Molecular Signals Controlling the Inhibition of Nodulation by Nitrate in Medicago truncatula

The nitrate transporter-sensor MtNPF6.8 regulates the branched chain amino acid/pantothenate metabolic pathway in barrel medic (Medicago truncatula Gaertn.) root tip

Nitrogen is the most limiting nutrient for plants, and it is preferentially absorbed in the form of nitrate by roots, which adapt to nitrate fluctuations by remodelling their architecture. Although core mechanisms of the response to nitrate availability are relatively well-known, signalling events controlling root growth and architecture have not all been identified, in particular in Legumes. However, the developmental effect of nitrate in Legumes is critical since external nitrate not only regulates root architecture but also N2-fixing nodule development. We have previously shown that in barrel medic (Medicago truncatula), the nitrate transporter MtNPF6.8 is required for nitrate sensitivity in root tip. However, uncertainty remains as to whether nitrogen metabolism itself is involved in the MtNPF6.8-mediated response. Here, we examine the metabolic effects of MtNPF6.8-dependent nitrate signalling using metabolomics and proteomics in WT and mtnpf6.8 root tips in presence or absence of nitrate. We found a reorchestration of metabolism due to the mutation, in favour of the branched chain amino acids/pantothenate metabolic pathway, and lipid catabolism via glyoxylate. That is, the mtnpf6.8 mutation was likely associated with a specific rerouting of acetyl-CoA production (glyoxylic cycle) and utilisation (pantothenate and branched chain amino acid synthesis). In agreement with our previous findings, class III peroxidases were confirmed as the main protein class responsive to nitrate, although in an MtNPF6.8-independent fashion. Our data rather suggest the involvement of other pathways within mtnpf6.8 root tips, such as Ca2+ signalling or cell wall methylation.

A comparative meta-proteomic pipeline for the identification of plasmodesmata proteins and regulatory conditions in diverse plant species

BACKGROUND

A major route for cell-to-cell signalling in plants is mediated by cell wall-embedded pores termed plasmodesmata forming the symplasm. Plasmodesmata regulate the plant development and responses to the environment; however, our understanding of what factors or regulatory cues affect their structure and permeability is still limited. In this paper, a meta-analysis was carried out for the identification of conditions affecting plasmodesmata transport and for the in silico prediction of plasmodesmata proteins in species for which the plasmodesmata proteome has not been experimentally determined.

RESULTS

Using the information obtained from experimental proteomes, an analysis pipeline (named plasmodesmata in silico proteome 1 or PIP1) was developed to rapidly generate candidate plasmodesmata proteomes for 22 plant species. Using the in silico proteomes to interrogate published transcriptomes, gene interaction networks were identified pointing to conditions likely affecting plasmodesmata transport capacity. High salinity, drought and osmotic stress regulate the expression of clusters enriched in genes encoding plasmodesmata proteins, including those involved in the metabolism of the cell wall polysaccharide callose. Experimental determinations showed restriction in the intercellular transport of the symplasmic reporter GFP and enhanced callose deposition in Arabidopsis roots exposed to 75-mM NaCl and 3% PEG (polyethylene glycol). Using PIP1 and transcriptome meta-analyses, candidate plasmodesmata proteins for the legume Medicago truncatula were generated, leading to the identification of Medtr1g073320, a novel receptor-like protein that localises at plasmodesmata. Expression of Medtr1g073320 affects callose deposition and the root response to infection with the soil-borne bacteria rhizobia in the presence of nitrate.

CONCLUSIONS

Our study shows that combining proteomic meta-analysis and transcriptomic data can be a valuable tool for the identification of new proteins and regulatory mechanisms affecting plasmodesmata function. We have created the freely accessible pipeline PIP1 as a resource for the screening of experimental proteomes and for the in silico prediction of PD proteins in diverse plant species.

