Legumes Regulate Symbiosis with Rhizobia via Their Innate Immune System.

Plant roots are constantly exposed to a diverse microbiota of pathogens and mutualistic partners. The host's immune system is an essential component for its survival, enabling it to monitor nearby microbes for potential threats and respond with a defence response when required. Current research suggests that the plant immune system has also been employed in the legume-rhizobia symbiosis as a means of monitoring different rhizobia strains and that successful rhizobia have evolved to overcome this system to infect the roots and initiate nodulation. With clear implications for host-specificity, the immune system has the potential to be an important target for engineering versatile crops for effective nodulation in the field. However, current knowledge of the interacting components governing this pathway is limited, and further research is required to build on what is currently known to improve our understanding. This review provides a general overview of the plant immune system's role in nodulation. With a focus on the cycles of microbe-associated molecular pattern-triggered immunity (MTI) and effector-triggered immunity (ETI), we highlight key molecular players and recent findings while addressing the current knowledge gaps in this area.

A nitrogen fixing symbiosis-specific pathway required for legume flowering.

Symbiotic nitrogen fixation boosts legume growth and production in nitrogen-poor soils. It has long been assumed that fixed nitrogen increases reproductive success, but until now, the regulatory mechanism was unknown. Here, we report a symbiotic flowering pathway that couples symbiotic and nutrient signals to the flowering induction pathway in legumes. We show that the symbiotic microRNA-microRNA172c (miR172c) and fixed nitrogen systemically and synergistically convey symbiotic and nutritional cues from roots to leaves to promote soybean (Glycine max) flowering. The combinations of symbiotic miR172c and local miR172c elicited by fixed nitrogen and development in leaves activate florigen-encoding FLOWERING LOCUS T (FT) homologs (GmFT2a/5a) by repressing TARGET OF EAT1-like 4a (GmTOE4a). Thus, FTs trigger reproductive development, which allows legumes to survive and reproduce under low-nitrogen conditions.

Spatiotemporal changes in gibberellin content are required for soybean nodulation.

The plant hormone gibberellin (GA) is required at different stages of legume nodule development, with its spatiotemporal distribution tightly regulated. Transcriptomic and bioinformatic analyses established that several key GA biosynthesis and catabolism enzyme encoding genes are critical to soybean (Glycine max) nodule formation. We examined the expression of several GA oxidase genes and used a Förster resonance energy transfer-based GA biosensor to determine the bioactive GA content of roots inoculated with DsRed-labelled Bradyrhizobium diazoefficiens. We manipulated the level of GA by genetically disrupting the expression of GA oxidase genes. Moreover, exogenous treatment of soybean roots with GA3 induced the expression of key nodulation genes and altered infection thread and nodule phenotypes. GmGA20ox1a, GmGA3ox1a, and GmGA2ox1a are upregulated in soybean roots inoculated with compatible B. diazoefficiens. GmGA20ox1a expression is predominately localized to the transient meristem of soybean nodules and coincides with the spatiotemporal distribution of bioactive GA occurring throughout nodule organogenesis. GmGA2ox1a exhibits a nodule vasculature-specific expression pattern, whereas GmGA3ox1a can be detected throughout the nodule and root. Disruptions in the level of GA resulted in aberrant rhizobia infection and reduced nodule numbers. Collectively, our results establish a central role for GAs in root hair infection by symbiotic rhizobia and in nodule organogenesis.

Role of hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase 1 in nodule development of soybean.

Autoregulation of nodulation (AON) plays a central role in nodulation by inhibiting the formation of excess number of legume root nodules. In this study, the effect of hydroxymethylglutaryl-coenzyme A reductase 1 (GmHMGR1) gene expression on nodulation and the AON system in Glycine max (L.) Merr was investigated. Wild-type soybean (cultivar Bragg) and its near-isogenic supernodulating mutant (nitrate tolerant symbiotic) nts1007 were selected to identify the expression pattern of this gene in rootlets after inoculation by its microsymbiont Bradyrhizobium. For further analysis, the full length of GmHMGR1 and its promoter were cloned after amplification by inverse-PCR and BAC library screening. Also, we constructed an intron hairpin RNA interference (ihpRNAi) and a GmHMGR1 promoter: ß-glucuronidase fusion constructs, consequently for suppression of GmHMGR1 and histochemical analysis in transgenic soybean hairy roots induced by Agrobacterium rhizogenes strain K599. The GmHMGR1 gene was functional during the early stages of nodulation with the AON system having a negative effect on GmHMGR1 expression and nodule formation in wild-type rootlets. GmHMGR1 was particularly expressed in the developing phloem within the root, nodules and nodule lenticels. Expression of GmHMGR1 in transgenic hairy roots was suppressed by RNAi silencing approximately 85% as compared to empty vector controls. This suggests that the GmHMGR1 gene has an important role in triggering nodule formation as its suppression caused a reduction of nodule formation in nts mutant lines with a deficient AON system.

