Two CCAAT-box-binding transcription factors redundantly regulate early steps of the legume-rhizobia endosymbiosis

The auxin phenylacetic acid induces NIN expression in the actinorhizal plant Datisca glomerata, whereas cytokinin acts antagonistically

All nitrogen-fixing root nodule symbioses of angiosperms-legume and actinorhizal symbioses-possess a common ancestor. Molecular processes for the induction of root nodules are modulated by phytohormones, as is the case of the first nodulation-related transcription factor NODULE INCEPTION (NIN), whose expression can be induced by exogenous cytokinin in legumes. The process of actinorhizal nodule organogenesis is less well understood. To study the changes exerted by phytohormones on the expression of the orthologs of CYCLOPS, NIN, and NF-YA1 in the actinorhizal host Datisca glomerata, an axenic hydroponic system was established and used to examine the transcriptional responses (RT-qPCR) in roots treated with the synthetic cytokinin 6-Benzylaminopurine (BAP), the natural auxin Phenylacetic acid (PAA), and the synthetic auxin 1-Naphthaleneacetic acid (NAA). The model legume Lotus japonicus was used as positive control. Molecular readouts for auxins and cytokinin were established: DgSAUR1 for PAA, DgGH3.1. for NAA, and DgARR9 for BAP. L. japonicus NIN was induced by BAP, PAA, and NAA in a dosage- and time-dependent manner. While expression of D. glomerata NIN2 could not be induced in roots, D. glomerata NIN1 was induced by PAA; this induction was abolished in the presence of exogenous BAP. Furthermore, the induction of DgNIN1 expression by PAA required ethylene and gibberellic acid. This study suggests that while cytokinin signaling is central for cortex-induced nodules of L. japonicus, it acts antagonistically to the induction of nodule primordia of D. glomerata by PAA in the root pericycle.

A conserved nuclear factor YC subunit, NF-YC3, is essential for arbuscule development

Establishing reciprocal symbiosis with arbuscular mycorrhizal (AM) fungi is an important evolutionary strategy of most terrestrial plants to adapt to environmental stresses, especially phosphate (Pi) deficiencies. Identifying the key genes essential for AM symbiosis in plants and dissecting their functional mechanisms will be helpful for the breeding of new crop varieties with enhanced nutrient uptake efficiency. Here, we report a nuclear factor YC subunit-encoding gene, OsNF-YC3, whose expression is specifically induced in arbuscule-containing cells, plays an essential role in AM symbiosis. Knockout of OsNF-YC3 resulted in stunted arbuscule morphology and substantially decreased P accumulation, while overexpressing OsNF-YC3 enhanced mycorrhization and Pi uptake efficiency. OsNF-YC3 is directly regulated by OsPHRs, the major regulators of Pi starvation responses. Chromatin immunoprecipitation sequencing analysis uncovered multiple genes with crucial roles in arbuscule development as its potential downstream targets, including the AM-specific Pi transporter gene OsPT11. OsNF-YC3 can form a heterotrimer with the other two NF-Y subunits, OsNF-YA11 and OsNF-YB11, in yeast. Loss of OsNF-YA11 function also severely impaired arbuscule development in its mutants. Overall, our results highlight an essential role of OsNF-YC3 and its potential interacting NF-Y subunit, OsNF-YA11, in regulating AM symbiosis and arbuscule development.

Insights into the genetic architecture of the reciprocal interspecific hybrids derived from Chrysanthemum dichrum and C. nankingense

Chrysanthemums are versatile ornamental plants, and improving leaf and flower traits is an important breeding objective. Distant hybridization is a powerful method for plant breeding and genetic improvement, whereas the genetic basis in interspecific F1 progeny of chrysanthemums needs to be better understood for breeding purposes. In this study, the leaf and floral traits of the 273 reciprocal interspecific F1 hybrids of diploid C. dichrum (YSJ) and C. nankingense (JHN) were analyzed along with their SNP-derived genetic structure to elucidate the influence of differences in genetic background between the parents on the hybrid performance. We then performed a genome-wide association analysis (GWAS) to reveal the investigated traits' genomic loci and candidate genes. Considerable phenotypic variation (8.81% ~ 55.78%) and heterosis with transgressive segregation in both directions were observed in the reciprocal progenies. We observed a higher level of phenotypic variation in JHN × YSJ rather than in YSJ × JHN. Also, a significant reciprocal effect was observed for most examined traits. Based on the SNP data, we separated the hybrid progenies into three groups (I, II, and III), albeit imperfectly dependent on the cross directions, except for some reciprocal hybrids clustering into group II. Group I from YSJ × JHN and Group III from YSJ × JHN differed with contrasting F ST and π ratios, indicating the genetic changes in the reciprocal populations. The outcome of GWAS via the IIIVmrMLM method detected 339 significant quantitative trait nucleotides (QTNs) and 40 suggestive QTNs, and the phenotypic variation explained by a single QTN ranged from 0.26% to 7.42%. Within 100 kb upstream and downstream of the important QTNs, we discovered 49 known genes and 39 new candidate genes for the investigated leaf and floral traits. Our study provides profound insights into the genetic architecture of reciprocal hybrid progenies of chrysanthemum species, facilitating future breeding activities.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01518-0.

Gibberellin dynamics governing nodulation revealed using GIBBERELLIN PERCEPTION SENSOR 2 in Medicago truncatula lateral organs

During nutrient scarcity, plants can adapt their developmental strategy to maximize their chance of survival. Such plasticity in development is underpinned by hormonal regulation, which mediates the relationship between environmental cues and developmental outputs. In legumes, endosymbiosis with nitrogen-fixing bacteria (rhizobia) is a key adaptation for supplying the plant with nitrogen in the form of ammonium. Rhizobia are housed in lateral root-derived organs termed nodules that maintain an environment conducive to Nitrogenase in these bacteria. Several phytohormones are important for regulating the formation of nodules, with both positive and negative roles proposed for gibberellin (GA). In this study, we determine the cellular location and function of bioactive GA during nodule organogenesis using a genetically encoded second-generation GA biosensor, GIBBERELLIN PERCEPTION SENSOR 2 in Medicago truncatula. We find endogenous bioactive GA accumulates locally at the site of nodule primordia, increasing dramatically in the cortical cell layers, persisting through cell divisions, and maintaining accumulation in the mature nodule meristem. We show, through misexpression of GA-catabolic enzymes that suppress GA accumulation, that GA acts as a positive regulator of nodule growth and development. Furthermore, increasing or decreasing GA through perturbation of biosynthesis gene expression can increase or decrease the size of nodules, respectively. This is unique from lateral root formation, a developmental program that shares common organogenesis regulators. We link GA to a wider gene regulatory program by showing that nodule-identity genes induce and sustain GA accumulation necessary for proper nodule formation.

CLAVATA signaling in plant-environment interactions

Plants must rapidly and dynamically adapt to changes in their environment. Upon sensing environmental signals, plants convert them into cellular signals, which elicit physiological or developmental changes that allow them to respond to various abiotic and biotic cues. Because plants can be simultaneously exposed to multiple environmental cues, signal integration between plant cells, tissues, and organs is necessary to induce specific responses. Recently, CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) peptides and their cognate CLAVATA-type receptors received increased attention for their roles in plant-environment interactions. CLE peptides are mobile signaling molecules, many of which are induced by a variety of biotic and abiotic stimuli. Secreted CLE peptides are perceived by receptor complexes on the surface of their target cells, which often include the leucine-rich repeat receptor-like kinase CLAVATA1. Receptor activation then results in cell-type and/or environment-specific responses. This review summarizes our current understanding of the diverse roles of environment-regulated CLE peptides in modulating plant responses to environmental cues. We highlight how CLE signals regulate plant physiology by fine-tuning plant-microbe interactions, nutrient homeostasis, and carbon allocation. Finally, we describe the role of CLAVATA receptors in the perception of environment-induced CLE signals and discuss how diverse CLE-CLAVATA signaling modules may integrate environmental signals with plant physiology and development.

