Legume NADPH Oxidases Have Crucial Roles at Different Stages of Nodulation

Defense and senescence interplay in legume nodules

Immunity and senescence play a crucial role in the functioning of the legume symbiotic nodules. The miss-regulation of one of these processes compromises the symbiosis leading to death of the endosymbiont and the arrest of the nodule functioning. The relationship between immunity and senescence has been extensively studied in plant organs where a synergistic response can be observed. However, the interplay between immunity and senescence in the symbiotic organ is poorly discussed in the literature and these phenomena are often mixed up. Recent studies revealed that the cooperation between immunity and senescence is not always observed in the nodule, suggesting complex interactions between these two processes within the symbiotic organ. Here, we discuss recent results on the interplay between immunity and senescence in the nodule and the specificities of this relationship during legume-rhizobium symbiosis.

PbRbohH/J mediates ROS generation to regulate the growth of pollen tube in pear

Respiratory burst oxidase homolog (Rboh) family genes play crucial functions in development and growth. However, comprehensive and systematic investigation of Rboh family members in Rosaceae and their specific functions during pear pollen development are still limited. In the study, 63 Rboh genes were identified from eight Rosaceae genomes (Malus domestica, Pyrus bretschneideri, Pyrus communis, Prunus persica, Rubus occidentalis, Fragaria vesca, Prunus mume and Prunus avium) and divided into seven main subfamilies (I-VII) according to phylogenetic and structural features. Different modes of gene duplication led to the expansion of Rboh family, with purifying selection playing a vital role in the evolution of Rboh genes. In addition, RNA sequencing and qRT-PCR results indicated that PbRbohH and PbRbohJ were specifically high-expressed in pear pollen. Subsequently, subcellular localization revealed that PbRbohH/J distributed at the plasma membrane. Furthermore, by pharmacological analysis and antisense oligodeoxynucleotide assay, PbRbohH/J were demonstrated to mediate the formation of reactive oxygen species (ROS) to manage pollen tube growth. In conclusion, our results provide useful insights into the functions, expression patterns, evolutionary history of the Rboh genes in pear and other Rosaceae species.

Dual RNA-Seq Analysis Pinpoints a Balanced Regulation between Symbiosis and Immunity in Medicago truncatula-Sinorhizobium meliloti Symbiotic Nodules

Legume-rhizobial symbiosis initiates the formation of root nodules, within which rhizobia reside and differentiate into bacteroids to convert nitrogen into ammonium, facilitating plant growth. This process raises a fundamental question: how is plant immunity modulated within nodules when exposed to a substantial number of foreign bacteria? In Medicago truncatula, a mutation in the NAD1 (Nodules with Activated Defense 1) gene exclusively results in the formation of necrotic nodules combined with activated immunity, underscoring the critical role of NAD1 in suppressing immunity within nodules. In this study, we employed a dual RNA-seq transcriptomic technology to comprehensively analyze gene expression from both hosts and symbionts in the nad1-1 mutant nodules at different developmental stages (6 dpi and 10 dpi). We identified 89 differentially expressed genes (DEGs) related to symbiotic nitrogen fixation and 89 DEGs from M. truncatula associated with immunity in the nad1-1 nodules. Concurrently, we identified 27 rhizobial DEGs in the fix and nif genes of Sinorhizobium meliloti. Furthermore, we identified 56 DEGs from S. meliloti that are related to stress responses to ROS and NO. Our analyses of nitrogen fixation-defective plant nad1-1 mutants with overactivated defenses suggest that the host employs plant immunity to regulate the substantial bacterial colonization in nodules. These findings shed light on the role of NAD1 in inhibiting the plant's immune response to maintain numerous rhizobial endosymbiosis in nodules.

TETRASPANIN 8-1 from Phaseolus vulgaris plays a key role during mutualistic interactions

Arbuscular mycorrhizal (AM) fungi and rhizobia form two of the most important plant-microbe associations for the assimilation of phosphorus (P) and nitrogen (N). Symbiont-derived signals are able to coordinate the infection process by triggering multiple responses in the plant root, such as calcium influxes and oscillations, increased reactive oxygen species (ROS), cytoskeletal rearrangements and altered gene expression. An examination was made of the role of tetraspanins, which are transmembrane proteins that self-organize into tetraspanin web regions, where they recruit specific proteins into platforms required for signal transduction, membrane fusion, cell trafficking, and ROS generation. In plant cells, tetraspanins are scaffolding proteins associated with root radial patterning, biotic and abiotic stress responses, cell fate determination, plasmodesmata and hormonal regulation. Some plant tetraspanins, such as Arabidopsis thaliana TETRASPANIN 8 and TETRASPANIN 9 (AtTET8 and AtTET9) are associated with exosomes during inter-kingdom communication. In this study, a homolog of AtTET8, PvTET8-1, in common bean (Phaseolus vulgaris L. var. Negro Jamapa) was examined in roots during interactions with Rhizobium tropici and Rhizophagus irregularis. The promoter of PvTET8-1 contained several cis-acting regulatory DNA elements potentially related to mutualistic interactions, and PvTET8-1 was transcriptionally activated during AM fungal and rhizobial associations. Silencing it decreased the size and number of nodules, nitrogen fixation, and mycorrhizal arbuscule formation, whereas overexpressing it increased the size and number of nodules, and mycorrhizal arbuscule formation but decreased nitrogen fixation. PvTET8-1 appears to be an important element in both of these mutualistic interactions, perhaps through its interaction with NADPH oxidase and the generation of ROS during the infection processes.

