Molecular characterization of microsymbionts associated with root nodules of <em>Crotalaria burhia</em> Buch.-Ham. ex Benth., a native keystone legume species from Thar Desert of India

373-384Establishment of legume-rhizobia symbiosis has ample agronomic and ecological significance. Characterization of native rhizobia could enhance our understanding of their natural distribution and co-evolution. The Great Indian Thar Desert is an ecologically significant unique habitat with its flora and fauna. Crotalaria spp. is an economically important legume widely distributed in the Thar Desert and can be considered its one of the bioresources, particularly for biological nitrogen fixation with their symbiotic rhizobia. Here, we examined the legume Crotalaria burhia Buch.-Ham. ex Benth. in search of potential novel rhizobial species. Out of 72 root nodule bacterial (RNB) strains isolated from C. burhia, 51 rhizobia-like strains were examined for genetic diversity based on ARDRA and RAPD patterns. BLASTn sequence similarity results based on 16S rRNA gene of selective thirteen strains representing four ARDRA types revealed that they were related to genera Ensifer, Rhizobium and Bradyrhizobium. In 16S rRNA gene phylogeny, five (CB5, CB17, CB36, CB44, CB56) strains were closer to Ensifer kostiensis, three (CB6, CB12, CB32) to E. terangae and CB11 showed similarity with E. kostiensis and E. saheli. Strain CB4 was similar to Bradyrhizobium yuanmingense and three (CB29, CB31, CB46) strains were closer to species of Rhizobium (R. etli, R. sullae and R. borbori respectively). Symbiotic (nodA and nifH) genes phylogeny of Ensifer sp. CB56 was incongruent and showed close similarity with E. fredii whereas sym gene phylogeny of Bradyrhizobium sp. CB4 was congruent with 16S rRNA gene phylogeny. In Rhizobium strains sym genes could not be amplified and they failed to nodulate host. Our study suggests that C.burhia is nodulated by diverse strains of Ensifer and Bradyrhizobium in alkaline soil of Thar Desert and these strains effectively cross-nodulated crop Vigna radiata

An invasive Mimosa in India does not adopt the symbionts of its native relatives

BACKGROUND AND AIMS: The large monophyletic genus Mimosa comprises approx. 500 species, most of which are native to the New World, with Central Brazil being the main centre of radiation. All Brazilian Mimosa spp. so far examined are nodulated by rhizobia in the betaproteobacterial genus Burkholderia. Approximately 10 Mya, transoceanic dispersal resulted in the Indian subcontinent hosting up to six endemic Mimosa spp. The nodulation ability and rhizobial symbionts of two of these, M. hamata and M. himalayana, both from north-west India, are here examined, and compared with those of M. pudica, an invasive species. METHODS: Nodules were collected from several locations, and examined by light and electron microscopy. Rhizobia isolated from them were characterized in terms of their abilities to nodulate the three Mimosa hosts. The molecular phylogenetic relationships of the rhizobia were determined by analysis of 16S rRNA, nifH and nodA gene sequences. KEY RESULTS: Both native Indian Mimosa spp. nodulated effectively in their respective rhizosphere soils. Based on 16S rRNA, nifH and nodA sequences, their symbionts were identified as belonging to the alphaproteobacterial genus Ensifer, and were closest to the ‘Old World’ Ensifer saheli, E. kostiensis and E. arboris. In contrast, the invasive M. pudica was predominantly nodulated by Betaproteobacteria in the genera Cupriavidus and Burkholderia. All rhizobial strains tested effectively nodulated their original hosts, but the symbionts of the native species could not nodulate M. pudica. CONCLUSIONS: The native Mimosa spp. in India are not nodulated by the Burkholderia symbionts of their South American relatives, but by a unique group of alpha-rhizobial microsymbionts that are closely related to the ‘local’ Old World Ensifer symbionts of other mimosoid legumes in north-west India. They appear not to share symbionts with the invasive M. pudica, symbionts of which are mostly beta-rhizobial