Mutualists Stabilize the Coexistence of Congeneric Legumes

Coexistence requires that stabilizing niche differences, which cause species to limit themselves more than others, outweigh relative fitness differences, which cause competitive exclusion. Interactions with shared mutualists, which can differentially affect host fitness and change in magnitude with host frequency, can satisfy these conditions for coexistence, yet empirical tests of mutualist effects on relative fitness and stabilizing niche differences are largely lacking within the framework of coexistence theory. Here, we show that N-fixing rhizobial mutualists mediate coexistence in four naturally co-occurring congeneric legume (Trifolium) species. Using experimental greenhouse communities, we quantified relative fitness and stabilizing niche differences for each species in the presence of rhizobia originating from conspecific or congeneric hosts. Rhizobia stabilized coexistence by increasing the self-limitation of Trifolium species grown with rhizobia isolated from conspecifics, thus allowing congeners to increase when rare. Greenhouse-measured invasion growth rates predicted natural, unmanipulated coexistence dynamics of Trifolium species over 2 years at our field sites. Our results demonstrate that interactions with shared mutualists can stabilize the coexistence of closely related species.

INCREASING NODULE SIZE1 Expression Is Required for Normal Rhizobial Symbiosis and Nodule Development

Nodulation is crucial for biological nitrogen fixation (BNF) in legumes, but the molecular mechanisms underlying BNF have remained elusive. Here, we cloned a candidate gene underlying a major nodulation quantitative trait locus in soybean (Glycine max), INCREASING NODULE SIZE1 (GmINS1). GmINS1 encodes a cell wall β-expansin and is expressed primarily in vascular bundles, along with cortical and parenchyma cells of nodules. Four single-nucleotide polymorphisms distinguishing the two parents were found in the GmINS1 promoter region. Among them, single-nucleotide polymorphism A/C has a significant effect on GmINS1 expression in the parental genotype P2, based on β-glucuronidase activity and promoter deletion analysis. The expression of GmINS1 and the P2 genotype promoter was strongly associated with nodule development, not only in the parents but also in 40 progeny lines and 40 genotypes selected from a soybean core collection. Overexpression of GmINS1 resulted in increases in the number, biomass, infection cell abundance, and nitrogenase activity of large nodules and subsequently changed the nitrogen content and biomass of soybean plants. GmINS1 suppression via RNA interference had the opposite effect. Double suppression of GmEXPB2 and GmINS1 dramatically inhibited soybean nodulation. Our results reveal that GmINS1 is a critical gene in nodule development and that GmEXPB2 and GmINS1 synergistically control nodulation in soybean. Our findings shed light on the genetic basis of soybean nodulation and provide a candidate gene for optimizing BNF capacity through molecular breeding in soybean.

Identification of Plant Compounds Involved in the Microbe-Plant Communication During the Coinoculation of Soybean with Bradyrhizobium elkanii and Delftia sp. strain JD2

Delftia sp. strain JD2 is a betaproteobacterium characterized as a plant growth-promoting bacterium with a 'helper' function, enhancing the performance of rhizobial inoculant strains during the coinoculation of alfalfa and clover. In this work we analyzed i) the effect of the coinoculation with Bradyrhizobium elkanii and Delftia sp. strain JD2 strains on the performance of soybean plants and ii) the production of a few secondary plant metabolites that would explain the positive effect of coinoculation on the growth and development of soybean plants. The results showed a beneficial effect of coinoculation on soybean growth, nodulation rate, and pulse yield, with the concomitant benefit for the agricultural economy. In addition, based on a metabolomics approach, we demonstrated that a different pattern of plant metabolites is being produced at different stages of plant growth. The new information suggests that the coinoculation of soybean changes the primary and secondary metabolism of the plant, including changes in the metabolic status of main and secondary nodules within the plant. The relevance of producing a different pattern of photosynthetic and photoprotective pigments, flavonoids, organic acids, and carbohydrates are discussed. Finally, we propose that JD2 could be used together with bradyrhizobia to manipulate the chemical composition of plant tissues, promoting the nutritional benefits and health of soybean.