Shoot-derived miR2111 controls legume root and nodule development.

Legumes control their nodule numbers through the autoregulation of nodulation (AON). Rhizobia infection stimulates the production of root-derived CLE peptide hormones that are translocated to the shoot where they regulate a new signal. We used soybean to demonstrate that this shoot-derived signal is miR2111, which is transported via phloem to the root where it targets transcripts of Too Much Love (TML), a negative regulator of nodulation. Shoot perception of rhizobia-induced CLE peptides suppresses miR2111 expression, resulting in TML accumulation in roots and subsequent inhibition of nodule organogenesis. Feeding synthetic mature miR2111 via the petiole increased nodule numbers per plant. Likewise, elevating miR2111 availability by over-expression promoted nodulation, while target mimicry of TML induced the opposite effect on nodule development in wild-type plants and alleviated the supernodulating and stunted root growth phenotypes of AON-defective mutants. Additionally, in non-nodulating wild-type plants, ectopic expression of miR2111 significantly enhanced lateral root emergence with a decrease in lateral root length and average root diameter. In contrast, hairy roots constitutively expressing the target mimic construct exhibited reduced lateral root density. Overall, these findings demonstrate that miR2111 is both the critical shoot-to-root factor that positively regulates root nodule development and also acts to shape root system architecture.

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

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

Hypernodulating soybean mutant line nod4 lacking 'Autoregulation of Nodulation' (AON) has limited root-to-shoot water transport capacity.

BACKGROUND AND AIMS: Although hypernodulating phenotype mutants of legumes, such as soybean, possess a high leaf N content, the large number of root nodules decreases carbohydrate availability for plant growth and seed yield. In addition, under conditions of high air vapour pressure deficit (VPD), hypernodulating plants show a limited capacity to replace water losses through transpiration, resulting in stomatal closure, and therefore decreased net photosynthetic rates. Here, we used hypernodulating (nod4) (282.33 ± 28.56 nodules per plant) and non-nodulating (nod139) (0 nodules per plant) soybean mutant lines to determine explicitly whether a large number of nodules reduces root hydraulic capacity, resulting in decreased stomatal conductance and net photosynthetic rates under high air VPD conditions. METHODS: Plants were either inoculated or not inoculated with Bradyrhizobium diazoefficiens (strain BR 85, SEMIA 5080) to induce nitrogen-fixing root nodules (where possible). Absolute root conductance and root conductivity, plant growth, leaf water potential, gas exchange, chlorophyll a fluorescence, leaf 'greenness' [Soil Plant Analysis Development (SPAD) reading] and nitrogen content were measured 37 days after sowing. KEY RESULTS: Besides the reduced growth of hypernodulating soybean mutant nod4, such plants showed decreased root capacity to supply leaf water demand as a consequence of their reduced root dry mass and root volume, which resulted in limited absolute root conductance and root conductivity normalized by leaf area. Thereby, reduced leaf water potential at 1300 h was observed, which contributed to depression of photosynthesis at midday associated with both stomatal and non-stomatal limitations. CONCLUSIONS: Hypernodulated plants were more vulnerable to VPD increases due to their limited root-to-shoot water transport capacity. However, greater CO2 uptake caused by the high N content can be partly compensated by the stomatal limitation imposed by increased VPD conditions.

Triarabinosylation is required for nodulation-suppressive CLE peptides to systemically inhibit nodulation in Pisum sativum.

Legumes form root nodules to house beneficial nitrogen-fixing rhizobia bacteria. However, nodulation is resource demanding; hence, legumes evolved a systemic signalling mechanism called autoregulation of nodulation (AON) to control nodule numbers. AON begins with the production of CLE peptides in the root, which are predicted to be glycosylated, transported to the shoot, and perceived. We synthesized variants of nodulation-suppressing CLE peptides to test their activity using petiole feeding to introduce CLE peptides into the shoot. Hydroxylated, monoarabinosylated, and triarabinosylated variants of soybean GmRIC1a and GmRIC2a were chemically synthesized and fed into recipient Pisum sativum (pea) plants, which were used due to the availability of key AON pathway mutants unavailable in soybean. Triarabinosylated GmRIC1a and GmRIC2a suppressed nodulation of wild-type pea, whereas no other peptide variant tested had this ability. Suppression also occurred in the supernodulating hydroxyproline O-arabinosyltransferase mutant, Psnod3, but not in the supernodulating receptor mutants, Pssym29, and to some extent, Pssym28. During our study, bioinformatic resources for pea became available and our analyses identified 40 CLE peptide-encoding genes, including orthologues of nodulation-suppressive CLE peptides. Collectively, we demonstrated that soybean nodulation-suppressive CLE peptides can function interspecifically in the AON pathway of pea and require arabinosylation for their activity.

