scholarly journals Involvement of the Azorhizobial Chromosome Partition Gene (parA) in the Onset of Bacteroid Differentiation during Sesbania rostrata Stem Nodule Development

2011 ◽  
Vol 77 (13) ◽  
pp. 4371-4382 ◽  
Author(s):  
Chi-Te Liu ◽  
Kyung-Bum Lee ◽  
Yu-Sheng Wang ◽  
Min-Hua Peng ◽  
Kung-Ta Lee ◽  
...  
Keyword(s):  
Cell Shape ◽  
Deletion Mutant ◽  
Nodule Development ◽  
Sesbania Rostrata ◽  
Gene Transcript ◽  
Azorhizobium Caulinodans ◽  
Free Living ◽  
Content Type ◽  
Chromosome Partitioning ◽  
Cellular Development

ABSTRACTAparAgene in-frame deletion mutant ofAzorhizobium caulinodansORS571 (ORS571-ΔparA) was constructed to evaluate the roles of the chromosome-partitioning gene on various bacterial traits and on the development of stem-positioned nodules. The ΔparAmutant showed a pleiomorphic cell shape phenotype and was polyploid, with differences in nucleoid sizes due to dramatic defects in chromosome partitioning. Upon inoculation of the ΔparAmutant onto the stem ofSesbania rostrata, three types of immature nodule-like structures with impaired nitrogen-fixing activity were generated. Most showed signs of bacteroid early senescence. Moreover, the ΔparAcells within the nodule-like structures exhibited multiple developmental-stage phenotypes. Since thebacAgene has been considered an indicator for bacteroid formation, we applied the expression pattern ofbacAas a nodule maturity index in this study. Our data indicate that thebacAgene expression isparAdependent in symbiosis. The presence of theparAgene transcript was inversely correlated with the maturity of nodule; the transcript was switched off in fully mature bacteroids. In summary, our experimental evidence demonstrates that theparAgene not only plays crucial roles in cellular development when the microbe is free-living but also negatively regulates bacteroid formation inS. rostratastem nodules.

scholarly journals The Sinorhizobium melilotintrXGene Is Involved in Succinoglycan Production, Motility, and Symbiotic Nodulation on Alfalfa

2013 ◽  
Vol 79 (23) ◽  
pp. 7150-7159 ◽  
Author(s):  
Dong Wang ◽  
Haiying Xue ◽  
Yiwen Wang ◽  
Ruochun Yin ◽  
Fang Xie ◽  
...  
Keyword(s):  
Sinorhizobium Meliloti ◽  
Nitrogen Starvation ◽  
Response Regulator ◽  
Insertion Mutant ◽  
Sesbania Rostrata ◽  
Symbiotic Relationship ◽  
Azorhizobium Caulinodans ◽  
Free Living ◽  
Content Type ◽  
Regulatory Systems

ABSTRACTRhizobia establish a symbiotic relationship with their host legumes to induce the formation of nitrogen-fixing nodules. This process is regulated by many rhizobium regulators, including some two-component regulatory systems (TCSs). NtrY/NtrX, a TCS that was first identified inAzorhizobium caulinodans, is required for free-living nitrogen metabolism and symbiotic nodulation onSesbania rostrata. However, its functions in a typical rhizobium such asSinorhizobium melilotiremain unclear. Here we found that theS. melilotiresponse regulator NtrX but not the histidine kinase NtrY is involved in the regulation of exopolysaccharide production, motility, and symbiosis with alfalfa. A plasmid insertion mutant ofntrXformed mucous colonies, which overproduced succinoglycan, an exopolysaccharide, by upregulating its biosynthesis genes. This mutant also exhibited motility defects due to reduced flagella and decreased expression of flagellins and regulatory genes. The regulation is independent of the known regulatory systems of ExoR/ExoS/ChvI, EmmABC, and ExpR. Alfalfa plants inoculated with thentrXmutant were small and displayed symptoms of nitrogen starvation. Interestingly, the deletion mutant ofntrYshowed a phenotype similar to that of the parent strain. These findings demonstrate that theS. melilotiNtrX is a new regulator of succinoglycan production and motility that is not genetically coupled with NtrY.

