Prevalance of multifunctional Azospirillum formosense strains in the rhizosphere of pearl millet across diverse edaphoclimatic regions of India

The present study aimed at selecting indigenous Azospirillum spp. strains exhibiting plant growth promoting traits and tolerance to osmotic stress, for their potential application as bioinoculants in rainfed pearl millet. Seventy-five putative Azospirillum spp. isolates were retrieved from the rhizosphere of pearl millet cultivated under diverse edaphoclimatic conditions of India using differential media. The isolates exhibited nitrogenase activity (range between 0.2 and 200 nmoles ethylene produced h − 1 ), indole acetic acid production (range between 2.5 and 28.3 µg ml − 1 ), siderophore production and mannitol tolerance (up to 1000 mM). The 16 S rRNA gene sequence (amplified using universal primers 8 F and 1492R) analysis with reference to NCBI nucleotide database revealed that majority of the isolates obtained from different locations exhibited maximum identity (93.31–99.32%) to A. formosense , whereas two isolates AIM1 and AIM45 exhibited maximum identity to A. soli (98.54% identity) and A. oryza e (93.86% identity), respectively. However, the homology below 97% indicated the possibility of novel strains/species among the isolates. Selected strains were evaluated by designing a vertical agar plate assembly (VAPA) for their effect on root traits and growth of pearl millet (RHB173) seedlings. Bacterial inoculation significantly improved plant biomass (shoot and root) and influenced root architecture (root branching and root hair density). Microscopic observations revealed extensive colonization on the root surface as well as in the endorhizosphere. The VAPA assay helped in selecting promising strains AIM19, AIM57, AIM80, AIM38 and AIM3 which were able to influence plant growth and root architecture, the trait helpful in survival under stressful conditions. The present study indicated the abundance of multifunctional A. formosense strains in the rhizosphere of pearl millet grown under diverse locations. Also, the VAPA assay demonstrated in the present study that it can be used for the screening of large number of isolates for early stage plant growth promotion studies.