Interaction Of Rhizobia Research Articles

GmRj2/Rfg1 control of soybean–rhizobium–soil compatibility

Coordinated evolution and mutual adaptation of soybean–rhizobium–soil (SRS) are crucial for soybean distribution, but the genetic mechanism involved had remained unclear. In a recent study, Li et al. identified a natural variant of the GmRj2/Rfg1 gene that affected the ability of soybean to adapt to distinct soil types by controlling soybean–rhizobium interaction, thus unravelling the mystery of SRS compatibility.

First Report of Tripartite Symbiosis Potential among Soybean, Bradyrhizobium japonicum, and Dark Septate Endophytes

Successful soybean and rhizobium interaction is characterized by the formation of root nodules. This symbiosis significantly affects the growth and yield of soybeans and has long been recognized as a key factor in sustainable agricultural systems. Naturally, soybeans could form a tripartite symbiotic relationship with rhizobium and arbuscular mycorrhizal fungi (AMF). However, there is no information regarding the symbiotic potential of soybean, nodulating bacteria, and dark septate endophytic (DSE) fungi. This study aims to delve into new tripartite symbiosis potential, shedding light on its efficacy in improving soybean growth while reducing environmental impacts. We examined the effect of incorporating two DSEs: Cladophialophora chaetospira SK51 (Cc) and Veronaeopsis simplex Y34 (Vs), into the classic soybean-rhizobium symbiosis. Under sterile conditions, the co-inoculation of DSEs with Bradyrhizobium japonicum IncB6 (Bj) significantly increased the nodule number and dry weight, leading to the acceleration of soybean vegetative growth. Soybean nodule numbers under co-inoculation treatments-CcBj and VsBj-were significantly increased by 77.6 and 43.6%, respectively, compared to the Bj treatment. Furthermore, under the CcBj and VsBj treatments, the soybean nodule dry mass was significantly increased by 455 and 363%, respectively, compared to Bj. This finding represents the first report of new beneficial tripartite symbiosis potential for soybean nodulation and vegetative growth.

Rhizobial nitrogen fixation efficiency shapes endosphere bacterial communities and Medicago truncatula host growth

BackgroundDespite the knowledge that the soil–plant–microbiome nexus is shaped by interactions amongst its members, very little is known about how individual symbioses regulate this shaping. Even less is known about how the agriculturally important symbiosis of nitrogen-fixing rhizobia with legumes is impacted according to soil type, yet this knowledge is crucial if we are to harness or improve it. We asked how the plant, soil and microbiome are modulated by symbiosis between the model legume Medicago truncatula and different strains of Sinorhizobium meliloti or Sinorhizobium medicae whose nitrogen-fixing efficiency varies, in three distinct soil types that differ in nutrient fertility, to examine the role of the soil environment upon the plant–microbe interaction during nodulation.ResultsThe outcome of symbiosis results in installment of a potentially beneficial microbiome that leads to increased nutrient uptake that is not simply proportional to soil nutrient abundance. A number of soil edaphic factors including Zn and Mo, and not just the classical N/P/K nutrients, group with microbial community changes, and alterations in the microbiome can be seen across different soil fertility types. Root endosphere emerged as the plant microhabitat more affected by this rhizobial efficiency-driven community reshaping, manifested by the accumulation of members of the phylum Actinobacteria. The plant in turn plays an active role in regulating its root community, including sanctioning low nitrogen efficiency rhizobial strains, leading to nodule senescence in particular plant–soil–rhizobia strain combinations.ConclusionsThe microbiome–soil–rhizobial dynamic strongly influences plant nutrient uptake and growth, with the endosphere and rhizosphere shaped differentially according to plant–rhizobial interactions with strains that vary in nitrogen-fixing efficiency levels. These results open up the possibility to select inoculation partners best suited for plant, soil type and microbial community.Dv-bX6Q_Z8DTbrQMTFWdQvVideo

Rhizobia: A Promising Source of Plant Growth-Promoting Molecules and Their Non-Legume Interactions: Examining Applications and Mechanisms

