NrcR, a New Transcriptional Regulator of Rhizobium tropici CIAT 899 Involved in the Legume Root-Nodule Symbiosis.

The establishment of nitrogen-fixing rhizobium-legume symbioses requires a highly complex cascade of events. In this molecular dialogue the bacterial NodD transcriptional regulators in conjunction with plant inducers, mostly flavonoids, are responsible for the biosynthesis and secretion of Nod factors which are key molecules for successful nodulation. Other transcriptional regulators related to the symbiotic process have been identified in rhizobial genomes, including negative regulators such as NolR.

Rhizobial strains isolated from nodules of Medicago marina in southwest Spain are abiotic-stress tolerant and symbiotically diverse.

The isolation and characterisation of nitrogen-fixing root nodule bacteria from Medicago marina, a tolerant legume species, were studied in two areas from southwest Spain. A total of 30 out of 82 isolates with distinct ERIC-PCR fingerprints were analysed on the basis of molecular (PCR-RFLP of the 16S-23S rDNA intergenic spacer region (IGS) with two endonucleases, analysis of the 16S rDNA and symbiotic nodC gene sequences, plasmid profiles and SDS-PAGE of LPS, including the partial sequence of the housekeeping gene glnII and the symbiotic gene nodA of some representatives), physiological (utilisation of sole carbon sources, tolerance to antibiotics, NaCl, heavy metals, temperature and pH) and symbiotic parameters (efficacy on M. marina, M.

The symbiotic biofilm of Sinorhizobium fredii SMH12, necessary for successful colonization and symbiosis of Glycine max cv Osumi, is regulated by Quorum Sensing systems and inducing flavonoids via NodD1.

Bacterial surface components, especially exopolysaccharides, in combination with bacterial Quorum Sensing signals are crucial for the formation of biofilms in most species studied so far. Biofilm formation allows soil bacteria to colonize their surrounding habitat and survive common environmental stresses such as desiccation and nutrient limitation. This mode of life is often essential for survival in bacteria of the genera Mesorhizobium, Sinorhizobium, Bradyrhizobium, and Rhizobium.

Plant growth promotion in cereal and leguminous agricultural important plants: from microorganism capacities to crop production.

Plant growth-promoting rhizobacteria (PGPR) are free-living bacteria which actively colonize plant roots, exerting beneficial effects on plant development. The PGPR may (i) promote the plant growth either by using their own metabolism (solubilizing phosphates, producing hormones or fixing nitrogen) or directly affecting the plant metabolism (increasing the uptake of water and minerals), enhancing root development, increasing the enzymatic activity of the plant or "helping" other beneficial microorganisms to enhance their action on the plants; (ii) or may promote the plant growth by suppressing plant pathogens. These abilities are of great agriculture importance in terms of improving soil fertility and crop yield, thus reducing the negative impact of chemical fertilizers on the environment.

Rice and bean AHL-mimic quorum-sensing signals specifically interfere with the capacity to form biofilms by plant-associated bacteria.

Many bacteria regulate their gene expression in response to changes in their population density in a process called quorum sensing (QS), which involves communication between cells mediated by small diffusible signal molecules termed autoinducers. n-acyl-homoserine-lactones (AHLs) are the most common autoinducers in proteobacteria. QS-regulated genes are involved in complex interactions between bacteria of the same or different species and even with some eukaryotic organisms.