Employing mutants to study the role of a three-membered bacterial consortium in growth promotion of Cajanus cajan

Abstract

Bacterial consortia, comprising plant growth promoting (PGP) rhizobacteria, are known to outcompete their impacts on plant attributes compared to their individual application. However, tracking of individual bacterial strains post application as consortium, remains challenging. The primary goal of this study was to develop an efficient method of tracking bioinoculants by generating spontaneous mutants of three different bacterial strains in an established consortium, using antibiotic-based screening, followed by their enumeration after application in Cajanus cajan. Mutants were generated for consortium members, viz. Azotobacter chroococcum (A), Priestia megaterium (formerly Bacillus megaterium) (B), and Pseudomonas sp. (P), against streptomycin, kanamycin and rifampicin, respectively. Those mutants having similar growth rates and PGP properties as compared to wild type bacterial strains were selected to test their efficacy in plant growth promotion. Selected mutant strains were applied as mono, dual and triple cultures to C. cajan grown hydroponically. Enumeration of mutant bacterial strains was carried out to check their viability. Bacterial colonization on roots was also analyzed. The application of triple (mutant) inoculants improved plant growth attributes significantly in comparison to mono and dual culture treatments and control. Cell enumeration revealed that the abundance of each bacterial strain increased till the 5th day of treatment. No significant change was observed later in their abundance for any treatment. The triple culture treatment showed greater abundance of bacterial mutant strains in comparison to mono- or dual cultures. To the best of our knowledge, this is the first mutant-based study to have reported the successful tracking and enumeration of bacterial consortium members, post their application in C. cajan.