Light-sensitive short hypocotyl genes confer symbiotic nodule identity in the legume Medicago truncatula

Legumes produce specialized root nodules that are distinct from lateral roots in morphology and function, with nodules intracellularly hosting nitrogen-fixing bacteria. We have previously shown that a lateral root program underpins nodule initiation, but there must be additional developmental regulators that confer nodule identity. Here, we show two members of the LIGHT-SENSITIVE SHORT HYPOCOTYL (LSH) transcription factor family, predominantly known to define shoot meristem complexity and organ boundaries, function as regulators of nodule organ identity. In parallel to the root initiation program, LSH1/LSH2 recruit a program into the root cortex that mediates the divergence into nodules, in particular with cell divisions in the mid-cortex. This includes regulation of auxin and cytokinin, promotion of NODULE ROOT1/2 and Nuclear Factor YA1, and suppression of the lateral root program. A principal outcome of LSH1/LSH2 function is the production of cells able to accommodate nitrogen-fixing bacteria, a key feature unique to nodules.

An emerging role of heterotrimeric G-proteins in nodulation and nitrogen sensing

MAIN CONCLUSION

A comprehensive understanding of nitrogen signaling cascades involving heterotrimeric G-proteins and their putative receptors can assist in the production of nitrogen-efficient plants. Plants are immobile in nature, so they must endure abiotic stresses including nutrient stress. Plant development and agricultural productivity are frequently constrained by the restricted availability of nitrogen in the soil. Non-legume plants acquire nitrogen from the soil through root membrane-bound transporters. In depleted soil nitrogen conditions, legumes are naturally conditioned to fix atmospheric nitrogen with the aid of nodulation elicited by nitrogen-fixing bacteria. Moreover, apart from the symbiotic nitrogen fixation process, nitrogen uptake from the soil can also be a significant secondary source to satisfy the nitrogen requirements of legumes. Heterotrimeric G-proteins function as molecular switches to help plant cells relay diverse stimuli emanating from external stress conditions. They are comprised of Gα, Gβ and Gγ subunits, which cooperate with several downstream effectors to regulate multiple plant signaling events. In the present review, we concentrate on signaling mechanisms that regulate plant nitrogen nutrition. Our review highlights the potential of heterotrimeric G-proteins, together with their putative receptors, to assist the legume root nodule symbiosis (RNS) cascade, particularly during calcium spiking and nodulation. Additionally, the functions of heterotrimeric G-proteins in nitrogen acquisition by plant roots as well as in improving nitrogen use efficiency (NUE) have also been discussed. Future research oriented towards heterotrimeric G-proteins through genome editing tools can be a game changer in the enhancement of the nitrogen fixation process. This will foster the precise manipulation and production of plants to ensure global food security in an era of climate change by enhancing crop productivity and minimizing reliance on external inputs.

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.

Linking plant functional genes to rhizosphere microbes: a review

The importance of rhizomicrobiome in plant development, nutrition acquisition and stress tolerance is unquestionable. Relevant plant genes corresponding to the above functions also regulate rhizomicrobiome construction. Deciphering the molecular regulatory network of plant-microbe interactions could substantially contribute to improving crop yield and quality. Here, the plant gene-related nutrient uptake, biotic and abiotic stress resistance, which may influence the composition and function of microbial communities, are discussed in this review. In turn, the influence of microbes on the expression of functional plant genes, and thereby plant growth and immunity, is also reviewed. Moreover, we have specifically paid attention to techniques and methods used to link plant functional genes and rhizomicrobiome. Finally, we propose to further explore the molecular mechanisms and signalling pathways of microbe-host gene interactions, which could potentially be used for managing plant health in agricultural systems.

Biological nitrogen fixation in cereal crops: Progress, strategies, and perspectives

Nitrogen is abundant in the atmosphere but is generally the most limiting nutrient for plants. The inability of many crop plants, such as cereals, to directly utilize freely available atmospheric nitrogen gas means that their growth and production often rely heavily on the application of chemical fertilizers, which leads to greenhouse gas emissions and the eutrophication of water. By contrast, legumes gain access to nitrogen through symbiotic association with rhizobia. These bacteria convert nitrogen gas into biologically available ammonia in nodules through a process termed symbiotic biological nitrogen fixation, which plays a decisive role in ecosystem functioning. Engineering cereal crops that can fix nitrogen like legumes or associate with nitrogen-fixing microbiomes could help to avoid the problems caused by the overuse of synthetic nitrogen fertilizer. With the development of synthetic biology, various efforts have been undertaken with the aim of creating so-called "N-self-fertilizing" crops capable of performing autonomous nitrogen fixation to avoid the need for chemical fertilizers. In this review, we briefly summarize the history and current status of engineering N-self-fertilizing crops. We also propose several potential biotechnological approaches for incorporating biological nitrogen fixation capacity into non-legume plants.

An automated system for interrogating the evolution of microbial endosymbiosis

Inter-kingdom endosymbiotic interactions between bacteria and eukaryotic cells are critical to human health and disease. However, the molecular mechanisms that drive the emergence of endosymbiosis remain obscure. Here, we describe the development of a microfluidic system, named SEER (S̲ystem for the E̲volution of E̲ndosymbiotic R̲elationships), that automates the evolutionary selection of bacteria with enhanced intracellular survival and persistence within host cells, hallmarks of endosymbiosis. Using this system, we show that a laboratory strain of Escherichia coli that initially possessed limited abilities to survive within host cells, when subjected to SEER selection, rapidly evolved to display a 55-fold enhancement in intracellular survival. Notably, molecular dissection of the evolved strains revealed that a single-point mutation in a flexible loop of CpxR, a gene regulator that controls bacterial stress responses, substantially contributed to this intracellular survival. Taken together, these results establish SEER as the first microfluidic system for investigating the evolution of endosymbiosis, show the importance of CpxR in endosymbiosis, and set the stage for evolving bespoke inter-kingdom endosymbiotic systems with novel or emergent properties.

The effects of ERN1 on gene expression during early rhizobial infection in Lotus japonicus

Legumes develop root nodules in association with compatible rhizobia to overcome nitrogen deficiency. Rhizobia enter the host legume, mainly through infection threads, and induce nodule primordium formation in the root cortex. Multiple transcription factors have been identified to be involved in the regulation of the establishment of root nodule symbiosis, including ERF Required for Nodulation1 (ERN1). ERN1 is involved in a transcription network with CYCLOPS and NODULE INCEPTION (NIN). Mutation of ERN1 often results in misshapen root hair tips, deficient infection thread formation, and immature root nodules. ERN1 directly activates the expression of ENOD11 in Medicago truncatula to assist cell wall remodeling and Epr3 in Lotus japonicus to distinguish rhizobial exopolysaccharide signals. However, aside from these two genes, it remains unclear which genes are regulated by LjERN1 or what role LjERN1 plays during root nodule symbiosis. Thus, we conducted RNA sequencing to compare the gene expression profiles of wild-type L. japonicus and Ljern1-6 mutants. In total, 234 differentially expressed genes were identified as candidate LjERN1 target genes. These genes were found to be associated with cell wall remodeling, signal transduction, phytohormone metabolism, and transcription regulation, suggesting that LjERN1 is involved in multiple processes during the early stages of the establishment of root nodule symbiosis. Many of these candidate genes including RINRK1 showed decreased expression levels in Ljnin-2 mutants based on a search of a public database, suggesting that LjERN1 and LjNIN coordinately regulate gene expression. Our data extend the current understanding of the pleiotropic role of LjERN1 in root nodule symbiosis.