Legume nodulation: The host controls the party.

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

A differential k-mer analysis pipeline for comparing RNA-Seq transcriptome and meta-transcriptome datasets without a reference.

Next-generation DNA sequencing technologies, such as RNA-Seq, currently dominate genome-wide gene expression studies. A standard approach to analyse this data requires mapping sequence reads to a reference and counting the number of reads which map to each gene. However, for many transcriptome studies, a suitable reference genome is unavailable, especially for meta-transcriptome studies which assay gene expression from mixed populations of organisms. Where a reference is unavailable, it is possible to generate a reference by the de novo assembly of the sequence reads. However, the high cost of generating high-coverage data for de novo assembly hinders this approach and more importantly the accurate assembly of such data is challenging, especially for meta-transcriptome data, and resulting assemblies frequently suffer from collapsed regions or chimeric sequences. As an alternative to the standard reference mapping approach, we have developed a k-mer-based analysis pipeline (DiffKAP) to identify differentially expressed reads between RNA-Seq datasets without the requirement for a reference. We compared the DiffKAP approach with the traditional Tophat/Cuffdiff method using RNA-Seq data from soybean, which has a suitable reference genome. We subsequently examined differential gene expression for a coral meta-transcriptome where no reference is available, and validated the results using qRT-PCR. We conclude that DiffKAP is an accurate method to study differential gene expression in complex meta-transcriptomes without the requirement of a reference genome.

Local and Systemic Effect of Cytokinins on Soybean Nodulation and Regulation of Their Isopentenyl Transferase (IPT) Biosynthesis Genes Following Rhizobia Inoculation.

Cytokinins are important regulators of cell proliferation and differentiation in plant development. Here, a role for this phytohormone group in soybean nodulation is shown through the exogenous application of cytokinins (6-benzylaminopurine, N6-(Δ2-isopentenyl)-adenine and trans-zeatin) via either root drenching or a petiole feeding technique. Overall, nodule numbers were reduced by treatment with high cytokinin concentrations, but increased with lower concentrations. This was especially evident when feeding the solutions directly into the vasculature via petiole feeding. These findings highlight the importance of cytokinin in nodule development. To further investigate the role of cytokinin in controlling nodule numbers, the IPT gene family involved in cytokinin biosynthesis was characterized in soybean. Bioinformatic analyses identified 17 IPT genes in the soybean genome and homeologous duplicate gene partners were subsequently identified including GmIPT5 and GmIPT6, the orthologs of LjIPT3. Expression of GmIPT5 was upregulated in the shoot in response to nodulation, but this was independent of a functional copy of the autoregulation of nodulation (AON) receptor, GmNARK, which suggests it is unlikely to have a role in the negative feedback system called AON. Legumes also control nodule numbers in the presence of soil nitrogen through nitrate-dependent regulation of nodulation, a locally acting pathway in soybean. Upon nitrate treatment to the root, the tandem duplicates GmIPT3 and GmIPT15 were upregulated in expression indicating a role for these genes in the plant's response to soil nitrogen, potentially including the nitrate-dependent regulation of legume nodulation pathway. Additional roles for cytokinin and their IPT biosynthetic genes in nodulation and the control of nodule numbers are discussed.

Author Correction: CLE peptide-encoding gene families in Medicago truncatula and Lotus japonicus, compared with those of soybean, common bean and Arabidopsis.

A correction to this article has been published and is linked from the HTML version of this paper. The error has not been fixed in the paper.

Arabinosylation Modulates the Growth-Regulating Activity of the Peptide Hormone CLE40a from Soybean.