The Nitrate Transporter MtNPF6.8 Is a Master Sensor of Nitrate Signal in the Primary Root Tip of Medicago truncatula

Nitrate is not only an essential nutrient for plants, but also a signal involved in plant development. We have previously shown in the model legume Medicago truncatula, that the nitrate signal, which restricts primary root growth, is mediated by MtNPF6.8, a nitrate transporter. Nitrate signal also induces changes in reactive oxygen species accumulation in the root tip due to changes in cell wall peroxidase (PODs) activity. Thus, it was interesting to determine the importance of the role of MtNPF6.8 in the regulation of the root growth by nitrate and identify the POD isoforms responsible for the changes in POD activity. For this purpose, we compared in M. truncatula a npf6.8 mutant and nitrate insensitive line deficient in MtNPF6.8 and the corresponding wild and sensitive genotype for their transcriptomic and proteomic responses to nitrate. Interestingly, only 13 transcripts and no protein were differently accumulated in the primary root tip of the npf6.8-3 mutant line in response to nitrate. The sensitivity of the primary root tip to nitrate appeared therefore to be strongly linked to the integrity of MtNPF6.8 which acts as a master mediator of the nitrate signal involved in the control of the root system architecture. In parallel, 7,259 and 493 genes responded, respectively, at the level of transcripts or proteins in the wild type, 196 genes being identified by both their transcript and protein. By focusing on these 196 genes, a concordance of expression was observed for most of them with 143 genes being up-regulated and 51 being down-regulated at the two gene expression levels. Their ontology analysis uncovered a high enrichment in POD genes, allowing the identification of POD candidates involved in the changes in POD activity previously observed in response to nitrate.

NIN-Like Proteins: Interesting Players in Rhizobia-Induced Nitrate Signaling Response During Interaction with Non-Legume Host Arabidopsis thaliana

Nitrogen is an essential macronutrient and a key cellular messenger. Plants have evolved refined molecular systems to sense the cellular nitrogen status. This is exemplified by the root nodule symbiosis between legumes and symbiotic rhizobia, where nitrate availability inhibits this mutualistic interaction. Additionally, nitrate also functions as a metabolic messenger, resulting in nitrate signaling cascades which intensively crosstalk with other physiological pathways. Nodule inception-like proteins (NLPs) are key players in nitrate signaling and regulate nitrate-dependent transcription during legume-rhizobia interactions. Nevertheless, the coordinated interplay between nitrate signaling pathways and rhizobacteria-induced responses remains to be elucidated. In our study, we investigated rhizobia-induced changes in the root system architecture of the non-legume host arabidopsis under different nitrate conditions. We demonstrate that rhizobium-induced lateral root growth and increased root hair length and density are regulated by a nitrate-related signaling pathway. Key players in this process are AtNLP4 and AtNLP5, because the corresponding mutants failed to respond to rhizobia. At the cellular level, AtNLP4 and AtNLP5 control a rhizobia-induced decrease in cell elongation rates, while additional cell divisions occurred independently of AtNLP4. In summary, our data suggest that root morphological responses to rhizobia are coordinated by a newly considered nitrate-related NLP pathway that is evolutionarily linked to regulatory circuits described in legumes.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

The impact of indigenous soil yeasts inoculation on bean (Phaseolus vulgaris) growth

To determine whether the use of soil yeasts as inoculum would intervene in improving bean (Phaseolus vulgaris) growth and its symbiotic parameters, various experiments have been carried out. The effect of inoculation by soil yeasts Meyerozyma guilliermondii GP MT258984, Debaryomyces hansenii DFA MT259039, Rhodotorula mucilaginosa LIA MT259358, Rhodotorula mucilaginosa LC MT252049, along with the strain Saccharomyces cerevisiae in their cell or metabolite forms, was investigated in non-sterile (sand) and agricultural substrates (soil), in association or not with mycorrhizal fungi. The results showed that the yeast could have a beneficial effect on the bean biomass directly without increasing its nodulation and mycorrhization, or indirectly by improving those symbiotic parameters. They also revealed a very important ecological aspect and the possibility of inoculating with indigenous soil yeasts to increase the number of fertilising microorganisms, such as mycorrhizae and rhizobia, without resorting to introducing exogenous strains.

Physiological impact of flavonoids on nodulation and ureide metabolism in legume plants

