Nod Factor Signaling Research Articles

Tyrosylprotein Sulfotransferase Positively Regulates Symbiotic Nodulation and Root Growth.

Posttranslational tyrosine sulfation of peptides and proteins is catalysed by tyrosylprotein sulfotransferases (TPSTs). In Arabidopsis, tyrosine sulfation is essential for the activities of peptide hormones, such as phytosulfokine (PSK) and root meristem growth factor (RGF). Here, we identified a TPST-encoding gene, MtTPST, from model legume Medicago truncatula. MtTPST expression was detected in all organs, with the highest level in root nodules. A promoter:GUS assay revealed that MtTPST was highly expressed in the root apical meristem, nodule primordium and nodule apical meristem. The loss-of-function mutant mttpst exhibited a stunted phenotype with short roots and reduced nodule number and size. Application of both of the sulfated peptides PSK and RGF3 partially restored the defective root length of mttpst. The reduction in symbiotic nodulation in mttpst was partially recovered by treatment with sulfated PSK peptide. MtTPST-PSK module functions downstream of the Nod factor signalling to promote nodule initiation via regulating accumulation and/or signalling of cytokinin and auxin. Additionally, the small-nodule phenotype of mttpst, which resulted from decreased apical meristematic activity, was partially complemented by sulfated RGF3 treatment. Together, these results demonstrate that MtTPST, through its substrates PSK, RGF3 and other sulfated peptide(s), positively regulates nodule development and root growth.

The type III effector NopL interacts with GmREM1a and GmNFR5 to promote symbiosis in soybean

The establishment of symbiotic interactions between leguminous plants and rhizobia requires complex cellular programming activated by Rhizobium Nod factors (NFs) as well as type III effector (T3E)-mediated symbiotic signaling. However, the mechanisms by which different signals jointly affect symbiosis are still unclear. Here we describe the mechanisms mediating the cross-talk between the broad host range rhizobia Sinorhizobium fredii HH103 T3E Nodulation Outer Protein L (NopL) effector and NF signaling in soybean. NopL physically interacts with the Glycine max Remorin 1a (GmREM1a) and the NFs receptor NFR5 (GmNFR5) and promotes GmNFR5 recruitment by GmREM1a. Furthermore, NopL and NF influence the expression of GmRINRK1, a receptor-like kinase (LRR-RLK) ortholog of the Lotus RINRK1, that mediates NF signaling. Taken together, our work indicates that S. fredii NopL can interact with the NF signaling cascade components to promote the symbiotic interaction in soybean.

The role of GmHSP23.9 in regulating soybean nodulation under elevated CO2 condition

Legume-rhizobia symbiosis offers a unique approach to increase leguminous crop yields. Previous studies have indicated that the number of soybean nodules are increased under elevated CO2 concentration. However, the underlying mechanism behind this phenomenon remains elusive. In this study, transcriptome analysis was applied to identify candidate genes involved in regulating soybean nodulation mediated by elevated CO2 concentration. Among the different expression genes (DEGs), we identified a gene encoding small heat shock protein (sHSP) called GmHSP23.9, which mainly expressed in soybean roots and nodules, and its expression was significantly induced by rhizobium USDA110 infection at 14 days after inoculation (DAI) under elevated CO2 conditions. We further investigated the role of GmHSP23.9 by generating transgenic composite plants carrying GmHSP23.9 overexpression (GmHSP23.9-OE), RNA interference (GmHSP23.9-RNAi), and CRISPR-Cas9 (GmHSP23.9-KO), and these modifications resulted in notable changes in nodule number and the root hairs deformation and suggesting that GmHSP23.9 function as an important positive regulator in soybean. Moreover, we found that altering the expression of GmHSP23.9 influenced the expression of genes involved in the Nod factor signaling pathway and AON signaling pathway to modulate soybean nodulation. Interestingly, we found that knocking down of GmHSP23.9 prevented the increase in the nodule number of soybean in response to elevated CO2 concentration. This research has successfully identified a crucial regulator that influences soybean nodulation under elevated CO2 level and shedding new light on the role of sHSPs in legume nodulation.

