Unraveling new molecular players involved in the autoregulation of nodulation in Medicago truncatula

Staying hungry: a roadmap to harnessing central regulators of symbiotic nitrogen fixation under fluctuating nitrogen availability

Legumes have evolved specific inventions to enhance nitrogen (N) acquisition by establishing symbiotic interactions with N-fixing rhizobial bacteria. Because symbiotic N fixation is energetically costly, legumes have developed sophisticated mechanisms to ensure carbon-nitrogen balance, in a variable environment, both locally and at the whole plant level, by monitoring nodule number, nodule development, and nodular nitrogenase activity, as well as controlling nodule senescence. Studies of the autoregulation of nodulation and regulation of nodulation by nodule inception (NIN) and NIN-LIKE PROTEINs (NLPs) have provided great insights into the genetic mechanisms underlying the nitrate-induced regulation of root nodulation for adapting to N availability in the rhizosphere. However, many aspects of N-induced pleiotropic regulation remain to be fully explained, such as N-triggered senescence in mature nodules. Wang et al. determined that this process is governed by a transcriptional network regulated by NAC-type transcription factors. Characterization and dissection of these soybean nitrogen-associated NAPs (SNAPs) transcription factor-mastered networks have yielded a roadmap for exploring how legumes rewire nodule functions across a range of N levels, laying the foundation for enhancing the growth of N-deprived crops in agricultural settings.

Mutation of BAM2 rescues the sunn hypernodulation phenotype in Medicago truncatula, suggesting that a signaling pathway like CLV1/BAM in Arabidopsis affects nodule number

The unique evolutionary adaptation of legumes for nitrogen-fixing symbiosis leading to nodulation is tightly regulated by the host plant. The autoregulation of nodulation (AON) pathway negatively regulates the number of nodules formed in response to the carbon/nitrogen metabolic status of the shoot and root by long-distance signaling to and from the shoot and root. Central to AON signaling in the shoots of Medicago truncatula is SUNN, a leucine-rich repeat receptor-like kinase with high sequence similarity with CLAVATA1 (CLV1), part of a class of receptors in Arabidopsis involved in regulating stem cell populations in the root and shoot. This class of receptors in Arabidopsis includes the BARELY ANY MERISTEM family, which, like CLV1, binds to CLE peptides and interacts with CLV1 to regulate meristem development. M. truncatula contains five members of the BAM family, but only MtBAM1 and MtBAM2 are highly expressed in the nodules 48 hours after inoculation. Plants carry mutations in individual MtBAMs, and several double BAM mutant combinations all displayed wild-type nodule number phenotypes. However, Mtbam2 suppressed the sunn-5 hypernodulation phenotype and partially rescued the short root length phenotype of sunn-5 when present in a sunn-5 background. Grafting determined that bam2 suppresses supernodulation from the roots, regardless of the SUNN status of the root. Overexpression of MtBAM2 in wild-type plants increases nodule numbers, while overexpression of MtBAM2 in some sunn mutants rescues the hypernodulation phenotype, but not the hypernodulation phenotypes of AON mutant rdn1-2 or crn. Relative expression measurements of the nodule transcription factor MtWOX5 downstream of the putative bam2 sunn-5 complex revealed disruption of meristem signaling; while both bam2 and bam2 sunn-5 influence MtWOX5 expression, the expression changes are in different directions. We propose a genetic model wherein the specific root interactions of BAM2/SUNN are critical for signaling in nodule meristem cell homeostasis in M. truncatula.

CEP hormones at the nexus of nutrient acquisition and allocation, root development, and plant-microbe interactions

A growing understanding is emerging of the roles of peptide hormones in local and long-distance signalling that coordinates plant growth and development as well as responses to the environment. C-TERMINALLY ENCODED PEPTIDE (CEP) signalling triggered by its interaction with CEP RECEPTOR 1 (CEPR1) is known to play roles in systemic nitrogen (N) demand signalling, legume nodulation, and root system architecture. Recent research provides further insight into how CEP signalling operates, which involves diverse downstream targets and interactions with other hormone pathways. Additionally, there is emerging evidence of CEP signalling playing roles in N allocation, root responses to carbon levels, the uptake of other soil nutrients such as phosphorus and sulfur, root responses to arbuscular mycorrhizal fungi, plant immunity, and reproductive development. These findings suggest that CEP signalling more broadly coordinates growth across the whole plant in response to diverse environmental cues. Moreover, CEP signalling and function appear to be conserved in angiosperms. We review recent advances in CEP biology with a focus on soil nutrient uptake, root system architecture and organogenesis, and roles in plant-microbe interactions. Furthermore, we address knowledge gaps and future directions in this research field.

TML1 AND TML2 SYNERGISTICALLY REGULATE NODULATION BUT NOT ARBUSCULAR MYCORRHIZA IN MEDICAGO TRUNCATULA

Two symbiotic processes, nodulation and arbuscular mycorrhiza, are primarily controlled by the plant's need for nitrogen (N) and phosphorus (P), respectively. Autoregulation of Nodulation (AON) and Autoregulation of Mycorrhization (AOM) share multiple components - plants that make too many nodules usually have higher arbuscule density. The protein TML (TOO MUCH LOVE) was shown to function in roots to maintain susceptibly to rhizobial infection under low N conditions and control nodule number through AON in Lotus japonicus. M. truncatula has two sequence homologs: MtTML1 and MtTML2. We report the generation of stable single and double mutants harboring multiple allelic variations in MtTML1 and MtTML2 using CRISPR-Cas9 targeted mutagenesis and screening of a transposon mutagenesis library. Plants containing single mutations in either gene produced twice the nodules of wild type plants whereas plants containing mutations in both genes displayed a synergistic effect, forming 20x more nodules and short roots compared to wild type plants. The synergistic effect on nodulation was maintained in the presence of 10mM nitrogen, but not observed in root length phenotypes. Examination of expression and heterozygote effects suggest genetic compensation may play a role in the observed synergy. However, plants with mutations in both TMLs had no detectable change in arbuscular mycorrhizal associations, suggesting that MtTMLs are specific to nodulation and nitrate signaling. The mutants created will be useful tools to dissect the mechanism of synergistic action of MtTML1 and MtTML2 in M. truncatula nodulation as well as the separation of AON from AOM.

