Nitrogen control in bacteria.

Genetic regulation of nitrogen fixation in rhizobia.

Initial stages in the Rhizobium–legume symbiosis can be thought of as a reciprocal molecular conversation: transmission of a gene inducer from legume host to bacterium, with ensuing bacterial synthesis of a morphogen that is transmitted to the plant, switching the developmental fate of the legume root. These signal molecules have a key role in determining bacterium–host specificity and the purified Nod factor compounds provide useful new tools to probe plant cell function.

Rhizobium-plant signal exchange

The Rhizobium-plant symbiosis.

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described.

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Detection of and Response to Signals Involved in Host-Microbe Interactions by Plant-Associated Bacteria

The presence of combined nitrogen in the soil suppresses the formation of nitrogen-fixing root nodules by Rhizobium. We demonstrate that bacterial genes determining early nodulation functions (nodABC) as well as the regulatory gene nodD3 are under nitrogen (NH4+) control. Our results suggest that the gene product of nodD3 has a role in mediating the ammonia regulation of early nod genes. The general nitrogen regulatory (ntr) system as well as a chromosomal locus mutated in Rhizobium meliloti were also found to be involved in the regulation of nod gene expression. A R. meliloti mutant with altered sensitivity to ammonia regulation was isolated, capable of more efficient nodulation of alfalfa than the wild-type strain in the presence of 2 mM ammonium sulfate.

The Rhizobium meliloti early nodulation genes (nodABC) are nitrogen-regulated: Isolation of a mutant strain with efficient nodulation capacity on alfalfa in the presence of ammonium

Agnes Bakos

Papers

The Rhizobium meliloti early nodulation genes (nodABC) are nitrogen-regulated: Isolation of a mutant strain with efficient nodulation capacity on alfalfa in the presence of ammonium

Availability and costs of medicines for the treatment of tuberculosis in Europe.