobium hidalgonense and Rhizobium redzepovicii as faba bean (Vicia faba L.) microsymbionts in Mexican soils.

As a legume crop widely cultured in the world, faba bean (Vicia faba L.) forms root nodules with diverse Rhizobium species in different regions. However, the symbionts associated with this plant in Mexico have not been studied. To investigate the diversity and species/symbiovar affiliations of rhizobia associated with faba bean in Mexico, rhizobia were isolated from this plant grown in two Mexican sites in the present study. Based upon the analysis of recA gene phylogeny, two genotypes were distinguished among a total of 35 isolates, and they were identified as Rhizobium hidalgonense and Rhizobium redzepovicii, respectively, by the whole genomic sequence analysis. Both the species harbored identical nod gene cluster and the same phylogenetic positions of nodC and nifH. So, all of them were identified into the symbiovar viciae. As a minor group, R. hidalgonense was only isolated from slightly acid soil and R. redzepovicii was the dominant group in both the acid and neutral soils. In addition, several genes related to resistance to metals (zinc, copper etc.) and metalloids (arsenic) were detected in genomes of the reference isolates, which might offer them some adaptation benefits. As conclusion, the community composition of faba bean rhizobia in Mexico was different from those reported in other regions. Furthermore, our study identified sv. viciae as the second symbiovar in the species R. redzepovicii. These results added novel evidence about the co-evolution, diversification and biogeographic patterns of rhizobia in association with their host legumes in distinct geographic regions.

Guidelines for the description of rhizobial symbiovars.

Rhizobia are bacteria that form nitrogen-fixing nodules in legume plants. The sets of genes responsible for both nodulation and nitrogen fixation are carried in plasmids or genomic islands that are often mobile. Different strains within a species sometimes have different host specificities, while very similar symbiosis genes may be found in strains of different species. These specificity variants are known as symbiovars, and many of them have been given names, but there are no established guidelines for defining or naming them. Here, we discuss the requirements for guidelines to describe symbiovars, propose a set of guidelines, provide a list of all symbiovars for which descriptions have been published so far, and offer a mechanism to maintain a list in the future.

Rhizobial-induced phosphatase GmPP2C61A positively regulates soybean nodulation.

Symbiotic nitrogen fixation (SNF) is crucial for legumes, providing them with the nitrogen necessary for plant growth and development. Nodulation is the first step in the establishment of SNF. However, the determinant genes in soybean nodulation and the understanding of the underlying molecular mechanisms governing nodulation are still limited. Herein, we identified a phosphatase, GmPP2C61A, which was specifically induced by rhizobia inoculation. Using transgenic hairy roots harboring GmPP2C61A::GUS, we showed that GmPP2C61A was mainly induced in epidermal cells following rhizobia inoculation. Functional analysis revealed that knockdown or knock-out of GmPP2C61A significantly reduced the number of nodules, while overexpression of GmPP2C61A promoted nodule formation. Additionally, GmPP2C61A protein was mainly localized in the cytoplasm and exhibited conserved phosphatase activity in vitro. Our findings suggest that phosphatase GmPP2C61A serves as a critical regulator in soybean nodulation, highlighting its potential significance in enhancing symbiotic nitrogen fixation.

Bradyrhizobium ontarionense sp. nov., a novel bacterial symbiont isolated from Aeschynomene indica (Indian jointvetch), harbours photosynthesis, nitrogen fixation and nitrous oxide (N2O) reductase genes.

A novel bacterial symbiont, strain A19T, was previously isolated from a root-nodule of Aeschynomene indica and assigned to a new lineage in the photosynthetic clade of the genus Bradyrhizobium. Here data are presented for the detailed genomic and taxonomic analyses of novel strain A19T. Emphasis is placed on the analysis of genes of practical or ecological significance (photosynthesis, nitrous oxide reductase and nitrogen fixation genes). Phylogenomic analysis of whole genome sequences as well as 50 single-copy core gene sequences placed A19T in a highly supported lineage distinct from described Bradyrhizobium species with B. oligotrophicum as the closest relative. The digital DNA-DNA hybridization and average nucleotide identity values for A19T in pair-wise comparisons with close relatives were far lower than the respective threshold values of 70% and ~ 96% for definition of species boundaries. The complete genome of A19T consists of a single 8.44 Mbp chromosome and contains a photosynthesis gene cluster, nitrogen-fixation genes and genes encoding a complete denitrifying enzyme system including nitrous oxide reductase implicated in the reduction of N2O, a potent greenhouse gas, to inert dinitrogen. Nodulation and type III secretion system genes, needed for nodulation by most rhizobia, were not detected. Data for multiple phenotypic tests complemented the sequence-based analyses. Strain A19T elicits nitrogen-fixing nodules on stems and roots of A. indica plants but not on soybeans or Macroptilium atropurpureum. Based on the data presented, a new species named Bradyrhizobium ontarionense sp. nov. is proposed with strain A19T (= LMG 32638T = HAMBI 3761T) as the type strain.

Co-inoculation with novel nodule-inhabiting bacteria reduces the benefits of legume-rhizobium symbiosis.

