The native Indian mimosoid tree Prosopis cineraria shares diverse root nodulating rhizobia symbionts with exotic species of Neltuma (ex-Prosopis)

Root nodule microsymbionts of native Prosopis cineraria and three exotic species of Neltuma (formerly Prosopis), N. juliflora, N. alba and N. pallida, were trapped from soils of different agro-climatic regions of India to analyze preferences of host plants towards their microsymbionts.

Ability of Prosopis/Neltuma species to nodulate in alkaline to acidic soils was determined through trap experiments. Nodule anatomy was examined using light and transmission electron microscopy. Symbionts were genetically characterized using multi locus gene sequence-based phylogenies of core and symbiotic genes. Host range was confirmed through cross-inoculation experiments.

Nodules of Prosopis and Neltuma species are of the mimosoid type with genetically diverse strains of Ensifer/Sinorhizobium, Mesorhizobium, Bradyrhizobium and Rhizobium as their microsymbionts. Only P. cineraria nodulated in acidic soils of Meghalaya wherein it adopted Bradyrhizobium as symbionts. In addition to large variability based on core genes, substantial diversity was observed in the nodA genes harbored by Sinorhizobium with clear incidences of horizontal gene transfer; several Sinorhizobium strains harbored dominant and typical “Indian mimosoid clade” nodA genes. Sinorhizobium is the dominant symbiont of Prosopis/Neltuma species and cross nodulates related Indian native mimosoids such as Mimosa himalayana and species of Vachellia.

Overlaps were seen in symbionts isolated from the native and exotic species suggesting that these hosts are open and inclusive towards a wide range of symbionts irrespective of their origin. Most tree rhizobia were incompatible with herbaceous papilionoid legumes. Invasive mesquites are assisted in their invasiveness by their ability to nodulate with native rhizobia.