The Phaseolus vulgaris Receptor-Like Kinase PvFER1 and the Small Peptides PvRALF1 and PvRALF6 Regulate Nodule Number as a Function of Nitrate Availability

Legumes associate with Gram-negative soil bacteria called rhizobia, resulting in the formation of a nitrogen-fixing organ, the nodule. Nodules are an important sink for photosynthates for legumes, so these plants have developed a systemic regulation mechanism that controls their optimal number of nodules, the so-called autoregulation of nodulation (AON) pathway, to balance energy costs with the benefits of nitrogen fixation. In addition, soil nitrate inhibits nodulation in a dose-dependent manner, through systemic and local mechanisms. The CLE family of peptides and their receptors are key to tightly controlling these inhibitory responses. In the present study, a functional analysis revealed that PvFER1, PvRALF1, and PvRALF6 act as positive regulators of the nodule number in growth medium containing 0 mM of nitrate but as negative regulators in medium with 2 and 5 mM of nitrate. Furthermore, the effect on nodule number was found to be consistent with changes in the expression levels of genes associated with the AON pathway and with the nitrate-mediated regulation of nodulation (NRN). Collectively, these data suggest that PvFER1, PvRALF1, and PvRALF6 regulate the optimal number of nodules as a function of nitrate availability.

Macrophage migration inhibitory factor MtMIF3 prevents the premature aging of Medicago truncatula nodules

Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine involved in immune response in animals. However, the role of MIFs in plants such as Medicago truncatula, particularly in symbiotic nitrogen fixation, remains unclear. An investigation of M. truncatula-Sinorhizobium meliloti symbiosis revealed that MtMIF3 was mainly expressed in the nitrogen-fixing zone of the nodules. Silencing MtMIF3 using RNA interference (Ri) technology resulted in increased nodule numbers but higher levels of bacteroid degradation in the infected cells of the nitrogen-fixing zone, suggesting that premature aging was induced in MtMIF3-Ri nodules. In agreement with this conclusion, the activities of nitrogenase, superoxide dismutase and catalase were lower than those in controls, but cysteine proteinase activity was increased in nodulated roots at 28 days postinoculation. In contrast, the overexpression of MtMIF3 inhibited nodule senescence. MtMIF3 is localized in the plasma membrane, nucleus, and cytoplasm, where it interacts with methionine sulfoxide reductase B (MsrB), which is also localized in the chloroplasts of tobacco leaf cells. Taken together, these results suggest that MtMIF3 prevents premature nodule aging and protects against oxidation by interacting with MtMsrB.

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

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

Nod factor perception: an integrative view of molecular communication during legume symbiosis

Compatible interaction between rhizobial Nod factors and host receptors enables initial recognition and signaling events during legume-rhizobia symbiosis. Molecular communication is a new paradigm of information relay, which uses chemical signals or molecules as dialogues for communication and has been witnessed in prokaryotes, plants as well as in animal kingdom. Understanding this fascinating relay of signals between plants and rhizobia during the establishment of a synergistic relationship for biological nitrogen fixation represents one of the hotspots in plant biology research. Predominantly, their interaction is initiated by flavonoids exuding from plant roots, which provokes changes in the expression profile of rhizobial genes. Compatible interactions promote the secretion of Nod factors (NFs) from rhizobia, which are recognised by cognate host receptors. Perception of NFs by host receptors initiates the symbiosis and ultimately leads to the accommodation of rhizobia within root nodules via a series of mutual exchange of signals. This review elucidates the bacterial and plant perspectives during the early stages of symbiosis, explicitly emphasizing the significance of NFs and their cognate NF receptors.

Rhizobial HmuSpSym as a heme-binding factor is required for optimal symbiosis between Mesorhizobium amorphae CCNWGS0123 and Robinia pseudoacacia

Nitrogen-fixing root nodules are formed by symbiotic association of legume hosts with rhizobia in nitrogen-deprived soils. Successful symbiosis is regulated by signals from both legume hosts and their rhizobial partners. HmuS is a heme degrading factor widely distributed in bacteria, but little is known about the role of rhizobial hmuS in symbiosis with legumes. Here, we found that inactivation of hmuS_pSym in the symbiotic plasmid of Mesorhizobium amorphae CCNWGS0123 disrupted rhizobial infection, primordium formation, and nitrogen fixation in symbiosis with Robinia pseudoacacia. Although there was no difference in bacteroids differentiation, infected plant cells were shrunken and bacteroids were disintegrated in nodules of plants infected by the ΔhmuS_pSym mutant strain. The balance of defence reaction was also impaired in ΔhmuS_pSym strain-infected root nodules. hmuS_pSym was strongly expressed in the nitrogen-fixation zone of mature nodules. Furthermore, the HmuS_pSym protein could bind to heme but not degrade it. Inactivation of hmuS_pSym led to significantly decreased expression levels of oxygen-sensing related genes in nodules. In summary, hmuS_pSym of M. amorphae CCNWGS0123 plays an essential role in nodule development and maintenance of bacteroid survival within R. pseudoacacia cells, possibly through heme-binding in symbiosis.

An NADPH oxidase regulates carbon metabolism and the cell cycle during root nodule symbiosis in common bean (Phaseolus vulgaris)

BACKGROUND

Rhizobium-legume symbiosis is a specific, coordinated interaction that results in the formation of a root nodule, where biological nitrogen fixation occurs. NADPH oxidases, or Respiratory Burst Oxidase Homologs (RBOHs) in plants, are enzymes that generate superoxide (O2 •-). Superoxide produces other reactive oxygen species (ROS); these ROS regulate different stages of mutualistic interactions. For example, changes in ROS levels are thought to induce ROS scavenging, cell wall remodeling, and changes in phytohormone homeostasis during symbiotic interactions. In common bean (Phaseolus vulgaris), PvRbohB plays a key role in the early stages of nodulation.

RESULTS

In this study, to explore the role of PvRbohB in root nodule symbiosis, we analyzed transcriptomic data from the roots of common bean under control conditions (transgenic roots without construction) and roots with downregulated expression of PvRbohB (by RNA interference) non-inoculated and inoculated with R. tropici. Our results suggest that ROS produced by PvRBOHB play a central role in infection thread formation and nodule organogenesis through crosstalk with flavonoids, carbon metabolism, cell cycle regulation, and the plant hormones auxin and cytokinin during the early stages of this process.

CONCLUSIONS

Our findings provide important insight into the multiple roles of ROS in regulating rhizobia-legume symbiosis.

