Strategies of Salt Tolerance in the Rhizobia-Legume Symbiosis

Abstract

Rhizobium bacteria are soil microorganisms which are able to induce root nodules in leguminous plants. In the nodules, rhizobium functions as a nitrogen fi xing organel so that leguminous plants can grow without externally supplied fixed nitrogen. This gives legumes a selective advantage to non-leguminous plants in an environment without or with low concentrations of fi xed nitrogen. This property can also be used in agriculture to grow (leguminous) plants without nitrogen fertilizer. Two organisms, a prokaryote (rhizobium) and a eukaryote (leguminous plant) together form a completely new organ, the symbiotic root nodule, which means that they have to communicate with one another. A complex series of events, coordinated by host and bacterial signalling, underlie and lead to the development of this symbiotic interaction. Rhizobia respond to fl avonoids1Departamento de Microbiologia, Facultad de Biologia, Universidad de Sevilla. Avd. de Reina Mercedes 6, 41012 Sevilla, Spain. 2Sylvius Laboratory, Institute of Biology, Leiden University. Sylviusweg. 70, 2333 BE, Leiden, The Netherlands. 3Departamento de Microbiologia y Parasitologia, Facultad de Farmacia, Universidad de Sevilla, Profesor Garcia Gonzalez, 41012 Sevilla, Spain. *Corresponding author: megiasg@us.esexuded by the leguminous plant roots by secreting Nod factors, which are the key molecular signals to induce nodule morphogenesis and allow the bacteria to initiate the symbiosis with the host plant.