Inoculation with Efficient Nitrogen Fixing and Indoleacetic Acid Producing Bacterial Microsymbiont Enhance Tolerance of the Model Legume Medicago truncatula to Iron Deficiency

The aim of this study was to assess the effect of symbiotic bacteria inoculation on the response of Medicago truncatula genotypes to iron deficiency. The present work was conducted on three Medicago truncatula genotypes: A17, TN8.20, and TN1.11. Three treatments were performed: control (C), direct Fe deficiency (DD), and induced Fe deficiency by bicarbonate (ID). Plants were nitrogen-fertilized (T) or inoculated with two bacterial strains: Sinorhizobium meliloti TII7 and Sinorhizobium medicae SII4. Biometric, physiological, and biochemical parameters were analyzed. Iron deficiency had a significant lowering effect on plant biomass and chlorophyll content in all Medicago truncatula genotypes. TN1.11 showed the highest lipid peroxidation and leakage of electrolyte under iron deficiency conditions, which suggest that TN1.11 was more affected than A17 and TN8.20 by Fe starvation. Iron deficiency affected symbiotic performance indices of all Medicago truncatula genotypes inoculated with both Sinorhizobium strains, mainly nodules number and biomass as well as nitrogen-fixing capacity. Nevertheless, inoculation with Sinorhizobium strains mitigates the negative effect of Fe deficiency on plant growth and oxidative stress compared to nitrogen-fertilized plants. The highest auxin producing strain, TII7, preserves relatively high growth and root biomass and length when inoculated to TN8.20 and A17. On the other hand, both TII7 and SII4 strains improve the performance of sensitive genotype TN1.11 through reduction of the negative effect of iron deficiency on chlorophyll and plant Fe content. The bacterial inoculation improved Fe-deficient plant response to oxidative stress via the induction of the activities of antioxidant enzymes.

Water availability moderates N2 fixation benefit from elevated [CO2 ]: A 2-year free-air CO2 enrichment study on lentil (Lens culinaris MEDIK.) in a water limited agroecosystem

Increased biomass and yield of plants grown under elevated [CO₂ ] often corresponds to decreased grain N concentration ([N]), diminishing nutritional quality of crops. Legumes through their symbiotic N₂ fixation may be better able to maintain biomass [N] and grain [N] under elevated [CO₂ ], provided N₂ fixation is stimulated by elevated [CO₂ ] in line with growth and yield. In Mediterranean-type agroecosystems, N₂ fixation may be impaired by drought, and it is unclear whether elevated [CO₂ ] stimulation of N₂ fixation can overcome this impact in dry years. To address this question, we grew lentil under two [CO₂ ] (ambient ~400 ppm and elevated ~550 ppm) levels in a free-air CO₂ enrichment facility over two growing seasons sharply contrasting in rainfall. Elevated [CO₂ ] stimulated N₂ fixation through greater nodule number (+27%), mass (+18%), and specific fixation activity (+17%), and this stimulation was greater in the high than in the low rainfall/dry season. Elevated [CO₂ ] depressed grain [N] (-4%) in the dry season. In contrast, grain [N] increased (+3%) in the high rainfall season under elevated [CO₂ ], as a consequence of greater post-flowering N₂ fixation. Our results suggest that the benefit for N₂ fixation from elevated [CO₂ ] is high as long as there is enough soil water to continue N₂ fixation during grain filling.

Evidence for Phosphate Starvation of Rhizobia without Terminal Differentiation in Legume Nodules

Phosphate homeostasis is tightly modulated in all organisms, including bacteria, which harbor both high- and low-affinity transporters acting under conditions of fluctuating phosphate levels. It was thought that nitrogen-fixing rhizobia, named bacteroids, inhabiting root nodules of legumes are not phosphate limited. Here, we show that the high-affinity phosphate transporter PstSCAB, rather than the low-affinity phosphate transporter Pit, is essential for effective nitrogen fixation of Sinorhizobium fredii in soybean nodules. Symbiotic and growth defects of the pst mutant can be effectively restored by knocking out PhoB, the transcriptional repressor of pit. The pst homologs of representative rhizobia were actively transcribed in bacteroids without terminal differentiation in nodules of diverse legumes (soybean, pigeonpea, cowpea, common bean, and Sophora flavescens) but exhibited a basal expression level in terminally differentiated bacteroids (alfalfa, pea, and peanut). Rhizobium leguminosarum bv. viciae Rlv3841 undergoes characteristic nonterminal and terminal differentiations in nodules of S. flavescens and pea, respectively. The pst mutant of Rlv3841 showed impaired adaptation to the nodule environment of S. flavescens but was indistinguishable from the wild-type strain in pea nodules. Taken together, root nodule rhizobia can be either phosphate limited or nonlimited regarding the rhizobial differentiation fate, which is a host-dependent feature.

