THE SESBANIA ROOT SYMBIONTS SINORHIZOBIUM-SAHELI AND S-TERANGA BV SESBANIE CAN FORM STEM NODULES ON SESBANIA-ROSTRATA, ALTHOUGH THEY ARE LESS ADAPTED TO STEM NODULATION THAN AZORHIZOBIUM-CAULINODANS

Sesbania species can establish symbiotic interactions with rhizobia fr om two taxonomically distant genera, including the Sesbania rostrata s tem-nodulating Azorhizobium sp. and Azorhizobium caulinodans and the n ewly described Sinorhizobium saheli and Sinorhizobium teranga by. sesb aniae, isolated from the roots of various Sesbania species, A collecti on of strains from both groups were analyzed for their symbiotic prope rties with different Sesbania species, S. saheli and S. teranga bv. se sbaniae strains were found to effectively stem nodulate Sesbania rostr ata, showing that stem nodulation is not restricted to Azorhizobium, S inorhizobia and azorhizobia, however, exhibited clear differences in o ther aspects of symbiosis, Unlike Azorhizobium, S. teranga bv. sesbani ae and S. saheli did not induce effective stem nodules on plants previ ously inoculated on the roots, although stem nodulation was arrested a t different stages. For Sesbania rostrata root nodulation, Sinorhizobi um appeared more sensitive than Azorhizobium to the presence of combin ed nitrogen, S. saheli and S. teranga bv. sesbaniae were effective sym bionts with all Sesbania species tested, while Azorhizobium strains fi xed nitrogen only in symbiosis with Sesbania rostrata. In a simple scr eening test, S. saheli and S. teranga bv. sesbaniae were incapable of asymbiotic nitrogenase activity. Thus, Azorhizobium can easily be dist inguished from Sinorhizobium among Sesbania symbionts on the basis of symbiotic and free-living nitrogen fixation, The ability of Azorhizobi um to overcome the systemic plant central appears to be a stem adaptat ion function, This last property, together with its host-specific symb iotic nitrogen fixation, makes Azorhizobium highly specialized for ste m nodulation of the aquatic legume Sesbania rostrata.