Characteristics and Research Progress of Legume Nodule Senescence

Delaying the nodule senescence of legume crops can prolong the time of nitrogen fixation and attenuate the lack of fertilizer in the later stage of legume crop cultivation, resulting in improved crop yield and reduced usage of nitrogen fertilizer. However, effective measures to delay the nodule senescence of legume crops in agriculture are relatively lacking. In the present review, we summarized the structural and physiological characteristics of nodule senescence, as well as the corresponding detection methods, providing technical support for the identification of nodule senescence phenotype. We then outlined the key genes currently known to be involved in the regulation of nodule senescence, offering the molecular genetic information for breeding varieties with delayed nodule senescence. In addition, we reviewed various abiotic factors affecting nodule senescence, providing a theoretical basis for the interaction between molecular genetics and abiotic factors in the regulation of nodule senescence. Finally, we briefly prospected research foci of nodule senescence in the future.

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.

Distinct signaling routes mediate intercellular and intracellular rhizobial infection in Lotus japonicus

Rhizobial infection of legume roots during the development of nitrogen-fixing root nodules can occur intracellularly, through plant-derived infection threads traversing cells, or intercellularly, via bacterial entry between epidermal plant cells. Although it is estimated that around 25% of all legume genera are intercellularly infected, the pathways and mechanisms supporting this process have remained virtually unexplored due to a lack of genetically amenable legumes that exhibit this form of infection. In this study, we report that the model legume Lotus japonicus is infected intercellularly by the IRBG74 strain, recently proposed to belong to the Agrobacterium clade of the Rhizobiaceae. We demonstrate that the resources available for L. japonicus enable insight into the genetic requirements and fine-tuning of the pathway governing intercellular infection in this species. Inoculation of L. japonicus mutants shows that Ethylene-responsive factor required for nodulation 1 (Ern1) and Leu-rich Repeat Receptor-Like Kinase (RinRK1) are dispensable for intercellular infection in contrast to intracellular infection. Other symbiotic genes, including nod factor receptor 5 (NFR5), symbiosis receptor-like kinase (SymRK), Ca2+/calmodulin dependent kinase (CCaMK), exopolysaccharide receptor 3 (Epr3), Cyclops, nodule inception (Nin), nodulation signaling pathway 1 (Nsp1), nodulation signaling pathway 2 (Nsp2), cystathionine-β-synthase (Cbs), and Vapyrin are equally important for both entry modes. Comparative RNAseq analysis of roots inoculated with IRBG74 revealed a distinctive transcriptome response compared with intracellular colonization. In particular, several cytokinin-related genes were differentially regulated. Corroborating this observation, cyp735A and ipt4 cytokinin biosynthesis mutants were significantly affected in their nodulation with IRBG74, whereas lhk1 cytokinin receptor mutants formed no nodules. These results indicate a differential requirement for cytokinin signaling during intercellular rhizobial entry and highlight distinct modalities of inter- and intracellular infection mechanisms in L. japonicus.

Potassium content diminishes in infected cells of Medicago truncatula nodules due to the mislocation of channels MtAKT1 and MtSKOR/GORK

Rhizobia establish a symbiotic relationship with legumes that results in the formation of root nodules, where bacteria encapsulated by a membrane of plant origin (symbiosomes), convert atmospheric nitrogen into ammonia. Nodules are more sensitive to ionic stresses than the host plant itself. We hypothesize that such a high vulnerability might be due to defects in ion balance in the infected tissue. Low temperature SEM (LTSEM) and X-ray microanalysis of Medicago truncatula nodules revealed a potassium (K⁺) decrease in symbiosomes and vacuoles during the life span of infected cells. To clarify K⁺ homeostasis in the nodule, we performed phylogenetic and gene expression analyses, and confocal and electron microscopy localization of two key plant Shaker K⁺ channels, AKT1 and SKOR/GORK. Phylogenetic analyses showed that the genome of some legume species, including the Medicago genus, contained one SKOR/GORK and one AKT1 gene copy, while other species contained more than one copy of each gene. Localization studies revealed mistargeting and partial depletion of both channels from the plasma membrane of M. truncatula mature nodule-infected cells that might compromise ion transport. We propose that root nodule-infected cells have defects in K⁺ balance due to mislocation of some plant ion channels, as compared with non-infected cells. The putative consequences are discussed.

Medicago truncatula Ferroportin2 mediates iron import into nodule symbiosomes

Iron is an essential cofactor for symbiotic nitrogen fixation, required by many of the enzymes involved, including signal transduction proteins, O₂ homeostasis systems, and nitrogenase itself. Consequently, host plants have developed a transport network to deliver essential iron to nitrogen-fixing nodule cells. Ferroportin family members in model legume Medicago truncatula were identified and their expression was determined. Yeast complementation assays, immunolocalization, characterization of a tnt1 insertional mutant line, and synchrotron-based X-ray fluorescence assays were carried out in the nodule-specific M. truncatula ferroportin Medicago truncatula nodule-specific gene Ferroportin2 (MtFPN2) is an iron-efflux protein. MtFPN2 is located in intracellular membranes in the nodule vasculature and in inner nodule tissues, as well as in the symbiosome membranes in the interzone and early-fixation zone of the nodules. Loss-of-function of MtFPN2 alters iron distribution and speciation in nodules, reducing nitrogenase activity and biomass production. Using promoters with different tissular activity to drive MtFPN2 expression in MtFPN2 mutants, we determined that expression in the inner nodule tissues is sufficient to restore the phenotype, while confining MtFPN2 expression to the vasculature did not improve the mutant phenotype. These data indicate that MtFPN2 plays a primary role in iron delivery to nitrogen-fixing bacteroids in M. truncatula nodules.