For over a century, the scientific community has had a comprehensive understanding of how rhizobia can promote the growth of legumes by forming nitrogen fixing nodules. Despite this knowledge, the interaction of rhizobia with non-legumes has remained largely ignored as a subject of study until more recent decades. In the last few years, research has shown that rhizobia can also associate with non-legume roots, which ultimately leads to the stimulation of growth through diverse direct and indirect mechanisms. For example, rhizobia can enhance growth through phytohormones production, the improvement of plant nutrient uptake, such as the solubilization of precipitated phosphorus, the production of siderophores to address iron needs, and also the reduction of ethylene levels through the ACC deaminase enzyme to cope with drought stress. Additionally, rhizobia can improve, indirectly, non-legume growth through biocontrol of pathogens and the induction of systemic resistance in the host plant. It can also increase root adherence to soil by releasing exopolysaccharides, which regulate water and soil nutrient movement. The objective of this review is to assess and analyze the existing knowledge and information regarding the mechanisms through which rhizobia promote the growth of non-legumes. By conducting a comprehensive analysis of these findings, we aim to gain new insights into the development of Rhizobium/non-legume interactions.

A thiamine transporter is required for biofilm formation by Rhizobium sp. IRBG74.

Rhizobium sp. IRBG74 is a nitrogen fixing symbiont of Sesbania cannabina and a growth promoting endophyte of rice thus making it a good model to compare rhizobial interactions with legumes and cereals. In this report, we show that Rhizobium sp. IRBG74 forms biofilms on the roots of S. cannabina and rice. A mutant defective in biofilm formation was identified by screening a transposon mutant library. The transposon insertion was in thiQ, part of the thiBPQ operon that encodes the components of a thiamine/thiamine pyrophosphate ABC transporter. Complementation with thiBPQ partially restored biofilm formation. Addition of thiamine in growth media led to repression of thiC expression in the wild type strain but not in the thiQ mutant. These results suggest that thiBPQ is involved in thiamine/TPP transport in Rhizobium sp. IRBG74. Using a GUS reporter, we show that the expression of thiC is significantly higher in biofilm as compared to cells in planktonic growth. Based on these results we propose that Rhizobium sp. IRBG74 is thiamine limited and requires thiamine transport for efficient biofilm formation and plant colonization. Thiamine synthesis in aerobic bacteria such as Rhizobium requires O2 and thus could be inhibited in the microaerobic/anaerobic conditions in biofilms.

Interaction of rhizobia with native AM fungi shaped biochar effect on soybean growth

While biochar amendment is widely recommended as an excellent agricultural management practice, the biochar effect on the productivity of legume crops with symbiotic microorganisms has been poorly elucidated. Hence, a pot experiment was conducted by growing soybean with or without commercial rhizobia inoculation in an arbuscular mycorrhizal (AM) fungi rich sandy soil amended with wheat straw biochar produced respectively at 350 °C, 450 °C, and 550 °C. We tested the effect of biochar on soybean–rhizobia symbiosis and its impact on plant growth, leaf gas exchange and mycorrhizal colonisation. In brief, rhizobia inoculation, without biochar, increased net photosynthetic rate by 81% and plant total biomass by 44% compared to no inoculation (the control). Without inoculation, amendment of low-temperature biochar (350 ºC) resulted in higher (by 51%) plant biomass than high-temperature (550 °C) biochar. With rhizobia inoculation, low-temperature biochar decreased plant biomass by 9.3% compared to high-temperature biochar. Further, soybean with rhizobia under high-temperature biochar decreased plant biomass by 17% compared to rhizobia inoculation only but increased by 19% compared to the control. Soybean leaf gas exchange capacity and root morphological traits were consistent with plant biomass but symbiotic activity showed small differences among all the treatments. Overall, N-fixing rhizobia could alter the biochar effect on soybean in soil rich in indigenous AM fungi, depending on the pyrolysis temperature of the biochar used. Low-temperature biochar stimulated soybean growth greater than high-temperature biochar, while rhizobia shifted biochar’s positive effect. • Pyrolysis temperature shaped biochar effect on soybean growth with and without rhizobia inoculation in a sandy soil. • Soybean growth without rhizobia inoculation is promoted rather by biochar-350 °C than by biochar-550 °C. • A tradeoff on soybean plant growth between biochar and rhizobia inoculation. • Soil inorganic N correlated with root morphology while leaf gas exchange responded to soil NO 3 - .

How does familiarity in rhizobial interactions impact the performance of invasive and native legumes?