Comparative RNA sequencing-based transcriptome profiling of ten grapevine rootstocks: shared and specific sets of genes respond to mycorrhizal symbiosis

Arbuscular mycorrhizal symbiosis improves water and nutrient uptake by plants and provides them other ecosystem services. Grapevine is one of the major crops in the world. Vitis vinifera scions generally are grafted onto a variety of rootstocks that confer different levels of resistance against different pests, tolerance to environmental stress, and influence the physiology of the scions. Arbuscular mycorrhizal fungi are involved in the root architecture and in the immune response to soil-borne pathogens. However, the fine-tuned regulation and the transcriptomic plasticity of rootstocks in response to mycorrhization are still unknown. We compared the responses of 10 different grapevine rootstocks to arbuscular mycorrhizal symbiosis (AMS) formed with Rhizophagus irregularis DAOM197198 using RNA sequencing-based transcriptome profiling. We have highlighted a few shared regulation mechanisms, but also specific rootstock responses to R. irregularis colonization. A set of 353 genes was regulated by AMS in all ten rootstocks. We also compared the expression level of this set of genes to more than 2000 transcriptome profiles from various grapevine varieties and tissues to identify a class of transcripts related to mycorrhizal associations in these 10 rootstocks. Then, we compared the response of the 351 genes upregulated by mycorrhiza in grapevine to their Medicago truncatula homologs in response to mycorrhizal colonization based on available transcriptomic studies. More than 97% of the 351 M. truncatula-homologous grapevine genes were expressed in at least one mycorrhizal transcriptomic study, and 64% in every single RNAseq dataset. At the intra-specific level, we described, for the first time, shared and specific grapevine rootstock genes in response to R. irregularis symbiosis. At the inter-specific level, we defined a shared subset of mycorrhiza-responsive genes.

Strigolactones repress nodule development and senescence in pea

Strigolactones are a class of phytohormones that are involved in many different plant developmental processes, including the rhizobium-legume nodule symbiosis. Although both positive and negative effects of strigolactones on the number of nodules have been reported, the influence of strigolactones on nodule development is still unknown. Here, by means of the ramosus (rms) mutants of Pisum sativum (pea) cv Terese, we investigated the impact of strigolactone biosynthesis (rms1 and rms5) and signaling (rms3 and rms4) mutants on nodule growth. The rms mutants had more red, that is, functional, and larger nodules than the wild-type plants. Additionally, the increased nitrogen fixation and senescence zones with consequently reduced meristematic and infection zones indicated that the rms nodules developed faster than the wild-type nodules. An enhanced expression of the nodule zone-specific molecular markers for meristem activity and senescence supported the enlarged, fast maturing nodules. Interestingly, the master nodulation regulator, NODULE INCEPTION, NIN, was strongly induced in nodules of all rms mutants but not prior to inoculation. Determination of sugar levels with both bulk and spatial metabolomics in roots and nodules, respectively, hints at slightly increased malic acid levels early during nodule primordia formation and reduced sugar levels at later stages, possibly the consequence of an increased carbon usage of the enlarged nodules, contributing to the enhanced senescence. Taken together, these results suggest that strigolactones regulate the development of nodules, which is probably mediated through NIN, and available plant sugars.

Analysis of the structure and function of the LYK cluster of Medicago truncatula A17 and R108

The establishment of the Legume-Rhizobia symbiosis is generally dependent on the production of rhizobial lipochitooligosaccharidic Nod factors (NFs) and their perception by plant Lysin Motif Receptor-Like Kinases (LysM-RLKs). In this study, we characterized a cluster of LysM-RLK genes implicated in strain-specific recognition in two highly divergent and widely-studied Medicago truncatula genotypes, A17 and R108. We then used reverse genetic approaches and biochemical analyses to study the function of selected genes in the clusters and the ability of their encoded proteins to bind NFs. Our study has revealed that the LYK cluster exhibits a high degree of variability among M. truncatula genotypes, which in A17 and R108 includes recent recombination events within the cluster and a transposon insertion in A17. The essential role of LYK3 in nodulation in A17 is not conserved in R108 despite similar sequences and good nodulation expression profiles. Although, LYK2, LYK5 and LYK5bis are not essential for nodulation of the two genotypes, some evidence points to accessory roles in nodulation, but not through high-affinity NF binding. This work shows that recent evolution in the LYK cluster provides a source of variation for nodulation, and potential robustness of signaling through genetic redundancy.