Soybean CEP6 Signaling Peptides Positively Regulate Nodulation

Nodulation is the most efficient nitrate assimilation system in the ecosystem, while excessive fertilization has an increased nitrate inhibition effect; deciphering the nitrate signal transduction mechanism in the process is of the utmost importance. In this study, genome-wide analyses of the GmCEP genes were applied to identify nodulation-related CEP genes; 22 GmCEP family members were identified, while GmCEP6 was mainly expressed in nodules and significantly responded to nitrate treatment and rhizobium infection, especially in later stages. Overexpression and CRISPR-Cas9 were used to validate its role in nodulation. We found that GmCEP6 overexpression significantly increased the nodule number, while GmCEP6 knock-out significantly decreased the nodule number, which suggests that GmCEP6 functions as a positive regulator in soybean nodulation. qRT-PCR showed that alterations in the expression of GmCEP6 affected the expression of marker genes in the Nod factor signaling pathway. Lastly, the function of GmCEP6 in nitrate inhibition of nodulation was analyzed; nodule numbers in the GmCEP6-overexpressed roots significantly increased under nitrogen treatments, which suggests that GmCEP6 functions in the resistance to nitrate inhibition. The study helps us understand that GmCEP6 promotes nodulation and participates in the regulation of nitrate inhibition of nodulation, which is of great significance for high efficiency utilization of nitrogen in soybeans.

RinRK1 enhances NF receptors accumulation in nanodomain-like structures at root-hair tip

Legume-rhizobia root-nodule symbioses involve the recognition of rhizobial Nod factor (NF) signals by NF receptors, triggering both nodule organogenesis and rhizobial infection. RinRK1 is induced by NF signaling and is essential for infection thread (IT) formation in Lotus japonicus. However, the precise mechanism underlying this process remains unknown. Here, we show that RinRK1 interacts with the extracellular domains of NF receptors (NFR1 and NFR5) to promote their accumulation at root hair tips in response to rhizobia or NFs. Furthermore, Flotillin 1 (Flot1), a nanodomain-organizing protein, associates with the kinase domains of NFR1, NFR5 and RinRK1. RinRK1 promotes the interactions between Flot1 and NF receptors and both RinRK1 and Flot1 are necessary for the accumulation of NF receptors at root hair tips upon NF stimulation. Our study shows that RinRK1 and Flot1 play a crucial role in NF receptor complex assembly within localized plasma membrane signaling centers to promote symbiotic infection.

Nitrogen and Nod factor signaling determine Lotus japonicus root exudate composition and bacterial assembly

Symbiosis with soil-dwelling bacteria that fix atmospheric nitrogen allows legume plants to grow in nitrogen-depleted soil. Symbiosis impacts the assembly of root microbiota, but it is unknown how the interaction between the legume host and rhizobia impacts the remaining microbiota and whether it depends on nitrogen nutrition. Here, we use plant and bacterial mutants to address the role of Nod factor signaling on Lotus japonicus root microbiota assembly. We find that Nod factors are produced by symbionts to activate Nod factor signaling in the host and that this modulates the root exudate profile and the assembly of a symbiotic root microbiota. Lotus plants with different symbiotic abilities, grown in unfertilized or nitrate-supplemented soils, display three nitrogen-dependent nutritional states: starved, symbiotic, or inorganic. We find that root and rhizosphere microbiomes associated with these states differ in composition and connectivity, demonstrating that symbiosis and inorganic nitrogen impact the legume root microbiota differently. Finally, we demonstrate that selected bacterial genera characterizing state-dependent microbiomes have a high level of accurate prediction.

The Bax inhibitor GmBI-1α interacts with a Nod factor receptor and plays a dual role in the legume-rhizobia symbiosis.

The gene networks surrounding Nod factor receptors that govern the symbiotic process between legumes and rhizobia remain largely unexplored. Here, we identify 13 novel GmNFR1α-associated proteins by yeast two-hybrid screening, and describe a potential interacting protein, GmBI-1α. GmBI-1α had the highest positive correlation with GmNFR1α in a co-expression network analysis, and its expression at the mRNA level in roots was enhanced by rhizobial infection. Moreover, GmBI-1α-GmNFR1α interaction was shown to occur in vitro and in vivo. The GmBI-1α protein was localized to multiple subcellular locations, including the endoplasmic reticulum and plasma membrane. Overexpression of GmBI-1α increased the nodule number in transgenic hairy roots or transgenic soybean, whereas down-regulation of GmBI-1α transcripts by RNA interference reduced the nodule number. In addition, the nodules in GmBI-1α-overexpressing plants became smaller in size and infected area with reduced nitrogenase activity. In GmBI-1α-overexpressing transgenic soybean, the elevated GmBI-1α also promoted plant growth and suppressed the expression of defense signaling-related genes. Infection thread analysis of GmBI-1α-overexpressing plants showed that GmBI-1α promoted rhizobial infection. Collectively, our findings support a GmNFR1α-associated protein in the Nod factor signaling pathway and shed new light on the regulatory mechanism of GmNFR1α in rhizobial symbiosis.