Legumes versus rhizobia: a model for ongoing conflict in symbiosis

Contents Summary 1199 I. Introduction 1199 II. Selecting beneficial symbionts: one problem, many solutions 1200 III. Control and conflict over legume nodulation 1201 IV. Control and conflict over nodule growth and senescence 1204 V. Conclusion 1204 Acknowledgements 1205 References 1205 SUMMARY: The legume-rhizobia association is a powerful model of the limits of host control over microbes. Legumes regulate the formation of root nodules that house nitrogen-fixing rhizobia and adjust investment into nodule development and growth. However, the range of fitness outcomes in these traits reveals intense conflicts of interest between the partners. New work that we review and synthesize here shows that legumes have evolved varied mechanisms of control over symbionts, but that host control is often subverted by rhizobia. An outcome of this conflict is that both legumes and rhizobia have evolved numerous traits that can improve their own short-term fitness in this interaction, but little evidence exists for any net improvement in the joint trait of nitrogen fixation.

Symbiotic characteristics of Bradyrhizobium diazoefficiens USDA 110 mutants associated with shrubby sophora (Sophora flavescens) and soybean (Glycine max)

Site-specific insertion plasmid pVO155 was used to knockout the genes involved in the alternation of host range of strain Bradyrhizobium diazoefficiens USDA 110 from its original determinate-nodule-forming host soybean (Glycine max), to promiscuous and indeterminate-nodule-forming shrubby legume sophora (Sophora flavescens). Symbiotic phenotypes of these mutants inoculated to these two legumes, were compared to those infected by wild-type strain USDA 110. Six genes of the total fourteen Tn5 transposon mutated genes were broken using the pVO155 plasmid. Both Tn5 and pVO155-inserted mutants could nodulate S. flavescens with different morphologies of low-efficient indeterminate nodules. One to several rod or irregular bacteroids, containing different contents of poly-β-hydroxybutyrate or polyphosphate were found within the symbiosomes in nodulated cells of S. flavescens infected by the pVO155-inserted mutants. Moreover, none of bacteroids were observed in the pseudonodules of S. flavescens, infected by wild-type strain USDA 110. These mutants had the nodulation ability with soybean but the symbiotic efficiency reduced to diverse extents. These findings enlighten the complicated interactions between rhizobia and legumes, i. e., mutation of genes involved in metabolic pathways, transporters, chemotaxis and mobility could alter the rhizobial entry and development of the bacteroid inside the nodules of a new host legume.

Variation in bradyrhizobial NopP effector determines symbiotic incompatibility with Rj2-soybeans via effector-triggered immunity

Genotype-specific incompatibility in legume-rhizobium symbiosis has been suggested to be controlled by effector-triggered immunity underlying pathogenic host-bacteria interactions. However, the rhizobial determinant interacting with the host resistance protein (e.g., Rj2) and the molecular mechanism of symbiotic incompatibility remain unclear. Using natural mutants of Bradyrhizobium diazoefficiens USDA 122, we identified a type III-secretory protein NopP as the determinant of symbiotic incompatibility with Rj2-soybean. The analysis of nopP mutations and variants in a culture collection reveal that three amino acid residues (R60, R67, and H173) in NopP are required for Rj2-mediated incompatibility. Complementation of rj2-soybean by the Rj2 allele confers the incompatibility induced by USDA 122-type NopP. In response to incompatible strains, Rj2-soybean plants activate defense marker gene PR-2 and suppress infection thread number at 2 days after inoculation. These results suggest that Rj2-soybeans monitor the specific variants of NopP and reject bradyrhizobial infection via effector-triggered immunity mediated by Rj2 protein.

