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Microbiology and Molecular Biology Reviews, June 2002, p. 203-222, Vol. 66, No. 2
1092-2172/02/$04.00+0     DOI: 10.1128/MMBR.66.2.203-222.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Key Role of Bacterial NH4+ Metabolism in Rhizobium-Plant Symbiosis

Eduardo J. Patriarca,* Rosarita Tatè, and Maurizio Iaccarino

International Institute of Genetics and Biophysics, Consiglio Nazionale delle Ricerche, 80125 Naples, Italy

Symbiotic nitrogen fixation is carried out in specialized organs, the nodules, whose formation is induced on leguminous host plants by bacteria belonging to the family Rhizobiaceae. Nodule development is a complex multistep process, which requires continued interaction between the two partners and thus the exchange of different signals and metabolites. NH4+ is not only the primary product but also the main regulator of the symbiosis: either as ammonium and after conversion into organic compounds, it regulates most stages of the interaction, from the production of nodule inducers to the growth, function, and maintenance of nodules. This review examines the adaptation of bacterial NH4+ metabolism to the variable environment generated by the plant, which actively controls and restricts bacterial growth by affecting oxygen and nutrient availability, thereby allowing a proficient interaction and at the same time preventing parasitic invasion. We describe the regulatory circuitry responsible for the downregulation of bacterial genes involved in NH4+ assimilation occurring early during nodule invasion. This is a key and necessary step for the differentiation of N2-fixing bacteroids (the endocellular symbiotic form of rhizobia) and for the development of efficient nodules.

* Corresponding author. Mailing address: IIGB-CNR, Via G. Marconi 10, 80125 Naples, Italy. Phone: 0039081 725 7257. Fax: 0039081 725 7202. E-mail: patriarca{at}iigb.na.cnr.it.

Microbiology and Molecular Biology Reviews, June 2002, p. 203-222, Vol. 66, No. 2
1092-2172/02/$04.00+0     DOI: 10.1128/MMBR.66.2.203-222.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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