Roles of a CCR4-NOT complex component GmNOT4-1 in regulating soybean nodulation.

Legume-rhizobial symbiotic nitrogen fixation is the most efficient nitrogen assimilation system in the ecosystem. In the special interaction between organ-root nodules, legumes supply rhizobial carbohydrates for their proliferation, while rhizobials provide host plants with absorbable nitrogen. Nodule initiation and formation require a complex molecular dialogue between legumes and rhizobia, which involves the accurate regulation of a series of legume genes. The CCR4-NOT complex is a conserved multi-subunit complex with functions regulating gene expression in many cellular processes. However, the functions of the CCR4-NOT complex in rhizobia-host interactions remain unclear. In this study, we identified seven members of the NOT4 family in soybean and further classified them into three subgroups. Bioinformatic analysis showed that NOT4s shared relatively conserved motifs and gene structures in each subgroup, while there were significant differences between NOT4s in the different subgroups. Expression profile analysis indicated that NOT4s may be involved in nodulation in soybean, as most of them were induced by Rhizobium infection and highly expressed in nodules. We further selected GmNOT4-1 to clarify the biological function of these genes in soybean nodulation. Interestingly, we found that either GmNOT4-1 overexpression or down-regulation of GmNOT4-1 by RNAi or CRISPR/Cas9 gene editing would suppress the number of nodules in soybean. Intriguingly, alterations in the expression of GmNOT4-1 repressed the expression of genes in the Nod factor signaling pathway. This research provides new insight into the function of the CCR4-NOT family in legumes and reveals GmNOT4-1 to be a potent gene for regulating symbiotic nodulation.

Phytosulfokine-δ: A Small Peptide, but a Big Player in Symbiosis Gene Regulation

Nitrogen availability is one of the critical determinants of agricultural yield. Biological nitrogen fixation, such as legume–rhizobia symbiotic association, might function as a solution to fix nitrogen. Using phytosulfokine (PSK)-α sequences as a query, Yu et al., 2022 performed a comprehensive genome-wide search of legume species to identify PSK-δ, a divergent pentapeptide differing in single amino acid. Furthermore, PSK-δ exhibited nodule-specific expression with lower expression in the root, substantiating the nodule-specific temporal expression and suggesting its role in nodule development and nitrogen fixation. Additionally, in planta functional characterization in Medicago truncatula using overexpression and Tnt1-insertion mutant analysis indicated the role of PSK-δ in symbiotic nodulation. Interestingly, a similar phenotype of MtPSKδ mutant (mtpskδ) with that of wild-type control led to the hypothesis of its functional redundancy with PSK-α in nodule organogenesis. Further investigation regarding its position in the Nod-factor signaling pathway revealed the downstream function of PSK-δ in association with MtENOD11 in regulating nodule formation.

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.

QTL mapping of genes related to Nod factor signalling using recombinant inbred lines of soybean (Glycine max)

AbstractSoybean, which originated in China, is an important oil crop. The soybean–rhizobium symbiotic relationship provides soybean plants with abundant nitrogen. The Nod factor, which is an early signalling factor, affects the establishment of this symbiosis. In this study, the generated Sinorhizobium fredii HH103ΩNodB mutant exhibited defective Nod factor synthesis. The nodulation of 30 core collections, including ‘Dongnong594’ and ‘Charleston’, by the NodB mutant differed from the nodulation by the wild‐type strain. Soybean recombinant inbred lines derived from a ‘Dongnong594’ × ‘Charleston’ cross were used for mapping soybean genes related to the Nod factor signalling pathway. Eleven QTLs related to nodule number and eight QTLs related to nodule dry weight were revealed. On the basis of a qRT‐PCR analysis, Glyma.03G120700, Glyma.08G296000 and Glyma.15G21560 were identified as candidate genes influencing the NF interaction. These genes were differentially expressed following the inoculations with the wild‐type rhizobium and the NodB mutant. This study provides important insights into the mechanism mediating soybean responses to the NF after the soybean–rhizobium symbiosis is established.

