The motility and chemosensory systems of Rhizobium leguminosarum, their role in symbiosis, and link to PTS regulation.

Motility and chemotaxis are crucial processes for soil bacteria and plant-microbe interactions. This applies to the symbiotic bacterium Rhizobium leguminosarum, where motility is driven by flagella rotation controlled by two chemotaxis systems, Che1 and Che2. The Che1 cluster is particularly important in free-living motility prior to the establishment of the symbiosis, with a che1 mutant delayed in nodulation and reduced in nodulation competitiveness.

NIN-like protein transcription factors regulate leghemoglobin genes in legume nodules.

Leghemoglobins enable the endosymbiotic fixation of molecular nitrogen (N) in legume nodules by channeling O for bacterial respiration while maintaining a micro-oxic environment to protect O-sensitive nitrogenase. We found that the NIN-like protein (NLP) transcription factors NLP2 and NIN directly activate the expression of leghemoglobins through a promoter motif, resembling a “double” version of the nitrate-responsive elements (NREs) targeted by other NLPs, that has conserved orientation and position across legumes. CRISPR knockout of the NRE-like element resulted in strongly decreased expression of the associated leghemoglobin.

Rhizobial Chemotaxis and Motility Systems at Work in the Soil.

Bacteria navigate their way often as individual cells through their chemical and biological environment in aqueous medium or across solid surfaces. They swim when starved or in response to physical and chemical stimuli. Flagella-driven chemotaxis in bacteria has emerged as a paradigm for both signal transduction and cellular decision-making.

Competition, Nodule Occupancy, and Persistence of Inoculant Strains: Key Factors in the -Legume Symbioses.

Biological nitrogen fixation by -legume symbioses represents an environmentally friendly and inexpensive alternative to the use of chemical nitrogen fertilizers in legume crops. Rhizobial inoculants, applied frequently as biofertilizers, play an important role in sustainable agriculture. However, inoculants often fail to compete for nodule occupancy against native rhizobia with inferior nitrogen-fixing abilities, resulting in low yields.

Regulation and Characterization of Mutants of in .

Symbiosis between and requires tight control of redox balance in order to maintain respiration under the microaerobic conditions required for nitrogenase while still producing the eight electrons and sixteen molecules of ATP needed for nitrogen fixation. FixABCX, a cluster of electron transfer flavoproteins essential for nitrogen fixation, is encoded on the Sym plasmid (pRL10), immediately upstream of , which encodes the general transcriptional regulator of nitrogen fixation. There is a symbiotically regulated NifA-dependent promoter upstream of (P), as well as an additional basal constitutive promoter driving background expression of (P).

Global control of bacterial nitrogen and carbon metabolism by a PTS-regulated switch.

The nitrogen-related phosphotransferase system (PTS) of bv. 3841 transfers phosphate from PEP via PtsP and NPr to two output regulators, ManX and PtsN. ManX controls central carbon metabolism via the tricarboxylic acid (TCA) cycle, while PtsN controls nitrogen uptake, exopolysaccharide production, and potassium homeostasis, each of which is critical for cellular adaptation and survival.

Optimizing legume symbioses by simultaneous measurement of rhizobial competitiveness and N fixation in nodules.

Legumes tend to be nodulated by competitive rhizobia that do not maximize nitrogen (N) fixation, resulting in suboptimal yields. Rhizobial nodulation competitiveness and effectiveness at N fixation are independent traits, making their measurement extremely time-consuming with low experimental throughput. To transform the experimental assessment of rhizobial competitiveness and effectiveness, we have used synthetic biology to develop reporter plasmids that allow simultaneous high-throughput measurement of N fixation in individual nodules using green fluorescent protein (GFP) and barcode strain identification (Plasmid ID) through next generation sequencing (NGS).

Formulation of a Highly Effective Inoculant for Common Bean Based on an Autochthonous Elite Strain of bv. , and Genomic-Based Insights Into Its Agronomic Performance.

Common bean is a poor symbiotic N-fixer, with a low response to inoculation owing to its promiscuous nodulation with competitive but inefficient resident rhizobia. Consequently, farmers prefer to fertilize them rather than rely on their capacity for Biological Nitrogen Fixation (BNF). However, when rhizobial inoculants are based on autochthonous strains, they often have superior BNF performance in the field due to their genetic adaptations to the local environment.

Genomic Diversity in the Endosymbiotic Bacterium Rhizobium leguminosarum.

bv. is a soil α-proteobacterium that establishes a diazotrophic symbiosis with different legumes of the tribe. The number of genome sequences from rhizobial strains available in public databases is constantly increasing, although complete, fully annotated genome structures from rhizobial genomes are scarce.

Understanding the holobiont: the interdependence of plants and their microbiome.

The holobiont is composed by the plant and its microbiome. In a similar way to ecological systems of higher organisms, the holobiont shows interdependent and complex dynamics [1,2]. While plants originate from seeds, the microbiome has a multitude of sources.

Construction of a marker system for the evaluation of competitiveness for legume nodulation in Rhizobium strains.

A marker system has been set up for the analysis of competitiveness of Rhizobium leguminosarum strains for legume nodulation. The strains generated incorporate gusA and celB marker genes at identical positions and allow efficient scoring of single and double infections. Based on this system, we have found that strain UPM791 outcompetes 3841 for nodulation in pea.

Endosymbiotic bacteria nodulating a new endemic lupine Lupinus mariae-josephi from alkaline soils in Eastern Spain represent a new lineage within the Bradyrhizobium genus.

Lupinus mariae-josephi is a recently described endemic Lupinus species from a small area in Eastern Spain where it thrives in soils with active lime and high pH. The L. mariae-josephi root symbionts were shown to be very slow-growing bacteria with different phenotypic and symbiotic characteristics from those of Bradyrhizobium strains nodulating other Lupinus.