Small post-translationally modified peptide hormones mediate crucial developmental and regulatory processes in plants. CLAVATA/ENDOSPERM-SURROUNDING REGION (CLE) genes are found throughout the plant kingdom and encode for 12-13 amino acid peptides that must often undergo post-translational proline hydroxylation and glycosylation with O-ß1,2-triarabinose moieties before they become functional. Apart from a few recent examples, a detailed understanding of the structure and function of most CLE hormones is yet to be uncovered. This is mainly owing to difficulties in isolating mature homogeneously modified CLE peptides from natural plant sources. In this study, we describe the efficient synthesis of a synthetic Araf3Hyp glycosylamino acid building block that was used to access a hitherto uninvestigated CLE hormone from soybean called GmCLE40a. Through the development and implementation of a novel in vivo root growth assay, we show that the synthetic triarabinosylated glycopeptide suppresses primary root growth in this important crop species.

CLE peptide-encoding gene families in Medicago truncatula and Lotus japonicus, compared with those of soybean, common bean and Arabidopsis.

CLE peptide hormones are critical regulators of many cell proliferation and differentiation mechanisms in plants. These 12-13 amino acid glycosylated peptides play vital roles in a diverse range of plant tissues, including the shoot, root and vasculature. CLE peptides are also involved in controlling legume nodulation. Here, the entire family of CLE peptide-encoding genes was identified in Medicago truncatula (52) and Lotus japonicus (53), including pseudogenes and non-functional sequences that were identified. An array of bioinformatic techniques were used to compare and contrast these complete CLE peptide-encoding gene families with those of fellow legumes, Glycine max and Phaseolus vulgaris, in addition to the model plant Arabidopsis thaliana. This approach provided insight into the evolution of CLE peptide families and enabled us to establish putative M. truncatula and L. japonicus orthologues. This includes orthologues of nodulation-suppressing CLE peptides and AtCLE40 that controls the stem cell population of the root apical meristem. A transcriptional meta-analysis was also conducted to help elucidate the function of the CLE peptide family members. Collectively, our analyses considerably increased the number of annotated CLE peptides in the model legume species, M. truncatula and L. japonicus, and substantially enhanced the knowledgebase of this critical class of peptide hormones.

Neodiversification of homeologous CLAVATA1-like receptor kinase genes in soybean leads to distinct developmental outcomes.

The CLAVATA pathway that regulates stem cell numbers of the shoot apical meristem has exclusively been studied in Arabidopsis; as such insight into other species is warranted. In this study, a GmCLV1A mutant (F-S562L) with altered lateral organ development, and two mutants of GmNARK, isolated from a Forrest M2 population (EMS-mutated soybean) were studied. GmCLV1A and GmNARK encode for LRR receptor kinases, and share 92% of protein sequence. While GmNARK is critical for systemic regulation of nodulation (new organ made on the root through symbiosis), we show that GmCLV1A functions locally and has no apparent function in nodulation or root development. However, a recessive, loss-of-function mutation (S562L) in a putative S-glycosylation site of GmCLV1A causes stem nodal identity alterations as well as flower and pod abnormalities (deformed flower and pod). The mutant also exhibits a homeotic phenotype, displaying abnormal leaf development/number, vein-derived leaf emergence, and a thick, faciated stem. The mutant phenotype is also temperature-sensitive. Interestingly, a novel truncated version of GmCLV1A was identified upstream of GmCLV1A that is absent from GmNARK, but is present upstream of the GmNARK orthologues, MtSUNN and PvNARK. Taken together, our findings indicate that GmCLV1A acts on shoot architecture, whereas GmNARK, functions in controlling nodule numbers.

Molecular Signals in Nodulation Control.

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

Genome-wide annotation and characterization of CLAVATA/ESR (CLE) peptide hormones of soybean (Glycine max) and common bean (Phaseolus vulgaris), and their orthologues of Arabidopsis thaliana.

CLE peptides are key regulators of cell proliferation and differentiation in plant shoots, roots, vasculature, and legume nodules. They are C-terminally encoded peptides that are post-translationally cleaved and modified from their corresponding pre-propeptides to produce a final ligand that is 12-13 amino acids in length. In this study, an array of bionformatic and comparative genomic approaches was used to identify and characterize the complete family of CLE peptide-encoding genes in two of the world's most important crop species, soybean and common bean. In total, there are 84 CLE peptide-encoding genes in soybean (considerably more than the 32 present in Arabidopsis), including three pseudogenes and two multi-CLE domain genes having six putative CLE domains each. In addition, 44 CLE peptide-encoding genes were identified in common bean. In silico characterization was used to establish all soybean homeologous pairs, and to identify corresponding gene orthologues present in common bean and Arabidopsis. The soybean CLE pre-propeptide family was further analysed and separated into seven distinct groups based on structure, with groupings strongly associated with the CLE domain sequence and function. These groups provide evolutionary insight into the CLE peptide families of soybean, common bean, and Arabidopsis, and represent a novel tool that can aid in the functional characterization of the peptides. Transcriptional evidence was also used to provide further insight into the location and function of all CLE peptide-encoding members currently available in gene atlases for the three species. Taken together, this in-depth analysis helped to identify and categorize the complete CLE peptide families of soybean and common bean, established gene orthologues within the two legume species, and Arabidopsis, and provided a platform to help compare, contrast, and identify the function of critical CLE peptide hormones in plant development.