scholarly journals CheY1 and CheY2 of Azorhizobium caulinodans ORS571 Regulate Chemotaxis and Competitive Colonization with the Host Plant

2020 ◽  
Vol 86 (15) ◽  
Author(s):  
Wei Liu ◽  
Xue Bai ◽  
Yan Li ◽  
Jun Min ◽  
Yachao Kong ◽  
...  
Keyword(s):  
Response Regulator ◽  
Bacterial Chemotaxis ◽  
Nodule Formation ◽  
Sesbania Rostrata ◽  
Azorhizobium Caulinodans ◽  
Response Regulators ◽  
Free Living ◽  
Content Type ◽  
Colonization Ability ◽  
Cell Phenotypes

ABSTRACT The genome of Azorhizobium caulinodans ORS571 encodes two chemotaxis response regulators: CheY1 and CheY2. cheY1 is located in a chemotaxis cluster (cheAWY1BR), while cheY2 is located 37 kb upstream of the cheAWY1BR cluster. To determine the contributions of CheY1 and CheY2, we compared the wild type (WT) and mutants in the free-living state and in symbiosis with the host Sesbania rostrata. Swim plate tests and capillary assays revealed that both CheY1 and CheY2 play roles in chemotaxis, with CheY2 having a more prominent role than CheY1. In an analysis of the swimming paths of free-swimming cells, the ΔcheY1 mutant exhibited decreased frequency of direction reversal, whereas the ΔcheY2 mutant appeared to change direction much more frequently than the WT. Exopolysaccharide (EPS) production in the ΔcheY1 and ΔcheY2 mutants was lower than that in the WT, but the ΔcheY2 mutant had more obvious EPS defects that were similar to those of the ΔcheY1 ΔcheY2 and Δeps1 mutants. During symbiosis, the levels of competitiveness for root colonization and nodule occupation of ΔcheY1 and ΔcheY2 mutants were impaired compared to those of the WT. Moreover, the competitive colonization ability of the ΔcheY2 mutant was severely impaired compared to that of the ΔcheY1 mutant. Taken together, the ΔcheY2 phenotypes are more severe than the ΔcheY1 phenotype in free-living and symbiotic states, and that of the double mutant resembles the ΔcheY2 single-mutant phenotype. These defects of ΔcheY1 and ΔcheY2 mutants were restored to the WT phenotype by complementation. These results suggest that there are different regulatory mechanisms of CheY1 and CheY2 and that CheY2 is a key chemotaxis regulator under free-living and symbiosis conditions. IMPORTANCE Azorhizobium caulinodans ORS571 is a motile soil bacterium that has the dual capacity to fix nitrogen both under free-living conditions and in symbiosis with Sesbania rostrata, forming nitrogen-fixing root and stem nodules. Bacterial chemotaxis to chemoattractants derived from host roots promotes infection and subsequent nodule formation by directing rhizobia to appropriate sites of infection. In this work, we identified and demonstrated that CheY2, a chemotactic response regulator encoded by a gene outside the chemotaxis cluster, is required for chemotaxis and multiple other cell phenotypes. CheY1, encoded by a gene in the chemotaxis cluster, also plays a role in chemotaxis. Two response regulators mediate bacterial chemotaxis and motility in different ways. This work extends the understanding of the role of multiple response regulators in Gram-negative bacteria.

scholarly journals A Chemotaxis Receptor Modulates Nodulation during the Azorhizobium caulinodans-Sesbania rostrata Symbiosis