A Functional General Stress Response of Bradyrhizobium diazoefficiens Is Required for Early Stages of Host Plant Infection

Phylogenetically diverse bacteria respond to various stress conditions by mounting a general stress response (GSR) resulting in the induction of protection or damage repair functions. In α-proteobacteria, the GSR is induced by a regulatory cascade consisting of the extracytoplasmic function (ECF) σ factor σEcfG, its anti-σ factor NepR, and the anti-anti-σ factor PhyR. We have reported previously that σEcfG and PhyR of Bradyrhizobium diazoefficiens (formerly named Bradyrhizobium japonicum), the nitrogen-fixing root nodule symbiont of soybean and related legumes, are required for efficient symbiosis; however, the precise role of the GSR remained undefined. Here, we analyze the symbiotic defects of a B. diazoefficiens mutant lacking σEcfG by comparing distinct infection stages of enzymatically or fluorescently tagged wild-type and mutant bacteria. Although root colonization and root hair curling were indistinguishable, the mutant was not competitive, and showed delayed development of emerging nodules and only a few infection threads. Consequently, many of the mutant-induced nodules were aborted, empty, or partially colonized. Congruent with these results, we found that σEcfG was active in bacteria present in root-hair-entrapped microcolonies and infection threads but not in root-associated bacteria and nitrogen-fixing bacteroids. We conclude that GSR-controlled functions are crucial for synchronization of infection thread formation, colonization, and nodule development.

Gibberellins promote nodule organogenesis but inhibit the infection stages of nodulation

Leguminous plant roots can form a symbiosis with soil-dwelling nitrogen-fixing rhizobia, leading to the formation of a new root organ, the nodule. Successful nodulation requires co-ordination of spatially separated events in the root, including infection in the root epidermis and nodule organogenesis deep in the root cortex. We show that the hormone gibberellin plays distinct roles in these epidermal and cortical programmes. We employed a unique set of genetic material in pea that includes severely gibberellin-deficient lines and della-deficient lines that enabled us to characterize all stages of infection and nodule development. We confirmed that gibberellin suppresses infection thread formation and show that it also promotes nodule organogenesis into nitrogen-fixing organs. In both cases, this is achieved through the action of DELLA proteins. This study therefore provides a mechanism to explain how both low and high gibberellin signalling can result in reduced nodule number and reveals a clear role for gibberellin in the maturation of nodules into nitrogen-fixing organs. We also demonstrate that gibberellin acts independently of ethylene in promoting nodule development.

Phosphatidylinositol 3-kinase function at very early symbiont perception: a local nodulation control under stress conditions?

Root hair curling is an early and essential morphological change required for the success of the symbiotic interaction between legumes and rhizobia. At this stage rhizobia grow as an infection thread within root hairs and are internalized into the plant cells by endocytosis, where the PI3K enzyme plays important roles. Previous observations show that stress conditions affect early stages of the symbiotic interaction, from 2 to 30 min post-inoculation, which we term as very early host responses, and affect symbiosis establishment. Herein, we demonstrated the relevance of the very early host responses for the symbiotic interaction. PI3K and the NADPH oxidase complex are found to have key roles in the microsymbiont recognition response, modulating the apoplastic and intracellular/endosomal ROS induction in root hairs. Interestingly, compared with soybean mutant plants that do not perceive the symbiont, we demonstrated that the very early symbiont perception under sublethal saline stress conditions induced root hair death. Together, these results highlight not only the importance of the very early host-responses on later stages of the symbiont interaction, but also suggest that they act as a mechanism for local control of nodulation capacity, prior to the abortion of the infection thread, preventing the allocation of resources/energy for nodule formation under unfavorable environmental conditions.