The ecologically and economically vital symbiosis between nitrogen-fixing rhizobia and leguminous plants is often thought of as a bi-partite interaction, yet studies increasingly show the prevalence of non-rhizobial endophytes (NREs) that occupy nodules alongside rhizobia. Yet, what impact these NREs have on plant or rhizobium fitness remains unclear. Here, we investigated four NRE strains found to naturally co-occupy nodules of the legume Medicago truncatula alongside Sinorhizobium meliloti in native soils. Our objectives were to (1) examine the direct and indirect effects of NREs on M. truncatula and S. meliloti fitness, and (2) determine whether NREs can re-colonize root and nodule tissues upon reinoculation. We identified one NRE strain (522) as a novel Paenibacillus species, another strain (717A) as a novel Bacillus species, and the other two (702A and 733B) as novel Pseudomonas species. Additionally, we found that two NREs (Bacillus 717A and Pseudomonas 733B) reduced the fitness benefits obtained from symbiosis for both partners, while the other two (522, 702A) had little effect. Lastly, we found that NREs were able to co-infect host tissues alongside S. meliloti. This study demonstrates that variation of NREs present in natural populations must be considered to better understand legume-rhizobium dynamics in soil communities.

Minimal gene set from Sinorhizobium (Ensifer) meliloti pSymA required for efficient symbiosis with Medicago.

Reduction of N2 gas to ammonia in legume root nodules is a key component of sustainable agricultural systems. Root nodules are the result of a symbiosis between leguminous plants and bacteria called rhizobia. Both symbiotic partners play active roles in establishing successful symbiosis and nitrogen fixation: while root nodule development is mostly controlled by the plant, the rhizobia induce nodule formation, invade, and perform N2 fixation once inside the plant cells. Many bacterial genes involved in the rhizobia-legume symbiosis are known, and there is much interest in engineering the symbiosis to include major nonlegume crops such as corn, wheat, and rice. We sought to identify and combine a minimal bacterial gene complement necessary and sufficient for symbiosis. We analyzed a model rhizobium, Sinorhizobium (Ensifer) meliloti, using a background strain in which the 1.35-Mb symbiotic megaplasmid pSymA was removed. Three regions representing 162 kb of pSymA were sufficient to recover a complete N2-fixing symbiosis with alfalfa, and a targeted assembly of this gene complement achieved high levels of symbiotic N2 fixation. The resulting gene set contained just 58 of 1,290 pSymA protein-coding genes. To generate a platform for future synthetic manipulation, the minimal symbiotic genes were reorganized into three discrete nod, nif, and fix modules. These constructs will facilitate directed studies toward expanding the symbiosis to other plant partners. They also enable forward-type approaches to identifying genetic components that may not be essential for symbiosis, but which modulate the rhizobium's competitiveness for nodulation and the effectiveness of particular rhizobia-plant symbioses.

Conditional sanctioning in a legume-Rhizobium mutualism.

Legumes are high in protein and form a valuable part of human diets due to their interaction with symbiotic nitrogen-fixing bacteria known as rhizobia. Plants house rhizobia in specialized root nodules and provide the rhizobia with carbon in return for nitrogen. However, plants usually house multiple rhizobial strains that vary in their fixation ability, so the plant faces an investment dilemma. Plants are known to sanction strains that do not fix nitrogen, but nonfixers are rare in field settings, while intermediate fixers are common. Here, we modeled how plants should respond to an intermediate fixer that was otherwise isogenic and tested model predictions using pea plants. Intermediate fixers were only tolerated when a better strain was not available. In agreement with model predictions, nodules containing the intermediate-fixing strain were large and healthy when the only alternative was a nonfixer, but nodules of the intermediate-fixing strain were small and white when the plant was coinoculated with a more effective strain. The reduction in nodule size was preceded by a lower carbon supply to the nodule even before differences in nodule size could be observed. Sanctioned nodules had reduced rates of nitrogen fixation, and in later developmental stages, sanctioned nodules contained fewer viable bacteria than nonsanctioned nodules. This indicates that legumes can make conditional decisions, most likely by comparing a local nodule-dependent cue of nitrogen output with a global cue, giving them remarkable control over their symbiotic partners.

Inter-species interaction of bradyrhizobia affects their colonization and plant growth promotion in Arachis hypogaea.

Bradyrhizobia are the principal symbiotic partner of the leguminous plant and take active part in biological nitrogen-fixation. The present investigation explores the underlying competition among different strains during colonization in host roots. Six distinct GFP and RFP-tagged Bradyrhizobium strains were engineered to track them inside the peanut roots either independently or in combination. The Bradyrhizobium strains require different time-spans ranging from 4 to 21 days post-infection (dpi) for successful colonization which further varies in presence of another strain. While most of the individual strains enhanced the shoot and root dry weight, number of nodules, and nitrogen fixation capabilities of the host plants, no significant enhancement of plant growth and nodulation efficiency was observed when they were allowed to colonize in combinations. However, if among the combinations one strains is SEMIA 6144, the co-infection results in higher growth and nodulation efficiency of the hosts. From the competition experiments it has been found that Bradyrhizobium japonicum SEMIA 6144 was found to be the most dominant strain for effective nodulation in peanut. The extent of biofilm and exopolysaccharide (EPS) production by these isolates, individually or in combinations, were envisaged to correlate whether these parameters have any impact on the symbiotic association. But the extent of colonization, growth-promotion and nitrogen-fixation ability drastically lowered when a strain present together with other Bradyrhizobium strain. Therefore, it is imperative to understand the interaction between two co-inoculating Bradyrhizobium species for nodulation followed by plant growth promotion to develop suitable consortia for enhancing BNF in peanut and possibly for other legumes.