2016 ◽  
Vol 82 (11) ◽  
pp. 3174-3184 ◽  
Author(s):  
Nan Jiang ◽  
Wei Liu ◽  
Yan Li ◽  
Hailong Wu ◽  
Zhenhai Zhang ◽  
...  
Keyword(s):  
Mutant Strain ◽  
Type Strain ◽  
Wild Type Strain ◽  
Bacterial Chemotaxis ◽  
Sesbania Rostrata ◽  
Nitrogen Fixing ◽  
Azorhizobium Caulinodans ◽  
Wild Type ◽  
Free Living ◽  
Content Type

ABSTRACTAzorhizobium caulinodansORS571 is a free-living nitrogen-fixing bacterium which can induce nitrogen-fixing nodules both on the root and the stem of its legume hostSesbania rostrata. This bacterium, which is an obligate aerobe that moves by means of a polar flagellum, possesses a single chemotaxis signal transduction pathway. The objective of this work was to examine the role that chemotaxis and aerotaxis play in the lifestyle of the bacterium in free-living and symbiotic conditions. In bacterial chemotaxis, chemoreceptors sense environmental changes and transmit this information to the chemotactic machinery to guide motile bacteria to preferred niches. Here, we characterized a chemoreceptor ofA. caulinodanscontaining an N-terminal PAS domain, named IcpB. IcpB is a soluble heme-binding protein that localized at the cell poles. AnicpBmutant strain was impaired in sensing oxygen gradients and in chemotaxis response to organic acids. Compared to the wild-type strain, theicpBmutant strain was also affected in the production of extracellular polysaccharides and impaired in flocculation. When inoculated alone, theicpBmutant induced nodules onS. rostrata, but the nodules formed were smaller and had reduced N2-fixing activity. TheicpBmutant failed to nodulate its host when inoculated competitively with the wild-type strain. Together, the results identify chemotaxis and sensing of oxygen by IcpB as key regulators of theA. caulinodans-S. rostratasymbiosis.IMPORTANCEBacterial chemotaxis has been implicated in the establishment of various plant-microbe associations, including that of rhizobial symbionts with their legume host. The exact signal(s) detected by the motile bacteria that guide them to their plant hosts remain poorly characterized.Azorhizobium caulinodansORS571 is a diazotroph that is a motile and chemotactic rhizobial symbiont ofSesbania rostrata, where it forms nitrogen-fixing nodules on both the roots and the stems of the legume host. We identify here a chemotaxis receptor sensing oxygen inA. caulinodansthat is critical for nodulation and nitrogen fixation on the stems and roots ofS. rostrata. These results identify oxygen sensing and chemotaxis as key regulators of theA. caulinodans-S. rostratasymbiosis.

scholarly journals Lon Protease of Azorhizobium caulinodans ORS571 Is Required for Suppression ofrebGene Expression

2012 ◽  
Vol 78 (17) ◽  
pp. 6251-6261 ◽  
Author(s):  
Azusa Nakajima ◽  
Toshihiro Aono ◽  
Shuhei Tsukada ◽  
Lowela Siarot ◽  
Tetsuhiro Ogawa ◽  
...  
Keyword(s):  
Nitrogen Fixation ◽  
Host Cells ◽  
Lon Protease ◽  
Bacterial Cells ◽  
Azorhizobium Caulinodans ◽  
Free Living ◽  
Content Type ◽  
Reverse Transcription Pcr ◽  
Nitrogen Fixation Activity ◽  
Fixation Activity