Nod factor receptor complex phosphorylates GmGEF2 to stimulate ROP signaling during nodulation

The establishment of the symbiotic interaction between rhizobia and legumes involves the Nod factor signaling pathway. Nod factor recognition occurs through two plant receptors, NFR1 and NFR5. However, the signal transduction mechanisms downstream of NFR1-NFR5-mediated Nod factor perception remain largely unknown. Here, we report that a small guanosine triphosphatase (GTPase), GmROP9, and a guanine nucleotide exchange factor, GmGEF2, are involved in the soybean-rhizobium symbiosis. We show that GmNFR1α phosphorylates GmGEF2a at its N-terminal S86, which stimulates guanosine diphosphate (GDP)-to-GTP exchange to activate GmROP9 and that the active form of GmROP9 can associate with both GmNFR1α and GmNFR5α. We further show that a scaffold protein, GmRACK1, interacts with active GmROP9 and contributes to root nodule symbiosis. Collectively, our results highlight the symbiotic role of GmROP9-GmRACK1 and support the hypothesis that rhizobial signals promote the formation of a protein complex comprising GmNFR1, GmNFR5, GmROP9, and GmRACK1 for symbiotic signal transduction in soybean.

Microrna Regulation of Nodule Zone-Specific Gene Expression In Soybean

Microrna Regulation of Nodule Zone-Specific Gene Expression In Soybean

Interaction of Symbiotic Rhizobia and Parasitic Root-Knot Nematodes in Legume Roots: From Molecular Regulation to Field Application.

Legumes form two types of root organs in response to signals from microbes, namely, nodules and root galls. In the field, these interactions occur concurrently and often interact with each other. The outcomes of these interactions vary and can depend on natural variation in rhizobia and nematode populations in the soil as well as abiotic conditions. While rhizobia are symbionts that contribute fixed nitrogen to their hosts, parasitic root-knot nematodes (RKN) cause galls as feeding structures that consume plant resources without a contribution to the plant. Yet, the two interactions share similarities, including rhizosphere signaling, repression of host defense responses, activation of host cell division, and differentiation, nutrient exchange, and alteration of root architecture. Rhizobia activate changes in defense and development through Nod factor signaling, with additional functions of effector proteins and exopolysaccharides. RKN inject large numbers of protein effectors into plant cells that directly suppress immune signaling and manipulate developmental pathways. This review examines the molecular control of legume interactions with rhizobia and RKN to elucidate shared and distinct mechanisms of these root-microbe interactions. Many of the molecular pathways targeted by both organisms overlap, yet recent discoveries have singled out differences in the spatial control of expression of developmental regulators that may have enabled activation of cortical cell division during nodulation in legumes. The interaction of legumes with symbionts and parasites highlights the importance of a comprehensive view of root-microbe interactions for future crop management and breeding strategies.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A Nod factor- and type III secretion system-dependent manner for Robinia pseudoacacia to establish symbiosis with Mesorhizobium amorphae CCNWGS0123.

Under nitrogen-limiting conditions, symbiotic nodulation promotes the growth of legume plants via the fixation of atmospheric nitrogen to ammonia by rhizobia in root nodules. The rhizobial Nod factor (NF) and type III secretion system (T3SS) are two key signaling pathways for establishing the legume-rhizobium symbiosis. However, whether NF signaling is involved in the nodulation of Robinia pseudoacacia and Mesorhizobium amorphae CCNWGS0123, and its symbiotic differences compared with T3SS signaling remain unclear. Therefore, to elucidate the function of NF signaling in nodulation, we mutated nodC in M. amorphae CCNWGS0123, which aborted NF synthesis. Compared with the plants inoculated with the wild type strain, the plants inoculated with the NF-deficient strain exhibited shorter shoots with etiolated leaves. These phenotypic characteristics were similar to those of the plants inoculated with the T3SS-deficient strain, which served as a Nod- (non-effective nodulation) control. The plants inoculated with both the NF- and T3SS-deficient strains formed massive root hair swellings, but no normal infection threads were detected. Sections of the nodules showed that inoculation with the NF- and T3SS-deficient strains induced small, white bumps without any rhizobia inside. Analyzing the accumulation of 6 plant hormones and the expression of 10 plant genes indicated that the NF- and T3SS-deficient strains activated plant defense reactions while suppressing plant symbiotic signaling during the perception and nodulation processes. The requirement for NF signaling appeared to be conserved in two other leguminous trees that can establish symbiosis with M. amorphae CCNWGS0123. In contrast, the function of the T3SS might differ among species, even within the same subfamily (Faboideae). Overall, this work demonstrated that nodulation of R. pseudoacacia and M. amorphae CCNWGS0123 was both NF and T3SS dependent.