GmYSL7 controls iron uptake, allocation, and cellular response of nodules in soybean

Iron (Fe) is essential for DNA synthesis, photosynthesis and respiration of plants. The demand for Fe substantially increases during legumes-rhizobia symbiotic nitrogen fixation because of the synthesis of leghemoglobin in the host and Fe-containing proteins in bacteroids. However, the mechanism by which plant controls iron transport to nodules remains largely unknown. Here we demonstrate that GmYSL7 serves as a key regulator controlling Fe uptake from root to nodule and distribution in soybean nodules. GmYSL7 is Fe responsive and GmYSL7 transports iron across the membrane and into the infected cells of nodules. Alterations of GmYSL7 substantially affect iron distribution between root and nodule, resulting in defective growth of nodules and reduced nitrogenase activity. GmYSL7 knockout increases the expression of GmbHLH300, a transcription factor required for Fe response of nodules. Overexpression of GmbHLH300 decreases nodule number, nitrogenase activity and Fe content in nodules. Remarkably, GmbHLH300 directly binds to the promoters of ENOD93 and GmLbs, which regulate nodule number and nitrogenase activity, and represses their transcription. Our data reveal a new role of GmYSL7 in controlling Fe transport from host root to nodule and Fe distribution in nodule cells, and uncover a molecular mechanism by which Fe affects nodule number and nitrogenase activity.

Transcription Factors Controlling the Rhizobium-Legume Symbiosis: Integrating Infection, Organogenesis and the Abiotic Environment

Legume roots engage in a symbiotic relationship with rhizobia, leading to the development of nitrogen-fixing nodules. Nodule development is a sophisticated process and is under the tight regulation of the plant. The symbiosis initiates with a signal exchange between the two partners, followed by the development of a new organ colonized by rhizobia. Over two decades of study have shed light on the transcriptional regulation of rhizobium-legume symbiosis. A large number of transcription factors (TFs) have been implicated in one or more stages of this symbiosis. Legumes must monitor nodule development amidst a dynamic physical environment. Some environmental factors are conducive to nodulation, whereas others are stressful. The modulation of rhizobium-legume symbiosis by the abiotic environment adds another layer of complexity and is also transcriptionally regulated. Several symbiotic TFs act as integrators between symbiosis and the response to the abiotic environment. In this review, we trace the role of various TFs involved in rhizobium-legume symbiosis along its developmental route and highlight the ones that also act as communicators between this symbiosis and the response to the abiotic environment. Finally, we discuss contemporary approaches to study TF-target interactions in plants and probe their potential utility in the field of rhizobium-legume symbiosis.

Expression and Variation of the Genes Involved in Rhizobium Nodulation in Red Clover

Red clover (Trifolium pratense L.) is an important forage crop and serves as a major contributor of nitrogen input in pasture settings because of its ability to fix atmospheric nitrogen. During the legume-rhizobial symbiosis, the host plant undergoes a large number of gene expression changes, leading to development of root nodules that house the rhizobium bacteria as they are converted into nitrogen-fixing bacteroids. Many of the genes involved in symbiosis are conserved across legume species, while others are species-specific with little or no homology across species and likely regulate the specific plant genotype/symbiont strain interactions. Red clover has not been widely used for studying symbiotic nitrogen fixation, primarily due to its outcrossing nature, making genetic analysis rather complicated. With the addition of recent annotated genomic resources and use of RNA-seq tools, we annotated and characterized a number of genes that are expressed only in nodule forming roots. These genes include those encoding nodule-specific cysteine rich peptides (NCRs) and nodule-specific Polycystin-1, Lipoxygenase, Alpha toxic (PLAT) domain proteins (NPDs). Our results show that red clover encodes one of the highest number of NCRs and ATS3-like/NPDs, which are postulated to increase nitrogen fixation efficiency, in the Inverted-Repeat Lacking Clade (IRLC) of legumes. Knowledge of the variation and expression of these genes in red clover will provide more insights into the function of these genes in regulating legume-rhizobial symbiosis and aid in breeding of red clover genotypes with increased nitrogen fixation efficiency.

MtNF-YC6 and MtNF-YC11 are involved in regulating the transcriptional program of arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal fungi are obligate symbionts that transfer mineral nutrients to host plants through arbuscules, a fungal structure specialized for exchange for photosynthetic products. MtNF-YC6 and MtNF-YC11, which encode the C subunits of nuclear factor Y (NF-Y) family in Medicago truncatula are induced specifically by arbuscular mycorrhizal symbiosis (AMS). A previous study showed that MtNF-YC6 and MtNF-YC11 are activated in cortical cells of mycorrhizal roots, but the gene functions were unknown. Herein, we identified both MtNF-YB17 and MtNF-YB12 as the interacting partners of MtNF-YC6 and MtNF-YC11 in yeast and plants. MtNF-YB17 was highly induced by AMS and activated in cortical cells only in mycorrhizal roots but MtNF-YB12 was not affected. The formation of B/C heterodimers led the protein complexes to transfer from the cytoplasm to the nucleus. Silencing MtNF-YC6 and C11 by RNA interference (RNAi) resulted in decreased colonization efficiency and arbuscule richness. Coincidently, genes associated with arbuscule development and degeneration in RNAi roots were also downregulated. In silico analysis showed CCAAT-binding motifs in the promoter regions of downregulated genes, further supporting the involvement of NF-Y complexes in transcriptional regulation of symbiosis. Taken together, this study identifies MtNF-YC6- or MtNF-YC11-containing protein complexes as novel transcriptional regulators of symbiotic program and provides a list of potential downstream target genes. These data will help to further dissect the AMS regulatory network.

NIPK, a protein pseudokinase that interacts with the C subunit of the transcription factor NF-Y, is involved in rhizobial infection and nodule organogenesis

Heterotrimeric Nuclear Factor Y (NF-Y) transcription factors are key regulators of the symbiotic program that controls rhizobial infection and nodule organogenesis. Using a yeast two-hybrid screening, we identified a putative protein kinase of Phaseolus vulgaris that interacts with the C subunit of the NF-Y complex. Physical interaction between NF-YC1 Interacting Protein Kinase (NIPK) and NF-YC1 occurs in the cytoplasm and the plasma membrane. Only one of the three canonical amino acids predicted to be required for catalytic activity is conserved in NIPK and its putative homologs from lycophytes to angiosperms, indicating that NIPK is an evolutionary conserved pseudokinase. Post-transcriptional silencing on NIPK affected infection and nodule organogenesis, suggesting NIPK is a positive regulator of the NF-Y transcriptional complex. In addition, NIPK is required for activation of cell cycle genes and early symbiotic genes in response to rhizobia, including NF-YA1 and NF-YC1. However, strain preference in co-inoculation experiments was not affected by NIPK silencing, suggesting that some functions of the NF-Y complex are independent of NIPK. Our work adds a new component associated with the NF-Y transcriptional regulators in the context of nitrogen-fixing symbiosis.

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.

At the Crossroads of Salinity and Rhizobium-Legume Symbiosis

Legume roots interact with soil bacteria rhizobia to develop nodules, de novo symbiotic root organs that host these rhizobia and are mini factories of atmospheric nitrogen fixation. Nodulation is a sophisticated developmental process and is sensitive to several abiotic factors, salinity being one of them. While salinity influences both the free-living partners, symbiosis is more vulnerable than other aspects of plant and microbe physiology, and the symbiotic interaction is strongly impaired even under moderate salinity. In this review, we tease apart the various known components of rhizobium-legume symbiosis and how they interact with salt stress. We focus primarily on the initial stages of symbiosis since we have a greater mechanistic understanding of the interaction at these stages.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

MtEFD and MtEFD2: Two transcription factors with distinct neofunctionalization in symbiotic nodule development