ABSTRACTBacterial Lon proteases play important roles in a variety of biological processes in addition to housekeeping functions. In this study, we focused on the Lon protease ofAzorhizobium caulinodans, which can fix nitrogen both during free-living growth and in stem nodules of the legumeSesbania rostrata. The nitrogen fixation activity of anA. caulinodanslonmutant in the free-living state was not significantly different from that of the wild-type strain. However, the stem nodules formed by thelonmutant showed little or no nitrogen fixation activity. By microscopic analyses, two kinds of host cells were observed in the stem nodules formed by thelonmutant. One type has shrunken host cells containing a high density of bacteria, and the other type has oval or elongated host cells containing a low density or no bacteria. This phenotype is similar to apraRmutant highly expressing therebgenes. Quantitative reverse transcription-PCR analyses revealed thatrebgenes were also highly expressed in thelonmutant. Furthermore, alon rebdouble mutant formed stem nodules showing higher nitrogen fixation activity than thelonmutant, and shrunken host cells were not observed in these stem nodules. These results suggest that Lon protease is required to suppress the expression of therebgenes and that high expression ofrebgenes in part causes aberrance in theA. caulinodans-S. rostratasymbiosis. In addition to the suppression ofrebgenes, it was found that Lon protease was involved in the regulation of exopolysaccharide production and autoagglutination of bacterial cells.

scholarly journals AcheZ-Like Gene inAzorhizobium caulinodansIs a Key Gene in the Control of Chemotaxis and Colonization of the Host Plant

2017 ◽  
Vol 84 (3) ◽  
Author(s):  
Xiaolin Liu ◽  
Wei Liu ◽  
Yu Sun ◽  
Chunlei Xia ◽  
Claudine Elmerich ◽  
...  
Keyword(s):  
Host Plant ◽  
Active Site ◽  
Deletion Mutant ◽  
Sinorhizobium Meliloti ◽  
Response Regulator ◽  
General Situation ◽  
Symbiotic Association ◽  
Azorhizobium Caulinodans ◽  
Content Type ◽  
In The Wild

ABSTRACTChemotaxis can provide bacteria with competitive advantages for survival in complex environments. The CheZ chemotaxis protein is a phosphatase, affecting the flagellar motor inEscherichia coliby dephosphorylating the response regulator phosphorylated CheY protein (CheY∼P) responsible for clockwise rotation. AcheZgene has been found inAzorhizobium caulinodansORS571, in contrast to other rhizobial species studied so far. The CheZ protein in strain ORS571 has a conserved motif similar to that corresponding to the phosphatase active site inE. coli. The construction of acheZdeletion mutant strain and ofcheZmutant strains carrying a mutation in residues of the putative phosphatase active site showed that strain ORS571 participates in chemotaxis and motility, causing a hyperreversal behavior. In addition, the properties of thecheZdeletion mutant revealed that ORS571 CheZ is involved in other physiological processes, since it displayed increased flocculation, biofilm formation, exopolysaccharide (EPS) production, and host root colonization. In particular, it was observed that the expression of severalexpgenes, involved in EPS synthesis, was upregulated in thecheZmutant compared to that in the wild type, suggesting that CheZ negatively controlsexpgene expression through an unknown mechanism. It is proposed that CheZ influences theAzorhizobium-plant association by negatively regulating early colonization via the regulation of EPS production. This report established that CheZ inA. caulinodansplays roles in chemotaxis and the symbiotic association with the host plant.IMPORTANCEChemotaxis allows bacteria to swim toward plant roots and is beneficial to the establishment of various plant-microbe associations. The level of CheY phosphorylation (CheY∼P) is central to the chemotaxis signal transduction. The mechanism of the signal termination of CheY∼P remains poorly characterized amongAlphaproteobacteria, except forSinorhizobium meliloti, which does not contain CheZ but which controls CheY∼P dephosphorylation through a phosphate sink mechanism.Azorhizobium caulinodansORS571, a microsymbiont ofSesbania rostrata, has an orphancheZgene besides twocheYgenes similar to those inS. meliloti. In addition to controlling the chemotaxis response, the CheZ-like protein in strain ORS571 is playing a role by decreasing bacterial adhesion to the host plant, in contrast to the general situation where chemotaxis-associated proteins promote adhesion. In this study, we identified a CheZ-like protein amongAlphaproteobacteriafunctioning in chemotaxis and theA. caulinodans-S. rostratasymbiosis.