Requirements of Qa-SNARE LjSYP132s for Nodulation and Seed Development in Lotus japonicus

SNAREs (soluble N-ethyl maleimide-sensitive factor attachment protein receptors) mediate membrane fusion of vesicle transport in eukaryotic cells. LjSYP132s are the members of Qa-SNAREs in Lotus japonicus. Two isoforms, LjSYP132a and LjSYP132b, are generated by alternative splicing. Immunoblot analysis detected strong expression of LjSYP132s in infected root nodules and seeds by posttranscriptional modification. In either LjSYP132a or LjSYP132b silenced roots (RNAi-LjSYP132a, RNAi-LjSYP132b), the infection thread (IT) was not elongated, suggesting that both LjSYP132a and LjSYP132b have a role in IT progression. The results were consistent with the data of qRT-PCR showing that both genes were expressed at the early stage of infection. However, during the nodulation, only LjSYP132a was induced. LjSYP132s protein was observed in the Mesorhizobium loti-inoculated roots of mutants, nfr1, castor and pollux, suggesting that LjSYP132s can be induced without Nod factor signaling. Accumulation of LjSYP132s in the peribacteroid membrane suggests the function of not only IT formation but also nutrient transport. In contrast, qRT-PCR showed that LjSYP132b was expressed in the seeds. A stable transgenic plant of LjSYP132b, R132b, was produced by RNAi silencing. In the R132b plants, small pods with a few seeds and abnormal tip growth of the pollen tubes were observed, suggesting that LjSYP132b has a role in pollen tube growth and nutrient transport in the plasma membrane of seeds.

Infection of Medicago truncatula by the Root-Knot Nematode Meloidogyne javanica Does Not Require Early Nodulation Genes.

Because of the developmental similarities between root nodules induced by symbiotic rhizobia and root galls formed by parasitic nematodes, we investigated the involvement of nodulation genes in the infection of Medicago truncatula by the root knot nematode (RKN), Meloidogyne javanica. We found that gall formation, including giant cell formation, pericycle and cortical cell division, as well as egg laying, occurred successfully in the non-nodulating mutants nfp1 (nod factor perception1), nin1 (nodule inception1) and nsp2 (nodulation signaling pathway2) and the cytokinin perception mutant cre1 (cytokinin receptor1). Gall and egg formation were significantly reduced in the ethylene insensitive, hypernodulating mutant skl (sickle), and to a lesser extent, in the low nodulation, abscisic acid insensitive mutant latd/nip (lateral root-organ defective/numerous infections and polyphenolics). Despite its supernodulation phenotype, the sunn4 (super numeric nodules4) mutant, which has lost the ability to autoregulate nodule numbers, did not form excessive numbers of galls. Co-inoculation of roots with nematodes and rhizobia significantly reduced nodule numbers compared to rhizobia-only inoculated roots, but only in the hypernodulation mutant skl. Thus, this effect is likely to be influenced by ethylene signaling, but is not likely explained by resource competition between galls and nodules. Co-inoculation with rhizobia also reduced gall numbers compared to nematode-only infected roots, but only in the wild type. Therefore, the protective effect of rhizobia on nematode infection does not clearly depend on nodule number or on Nod factor signaling. Our study demonstrates that early nodulation genes that are essential for successful nodule development are not necessary for nematode-induced gall formation, that gall formation is not under autoregulation of nodulation control, and that ethylene signaling plays a positive role in successful RKN parasitism in M. truncatula.

Mutational analysis indicates that abnormalities in rhizobial infection and subsequent plant cell and bacteroid differentiation in pea (Pisum sativum) nodules coincide with abnormal cytokinin responses and localization.