MicroRNA167-Directed Regulation of the Auxin Response Factors GmARF8a and GmARF8b Is Required for Soybean Nodulation and Lateral Root Development.

Legume root nodules convert atmospheric nitrogen gas into ammonium through symbiosis with a prokaryotic microsymbiont broadly called rhizobia. Auxin signaling is required for determinant nodule development; however, the molecular mechanism of auxin-mediated nodule formation remains largely unknown. Here, we show in soybean (Glycine max) that the microRNA miR167 acts as a positive regulator of lateral root organs, namely nodules and lateral roots. miR167c expression was up-regulated in the vasculature, pericycle, and cortex of soybean roots following inoculation with Bradyrhizobium japonicum strain USDA110 (the microsymbiont). It was found to positively regulate nodule numbers directly by repressing the target genes GmARF8a and GmARF8b (homologous genes of Arabidopsis [Arabidopsis thaliana] AtARF8 that encode auxin response factors). Moreover, the expression of miR167 and its targets was up- and down-regulated by auxin, respectively. The miR167-GmARF8 module also positively regulated nodulation efficiency under low microsymbiont density, a condition often associated with environmental stress. The regulatory role of miR167 on nodule initiation was dependent on the Nod factor receptor GmNFR1α, and it acts upstream of the nodulation-associated genes nodule inception, nodulation signaling pathway1, early nodulin40-1, NF-YA1 (previously known as HAEM activator protein2-1), and NF-YA2. miR167 also promoted lateral root numbers. Collectively, our findings establish a key role for the miR167-GmARF8 module in auxin-mediated nodule and lateral root formation in soybean.

Scanning the effects of ethyl methanesulfonate on the whole genome of Lotus japonicus using second-generation sequencing analysis.

Genetic structure can be altered by chemical mutagenesis, which is a common method applied in molecular biology and genetics. Second-generation sequencing provides a platform to reveal base alterations occurring in the whole genome due to mutagenesis. A model legume, Lotus japonicus ecotype Miyakojima, was chemically mutated with alkylating ethyl methanesulfonate (EMS) for the scanning of DNA lesions throughout the genome. Using second-generation sequencing, two individually mutated third-generation progeny (M3, named AM and AS) were sequenced and analyzed to identify single nucleotide polymorphisms and reveal the effects of EMS on nucleotide sequences in these mutant genomes. Single-nucleotide polymorphisms were found in every 208 kb (AS) and 202 kb (AM) with a bias mutation of G/C-to-A/T changes at low percentage. Most mutations were intergenic. The mutation spectrum of the genomes was comparable in their individual chromosomes; however, each mutated genome has unique alterations, which are useful to identify causal mutations for their phenotypic changes. The data obtained demonstrate that whole genomic sequencing is applicable as a high-throughput tool to investigate genomic changes due to mutagenesis. The identification of these single-point mutations will facilitate the identification of phenotypically causative mutations in EMS-mutated germplasm.

Identification of the primary lesion of toxic aluminum in plant roots.

Despite the rhizotoxicity of aluminum (Al) being identified over 100 years ago, there is still no consensus regarding the mechanisms whereby root elongation rate is initially reduced in the approximately 40% of arable soils worldwide that are acidic. We used high-resolution kinematic analyses, molecular biology, rheology, and advanced imaging techniques to examine soybean (Glycine max) roots exposed to Al. Using this multidisciplinary approach, we have conclusively shown that the primary lesion of Al is apoplastic. In particular, it was found that 75 µm Al reduced root growth after only 5 min (or 30 min at 30 µm Al), with Al being toxic by binding to the walls of outer cells, which directly inhibited their loosening in the elongation zone. An alteration in the biosynthesis and distribution of ethylene and auxin was a second, slower effect, causing both a transient decrease in the rate of cell elongation after 1.5 h but also a longer term gradual reduction in the length of the elongation zone. These findings show the importance of focusing on traits related to cell wall composition as well as mechanisms involved in wall loosening to overcome the deleterious effects of soluble Al.