A heterotrophic semisolid medium was used with two sensitive assay methods, C(2)H(2) reduction and O(2)-dependent tritium uptake, to determine nitrogenase and hydrogenase activities, respectively. Organisms known to be positive for both activities showed hydrogenase activity in both the presence and absence of 1% C(2)H(2), and thus, it was possible to test a single culture for both activities. ...

Members of the Leguminosae form the largest plant family on Earth, with around 18,000 species. The success of legumes can largely be attributed to their ability to form a nitrogen-fixing symbiosis with specific bacteria known as rhizobia, manifested by the development of nodules on the plant roots in which the bacteria fix atmospheric nitrogen, a major contributor to the global nitrogen cycle. ...

To improve their nutrition, most plants associate with soil microorganisms, particularly fungi, to form mycorrhizae. A few lineages, including actinorhizal plants and legumes are also able to interact with nitrogen-fixing bacteria hosted intracellularly inside root nodules. Fossil and molecular data suggest that the molecular mechanisms involved in these root nodule symbioses (RNS) have been pa...

Sinorhizobium meliloti exists either in a free-living state in the soil or in symbiosis within legume nodules, where the bacteria differentiate into nitrogen-fixing bacteroids. Expression of genes involved in nitrogen fixation and associated respiration is governed by two intermediate regulators, NifA and FixK, respectively, which are controlled by a two-component regulatory system FixLJ in res...

Legumes have evolved symbiotic interactions with rhizobial bacteria to efficiently utilize nitrogen. Recent progress in symbiosis has revealed several key components of host plants required for nitrogen-fixing nodule organogenesis, in which complicated metabolic and signaling pathways in the host plant are reprogrammed to generate nodules in the cortex upon perception of the rhizobial Nod facto...

We examined the genetic structure and symbiotic characteristics of Bradyrhizobium isolates recovered from four legume species (Lupinus albus [white lupine], Lupinus angustifolius [blue lupine], Ornithopus compressus [yellow serradella], and Macroptilium atropurpureum [sirato]) grown in an Oregon soil. We established that multilocus enzyme electrophoresis (MLEE) can provide insights into the gen...

cgMT1 is a metallothionein (MT)-like gene that was isolated from a cDNA library of young nitrogen-fixing nodules resulting from the symbiotic interaction between Frankia spp. and the actinorhizal tree Casuarina glauca. cgMT1 is highly transcribed in the lateral roots and nitrogen-fixing cells of actinorhizal nodules; it encodes a class I type 1 MT. To obtain insight into the function of cgMT1, ...

In legume nodules, symbiosomes containing endosymbiotic rhizobial bacteria act as temporary plant organelles that are responsible for nitrogen fixation, these bacteria develop mutual metabolic dependence with the host legume. In most legumes, the rhizobia infect post-mitotic cells that have lost their ability to divide, although in some nodules cells do maintain their mitotic capacity after inf...

Eukaryotes often form symbioses with microorganisms. Among these, associations between plants and nitrogen-fixing bacteria are responsible for the nitrogen input into various ecological niches. Plants of many different families have evolved the capacity to develop root or stem nodules with diverse genera of soil bacteria. Of these, symbioses between legumes and rhizobia (Azorhizobium, Bradyrhiz...

The occurrence in Azospirillum brasilense of genes that code for exopolysaccharide (EPS) synthesis was investigated through complementation studies of Rhizobium meliloti Exo- mutants. These mutants are deficient in the synthesis of the major acidic EPS of Rhizobium species and form empty, non-nitrogen-fixing root nodules on alfalfa (J. A. Leigh, E. R. Signer, and G. C. Walker, Proc. Natl. Acad....