Mutualisms can be disrupted when non-native plants are introduced into novel environments, potentially impacting their establishment success. Introduced species can reassemble mutualisms by forming novel associations with resident biota or by maintaining familiar associations when they are co-introduced with their mutualists. Invasive AustralianAcaciaspecies in South Africa have formed nitrogen-fixing rhizobium mutualisms using both pathways.Here we examined the contributions of novel vs familiar rhizobial associations to the performance ofAcacia salignaacross different soils within South Africa’s Core Cape Subregion (CCR), and the concomitant impacts of exotic rhizobia on the endemic legume,Psoralea pinnata. We grew each legume with and without AustralianBradyrhizobiumstrains across various CCR soil types in a glasshouse. We identified root nodule rhizobium communities associating with seedlings grown in each treatment combination using next-generation sequencing (NGS) techniques.Our results show that different CCR soils affected growth performances of seedlings for both species while the addition of Australian bradyrhizobia affected growth performances ofA. saligna, but notP. pinnata. NGS data revealed that each legume associated mostly with their familiar rhizobial partners, regardless of soil conditions or inoculum treatment.Acacia salignapredominantly associated with Australian bradyrhizobia, even when grown in soils without inoculum, whileP. pinnatalargely associated with native South AfricanMesorhizobiumstrains.Our study suggests that exotic Australian bradyrhizobia are already present and widespread in pristine CCR soils, and that mutualist limitation is not an impediment to further acacia invasion in the region. The ability ofP. pinnatato sanction AustralianBradyrhizobiumstrains suggests that this species may be a good candidate for restoration efforts following the removal of acacias in CCR habitats.

Genome wide analysis of DWARF27 genes in soybean and functional characterization of GmD27c reveals eminent role of strigolactones in rhizobia interaction and nodulation in Glycine max.

Strigolactones (SLs) are newly identified hormones and their biosynthesis is stimulated under phosphate deprivation and accomplished by the action of several enzymes, including the beta-carotene isomerase DWARF27 (D27). Expression of D27 is well renowned to respond to phosphate insufficiency. However, the identification and functional analysis of the carotenoid isomerase D27 genes are not elucidated in soybean. A total of six D27 genes were identified in the soybean genome and designated on the basis of chromosomal localization. According to the findings, these genes were irregularly distributed on chromosomes, and segmental repetition led to the expansion of the soybean GmD27 gene family. Based on a neighbor-joining phylogenetic tree, the predicted D27 proteins of soybean were divided into three clades. Based on RNA seq data analysis, GmD27 genes were differently expressed in various tissues but GmD27c was the highest. Therefore, GmD27c was chosen for the additional functional study due to its rather obvious transcription in nodulation and roots. RT-qPCR results showed that GmD27c was highly expressed in different nodule stages and in response to rhizobia infection. Functional characterization of GmD27c revealed that overexpression of GmD27c led to higher nodule number, while GmD27c knockdown caused fewer nodules compared to GUS control. Furthermore, GmD27c overexpressed and knockdown lines oppositely regulated the expression of numerous nodulation genes, which are vital for the development of nodules. This study not only discovered that SL biosynthesis and signaling pathway genes are conserved, but it also revealed that SL biosynthesis gene GmD27c and legume rhizobia have close interactions in controlling plant nodule number.

Mycorrhizal and rhizobial interactions influence model grassland plant community structure and productivity.

Arbuscular mycorrhizal (AM) fungi and rhizobium are likely important drivers of plant coexistence and grassland productivity due to complementary roles in supplying limiting nutrients. However, the interactive effects of mycorrhizal and rhizobial associations on plant community productivity and competitive dynamics remain unclear. To address this, we conducted a greenhouse experiment to determine the influences of these key microbial functional groups on communities comprising three plant species by comparing plant communities grown with or without each symbiont. We also utilized N-fertilization and clipping treatments to explore potential shifts in mycorrhizal and rhizobial benefits across abiotic and biotic conditions. Our research suggests AM fungi and rhizobium co-inoculation was strongly facilitative for plant community productivity and legume (Medicago sativa) growth and nodulation. Plant competitiveness shifted in the presence of AM fungi and rhizobium, favoring M. sativa over a neighboring C4 grass (Andropogon gerardii) and C3 forb (Ratibida pinnata). This may be due to rhizobial symbiosis as well as the relatively greater mycorrhizal growth response of M. sativa, compared to the other model plants. Clipping and N-fertilization altered relative costs and benefits of both symbioses, presumably by altering host-plant nitrogen and carbon dynamics, leading to a relative decrease in mycorrhizal responsiveness and proportional biomass of M. sativa relative to the total biomass of the entire plant community, with a concomitant relative increase in A. gerardii and R. pinnata proportional biomass. Our results demonstrate a strong influence of both microbial symbioses on host-plant competitiveness and community dynamics across clipping and N-fertilization treatments, suggesting the symbiotic rhizosphere community is critical for legume establishment in grasslands.