A Methionine Sulfoxide Reductase B Is Required for the Establishment of Astragalus sinicus-Mesorhizobium Symbiosis

Methionine sulfoxide reductase B (MsrB) is involved in oxidative stress or defense responses in plants. However, little is known about its role in legume-rhizobium symbiosis. In this study, an MsrB gene was identified from Astragalus sinicus and its function in symbiosis was characterized. AsMsrB was induced under phosphorus starvation and displayed different expression patterns under symbiotic and nonsymbiotic conditions. Hydrogen peroxide or methyl viologen treatment enhanced the transcript level of AsMsrB in roots and nodules. Subcellular localization showed that AsMsrB was localized in the cytoplasm of onion epidermal cells and co-localized with rhizobia in nodules. Plants with AsMsrB-RNAi hairy roots exhibited significant decreases in nodule number, nodule nitrogenase activity and fresh weight of the aerial part, as well as an abnormal nodule and symbiosome development. Statistical analysis of infection events showed that plants with AsMsrB-RNAi hairy roots had significant decreases in the number of root hair curling events, infection threads and nodule primordia compared with the control. The content of hydrogen peroxide increased in AsMsrB-RNAi roots but decreased in AsMsrB overexpression roots at the early stage of infection. The transcriptome analysis showed synergistic modulations of the expression of genes involved in reactive oxygen species generation and scavenging, defense and pathogenesis and early nodulation. In addition, a candidate protein interacting with AsMsrB was identified and confirmed by bimolecular fluorescence complementation. Taken together, our results indicate that AsMsrB plays an essential role in nodule development and symbiotic nitrogen fixation by affecting the redox homeostasis in roots and nodules.

Interaction between TaNOX7 and TaCDPK13 Contributes to Plant Fertility and Drought Tolerance by Regulating ROS Production

Reactive oxygen species (ROS) homeostasis is critical for both physiological processes and stress responses of plants. NADPH oxidases (NOXs) are the key producers of ROS in plants. However, their functions in ROS homeostasis and plant growth regulation in wheat (Triticum aestivum) are little investigated. Here, we cloned and characterized a NOX isoform TaNOX7 in wheat. Overexpression of TaNOX7 in rice led to enhanced root length, ROS production, drought tolerance as well as bigger panicles and higher yield but shorter growth period duration. Further results indicate that TaCDPK13, a member of calcium-dependent protein kinases (CDPKs), can directly interact with TaNOX7 and enhance ROS production in plants. These results demonstrate that TaNOX7 plays crucial roles in wheat development, fertility, and drought tolerance via interaction with TaCDPK13, which may act as an upstream regulator of TaNOX7 to regulate ROS production in wheat.

Genome-Wide Identification of the CrRLK1L Subfamily and Comparative Analysis of Its Role in the Legume-Rhizobia Symbiosis

The plant receptor-like-kinase subfamily CrRLK1L has been widely studied, and CrRLK1Ls have been described as crucial regulators in many processes in Arabidopsis thaliana (L.), Heynh. Little is known, however, about the functions of these proteins in other plant species, including potential roles in symbiotic nodulation. We performed a phylogenetic analysis of CrRLK1L subfamily receptors of 57 different plant species and identified 1050 CrRLK1L proteins, clustered into 11 clades. This analysis revealed that the CrRLK1L subfamily probably arose in plants during the transition from chlorophytes to embryophytes and has undergone several duplication events during its evolution. Among the CrRLK1Ls of legumes and A. thaliana, protein structure, gene structure, and expression patterns were highly conserved. Some legume CrRLK1L genes were active in nodules. A detailed analysis of eight nodule-expressed genes in Phaseolus vulgaris L. showed that these genes were differentially expressed in roots at different stages of the symbiotic process. These data suggest that CrRLK1Ls are both conserved and underwent diversification in a wide group of plants, and shed light on the roles of these genes in legume–rhizobia symbiosis.

NADPH Oxidases: The Vital Performers and Center Hubs during Plant Growth and Signaling

NADPH oxidases (NOXs), mostly known as respiratory burst oxidase homologs (RBOHs), are the key producers of reactive oxygen species (ROS) in plants. A lot of literature has addressed ROS signaling in plant development regulation and stress responses as well as on the enzyme's structure, evolution, function, regulation and associated mechanisms, manifesting the role of NOXs/RBOHs as the vital performers and center hubs during plant growth and signaling. This review focuses on recent advances of NOXs/RBOHs on cell growth, hormone interaction, calcium signaling, abiotic stress responses, and immunity. Several primary particles, including Ca2+, CDPKs, BIK1, ROPs/RACs, CERK, FER, ANX, SnRK and SIK1-mediated regulatory mechanisms, are fully summarized to illustrate the signaling behavior of NOXs/RBOHs and their sophisticated and dexterous crosstalks. Diverse expression and activation regulation models endow NOXs/RBOHs powerful and versatile functions in plants to maintain innate immune homeostasis and development integrity. NOXs/RBOHs and their related regulatory items are the ideal targets for crop improvement in both yield and quality during agricultural practices.

Nicotianamine Synthase 2 Is Required for Symbiotic Nitrogen Fixation in Medicago truncatula Nodules

Symbiotic nitrogen fixation carried out by the interaction between legumes and diazotrophic bacteria known as rhizobia requires relatively large levels of transition metals. These elements are cofactors of many key enzymes involved in this process. Metallic micronutrients are obtained from soil by the roots and directed to sink organs by the vasculature, in a process mediated by a number of metal transporters and small organic molecules that facilitate metal delivery in the plant fluids. Among the later, nicotianamine is one of the most important. Synthesized by nicotianamine synthases (NAS), this molecule forms metal complexes participating in intracellular metal homeostasis and long-distance metal trafficking. Here we characterized the NAS2 gene from model legume Medicago truncatula. MtNAS2 is located in the root vasculature and in all nodule tissues in the infection and fixation zones. Symbiotic nitrogen fixation requires of MtNAS2 function, as indicated by the loss of nitrogenase activity in the insertional mutant nas2-1, phenotype reverted by reintroduction of a wild-type copy of MtNAS2. This would result from the altered iron distribution in nas2-1 nodules shown with X-ray fluorescence. Moreover, iron speciation is also affected in these nodules. These data suggest a role of nicotianamine in iron delivery for symbiotic nitrogen fixation.