Legume plants from Fabaceae family (phylogenetic group composed by three subfamilies: Caesalpinioideae, Mimosoideae, and Papilionoideae) can fix atmospheric nitrogen (N₂) into ammonia (NH₃) by the symbiotic relationship with rhizobia bacteria. These bacteria respond chemotactically to certain compounds released by plants such as sugars, amino acids and organic acids. Root secretion of isoflavonoids acts as inducers for nod genes in rhizobia and ABC transporters and ICHG (isoflavone conjugates hydrolyzing beta-glucosidase) at apoplast are related to the exudation of genistein and daidzein in soybean roots. Biological nitrogen fixation (BNF) occurs inside the nodule by the action of nitrogenase enzyme, which fixes N₂ into NH₃, which is converted into ureides (allantoin and allantoic acid). In this review, we bring together the latest findings on flavonoids biosynthesis and ureide metabolism in several legume plant species. We emphasize how flavonoids induce nod genes in rhizobia, affecting chemotaxis, nodulation, ureide production, growth and yield of legume plants. Mainly, isoflavonoids daidzein and genistein are responsible for nod genes activation in the rhizobia bacteria. Flavonoids also play an important role during nodule organogenesis by acting as auxin transporter inhibitors in root cells, especially in indeterminate nodules. The ureides are the main N transport form in tropical legumes and they are catabolized in leaves and other sink tissues to produce amino acids and proteins needed for plant growth and yield.

Structural Modeling and in planta Complementation Studies Link Mutated Residues of the Medicago truncatula Nitrate Transporter NPF1.7 to Functionality in Root Nodules

Symbiotic nitrogen fixation is a complex and regulated process that takes place in root nodules of legumes and allows legumes to grow in soils that lack nitrogen. Nitrogen is mostly acquired from the soil as nitrate and its level in the soil affects nodulation and nitrogen fixation. The mechanism(s) by which legumes modulate nitrate uptake to regulate nodule symbiosis remain unclear. In Medicago truncatula, the MtNPF1.7 transporter has been shown to control nodulation, symbiosis, and root architecture. MtNPF1.7 belongs to the nitrate/peptide transporter family and is a symporter with nitrate transport driven by proton(s). In this study we combined in silico structural predictions with in planta complementation of the severely defective mtnip-1 mutant plants to understand the role of a series of distinct amino acids in the transporter's function. Our results support hypotheses about the functional importance of the ExxE(R/K) motif including an essential role for the first glutamic acid of the motif in proton(s) and possibly substrate transport. Results reveal that Motif A, a motif conserved among major facilitator transport (MFS) proteins, is essential for function. We hypothesize that it participates in intradomain packing of transmembrane helices and stabilizing one conformation during transport. Our results also question the existence of a putative TMH4-TMH10 salt bridge. These results are discussed in the context of potential nutrient transport functions for MtNPF1.7. Our findings add to the knowledge of the mechanism of alternative conformational changes as well as symport transport in NPFs and enhance our knowledge of the mechanisms for nitrate signaling.

The Effect of Exogenous Nitrate on LCO Signalling, Cytokinin Accumulation, and Nodule Initiation in Medicago truncatula

Nitrogen fixation by rhizobia is a highly energy-demanding process. Therefore, nodule initiation in legumes is tightly regulated. Environmental nitrate is a potent inhibitor of nodulation. However, the precise mechanism by which this agent (co)regulates the inhibition of nodulation is not fully understood. Here, we demonstrate that in Medicago truncatula the lipo-chitooligosaccharide-induced accumulation of cytokinins is reduced in response to the application of exogenous nitrate. Under permissive nitrate conditions, perception of rhizobia-secreted signalling molecules leads to an increase in the level of four cytokinins (i.e., iP, iPR, tZ, and tZR). However, under high-nitrate conditions, this increase in cytokinins is reduced. The ethylene-insensitive mutant Mtein2/sickle, as well as wild-type plants grown in the presence of the ethylene biosynthesis inhibitor 2-aminoethoxyvinyl glycine (AVG), is resistant to the inhibition of nodulation by nitrate. This demonstrates that ethylene biosynthesis and perception are required to inhibit nodule organogenesis under high-nitrate conditions.

Co-Inoculation of Mesorhizobium ciceri with Either Bacillus sp. or Enterobacter aerogenes on Chickpea Improves Growth and Productivity in Phosphate-Deficient Soils in Dry Areas of a Mediterranean Region

Biological nitrogen fixation requires a large amount of phosphorus (P). However, most of the soils are P-deficient and the extensive use of P- chemical fertilizers constitute a serious threat to the environment. In this context, two field experiments were carried out to investigate the effect of co-inoculation of Mesorhizobium ciceri with phosphate solubilizing bacteria (PSB), Bacillus sp., and Enterobacter aerogenes, on chickpea as an alternative to chemical nitrogen (N) and phosphorous fertilizers in P-deficient soils in dry areas of Morocco. The results revealed that combined inoculation of chickpea with rhizobia and PSB showed a significant enhancement of chickpea nodulation, biomass production, yields and N, P, and protein content in grains as compared to single inoculation or single application of N or P. A significantly higher increase was obtained by inoculating chickpea with Mesorhizobium sp. MA72 combined with E. aerogenes P1S6. This combination allowed an enhancement of more than 270% in nodulation, 192% in shoot dry weight and 242% in grain yield. The effect of this combination was equivalent to the effect of combined application of N and P fertilizers. Formulation of biofertilizers based on tasted strains could be used for chickpea co-inoculation in P-deficient soils for an eco-friendly sustainable production of chickpea.