Rhizobium-legume nitrogen-fixing symbiosis involves the formation of a specific organ, the root nodule, which provides bacteria with the proper cellular environment for atmospheric nitrogen fixation. Coordinated differentiation of plant and bacterial cells is an essential step of nodule development, for which few transcriptional regulators have been characterized. Medicago truncatula ETHYLENE RESPONSE FACTOR REQUIRED FOR NODULE DIFFERENTIATION (MtEFD) encodes an APETALA2/ETHYLENE RESPONSIVE FACTOR (ERF) transcription factor, the mutation of which leads to both hypernodulation and severe defects in nodule development. MtEFD positively controls a negative regulator of cytokinin signaling, the RESPONSE REGULATOR 4 (MtRR4) gene. Here we showed that that the Mtefd-1 mutation affects both plant and bacterial endoreduplication in nodules, as well as the expression of hundreds of genes in young and mature nodules, upstream of known regulators of symbiotic differentiation. MtRR4 expressed with the MtEFD promoter complemented Mtefd-1 hypernodulation but not the nodule differentiation phenotype. Unexpectedly, a nonlegume homolog of MtEFD, AtERF003 in Arabidopsis (Arabidopsis thaliana), could efficiently complement both phenotypes of Mtefd-1, in contrast to the MtEFD paralog MtEFD2 expressed in the root and nodule meristematic zone. A domain swap experiment showed that MtEFD2 differs from MtEFD by its C-terminal fraction outside the DNA binding domain. Furthermore, clustered regularly interspaced short palindromic repeats-CRISPR associated protein 9 (CRISPR-Cas9) mutagenesis of MtEFD2 led to a reduction in the number of nodules formed in Mtefd-1, with downregulation of a set of genes, including notably NUCLEAR FACTOR-YA1 (MtNF-YA1) and MtNF-YB16, which are essential for nodule meristem establishment. We, therefore, conclude that nitrogen-fixing symbiosis recruited two proteins originally expressed in roots, MtEFD and MtEFD2, with distinct functions and neofunctionalization processes for each of them.

Glycine max Homologs of DOESN'T MAKE INFECTIONS 1, 2, and 3 Function to Impair Heterodera glycines Parasitism While Also Regulating Mitogen Activated Protein Kinase Expression

Glycine max root cells developing into syncytia through the parasitic activities of the pathogenic nematode Heterodera glycines underwent isolation by laser microdissection (LM). Microarray analyses have identified the expression of a G. max DOESN'T MAKE INFECTIONS3 (DMI3) homolog in syncytia undergoing parasitism but during a defense response. DMI3 encodes part of the common symbiosis pathway (CSP) involving DMI1, DMI2, and other CSP genes. The identified DMI gene expression, and symbiosis role, suggests the possible existence of commonalities between symbiosis and defense. G. max has 3 DMI1, 12 DMI2, and 2 DMI3 paralogs. LM-assisted gene expression experiments of isolated syncytia under further examination here show G. max DMI1-3, DMI2-7, and DMI3-2 expression occurring during the defense response in the H. glycines-resistant genotypes G.max [Peking/PI548402] and G.max [PI88788] indicating a broad and consistent level of expression of the genes. Transgenic overexpression (OE) of G. max DMI1-3, DMI2-7, and DMI3-2 impairs H. glycines parasitism. RNA interference (RNAi) of G. max DMI1-3, DMI2-7, and DMI3-2 increases H. glycines parasitism. The combined opposite outcomes reveal a defense function for these genes. Prior functional transgenic analyses of the 32-member G. max mitogen activated protein kinase (MAPK) gene family has determined that 9 of them act in the defense response to H. glycines parasitism, referred to as defense MAPKs. RNA-seq analyses of root RNA isolated from the 9 G. max defense MAPKs undergoing OE or RNAi reveal they alter the relative transcript abundances (RTAs) of specific DMI1, DMI2, and DMI3 paralogs. In contrast, transgenically-manipulated DMI1-3, DMI2-7, and DMI3-2 expression influences MAPK3-1 and MAPK3-2 RTAs under certain circumstances. The results show G. max homologs of the CSP, and defense pathway are linked, apparently involving co-regulated gene expression.

The Medicago truncatula IEF Gene Is Crucial for the Progression of Bacterial Infection During Symbiosis

Legumes are able to meet their nitrogen need by establishing nitrogen-fixing symbiosis with rhizobia. Nitrogen fixation is performed by rhizobia, which has been converted to bacteroids, in newly formed organs, the root nodules. In the model legume Medicago truncatula, nodule cells are invaded by rhizobia through transcellular tubular structures called infection threads (ITs) that are initiated at the root hairs. Here, we describe a novel M. truncatula early symbiotic mutant identified as infection-related epidermal factor (ief), in which the formation of ITs is blocked in the root hair cells and only nodule primordia are formed. We show that the function of MtIEF is crucial for the bacterial infection in the root epidermis but not required for the nodule organogenesis. The IEF gene that appears to have been recruited for a symbiotic function after the duplication of a flower-specific gene is activated by the ERN1-branch of the Nod factor signal transduction pathway and independent of the NIN activity. The expression of MtIEF is induced transiently in the root epidermal cells by the rhizobium partner or Nod factors. Although its expression was not detectable at later stages of symbiosis, complementation experiments indicate that MtIEF is also required for the proper invasion of the nodule cells by rhizobia. The gene encodes an intracellular protein of unknown function possessing a coiled-coil motif and a plant-specific DUF761 domain. The IEF protein interacts with RPG, another symbiotic protein essential for normal IT development, suggesting that combined action of these proteins plays a role in nodule infection.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Identification of conserved and new miRNAs that affect nodulation and strain selectivity in the Phaseolus vulgaris-Rhizobium etli symbiosis through differential analysis of host small RNAs

Phaseolus vulgaris plants from the Mesoamerican centre of genetic diversification establish a preferential and more efficient root nodule symbiosis with sympatric Rhizobium etli strains. This is mediated by changes in host gene expression, which might occur either at the transcriptional or at the post-transcriptional level. However, the implication of small RNA (sRNA)-mediated control of gene expression in strain selectivity has remained elusive. sRNA sequencing was used to identify host microRNAs (miRNAs) differentially regulated in roots at an early stage of the symbiotic interaction, which were further characterized by applying a reverse genetic approach. In silico analysis identified known and new miRNAs that accumulated to a greater extent in the preferential and more efficient interaction. One of them, designated as Pvu-miR5924, participates in the mechanisms that determine the selection of R. etli strains that will colonize the nodules. In addition, the functional analysis of Pvu-miR390b verified that this miRNA is a negative modulator of nodule formation and bacterial infection. This study not only extended the list of miRNAs identified in P. vulgaris but also enabled the identification of miRNAs that play relevant functions in nodule formation, rhizobial infection and the selection of the rhizobial strains that will occupy the nodule.

Engineered CaM2 modulates nuclear calcium oscillation and enhances legume root nodule symbiosis

Oscillations in intracellular calcium concentration play an essential role in the regulation of multiple cellular processes. In plants capable of root endosymbiosis with nitrogen-fixing bacteria and/or arbuscular mycorrhizal fungi, nuclear localized calcium oscillations are essential to transduce the microbial signal. Although the ion channels required to generate the nuclear localized calcium oscillations have been identified, their mechanisms of regulation are unknown. Here, we combined proteomics and engineering approaches to demonstrate that the calcium-bound form of the calmodulin 2 (CaM2) associates with CYCLIC NUCLEOTIDE GATED CHANNEL 15 (CNGC15s), closing the channels and providing the negative feedback to sustain the oscillatory mechanism. We further unraveled that the engineered CaM2 accelerates early endosymbioses and enhanced root nodule symbiosis but not arbuscular mycorrhization.

The key physiological event essential to the establishment of nitrogen-fixing bacteria and phosphate-delivering arbuscular mycorrhizal symbioses is the induction of nuclear calcium oscillations that are required for endosymbioses. These regular fluctuations in nucleoplasmic calcium concentrations are generated by ion channels and a pump located at the nuclear envelope, including the CYCLIC NUCLEOTIDE GATED CHANNEL 15 (CNGC15). However, how the CNGC15s are regulated in planta to sustain a calcium oscillatory mechanism remains unknown. Here, we demonstrate that the CNGC15s are regulated by the calcium-bound form of the calmodulin 2 (holo-CaM2), which, upon release of calcium, provides negative feedback to close the CNGC15s. Combining structural and evolutionary analyses of CaM residues with bioinformatic analysis, we engineered a holo-CaM2 with an increased affinity for CNGC15s. In planta, the expression of the engineered holo-CaM2 accelerates the calcium oscillation frequency, early endosymbioses signaling and is sufficient to sustain over time an enhanced root nodule symbiosis but not an increased arbuscular mycorrhization. Together, these results reveal that holo-CaM2 is a component of endosymbiosis signaling required to modulate CNGC15s activity and the downstream root nodule symbiosis pathway.