Natural variation of GmRj2/Rfg1 determines symbiont differentiation in soybean

Symbiotic nitrogen fixation (SNF) provides much of the N utilized by leguminous plants throughout growth and development. Legumes may simultaneously establish symbiosis with different taxa of microbial symbionts. Yet, the mechanisms used to steer associations toward symbionts that are most propitious across variations in soil types remain mysterious. Here, we demonstrate that GmRj2/Rfg1 is responsible for regulating symbiosis with multiple taxa of soybean symbionts. In our experiments, the GmRj2/Rfg1SC haplotype favored association with Bradyrhizobia, which is mostly distributed in acid soils, whereas the GmRj2/Rfg1HH haplotype and knockout mutants of GmRj2/Rfg1SC associated equally with Bradyrhizobia and Sinorhizobium. Association between GmRj2/Rfg1 and NopP, furthermore, appeared to be involved in symbiont selection. Furthermore, geographic distribution analysis of 1,821 soybean accessions showed that GmRj2/Rfg1SC haplotypes were enriched in acidic soils where Bradyrhizobia were the dominant symbionts, whereas GmRj2/Rfg1HH haplotypes were most prevalent in alkaline soils dominated by Sinorhizobium, and neutral soils harbored no apparent predilections toward either haplotype. Taken together, our results suggest that GmRj2/Rfg1 regulates symbiosis with different symbionts and is a strong determinant of soybean adaptability across soil regions. As a consequence, the manipulation of the GmRj2/Rfg1 genotype or application of suitable symbionts according to the haplotype at the GmRj2/Rfg1 locus might be suitable strategies to explore for increasing soybean yield through the management of SNF.

A Medicago truncatula Autoregulation of Nodulation Mutant Transcriptome Analysis Reveals Disruption of the SUNN Pathway Causes Constitutive Expression Changes in Some Genes, but Overall Response to Rhizobia Resembles Wild-Type, Including Induction of TML1 and TML2

Nodule number regulation in legumes is controlled by a feedback loop that integrates nutrient and rhizobia symbiont status signals to regulate nodule development. Signals from the roots are perceived by shoot receptors, including a CLV1-like receptor-like kinase known as SUNN in Medicago truncatula. In the absence of functional SUNN, the autoregulation feedback loop is disrupted, resulting in hypernodulation. To elucidate early autoregulation mechanisms disrupted in SUNN mutants, we searched for genes with altered expression in the loss-of-function sunn-4 mutant and included the rdn1-2 autoregulation mutant for comparison. We identified constitutively altered expression of small groups of genes in sunn-4 roots and in sunn-4 shoots. All genes with verified roles in nodulation that were induced in wild-type roots during the establishment of nodules were also induced in sunn-4, including autoregulation genes TML2 and TML1. Only an isoflavone-7-O-methyltransferase gene was induced in response to rhizobia in wild-type roots but not induced in sunn-4. In shoot tissues of wild-type, eight rhizobia-responsive genes were identified, including a MYB family transcription factor gene that remained at a baseline level in sunn-4; three genes were induced by rhizobia in shoots of sunn-4 but not wild-type. We cataloged the temporal induction profiles of many small secreted peptide (MtSSP) genes in nodulating root tissues, encompassing members of twenty-four peptide families, including the CLE and IRON MAN families. The discovery that expression of TML2 in roots, a key factor in inhibiting nodulation in response to autoregulation signals, is also triggered in sunn-4 in the section of roots analyzed, suggests that the mechanism of TML regulation of nodulation in M. truncatula may be more complex than published models.

The Medicago SymCEP7 hormone increases nodule number via shoots without compromising lateral root number

Legumes acquire soil nutrients through nitrogen-fixing root nodules and lateral roots. To balance the costs and benefits of nodulation, legumes negatively control root nodule number by autoregulatory and hormonal pathways. How legumes simultaneously coordinate root nodule and lateral root development to procure nutrients remains poorly understood. In Medicago (Medicago truncatula), a subset of mature C-TERMINALLY ENCODED PEPTIDE (CEP) hormones can systemically promote nodule number, but all CEP hormones tested to date negatively regulate lateral root number. Here we showed that Medicago CEP7 produces a mature peptide, SymCEP7, that promotes nodulation from the shoot without compromising lateral root number. Rhizobial inoculation induced CEP7 in the susceptible root nodulation zone in a Nod factor-dependent manner, and, in contrast to other CEP genes, its transcription level was elevated in the ethylene signaling mutant sickle. Using mass spectrometry, fluorescence microscopy and expression analysis, we demonstrated that SymCEP7 activity requires the COMPACT ROOT ARCHITECTURE 2 receptor and activates the shoot-to-root systemic effector, miR2111. Shoot-applied SymCEP7 rapidly promoted nodule number in the pM to nM range at concentrations up to five orders of magnitude lower than effects mediated by root-applied SymCEP7. Shoot-applied SymCEP7 also promoted nodule number in White Clover (Trifolium repens) and Lotus (Lotus japonicus), which suggests that this biological function may be evolutionarily conserved. We propose that SymCEP7 acts in the Medicago shoot to counter balance the autoregulation pathways induced rapidly by rhizobia to enable nodulation without compromising lateral root growth, thus promoting the acquisition of nutrients other than nitrogen to support their growth.