Characterisation of Medicago truncatula CLE34 and CLE35 in nitrate and rhizobia regulation of nodulation

Legumes form a symbiosis with atmospheric nitrogen (N₂ )-fixing soil rhizobia, resulting in new root organs called nodules that enable N₂ -fixation. Nodulation is a costly process that is tightly regulated by the host through autoregulation of nodulation (AON) and nitrate-dependent regulation of nodulation. Both pathways require legume-specific CLAVATA/ESR-related (CLE) peptides. Nitrogen-induced nodulation-suppressing CLE peptides have not previously been investigated in Medicago truncatula, for which only rhizobia-induced MtCLE12 and MtCLE13 have been characterised. Here, we report on novel peptides MtCLE34 and MtCLE35 in nodulation control. The nodulation-suppressing CLE peptides of five legume species were classified into three clades based on sequence homology and phylogeny. This approached identified MtCLE34 and MtCLE35 and four new CLE peptide orthologues of Pisum sativum. Whereas MtCLE12 and MtCLE13 are induced by rhizobia, MtCLE34 and MtCLE35 respond to both rhizobia and nitrate. MtCLE34 was identified as a pseudogene lacking a functional CLE-domain. MtCLE35 was found to inhibit nodulation in a SUNN- and RDN1-dependent manner via overexpression analysis. Together, our findings indicate that MtCLE12 and MtCLE13 have a specific role in AON, while MtCLE35 regulates nodule numbers in response to both rhizobia and nitrate. MtCLE34 likely had a similar role to MtCLE35, but its function was lost due to a premature nonsense mutation.

The role of CLAVATA signalling in the negative regulation of mycorrhizal colonization and nitrogen response of tomato

Plants form mutualistic nutrient-acquiring symbioses with microbes, including arbuscular mycorrhizal fungi. The formation of these symbioses is costly, and plants employ a negative feedback loop termed autoregulation of mycorrhizae (AOM) to limit formation of arbuscular mycorrhizae (AM). We provide evidence for the role of one leucine-rich repeat receptor-like kinase (FAB), a hydroxyproline O-arabinosyltransferase enzyme (FIN), and additional evidence for one receptor-like protein (SlCLV2) in the negative regulation of AM formation in tomato. Reciprocal grafting experiments suggest that the FAB gene acts locally in the root, while the SlCLV2 gene may act in both the root and the shoot. External nutrients including phosphate and nitrate can also strongly suppress AM formation. We found that FAB and FIN are required for nitrate suppression of AM but are not required for the powerful suppression of AM colonization by phosphate. This parallels some of the roles of legume homologues in the autoregulation of the more recently evolved symbioses with nitrogen-fixing bacteria leading to nodulation. This deep homology in the symbiotic role of these genes suggests that in addition to the early signalling events that lead to the establishment of AM and nodulation, the autoregulation pathway might also be considered part of the common symbiotic toolkit that enabled plants to form beneficial symbioses.

Recent development and new insight of diversification and symbiosis specificity of legume rhizobia: mechanism and application

Currently, symbiotic rhizobia (sl., rhizobium) refer to the soil bacteria in α- and β-Proteobacteria that can induce root and/or stem nodules on some legumes and a few of nonlegumes. In the nodules, rhizobia convert the inert dinitrogen gas (N₂ ) into ammonia (NH₃ ) and supply them as nitrogen nutrient to the host plant. In general, this symbiotic association presents specificity between rhizobial and leguminous species, and most of the rhizobia use lipochitooligosaccharides, so called Nod factor (NF), for cooperating with their host plant to initiate the formation of nodule primordium and to inhibit the plant immunity. Besides NF, effectors secreted by type III secretion system (T3SS), exopolysaccharides and many microbe-associated molecular patterns in the rhizobia also play important roles in nodulation and immunity response between rhizobia and legumes. However, the promiscuous hosts like Glycine max and Sophora flavescens can nodulate with various rhizobial species harbouring diverse symbiosis genes in different soils, meaning that the nodulation specificity/efficiency might be mainly determined by the host plants and regulated by the soil conditions in a certain cases. Based on previous studies on rhizobial application, we propose a '1+n-N' model to promote the function of symbiotic nitrogen fixation (SNF) in agricultural practice, where '1' refers to appreciate rhizobium; '+n' means the addition of multiple trace elements and PGPR bacteria; and '-N' implies the reduction of chemical nitrogen fertilizer. Finally, open questions in the SNF field are raised to future think deeply and researches.