Spatiotemporal cytokinin response imaging and ISOPENTENYLTRANSFERASE 3 function in Medicago nodule development

Most legumes can establish a symbiotic association with soil rhizobia that trigger the development of root nodules. These nodules host the rhizobia and allow them to fix nitrogen efficiently. The perception of bacterial lipo-chitooligosaccharides (LCOs) in the epidermis initiates a signaling cascade that allows rhizobial intracellular infection in the root and de-differentiation and activation of cell division that gives rise to the nodule. Thus, nodule organogenesis and rhizobial infection need to be coupled in space and time for successful nodulation. The plant hormone cytokinin (CK) contributes to the coordination of this process, acting as an essential positive regulator of nodule organogenesis. However, the temporal regulation of tissue-specific CK signaling and biosynthesis in response to LCOs or Sinorhizobium meliloti inoculation in Medicago truncatula remains poorly understood. In this study, using a fluorescence-based CK sensor (pTCSn::nls:tGFP), we performed a high-resolution tissue-specific temporal characterization of the sequential activation of CK response during root infection and nodule development in M. truncatula after inoculation with S. meliloti. Loss-of-function mutants of the CK-biosynthetic gene ISOPENTENYLTRANSFERASE 3 (IPT3) showed impairment of nodulation, suggesting that IPT3 is required for nodule development in M. truncatula. Simultaneous live imaging of pIPT3::nls:tdTOMATO and the CK sensor showed that IPT3 induction in the pericycle at the base of nodule primordium contributes to CK biosynthesis, which in turn promotes expression of positive regulators of nodule organogenesis in M. truncatula.

Regulation of the Later Stages of Nodulation Stimulated by IPD3/CYCLOPS Transcription Factor and Cytokinin in Pea Pisum sativum L

The IPD3/CYCLOPS transcription factor was shown to be involved in the regulation of nodule primordia development and subsequent stages of nodule differentiation. In contrast to early stages, the stages related to nodule differentiation remain less studied. Recently, we have shown that the accumulation of cytokinin at later stages may significantly impact nodule development. This conclusion was based on a comparative analysis of cytokinin localization between pea wild type and ipd3/cyclops mutants. However, the role of cytokinin at these later stages of nodulation is still far from understood. To determine a set of genes involved in the regulation of later stages of nodule development connected with infection progress, intracellular accommodation, as well as plant tissue and bacteroid differentiation, the RNA-seq analysis of pea mutant SGEFix--2 (sym33) nodules impaired in these processes compared to wild type SGE nodules was performed. To verify cytokinin's influence on late nodule development stages, the comparative RNA-seq analysis of SGEFix--2 (sym33) mutant plants treated with cytokinin was also conducted. Findings suggest a significant role of cytokinin in the regulation of later stages of nodule development.

Genome-wide screening and identification of nuclear Factor-Y family genes and exploration their function on regulating abiotic and biotic stress in potato (Solanum tuberosum L.)

The Nuclear Factor-Y (NF-Y) transcription factor (TF), which includes three distinct subunits (NF-YA, NF-YB and NF-YC), is known to manipulate various aspects of plant growth, development, and stress responses. Although the NF-Y gene family was well studied in many species, little is known about their functions in potato. In this study, a total of 37 potato NF-Y genes were identified, including 11 StNF-YAs, 20 StNF-YBs, and 6 StNF-YCs. The genetic features of these StNF-Y genes were investigated by comparing their evolutionary relationship, intron/exon organization and motif distribution pattern. Multiple alignments showed that all StNF-Y proteins possessed clearly conserved core regions that were flanked by non-conserved sequences. Gene duplication analysis indicated that nine StNF-Y genes were subjected to tandem duplication and eight StNF-Ys arose from segmental duplication events. Synteny analysis suggested that most StNF-Y genes (33 of 37) were orthologous to potato's close relative tomato (Solanum lycopersicum L.). Tissue-specific expression of the StNF-Y genes suggested their potential roles in controlling potato growth and development. The role of StNF-Ys in regulating potato responses to abiotic stress (ABA, drought and salinity) was also confirmed: twelve StNF-Y genes were up-regulated and another two were down-regulated under different abiotic treatments. In addition, genes responded differently to pathogen challenges, suggesting that StNF-Y genes may play distinct roles under certain biotic stress. In summary, insights into the evolution of NF-Y family members and their functions in potato development and stress responses are provided.

Phosphate starvation responsive GmSPX5 mediates nodule growth through interaction with GmNF-YC4 in soybean (Glycine max)

Phosphorus (P) deficiency adversely affects nodule development as reflected by reduced nodule fresh weight in legume plants. Though mechanisms underlying nodule adaptation to P deficiency have been studied extensively, it remains largely unknown which regulator mediates nodule adaptation to P deficiency. In this study, GUS staining and quantitative reverse transcription-PCR analysis reveal that the SPX member GmSPX5 is preferentially expressed in soybean (Glycine max) nodules. Overexpression of GmSPX5 enhanced soybean nodule development particularly under phosphate (Pi) sufficient conditions. However, the Pi concentration was not affected in soybean tissues (i.e., leaves, roots, and nodules) of GmSPX5 overexpression or suppression lines, which distinguished it from other well-known SPX members functioning in control of Pi homeostasis in plants. Furthermore, GmSPX5 was observed to interact with the transcription factor GmNF-YC4 in vivo and in vitro. Overexpression of either GmSPX5 or GmNF-YC4 significantly upregulated the expression levels of five asparagine synthetase-related genes (i.e., GmASL2-6) in soybean nodules. Meanwhile, yeast one-hybrid and luciferase activity assays strongly suggested that interactions of GmSPX5 and GmNF-YC4 activate GmASL6 expression through enhancing GmNF-YC4 binding of the GmASL6 promoter. These results not only demonstrate the GmSPX5-GmNF-YC4-GmASL6 regulatory pathway mediating soybean nodule development, but also considerably improve our understanding of SPX functions in legume crops.

Light-induced mobile factors from shoots regulate rhizobium-triggered soybean root nodulation

Symbiotic nitrogen fixation is an energy-expensive process, and the light available to plants has been proposed to be a primary influencer. We demonstrate that the light-induced soybean TGACG-motif binding factor 3/4 (GmSTF3/4) and FLOWERING LOCUS T (GmFTs), which move from shoots to roots, interdependently induce nodule organogenesis. Rhizobium-activated calcium- and calmodulin-dependent protein kinase (CCaMK) phosphorylates GmSTF3, triggering GmSTF3–GmFT2a complex formation, which directly activates expression of nodule inception (NIN) and nuclear factor Y (NF-YA1 and NF-YB1). Accordingly, the CCaMK–STF–FT module integrates aboveground light signals with underground symbiotic signaling, ensuring that the host plant informs its roots that the aboveground environment is prepared to sustainably supply the carbohydrate necessary for symbiosis. These results suggest approaches that could enhance the balance of carbon and nitrogen in the biosphere.

Translational regulation in pathogenic and beneficial plant-microbe interactions

Plants are surrounded by a vast diversity of microorganisms. Limiting pathogenic microorganisms is crucial for plant survival. On the other hand, the interaction of plants with beneficial microorganisms promotes their growth or allows them to overcome nutrient deficiencies. Balancing the number and nature of these interactions is crucial for plant growth and development, and thus, for crop productivity in agriculture. Plants use sophisticated mechanisms to recognize pathogenic and beneficial microorganisms and genetic programs related to immunity or symbiosis. Although most research has focused on characterizing changes in the transcriptome during plant-microbe interactions, the application of techniques such as Translating Ribosome Affinity Purification (TRAP) and Ribosome profiling allowed examining the dynamic association of RNAs to the translational machinery, highlighting the importance of the translational level of control of gene expression in both pathogenic and beneficial interactions. These studies revealed that the transcriptional and the translational responses are not always correlated, and that translational control operates at cell-specific level. In addition, translational control is governed by cis-elements present in the 5'mRNA leader of regulated mRNAs, e.g. upstream open reading frames (uORFs) and sequence-specific motifs. In this review, we summarize and discuss the recent advances made in the field of translational control during pathogenic and beneficial plant-microbe interactions.