Control of the rhizobium-legume symbiosis by the plant nitrogen demand is tightly integrated at the whole plant level and requires inter-organ systemic signaling

Symbiotic nodules formed on legume roots with rhizobia fix atmospheric N₂. Bacteria reduce N₂ to NH₄ ⁺ that is assimilated into amino acids by the plant. In return, the plant provides photosynthates to fuel the symbiotic nitrogen fixation. Symbiosis is tightly adjusted to the whole plant nutritional demand and to the plant photosynthetic capacities, but regulatory circuits behind this control remain poorly understood. The use of split-root systems combined with biochemical, physiological, metabolomic, transcriptomic, and genetic approaches revealed that multiple pathways are acting in parallel. Systemic signaling mechanisms of the plant N demand are required for the control of nodule organogenesis, mature nodule functioning, and nodule senescence. N-satiety/N-deficit systemic signaling correlates with rapid variations of the nodules' sugar levels, tuning symbiosis by C resources allocation. These mechanisms are responsible for the adjustment of plant symbiotic capacities to the mineral N resources. On the one hand, if mineral N can satisfy the plant N demand, nodule formation is inhibited, and nodule senescence is activated. On the other hand, local conditions (abiotic stresses) may impair symbiotic activity resulting in plant N limitation. In these conditions, systemic signaling may compensate the N deficit by stimulating symbiotic root N foraging. In the past decade, several molecular components of the systemic signaling pathways controlling nodule formation have been identified, but a major challenge remains, that is, to understand their specificity as compared to the mechanisms of non-symbiotic plants that control root development and how they contribute to the whole plant phenotypes. Less is known about the control of mature nodule development and functioning by N and C nutritional status of the plant, but a hypothetical model involving the sucrose allocation to the nodule as a systemic signaling process, the oxidative pentose phosphate pathway, and the redox status as potential effectors of this signaling is emerging. This work highlights the importance of organism integration in plant biology.

Local and systemic targets of the MtCLE35-SUNN pathway in the roots of Medicago truncatula

CLE (CLAVATA3/ENDOSPERM SURROUNDING REGION-related) peptides are systemic regulators of legume-rhizobium symbiosis that negatively control the number of nitrogen-fixing nodules. CLE peptides are produced in the root in response to rhizobia inoculation and/or nitrate treatment and are transported to the shoot where they are recognized by the CLV1-like (CLAVATA1-like) receptor kinase. As a result, a shoot-derived signaling pathway is activated that inhibits subsequent nodule development in the root. In Medicago truncatula, MtCLE35 is activated in response to rhizobia and nitrate treatment and the overexpression of this gene systemically inhibits nodulation. The inhibitory effect of MtCLE35 overexpression is dependent on the CLV1-like receptor kinase MtSUNN (SUPER NUMERIC NODULES), suggesting that MtSUNN could be involved in the reception of the MtCLE35 peptide. Yet little is known about the downstream genes regulated by a MtCLE35-activated response in the root. In order to identify genes whose expression levels could be regulated by the MtCLE35-MtSUNN pathway, we performed a MACE-Seq (Massive Analysis of cDNA Ends) transcriptomic analysis of MtCLE35-overexpressing roots. Among upregulated genes, the gene MtSUNN that encodes a putative receptor of MtCLE35 was detected. Moreover, we found that MtSUNN, as well as several other differentially expressed genes, were upregulated locally in MtCLE35-overexpressing roots whereas the MtTML1 and MtTML2 genes were upregulated systemically. Our data suggest that MtCLE35 has both local and systemic effects on target genes in the root.

Cell-specific pathways recruited for symbiotic nodulation in the Medicago truncatula legume

Medicago truncatula is a model legume species that has been studied for decades to understand the symbiotic relationship between legumes and soil bacteria collectively named rhizobia. This symbiosis called nodulation is initiated in roots with the infection of root hair cells by the bacteria, as well as the initiation of nodule primordia from root cortical, endodermal, and pericycle cells, leading to the development of a new root organ, the nodule, where bacteria fix and assimilate the atmospheric dinitrogen for the benefit of the plant. Here, we report the isolation and use of the nuclei from mock and rhizobia-inoculated roots for the single nuclei RNA-seq (sNucRNA-seq) profiling to gain a deeper understanding of early responses to rhizobial infection in Medicago roots. A gene expression map of the Medicago root was generated, comprising 25 clusters, which were annotated as specific cell types using 119 Medicago marker genes and orthologs to Arabidopsis cell-type marker genes. A focus on root hair, cortex, endodermis, and pericycle cell types, showing the strongest differential regulation in response to a short-term (48 h) rhizobium inoculation, revealed not only known genes and functional pathways, validating the sNucRNA-seq approach, but also numerous novel genes and pathways, allowing a comprehensive analysis of early root symbiotic responses at a cell type-specific level.

Deciphering the role of SPL12 and AGL6 from a genetic module that functions in nodulation and root regeneration in Medicago sativa

Our results show that SPL12 plays a crucial role in regulating nodule development in Medicago sativa L. (alfalfa), and that AGL6 is targeted and downregulated by SPL12. Root architecture in plants is critical because of its role in controlling nutrient cycling, water use efficiency and response to biotic and abiotic stress factors. The small RNA, microRNA156 (miR156), is highly conserved in plants, where it functions by silencing a group of SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors. We previously showed that transgenic Medicago sativa (alfalfa) plants overexpressing miR156 display increased nodulation, improved nitrogen fixation and enhanced root regenerative capacity during vegetative propagation. In alfalfa, transcripts of eleven SPLs, including SPL12, are targeted for cleavage by miR156. In this study, we characterized the role of SPL12 in root architecture and nodulation by investigating the transcriptomic and phenotypic changes associated with altered transcript levels of SPL12, and by determining SPL12 regulatory targets using SPL12-silencing and -overexpressing alfalfa plants. Phenotypic analyses showed that silencing of SPL12 in alfalfa caused an increase in root regeneration, nodulation, and nitrogen fixation. In addition, AGL6 which encodes AGAMOUS-like MADS box transcription factor, was identified as being directly targeted for silencing by SPL12, based on Next Generation Sequencing-mediated transcriptome analysis and chromatin immunoprecipitation assays. Taken together, our results suggest that SPL12 and AGL6 form a genetic module that regulates root development and nodulation in alfalfa.