CLE peptide tri-arabinosylation and peptide domain sequence composition are essential for SUNN-dependent autoregulation of nodulation in Medicago truncatula

MtCLE12 and MtCLE13 encode CLAVATA3/EMBRYO-SURROUNDING REGION RELATED (CLE) peptides which regulate autoregulation of nodulation (AON) in Medicago through the shoot receptor, SUNN (SUPER NUMERIC NODULES). Genetics suggests RDN1 (ROOT-DETERMINED NODULATION 1) arabinosylates MtCLE12 to enable SUNN perception. The functional structures of MtCLE12 and MtCLE13 peptides, however, remain elusive. We combined genetic and chemical synthesis approaches to determine if glyco-modifications of three nodule-expressed CLE peptides are essential for AON. We also examined how root and shoot applied AON-CLEs inhibit nodulation. MtCLE12, MtCLE13 and MtCLE42 peptides were synthesized with hydroxylation, mono-arabinosylation or tri-arabinosylation (TaP) at proline 7. Only MtCLE12-TaP and MtCLE13-TaP peptides induced AON in wild-type (WT) and rdn1-1, but not in sunn-4. The application of MtCLE13-TaP to cotyledons 1 d before rhizobial inoculation completely inhibited both rhizobial infection and nodulation. By contrast, MtCLE12-TaP induced significant AON without abolishing rhizobial infection. The results indicate that key CLE domain amino acids and TaP modifications to MtCLE12 and MtCLE13 are essential for SUNN-dependent AON. We also show evidence that RDN1 does not tri-arabinosylate MtCLE13. Finally, MtCLE13-TaP can induce a strong AON response in shoots that inhibits the entire symbiotic processes in roots. We present a new model for AON in Medicago.

Small peptide signaling pathways modulating macronutrient utilization in plants

Root system architecture (RSA) and physiological functions define macronutrient uptake efficiency. Small signaling peptides (SSPs), that act in manners similar to hormones, and their cognate receptors transmit signals both locally and systemically. Several SSPs controlling morphological and physiological traits of roots have been identified to be associated with macronutrient uptake. Recent development in plant genome research has provided an avenue toward systems-based identification and prediction of additional SSPs. This review highlights recent studies on SSP pathways important for optimization of macronutrient uptake and provides new insights into the diversity of SSPs regulated in response to changes in macronutrient availabilities.

Nitrate transporters: an overview in legumes

The nitrate transporters, belonging to NPF and NRT2 families, play critical roles in nitrate signaling, root growth and nodule development in legumes. Nitrate plays an essential role during plant development as nutrient and also as signal molecule, in both cases working via the activity of nitrate transporters. To date, few studies on NRT2 or NPF nitrate transporters in legumes have been reported, and most of those concern Lotus japonicus and Medicago truncatula. A molecular characterization led to the identification of 4 putative LjNRT2 and 37 putative LjNPF gene sequences in L. japonicus. In M. truncatula, the NRT2 family is composed of 3 putative members. Using the new genome annotation of M. truncatula (Mt4.0), we identified, for this review, 97 putative MtNPF sequences, including 32 new sequences relative to previous studies. Functional characterization has been published for only two MtNPF genes, encoding nitrate transporters of M. truncatula. Both transporters have a role in root system development via abscisic acid signaling: MtNPF6.8 acts as a nitrate sensor during the cell elongation of the primary root, while MtNPF1.7 contributes to the cellular organization of the root tip and nodule formation. An in silico expression study of MtNPF genes confirmed that NPF genes are expressed in nodules, as previously shown for L. japonicus, suggesting a role for the corresponding proteins in nitrate transport, or signal perception in nodules. This review summarizes our knowledge of legume nitrate transporters and discusses new roles for these proteins based on recent discoveries.

Transcriptional analysis of genes involved in competitive nodulation in Bradyrhizobium diazoefficiens at the presence of soybean root exudates

Nodulation competition is a key factor that limits symbiotic nitrogen fixation between rhizobia and their host legumes. Soybean root exudates (SREs) are thought to act as signals that influence Bradyrhizobium ability to colonize roots and to survive in the rhizosphere, and thus they act as a key determinant of nodulation competitiveness. In order to find the competitiveness-related genes in B. diazoefficiens, the transcriptome of two SREs treated B. diazoefficiens with completely different nodulation abilities (B. diazoefficiens 4534 and B. diazoefficiens 4222) were sequenced and compared. In SREs treated strain 4534 (SREs-4534), 253 unigenes were up-regulated and 204 unigenes were down-regulated. In SREs treated strain 4534 (SREs-4222), the numbers of up- and down-regulated unigenes were 108 and 185, respectively. There were considerable differences between the SREs-4534 and SREs-4222 gene expression profiles. Some differentially expressed genes are associated with a two-component system (i.g., nodW, phyR-σEcfG), bacterial chemotaxis (i.g., cheA, unigene04832), ABC transport proteins (i.g., unigene02212), IAA (indole-3-acetic acid) metabolism (i.g., nthA, nthB), and metabolic fitness (i.g., put.), which may explain the higher nodulation competitiveness of B. diazoefficiens in the rhizosphere. Our results provide a comprehensive transcriptomic resource for SREs treated B. diazoefficiens and will facilitate further studies on competitiveness-related genes in B. diazoefficiens.