Structure and Development of the Legume-Rhizobial Symbiotic Interface in Infection Threads

The intracellular infection thread initiated in a root hair cell is a unique structure associated with Rhizobium-legume symbiosis. It is characterized by inverted tip growth of the plant cell wall, resulting in a tunnel that allows invasion of host cells by bacteria during the formation of the nitrogen-fixing root nodule. Regulation of the plant-microbial interface is essential for infection thread growth. This involves targeted deposition of the cell wall and extracellular matrix and tight control of cell wall remodeling. This review describes the potential role of different actors such as transcription factors, receptors, and enzymes in the rearrangement of the plant-microbial interface and control of polar infection thread growth. It also focuses on the composition of the main polymers of the infection thread wall and matrix and the participation of reactive oxygen species (ROS) in the development of the infection thread. Mutant analysis has helped to gain insight into the development of host defense reactions. The available data raise many new questions about the structure, function, and development of infection threads.

Early stages of legume-rhizobia symbiosis are controlled by ABCG-mediated transport of active cytokinins

Growing evidence has highlighted the essential role of plant hormones, notably, cytokinins (CKs), in nitrogen-fixing symbiosis, both at early and late nodulation stages^1,2. Despite numerous studies showing the central role of CK in nodulation, the importance of CK transport in the symbiosis is unknown. Here, we show the role of ABCG56, a full-size ATP-binding cassette (ABC) transporter in the early stages of the nodulation. MtABCG56 is expressed in roots and nodules and its messenger RNA levels increase upon treatment with symbiotic bacteria, isolated Nod factor and CKs, accumulating within the epidermis and root cortex. MtABCG56 exports bioactive CKs in an ATP-dependent manner over the plasma membrane and its disruption results in an impairment of nodulation. Our data indicate that ABCG-mediated cytokinin transport is important for proper establishment of N-fixing nodules.

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.

NIN-like protein 7 promotes nitrate-mediated lateral root development by activating transcription of TRYPTOPHAN AMINOTRANSFERASE RELATED 2

Nitrate is essential for plant growth and development. When nitrate availability is low, plants produce more lateral roots (LRs) to seek nitrate from the soil. In this study, by DNA electrophoretic mobility shift and luciferase assays, it was showed that NIN-like protein 7 (NLP7) transcription factor activated expression of TAR2 by directly binding to its promoter. Finally, through genetic analysis, it was speculated that NLP7 regulated LR development through TAR2. In conclusion, NLP7 binds to the TAR2 promoter and activates TAR2 expression, thereby promoting nitrate-dependent LR development.

Lotus japonicus Nuclear Factor YA1, a nodule emergence stage-specific regulator of auxin signalling

Organogenesis of legume root nodules begins with the nodulation factor-dependent stimulation of compatible root cells to initiate divisions, signifying an early nodule primordium formation event. This is followed by cellular differentiation, including cell expansion and vascular bundle formation, and we previously showed that Lotus japonicus NF-YA1 is essential for this process, presumably by regulating three members of the SHORT INTERNODES/STYLISH (STY) transcription factor gene family. In this study, we used combined genetics, genomics and cell biology approaches to characterize the role of STY genes during root nodule formation and to test a hypothesis that they mediate nodule development by stimulating auxin signalling. We show here that L. japonicus STYs are required for nodule emergence. This is attributed to the NF-YA1-dependent regulatory cascade, comprising STY genes and their downstream targets, YUCCA1 and YUCCA11, involved in a local auxin biosynthesis at the post-initial cell division stage. An analogous NF-YA1/STY regulatory module seems to operate in Medicago truncatula in association with the indeterminate nodule patterning. Our data define L. japonicus and M. truncatula NF-YA1 genes as important nodule emergence stage-specific regulators of auxin signalling while indicating that the inductive stage and subsequent formation of early nodule primordia are mediated through an independent mechanism(s).

Nitrogen and Phosphorus Signaling and Transport During Legume-Rhizobium Symbiosis

Nitrogen (N) and phosphorus (P) are the two predominant mineral elements, which are not only essential for plant growth and development in general but also play a key role in symbiotic N fixation in legumes. Legume plants have evolved complex signaling networks to respond to both external and internal levels of these macronutrients to optimize symbiotic N fixation in nodules. Inorganic phosphate (Pi) and nitrate (NO₃ ^-) are the two major forms of P and N elements utilized by plants, respectively. Pi starvation and NO₃ ^- application both reduce symbiotic N fixation via similar changes in the nodule gene expression and invoke local and long-distance, systemic responses, of which N-compound feedback regulation of rhizobial nitrogenase activity appears to operate under both conditions. Most of the N and P signaling and transport processes have been investigated in model organisms, such as Medicago truncatula, Lotus japonicus, Glycine max, Phaseolus vulgaris, Arabidopsis thaliana, Oryza sativa, etc. We attempted to discuss some of these processes wherever appropriate, to serve as references for a better understanding of the N and P signaling and transport during symbiosis.

No Home without Hormones: How Plant Hormones Control Legume Nodule Organogenesis

The establishment of symbiotic nitrogen fixation requires the coordination of both nodule development and infection events. Despite the evolution of a variety of anatomical structures, nodule organs serve a common purpose in establishing a localized area that facilitates efficient nitrogen fixation. As in all plant developmental processes, the establishment of a new nodule organ is regulated by plant hormones. During nodule initiation, regulation of plant hormone signaling is one of the major targets of symbiotic signaling. We review the role of major developmental hormones in the initiation of the nodule organ and argue that the manipulation of plant hormones is a key requirement for engineering nitrogen fixation in non-legumes as the basis for improved food security and sustainability.

Transcriptional regulation of NIN expression by IPN2 is required for root nodule symbiosis in Lotus japonicus

Expression of Nodule Inception (NIN) is essential for initiation of legume-rhizobial symbiosis. An existing model regarding the regulation of NIN expression involves two GRAS transcription factors - NSP1 (Nodulation Signaling Pathway 1) and NSP2. NSP2 forms a complex with NSP1 to directly bind to NIN promoter. However, rhizobial treatment-induced NIN expression could still be detected in the nsp1 mutant plants, suggesting that other proteins must be involved in the regulation of NIN expression. A combination of molecular, biochemical and genetic analyses was used to investigate the molecular basis of IPN2 in regulating root development and NIN expression in Lotus japonicus. In this study, we identified that IPN2 is a close homolog of Arabidopsis APL (ALTERED PHLOEM DEVELOPMENT) with essential function in root development. However, Lotus IPN2 has a different expression pattern compared with the Arabidopsis APL gene. IPN2 binds to the IPN2-responsive cis element (IPN2-RE) of NIN promoter and activates NIN expression. IPN2, NSP1 and NSP2 form a protein complex to directly target NIN promoter and activate NIN expression in the legume-rhizobial symbiosis. Our data refine the regulatory model of NIN expression that NSP2 works together with NSP1 and IPN2 to activate the NIN gene allowing nodulation in L. japonicus.