Interaction of Nitrogen and Rhizobium on Photosynthetic and Yield Components of Red Beans (Sayad Cultivar) in Azna

Interaction of Nitrogen and Rhizobium on Photosynthetic and Yield Components of Red Beans (Sayad Cultivar) in Azna

SCREENING OF MODERN CHICKPEA VARIETIES BY RESPONSE TO BACTERIZATION

Objective. To screen modern varieties of chickpeas of Ukrainian selection by the response to bacterization, to identify varieties characterized by high increases in symbiotic activity and yield. Methods. Biotesting, gas-chromatographic, field experiment, statistical. Results. The response to bacterization of chickpea seeds of different varieties was studied in a field experiment in the southern region of Ukraine (Region of Odesa). As a result of inoculation of chickpea seeds with a bacterial suspension of an effective strain of Mesorhizobium ciceri ND-64, a significant increase in nodulation activity was registered. The highest increase in the number of nodules (51.6 % and 53 %) was reported for the varieties Odysei and Triumf. Upon seed inoculation with the studied strain, a significant increase in the weight of nodules of the Triumf variety (2.3 times) was shown. In the control variants, the nitrogenase activity in the nodules of Skarb variety plants was the highest (425 nmol C2H4/plant per hour), and in the Budzhak variety it was the lowest (159 nmol C2H4/plant per hour). Inoculation of seeds with bacterial suspension of M. ciceri ND-64 contributed to a significant increase in the studied parameter in the varieties Budzhak, Pamiat, Odysei and Triumf. The latter was characterized by the most significant response to inoculation with an effective strain in terms of nodulation and nitrogenase activity. Symbiotic parameters showed the weakest ability to respond to inoculation in terms of symbiotic parameters in Skarb variety. Pamiat variety was characterised by the maximum number of seeds per plant (18.5 units/plant). However, as a result of inoculation of chickpea seeds with M. ciceri ND-64, the number of seeds increased the most in the variety Skarb and amounted to 28.7 units/plant. In terms of seed weight, the Triumph variety was superior (9.5 g/plant). The yield of all studied varieties of chickpeas after bacterization of seeds with M. ciceri ND-64 significantly exceeded the control variants (without inoculation) by 21–56 %. The highest increase in control was observed for the varieties Triumph (0.49 t/ha) and Skarb (0.56 t/ha). The ability of nodule bacteria M. ciceri ND-64 to intensive synthesis of phytohormones and the formation of highly effective symbiosis with chickpea plants of different varieties provided a complementary interaction of rhizobia with plants and a significant increase in crop yield. Conclusion: Due to the interaction with aboriginal nodule bacteria, the highest parameters of nodulation and nitrogenase activity were typical for the varieties Triumf and Skarb, yields for Triumf and Budzhak. Screening of varieties by the response to inoculation with an effective strain of M. ciceri ND-64 indicates an increase in symbiotic activity in varieties Triumf, Budzhak (large-seeded), Pamiat (medium-seeded). The maximum increase in yield as a result of inoculation was found in chickpea varieties Skarb and Triumf. M. ciceri ND-64 are capable of active synthesis of substances of phytohormonal nature, which contributes to the effective interaction between rhizobia and chickpea plants.

Phospholipase D- and phosphatidic acid-mediated phospholipid metabolism and signaling modulate symbiotic interaction and nodulation in soybean (Glycine max).