A dual legume-rhizobium transcriptome of symbiotic nodule senescence reveals coordinated plant and bacterial responses

Senescence determines plant organ lifespan depending on aging and environmental cues. During the endosymbiotic interaction with rhizobia, legume plants develop a specific organ, the root nodule, which houses nitrogen (N)-fixing bacteria. Unlike earlier processes of the legume-rhizobium interaction (nodule formation, N fixation), mechanisms controlling nodule senescence remain poorly understood. To identify nodule senescence-associated genes, we performed a dual plant-bacteria RNA sequencing approach on Medicago truncatula-Sinorhizobium meliloti nodules having initiated senescence either naturally (aging) or following an environmental trigger (nitrate treatment or salt stress). The resulting data allowed the identification of hundreds of plant and bacterial genes differentially regulated during nodule senescence, thus providing an unprecedented comprehensive resource of new candidate genes associated with this process. Remarkably, several plant and bacterial genes related to the cell cycle and stress responses were regulated in senescent nodules, including the rhizobial RpoE2-dependent general stress response. Analysis of selected core nodule senescence plant genes allowed showing that MtNAC969 and MtS40, both homologous to leaf senescence-associated genes, negatively regulate the transition between N fixation and senescence. In contrast, overexpression of a gene involved in the biosynthesis of cytokinins, well-known negative regulators of leaf senescence, may promote the transition from N fixation to senescence in nodules.

A rulebook for peptide control of legume-microbe endosymbioses

Plants engage in mutually beneficial relationships with microbes, such as arbuscular mycorrhizal fungi or nitrogen-fixing rhizobia, for optimized nutrient acquisition. In return, the microbial symbionts receive photosynthetic carbon from the plant. Both symbioses are regulated by the plant nutrient status, indicating the existence of signaling pathways that allow the host to fine-tune its interactions with the beneficial microbes depending on its nutrient requirements. Peptide hormones coordinate a plethora of developmental and physiological processes and, recently, various peptide families have gained special attention as systemic and local regulators of plant-microbe interactions and nutrient homeostasis. In this review, we identify five 'rules' or guiding principles that govern peptide function during symbiotic plant-microbe interactions, and highlight possible points of integration with nutrient acquisition pathways.

Shoot-to-root translocated GmNN1/FT2a triggers nodulation and regulates soybean nitrogen nutrition

Symbiotic nitrogen fixation (SNF) provides sufficient nitrogen (N) to meet most legume nutrition demands. In return, host plants feed symbionts carbohydrates produced in shoots. However, the molecular dialogue between shoots and symbionts remains largely mysterious. Here, we report the map-based cloning and characterization of a natural variation in GmNN1, the ortholog of Arabidopsis thaliana FLOWERING LOCUS T (FT2a) that simultaneously triggers nodulation in soybean and modulates leaf N nutrition. A 43-bp insertion in the promoter region of GmNN1/FT2a significantly decreased its transcription level and yielded N deficiency phenotypes. Manipulating GmNN1/GmFT2a significantly enhanced soybean nodulation, plant growth, and N nutrition. The near-isogenic lines (NILs) carrying low mRNA abundance alleles of GmNN1/FT2a, along with stable transgenic soybeans with CRISPR/Cas9 knockouts of GmNN1/FT2a, had yellower leaves, lower N concentrations, and fewer nodules than wild-type control plants. Grafting together with split-root experiments demonstrated that only shoot GmNN1/FT2a was responsible for regulating nodulation and thereby N nutrition through shoot-to-root translocation, and this process depends on rhizobial infection. After translocating into roots, shoot-derived GmNN1/FT2a was found to interact with GmNFYA-C (nuclear factor-Y subunit A-C) to activate symbiotic signaling through the previously reported GmNFYA-C-ENOD40 module. In short, the description of the critical soybean nodulation regulatory pathway outlined herein sheds novel insights into the shoot-to-root signaling required for communications between host plants and root nodulating symbionts.

Symbiotic nitrogen fixation provides a vital nitrogen source in agroecosystems but nodulation is tightly controlled by a long-distance signaling system. This study uses map-based cloning to reveal GmNN1/FT2a as a new shoot-to-root mobile protein that significantly regulates nodule formation and thus nitrogen nutrition in soybean.

Progress in the Self-Regulation System in Legume Nodule Development-AON (Autoregulation of Nodulation)

The formation and development of legumes nodules requires a lot of energy. Legumes must strictly control the number and activity of nodules to ensure efficient energy distribution. The AON system can limit the number of rhizobia infections and nodule numbers through the systemic signal pathway network that the aboveground and belowground parts participate in together. It can also promote the formation of nodules when plants are deficient in nitrogen. The currently known AON pathway includes four parts: soil NO₃⁻ signal and Rhizobium signal recognition and transmission, CLE-SUNN is the negative regulation pathway, CEP-CRA2 is the positive regulation pathway and the miR2111/TML module regulates nodule formation and development. In order to ensure the biological function of this important approach, plants use a variety of plant hormones, polypeptides, receptor kinases, transcription factors and miRNAs for signal transmission and transcriptional regulation. This review summarizes and discusses the research progress of the AON pathway in Legume nodule development.