Mutant analysis in the nonlegume Parasponia andersonii identifies NIN and NF-YA1 transcription factors as a core genetic network in nitrogen-fixing nodule symbioses

●Nitrogen-fixing nodulation occurs in 10 taxonomic lineages, with either rhizobia or Frankia bacteria. To establish such an endosymbiosis, two processes are essential: nodule organogenesis and intracellular bacterial infection. In the legume-rhizobium endosymbiosis, both processes are guarded by the transcription factor NODULE INCEPTION (NIN) and its downstream target genes of the NUCLEAR FACTOR Y (NF-Y) complex. ●It is hypothesized that nodulation has a single evolutionary origin c. 110 Ma, followed by many independent losses. Despite a significant body of knowledge of the legume-rhizobium symbiosis, it remains elusive which signalling modules are shared between nodulating species in different taxonomic clades. We used Parasponia andersonii to investigate the role of NIN and NF-YA genes in rhizobium nodulation in a nonlegume system. ●Consistent with legumes, P. andersonii PanNIN and PanNF-YA1 are coexpressed in nodules. By analyzing single, double and higher-order CRISPR-Cas9 knockout mutants, we show that nodule organogenesis and early symbiotic expression of PanNF-YA1 are PanNIN-dependent and that PanNF-YA1 is specifically required for intracellular rhizobium infection. ●This demonstrates that NIN and NF-YA1 have conserved symbiotic functions. As Parasponia and legumes diverged soon after the birth of the nodulation trait, we argue that NIN and NF-YA1 represent core transcriptional regulators in this symbiosis.

A Roadmap toward Engineered Nitrogen-Fixing Nodule Symbiosis

In the late 19th century, it was discovered that legumes can establish a root nodule endosymbiosis with nitrogen-fixing rhizobia. Soon after, the question was raised whether it is possible to transfer this trait to non-leguminous crops. In the past century, an ever-increasing amount of knowledge provided unique insights into the cellular, molecular, and genetic processes controlling this endosymbiosis. In addition, recent phylogenomic studies uncovered several genes that evolved to function specifically to control nodule formation and bacterial infection. However, despite this massive body of knowledge, the long-standing objective to engineer the nitrogen-fixing nodulation trait on non-leguminous crop plants has not been achieved yet. In this review, the unsolved questions and engineering strategies toward nitrogen-fixing nodulation in non-legume plants are discussed and highlighted.

A protein complex required for polar growth of rhizobial infection threads

During root nodule symbiosis, intracellular accommodation of rhizobia by legumes is a prerequisite for nitrogen fixation. For many legumes, rhizobial colonization initiates in root hairs through transcellular infection threads. In Medicago truncatula, VAPYRIN (VPY) and a putative E3 ligase LUMPY INFECTIONS (LIN) are required for infection thread development but their cellular and molecular roles are obscure. Here we show that LIN and its homolog LIN-LIKE interact with VPY and VPY-LIKE in a subcellular complex localized to puncta both at the tip of the growing infection thread and at the nuclear periphery in root hairs and that the punctate accumulation of VPY is positively regulated by LIN. We also show that an otherwise nuclear and cytoplasmic exocyst subunit, EXO70H4, systematically co-localizes with VPY and LIN during rhizobial infection. Genetic analysis shows that defective rhizobial infection in exo70h4 is similar to that in vpy and lin. Our results indicate that VPY, LIN and EXO70H4 are part of the symbiosis-specific machinery required for polar growth of infection threads.

The PvNF-YA1 and PvNF-YB7 Subunits of the Heterotrimeric NF-Y Transcription Factor Influence Strain Preference in the Phaseolus vulgaris-Rhizobium etli Symbiosis

Transcription factors of the Nuclear Factor Y (NF-Y) family play essential functions in plant development and plasticity, including the formation of lateral root organs such as lateral root and symbiotic nodules. NF-Ys mediate transcriptional responses by acting as heterotrimers composed of three subunits, NF-YA, NF-YB, and NF-YC, which in plants are encoded by relatively large gene families. We have previously shown that, in the Phaseolus vulgaris × Rhizobium etli interaction, the PvNF-YC1 subunit is involved not only in the formation of symbiotic nodules, but also in the preference exhibited by the plant for rhizobial strains that are more efficient and competitive in nodule formation. PvNF-YC1 forms a heterotrimer with the PvNF-YA1 and PvNF-YB7 subunits. Here, we used promoter:reporter fusions to show that both PvNF-YA1 and PvNF-YB7 are expressed in symbiotic nodules. In addition, we report that knock-down of PvNF-YA1 and its close paralog PvNF-YA9 abolished nodule formation by either high or low efficient strains and arrested rhizobial infection. On the other hand, knock-down of PvNF-YB7 only affected the symbiotic outcome of the high efficient interaction, suggesting that other symbiotic NF-YB subunits might be involved in the more general mechanisms of nodule formation. More important, we present functional evidence supporting that both PvNF-YA1 and PvNF-YB7 are part of the mechanisms that allow P. vulgaris plants to discriminate and select those bacterial strains that perform better in nodule formation, most likely by acting in the same heterotrimeric complex that PvNF-YC1.

LACK OF SYMBIONT ACCOMMODATION controls intracellular symbiont accommodation in root nodule and arbuscular mycorrhizal symbiosis in Lotus japonicus

Nitrogen-fixing rhizobia and arbuscular mycorrhizal fungi (AMF) form symbioses with plant roots and these are established by precise regulation of symbiont accommodation within host plant cells. In model legumes such as Lotus japonicus and Medicago truncatula, rhizobia enter into roots through an intracellular invasion system that depends on the formation of a root-hair infection thread (IT). While IT-mediated intracellular rhizobia invasion is thought to be the most evolutionarily derived invasion system, some studies have indicated that a basal intercellular invasion system can replace it when some nodulation-related factors are genetically modified. In addition, intracellular rhizobia accommodation is suggested to have a similar mechanism as AMF accommodation. Nevertheless, our understanding of the underlying genetic mechanisms is incomplete. Here we identify a L. japonicus nodulation-deficient mutant, with a mutation in the LACK OF SYMBIONT ACCOMMODATION (LAN) gene, in which root-hair IT formation is strongly reduced, but intercellular rhizobial invasion eventually results in functional nodule formation. LjLAN encodes a protein that is homologous to Arabidopsis MEDIATOR 2/29/32 possibly acting as a subunit of a Mediator complex, a multiprotein complex required for gene transcription. We also show that LjLAN acts in parallel with a signaling pathway including LjCYCLOPS. In addition, the lan mutation drastically reduces the colonization levels of AMF. Taken together, our data provide a new factor that has a common role in symbiont accommodation process during root nodule and AM symbiosis.

Symbiosis between plants and beneficial microbes such as nitrogen-fixing bacteria and arbuscular mycorrhizal fungi has enabled plant colonization of new environments. Root nodule symbiosis with nitrogen-fixing rhizobia enables sessile plants to survive in a nitrogen-deficient environment. To establish the symbiosis, host plant cells need to accommodate rhizobia during nodule development, a process mediated by a plant-derived intracellular structure called the infection thread (IT). In this study, we show that LACK OF SYMBIONT ACCOMMODATION (LAN) is involved in intracellular rhizobia accommodation in the model leguminous plant Lotus japonicus. LjLAN encodes a putative subunit of Mediator complex, a multiprotein complex that has a fundamental role as an activator of gene transcription. Mutation analysis suggests that LjLAN is required for root hair IT formation, which enables swift and efficient rhizobial accommodation. Moreover, we show that LjLAN is required for symbiosis with arbuscular mycorrhizal fungi. These data add a new component to the molecular mechanism relevant to the establishment of root nodule and arbuscular mycorrhizal symbiosis.