Cytokinin Biosynthesis Promotes Cortical Cell Responses during Nodule Development

Legume mutants have shown the requirement for receptor-mediated cytokinin signaling in symbiotic nodule organogenesis. While the receptors are central regulators, cytokinin also is accumulated during early phases of symbiotic interaction, but the pathways involved have not yet been fully resolved. To identify the source, timing, and effect of this accumulation, we followed transcript levels of the cytokinin biosynthetic pathway genes in a sliding developmental zone of Lotus japonicus roots. LjIpt2 and LjLog4 were identified as the major contributors to the first cytokinin burst. The genetic dependence and Nod factor responsiveness of these genes confirm that cytokinin biosynthesis is a key target of the common symbiosis pathway. The accumulation of LjIpt2 and LjLog4 transcripts occurs independent of the LjLhk1 receptor during nodulation. Together with the rapid repression of both genes by cytokinin, this indicates that LjIpt2 and LjLog4 contribute to, rather than respond to, the initial cytokinin buildup. Analysis of the cytokinin response using the synthetic cytokinin sensor, TCSn, showed that this response occurs in cortical cells before spreading to the epidermis in L. japonicus While mutant analysis identified redundancy in several biosynthesis families, we found that mutation of LjIpt4 limits nodule numbers. Overexpression of LjIpt3 or LjLog4 alone was insufficient to produce the robust formation of spontaneous nodules. In contrast, overexpressing a complete cytokinin biosynthesis pathway leads to large, often fused spontaneous nodules. These results show the importance of cytokinin biosynthesis in initiating and balancing the requirement for cortical cell activation without uncontrolled cell proliferation.

ROOT DETERMINED NODULATION1 Is Required for M. truncatula CLE12, But Not CLE13, Peptide Signaling through the SUNN Receptor Kinase

The combinatorial interaction of a receptor kinase and a modified CLE peptide is involved in several developmental processes in plants, including autoregulation of nodulation (AON), which allows legumes to limit the number of root nodules formed based on available nitrogen and previous rhizobial colonization. Evidence supports the modification of CLE peptides by enzymes of the hydroxyproline O-arabinosyltransferase (HPAT/RDN) family. Here, we show by grafting and genetic analysis in Medicago truncatula that, in the AON pathway, RDN1, functioning in the root, acts upstream of the receptor kinase SUNN, functioning in the shoot. As expected for a glycosyltransferase, we found that RDN1 and RDN2 proteins are localized to the Golgi, as was shown previously for AtHPAT1. Using composite plants with transgenic hairy roots, we show that RDN1 and RDN2 orthologs from dicots as well as a related RDN gene from rice (Oryza sativa) can rescue the phenotype of rdn1-2 when expressed constitutively, but the less related MtRDN3 cannot. The timing of the induction of MtCLE12 and MtCLE13 peptide genes (negative regulators of AON) in nodulating roots is not altered by the mutation of RDN1 or SUNN, although expression levels are higher. Plants with transgenic roots constitutively expressing MtCLE12 require both RDN1 and SUNN to prevent nodule formation, while plants constitutively expressing MtCLE13 require only SUNN, suggesting that the two CLEs have different requirements for function. Combined with previous work, these data support a model in which RDN1 arabinosylates MtCLE12, and this modification is necessary for the transport and/or reception of the AON signal by the SUNN kinase.