Molecular Weapons Contribute to Intracellular Rhizobia Accommodation Within Legume Host Cell

The interaction between legumes and bacteria of rhizobia type results in a beneficial symbiotic relationship characterized by the formation of new root organs, called nodules. Within these nodules the bacteria, released in plant cells, differentiate into bacteroids and fix atmospheric nitrogen through the nitrogenase activity. This mutualistic interaction has evolved sophisticated signaling networks to allow rhizobia entry, colonization, bacteroid differentiation and persistence in nodules. Nodule cysteine rich (NCR) peptides, reactive oxygen species (ROS), reactive nitrogen species (RNS), and toxin-antitoxin (TA) modules produced by the host plants or bacterial microsymbionts have a major role in the control of the symbiotic interaction. These molecules described as weapons in pathogenic interactions have evolved to participate to the intracellular bacteroid accommodation by escaping control of plant innate immunity and adapt the functioning of the nitrogen-fixation to environmental signalling cues.

Transcriptome analysis of the differential effect of the NADPH oxidase gene RbohB in Phaseolus vulgaris roots following Rhizobium tropici and Rhizophagus irregularis inoculation

BACKGROUND

Reactive oxygen species (ROS) are generated by NADPH oxidases known as respiratory burst oxidase homologs (RBOHs) in plants. ROS regulate various cellular processes, including the mutualistic interactions between legumes and nitrogen-fixing bacteria or arbuscular mycorrhizal (AM) fungi. Rboh is a multigene family comprising nine members (RbohA-I) in common bean (Phaseolus vulgaris). The RNA interference-mediated silencing of RbohB (PvRbohB-RNAi) in this species diminished its ROS production and greatly impaired nodulation. By contrast, the PvRbohB-RNAi transgenic roots showed early hyphal root colonization with enlarged fungal hypopodia; therefore, we proposed that PvRbohB positively regulates rhizobial infection (Rhizobium tropici) and inhibits AM colonization by Rhizophagus irregularis in P. vulgaris.

RESULTS

To corroborate this hypothesis, an RNA-Seq transcriptomic analysis was performed to identify the differentially expressed genes in the PvRbohB-RNAi roots inoculated with Rhizobium tropici or Rhizophagus irregularis. We found that, in the early stages, root nodule symbioses generated larger changes of the transcriptome than did AM symbioses in P. vulgaris. Genes related to ROS homeostasis and cell wall flexibility were markedly upregulated in the early stages of rhizobial colonization, but not during AM colonization. Compared with AM colonization, the rhizobia induced the expression of a greater number of genes encoding enzymes involved in the metabolism of auxins, cytokinins, and ethylene, which were typically repressed in the PvRbohB-RNAi roots.

CONCLUSIONS

Our research provides substantial insights into the genetic interaction networks in the early stages of rhizobia and AM symbioses with P. vulgaris, as well as the differential roles that RbohB plays in processes related to ROS scavenging, cell wall remodeling, and phytohormone homeostasis during nodulation and mycorrhization in this legume.

CRISPR/Cas9 knockout of leghemoglobin genes in Lotus japonicus uncovers their synergistic roles in symbiotic nitrogen fixation

Legume nodules contain high concentrations of leghemoglobins (Lbs) encoded by several genes. The reason for this multiplicity is unknown. CRISPR/Cas9 technology was used to generate stable mutants of the three Lbs of Lotus japonicus. The phenotypes were characterized at the physiological, biochemical and molecular levels. Nodules of the triple mutants were examined by electron microscopy and subjected to RNA-sequencing (RNA-seq) analysis. Complementation studies revealed that Lbs function synergistically to maintain optimal N₂ fixation. The nodules of the triple mutants overproduced superoxide radicals and hydrogen peroxide, which was probably linked to activation of NADPH oxidases and changes in superoxide dismutase isoforms expression. The mutant nodules showed major ultrastructural alterations, including vacuolization, accumulation of poly-β-hydroxybutyrate and disruption of mitochondria. RNA-seq of c. 20 000 genes revealed significant changes in expression of carbon and nitrogen metabolism genes, transcription factors, and proteinases. Lb-deficient nodules had c. 30-50-fold less heme but similar transcript levels of heme biosynthetic genes, suggesting a post-translational regulatory mechanism of heme synthesis. We conclude that Lbs act additively in nodules and that the lack of Lbs results in early nodule senescence. Our observations also provide insight into the reprogramming of the gene expression network associated with Lb deficiency, probably as a result of uncontrolled intracellular free O₂ concentration.

Genetic Diversity Linked to Haplotype Variation in the World Core Collection of Trifolium subterraneum for Boron Toxicity Tolerance Provides Valuable Markers for Pasture Breeding

In alkaline soils in arid and semi-arid areas toxic concentrations of the micronutrient boron (B) are problematic for many cereal and legume crops. Molecular markers have been developed for B toxicity in cereals and Medicago. There is a need for such tools in clovers-Trifolium. To this end, we undertook a genome-wide association study (GWAS) with a diversity panel of subterranean clover (Trifolium subterraneum L.), an established model pasture legume for genetic and genomic analyses for the genus. The panel comprised 124 T. subterraneum genotypes (97 core collection accessions and 27 Australian cultivars). Substantial and useful diversity in B toxicity tolerance was found in T. subterraneum. Such variation was continuously distributed and exhibited a high broad sense heritability H ² = 0.92. Among the subspecies of T. subterraneum, ssp. brachycalycinum was most susceptible to B toxicity (P < 0.05). From the GWAS, the most important discoveries were single-nucleotide polymorphisms (SNPs) located on Chr 1, 2, and 3, which mapped to haplotype blocks providing potential genes for a B toxicity tolerance assay and meriting further investigation. A SNP identified on Chr 1 aligned with Medicago truncatula respiratory burst oxidase-like protein (TSub_ g2235). This protein is known to respond to abiotic and biotic stimuli. The identification of these novel potential genes and their use to design markers for marker-assisted selection offer a pathway in pasture legumes to manage B toxicity by exploiting B tolerance.