Genetic Factors Associated With Nodulation and Nitrogen Derived From Atmosphere in a Middle American Common Bean Panel

Among grain legume crops, common beans (Phaseolus vulgaris L.) are considered to have poor biological nitrogen (N₂) fixation (BNF) capabilities although variation in N₂ fixing capabilities exists within the species. The availability of genetic panel varying in BNF capacity and a large-scale single nucleotide polymorphism (SNP) data set for common bean provided an opportunity to discover genetic factors associated with N₂ fixation among genotypes in the Middle American gene pool. Using nodulation and percentage of N₂-derived from atmosphere (%NDFA) data collected from field trials, at least 11 genotypes with higher levels of BNF capacity were identified. Genome-wide association studies (GWASs) detected both major and minor effects that control these traits. A major nodulation interval at Pv06:28.0-28.27 Mbp was discovered. In this interval, the peak SNP was located within a small GTPase that positively regulates cellular polarity and growth of root hair tips. Located 20 kb upstream of this peak SNP is an auxin-responsive factor AUX/indole acetic auxin (IAA)-related gene involved in auxin transportation during root nodulation. For %NDFA, nitrate (NO₃ ^-) transporters, NRT1:2 and NRT1.7 (Pv02:8.64), squamosa promoter binding transcriptome factor (Pv08:28.42), and multi-antimicrobial extrusion protein (MATE) efflux family protein (Pv06:10.91) were identified as candidate genes. Three additional QTLs were identified on chromosomes Pv03:5.24, Pv09:25.89, and Pv11: 32.89 Mbp. These key candidate genes from both traits were integrated with previous results on N₂ fixation to describe a BNF pathway.

Transfer cells mediate nitrate uptake to control root nodule symbiosis

Root nodule symbiosis enables nitrogen fixation in legumes and, therefore, improves crop production for sustainable agriculture^1,2. Environmental nitrate levels affect nodulation and nitrogen fixation, but the mechanisms by which legume plants modulate nitrate uptake to regulate nodule symbiosis remain unclear¹. Here, we identify a member of the Medicago truncatula nitrate peptide family (NPF), NPF7.6, which is expressed specifically in the nodule vasculature. NPF7.6 localizes to the plasma membrane of nodule transfer cells (NTCs), where it functions as a high-affinity nitrate transporter. Transfer cells show characteristic wall ingrowths that enhance the capacity for membrane transport at the apoplasmic-symplasmic interface between the vasculature and surrounding tissues³. Importantly, knockout of NPF7.6 using CRISPR-Cas9 resulted in developmental defects of the nodule vasculature, with excessive expansion of NTC plasma membranes. npf7.6 nodules showed severely compromised nitrate responsiveness caused by an attenuated ability to transport nitrate. Moreover, npf7.6 nodules exhibited disturbed nitric oxide homeostasis and a notable decrease in nitrogenase activity. Our findings indicate that NPF7.6 has been co-opted into a regulatory role in nodulation, functioning in nitrate uptake through NTCs to fine-tune nodule symbiosis in response to fluctuating environmental nitrate status. These observations will inform efforts to optimize nitrogen fixation in legume crops.