Callose-Regulated Symplastic Communication Coordinates Symbiotic Root Nodule Development

The formation of nitrogen-fixing nodules in legumes involves the initiation of synchronized programs in the root epidermis and cortex to allow rhizobial infection and nodule development. In this study, we provide evidence that symplastic communication, regulated by callose turnover at plasmodesmata (PD), is important for coordinating nodule development and infection in Medicago truncatula. Here, we show that rhizobia promote a reduction in callose levels in inner tissues where nodules initiate. This downregulation coincides with the localized expression of M. truncatula β-1,3-glucanase 2 (MtBG2), encoding a novel PD-associated callose-degrading enzyme. Spatiotemporal analyses revealed that MtBG2 expression expands from dividing nodule initials to rhizobia-colonized cortical and epidermal tissues. As shown by the transport of fluorescent molecules in vivo, symplastic-connected domains are created in rhizobia-colonized tissues and enhanced in roots constitutively expressing MtBG2. MtBG2-overexpressing roots additionally displayed reduced levels of PD-associated callose. Together, these findings suggest an active role for MtBG2 in callose degradation and in the formation of symplastic domains during sequential nodule developmental stages. Interfering with symplastic connectivity led to drastic nodulation phenotypes. Roots ectopically expressing β-1,3-glucanases (including MtBG2) exhibited increased nodule number, and those expressing MtBG2 RNAi constructs or a hyperactive callose synthase (under symbiotic promoters) showed defective nodulation phenotypes. Obstructing symplastic connectivity appears to block a signaling pathway required for the expression of NODULE INCEPTION (NIN) and its target NUCLEAR FACTOR-YA1 (NF-YA1) in the cortex. We conclude that symplastic intercellular communication is proactively enhanced by rhizobia, and this is necessary for appropriate coordination of bacterial infection and nodule development.

NUCLEAR FACTOR-Y: still complex after all these years?

The NUCLEAR FACTOR-Y (NF-Y) families of transcription factors are important regulators of plant development and physiology. Though NF-Y regulatory roles have recently been suggested for numerous aspects of plant biology, their roles in flowering time, early seedling development, stress responses, hormone signaling, and nodulation are the best characterized. The past few years have also seen significant advances in our understanding of the mechanistic function of the NF-Y, and as such, increasingly complex and interesting questions are now more approachable. This review will primarily focus on these developmental, physiological, and mechanistic roles of the NF-Y in recent research.

Development of endogenous promoters that drive high-level expression of introduced genes in the model diatom Phaeodactylum tricornutum

The marine diatom Phaeodactylum tricornutum is attractive for basic and applied diatom research. We isolated putative endogenous gene promoters derived from genes that are highly expressed in P. tricornutum: the fucoxanthin chlorophyll a/c-binding protein (FCP) C gene, the vacuolar ATP synthase 16-kDa proteolipid subunit (V-ATPase C) gene, the clumping factor A gene and the solute carrier family 34 member 2 gene. Five putative promoter regions were isolated, linked to an antibiotic resistance gene (Sh ble) and transformed into P. tricornutum. Using quantitative RT-PCR, the promoter activities in the transformants were analyzed and compared to that of the diatom endogenous gene promoter, the FCP A gene promoter which has been used for the transformation of P. tricornutum. Among the five isolated potential promoters, the activity of the V-ATPase C gene promoter was approximately 2.73 times higher than that of the FCP A gene promoter. The V-ATPase C gene promoter drove the expression of Sh ble mRNA transcripts under both light and dark conditions at the stationary phase. These results suggest that the V-ATPase C gene promoter is a novel tool for the genetic engineering of P. tricornutum.

Genome-wide characterization and expression analysis of citrus NUCLEAR FACTOR-Y (NF-Y) transcription factors identified a novel NF-YA gene involved in drought-stress response and tolerance

Nuclear factor Y (NF-Y) is a ubiquitous transcription factor found in eukaryotes. It is composed of three distinct subunits called NF-YA, NF-YB and NF-YC. NF-Ys have been identified as key regulators of multiple pathways in the control of development and tolerance to biotic and abiotic factors. The present study aimed to identify and characterize the complete repertoire of genes coding for NF-Y in citrus, as well as to perform the functional characterization of one of its members, namely CsNFYA5, in transgenic tobacco plants. A total of 22 genes coding for NF-Y were identified in the genomes of sweet orange (Citrus sinensis) and Clementine mandarin (C. clementina), including six CsNF-YAs, 11 CsNF-YBs and five CsNF-YCs. Phylogenetic analyses showed that there is a NF-Y orthologous in the Clementine genome for each sweet orange NF-Y gene; this was not observed when compared to Arabidopsis thaliana. CsNF-Y proteins shared the same conserved domains with their orthologous proteins in other organisms, including mouse. Analysis of gene expression by RNA-seq and EST data demonstrated that CsNF-Ys have a tissue-specific and stress inducible expression profile. qRT-PCR analysis revealed that CsNF-YA5 exhibits differential expression in response to water deficit in leaves and roots of citrus plants. Overexpression of CsNF-YA5 in transgenic tobacco plants contributed to the reduction of H2O2 production under dehydration conditions and increased plant growth and photosynthetic rate under normal conditions and drought stress. These biochemical and physiological responses to drought stress promoted by CsNF-YA5 may confer a productivity advantage in environments with frequent short-term soil water deficit.

Transcription factors network in root endosymbiosis establishment and development

Root endosymbioses are mutualistic interactions between plants and the soil microorganisms (Fungus, Frankia or Rhizobium) that lead to the formation of nitrogen-fixing root nodules and/or arbuscular mycorrhiza. These interactions enable many species to survive in different marginal lands to overcome the nitrogen-and/or phosphorus deficient environment and can potentially reduce the chemical fertilizers used in agriculture which gives them an economic, social and environmental importance. The formation and the development of these structures require the mediation of specific gene products among which the transcription factors play a key role. Three of these transcription factors, viz., CYCLOPS, NSP1 and NSP2 are well conserved between actinorhizal, legume, non-legume and mycorrhizal symbioses. They interact with DELLA proteins to induce the expression of NIN in nitrogen fixing symbiosis or RAM1 in mycorrhizal symbiosis. Recently, the small non coding RNA including micro RNAs (miRNAs) have emerged as major regulators of root endosymbioses. Among them, miRNA171 targets NSP2, a TF conserved in actinorhizal, legume, non-legume and mycorrhizal symbioses. This review will also focus on the recent advances carried out on the biological function of others transcription factors during the root pre-infection/pre-contact, infection or colonization. Their role in nodule formation and AM development will also be described.

IPD3 and IPD3L Function Redundantly in Rhizobial and Mycorrhizal Symbioses

Legume plants form symbiotic associations with either nitrogen-fixing bacteria or arbuscular mycorrhizal (AM) fungi, which are regulated by a set of common symbiotic signaling pathway genes. Central to the signaling pathway is the activation of the DMI3/IPD3 protein complex by Ca²⁺ oscillations, and the initiation of nodule organogenesis and mycorrhizal symbiosis. DMI3 is essential for rhizobial infection and nodule organogenesis; however, ipd3 mutants have been shown to be impaired only in infection thread formation but not in root nodule organogenesis in Medicago truncatula. We identified an IPD3-like (IPD3L) gene in the M. truncatula genome. A single ipd3l mutant exhibits a normal root nodule phenotype. The ipd3l/ipd3-2 double mutant is completely unable to initiate infection threads and nodule primordia. IPD3L can functionally replace IPD3 when expressed under the control of the IPD3 promoter, indicating functional redundancy between these two transcriptional regulators. We constructed a version of IPD3 that was phosphomimetic with respect to two conserved serine residues (IPD3-2D). This was sufficient to trigger root nodule organogenesis, but the increased multisite phosphorylation of IPD3 (IPD3-8D) led to low transcriptional activity, suggesting that the phosphorylation levels of IPD3 fine-tune its transcriptional activity in the root nodule symbiosis. Intriguingly, the phosphomimetic version of IPD3 triggers spontaneous root-like nodules on the roots of dmi3-1 and dmi2-1 (DMI2 is an LRR-containing receptor-like kinase gene which is required for Ca²⁺ spiking), but not on the roots of wild-type or ipd3l ipd3-2 plants. In addition, fully developed arbuscules were formed in the ipd3l ipd3-2 mutants but not the ccamk/dmi3-1 mutants. Collectively, our data indicate that, in addition to IPD3 and IPD3L, another new genetic component or other new phosphorylation sites of IPD3 function downstream of DMI3 in rhizobial and mycorrhizal symbioses.