Nitrogen represses haustoria formation through abscisic acid in the parasitic plant Phtheirospermum japonicum

Parasitic plants are globally prevalent pathogens that withdraw nutrients from their host plants using an organ known as the haustorium. The external environment including nutrient availability affects the extent of parasitism and to understand this phenomenon, we investigated the role of nutrients and found that nitrogen is sufficient to repress haustoria formation in the root parasite Phtheirospermum japonicum. Nitrogen increases levels of abscisic acid (ABA) in P. japonicum and prevents the activation of hundreds of genes including cell cycle and xylem development genes. Blocking ABA signaling overcomes nitrogen’s inhibitory effects indicating that nitrogen represses haustoria formation by increasing ABA. The effect of nitrogen appears more widespread since nitrogen also inhibits haustoria in the obligate root parasite Striga hermonthica. Together, our data show that nitrogen acts as a haustoria repressing factor and suggests a mechanism whereby parasitic plants use nitrogen availability in the external environment to regulate the extent of parasitism.

Parasitic plants obtain nutrients from their hosts. Here the authors show that nitrogen sufficiency suppresses parasitism in the root parasite Phtheirospermum japonicum by increasing levels of the phytohormone ABA suggesting that the degree of parasitism is regulated by nutrient availability.

Understanding Nod factor signalling paves the way for targeted engineering in legumes and non-legumes

Legumes evolved LysM receptors for recognition of rhizobial Nod factors and initiation of signalling pathways for nodule organogenesis and infection. Intracellularly hosted bacteria are supplied with carbon resources in exchange for fixed nitrogen. Nod factor recognition is crucial for initial signalling, but is reiterated in growing roots initiating novel symbiotic events, and in developing primordia until symbiosis is well-established. Understanding how this signalling coordinates the entire process from cellular to plant level is key for de novo engineering in non-legumes and for improved efficiency in legumes. Here we discuss how recent studies bring new insights into molecular determinants of specificity and sensitivity in Nod factor signalling in legumes, and present some of the unknowns and challenges for engineering.

Nitrate-induced CLE35 signaling peptides inhibit nodulation through the SUNN receptor and miR2111 repression

Legume plants form nitrogen (N)-fixing symbiotic nodules when mineral N is limiting in soils. As N fixation is energetically costly compared to mineral N acquisition, these N sources, and in particular nitrate, inhibit nodule formation and N fixation. Here, in the model legume Medicago truncatula, we characterized a CLAVATA3-like (CLE) signaling peptide, MtCLE35, the expression of which is upregulated locally by high-N environments and relies on the Nodule Inception-Like Protein (NLP) MtNLP1. MtCLE35 inhibits nodule formation by affecting rhizobial infections, depending on the Super Numeric Nodules (MtSUNN) receptor. In addition, high N or the ectopic expression of MtCLE35 represses the expression and accumulation of the miR2111 shoot-to-root systemic effector, thus inhibiting its positive effect on nodulation. Conversely, ectopic expression of miR2111 or downregulation of MtCLE35 by RNA interference increased miR2111 accumulation independently of the N environment, and thus partially bypasses the nodulation inhibitory action of nitrate. Overall, these results demonstrate that the MtNLP1-dependent, N-induced MtCLE35 signaling peptide acts through the MtSUNN receptor and the miR2111 systemic effector to inhibit nodulation.

Nitrogen Systemic Signaling: From Symbiotic Nodulation to Root Acquisition

Plant nutrient acquisition is tightly regulated by resource availability and metabolic needs, implying the existence of communication between roots and shoots to ensure their integration at the whole-plant level. Here, we focus on systemic signaling pathways controlling nitrogen (N) nutrition, achieved both by the root import of mineral N and, in legume plants, through atmospheric N fixation by symbiotic bacteria inside dedicated root nodules. We explore features conserved between systemic pathways repressing or enhancing symbiotic N fixation and the regulation of mineral N acquisition by roots, as well as their integration with other environmental factors, such as phosphate, light, and CO₂ availability.

The interplay of phloem-mobile signals in plant development and stress response

Plants integrate a variety of biotic and abiotic factors for optimal growth in their given environment. While some of these responses are local, others occur distally. Hence, communication of signals perceived in one organ to a second, distal part of the plant and the coordinated developmental response require an intricate signaling system. To do so, plants developed a bipartite vascular system that mediates the uptake of water, minerals, and nutrients from the soil; transports high-energy compounds and building blocks; and traffics essential developmental and stress signals. One component of the plant vasculature is the phloem. The development of highly sensitive mass spectrometry and molecular methods in the last decades has enabled us to explore the full complexity of the phloem content. As a result, our view of the phloem has evolved from a simple transport path of photoassimilates to a major highway for pathogens, hormones and developmental signals. Understanding phloem transport is essential to comprehend the coordination of environmental inputs with plant development and, thus, ensure food security. This review discusses recent developments in its role in long-distance signaling and highlights the role of some of the signaling molecules. What emerges is an image of signaling paths that do not just involve single molecules but rather, quite frequently an interplay of several distinct molecular classes, many of which appear to be transported and acting in concert.

miR169c-NFYA-C-ENOD40 modulates nitrogen inhibitory effects in soybean nodulation

