Genome sequencing of Mesorhizobium Spp. NI-7, an efficient nitrogen-fixing microsymbiont of chickpea with potential to unravel the molecular mechanisms of symbiotic nitrogen fixation in legumes

Abstract

Root nodule symbiosis is a major pillar of sustainable agriculture. The newly formed symbiotic organ in the legume root harbours rhizobacteria, which can fix atmospheric nitrogen into a bioavailable and reduced form, ammonia. Previously, we reported the isolation of an efficient Mesorhizobium spp. NI-7, from the interior of chickpea nodules. Here, we report the draft genome sequence of the Mesorhizobium spp. NI-7 and the comparative genomics among different Mesorhizobium strains that have adopted symbiosis during chickpea domestication. The draft genome of Mesorhizobium spp. NI-7 consists of a single 4.28 Mbp chromosome and a 359 Kbp plasmid. The 16 S rDNA sequence based phylogenetic analysis highlighted that Mesorhizobium spp. NI-7 belongs to a diverse Mesorhizobium clade that evolved during the domestication of chickpea. Comparative genomics among several Mesorhizobium strains identified 2193 common orthologous groups and several unique orthologous groups among the different Mesorhizobium pairs. The draft genome contains the essential nitrogen fixation genes along with the genes required for the nutrient exchange from the plant to the symbiont. Additionally, part of the symbiotic NOD-factor operon and Type III secretion system were also detected in the Mesorhizobium spp. NI-7 draft genome. The comparative genomics among the Mesorhizobium strains identified a subset of rhizobial genes that would have evolved during chickpea-Mesorhizobium adaptation to the Indian sub-continent. These genes are unique targets that can be validated in the future to understand the chickpea and Mesorhizobium adaptation. In summary, the draft genome sequencing of Mesorhizobium spp. NI-7 will equip the plant-microbe community with a chickpea-compatible Mesorhizobium strain isolated from India, suitable for both fundamental and advanced research on nodulation in chickpea, as well as for promoting sustainable agriculture in a comprehensive manner.