Symbiotic rhizobium-legume interactions, such as root hair curling, rhizobial invasion, infection thread expansion, cell division and proliferation of nitrogen-fixing bacteroids, and nodule formation, involve extensive membrane synthesis, lipid remodeling and cytoskeleton dynamics. However, little is known about these membrane-cytoskeleton interfaces and related genes. Here, we report the roles of a major root phospholipase D (PLD), PLDα1, and its enzymatic product, phosphatidic acid (PA), in rhizobium-root interaction and nodulation. PLDα1 was activated and the PA content transiently increased in roots after rhizobial infection. Levels of PLDα1 transcript and PA, as well as actin and tubulin cytoskeleton-related gene expression, changed markedly during root-rhizobium interactions and nodule development. Pre-treatment of the roots of soybean seedlings with n-butanol suppressed the generation of PLD-derived PA, the expression of early nodulation genes and nodule numbers. Overexpression or knockdown of GmPLDα1 resulted in changes in PA levels, glycerolipid profiles, nodule numbers, actin cytoskeleton dynamics, early nodulation gene expression and hormone levels upon rhizobial infection compared with GUS roots. The transcript levels of cytoskeleton-related genes, such as GmACTIN, GmTUBULIN, actin capping protein 1 (GmCP1) and microtubule-associating protein (GmMAP1), were modified in GmPLDα1-altered hairy roots compared with those of GUS roots. Phosphatidic acid physically bound to GmCP1 and GmMAP1, which could be related to cytoskeletal changes in rhizobium-infected GmPLDα1 mutant roots. These data suggest that PLDα1 and PA play important roles in soybean-rhizobium interaction and nodulation. The possible underlying mechanisms, including PLDα1- and PA-mediated lipid signaling, membrane remodeling, cytoskeleton dynamics and related hormone signaling, are discussed herein.

RNA Sequencing-Associated Study Identifies GmDRR1 as Positively Regulating the Establishment of Symbiosis in Soybean.

In soybean (Glycine max)-rhizobium interactions, the type III secretion system (T3SS) of rhizobium plays a key role in regulating host specificity. However, the lack of information on the role of T3SS in signaling networks limits our understanding of symbiosis. Here, we conducted an RNA sequencing analysis of three soybean chromosome segment substituted lines, one female parent and two derived lines with different chromosome-substituted segments of wild soybean and opposite nodulation patterns. By analyzing chromosome-linked differentially expressed genes in the substituted segments and quantitative trait loci (QTL)-assisted selection in the substituted-segment region, genes that may respond to type III effectors to mediate plant immunity-related signaling were identified. To narrow down the number of candidate genes, QTL assistant was used to identify the candidate region consistent with the substituted segments. Furthermore, one candidate gene, GmDRR1, was identified in the substituted segment. To investigate the role of GmDRR1 in symbiosis establishment, GmDRR1-overexpression and RNA interference soybean lines were constructed. The nodule number increased in the former compared with wild-type soybean. Additionally, the T3SS-regulated effectors appeared to interact with the GmDDR1 signaling pathway. This finding will allow the detection of T3SS-regulated effectors involved in legume-rhizobium interactions.

Symbiotic Outcome Modified by the Diversification from 7 to over 700 Nodule-Specific Cysteine-Rich Peptides.

Legume-rhizobium symbiosis represents one of the most successfully co-evolved mutualisms. Within nodules, the bacterial cells undergo distinct metabolic and morphological changes and differentiate into nitrogen-fixing bacteroids. Legumes in the inverted repeat lacking clade (IRLC) employ an array of defensin-like small secreted peptides (SSPs), known as nodule-specific cysteine-rich (NCR) peptides, to regulate bacteroid differentiation and activity. While most NCRs exhibit bactericidal effects in vitro, studies confirm that inside nodules they target the bacterial cell cycle and other cellular pathways to control and extend rhizobial differentiation into an irreversible (or terminal) state where the host gains control over bacteroids. While NCRs are well established as positive regulators of effective symbiosis, more recent findings also suggest that NCRs affect partner compatibility. The extent of bacterial differentiation has been linked to species-specific size and complexity of the NCR gene family that varies even among closely related species, suggesting a more recent origin of NCRs followed by rapid expansion in certain species. NCRs have diversified functionally, as well as in their expression patterns and responsiveness, likely driving further functional specialisation. In this review, we evaluate the functions of NCR peptides and their role as a driving force underlying the outcome of rhizobial symbiosis, where the plant is able to determine the outcome of rhizobial interaction in a temporal and spatial manner.

Hydrogen sulfide and rhizobia synergistically regulate nitrogen (N) assimilation and remobilization during N deficiency-induced senescence in soybean.

