Diversity in a promiscuous group of rhizobia from three Sesbania spp. colonizing ecologically distinct habitats of the semi-arid Delhi region.

Sesbania-rhizobia associations have immense significance in soil amelioration programs for diverse habitats. Diversity in symbiotic properties, LPS profiles, Sym plasmid and rhizobiophage sensitivity of 28 root- and stem-nodulating bacterial isolates of three Sesbania species (S. sesban, S. aegyptica and S. rostrata) inhabiting six ecologically distinct sites of semi-arid Delhi region was analyzed. The isolates were highly promiscuous among the symbiotic partners (Sesbania spp.). The root nodules formed by all the isolates were morphologically similar but they differed in their symbiotic efficiency and effectiveness. 16S rDNA sequence analyses revealed that root nodule isolates of sesbanias belong to diverse rhizobial taxa (Sinorhizobium saheli, S. meliloti, Rhizobium huautlense) whereas stem-nodule isolates were strictly Azorhizobium caulinodans. Sinorhizobium spp. seem to dominate as microsymbiont partner of Sesbania in the Delhi region. The genetic diversity revealed by cluster analyses based on NPC-PCR reflects sorting of isolates across the ecological gradient. Parallel diversity was also observed in the grouping based on LPS profiles and sym plasmid (NPC-PCR). Segregation of different rhizobial taxa into distinct types/clusters based on LPS and NPC-PCR analyses suggest its significance in the circumscription of the taxa. However, subtypes and subclusters showed their sorting across the ecological gradients. Sesbania rhizobia showed extremely high specificity to rhizobiophages. Enormous diversity in LPS profiles and high specificity of rhizobiophages might be the result of environmental selection pressures operating in ecologically distinct habitats. The ability of sesbanias to enter into effective symbioses with different rhizobial taxa and colonize diverse habitats with various biotic and abiotic stresses appears to contribute to its wide ecological amplitude.