The widely distributed legume tree Vachellia (Acacia) nilotica subsp. indica is nodulated by genetically diverse Ensifer strains in India

Vachellia (syn. Acacia) nilotica subsp. indica is a widespread legume tree in India of ecological and economic importance due to its ability to nodulate and fix nitrogen. The structure of V. nilotica nodules was examined and diversity of its rhizobial symbionts in various locations in India was assessed. More than a hundred strains were purified from indeterminate Vachellia-type root nodules on seedlings grown in alkaline soils from different regions of Thar Desert in Western Rajasthan and in neighbouring states (Gujarat, Haryana, Punjab). Plants grown in acidic soils of Meghalaya and Nagaland failed to nodulate. Twenty-one strains identified as species of Ensifer (Sinorhizobium) based on their 16S rRNA gene sequences and grouped into seven recA-types. Eight strains were characterized using two additional housekeeping (glnII, dnaK) and symbiotic (nodA, nifH) genes. A concatenated core gene phylogeny revealed that V. nilotica-Ensifer strains are novel and genetically diversified from E. saheli, E. arboris, E. terangae and E. americanus. Most are distinct from Ensifer strains previously reported in mimosoid, caesalpinioid and papilionoid legume species of W. Rajasthan. These Ensifer strains harboured typical mimosoid-type symbiotic genes that were diversified from E. arboris, E. kostiensis and E. terangae. They nodulated other mimosoids (Prosopis cineraria, Mimosa hamata, Senegalia senegal and Vachellia spp.), but failed to make symbiotic interactions with a papilionoid crop species, Vigna radiata, suggesting their specificity towards mimosoids. Vachellia nilotica is quite promiscuous, fixing nitrogen in symbiosis with diverse strains of Ensifer, and has successfully invaded various agro-climatic regions, thus indicating that it could be exploited for agroforestry projects.