Hydrogen sulfide (H2 S) is emerging as an important signalling molecule that regulates plant growth and abiotic stress responses. However, the roles of H2 S in symbiotic nitrogen (N) assimilation and remobilization have not been characterized. Therefore, we examined how H2 S influences the soybean (Glycine max)/rhizobia interaction in terms of symbiotic N fixation and mobilization during N deficiency-induced senescence. H2 S enhanced biomass accumulation and delayed leaf senescence through effects on nodule numbers, leaf chlorophyll contents, leaf N resorption efficiency, and the N contents in different tissues. Moreover, grain numbers and yield were regulated by H2 S and rhizobia, together with N accumulation in the organs, and N use efficiency. The synergistic effects of H2 S and rhizobia were also demonstrated by effects on the enzyme activities, protein abundances, and gene expressions associated with N metabolism, and senescence-associated genes (SAGs) expression in soybeans grown under conditions of N deficiency. Taken together, these results show that H2 S and rhizobia accelerate N assimilation and remobilization by regulation of the expression of SAGs during N deficiency-induced senescence. Thus, H2 S enhances the vegetative and reproductive growth of soybean, presumably through interactions with rhizobia under conditions of N deficiency.

PENGARUH INTERAKSI PERLAKUAN Rhizobium DAN PEMUPUKAN NITROGEN TERHADAP INDEKS PANEN TERHADAP TIGA VARIETAS KEDELAI

The use of nitrogen chemical fertilizers tends to be high in soils with low nitrogen; this can affect the balance of the global N cycle. One alternative to minimize the above problem is to optimize the utilization of biological nitrogen fixation. The purpose of this study was to determine the effect of increased nitrogen uptake of yields and some soybean varieties. The study was conducted using a split plot design that consisted of three factors where the first factor was Rhizobium (R) inoculation, the second factor was soybean (V) variety, and the third factor was N (N) fertilizer. Results of this study showed that the N fertilization treatment gave a significant response while Rhizobium (R) treatment and variety (V) did not provide a significant response to the harvest index parameters. The interaction of varieties and N fertilization (VxN) gave a significant response while the interaction of Rhizobium and varieties (RxV), Rhizobium and N (RxN), Rhizobium fertilization, varieties and N (RxVxN) fertilization did not provide a significant response to the harvest index. The highest Index was Wilis variety that was harvested without N (V3N0) fertilization but was not significantly different from V3N1, V3N2 and V3N3. The production of soybean seeds had not been able to increase ESN because the plant dry weight and N uptake were not comparable.  Â

Expression profiling of miRNAs indicates crosstalk between phytohormonal response and rhizobial infection in chickpea

Legumes develop root nodules in which bacteria fix nitrogen for plants. The phytohormones auxin and cytokinin regulate nodule organogenesis by recruiting various genes to effect symbiosis. Moreover, these genes are regulated by the action of microRNAs also. To understand the complex regulatory network involving miRNAs in response to phytohormones and rhizobial interactions in chickpea roots, a miRNA expression profiling was performed. Indole acetic acid and 6-benzylaminopurine at concentrations of 0.1, 1 and 10 µM were used for auxin and cytokinin exogenous treatment and Mesorhizobium ciceri to study rhizobial interaction with chickpea root. Expression profiling of a set of 11 miRNAs was performed. Further, the targets of the candidate miRNAs were identified, followed by functional annotation. This analysis revealed that cat-miR160, cat-miR164, cat-miR396 and cat-miR398 were responsive to auxin and cytokinin. cat-miR319 was found to be only auxin responsive and is known to regulate auxin signalling by targeting TEOSINTE BRANCHED/CYCLOIDEA/PCF (TCP) which interacts with auxin inducible genes. Further, cytokinin elicited a response at very low concentration of 0.1 µM, and most of the miRNAs investigated were responsive to cytokinin. Interactome analysis revealed that cat-miR164 and cat-miR168 work in conjunction to regulate auxin signalling. Interestingly, cat-miR169 and cat-miR482 were low expressing during auxin treatment and M. ciceri infection but their expression spiked during cytokinin treatment, indicating a cytokinin mediated mode of action. The miRNA expression profiling in response to phytohormones and rhizobia and the reported function of their target genes suggested a crosstalk among the phytohormonal responses during chickpea nodulation.