scholarly journals A Novel Regulatory Pathway for K+ Uptake in the Legume Symbiont Azorhizobium caulinodans in Which TrkJ Represses the kdpFABC Operon at High Extracellular K+ Concentrations

2017 ◽  
Vol 83 (19) ◽  
Author(s):  
Lowela Siarot ◽  
Hiroki Toyazaki ◽  
Makoto Hidaka ◽  
Keigo Kurumisawa ◽  
Tomoki Hirakawa ◽  
...  
Keyword(s):  
Escherichia Coli ◽  
Uptake System ◽  
Azorhizobium Caulinodans ◽  
Free Living ◽  
Content Type ◽  
High K ◽  
Living State ◽  
In The Wild ◽  
Low K ◽  
Distinct Locus

ABSTRACT Bacteria have multiple K+ uptake systems. Escherichia coli, for example, has three types of K+ uptake systems, which include the low-K+-inducible KdpFABC system and two constitutive systems, Trk (TrkAG and TrkAH) and Kup. Azorhizobium caulinodans ORS571, a rhizobium that forms nitrogen-fixing nodules on the stems and roots of Sesbania rostrata, also has three types of K+ uptake systems. Through phylogenetic analysis, we found that A. caulinodans has two genes homologous to trkG and trkH, designated trkI and trkJ. We also found that trkI is adjacent to trkA in the genome and these two genes are transcribed as an operon; however, trkJ is present at a distinct locus. Our results demonstrated that trkAI, trkJ, and kup were expressed in the wild-type stem nodules, whereas kdpFABC was not. Interestingly, Δkup and Δkup ΔkdpA mutants formed Fix– nodules, while the Δkup ΔtrkA ΔtrkI ΔtrkJ mutant formed Fix+ nodules, suggesting that with the additional deletion of Trk system genes in the Δkup mutant, Fix+ nodule phenotypes were recovered. kdpFABC of the Δkup ΔtrkJ mutant was expressed in stem nodules, but not in the free-living state, under high-K+ conditions. However, kdpFABC of the Δkup ΔtrkA ΔtrkI ΔtrkJ mutant was highly expressed even under high-K+ conditions. The cytoplasmic K+ levels in the Δkup ΔtrkA ΔtrkI mutant, which did not express kdpFABC under high-K+ conditions, were markedly lower than those in the Δkup ΔtrkA ΔtrkI ΔtrkJ mutant. Taking all these results into consideration, we propose that TrkJ is involved in the repression of kdpFABC in response to high external K+ concentrations and that the TrkAI system is unable to function in stem nodules. IMPORTANCE K+ is a major cytoplasmic cation in prokaryotic and eukaryotic cells. Bacteria have multiple K+ uptake systems to control the cytoplasmic K+ levels. In many bacteria, the K+ uptake system KdpFABC is expressed under low-K+ conditions. For years, many researchers have argued over how bacteria sense K+ concentrations. Although KdpD of Escherichia coli is known to sense both cytoplasmic and extracellular K+ concentrations, the detailed mechanism of K+ sensing is still unclear. In this study, we propose that the transmembrane TrkJ protein of Azorhizobium caulinodans acts as a sensor for the extracellular K+ concentration and that high extracellular K+ concentrations repress the expression of KdpFABC via TrkJ.

scholarly journals Characterization of theSinorhizobium melilotiHslUV and ClpXP Protease Systems in Free-Living and Symbiotic States

2019 ◽  
Vol 201 (7) ◽  
Author(s):  
Aaron J. Ogden ◽  
Jacqueline M. McAleer ◽  
Michael L. Kahn
Keyword(s):  
Quality Control ◽  
Nitrogen Fixation ◽  
Medicago Sativa ◽  
Soil Bacteria ◽  
Symbiotic Nitrogen Fixation ◽  
Nodule Development ◽  
Stress Adaptation ◽  
Free Living ◽  
Content Type ◽  
Metabolic Remodeling