The Endophytic Fungus Phomopsis liquidambari Increases Nodulation and N2 Fixation in Arachis hypogaea by Enhancing Hydrogen Peroxide and Nitric Oxide Signalling

The continuous cropping obstacles in monoculture fields are a major production constraint for peanuts. Application of the endophytic fungus Phomopsis liquidambari has increased peanut yields, and nodulation and N₂ fixation increases have been considered as important factors for P. liquidambari infection-improved peanut yield. However, the mechanisms involved in this process remain unknown. This work showed that compared with only Bradyrhizobium inoculation, co-inoculation with P. liquidambari significantly elevated endogenous H₂O₂ and NO levels in peanut roots. Pre-treatment of seedlings with specific scavengers of H₂O₂ (CAT) and NO (cPTIO) blocked P. liquidambari-induced nodulation and N₂ fixation. CAT not only suppressed the P. liquidambari-induced nodulation and N₂ fixation, but also suppressed the enhanced H₂O₂ and NO generation. Nevertheless, the cPTIO did not significantly inhibit the induced H₂O₂ biosynthesis, implying that H₂O₂ acted upstream of NO production. These results were confirmed by observations that exogenous H₂O₂ and sodium nitroprusside (SNP) reversed the inhibition of P. liquidambari-increased nodulation and N₂ fixation by the specific scavengers. The transcriptional activities of the symbiosis-related genes SymRK and CCaMK of peanut-Bradyrhizobium interactions also increased significantly in response to P. liquidambari, H₂O₂ and SNP treatments. The pot experiment further confirmed that the P. liquidambari infection-enhanced H₂O₂ and NO signalling pathways were significantly related to the increase in peanut nodulation and N₂ fixation. This is the first report that endophytic fungus P. liquidambari can increase peanut-Bradyrhizobium interactions via enhanced H₂O₂/NO-dependent signalling crosstalk, which is conducive to the alleviation of continuous cropping obstacles via an increase in nodulation and N₂ fixation.

Comparative conventional and phenomics approaches to assess symbiotic effectiveness of Bradyrhizobia strains in soybean (Glycine max L. Merrill) to drought

Symbiotic effectiveness of rhizobitoxine (Rtx)-producing strains of Bradyrhizobium spp. in soybean (cultivar NRC-37/Ahilya-4) under limited soil moisture conditions was evaluated using phenomics tools such as infrared(IR) thermal and visible imaging. Red, green and blue (RGB) colour pixels were standardized to analyse a total of 1017 IR thermal and 692 visible images. Plants inoculated with the Rtx-producing strains B. elkanii USDA-61 and USDA-94 and successive inoculation by B. diazoefficiens USDA-110 resulted in cooler canopy temperatures and increased canopy greenness. The results of the image analysis of plants inoculated with Rtx-producing strains were correlated with effective nodulation, improved photosynthesis, plant nitrogen status and yield parameters. Principal component analysis (PCA) revealed the reliability of the phenomics approach over conventional destructive approaches in assessing the symbiotic effectiveness of Bradyrhizobium strains in soybean plants under watered (87.41-89.96%) and water-stressed (90.54-94.21%) conditions. Multivariate cluster analysis (MCA) revealed two distinct clusters denoting effective (Rtx) and ineffective (non-Rtx) Bradyrhizobium inoculation treatments in soybean. Furthermore, correlation analysis showed that this phenotyping approach is a dependable alternative for screening drought tolerant genotypes or drought resilience symbiosis. This is the first report on the application of non-invasive phenomics techniques, particularly RGB-based image analysis, in assessing plant-microbe symbiotic interactions to impart abiotic stress tolerance.