Regulation of Resource Partitioning Coordinates Nitrogen and Rhizobia Responses and Autoregulation of Nodulation in Medicago truncatula

Understanding how plants respond to nitrogen in their environment is crucial for determining how they use it and how the nitrogen use affects other processes related to plant growth and development. Under nitrogen limitation the activity and affinity of uptake systems is increased in roots, and lateral root formation is regulated in order to adapt to low nitrogen levels and scavenge from the soil. Plants in the legume family can form associations with rhizobial nitrogen-fixing bacteria, and this association is tightly regulated by nitrogen levels. The effect of nitrogen on nodulation has been extensively investigated, but the effects of nodulation on plant nitrogen responses remain largely unclear. In this study, we integrated molecular and phenotypic data in the legume Medicago truncatula and determined that genes controlling nitrogen influx are differently expressed depending on whether plants are mock or rhizobia inoculated. We found that a functional autoregulation of nodulation pathway is required for roots to perceive, take up, and mobilize nitrogen as well as for normal root development. Our results together revealed that autoregulation of nodulation, root development, and the location of nitrogen are processes balanced by the whole plant system as part of a resource-partitioning mechanism.

Legume nodulation: The host controls the party

Global demand to increase food production and simultaneously reduce synthetic nitrogen fertilizer inputs in agriculture are underpinning the need to intensify the use of legume crops. The symbiotic relationship that legume plants establish with nitrogen-fixing rhizobia bacteria is central to their advantage. This plant-microbe interaction results in newly developed root organs, called nodules, where the rhizobia convert atmospheric nitrogen gas into forms of nitrogen the plant can use. However, the process of developing and maintaining nodules is resource intensive; hence, the plant tightly controls the number of nodules forming. A variety of molecular mechanisms are used to regulate nodule numbers under both favourable and stressful growing conditions, enabling the plant to conserve resources and optimize development in response to a range of circumstances. Using genetic and genomic approaches, many components acting in the regulation of nodulation have now been identified. Discovering and functionally characterizing these components can provide genetic targets and polymorphic markers that aid in the selection of superior legume cultivars and rhizobia strains that benefit agricultural sustainability and food security. This review addresses recent findings in nodulation control, presents detailed models of the molecular mechanisms driving these processes, and identifies gaps in these processes that are not yet fully explained.

Nitrate-mediated control of root nodule symbiosis

Nitrogen is an indispensable inorganic nutrient that is required by plants throughout their life. Root nodule symbiosis (RNS) is an important strategy mainly adopted by legumes to enhance nitrogen acquisition, where several key processes required for the establishment of the symbiosis, are pleiotropically controlled by nitrate availability in soil. Although the autoregulation of nodulation (AON), a systemic long-range signaling, has been suggested to be implicated in nitrate-induced control of RNS, AON alone is insufficient to fully explain the pleiotropic regulation that is induced by nitrate. A recent elucidation of the function of a NIN-LIKE PROTEIN transcription factor has provided greater insights into the genetic mechanisms underlying nitrate-induced control of RNS in varying nitrate environments.

Auxin transport, metabolism, and signalling during nodule initiation: indeterminate and determinate nodules

Most legumes can form a unique type of lateral organ on their roots: root nodules. These structures host symbiotic nitrogen-fixing bacteria called rhizobia. Several different types of nodules can be found in nature, but the two best-studied types are called indeterminate and determinate nodules. These two types differ with respect to the presence or absence of a persistent nodule meristem, which consistently correlates with the cortical cell layers giving rise to the nodule primordia. Similar to other plant developmental processes, auxin signalling overlaps with the site of organ initiation and meristem activity. Here, we review how auxin contributes to early nodule development. We focus on changes in auxin transport, signalling, and metabolism during nodule initiation, describing both experimental evidence and computer modelling. We discuss how indeterminate and determinate nodules may differ in their mechanisms for generating localized auxin response maxima and highlight outstanding questions for future research.

Spatio-temporal control of mutualism in legumes helps spread symbiotic nitrogen fixation

Mutualism is of fundamental importance in ecosystems. Which factors help to keep the relationship mutually beneficial and evolutionarily successful is a central question. We addressed this issue for one of the most significant mutualistic interactions on Earth, which associates plants of the leguminosae family and hundreds of nitrogen (N2)-fixing bacterial species. Here we analyze the spatio-temporal dynamics of fixers and non-fixers along the symbiotic process in the Cupriavidus taiwanensis-Mimosa pudica system. N2-fixing symbionts progressively outcompete isogenic non-fixers within root nodules, where N2-fixation occurs, even when they share the same nodule. Numerical simulations, supported by experimental validation, predict that rare fixers will invade a population dominated by non-fixing bacteria during serial nodulation cycles with a probability that is function of initial inoculum, plant population size and nodulation cycle length. Our findings provide insights into the selective forces and ecological factors that may have driven the spread of the N2-fixation mutualistic trait.

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.