Legume crops contribute a great portion of clean nitrogen (N) to agro-ecosystems through symbiotic N2 fixation in the nodule; however, the nodulation is always inhibited by high N availability which is known as the N inhibitory effect through largely unknown mechanisms. We functionally investigated miR169c-GmNFYA-C-GmENOD40 under multiple N conditions in soybean (Glycine max) (ENOD, Early Nodulin; NFYA, Nuclear Factor-Y Subunit A). We elucidated their regulatory roles in soybean nodulation through analyzing expression patterns, micro-messenger RNA (miRNA-mRNA) interactions, phenotypes of transgenic soybean plants and genetic interactions. We found that miR169c expression was induced by high N, whereas its target GmNFYA-C was preferentially expressed in nodules and induced by rhizobium inoculation. Overexpression of miR169c inhibited nodulation through targeting 3'-UTR of GmNFYA-C, whereas knockout miR169c through CRISPR-cas9 promoted nodulation. However, overexpression of GmNFYA-C promoted soybean nodulation through facilitating rhizobium infection and increasing the expression of symbiotic signaling gene GmENOD40. Besides, GmNFYA-C directly induced the expression of GmENOD40. In addition, overexpression of GmNFYA-C without the target site of miR169c partially attenuated the inhibitory effect of high N on soybean nodulation. We discovered a new regulatory pathway involving the miR169c-NFYA-C-ENOD40 module that regulates soybean nodulation in response to N availability. This pathway provides substantial new insights into the mechanisms underlying the N inhibitory effect on nodulation.

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.

The Regulation of Nodule Number in Legumes Is a Balance of Three Signal Transduction Pathways

Nitrogen is a major determinant of plant growth and productivity and the ability of legumes to form a symbiotic relationship with nitrogen-fixing rhizobia bacteria allows legumes to exploit nitrogen-poor niches in the biosphere. But hosting nitrogen-fixing bacteria comes with a metabolic cost, and the process requires regulation. The symbiosis is regulated through three signal transduction pathways: in response to available nitrogen, at the initiation of contact between the organisms, and during the development of the nodules that will host the rhizobia. Here we provide an overview of our knowledge of how the three signaling pathways operate in space and time, and what we know about the cross-talk between symbiotic signaling for nodule initiation and organogenesis, nitrate dependent signaling, and autoregulation of nodulation. Identification of common components and points of intersection suggest directions for research on the fine-tuning of the plant's response to rhizobia.

Systemic Optimization of Legume Nodulation: A Shoot-Derived Regulator, miR2111

Long-distance signaling between the shoot and roots of land plants plays a crucial role in ensuring their growth and development in a fluctuating environment, such as with soil nutrient deficiencies. MicroRNAs (miRNAs) are considered to contribute to such environmental adaptation via long-distance signaling since several miRNAs are transported between the shoot and roots in response to various soil nutrient changes. Leguminous plants adopt a shoot-mediated long-distance signaling system to maintain their mutualism with symbiotic nitrogen-fixing rhizobia by optimizing the number of symbiotic organs and root nodules. Recently, the involvement and importance of shoot-derived miR2111 in regulating nodule numbers have become evident. Shoot-derived miR2111 can systemically enhance rhizobial infection, and its accumulation is quickly suppressed in response to rhizobial inoculation and high-concentration nitrate application. In this mini-review, we briefly summarize the recent progress on the systemic optimization of nodulation in response to external environments, with a focus on systemic regulation via miR2111.

Shoot Extracts from Two Low Nodulation Mutants Significantly Reduce Nodule Number in Pea

E107 and E132 are pea mutants that nodulate poorly. Because they have a shoot-controlled nodulation phenotype, we asked if their mutated genes were implicated in the autoregulation of nodulation (AON), a mechanism which consists of two systemic circuits, the positive CEP/CRA2 and the negative CLE/SUNN, coordinated via NIN and miR2111. We further characterized the mutants’ phenotype by studying nodule distribution and nodulation efficiency. E107 was similar to wild-type (WT) in its nodule distribution, but E132 had an extended nodulation zone with nodules forming distally on its lateral roots. Moreover, we tested whether their shoots produced a compound inhibitory to nodulation. We made ethyl-acetate extracts of roots and shoots of both mutants and WT, which we applied to rhizobia-inoculated WT seedlings and to pure rhizobial cultures. Whereas free-living bacteria were unaffected by any of the extracts, WT treated with shoot extracts from either inoculated mutant had fewer nodules than that of control. E107 and E132 shoot extracts led to a 50% and a 35% reduction in nodule number, respectively. We propose that E107 and E132 belong to a new sub-class of AON mutants, i.e., hypo-nodulators, and that their respective gene products are acting in the AON descending branch, upstream of TML signaling.

MIR2111-5 locus and shoot-accumulated mature miR2111 systemically enhance nodulation depending on HAR1 in Lotus japonicus

Legumes utilize a shoot-mediated signaling system to maintain a mutualistic relationship with nitrogen-fixing bacteria in root nodules. In Lotus japonicus, shoot-to-root transfer of microRNA miR2111 that targets TOO MUCH LOVE, a nodulation suppressor in roots, has been proposed to explain the mechanism underlying nodulation control from shoots. However, the role of shoot-accumulating miR2111s for the systemic regulation of nodulation was not clearly shown. Here, we find L. japonicus has seven miR2111 loci, including those mapped through RNA-seq. MIR2111-5 expression in leaves is the highest among miR2111 loci and repressed after rhizobial infection depending on a shoot-acting HYPERNODULATION ABERRANT ROOT FORMATION1 (HAR1) receptor. MIR2111-5 knockout mutants show significantly decreased nodule numbers and miR2111 levels. Furthermore, grafting experiments using transformants demonstrate scions with altered miR2111 levels influence nodule numbers in rootstocks in a dose-dependent manner. Therefore, miR2111 accumulation in leaves through MIR2111-5 expression is required for HAR1-dependent systemic optimization of nodule number.

