Next generation bioformulation prepared by amalgamating Bradyrhizobium, cell free culture supernatant, and exopolysaccharides enhances the indigenous rhizospheric rhizobial population, nodulation, and productivity of pigeon pea

Addition of living cells for plant growth promotion has several constraints including erratic and inconsistent field performance, poor shelf life, and reduced population due to tough competition with resident microbes. Due to the focus on employing cell-based formulation, bacterial cell free culture supernatant (CFCS), and metabolites produced by them, have been largely ignored. Hence, present study aimed to enhance productivity and nodulation of Cajanus cajan (pigeon pea) by using diverse formulations developed from Bradyrhizobium strain IC-4059 (cell based formulation), its CFCS (supernatant based), and exopolysaccharides (EPS) (metabolite based), blended formulation (cells with CFCS; cells with metabolite; metabolite with CFCS), and combinational formulation (IC-4059 + EPS + CFCS). Efficacy of developed formulations was monitored initially in tube assay, followed by field experiment with Pusa 991 pigeon pea cultivar in randomized block design. Growth promoting metabolites present in CFCS and EPS were analyzed by metabolomic approach using UPLC-MS and HPLC, respectively. In both tube assay and field experiment, seeds receiving treatment with combination of IC-4059 cells plus CFCS and EPS performed best, followed by blend IC-4059 CFCS + EPS, and IC-4059 cells + EPS, in terms of plant parameters. During field experiment seeds treated with combination showed significant enhancement in shoot length, root length, dry weight, protein content, and nodule number by 1.2, 1.3, 1.2, 1.1 and 2.0-fold, respectively, in comparison with rhizobia-based formulation. The prominent compounds identified by UPLC-MS were phosphatidylserine, phosphatidylcholine, FMC-5, pantoic acid, ascorbic acid, benzoic acid, 4-pentyl-4-formylphenyl ester, benzimidazole, and phosphatidylethanolamine. The study disseminates the use of combinational formulation developed from Bradyrhizobium sp. IC-4059, its CFCS, and its EPS for augmenting nodulation and growth of pigeon pea crop in field in comparison with application of rhizobial cell alone, as next generation of bioformulations.