Nitric oxide signaling, metabolism and toxicity in nitrogen-fixing symbiosis

Interactions between legumes and rhizobia lead to the establishment of a symbiotic relationship characterized by the formation of a new organ, the nodule, which facilitates the fixation of atmospheric nitrogen (N2) by nitrogenase through the creation of a hypoxic environment. Significant amounts of nitric oxide (NO) accumulate at different stages of nodule development, suggesting that NO performs specific signaling and/or metabolic functions during symbiosis. NO, which regulates nodule gene expression, accumulates to high levels in hypoxic nodules. NO accumulation is considered to assist energy metabolism within the hypoxic environment of the nodule via a phytoglobin-NO-mediated respiration process. NO is a potent inhibitor of the activity of nitrogenase and other plant and bacterial enzymes, acting as a developmental signal in the induction of nodule senescence. Hence, key questions concern the relative importance of the signaling and metabolic functions of NO versus its toxic action and how NO levels are regulated to be compatible with nitrogen fixation functions. This review analyses these paradoxical roles of NO at various stages of symbiosis, and highlights the role of plant phytoglobins and bacterial hemoproteins in the control of NO accumulation.

Differential tetraspanin genes expression and subcellular localization during mutualistic interactions in Phaseolus vulgaris

Arbuscular mycorrhizal fungi and rhizobia association with plants are two of the most successful plant-microbe associations that allow the assimilation of P and N by plants, respectively. These mutualistic interactions require a molecular dialogue, i.e., legume roots exude flavonoids or strigolactones which induce the Nod factors or Myc factors synthesis and secretion from the rhizobia or fungi, respectively. These Nod or Myc factors trigger several responses in the plant root, including calcium oscillations, and reactive oxygen species (ROS). Furthermore, superoxide and H₂O₂ have emerged as key components that regulate the transitions from proliferation to differentiation in the plant meristems. Similar to the root meristem, the nodule meristem accumulates superoxide and H₂O₂. Tetraspanins are transmembrane proteins that organize into tetraspanin web regions, where they recruit specific proteins into platforms required for signal transduction, membrane fusion, cell trafficking and ROS generation. Plant tetraspanins are scaffolding proteins associated with root radial patterning, biotic and abiotic stress responses, cell fate determination, and hormonal regulation and recently have been reported as a specific marker of exosomes in animal and plant cells and key players at the site of plant fungal infection. In this study, we conducted transcriptional profiling of the tetraspanin family in common bean (Phaseolus vulgaris L. var. Negro Jamapa) to determine the specific expression patterns and subcellular localization of tetraspanins during nodulation or under mycorrhizal association. Our results demonstrate that the tetraspanins are transcriptionally modulated during the mycorrhizal association, but are also expressed in the infection thread and nodule meristem development. Subcellular localization indicates that tetraspanins have a key role in vesicular trafficking, cell division, and root hair polar growth.

Genomic, molecular evolution, and expression analysis of NOX genes in soybean (Glycine max)

Reactive oxygen species (ROS) are versatile signaling molecules in sensing stresses and play critical roles in signaling and development. Plasma membrane NADPH oxidases (NOXs) are key producers of ROS, and play important roles in the regulation of plant-pathogen interactions. Here, we performed a comprehensive analysis of the NOX gene family in the soybean genome (Glycine max) and 17 NOX (GmNOX) genes were identified. Structural analysis revealed that the GmNOX proteins in soybean were as conserved as those in other plants. 8 duplicated gene pairs were formed by a Glycine-specific whole-genome duplication (WGD) event approximately 13 million years ago (Mya). The Ka/Ks ratios of GmNOX genes ranged from 0.04 to 0.28, suggesting that the GmNOX family had undergone purifying selection in soybean. Gene expression patterns showed different expression of these duplicate genes, suggesting that the GmNOXs were retained by substantial subfunctionalization during the soybean evolutionary processes. Subsequently, the expression of GmNOXs in response to drought and phytohormones were characterized via qPCR. Importantly, four GmNOXs showed strong expression in nodules, pointing to their probable involvement in nodulation. Thus, our results shed light on the evolutionary history of this family in soybean and contribute to the functional characterization of GmNOX genes in soybean.

LncRNA33732-respiratory burst oxidase module associated with WRKY1 in tomato- Phytophthora infestans interactions

Our previous studies indicated that tomato WRKY1 transcription factor acts as a positive regulator during tomato resistance to Phytophthora infestans. However, the molecular mechanism of WRKY1-mediated resistance regulation remains unclear. Here, we used a comparative transcriptome analysis between wild-type and WRKY1-overexpressing tomato plants to identify differentially expressed genes (DEGs) and long non-coding RNAs (DELs), and we examined long non-coding RNA (lncRNA)-gene networks. The promoter sequences of the upregulated DEGs and DELs were analyzed. Among 1073 DEGs and 199 DELs, 1 kb 5'-upstream regions of 59 DEGs and 22 DELs contain the W-box, the target sequence of the WRKY1. The results of promoter-β-glucuronidase (GUS) fusion and yeast one-hybrid assay showed that lncRNA33732 was activated by WRKY1 through sequence-specific interactions with the W-box element in its promoter. The overexpression and silencing analysis of lncRNA33732 in tomato showed that lncRNA33732 acts as a positive regulator and enhanced tomato resistance to P. infestans by induction of the expression of respiratory burst oxidase (RBOH) and increase in the accumulation of H₂ O₂ . When the expression of RBOH gene was inhibited in tomato plants, H₂ O₂ accumulation decreased and resistance were impaired. These findings suggest that lncRNA33732 activated by WRKY1 induces RBOH expression to increase H₂ O₂ accumulation in early defense reaction of tomato to P. infestans attack. Our results provide insights into the WRKY1-lncRNA33732-RBOH module involved in the regulation of H₂ O₂ accumulation and resistance to P. infestans, as well as provide candidates to enhance broad-spectrum resistance to pathogens in tomato.