ABSTRACTSymbiotic nitrogen fixation (SNF) in the interaction between the soil bacteriaSinorhizobium melilotiand legume plantMedicago sativais carried out in specialized root organs called nodules. During nodule development, each symbiont must drastically alter their proteins, transcripts, and metabolites in order to support nitrogen fixation. Moreover, bacteria within the nodules are under stress, including challenges by plant antimicrobial peptides, low pH, limited oxygen availability, and strongly reducing conditions, all of which challenge proteome integrity.S. melilotistress adaptation, proteome remodeling, and quality control are controlled in part by the large oligomeric protease complexes HslUV and ClpXP1. To improve understanding of the roles ofS. melilotiHslUV and ClpXP1 under free-living conditions and in symbiosis withM. sativa, we generated ΔhslU, ΔhslV, ΔhslUV, and ΔclpP1knockout mutants. The shoot dry weight ofM. sativaplants inoculated with each deletion mutant was significantly reduced, suggesting a role in symbiosis. Further, slower free-living growth of the ΔhslUVand ΔclpP1mutants suggests that HslUV and ClpP1 were involved in adapting to heat stress, the while ΔhslUand ΔclpP1mutants were sensitive to kanamycin. All deletion mutants produced less exopolysaccharide and succinoglycan, as shown by replicate spot plating and calcofluor binding. We also generated endogenous C-terminal enhanced green fluorescent protein (eGFP) fusions to HslU, HslV, ClpX, and ClpP1 inS. meliloti. Using anti-eGFP antibodies, native coimmunoprecipitation experiments with proteins from free-living and nodule tissues were performed and analyzed by mass spectrometry. The results suggest that HslUV and ClpXP were closely associated with ribosomal and proteome quality control proteins, and they identified several novel putative protein-protein interactions.IMPORTANCESymbiotic nitrogen fixation (SNF) is the primary means by which biologically available nitrogen enters the biosphere, and it is therefore a critical component of the global nitrogen cycle and modern agriculture. SNF is the result of highly coordinated interactions between legume plants and soil bacteria collectively referred to as rhizobia, e.g.,Medicago sativaandS. meliloti, respectively. Accomplishing SNF requires significant proteome changes in both organisms to create a microaerobic environment suitable for high-level bacterial nitrogenase activity. The bacterial protease systems HslUV and ClpXP are important in proteome quality control, in metabolic remodeling, and in adapting to stress. This work shows thatS. melilotiHslUV and ClpXP are involved in SNF, in exopolysaccharide production, and in free-living stress adaptation.

scholarly journals Bradyrhizobium diazoefficiens Requires Chemical Chaperones To Cope with Osmotic Stress during Soybean Infection

mBio ◽  
2021 ◽  
Vol 12 (2) ◽  
Author(s):  
Raphael Ledermann ◽  
Barbara Emmenegger ◽  
Jean-Malo Couzigou ◽  
Nicola Zamboni ◽  
Patrick Kiefer ◽  
...  
Keyword(s):  
Stress Response ◽  
Environmental Changes ◽  
Compatible Solutes ◽  
Nodule Development ◽  
Free Living ◽  
Chemical Chaperone ◽  
Content Type ◽  
General Stress Response ◽  
General Stress ◽  
Host Infection