Recent findings indicate that Nod factor signalling is tightly interconnected with phytohormonal regulation that affects the development of nodules. Since the mechanisms of this interaction are still far from understood, here the distribution of cytokinin and auxin in pea (Pisum sativum) nodules was investigated. In addition, the effect of certain mutations blocking rhizobial infection and subsequent plant cell and bacteroid differentiation on cytokinin distribution in nodules was analysed. Patterns of cytokinin and auxin in pea nodules were profiled using both responsive genetic constructs and antibodies. In wild-type nodules, cytokinins were found in the meristem, infection zone and apical part of the nitrogen fixation zone, whereas auxin localization was restricted to the meristem and peripheral tissues. We found significantly altered cytokinin distribution in sym33 and sym40 pea mutants defective in IPD3/CYCLOPS and EFD transcription factors, respectively. In the sym33 mutants impaired in bacterial accommodation and subsequent nodule differentiation, cytokinin localization was mostly limited to the meristem. In addition, we found significantly decreased expression of LOG1 and A-type RR11 as well as KNOX3 and NIN genes in the sym33 mutants, which correlated with low cellular cytokinin levels. In the sym40 mutant, cytokinins were detected in the nodule infection zone but, in contrast to the wild type, they were absent in infection droplets. In conclusion, our findings suggest that enhanced cytokinin accumulation during the late stages of symbiosis development may be associated with bacterial penetration into the plant cells and subsequent plant cell and bacteroid differentiation.

The Medicago truncatula PIN2 auxin transporter mediates basipetal auxin transport but is not necessary for nodulation.

The development of root nodules leads to an increased auxin response in early nodule primordia, which is mediated by changes in acropetal auxin transport in some legumes. Here, we investigated the role of root basipetal auxin transport during nodulation. Rhizobia inoculation significantly increased basipetal auxin transport in both Medicago truncatula and Lotus japonicus. In M. truncatula, this increase was dependent on functional Nod factor signalling through NFP, NIN, and NSP2, as well as ethylene signalling through SKL. To test whether increased basipetal auxin transport is required for nodulation, we examined a loss-of-function mutant of the M. truncatula PIN2 gene. The Mtpin2 mutant exhibited a reduction in basipetal auxin transport and an agravitropic phenotype. Inoculation of Mtpin2 roots with rhizobia still led to a moderate increase in basipetal auxin transport, but the mutant nodulated normally. No clear differences in auxin response were observed during nodule development. Interestingly, inoculation of wild-type roots increased lateral root numbers, whereas inoculation of Mtpin2 mutants resulted in reduced lateral root numbers compared with uninoculated roots. We conclude that the MtPIN2 auxin transporter is involved in basipetal auxin transport, that its function is not essential for nodulation, but that it plays an important role in the control of lateral root development.

Matching population diversity of rhizobial nodA and legume NFR5 genes in plant-microbe symbiosis.

We hypothesized that population diversities of partners in nitrogen‐fixing rhizobium–legume symbiosis can be matched for “interplaying” genes. We tested this hypothesis using data on nucleotide polymorphism of symbiotic genes encoding two components of the plant–bacteria signaling system: (a) the rhizobial nodA acyltransferase involved in the fatty acid tail decoration of the Nod factor (signaling molecule); (b) the plant NFR5 receptor required for Nod factor binding. We collected three wild‐growing legume species together with soil samples adjacent to the roots from one large 25‐year fallow: Vicia sativa, Lathyrus pratensis, and Trifolium hybridum nodulated by one of the two Rhizobium leguminosarum biovars (viciae and trifolii). For each plant species, we prepared three pools for DNA extraction and further sequencing: the plant pool (30 plant indiv.), the nodule pool (90 nodules), and the soil pool (30 samples). We observed the following statistically significant conclusions: (a) a monotonic relationship between the diversity in the plant NFR5 gene pools and the nodule rhizobial nodA gene pools; (b) higher topological similarity of the NFR5 gene tree with the nodA gene tree of the nodule pool, than with the nodA gene tree of the soil pool. Both nonsynonymous diversity and Tajima's D were increased in the nodule pools compared with the soil pools, consistent with relaxation of negative selection and/or admixture of balancing selection. We propose that the observed genetic concordance between NFR5 gene pools and nodule nodA gene pools arises from the selection of particular genotypes of the nodA gene by the host plant.