Bacterial Biosensors for in Vivo Spatiotemporal Mapping of Root Secretion

Plants engineer the rhizosphere to their advantage by secreting various nutrients and secondary metabolites. Coupling transcriptomic and metabolomic analyses of the pea (Pisum sativum) rhizosphere, a suite of bioreporters has been developed in Rhizobium leguminosarum bv viciae strain 3841, and these detect metabolites secreted by roots in space and time. Fourteen bacterial lux fusion bioreporters, specific for sugars, polyols, amino acids, organic acids, or flavonoids, have been validated in vitro and in vivo. Using different bacterial mutants (nodC and nifH), the process of colonization and symbiosis has been analyzed, revealing compounds important in the different steps of the rhizobium-legume association. Dicarboxylates and sucrose are the main carbon sources within the nodules; in ineffective (nifH) nodules, particularly low levels of sucrose were observed, suggesting that plant sanctions affect carbon supply to nodules. In contrast, high myo-inositol levels were observed prior to nodule formation and also in nifH senescent nodules. Amino acid biosensors showed different patterns: a γ-aminobutyrate biosensor was active only inside nodules, whereas the phenylalanine bioreporter showed a high signal also in the rhizosphere. The bioreporters were further validated in vetch (Vicia hirsuta), producing similar results. In addition, vetch exhibited a local increase of nod gene-inducing flavonoids at sites where nodules developed subsequently. These bioreporters will be particularly helpful in understanding the dynamics of root exudation and the role of different molecules secreted into the rhizosphere.

Agglomeration Determines Effects of Carbonaceous Nanomaterials on Soybean Nodulation, Dinitrogen Fixation Potential, and Growth in Soil

The potential effects of carbonaceous nanomaterials (CNMs) on agricultural plants are of concern. However, little research has been performed using plants cultivated to maturity in soils contaminated with various CNMs at different concentrations. Here, we grew soybean for 39 days to seed production in soil amended with 0.1, 100, or 1000 mg kg-1 of either multiwalled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), or carbon black (CB) and studied plant growth, nodulation, and dinitrogen (N2) fixation potential. Plants in all CNM treatments flowered earlier (producing 60% to 372% more flowers when reproduction started) than the unamended controls. The low MWCNT-treated plants were shorter (by 15%) with slower leaf cover expansion (by 26%) and less final leaf area (by 24%) than the controls. Nodulation and N2 fixation potential appeared negatively impacted by CNMs, with stronger effects at lower CNM concentrations. All CNM treatments reduced the whole-plant N2 fixation potential, with the highest reductions (by over 91%) in the low and medium CB and the low MWCNT treatments. CB and GNPs appeared to accumulate inside nodules as observed by transmission electron microscopy. CNM dispersal in aqueous soil extracts was studied to explain the inverse dose-response relationships, showing that CNMs at higher concentrations were more agglomerated (over 90% CNMs settled as agglomerates >3 μm after 12 h) and therefore proportionally less bioavailable. Overall, our findings suggest that lower concentrations of CNMs in soils could be more impactful to leguminous N2 fixation, owing to greater CNM dispersal and therefore increased bioavailability at lower concentrations.

Adaptive strategies for nitrogen metabolism in phosphate deficient legume nodules

Legumes play a significant role in natural and agricultural ecosystems. They can fix atmospheric N₂ and contribute the fixed N to soils and plant N budgets. In legumes, the availability of P does not only affect nodule development, but also N acquisition and metabolism. For legumes as an important source of plant proteins, their capacity to metabolise N during P deficiency is critical for their benefits to agriculture and the natural environment. In particular for farming, rock P is a non-renewable source of which the world has about 60-80 years of sustainable extraction of this P left. The global production of legume crops would be devastated during a scarcity of P fertiliser. Legume nodules have a high requirement for mineral P, which makes them vulnerable to soil P deficiencies. In order to maintain N metabolism, the nodules have evolved several strategies to resist the immediate effects of P limitation and to respond to prolonged P deficiency. In legumes nodules, N metabolism is determined by several processes involving the acquisition, assimilation, export, and recycling of N in various forms. Although these processes are integrated, the current literature lacks a clear synthesis of how legumes respond to P stress regarding its impact on N metabolism. In this review, we synthesise the current state of knowledge on how legumes maintain N metabolism during P deficiency. Moreover, we discuss the potential importance of two additional alterations to N metabolism during P deficiency. Our goals are to place these newly proposed mechanisms in perspective with other known adaptations of N metabolism to P deficiency and to discuss their practical benefits during P deficiency in legumes.

Molecular Signals in Nodulation Control

Our world is facing major problems relating to food production. According to an August 30, 2015 program of LANDLINE (Australian Broadcasting Corporation, Australia),we lose 120,000,000 hectares of agricultural land per year due to population growth, associated urbanisation, and desertification.