Mutualistic co-evolution of T3SSs during the establishment of symbiotic relationships between Vigna radiata and Bradyrhizobia

This study supports the idea that the evolution of type III secretion system (T3SS) is one of the factors that controls Vigna radiata-bradyrhizobia symbiosis. Based on phylogenetic tree data and gene arrangements, it seems that the T3SSs of the Thai bradyrhizobial strains SUTN9-2, DOA1, and DOA9 and the Senegalese strain ORS3257 may share the same origin. Therefore, strains SUTN9-2, DOA1, DOA9, and ORS3257 may have evolved their T3SSs independently from other bradyrhizobia, depending on biological and/or geological events. For functional analyses, the rhcJ genes of ORS3257, SUTN9-2, DOA9, and USDA110 were disrupted. These mutations had cultivar-specific effects on nodulation properties. The T3SSs of ORS3257 and DOA9 showed negative effects on V. radiata nodulation, while the T3SS of SUTN9-2 showed no effect on V. radiata symbiosis. In the roots of V. radiata CN72, the expression levels of the PR1 gene after inoculation with ORS3257 and DOA9 were significantly higher than those after inoculation with ORS3257 ΩT3SS, DOA9 ΩT3SS, and SUTN9-2. The T3Es from ORS3257 and DOA9 could trigger PR1 expression, which ultimately leads to abort nodulation. In contrast, the T3E from SUTN9-2 reduced PR1 expression. It seems that the mutualistic relationship between SUTN9-2 and V. radiata may have led to the selection of the most well-adapted combination of T3SS and symbiotic bradyrhizobial genotype.

NADPH Oxidase (Rboh) Activity is Up Regulated during Sweet Pepper (Capsicum annuum L.) Fruit Ripening

In plants, NADPH oxidase (NOX) is also known as a respiratory burst oxidase homolog (Rboh). This highly important enzyme, one of the main enzymatic sources of superoxide radicals (O₂•⁻), is involved in the metabolism of reactive oxygen and nitrogen species (ROS and RNS), which is active in the non-climacteric pepper (Capsicum annuum L.) fruit. We used sweet pepper fruits at two ripening stages (green and red) to biochemically analyze the O₂•⁻-generating Rboh activity and the number of isozymes during this physiological process. Malondialdehyde (MDA) content, an oxidative stress marker, was also assayed as an index of lipid peroxidation. In red fruits, MDA was observed to increase 2-fold accompanied by a 5.3-fold increase in total Rboh activity. Using in-gel assays of Rboh activity, we identified a total of seven CaRboh isozymes (I–VII) which were differentially modulated during ripening. CaRboh-III and CaRboh-I were the most prominent isozymes in green and red fruits, respectively. An in vitro assay showed that CaRboh activity is inhibited in the presence of nitric oxide (NO) donors, peroxynitrite (ONOO⁻) and glutathione (GSH), suggesting that CaRboh can undergo S-nitrosation, Tyr-nitration, and glutathionylation, respectively. In summary, this study provides a basic biochemical characterization of CaRboh activity in pepper fruits and indicates that this O₂•⁻-generating Rboh is involved in nitro-oxidative stress associated with sweet pepper fruit ripening.

Suppression of innate immunity mediated by the CDPK-Rboh complex is required for rhizobial colonization in Medicago truncatula nodules

Suppression of innate immunity is essential for rhizobial infection and colonization in compatible interactions with leguminous plants. In Medicago nad1 mutant plants, innate immunity is excessively activated, resulting in necrotic cell death after rhizobia are released from infection threads into symbiotic cells, suggesting that innate immunity plays a critical role in regulating bacteroid persistence. In this study, we identified three respiratory burst oxidase homologs (Rboh) and one calcium-dependent protein kinase (CDPK) as key factors for the activation of immunity in Medicago nodules using genetic and biochemical methods. Knock-out of either MtRbohB or MtRbohD in nad1-1 mutant plants produced effective nodules with intact symbiotic cells, while knock-out of MtRbohC decreased brown pigment deposition, leading to less necrosis in nad1-1 mutant nodules. MtCDPK5 directly phosphorylated MtRbohB, MtRbohC and MtRbohD, which triggered immune responses in plants. Knock-out of MtCDPK5 in nad1-1 mutant plants partially restored nitrogen-fixing nodules. Overexpression of the constitutively activated variant MtCDPK5^VK under the control of the NAD1 promoter elicited strong immune responses, resulting in ineffective nodules in wild-type plants. Our data provide direct evidence that host plants utilize innate immunity to regulate rhizobial colonization in symbiotic cells in Medicago truncatula.

Chromium(VI) Toxicity in Legume Plants: Modulation Effects of Rhizobial Symbiosis

Most legume species have the ability to establish a symbiotic relationship with soil nitrogen-fixing rhizobacteria that promote plant growth and productivity. There is an increasing evidence of reactive oxygen species (ROS) important role in formation of legume-rhizobium symbiosis and nodule functioning. Environmental pollutants such as chromium compounds can cause damage to rhizobia, legumes, and their symbiosis. In plants, toxic effects of chromium(VI) compounds are associated with the increased production of ROS and oxidative stress development as well as with inhibition of pigment synthesis and modification of virtually all cellular components. These metabolic changes result in inhibition of seed germination and seedling development as well as reduction of plant biomass and crop yield. However, if plants establish symbiosis with rhizobia, heavy metals are accumulated preferentially in nodules decreasing the toxicity of metals to the host plant. This review summarizes data on toxic effects of chromium on legume plants and legume-rhizobium symbiosis. In addition, we discussed the role of oxidative stress in both chromium toxicity and formation of rhizobial symbiosis and use of nodule bacteria for minimizing toxic effects of chromium on plants.

NAD1 Controls Defense-Like Responses in Medicago truncatula Symbiotic Nitrogen Fixing Nodules Following Rhizobial Colonization in a BacA-Independent Manner

Legumes form endosymbiotic interaction with host compatible rhizobia, resulting in the development of nitrogen-fixing root nodules. Within symbiotic nodules, rhizobia are intracellularly accommodated in plant-derived membrane compartments, termed symbiosomes. In mature nodule, the massively colonized cells tolerate the existence of rhizobia without manifestation of visible defense responses, indicating the suppression of plant immunity in the nodule in the favur of the symbiotic partner. Medicago truncatulaDNF2 (defective in nitrogen fixation 2) and NAD1 (nodules with activated defense 1) genes are essential for the control of plant defense during the colonization of the nitrogen-fixing nodule and are required for bacteroid persistence. The previously identified nodule-specific NAD1 gene encodes a protein of unknown function. Herein, we present the analysis of novel NAD1 mutant alleles to better understand the function of NAD1 in the repression of immune responses in symbiotic nodules. By exploiting the advantage of plant double and rhizobial mutants defective in establishing nitrogen-fixing symbiotic interaction, we show that NAD1 functions following the release of rhizobia from the infection threads and colonization of nodule cells. The suppression of plant defense is self-dependent of the differentiation status of the rhizobia. The corresponding phenotype of nad1 and dnf2 mutants and the similarity in the induction of defense-associated genes in both mutants suggest that NAD1 and DNF2 operate close together in the same pathway controlling defense responses in symbiotic nodules.

