World Journal of Microbiology and Biotechnology 9, 444-454 Special Topic Review Control of the expression of bacterial genes involved in symbiotic nitrogen fixation M. Megias,* J.L. Folch and C. Sousa Several genera of NJixing bacteria establish symbiotic associations with plants. Among these, the genus Rhizobiurn has the most signiticant contribution, in terms of yield, in many important crop plants. The establishment of the Rhizobiunrlegume symbiosis is a very complex process involving many genes which need to be co-ordinately regulated. In the lirst instance, plant signal molecules, known to be flavonoids, trigger the expression of host-specific genes in the bacterial partner through the action of the regulatory NodD protein. In response to these signak, RGzo6ium bacteria synthesize llpo-oligosaccharide molecules which in turn cause cell differentiation and nodule development. Once the nodule has formed, Rhizobium cells differentiate into bacteroids and another set of genes is activated. These genes, designated nif and fix, are responsible for N, fixation. In this system, several regulatory proteins are involved in a complex manner, the most important being NifA and a two component (FixK and FixL) regulatory system. Our knowledge about the establishment of these symbioses has advanced recently, although there are many questions yet to be solved. R$ WOT&: Bacteria, expression, genes, nitrogen fixation, symbiotic Bacteria-plant interactions range from the pathogenic, in which bacteria parasitize a plant, to the symbiotic, in which two organisms help each other to survive under otherwise difficult environmental conditions. Bacteria of the genus Rhizobium are able to interact symbiotically with specific legume host plants, leading to the formation of N,-fixing root nodules. Nodules are specialized organs, within which the rhizobia, differentiated into bacteroids, utilize plant- provided C compounds and reduce atmospheric N, to ammonia for its assimilation by the plant (Long 1989). In return, the bacteria obtain C sources (citric acid cycle intermediates like malate and succinate).As this association is normally of mutual benefit to both the invader (Rhizobium cells) and the invaded (plant host), it is called symbiosis. The incorporation of atmospheric N, into organic material which results from the Rhizobiurn-legume symbiosis is estimated to account for one third of the total N needed for world agriculture (Economou & Downie 1992). This unique intracellular association contributes significantly towards agricultural yield. The agronomic importance of this symbiotic association has thus justified extensive research M. Meglas, J. L. Folch and C. Sousa are with the Departamento de Microbiologla y Parasitologia, Univenidad de Sevilla, Apdo 674, 41012 Sevilla, Spain; fax: +34 64 626162. ‘Corresponding author. @I 7993 Rapid Communications of Oxford Ltd over the past 100 years and the association is probably one of the best known of all plant-microbe interactions (Economou & Downie 1992; Fischer & Long 1992). Nitrogen-fixing bacteria are taxonomically diverse and occupy various ecological niches. Some genera in the Rhizobiaceae (Rhizobium, Brudyrhizobitrmand Az.orhizobium) nodulate legumes whereas other members (Agrobacterium) are plant pathogens (Young & Johnston 1989). In an agricultural context, the most important association is that between members of the Leguminosae and Rhizobium, so not surprisingly this has received the most detailed study and will be the subject of this review. Each bacterial species is restricted to its specificgroup of host plants, thus defining ‘cross-inoculation groups’ in which the bacteria are classified according to their ability to establish symbiosis with a particular group of plants. Successful nodulation is a host-specific process in the sense that Pisum and Viciu species are host plants for R. leguminosarum biovar (bv.) viciae, Medicago and Melilotus are hosts for R. meliloti, and Trifolium spp. are hosts for R. legtlminosammbv. trifolii. Members of the genus Rhizobittm are relatively fast growing strains and have a narrow host range. On the other hand, the genus Bradyrhizobium contains a group of slow-growing rhizobia that generally nodulate quite a wide range of host plants.