The CLE53-SUNN genetic pathway negatively regulates arbuscular mycorrhiza root colonization in Medicago truncatula

Plants and arbuscular mycorrhizal fungi (AMF) engage in mutually beneficial symbioses based on a reciprocal exchange of nutrients. The beneficial character of the symbiosis is maintained through a mechanism called autoregulation of mycorrhization (AOM). AOM includes root-to-shoot-to-root signaling; however, the molecular details of AOM are poorly understood. AOM shares many features of autoregulation of nodulation (AON) where several genes are known, including the receptor-like kinase SUPER NUMERIC NODULES (SUNN), root-to-shoot mobile CLAVATA3/ENDOSPERM SURROUNDING REGION (ESR)-RELATED (CLE) peptides, and the hydroxyproline O-arabinosyltransferase ROOT DETERMINED NODULATION1 (RDN1) required for post-translational peptide modification. In this work, CLE53 was identified to negatively regulate AMF symbiosis in a SUNN- and RDN1-dependent manner. CLE53 expression was repressed at low phosphorus, while it was induced by AMF colonization and high phosphorus. CLE53 overexpression reduced AMF colonization in a SUNN- and RDN1 dependent manner, while cle53, rdn1, and sunn mutants were more colonized than the wild type. RNA-sequencing identified 700 genes with SUNN-dependent regulation in AMF-colonized plants, providing a resource for future identification of additional AOM genes. Disruption of AOM genes in crops potentially constitutes a novel route for improving AMF-derived phosphorus uptake in agricultural systems with high phosphorus levels.

A linkage disequilibrium-based statistical test for Genome-Wide Epistatic Selection Scans in structured populations

The quest for signatures of selection using single nucleotide polymorphism (SNP) data has proven efficient to uncover genes involved in conserved and/or adaptive molecular functions, but none of the statistical methods were designed to identify interacting alleles as targets of selective processes. Here, we propose a statistical test aimed at detecting epistatic selection, based on a linkage disequilibrium (LD) measure accounting for population structure and heterogeneous relatedness between individuals. SNP-based ([Formula: see text]) and window-based ([Formula: see text]) statistics fit a Student distribution, allowing to test the significance of correlation coefficients. As a proof of concept, we use SNP data from the Medicago truncatula symbiotic legume plant and uncover a previously unknown gene coadaptation between the MtSUNN (Super Numeric Nodule) receptor and the MtCLE02 (CLAVATA3-Like) signaling peptide. We also provide experimental evidence supporting a MtSUNN-dependent negative role of MtCLE02 in symbiotic root nodulation. Using human HGDP-CEPH SNP data, our new statistical test uncovers strong LD between SLC24A5 (skin pigmentation) and EDAR (hairs, teeth, sweat glands development) world-wide, which persists after correction for population structure and relatedness in Central South Asian populations. This result suggests that epistatic selection or coselection could have contributed to the phenotypic make-up in some human populations. Applying this approach to genome-wide SNP data will facilitate the identification of coadapted gene networks in model or non-model organisms.

Compact Root Architecture 2 Promotes Root Competence for Nodulation through the miR2111 Systemic Effector

Nitrogen-deprived legume plants form new root organs, the nodules, following a symbiosis with nitrogen-fixing rhizobial bacteria [1]. Because this interaction is beneficial for the plant but has a high energetic cost, nodulation is tightly controlled by host plants through systemic pathways (acting at long distance) to promote or limit rhizobial infections and nodulation depending on earlier infections and on nitrogen availability [2]. In the Medicago truncatula model legume, CLE12 (Clavata3/Embryo surrounding region 12) and CLE13 signaling peptides produced in nodulated roots act in shoots through the SUNN (Super Numeric Nodule) receptor to negatively regulate nodulation and therefore autoregulate nodule number [3-5]. Conversely, CEP (C-terminally Encoded Peptide) signaling peptides produced in nitrogen-starved roots act in shoots through the CRA2 (Compact Root Architecture 2) receptor to promote nodulation already in the absence of rhizobia [6-9]. We show in this study that a downstream shoot-to-root signaling effector of these systemic pathways is the shoot-produced miR2111 microRNA [10] that negatively regulates TML1 (Too Much Love 1) and TML2 [11] transcripts accumulation in roots, ultimately promoting nodulation. Low nitrogen conditions and CEP1 signaling peptides induce in the absence of rhizobia the production of miR2111 depending on CRA2 activity in shoots, thus favoring root competence for nodulation. Together with the SUNN pathway negatively regulating the same miR2111 systemic effector when roots are nodulated, this allows a dynamic fine-tuning of the nodulation capacity of legume roots by nitrogen availability and rhizobial cues.

Celebrating 20 Years of Genetic Discoveries in Legume Nodulation and Symbiotic Nitrogen Fixation

Since 1999, various forward- and reverse-genetic approaches have uncovered nearly 200 genes required for symbiotic nitrogen fixation (SNF) in legumes. These discoveries advanced our understanding of the evolution of SNF in plants and its relationship to other beneficial endosymbioses, signaling between plants and microbes, the control of microbial infection of plant cells, the control of plant cell division leading to nodule development, autoregulation of nodulation, intracellular accommodation of bacteria, nodule oxygen homeostasis, the control of bacteroid differentiation, metabolism and transport supporting symbiosis, and the control of nodule senescence. This review catalogs and contextualizes all of the plant genes currently known to be required for SNF in two model legume species, Medicago truncatula and Lotus japonicus, and two crop species, Glycine max (soybean) and Phaseolus vulgaris (common bean). We also briefly consider the future of SNF genetics in the era of pan-genomics and genome editing.