A Method for Selection and Characterization of Rhizosphere-Competent Bacteria of Chickpea

Abstract.

A greenhouse assay was developed to evaluate the root-colonizing capability of the native chickpea rhizospheric bacterial population. In this assay system, screening time was reduced on two counts. First, spontaneous chromosomal rifampicin-resistant (Rif^r) strains were directly inoculated to seeds without any check for the stability of the mutation, and second, no attempts were made to taxonomically identify all the strains being screened for chickpea rhizosphere competence. Only two chickpea rhizosphere-competent Rif^r strains from the group of six good chickpea rhizosphere colonizers forming 10⁷ to 10⁸ colony-forming units (cfu)/g root were taxonomically identified as Pseudomonas fluorescens NB13R and Pseudomonas spp. NB49R, after screening 49 bacteria. Both the strains showed no difference from their corresponding wild-type strains P. fluorescens NB13 and Pseudomonas spp. NB49 in terms of chickpea rhizosphere competence. Isogenic or equally rhizospheric competitive second non-isogenic bacterial isolate, when present in tenfold higher amount, pre-empted the colonization of the soil by the bacterium, which was present in smaller ratio. These findings indicate that the isogenic or equally rhizospheric competitive second non-isogenic Rif^r strains should be compared for their survival and competition with that of the isogenic parent and with each other for specific ecological niche, before using a mixture of isolates, for stable and consistent biological seed treatment to control soilborn pathogens or pests or to promote plant growth.