Respiratory Burst Oxidase Homolog Gene A Is Crucial for Rhizobium Infection and Nodule Maturation and Function in Common Bean

Reactive oxygen species (ROS) produced by respiratory burst oxidase homologs (RBOHs) regulate numerous plant cell processes, including the symbiosis between legumes and nitrogen-fixing bacteria. Rapid and transient ROS production was reported after Phaseolus vulgaris root hairs were treated with Nod factors, indicating the presence of a ROS-associated molecular signature in the symbiosis signaling pathway. Rboh is a multigene family containing nine members (RbohA-I) in P. vulgaris. RNA interference of RbohB suppresses ROS production and attenuates rhizobial infection thread (IT) progression in P. vulgaris root hairs. However, the roles of other Rboh members in symbiotic interactions are largely unknown. In this study, we characterized the role of the NADPH oxidase-encoding gene RbohA (Phvulv091020621) in the P. vulgaris-Rhizobium tropici symbiosis. The spatiotemporal activity of the RbohA promoter colocalized with growing ITs and was associated with vascular bundles in developing nodules. Subcellular localization studies indicated that RBOHA was localized in the plasma membrane of P. vulgaris root hairs. After rhizobial inoculation, PvRBOHA was mainly distributed in the infection pocket and, to a lesser extent, throughout the IT. In PvRbohA RNAi lines, the rhizobial infection events were significantly reduced and, in successful infections, IT progression was arrested within the root hair, but did not impede cortical cell division. PvRbohA-RNAi nodules failed to fix nitrogen, since the infected cells in the few nodules formed were empty. RbohA-dependent ROS production and upregulation of several antioxidant enzymes was attenuated in rhizobia-inoculated PvRbohA-RNAi roots. These combined results indicate that PvRbohA is crucial for effective Rhizobium infection and its release into the nodule cells. This oxidase is partially or indirectly required to promote nodule organogenesis, altering the expression of auxin- and cyclin-related genes and genes involved in cell growth and division.

The Nitrate Transporter Family Protein LjNPF8.6 Controls the N-Fixing Nodule Activity

N-fixing nodules are new organs formed on legume roots as a result of the beneficial interaction with soil bacteria, rhizobia. The nodule functioning is still a poorly characterized step of the symbiotic interaction, as only a few of the genes induced in N-fixing nodules have been functionally characterized. We present here the characterization of a member of the Lotus japonicus nitrate transporter1/peptide transporter family, LjNPF8.6 The phenotypic characterization carried out in independent L. japonicus LORE1 insertion lines indicates a positive role of LjNPF8.6 on nodule functioning, as knockout mutants display N-fixation deficiency (25%) and increased nodular superoxide content. The partially compromised nodule functioning induces two striking phenotypes: anthocyanin accumulation already displayed 4 weeks after inoculation and shoot biomass deficiency, which is detected by long-term phenotyping. LjNPF8.6 achieves nitrate uptake in Xenopus laevis oocytes at both 0.5 and 30 mm external concentrations, and a possible role as a nitrate transporter in the control of N-fixing nodule activity is discussed.

The Role of Plant Innate Immunity in the Legume-Rhizobium Symbiosis

A classic view of the evolution of mutualism is that it derives from a pathogenic relationship that attenuated over time to a situation in which both partners can benefit. If this is the case for rhizobia, then one might uncover features of the symbiosis that reflect this earlier pathogenic state. For example, as with plant pathogens, it is now generally assumed that rhizobia actively suppress the host immune response to allow infection and symbiosis establishment. Likewise, the host has retained mechanisms to control the nutrient supply to the symbionts and the number of nodules so that they do not become too burdensome. The open question is whether such events are strictly ancillary to the central symbiotic nodulation factor signaling pathway or are essential for rhizobial host infection. Subsequent to these early infection events, plant immune responses can also be induced inside nodules and likely play a role in, for example, nodule senescence. Thus, a balanced regulation of innate immunity is likely required throughout rhizobial infection, symbiotic establishment, and maintenance. In this review, we discuss the significance of plant immune responses in the regulation of symbiotic associations with rhizobia, as well as rhizobial evasion of the host immune system.

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.

Migration of endophytic diazotroph Azorhizobium caulinodans ORS571 inside wheat (Triticum aestivum L) and its effect on microRNAs

Azorhizobium caulinodans ORS571, a novel rhizobium, forms endosymbionts with its nature host Sesbania rostrata, a semi-aquatic leguminous tree. Recent studies showed that A. caulinodans ORS571, as endophytic rhizobium, disseminated and colonized inside of cereal plants. However, how this rhizobium infects monocot plants and the regulatory mechanism remains unknown. MicroRNAs (miRNAs) are small, endogenous RNAs that regulate gene expression at the post-transcriptional levels. In this study, we employed laser scanning confocal microscope to monitor the pathway that rhizobium invade wheat; we also investigated the potential role of miRNAs during A. caulinodans ORS571 infecting wheat. Our results showed that gfp-labeled A. caulinodans ORS571 infected wheat root hairs and emerged lateral roots, then disseminated and colonized within roots and migrated to other plant tissues, such as stems and leaves. Endophytic rhizobium induced the aberrant expression of miRNAs in wheat with a tissue- and time-dependent manner with a peak at 12-24 h after rhizobium infection. Some miRNAs, such as miR167 and miR393 responded more in roots than that in shoots. In contrast, miR171 responded higher in shoots than that in roots. These results suggested that miRNAs could be responsive to A. caulinodans ORS571 infection and played important role in plant growth, nutrient metabolisms, and wheat-rhizobium interactions.