Selection of Bradyrhizobium or Ensifer symbionts by the native Indian caesalpinioid legume Chamaecrista pumila depends on soil pH and other edaphic and climatic factors

Nodules of Chamaecrista pumila growing in several locations in India were sampled for anatomical studies and for characterization of their rhizobial microsymbionts. Regardless of their region of origin, the nodules were indeterminate with their bacteroids contained within symbiosomes which were surrounded by pectin. More than 150 strains were isolated from alkaline soils from the Thar Desert (Rajasthan), wet-acidic soils of Shillong (Meghalaya), and from trap experiments using soils from four other states with different agro-ecological regions. Molecular phylogenetic analysis based on five housekeeping (rrs, recA, glnII, dnaK andatpD) and two symbiotic (nodA and nifH) genes was performed for selected strains. Chamaecrista pumila was shown to be nodulated by niche-specific diverse strains of either Ensifer or Bradyrhizobium in alkaline (Thar Desert) to neutral (Tamil Nadu) soils and only Bradyrhizobium strains in acidic (Shillong) soils. Concatenated core gene phylogenies showed four novel Ensifer-MLSA types and nine Bradyrhizobium-MLSA types. Genetically diverse Ensifer strains harbored similar sym genes which were novel. In contrast, significant symbiotic diversity was observed in the Bradyrhizobium strains. The C. pumila strains cross-nodulated Vigna radiata and some wild papilionoid and mimosoid legumes. It is suggested that soil pH and moisture level played important roles in structuring the C. pumila microsymbiont community.