Genome-based identification and plant growth promotion of a nitrogen-fixing strain isolated from soil

Abstract:

[Objective] The aim of this study is to obtain efficient nitrogen-fixing strains and fully utilize soil nitrogen-fixing microbial resources.[Methods] The Entisol with superior nitrogen fixation capability was selected to isolate nitrogen-fixing strains via enrichment and purification procedure. The identification of newly isolated strain at species level was performed by means of 16S rRNA gene phylogenetic analysis and whole genome correlation index comparison. Quantitative determination of nitrogen fixation activity of newly isolated strain was performed by acetylene reduction assay and ¹⁵N₂ tracer method. The plant growth promotion potentials of the nitrogen-fixing strain were preliminarily determined through culture characteristics and inoculation effects.[Results] A strain, designated P208, which could grow rapidly in nitrogen-free medium, was isolated from Entisol. Phylogenetic analyses based on 16S rRNA gene and 92 core genes revealed that strain P208 was most phylogenetically closely related to Azotobacter chroococcum IAM 12666^T (=ATCC 9043^T) (99.79% 16S rRNA gene similarities). The average nucleotide identity (ANI), average amino acid identity (AAI) and digital DNA-DNA hybridization (dDDH) values based on genome sequences of strain P208 and A. chroococcum ATCC 9043^T were higher than the thresholds for species circumscription (ANI>95%-96%, AAI>95%-96%, dDDH>70%), and the maximal unique matches index (MUMi) value was lower than the species circumscription threshold (<0.33). Therefore, The newly isolated strain P208 could be identified as A. chroococcum. The nitrogen fixation activity of A. chroococcum P208 was 2.61 times that of the type strain A. chroococcum ATCC 9043^T. In addition to nitrogen fixation, A. chroococcum P208 had plant growth promotion potentials in cultures, such as IAA production, phosphate solubilization and siderophore production. Inoculation with A. chroococcum P208 could promote root growth of rice and wheat seedlings under controlled conditions.[Conclusion]] A strain with superior nitrogen fixation and plant growth promotion potential was isolated and purified from the paddy soil with superior nitrogen fixation capability. It has relatively good development and application potential value, and could provide microbial resource for the study and utilization of biological nitrogen fixation.