ABSTRACT When engaging in symbiosis with legume hosts, rhizobia are confronted with environmental changes, including nutrient availability and stress exposure. Genetic circuits allow responding to these environmental stimuli to optimize physiological adaptations during the switch from the free-living to the symbiotic life style. A pivotal regulatory system of the nitrogen-fixing soybean endosymbiont Bradyrhizobium diazoefficiens for efficient symbiosis is the general stress response (GSR), which relies on the alternative sigma factor σEcfG. However, the GSR-controlled process required for symbiosis has not been identified. Here, we demonstrate that biosynthesis of trehalose is under GSR control, and mutants lacking the respective biosynthetic genes otsA and/or otsB phenocopy GSR-deficient mutants under symbiotic and selected free-living stress conditions. The role of trehalose as a cytoplasmic chemical chaperone and stress protectant can be functionally replaced in an otsA or otsB mutant by introducing heterologous genetic pathways for biosynthesis of the chemically unrelated compatible solutes glycine betaine and (hydroxy)ectoine. Alternatively, uptake of exogenously provided trehalose also restores efficient symbiosis and tolerance to hyperosmotic and hyperionic stress of otsA mutants. Hence, elevated cytoplasmic trehalose levels resulting from GSR-controlled biosynthesis are crucial for B. diazoefficiens cells to overcome adverse conditions during early stages of host infection and ensure synchronization with root nodule development. IMPORTANCE The Bradyrhizobium-soybean symbiosis is of great agricultural significance and serves as a model system for fundamental research in bacterium-plant interactions. While detailed molecular insight is available about mutual recognition and early nodule organogenesis, our understanding of the host-imposed conditions and the physiology of infecting rhizobia during the transition from a free-living state in the rhizosphere to endosymbiotic bacteroids is currently limited. In this study, we show that the requirement of the rhizobial general stress response (GSR) during host infection is attributable to GSR-controlled biosynthesis of trehalose. Specifically, trehalose is crucial for an efficient symbiosis by acting as a chemical chaperone to protect rhizobia from osmostress during host infection.

scholarly journals FixJ family regulator AcfR of Azorhizobium caulinodans is involved in symbiosis with the host plant

2021 ◽  
Vol 21 (1) ◽  
Author(s):  
Wei Liu ◽  
Xue Bai ◽  
Yan Li ◽  
Haikun Zhang ◽  
Xiaoke Hu
Keyword(s):  
Signal Transduction ◽  
Host Plant ◽  
Type Strain ◽  
Wild Type Strain ◽  
Response Regulator ◽  
Azorhizobium Caulinodans ◽  
Wild Type ◽  
Response Regulators ◽  
Free Living ◽  
Two Component

Abstract Background A wide variety of bacterial adaptative responses to environmental conditions are mediated by signal transduction pathways. Two-component signal transduction systems are one of the predominant means used by bacteria to sense the signals of the host plant and adjust their interaction behaviour. A total of seven open reading frames have been identified as putative two-component response regulators in the gram-negative nitrogen-fixing bacteria Azorhizobium caulinodans ORS571. However, the biological functions of these response regulators in the symbiotic interactions between A. caulinodans ORS571 and the host plant Sesbania rostrata have not been elucidated to date. Results In this study, we identified and investigated a two-component response regulator, AcfR, with a phosphorylatable N-terminal REC (receiver) domain and a C-terminal HTH (helix-turn-helix) LuxR DNA-binding domain in A. caulinodans ORS571. Phylogenetic analysis showed that AcfR possessed close evolutionary relationships with NarL/FixJ family regulators. In addition, six histidine kinases containing HATPase_c and HisKA domains were predicted to interact with AcfR. Furthermore, the biological function of AcfR in free-living and symbiotic conditions was elucidated by comparing the wild-type strain and the ΔacfR mutant strain. In the free-living state, the cell motility behaviour and exopolysaccharide production of the ΔacfR mutant were significantly reduced compared to those of the wild-type strain. In the symbiotic state, the ΔacfR mutant showed a competitive nodule defect on the stems and roots of the host plant, suggesting that AcfR can provide A. caulinodans with an effective competitive ability for symbiotic nodulation. Conclusions Our results showed that AcfR, as a response regulator, regulates numerous phenotypes of A. caulinodans under the free-living conditions and in symbiosis with the host plant. The results of this study help to elucidate the involvement of a REC + HTH_LuxR two-component response regulator in the Rhizobium-host plant interaction.

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