Conjugative transfer between Rhizobium etli endosymbionts inside the root nodule.

Since the discovery that biological nitrogen fixation ensues in nodules resulting from the interaction of rhizobia with legumes, nodules were thought to be exclusive for hosting nitrogen-fixing and plant growth promoting bacteria. In this work, we uncover a novel function of nodules, as a niche permissive to acquisition of plasmids via conjugative transfer. We used Rhizobium etli CFN42, which nodulates Phaseolus vulgaris. The genome of R. etli CFN42 contains a chromosome and six plasmids. pRet42a is a conjugative plasmid regulated by Quorum-Sensing (QS), and pRet42d is the symbiotic plasmid. Here, using confocal microscopy and flow cytometry, we show that pRet42a transfers on the root's surface, and unexpectedly, inside the nodules. Conjugation still took place inside nodules, even when it was restricted on the plant surface by placing the QS traI regulator under the promoter of the nitrogenase gene, which is only expressed inside the nodules, or by inhibiting the QS transcriptional induction of transfer genes with a traM antiactivator on an unstable vector maintained on the plant surface and lost inside the nodules. These results conclusively confirm the occurrence of conjugation in these structures, defining them as a protected environment for bacterial diversification.

The role of endogenous thiamine produced via THIC in root nodule symbiosis in Lotus japonicus

Thiamine is a pivotal primary metabolite which is indispensable to all organisms. Although its biosynthetic pathway has been well documented, the mechanism by which thiamine influences the legume-rhizobium symbiosis remains uncertain. Here, we used overexpressing transgenic plants, mutants and grafting experiments to investigate the roles played by thiamine in Lotus japonicus nodulation. ljthic mutants displayed lethal phenotypes and the defect could be overcome by supplementation of thiamine or by overexpression of LjTHIC. Reciprocal grafting between L. japonicus wild-type Gifu B-129 and ljthic showed that the photosynthetic products of the aerial part made a major contribution to overcoming the nodulation defect in ljthic. Overexpression of LjTHIC in Lotus japonicus (OE-LjTHIC) decreased shoot growth and increased the activity of the enzymes 2-oxoglutarate dehydrogenase and pyruvate dehydrogenase. OE-LjTHIC plants exhibited an increase in the number of infection threads and also developed more nodules, which were of smaller size but unchanged nitrogenase activity compared to the wildtype. Taken together, our results suggest that endogenous thiamine produced via LjTHIC acts as an essential nutrient provided by the host plant for rhizobial infection and nodule growth in the Lotus japonicus – rhizobium interaction.

New Insights into Soybean Biological Nitrogen Fixation

Core Ideas As N2 fixation (%) increased, seed production per N2 fixation decreased. The N‐gap between crop N uptake and N supplied by N2 fixation rose when contribution from biological N2 fixation increased. The partial N balance revealed negative values across all N derived from the atmosphere levels. Yield was negatively related to partial N balance when N derived from the atmosphere was below 42%. Soybean biological N2 fixation (BNF) relationships with fertilizer N and yield response have been comprehensively reviewed in the scientific literature. However, the study of the N‐gap between N uptake and N supplied by N2 fixation, and the partial N balance (fixed N in aboveground biomass – N seeds) needs further investigation. Therefore, the goals of this synthesis–analysis were to (i) quantify seed production per unit of fixed N under different amounts of N derived from the atmosphere (NDFA, %), (ii) study the N‐gap and explore limitations of N2 fixation (kg ha−1) for satisfying plant N demand, and (iii) calculate a partial N balance for soybean and determine its relationship with the N2 fixation process. Data was gathered from 1955 through 2016 using studies reporting BNF, seed yield, and plant N uptake (n = 733 data points). The main outcomes of this review were (i) as NDFA increased, seed production per N2 fixation decreased (from 0.033 to 0.017 Mg yield kg−1 N from low, 28%, to high, 80%, NDFA); (ii) N‐gap increased faster when NDFA values were above 80% and after plant N content was above 370 kg N ha−1 suggesting that the crop needs additional N for coping yield potential; and (iii) when excluding roots, the partial N balance calculation revealed negative values across all NDFA levels. Future studies should consider a holistic approach to quantify the contribution of BNF in overall N cycling, including N contribution from roots, and to better understand the soil × plant × rhizobia interactions.